id
stringlengths 54
56
| text
stringlengths 0
1.34M
| source
stringclasses 1
value | added
stringdate 2025-03-18 00:34:10
2025-03-18 00:39:48
| created
stringlengths 3
51
⌀ | metadata
dict |
|---|---|---|---|---|---|
https://oercommons.org/courseware/lesson/78290/overview
|
French Level 2, Activity 08: Jeu de révision : Jeopardy / Jeopardy Review Game (Online)
Overview
Students will play charades and jeopardy to review
Activity Information
Did you know that you can access the complete collection of Pathways Project French activities in our new Let’s Chat! French pressbook? View the book here: https://boisestate.pressbooks.pub/pathwaysfrench
Please Note: Many of our activities were created by upper-division students at Boise State University and serve as a foundation that our community of practice can build upon and refine. While they are polished, we welcome and encourage collaboration from language instructors to help modify grammar, syntax, and content where needed. Kindly contact pathwaysproject@boisestate.edu with any suggestions and we will update the content in a timely manner.
Jeopardy Review / Jeu de révision
Description
In this activity students will be reviewing various grammar structures and vocabulary by playing a game of Jeopardy.
Semantic Topics
Review, Jeopardy, vocabulary, grammar, révision, vocabulaire, grammaire
Materials Needed
Would you like to make changes to the materials?
Access the template(s) below:
- -RE verb cards (Canva Template, free account required)
Warm-Up
Warm-Up
Students will play charades to review -RE verbs.
1. View the -RE verbs on Canva, if you need ideas and private message the verb to the student who will be doing the charade.
"Pour commencer, nous allons jouer à des charades. Quand ce sera votre tour, je vous enverrai en privé le verbe que vous allez mettre en scène." ("To start, we are going to play charades. When it is your turn, I will privately message you the verb that you are going to be acting out.")
2. Have students take turns playing out the verbs
Main Activity
Main Activity
*Note: If you'd like to customize this Jeopardy game, click "Edit." Once you've clicked edit, you will see an option that says, "If this isn't your template (or you forgot the password) then you can clone this template and edit the clone. Create a password for your clone below."
Un jeu de révision : Jeopardy
1. Today, we are going to play Jeopardy to review chapter six.
Aujourd'hui nous allons jouer Jeopardy pour revoir le chapitre six.
2. Split the group into two teams.
Nous allons diviser le groupe en deux équipes.
3. Choose a category and answer the question in your teams. The team with the most points wins.
Choisissez une catégorie et répondez à la question avec votre équipe. L'équipe avec le maximum de points, gagne!
Jeopardy Answers:
La vocabulaire:
- aliments aux fêtes: bière, vin, gâteaux, bonbons, dessert, champagne, la glace, biscuit
- vêtements: chemisier, gants, jean, jupe, manteau, pantalon, pull, sous-vêtement, chemise, chaussures, chaussettes, chapeau, écharpe,
Passé composé:
- Nous avons parlé à notre mère
- J’ai oublié mes devoirs
- Elle n’a pas acheté de biscuit hier
- Les filles ont beaucoup travaillé
- Tu n’as pas encore visité notre ville
Les objets indirects:
- Ophélie parle à sa mère
- Marc lui parle
- J’envoie des cadeaux à mes amis
- Ils vont me donner une cravate
- Notre prof ne nous a pas envoyé d’email
L’adjectif démonstratifs:
- ce
- cette
- cet
- ces
- cet
La culture:
- La fondation de la république française en 1789
- La France, le Canada, les États-unis (Nouvelle-Orléans), la Martinique, Haïti, la Guadeloupe
- Les défilés de musique, les masques, les costumes, les chars fleuris, la carême, Mardi gras
- Les fringues, look, vintage, BCBG, Ringard(e), être bien/mal sapé, être sur son 31
- L’hymne national français
Wrap-Up
Wrap-Up
End of Activity
- Can-Do statement check-in… “Where are we?”
- Read can-do statements and have students evaluate their confidence.
- Encourage students to be honest in their self evaluation
- Pay attention, and try to use feedback for future activities!
|
oercommons
|
2025-03-18T00:39:09.082611
|
Camille Daw
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/78290/overview",
"title": "French Level 2, Activity 08: Jeu de révision : Jeopardy / Jeopardy Review Game (Online)",
"author": "Mimi Fahnstrom"
}
|
https://oercommons.org/courseware/lesson/123331/overview
|
PAST, PRESENT AND FUTURE OF INFORMATION RETRIEVAL EXPERIMENTS
Overview
Information retrieval (IR) evaluation trials are essential for determining the accuracy and efficiency of IR models, algorithms, and systems. These tests aim to assess a system's ability to retrieve pertinent data in response to user enquiries. These studies, which frequently involve a test collection that includes documents, queries, and relevance judgements, are intended to determine how well a system works on a specific set of activities.
PAST, PRESENT AND FUTURE OF INFORMATION RETRIEVAL EXPERIMENTS
History of IR Experiments
Since the early days of computerization, when the possibility of automating document indexing and retrieval was first recognized, information retrieval has existed as a field for fifty years. In fact, the field’s first research was conducted without the aid of computers. The discipline has had a strong experimental heritage from its inception. One of the things that sets information retrieval apart from its more theoretically orientated father, information science, is its emphasis on empirical validation and evaluation. As old as information retrieval itself is the history of information retrieval evaluation, which is the focus of this thesis. One of the discipline’s advantages had been its long history of experimentation. But one could argue that an overabundance of empiricism has limited the field’s scope. In the meantime, current experimental approaches face significant problems due to the growth of the web and the significance of web search engines.
Types of IR Experiments
There are five types of IR experiments, these are:
- The Cranfield Tests (Cranfield 1 and 2)
- Smart Retrieval experiments
- MEDLARS Test
- The STAIRS project
- TREC Experiment: The Text Retrieval Conference
The Cranfield Tests:
Cranfield Test 1
The Cranfield 1 study, led by C. W. Cleverdon, was the first comprehensive assessment of information retrieval systems conducted in Cranfield, UK. Cleverdon's 1962 report on the first Cranfield Study, which started in 1957.
Parameters of the System
18,000 indexed items and 1200 search topics were used in the investigation. The documents were selected evenly from the general public, with half being research reports and the other half being magazine pieces. Field of high-speed aerodynamics, which is a subfield of aeronautics.
Three indexes were selected: one with prior indexing experience, one with subject understanding, and one straight out of library school with no prior indexing or subject knowledge.
Each indexer was instructed to spend 2, 4, 8, 12, and 16 minutes indexing each source page five times. Thus, a set of 6000 indexed items was created from 100 source documents (100 documents X 3 indexers X 4 systems X 5 times). The system operated on a total of 18,000 (6000 X 3 phases) indexed items since each of these 6000 items was examined in three different locations. In order to determine whether the level of performance rose as system personnel's expertise increased, the test was divided into three parts.
Significance
In many respects, the Cranfield 1 test results ran counter to popular wisdom about the nature of information retrieval systems. The test demonstrated that an indexer’s experience and subject-matter background have no bearing on a system's performance. It demonstrated that systems that arrange documents according to a faceted classification scheme perform worse than the uniterm and alphabetical index systems. It established for the first time the approaches that could be used effectively in assessing information retrieval systems and identified the key elements that influence their performance. Furthermore, it demonstrated the negative relationship between recall and precision, which are the two most crucial factors in assessing the effectiveness of information retrieval systems.
Significant results from Cranfield’s study include the following:
- Non-technical indexers could produce high-quality indexing;
- Indexing times longer than four minutes did not significantly improve performance.
- The system’s recall and precision rates were 70–90% and 8–20%, respectively.
- A 3% decrease in recall could result from a 1% increase in precision.
- All four indexing techniques provided performance that was largely comparable; recall and precision were inversely correlated.
Methodology
- Several individuals from various organizations were asked to choose documents from the collection and, in each instance, to formulate a question that the document would address.
- The project made use of pre-made enquiries that were created prior to the start of the real search. In all, 400 queries were created, and the system handled each one in its three stages. As a result, the system processed 1200 search requests in total.
- The indexers were given the questions.
Results
- With a recall percentage ranging from 60% to 90% and an overall average of 80%, all four systems were functioning effectively.
- The following were the average recall ratios for the various systems:
- 81.5% of Index alphabetically
- 74% of faceted classification
- 76% of the UDC Scheme
- 82% of Uniterm Indexing
- The faceted classification scheme's recall factor then raised to 83% after the facet sequence was changed. The chance of recall increased as indexing time increased. The following were the recall ratios for various timings:
Times (in minutes) Recall (%)
2 73
4 80
8 74
12 83
16 84
It was challenging to explain the apparent decline in efficiency at the 8 minutes level. Cleverdon himself is unable to provide an explanation.
- The retrieval of documents indexed by the three distinct indexers did not differ significantly. Stated differently, there was no discernible variation in the three indexers' performances.
- It was found that retrieving papers in general aeronautics domains had success rates that were 4-5% higher than those in specialized fields like high-speed aerodynamics.
- The third group’s 6000 item success rate was 3-4% higher than the second group's, indicating that the third group’s papers were better indexed. In other words, despite lacking subject knowledge, skilled indexers without prior indexing experience were able to consistently produce high-quality indexing work.
Cranfield Test 2
The shortcomings of Cranfield 1 made additional research necessary. Cranfield 2 the second phase of the Cranfield studies started in 1963 and ended in 1966.The Cranfield 2 was a controlled experiment designed to look into the elements of index languages and how they affect retrieval system performance. The impact of the different index language devices on a retrieval system's recall and precision was assessed in Cranfield 2. In order to evaluate the impact, this study varied each component while holding the others constant. By introducing real-world scenarios and enabling feedback mechanisms between indexers and users, Cranfield 2 addressed some of the shortcomings of Cranfield I.
Scope
The Cranfield 2 test was created for information retrieval research, specifically to assess how well IR systems can employ user queries to extract pertinent information from a vast collection of documents. Its scope consists of:
- It examines a number of retrieval models, including probabilistic, vector space, and Boolean search models.
- A collection of technical papers, journals, or abstracts that are pertinent to a certain field in Cranfield's instance, the military or aeronautics is known as a document collection.
- A collection of search terms that are used to ask the document collection for information.
Methodology
The following crucial steps are part of the Cranfield 2 test methodology:
- Documents from a predetermined corpus are used. It usually contains 1,400 items (research papers, articles, etc.) for Cranfield 2.
- The retrieval system is tested using a set of 75 queries. These queries are examples of common search terms that a user might use to find specific information in the corpus of documents.
- The relevancy of the documents for each inquiry is assessed by human assessors. A document's relevance to a particular query is indicated by its marking. Though occasionally a graded scale is employed, this assessment is usually binary (relevant or not relevant).
- The responsibility of retrieving materials in answer to each inquiry falls to the information retrieval system (the testing subject).
- Performance metrics are calculated by comparing the recovered documents to the relevance judgements.
- To evaluate the retrieval system's performance, the results are examined. It is possible to compare several models or methods.
Results
The Cranfield 2 results were rather surprising because, in addition to confirming the inverse relationships between recall and precision, they displayed that:
- Using a natural language single term index, like Uniterm, based on words found in document texts, produced the best performance result;
- Efficiency decreased as a result of the natural language construction of term classes or groups that went beyond the stage of actual synonyms or word forms;
- The basic accuracy of coordination was more effective than the use of precision devices like partitioning or intermixing;
- It was proposed that removing synonyms is beneficial and that terms extracted from texts might be used effectively in a post-coordinate index with little control. However, any attempts to regulate the vocabulary are likely to make it less effective;
- When ideas were employed for indexing, the addition of superordinate, subordinate, and collateral classes to the original concepts made the system performance worse.
- The performance declined when both narrower and broader terms were added to the thesaurus’s-controlled languages; and
- Index languages derived from titles outperformed those derived from abstracts.
Present use of Cranfield 2 Test
- Against evaluate the performance of traditional IR models (such as the Boolean or Vector Space Model) against more recent methods, some researchers continue to employ the Cranfield 2 test. It is employed to illustrate how IR approaches have evolved and changed throughout time.
- The Cranfield 2 test is used in introductory IR research, where the objective is to illustrate and clarify fundamental IR ideas. In order to comprehend how relevance and evaluation in information retrieval were handled in the past, more recent academics in the subject could utilize it as a historical case study.
- The Cranfield 2 dataset may be used as part of baseline studies in some research publications that concentrate on the assessment of IR systems or the creation of new metrics in order to demonstrate how their suggested approaches stack up against conventional systems.
- The Cranfield 2 test is no longer the main instrument used to assess contemporary IR systems. Nonetheless, its impact endures in the field’s foundational, historical, and educational facets. More like a legacy benchmark, it aids academics in contrasting early retrieval models with more recent methods. Advanced analytics, real-world dynamic material, and bigger datasets are now all part of modern IR evaluation.
Future of Cranfield tests
The Cranfield test will probably need to be significantly modified in the future to address the difficulties of contemporary information retrieval. More real-world complexity, such as personalized search, multimodal data, and interactive evaluation, will be incorporated into the conventional framework of controlled evaluation based on precision and recall.
Future Cranfield-style experiments will also need to adopt new evaluation measures, concentrate on ethics and justice, and consider the environmental sustainability of retrieval systems as AI and deep learning continue to propel advances in IR. With these modifications, the Cranfield test will continue to be a useful instrument for assessing IR systems in a quickly evolving technical environment.
Smart Retrieval experiment
Gerard Salton evaluated the several searching options provided by the SMART retrieval system under laboratory conditions. The system was introduced in 1964 and is based on the processing of abstracts in natural language forms.
For the development and assessment of automated retrieval methods, the SMART retrieval system provided a special experimental setting. In the SMART system, a set of weighted terms (also known as term vectors) represented documents, and a term assignment array represented a group of documents. Every term had a weight, which was zero if an index term wasn’t actually given to a document and positive if it was. Similar to this, a vector of query phrases was used to represent a single question. The document vector was created via automatic indexing with the term discrimination model.
The capacity of index terms to raise the average dissimilarity of document descriptions in a database was the basis for this model’s evaluation. The average dissimilarity of the documents in the collection rose with a good indexing word and fell with a bad one. Following that, terms were given discrimination values based on how much they increased or decreased the average document dissimilarity. The degree of similarity between the query and document vectors served as the basis for retrieval.
Scope
- Enhances accuracy and relevance by utilising cutting-edge methods such as machine learning, natural language processing (NLP), and semantic comprehension. Smart retrieval interprets the meaning of searches by doing more than just matching keywords.
- Unlike conventional retrieval systems that depend on keyword searches or exact matches, semantic search efforts aim to comprehend the meaning of a query. To do this, relationships between concepts are mapped using knowledge graphs and deep learning.
- Grouping related papers, files, or data points together according to patterns using clever retrieval procedures, which enables the system to provide more grouped and pertinent results.
Methodology
A collection of 1268 abstracts in the field of library science and documentation, largely published in American documentation, 1963-1964, and also in some other. For this experiment, journals were used. Eight distinct individuals with knowledge of the topic, either as librarians or library science students, were requested to create 48 distinct search queries in the documentation area using clear, grammatically accurate English. After each of the eight individuals submitted their inquiry, a total of 48 queries were examined utilizing the several search options provided by the SMART system against a file containing 1268 data.
Next, after receiving the content of the document abstracts, each participant was asked to rate each abstract's applicability to each of his six questions. The relevance feedback was a crucial component of the SMART trial. The system recalculates the weight of the items in the database if the user can specify which items are relevant and which are not in an initial output. This is accomplished by giving the qualities connected to the relevant items more weight and decreasing the weights connected to the non-relevant ones. Four sets of evaluations were contrasted.
Results
Under typical conditions, it was discovered that assessing performance for a range of processing techniques necessitated looking at the order of the associated recall-precision curves rather than a thorough comparison of the actual values for precision and recall. It was observed from a ranking of the recall-precision graphs produced by the various processing techniques, that Changes in the relevance judgements had no effect on the relative performance of the different retrieval methods, despite the fact that the groups’ general consistency of relevance agreements was not very high. The ranking of alternative search methods was the same across four sets of relevance evaluations. More precisely, the thesaurus process outperformed the word stem match by a little margin, while the word form process was shown to be weaker than the other two processes.
Future of SMART Retrieval Experiment
Keyword-based, static models will give way to intelligent, interactive, personalized, and multimodal systems powered by AI, deep learning, and natural language processing in the future of SMART retrieval research. In addition to retrieving pertinent data, these future technologies will enhance user experience, adjust to contextual and individual demands, and guarantee accuracy, fairness, and practical applicability. These advancements will be influenced by the SMART foundation, which will assist contemporary IR systems in tackling information retrieval problems that are becoming more intricate, varied, and dynamic.
MEDLARS Test
Small collections were used for the majority of the evaluation investigations. The National Library’s Medical Literature Analysis and Retrieval System (MEDLARS) performance was founded on the extensive MEDLARS database, which at the time of its creation in August 1966–July 1967 had 7,50,000 records pertaining to medical literature on magnetic tape. It was the first significant assessment of an operating system retrieval system. Monthly editions of Index Medicus were produced from the MEDLARS tape, and terms used to index the articles' subjects were taken from a Medical Subject Headings thesaurus (MeSH), which at the time included roughly 7000 primary subject headings.
Scope
The National Library of Medicine (NLM) created MEDLARS (Medical Literature Analysis and Retrieval System) in the early 1960s. Its goal was to make it easier to find information on medical and health-related publications. In order to facilitate access to vital medical information for researchers, medical professionals, and institutions, the primary objective was to index and catalogue biomedical literature.
In order to give academics and medical professionals access to vital medical information, MEDLARS was created to index and retrieve medical literature. Its goal was to provide access to information and included a broad range of biomedical topics.
Methodology
Initially, a sample work statement was created that included a list of questions to be addressed in the MEDLARS study. It was determined that 300 evaluated queries that is, fully analyzable test search requests were required to offer a sufficient test. As far as feasible, the variety of questions should reflect the typical need for information on various topics covered in medical literature, such as illnesses, medications, public health, and so forth. Stratified sampling of the medical facilities from which demands had originated in 1965 and answering enquiries from the sample facilities allowed for representativeness throughout the course of a year. Additionally, it was agreed to include all types of users (government, academic, research, pharmaceutical, and clinical) in the exam and to require them to provide a specific number of test questions. This is how the twenty-one user groups were chosen. After the user group submitted about 410 queries, 302 of them were thoroughly assessed and utilized in the MEDLARS test.
After receiving the submitted queries, MEDLARS personnel used a suitable combination of MeSH terms to create a search formulation (also known as a query designation) for each query. A computer search was then proceeded with as usual. Each user was then requested to submit a list of recent publications that he believed were pertinent to his question. A computer output of references was the outcome of a search. 25 to 30 things were chosen at random from the list, and photocopies of these were given to the searcher for relevance evaluation because the total number of items retrieved could be high (some searches returned more than 500 references).
Each object that was retrieved was to be marked by the user using the following scales:
H1 – of major value;
H2 – of minor value;
W1 – of no value;
W2 – of undetermined value;
The following formula was used to determine the search precision based on these figures:
Ratio of precision = ((H1 + H2)/L) X 100
(The number of sample items retrieved is indicated by L.)
Results
An average of 175 references were returned for each search, with an overall accuracy ratio of 50.4%. This means that roughly 87 of the 175 references that were typically returned were deemed irrelevant. The recall ratio for overall was 57.7%, according to an indirect calculation. An overall recall ratio of 57.7% suggests that almost 150 references should have been located, but 62 were overlooked, based on an average search and the assumption that roughly 88 of the references found were pertinent. Nevertheless, each of the 302 searches' memory and precision ratios were examined, and the MEDLARS test was used to average the individual ratios. Here the results are:
Overall Major value
Recall ratio 57.7% 65.2%
Precision ratio 50.4% 25.7%
Present application of MEDLARS
Although MEDLARS is no longer in use, the MEDLINE database, which developed from MEDLARS, carries on its heritage. Modern systems like PubMed were developed in part thanks to the fundamental technology and ideas outlined by MEDLARS.
- MEDLINE and PubMed: Millions of citations and abstracts from biomedical literature are available through PubMed, a free online search engine run by the National Library of Medicine, making MEDLINE the principal database for medical literature today. Researchers, physicians, and students can look for excellent, peer-reviewed journal articles in a variety of medical specialties using PubMed. Originally created for MEDLARS, the MeSH system is still in use today to help with efficient information retrieval and searching in PubMed and MEDLINE.
- Connectivity to Other Databases: Contemporary information retrieval systems are a reflection of the concepts and procedures established in MEDLARS. Similar indexing, keyword labelling, and standardized terminologies are already common in many contemporary biomedical databases. For example, PubMed’s connections to other health databases like Cochrane, EMBASE, and CINAHL enhance access to thorough medical data.
Future of MEDLARS
- PubMed, MEDLINE, and other research databases are already using machine learning (ML) and artificial intelligence (AI) technology to enhance recommendation systems, search accuracy, and personalization.
- More advanced methods for locating research papers, clinical trials, and other medical material will be made available by AI-powered search engines, improving the effectiveness and accuracy of search results.
- Making sure that medical literature databases like PubMed are available in numerous languages, interact with international repositories, and promote cooperation across various healthcare systems will become increasingly important as international collaboration in medical research grows.
- More global collaborations and interconnected databases may be a part of new systems, which would make clinical and research data widely available and usable.
- More interactive interfaces with the ability to refine searches with voice commands, visual searching tools, and even virtual assistants are probably in store for medical literature search engines in the future.
- Researchers and physicians will be able to search databases using natural language processing (NLP) technology, which will make it simpler to retrieve pertinent information.
The STAIRS project
Blair and Maron (1985) released a report on a large-scale experiment designed to assess a full-text search and retrieval system’s retrieval efficacy. The Storage and Information Retrieval System (STAIRS) Study is the name given to this.
Methodology
Nearly 40,000 documents, or about 350,000 pages of hard copy text used in the defence of a major corporate lawsuit, made up the database that the STAIRS study looked at. One significant aspect of STAIRS was that attorneys using the system for litigation support required that 75% of all the documents pertinent to a particular request be retrievable. The primary purpose of the STAIRS evaluation was to determine how well the system could retrieve all of the documents and only those that were pertinent to a particular request using precision and recall metrics.
By dividing the total number of “vital”, “satisfactory,” and “marginally relevant” documents by the total number of documents recovered, the precision of the STAIRS Project was determined. The recall was calculated using a sampling technique. Samples were drawn at random and the solicitors assessed these. It was estimated that there was a total of pertinent papers in these subsets.
Results
Of the 51 requests, 40 had their recall and precision values calculated, while the remaining 11 were utilised to test sampling techniques and take potential bias into consideration when evaluating retrieval and sample testing. In percentage terms, accuracy became more significant than 100%. The customer naturally wishes to reformulate the query by adding more and more search phrases to bring the output size down to a tolerable level, as they point out that it is impractical for the user to peruse a retrieved set of several thousand documents. This is their attempt to figure out why, in response to a request, STAIRS could only get one of five relevant objects.
Future of The Stairs Project
Semantic, user-centered, and scalable retrieval systems are the way of the future for the Stairs Project in information retrieval. Future systems will expand on the basic work of the Stairs Project by combining AI, NLP, machine learning, and multimodal data retrieval as more data becomes connected and accessible in various formats. In addition to resolving privacy, bias, and ethical issues, they will be able to deliver more precise, contextually aware, and tailored search results across ever-more complex and linked datasets. To put it briefly, Stairs-like systems will develop to accommodate the expanding volume and complexity of the data-driven world, enhancing our ability to access and utilize information in various fields.
TREC Experiment: The Text Retrieval Conference
It has been noted that practically all of the previous evaluation studies were unconnected to the real-life situation and were based on a small data collection. The main challenge for IR researchers was to base the evaluation tests on a sizable test collection that mirrored real-world scenarios and had the infrastructure to support them. Under these conditions, the TREC (Text Retrieval Conference) series of information retrieval experiment was established in 1991 to allow IR researchers to progress from small data collection to larger experiments. The TREC series is run by the National Institute for Science and Technology (NIST) and funded by the Defence Advanced Research Project Agency (DARPA, USA).
Since its start, the TREC series of experiments has attracted the interest of LIS professionals worldwide and demonstrated that international cooperation and efforts may yield important research findings.
Scope
In several TREC studies, a broad variety of information retrieval techniques were examined (i.e. from TREC 1 in 1992 to TREC 12 in 2003). Boolean search, statistical and probabilistic indexing, and term weighting strategies are a few noteworthy examples. Other examples include: retrieving passages or paragraphs; combining the results of multiple searches; retrieving based on previous relevance assessments; indexing phrases based on natural language and statistics; expanding and reducing queries; searching using strings and concepts; searching using dictionaries; question-answering; content-based multimedia retrieval.
Methodology
To oversee the TREC activities, a program committee of representatives from academia, industry, and government was established. NIST supplied a test set of papers and questions for every TREC. After TREC participants ran their own retrieval system on the data, they sent a list of the top-ranked documents they had recovered to NIST, where the results were reviewed after the retrieved documents were judged to be correct and the individual findings were combined. A workshop serves as a platform for participants to share their experiences when the TREC cycle comes to a finish. The most recent TREC workshop, which is held annually, was the 12th in the series and took place at NIST in 2003.
Results
Results from the TREC series of tests have been quite noteworthy and intriguing. Every experiment’s result, together with particular reports, are routinely posted on the TREC website (http://trec.nist.gov). Several significant conclusions from the TREC experiments:
- ‘pooling’ was determined to be more than sufficient for test collecting in producing the sample results for relevance judgements.
- It appears that automatic query generation from natural language questions performs as well as or better than manual query construction, which is encouraging for organizations who advocate for the use of straightforward NLP interfaces for retrieval systems.
- When the number of documents per subject increased from 200 to 1000 and the database size increased from 1 GB to 3 gigabytes, TREC 2 showed a significant improvement in retrieval performance over TREC 1 in terms of the routing task.
- Despite several experimental designs, the level of performance remained the same. For example, some groups used the topic statements to automatically produce queries, while others did so manually; Many systems lacked relevance feedback; the computer platforms employed ranged from personal computers to supercomputers.
- Variations in the precision-recall curve were negligible.
- Despite the comparable precision-recall results, the actual documents retrieved showed a significant amount of variation.
Present application of TREC
- TREC is still a crucial event for assessing how well information retrieval systems and search engine’s function. To enable comparisons between various methods, researchers and businesses submit their systems to TREC for benchmarking against standardised test datasets. This is particularly important for search engines that are utilised in domains like: Internet search, e.g., Bing, Google; Enterprise search, for business databases; specialised search, in the domains of science, medicine, or law.
- As the use of multimedia (audio, video, and image) becomes more prevalent, TREC’s Multimedia Retrieval tracks aid in evaluating systems that can index and retrieve non-textual data in response to queries.
- To give an example, the Legal Information Retrieval track is frequently used to compare systems made for the legal sector, where it is essential to locate pertinent statutes, case law, and documents. Courts, legal technology firms, and law firms all depend on this application.
- Personalised search systems that consider user preferences and behaviour have become a greater emphasis of TREC. Systems that can adjust and customise outcomes based on user data are becoming increasingly crucial as desire in personalised experiences grows.
Future of TREC
Machine learning, multimodal data, personalization, ethics, and real-time retrieval are all expected to present new opportunities and difficulties for TREC in information retrieval in the future. With a greater emphasis on the systems’ practicality, TREC will remain a vital platform for assessing and comparing the most advanced IR methodologies. This entails broadening the definition of IR to encompass cross-modal, cross-lingual, and customized search experiences in addition to text-based retrieval. TREC will continue to be a major force behind innovation in IR as the discipline develops, assisting practitioners and researchers in testing and improving systems that will influence information retrieval in the future.
Reference
- Chowdhury, G.G. (2010). Introduction to Modern Information Retrieval (3rd ed.). London: Facet publishing.
- https://www.egyankosh.ac.in/bitstream/123456789/76420/1/Unit-5.pdf
- TREC: Experiment and Evaluation in Information Retrieval. Retrieved from https://aclanthology.org/J06-4008.pdf
- Ellen M. Voorhees. (n.d.). TREC: Improving information access through evaluation. Retrieved from https://doi.org/10.1002/bult.2003.1720320105
- The SMART information retrieval project. Retrieved from https://DOI:10.3115/1075671.1075771
- STAIRS Redux: Thoughts on the STAIRS Evaluation, Ten Years after. Retrieved from
https://yunus.hacettepe.edu.tr/~tonta/courses/spring2008/bby703/Blair.pdf
- Retrieved from https://trec.nist.gov/
- The Information Retrieval Experiment Platform. Retrieved from
https://arxiv.org/pdf/2305.18932v1
- Recent Developments in the Evaluation of Information Retrieval Systems: Moving Towards Diversity and Practical Relevance. Retrieved from
https://www.researchgate.net/publication/220166136
- SMART Information Retrieval System. Retrieved from
https://en.wikipedia.org/wiki/SMART_Information_Retrieval_System
- Cranfield experiments. Retrieved from
https://en.wikipedia.org/wiki/Cranfield_experiments
- Retrieved from https://ebooks.inflibnet.ac.in/lisp7/chapter/evaluation-and-measurement-of-information-retrieval-system/
- Retrieved from https://www.semanticscholar.org/paper/The-Cranfield-tests-Jones/5f321ab884bd97d58784dd1f6b08f054ec256d57
- Retrieved from https://trec.nist.gov/data/interactive.html
|
oercommons
|
2025-03-18T00:39:09.183168
|
12/22/2024
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/123331/overview",
"title": "PAST, PRESENT AND FUTURE OF INFORMATION RETRIEVAL EXPERIMENTS",
"author": "SULTANA KHATUN SHEIKH"
}
|
https://oercommons.org/courseware/lesson/105018/overview
|
Mushroom Cultivation
Overview
Mushroompreneurship refers to the entrepreneurial pursuit of starting and running a business related to mushroom cultivation, production, and sale. It involves leveraging the growing demand for mushrooms in various industries, including food, medicine, cosmetics, and agriculture. Mushroompreneurship offers several opportunities for aspiring entrepreneurs due to the numerous benefits and market potential of mushrooms
Mushroompreneurship
Mushroompreneurship refers to the entrepreneurial pursuit of starting and running a business related to mushroom cultivation, production, and sale. It involves leveraging the growing demand for mushrooms in various industries, including food, medicine, cosmetics, and agriculture. Mushroompreneurship offers several opportunities for aspiring entrepreneurs due to the numerous benefits and market potential of mushrooms
|
oercommons
|
2025-03-18T00:39:09.201867
|
06/09/2023
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/105018/overview",
"title": "Mushroom Cultivation",
"author": "S.Iruthaya Kalai Selvam"
}
|
https://oercommons.org/courseware/lesson/112677/overview
|
تكنولوجيا تعلم ومعلومات
Overview
يهدف الموقع إلى جمع الكتب الطبية من الستوى الأول حتى الخامس كلية الطب / جامعة إب
يهدف إلى تسهيل تحميل الكتب عبر الإنترنت للمتعلمين
تكنولوجيا تعلم ومعلومات
يهدف الموقع على جمع الكتب الطبية من الستوى الأول حتى الخامس كلية الطب / جامعة إب
بغرض تسهيل تحميل الكتب عبر الإنترنت للمتعلمين
|
oercommons
|
2025-03-18T00:39:09.214301
|
baraah mustafa
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/112677/overview",
"title": "تكنولوجيا تعلم ومعلومات",
"author": "Student Guide"
}
|
https://oercommons.org/courseware/lesson/116969/overview
|
FAD Syllabus: UNCC HIST10160
Overview
Syllabus shared by a UNC System faculty member.
Sample Syllabus
History 1160: United States History to 1865
Fall 2017, CHHS376
Professor: [NAME]
Office Hours: Tuesday 8-9; Thursday 11-12 or by appt. Email: [FACULTY EMAIL ADDRESS]
Teaching Assistant: [NAME] Office Hours: Tuesday 11-1 [FACULTY EMAIL ADDRESS]
Course Description
This course is an introduction to American history from the time of the original Indian settlements to the Civil War. Important themes in this course include the political, class, gender, and racial identities of Americans. We will explore in depth the contributions of both elites and non-elites to the history of America in this period. While we cannot cover every topic in US history before 1865, we will examine the rise and growth of slavery, religious and intellectual movements, the rise of democracy, the growth and effects of capitalism, and the coming of the Civil War.
Course Goals
This course aims to help you answer the following questions: What were the primary developments that spurred European migration to North America? What was Native American life like before European migration, and how did this migration affect Native American life? Why did Europeans turn to African slavery to fulfill their labor needs? What role did slavery play in the development of the nation? What were the ideological influences on early Americans, both religious and secular? Why did the colonies revolt against Great Britain? How did different groups use the legacy of the Revolution in the years leading up to the Civil War?
This course also aims to help you refine your analytical skills with a focus on enabling you to clearly and effectively communicate ideas verbally and in writing. You will be asked to read a broad range of primary and secondary sources, sifting through complex information and arguments, and think critically about the material. Our discussions will provide you a forum to ask questions, debate ideas, and collaborate with your peers.
Course Format
This course will meet on Tuesdays and Thursdays and will be comprised of lectures and discussions of the readings.
Course Requirements Pop Quizzes (35 percent) Short Essays (20 percent) Midterm Exam (20 percent) Final Exam (25 percent)
In order to succeed in this class, you will need to do three things. First, you need to show up in lecture and discussions. Second, you need to do the assigned reading and writing assignments. Finally, you should come to class prepared to ask questions raised by the reading or lecture and discuss your ideas with other students and the professor. Like any course, the more effort you put forth, the more you will get out of this class.
Every student in the class is responsible for following the syllabus and doing all of the work as spelled out in the syllabus. If there are any changes concerning reading or writing assignments- I will alert the class to the changes via email and revise the syllabus on Canvas. If you joined my class late- you are responsible for all of the material we covered before you joined. If you are absent from a class- you are responsible for the material we covered during your absence. Being responsible includes doing the reading and getting the class notes from someone in class. There is no need to ask me what work you missed- just consult the syllabus and get the class notes from someone in class.
Assignments
Pop quizzes will be administered randomly eight times throughout the semester. These quizzes will be very short, just one or two questions, and will test your comprehension of the reading for that day. There will not be quizzes on days where short essays are also due. The best way to prepare for these is to take good notes on all of your reading assignments and review them before each class. Quizzes will be graded 0, 50, or 100. Quizzes will be open-notes, but not open-book. I will drop one quiz grade.
Throughout the semester you will write two short essays (3 pages) on a topic related to the texts for that day. No late papers will be accepted and essays will be due prior to the class where we discuss the corresponding reading. Short essays will be graded on a 10 point scale, and all essays will be submitted on Canvas. You can find the essay topics on Canvas as well.
There will be a midterm exam covering the material from the first half of the course. The exam will be comprised of identifications and an essay.
There will be a final exam covering the material from the entire course. The final will also be comprised of identifications and an essay.
Canvas
The syllabus, assignments, select readings and announcements will be posted on the Canvas site for our class. To access Canvas go to canvas.uncc.edu.
Attendance
There is no attendance policy. To get an excused absence from class and to be given a makeup assignment, however, you must go to the Dean of Students Office and provide them with documentation. I do not offer makeup assignments without an excused absence from this office.
Extra Credit
There is no extra credit for this class. Your grade will be determined solely by the assignments listed in this syllabus.
Readings
The following books are required for this course and are available either online, at the campus bookstore, or at Gray’s College bookstore. Failing to get the required texts on time will not excuse you from any assignment in the class.
Olaudah Equiano, The Interesting Narrative of Olaudah Equiano (Bedford/St. Martin’s, 3rd edition)
Paul E. Johnson and Sean Wilentz, The Kingdom of Matthias: A Story of Sex and Salvation in 19th-Century America (Oxford University Press, Updated Edition)
Eve Kornfeld, Creating an American Culture, 1775-1800 (Bedford/St. Martin’s)
Michael P. Johnson, Reading the American Past, Selected Historical Documents: Volume I, to 1877 (5th edition)
ACADEMIC INTEGRITY STATEMENT
Academic honesty and integrity are essential to the existence and growth of an academic community. Without maintenance of high standards of honesty, members of the instructional faculty are defrauded, students are unfairly treated, and society itself is poorly served.
Maintaining the academic standards of honesty and integrity is ultimately the formal responsibility of the instructional faculty; and this responsibility is shared by all members of the academic community.
Students have the responsibility to know and observe UNC Charlotte’s “The Code of Student Academic Integrity. The code is on the web at: http://www.legal.uncc.edu/policies/ps-105.html
If you commit plagiarism on any of the assignments in this course, you will automatically receive an F for the class and will be reported to the Academic Integrity Board. There will be no exceptions to this rule.
DISABILITY STATEMENT
Consistent with the requirements of the Acts, the University and all members of the faculty and staff shall operate its programs, activities, and services to ensure that no qualified individual with a disability shall be excluded from participation in, be denied the benefits of, or be subjected to discrimination under any such program, activity, or service solely by reason of his/her disability. Students with documented disabilities requiring accommodation in this course should contact Disability Services in Fretwell 230.
SCHEDULE OF CLASSES
Week 1
Tuesday, 8/22 Introduction
Thursday, 8/24 Native Americans to 1500; Discussion, Johnson, Ch. 1
Week 2
Tuesday, 8/29 Expanding Europe; Discussion, Johnson, Ch. 2 Thursday, 8/31 Founding Virginia; Discussion, Johnson, Ch. 3
Week 3
Tuesday, 9/5 Founding New England; Discussion, Johnson, Ch. 4
Thursday, 9/7 The Enlightenment and Great Awakening; Discussion, Johnson, Ch. 5, Doc. 5-3
Week 4
Tuesday, 9/12 The Consumer Revolution and Colonial Society; Discussion, “Colonial America at Mid-Century” (Canvas) and Johnson, Ch. 5, Doc. 5-2
Thursday, 9/14 The Colonial Crisis; Discussion, Johnson, Ch. 6
Week 5
Tuesday, 9/19 The American Revolution, Part I; Discussion, Johnson, Ch. 7, Doc. 7-1 Thursday, 9/21 The American Revolution, Part II; Discussion, Johnson, Ch. 7, Docs. 7-2
thru 7-5
Week 6
Tuesday, 9/26 18th Century Slavery; Discussion, The Interesting Narrative of Olaudah Equiano Short Essay Due
Thursday, 9/28 No Class
Week 7
Tuesday, 10/3 Choosing the Constitution; Discussion, The Constitution (Canvas) and Johnson, Ch. 8, Docs. 8-4 and 8-5
Thursday, 10/5 Politics and Society in the 1790s; Discussion, Johnson, Ch. 9
Week 8
Tuesday, 10/10 No Class, Student Recess
Thursday, 10/12 Midterm Exam
Week 9
Tuesday, 10/17 Creating an American Culture; Discussion, Kornfeld, Creating an American Culture Short Essay Due
Thursday, 10/19 Jefferson’s Presidency to the War of 1812
Week 10
Tuesday, 10/24 Democratic Politics and the Second Party System: Discussion, “The Emergence of Southern Nationalism” (Canvas) and Johnson, Ch. 11, Doc. 11-1
Thursday, 10/26 No Class
Week 11
Tuesday, 10/31 The Market Revolution and the Second Great Awakening Thursday, 11/2 Discussion, Johnson and Wilentz, The Kingdom of Matthias
Week 12
Tuesday, 11/7 Abolitionism; Discussion, Frederick Douglass, “What to the Slave is the Fourth of July?” (Canvas) and Johnson, Ch. 11, Doc. 11-4
Thursday, 11/9 The Women’s Rights Movement; Discussion, Johnson, Ch. 11, Doc. 11-5 and Ch. 12, Docs. 12-4 and 12-5
Week 13
Tuesday, 11/14 The Slave South: Film, Slavery and the Making of America
Thursday, 11/16 Manifest Destiny and the Mexican-American War
Week 14
Tuesday, 11/21 No Class
Thursday, 11/23 No Class, Thanksgiving Break
Week 15
Tuesday, 11/28 The Coming of the Civil War and Secessionism; Discussion, Johnson, Ch.
14
Thursday, 11/30 The Civil War and Emancipation; Discussion, Johnson, Ch. 15
Week 16
Tuesday, 12/5 Final Exam Review
Final Exam: Thursday, December 14, 8-10:30AM
The instructor reserves the right to make revisions or addendums to the syllabus at any time throughout the duration of the semester. If any changes do occur, students will be notified and allowed adequate time to make any necessary adjustments
|
oercommons
|
2025-03-18T00:39:09.262337
|
06/18/2024
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/116969/overview",
"title": "FAD Syllabus: UNCC HIST10160",
"author": "UNC System"
}
|
https://oercommons.org/courseware/lesson/116965/overview
|
FAD Syllabus: UNCA HIST312
Overview
Syllabus shared by a UNC System faculty member.
Sample Syllabus
HIST 312 - U.S. Constitution in Context
[NAME]
MW 3:15 – 4:55
Office: Email:
Office Hours: 11:30 – 2:00 Monday, Tuesday, Wednesday, and by appointment.
Texts: The U.S. Constitution https://www.archives.gov/founding-docs/constitution-transcript The Declaration of Independence https://www.archives.gov/founding-docs/declaration-transcript The Bill of Rights https://www.archives.gov/founding-docs/bill-of-rights-transcript Amendments to the Constitution 11 – 27
https://www.archives.gov/founding-docs/amendments-11-27
Magna Carta, Mayflower Compact, Virginia Declaration of Rights, The Federalist Papers,
Emancipation Proclamation
https://constitutioncenter.org/learn/educational-resources/historical-documents Supreme Court Cases https://caselaw.findlaw.com/court/us-supreme-court Fruchtman, Ame
Course Requirements:
Exams: There will be three exams, one for each major section of the course. Exams will include a choice essays. You will be expected to be prepared to deal with material from lectures, class discussions, and the textbooks. Exam dates and grade weight of each exam are as follows:
Exam #1 – September 25 - 15%
Exam #2 – October 30 - 20%
Exam #3 – December 11, 11:30 a.m. - 20%
Primary Source Research Paper: Choose an important Supreme Court case (up until 1963) and:
- Give the background and major players in the case using secondary sources.
- Using online newspaper sources gauge the reaction among at least five different newspapers to that case. You should choose newspapers from various parts of the country and from various political perspectives. The best places to go are the Library of Congress site Chronicling America, the New York Times – Historical Newspaper Collection (available under Online Resources on the library website), and America’s Historical Newspapers (goes up to 1876 and can also be found at Online Resources).
- Analyze your information and arrive at some conclusions about the way this case was perceived at the time and how that view may have changed over the years. Paper should be 6 to 8 pages with footnotes and bibliography. 20% of course grade and due November 15.
Reflection Journals: Each student will keep a reflection journal which will be submitted to my email address (FACULTY MEMBER EMAIL ADDRESS) every Wednesday before midnight (Except for weeks when we have an exam, comparative essay, or break). Approximately one to three pages in length, each entry should reflect (your entry should not just repeat something you heard or read, but should demonstrate reflection and rumination) on a reading or topic of discussion from the
previous week’s course material that made you think. Feel free to make it light, even funny, to speculate, and let your imagination go, but make sure you make specific connections to course material. I’ll grade each on a one to ten-point scale. You’ll have 13 opportunities and I’ll take the 10 use your 10 best journals to determine your final grade. 25% of course grade.
Professor's Responsibilities
- Arrive at each class meeting on time and well prepared.
- Grade each student's work fairly and promptly (I always attempt to return assignments within one week of the date that they were received--I will notify you if I will be unable to do so).
- Treat each student with respect and courtesy.
- Faithfully meet office hours and honor appointments with students.
- Communicate with students clearly and listen to students attentively.
Student Responsibilities
- Attend all classes. Students cannot expect to be successful if they miss class. Students will be penalized 2 points on their final average for each unexcused absence in excess of three (3).
- Arrive at class on time. It is very distracting to have students walking in to class late.
- Come to class well prepared. Students should plan on spending at least two hours of preparation for each hour in class in reading, writing, and ruminating.
- Take all exams and turn in all papers at the assigned time. Make-up exams will only be given to students with legitimate, documented excuses. Late papers will be penalized at a rate of five
(5) points per day.
- Do you own work. While students are encouraged to cooperate and study together, each individual is required to do their own work. Cheating and/or plagiarism will not be tolerated.
- Actively participate in the life of the class. I have designed this class as a learning community. Each student should participate in the life of the community by learning other classmates' names, working diligently in groups, studying with others outside of class, and contributing to classroom discussion.
- Treat the professor with respect and courtesy. Turn off your cell phone before class (or better yet, leave it at home) and avoid other distracting behaviors. Laptops and tablets are only allowed in class when students are instructed to bring them for research and classwork purposes.
- Communicate clearly and promptly with the professor. I am much more sympathetic when students inform me that they must miss class or will be late with an assignment if I am notified well in advance. Students should not hesitate to speak to the professor if they are experiencing difficulty in the class or if they have special problems of which the professor needs to be aware.
Course Schedule:
August
21 Course Introduction
23 The Roots of the Constitution Chap. 1 -2 Magna Carta
Mayflower Compact
28 Cont.
30 The American Revolution and the Articles of Confederation
September
4 No Class – Labor Day
Chaps 4 – 5
Dec. of Independence
6 Construction and Ratification Chaps. 6 – 7 Federalist Papers, 10, 39, 51
11 The Constitution U.S. Constitution
13 Cont.
18 The Bill of Rights Virginia Declaration of Rights Bill of Rights Federalist 29
20 The Constitution and Religion https://wallbuilders.com/america-christian-nation/
https://www.huffpost.com/entry/founding-fathers-we-are-n_b_6761840 https://www.thegospelcoalition.org/blogs/evangelical-history/america-as-a
-christian-nation-a-conversation-with-mark-noll-and-george-marsden/
25 Exam #1
27 Marshall Court – Judicial Review and Economic Issues Chaps. 8, 10 – 11 October
2 Marshall – Federal/State Relations and Nullification
4 The Taney Court
9 No Class – Fall Break
11 Slavery and the Constitution Chaps. 14 - 15
16 The Constitution, Civil War, and Reconstruction Chap. 16
18 Cont. 13th, 14th, 15th
Amendments
23 Cont. Chap. 17 -18
25 Exam #2
30 The Constitution and the Industrial Revolution Moodle – “The Constitution in the Age of Industrialization”
November
1 The Constitution and Jim Crow Moodle –“The Court and Civil Rights
6 Cont.
November
8 The Constitution in the Progressive Era
13 The Constitution, Civil Liberties, and War Moodle – “The Development of Modern Civil Liberties Law”
15 The Constitution and the New Deal
20 The Warren Court and Civil Rights
22 No Class - Thanksgiving
27 Cont.
29 Religion, Speech and the Court in the 1960s
December
4 The Environment, Privacy, Marriage, and Political “Speech”
8 Final Exam
Other Important Pieces of Info:
Office of Academic Accessibility
UNC-Asheville values the diversity of our student body as a strength and a critical component of our dynamic community. Students with disabilities or temporary injuries/conditions may require accommodations due to barriers in the structure of facilities, course design, technology used for curricular purposes, or other campus resources.
Students who experience a barrier to full access to this class should let the professor know, and/or make an appointment to meet with the Office of Academic Accessibility as soon as possible. To make an appointment, call 828.232.5050; email academicaccess@unca.edu; use this link https://uncaoaaintake.youcanbook.me/; or drop by the Academic Accessibility Office, room 005 in the One Stop suite (lower level of Ramsey Library). Learn more about the process of registering, and the services available through the Office of Academic Accessibility
here: https://oaa.unca.edu/
While students may disclose disability at any point in the semester, students who receive Letters of Accommodation are strongly encouraged to request, obtain and present these to their professors as early in the semester as possible so that accommodations can be made in a timely manner. It is the student’s responsibility to follow this process each semester.
Sexual Harassment and Misconduct
All members of the University community are expected to engage in conduct that contributes to the culture of integrity and honor upon which the University of North Carolina at Asheville is grounded. Acts of sexual misconduct, sexual harassment, dating violence, domestic violence and stalking jeopardize the health and welfare of our campus community and the larger community as a whole and will not be tolerated. The University has established procedures for preventing and investigating allegations of sexual misconduct, sexual harassment, dating violence, domestic violence and stalking that are compliant with Title IX federal regulations. To learn more about these procedures or to report an incident of sexual misconduct, go to titleix.unca.edu. Students may also report incidents to an instructor, faculty or staff member, who are required by law to notify the Title IX Office.
Academic Alerts
Faculty at UNCA are encouraged to use the university's Academic Alert system to communicate with students about their progress in courses. Academic Alerts can reflect that a student’s performance is satisfactory at the time the alert is submitted, or they can indicate concerns (e.g., academic difficulty, attendance problems, or other concerns). Professors use the alert system because they are invested in student success and want to encourage open conversations about how students can improve their learning, and students who respond to alerts quickly are consistently more likely to earn credit for the course. Please note, professors of 100-level courses are required to submit at least one alert about each student on or before the seventh week of classes.
When a faculty member submits an alert that expresses a concern, the student receives an email from Academic Advising notifying them of the alert and subsequent registration hold on their account. To clear the hold, the student must complete a short Google Response Form included in the alert e-mail; the results will be shared with their instructor and advising staff. Instructors may also request to meet with the student to discuss the alert.
Questions about the Academic Alert system can be directed to [STAFF MEMBER NAME] (STAFF MEMBER EMAIL ADDRESS) in OneStop Advising and Learning Support.
University Writing Center
The University Writing Center (UWC) supports writers in one-on-one sessions lasting 10 to 45 minutes. Consultants can help writers organize ideas, document sources, and revise prose. If you visit the UWC, bring a copy of your assignment, any writing or notes you may have, and the sources you are working with. Make an appointment by visiting writingcenter.unca.edu and clicking on "Schedule an Appointment," or drop in during open hours Monday-Friday.
|
oercommons
|
2025-03-18T00:39:09.382060
|
06/18/2024
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/116965/overview",
"title": "FAD Syllabus: UNCA HIST312",
"author": "UNC System"
}
|
https://oercommons.org/courseware/lesson/94735/overview
|
Kilmanagh Meadows
Overview
Kilmanagh Meadows involves a two-party negotiation between neighbours over a dispute relating to noise pollution, boundary walls, and maintenance costs. This simulation can also be adapted for use as a mediation simulation. This simulation is designed for teaching and learning at third level, and works best where students have some experience and knowledge of negotiation and/or alternative dispute resolution (mediation).
Kilmanagh Meadows allows students to move away from positional bargaining towards creating opportunities for mutual gain and value creation. Maintaining the relationship is a key concern for both parties, and students may choose to prioritise this in the negotiation. This simulation has a large ZOPA, allowing for a wide range of outcomes.
Teaching Notes include General Instructions, Confidential Instructions for Liam MacManus and Confidential Instructions for James Canty.
Kilmanagh Meadows
Kilmanagh Meadows involves a two-party negotiation between neighbours over a dispute relating to noise pollution, boundary walls, and maintenance costs. This simulation can also be adapted for use as a mediation simulation. This simulation is designed for teaching and learning at third level, and works best where students have some experience and knowledge of negotiation and/or alternative dispute resolution (mediation).
Kilmanagh Meadows allows students to move away from positional bargaining towards creating opportunities for mutual gain and value creation. Maintaining the relationship is a key concern for both parties, and students may choose to prioritise this in the negotiation. This simulation has a large ZOPA, allowing for a wide range of outcomes.
Teaching Notes include General Instructions, Confidential Instructions for Liam MacManus and Confidential Instructions for James Canty.
|
oercommons
|
2025-03-18T00:39:09.401702
|
06/29/2022
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/94735/overview",
"title": "Kilmanagh Meadows",
"author": "Nessa Boland"
}
|
https://oercommons.org/courseware/lesson/77368/overview
|
French Level 4, Activity 04: En plein air (Protégez la faune) / Outdoors (Protect the Fauna) (Online)
Overview
In this activity students will practice proposing new laws to protect wildlife and defending these laws by explaining its pros and cons. They will also practice asking elaborating questions.
Activity Information
Did you know that you can access the complete collection of Pathways Project French activities in our new Let’s Chat! French pressbook? View the book here: https://boisestate.pressbooks.pub/pathwaysfrench
Please Note: Many of our activities were created by upper-division students at Boise State University and serve as a foundation that our community of practice can build upon and refine. While they are polished, we welcome and encourage collaboration from language instructors to help modify grammar, syntax, and content where needed. Kindly contact pathwaysproject@boisestate.edu with any suggestions and we will update the content in a timely manner.
Outdoors (Protect the Fauna) / En plein air (Protégez la faune)
Description
In this activity students will practice proposing new laws to protect wildlife and defending these laws by explaining its pros and cons. They will also practice asking elaborating questions.
Semantic Topics
Outdoors, nature, protecting nature, wildlife, en plein air, protégez la nature, la faune et flore, les questions complexes, complex questions
Products
Outdoors
Practices
Protecting the environment, being ecologically conscious.
Perspectives
France places a high level of importance on environmental concerns, and their government is leading advocate for environmental laws.
NCSSFL-ACTFL World-Readiness Standards
- Standard 1.1: Students engage in conversations or correspondence in French to provide and obtain information, express feelings and emotions, and exchange opinions.
- Standard 1.2: Students understand and interpret spoken and written French on a variety of topics.
- Standard 2.1: Students demonstrate an understanding of the relationship between the practices and perspectives of the cultures of the francophone world.
- Standard 3.1: Students reinforce and further their knowledge of other disciplines through French.
Idaho State Content Standards
- COMM 1.1: Interact and negotiate meaning (spoken, signed, written conversation) to share information, reactions, feelings, and opinions.
- COMM 2.1: Understand, interpret, and analyze what is heard, read, or viewed on a variety of topics.
- COMM 3.1: Present information, concepts, and ideas to inform, explain, persuade, and narrate on a variety of topics using appropriate media in the target language.
- CLTR 1.1: Analyze the cultural practices/patterns of behavior accepted as the societal norm in the target culture.
- CLTR 1.2: Explain the relationship between cultural practices/behaviors and the perspectives that represent the target culture’s view of the world.
NCSSFL-ACTFL Can-Do Statements
- I can exchange information on my favorite animal and its habitat.
- I can propose ideas for laws that could protect wildlife.
- I can discuss the pros and cons of a proposed law.
Materials Needed
Warm-Up
Warm-Up
1. Begin the activity by opening the Google presentation and introducing the Can-Do statements.
2. For the warm up activity ask students what their favorite animal is and why.
- Quel est votre animal préféré ? Pourquoi ?
3. Then have students describe this animal's habitat. Is this animal endangered? If yes, why?
- Décrivez le climat de l’habitat de votre animal préféré. Cet animal est-il une espèce menacée ? Si oui, pourquoi?
Main Activity
Main Activity
In this activity you will put the students in pairs, and have one student propose a new law to protect the wildlife and the other student play the role of an environmental protection committee member who evaluates the pros/cons of said law. After 10 minutes, the roles will switch.
Dans cette activité, vous aurez un partenaire. Partenaire 1 va proposer une nouvelle loi pour protéger la faune et Partenaire 2 va jouer le rôle d'un membre du comité de protection de l'environnement qui évalue les avantages/inconvénients la loi. Après 10 minutes, les rôles changeront.
For small groups : (4 or less)
1. Put students into pairs. Have Partner 1 propose a new environmental law to protect the wildlife and Partner 2 play the role of an environmental protection committee member.
2. Remind the students that the committee member’s job is to ask elaborating questions: N'oubliez pas de demander des questions si vous êtes un membre du comité de protection de l'environnement !
- Pourquoi cette loi est-elle importante ? (Why is this law important?)
- Quels sont les avantages de cette loi ? (What are the benefits of this law?)
- Est-ce qu’il y a des inconvénients ? (Are there any cons?)
3. After ~10 minutes, have Partner 1 and Partner 2 switch roles.
For large groups : (5 or more)
1. Put students into pairs.
2. Before sending them to breakout rooms, explain the rules of the activity.
Partenaire 1 va proposer une nouvelle loi pour protéger la faune et Partenaire 2 va jouer le rôle d'un membre du comité de protection de l'environnement qui évalue les avantages/inconvénients la loi. Après 10 minutes, les rôles changeront. (Partner 1 will propose a new environmental to protect the wildlife law and Partner 2 will play the role of an environmental protection committee member)
3. Remind the students that the committee member’s job is to ask elaborating questions: N'oubliez pas de demander des questions si vous êtes un membre du comité de protection de l'environnement !
- Pourquoi cette loi est-elle importante ? (Why is this law important?)
- Quels sont les avantages de cette loi ? (What are the benefits of this law?)
- Est-ce qu’il y a des inconvénients ? (Are there any cons?)
*Copy-paste these questions in the chat if necessary.
4. After 10 minutes, have the students switch roles.
*You can have them return to the main room and give them new partners if you’d like, but this is not required.
5. Once each student has played both roles, have them return to the main room and share one of the laws that they discussed with their partner.
Vous devez partager l'une des lois que vous avez discuter avec votre partenaire.
Wrap-Up
Wrap-Up
Ask the following question(s) to finish the activity:
- Que pouvons-nous faire pour protéger la faune locale ? (What can we do to protect the local wildlife?)
Cultural Resources
Which species conservationists are looking to protect in France
End of Activity
- Can-Do statement check-in… “Where are we?”
- Read can-do statements and have students evaluate their confidence.
- Encourage students to be honest in their self evaluation
- Pay attention, and try to use feedback for future activities!
NCSSFL-ACTFL Can-Do Statements
- I can exchange information on my favorite animal and its habitat.
- I can propose ideas for laws that could protect wildlife.
- I can discuss the pros and cons of a proposed law.
|
oercommons
|
2025-03-18T00:39:09.441378
|
Camille Daw
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/77368/overview",
"title": "French Level 4, Activity 04: En plein air (Protégez la faune) / Outdoors (Protect the Fauna) (Online)",
"author": "Mimi Fahnstrom"
}
|
https://oercommons.org/courseware/lesson/106565/overview
|
Goal Writing
Overview
The following resource provides potential courses regarding goal writing for new hires.
Goal Writing
The following two courses are paid courses that can assist new hires in writing effecting effective goals to provide quality services to students:
The Well Equipped Therapist (13.5 hour full course-- paid)
Creating Goals that are Easy to Monitor (65 min course- paid)
Additionally, the AOTA, APTA, and ASHA have issued a joint statement regarding collaborative goals within school-based practice:
Joint Statement on Interprofessional Collaborative Goals in School-Based Practice
|
oercommons
|
2025-03-18T00:39:09.459475
|
07/10/2023
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/106565/overview",
"title": "Goal Writing",
"author": "Nathaniel Baniqued"
}
|
https://oercommons.org/courseware/lesson/66016/overview
|
Dr. Anil Chidrawar
Overview
Phenols are aromatic hydroxy compounds in which one or more hydroxy groups are directly attached to the aromatic ring.
Phenols
Definition of phenol
Classification of phenols
Preparation of phenols
The chemical reaction of phenols
Acidic character of phenol
|
oercommons
|
2025-03-18T00:39:09.477098
|
05/03/2020
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/66016/overview",
"title": "Dr. Anil Chidrawar",
"author": "Dr. Anil Chidrawar"
}
|
https://oercommons.org/courseware/lesson/79493/overview
|
French Level 4, Activity 12: Pictionary Review (Online)
Overview
In this activity students will review vocabulary related to the performing arts and theater by playing a game of Pictionary.
Activity Information
Did you know that you can access the complete collection of Pathways Project French activities in our new Let’s Chat! French pressbook? View the book here: https://boisestate.pressbooks.pub/pathwaysfrench
Please Note: Many of our activities were created by upper-division students at Boise State University and serve as a foundation that our community of practice can build upon and refine. While they are polished, we welcome and encourage collaboration from language instructors to help modify grammar, syntax, and content where needed. Kindly contact pathwaysproject@boisestate.edu with any suggestions and we will update the content in a timely manner.
Pictionary Review
Description
In this activity students will review vocabulary related to the performing arts and theater by playing a game of Pictionary.
Semantic Topics
Pictionary, review, performing arts, theater, revue, théâtre, arts du spectacle
Products
Theater pieces, performances
Practices
Supporting the performing arts, attending musical theater performances, orchestra concerts, ballets, etc.
Perspectives
How are the performing arts valued in French culture?
Materials Needed
- Word Bank
- White Boards (via Zoom)
- Random Number Generator (search on Google)
Main Activity
Main Activity
Important Notes:
- Make sure that you have screen sharing enabled for all participants. In order to play Pictionary, all students need to be able to share their own Whiteboard through Zoom.
- Try not to repeat vocab words. If the number generator generates a number that’s already been done, try again until you get an unused number.
- It might be helpful to highlight numbers that have already been done on the Word Bank document. Just make sure to undo any modifications once you’ve finished the activity.
Aujourd'hui, nous allons réviser le vocabulaire du théâtre en jouant Pictionary.
1. Pull up a random number generator. This can be found simply by googling “Random Number Generator”
- Set the max. to 40
2. Choose a student to start the game of Pictionary, and use the random number generator to assign them a number between 1 and 40.
- This number corresponds to a vocabulary term from the Word Bank document.
- Send this vocab word to the student directly via the chat. *Make sure that you send it to only them and not everyone*
Quand c'est votre tour, vous allez choisir un nombre entre 1 et 40. Selon ce nombre, je vais vous envoyer un mot dans le chat.
3. Once they have their word, they will then share their screen and select “Whiteboard”
Partagez votre écran et choisissez "Whiteboard."
4. From there, they will choose “Draw” from the taskbar and begin drawing their vocab word.
Puis, vous allez choisir "Draw" de la barre des tâches et commencer à dessiner votre mot.
5. The other students will try and guess the word based on the drawing.
Les autres doivent essayer de diviner le mot selon le dessin.
6. If they’re having a hard time figuring it out, encourage them to ask their classmate questions.
S'il est difficile de diviner le mot, demandez des questions à votre camarade de classe. Voilà quelques exemples :
- Examples of questions:
- Ce mot, est-ce qu'il est masculin ou feminin ? (Is the word masculine or feminine?)
- Ce mot, est-ce qu'il est un nom ou un verbe ? (Is the word a noun or a verb?)
7. Repeat these steps until all the vocab words have been guessed or until time runs out.
Nous allons répéter ces étapes jusqu'à la fin de l'activité.
Wrap-Up
Wrap-Up
Ask the following question(s) to finish the activity:
- Avez-vous des questions ? (Have any questions?)
Cultural Resources
https://www.whatparis.com/theater-history-paris.html
http://www.discoverfrance.net/France/Theatre/DF_theatre.shtml
|
oercommons
|
2025-03-18T00:39:09.505590
|
Camille Daw
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/79493/overview",
"title": "French Level 4, Activity 12: Pictionary Review (Online)",
"author": "Mimi Fahnstrom"
}
|
https://oercommons.org/courseware/lesson/80344/overview
|
Like Water for Chocolate
Overview
This is a PPT presentation on the novel Like Water for Chocolate. It is designed to introduce students to the novel before reading.
Like Water for Chocolate Novel Overview, Background and Characters
This is a PPT presentation on Like Water for Chocolate. It provides the author background, character introductions, and information on magical realism.
|
oercommons
|
2025-03-18T00:39:09.521842
|
05/13/2021
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/80344/overview",
"title": "Like Water for Chocolate",
"author": "Trudi Mullerworth"
}
|
https://oercommons.org/courseware/lesson/103490/overview
|
Fear in Humans and Nonhuman Animals
Overview
Fear is a physiological, behavioral, and emotional reaction to stimuli that an animal encounters (Horwitz, 2013). In many species across the animal kingdom, the physiological reaction results in an increase in heart rate, increased respiratory rate (panting), sweating, trembling, pacing, and possibly urination and defecation. Fear is an appropriate response to environments and stimuli that are potentially harmful to the animal, which could utilize a creature’s fight or flight response and preserve its life. In this resource, fear of human and nonhuman animals, along with potential treatments for fear disorders, is explored and reviewed.
Glossary
Glossary
Respiratory rate
Fear
Fear stimulation
Secretion
Hormone
Limbic system
Fear circuits
Keystone area
PTSD
Somatosensory cortices
Cognitive Behavioral Therapy (CBT)
Claustrophobia
anxiety-based illnesses
Pharmacogenetics
Optogenetics
selective serotonin reuptake inhibitors (SSRIs)
Objectives
Objectives
Understand the neurological background of fear among people
Learn about the fear in a hormonal context - how it is stimulated and what is the trigger that cues fear stimulation
Validate if the fear exists among different species out of humans and in what condition their fear stimulates
Introduction
Introduction
Skyler Martinez
Lehigh University
Fear is a physiological, behavioral, and emotional reaction to stimuli that an animal encounters (Horwitz, 2013). In many species across the animal kingdom, the physiological reaction results in an increase in heart rate, increased respiratory rate (panting), sweating, trembling, pacing, and possibly urination and defecation. Fear is an appropriate response to environments and stimuli that are potentially harmful to the animal, which could utilize a creature’s fight or flight response and preserve its life.
Stimuli and Perception (+Hormones)
Stimuli and Perception (+Hormones)
Jay Kang
Lehigh University
It is well known that hormone secretion is not directly related to fear, fear stimulates the secretion of stress hormones such as epinephrine and cortisol, and they help the body to deal with whatever was causing the fear. In fact, the research by Stark has found that even among humans, the release of cortisol differs among males and females. The experiment was held with males in a two-group, receiving 30mg of cortisol or placebo, to experience fear stimulating paradigm. The result has shown that cortisol has restricted the learning process in males. However, the opposite result has been shown among the females. (Stark, 2006). This research indicates that even among the hormones that are responsible to fear stimulation, we have to acknowledge that these
Furthermore, recent research has shown a new view among the studies. Research by Maldonado suggested that hormones such as corticotropin-releasing hormone, adrenocorticotropic hormone, and glucocorticoids play roles in fear and anxiety roles. Furthermore, the limbic system stimulates fear circuits, functioning as a keystone area of the brain for fear circuits. (Maldonado, 2022) More specifically among PTSD patients a mental disorder which is specifically related to the fear of learning through exceptionally stressful conditions. In fact, the endocannabinoid system plays a key role in fear-related brain circuits by modulating these memories (Maldonado, 2022).
Fear-related signals and responses
Fear-related signals and responses
Skyler Martinez
Lehigh University
An interesting discovery related to complex fear systems can be found in the visual recognition of fearful stimuli. In a study that examined fearful faces as salient stimuli, indicating the presence of threat in the surrounding, there was evidence that fearful faces are preferentially processed in the fear system and connected sensory cortices. This is so even when they are presented outside of the participant's awareness or are irrelevant to the task being performed. Considering this, evidence was collected that showed the somatosensory cortices prioritize fear-related stimuli, primarily to the extent that tactile processing is enhanced in the presence of fearful faces (stimuli) (Bertini et. al, 2021).
Fear-based treatments (learning/unlearning)
Fear-based treatments (learning/unlearning)
Skyler Martinez
Lehigh University
Cognitive behavioral therapy (CBT) is a form of psychological treatment that has been demonstrated to be effective for a range of problems including depression, anxiety disorders, alcohol and drug use problems, marital problems, eating disorders, and severe mental illness (APA, 2017). Exposure therapy, one of many treatments under the CBT umbrella, is the most well-known medium of treatment for phobias and irrational fears. A proper example of cognitive behavioral therapy (CBT) and exposure therapy can be seen in the video below, with Dr. Reid Wilson treating a female patient suffering from claustrophobia. Conducting two notable sessions with the patient, the video shows how exposure therapy is easily considered one of the most effective ways to get long-lasting results in the treatment of specific phobias.
Exposure Therapy for Phobias Video with Reid Wilson
Illustration and schematic of key brain regions involved in fear and extinction learning (Milton et. al, 2019)
Striving for advancements in treatment for fear-related disorders, such as trauma and anxiety-based illnesses, pharmacogenetics, and optogenetics have allowed greater resolution in understanding the neural circuits that underlie fear (see image above). Implications for improving treatment have been made by researchers based on an expanded understanding of these fear circuits. Chronic pharmacological therapy, such as the utilzation of selective serotonin reuptake inhibitors (SSRIs), continues to be implied as an effective approach with observations showing enhanced acquisition of fear learning and delayed extinction learning in rats (Milton, 2019).
Regarding new approaches, there is great interest expressed in directly targeting maladaptive fear memories that contribute to disorders such as post-traumatic stress disorder PTSD. This would allow the treatment of PTSD in patients with remote trauma memories and would avoid ethical issues such as obtaining informed consent in acutely traumatized patients (Milton, 2019). Additionally, the use of behavioral interference techniques rather than a pharmacological amnestic agent had also been introduced by the research team. Their final suggestion to investigate would be proposing treatments that interfere with the reconsolidation of the cue representation that triggers intrusive thoughts and involuntary flashbacks in PTSD (Milton, 2019). Looking to the future, further investigation and experimentation in these approaches may yield promising results for the betterment and treatment of fear-disorder patients, notably those suffering from PTSD and related illnesses.
Animal’s Fear of Humans
Fear observed and expressed in nonhuman animals
Animal’s Fear of Humans
Karla Deleon
Lehigh University
In non-human animals, fear is defined as a psychological, behavioral, and emotional reaction to stimuli that an animal encounters. Stimuli that can cause an animal to express the emotion of fear can be from other animals, humans, noises, etc. When presented with a stimulus, the animal’s parasympathetic system kicks in and can cause autonomic changes such as an increase in heart rate and respiratory rates. Other signs of fear include sweating, trembling, pacing, and changes in the body’s posture (lowering of the body or fleeing/hiding).
As mentioned, animals can be fearful of humans. In a study done on domestic animals, their fear of humans can develop stress for the animals, which can then cause lost production. Much of this can be due to the fact of how the animal is handled by a person; if it is in a manner that seems unpleasant, it can lead to fear.
Video: Canine body language: fear
Pheromonal fear of animals
Pheromonal fear of animals
Jay Kang
Lehigh University
If you have seen National Geographics, you would have witnessed some examples that certain animals recognizing their predator without visually seeing them. This gives a range of thought if there are other communication tools within animals that may shape fear action. Then, the research by Bredy has suggested that pheromone communication may carve a social modulation of associative fear learning among animals. Through the experiment using rodent species, they found that fear of learning or extinction on subsequent extinction learning under the same species supports the effect of associative learning while it was not easily explained by changes in coping strategy (Bredy, 2008). This suggested that animals communicate information transferred through olfactory or pheromone cues (Bredy, 2008).
References and Works Cited
References and Works Cited
Bertini, C., & Ladavas, E. (n.d.). Fear-related signals are prioritised in visual, somatosensory and spatial systems. Neuropsychologia. Retrieved May 4, 2023, from https://pubmed.ncbi.nlm.nih.gov/33253690/
Society of Clinical Psychology . (n.d.). What is cognitive behavioral therapy? American Psychological Association. Retrieved May 4, 2023, from https://www.apa.org/ptsd-guideline/patients-and-families/cognitive-behavioral#:~:text=Cognitive%20behavioral%20therapy%20(CBT)%20is,disorders%2C%20and%20severe%20mental%20illness.
Wilson, R. (2012, November 13). Exposure therapy for phobias video with Reid Wilson. PsychotherapyNet. Retrieved May 4, 2023, from https://www.youtube.com/watch?v=0jZdzjAif60&ab_channel=PsychotherapyNet
Alexandra Kredlow, M., Fenster, R. J., Laurent, E. S., Ressler, K. J., & Phelps, E. A. (2021). Prefrontal cortex, amygdala, and threat processing: Implications for PTSD. Neuropsychopharmacology, 47(1), 247–259. https://doi.org/10.1038/s41386-021-01155-7
Stark, R., Wolf, O. T., Tabbert, K., Kagerer, S., Zimmermann, M., Kirsch, P., Schienle, A., & Vaitl, D. (2006). Influence of the stress hormone cortisol on fear conditioning in humans: Evidence for sex differences in the response of the prefrontal cortex. NeuroImage, 32(3), 1290–1298. https://doi.org/10.1016/j.neuroimage.2006.05.046
AL;, M. (n.d.). Fear not: Recent advances in understanding the neural basis of fear memories and implications for treatment development. F1000Research. Retrieved May 4, 2023, from https://pubmed.ncbi.nlm.nih.gov/31824654/
Bredy, T. W., & Barad, M. (2008). Social modulation of associative fear learning by pheromone communication. Learning & Memory, 16(1), 12–18. https://doi.org/10.1101/lm.1226009
Aztec Animal Clinic. (2016, August 3). Fears, Phobias and Anxiety - Aztec Animal Clinic. https://aztecanimalclinic.com/resources/pet-care-library/canine/fears-phobias-anxiety/#:~:text=Fear%20is%20a%20physiological%2C%20behavioral,and%20possibly%20urination%20and%20defecation.
Rushen, J., Taylor, A., & De Passillé, A. (1999). Domestic animals’ fear of humans and its effect on their welfare. Applied Animal Behaviour Science, 65(3), 285–303. https://doi.org/10.1016/s0168-159
|
oercommons
|
2025-03-18T00:39:09.548840
|
Health, Medicine and Nursing
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/103490/overview",
"title": "Fear in Humans and Nonhuman Animals",
"author": "Chemistry"
}
|
https://oercommons.org/courseware/lesson/58148/overview
|
Resume Development
The Professional Portfolio
Unit I: Introduction to the Pre-Professional - PEP 410 - Spring 2020 - Lincoln University of Missouri
Overview
I. Building the Pre-Professional Experience
II. Professional Associations
III. Professional Development/Networking
IV. Developing Job Search Documents
V. Professional Portfolio
Building the Pre-Professional Experience
What is a Pre-Professional Experience?
Pre-professional work experience is a great transition step from college to career, especially in industries where experience is a virtual must-have for entry-level positions. Pre-professional opportunities help students obtain valuable industry- and job-specific knowledge and skills that pique employer interest.
The Lincoln University School of Education continuously supports you, our students, with their educational and professional goals through experiences beyond the classroom. Regardless of how you entered the degree program, the educational opportunities offered to you will greatly impact your success as a professional. These are opportunities for you to take your theoretical knowledge (textbook and theories) and apply them in a practical setting (use the knowledge in a hands-on application).
This combination of theoretical knowledge and practical experience will require a considerable investment of time and effort. To maximize your investment, the following areas of experiences have been identified for you:
- Teaching Observations
- Athletic Training/Coaching Observations
- Teaching Practicum Opportunities
- Internships
- Research Experience for Undergraduates
- Service Learning
- Volunteer Opportunities
- Student Organizations
- Student Teaching
The goal for you as a Lincoln University student and future Educational Professional is to get involved in as many of the above experiences as possible prior to entering the field, but ultimately to obtain a student teaching experience.
Complete Assignment Pre-Professional Experience (Canvas)
Professional Associations
Joining a Professional Association provides members with a competitive advantage because they become active, informed members of their industry. Many association members who lead busy professional lives depend on their association to brief them on important industry trends, new legislative rulings, and advances in technology.
A professional association (also called a professional organization or professional society) seeks to further a profession, the interests of individuals engaged in that profession and the public interest.
What are the Benefits of Joining a Professional Organization?
- Continuing Education
- Job Prospects
- Mentoring Programs
- Networking Opportunities
- Access to Resources
- Learning New Perspectives
- Professional Development
- Liability Insurance
Read the following Article:
https://www.onelegal.com/blog/10-benefits-joining-professional-association/
Why Should You Join a Professional Organization?
If you are interested in furthering your career, joining a professional association is a good start. There are associations for nearly every profession or area of interest, and many have national, state and regional chapters available to join. An association is a synergistic group, meaning that the effect of a collection of people is greater than just one person. So, how exactly can becoming part of this synergistic group help further your career goals? Here are some benefits of joining an association.
Enhance Your Network:
For most people, creating professional relationships is important, and joining a group allows you to have a sense of security and trust. From this, you can support and help one another in reaching your professional goals. Associations sponsor numerous events throughout the year that allow you to connect with your peers. You can share ideas, ask for advice, volunteer to be a speaker or become a member of a committee. Since most associations have national or local conferences, you can participate and have the opportunity to learn about breaking news in your career, learn "best practices" or new ideas, hear about key achievers in your field and also meet and brainstorm with others who are also looking to share and learn new information.
Another benefit of enhancing your network is that you may find a mentor to help you with your professional needs or you may be able to become a mentor to someone else. As an educator, giving back can be the greatest reward and benefit. Participating in forums, chat groups or discussion boards sponsored by the association is also a great way to grow your network. This allows you to use your peers as sounding boards and often make some great friends with the same interests as you.
Take Charge of Your Career:
Another important reason to consider membership to a professional organization is to take advantage of their career resources. Associations often have job listings online or in print available only to their members. This is a great way to find targeted job postings for your area of interest. Additionally, many associations have career resources available such as tips on effective resumes or cover letters, job searching strategies, and negotiating techniques. Some associations even have panels of experts that you can contact for specific questions on career issues. Other benefits include information about seminars, training or certification classes that may be suitable for you. Often these classes can be done through web or podcasts, so you don't even have to leave your home. And don't forget, listing your association membership on your resume is impressive to current or future employers as it shows that you are dedicated to staying connected in your profession.
Broaden Your Knowledge:
Most associations provide an enormous amount of access to resource information such as case studies, articles, white papers and books written by experts in your field or area of interest. Also, major journal, magazine and newsletter access is provided as a part of your membership privileges. Another reason to join an association is to learn more or stay informed about issues in diversity. For example, Academic360.com includes a list of associations and articles that provide valuable information such as resource guides for diversity, affirmative action, and advocacy, as well as information on new and proposed regulations related to diversity. Associations also provide a source for scholarship information, links to publications, and awards for persons achieving excellence in their field. No matter what your field is, staying on top of all of these issues is important.
So, whether you are looking to learn about job postings in your field, network in your professional community, gain access to current events in your career area, or just have some fun while meeting new people, joining a professional association is a step in the right direction!
Complete Assignment Professional Organizations (See Canvas)
Professional Development/Networking
Networking has become an essential aspect of your job search in today’s competitive market. You must develop relationships and connections within your network to have more opportunities to advance your career. You should build your “professional network” by joining local, state, and national organizations, attending conferences and networking events and building professional relationships every chance you get.
Why Networking is Important
Having a well-established network has become an important part of our lives. The easiest way to expand your network is to build on relationships with people you know; family, friends, classmates, colleagues, and acquaintances.
We are all expanding our networks daily. Think about it, who would you ask if you needed an electrician, a plumber, a dry-waller, a painter or landscaper? You would likely ask a family member, friend or close colleague if they know anyone they would recommend or maybe you would go to Angie’s List or the Better Business Bureau. These are all ways of networking.
Professional Networking
Professional Networking has become an essential aspect of your job search. Even if you are well established in your job and have no plans of moving or advancing your career soon, networking has proven to be a valuable tool. Today, studies have shown that up to 80% of jobs are never advertised – they are filled by word of mouth. So, it’s who you know and who knows you that matters. You must develop relationships and connections within your network to have more opportunities to advance your career. Attending meetings and social events hosted by your professional association is a great way to connect with people in your field.
Volunteering
We all have things we're passionate about and special skills and interests. Volunteering is another great way to meet people and broaden your network. There are always ways to make connections with individuals in your professional organization and it could prove to be a great resource for advancing your career. Remember to add your volunteering activities to your resume.
Network Your Way to Success
Any expert will tell you that networking is one of the best ways to advance your career, and it's also a good source of support for everyday job concerns. Employers, especially those with good diversity programs, also recognize the value of networking, and there are official -- as well as unofficial -- networks for virtually every group.
"A lot of people of color find these networks especially important," says Cornelia Gamlem, president of the GEMS Group, a human resource consulting firm in Herndon, Virginia. "They can be a kind of balance in understanding whether a situation is unique or if it's something other people have gone through as well. [Networks] can help people avoid that feeling of being isolated and overcome problems all on their own."
But, remember how you network is just as important as whether you network. Here are some rules to network by:
Get an Early Start
The sooner you start creating a network, the faster you'll progress in your career. Many professional societies have student chapters in colleges and universities. Making connections early will give you a head start on your job search. Keep your eyes open for networking opportunities as soon as you've landed a job.
Look Before You Leap
"Be careful of whom you ally yourself with," warns Mary Jane Sinclair, president of MJS Associates in Morristown, New Jersey. "They may be using you to advance an issue." Sinclair uses an example of a young college grad who joined an in-company women's network. However, rather than advancing the members' cause, this network was more interested in taking on management. "This woman was viewed by management as a troublemaker," Sinclair says. Once you've taken a job, carefully find the networks that will be most beneficial to you and your career.
If at First You Don't Succeed, Try Again
Unfortunately, there isn't always an obvious network to join. For instance, if you're an African American woman in a sea of white colleagues, it may not be easy to align yourself with others in the company. See if there's a local professional organization with African American members. Or seek out people in your community. Don't just limit yourself to racial or gender categories.
Cast a Wide Net
"Look for support wherever you find it," Sinclair says. "Networking really works best when the group's common interest isn't just race or gender, but the success of each member in the group." Establishing a broad network enables you to turn to different groups, depending on your professional challenges. "Without a broad-based network, there's no one to turn to in a time of crisis," Sinclair says. "The broader you cast your net, the broader your catch will be."
Read the Following Article:
Developing Job Search Documents
Finding the job you want is a process that takes multiple steps and involves many decisions.
To be successful, job seekers need relevant information and well-developed job-hunting skills that allow them to be strategic in their search. Whether you are looking for an internship, job, or making a career change, the principles and the process of conducting a successful job search are the same.
The following tips are designed to guide you through the broad process of identifying job targets to the specific steps of finding openings and applying for positions. Each strategy is equally important, but the sequence of steps may vary. A successful job search plan may involve doing all the activities simultaneously. Be prepared to revisit any step, evaluating and adjusting your strategy as your search evolves and changes.
THE RESUME
Your resume represents the quality of your work and is the first sample that a prospective employer will review. To create a positive impression, it should:
- Be professional in content and appearance
- Be well organized
- Communicate your information clearly and effectively
- Show achievement
- Be honest and accurate
- Demonstrate what you can do for the employer
Think of your resume as a custom-designed marketing tool developed to capture the attention of a prospective employer – within 20 seconds! That’s how long it takes a reader to formulate a first impression and that's how long you have to create an impact! Communicating your professional qualifications quickly and effectively will determine your chances of being considered for an interview.
A resume is a descriptive summary of your background, concisely written and attractively presented. It should focus the reader on your strongest points in relation to your current career goals. Follow the rules of grammar, punctuation, and structure. To be effective, customize your standard resume for a specific position or industry.
Take time to prepare your resume by reviewing your achievements, skills, experiences, and strengths. Be sure to highlight the skills and achievements that relate to the positions you are targeting.
Read the following Article:
See Attachment: Resume Development
PROFESSIONAL REFERENCES
At some point in the job search process, most employers require the names of certain individuals who can attest to your qualifications for employment. Prepare a separate one-page reference sheet to have available during an interview or when sending a resume, if requested.
Select three to five people who you believe will provide a positive reference for you. These people may include former or current employers/supervisors, professors, coaches or others who have observed you in a leadership, academic or professional capacity. Do not select relatives or friend. Make selections based on a person’s ability to make objective comments regarding your work ethic, your responsibility level, your sense of creativity and initiative. When deciding who to ask, try to have a mix of professors, supervisors, and others.
Remember to contact your references in advance for permission to use their name and for their preferred mode of contact, (e.g. e-mail, phone). Provide these people with a copy of your resume and tell them about the types of positions you are trying to obtain. Remind them about the skills and experiences that make you a good match for the job. Ask them to write a recommendation letter so you can attach it to your application materials when completing an application.
THE APPLICATION/COVER LETTER
Employers receive hundreds of resumes from job applicants. To make a great first impression, your application/cover letter needs to be well-written to grab the employer’s attention. As soon as the envelope (or e-mail) is opened an indelible impact is made. Your cover letter often determines whether the resume is even read!
A cover letter is a personal marketing component of the application process. It is your opportunity to introduce yourself, point out your specific job-related qualifications, and demonstrate your written communication skills. You want the employer to be interested in what you have to say and want to learn more about how you match the qualifications for their specific job opening.
A cover letter should always be tailored to the specific position. This letter should be one page, no more than three or four paragraphs and designed to:
- Market your skills, related experience, and accomplishments
- Introduce yourself and establish yourself as a high-value candidate
- Generate interest in meeting you
Remember, cover letters should ALWAYS accompany a resume, even if one is not specifically requested. A good cover letter takes time to write, but in the long run, it will be worth the extra time and effort. It is the first sample of your work that the employer will read.
Read the following Article:
See Attachment: Application Letter Development
Professional Portfolio
When and why do you use Professional Portfolios?
During a job search, the Professional Portfolio showcases your work to potential employers. It presents evidence of your relevant skills and abilities. Portfolios are also helpful for independent contractors, consultants, business owners, and professional educators who need to provide work samples to potential clients or employers.
Once you've identified an internship or job, the common practice of applying is submitting your resume and application/cover letter. A Professional Portfolio provides a potential employer with even more information and specific examples of your work. An impressive portfolio can often catch the interest of employers, which is your goal when applying for an internship or job, and then hopefully make them want to call you in for an interview.
A Professional Portfolio provides potential employers with a complete picture of a job candidate's abilities. It should include your experience, accomplishments, skills, education, interests, and professional goals and objectives. You may send your portfolio prior to an interview, bring it with you during the interview process, and you may decide to leave it with the decision-maker in the hiring process for follow-up information.
Here are some helpful tips on how to make a professional portfolio:
Collect Examples of Your Work
Creating a professional portfolio begins by collecting examples of your work. These examples may include evaluations, reports, surveys, specific materials you have designed for a college course or previous employer, graphs, press releases, artwork, examples of spreadsheets, etc., that you designed to complete certain projects or to improve the flow of the work.
If you are currently completing an internship or student-teaching, be sure to include some of the work you are doing right now. Teachers can put together exceptional portfolios by highlighting projects they introduced in the classroom as well as the lesson plans they created for the class. Portfolios are a great chance to show your creativity and the nice thing is that no two portfolios are alike.
Include Photos of Yourself Working
Including photos of yourself working on specific academic projects or in previous internships or community service work will help the employer to see you in action. Sometimes these visual pictures can say a thousand words and don’t often need any explanation. If you are currently completing an internship or volunteering for a company, be sure to take some photos to keep your portfolio current and to show employers what you're doing right now.
Include Info About Educational Projects
If you have previously worked with educational projects or initiatives at a school, be sure to include information about them. This may include standards-based grading, Curriculum Development, Units of Instruction, Varying Delivery Methods, and Lesson Plans for new or creative lesson ideas.
Include Positive Correspondence
Consider including any positive correspondence that you have received in the past from teachers, professors, previous employers describing your hard work and professionalism. Having professionals in the field commenting on the outstanding work you contributed to any project can make a real positive impact on any employer. Be sure to include an example of the outstanding piece of work (college paper, artwork, lesson plan) so they can see it and evaluate it for themselves. It is important to understand that if you don’t highlight your successful experiences no one will know and nobody else will.
Demonstrate Your Skills
If you have a video of you performing job-related skills, such as teaching a lesson or coaching a practice session be sure to include the links to showcase the work you have done. Some students will include a DVD or CD attached to a plastic sleeve on the front or back cover of the portfolio.
Create Clear Concise Documents That Are Organized
You want your portfolio to look professional. Creating clear, concise documents that are organized well will let the employer know that you are serious about the job. Be sure to always keep copies of your work and make sure that you keep updating your portfolio so that some of the examples of your work are recent which can also illustrate the growth you’ve made over the years.
Remember to keep your Professional Portfolio updated. This is important to do even after you’ve gotten the job. Down the road, you may be in the job market again looking for a new job and the last thing you want is to only have an outdated portfolio that needs to be re-created from scratch.
Read the following Article:
See Attachment: The Professional Portfolio
|
oercommons
|
2025-03-18T00:39:09.607792
|
Chad Kish
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/58148/overview",
"title": "Unit I: Introduction to the Pre-Professional - PEP 410 - Spring 2020 - Lincoln University of Missouri",
"author": "Dylan Martin"
}
|
https://oercommons.org/courseware/lesson/80017/overview
|
Syllabus PO 102 United States Government
Overview
This is the 2021 Spring Semester PO 102 United States Government Syllabus taught at Bennett College in Greensboro, NC.
Syllabus PO 102 US Government Spring 2021
The Spring 2021 Syllabus for PO 102 United States Government is uploaded in the Section Resources below.
|
oercommons
|
2025-03-18T00:39:09.626410
|
05/08/2021
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/80017/overview",
"title": "Syllabus PO 102 United States Government",
"author": "Gwendolyn Bookman"
}
|
https://oercommons.org/courseware/lesson/106788/overview
|
Wellness and Prevention
Overview
This resource provides an ECHOES recording regarding Every Moment Counts, a program designed to build capacity of school personnel and families with mental health/awareness. While currently there are few resources, physical health/wellness, and mental health/wellbeing are part of the PT and OT scopes of practices. A discussion with the new therapist may center around district priorities and the district's right of assignment of the contracted OT/PT time.
Wellness and Prevention
The following ECHO recording discusses the program Every Moment Counts which is a program designed to build capacity for school personnel and families regarding the promotion of positive mental health.
Building Communities of Practice to Foster Implementation of Every Moment Counts (75 min recorded session)
While currently there are few resources, physical health/wellness, and mental health/wellbeing are part of the PT and OT scopes of practices. A discussion with the new therapist may center around district priorities and the district's right of assignment of the contracted OT/PT time.
|
oercommons
|
2025-03-18T00:39:09.639435
|
07/17/2023
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/106788/overview",
"title": "Wellness and Prevention",
"author": "Nathaniel Baniqued"
}
|
https://oercommons.org/courseware/lesson/60661/overview
|
Friends and Peer Groups Presentation
Overview
This is our video presentation on friends and peer gorups for our physchology class. We covered how we approached the topic as well as what our learning objectives and and learning material we used. We hope you enjoy our video and take something away from it.
Friends and Peers - Chase Dreksler and Cooper Wilcox - PSYC 310
|
oercommons
|
2025-03-18T00:39:09.651464
|
12/12/2019
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/60661/overview",
"title": "Friends and Peer Groups Presentation",
"author": "Cooper Wilcox"
}
|
https://oercommons.org/courseware/lesson/106262/overview
|
CAT final draft
Phonemic Awareness in Early Intervention
Overview
What is the current state of the research on phonemic awareness treatment efficacy?
What is the current state of the research on phonemic awareness treatment efficacy?
What is the current state of the research on phonemic awareness treatment efficacy?
|
oercommons
|
2025-03-18T00:39:09.669058
|
07/03/2023
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/106262/overview",
"title": "Phonemic Awareness in Early Intervention",
"author": "Lauren Sprague"
}
|
https://oercommons.org/courseware/lesson/38042/overview
|
Team GBSD! OSP NGSS4Oregon Module #2 - Talk & Equity
Overview
The Oregon Science Project Module #2 is designed for K-12 and nonformal educators who want to learn more about NGSS, with an emphasis on the central role student discourse and talk play in the K-12 NGSS classroom. It is designed to provide 6-12 hours of work and asks learners to create something new to contribute to the work.
Who talks and why?
Engaging All Students
Why Is Science Talk Important? Individual Work
C
Module #2 Components:
Task #1 - Why is Science Talk Important? Individual Work
Task #2 - Why is Science Talk Important? Group Reflection and Discussion
Task #3 - What Does Science Talk in the Classroom Look Like? Individual Work
Task #4 - What Does Science Talk in the Classroom Look Like? Group Reflection and Discussion
Task #5 - How Do We Increase Science Talk? How Do We Show Others? Individual Work
Task #6 - How Do We Increase Science Talk? How Do We Show Others? Group Reflection and Discussion
On Your Own:
Components: Readings, visuals, and survey response to prepare for Task #2 Relevance: Choose between primary, elementary, and secondary options Preparation: This individual work portion prepares you to engage in reflective discussion with a small group in Task #2
Questions driving our work together in this module:
Q: Why is it important to engage all of our students in science talk?
Q: How do students engage in talk during science in your classroom (what protocols, norms, or framing do you use)?
Q: How would you like them to engage?
Students' attitude, motivation, and identity greatly impact how, and if, they participate productively in science in the classroom. The impact of these traits on student learning vary greatly K-12. Research also shows that it is the teacher's framing of the classroom that is essentail for promoting students' feeling of belonging and participation necessary for them to share their ideas and make their thinking public.
"I can do science."
"I want to do science."
"I belong."
Please click on the resources below that best relate to your practice and interests. As you engage with them, think about how you frame your classroom to promote productive participation for your students, and what is needed to include more students. You will need to use these resources to complete Survey #1 at the end of this task below. Once you have completed that survey, you can proceed to Task #2.
Primary Resources to Complete this Task
Upper Elementary Resources to Complete this Task
Secondary Resources to Complete this Task
Survey #1 - complete after engaging with relevant resources
Why is Science Talk Important? Group Reflection and Discussion
c
In A Small Group:
Components: Survey with question prompts to drive reflective discussion. Every person completes their own survey. Relevance: Although the resources vary by grade level, this group task is not grade-level specific.
Bring your thinking and reflections from Task 1 so you are ready to contribute to the group discussion. Please collaboratively complete the survey by discussing questions together as each of you fills out your own survey.
Survey #2 Why is Science Talk Important? Group Reflection and Discussion
What Does Science Talk in the Classroom Look Like? Individual Work
c
On Your Own:
Components: Grade-appropriate video examples and resources. (NO SURVEY) Relevance: Choose between primary, elementary, and secondary options. Preparation: This individual work portion prepares you to engage in reflective group discussion.
"Instruction can be designed in ways that foster a positive orientation toward science and promote productive participation in science classrooms. Such approaches include offering choice, providing meaningful tasks and an appropriate level of challenge, giving students authority over their learning while making sure their work can be examined by others, and making sure they have access to the resources they need to evaluate their claims and communicate them to others." - Taking Science to School.
Questions from prior work continues to drive your discussion and should be considered as you engage with the materials below:
Q: Why is it important to engage all of our students in science talk?
Q: How do students engage in talk during science in your classroom (what protocols, norms, or framing do you use)?
Q: How would you like them to engage?
Please select the grade level that is most relevant for your practice and watch all video segments and engage with any readings or articles. Be ready to bring your observations and questions to your small group discussion in Task #4.
As you engage, make connections to your own practice and your vision for increased productive participation by more of your students.
Primary Grades
Upper Elementary
Talk Moves Primer (read pages 7-11)
Secondary
Discourse Primer (read pages 5-14 paying attention to "discourse moves")
There is no survey for this task. Be ready to engage in active discussion around what talk looks like for the next task.
What Does Science Talk in the Classroom Look Like? Group Reflection and Discussion
c
In A Small Group:
Components: Survey with question prompts to drive reflective discussion. Every person completes their own survey. Relevance: Although the resources vary by grade level, this group task is not grade-level specific.
As a small group, please collaboratively complete the survey by discussing questions together as each of you fills out your own survey. Be sure to bring in your impressions, observations, and wonderings prompted by the resources in Task #3.
Survey #3 Group Reflection and Individual Survey
How Do We Increase Student Science Talk? How Do We Show Others? Individual Work
c
On Your Own:
Components: Blog post reading, task analysis survey, exploration of gradeband NGSS storylines. Preparation: This individual work portion prepares you to engage in reflective group discussion.
When we think of framing we are referring to "a set of expectations an individual has about the situation in which she finds herself that affects what she notices and how she thinks to act." - Resources, Framing, and Transfer
Please read this short blog post comparing two different classrooms using the idea of framing to set the context for student exploration, learning, and understanding of what they are learning in science as envisioned by the NGSS.
Look at these norms and think of your own classroom. As you set the context and frame your classroom for productive participation, look closely to see how you are asking students to productively participate. Below is an example from the Inquiry Project where teachers worked collaboratively when approaching their students to develop norms for equitable participation.
Please complete this task analysis survey by clicking here or the image of the survey below on your own by imagining a hypothetical group of students. Please consider a group of students engaged in the task who are similar to students you work with in your own practice. How can the NGSS practices guide planning for rich language use and development by students? One tool that can help us is a task analysis process.
Please read the first pages of a relevant grade and/or core idea storyline below in preparation to think about a relevant task to create and analyze that could provide opportunities for productive participation by students by engaging them in NGSS practices. Remember, the task should be very small - requiring only 10-20 minutes of work by students. Any larger grain size of task and the task analysis is no longer a useful tool. We are having you use the storyline as a tool because it covers the core ideas of your grade(s) and lets us connect to our ideas of hands-on explorations. You are also welcome to go further into the documents and work from a performance expectation, but the task for this must be at a very small grain size in comparison to the gigantic performance expectations. You will be crafting the task with the support of your small group in Task #6.
NGSS Storylines
How Do We Increase Student Science Talk? How Do We Show Others? Group Reflection and Discussion
c
In a Small Group:
Components: Two surveys to drive reflection and creation.
Collaboratively complete Survey #4. Utilizing your experience learning more about framing, productive partcipation norms, and task analysis please collaboratively go through Survey #4 below. One at a time each participant should share their draft ideas for a task they planned on their own in Task 5. You will submit your task (remember small grain size!) on this survey and you will be able to see others' tasks as well.
Survey #4: Collaborative Survey for Task Creation
Here is an image of the 3 dimensions to quickly reference as you create your tasks.
Collaboratively complete Survey #5. Each person will have the group analyze their newly created task and then each person submits their survey for their task only. Directions on survey.
Survey #5: Collaborative Survey for Analysis of Your Tasks
Once everyone has completed the surveys and the discussion has wrapped up:
Look at the collective responses and discuss how you could use this in your practice to communicate the importance of talk in the science classroom.
|
oercommons
|
2025-03-18T00:39:09.695942
|
Jennie Richard
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/38042/overview",
"title": "Team GBSD! OSP NGSS4Oregon Module #2 - Talk & Equity",
"author": "Module"
}
|
https://oercommons.org/courseware/lesson/123839/overview
|
NMR practice quiz
Overview
A practice quiz from 2019
1) alkene addition
2) Friedel-Crafts
3) simple NMR
NMR practice quiz
Alkene + X2
Friedel-Crafts
simple NMR
|
oercommons
|
2025-03-18T00:39:09.711847
|
01/16/2025
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/123839/overview",
"title": "NMR practice quiz",
"author": "Shallee Page"
}
|
https://oercommons.org/courseware/lesson/79411/overview
|
The importance of music in different cultures
Overview
Objective: The importance of music in different cultures
Level of culture: All 3 levels of culture
Skills: Speaking and Listening
1. The teacher chooses different countries (including , for instance, Armenia, the UK,
and the USA) and plays three different pieces of music.
2. The class is divided in three groups, and each one selects a country.
3. The students gather information, listen carefully to every piece, write down the pecularities of music and do the research about the musical traditions in countries. They can use computer, audio-visual materials to do the research.
4. Finally, the students show the presentation before the class and discuss the questions asked by the classmates.
Here we have got all three levels of culture included into the process. The students; show their own culture by presenting the source culture, then comes the target culture (the UK musical traditions) and then comes the international culture bringnig international interaction.
I made this activity an OER, by marking it “CC BY 4.0” and, thus allowing you to “reuse, revise, remix, and redistribute it “.
The importance of music in different cultures
Objective: The importance of music in different cultures
Level of culture: All 3 levels of culture
Skills: Speaking and Listening
1. The teacher chooses different countries (including , for instance, Armenia, the UK,
and the USA) and plays three different pieces of music.
2. The class is divided in three groups, and each one selects a country.
3. The students gather information, listen carefully to every piece, write down the pecularities of music and do the research about the musical traditions in countries. They can use computer, audio-visual materials to do the research.
4. Finally, the students show the presentation before the class and discuss the questions asked by the classmates.
Here we have got all three levels of culture included into the process. The students; show their own culture by presenting the source culture, then comes the target culture (the UK musical traditions) and then comes the international culture bringnig international interaction.
I made this activity an OER, by marking it “CC BY 4.0” and, thus allowing you to “reuse, revise, remix, and redistribute it “.
|
oercommons
|
2025-03-18T00:39:09.724887
|
04/19/2021
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/79411/overview",
"title": "The importance of music in different cultures",
"author": "Zhanna Kazazyan"
}
|
https://oercommons.org/courseware/lesson/86389/overview
|
Nuclear Chemistry
Overview
Nuclear Chemistry topics covered from chemistry text book.
Chemistry
PowerPoints cover basics of Nuclear Chemistry.
|
oercommons
|
2025-03-18T00:39:09.740550
|
10/02/2021
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/86389/overview",
"title": "Nuclear Chemistry",
"author": "Shibin Chacko"
}
|
https://oercommons.org/courseware/lesson/115668/overview
|
exercise and the brain Overview exercise exercise and the brain exercise PDF Exercise and the brain chapter Download View
|
oercommons
|
2025-03-18T00:39:09.765607
|
05/01/2024
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/115668/overview",
"title": "exercise and the brain",
"author": "Nolan Jetter"
}
|
https://oercommons.org/courseware/lesson/116884/overview
|
Class Activity
Child Psychology Class Participation Activity and Capstone Project
Overview
This resource comprises a series of educational materials specifically designed for instructional use in the PSYC 310 Child Psychology course at Colorado Mesa University. Included within this resource are 12 Class Participation Activities and a comprehensive Capstone Project.
These materials are stored as downloadable documents in a Google Drive folder, accessible for educators and students alike. They are ideal for enhancing course content in undergraduate Child Psychology or Child Development classes. Educators are encouraged to freely adapt these materials to fit their specific instructional needs.
Please feel free to contact me at aniu@coloradomesa.edu if you encounter any issues, such as typographical errors, or if you have questions about adaptation. I am committed to continually improving these resources and appreciate all constructive feedback.
Colorado Mesa University - PSYC 310 Child Psychology - 2024 Spring
This suite of educational materials, designed for the PSYC 310 Child Psychology course at Colorado Mesa University, includes both Class Participation Activities and a Capstone Project. These resources are part of the "Level 2- Adapt" OER project, supported by the More CMU MORE grant from the Colorado Department of Higher Education, and utilize the adapted OER textbook "Early Childhood Development" by Alexa Johnson et al., provided by College of the Canyons.
Class Participation Activities: This collection features 12 engaging activities aligned with the first 12 chapters of the textbook, covering essential topics such as child psychology theories, genetic foundations, parenting styles, and attachment. Each activity is detailed with objectives, topic coverage, necessary materials, and instructions, making them ideal for introductory undergraduate Child Psychology or Child Development classes.
Capstone Project: "Applying Child Psychology in Real-World Settings" allows students to practically apply their learning to real-life scenarios of their interest. It includes phases of exploration, proposal development, theoretical essay writing, and a final presentation, providing a structured approach to integrate research findings both theoretically and practically.
These materials are designed to enrich the learning experience, enhance student engagement, and provide educators with ready-to-use resources that support both theoretical understanding and practical application in Child Psychology.
Reference for textbook: Johnson, A., Ricardo, A., Rymond, D., & Paris, J. (2019). Early Childhood Development. College of the Canyons. https://oercommons.org/courses/child-growth-and-development
|
oercommons
|
2025-03-18T00:39:09.784085
|
06/17/2024
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/116884/overview",
"title": "Child Psychology Class Participation Activity and Capstone Project",
"author": "Yanzhuo Niu"
}
|
https://oercommons.org/courseware/lesson/122192/overview
|
Democracy In Rwanda
Overview
Rwanda is the "Land of a Thousand Hills" full of greenery and fertility. The nation is landlocked boarded by Uganda, Tanzania, Burundi, and the Democratic Republic of the Congo. The country has a growing population with over 13 million people one of the highest population densities in Africa. Many of its citizens are young people born after The 1994 Genocide Against The Tutsi and are jobless and in poverty. Kinyarwanda is the country's official language. Rwanda is a very religious nation and is known as one of the most politically stable countries in the continent of Africa. Rwanda is known for its cleanliness, and banning plastic bags, with a collective effort for community unity and organization. The citizens of Rwanda gather together for Umuganda a traditional practice and a national holiday that takes place the last Saturday of every month from 8-11 am, where everyone comes together to clean, repair, and build in their communities.This Open Educational Resource will navigate the democratic attributes of Rwanda and the lack thereof.
Learning Goals
Essential Question:
Is Rwanda a Democracy?
You will be able to:
Describe the attributes of Rwanda’s Democracy or Lack Thereof.
History of Rwanda
The Kingdom of Rwanda was a Monarchy, known as the Mwami. The Monarchy emerged in the 15th century. Germany's colonial rule impacted the Kingdom in the 1890's. 1916 Rwanda became a Belgian mandate after Germany's defeat in World War I. By the 1950s Rwanda's governance system had completely shifted away from its traditional practices and became a complete colonial administration. The three main societal social groups Hutu, Twa, and the Tutsi were determined by colonial leaders. Creating a "Tutsi Elite" and a "Hutu Majority" the male-led political parties within these identity factions were so determined and encouraged in their beliefs that ignited the first phase of the genocide in 1959 (Prunier). Violence between and against these groups escalated through propaganda and politics leading to the 1994 Genocide Against The Tutsi.
How Does Rwanda's Democracy Function?
Today, Rwanda has a strong governmental structure. Its constitution is centered on reconciliation and peacebuilding. The country has executive, legislative, and judicial branches, as well as several ministries departments. The Ministries of Health, education, agriculture, animal resources, ministry of infrastructure, finance, and Economic Planning have full control of their focuses. The Rwandan Governance Board monitors compliance and good governance in the private, public, and non-governmental sectors of Rwanda.
The Country is divided into four provinces; the Northern Province, Southern Province, Eastern Province, and Western Province. The City of Kigali is the capital of Rwanda and is considered a decentralized governance. Within each province, there are subdivisions of 30 districts, which are further subdivided into sectors, cells, and villages. The majority of the provinces are rural and are catered to by the government through needed health, food, and farming programs and benefits.
Umudogudo is the village gathering where leaders navigate issues together in their neighborhoods. Rwanda ensures that everyone has health insurance. Health insurance is half a dollar for a year. Your economic category (Ubudeje) will impact how much you have to pay for insurance. The wealthy pay 75% and middle class pays 50% and Graduates from university pay 15% with the government paying the remainder. For the improvised communities the government pays 100%.
This is Minubuwe the Ministry of National Unity & Civic Engagement in Kigali, Rwanda
Leadership in Rwanda
The Leadership in Rwanda has been consistent since after the 1994 Genocide Against The Tutsi, with Paul Kagame being President. A great attribute in leadership is Rwanda's focused on women holding positions of power. Rape and Gender Based Violence was used a tool in the genocide. Now, women are being uplifted in Rwanda's government holding 61.3% of seats in Rwanda's Parliament. Which is the highest percentage in the world. Women also occupy 50% of the positions in the President's cabinet. However, it is known that Rwanda's "increased representation of women has not led to greater statutory protection of women's rights, nor has it led to a more democratic political terrain." (Burnet)
Political Parties:
RPF
Liberal Party
Green Party
Opposition:
FDLR
Interhamwe
Conflicts & Concerns of Rwanda's Democracy
Rwanda has many democratic features such as elections, institutions, and economic development through partnerships, gentrification, agriculture, and tourism. However, Rwanda's President Paul Kagame has been in power since 2000. There have been very intense limitations on any of his opposition parties. The country does not have freedom of speech in the media. Rwanda is a very civil society with little activism or criticism of the government at all.
It has only been 30 years since the Genocide and the country is still recovering and remains weary of conflict. Many people do not trust anyone, rightfully so, there is an overall lack of vulnerability. This 30th anniversary will also mark the release dates for many perpetrators of the genocide.
The government of Rwanda plays a significant role in the ongoing genocide in the Democratic Republic Of the Congo.
My Personal Experience In Rwanda
Ms.McCullough's Observations Of Democracy In Rwanda.
Jocelyn McCullough studied abroad in Rwanda in Spring 2024, while she was there she reached every point of the country. Meeting all types of citizens and leaders, she was able to navigate the country and learn a lot about its functionality.
Key Takeaways
Sources
Works Cited
Buckley-Zistel, Susanne. “What Caused the Rwandan Genocide? - Susanne Buckley-Zistel.” YouTube, 27 June 2023, youtu.be/MF7EbUGlaOU?si=xVUPjBGjSrgpgPcg. Accessed 2 Dec. 2024.
Burnet, Jennie E., "Women Have Found Respect: Gender Quotas, Symbolic Representation and Female Empowerment in Rwanda" (2011). Anthropology Faculty Publications. 3.https://scholarworks.gsu.edu/anthro_facpub/3
EBSCO Publishing: eBook Collection (EBSCOhost) - printed on 9/23/2020 9:12 AM via SIT GRADUATEINSTITUTE/SIT STUDY ABROADAN: 2175846; Uvin, Peter.; Aiding Violence: The Development Enterprise in RwandaAccount: s9324602
Gérard Prunier. The Rwanda Crisis : History of a Genocide. New York, Columbia University Press, 1997.
Harding, Robin . “Who Is Democracy Good For? Elections, Rural Bias, and Health and Education Outcomes in Sub-Saharan Africa.” University of Oxford, 27 Dec. 2018.
Newbury, Catharine. “Background to Genocide: Rwanda.” Issue: A Journal of Opinion 23, no. 2 (1995): 12–17. https://doi.org/10.2307/1166500.
News Global, TLDR. “Will War Break out between Rwanda and the DRC?” Www.youtube.com, www.youtube.com/watch?v=uxWahUxx2i8.
|
oercommons
|
2025-03-18T00:39:09.805605
|
11/25/2024
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/122192/overview",
"title": "Democracy In Rwanda",
"author": "Jocelyn McCullough"
}
|
https://oercommons.org/courseware/lesson/115658/overview
|
Exercise and the Brain
Overview
This chapter details the effects that exercise on neuroplasticity, mental health, and neuroprotection.
Exercise and the Brain
See attached file for content
|
oercommons
|
2025-03-18T00:39:09.822901
|
Jessica Jones
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/115658/overview",
"title": "Exercise and the Brain",
"author": "Lesson"
}
|
https://oercommons.org/courseware/lesson/124454/overview
|
Worksheet Idea
Archaeology in a Box - Elementary School - Remixed
Overview
Lesson plan supporting elementary-school aged students.
Background Information
Objective
Students will learn to identify and explain what each 3D printed manipulative is and its historical function and the human connection in a 40–45-minute period.
Standards
Career Readiness Standards:
13.1.1-5.E Using school resources to learn about various jobs.
12.1.6-8.E meeting and talking to community members to learn about jobs.
History Standards:
8.2.7.B Identify the role of local community as related to significant historical documents, artifacts, and places critical to Pennsylvania history.
8.3.7.B Examine the importance of significant historical documents, artifacts, and places critical to United States History.
Process
- Ask students,
- What is an Artifact?
- What is an archaeologist?
- What do archaeologist look for? (Size shape material)
- Split students into 5-6 groups.
- Give each group 1 artifact.
- Provide students with questions worksheet.
- Student answer questions on worksheet.
- Pass artifacts clockwise.
- Repeat 5 –6 for all artifacts.
- Explain artifacts and what we know about the articles.
- Explain were archologist able to figure out about the artifact and the people who used them.
- Student are expected to follow along filling out their worksheet.
- Ask students if they remember Otzi the Icemen? What was he found with? If you remember Otzi the Iceman and the artifacts that were found with him now it is your turn to create a story about the person who once owned the artifact in front of you.
- Then share with the group around you.
- If wanted; share with the class.
|
oercommons
|
2025-03-18T00:39:09.847540
|
Thomas McClain
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/124454/overview",
"title": "Archaeology in a Box - Elementary School - Remixed",
"author": "Lesson Plan"
}
|
https://oercommons.org/courseware/lesson/119248/overview
|
Teamwork and Interprofessional Collaborator Microcredential -2024- Common Cartridge v. 1.2
Teamwork and Interprofessional Collaborator Micro-credential Course Files
Teamwork and Interprofessional Collaborator Micro-credential ( Part 3 of Professionalism in Healthcare Series)
Overview
This resource includes a SCORM package, course files, and a collection of resources on how to use the materials of the Teamwork and Interprofessional Collaborator Micro-credential which is part 3 of the Professionalism in Healthcare Series.
Within this resource you can find Powerpoint lectures, videos, article links, assessments, activities, and other resources.
Teamwork and interpersonal collaboration are essential skills in healthcare settings, as they contribute to the overall quality of patient care and enhance the working environment for healthcare professionals. These learning outcomes aim to prepare healthcare professionals to function effectively in a collaborative and dynamic healthcare environment, ultimately improving patient outcomes and the overall quality of care.
Teamwork & Interprofessional Collaborator Micro-credential (Part 3 - Professionalism in Healthcare Series)
This micro-credential completes the Professionalism in Healthcare Series.
This microcredential aims to prepare healthcare professionals to function effectively in a collaborative and dynamic healthcare environment, topics include: healthcare teams roles and responsibilities, barriers, communication with teams and team leaders/supervisors. Teamwork and interpersonal collaboration are essential skills in healthcare settings, as they contribute to the overall quality of patient care and enhance the working environment for healthcare professionals. The learning outcomes aim to prepare healthcare professionals to function effectively in a collaborative and dynamic healthcare environment, ultimately improving patient outcomes and the overall quality of care.
Skills: Team Communication | Collaborative Decision Making | Accountability | Conflict Resolution | Problem-solving | Professionalism
This course is fully online, self-led, and takes around 2-3 hours to fully complete at your own pace. It is a professional enhancement level micro-credential. This course could be modified to be instructor led.
Materials and Methods used:
This course has been developed in collaboration with healthcare employers and industry professionals.
Content mostly consists of powerpoint lectures, supplementary videos, links to articles, and activities and resources. There are multiple choice assessments as well.
SCORM file
This Common Cartridge file is designed to directly embed the entire course and its content into a compatible Learning Management System (LMS). It was used in Blackboard Ultra and is formatted to SCORM 1.2.
Course Files (ZIP Folder):
This folder contains all course files, including PowerPoint presentations, images, and external documents. It also includes a course roadmap, which outlines the intended sequence for building the course from scratch. The embedded links document provides all resource links used within the micro-credential.
Resources and Guidance Documents (ZIP Folder):
This folder contains instructional resources and guidance for instructors on how to effectively use the provided materials.
GRANT DISCLAIMER
The total cost of CT Statewide Healthcare Industry Pathway project (CT SHIP) was $6.9M. $3.4M (49%) was funded through a U.S. Department of Labor – Employment and Training Administration grant and another $3.5M (51%) was committed through non-federal state and local resources.
The Workforce product was funded by the grant awarded by the U.S Department of Labor's Employment and Training Administration. The product was created by the grantee and does not necessarily reflect the official position of the U.S Department of Labor. The U.S Department of Labor makes no guarantees, warranties, or assurances of any kind, express or implied, with respect to such information, including any information on any linked sites and include, but not limited to, the accuracy of the information or its completeness, timeliness, usefulness, adequacy, continued availability, or ownership.
|
oercommons
|
2025-03-18T00:39:09.869834
|
08/27/2024
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/119248/overview",
"title": "Teamwork and Interprofessional Collaborator Micro-credential ( Part 3 of Professionalism in Healthcare Series)",
"author": "Renee Dunbar"
}
|
https://oercommons.org/courseware/lesson/106234/overview
|
Education Standards
Plot Coaster Lesson for 6th Grade
Overview
This resource details plot for students in 6th grade. It uses the plot diagram, but focuses on making the students see it as a roller coaster instead of a moutain.
Introduction
| Anticipatory Set | 1. Using a pencil and paper, describe how it feels to ride on a roller coaster. If you have never ridden on one before, describe what happens on a roller coaster. Teacher Note: Give the students some time to describe. Ask for volunteers to read what they wrote down. Connect it to plot by saying that coasters have a starting point, middle, and end. They also are interesting because of all of the twists and turns involved just like a story that has a solid plot. |
| Instructional Activities (may take a few class periods/blocks) | Using the Plot Coaster resource provided, have student take notes. Ensure that exposition, confict, rising action, climax, falling action, and resolution are all defined the way you wish to describe it to your students. Teacher Note: For climax, make sure to mention that although it looks like it happens in the middle, that is not always the case. Once students have the notes, direct their attention to a YouTube video of your choosing. I usually stick to pixar shorts because they are easy to follow. Have the students watch it completely, then go back and pause it at the specific parts. Have students list what they think each part represents on the plot coaster and why. Group students together and provide a blank plot coaster. Put on another video and replay it a few times. Instruct the students to wrie down the parts of plot. Once done, make sure to go over it. Have students get into groups once again. Choose a short story to read. The students now will be tasked with plotting the story on their own, but they will create their own roller coasters. You can have the students create roller coasters out of recycled objects or simply on paper. The students must label the parts of plot with the examples from the story. |
| Closure | Have students share their creations to the class OR have them do a gallery walk and leave comments about what they like on a piece of paper by the students' work. |
|
oercommons
|
2025-03-18T00:39:09.891569
|
07/03/2023
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/106234/overview",
"title": "Plot Coaster Lesson for 6th Grade",
"author": "Paige Krempasky"
}
|
https://oercommons.org/courseware/lesson/108004/overview
|
About WGS at UMD
Adrienne Rich, Claiming an Education
Beverley Tatum, "Defining Racism"
Carol Adams' slideshow
Class Differences
Disability representation in the media
Dorothy Allison, A Question of Class
Feminism: The Second Wave
How Professional Black Women Suffer From Racial Stressors
"I have never felt sexual desire."
Introduction to binary systems
Introduction to Gender, Eckert et al. (edited)
Introduction to sexualities
Lighting the Way: Historic Women of the South Coast
Media
More Than an Afterthought: Authentically Representing Intersectionality in Media
"Our Love is Radical"
Overview of the first wave of feminism from the National Women's History Museum
Peggy McIntosh, "White Privilege
Quick Facts About the Gender Wage Gap
Rachel Pollack on creating a trans super hero
Ravyn Wngz - read and watch video of her speech
The 1968 Miss America Protest
The Basic Facts About Women in Poverty
The influence of media on views of gender
The Lesbian until Graduation
The Unequal Toll of Toxic Stress
Transgender and intersex folks
'We Must Educate Health Providers' about Black Women's experience
"What is Intersectionality? All of who I am"
What Is the Gender Wage Gap in Your State?
Women are calling out 'medical gaslighting'
Women's and Gender Studies
Overview
Introduction to Women's and Gender Studies textbook with primary source readings.
This resource was supported by funding from the OER Creator Program at UMass Dartmouth.
What Is Women's and Gender Studies and Why Should We Learn about it?
Introduction
In this first section you will be introduced to the field of Women's and Gender Studies: what it is and why we should all study it. Women's Studies as a distinct academic entity started in 1970 at what is now San Diego State University. That may seem a while ago, but consider that the other academic fields have a very long history; for example, the study of literature began in the nineteenth century and philosophy as a discipline began circa 400 B.C.E. Interestingly, Ancient Greek philosopher Plato considered the equality of the sexes as far back as c. 375 B.C.E.
The reading from Adrienne Rich, and the TEDx talk by Chimamanda Ngozi Adichie introduce you to the basic tenets of feminism. By definition, Women's and Gender Studies is a feminist discipline, one that examines social inequalities. We should recognize, however, that feminism is not just for women, as Adichie says, "We should all be feminists."
"We should all be feminists." Chimamanda Ngozi Adichie
The first reading (attached below) is the UMass Dartmouth's Women's and Gender Studies (WGS) webpage. Here we learn that "WGS is an interdisciplinary academic field that fosters active analysis of how gender (together with race, class, sexuality, etc.) affects our lived lives...WGS students reflect on how gender structures societies past and present and how it affects people at the individual and group levels; they study the historical factors that have shaped the status of women from varying backgrounds and cultures; and they explore paths to achieve equality for all people."
The second reading (attached below) is "Claiming an Education" by Adrienne Rich (1929-2012), who was an American poet, essayist and feminist. In this talk given at Douglass College convocation in 1977, Rich emphasizes the importance of "claiming" your education (for everyone) rather than passively receiving it. Part of WGS is to critically consider what we are told is "knowledge," when that knowledge may, in fact, be biased or solely based on the experiences of men or white people. For example, your High School education in American history may have focused on "great men" or the history of white people. This lop-sided view of the world is what WGS courses aim to address and redress. But, as Rich states, such courses go hand-in-hand with a change in your attitude towards yourself and your world: "taking responsibility for yourself." By this responsibility, Rich means to shed the notion that women should take second place to others and instead learn to speak for themselves and respect themselves (rather than seeing your value solely in relation to men).
In essence, this change in attitude is a "feminist" attitude. This attitude is one men should adopt too and they will also benefit from women adopting it. It is only when we understand how to respect ourselves that we can truly understand how to respect others. Yes, it may seem appealing to have someone playing the traditional role of a "wife" for you, but they are only playing this role and only doing so out of socialization - it is just an illusion of care.
So, the claim is that feminism is for everybody.
Unfortunately, there are many misinterpretations of feminism, so, finally, watch Nigerian-born novelist Chimamanda Ngozi Adichie's 2013 TEDx talk, "We should all be feminists," on growing up to discover feminism.
Points to consider:
- Can you see how WGS connects to your other areas of study?
- Does Rich simply mean a formal education?
- Do you agree with Adichie's claim that we should all be feminists?
Intersectionality
A simple diagram of intersectionality.
Introduction
So, if this course is an introduction to Women's and Gender Studies, we need to ask what is "gender" and why is it so central in our lives? However, before we discuss "what" gender is (and note that not everyone sees themselves as having a particular gender), we should explore the concept of "intersectionality." Otherwise, we will produce a confusing "additive" picture of human social identity: gender + ethnicity + sexuality + ?, etc. Instead, our identities are "intersectional," they comprise multiple social aspects that intersect, and this concept is the focus of this section.
In essence, the concept of intersectionality is easy to understand: we are not just women (for example), we are also raced, classed, etc. Moreover, these intersections are not just conceptual but play out in our lived lives: our pay, stereotypes other people may have of us, etc. However, once we have initially understood the concept, it is important to understand its complexities and explanatory power. And the first reading (attached below) raises some core themes of intersectionality.
Kimberlé Crenshaw is credited with coining the phrase, although African American feminists have long recognized how their race intersects with their gender to produce different oppressions from white women. In this video Crenshaw explains intersectionality and how it can play out specifically in education and educational institutions.
When you try to visualize intersectionality, it is crucial NOT to think of it as a tootsie roll: separate oppressions rolled together. The venn diagram at the top of the page can help you visualize intersecting oppressions. Another useful image is to think of yourself as standing at a crossroads. You are in greater danger of being in an accident than if you are standing at a one-lane road. Moreover, the more roads intersect where you are standing, the more vulnerable you are to harm. As Crenshaw says, the concept of intersectionality helps us analyze our oppressions, vulnerabilities, privileges, and advantages.
“The way we imagine discrimination or disempowerment is often more complicated for people who are subjected to multiple forms of exclusion. The good news is that intersectionality provides us a way to see it." Kimberlé Crenshaw
On the micro or individual level, intersectionality helps us identify "all of who I am." To locate myself politically and socially means to identify specific factors about my identity (identity markers), such as gender, race, or nationality. Another factor of this location is to identify oppression, power and unearned privilege. Intersectionality helps us think about how we may be oppressed in one category but privileged in another; for example, a heterosexual middle-class woman of color will benefit from class privilege and sexuality identity privilege, while potentially experiencing racism, sexism, and race-gender discrimination.
Beyond the individual or personal level, intersectionality also offers a framework for analysis. As we shall see, we need to apply the concept of intersectionality to anything considered a "women's issue," such as disparities in pay, cultural ideals of beauty, representations of women in the mainstream media, etc. Using the lens of intersectionality, however, we understand that we should look beyond "gender" and ask about pay disparities for women of color (see section 9); whether cultural ideals of beauty are - essentially - ideals for white women (see section 8); and how - or whether - groups such as women with disabilities, queer women, etc. are represented in the mainstream media.
Moreover, as Crenshaw argues elsewhere, intersectionality gives us a "frame" or a "narrative" in order to name a problem that may otherwise be hidden from view. Crenshaw states that we currently recognize the problem of state-sanctioned violence against African American men. We also recognize the problem of violence against women in general. What may be hidden from our view is the violence experienced by African American women because they are African American women, and Crenshaw challenges us to learn about and speak about these women. Crenshaw is a co-creator of the "Say Her Name" initiative in an effort to include women in the national conversation about race and policing.
Note: this video may be triggering.
Thirdly, as we can see with the diagram below, an intersectional lens allows us to see that the feminist movement's goal of ending sexism will never be complete unless we also end racism, homophobia, transphobia, classism, etc., as they are intertwined with sexism. And this is an issue we shall return to in sections 10 and 11.
Finally, the disadvantages or privileges produced by the intersecting oppressions or freedoms in our lives need to be portrayed in the mainstream media (TV, movies, commercials, music videos, etc.) we consume. While there is nothing wrong with escapism or fantasy, it is important not to erase or marginalize people. If you never see yourself, or your frustrations and successes, portrayed in the media, it is difficult to feel validated or even as if you exist. In the second reading (attached below), "More Than an Afterthought," Yalda Tehranian demonstrates the typical patterns of the representation of Black women in mainstream media, from lacking femininity, to being portrayed as one-dimensional, for example.
Points to consider:
- Can you identify two or three of your own social identities and how they intersect to produce your lived experiences or advantage or disadvantage?
- Now that you understand the concept of intersectionality, can you analyze your favorite TV show, movie or music video using an intersectional lens.
- We are now able to create our own content on social media (Tiktok, Instagram, etc.), do you think that offers an opportunity for greater, and better, representation of groups that were previously marginalized in mainstream media?
Useful terms when discussing intersectionality:
Stereotype: A widely held but fixed and oversimplified image or idea of a person or thing. In itself, a stereotype is not necessarily bad. It is just a generalization about how a group of people behave. Stereotypes can be useful as a rule of thumb when we want to make quick decisions. However...
Controlling Image: Stereotypes can have a specific function: to maintain domination over subordinate groups. Even if the stereotype is a “positive" one, it dictates what kinds of behaviors are “normal” and penalizes those who step outside of or resist that image. Furthermore, as these images are set by the dominant groups, individuals or subordinate groups may find that they have to work within these images to fulfill their economic and social needs. What is fascinating/disturbing is that often these controlling images do not make much sense: Asian women are stereotyped as both submissive AND hypersexual??? Black men are stereotyped as both aggressive AND childlike??? But that is part of the point: you can never win.
Dominant narrative. Narratives help us make sense of our lives and our realities, and, as such, they can be positive. However, a "dominant narrative" is where your story/history is told from the perspective of the dominant culture and serves to further or support the needs and interests of the dominant social group.
Oppression: Marilyn Frye argues that oppression is more than being restricted or constrained. It is experiencing a "double bind," where you cannot win; for example, women are encouraged to look attractive, but then blamed if they are assaulted or attract unwelcome attention. Frye's enduring metaphor is of the bird cage. When the bird looks out through the bars, it may think it is free because it does not see the complex system of wires constructing the cage.
Iris Marion Young identifies five "faces" of oppression of a group: exploitation; marginalization (relegation to a lower social standing or the outer edge of society); powerlessness; cultural imperialism (the imposition of the cultural values of the dominant class, such as the view that heterosexuality is "normal and better"); and violence (the most evident form of oppression).
Even if the oppressed recognize their oppression, they may be forbidden or prevented from voicing it. This silencing can also be internalized if the oppressed accept their inferiority as a natural fact of life. This internalized silencing need not happen directly; it can also happen indirectly through negative, dehumanizing images that become internalized, which is why a critical analysis of representation in the media is so vital.
Gender
Introduction
The picture above is of a pair of twins. The baby on the left was assigned female at birth (AFAB) and the baby on the right was assigned male at birth (AMAB). Their parents have already begun "gendering" them through clothing, with the little girl wearing pink and a flower and the little boy wearing blue and a button. And this process of "gendering" is the subject of this section, as we ask what it is, why do we do it, and why does it appear so important?
“The problem with gender is that it prescribes how we should be rather than recognizing how we are.” Chimamanda Ngozi Adichie
Humans do naturally - and usually unproblematically - order the world in terms of binaries; for example, night/day or human/animal. However, we also acknowledge that such binaries are not completely fixed: night everntually changes to day and humans are also categorized as animals. Where this ordering becomes problematic is with socially constructed categories like gender or race. The problem, for example, with the man-woman gender binary is that it is seen as fixed and natural, and yet - paradoxically - requires social enforcement. However, we now regnize that gender is not a binary but on a spectrum; indeed, some people do not self-identify as being on this spectrum as doing so implicitly reinforces the gender binary. The reading (attached below), "Introduction to Binary Systems," explores the problems of these binary social systems further.
The reading (attached below), Eckert et al.'s "Introduction to Gender," offers a clear explanation of how gendering takes place. We are not born with a particular gender but our gender is a social construction, produced by our parents, society, institutions, media, and even ourselves.
Key concepts:
Sex refers to a set of biological attributes in humans and animals. It is primarily associated with physical and physiological features including chromosomes, gene expression, hormone levels and function, and reproductive/sexual anatomy. Sex is usually categorized as female or male but there is a far greater variation in the biological attributes that comprise sex than is often acknowledged. Thus, the definition of the biological categories of male and female is ultimately social.
Gender is a social construct: it is made.
“Gender is the very process of creating a dichotomy [binary] by effacing similarity and elaborating on difference, and where there are biological differences, these differences are exaggerated and extended in the service of constructing gender.” Eckert et al.
Gender "policing" or "ordering." If gender flowed naturally from our assigned sex, then there would be little or no need to reinforce it through clothing, behaviors, etc. Yet our gender is policed by other people (even strangers on the street), institutions (e.g., workplace dress codes), etc. Ideed, we may even self-police.
The system of gender intersects with other systems. The binary system of gender is only coherent within the system of heterosexuality, which, in its turn, supports the binary gender system. But, as we shall see, the system of heterosexuality is neither fixed nor defined. In addition, the system of capitalism supports these two systems: expensive weddings, St. Valentine's day, etc.
The gender system is a system of inequality. If we look at, for example, sterotypical gender characteristics, we can see that they are not truly pairs of opposites; rather, one has more socio-cultural value than the other: men are seen as rational and natural leaders, etc., while women are emotional, who take on caring roles at home or in their career choices. Some feminists in the 1970s did argue that we should revalue characteristics and roles associated with women or the feminine as a way to equality; however, that approach does not question the construction of gendering itself or the initial devaluation of women and the feminine. Certainly, people do step out of their gender roles and socially expected characteristics, but they may encounter gender policing; for example, male nurses may have their heterosexuality questioned and female CEOs are likely to be consider a b*tch. Finally, the gender system produces inequality in that not everyone identifies/can identify as having a particular gender. Thus, gender in itself is a privilege.
Michael Kimmel is best known for his research on men and masculnities.Here he talks about his own gender and race-gender privilege.
The final reading (attached below) is on people who are transgender or intersex, which is an increasing percentage of the population. A 2022 Pew Research Center survey in the U.S. found that 1.6% of Americans identify as transgender or non-binary, and 5.1% of younger Americans (under 30-years-old) identify as transgender or non-binary. (all %’s approx.)
In the video below, model and transgender advocate Geena Rocero explains why she needed to break free of her assigned sex and the importance of her self-identification as a woman to her personal truth and her human dignity.
Points to consider:
- Now you recognize that gender is socially constructed, can you see its production in your peers, the media you consume, your family, yourself, etc.?
- Can you offer some examples of the way the system of gender interconnects with (and mutually supports) other systems of oppression?
- Can you identify some other ways that the gender system is a system of inequality?
Finally, let us continue the rest of this course thinking "beyond the binary." Currently, there is no official symbol for non-binary. However, Jonathan R. is credited for creating this symbol on Tumblr.
In addition, here is a range of gender symbols that include gender resistant, gender fluid, etc. options. A crucial takeaway is that these terms are for somone's identity, their sense of self; they may not bear any connection to who that person is attracted to (if at all), and we shall consider sexuality in the next section.
Sexuality
Above is the Pride Progress Flag. It was created by graphic designer Daniel Quasar in 2018 to represent marginalized LGBTQ+ communities of color and transgender individuals. The flag is a symbol of inclusion and progress. More recent versions also include a purple circle for intersex individuals.
Introduction
The term "sexuality" encompasses our sexual identity (how we identify), who we desire (who we are attracted to), and our experiences (we will also discuss asexuality). The traditional ideology of human sexuality assumes that heterosexuality is innate/inborn and is the primary form of sexual relationship among humans. This assumption that a sexual relationship between people of the opposite sex is natural ("opposites attract") therefore justifies the resulting social reinforcement of heterosexuality. However, we need to ask why - if it is so natural - do heterosexual relations need to be socially, culturally, and even legally in some cases reinforced? In fact, just as society (parents, religion, friends, complete strangers on the street) goes to tremendous efforts to "police" our gender, so our sexual behaviors, desires, choices, and partners are "policed."
As with gender, we should not look to biology or science to explain and understand human sexuality (research into the so-called "gay gene" have been fruitless); rather we should understand that heterosexuality is an ideological system, which functions - in part - to maintain the system of gender (and vice versa); for example, passivity and vanity are encouraged in women to make them more "marketable" to men. While this marketability may not be urgent as women no longer need to marry for economic security, the ideology has been absorbed into our culture and can be seen in, for example, the messaging about women in mainstream media. Moreover, so-called "normal" heterosexual identity and behaviors can function to further oppress some social groups; for example Black and Latinx people are often stereotyped as problematically oversexed ("Latin lovers," etc.), while Asian men are often framed as sexless.
The system of heterosexuality is in many ways invisible, as it has become so normalized that human sexuality itself has become identified with it. As heterosexuality is assumed to be "natural," then homosexuality (seen as the opposite) has traditionally been labelled as "unnatural." This is one of the reasons that gay rights advocates have claimed that they are "born this way." And, initially, this claim provides a powerful counter to attempts to, for example, "convert" gay and lesbian folks as well as to protect them from discrimination. However, as we shall see with the talk by Lisa Diamond, the claim may ultimately do the community a disservice, moreover, it neglects the wide variety of human sexuality. We are complex beings, and it is important that this complexity is reflected in our accounts of human sexuality and romantic love, and the first reading, "Introduction to Sexualities" (attached below) provides an introduction to this complexity.
Here is Lady Gaga's famous anthem to queerness and acceptance of difference, sentiments we can applaud, but is it actually true we are born this way?
However, no matter how powerful this message, it is important to hold it up to scrutiny.
“Just because an argument is politically expedient, doesn’t make it true.” Jane Ward, professor of feminist, queer, and heterosexuality studies.
Diamond explains that the "born this way" argument is not scientifically accurate, is not legally necessary, and - perhaps surprisingly - it is unjust. While the "born this way" argument may have had its uses in the past, Diamond claims that is it now time to retire it.
Moreover, a focus on whether LGBT folks are born this way, neglects the possibility that human sexuality is fluid. Even if your identity as gay or lesbian or bisexual is socially accepted, it is assumed that you must always remain that way.
“I don’t think I was born gay. I don’t think I was born straight. I was born the way all of us are born: as a human being with a seemingly infinite capacity…to play with limiting categories, to challenge them and topple them, to cultivate my tastes and preferences, and, most importantly, to love and receive love.” Brandon Ambrosino, writer.
However, fluidity does not fit in well with the contemporary dominant narrative of human sexuality. For example, a man who was in a heterosexual relationship for years and now is dating other men must have been "gay all along and just deceiving himself." No doubt you have heard of the trope of the lesbian-until-graduation, but how much actual truth is there in this image? While the second reading, "Lesbian until Graduation" (attached below) on this trope is rather caustic, it does offer some useful facts and an interesting analysis of sexuality and social class (and the latter social identity is the focus of the next section).
“The biggest question facing asexual people is how to live in a world that is very sexualized...They may face discrimination, or at least some level of disconnection, from the sexualized world.” Anthony Bogaert, professor of community health sciences and psychology.
Even if we can move from the rigid binary categories of gay/straight to acceptance of sexual fluidity, another category of human sexual life - asexuality - may not be recognized within our discourse, yet, according to the Williams Institute at UCLA, as much as 1.7% of the population identify as asexual. The final reading in this section, "I have never felt sexual desire," (attached below) discusses the misunderstandings about this particular sexual identity and clarifies its parameters. And here we can see asexual activist model Yasmin Benoit addressing the issue of "dressing the part."
Points to consider:
- What are the privileges associated with heterosexuality?
- What are the privileges associated with having a fixed sexual identity?
- What are the privileges associated with having a sexual identity that is socially recognized?
Social Class
Introduction
- Do you believe anyone can reach "the American dream" simply by working hard enough?
Social class can be a difficult concept in the United States, as many people believe that we can raise our socio-economic class simply by working hard enough. While some (few) people do achieve "the American dream" (home ownership, nice car, etc.) through hard work alone, good luck (and an absence of bad luck) can play a major factor for these people. Education, social networks, family support, a lack of social barriers, etc. are, instead, typically central factors to achieving social and economic "success."
If you come from a different nationality or your parents do, then you may have a different understanding of social class and success. For example, in some Latin American countries, it is rare for people to be able to move out of the social class they were born into. In the United Kingdom, the markers of the SEVEN social classes run deep, everything from where you shop for groceries to your accent: it is as much about attitudes and values as money.
While we may expect the binary associated with social class to be rich/poor, in the United States most people claim that they are middle class, which is essentially "code" for hard-working and morally upstanding. In other words, middle-class status is not so much about economic status as it is about holding a particular set of values. The middle class is placed in opposition to "the poor," who are framed as lazy or as cheating the government (excessive weath does not appear to be problematic on this binary).
“I have learned with great difficulty that the vast majority of people believe that poverty is a voluntary condition.” Dorothy Allison, novelist.
The dominant narratives of welfare recipients tend towards blaming them for their own poverty and trafficking in the stereotype of women having multiple children by different fathers in order to milk the system. Anyone who actually has a child or works caring for children would know that having a job may be the easier choice...However, the reality of the American welfare system is far different from these dominant narratives. The 1996 Personal Responsibility/Work Opportunities Reconciliation Act (PRWORA) limits receipt of welfare to 5 years or 60 months, and able-bodied recipients are required to work or job-train while they receive checks.
Indeed, recent studies show that 40% of middle-class Americans are just one missed paycheck away from homelessness, and, during the pandemic, there was a rapid increase in middle-class households using community food support (like food banks). Moreover, the gap between the rich and the rest of us is increasing. In 2023, 69% of the total wealth in the United States was owned by the top 10% of earners. In comparison, the lowest 50% of earners only owned 2.4% of the total wealth (Statista).
- Do you think social class in America is defined solely by money?
Class is an important social identity for this course, but we are not merely talking about economic resources:
Key terms: (Source: Teaching for Diversity and Social Justice, Third Edition. © Taylor & Francis 2015)
Economic capital refers to the tangible material resources of income and wealth
Social capital refers to the social networks one is part of and to which one has ready access, insofar as they have the potential to translate into material resources.
Cultural capital is a term coined by the late sociologist Pierre Bourdieu to address non-monetary class differences such as tastes in music or knowledge of high culture. Bourdieu argued that even if a previously poor person achieves economic mobility and becomes middle-class, there are still markers of her former status in the way she carries herself and the things she knows. Non-material resources such as knowledge, language, way of life and self-presentation can act as personal markers of class, and influence economic opportunity as well as quality of life. (Horvat, 2001; Swartz, 1997).
The first reading, "Class Differences," (attached below) on the psychology of class studies how we see our lives through the lens of class. In the second reading, "A Question of Class," (attached below) novelist Dorothy Allison describes growing up in poverty, while also providing an illustration of the intersectionality of privilege and disadvantage in her life.
The Silence of the Lambs is a well-known thriller. Hannibal Lecter is a forensic psychiatrist imprisoned for murder and then eating his victims. In the clip below he uses his insight into FBI agent Starling's lack of cultural and social capital to expose her class insecurities and to exert control over her. Can you see how he does this?
The third central element of our discussion of social class is the "Myth of Meritocracy." This myth is the popular belief that hard work and talent will always be rewarded by upward economic and social mobility and that an individual's success or failure reflects their merit or lack of merit. Yes, the idea is appealing - if you just work hard enough, you will succeed - but that belief is to ignore the social contexts in which we live, and our social locations (gender, race, etc.).
“Our leaders sell the myth as a utopian system of fairness – but merit has been manipulated to privilege the wealthy.” Jo Littler, The Guardian
The following are excerpts from President Trump's first speech to congress for your analysis. NOTE: This is not intended to be anti-Trump; there are plenty of other world leaders who maintain the myth as well.
“…I am going to bring back millions of jobs. Protecting our workers also means reforming our system of legal immigration. The current, outdated system depresses wages for our poorest workers, and puts great pressure on taxpayers. Nations around the world, like Canada, Australia and many others — have a merit-based immigration system. It is a basic principle that those seeking to enter a country ought to be able to support themselves financially. Yet, in America, we do not enforce this rule, straining the very public resources that our poorest citizens rely upon. According to the National Academy of Sciences, our current immigration system costs America’s taxpayers many billions of dollars a year. Switching away from this current system of lower-skilled immigration, and instead adopting a merit-based system, will have many benefits: it will save countless dollars, raise workers’ wages, and help struggling families — including immigrant families — enter the middle class…”
- Is this speech truly about the protection of U.S. workers? Or is it a policy speech about immigration wrapped up in the language of meritocracy? Does Trump's appeal to merit also contain an implicit appeal to racism?
Points to consider:
- How important do you think social capital and cultural capital are to economic and social success in contemporary America?
- How do you think the narrative of America as the "land of opportunity" play into the myth of meritocracy?
- In what ways does the "myth of meritocracy" justify the status quo and serve to defect attention away from structural inequalities?
Finally, race privilege and racism are intertwined with social class, and we shall explore these interconnections in the next section. We will also turn to a discussion of gender and race-gender pay inequalities in section nine.
Race, Racism, and Privilege
Introduction
Race does not exist.
Often people can find this claim hard to accept because it seems to contradict what they see around them. But race is not biologically real, which is different from saying “I do not see race,” which is a claim about one's lack of prejudices. The claim that there are common physical (and psychological or even moral) characteristics that form natural separations among people is false. Like gender, race is a social construction (although there is no agreement among race theorists about how this construction works). So, in THAT sense it does exist.
Interestingly, some race theorists argue that the claim that race is a social construct is a claim that it is a cultural construct: a participation in distinctive ways of life. Thus, even after white supremacy, and its accompanying racism, have disappeared, the cultural construct of Black culture can remain.
Originally, the term "race" was equivalent to nationality; for example, the Irish were considered a different race from the English or the Scottish. The modern concept of race appeared in the mid-C17th as a way of justifying the forced enslavement of Africans. Scientists and philosophers then aimed retroactively to "discover" and justify "racial differences."
The fact that your racial assignation depends on your geographical or historical location is a clear demonstration of the social construction of race; for example, in the United Kingdom Arabic people have their own separate ethnic group, whereas in the United States anyone with origins in the Middle East or North Africa is defined as white. Historian Noel Ignatiev in "How the Irish Became White (1995)," explains, somewhat controversially, how the Catholic Irish went from oppressed group to oppressing group. Originally, when the Irish came to America in the eighteenth and nineteenth centuries, they typically occupied the lowest rungs of the social ladder and were seen as a separate race from the white colonists and their descendants; indeed, in the South, Irish laborers were often given work that was considered too dangerous for an enslaved person to do. However, Ignatiev argues, partially through adopting the white culture of oppression of African Americans, Irish migrants eventually became defined as white.
Interestingly for the UMass Dartmouth community, as many of our students are Portuguese American, the racial/ethnic classification of Portuguese Americans remains vague. For a while they were federally classified as Hispanic, but now they are classified as white. The state of Florida, however, still classifies Portuguese Americans as Hispanic, while California does not. In Massachusetts there are laws and regulations that treat them as a separate minority group for some purposes; for example, through a “Portuguese business enterprise" status, which makes them eligible for special consideration on specific projects.
Even if race DOES biologically exist, the problem is that psychological and even moral characteristics are associated with different races. In other words, racial categories are NOT value neutral; rather, race is about political and social domination: the creation of “superior” and “inferior” groups. The historical construction of race has created systems of global socio-economic privilege for the dominant white “superior” group.
Racial categorization has been crucial in the United States, initially for supporting the slave economy, and then for maintaining the lowered social status of African Americans, a lowered status that served to advantage white Americans. Initially, for example, children born to a Black mother (regardless of the race of the father) inherited her enslaved status, thus providing a continuous supply of enslaved workers. After Emancipation, "Black codes" were laws enacted to control newly freed Black Americans. For example, vagrancy laws in the South meant that unemployed Black Americans were subject to arrest and forced labor as a punishment, whereas in South Carolina Black Americans were restricted to farming, manual labor or domestic service and forbidden from being artisans or having their own business. Needless to say, Black Americans in the South could do little to change these laws, as they were often restricted from voting or holding office. While the restrictions on Black Americans are no longer explicit nowadays, restrictions on their socio-economic status continue to maintain their oppression and advantage white Americans as a group. For example, racial disparities in access to a good education mean that Black Americans are more likely to be funneled into lower-paid jobs, such as care home workers, while white Americans as a group will benefit from having affordable health care for their aging parents, freeing this group of Americans to take high-paid jobs.
The concept of "differential racialization" is related to the concept of race as a social construct, this concept calls attention to the ways in which the dominant society racializes different minority groups in different ways at different times in response to its shifting needs for domination and marginalization. For example, during the era of slavery Black people were often characterized as childlike, thus "justifying" their control by white people through enslavement. Nowadays, Black Americans, especially men, are often stereotyped as "criminal," thus "justifying" state control through the prison-industrial complex.
Reminder: The analytic lens of intersectionality shows us that nobody is just “raced”; rather, they are also gendered, etc. This makes it difficult to unpack discriminations and experiences and thus dismantle racism. However, it is important to recognize the multi-layered experiences of individuals in order to uproot oppressions.
In your first reading (attached below), Desmond-Harris offers "11 ways that race isn't real." In addition, she includes a brief video to watch debunking the myth of "race."
Even though race is not real, there is no doubt that racial oppression and racism are very real, playing out in state-sanctioned violence, stereotypes in popular culture, disparity in pay, and even the environments in which we live. Racism is “normalized”: it has become an ordinary experience of most people of color. Racism covers a broad range of experiences from unequal pay to microaggressions to police harassment and brutality. This very normality makes racism harder to recognize and even harder to address.
In the second reading, "Defining Racism," (attached below) Beverley Tatum explains how stereotypes, omissions, and distortions all contribute to the development of prejudice, which is hard for any of us to escape as we live in a racist society; indeed, even people of color can internalize such prejudice.
“Racism is a system of advantage based on race.” David Wellman quoted in Beverley Tatum
However, Tatum thinks it is important to distinguish prejudice from racism, which she defines as a system of advantage based on race. And the third reading, "White Privilege: Unpacking the Invisible Knapsack," is the well-known list of the advantages of white privilege compiled by Peggy McIntosh, although, obviously, not all white people benefit equally or in the same way, as gender, socio-economic states, age, etc. will all play into access to power and status.
Forms of racism (often categorized as follows):
Interpersonal racism: This form of racism is what people most often think about when we discuss racism. Interpersonal racism is not just slurs or overt actions and speech, it can also be microaggressions, medical gaslighting (which we will discuss in section 7), etc.
Institutional racism: Systems of power that unfairly disadvantage racial minorities while often advantaging white people; for example, lower funding for health clinics in areas with a larger population of racial minorities.
Structural racism: Structural racism is sometimes used interchangeably with systemic racism. All the ways our society encourages racism, such as employment and the criminal justice system. This form of racism serves to maintain white supremacy.
These three forms of racism do not operate in isolation. For example, interpersonal racism can affect Black people's health due to stress, while institutional racism can limit their access to health care to deal with the effects of this stress, while structural racism reinforces and "justifies" racist views held by medical professionals.
- Can you think of other examples?
“Racism and other systems of inequity structure open/closed signs in our society…[which creates] a dual system of reality where on the inside it is difficult for us to recognize any system of inequity that is privileging us.” Camara Jones
Finally, in the video below Camara Jones brings many of these points together in her talk on race and racism, using allegories to explain: (1) race as a social classification, not a biological descriptor; (2) the dual reality in our society produced by racism; (3) the three different levels of racism; and (4) how to act to be actively anti-racist, which is different from being "non-racist" or "not-racist." Indeed, while many people may think that being "non-racist" or "not-racist" is the opposite of being racist, being anti-racist is the true opposite of being racist, and, as such, requires action.
Points to consider:
- If asked, could you put into your own words how race is not real?
- Can you see how hard it is to escape absorbing prejudicial attiitudes in a racist society?
- Which of Peggy McIntosh's privileges of being racialized-as-white did you find most striking?
- Can you see some ways that race and social class intersect?
- How do you think we can be actively anti-racist?
Health, Gender, and Race
Introduction
Health and health care are feminist issues. Not only do women provide the "invisible" support that helps maintain the formal health care system, but women and people of color tend to fill the lower-paid "care" jobs in this system. It can be easy to focus on "personal responsibility" for illnesses like diabetes, which are bound up with (often mistaken) assumptions about weight and diet. Yet stress and lack of access to good healthcare (factors that are beyond our control) are also major factors in illnesses like diabetes. It is important to recognize that racism, interpersonal as well as institutional, is a significant stressor as well as an indicator of access to good quality healthcare, and this issue was raised in the previous section. Connected to the stressors and expectations of daily living, we will also look at the "Superwoman" role for Black women, and ask whether it is an empowering or oppressive ideal (this article on the "Superwoman" role is fascinating, by the way).
Our focus in this section is on health and women of color; however, this is not to say that damaging stress is not experienced by other racialized communities or that white women do not experience such stress. The main reason for our focus in this class is that most of the available research is on women of color, especially Black women.
Needless to say, there are many other issues connected to gender, race, etc. and health/health care. In fact, UMass Dartmouth now offers an entire major built around these issues called "Health and Society."
We saw Dr. Camara Jones in the previous section use analogies to explain race and racism. Here she uses the analogy of a cliff to show the impact of social conditions on health—including racism, poverty, and other inequities. She calls for communities and health professionals to take action on those social conditions in order to eliminate health disparities.
Gaslighting: The term "gaslighting" comes from comes from a 1938 play and a 1944 movie in which a husband tries to make his wife go mad by claiming she is hallucinating and imagining things. One of the things he does is to turn the gas lights up and down, saying that the changes are in her imagination. Here is a great scene from the movie...
"Medical gaslighting" is now a recognized phenomenon, where women and people of color are more likely to have their symptoms dismissed by medical practitioners. And your first reading, "Women are Calling Out 'Medical Gaslighting,'" (attached below) provides an introduction to this issue (you can also listen to this article by clicking on the audio link in the attached reading).
The second reading, "The Unequal Toll of Toxic Stress," (attached below) traces the roots of gender inequality and race-gender inequality to early childhood stressors.
Attached next is an audio recording, "How Professional Black Women Still Suffer From A Legacy of Lingering Bias." Click on "Listen" to hear Marlene Harris-Taylor describe how many professional Black women are facing an under-current of stress from racial bias, which has very real implications for their health.
Our final reading,"'We Must Educate Health Providers' about Black Women's Experience," (attached below) is on stress-related health disparities. In an interview with Medical News Today, Professor Cheryl Giscombé, an expert on stress-related health disparities among African Americans, explains a source of pressure that many African American women experience: the obligation to project an image of strength or that of fulfilling a ‘superwoman’ role.
The song and video below from Alicia Keys demonstrate just how much the "superwoman" role or ideal has filtered into our cultural consciousness.
Points to consider:
- Do you see the "superwoman" schema as empowering or oppressive or perhaps as both?
- Were you surprised to find that is a relation between social experiences of oppression, such as racism and sexism, and physical and mental health?
- While you may have come across the phenomenon of gaslighting in personal relationships before, were you aware that medical gaslighting is a phenomenon that affects the treatment and diagnosis of illness, particularly of women and people of color?
Media
Introduction
Before you begin this section, think about how women, men, and/or people of color are portrayed in the mainstream media. For example, do you see older women with facial lines and grey hair represented as having social power? Do you see gay men who are NOT living privileged lives in NYC? Do you see Black men (who are not former President Obama) having political power?
The visibility in the media of some culturally dominant groups, and the concomitant invisibility of some marginalized groups, means that we may not recognize the problematic nature of the systems and structures that hold in place this domination or marginalization.
“We have to constantly critique white supremacist patriarchal culture because it is normalized by mass media and rendered unproblematic.” bell hooks
In the first reading, "Media," (attached below) the authors discuss just how much we are exposed to media everyday (advertising, music videos, movies, etc.). While we may like to believe we are not influenced by advertising, recent studies have shown that children as young as two-years-old can recognize brand logos for corporations like McDonalds, potentially laying the foundations for a loyal consumer base in the future.
Media reflects our culture, but it can also create that culture. The example the first reading uses is of Disney movies that you may have watched as a child. The movies tell narratives about normative gender and sexuality. Even the Harry Potter series (both books and films), which introduced us to an amazing fantasy world of creatures and magic, ends with the characters pairing into rather imbalanced couples and leading lives of heteronormativity.
The media, especially mainstream media, also reproduces racialized and gendered normative beauty ideals. You may already be familiar with the problems of female body ideals, but there is also growing concern about the effects of male body ideals on young men leading, in particular, to an obsessive desire to be more muscular.
Some social groups either have little to no representation in the media at all OR these social groups are plot twists or used for comedy: "symbolic annihilation." For example, the trans woman as a plot twist (watch the video in this first reading) or the Black character being the first one to die in a horror movie (which has now become such a well-known trope that it is itself a self-referential joke).
In the second reading, "Gendered Media: The Influence of Media on Views of Gender," author Julia T. Wood, identifies three themes in the way gender is represented in the mainstream media: women are underrepresented; protrayals of men and women are often stereotypical, thus reinforcing societal views of gender; and depictions of relationships between men and women often emphasize traditional roles and normalize violence against women.
The work of Carol J. Adams is recognized for the originality of its comparisons of the literal consumption of meat and the visual consumption of women (by men), especially in the advertising industry, and the third reading is a slideshow by Adams that illustrates her theory. The advertising industry has frequently been critiqued for its sexual objectification of women, but here Adams shows how this objectification intersects with the objectification of animals, thus reinforcing the disposability of women (like animals, they can be thrown away) and the sexualizing of meat-eating (animals are portrayed as actually wanting to be eaten). Problematically, once a woman has become an object (objectified), then violent or abusive treatment becomes normalized: it becomes acceptable to treat her like a thing. What Adams shows that if a woman is objectified as animal in some way, then violence becomes justifiable, as humans are allowed to dominate animals. Note: how many of the images in Adams' slideshow demonstrate the "fragmentation" of women (like animals) into body parts.
- Now that you have considered Adams' slideshow, what do you make of this parody intended to sell a Chicken Cookbook? What hidden messages about feminization, sexualization, dehumanization, and control can you see?
Feminist theorizing offers, among other things, an analysis of, and response to, the ways that cultural ideas of the body have served to oppress women. An inability to achieve such cultural ideals of the body for "normal" women is tied to assumptions about a lack of self-control and of personal responsibility; for example, think of the discourse around obesity and the "policing" of overweight people. It is similar to the thinking of "American individualism" that is connected to class status: anyone can achieve the goal of thinness (which is not necessarily the same as health) if they just try hard enough.
This analysis, in its turn, can illuminate the ways that these ideals serve to marginalize people with physical disabilities. People with disabilities represent the existence of the opposite of "normal," as they are culturally understood to lack independence and to have a body they cannot control. Failure to achieve these (impossible) cultural ideals in so-called "normal" women can lead to a feeling of alienation from one's own body and a devaluing of those who are less than perfect. For women with physical disabilities, this failure is so magnified that they are not always perceived as "real" women: women who can have sexual relationships, children, etc.
Much the same follows for women with so-called "invisible" disabilities (for example, mental illness). Women who are not compliant in their expected social/cultural roles may be marginalized or "policed" by the medical establishment. Moreover, as we have seen, oppresions and traumas (such as concerns for safety and the cultural ideal of the superwoman) that may lead to psychiatric problems are often not taken seriously.
If someone fails to achieve these (impossible) cultural ideals, then they may experience "shaming": either shaming by others or self-shaming. The media often plays an outsized role in this shaming, especially through its visual presentations of women who have been photoshopped. To paraphrase Black feminist bell hooks, we need to realize that such shaming is a tool of societal control that runs so deep it can often produce mental, emotional, and political paralysis, Interestingly, some contemporary (fourth-wave) feminists are fighting back at body shaming through online activism centered on body positivity, and, in this way, they fight the trauma-induced paralysis of body shaming. See, for example, Jameela Jamil's Instagram account I_Weigh, where she she challenges defining weight in terms of pounds and ounces, replacing the definition in terms of our contributions to society and what we value in our lives.
The final reading, "A Group Left Behind," is on the invisibility of disability in the media, even though approximately 20% of people in America have a disability. Make sure to click on the links for the video clips. Even if people with disabilities are represented in mainstream media, their disability can be connected to their life of crime or an explanation for their murderous villainy.
- Can you think of an example where the disability of a character in a movie is connected to their evil actions and/or evil nature?
However, it would appear that small changes are happening. Recent TV series have offered us authentic depictions of people with disabilities, such as Isaac in Sex Education or Rebecca Hall-Yoshida in Never Have I Ever. Moreover, there is starting to be a recognition of the beauty of people with disabilities. Here is an image of Jillian Mercado, an in-demand model who uses a wheelchair.
When we think of disability, we most likely use the “medical model” of disability: “a physical or mental condition that limits a person’s movements, senses of activities.” Oxford English Dictionary. But what if we step back and see disability less as an individual/medical issue and more as one that society has contributed to? It is society’s insistence on putting curbs and stairs everywhere, for example, that is problematic for someone who uses a wheelchair. Consider UMass Dartmouth! In other words, the problem is not with the person but with the lack of accessibility in society. Or, as some disability theorists put it, disability is a social construct. Moreover, making the world more accessible can help everyone. For example, accessible buildings can help a parent with a stroller, while "closed captions" can help students concentrate/take in more.
Points to consider:
- The majority of the discussion about representation of women and other marginalized groups in the media is about their underrepresentation or misrepresentation in mainstream media. Now that anyone with a smartphone and an internet connection can make their own media, do you think that these problems with representation could disappear?
- Julia T. Wood identifies three themes in the way gender is represented in mainstream media, can you see any one of these themes in the mainstream media you typically consume? (For example, movies, TV, music, videogames, etc.).
- Can you find an example in advertising that demonstrates the sexualization of meat-eating?
- How frequently do you see people with disabilities portrayed in the mainstream media you consume?
Work and Poverty
Introduction
“Gender biases and inequalities that have placed women in low-wage occupations, such as differences in jobs and hours worked, as well as women’s disproportionate caregiving responsibilities, contribute to the gender wage gap.” World Bank, 2023
In 1945 Congress introduced the Women's Equal Pay Act, which failed to pass. It was not until 1963 that the Equal Pay Act was passed in the U.S., which guarantees equal pay for equal work for men and women in the same workplace (this work need not be identical; it must be substantially equal).
Yet the stark fact is that there is a gap between the overall pay for women and the overall pay for men, with women in full-time work earning on average 84% of what men earned in 2024 (U.S. Dept. of Labor). This gap is substantially more for women of color, with Black women earning approximately 70% as much as white men and Hispanic women earning 65% as much (Pew Research Center, 2022). The gap is narrowing, but very slowly, and it is estimated that the pay gap will remain until 2059.
Researchers have also uncovered evidence of discrimination in the hiring of other marginalized groups, such as LGBTQ+ people or those with disabilities, such discrimination may produce differences in earnings by excluding these groups of workers from opportunities. Interestingly, some women may have a false consciousness about gender discrimination and/or sexual harassment at work, believing it to be individual-to-individual, not systemic.
Multiple interconnecting factors lead to the gender wage gap. "Occupational segregation," where women are overrepresented in lower-paid occupations such as nurses, primary school teachers, retail or childcare workers, the so-called "feminized" occupations. Moreover, overt discrimination or socialization may channel women into these lower-paid occupations. Even though there is nominal gender equality in the U.S., women disproportionately shoulder the burden of childcare, and they are more likely to seek employment with flexible hours and little to no evening or weekend work commitments. Unfortunately, the work culture in the U.S. values the "always-on" employee, while more flexible employment with shorter hours tends to receive lower compensation. Connected to women's cultural role as caretaker is the fact that women are more likely than men to take time out of work to care for elderly parents or sick children. Any significant break from work responsibilities means that the "clock" is also stopped on eligibility for promotion and pay raises, or they will have less work experience to offer a prospective new employer.
We should not underestimate the importance of this final factor. According to the 2023 Nobel Prize winner for economics, Claudia Goldin, differences in education and occupational choices historically provided the explanation for the gender pay gap. However, the continuation of the pay gap in the modern workforce between men and women in the same occupation arises with the birth of the first child, a "motherhood effect," a pattern that can be seen in other countries. Structural barriers, such as access to education and career opportunities, have now been removed; instead, policy makers should now focus on what is needed for women to work productively and profitably after having children if we want to close the pay gap.
- Can you see how closing the gender pay gap would not just benefit women?
Economic justice is not just a moral and political requirement; it is a prerequisite for economic growth. If all working women were paid the same as comparable men (men of the same age, education, geographical location, and number of hours worked), this would amount to an earnings increase of $482.2 billion, or 2.8 percent of this country’s gross domestic product (GDP) (2014 estimate). An increase that would boost taxes, local economies, and consumer spending (Status of Women In the States). A more recent report (2023) by Moody's Analytics argues that narrowing the gender pay gap could boost the global economy by 7% (7 trillion).
Significantly, equal comparable pay in the US would reduce women's poverty; for example, the 2014 poverty rate in Massachusetts was 7.2%, but it would be reduced to 3.2% if the gender pay gap was closed. The 2014 poverty rate for working single mothers in Massachusetts was 24.6%, but would be reduced to 13.1% if the pay gap was closed. The Massachusetts state economy would be boosted by 3% if there was gender equity in pay.
The "glass ceiling" is the invisible social and cultural barrier that prevents the promotion of women and other marginalized groups to top-level positions. The "sticky floor," on the other hand, describes a discriminatory pattern of employment that keeps women and other marginalized groups at the ground level. Typically, these workers are "pink collar" - clerical or service jobs - and approximately half of working women hold pink collar work as opposed to one-sixth of working men.
In the first reading, "Quick Facts About the Gender Wage Gap" (attached below), Robin Bleiweis identifies the main drivers of the wage gap and explains its impact on women and their families.
In the second reading, "What Is the Gender Wage Gap in Your State?" (attached below), we see that there is a pay gap between the earnings of men and women in Massachusetts. This gap is especially troubling considering that Massachusetts was the first state in the country to pass an equal pay law. In 2018 this law was updated to clarify unlawful wage discrimination and to add further protections to ensure equity in the workplace. In addition, it is anticipated that Massachusetts will pass a pay transparency bill in 2023. Pay transparency can reduce pay gaps by preventing unintended hiring bias and discrimination, while empowering women to negotiate their pay, moreover, a culture of openess has been shown to benefit the business itself.
- Given that Massachusetts has a (well-deserved) reputation for being a progessive state, were you surprised to see how its gender pay gap ranked compared to other states?
If you click on Massachusetts on the map, you should be able to find the answer for the gap.
The third reading is "The Basic Facts About Women in Poverty" (attached below), which explains the major reasons why more women than men, and especially women of color, live in poverty.
The "feminization of poverty" was a term first used in the 1970s by researcher Diana Pearce to frame gender and poverty in the United States, which she saw as rooted in the lack of government support for divorced and single women. The term gained global recognition at the Fourth United Nations Conference on Women in 1995. The feminization of poverty is a deceptively simple concept, but the causes of women's poverty are complex. Most of the world's poor are women, but, as with health, the notion of personal responsibility can cloud over the structural inequalities that lead to this poverty. Among the main structural inequalities are the facts that women are often responsible for the unpaid labor of childcare; that they may lack opportunities or access to education to raise themselves up economically; their vulnerability to gender-based violence; and that they are often paid less than their male counterparts.
“Not only is gender still correlated with poverty, but…gender is an increasingly important factor underlying current poverty trends, and that is true for all racial and ethnic groups.” Diana Pearce, 1989.
Points to consider:
- Could you find an explanation for the pay gap in Massachusetts?
- Whatever your gender identity, do you think your future career/occupation is gendered?
- Despite the fact that pay equality laws have been passed and the existence of the gender wage gap has been formally recognized, the gender wage gap remains. What do you think is the most compelling reason for its continuation?
Feminist Movements
Introduction
The readings talk about "waves" of feminism but acknowledge that there are no clear delineations between these waves; rather, the image of waves is a useful metaphor. Roughly speaking, the first wave in the U.S. was the nineteenth century up until the ratification of the 19th amendment in 1920 (image above), which granted women the right to vote.
However, it is important to recognize that prior to the nineteenth century, there had been centuries of criticism by both men and women of the lower status of women, their oppressions, and their mistreatment. For example, the equality of the sexes is part of the discussion in Plato's Republic (circa 375 BCE) of the ideal state, whereas Chinese woman philosopher Ban Zhao (45/49 - 117/120) framed Confucian philosophical precepts for the needs of women.
As we shall see, women's suffrage was only part of the solution for the equality of the sexes, and the struggle continues in various formulations until the present day. Reviewing our histories offers us the benefit of hindsight, as we should ask WHO is, and WHO should be, part of the struggle? And WHO are/should be our allies?
The first reading, "Overview of the first wave of feminism" (attached below), is a good resource as it aims to be inclusive, giving a more diverse picture of the early feminist movement than we often see in standard history books.
The second reading (attached below) is from "Lighting the Way," a local history project that tells the stories of historic women from the South Coast of Massachusetts. Some of these women were clearly feminist activists, while others embodied a "lived feminism" by breaking gender barriers. Click on the drop down menu under "profiles" and you can see their different contributions of these historic women by topic, such as, abolition, activism, voting rights, and women's rights, or by geographical location, such as Dartmouth or New Bedford.
The third reading is "Feminism: The Second Wave" (attached below). The "second wave" of feminism covers the period from roughly the 1960s to the 1980s in both the U.S. and Europe. A central reason that this second wave arose in the United States was the recognition that, despite improvements in the legal and civil rights of women, they had not yet achieved true equality. This second wave was not a unified movement; however, two major approaches can be identified: a primarily liberal feminist approach and a more radical feminist approach.
The liberal approach is typified by the National Organization of Women (NOW), which was officially founded in 1966 to campaign for the equal rights of women in all areas of society, such as employment, education, and family. We have these liberal activists to thank for their work on, for example, The Equal Pay Act of 1963, which makes it illegal for private employers to pay different rates based on gender. The second - radical - group was composed of a variety of connected groups who had been part of other protest movements in the 1960s but who had recognized that even these progressive movements were sexist or male-dominated. We owe radical feminists thanks for their work on women's safety; for example, the creation of shelters for domestic abuse survivors. Perhaps the best known action of these radical feminists is the 1968 Miss America protest, which is discussed in detail in your fourth reading (attached below). View a collection of photos from the 1968 Miss America Protests from Duke University.
Here is a flyer about the pageant, which really reflects the anger and frustration of the Miss America protesters. The comparison of the treatment of women to chattel slavery began in the 1850s. While it is certainly a powerful rhetorical move, do you have concerns about this comparison?
Prior to strategizing and performing political action, consciouness raising was a central strategy during the second wave of feminism, used particularly by radical feminists. Driven by the insight, "the personal is political," small groups of women would meet to share what they had previously seen as merely personal or individual experiences. This sharing would then lead the group to see that there were common themes in their experiences due to the structural and institutionalized nature of women's oppression.
However, in the second wave, feminism was still implicitly a white, heterosexual, cisgendered woman's movement. Even though many individual liberal feminists and radical feminists were progressive and anti-racist, the groups themselves engaged in racism through not explicitly including women of color and their needs. Whereas in 1970 Betty Friedan, the president of the (liberal feminist) National Organization for Women, described lesbians as a "lavender menace" who threatened the feminist movement by potentially stereotying feminists as just man-hating lesbians. In response, women in the more radical Gay Liberation Front crashed the NOW congress, wearing t-shirts emblazoned with "lavender menace" and demanded to be heard. NOW members agreed, and within a year voted to respect lesbian rights. While we may think anti-trans feminism (also more euphemistically known as "gender critical feminism") is a modern phenomenon, it has its roots in the criticism by some 1970s radical feminists of trans folks, when these radical feminists claimed that trans men were betraying other women and trans women were invading women's safe spaces. Don't forget to take a look at a photo archived by the New York Public Library of Rita Mae Brown, in a Lavender Menace t-shirt, at the Lavender Menace Action.
By the 1980s more women of color began to participate in the feminist movement and in feminist theorizing. Because of mainstream feminism's legacy of racism, some women of color reject the label "feminist," and prefer another label, such as "womanist."
The "third wave" of feminism began in the United States in the 1980s and continues until today, or, as some feminists claim, up until the 2010s, which is when the "fourth wave" of feminism began. The distinct characteristics of this third wave aimed/aims towards increased inclusion in terms of geography, race, ethnicity, sexuality, gender, etc., and third wave feminists recognize that there is no universal "women's experience"; rather, they emphasize the intersectionality we discussed in section two of this work.
The central achievement of the third wave of feminism is its expansion of the notion of sexual liberation. Whereas the second wave framed sexual liberation in terms of freeing women to explore their own sexuality through, for example, easy access to birth control or removing the stigma of lesbianism, third-wave feminism understands gender and sexuality as social constructs, and thus liberation is becoming free to explore and express our authentic gender and sexual identities.
While third-wave feminists continue to aim for the liberation of women, they do not see this as a separate task from the liberation of other subordinate groups. Third wave feminism is a coalition politics formed from diverse communities; it is a collective voice, not a single, unified voice. As such, it resists a too-easy or oversimplified definition.
What, however, is the strength of third-wave feminism is also its weakness, as it has no particular theoretical identity, even though it has identifiable key issues.
Is there a "fourth wave of feminism"? It really depends on who you ask. Some feminist theorists consider contemporary activism as a development of the third wave, while others see it as a distinct entity that began in the 2010s. UMass Dartmouth's WGS department falls into the latter category and offers a course on the fourth wave. One of the defining characteristics of the fourth wave is the use of social media for activism. While consciousness raising groups have fallen out of fashion as a political strategy since the second wave, some theorists suggest that their place has been taken by feminist websites, hashtags, etc. which allow their audiences to recognize their own oppressions, speak out against these oppressions, and join with others (virtually or IRL) in resisting them.
Below is the enduring meme of the 2017 "Women's March on Washington": the "pussy" hat. Participants marched to demonstrate the inauguration of President Donald Trump after his offensive remarks about women, a march that was sparked by a FaceBook post from Teresa Shook. Similarly, #BLM began as a FaceBook post by Alicia Garza, one of the co-founders of the Black Lives Matter movement. In addition to its primarily virtual location, fourth-wave feminism is explicitly queer, trans inclusive, sex positive, and body positive.
Points to consider:
- Where and when did you first learn about the feminist movements or is this information new to you?
- Why is it important for the feminist movement to be inclusive?
- Do you follow any fourth-wave feminist social media?
Our Transcestors
The Trans Flag
Introduction
"Trans people…have no choice but to become experts in resistance." Mariah Moore, trans activist
Within the already marginalized LGBTQ+ community, trans folk are often the most marginalized. As such, they may have what is termed "epistemic privilege": a knowledge that is gained from the particular standpoint of living at multiple intersections of oppression, and this is something we will see with Pauli Murray and Ravyn Wngz. In this final section of the course we will discuss the contributions of trans activists (aka "our transcestors") to the feminist movement, and to social justice movements more generally. We will also see how their work links social justice movements such as feminism, anti-racism, disability rights, etc. Indeed, as the first reading, "Our Love is Radical" (attached below), demonstrates, trans activists have been central to the battle for social justice for decades. Mariah Moore, a trans activist, says activism naturally comes out of trans folk. This seems less of an essentialist claim and more of a claim about their social/political location: they need to fight for freedom or be subsumed. There is no choice but to fight/resist.
- Why should we move trans activists from the margins to the center?
With Adrienne Rich at the very beginning of the course we talked about the dominant (white) male eye through which the (political, social, etc.) world is seen. If we see the world through this white male eye, then we do not have the knowledges gained from the experiences of women and/or people of color, etc. Seeing through this eye means that we neglect, for example, the histories of Indigenous peoples and enslaved Africans who were part of the history of this country, or we may not recognize the contributions of women or people of color to such fields as literature or science; instead, we will be looking for their connections to white male figures in order to legitimize them. Moreover, legal gains for trans folks have often translated into gains for other marginalized groups, so placing trans folks at the center may translate into literal changes.
In this first reading, we were introduced to Pauli Murray. Here is the official trailer from a recent documentary about Pauli Murray.
While Murray's contributions to social justice are - rightly - becoming more visible, this visibility often happens at the expense of her intersectionality. The United States Mint celebrates Murray on a quarter (2024) as part of its "American Women Quarters" program. The Mint describes Murray as "a poet, writer, activist, lawyer, and Episcopal priest."
- Can you see what this description leaves out about Murray's identity? Why is this invisibility concerning?
In the second reading (attached below), we are introduced to contemporary trans activist of color, Ravyn Wngz (make sure to watch the video of her speech as well). In the article we find that, growing up, Wngz did not know of the existence of trans activists of color, leaving her without role models. In the video of her speech she expresses to the audience her frustration at their inability to draw attention to, and create an understanding of, Black rights, when the humanity of her community should be self-evident.
Finally, we all need heroes in popular culture as well as in real life. The third reading (attached below) is an illustrated interview with Rachel Pollack, who created "Coagula," the first trans super hero for DC Comics. Pollack's work is important because it also "normalizes" trans folk for popular culture. Too often a trans individual is the plot twist or the punchline, as we saw in the section on media.
Points to consider: (thank you to my Spring 2023 WGS 101 students for suggesting these questions)
- Why are the contributions of our trancestors not included in our history books and what might have changed if they were?
- How can we honor our transcestors and acknowledge their contributions to the feminist movement?
- How would the world look or what would change if someone like Ravyn Wingz or Pauli Murray was seeing from the center?
- Do you think our transcestors would be proud of what American feminism has become?
Acknowledgements
This resource was supported by funding from the OER Creator Program at UMass Dartmouth.
https://www.umassd.edu/faculty/heoa-textbook/oer-resources
Section 1: Introduction to Women's and Gender Studies
UMass Dartmouth Women's and Gender Studies website. https://www.umassd.edu/cas/wms/
Adrienne Rich, "Claiming an Education." Speech delivered at convocation of Douglass College, 1977. https://net-workingworlds.weebly.com/uploads/1/5/1/5/15155460/rich-claiming_an_education-1.pdf
Chimamanda Ngozi Adichie, "We should all be feminists," TEDx 2012.
https://www.ted.com/talks/chimamanda_ngozi_adichie_we_should_all_be_feminists?language=en
Section 2: Intersectionality
Image: Womankind Worlwide, "Intersectionality 101: what is it and why is it important?" https://www.womankind.org.uk/intersectionality-101-what-is-it-and-why-is-it-important/
"What is Intersectionality? All of Who I am." The Conversation. 2019. https://theconversation.com/what-is-intersectionality-all-of-who-i-am-105639
Kimberlé Crenshaw, "What is Intersectionality?" https://www.youtube.com/watch?v=ViDtnfQ9FHc&t=24s
"#SAYHERNAME" https://www.youtube.com/watch?v=49-jhS-H50c&t=32s
"Intersecting Axes of Privilege, Domination, and Oppression." Adapted from Kathryn Pauly Morgan, "Describing the Emperor's New Clothes: Three Myths of Eductional (In)Equality." In The Gender Question in Education: Theory, Pedagogy & Politics, Ann Diller, et al. Westview, 1996.
Yelda Tehranian. "More than an Afterthought: Authentically Representing Intersetionality in Media." Center for Scholars and Storytellers. https://www.scholarsandstorytellers.com/blog/diversity-in-hollywood-the-importance-of-representing-intersectional-identities#:~:text=While%20the%20show%20had%20limited,Moonlight%2C%20and%20Girls'%20Trip.
Marilyn Frye, "Oppression." The Politics of Reality: Essays in Feminist Theory. The Crossings Press, 1983.
Iris Marion Young, "Five Faces of Oppression," in Oppression, Privilege, and Resistance, eds Heldke and O'Connor, McGraw-Hill, 2004.
Section 3: Gender
Image: iStock. Katrinaelena. Stock photo ID:474374621.
"Introduction to Binary Systems," in Introduction to Women, Gender, Sexuality Studies." Kang et al. 2017.
http://openbooks.library.umass.edu/introwgss/chapter/introduction-binary-systems/
"An Introduction to Gender" (excerpt edited for length), Eckert and Mcconnell-Ginet in Language and Gender, 2nd edition, Cambridge University Press, 2013. https://assets.cambridge.org/97811070/29057/excerpt/9781107029057_excerpt.pdf
Michael Kimmel: On Gender. https://www.youtube.com/watch?v=JgaOK74HqiA
"Gender and Sex - Transgender and Intersex," in Introduction to Women, Gender, Sexuality Studies." Kang et al. 2017. http://openbooks.library.umass.edu/introwgss/chapter/gender-and-sex-transgender-and-intersex/
Geena Rocero, "Why I Must Come Out." 2014 TEDTalk. https://www.youtube.com/watch?v=mCZCok_u37w
Gender icons: iStock-1134593976.jpg
Section 4: Sexuality
Image: iStock-1134593976.jpg
"Sexualities," in Introduction to Women, Gender, Sexuality Studies." Kang et al., 2017. http://openbooks.library.umass.edu/introwgss/chapter/sexualities/
Lady Gaga, "Born this Way" (official music video), 2011. https://www.youtube.com/watch?v=wV1FrqwZyKw
Lisa Diamond, "Why the "born this way" argument doesn't advance LGBT equality." TEDx, 2018. https://www.youtube.com/watch?v=RjX-KBPmgg4
Rachel, "The Lesbian Until Graduation." Autostraddle, 2011. https://www.autostraddle.com/the-lesbian-until-graduation-now-a-new-york-times-most-emailed-article-81758/
Yasmin Benoit, "Twisted Expectations to Dress the Part." https://www.youtube.com/watch?v=xQXkXFbIvBo
Section 5: Social Class
Image: iStock-1257170453
"Key Terms." From Teaching for Diversity and Social Justice, Third Edition. © Taylor & Francis 2015.
"Wealth distribution in the United States in the second quarter of 2023." Statista, 2023. https://www.statista.com/statistics/203961/wealth-distribution-for-the-us/
The Silence of the Lambs. MGM, 1991. https://www.youtube.com/watch?v=EnMV60UDuF0
Jo Littler, "Meritocracy: the great delusion that ingrains inequality." The Guardian. 3/20/2017 https://www.theguardian.com/commentisfree/2017/mar/20/meritocracy-inequality-theresa-may-donald-trump
President Donald J. Trump's address to a joint session of Congress, 2017 https://edition.cnn.com/2017/02/28/politics/donald-trump-speech-transcript-full-text/
Section 6: Race, Racism, and Privilege
Image: iStock photo ID: 1473513712. Meeko Media
Noel Ignatiev, How The Irish Became White. Routledge, 1995.
Jenée Desmond-Harris, "11 ways race isn't real," Vox. 10/10/2014.
Beverley Daniel Tatum, "Defining Racism, Can We Talk?" from "Why Are All the Black Kids Sitting Together in the Cafeteria?" and Other Conversations about Race, Perseus Books 1997: 3-13. https://wmbranchout.files.wordpress.com/2011/12/defining-racism-beverly-daniel-tatum.pdf
Peggy McIntosh, "White Privilege: Unpacking the Invisible Knapsack." https://psychology.umbc.edu/wp-content/uploads/sites/57/2016/10/White-Privilege_McIntosh-1989.pdf
Camara Jones, Allegories on Race and Racism. TEDx, 2014. https://youtu.be/GNhcY6fTyBM
Section 7: Health, Gender, and Race
Camara Jones, "The Cliff of Good Health." 2018. https://www.youtube.com/watch?v=to7Yrl50iHI
Gaslight (1944). Movieclips. https://www.youtube.com/watch?v=kFhDGoJh4O4
Melinda Wenner Moyer, "Women are Calling Out 'Medical Gaslighting'" The New York Times. 3/28/2022. https://www.nytimes.com/2022/03/28/well/live/gaslighting-doctors-patients-health.html
CAP Report. "The Unequal Toll of Toxic Stress." 11/17/2017. https://www.americanprogress.org/article/unequal-toll-toxic-stress/
Marlene Harris-Taylor, "From Overt to Covert Racism - How Professional Black Women Still Suffer From A Lingering Legacy of Bias." Ideastream Public Media. 11/17/2017. https://www.ideastream.org/news/from-overt-to-covert-racism-how-professional-black-women-still-suffer-from-a-lingering-legacy-of-bias
Medical News Today. "'We must educate health providers' about Black women's experience." 7/31/2020. https://www.medicalnewstoday.com/articles/we-must-educate-doctors-about-black-womens-experience-says-expert#1
Alicia Keys, "Superwoman." 10/03/2009. https://www.youtube.com/watch?v=-AphKUK8twg
Section 8: Media
"Media," in Introduction to Women, Gender, Sexuality Studies." Kang et al. 2017. http://openbooks.library.umass.edu/introwgss/chapter/media/
Julia T. Wood, "Gendered Media: The Influence of Media on Views of Gender." https://pages.nyu.edu/jackson/causes.of.gender.inequality/Readings/Wood%20-%20Gendered%20Media%20-%2094.pdf
Promotional trailer for F.L. Fowler, Fifty Shades of Chicken: A Parody in a Cookbook. Clarkson Potter, 2012. https://www.youtube.com/watch?v=Oa3eC02delM&t=110s
Image of Jillian Mercado, Everett Collection Inc / Alamy Stock Photo (worldwide usage in presentation or newsletter purchased).
Section 9: Work and Poverty
Jessica Schieder and Elise Gould, "Women's work" and the gender pay gap." Economic Policy Institute, 2016.
Data on economic impact of comparable pay; https://statusofwomendata.org/featured/the-economic-impact-of-equal-pay-by-state
Rakesh Kochhar, "The Enduring Grip of the Gender Pay Gap," Pew Research Center, 03/01/2023. https://www.pewresearch.org/social-trends/2023/03/01/the-enduring-grip-of-the-gender-pay-gap/
Robin Bleiweis, "Quick Facts About the Gender Wage Gap," CAP fact sheet, 3/24/2020. https://www.americanprogress.org/article/quick-facts-gender-wage-gap/
Megan Wisnierwski, "What Is the Gender Wage Gap in Your State?" 03/01/2022. https://www.census.gov/library/stories/2022/03/what-is-the-gender-wage-gap-in-your-state.html (United States Census Bureau).
Robin Bleiweis, et al., "The Basic Facts About Women in Poverty," CAP fact sheet, 08/03/202. https://www.americanprogress.org/article/basic-facts-women-poverty/
Diana Pearce, "The Feminization of Poverty: A Second Look." Paper presented to the ASA, 1989: 2.
Section 10: Feminist Movements
Istock Stock photo ID:172640647
Section 11: Our Transcestors
Sam Levin, "'Our love is radical': why trans activists lead the way in protest movements." The Guardian. 9/29/2020. https://www.theguardian.com/us-news/2020/sep/29/trans-activists-civil-rights-lgbt-pauli-murray
My Name Is Pauli Murray, official trailer, Amazon Prime Movies. 8/30/2021. https://www.youtube.com/watch?v=Uh4r95VBU2Q
Ravyn Wngz, "As a queer, trans and Afro-Indigenous woman, I believed that I could never be a representative of Black liberation," Maclean's, 7/31/2020.
Annie Mok, "A Superhero on Your Own Terms: An Interview With Rachel Pollack," The Nib. 09/13/2019.
_____________________________________________________________________________________________
All images courtesy of Pixabay.com unless otherwise noted.
|
oercommons
|
2025-03-18T00:39:10.014501
|
Catherine Gardner
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/108004/overview",
"title": "Women's and Gender Studies",
"author": "Textbook"
}
|
https://oercommons.org/courseware/lesson/100538/overview
|
Educator PD: Safety Cues & Brain States
Overview
Psychological safety in education encourages student collaboration, creativity, critical thinking, and discourse. Learn about how building in safety cues to the classroom can support learning in classrooms. This presentation can be 1 or 3 hour learning experience. Shift and change the content as needed to fit your needs. Read the speaker notes for information regarding the challenge model structure and the content. Prior knowledg in this area is not necessary. Learn with your staff while you facilitate the PD in your school.
PD: Safety Cues & Brain States
Explore safety cues and brain states with other educators as we work to co-create our classrooms with psychological safety.
Challenge: How can we use structures & supports co-create safety cues in our classrooms?
Initial thoughts
Multiple Perspectives
Reflection
Flipped Resources
Revised Thinking
Report Out
LTP
|
oercommons
|
2025-03-18T00:39:10.039254
|
Missy Widmann
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/100538/overview",
"title": "Educator PD: Safety Cues & Brain States",
"author": "Teaching/Learning Strategy"
}
|
https://oercommons.org/courseware/lesson/87989/overview
|
Spanish Civil War
Overview
The Spanish Civil War
The onset of the Great Depression destabilized the economy of Spain and resulted in the collapse of the Spanish monarchy in 1931. After the establishment of a Republic, civil war erupted between Communists and Socialists on the left and the Spanish army on the right under the leadership of Francisco Franco. By 1939 Franco defeated his enemies and established a military dictatorship.
Learning Objectives
- Examine the development of Franco’s Fascist Spain.
Key Terms / Key Concepts
Falangism: a Fascist movement founded in Spain in 1933; the one legal party in Spain under the regime of Franco
Francisco Franco: a Spanish general who ruled over Spain as a dictator for 36 years from 1939 until his death (He took control of Spain from the government of the Second Spanish Republic after winning the Civil War, and was in power until 1978, when the Spanish Constitution of 1978 went into effect.)
personality cult: when an individual uses mass media, propaganda, or other methods to create an idealized, heroic, and at times worshipful image, often through unquestioned flattery and praise
Spanish Civil War: a war from 1936 to 1939 between the Republicans (loyalists to the democratic, left leaning and relatively urban Second Spanish Republic along with Anarchists and Communists) and forces loyal to General Francisco Franco (Nationalists, Falangists, and Carlists - a largely aristocratic conservative group)
Francisco Franco: El Caudillo
Francisco Franco (December 4, 1892 – November 20, 1975) was a Spanish general who ruled over Spain as a dictator for 36 years from 1939 until his death. As a conservative and a monarchist military officer, he opposed the abolition of the monarchy and the establishment of a republic in 1931. With the 1936 elections, the conservative Spanish Confederation of Autonomous Right-wing Groups lost by a narrow margin and the leftist Popular Front came to power. This Popular Front was an alliance between Spanish Liberals and Communists. Intending to overthrow the republic, Franco worked with other like-minded generals in attempting a failed coup that precipitated the Spanish Civil War (1936 – 1939). With the death of the other generals during this war, Franco quickly became his faction’s only leader. After securing his position as military dictator, Franco eventually in 1947, restored the Spanish monarchy in name only with himself as regent.
During the Civil War, Franco gained military support from various regimes and groups, especially Nazi Germany and the Fascist Italy. The opposition—or the Republican side—was supported by Spanish communists and anarchists, as well as the Soviet Union, Mexico, and the International Brigades. These brigades included volunteers from around the world who supported the Republic.
Leaving half a million people dead, the war was eventually won by Franco in 1939. He established a military dictatorship, which he defined as a totalitarian state. Franco proclaimed himself Head of State and Government under the title El Caudillo, a term similar to Il Duce (Italian) for Benito Mussolini and Der Führer (German) for Adolf Hitler. Under Franco, Spain became a one-party state, as the various conservative and royalist factions were merged into the fascist party and other political parties were outlawed.
Franco’s regime committed a series of violent human rights abuses against the Spanish people, which included the establishment of concentration camps and the use of forced labor and executions, mostly against political and ideological enemies, causing an estimated 200,000 to 400,000 deaths in more than 190 concentration camps over the course of his 36 years as dictator (1939 – 1975). During the last several decades of his regime, the number of executions declined considerably
During World War II, Spain sympathized with its fellow Fascist European states, the Axis powers, Germany and Italy. Spain’s entry into the war on the Axis side was prevented largely by British Secret Intelligence Service (MI-6) efforts that included up to $200 million in bribes for Spanish officials to keep the regime from getting involved. Franco was also able to take advantage of the resources of the Axis Powers, while choosing to avoid becoming heavily involved in the Second World War.
Ideology of Francoist Spain
The consistent points in Francoism included authoritarianism, nationalism, national Catholicism, militarism, conservatism, anti-communism, and anti-liberalism. The Spanish State was authoritarian. It suppressed non-government trade unions and all political opponents across the political spectrum often through police repression. Most country towns and rural areas were patrolled by pairs of Guardia Civil—a military police made up of civilians, which functioned as a chief means of social control. Larger cities and capitals were mostly under the heavily armed Policía Armada, commonly called grises due to their grey uniforms.
The Spanish state also enjoyed the broad support of the Roman Catholic Church. Many traditional Spanish Roman Catholics were relieved that Franco’s forces had crushed the atheistic, anti-clerical (anti-priests), Communists. Franco was also the focus of a personality cult which taught that he had been sent by Divine Providence to save the country from chaos and poverty.
Franco’s Spanish nationalism promoted a unitary national identity by repressing Spain’s cultural diversity. Bullfighting and flamenco were promoted as national traditions, while those traditions not considered Spanish were suppressed. Franco’s view of Spanish tradition was somewhat artificial and arbitrary: while some regional traditions were suppressed, Flamenco, an Andalusian tradition, was considered part of a larger, national identity. All cultural activities were subject to censorship, and many were forbidden entirely, often in an erratic manner.
Francoism professed a strong devotion to militarism, hypermasculinity, and the traditional role of women in society. A woman was to be loving to her parents and brothers and faithful to her husband, as well as reside with her family. Official propaganda confined women’s roles to family care and motherhood. Most progressive laws passed by the Second Republic were declared void. Women could not become judges, testify in trial, or become university professors.
The Civil War had ravaged the Spanish economy. Infrastructure had been damaged, workers killed, and daily business severely hampered. For more than a decade after Franco’s victory, the economy improved little. Franco initially pursued a policy of autarky, cutting off almost all international trade. The policy had devastating effects, and the economy stagnated. Only black marketeers could enjoy an evident affluence.
Up to 200,000 people died of starvation during the early years of Francoism, a period known as Los Años de Hambre (the Years of Hunger). This period coincided with the ravages of World War II (1939 – 1945).
Falangism: Spanish Fascism
Falangism was the official fascist ideology of Franco’s military dictatorship. Falangism was the political ideology of the Falange Española de las JONS when this political party was formed in Spain in 1934. Afterwards in 1937, Franco reformed this party as the Falange Española Tradicionalista y de las Juntas de Ofensiva Nacional Sindicalista (both known simply as the “Falange”). This new party remained the official party of the Spanish state until the collapse of this fascist regime soon after Franco’s death in 1975, Under the leadership of Franco, many of the more radical elements of Falangism considered fascist were diluted, and the party largely became an authoritarian, conservative ideology connected with Francoist Spain. Opponents of Franco’s changes to the party’s ideology included former Falange leader Manuel Hedilla. Falangism placed a strong emphasis on Catholic religious identity, though it held some secular views on the Church’s direct influence in society, as it believed that the state should have the supreme authority over the nation. Falangism emphasized the need for authority, hierarchy, and order in society. Falangism was also anti-communist, anti-capitalist, anti-democratic, and anti-liberal. Under Franco’s leadership, however, the Falange abandoned its original anti-capitalist tendencies, declaring the ideology to be fully compatible with capitalism.
The Falange’s original manifesto, the “Twenty-Seven Points,” declared that Falangism supported the unity of Spain and the elimination of regional separatism that existed among the Basques and Catalans of Northwestern and Northeastern Spain. This manifesto established a dictatorship led by the Falange and used violence to regenerate Spain. It also promoted the revival and development of the Spanish Empire overseas and championed a social revolution to create a national syndicalist economy. Syndicalists hoped to transfer the ownership and control of the means of production (i.e., factories) and distribution to state controlled workers' unions. This new economy was to mutually organize and control economic activity, agrarian reform, industrial expansion, while respecting private property except for nationalizing credit facilities (i.e., banks) to prevent capitalist usury (charging interest on loans). It criminalized strikes by employees and lockouts by employers as illegal acts. Falangism supported the state to have jurisdiction of setting wages. The Franco-era Falange supported the development of workers cooperatives (employee-owned businesses) such as the Mondragon Corporation in 1956, because it bolstered the Francoist claim of the nonexistence of an oppressed working class in Spain during his rule. The Mondragon Corporation still operates in Spain today, but the Falange Española Tradicionalista y de las Juntas de Ofensiva Nacional Sindicalista dissolved in 1977 soon after Franco’s death in 1975.
Attributions
Title Image
https://commons.wikimedia.org/wiki/File:Condor_Legion_marching_during_the_Spanish_Civil_War.jpg
Photo of a victory parade of Spanish national troops and the German Condor Legion in honor of General Francisco Franco in the festively decorated streets of Ciudad de Leon, Castile and Leon on May 22, 1939 - Unknown authorUnknown author, Public domain, via Wikimedia Commons
Adapted from:
https://courses.lumenlearning.com/boundless-worldhistory/chapter/the-rise-of-fascism/
|
oercommons
|
2025-03-18T00:39:10.069413
|
Neil Greenwood
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/87989/overview",
"title": "Statewide Dual Credit World History, The Catastrophe of the Modern Era: 1919-Present CE, Chapter 13: Post WWI, Spanish Civil War",
"author": "Anna McCollum"
}
|
https://oercommons.org/courseware/lesson/58764/overview
|
The Cardiovascular System: Blood
Overview
The Cardiovascular System: Blood
Introduction
Figure 18.1 Blood Cells A single drop of blood contains millions of red blood cells, white blood cells, and platelets. One of each type is shown here, isolated from a scanning electron micrograph.
CHAPTER OBJECTIVES
After studying this chapter, you will be able to:
- Identify the primary functions of blood, its fluid and cellular components, and its physical characteristics
- Identify the most important proteins and other solutes present in blood plasma
- Describe the formation of the formed element components of blood
- Discuss the structure and function of red blood cells and hemoglobin
- Classify and characterize white blood cells
- Describe the structure of platelets and explain the process of hemostasis
- Explain the significance of AB and Rh blood groups in blood transfusions
- Discuss a variety of blood disorders
Single-celled organisms do not need blood. They obtain nutrients directly from and excrete wastes directly into their environment. The human organism cannot do that. Our large, complex bodies need blood to deliver nutrients to and remove wastes from our trillions of cells. The heart pumps blood throughout the body in a network of blood vessels. Together, these three components—blood, heart, and vessels—makes up the cardiovascular system. This chapter focuses on the medium of transport: blood.
An Overview of Blood
- Identify the primary functions of blood in transportation, defense, and maintenance of homeostasis
- Name the fluid component of blood and the three major types of formed elements, and identify their relative proportions in a blood sample
- Discuss the unique physical characteristics of blood
- Identify the composition of blood plasma, including its most important solutes and plasma proteins
Recall that blood is a connective tissue. Like all connective tissues, it is made up of cellular elements and an extracellular matrix. The cellular elements—referred to as the formed elements—include red blood cells (RBCs), white blood cells (WBCs), and cell fragments called platelets. The extracellular matrix, called plasma, makes blood unique among connective tissues because it is fluid. This fluid, which is mostly water, perpetually suspends the formed elements and enables them to circulate throughout the body within the cardiovascular system.
Functions of Blood
The primary function of blood is to deliver oxygen and nutrients to and remove wastes from body cells, but that is only the beginning of the story. The specific functions of blood also include defense, distribution of heat, and maintenance of homeostasis.
Transportation
Nutrients from the foods you eat are absorbed in the digestive tract. Most of these travel in the bloodstream directly to the liver, where they are processed and released back into the bloodstream for delivery to body cells. Oxygen from the air you breathe diffuses into the blood, which moves from the lungs to the heart, which then pumps it out to the rest of the body. Moreover, endocrine glands scattered throughout the body release their products, called hormones, into the bloodstream, which carries them to distant target cells. Blood also picks up cellular wastes and byproducts, and transports them to various organs for removal. For instance, blood moves carbon dioxide to the lungs for exhalation from the body, and various waste products are transported to the kidneys and liver for excretion from the body in the form of urine or bile.
Defense
Many types of WBCs protect the body from external threats, such as disease-causing bacteria that have entered the bloodstream in a wound. Other WBCs seek out and destroy internal threats, such as cells with mutated DNA that could multiply to become cancerous, or body cells infected with viruses.
When damage to the vessels results in bleeding, blood platelets and certain proteins dissolved in the plasma, the fluid portion of the blood, interact to block the ruptured areas of the blood vessels involved. This protects the body from further blood loss.
Maintenance of Homeostasis
Recall that body temperature is regulated via a classic negative-feedback loop. If you were exercising on a warm day, your rising core body temperature would trigger several homeostatic mechanisms, including increased transport of blood from your core to your body periphery, which is typically cooler. As blood passes through the vessels of the skin, heat would be dissipated to the environment, and the blood returning to your body core would be cooler. In contrast, on a cold day, blood is diverted away from the skin to maintain a warmer body core. In extreme cases, this may result in frostbite.
Blood also helps to maintain the chemical balance of the body. Proteins and other compounds in blood act as buffers, which thereby help to regulate the pH of body tissues. Blood also helps to regulate the water content of body cells.
Composition of Blood
You have probably had blood drawn from a superficial vein in your arm, which was then sent to a lab for analysis. Some of the most common blood tests—for instance, those measuring lipid or glucose levels in plasma—determine which substances are present within blood and in what quantities. Other blood tests check for the composition of the blood itself, including the quantities and types of formed elements.
One such test, called a hematocrit, measures the percentage of RBCs, clinically known as erythrocytes, in a blood sample. It is performed by spinning the blood sample in a specialized centrifuge, a process that causes the heavier elements suspended within the blood sample to separate from the lightweight, liquid plasma (Figure 18.2). Because the heaviest elements in blood are the erythrocytes, these settle at the very bottom of the hematocrit tube. Located above the erythrocytes is a pale, thin layer composed of the remaining formed elements of blood. These are the WBCs, clinically known as leukocytes, and the platelets, cell fragments also called thrombocytes. This layer is referred to as the buffy coat because of its color; it normally constitutes less than 1 percent of a blood sample. Above the buffy coat is the blood plasma, normally a pale, straw-colored fluid, which constitutes the remainder of the sample.
The volume of erythrocytes after centrifugation is also commonly referred to as packed cell volume (PCV). In normal blood, about 45 percent of a sample is erythrocytes. The hematocrit of any one sample can vary significantly, however, about 36–50 percent, according to gender and other factors. Normal hematocrit values for females range from 37 to 47, with a mean value of 41; for males, hematocrit ranges from 42 to 52, with a mean of 47. The percentage of other formed elements, the WBCs and platelets, is extremely small so it is not normally considered with the hematocrit. So the mean plasma percentage is the percent of blood that is not erythrocytes: for females, it is approximately 59 (or 100 minus 41), and for males, it is approximately 53 (or 100 minus 47).
Figure 18.2 Composition of Blood The cellular elements of blood include a vast number of erythrocytes and comparatively fewer leukocytes and platelets. Plasma is the fluid in which the formed elements are suspended. A sample of blood spun in a centrifuge reveals that plasma is the lightest component. It floats at the top of the tube separated from the heaviest elements, the erythrocytes, by a buffy coat of leukocytes and platelets. Hematocrit is the percentage of the total sample that is comprised of erythrocytes. Depressed and elevated hematocrit levels are shown for comparison.
Characteristics of Blood
When you think about blood, the first characteristic that probably comes to mind is its color. Blood that has just taken up oxygen in the lungs is bright red, and blood that has released oxygen in the tissues is a more dusky red. This is because hemoglobin is a pigment that changes color, depending upon the degree of oxygen saturation.
Blood is viscous and somewhat sticky to the touch. It has a viscosity approximately five times greater than water. Viscosity is a measure of a fluid’s thickness or resistance to flow, and is influenced by the presence of the plasma proteins and formed elements within the blood. The viscosity of blood has a dramatic impact on blood pressure and flow. Consider the difference in flow between water and honey. The more viscous honey would demonstrate a greater resistance to flow than the less viscous water. The same principle applies to blood.
The normal temperature of blood is slightly higher than normal body temperature—about 38 °C (or 100.4 °F), compared to 37 °C (or 98.6 °F) for an internal body temperature reading, although daily variations of 0.5 °C are normal. Although the surface of blood vessels is relatively smooth, as blood flows through them, it experiences some friction and resistance, especially as vessels age and lose their elasticity, thereby producing heat. This accounts for its slightly higher temperature.
The pH of blood averages about 7.4; however, it can range from 7.35 to 7.45 in a healthy person. Blood is therefore somewhat more basic (alkaline) on a chemical scale than pure water, which has a pH of 7.0. Blood contains numerous buffers that actually help to regulate pH.
Blood constitutes approximately 8 percent of adult body weight. Adult males typically average about 5 to 6 liters of blood. Females average 4–5 liters.
Blood Plasma
Like other fluids in the body, plasma is composed primarily of water: In fact, it is about 92 percent water. Dissolved or suspended within this water is a mixture of substances, most of which are proteins. There are literally hundreds of substances dissolved or suspended in the plasma, although many of them are found only in very small quantities.
INTERACTIVE LINK
Visit this site for a list of normal levels established for many of the substances found in a sample of blood. Serum, one of the specimen types included, refers to a sample of plasma after clotting factors have been removed. What types of measurements are given for levels of glucose in the blood?
Plasma Proteins
About 7 percent of the volume of plasma—nearly all that is not water—is made of proteins. These include several plasma proteins (proteins that are unique to the plasma), plus a much smaller number of regulatory proteins, including enzymes and some hormones. The major components of plasma are summarized in Figure 18.3.
The three major groups of plasma proteins are as follows:
- Albumin is the most abundant of the plasma proteins. Manufactured by the liver, albumin molecules serve as binding proteins—transport vehicles for fatty acids and steroid hormones. Recall that lipids are hydrophobic; however, their binding to albumin enables their transport in the watery plasma. Albumin is also the most significant contributor to the osmotic pressure of blood; that is, its presence holds water inside the blood vessels and draws water from the tissues, across blood vessel walls, and into the bloodstream. This in turn helps to maintain both blood volume and blood pressure. Albumin normally accounts for approximately 54 percent of the total plasma protein content, in clinical levels of 3.5–5.0 g/dL blood.
- The second most common plasma proteins are the globulins. A heterogeneous group, there are three main subgroups known as alpha, beta, and gamma globulins. The alpha and beta globulins transport iron, lipids, and the fat-soluble vitamins A, D, E, and K to the cells; like albumin, they also contribute to osmotic pressure. The gamma globulins are proteins involved in immunity and are better known as an antibodies or immunoglobulins. Although other plasma proteins are produced by the liver, immunoglobulins are produced by specialized leukocytes known as plasma cells. (Seek additional content for more information about immunoglobulins.) Globulins make up approximately 38 percent of the total plasma protein volume, in clinical levels of 1.0–1.5 g/dL blood.
- The least abundant plasma protein is fibrinogen. Like albumin and the alpha and beta globulins, fibrinogen is produced by the liver. It is essential for blood clotting, a process described later in this chapter. Fibrinogen accounts for about 7 percent of the total plasma protein volume, in clinical levels of 0.2–0.45 g/dL blood.
Other Plasma Solutes
In addition to proteins, plasma contains a wide variety of other substances. These include various electrolytes, such as sodium, potassium, and calcium ions; dissolved gases, such as oxygen, carbon dioxide, and nitrogen; various organic nutrients, such as vitamins, lipids, glucose, and amino acids; and metabolic wastes. All of these nonprotein solutes combined contribute approximately 1 percent to the total volume of plasma.
Figure 18.3 Major Blood Components
CAREER CONNECTION
Phlebotomy and Medical Lab Technology
Phlebotomists are professionals trained to draw blood (phleb- = “a blood vessel”; -tomy = “to cut”). When more than a few drops of blood are required, phlebotomists perform a venipuncture, typically of a surface vein in the arm. They perform a capillary stick on a finger, an earlobe, or the heel of an infant when only a small quantity of blood is required. An arterial stick is collected from an artery and used to analyze blood gases. After collection, the blood may be analyzed by medical laboratories or perhaps used for transfusions, donations, or research. While many allied health professionals practice phlebotomy, the American Society of Phlebotomy Technicians issues certificates to individuals passing a national examination, and some large labs and hospitals hire individuals expressly for their skill in phlebotomy.
Medical or clinical laboratories employ a variety of individuals in technical positions:
- Medical technologists (MT), also known as clinical laboratory technologists (CLT), typically hold a bachelor’s degree and certification from an accredited training program. They perform a wide variety of tests on various body fluids, including blood. The information they provide is essential to the primary care providers in determining a diagnosis and in monitoring the course of a disease and response to treatment.
- Medical laboratory technicians (MLT) typically have an associate’s degree but may perform duties similar to those of an MT.
- Medical laboratory assistants (MLA) spend the majority of their time processing samples and carrying out routine assignments within the lab. Clinical training is required, but a degree may not be essential to obtaining a position.
Production of the Formed Elements
- Trace the generation of the formed elements of blood from bone marrow stem cells
- Discuss the role of hemopoietic growth factors in promoting the production of the formed elements
The lifespan of the formed elements is very brief. Although one type of leukocyte called memory cells can survive for years, most erythrocytes, leukocytes, and platelets normally live only a few hours to a few weeks. Thus, the body must form new blood cells and platelets quickly and continuously. When you donate a unit of blood during a blood drive (approximately 475 mL, or about 1 pint), your body typically replaces the donated plasma within 24 hours, but it takes about 4 to 6 weeks to replace the blood cells. This restricts the frequency with which donors can contribute their blood. The process by which this replacement occurs is called hemopoiesis, or hematopoiesis (from the Greek root haima- = “blood”; -poiesis = “production”).
Sites of Hemopoiesis
Prior to birth, hemopoiesis occurs in a number of tissues, beginning with the yolk sac of the developing embryo, and continuing in the fetal liver, spleen, lymphatic tissue, and eventually the red bone marrow. Following birth, most hemopoiesis occurs in the red marrow, a connective tissue within the spaces of spongy (cancellous) bone tissue. In children, hemopoiesis can occur in the medullary cavity of long bones; in adults, the process is largely restricted to the cranial and pelvic bones, the vertebrae, the sternum, and the proximal epiphyses of the femur and humerus.
Throughout adulthood, the liver and spleen maintain their ability to generate the formed elements. This process is referred to as extramedullary hemopoiesis (meaning hemopoiesis outside the medullary cavity of adult bones). When a disease such as bone cancer destroys the bone marrow, causing hemopoiesis to fail, extramedullary hemopoiesis may be initiated.
Differentiation of Formed Elements from Stem Cells
All formed elements arise from stem cells of the red bone marrow. Recall that stem cells undergo mitosis plus cytokinesis (cellular division) to give rise to new daughter cells: One of these remains a stem cell and the other differentiates into one of any number of diverse cell types. Stem cells may be viewed as occupying a hierarchal system, with some loss of the ability to diversify at each step. The totipotent stem cell is the zygote, or fertilized egg. The totipotent (toti- = “all”) stem cell gives rise to all cells of the human body. The next level is the pluripotent stem cell, which gives rise to multiple types of cells of the body and some of the supporting fetal membranes. Beneath this level, the mesenchymal cell is a stem cell that develops only into types of connective tissue, including fibrous connective tissue, bone, cartilage, and blood, but not epithelium, muscle, and nervous tissue. One step lower on the hierarchy of stem cells is the hemopoietic stem cell, or hemocytoblast. All of the formed elements of blood originate from this specific type of cell.
Hemopoiesis begins when the hemopoietic stem cell is exposed to appropriate chemical stimuli collectively called hemopoietic growth factors, which prompt it to divide and differentiate. One daughter cell remains a hemopoietic stem cell, allowing hemopoiesis to continue. The other daughter cell becomes either of two types of more specialized stem cells (Figure 18.4):
- Lymphoid stem cells give rise to a class of leukocytes known as lymphocytes, which include the various T cells, B cells, and natural killer (NK) cells, all of which function in immunity. However, hemopoiesis of lymphocytes progresses somewhat differently from the process for the other formed elements. In brief, lymphoid stem cells quickly migrate from the bone marrow to lymphatic tissues, including the lymph nodes, spleen, and thymus, where their production and differentiation continues. B cells are so named since they mature in the bone marrow, while T cells mature in the thymus.
- Myeloid stem cells give rise to all the other formed elements, including the erythrocytes; megakaryocytes that produce platelets; and a myeloblast lineage that gives rise to monocytes and three forms of granular leukocytes: neutrophils, eosinophils, and basophils.
Figure 18.4 Hematopoietic System of Bone Marrow Hemopoiesis is the proliferation and differentiation of the formed elements of blood.
Lymphoid and myeloid stem cells do not immediately divide and differentiate into mature formed elements. As you can see in Figure 18.4, there are several intermediate stages of precursor cells (literally, forerunner cells), many of which can be recognized by their names, which have the suffix -blast. For instance, megakaryoblasts are the precursors of megakaryocytes, and proerythroblasts become reticulocytes, which eject their nucleus and most other organelles before maturing into erythrocytes.
Hemopoietic Growth Factors
Development from stem cells to precursor cells to mature cells is again initiated by hemopoietic growth factors. These include the following:
- Erythropoietin (EPO) is a glycoprotein hormone secreted by the interstitial fibroblast cells of the kidneys in response to low oxygen levels. It prompts the production of erythrocytes. Some athletes use synthetic EPO as a performance-enhancing drug (called blood doping) to increase RBC counts and subsequently increase oxygen delivery to tissues throughout the body. EPO is a banned substance in most organized sports, but it is also used medically in the treatment of certain anemia, specifically those triggered by certain types of cancer, and other disorders in which increased erythrocyte counts and oxygen levels are desirable.
- Thrombopoietin, another glycoprotein hormone, is produced by the liver and kidneys. It triggers the development of megakaryocytes into platelets.
- Cytokines are glycoproteins secreted by a wide variety of cells, including red bone marrow, leukocytes, macrophages, fibroblasts, and endothelial cells. They act locally as autocrine or paracrine factors, stimulating the proliferation of progenitor cells and helping to stimulate both nonspecific and specific resistance to disease. There are two major subtypes of cytokines known as colony-stimulating factors and interleukins.
- Colony-stimulating factors (CSFs) are glycoproteins that act locally, as autocrine or paracrine factors. Some trigger the differentiation of myeloblasts into granular leukocytes, namely, neutrophils, eosinophils, and basophils. These are referred to as granulocyte CSFs. A different CSF induces the production of monocytes, called monocyte CSFs. Both granulocytes and monocytes are stimulated by GM-CSF; granulocytes, monocytes, platelets, and erythrocytes are stimulated by multi-CSF. Synthetic forms of these hormones are often administered to patients with various forms of cancer who are receiving chemotherapy to revive their WBC counts.
- Interleukins are another class of cytokine signaling molecules important in hemopoiesis. They were initially thought to be secreted uniquely by leukocytes and to communicate only with other leukocytes, and were named accordingly, but are now known to be produced by a variety of cells including bone marrow and endothelium. Researchers now suspect that interleukins may play other roles in body functioning, including differentiation and maturation of cells, producing immunity and inflammation. To date, more than a dozen interleukins have been identified, with others likely to follow. They are generally numbered IL-1, IL-2, IL-3, etc.
EVERYDAY CONNECTION
Blood Doping
In its original intent, the term blood doping was used to describe the practice of injecting by transfusion supplemental RBCs into an individual, typically to enhance performance in a sport. Additional RBCs would deliver more oxygen to the tissues, providing extra aerobic capacity, clinically referred to as VO2 max. The source of the cells was either from the recipient (autologous) or from a donor with compatible blood (homologous). This practice was aided by the well-developed techniques of harvesting, concentrating, and freezing of the RBCs that could be later thawed and injected, yet still retain their functionality. These practices are considered illegal in virtually all sports and run the risk of infection, significantly increasing the viscosity of the blood and the potential for transmission of blood-borne pathogens if the blood was collected from another individual.
With the development of synthetic EPO in the 1980s, it became possible to provide additional RBCs by artificially stimulating RBC production in the bone marrow. Originally developed to treat patients suffering from anemia, renal failure, or cancer treatment, large quantities of EPO can be generated by recombinant DNA technology. Synthetic EPO is injected under the skin and can increase hematocrit for many weeks. It may also induce polycythemia and raise hematocrit to 70 or greater. This increased viscosity raises the resistance of the blood and forces the heart to pump more powerfully; in extreme cases, it has resulted in death. Other drugs such as cobalt II chloride have been shown to increase natural EPO gene expression. Blood doping has become problematic in many sports, especially cycling. Lance Armstrong, winner of seven Tour de France and many other cycling titles, was stripped of his victories and admitted to blood doping in 2013.
INTERACTIVE LINK
Watch this video to see doctors discuss the dangers of blood doping in sports. What are the some potential side effects of blood doping?
Bone Marrow Sampling and Transplants
Sometimes, a healthcare provider will order a bone marrow biopsy, a diagnostic test of a sample of red bone marrow, or a bone marrow transplant, a treatment in which a donor’s healthy bone marrow—and its stem cells—replaces the faulty bone marrow of a patient. These tests and procedures are often used to assist in the diagnosis and treatment of various severe forms of anemia, such as thalassemia major and sickle cell anemia, as well as some types of cancer, specifically leukemia.
In the past, when a bone marrow sample or transplant was necessary, the procedure would have required inserting a large-bore needle into the region near the iliac crest of the pelvic bones (os coxae). This location was preferred, since its location close to the body surface makes it more accessible, and it is relatively isolated from most vital organs. Unfortunately, the procedure is quite painful.
Now, direct sampling of bone marrow can often be avoided. In many cases, stem cells can be isolated in just a few hours from a sample of a patient’s blood. The isolated stem cells are then grown in culture using the appropriate hemopoietic growth factors, and analyzed or sometimes frozen for later use.
For an individual requiring a transplant, a matching donor is essential to prevent the immune system from destroying the donor cells—a phenomenon known as tissue rejection. To treat patients with bone marrow transplants, it is first necessary to destroy the patient’s own diseased marrow through radiation and/or chemotherapy. Donor bone marrow stem cells are then intravenously infused. From the bloodstream, they establish themselves in the recipient’s bone marrow.
Erythrocytes
- Describe the anatomy of erythrocytes
- Discuss the various steps in the lifecycle of an erythrocyte
- Explain the composition and function of hemoglobin
The erythrocyte, commonly known as a red blood cell (or RBC), is by far the most common formed element: A single drop of blood contains millions of erythrocytes and just thousands of leukocytes. Specifically, males have about 5.4 million erythrocytes per microliter (µL) of blood, and females have approximately 4.8 million per µL. In fact, erythrocytes are estimated to make up about 25 percent of the total cells in the body. As you can imagine, they are quite small cells, with a mean diameter of only about 7–8 micrometers (µm) (Figure 18.5). The primary functions of erythrocytes are to pick up inhaled oxygen from the lungs and transport it to the body’s tissues, and to pick up some (about 24 percent) carbon dioxide waste at the tissues and transport it to the lungs for exhalation. Erythrocytes remain within the vascular network. Although leukocytes typically leave the blood vessels to perform their defensive functions, movement of erythrocytes from the blood vessels is abnormal.
Figure 18.5 Summary of Formed Elements in Blood
Shape and Structure of Erythrocytes
As an erythrocyte matures in the red bone marrow, it extrudes its nucleus and most of its other organelles. During the first day or two that it is in the circulation, an immature erythrocyte, known as a reticulocyte, will still typically contain remnants of organelles. Reticulocytes should comprise approximately 1–2 percent of the erythrocyte count and provide a rough estimate of the rate of RBC production, with abnormally low or high rates indicating deviations in the production of these cells. These remnants, primarily of networks (reticulum) of ribosomes, are quickly shed, however, and mature, circulating erythrocytes have few internal cellular structural components. Lacking mitochondria, for example, they rely on anaerobic respiration. This means that they do not utilize any of the oxygen they are transporting, so they can deliver it all to the tissues. They also lack endoplasmic reticula and do not synthesize proteins. Erythrocytes do, however, contain some structural proteins that help the blood cells maintain their unique structure and enable them to change their shape to squeeze through capillaries. This includes the protein spectrin, a cytoskeletal protein element.
Erythrocytes are biconcave disks; that is, they are plump at their periphery and very thin in the center (Figure 18.6). Since they lack most organelles, there is more interior space for the presence of the hemoglobin molecules that, as you will see shortly, transport gases. The biconcave shape also provides a greater surface area across which gas exchange can occur, relative to its volume; a sphere of a similar diameter would have a lower surface area-to-volume ratio. In the capillaries, the oxygen carried by the erythrocytes can diffuse into the plasma and then through the capillary walls to reach the cells, whereas some of the carbon dioxide produced by the cells as a waste product diffuses into the capillaries to be picked up by the erythrocytes. Capillary beds are extremely narrow, slowing the passage of the erythrocytes and providing an extended opportunity for gas exchange to occur. However, the space within capillaries can be so minute that, despite their own small size, erythrocytes may have to fold in on themselves if they are to make their way through. Fortunately, their structural proteins like spectrin are flexible, allowing them to bend over themselves to a surprising degree, then spring back again when they enter a wider vessel. In wider vessels, erythrocytes may stack up much like a roll of coins, forming a rouleaux, from the French word for “roll.”
Figure 18.6 Shape of Red Blood Cells Erythrocytes are biconcave discs with very shallow centers. This shape optimizes the ratio of surface area to volume, facilitating gas exchange. It also enables them to fold up as they move through narrow blood vessels.
Hemoglobin
Hemoglobin is a large molecule made up of proteins and iron. It consists of four folded chains of a protein called globin, designated alpha 1 and 2, and beta 1 and 2 (Figure 18.7a). Each of these globin molecules is bound to a red pigment molecule called heme, which contains an ion of iron (Fe2+) (Figure 18.7b).
Figure 18.7 Hemoglobin (a) A molecule of hemoglobin contains four globin proteins, each of which is bound to one molecule of the iron-containing pigment heme. (b) A single erythrocyte can contain 300 million hemoglobin molecules, and thus more than 1 billion oxygen molecules.
Each iron ion in the heme can bind to one oxygen molecule; therefore, each hemoglobin molecule can transport four oxygen molecules. An individual erythrocyte may contain about 300 million hemoglobin molecules, and therefore can bind to and transport up to 1.2 billion oxygen molecules (see Figure 18.7b).
In the lungs, hemoglobin picks up oxygen, which binds to the iron ions, forming oxyhemoglobin. The bright red, oxygenated hemoglobin travels to the body tissues, where it releases some of the oxygen molecules, becoming darker red deoxyhemoglobin, sometimes referred to as reduced hemoglobin. Oxygen release depends on the need for oxygen in the surrounding tissues, so hemoglobin rarely if ever leaves all of its oxygen behind. In the capillaries, carbon dioxide enters the bloodstream. About 76 percent dissolves in the plasma, some of it remaining as dissolved CO2, and the remainder forming bicarbonate ion. About 23–24 percent of it binds to the amino acids in hemoglobin, forming a molecule known as carbaminohemoglobin. From the capillaries, the hemoglobin carries carbon dioxide back to the lungs, where it releases it for exchange of oxygen.
Changes in the levels of RBCs can have significant effects on the body’s ability to effectively deliver oxygen to the tissues. Ineffective hematopoiesis results in insufficient numbers of RBCs and results in one of several forms of anemia. An overproduction of RBCs produces a condition called polycythemia. The primary drawback with polycythemia is not a failure to directly deliver enough oxygen to the tissues, but rather the increased viscosity of the blood, which makes it more difficult for the heart to circulate the blood.
In patients with insufficient hemoglobin, the tissues may not receive sufficient oxygen, resulting in another form of anemia. In determining oxygenation of tissues, the value of greatest interest in healthcare is the percent saturation; that is, the percentage of hemoglobin sites occupied by oxygen in a patient’s blood. Clinically this value is commonly referred to simply as “percent sat.”
Percent saturation is normally monitored using a device known as a pulse oximeter, which is applied to a thin part of the body, typically the tip of the patient’s finger. The device works by sending two different wavelengths of light (one red, the other infrared) through the finger and measuring the light with a photodetector as it exits. Hemoglobin absorbs light differentially depending upon its saturation with oxygen. The machine calibrates the amount of light received by the photodetector against the amount absorbed by the partially oxygenated hemoglobin and presents the data as percent saturation. Normal pulse oximeter readings range from 95–100 percent. Lower percentages reflect hypoxemia, or low blood oxygen. The term hypoxia is more generic and simply refers to low oxygen levels. Oxygen levels are also directly monitored from free oxygen in the plasma typically following an arterial stick. When this method is applied, the amount of oxygen present is expressed in terms of partial pressure of oxygen or simply pO2 and is typically recorded in units of millimeters of mercury, mm Hg.
The kidneys filter about 180 liters (~380 pints) of blood in an average adult each day, or about 20 percent of the total resting volume, and thus serve as ideal sites for receptors that determine oxygen saturation. In response to hypoxemia, less oxygen will exit the vessels supplying the kidney, resulting in hypoxia (low oxygen concentration) in the tissue fluid of the kidney where oxygen concentration is actually monitored. Interstitial fibroblasts within the kidney secrete EPO, thereby increasing erythrocyte production and restoring oxygen levels. In a classic negative-feedback loop, as oxygen saturation rises, EPO secretion falls, and vice versa, thereby maintaining homeostasis. Populations dwelling at high elevations, with inherently lower levels of oxygen in the atmosphere, naturally maintain a hematocrit higher than people living at sea level. Consequently, people traveling to high elevations may experience symptoms of hypoxemia, such as fatigue, headache, and shortness of breath, for a few days after their arrival. In response to the hypoxemia, the kidneys secrete EPO to step up the production of erythrocytes until homeostasis is achieved once again. To avoid the symptoms of hypoxemia, or altitude sickness, mountain climbers typically rest for several days to a week or more at a series of camps situated at increasing elevations to allow EPO levels and, consequently, erythrocyte counts to rise. When climbing the tallest peaks, such as Mt. Everest and K2 in the Himalayas, many mountain climbers rely upon bottled oxygen as they near the summit.
Lifecycle of Erythrocytes
Production of erythrocytes in the marrow occurs at the staggering rate of more than 2 million cells per second. For this production to occur, a number of raw materials must be present in adequate amounts. These include the same nutrients that are essential to the production and maintenance of any cell, such as glucose, lipids, and amino acids. However, erythrocyte production also requires several trace elements:
- Iron. We have said that each heme group in a hemoglobin molecule contains an ion of the trace mineral iron. On average, less than 20 percent of the iron we consume is absorbed. Heme iron, from animal foods such as meat, poultry, and fish, is absorbed more efficiently than non-heme iron from plant foods. Upon absorption, iron becomes part of the body’s total iron pool. The bone marrow, liver, and spleen can store iron in the protein compounds ferritin and hemosiderin. Ferroportin transports the iron across the intestinal cell plasma membranes and from its storage sites into tissue fluid where it enters the blood. When EPO stimulates the production of erythrocytes, iron is released from storage, bound to transferrin, and carried to the red marrow where it attaches to erythrocyte precursors.
- Copper. A trace mineral, copper is a component of two plasma proteins, hephaestin and ceruloplasmin. Without these, hemoglobin could not be adequately produced. Located in intestinal villi, hephaestin enables iron to be absorbed by intestinal cells. Ceruloplasmin transports copper. Both enable the oxidation of iron from Fe2+ to Fe3+, a form in which it can be bound to its transport protein, transferrin, for transport to body cells. In a state of copper deficiency, the transport of iron for heme synthesis decreases, and iron can accumulate in tissues, where it can eventually lead to organ damage.
- Zinc. The trace mineral zinc functions as a co-enzyme that facilitates the synthesis of the heme portion of hemoglobin.
- B vitamins. The B vitamins folate and vitamin B12 function as co-enzymes that facilitate DNA synthesis. Thus, both are critical for the synthesis of new cells, including erythrocytes.
Erythrocytes live up to 120 days in the circulation, after which the worn-out cells are removed by a type of myeloid phagocytic cell called a macrophage, located primarily within the bone marrow, liver, and spleen. The components of the degraded erythrocytes’ hemoglobin are further processed as follows:
- Globin, the protein portion of hemoglobin, is broken down into amino acids, which can be sent back to the bone marrow to be used in the production of new erythrocytes. Hemoglobin that is not phagocytized is broken down in the circulation, releasing alpha and beta chains that are removed from circulation by the kidneys.
- The iron contained in the heme portion of hemoglobin may be stored in the liver or spleen, primarily in the form of ferritin or hemosiderin, or carried through the bloodstream by transferrin to the red bone marrow for recycling into new erythrocytes.
- The non-iron portion of heme is degraded into the waste product biliverdin, a green pigment, and then into another waste product, bilirubin, a yellow pigment. Bilirubin binds to albumin and travels in the blood to the liver, which uses it in the manufacture of bile, a compound released into the intestines to help emulsify dietary fats. In the large intestine, bacteria breaks the bilirubin apart from the bile and converts it to urobilinogen and then into stercobilin. It is then eliminated from the body in the feces. Broad-spectrum antibiotics typically eliminate these bacteria as well and may alter the color of feces. The kidneys also remove any circulating bilirubin and other related metabolic byproducts such as urobilins and secrete them into the urine.
The breakdown pigments formed from the destruction of hemoglobin can be seen in a variety of situations. At the site of an injury, biliverdin from damaged RBCs produces some of the dramatic colors associated with bruising. With a failing liver, bilirubin cannot be removed effectively from circulation and causes the body to assume a yellowish tinge associated with jaundice. Stercobilins within the feces produce the typical brown color associated with this waste. And the yellow of urine is associated with the urobilins.
The erythrocyte lifecycle is summarized in Figure 18.8.
Figure 18.8 Erythrocyte Lifecycle Erythrocytes are produced in the bone marrow and sent into the circulation. At the end of their lifecycle, they are destroyed by macrophages, and their components are recycled.
Disorders of Erythrocytes
The size, shape, and number of erythrocytes, and the number of hemoglobin molecules can have a major impact on a person’s health. When the number of RBCs or hemoglobin is deficient, the general condition is called anemia. There are more than 400 types of anemia and more than 3.5 million Americans suffer from this condition. Anemia can be broken down into three major groups: those caused by blood loss, those caused by faulty or decreased RBC production, and those caused by excessive destruction of RBCs. Clinicians often use two groupings in diagnosis: The kinetic approach focuses on evaluating the production, destruction, and removal of RBCs, whereas the morphological approach examines the RBCs themselves, paying particular emphasis to their size. A common test is the mean corpuscle volume (MCV), which measures size. Normal-sized cells are referred to as normocytic, smaller-than-normal cells are referred to as microcytic, and larger-than-normal cells are referred to as macrocytic. Reticulocyte counts are also important and may reveal inadequate production of RBCs. The effects of the various anemias are widespread, because reduced numbers of RBCs or hemoglobin will result in lower levels of oxygen being delivered to body tissues. Since oxygen is required for tissue functioning, anemia produces fatigue, lethargy, and an increased risk for infection. An oxygen deficit in the brain impairs the ability to think clearly, and may prompt headaches and irritability. Lack of oxygen leaves the patient short of breath, even as the heart and lungs work harder in response to the deficit.
Blood loss anemias are fairly straightforward. In addition to bleeding from wounds or other lesions, these forms of anemia may be due to ulcers, hemorrhoids, inflammation of the stomach (gastritis), and some cancers of the gastrointestinal tract. The excessive use of aspirin or other nonsteroidal anti-inflammatory drugs such as ibuprofen can trigger ulceration and gastritis. Excessive menstruation and loss of blood during childbirth are also potential causes.
Anemias caused by faulty or decreased RBC production include sickle cell anemia, iron deficiency anemia, vitamin deficiency anemia, and diseases of the bone marrow and stem cells.
- A characteristic change in the shape of erythrocytes is seen in sickle cell disease (also referred to as sickle cell anemia). A genetic disorder, it is caused by production of an abnormal type of hemoglobin, called hemoglobin S, which delivers less oxygen to tissues and causes erythrocytes to assume a sickle (or crescent) shape, especially at low oxygen concentrations (Figure 18.9). These abnormally shaped cells can then become lodged in narrow capillaries because they are unable to fold in on themselves to squeeze through, blocking blood flow to tissues and causing a variety of serious problems from painful joints to delayed growth and even blindness and cerebrovascular accidents (strokes). Sickle cell anemia is a genetic condition particularly found in individuals of African descent.
Figure 18.9 Sickle Cells Sickle cell anemia is caused by a mutation in one of the hemoglobin genes. Erythrocytes produce an abnormal type of hemoglobin, which causes the cell to take on a sickle or crescent shape. (credit: Janice Haney Carr)
- Iron deficiency anemia is the most common type and results when the amount of available iron is insufficient to allow production of sufficient heme. This condition can occur in individuals with a deficiency of iron in the diet and is especially common in teens and children as well as in vegans and vegetarians. Additionally, iron deficiency anemia may be caused by either an inability to absorb and transport iron or slow, chronic bleeding.
- Vitamin-deficient anemias generally involve insufficient vitamin B12 and folate.
- Megaloblastic anemia involves a deficiency of vitamin B12 and/or folate, and often involves diets deficient in these essential nutrients. Lack of meat or a viable alternate source, and overcooking or eating insufficient amounts of vegetables may lead to a lack of folate.
- Pernicious anemia is caused by poor absorption of vitamin B12 and is often seen in patients with Crohn’s disease (a severe intestinal disorder often treated by surgery), surgical removal of the intestines or stomach (common in some weight loss surgeries), intestinal parasites, and AIDS.
- Pregnancies, some medications, excessive alcohol consumption, and some diseases such as celiac disease are also associated with vitamin deficiencies. It is essential to provide sufficient folic acid during the early stages of pregnancy to reduce the risk of neurological defects, including spina bifida, a failure of the neural tube to close.
- Assorted disease processes can also interfere with the production and formation of RBCs and hemoglobin. If myeloid stem cells are defective or replaced by cancer cells, there will be insufficient quantities of RBCs produced.
- Aplastic anemia is the condition in which there are deficient numbers of RBC stem cells. Aplastic anemia is often inherited, or it may be triggered by radiation, medication, chemotherapy, or infection.
- Thalassemia is an inherited condition typically occurring in individuals from the Middle East, the Mediterranean, African, and Southeast Asia, in which maturation of the RBCs does not proceed normally. The most severe form is called Cooley’s anemia.
- Lead exposure from industrial sources or even dust from paint chips of iron-containing paints or pottery that has not been properly glazed may also lead to destruction of the red marrow.
- Various disease processes also can lead to anemias. These include chronic kidney diseases often associated with a decreased production of EPO, hypothyroidism, some forms of cancer, lupus, and rheumatoid arthritis.
In contrast to anemia, an elevated RBC count is called polycythemia and is detected in a patient’s elevated hematocrit. It can occur transiently in a person who is dehydrated; when water intake is inadequate or water losses are excessive, the plasma volume falls. As a result, the hematocrit rises. For reasons mentioned earlier, a mild form of polycythemia is chronic but normal in people living at high altitudes. Some elite athletes train at high elevations specifically to induce this phenomenon. Finally, a type of bone marrow disease called polycythemia vera (from the Greek vera = “true”) causes an excessive production of immature erythrocytes. Polycythemia vera can dangerously elevate the viscosity of blood, raising blood pressure and making it more difficult for the heart to pump blood throughout the body. It is a relatively rare disease that occurs more often in men than women, and is more likely to be present in elderly patients those over 60 years of age.
Leukocytes and Platelets
- Describe the general characteristics of leukocytes
- Classify leukocytes according to their lineage, their main structural features, and their primary functions
- Discuss the most common malignancies involving leukocytes
- Identify the lineage, basic structure, and function of platelets
The leukocyte, commonly known as a white blood cell (or WBC), is a major component of the body’s defenses against disease. Leukocytes protect the body against invading microorganisms and body cells with mutated DNA, and they clean up debris. Platelets are essential for the repair of blood vessels when damage to them has occurred; they also provide growth factors for healing and repair. See Figure 18.5 for a summary of leukocytes and platelets.
Characteristics of Leukocytes
Although leukocytes and erythrocytes both originate from hematopoietic stem cells in the bone marrow, they are very different from each other in many significant ways. For instance, leukocytes are far less numerous than erythrocytes: Typically there are only 5000 to 10,000 per µL. They are also larger than erythrocytes and are the only formed elements that are complete cells, possessing a nucleus and organelles. And although there is just one type of erythrocyte, there are many types of leukocytes. Most of these types have a much shorter lifespan than that of erythrocytes, some as short as a few hours or even a few minutes in the case of acute infection.
One of the most distinctive characteristics of leukocytes is their movement. Whereas erythrocytes spend their days circulating within the blood vessels, leukocytes routinely leave the bloodstream to perform their defensive functions in the body’s tissues. For leukocytes, the vascular network is simply a highway they travel and soon exit to reach their true destination. When they arrive, they are often given distinct names, such as macrophage or microglia, depending on their function. As shown in Figure 18.10, they leave the capillaries—the smallest blood vessels—or other small vessels through a process known as emigration(from the Latin for “removal”) or diapedesis (dia- = “through”; -pedan = “to leap”) in which they squeeze through adjacent cells in a blood vessel wall.
Once they have exited the capillaries, some leukocytes will take up fixed positions in lymphatic tissue, bone marrow, the spleen, the thymus, or other organs. Others will move about through the tissue spaces very much like amoebas, continuously extending their plasma membranes, sometimes wandering freely, and sometimes moving toward the direction in which they are drawn by chemical signals. This attracting of leukocytes occurs because of positive chemotaxis (literally “movement in response to chemicals”), a phenomenon in which injured or infected cells and nearby leukocytes emit the equivalent of a chemical “911” call, attracting more leukocytes to the site. In clinical medicine, the differential counts of the types and percentages of leukocytes present are often key indicators in making a diagnosis and selecting a treatment.
Figure 18.10 Emigration Leukocytes exit the blood vessel and then move through the connective tissue of the dermis toward the site of a wound. Some leukocytes, such as the eosinophil and neutrophil, are characterized as granular leukocytes. They release chemicals from their granules that destroy pathogens; they are also capable of phagocytosis. The monocyte, an agranular leukocyte, differentiates into a macrophage that then phagocytizes the pathogens.
Classification of Leukocytes
When scientists first began to observe stained blood slides, it quickly became evident that leukocytes could be divided into two groups, according to whether their cytoplasm contained highly visible granules:
- Granular leukocytes contain abundant granules within the cytoplasm. They include neutrophils, eosinophils, and basophils (you can view their lineage from myeloid stem cells in Figure 18.4).
- While granules are not totally lacking in agranular leukocytes, they are far fewer and less obvious. Agranular leukocytes include monocytes, which mature into macrophages that are phagocytic, and lymphocytes, which arise from the lymphoid stem cell line.
Granular Leukocytes
We will consider the granular leukocytes in order from most common to least common. All of these are produced in the red bone marrow and have a short lifespan of hours to days. They typically have a lobed nucleus and are classified according to which type of stain best highlights their granules (Figure 18.11).
Figure 18.11 Granular Leukocytes A neutrophil has small granules that stain light lilac and a nucleus with two to five lobes. An eosinophil’s granules are slightly larger and stain reddish-orange, and its nucleus has two to three lobes. A basophil has large granules that stain dark blue to purple and a two-lobed nucleus.
The most common of all the leukocytes, neutrophils will normally comprise 50–70 percent of total leukocyte count. They are 10–12 µm in diameter, significantly larger than erythrocytes. They are called neutrophils because their granules show up most clearly with stains that are chemically neutral (neither acidic nor basic). The granules are numerous but quite fine and normally appear light lilac. The nucleus has a distinct lobed appearance and may have two to five lobes, the number increasing with the age of the cell. Older neutrophils have increasing numbers of lobes and are often referred to as polymorphonuclear (a nucleus with many forms), or simply “polys.” Younger and immature neutrophils begin to develop lobes and are known as “bands.”
Neutrophils are rapid responders to the site of infection and are efficient phagocytes with a preference for bacteria. Their granules include lysozyme, an enzyme capable of lysing, or breaking down, bacterial cell walls; oxidants such as hydrogen peroxide; and defensins, proteins that bind to and puncture bacterial and fungal plasma membranes, so that the cell contents leak out. Abnormally high counts of neutrophils indicate infection and/or inflammation, particularly triggered by bacteria, but are also found in burn patients and others experiencing unusual stress. A burn injury increases the proliferation of neutrophils in order to fight off infection that can result from the destruction of the barrier of the skin. Low counts may be caused by drug toxicity and other disorders, and may increase an individual’s susceptibility to infection.
Eosinophils typically represent 2–4 percent of total leukocyte count. They are also 10–12 µm in diameter. The granules of eosinophils stain best with an acidic stain known as eosin. The nucleus of the eosinophil will typically have two to three lobes and, if stained properly, the granules will have a distinct red to orange color.
The granules of eosinophils include antihistamine molecules, which counteract the activities of histamines, inflammatory chemicals produced by basophils and mast cells. Some eosinophil granules contain molecules toxic to parasitic worms, which can enter the body through the integument, or when an individual consumes raw or undercooked fish or meat. Eosinophils are also capable of phagocytosis and are particularly effective when antibodies bind to the target and form an antigen-antibody complex. High counts of eosinophils are typical of patients experiencing allergies, parasitic worm infestations, and some autoimmune diseases. Low counts may be due to drug toxicity and stress.
Basophils are the least common leukocytes, typically comprising less than one percent of the total leukocyte count. They are slightly smaller than neutrophils and eosinophils at 8–10 µm in diameter. The granules of basophils stain best with basic (alkaline) stains. Basophils contain large granules that pick up a dark blue stain and are so common they may make it difficult to see the two-lobed nucleus.
In general, basophils intensify the inflammatory response. They share this trait with mast cells. In the past, mast cells were considered to be basophils that left the circulation. However, this appears not to be the case, as the two cell types develop from different lineages.
The granules of basophils release histamines, which contribute to inflammation, and heparin, which opposes blood clotting. High counts of basophils are associated with allergies, parasitic infections, and hypothyroidism. Low counts are associated with pregnancy, stress, and hyperthyroidism.
Agranular Leukocytes
Agranular leukocytes contain smaller, less-visible granules in their cytoplasm than do granular leukocytes. The nucleus is simple in shape, sometimes with an indentation but without distinct lobes. There are two major types of agranulocytes: lymphocytes and monocytes (see Figure 18.4).
Lymphocytes are the only formed element of blood that arises from lymphoid stem cells. Although they form initially in the bone marrow, much of their subsequent development and reproduction occurs in the lymphatic tissues. Lymphocytes are the second most common type of leukocyte, accounting for about 20–30 percent of all leukocytes, and are essential for the immune response. The size range of lymphocytes is quite extensive, with some authorities recognizing two size classes and others three. Typically, the large cells are 10–14 µm and have a smaller nucleus-to-cytoplasm ratio and more granules. The smaller cells are typically 6–9 µm with a larger volume of nucleus to cytoplasm, creating a “halo” effect. A few cells may fall outside these ranges, at 14–17 µm. This finding has led to the three size range classification.
The three major groups of lymphocytes include natural killer cells, B cells, and T cells. Natural killer (NK) cells are capable of recognizing cells that do not express “self” proteins on their plasma membrane or that contain foreign or abnormal markers. These “nonself” cells include cancer cells, cells infected with a virus, and other cells with atypical surface proteins. Thus, they provide generalized, nonspecific immunity. The larger lymphocytes are typically NK cells.
B cells and T cells, also called B lymphocytes and T lymphocytes, play prominent roles in defending the body against specific pathogens (disease-causing microorganisms) and are involved in specific immunity. One form of B cells (plasma cells) produces the antibodies or immunoglobulins that bind to specific foreign or abnormal components of plasma membranes. This is also referred to as humoral (body fluid) immunity. T cells provide cellular-level immunity by physically attacking foreign or diseased cells. A memory cell is a variety of both B and T cells that forms after exposure to a pathogen and mounts rapid responses upon subsequent exposures. Unlike other leukocytes, memory cells live for many years. B cells undergo a maturation process in the bone marrow, whereas T cells undergo maturation in the thymus. This site of the maturation process gives rise to the name B and T cells. The functions of lymphocytes are complex and will be covered in detail in the chapter covering the lymphatic system and immunity. Smaller lymphocytes are either B or T cells, although they cannot be differentiated in a normal blood smear.
Abnormally high lymphocyte counts are characteristic of viral infections as well as some types of cancer. Abnormally low lymphocyte counts are characteristic of prolonged (chronic) illness or immunosuppression, including that caused by HIV infection and drug therapies that often involve steroids.
Monocytes originate from myeloid stem cells. They normally represent 2–8 percent of the total leukocyte count. They are typically easily recognized by their large size of 12–20 µm and indented or horseshoe-shaped nuclei. Macrophages are monocytes that have left the circulation and phagocytize debris, foreign pathogens, worn-out erythrocytes, and many other dead, worn out, or damaged cells. Macrophages also release antimicrobial defensins and chemotactic chemicals that attract other leukocytes to the site of an infection. Some macrophages occupy fixed locations, whereas others wander through the tissue fluid.
Abnormally high counts of monocytes are associated with viral or fungal infections, tuberculosis, and some forms of leukemia and other chronic diseases. Abnormally low counts are typically caused by suppression of the bone marrow.
Lifecycle of Leukocytes
Most leukocytes have a relatively short lifespan, typically measured in hours or days. Production of all leukocytes begins in the bone marrow under the influence of CSFs and interleukins. Secondary production and maturation of lymphocytes occurs in specific regions of lymphatic tissue known as germinal centers. Lymphocytes are fully capable of mitosis and may produce clones of cells with identical properties. This capacity enables an individual to maintain immunity throughout life to many threats that have been encountered in the past.
Disorders of Leukocytes
Leukopenia is a condition in which too few leukocytes are produced. If this condition is pronounced, the individual may be unable to ward off disease. Excessive leukocyte proliferation is known as leukocytosis. Although leukocyte counts are high, the cells themselves are often nonfunctional, leaving the individual at increased risk for disease.
Leukemia is a cancer involving an abundance of leukocytes. It may involve only one specific type of leukocyte from either the myeloid line (myelocytic leukemia) or the lymphoid line (lymphocytic leukemia). In chronic leukemia, mature leukocytes accumulate and fail to die. In acute leukemia, there is an overproduction of young, immature leukocytes. In both conditions the cells do not function properly.
Lymphoma is a form of cancer in which masses of malignant T and/or B lymphocytes collect in lymph nodes, the spleen, the liver, and other tissues. As in leukemia, the malignant leukocytes do not function properly, and the patient is vulnerable to infection. Some forms of lymphoma tend to progress slowly and respond well to treatment. Others tend to progress quickly and require aggressive treatment, without which they are rapidly fatal.
Platelets
You may occasionally see platelets referred to as thrombocytes, but because this name suggests they are a type of cell, it is not accurate. A platelet is not a cell but rather a fragment of the cytoplasm of a cell called a megakaryocyte that is surrounded by a plasma membrane. Megakaryocytes are descended from myeloid stem cells (see Figure 18.4) and are large, typically 50–100 µm in diameter, and contain an enlarged, lobed nucleus. As noted earlier, thrombopoietin, a glycoprotein secreted by the kidneys and liver, stimulates the proliferation of megakaryoblasts, which mature into megakaryocytes. These remain within bone marrow tissue (Figure 18.12) and ultimately form platelet-precursor extensions that extend through the walls of bone marrow capillaries to release into the circulation thousands of cytoplasmic fragments, each enclosed by a bit of plasma membrane. These enclosed fragments are platelets. Each megakarocyte releases 2000–3000 platelets during its lifespan. Following platelet release, megakaryocyte remnants, which are little more than a cell nucleus, are consumed by macrophages.
Platelets are relatively small, 2–4 µm in diameter, but numerous, with typically 150,000–160,000 per µL of blood. After entering the circulation, approximately one-third migrate to the spleen for storage for later release in response to any rupture in a blood vessel. They then become activated to perform their primary function, which is to limit blood loss. Platelets remain only about 10 days, then are phagocytized by macrophages.
Platelets are critical to hemostasis, the stoppage of blood flow following damage to a vessel. They also secrete a variety of growth factors essential for growth and repair of tissue, particularly connective tissue. Infusions of concentrated platelets are now being used in some therapies to stimulate healing.
Disorders of Platelets
Thrombocytosis is a condition in which there are too many platelets. This may trigger formation of unwanted blood clots (thrombosis), a potentially fatal disorder. If there is an insufficient number of platelets, called thrombocytopenia, blood may not clot properly, and excessive bleeding may result.
Figure 18.12 Platelets Platelets are derived from cells called megakaryocytes.
INTERACTIVE LINK
Figure 18.13 Leukocytes (Micrographs provided by the Regents of University of Michigan Medical School © 2012)
View University of Michigan Webscopes at http://virtualslides.med.umich.edu/Histology/Cardiovascular%20System/081-2_HISTO_40X.svs/view.apml?cwidth=860&cheight=733&chost=virtualslides.med.umich.edu&listview=1&title=&csis=1 and explore the blood slides in greater detail. The Webscope feature allows you to move the slides as you would with a mechanical stage. You can increase and decrease the magnification. There is a chance to review each of the leukocytes individually after you have attempted to identify them from the first two blood smears. In addition, there are a few multiple choice questions.
Are you able to recognize and identify the various formed elements? You will need to do this is a systematic manner, scanning along the image. The standard method is to use a grid, but this is not possible with this resource. Try constructing a simple table with each leukocyte type and then making a mark for each cell type you identify. Attempt to classify at least 50 and perhaps as many as 100 different cells. Based on the percentage of cells that you count, do the numbers represent a normal blood smear or does something appear to be abnormal?
Hemostasis
- Describe the three mechanisms involved in hemostasis
- Explain how the extrinsic and intrinsic coagulation pathways lead to the common pathway, and the coagulation factors involved in each
- Discuss disorders affecting hemostasis
Platelets are key players in hemostasis, the process by which the body seals a ruptured blood vessel and prevents further loss of blood. Although rupture of larger vessels usually requires medical intervention, hemostasis is quite effective in dealing with small, simple wounds. There are three steps to the process: vascular spasm, the formation of a platelet plug, and coagulation (blood clotting). Failure of any of these steps will result in hemorrhage—excessive bleeding.
Vascular Spasm
When a vessel is severed or punctured, or when the wall of a vessel is damaged, vascular spasm occurs. In vascular spasm, the smooth muscle in the walls of the vessel contracts dramatically. This smooth muscle has both circular layers; larger vessels also have longitudinal layers. The circular layers tend to constrict the flow of blood, whereas the longitudinal layers, when present, draw the vessel back into the surrounding tissue, often making it more difficult for a surgeon to locate, clamp, and tie off a severed vessel. The vascular spasm response is believed to be triggered by several chemicals called endothelins that are released by vessel-lining cells and by pain receptors in response to vessel injury. This phenomenon typically lasts for up to 30 minutes, although it can last for hours.
Formation of the Platelet Plug
In the second step, platelets, which normally float free in the plasma, encounter the area of vessel rupture with the exposed underlying connective tissue and collagenous fibers. The platelets begin to clump together, become spiked and sticky, and bind to the exposed collagen and endothelial lining. This process is assisted by a glycoprotein in the blood plasma called von Willebrand factor, which helps stabilize the growing platelet plug. As platelets collect, they simultaneously release chemicals from their granules into the plasma that further contribute to hemostasis. Among the substances released by the platelets are:
- adenosine diphosphate (ADP), which helps additional platelets to adhere to the injury site, reinforcing and expanding the platelet plug
- serotonin, which maintains vasoconstriction
- prostaglandins and phospholipids, which also maintain vasoconstriction and help to activate further clotting chemicals, as discussed next
A platelet plug can temporarily seal a small opening in a blood vessel. Plug formation, in essence, buys the body time while more sophisticated and durable repairs are being made. In a similar manner, even modern naval warships still carry an assortment of wooden plugs to temporarily repair small breaches in their hulls until permanent repairs can be made.
Coagulation
Those more sophisticated and more durable repairs are collectively called coagulation, the formation of a blood clot. The process is sometimes characterized as a cascade, because one event prompts the next as in a multi-level waterfall. The result is the production of a gelatinous but robust clot made up of a mesh of fibrin—an insoluble filamentous protein derived from fibrinogen, the plasma protein introduced earlier—in which platelets and blood cells are trapped. Figure 18.14 summarizes the three steps of hemostasis.
Figure 18.14 Hemostasis (a) An injury to a blood vessel initiates the process of hemostasis. Blood clotting involves three steps. First, vascular spasm constricts the flow of blood. Next, a platelet plug forms to temporarily seal small openings in the vessel. Coagulation then enables the repair of the vessel wall once the leakage of blood has stopped. (b) The synthesis of fibrin in blood clots involves either an intrinsic pathway or an extrinsic pathway, both of which lead to a common pathway. (credit a: Kevin MacKenzie)
Clotting Factors Involved in Coagulation
In the coagulation cascade, chemicals called clotting factors (or coagulation factors) prompt reactions that activate still more coagulation factors. The process is complex, but is initiated along two basic pathways:
- The extrinsic pathway, which normally is triggered by trauma.
- The intrinsic pathway, which begins in the bloodstream and is triggered by internal damage to the wall of the vessel.
Both of these merge into a third pathway, referred to as the common pathway (see Figure 18.14b). All three pathways are dependent upon the 12 known clotting factors, including Ca2+ and vitamin K (Table 18.1). Clotting factors are secreted primarily by the liver and the platelets. The liver requires the fat-soluble vitamin K to produce many of them. Vitamin K (along with biotin and folate) is somewhat unusual among vitamins in that it is not only consumed in the diet but is also synthesized by bacteria residing in the large intestine. The calcium ion, considered factor IV, is derived from the diet and from the breakdown of bone. Some recent evidence indicates that activation of various clotting factors occurs on specific receptor sites on the surfaces of platelets.
The 12 clotting factors are numbered I through XIII according to the order of their discovery. Factor VI was once believed to be a distinct clotting factor, but is now thought to be identical to factor V. Rather than renumber the other factors, factor VI was allowed to remain as a placeholder and also a reminder that knowledge changes over time.
Clotting Factors
| Factor number | Name | Type of molecule | Source | Pathway(s) |
|---|---|---|---|---|
| I | Fibrinogen | Plasma protein | Liver | Common; converted into fibrin |
| II | Prothrombin | Plasma protein | Liver* | Common; converted into thrombin |
| III | Tissue thromboplastin or tissue factor | Lipoprotein mixture | Damaged cells and platelets | Extrinsic |
| IV | Calcium ions | Inorganic ions in plasma | Diet, platelets, bone matrix | Entire process |
| V | Proaccelerin | Plasma protein | Liver, platelets | Extrinsic and intrinsic |
| VI | Not used | Not used | Not used | Not used |
| VII | Proconvertin | Plasma protein | Liver * | Extrinsic |
| VIII | Antihemolytic factor A | Plasma protein factor | Platelets and endothelial cells | Intrinsic; deficiency results in hemophilia A |
| IX | Antihemolytic factor B (plasma thromboplastin component) | Plasma protein | Liver* | Intrinsic; deficiency results in hemophilia B |
| X | Stuart–Prower factor (thrombokinase) | Protein | Liver* | Extrinsic and intrinsic |
| XI | Antihemolytic factor C (plasma thromboplastin antecedent) | Plasma protein | Liver | Intrinsic; deficiency results in hemophilia C |
| XII | Hageman factor | Plasma protein | Liver | Intrinsic; initiates clotting in vitro also activates plasmin |
| XIII | Fibrin-stabilizing factor | Plasma protein | Liver, platelets | Stabilizes fibrin; slows fibrinolysis |
Table 18.1 *Vitamin K required.
Extrinsic Pathway
The quicker responding and more direct extrinsic pathway (also known as the tissue factor pathway) begins when damage occurs to the surrounding tissues, such as in a traumatic injury. Upon contact with blood plasma, the damaged extravascular cells, which are extrinsic to the bloodstream, release factor III (thromboplastin). Sequentially, Ca2+ then factor VII (proconvertin), which is activated by factor III, are added, forming an enzyme complex. This enzyme complex leads to activation of factor X (Stuart–Prower factor), which activates the common pathway discussed below. The events in the extrinsic pathway are completed in a matter of seconds.
Intrinsic Pathway
The intrinsic pathway (also known as the contact activation pathway) is longer and more complex. In this case, the factors involved are intrinsic to (present within) the bloodstream. The pathway can be prompted by damage to the tissues, resulting from internal factors such as arterial disease; however, it is most often initiated when factor XII (Hageman factor) comes into contact with foreign materials, such as when a blood sample is put into a glass test tube. Within the body, factor XII is typically activated when it encounters negatively charged molecules, such as inorganic polymers and phosphate produced earlier in the series of intrinsic pathway reactions. Factor XII sets off a series of reactions that in turn activates factor XI (antihemolytic factor C or plasma thromboplastin antecedent) then factor IX (antihemolytic factor B or plasma thromboplasmin). In the meantime, chemicals released by the platelets increase the rate of these activation reactions. Finally, factor VIII (antihemolytic factor A) from the platelets and endothelial cells combines with factor IX (antihemolytic factor B or plasma thromboplasmin) to form an enzyme complex that activates factor X (Stuart–Prower factor or thrombokinase), leading to the common pathway. The events in the intrinsic pathway are completed in a few minutes.
Common Pathway
Both the intrinsic and extrinsic pathways lead to the common pathway, in which fibrin is produced to seal off the vessel. Once factor X has been activated by either the intrinsic or extrinsic pathway, the enzyme prothrombinase converts factor II, the inactive enzyme prothrombin, into the active enzyme thrombin. (Note that if the enzyme thrombin were not normally in an inactive form, clots would form spontaneously, a condition not consistent with life.) Then, thrombin converts factor I, the soluble fibrinogen, into the insoluble fibrin protein strands. Factor XIII then stabilizes the fibrin clot.
Fibrinolysis
The stabilized clot is acted upon by contractile proteins within the platelets. As these proteins contract, they pull on the fibrin threads, bringing the edges of the clot more tightly together, somewhat as we do when tightening loose shoelaces (see Figure 18.14a). This process also wrings out of the clot a small amount of fluid called serum, which is blood plasma without its clotting factors.
To restore normal blood flow as the vessel heals, the clot must eventually be removed. Fibrinolysis is the gradual degradation of the clot. Again, there is a fairly complicated series of reactions that involves factor XII and protein-catabolizing enzymes. During this process, the inactive protein plasminogen is converted into the active plasmin, which gradually breaks down the fibrin of the clot. Additionally, bradykinin, a vasodilator, is released, reversing the effects of the serotonin and prostaglandins from the platelets. This allows the smooth muscle in the walls of the vessels to relax and helps to restore the circulation.
Plasma Anticoagulants
An anticoagulant is any substance that opposes coagulation. Several circulating plasma anticoagulants play a role in limiting the coagulation process to the region of injury and restoring a normal, clot-free condition of blood. For instance, a cluster of proteins collectively referred to as the protein C system inactivates clotting factors involved in the intrinsic pathway. TFPI (tissue factor pathway inhibitor) inhibits the conversion of the inactive factor VII to the active form in the extrinsic pathway. Antithrombin inactivates factor X and opposes the conversion of prothrombin (factor II) to thrombin in the common pathway. And as noted earlier, basophils release heparin, a short-acting anticoagulant that also opposes prothrombin. Heparin is also found on the surfaces of cells lining the blood vessels. A pharmaceutical form of heparin is often administered therapeutically, for example, in surgical patients at risk for blood clots.
INTERACTIVE LINK
View these animations to explore the intrinsic, extrinsic, and common pathways that are involved the process of coagulation. The coagulation cascade restores hemostasis by activating coagulation factors in the presence of an injury. How does the endothelium of the blood vessel walls prevent the blood from coagulating as it flows through the blood vessels?
Disorders of Clotting
Either an insufficient or an excessive production of platelets can lead to severe disease or death. As discussed earlier, an insufficient number of platelets, called thrombocytopenia, typically results in the inability of blood to form clots. This can lead to excessive bleeding, even from minor wounds.
Another reason for failure of the blood to clot is the inadequate production of functional amounts of one or more clotting factors. This is the case in the genetic disorder hemophilia, which is actually a group of related disorders, the most common of which is hemophilia A, accounting for approximately 80 percent of cases. This disorder results in the inability to synthesize sufficient quantities of factor VIII. Hemophilia B is the second most common form, accounting for approximately 20 percent of cases. In this case, there is a deficiency of factor IX. Both of these defects are linked to the X chromosome and are typically passed from a healthy (carrier) mother to her male offspring, since males are XY. Females would need to inherit a defective gene from each parent to manifest the disease, since they are XX. Patients with hemophilia bleed from even minor internal and external wounds, and leak blood into joint spaces after exercise and into urine and stool. Hemophilia C is a rare condition that is triggered by an autosomal (not sex) chromosome that renders factor XI nonfunctional. It is not a true recessive condition, since even individuals with a single copy of the mutant gene show a tendency to bleed. Regular infusions of clotting factors isolated from healthy donors can help prevent bleeding in hemophiliac patients. At some point, genetic therapy will become a viable option.
In contrast to the disorders characterized by coagulation failure is thrombocytosis, also mentioned earlier, a condition characterized by excessive numbers of platelets that increases the risk for excessive clot formation, a condition known as thrombosis. A thrombus (plural = thrombi) is an aggregation of platelets, erythrocytes, and even WBCs typically trapped within a mass of fibrin strands. While the formation of a clot is normal following the hemostatic mechanism just described, thrombi can form within an intact or only slightly damaged blood vessel. In a large vessel, a thrombus will adhere to the vessel wall and decrease the flow of blood, and is referred to as a mural thrombus. In a small vessel, it may actually totally block the flow of blood and is termed an occlusive thrombus. Thrombi are most commonly caused by vessel damage to the endothelial lining, which activates the clotting mechanism. These may include venous stasis, when blood in the veins, particularly in the legs, remains stationary for long periods. This is one of the dangers of long airplane flights in crowded conditions and may lead to deep vein thrombosis or atherosclerosis, an accumulation of debris in arteries. Thrombophilia, also called hypercoagulation, is a condition in which there is a tendency to form thrombosis. This may be familial (genetic) or acquired. Acquired forms include the autoimmune disease lupus, immune reactions to heparin, polycythemia vera, thrombocytosis, sickle cell disease, pregnancy, and even obesity. A thrombus can seriously impede blood flow to or from a region and will cause a local increase in blood pressure. If flow is to be maintained, the heart will need to generate a greater pressure to overcome the resistance.
When a portion of a thrombus breaks free from the vessel wall and enters the circulation, it is referred to as an embolus. An embolus that is carried through the bloodstream can be large enough to block a vessel critical to a major organ. When it becomes trapped, an embolus is called an embolism. In the heart, brain, or lungs, an embolism may accordingly cause a heart attack, a stroke, or a pulmonary embolism. These are medical emergencies.
Among the many known biochemical activities of aspirin is its role as an anticoagulant. Aspirin (acetylsalicylic acid) is very effective at inhibiting the aggregation of platelets. It is routinely administered during a heart attack or stroke to reduce the adverse effects. Physicians sometimes recommend that patients at risk for cardiovascular disease take a low dose of aspirin on a daily basis as a preventive measure. However, aspirin can also lead to serious side effects, including increasing the risk of ulcers. A patient is well advised to consult a physician before beginning any aspirin regimen.
A class of drugs collectively known as thrombolytic agents can help speed up the degradation of an abnormal clot. If a thrombolytic agent is administered to a patient within 3 hours following a thrombotic stroke, the patient’s prognosis improves significantly. However, some strokes are not caused by thrombi, but by hemorrhage. Thus, the cause must be determined before treatment begins. Tissue plasminogen activator is an enzyme that catalyzes the conversion of plasminogen to plasmin, the primary enzyme that breaks down clots. It is released naturally by endothelial cells but is also used in clinical medicine. New research is progressing using compounds isolated from the venom of some species of snakes, particularly vipers and cobras, which may eventually have therapeutic value as thrombolytic agents.
Blood Typing
- Describe the two basic physiological consequences of transfusion of incompatible blood
- Compare and contrast ABO and Rh blood groups
- Identify which blood groups may be safely transfused into patients with different ABO types
- Discuss the pathophysiology of hemolytic disease of the newborn
Blood transfusions in humans were risky procedures until the discovery of the major human blood groups by Karl Landsteiner, an Austrian biologist and physician, in 1900. Until that point, physicians did not understand that death sometimes followed blood transfusions, when the type of donor blood infused into the patient was incompatible with the patient’s own blood. Blood groups are determined by the presence or absence of specific marker molecules on the plasma membranes of erythrocytes. With their discovery, it became possible for the first time to match patient-donor blood types and prevent transfusion reactions and deaths.
Antigens, Antibodies, and Transfusion Reactions
Antigens are substances that the body does not recognize as belonging to the “self” and that therefore trigger a defensive response from the leukocytes of the immune system. (Seek more content for additional information on immunity.) Here, we will focus on the role of immunity in blood transfusion reactions. With RBCs in particular, you may see the antigens referred to as isoantigens or agglutinogens (surface antigens) and the antibodies referred to as isoantibodies or agglutinins. In this chapter, we will use the more common terms antigens and antibodies.
Antigens are generally large proteins, but may include other classes of organic molecules, including carbohydrates, lipids, and nucleic acids. Following an infusion of incompatible blood, erythrocytes with foreign antigens appear in the bloodstream and trigger an immune response. Proteins called antibodies (immunoglobulins), which are produced by certain B lymphocytes called plasma cells, attach to the antigens on the plasma membranes of the infused erythrocytes and cause them to adhere to one another.
- Because the arms of the Y-shaped antibodies attach randomly to more than one nonself erythrocyte surface, they form clumps of erythrocytes. This process is called agglutination.
- The clumps of erythrocytes block small blood vessels throughout the body, depriving tissues of oxygen and nutrients.
- As the erythrocyte clumps are degraded, in a process called hemolysis, their hemoglobin is released into the bloodstream. This hemoglobin travels to the kidneys, which are responsible for filtration of the blood. However, the load of hemoglobin released can easily overwhelm the kidney’s capacity to clear it, and the patient can quickly develop kidney failure.
More than 50 antigens have been identified on erythrocyte membranes, but the most significant in terms of their potential harm to patients are classified in two groups: the ABO blood group and the Rh blood group.
The ABO Blood Group
Although the ABO blood group name consists of three letters, ABO blood typing designates the presence or absence of just two antigens, A and B. Both are glycoproteins. People whose erythrocytes have A antigens on their erythrocyte membrane surfaces are designated blood type A, and those whose erythrocytes have B antigens are blood type B. People can also have both A and B antigens on their erythrocytes, in which case they are blood type AB. People with neither A nor B antigens are designated blood type O. ABO blood types are genetically determined.
Normally the body must be exposed to a foreign antigen before an antibody can be produced. This is not the case for the ABO blood group. Individuals with type A blood—without any prior exposure to incompatible blood—have preformed antibodies to the B antigen circulating in their blood plasma. These antibodies, referred to as anti-B antibodies, will cause agglutination and hemolysis if they ever encounter erythrocytes with B antigens. Similarly, an individual with type B blood has pre-formed anti-A antibodies. Individuals with type AB blood, which has both antigens, do not have preformed antibodies to either of these. People with type O blood lack antigens A and B on their erythrocytes, but both anti-A and anti-B antibodies circulate in their blood plasma.
Rh Blood Groups
The Rh blood group is classified according to the presence or absence of a second erythrocyte antigen identified as Rh. (It was first discovered in a type of primate known as a rhesus macaque, which is often used in research, because its blood is similar to that of humans.) Although dozens of Rh antigens have been identified, only one, designated D, is clinically important. Those who have the Rh D antigen present on their erythrocytes—about 85 percent of Americans—are described as Rh positive (Rh+) and those who lack it are Rh negative (Rh−). Note that the Rh group is distinct from the ABO group, so any individual, no matter their ABO blood type, may have or lack this Rh antigen. When identifying a patient’s blood type, the Rh group is designated by adding the word positive or negative to the ABO type. For example, A positive (A+) means ABO group A blood with the Rh antigen present, and AB negative (AB−) means ABO group AB blood without the Rh antigen.
Table 18.2 summarizes the distribution of the ABO and Rh blood types within the United States.
Summary of ABO and Rh Blood Types within the United States
| Blood Type | African-Americans | Asian-Americans | Caucasian-Americans | Latino/Latina-Americans |
|---|---|---|---|---|
| A+ | 24 | 27 | 33 | 29 |
| A− | 2 | 0.5 | 7 | 2 |
| B+ | 18 | 25 | 9 | 9 |
| B− | 1 | 0.4 | 2 | 1 |
| AB+ | 4 | 7 | 3 | 2 |
| AB− | 0.3 | 0.1 | 1 | 0.2 |
| O+ | 47 | 39 | 37 | 53 |
| O− | 4 | 1 | 8 | 4 |
Table 18.2
n contrast to the ABO group antibodies, which are preformed, antibodies to the Rh antigen are produced only in Rh− individuals after exposure to the antigen. This process, called sensitization, occurs following a transfusion with Rh-incompatible blood or, more commonly, with the birth of an Rh+ baby to an Rh− mother. Problems are rare in a first pregnancy, since the baby’s Rh+cells rarely cross the placenta (the organ of gas and nutrient exchange between the baby and the mother). However, during or immediately after birth, the Rh− mother can be exposed to the baby’s Rh+ cells (Figure 18.15). Research has shown that this occurs in about 13−14 percent of such pregnancies. After exposure, the mother’s immune system begins to generate anti-Rh antibodies. If the mother should then conceive another Rh+ baby, the Rh antibodies she has produced can cross the placenta into the fetal bloodstream and destroy the fetal RBCs. This condition, known as hemolytic disease of the newborn (HDN) or erythroblastosis fetalis, may cause anemia in mild cases, but the agglutination and hemolysis can be so severe that without treatment the fetus may die in the womb or shortly after birth.
Figure 18.15 Erythroblastosis Fetalis The first exposure of an Rh− mother to Rh+ erythrocytes during pregnancy induces sensitization. Anti-Rh antibodies begin to circulate in the mother’s bloodstream. A second exposure occurs with a subsequent pregnancy with an Rh+ fetus in the uterus. Maternal anti-Rh antibodies may cross the placenta and enter the fetal bloodstream, causing agglutination and hemolysis of fetal erythrocytes.
A drug known as RhoGAM, short for Rh immune globulin, can temporarily prevent the development of Rh antibodies in the Rh−mother, thereby averting this potentially serious disease for the fetus. RhoGAM antibodies destroy any fetal Rh+ erythrocytes that may cross the placental barrier. RhoGAM is normally administered to Rh− mothers during weeks 26−28 of pregnancy and within 72 hours following birth. It has proven remarkably effective in decreasing the incidence of HDN. Earlier we noted that the incidence of HDN in an Rh+ subsequent pregnancy to an Rh− mother is about 13–14 percent without preventive treatment. Since the introduction of RhoGAM in 1968, the incidence has dropped to about 0.1 percent in the United States.
Determining ABO Blood Types
Clinicians are able to determine a patient’s blood type quickly and easily using commercially prepared antibodies. An unknown blood sample is allocated into separate wells. Into one well a small amount of anti-A antibody is added, and to another a small amount of anti-B antibody. If the antigen is present, the antibodies will cause visible agglutination of the cells (Figure 18.16). The blood should also be tested for Rh antibodies.
Figure 18.16 Cross Matching Blood Types This sample of a commercially produced “bedside” card enables quick typing of both a recipient’s and donor’s blood before transfusion. The card contains three reaction sites or wells. One is coated with an anti-A antibody, one with an anti-B antibody, and one with an anti-D antibody (tests for the presence of Rh factor D). Mixing a drop of blood and saline into each well enables the blood to interact with a preparation of type-specific antibodies, also called anti-seras. Agglutination of RBCs in a given site indicates a positive identification of the blood antigens, in this case A and Rh antigens for blood type A+. For the purpose of transfusion, the donor’s and recipient’s blood types must match.
ABO Transfusion Protocols
To avoid transfusion reactions, it is best to transfuse only matching blood types; that is, a type B+ recipient should ideally receive blood only from a type B+ donor and so on. That said, in emergency situations, when acute hemorrhage threatens the patient’s life, there may not be time for cross matching to identify blood type. In these cases, blood from a universal donor—an individual with type O− blood—may be transfused. Recall that type O erythrocytes do not display A or B antigens. Thus, anti-A or anti-B antibodies that might be circulating in the patient’s blood plasma will not encounter any erythrocyte surface antigens on the donated blood and therefore will not be provoked into a response. One problem with this designation of universal donor is if the O− individual had prior exposure to Rh antigen, Rh antibodies may be present in the donated blood. Also, introducing type O blood into an individual with type A, B, or AB blood will nevertheless introduce antibodies against both A and B antigens, as these are always circulating in the type O blood plasma. This may cause problems for the recipient, but because the volume of blood transfused is much lower than the volume of the patient’s own blood, the adverse effects of the relatively few infused plasma antibodies are typically limited. Rh factor also plays a role. If Rh− individuals receiving blood have had prior exposure to Rh antigen, antibodies for this antigen may be present in the blood and trigger agglutination to some degree. Although it is always preferable to cross match a patient’s blood before transfusing, in a true life-threatening emergency situation, this is not always possible, and these procedures may be implemented.
A patient with blood type AB+ is known as the universal recipient. This patient can theoretically receive any type of blood, because the patient’s own blood—having both A and B antigens on the erythrocyte surface—does not produce anti-A or anti-B antibodies. In addition, an Rh+ patient can receive both Rh+ and Rh− blood. However, keep in mind that the donor’s blood will contain circulating antibodies, again with possible negative implications. Figure 18.17 summarizes the blood types and compatibilities.
At the scene of multiple-vehicle accidents, military engagements, and natural or human-caused disasters, many victims may suffer simultaneously from acute hemorrhage, yet type O blood may not be immediately available. In these circumstances, medics may at least try to replace some of the volume of blood that has been lost. This is done by intravenous administration of a saline solution that provides fluids and electrolytes in proportions equivalent to those of normal blood plasma. Research is ongoing to develop a safe and effective artificial blood that would carry out the oxygen-carrying function of blood without the RBCs, enabling transfusions in the field without concern for incompatibility. These blood substitutes normally contain hemoglobin- as well as perfluorocarbon-based oxygen carriers.
Figure 18.17 ABO Blood Group This chart summarizes the characteristics of the blood types in the ABO blood group. See the text for more on the concept of a universal donor or recipient.
Key Terms
- ABO blood group
- blood-type classification based on the presence or absence of A and B glycoproteins on the erythrocyte membrane surface
- agglutination
- clustering of cells into masses linked by antibodies
- agranular leukocytes
- leukocytes with few granules in their cytoplasm; specifically, monocytes, lymphocytes, and NK cells
- albumin
- most abundant plasma protein, accounting for most of the osmotic pressure of plasma
- anemia
- deficiency of red blood cells or hemoglobin
- antibodies
- (also, immunoglobulins or gamma globulins) antigen-specific proteins produced by specialized B lymphocytes that protect the body by binding to foreign objects such as bacteria and viruses
- anticoagulant
- substance such as heparin that opposes coagulation
- antithrombin
- anticoagulant that inactivates factor X and opposes the conversion of prothrombin (factor II) into thrombin in the common pathway
- B lymphocytes
- (also, B cells) lymphocytes that defend the body against specific pathogens and thereby provide specific immunity
- basophils
- granulocytes that stain with a basic (alkaline) stain and store histamine and heparin
- bilirubin
- yellowish bile pigment produced when iron is removed from heme and is further broken down into waste products
- biliverdin
- green bile pigment produced when the non-iron portion of heme is degraded into a waste product; converted to bilirubin in the liver
- blood
- liquid connective tissue composed of formed elements—erythrocytes, leukocytes, and platelets—and a fluid extracellular matrix called plasma; component of the cardiovascular system
- bone marrow biopsy
- diagnostic test of a sample of red bone marrow
- bone marrow transplant
- treatment in which a donor’s healthy bone marrow with its stem cells replaces diseased or damaged bone marrow of a patient
- buffy coat
- thin, pale layer of leukocytes and platelets that separates the erythrocytes from the plasma in a sample of centrifuged blood
- carbaminohemoglobin
- compound of carbon dioxide and hemoglobin, and one of the ways in which carbon dioxide is carried in the blood
- clotting factors
- group of 12 identified substances active in coagulation
- coagulation
- formation of a blood clot; part of the process of hemostasis
- colony-stimulating factors (CSFs)
- glycoproteins that trigger the proliferation and differentiation of myeloblasts into granular leukocytes (basophils, neutrophils, and eosinophils)
- common pathway
- final coagulation pathway activated either by the intrinsic or the extrinsic pathway, and ending in the formation of a blood clot
- cross matching
- blood test for identification of blood type using antibodies and small samples of blood
- cytokines
- class of proteins that act as autocrine or paracrine signaling molecules; in the cardiovascular system, they stimulate the proliferation of progenitor cells and help to stimulate both nonspecific and specific resistance to disease
- defensins
- antimicrobial proteins released from neutrophils and macrophages that create openings in the plasma membranes to kill cells
- deoxyhemoglobin
- molecule of hemoglobin without an oxygen molecule bound to it
- diapedesis
- (also, emigration) process by which leukocytes squeeze through adjacent cells in a blood vessel wall to enter tissues
- embolus
- thrombus that has broken free from the blood vessel wall and entered the circulation
- emigration
- (also, diapedesis) process by which leukocytes squeeze through adjacent cells in a blood vessel wall to enter tissues
- eosinophils
- granulocytes that stain with eosin; they release antihistamines and are especially active against parasitic worms
- erythrocyte
- (also, red blood cell) mature myeloid blood cell that is composed mostly of hemoglobin and functions primarily in the transportation of oxygen and carbon dioxide
- erythropoietin (EPO)
- glycoprotein that triggers the bone marrow to produce RBCs; secreted by the kidney in response to low oxygen levels
- extrinsic pathway
- initial coagulation pathway that begins with tissue damage and results in the activation of the common pathway
- ferritin
- protein-containing storage form of iron found in the bone marrow, liver, and spleen
- fibrin
- insoluble, filamentous protein that forms the structure of a blood clot
- fibrinogen
- plasma protein produced in the liver and involved in blood clotting
- fibrinolysis
- gradual degradation of a blood clot
- formed elements
- cellular components of blood; that is, erythrocytes, leukocytes, and platelets
- globin
- heme-containing globular protein that is a constituent of hemoglobin
- globulins
- heterogeneous group of plasma proteins that includes transport proteins, clotting factors, immune proteins, and others
- granular leukocytes
- leukocytes with abundant granules in their cytoplasm; specifically, neutrophils, eosinophils, and basophils
- hematocrit
- (also, packed cell volume) volume percentage of erythrocytes in a sample of centrifuged blood
- heme
- red, iron-containing pigment to which oxygen binds in hemoglobin
- hemocytoblast
- hemopoietic stem cell that gives rise to the formed elements of blood
- hemoglobin
- oxygen-carrying compound in erythrocytes
- hemolysis
- destruction (lysis) of erythrocytes and the release of their hemoglobin into circulation
- hemolytic disease of the newborn (HDN)
- (also, erythroblastosis fetalis) disorder causing agglutination and hemolysis in an Rh+ fetus or newborn of an Rh− mother
- hemophilia
- genetic disorder characterized by inadequate synthesis of clotting factors
- hemopoiesis
- production of the formed elements of blood
- hemopoietic growth factors
- chemical signals including erythropoietin, thrombopoietin, colony-stimulating factors, and interleukins that regulate the differentiation and proliferation of particular blood progenitor cells
- hemopoietic stem cell
- type of pluripotent stem cell that gives rise to the formed elements of blood (hemocytoblast)
- hemorrhage
- excessive bleeding
- hemosiderin
- protein-containing storage form of iron found in the bone marrow, liver, and spleen
- hemostasis
- physiological process by which bleeding ceases
- heparin
- short-acting anticoagulant stored in mast cells and released when tissues are injured, opposes prothrombin
- hypoxemia
- below-normal level of oxygen saturation of blood (typically <95 percent)
- immunoglobulins
- (also, antibodies or gamma globulins) antigen-specific proteins produced by specialized B lymphocytes that protect the body by binding to foreign objects such as bacteria and viruses
- interleukins
- signaling molecules that may function in hemopoiesis, inflammation, and specific immune responses
- intrinsic pathway
- initial coagulation pathway that begins with vascular damage or contact with foreign substances, and results in the activation of the common pathway
- leukemia
- cancer involving leukocytes
- leukocyte
- (also, white blood cell) colorless, nucleated blood cell, the chief function of which is to protect the body from disease
- leukocytosis
- excessive leukocyte proliferation
- leukopenia
- below-normal production of leukocytes
- lymphocytes
- agranular leukocytes of the lymphoid stem cell line, many of which function in specific immunity
- lymphoid stem cells
- type of hemopoietic stem cells that gives rise to lymphocytes, including various T cells, B cells, and NK cells, all of which function in immunity
- lymphoma
- form of cancer in which masses of malignant T and/or B lymphocytes collect in lymph nodes, the spleen, the liver, and other tissues
- lysozyme
- digestive enzyme with bactericidal properties
- macrophage
- phagocytic cell of the myeloid lineage; a matured monocyte
- megakaryocyte
- bone marrow cell that produces platelets
- memory cell
- type of B or T lymphocyte that forms after exposure to a pathogen
- monocytes
- agranular leukocytes of the myeloid stem cell line that circulate in the bloodstream; tissue monocytes are macrophages
- myeloid stem cells
- type of hemopoietic stem cell that gives rise to some formed elements, including erythrocytes, megakaryocytes that produce platelets, and a myeloblast lineage that gives rise to monocytes and three forms of granular leukocytes (neutrophils, eosinophils, and basophils)
- natural killer (NK) cells
- cytotoxic lymphocytes capable of recognizing cells that do not express “self” proteins on their plasma membrane or that contain foreign or abnormal markers; provide generalized, nonspecific immunity
- neutrophils
- granulocytes that stain with a neutral dye and are the most numerous of the leukocytes; especially active against bacteria
- oxyhemoglobin
- molecule of hemoglobin to which oxygen is bound
- packed cell volume (PCV)
- (also, hematocrit) volume percentage of erythrocytes present in a sample of centrifuged blood
- plasma
- in blood, the liquid extracellular matrix composed mostly of water that circulates the formed elements and dissolved materials throughout the cardiovascular system
- plasmin
- blood protein active in fibrinolysis
- platelet plug
- accumulation and adhesion of platelets at the site of blood vessel injury
- platelets
- (also, thrombocytes) one of the formed elements of blood that consists of cell fragments broken off from megakaryocytes
- pluripotent stem cell
- stem cell that derives from totipotent stem cells and is capable of differentiating into many, but not all, cell types
- polycythemia
- elevated level of hemoglobin, whether adaptive or pathological
- polymorphonuclear
- having a lobed nucleus, as seen in some leukocytes
- positive chemotaxis
- process in which a cell is attracted to move in the direction of chemical stimuli
- red blood cells (RBCs)
- (also, erythrocytes) one of the formed elements of blood that transports oxygen
- reticulocyte
- immature erythrocyte that may still contain fragments of organelles
- Rh blood group
- blood-type classification based on the presence or absence of the antigen Rh on the erythrocyte membrane surface
- serum
- blood plasma that does not contain clotting factors
- sickle cell disease
- (also, sickle cell anemia) inherited blood disorder in which hemoglobin molecules are malformed, leading to the breakdown of RBCs that take on a characteristic sickle shape
- T lymphocytes
- (also, T cells) lymphocytes that provide cellular-level immunity by physically attacking foreign or diseased cells
- thalassemia
- inherited blood disorder in which maturation of RBCs does not proceed normally, leading to abnormal formation of hemoglobin and the destruction of RBCs
- thrombin
- enzyme essential for the final steps in formation of a fibrin clot
- thrombocytes
- platelets, one of the formed elements of blood that consists of cell fragments broken off from megakaryocytes
- thrombocytopenia
- condition in which there are too few platelets, resulting in abnormal bleeding (hemophilia)
- thrombocytosis
- condition in which there are too many platelets, resulting in abnormal clotting (thrombosis)
- thrombopoietin
- hormone secreted by the liver and kidneys that prompts the development of megakaryocytes into thrombocytes (platelets)
- thrombosis
- excessive clot formation
- thrombus
- aggregation of fibrin, platelets, and erythrocytes in an intact artery or vein
- tissue factor
- protein thromboplastin, which initiates the extrinsic pathway when released in response to tissue damage
- totipotent stem cell
- embryonic stem cell that is capable of differentiating into any and all cells of the body; enabling the full development of an organism
- transferrin
- plasma protein that binds reversibly to iron and distributes it throughout the body
- universal donor
- individual with type O− blood
- universal recipient
- individual with type AB+ blood
- vascular spasm
- initial step in hemostasis, in which the smooth muscle in the walls of the ruptured or damaged blood vessel contracts
- white blood cells (WBCs)
- (also, leukocytes) one of the formed elements of blood that provides defense against disease agents and foreign materials
Chapter Review
18.1 An Overview of Blood
Blood is a fluid connective tissue critical to the transportation of nutrients, gases, and wastes throughout the body; to defend the body against infection and other threats; and to the homeostatic regulation of pH, temperature, and other internal conditions. Blood is composed of formed elements—erythrocytes, leukocytes, and cell fragments called platelets—and a fluid extracellular matrix called plasma. More than 90 percent of plasma is water. The remainder is mostly plasma proteins—mainly albumin, globulins, and fibrinogen—and other dissolved solutes such as glucose, lipids, electrolytes, and dissolved gases. Because of the formed elements and the plasma proteins and other solutes, blood is sticky and more viscous than water. It is also slightly alkaline, and its temperature is slightly higher than normal body temperature.
18.2 Production of the Formed Elements
Through the process of hemopoiesis, the formed elements of blood are continually produced, replacing the relatively short-lived erythrocytes, leukocytes, and platelets. Hemopoiesis begins in the red bone marrow, with hemopoietic stem cells that differentiate into myeloid and lymphoid lineages. Myeloid stem cells give rise to most of the formed elements. Lymphoid stem cells give rise only to the various lymphocytes designated as B and T cells, and NK cells. Hemopoietic growth factors, including erythropoietin, thrombopoietin, colony-stimulating factors, and interleukins, promote the proliferation and differentiation of formed elements.
18.3 Erythrocytes
The most abundant formed elements in blood, erythrocytes are red, biconcave disks packed with an oxygen-carrying compound called hemoglobin. The hemoglobin molecule contains four globin proteins bound to a pigment molecule called heme, which contains an ion of iron. In the bloodstream, iron picks up oxygen in the lungs and drops it off in the tissues; the amino acids in hemoglobin then transport carbon dioxide from the tissues back to the lungs. Erythrocytes live only 120 days on average, and thus must be continually replaced. Worn-out erythrocytes are phagocytized by macrophages and their hemoglobin is broken down. The breakdown products are recycled or removed as wastes: Globin is broken down into amino acids for synthesis of new proteins; iron is stored in the liver or spleen or used by the bone marrow for production of new erythrocytes; and the remnants of heme are converted into bilirubin, or other waste products that are taken up by the liver and excreted in the bile or removed by the kidneys. Anemia is a deficiency of RBCs or hemoglobin, whereas polycythemia is an excess of RBCs.
18.4 Leukocytes and Platelets
Leukocytes function in body defenses. They squeeze out of the walls of blood vessels through emigration or diapedesis, then may move through tissue fluid or become attached to various organs where they fight against pathogenic organisms, diseased cells, or other threats to health. Granular leukocytes, which include neutrophils, eosinophils, and basophils, originate with myeloid stem cells, as do the agranular monocytes. The other agranular leukocytes, NK cells, B cells, and T cells, arise from the lymphoid stem cell line. The most abundant leukocytes are the neutrophils, which are first responders to infections, especially with bacteria. About 20–30 percent of all leukocytes are lymphocytes, which are critical to the body’s defense against specific threats. Leukemia and lymphoma are malignancies involving leukocytes. Platelets are fragments of cells known as megakaryocytes that dwell within the bone marrow. While many platelets are stored in the spleen, others enter the circulation and are essential for hemostasis; they also produce several growth factors important for repair and healing.
18.5 Hemostasis
Hemostasis is the physiological process by which bleeding ceases. Hemostasis involves three basic steps: vascular spasm, the formation of a platelet plug, and coagulation, in which clotting factors promote the formation of a fibrin clot. Fibrinolysis is the process in which a clot is degraded in a healing vessel. Anticoagulants are substances that oppose coagulation. They are important in limiting the extent and duration of clotting. Inadequate clotting can result from too few platelets, or inadequate production of clotting factors, for instance, in the genetic disorder hemophilia. Excessive clotting, called thrombosis, can be caused by excessive numbers of platelets. A thrombus is a collection of fibrin, platelets, and erythrocytes that has accumulated along the lining of a blood vessel, whereas an embolus is a thrombus that has broken free from the vessel wall and is circulating in the bloodstream.
18.6 Blood Typing
Antigens are nonself molecules, usually large proteins, which provoke an immune response. In transfusion reactions, antibodies attach to antigens on the surfaces of erythrocytes and cause agglutination and hemolysis. ABO blood group antigens are designated A and B. People with type A blood have A antigens on their erythrocytes, whereas those with type B blood have B antigens. Those with AB blood have both A and B antigens, and those with type O blood have neither A nor B antigens. The blood plasma contains preformed antibodies against the antigens not present on a person’s erythrocytes.
A second group of blood antigens is the Rh group, the most important of which is Rh D. People with Rh− blood do not have this antigen on their erythrocytes, whereas those who are Rh+ do. About 85 percent of Americans are Rh+. When a woman who is Rh− becomes pregnant with an Rh+ fetus, her body may begin to produce anti-Rh antibodies. If she subsequently becomes pregnant with a second Rh+ fetus and is not treated preventively with RhoGAM, the fetus will be at risk for an antigen-antibody reaction, including agglutination and hemolysis. This is known as hemolytic disease of the newborn.
Cross matching to determine blood type is necessary before transfusing blood, unless the patient is experiencing hemorrhage that is an immediate threat to life, in which case type O− blood may be transfused.
Interactive Link Questions
Visit this site for a list of normal levels established for many of the substances found in a sample of blood. Serum, one of the specimen types included, refers to a sample of plasma after clotting factors have been removed. What types of measurements are given for levels of glucose in the blood?
2.Watch this video to see doctors discuss the dangers of blood doping in sports. What are the some potential side effects of blood doping?
3.Figure 18.13 Are you able to recognize and identify the various formed elements? You will need to do this is a systematic manner, scanning along the image. The standard method is to use a grid, but this is not possible with this resource. Try constructing a simple table with each leukocyte type and then making a mark for each cell type you identify. Attempt to classify at least 50 and perhaps as many as 100 different cells. Based on the percentage of cells that you count, do the numbers represent a normal blood smear or does something appear to be abnormal?
4.View these animations to explore the intrinsic, extrinsic, and common pathways that are involved the process of coagulation. The coagulation cascade restores hemostasis by activating coagulation factors in the presence of an injury. How does the endothelium of the blood vessel walls prevent the blood from coagulating as it flows through the blood vessels?
Review Questions
Which of the following statements about blood is true?
- Blood is about 92 percent water.
- Blood is slightly more acidic than water.
- Blood is slightly more viscous than water.
- Blood is slightly more salty than seawater.
Which of the following statements about albumin is true?
- It draws water out of the blood vessels and into the body’s tissues.
- It is the most abundant plasma protein.
- It is produced by specialized leukocytes called plasma cells.
- All of the above are true.
Which of the following plasma proteins is not produced by the liver?
- fibrinogen
- alpha globulin
- beta globulin
- immunoglobulin
Which of the formed elements arise from myeloid stem cells?
- B cells
- natural killer cells
- platelets
- all of the above
Which of the following statements about erythropoietin is true?
- It facilitates the proliferation and differentiation of the erythrocyte lineage.
- It is a hormone produced by the thyroid gland.
- It is a hemopoietic growth factor that prompts lymphoid stem cells to leave the bone marrow.
- Both a and b are true.
Interleukins are associated primarily with which of the following?
- production of various lymphocytes
- immune responses
- inflammation
- all of the above
Which of the following statements about mature, circulating erythrocytes is true?
- They have no nucleus.
- They are packed with mitochondria.
- They survive for an average of 4 days.
- All of the above
A molecule of hemoglobin ________.
- is shaped like a biconcave disk packed almost entirely with iron
- contains four glycoprotein units studded with oxygen
- consists of four globin proteins, each bound to a molecule of heme
- can carry up to 120 molecules of oxygen
The production of healthy erythrocytes depends upon the availability of ________.
- copper
- zinc
- vitamin B12
- copper, zinc, and vitamin B12
Aging and damaged erythrocytes are removed from the circulation by ________.
- myeoblasts
- monocytes
- macrophages
- mast cells
A patient has been suffering for 2 months with a chronic, watery diarrhea. A blood test is likely to reveal ________.
- a hematocrit below 30 percent
- hypoxemia
- anemia
- polycythemia
The process by which leukocytes squeeze through adjacent cells in a blood vessel wall is called ________.
- leukocytosis
- positive chemotaxis
- emigration
- cytoplasmic extending
Which of the following describes a neutrophil?
- abundant, agranular, especially effective against cancer cells
- abundant, granular, especially effective against bacteria
- rare, agranular, releases antimicrobial defensins
- rare, granular, contains multiple granules packed with histamine
T and B lymphocytes ________.
- are polymorphonuclear
- are involved with specific immune function
- proliferate excessively in leukopenia
- are most active against parasitic worms
A patient has been experiencing severe, persistent allergy symptoms that are reduced when she takes an antihistamine. Before the treatment, this patient was likely to have had increased activity of which leukocyte?
- basophils
- neutrophils
- monocytes
- natural killer cells
Thrombocytes are more accurately called ________.
- clotting factors
- megakaryoblasts
- megakaryocytes
- platelets
The first step in hemostasis is ________.
- vascular spasm
- conversion of fibrinogen to fibrin
- activation of the intrinsic pathway
- activation of the common pathway
Prothrombin is converted to thrombin during the ________.
- intrinsic pathway
- extrinsic pathway
- common pathway
- formation of the platelet plug
Hemophilia is characterized by ________.
- inadequate production of heparin
- inadequate production of clotting factors
- excessive production of fibrinogen
- excessive production of platelets
The process in which antibodies attach to antigens, causing the formation of masses of linked cells, is called ________.
- sensitization
- coagulation
- agglutination
- hemolysis
People with ABO blood type O ________.
- have both antigens A and B on their erythrocytes
- lack both antigens A and B on their erythrocytes
- have neither anti-A nor anti-B antibodies circulating in their blood plasma
- are considered universal recipients
Hemolytic disease of the newborn is a risk during a subsequent pregnancy in which ________.
- a type AB mother is carrying a type O fetus
- a type O mother is carrying a type AB fetus
- an Rh+ mother is carrying an Rh− fetus
- an Rh− mother is carrying a second Rh+ fetus
Critical Thinking Questions
A patient’s hematocrit is 42 percent. Approximately what percentage of the patient’s blood is plasma?
28.Why would it be incorrect to refer to the formed elements as cells?
29.True or false: The buffy coat is the portion of a blood sample that is made up of its proteins.
30.Myelofibrosis is a disorder in which inflammation and scar tissue formation in the bone marrow impair hemopoiesis. One sign is an enlarged spleen. Why?
31.Would you expect a patient with a form of cancer called acute myelogenous leukemia to experience impaired production of erythrocytes, or impaired production of lymphocytes? Explain your choice.
32.A young woman has been experiencing unusually heavy menstrual bleeding for several years. She follows a strict vegan diet (no animal foods). She is at risk for what disorder, and why?
33.A patient has thalassemia, a genetic disorder characterized by abnormal synthesis of globin proteins and excessive destruction of erythrocytes. This patient is jaundiced and is found to have an excessive level of bilirubin in his blood. Explain the connection.
34.One of the more common adverse effects of cancer chemotherapy is the destruction of leukocytes. Before his next scheduled chemotherapy treatment, a patient undergoes a blood test called an absolute neutrophil count (ANC), which reveals that his neutrophil count is 1900 cells per microliter. Would his healthcare team be likely to proceed with his chemotherapy treatment? Why?
35.A patient was admitted to the burn unit the previous evening suffering from a severe burn involving his left upper extremity and shoulder. A blood test reveals that he is experiencing leukocytosis. Why is this an expected finding?
36.A lab technician collects a blood sample in a glass tube. After about an hour, she harvests serum to continue her blood analysis. Explain what has happened during the hour that the sample was in the glass tube.
37.Explain why administration of a thrombolytic agent is a first intervention for someone who has suffered a thrombotic stroke.
38.Following a motor vehicle accident, a patient is rushed to the emergency department with multiple traumatic injuries, causing severe bleeding. The patient’s condition is critical, and there is no time for determining his blood type. What type of blood is transfused, and why?
39.In preparation for a scheduled surgery, a patient visits the hospital lab for a blood draw. The technician collects a blood sample and performs a test to determine its type. She places a sample of the patient’s blood in two wells. To the first well she adds anti-A antibody. To the second she adds anti-B antibody. Both samples visibly agglutinate. Has the technician made an error, or is this a normal response? If normal, what blood type does this indicate?
|
oercommons
|
2025-03-18T00:39:10.188138
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/58764/overview",
"title": "Anatomy and Physiology, Fluids and Transport",
"author": null
}
|
https://oercommons.org/courseware/lesson/58765/overview
|
The Cardiovascular System: The Heart
Overview
The Cardiovascular System: The Heart
Introduction
Figure 19.1 Human Heart This artist’s conception of the human heart suggests a powerful engine—not inappropriate for a muscular pump that keeps the body continually supplied with blood. (credit: Patrick J. Lynch)
CHAPTER OBJECTIVES
After studying this chapter, you will be able to:
- Identify and describe the interior and exterior parts of the human heart
- Describe the path of blood through the cardiac circuits
- Describe the size, shape, and location of the heart
- Compare cardiac muscle to skeletal and smooth muscle
- Explain the cardiac conduction system
- Describe the process and purpose of an electrocardiogram
- Explain the cardiac cycle
- Calculate cardiac output
- Describe the effects of exercise on cardiac output and heart rate
- Name the centers of the brain that control heart rate and describe their function
- Identify other factors affecting heart rate
- Describe fetal heart development
In this chapter, you will explore the remarkable pump that propels the blood into the vessels. There is no single better word to describe the function of the heart other than “pump,” since its contraction develops the pressure that ejects blood into the major vessels: the aorta and pulmonary trunk. From these vessels, the blood is distributed to the remainder of the body. Although the connotation of the term “pump” suggests a mechanical device made of steel and plastic, the anatomical structure is a living, sophisticated muscle. As you read this chapter, try to keep these twin concepts in mind: pump and muscle.
Although the term “heart” is an English word, cardiac (heart-related) terminology can be traced back to the Latin term, “kardia.” Cardiology is the study of the heart, and cardiologists are the physicians who deal primarily with the heart.
Heart Anatomy
- Describe the location and position of the heart within the body cavity
- Describe the internal and external anatomy of the heart
- Identify the tissue layers of the heart
- Relate the structure of the heart to its function as a pump
- Compare systemic circulation to pulmonary circulation
- Identify the veins and arteries of the coronary circulation system
- Trace the pathway of oxygenated and deoxygenated blood thorough the chambers of the heart
The vital importance of the heart is obvious. If one assumes an average rate of contraction of 75 contractions per minute, a human heart would contract approximately 108,000 times in one day, more than 39 million times in one year, and nearly 3 billion times during a 75-year lifespan. Each of the major pumping chambers of the heart ejects approximately 70 mL blood per contraction in a resting adult. This would be equal to 5.25 liters of fluid per minute and approximately 14,000 liters per day. Over one year, that would equal 10,000,000 liters or 2.6 million gallons of blood sent through roughly 60,000 miles of vessels. In order to understand how that happens, it is necessary to understand the anatomy and physiology of the heart.
Location of the Heart
The human heart is located within the thoracic cavity, medially between the lungs in the space known as the mediastinum. Figure 19.2 shows the position of the heart within the thoracic cavity. Within the mediastinum, the heart is separated from the other mediastinal structures by a tough membrane known as the pericardium, or pericardial sac, and sits in its own space called the pericardial cavity. The dorsal surface of the heart lies near the bodies of the vertebrae, and its anterior surface sits deep to the sternum and costal cartilages. The great veins, the superior and inferior venae cavae, and the great arteries, the aorta and pulmonary trunk, are attached to the superior surface of the heart, called the base. The base of the heart is located at the level of the third costal cartilage, as seen in Figure 19.2. The inferior tip of the heart, the apex, lies just to the left of the sternum between the junction of the fourth and fifth ribs near their articulation with the costal cartilages. The right side of the heart is deflected anteriorly, and the left side is deflected posteriorly. It is important to remember the position and orientation of the heart when placing a stethoscope on the chest of a patient and listening for heart sounds, and also when looking at images taken from a midsagittal perspective. The slight deviation of the apex to the left is reflected in a depression in the medial surface of the inferior lobe of the left lung, called the cardiac notch.
Figure 19.2 Position of the Heart in the Thorax The heart is located within the thoracic cavity, medially between the lungs in the mediastinum. It is about the size of a fist, is broad at the top, and tapers toward the base.
EVERYDAY CONNECTION
CPR
The position of the heart in the torso between the vertebrae and sternum (see Figure 19.2 for the position of the heart within the thorax) allows for individuals to apply an emergency technique known as cardiopulmonary resuscitation (CPR) if the heart of a patient should stop. By applying pressure with the flat portion of one hand on the sternum in the area between the line at T4 and T9 (Figure 19.3), it is possible to manually compress the blood within the heart enough to push some of the blood within it into the pulmonary and systemic circuits. This is particularly critical for the brain, as irreversible damage and death of neurons occur within minutes of loss of blood flow. Current standards call for compression of the chest at least 5 cm deep and at a rate of 100 compressions per minute, a rate equal to the beat in “Staying Alive,” recorded in 1977 by the Bee Gees. If you are unfamiliar with this song, a version is available on www.youtube.com. At this stage, the emphasis is on performing high-quality chest compressions, rather than providing artificial respiration. CPR is generally performed until the patient regains spontaneous contraction or is declared dead by an experienced healthcare professional.
When performed by untrained or overzealous individuals, CPR can result in broken ribs or a broken sternum, and can inflict additional severe damage on the patient. It is also possible, if the hands are placed too low on the sternum, to manually drive the xiphoid process into the liver, a consequence that may prove fatal for the patient. Proper training is essential. This proven life-sustaining technique is so valuable that virtually all medical personnel as well as concerned members of the public should be certified and routinely recertified in its application. CPR courses are offered at a variety of locations, including colleges, hospitals, the American Red Cross, and some commercial companies. They normally include practice of the compression technique on a mannequin.
Figure 19.3 CPR Technique If the heart should stop, CPR can maintain the flow of blood until the heart resumes beating. By applying pressure to the sternum, the blood within the heart will be squeezed out of the heart and into the circulation. Proper positioning of the hands on the sternum to perform CPR would be between the lines at T4 and T9.
INTERACTIVE LINK
Visit the American Heart Association website to help locate a course near your home in the United States. There are also many other national and regional heart associations that offer the same service, depending upon the location.
Shape and Size of the Heart
The shape of the heart is similar to a pinecone, rather broad at the superior surface and tapering to the apex (see Figure 19.2). A typical heart is approximately the size of your fist: 12 cm (5 in) in length, 8 cm (3.5 in) wide, and 6 cm (2.5 in) in thickness. Given the size difference between most members of the sexes, the weight of a female heart is approximately 250–300 grams (9 to 11 ounces), and the weight of a male heart is approximately 300–350 grams (11 to 12 ounces). The heart of a well-trained athlete, especially one specializing in aerobic sports, can be considerably larger than this. Cardiac muscle responds to exercise in a manner similar to that of skeletal muscle. That is, exercise results in the addition of protein myofilaments that increase the size of the individual cells without increasing their numbers, a concept called hypertrophy. Hearts of athletes can pump blood more effectively at lower rates than those of nonathletes. Enlarged hearts are not always a result of exercise; they can result from pathologies, such as hypertrophic cardiomyopathy. The cause of an abnormally enlarged heart muscle is unknown, but the condition is often undiagnosed and can cause sudden death in apparently otherwise healthy young people.
Chambers and Circulation through the Heart
The human heart consists of four chambers: The left side and the right side each have one atrium and one ventricle. Each of the upper chambers, the right atrium (plural = atria) and the left atrium, acts as a receiving chamber and contracts to push blood into the lower chambers, the right ventricle and the left ventricle. The ventricles serve as the primary pumping chambers of the heart, propelling blood to the lungs or to the rest of the body.
There are two distinct but linked circuits in the human circulation called the pulmonary and systemic circuits. Although both circuits transport blood and everything it carries, we can initially view the circuits from the point of view of gases. The pulmonary circuit transports blood to and from the lungs, where it picks up oxygen and delivers carbon dioxide for exhalation. The systemic circuit transports oxygenated blood to virtually all of the tissues of the body and returns relatively deoxygenated blood and carbon dioxide to the heart to be sent back to the pulmonary circulation.
The right ventricle pumps deoxygenated blood into the pulmonary trunk, which leads toward the lungs and bifurcates into the left and right pulmonary arteries. These vessels in turn branch many times before reaching the pulmonary capillaries, where gas exchange occurs: Carbon dioxide exits the blood and oxygen enters. The pulmonary trunk arteries and their branches are the only arteries in the post-natal body that carry relatively deoxygenated blood. Highly oxygenated blood returning from the pulmonary capillaries in the lungs passes through a series of vessels that join together to form the pulmonary veins—the only post-natal veins in the body that carry highly oxygenated blood. The pulmonary veins conduct blood into the left atrium, which pumps the blood into the left ventricle, which in turn pumps oxygenated blood into the aorta and on to the many branches of the systemic circuit. Eventually, these vessels will lead to the systemic capillaries, where exchange with the tissue fluid and cells of the body occurs. In this case, oxygen and nutrients exit the systemic capillaries to be used by the cells in their metabolic processes, and carbon dioxide and waste products will enter the blood.
The blood exiting the systemic capillaries is lower in oxygen concentration than when it entered. The capillaries will ultimately unite to form venules, joining to form ever-larger veins, eventually flowing into the two major systemic veins, the superior vena cava and the inferior vena cava, which return blood to the right atrium. The blood in the superior and inferior venae cavae flows into the right atrium, which pumps blood into the right ventricle. This process of blood circulation continues as long as the individual remains alive. Understanding the flow of blood through the pulmonary and systemic circuits is critical to all health professions (Figure 19.4).
Figure 19.4 Dual System of the Human Blood Circulation Blood flows from the right atrium to the right ventricle, where it is pumped into the pulmonary circuit. The blood in the pulmonary artery branches is low in oxygen but relatively high in carbon dioxide. Gas exchange occurs in the pulmonary capillaries (oxygen into the blood, carbon dioxide out), and blood high in oxygen and low in carbon dioxide is returned to the left atrium. From here, blood enters the left ventricle, which pumps it into the systemic circuit. Following exchange in the systemic capillaries (oxygen and nutrients out of the capillaries and carbon dioxide and wastes in), blood returns to the right atrium and the cycle is repeated.
Membranes, Surface Features, and Layers
Our exploration of more in-depth heart structures begins by examining the membrane that surrounds the heart, the prominent surface features of the heart, and the layers that form the wall of the heart. Each of these components plays its own unique role in terms of function.
Membranes
The membrane that directly surrounds the heart and defines the pericardial cavity is called the pericardium or pericardial sac. It also surrounds the “roots” of the major vessels, or the areas of closest proximity to the heart. The pericardium, which literally translates as “around the heart,” consists of two distinct sublayers: the sturdy outer fibrous pericardium and the inner serous pericardium. The fibrous pericardium is made of tough, dense connective tissue that protects the heart and maintains its position in the thorax. The more delicate serous pericardium consists of two layers: the parietal pericardium, which is fused to the fibrous pericardium, and an inner visceral pericardium, or epicardium, which is fused to the heart and is part of the heart wall. The pericardial cavity, filled with lubricating serous fluid, lies between the epicardium and the pericardium.
In most organs within the body, visceral serous membranes such as the epicardium are microscopic. However, in the case of the heart, it is not a microscopic layer but rather a macroscopic layer, consisting of a simple squamous epithelium called a mesothelium, reinforced with loose, irregular, or areolar connective tissue that attaches to the pericardium. This mesothelium secretes the lubricating serous fluid that fills the pericardial cavity and reduces friction as the heart contracts. Figure 19.5illustrates the pericardial membrane and the layers of the heart.
Figure 19.5 Pericardial Membranes and Layers of the Heart Wall The pericardial membrane that surrounds the heart consists of three layers and the pericardial cavity. The heart wall also consists of three layers. The pericardial membrane and the heart wall share the epicardium.
DISORDERS OF THE...
Heart: Cardiac Tamponade
If excess fluid builds within the pericardial space, it can lead to a condition called cardiac tamponade, or pericardial tamponade. With each contraction of the heart, more fluid—in most instances, blood—accumulates within the pericardial cavity. In order to fill with blood for the next contraction, the heart must relax. However, the excess fluid in the pericardial cavity puts pressure on the heart and prevents full relaxation, so the chambers within the heart contain slightly less blood as they begin each heart cycle. Over time, less and less blood is ejected from the heart. If the fluid builds up slowly, as in hypothyroidism, the pericardial cavity may be able to expand gradually to accommodate this extra volume. Some cases of fluid in excess of one liter within the pericardial cavity have been reported. Rapid accumulation of as little as 100 mL of fluid following trauma may trigger cardiac tamponade. Other common causes include myocardial rupture, pericarditis, cancer, or even cardiac surgery. Removal of this excess fluid requires insertion of drainage tubes into the pericardial cavity. Premature removal of these drainage tubes, for example, following cardiac surgery, or clot formation within these tubes are causes of this condition. Untreated, cardiac tamponade can lead to death.
Surface Features of the Heart
Inside the pericardium, the surface features of the heart are visible, including the four chambers. There is a superficial leaf-like extension of the atria near the superior surface of the heart, one on each side, called an auricle—a name that means “ear like”—because its shape resembles the external ear of a human (Figure 19.6). Auricles are relatively thin-walled structures that can fill with blood and empty into the atria or upper chambers of the heart. You may also hear them referred to as atrial appendages. Also prominent is a series of fat-filled grooves, each of which is known as a sulcus (plural = sulci), along the superior surfaces of the heart. Major coronary blood vessels are located in these sulci. The deep coronary sulcus is located between the atria and ventricles. Located between the left and right ventricles are two additional sulci that are not as deep as the coronary sulcus. The anterior interventricular sulcus is visible on the anterior surface of the heart, whereas the posterior interventricular sulcus is visible on the posterior surface of the heart. Figure 19.6 illustrates anterior and posterior views of the surface of the heart.
Figure 19.6 External Anatomy of the Heart Inside the pericardium, the surface features of the heart are visible.
Layers
The wall of the heart is composed of three layers of unequal thickness. From superficial to deep, these are the epicardium, the myocardium, and the endocardium (see Figure 19.5). The outermost layer of the wall of the heart is also the innermost layer of the pericardium, the epicardium, or the visceral pericardium discussed earlier.
The middle and thickest layer is the myocardium, made largely of cardiac muscle cells. It is built upon a framework of collagenous fibers, plus the blood vessels that supply the myocardium and the nerve fibers that help regulate the heart. It is the contraction of the myocardium that pumps blood through the heart and into the major arteries. The muscle pattern is elegant and complex, as the muscle cells swirl and spiral around the chambers of the heart. They form a figure 8 pattern around the atria and around the bases of the great vessels. Deeper ventricular muscles also form a figure 8 around the two ventricles and proceed toward the apex. More superficial layers of ventricular muscle wrap around both ventricles. This complex swirling pattern allows the heart to pump blood more effectively than a simple linear pattern would. Figure 19.7 illustrates the arrangement of muscle cells.
Figure 19.7 Heart Musculature The swirling pattern of cardiac muscle tissue contributes significantly to the heart’s ability to pump blood effectively.
Although the ventricles on the right and left sides pump the same amount of blood per contraction, the muscle of the left ventricle is much thicker and better developed than that of the right ventricle. In order to overcome the high resistance required to pump blood into the long systemic circuit, the left ventricle must generate a great amount of pressure. The right ventricle does not need to generate as much pressure, since the pulmonary circuit is shorter and provides less resistance. Figure 19.8illustrates the differences in muscular thickness needed for each of the ventricles.
Figure 19.8 Differences in Ventricular Muscle Thickness The myocardium in the left ventricle is significantly thicker than that of the right ventricle. Both ventricles pump the same amount of blood, but the left ventricle must generate a much greater pressure to overcome greater resistance in the systemic circuit. The ventricles are shown in both relaxed and contracting states. Note the differences in the relative size of the lumens, the region inside each ventricle where the blood is contained.
The innermost layer of the heart wall, the endocardium, is joined to the myocardium with a thin layer of connective tissue. The endocardium lines the chambers where the blood circulates and covers the heart valves. It is made of simple squamous epithelium called endothelium, which is continuous with the endothelial lining of the blood vessels (see Figure 19.5).
Once regarded as a simple lining layer, recent evidence indicates that the endothelium of the endocardium and the coronary capillaries may play active roles in regulating the contraction of the muscle within the myocardium. The endothelium may also regulate the growth patterns of the cardiac muscle cells throughout life, and the endothelins it secretes create an environment in the surrounding tissue fluids that regulates ionic concentrations and states of contractility. Endothelins are potent vasoconstrictors and, in a normal individual, establish a homeostatic balance with other vasoconstrictors and vasodilators.
Internal Structure of the Heart
Recall that the heart’s contraction cycle follows a dual pattern of circulation—the pulmonary and systemic circuits—because of the pairs of chambers that pump blood into the circulation. In order to develop a more precise understanding of cardiac function, it is first necessary to explore the internal anatomical structures in more detail.
Septa of the Heart
The word septum is derived from the Latin for “something that encloses;” in this case, a septum (plural = septa) refers to a wall or partition that divides the heart into chambers. The septa are physical extensions of the myocardium lined with endocardium. Located between the two atria is the interatrial septum. Normally in an adult heart, the interatrial septum bears an oval-shaped depression known as the fossa ovalis, a remnant of an opening in the fetal heart known as the foramen ovale. The foramen ovale allowed blood in the fetal heart to pass directly from the right atrium to the left atrium, allowing some blood to bypass the pulmonary circuit. Within seconds after birth, a flap of tissue known as the septum primum that previously acted as a valve closes the foramen ovale and establishes the typical cardiac circulation pattern.
Between the two ventricles is a second septum known as the interventricular septum. Unlike the interatrial septum, the interventricular septum is normally intact after its formation during fetal development. It is substantially thicker than the interatrial septum, since the ventricles generate far greater pressure when they contract.
The septum between the atria and ventricles is known as the atrioventricular septum. It is marked by the presence of four openings that allow blood to move from the atria into the ventricles and from the ventricles into the pulmonary trunk and aorta. Located in each of these openings between the atria and ventricles is a valve, a specialized structure that ensures one-way flow of blood. The valves between the atria and ventricles are known generically as atrioventricular valves. The valves at the openings that lead to the pulmonary trunk and aorta are known generically as semilunar valves. The interventricular septum is visible in Figure 19.9. In this figure, the atrioventricular septum has been removed to better show the bicupid and tricuspid valves; the interatrial septum is not visible, since its location is covered by the aorta and pulmonary trunk. Since these openings and valves structurally weaken the atrioventricular septum, the remaining tissue is heavily reinforced with dense connective tissue called the cardiac skeleton, or skeleton of the heart. It includes four rings that surround the openings between the atria and ventricles, and the openings to the pulmonary trunk and aorta, and serve as the point of attachment for the heart valves. The cardiac skeleton also provides an important boundary in the heart electrical conduction system.
Figure 19.9 Internal Structures of the Heart This anterior view of the heart shows the four chambers, the major vessels and their early branches, as well as the valves. The presence of the pulmonary trunk and aorta covers the interatrial septum, and the atrioventricular septum is cut away to show the atrioventricular valves.
DISORDERS OF THE...
Heart: Heart Defects
One very common form of interatrial septum pathology is patent foramen ovale, which occurs when the septum primum does not close at birth, and the fossa ovalis is unable to fuse. The word patent is from the Latin root patens for “open.” It may be benign or asymptomatic, perhaps never being diagnosed, or in extreme cases, it may require surgical repair to close the opening permanently. As much as 20–25 percent of the general population may have a patent foramen ovale, but fortunately, most have the benign, asymptomatic version. Patent foramen ovale is normally detected by auscultation of a heart murmur (an abnormal heart sound) and confirmed by imaging with an echocardiogram. Despite its prevalence in the general population, the causes of patent ovale are unknown, and there are no known risk factors. In nonlife-threatening cases, it is better to monitor the condition than to risk heart surgery to repair and seal the opening.
Coarctation of the aorta is a congenital abnormal narrowing of the aorta that is normally located at the insertion of the ligamentum arteriosum, the remnant of the fetal shunt called the ductus arteriosus. If severe, this condition drastically restricts blood flow through the primary systemic artery, which is life threatening. In some individuals, the condition may be fairly benign and not detected until later in life. Detectable symptoms in an infant include difficulty breathing, poor appetite, trouble feeding, or failure to thrive. In older individuals, symptoms include dizziness, fainting, shortness of breath, chest pain, fatigue, headache, and nosebleeds. Treatment involves surgery to resect (remove) the affected region or angioplasty to open the abnormally narrow passageway. Studies have shown that the earlier the surgery is performed, the better the chance of survival.
A patent ductus arteriosus is a congenital condition in which the ductus arteriosus fails to close. The condition may range from severe to benign. Failure of the ductus arteriosus to close results in blood flowing from the higher pressure aorta into the lower pressure pulmonary trunk. This additional fluid moving toward the lungs increases pulmonary pressure and makes respiration difficult. Symptoms include shortness of breath (dyspnea), tachycardia, enlarged heart, a widened pulse pressure, and poor weight gain in infants. Treatments include surgical closure (ligation), manual closure using platinum coils or specialized mesh inserted via the femoral artery or vein, or nonsteroidal anti-inflammatory drugs to block the synthesis of prostaglandin E2, which maintains the vessel in an open position. If untreated, the condition can result in congestive heart failure.
Septal defects are not uncommon in individuals and may be congenital or caused by various disease processes. Tetralogy of Fallot is a congenital condition that may also occur from exposure to unknown environmental factors; it occurs when there is an opening in the interventricular septum caused by blockage of the pulmonary trunk, normally at the pulmonary semilunar valve. This allows blood that is relatively low in oxygen from the right ventricle to flow into the left ventricle and mix with the blood that is relatively high in oxygen. Symptoms include a distinct heart murmur, low blood oxygen percent saturation, dyspnea or difficulty in breathing, polycythemia, broadening (clubbing) of the fingers and toes, and in children, difficulty in feeding or failure to grow and develop. It is the most common cause of cyanosis following birth. The term “tetralogy” is derived from the four components of the condition, although only three may be present in an individual patient: pulmonary infundibular stenosis (rigidity of the pulmonary valve), overriding aorta (the aorta is shifted above both ventricles), ventricular septal defect (opening), and right ventricular hypertrophy (enlargement of the right ventricle). Other heart defects may also accompany this condition, which is typically confirmed by echocardiography imaging. Tetralogy of Fallot occurs in approximately 400 out of one million live births. Normal treatment involves extensive surgical repair, including the use of stents to redirect blood flow and replacement of valves and patches to repair the septal defect, but the condition has a relatively high mortality. Survival rates are currently 75 percent during the first year of life; 60 percent by 4 years of age; 30 percent by 10 years; and 5 percent by 40 years.
In the case of severe septal defects, including both tetralogy of Fallot and patent foramen ovale, failure of the heart to develop properly can lead to a condition commonly known as a “blue baby.” Regardless of normal skin pigmentation, individuals with this condition have an insufficient supply of oxygenated blood, which leads to cyanosis, a blue or purple coloration of the skin, especially when active.
Septal defects are commonly first detected through auscultation, listening to the chest using a stethoscope. In this case, instead of hearing normal heart sounds attributed to the flow of blood and closing of heart valves, unusual heart sounds may be detected. This is often followed by medical imaging to confirm or rule out a diagnosis. In many cases, treatment may not be needed. Some common congenital heart defects are illustrated in Figure 19.10.
Figure 19.10 Congenital Heart Defects (a) A patent foramen ovale defect is an abnormal opening in the interatrial septum, or more commonly, a failure of the foramen ovale to close. (b) Coarctation of the aorta is an abnormal narrowing of the aorta. (c) A patent ductus arteriosus is the failure of the ductus arteriosus to close. (d) Tetralogy of Fallot includes an abnormal opening in the interventricular septum.
Right Atrium
The right atrium serves as the receiving chamber for blood returning to the heart from the systemic circulation. The two major systemic veins, the superior and inferior venae cavae, and the large coronary vein called the coronary sinus that drains the heart myocardium empty into the right atrium. The superior vena cava drains blood from regions superior to the diaphragm: the head, neck, upper limbs, and the thoracic region. It empties into the superior and posterior portions of the right atrium. The inferior vena cava drains blood from areas inferior to the diaphragm: the lower limbs and abdominopelvic region of the body. It, too, empties into the posterior portion of the atria, but inferior to the opening of the superior vena cava. Immediately superior and slightly medial to the opening of the inferior vena cava on the posterior surface of the atrium is the opening of the coronary sinus. This thin-walled vessel drains most of the coronary veins that return systemic blood from the heart. The majority of the internal heart structures discussed in this and subsequent sections are illustrated in Figure 19.9.
While the bulk of the internal surface of the right atrium is smooth, the depression of the fossa ovalis is medial, and the anterior surface demonstrates prominent ridges of muscle called the pectinate muscles. The right auricle also has pectinate muscles. The left atrium does not have pectinate muscles except in the auricle.
The atria receive venous blood on a nearly continuous basis, preventing venous flow from stopping while the ventricles are contracting. While most ventricular filling occurs while the atria are relaxed, they do demonstrate a contractile phase and actively pump blood into the ventricles just prior to ventricular contraction. The opening between the atrium and ventricle is guarded by the tricuspid valve.
Right Ventricle
The right ventricle receives blood from the right atrium through the tricuspid valve. Each flap of the valve is attached to strong strands of connective tissue, the chordae tendineae, literally “tendinous cords,” or sometimes more poetically referred to as “heart strings.” There are several chordae tendineae associated with each of the flaps. They are composed of approximately 80 percent collagenous fibers with the remainder consisting of elastic fibers and endothelium. They connect each of the flaps to a papillary muscle that extends from the inferior ventricular surface. There are three papillary muscles in the right ventricle, called the anterior, posterior, and septal muscles, which correspond to the three sections of the valves.
When the myocardium of the ventricle contracts, pressure within the ventricular chamber rises. Blood, like any fluid, flows from higher pressure to lower pressure areas, in this case, toward the pulmonary trunk and the atrium. To prevent any potential backflow, the papillary muscles also contract, generating tension on the chordae tendineae. This prevents the flaps of the valves from being forced into the atria and regurgitation of the blood back into the atria during ventricular contraction. Figure 19.11shows papillary muscles and chordae tendineae attached to the tricuspid valve.
Figure 19.11 Chordae Tendineae and Papillary Muscles In this frontal section, you can see papillary muscles attached to the tricuspid valve on the right as well as the mitral valve on the left via chordae tendineae. (credit: modification of work by “PV KS”/flickr.com)
The walls of the ventricle are lined with trabeculae carneae, ridges of cardiac muscle covered by endocardium. In addition to these muscular ridges, a band of cardiac muscle, also covered by endocardium, known as the moderator band (see Figure 19.9) reinforces the thin walls of the right ventricle and plays a crucial role in cardiac conduction. It arises from the inferior portion of the interventricular septum and crosses the interior space of the right ventricle to connect with the inferior papillary muscle.
When the right ventricle contracts, it ejects blood into the pulmonary trunk, which branches into the left and right pulmonary arteries that carry it to each lung. The superior surface of the right ventricle begins to taper as it approaches the pulmonary trunk. At the base of the pulmonary trunk is the pulmonary semilunar valve that prevents backflow from the pulmonary trunk.
Left Atrium
After exchange of gases in the pulmonary capillaries, blood returns to the left atrium high in oxygen via one of the four pulmonary veins. While the left atrium does not contain pectinate muscles, it does have an auricle that includes these pectinate ridges. Blood flows nearly continuously from the pulmonary veins back into the atrium, which acts as the receiving chamber, and from here through an opening into the left ventricle. Most blood flows passively into the heart while both the atria and ventricles are relaxed, but toward the end of the ventricular relaxation period, the left atrium will contract, pumping blood into the ventricle. This atrial contraction accounts for approximately 20 percent of ventricular filling. The opening between the left atrium and ventricle is guarded by the mitral valve.
Left Ventricle
Recall that, although both sides of the heart will pump the same amount of blood, the muscular layer is much thicker in the left ventricle compared to the right (see Figure 19.8). Like the right ventricle, the left also has trabeculae carneae, but there is no moderator band. The mitral valve is connected to papillary muscles via chordae tendineae. There are two papillary muscles on the left—the anterior and posterior—as opposed to three on the right.
The left ventricle is the major pumping chamber for the systemic circuit; it ejects blood into the aorta through the aortic semilunar valve.
Heart Valve Structure and Function
A transverse section through the heart slightly above the level of the atrioventricular septum reveals all four heart valves along the same plane (Figure 19.12). The valves ensure unidirectional blood flow through the heart. Between the right atrium and the right ventricle is the right atrioventricular valve, or tricuspid valve. It typically consists of three flaps, or leaflets, made of endocardium reinforced with additional connective tissue. The flaps are connected by chordae tendineae to the papillary muscles, which control the opening and closing of the valves.
Figure 19.12 Heart Valves With the atria and major vessels removed, all four valves are clearly visible, although it is difficult to distinguish the three separate cusps of the tricuspid valve.
Emerging from the right ventricle at the base of the pulmonary trunk is the pulmonary semilunar valve, or the pulmonary valve; it is also known as the pulmonic valve or the right semilunar valve. The pulmonary valve is comprised of three small flaps of endothelium reinforced with connective tissue. When the ventricle relaxes, the pressure differential causes blood to flow back into the ventricle from the pulmonary trunk. This flow of blood fills the pocket-like flaps of the pulmonary valve, causing the valve to close and producing an audible sound. Unlike the atrioventricular valves, there are no papillary muscles or chordae tendineae associated with the pulmonary valve.
Located at the opening between the left atrium and left ventricle is the mitral valve, also called the bicuspid valve or the left atrioventricular valve. Structurally, this valve consists of two cusps, known as the anterior medial cusp and the posterior medial cusp, compared to the three cusps of the tricuspid valve. In a clinical setting, the valve is referred to as the mitral valve, rather than the bicuspid valve. The two cusps of the mitral valve are attached by chordae tendineae to two papillary muscles that project from the wall of the ventricle.
At the base of the aorta is the aortic semilunar valve, or the aortic valve, which prevents backflow from the aorta. It normally is composed of three flaps. When the ventricle relaxes and blood attempts to flow back into the ventricle from the aorta, blood will fill the cusps of the valve, causing it to close and producing an audible sound.
In Figure 19.13a, the two atrioventricular valves are open and the two semilunar valves are closed. This occurs when both atria and ventricles are relaxed and when the atria contract to pump blood into the ventricles. Figure 19.13b shows a frontal view. Although only the left side of the heart is illustrated, the process is virtually identical on the right.
Figure 19.13 Blood Flow from the Left Atrium to the Left Ventricle (a) A transverse section through the heart illustrates the four heart valves. The two atrioventricular valves are open; the two semilunar valves are closed. The atria and vessels have been removed. (b) A frontal section through the heart illustrates blood flow through the mitral valve. When the mitral valve is open, it allows blood to move from the left atrium to the left ventricle. The aortic semilunar valve is closed to prevent backflow of blood from the aorta to the left ventricle.
Figure 19.14a shows the atrioventricular valves closed while the two semilunar valves are open. This occurs when the ventricles contract to eject blood into the pulmonary trunk and aorta. Closure of the two atrioventricular valves prevents blood from being forced back into the atria. This stage can be seen from a frontal view in Figure 19.14b.
Figure 19.14 Blood Flow from the Left Ventricle into the Great Vessels (a) A transverse section through the heart illustrates the four heart valves during ventricular contraction. The two atrioventricular valves are closed, but the two semilunar valves are open. The atria and vessels have been removed. (b) A frontal view shows the closed mitral (bicuspid) valve that prevents backflow of blood into the left atrium. The aortic semilunar valve is open to allow blood to be ejected into the aorta.
When the ventricles begin to contract, pressure within the ventricles rises and blood flows toward the area of lowest pressure, which is initially in the atria. This backflow causes the cusps of the tricuspid and mitral (bicuspid) valves to close. These valves are tied down to the papillary muscles by chordae tendineae. During the relaxation phase of the cardiac cycle, the papillary muscles are also relaxed and the tension on the chordae tendineae is slight (see Figure 19.13b). However, as the myocardium of the ventricle contracts, so do the papillary muscles. This creates tension on the chordae tendineae (see Figure 19.14b), helping to hold the cusps of the atrioventricular valves in place and preventing them from being blown back into the atria.
The aortic and pulmonary semilunar valves lack the chordae tendineae and papillary muscles associated with the atrioventricular valves. Instead, they consist of pocket-like folds of endocardium reinforced with additional connective tissue. When the ventricles relax and the change in pressure forces the blood toward the ventricles, the blood presses against these cusps and seals the openings.
INTERACTIVE LINK
Visit this site to observe an echocardiogram of actual heart valves opening and closing. Although much of the heart has been “removed” from this gif loop so the chordae tendineae are not visible, why is their presence more critical for the atrioventricular valves (tricuspid and mitral) than the semilunar (aortic and pulmonary) valves?
DISORDERS OF THE...
Heart Valves
When heart valves do not function properly, they are often described as incompetent and result in valvular heart disease, which can range from benign to lethal. Some of these conditions are congenital, that is, the individual was born with the defect, whereas others may be attributed to disease processes or trauma. Some malfunctions are treated with medications, others require surgery, and still others may be mild enough that the condition is merely monitored since treatment might trigger more serious consequences.
Valvular disorders are often caused by carditis, or inflammation of the heart. One common trigger for this inflammation is rheumatic fever, or scarlet fever, an autoimmune response to the presence of a bacterium, Streptococcus pyogenes, normally a disease of childhood.
While any of the heart valves may be involved in valve disorders, mitral regurgitation is the most common, detected in approximately 2 percent of the population, and the pulmonary semilunar valve is the least frequently involved. When a valve malfunctions, the flow of blood to a region will often be disrupted. The resulting inadequate flow of blood to this region will be described in general terms as an insufficiency. The specific type of insufficiency is named for the valve involved: aortic insufficiency, mitral insufficiency, tricuspid insufficiency, or pulmonary insufficiency.
If one of the cusps of the valve is forced backward by the force of the blood, the condition is referred to as a prolapsed valve. Prolapse may occur if the chordae tendineae are damaged or broken, causing the closure mechanism to fail. The failure of the valve to close properly disrupts the normal one-way flow of blood and results in regurgitation, when the blood flows backward from its normal path. Using a stethoscope, the disruption to the normal flow of blood produces a heart murmur.
Stenosis is a condition in which the heart valves become rigid and may calcify over time. The loss of flexibility of the valve interferes with normal function and may cause the heart to work harder to propel blood through the valve, which eventually weakens the heart. Aortic stenosis affects approximately 2 percent of the population over 65 years of age, and the percentage increases to approximately 4 percent in individuals over 85 years. Occasionally, one or more of the chordae tendineae will tear or the papillary muscle itself may die as a component of a myocardial infarction (heart attack). In this case, the patient’s condition will deteriorate dramatically and rapidly, and immediate surgical intervention may be required.
Auscultation, or listening to a patient’s heart sounds, is one of the most useful diagnostic tools, since it is proven, safe, and inexpensive. The term auscultation is derived from the Latin for “to listen,” and the technique has been used for diagnostic purposes as far back as the ancient Egyptians. Valve and septal disorders will trigger abnormal heart sounds. If a valvular disorder is detected or suspected, a test called an echocardiogram, or simply an “echo,” may be ordered. Echocardiograms are sonograms of the heart and can help in the diagnosis of valve disorders as well as a wide variety of heart pathologies.
INTERACTIVE LINK
Visit this site for a free download, including excellent animations and audio of heart sounds.
CAREER CONNECTION
Cardiologist
Cardiologists are medical doctors that specialize in the diagnosis and treatment of diseases of the heart. After completing 4 years of medical school, cardiologists complete a three-year residency in internal medicine followed by an additional three or more years in cardiology. Following this 10-year period of medical training and clinical experience, they qualify for a rigorous two-day examination administered by the Board of Internal Medicine that tests their academic training and clinical abilities, including diagnostics and treatment. After successful completion of this examination, a physician becomes a board-certified cardiologist. Some board-certified cardiologists may be invited to become a Fellow of the American College of Cardiology (FACC). This professional recognition is awarded to outstanding physicians based upon merit, including outstanding credentials, achievements, and community contributions to cardiovascular medicine.
INTERACTIVE LINK
Visit this site to learn more about cardiologists.
CAREER CONNECTION
Cardiovascular Technologist/Technician
Cardiovascular technologists/technicians are trained professionals who perform a variety of imaging techniques, such as sonograms or echocardiograms, used by physicians to diagnose and treat diseases of the heart. Nearly all of these positions require an associate degree, and these technicians earn a median salary of $49,410 as of May 2010, according to the U.S. Bureau of Labor Statistics. Growth within the field is fast, projected at 29 percent from 2010 to 2020.
There is a considerable overlap and complementary skills between cardiac technicians and vascular technicians, and so the term cardiovascular technician is often used. Special certifications within the field require documenting appropriate experience and completing additional and often expensive certification examinations. These subspecialties include Certified Rhythm Analysis Technician (CRAT), Certified Cardiographic Technician (CCT), Registered Congenital Cardiac Sonographer (RCCS), Registered Cardiac Electrophysiology Specialist (RCES), Registered Cardiovascular Invasive Specialist (RCIS), Registered Cardiac Sonographer (RCS), Registered Vascular Specialist (RVS), and Registered Phlebology Sonographer (RPhS).
INTERACTIVE LINK
Visit this site for more information on cardiovascular technologists/technicians.
Coronary Circulation
You will recall that the heart is a remarkable pump composed largely of cardiac muscle cells that are incredibly active throughout life. Like all other cells, a cardiomyocyte requires a reliable supply of oxygen and nutrients, and a way to remove wastes, so it needs a dedicated, complex, and extensive coronary circulation. And because of the critical and nearly ceaseless activity of the heart throughout life, this need for a blood supply is even greater than for a typical cell. However, coronary circulation is not continuous; rather, it cycles, reaching a peak when the heart muscle is relaxed and nearly ceasing while it is contracting.
Coronary Arteries
Coronary arteries supply blood to the myocardium and other components of the heart. The first portion of the aorta after it arises from the left ventricle gives rise to the coronary arteries. There are three dilations in the wall of the aorta just superior to the aortic semilunar valve. Two of these, the left posterior aortic sinus and anterior aortic sinus, give rise to the left and right coronary arteries, respectively. The third sinus, the right posterior aortic sinus, typically does not give rise to a vessel. Coronary vessel branches that remain on the surface of the artery and follow the sulci are called epicardial coronary arteries.
The left coronary artery distributes blood to the left side of the heart, the left atrium and ventricle, and the interventricular septum. The circumflex artery arises from the left coronary artery and follows the coronary sulcus to the left. Eventually, it will fuse with the small branches of the right coronary artery. The larger anterior interventricular artery, also known as the left anterior descending artery (LAD), is the second major branch arising from the left coronary artery. It follows the anterior interventricular sulcus around the pulmonary trunk. Along the way it gives rise to numerous smaller branches that interconnect with the branches of the posterior interventricular artery, forming anastomoses. An anastomosis is an area where vessels unite to form interconnections that normally allow blood to circulate to a region even if there may be partial blockage in another branch. The anastomoses in the heart are very small. Therefore, this ability is somewhat restricted in the heart so a coronary artery blockage often results in death of the cells (myocardial infarction) supplied by the particular vessel.
The right coronary artery proceeds along the coronary sulcus and distributes blood to the right atrium, portions of both ventricles, and the heart conduction system. Normally, one or more marginal arteries arise from the right coronary artery inferior to the right atrium. The marginal arteries supply blood to the superficial portions of the right ventricle. On the posterior surface of the heart, the right coronary artery gives rise to the posterior interventricular artery, also known as the posterior descending artery. It runs along the posterior portion of the interventricular sulcus toward the apex of the heart, giving rise to branches that supply the interventricular septum and portions of both ventricles. Figure 19.15 presents views of the coronary circulation from both the anterior and posterior views.
Figure 19.15 Coronary Circulation The anterior view of the heart shows the prominent coronary surface vessels. The posterior view of the heart shows the prominent coronary surface vessels.
DISEASES OF THE...
Heart: Myocardial Infarction
Myocardial infarction (MI) is the formal term for what is commonly referred to as a heart attack. It normally results from a lack of blood flow (ischemia) and oxygen (hypoxia) to a region of the heart, resulting in death of the cardiac muscle cells. An MI often occurs when a coronary artery is blocked by the buildup of atherosclerotic plaque consisting of lipids, cholesterol and fatty acids, and white blood cells, primarily macrophages. It can also occur when a portion of an unstable atherosclerotic plaque travels through the coronary arterial system and lodges in one of the smaller vessels. The resulting blockage restricts the flow of blood and oxygen to the myocardium and causes death of the tissue. MIs may be triggered by excessive exercise, in which the partially occluded artery is no longer able to pump sufficient quantities of blood, or severe stress, which may induce spasm of the smooth muscle in the walls of the vessel.
In the case of acute MI, there is often sudden pain beneath the sternum (retrosternal pain) called angina pectoris, often radiating down the left arm in males but not in female patients. Until this anomaly between the sexes was discovered, many female patients suffering MIs were misdiagnosed and sent home. In addition, patients typically present with difficulty breathing and shortness of breath (dyspnea), irregular heartbeat (palpations), nausea and vomiting, sweating (diaphoresis), anxiety, and fainting (syncope), although not all of these symptoms may be present. Many of the symptoms are shared with other medical conditions, including anxiety attacks and simple indigestion, so differential diagnosis is critical. It is estimated that between 22 and 64 percent of MIs present without any symptoms.
An MI can be confirmed by examining the patient’s ECG, which frequently reveals alterations in the ST and Q components. Some classification schemes of MI are referred to as ST-elevated MI (STEMI) and non-elevated MI (non-STEMI). In addition, echocardiography or cardiac magnetic resonance imaging may be employed. Common blood tests indicating an MI include elevated levels of creatine kinase MB (an enzyme that catalyzes the conversion of creatine to phosphocreatine, consuming ATP) and cardiac troponin (the regulatory protein for muscle contraction), both of which are released by damaged cardiac muscle cells.
Immediate treatments for MI are essential and include administering supplemental oxygen, aspirin that helps to break up clots, and nitroglycerine administered sublingually (under the tongue) to facilitate its absorption. Despite its unquestioned success in treatments and use since the 1880s, the mechanism of nitroglycerine is still incompletely understood but is believed to involve the release of nitric oxide, a known vasodilator, and endothelium-derived releasing factor, which also relaxes the smooth muscle in the tunica media of coronary vessels. Longer-term treatments include injections of thrombolytic agents such as streptokinase that dissolve the clot, the anticoagulant heparin, balloon angioplasty and stents to open blocked vessels, and bypass surgery to allow blood to pass around the site of blockage. If the damage is extensive, coronary replacement with a donor heart or coronary assist device, a sophisticated mechanical device that supplements the pumping activity of the heart, may be employed. Despite the attention, development of artificial hearts to augment the severely limited supply of heart donors has proven less than satisfactory but will likely improve in the future.
MIs may trigger cardiac arrest, but the two are not synonymous. Important risk factors for MI include cardiovascular disease, age, smoking, high blood levels of the low-density lipoprotein (LDL, often referred to as “bad” cholesterol), low levels of high-density lipoprotein (HDL, or “good” cholesterol), hypertension, diabetes mellitus, obesity, lack of physical exercise, chronic kidney disease, excessive alcohol consumption, and use of illegal drugs.
Coronary Veins
Coronary veins drain the heart and generally parallel the large surface arteries (see Figure 19.15). The great cardiac vein can be seen initially on the surface of the heart following the interventricular sulcus, but it eventually flows along the coronary sulcus into the coronary sinus on the posterior surface. The great cardiac vein initially parallels the anterior interventricular artery and drains the areas supplied by this vessel. It receives several major branches, including the posterior cardiac vein, the middle cardiac vein, and the small cardiac vein. The posterior cardiac vein parallels and drains the areas supplied by the marginal artery branch of the circumflex artery. The middle cardiac vein parallels and drains the areas supplied by the posterior interventricular artery. The small cardiac vein parallels the right coronary artery and drains the blood from the posterior surfaces of the right atrium and ventricle. The coronary sinus is a large, thin-walled vein on the posterior surface of the heart lying within the atrioventricular sulcus and emptying directly into the right atrium. The anterior cardiac veins parallel the small cardiac arteries and drain the anterior surface of the right ventricle. Unlike these other cardiac veins, it bypasses the coronary sinus and drains directly into the right atrium.
DISORDERS OF THE...
Heart: Coronary Artery Disease
Coronary artery disease is the leading cause of death worldwide. It occurs when the buildup of plaque—a fatty material including cholesterol, connective tissue, white blood cells, and some smooth muscle cells—within the walls of the arteries obstructs the flow of blood and decreases the flexibility or compliance of the vessels. This condition is called atherosclerosis, a hardening of the arteries that involves the accumulation of plaque. As the coronary blood vessels become occluded, the flow of blood to the tissues will be restricted, a condition called ischemia that causes the cells to receive insufficient amounts of oxygen, called hypoxia. Figure 19.16 shows the blockage of coronary arteries highlighted by the injection of dye. Some individuals with coronary artery disease report pain radiating from the chest called angina pectoris, but others remain asymptomatic. If untreated, coronary artery disease can lead to MI or a heart attack.
Figure 19.16 Atherosclerotic Coronary Arteries In this coronary angiogram (X-ray), the dye makes visible two occluded coronary arteries. Such blockages can lead to decreased blood flow (ischemia) and insufficient oxygen (hypoxia) delivered to the cardiac tissues. If uncorrected, this can lead to cardiac muscle death (myocardial infarction).
The disease progresses slowly and often begins in children and can be seen as fatty “streaks” in the vessels. It then gradually progresses throughout life. Well-documented risk factors include smoking, family history, hypertension, obesity, diabetes, high alcohol consumption, lack of exercise, stress, and hyperlipidemia or high circulating levels of lipids in the blood. Treatments may include medication, changes to diet and exercise, angioplasty with a balloon catheter, insertion of a stent, or coronary bypass procedure.
Angioplasty is a procedure in which the occlusion is mechanically widened with a balloon. A specialized catheter with an expandable tip is inserted into a superficial vessel, normally in the leg, and then directed to the site of the occlusion. At this point, the balloon is inflated to compress the plaque material and to open the vessel to increase blood flow. Then, the balloon is deflated and retracted. A stent consisting of a specialized mesh is typically inserted at the site of occlusion to reinforce the weakened and damaged walls. Stent insertions have been routine in cardiology for more than 40 years.
Coronary bypass surgery may also be performed. This surgical procedure grafts a replacement vessel obtained from another, less vital portion of the body to bypass the occluded area. This procedure is clearly effective in treating patients experiencing a MI, but overall does not increase longevity. Nor does it seem advisable in patients with stable although diminished cardiac capacity since frequently loss of mental acuity occurs following the procedure. Long-term changes to behavior, emphasizing diet and exercise plus a medicine regime tailored to lower blood pressure, lower cholesterol and lipids, and reduce clotting are equally as effective.
Cardiac Muscle and Electrical Activity
- Describe the structure of cardiac muscle
- Identify and describe the components of the conducting system that distributes electrical impulses through the heart
- Compare the effect of ion movement on membrane potential of cardiac conductive and contractile cells
- Relate characteristics of an electrocardiogram to events in the cardiac cycle
- Identify blocks that can interrupt the cardiac cycle
Recall that cardiac muscle shares a few characteristics with both skeletal muscle and smooth muscle, but it has some unique properties of its own. Not the least of these exceptional properties is its ability to initiate an electrical potential at a fixed rate that spreads rapidly from cell to cell to trigger the contractile mechanism. This property is known as autorhythmicity. Neither smooth nor skeletal muscle can do this. Even though cardiac muscle has autorhythmicity, heart rate is modulated by the endocrine and nervous systems.
There are two major types of cardiac muscle cells: myocardial contractile cells and myocardial conducting cells. The myocardial contractile cells constitute the bulk (99 percent) of the cells in the atria and ventricles. Contractile cells conduct impulses and are responsible for contractions that pump blood through the body. The myocardial conducting cells (1 percent of the cells) form the conduction system of the heart. Except for Purkinje cells, they are generally much smaller than the contractile cells and have few of the myofibrils or filaments needed for contraction. Their function is similar in many respects to neurons, although they are specialized muscle cells. Myocardial conduction cells initiate and propagate the action potential (the electrical impulse) that travels throughout the heart and triggers the contractions that propel the blood.
Structure of Cardiac Muscle
Compared to the giant cylinders of skeletal muscle, cardiac muscle cells, or cardiomyocytes, are considerably shorter with much smaller diameters. Cardiac muscle also demonstrates striations, the alternating pattern of dark A bands and light I bands attributed to the precise arrangement of the myofilaments and fibrils that are organized in sarcomeres along the length of the cell (Figure 19.17a). These contractile elements are virtually identical to skeletal muscle. T (transverse) tubules penetrate from the surface plasma membrane, the sarcolemma, to the interior of the cell, allowing the electrical impulse to reach the interior. The T tubules are only found at the Z discs, whereas in skeletal muscle, they are found at the junction of the A and I bands. Therefore, there are one-half as many T tubules in cardiac muscle as in skeletal muscle. In addition, the sarcoplasmic reticulum stores few calcium ions, so most of the calcium ions must come from outside the cells. The result is a slower onset of contraction. Mitochondria are plentiful, providing energy for the contractions of the heart. Typically, cardiomyocytes have a single, central nucleus, but two or more nuclei may be found in some cells.
Cardiac muscle cells branch freely. A junction between two adjoining cells is marked by a critical structure called an intercalated disc, which helps support the synchronized contraction of the muscle (Figure 19.17b). The sarcolemmas from adjacent cells bind together at the intercalated discs. They consist of desmosomes, specialized linking proteoglycans, tight junctions, and large numbers of gap junctions that allow the passage of ions between the cells and help to synchronize the contraction (Figure 19.17c). Intercellular connective tissue also helps to bind the cells together. The importance of strongly binding these cells together is necessitated by the forces exerted by contraction.
Figure 19.17 Cardiac Muscle (a) Cardiac muscle cells have myofibrils composed of myofilaments arranged in sarcomeres, T tubules to transmit the impulse from the sarcolemma to the interior of the cell, numerous mitochondria for energy, and intercalated discs that are found at the junction of different cardiac muscle cells. (b) A photomicrograph of cardiac muscle cells shows the nuclei and intercalated discs. (c) An intercalated disc connects cardiac muscle cells and consists of desmosomes and gap junctions. LM × 1600. (Micrograph provided by the Regents of the University of Michigan Medical School © 2012)
Cardiac muscle undergoes aerobic respiration patterns, primarily metabolizing lipids and carbohydrates. Myoglobin, lipids, and glycogen are all stored within the cytoplasm. Cardiac muscle cells undergo twitch-type contractions with long refractory periods followed by brief relaxation periods. The relaxation is essential so the heart can fill with blood for the next cycle. The refractory period is very long to prevent the possibility of tetany, a condition in which muscle remains involuntarily contracted. In the heart, tetany is not compatible with life, since it would prevent the heart from pumping blood.
EVERYDAY CONNECTION
Repair and Replacement
Damaged cardiac muscle cells have extremely limited abilities to repair themselves or to replace dead cells via mitosis. Recent evidence indicates that at least some stem cells remain within the heart that continue to divide and at least potentially replace these dead cells. However, newly formed or repaired cells are rarely as functional as the original cells, and cardiac function is reduced. In the event of a heart attack or MI, dead cells are often replaced by patches of scar tissue. Autopsies performed on individuals who had successfully received heart transplants show some proliferation of original cells. If researchers can unlock the mechanism that generates new cells and restore full mitotic capabilities to heart muscle, the prognosis for heart attack survivors will be greatly enhanced. To date, myocardial cells produced within the patient (in situ) by cardiac stem cells seem to be nonfunctional, although those grown in Petri dishes (in vitro) do beat. Perhaps soon this mystery will be solved, and new advances in treatment will be commonplace.
Conduction System of the Heart
If embryonic heart cells are separated into a Petri dish and kept alive, each is capable of generating its own electrical impulse followed by contraction. When two independently beating embryonic cardiac muscle cells are placed together, the cell with the higher inherent rate sets the pace, and the impulse spreads from the faster to the slower cell to trigger a contraction. As more cells are joined together, the fastest cell continues to assume control of the rate. A fully developed adult heart maintains the capability of generating its own electrical impulse, triggered by the fastest cells, as part of the cardiac conduction system. The components of the cardiac conduction system include the sinoatrial node, the atrioventricular node, the atrioventricular bundle, the atrioventricular bundle branches, and the Purkinje cells (Figure 19.18).
Figure 19.18 Conduction System of the Heart Specialized conducting components of the heart include the sinoatrial node, the internodal pathways, the atrioventricular node, the atrioventricular bundle, the right and left bundle branches, and the Purkinje fibers.
Sinoatrial (SA) Node
Normal cardiac rhythm is established by the sinoatrial (SA) node, a specialized clump of myocardial conducting cells located in the superior and posterior walls of the right atrium in close proximity to the orifice of the superior vena cava. The SA node has the highest inherent rate of depolarization and is known as the pacemaker of the heart. It initiates the sinus rhythm, or normal electrical pattern followed by contraction of the heart.
This impulse spreads from its initiation in the SA node throughout the atria through specialized internodal pathways, to the atrial myocardial contractile cells and the atrioventricular node. The internodal pathways consist of three bands (anterior, middle, and posterior) that lead directly from the SA node to the next node in the conduction system, the atrioventricular node (see Figure 19.18). The impulse takes approximately 50 ms (milliseconds) to travel between these two nodes. The relative importance of this pathway has been debated since the impulse would reach the atrioventricular node simply following the cell-by-cell pathway through the contractile cells of the myocardium in the atria. In addition, there is a specialized pathway called Bachmann’s bundle or the interatrial band that conducts the impulse directly from the right atrium to the left atrium. Regardless of the pathway, as the impulse reaches the atrioventricular septum, the connective tissue of the cardiac skeleton prevents the impulse from spreading into the myocardial cells in the ventricles except at the atrioventricular node. Figure 19.19illustrates the initiation of the impulse in the SA node that then spreads the impulse throughout the atria to the atrioventricular node.
Figure 19.19 Cardiac Conduction (1) The sinoatrial (SA) node and the remainder of the conduction system are at rest. (2) The SA node initiates the action potential, which sweeps across the atria. (3) After reaching the atrioventricular node, there is a delay of approximately 100 ms that allows the atria to complete pumping blood before the impulse is transmitted to the atrioventricular bundle. (4) Following the delay, the impulse travels through the atrioventricular bundle and bundle branches to the Purkinje fibers, and also reaches the right papillary muscle via the moderator band. (5) The impulse spreads to the contractile fibers of the ventricle. (6) Ventricular contraction begins.
The electrical event, the wave of depolarization, is the trigger for muscular contraction. The wave of depolarization begins in the right atrium, and the impulse spreads across the superior portions of both atria and then down through the contractile cells. The contractile cells then begin contraction from the superior to the inferior portions of the atria, efficiently pumping blood into the ventricles.
Atrioventricular (AV) Node
The atrioventricular (AV) node is a second clump of specialized myocardial conductive cells, located in the inferior portion of the right atrium within the atrioventricular septum. The septum prevents the impulse from spreading directly to the ventricles without passing through the AV node. There is a critical pause before the AV node depolarizes and transmits the impulse to the atrioventricular bundle (see Figure 19.19, step 3). This delay in transmission is partially attributable to the small diameter of the cells of the node, which slow the impulse. Also, conduction between nodal cells is less efficient than between conducting cells. These factors mean that it takes the impulse approximately 100 ms to pass through the node. This pause is critical to heart function, as it allows the atrial cardiomyocytes to complete their contraction that pumps blood into the ventricles before the impulse is transmitted to the cells of the ventricle itself. With extreme stimulation by the SA node, the AV node can transmit impulses maximally at 220 per minute. This establishes the typical maximum heart rate in a healthy young individual. Damaged hearts or those stimulated by drugs can contract at higher rates, but at these rates, the heart can no longer effectively pump blood.
Atrioventricular Bundle (Bundle of His), Bundle Branches, and Purkinje Fibers
Arising from the AV node, the atrioventricular bundle, or bundle of His, proceeds through the interventricular septum before dividing into two atrioventricular bundle branches, commonly called the left and right bundle branches. The left bundle branch has two fascicles. The left bundle branch supplies the left ventricle, and the right bundle branch the right ventricle. Since the left ventricle is much larger than the right, the left bundle branch is also considerably larger than the right. Portions of the right bundle branch are found in the moderator band and supply the right papillary muscles. Because of this connection, each papillary muscle receives the impulse at approximately the same time, so they begin to contract simultaneously just prior to the remainder of the myocardial contractile cells of the ventricles. This is believed to allow tension to develop on the chordae tendineae prior to right ventricular contraction. There is no corresponding moderator band on the left. Both bundle branches descend and reach the apex of the heart where they connect with the Purkinje fibers (see Figure 19.19, step 4). This passage takes approximately 25 ms.
The Purkinje fibers are additional myocardial conductive fibers that spread the impulse to the myocardial contractile cells in the ventricles. They extend throughout the myocardium from the apex of the heart toward the atrioventricular septum and the base of the heart. The Purkinje fibers have a fast inherent conduction rate, and the electrical impulse reaches all of the ventricular muscle cells in about 75 ms (see Figure 19.19, step 5). Since the electrical stimulus begins at the apex, the contraction also begins at the apex and travels toward the base of the heart, similar to squeezing a tube of toothpaste from the bottom. This allows the blood to be pumped out of the ventricles and into the aorta and pulmonary trunk. The total time elapsed from the initiation of the impulse in the SA node until depolarization of the ventricles is approximately 225 ms.
Membrane Potentials and Ion Movement in Cardiac Conductive Cells
Action potentials are considerably different between cardiac conductive cells and cardiac contractive cells. While Na+ and K+play essential roles, Ca2+ is also critical for both types of cells. Unlike skeletal muscles and neurons, cardiac conductive cells do not have a stable resting potential. Conductive cells contain a series of sodium ion channels that allow a normal and slow influx of sodium ions that causes the membrane potential to rise slowly from an initial value of −60 mV up to about –40 mV. The resulting movement of sodium ions creates spontaneous depolarization (or prepotential depolarization). At this point, calcium ion channels open and Ca2+ enters the cell, further depolarizing it at a more rapid rate until it reaches a value of approximately +15 mV. At this point, the calcium ion channels close and K+ channels open, allowing outflux of K+ and resulting in repolarization. When the membrane potential reaches approximately −60 mV, the K+ channels close and Na+ channels open, and the prepotential phase begins again. This phenomenon explains the autorhythmicity properties of cardiac muscle (Figure 19.20).
Figure 19.20 Action Potential at the SA Node The prepotential is due to a slow influx of sodium ions until the threshold is reached followed by a rapid depolarization and repolarization. The prepotential accounts for the membrane reaching threshold and initiates the spontaneous depolarization and contraction of the cell. Note the lack of a resting potential.
Membrane Potentials and Ion Movement in Cardiac Contractile Cells
There is a distinctly different electrical pattern involving the contractile cells. In this case, there is a rapid depolarization, followed by a plateau phase and then repolarization. This phenomenon accounts for the long refractory periods required for the cardiac muscle cells to pump blood effectively before they are capable of firing for a second time. These cardiac myocytes normally do not initiate their own electrical potential but rather wait for an impulse to reach them.
Contractile cells demonstrate a much more stable resting phase than conductive cells at approximately −80 mV for cells in the atria and −90 mV for cells in the ventricles. Despite this initial difference, the other components of their action potentials are virtually identical. In both cases, when stimulated by an action potential, voltage-gated channels rapidly open, beginning the positive-feedback mechanism of depolarization. This rapid influx of positively charged ions raises the membrane potential to approximately +30 mV, at which point the sodium channels close. The rapid depolarization period typically lasts 3–5 ms. Depolarization is followed by the plateau phase, in which membrane potential declines relatively slowly. This is due in large part to the opening of the slow Ca2+ channels, allowing Ca2+ to enter the cell while few K+ channels are open, allowing K+ to exit the cell. The relatively long plateau phase lasts approximately 175 ms. Once the membrane potential reaches approximately zero, the Ca2+ channels close and K+ channels open, allowing K+ to exit the cell. The repolarization lasts approximately 75 ms. At this point, membrane potential drops until it reaches resting levels once more and the cycle repeats. The entire event lasts between 250 and 300 ms (Figure 19.21).
The absolute refractory period for cardiac contractile muscle lasts approximately 200 ms, and the relative refractory period lasts approximately 50 ms, for a total of 250 ms. This extended period is critical, since the heart muscle must contract to pump blood effectively and the contraction must follow the electrical events. Without extended refractory periods, premature contractions would occur in the heart and would not be compatible with life.
Figure 19.21 Action Potential in Cardiac Contractile Cells (a) Note the long plateau phase due to the influx of calcium ions. The extended refractory period allows the cell to fully contract before another electrical event can occur. (b) The action potential for heart muscle is compared to that of skeletal muscle.
Calcium Ions
Calcium ions play two critical roles in the physiology of cardiac muscle. Their influx through slow calcium channels accounts for the prolonged plateau phase and absolute refractory period that enable cardiac muscle to function properly. Calcium ions also combine with the regulatory protein troponin in the troponin-tropomyosin complex; this complex removes the inhibition that prevents the heads of the myosin molecules from forming cross bridges with the active sites on actin that provide the power stroke of contraction. This mechanism is virtually identical to that of skeletal muscle. Approximately 20 percent of the calcium required for contraction is supplied by the influx of Ca2+ during the plateau phase. The remaining Ca2+ for contraction is released from storage in the sarcoplasmic reticulum.
Comparative Rates of Conduction System Firing
The pattern of prepotential or spontaneous depolarization, followed by rapid depolarization and repolarization just described, are seen in the SA node and a few other conductive cells in the heart. Since the SA node is the pacemaker, it reaches threshold faster than any other component of the conduction system. It will initiate the impulses spreading to the other conducting cells. The SA node, without nervous or endocrine control, would initiate a heart impulse approximately 80–100 times per minute. Although each component of the conduction system is capable of generating its own impulse, the rate progressively slows as you proceed from the SA node to the Purkinje fibers. Without the SA node, the AV node would generate a heart rate of 40–60 beats per minute. If the AV node were blocked, the atrioventricular bundle would fire at a rate of approximately 30–40 impulses per minute. The bundle branches would have an inherent rate of 20–30 impulses per minute, and the Purkinje fibers would fire at 15–20 impulses per minute. While a few exceptionally trained aerobic athletes demonstrate resting heart rates in the range of 30–40 beats per minute (the lowest recorded figure is 28 beats per minute for Miguel Indurain, a cyclist), for most individuals, rates lower than 50 beats per minute would indicate a condition called bradycardia. Depending upon the specific individual, as rates fall much below this level, the heart would be unable to maintain adequate flow of blood to vital tissues, initially resulting in decreasing loss of function across the systems, unconsciousness, and ultimately death.
Electrocardiogram
By careful placement of surface electrodes on the body, it is possible to record the complex, compound electrical signal of the heart. This tracing of the electrical signal is the electrocardiogram (ECG), also commonly abbreviated EKG (K coming kardiology, from the German term for cardiology). Careful analysis of the ECG reveals a detailed picture of both normal and abnormal heart function, and is an indispensable clinical diagnostic tool. The standard electrocardiograph (the instrument that generates an ECG) uses 3, 5, or 12 leads. The greater the number of leads an electrocardiograph uses, the more information the ECG provides. The term “lead” may be used to refer to the cable from the electrode to the electrical recorder, but it typically describes the voltage difference between two of the electrodes. The 12-lead electrocardiograph uses 10 electrodes placed in standard locations on the patient’s skin (Figure 19.22). In continuous ambulatory electrocardiographs, the patient wears a small, portable, battery-operated device known as a Holter monitor, or simply a Holter, that continuously monitors heart electrical activity, typically for a period of 24 hours during the patient’s normal routine.
Figure 19.22 Standard Placement of ECG Leads In a 12-lead ECG, six electrodes are placed on the chest, and four electrodes are placed on the limbs.
A normal ECG tracing is presented in Figure 19.23. Each component, segment, and interval is labeled and corresponds to important electrical events, demonstrating the relationship between these events and contraction in the heart.
There are five prominent points on the ECG: the P wave, the QRS complex, and the T wave. The small P wave represents the depolarization of the atria. The atria begin contracting approximately 25 ms after the start of the P wave. The large QRS complex represents the depolarization of the ventricles, which requires a much stronger electrical signal because of the larger size of the ventricular cardiac muscle. The ventricles begin to contract as the QRS reaches the peak of the R wave. Lastly, the T wave represents the repolarization of the ventricles. The repolarization of the atria occurs during the QRS complex, which masks it on an ECG.
The major segments and intervals of an ECG tracing are indicated in Figure 19.23. Segments are defined as the regions between two waves. Intervals include one segment plus one or more waves. For example, the PR segment begins at the end of the P wave and ends at the beginning of the QRS complex. The PR interval starts at the beginning of the P wave and ends with the beginning of the QRS complex. The PR interval is more clinically relevant, as it measures the duration from the beginning of atrial depolarization (the P wave) to the initiation of the QRS complex. Since the Q wave may be difficult to view in some tracings, the measurement is often extended to the R that is more easily visible. Should there be a delay in passage of the impulse from the SA node to the AV node, it would be visible in the PR interval. Figure 19.24 correlates events of heart contraction to the corresponding segments and intervals of an ECG.
INTERACTIVE LINK
Visit this site for a more detailed analysis of ECGs.
Figure 19.23 Electrocardiogram A normal tracing shows the P wave, QRS complex, and T wave. Also indicated are the PR, QT, QRS, and ST intervals, plus the P-R and S-T segments.
Figure 19.24 ECG Tracing Correlated to the Cardiac Cycle This diagram correlates an ECG tracing with the electrical and mechanical events of a heart contraction. Each segment of an ECG tracing corresponds to one event in the cardiac cycle.
EVERYDAY CONNECTION
ECG Abnormalities
Occassionally, an area of the heart other than the SA node will initiate an impulse that will be followed by a premature contraction. Such an area, which may actually be a component of the conduction system or some other contractile cells, is known as an ectopic focus or ectopic pacemaker. An ectopic focus may be stimulated by localized ischemia; exposure to certain drugs, including caffeine, digitalis, or acetylcholine; elevated stimulation by both sympathetic or parasympathetic divisions of the autonomic nervous system; or a number of disease or pathological conditions. Occasional occurances are generally transitory and nonlife threatening, but if the condition becomes chronic, it may lead to either an arrhythmia, a deviation from the normal pattern of impulse conduction and contraction, or to fibrillation, an uncoordinated beating of the heart.
While interpretation of an ECG is possible and extremely valuable after some training, a full understanding of the complexities and intricacies generally requires several years of experience. In general, the size of the electrical variations, the duration of the events, and detailed vector analysis provide the most comprehensive picture of cardiac function. For example, an amplified P wave may indicate enlargement of the atria, an enlarged Q wave may indicate a MI, and an enlarged suppressed or inverted Q wave often indicates enlarged ventricles. T waves often appear flatter when insufficient oxygen is being delivered to the myocardium. An elevation of the ST segment above baseline is often seen in patients with an acute MI, and may appear depressed below the baseline when hypoxia is occurring.
As useful as analyzing these electrical recordings may be, there are limitations. For example, not all areas suffering a MI may be obvious on the ECG. Additionally, it will not reveal the effectiveness of the pumping, which requires further testing, such as an ultrasound test called an echocardiogram or nuclear medicine imaging. It is also possible for there to be pulseless electrical activity, which will show up on an ECG tracing, although there is no corresponding pumping action. Common abnormalities that may be detected by the ECGs are shown in Figure 19.25.
Figure 19.25 Common ECG Abnormalities (a) In a second-degree or partial block, one-half of the P waves are not followed by the QRS complex and T waves while the other half are. (b) In atrial fibrillation, the electrical pattern is abnormal prior to the QRS complex, and the frequency between the QRS complexes has increased. (c) In ventricular tachycardia, the shape of the QRS complex is abnormal. (d) In ventricular fibrillation, there is no normal electrical activity. (e) In a third-degree block, there is no correlation between atrial activity (the P wave) and ventricular activity (the QRS complex).
INTERACTIVE LINK
Visit this site for a more complete library of abnormal ECGs.
EVERYDAY CONNECTION
External Automated Defibrillators
In the event that the electrical activity of the heart is severely disrupted, cessation of electrical activity or fibrillation may occur. In fibrillation, the heart beats in a wild, uncontrolled manner, which prevents it from being able to pump effectively. Atrial fibrillation (see Figure 19.25b) is a serious condition, but as long as the ventricles continue to pump blood, the patient’s life may not be in immediate danger. Ventricular fibrillation (see Figure 19.25d) is a medical emergency that requires life support, because the ventricles are not effectively pumping blood. In a hospital setting, it is often described as “code blue.” If untreated for as little as a few minutes, ventricular fibrillation may lead to brain death. The most common treatment is defibrillation, which uses special paddles to apply a charge to the heart from an external electrical source in an attempt to establish a normal sinus rhythm (Figure 19.26). A defibrillator effectively stops the heart so that the SA node can trigger a normal conduction cycle. Because of their effectiveness in reestablishing a normal sinus rhythm, external automated defibrillators (EADs) are being placed in areas frequented by large numbers of people, such as schools, restaurants, and airports. These devices contain simple and direct verbal instructions that can be followed by nonmedical personnel in an attempt to save a life.
Figure 19.26 Defibrillators (a) An external automatic defibrillator can be used by nonmedical personnel to reestablish a normal sinus rhythm in a person with fibrillation. (b) Defibrillator paddles are more commonly used in hospital settings. (credit b: “widerider107”/flickr.com)
A heart block refers to an interruption in the normal conduction pathway. The nomenclature for these is very straightforward. SA nodal blocks occur within the SA node. AV nodal blocks occur within the AV node. Infra-Hisian blocks involve the bundle of His. Bundle branch blocks occur within either the left or right atrioventricular bundle branches. Hemiblocks are partial and occur within one or more fascicles of the atrioventricular bundle branch. Clinically, the most common types are the AV nodal and infra-Hisian blocks.
AV blocks are often described by degrees. A first-degree or partial block indicates a delay in conduction between the SA and AV nodes. This can be recognized on the ECG as an abnormally long PR interval. A second-degree or incomplete block occurs when some impulses from the SA node reach the AV node and continue, while others do not. In this instance, the ECG would reveal some P waves not followed by a QRS complex, while others would appear normal. In the third-degree or complete block, there is no correlation between atrial activity (the P wave) and ventricular activity (the QRS complex). Even in the event of a total SA block, the AV node will assume the role of pacemaker and continue initiating contractions at 40–60 contractions per minute, which is adequate to maintain consciousness. Second- and third-degree blocks are demonstrated on the ECG presented in Figure 19.25.
When arrhythmias become a chronic problem, the heart maintains a junctional rhythm, which originates in the AV node. In order to speed up the heart rate and restore full sinus rhythm, a cardiologist can implant an artificial pacemaker, which delivers electrical impulses to the heart muscle to ensure that the heart continues to contract and pump blood effectively. These artificial pacemakers are programmable by the cardiologists and can either provide stimulation temporarily upon demand or on a continuous basis. Some devices also contain built-in defibrillators.
Cardiac Muscle Metabolism
Normally, cardiac muscle metabolism is entirely aerobic. Oxygen from the lungs is brought to the heart, and every other organ, attached to the hemoglobin molecules within the erythrocytes. Heart cells also store appreciable amounts of oxygen in myoglobin. Normally, these two mechanisms, circulating oxygen and oxygen attached to myoglobin, can supply sufficient oxygen to the heart, even during peak performance.
Fatty acids and glucose from the circulation are broken down within the mitochondria to release energy in the form of ATP. Both fatty acid droplets and glycogen are stored within the sarcoplasm and provide additional nutrient supply. (Seek additional content for more detail about metabolism.)
Cardiac Cycle
- Describe the relationship between blood pressure and blood flow
- Summarize the events of the cardiac cycle
- Compare atrial and ventricular systole and diastole
- Relate heart sounds detected by auscultation to action of heart’s valves
The period of time that begins with contraction of the atria and ends with ventricular relaxation is known as the cardiac cycle(Figure 19.27). The period of contraction that the heart undergoes while it pumps blood into circulation is called systole. The period of relaxation that occurs as the chambers fill with blood is called diastole. Both the atria and ventricles undergo systole and diastole, and it is essential that these components be carefully regulated and coordinated to ensure blood is pumped efficiently to the body.
Figure 19.27 Overview of the Cardiac Cycle The cardiac cycle begins with atrial systole and progresses to ventricular systole, atrial diastole, and ventricular diastole, when the cycle begins again. Correlations to the ECG are highlighted.
Pressures and Flow
Fluids, whether gases or liquids, are materials that flow according to pressure gradients—that is, they move from regions that are higher in pressure to regions that are lower in pressure. Accordingly, when the heart chambers are relaxed (diastole), blood will flow into the atria from the veins, which are higher in pressure. As blood flows into the atria, the pressure will rise, so the blood will initially move passively from the atria into the ventricles. When the action potential triggers the muscles in the atria to contract (atrial systole), the pressure within the atria rises further, pumping blood into the ventricles. During ventricular systole, pressure rises in the ventricles, pumping blood into the pulmonary trunk from the right ventricle and into the aorta from the left ventricle. Again, as you consider this flow and relate it to the conduction pathway, the elegance of the system should become apparent.
Phases of the Cardiac Cycle
At the beginning of the cardiac cycle, both the atria and ventricles are relaxed (diastole). Blood is flowing into the right atrium from the superior and inferior venae cavae and the coronary sinus. Blood flows into the left atrium from the four pulmonary veins. The two atrioventricular valves, the tricuspid and mitral valves, are both open, so blood flows unimpeded from the atria and into the ventricles. Approximately 70–80 percent of ventricular filling occurs by this method. The two semilunar valves, the pulmonary and aortic valves, are closed, preventing backflow of blood into the right and left ventricles from the pulmonary trunk on the right and the aorta on the left.
Atrial Systole and Diastole
Contraction of the atria follows depolarization, represented by the P wave of the ECG. As the atrial muscles contract from the superior portion of the atria toward the atrioventricular septum, pressure rises within the atria and blood is pumped into the ventricles through the open atrioventricular (tricuspid, and mitral or bicuspid) valves. At the start of atrial systole, the ventricles are normally filled with approximately 70–80 percent of their capacity due to inflow during diastole. Atrial contraction, also referred to as the “atrial kick,” contributes the remaining 20–30 percent of filling (see Figure 19.27). Atrial systole lasts approximately 100 ms and ends prior to ventricular systole, as the atrial muscle returns to diastole.
Ventricular Systole
Ventricular systole (see Figure 19.27) follows the depolarization of the ventricles and is represented by the QRS complex in the ECG. It may be conveniently divided into two phases, lasting a total of 270 ms. At the end of atrial systole and just prior to atrial contraction, the ventricles contain approximately 130 mL blood in a resting adult in a standing position. This volume is known as the end diastolic volume (EDV) or preload.
Initially, as the muscles in the ventricle contract, the pressure of the blood within the chamber rises, but it is not yet high enough to open the semilunar (pulmonary and aortic) valves and be ejected from the heart. However, blood pressure quickly rises above that of the atria that are now relaxed and in diastole. This increase in pressure causes blood to flow back toward the atria, closing the tricuspid and mitral valves. Since blood is not being ejected from the ventricles at this early stage, the volume of blood within the chamber remains constant. Consequently, this initial phase of ventricular systole is known as isovolumic contraction, also called isovolumetric contraction (see Figure 19.27).
In the second phase of ventricular systole, the ventricular ejection phase, the contraction of the ventricular muscle has raised the pressure within the ventricle to the point that it is greater than the pressures in the pulmonary trunk and the aorta. Blood is pumped from the heart, pushing open the pulmonary and aortic semilunar valves. Pressure generated by the left ventricle will be appreciably greater than the pressure generated by the right ventricle, since the existing pressure in the aorta will be so much higher. Nevertheless, both ventricles pump the same amount of blood. This quantity is referred to as stroke volume. Stroke volume will normally be in the range of 70–80 mL. Since ventricular systole began with an EDV of approximately 130 mL of blood, this means that there is still 50–60 mL of blood remaining in the ventricle following contraction. This volume of blood is known as the end systolic volume (ESV).
Ventricular Diastole
Ventricular relaxation, or diastole, follows repolarization of the ventricles and is represented by the T wave of the ECG. It too is divided into two distinct phases and lasts approximately 430 ms.
During the early phase of ventricular diastole, as the ventricular muscle relaxes, pressure on the remaining blood within the ventricle begins to fall. When pressure within the ventricles drops below pressure in both the pulmonary trunk and aorta, blood flows back toward the heart, producing the dicrotic notch (small dip) seen in blood pressure tracings. The semilunar valves close to prevent backflow into the heart. Since the atrioventricular valves remain closed at this point, there is no change in the volume of blood in the ventricle, so the early phase of ventricular diastole is called the isovolumic ventricular relaxation phase, also called isovolumetric ventricular relaxation phase (see Figure 19.27).
In the second phase of ventricular diastole, called late ventricular diastole, as the ventricular muscle relaxes, pressure on the blood within the ventricles drops even further. Eventually, it drops below the pressure in the atria. When this occurs, blood flows from the atria into the ventricles, pushing open the tricuspid and mitral valves. As pressure drops within the ventricles, blood flows from the major veins into the relaxed atria and from there into the ventricles. Both chambers are in diastole, the atrioventricular valves are open, and the semilunar valves remain closed (see Figure 19.27). The cardiac cycle is complete.
Figure 19.28 illustrates the relationship between the cardiac cycle and the ECG.
Figure 19.28 Relationship between the Cardiac Cycle and ECG Initially, both the atria and ventricles are relaxed (diastole). The P wave represents depolarization of the atria and is followed by atrial contraction (systole). Atrial systole extends until the QRS complex, at which point, the atria relax. The QRS complex represents depolarization of the ventricles and is followed by ventricular contraction. The T wave represents the repolarization of the ventricles and marks the beginning of ventricular relaxation.
Heart Sounds
One of the simplest, yet effective, diagnostic techniques applied to assess the state of a patient’s heart is auscultation using a stethoscope.
In a normal, healthy heart, there are only two audible heart sounds: S1 and S2. S1 is the sound created by the closing of the atrioventricular valves during ventricular contraction and is normally described as a “lub,” or first heart sound. The second heart sound, S2, is the sound of the closing of the semilunar valves during ventricular diastole and is described as a “dub” (Figure 19.29). In both cases, as the valves close, the openings within the atrioventricular septum guarded by the valves will become reduced, and blood flow through the opening will become more turbulent until the valves are fully closed. There is a third heart sound, S3, but it is rarely heard in healthy individuals. It may be the sound of blood flowing into the atria, or blood sloshing back and forth in the ventricle, or even tensing of the chordae tendineae. S3 may be heard in youth, some athletes, and pregnant women. If the sound is heard later in life, it may indicate congestive heart failure, warranting further tests. Some cardiologists refer to the collective S1, S2, and S3 sounds as the “Kentucky gallop,” because they mimic those produced by a galloping horse. The fourth heart sound, S4, results from the contraction of the atria pushing blood into a stiff or hypertrophic ventricle, indicating failure of the left ventricle. S4 occurs prior to S1 and the collective sounds S4, S1, and S2 are referred to by some cardiologists as the “Tennessee gallop,” because of their similarity to the sound produced by a galloping horse with a different gait. A few individuals may have both S3 and S4, and this combined sound is referred to as S7.
Figure 19.29 Heart Sounds and the Cardiac Cycle In this illustration, the x-axis reflects time with a recording of the heart sounds. The y-axis represents pressure.
The term murmur is used to describe an unusual sound coming from the heart that is caused by the turbulent flow of blood. Murmurs are graded on a scale of 1 to 6, with 1 being the most common, the most difficult sound to detect, and the least serious. The most severe is a 6. Phonocardiograms or auscultograms can be used to record both normal and abnormal sounds using specialized electronic stethoscopes.
During auscultation, it is common practice for the clinician to ask the patient to breathe deeply. This procedure not only allows for listening to airflow, but it may also amplify heart murmurs. Inhalation increases blood flow into the right side of the heart and may increase the amplitude of right-sided heart murmurs. Expiration partially restricts blood flow into the left side of the heart and may amplify left-sided heart murmurs. Figure 19.30 indicates proper placement of the bell of the stethoscope to facilitate auscultation.
Figure 19.30 Stethoscope Placement for Auscultation Proper placement of the bell of the stethoscope facilitates auscultation. At each of the four locations on the chest, a different valve can be heard.
Cardiac Physiology
- Relate heart rate to cardiac output
- Describe the effect of exercise on heart rate
- Identify cardiovascular centers and cardiac reflexes that regulate heart function
- Describe factors affecting heart rate
- Distinguish between positive and negative factors that affect heart contractility
- Summarize factors affecting stroke volume and cardiac output
- Describe the cardiac response to variations in blood flow and pressure
The autorhythmicity inherent in cardiac cells keeps the heart beating at a regular pace; however, the heart is regulated by and responds to outside influences as well. Neural and endocrine controls are vital to the regulation of cardiac function. In addition, the heart is sensitive to several environmental factors, including electrolytes.
Resting Cardiac Output
Cardiac output (CO) is a measurement of the amount of blood pumped by each ventricle in one minute. To calculate this value, multiply stroke volume (SV), the amount of blood pumped by each ventricle, by heart rate (HR), in contractions per minute (or beats per minute, bpm). It can be represented mathematically by the following equation:
CO = HR × SV
SV is normally measured using an echocardiogram to record EDV and ESV, and calculating the difference: SV = EDV – ESV. SV can also be measured using a specialized catheter, but this is an invasive procedure and far more dangerous to the patient. A mean SV for a resting 70-kg (150-lb) individual would be approximately 70 mL. There are several important variables, including size of the heart, physical and mental condition of the individual, sex, contractility, duration of contraction, preload or EDV, and afterload or resistance. Normal range for SV would be 55–100 mL. An average resting HR would be approximately 75 bpm but could range from 60–100 in some individuals.
Using these numbers, the mean CO is 5.25 L/min, with a range of 4.0–8.0 L/min. Remember, however, that these numbers refer to CO from each ventricle separately, not the total for the heart. Factors influencing CO are summarized in Figure 19.31.
Figure 19.31 Major Factors Influencing Cardiac Output Cardiac output is influenced by heart rate and stroke volume, both of which are also variable.
SVs are also used to calculate ejection fraction, which is the portion of the blood that is pumped or ejected from the heart with each contraction. To calculate ejection fraction, SV is divided by EDV. Despite the name, the ejection fraction is normally expressed as a percentage. Ejection fractions range from approximately 55–70 percent, with a mean of 58 percent.
Exercise and Maximum Cardiac Output
In healthy young individuals, HR may increase to 150 bpm during exercise. SV can also increase from 70 to approximately 130 mL due to increased strength of contraction. This would increase CO to approximately 19.5 L/min, 4–5 times the resting rate. Top cardiovascular athletes can achieve even higher levels. At their peak performance, they may increase resting CO by 7–8 times.
Since the heart is a muscle, exercising it increases its efficiency. The difference between maximum and resting CO is known as the cardiac reserve. It measures the residual capacity of the heart to pump blood.
Heart Rates
HRs vary considerably, not only with exercise and fitness levels, but also with age. Newborn resting HRs may be 120 bpm. HR gradually decreases until young adulthood and then gradually increases again with age.
Maximum HRs are normally in the range of 200–220 bpm, although there are some extreme cases in which they may reach higher levels. As one ages, the ability to generate maximum rates decreases. This may be estimated by taking the maximal value of 220 bpm and subtracting the individual’s age. So a 40-year-old individual would be expected to hit a maximum rate of approximately 180, and a 60-year-old person would achieve a HR of 160.
DISORDERS OF THE...
Heart: Abnormal Heart Rates
For an adult, normal resting HR will be in the range of 60–100 bpm. Bradycardia is the condition in which resting rate drops below 60 bpm, and tachycardia is the condition in which the resting rate is above 100 bpm. Trained athletes typically have very low HRs. If the patient is not exhibiting other symptoms, such as weakness, fatigue, dizziness, fainting, chest discomfort, palpitations, or respiratory distress, bradycardia is not considered clinically significant. However, if any of these symptoms are present, they may indicate that the heart is not providing sufficient oxygenated blood to the tissues. The term relative bradycardia may be used with a patient who has a HR in the normal range but is still suffering from these symptoms. Most patients remain asymptomatic as long as the HR remains above 50 bpm.
Bradycardia may be caused by either inherent factors or causes external to the heart. While the condition may be inherited, typically it is acquired in older individuals. Inherent causes include abnormalities in either the SA or AV node. If the condition is serious, a pacemaker may be required. Other causes include ischemia to the heart muscle or diseases of the heart vessels or valves. External causes include metabolic disorders, pathologies of the endocrine system often involving the thyroid, electrolyte imbalances, neurological disorders including inappropriate autonomic responses, autoimmune pathologies, over-prescription of beta blocker drugs that reduce HR, recreational drug use, or even prolonged bed rest. Treatment relies upon establishing the underlying cause of the disorder and may necessitate supplemental oxygen.
Tachycardia is not normal in a resting patient but may be detected in pregnant women or individuals experiencing extreme stress. In the latter case, it would likely be triggered by stimulation from the limbic system or disorders of the autonomic nervous system. In some cases, tachycardia may involve only the atria. Some individuals may remain asymptomatic, but when present, symptoms may include dizziness, shortness of breath, lightheadedness, rapid pulse, heart palpations, chest pain, or fainting (syncope). While tachycardia is defined as a HR above 100 bpm, there is considerable variation among people. Further, the normal resting HRs of children are often above 100 bpm, but this is not considered to be tachycardia Many causes of tachycardia may be benign, but the condition may also be correlated with fever, anemia, hypoxia, hyperthyroidism, hypersecretion of catecholamines, some cardiomyopathies, some disorders of the valves, and acute exposure to radiation. Elevated rates in an exercising or resting patient are normal and expected. Resting rate should always be taken after recovery from exercise. Treatment depends upon the underlying cause but may include medications, implantable cardioverter defibrillators, ablation, or surgery.
Correlation Between Heart Rates and Cardiac Output
Initially, physiological conditions that cause HR to increase also trigger an increase in SV. During exercise, the rate of blood returning to the heart increases. However as the HR rises, there is less time spent in diastole and consequently less time for the ventricles to fill with blood. Even though there is less filling time, SV will initially remain high. However, as HR continues to increase, SV gradually decreases due to decreased filling time. CO will initially stabilize as the increasing HR compensates for the decreasing SV, but at very high rates, CO will eventually decrease as increasing rates are no longer able to compensate for the decreasing SV. Consider this phenomenon in a healthy young individual. Initially, as HR increases from resting to approximately 120 bpm, CO will rise. As HR increases from 120 to 160 bpm, CO remains stable, since the increase in rate is offset by decreasing ventricular filling time and, consequently, SV. As HR continues to rise above 160 bpm, CO actually decreases as SV falls faster than HR increases. So although aerobic exercises are critical to maintain the health of the heart, individuals are cautioned to monitor their HR to ensure they stay within the target heart rate range of between 120 and 160 bpm, so CO is maintained. The target HR is loosely defined as the range in which both the heart and lungs receive the maximum benefit from the aerobic workout and is dependent upon age.
Cardiovascular Centers
Nervous control over HR is centralized within the two paired cardiovascular centers of the medulla oblongata (Figure 19.32). The cardioaccelerator regions stimulate activity via sympathetic stimulation of the cardioaccelerator nerves, and the cardioinhibitory centers decrease heart activity via parasympathetic stimulation as one component of the vagus nerve, cranial nerve X. During rest, both centers provide slight stimulation to the heart, contributing to autonomic tone. This is a similar concept to tone in skeletal muscles. Normally, vagal stimulation predominates as, left unregulated, the SA node would initiate a sinus rhythm of approximately 100 bpm.
Both sympathetic and parasympathetic stimulations flow through a paired complex network of nerve fibers known as the cardiac plexus near the base of the heart. The cardioaccelerator center also sends additional fibers, forming the cardiac nerves via sympathetic ganglia (the cervical ganglia plus superior thoracic ganglia T1–T4) to both the SA and AV nodes, plus additional fibers to the atria and ventricles. The ventricles are more richly innervated by sympathetic fibers than parasympathetic fibers. Sympathetic stimulation causes the release of the neurotransmitter norepinephrine (NE) at the neuromuscular junction of the cardiac nerves. NE shortens the repolarization period, thus speeding the rate of depolarization and contraction, which results in an increase in HR. It opens chemical- or ligand-gated sodium and calcium ion channels, allowing an influx of positively charged ions.
NE binds to the beta-1 receptor. Some cardiac medications (for example, beta blockers) work by blocking these receptors, thereby slowing HR and are one possible treatment for hypertension. Overprescription of these drugs may lead to bradycardia and even stoppage of the heart.
Figure 19.32 Autonomic Innervation of the Heart Cardioaccelerator and cardioinhibitory areas are components of the paired cardiac centers located in the medulla oblongata of the brain. They innervate the heart via sympathetic cardiac nerves that increase cardiac activity and vagus (parasympathetic) nerves that slow cardiac activity.
Parasympathetic stimulation originates from the cardioinhibitory region with impulses traveling via the vagus nerve (cranial nerve X). The vagus nerve sends branches to both the SA and AV nodes, and to portions of both the atria and ventricles. Parasympathetic stimulation releases the neurotransmitter acetylcholine (ACh) at the neuromuscular junction. ACh slows HR by opening chemical- or ligand-gated potassium ion channels to slow the rate of spontaneous depolarization, which extends repolarization and increases the time before the next spontaneous depolarization occurs. Without any nervous stimulation, the SA node would establish a sinus rhythm of approximately 100 bpm. Since resting rates are considerably less than this, it becomes evident that parasympathetic stimulation normally slows HR. This is similar to an individual driving a car with one foot on the brake pedal. To speed up, one need merely remove one’s foot from the break and let the engine increase speed. In the case of the heart, decreasing parasympathetic stimulation decreases the release of ACh, which allows HR to increase up to approximately 100 bpm. Any increases beyond this rate would require sympathetic stimulation. Figure 19.33 illustrates the effects of parasympathetic and sympathetic stimulation on the normal sinus rhythm.
Figure 19.33 Effects of Parasympathetic and Sympathetic Stimulation on Normal Sinus Rhythm The wave of depolarization in a normal sinus rhythm shows a stable resting HR. Following parasympathetic stimulation, HR slows. Following sympathetic stimulation, HR increases.
Input to the Cardiovascular Center
The cardiovascular center receives input from a series of visceral receptors with impulses traveling through visceral sensory fibers within the vagus and sympathetic nerves via the cardiac plexus. Among these receptors are various proprioreceptors, baroreceptors, and chemoreceptors, plus stimuli from the limbic system. Collectively, these inputs normally enable the cardiovascular centers to regulate heart function precisely, a process known as cardiac reflexes. Increased physical activity results in increased rates of firing by various proprioreceptors located in muscles, joint capsules, and tendons. Any such increase in physical activity would logically warrant increased blood flow. The cardiac centers monitor these increased rates of firing, and suppress parasympathetic stimulation and increase sympathetic stimulation as needed in order to increase blood flow.
Similarly, baroreceptors are stretch receptors located in the aortic sinus, carotid bodies, the venae cavae, and other locations, including pulmonary vessels and the right side of the heart itself. Rates of firing from the baroreceptors represent blood pressure, level of physical activity, and the relative distribution of blood. The cardiac centers monitor baroreceptor firing to maintain cardiac homeostasis, a mechanism called the baroreceptor reflex. With increased pressure and stretch, the rate of baroreceptor firing increases, and the cardiac centers decrease sympathetic stimulation and increase parasympathetic stimulation. As pressure and stretch decrease, the rate of baroreceptor firing decreases, and the cardiac centers increase sympathetic stimulation and decrease parasympathetic stimulation.
There is a similar reflex, called the atrial reflex or Bainbridge reflex, associated with varying rates of blood flow to the atria. Increased venous return stretches the walls of the atria where specialized baroreceptors are located. However, as the atrial baroreceptors increase their rate of firing and as they stretch due to the increased blood pressure, the cardiac center responds by increasing sympathetic stimulation and inhibiting parasympathetic stimulation to increase HR. The opposite is also true.
Increased metabolic byproducts associated with increased activity, such as carbon dioxide, hydrogen ions, and lactic acid, plus falling oxygen levels, are detected by a suite of chemoreceptors innervated by the glossopharyngeal and vagus nerves. These chemoreceptors provide feedback to the cardiovascular centers about the need for increased or decreased blood flow, based on the relative levels of these substances.
The limbic system can also significantly impact HR related to emotional state. During periods of stress, it is not unusual to identify higher than normal HRs, often accompanied by a surge in the stress hormone cortisol. Individuals experiencing extreme anxiety may manifest panic attacks with symptoms that resemble those of heart attacks. These events are typically transient and treatable. Meditation techniques have been developed to ease anxiety and have been shown to lower HR effectively. Doing simple deep and slow breathing exercises with one’s eyes closed can also significantly reduce this anxiety and HR.
DISORDERS OF THE...
Heart: Broken Heart Syndrome
Extreme stress from such life events as the death of a loved one, an emotional break up, loss of income, or foreclosure of a home may lead to a condition commonly referred to as broken heart syndrome. This condition may also be called Takotsubo cardiomyopathy, transient apical ballooning syndrome, apical ballooning cardiomyopathy, stress-induced cardiomyopathy, Gebrochenes-Herz syndrome, and stress cardiomyopathy. The recognized effects on the heart include congestive heart failure due to a profound weakening of the myocardium not related to lack of oxygen. This may lead to acute heart failure, lethal arrhythmias, or even the rupture of a ventricle. The exact etiology is not known, but several factors have been suggested, including transient vasospasm, dysfunction of the cardiac capillaries, or thickening of the myocardium—particularly in the left ventricle—that may lead to the critical circulation of blood to this region. While many patients survive the initial acute event with treatment to restore normal function, there is a strong correlation with death. Careful statistical analysis by the Cass Business School, a prestigious institution located in London, published in 2008, revealed that within one year of the death of a loved one, women are more than twice as likely to die and males are six times as likely to die as would otherwise be expected.
Other Factors Influencing Heart Rate
Using a combination of autorhythmicity and innervation, the cardiovascular center is able to provide relatively precise control over HR. However, there are a number of other factors that have an impact on HR as well, including epinephrine, NE, and thyroid hormones; levels of various ions including calcium, potassium, and sodium; body temperature; hypoxia; and pH balance (Table 19.1 and Table 19.2). After reading this section, the importance of maintaining homeostasis should become even more apparent.
Major Factors Increasing Heart Rate and Force of Contraction
| Factor | Effect |
|---|---|
| Cardioaccelerator nerves | Release of norepinephrine by cardioinhibitory nerves |
| Proprioreceptors | Increased firing rates of proprioreceptors (e.g. during exercise) |
| Chemoreceptors | Chemoreceptors sensing decreased levels of O2 or increased levels of H+, CO2 and lactic acid |
| Baroreceptors | Decreased firing rates of baroreceptors (indicating falling blood volume/pressure) |
| Limbic system | Anticipation of physical exercise or strong emotions by the limbic system |
| Catecholamines | Increased epinephrine and norepinephrine release by the adrenal glands |
| Thyroid hormones | Increased T3 and T4 in the blood (released by thyroid) |
| Calcium | Increase in calcium ions in the blood |
| Potassium | Decrease in potassium ions in the blood |
| Sodium | Decrease in sodium ions in the blood |
| Body temperature | Increase in body temperature |
| Nicotine and caffeine | Presence of nicotine, caffeine or other stimulants |
Table 19.1
Factors Decreasing Heart Rate and Force of Contraction
| Factor | Effect |
|---|---|
| Cardioinhibitor nerves (vagus) | Release of acetylcholine by cardioaccelerator nerves |
| Proprioreceptors | Decreased firing rates of proprioreceptors (e.g. during rest) |
| Chemoreceptors | Chemoreceptors sensing increased levels of O2 or decreased levels of H+, CO2 and lactic acid |
| Baroreceptors | Increased firing rates of baroreceptors (indicating rising blood volume/pressure) |
| Limbic system | Anticipation of relaxation by the limbic system |
| Catecholamines | Increased epinephrine and norepinephrine release by the adrenal glands |
| Thyroid hormones | Decreased T3 and T4 in the blood (released by thyroid) |
| Calcium | Increase in calcium ions in the blood |
| Potassium | Increase in potassium ions in the blood |
| Sodium | Increase in sodium ions in the blood |
| Body temperature | Decrease in body temperature |
| Opiates and tranquilizers | Presence of opiates (heroin), tranquilizers or other depressants |
Table 19.2
Epinephrine and Norepinephrine
The catecholamines, epinephrine and NE, secreted by the adrenal medulla form one component of the extended fight-or-flight mechanism. The other component is sympathetic stimulation. Epinephrine and NE have similar effects: binding to the beta-1 receptors, and opening sodium and calcium ion chemical- or ligand-gated channels. The rate of depolarization is increased by this additional influx of positively charged ions, so the threshold is reached more quickly and the period of repolarization is shortened. However, massive releases of these hormones coupled with sympathetic stimulation may actually lead to arrhythmias. There is no parasympathetic stimulation to the adrenal medulla.
Thyroid Hormones
In general, increased levels of thyroid hormone, or thyroxin, increase cardiac rate and contractility. The impact of thyroid hormone is typically of a much longer duration than that of the catecholamines. The physiologically active form of thyroid hormone, T3 or triiodothyronine, has been shown to directly enter cardiomyocytes and alter activity at the level of the genome. It also impacts the beta adrenergic response similar to epinephrine and NE described above. Excessive levels of thyroxin may trigger tachycardia.
Calcium
Calcium ion levels have great impacts upon both HR and contractility; as the levels of calcium ions increase, so do HR and contractility. High levels of calcium ions (hypercalcemia) may be implicated in a short QT interval and a widened T wave in the ECG. The QT interval represents the time from the start of depolarization to repolarization of the ventricles, and includes the period of ventricular systole. Extremely high levels of calcium may induce cardiac arrest. Drugs known as calcium channel blockers slow HR by binding to these channels and blocking or slowing the inward movement of calcium ions.
Caffeine and Nicotine
Caffeine and nicotine are not found naturally within the body. Both of these nonregulated drugs have an excitatory effect on membranes of neurons in general and have a stimulatory effect on the cardiac centers specifically, causing an increase in HR. Caffeine works by increasing the rates of depolarization at the SA node, whereas nicotine stimulates the activity of the sympathetic neurons that deliver impulses to the heart.
Although it is the world’s most widely consumed psychoactive drug, caffeine is legal and not regulated. While precise quantities have not been established, “normal” consumption is not considered harmful to most people, although it may cause disruptions to sleep and acts as a diuretic. Its consumption by pregnant women is cautioned against, although no evidence of negative effects has been confirmed. Tolerance and even physical and mental addiction to the drug result in individuals who routinely consume the substance.
Nicotine, too, is a stimulant and produces addiction. While legal and nonregulated, concerns about nicotine’s safety and documented links to respiratory and cardiac disease have resulted in warning labels on cigarette packages.
Factors Decreasing Heart Rate
HR can be slowed when a person experiences altered sodium and potassium levels, hypoxia, acidosis, alkalosis, and hypothermia (see Table 19.1). The relationship between electrolytes and HR is complex, but maintaining electrolyte balance is critical to the normal wave of depolarization. Of the two ions, potassium has the greater clinical significance. Initially, both hyponatremia (low sodium levels) and hypernatremia (high sodium levels) may lead to tachycardia. Severely high hypernatremia may lead to fibrillation, which may cause CO to cease. Severe hyponatremia leads to both bradycardia and other arrhythmias. Hypokalemia (low potassium levels) also leads to arrhythmias, whereas hyperkalemia (high potassium levels) causes the heart to become weak and flaccid, and ultimately to fail.
Acidosis is a condition in which excess hydrogen ions are present, and the patient’s blood expresses a low pH value. Alkalosis is a condition in which there are too few hydrogen ions, and the patient’s blood has an elevated pH. Normal blood pH falls in the range of 7.35–7.45, so a number lower than this range represents acidosis and a higher number represents alkalosis. Recall that enzymes are the regulators or catalysts of virtually all biochemical reactions; they are sensitive to pH and will change shape slightly with values outside their normal range. These variations in pH and accompanying slight physical changes to the active site on the enzyme decrease the rate of formation of the enzyme-substrate complex, subsequently decreasing the rate of many enzymatic reactions, which can have complex effects on HR. Severe changes in pH will lead to denaturation of the enzyme.
The last variable is body temperature. Elevated body temperature is called hyperthermia, and suppressed body temperature is called hypothermia. Slight hyperthermia results in increasing HR and strength of contraction. Hypothermia slows the rate and strength of heart contractions. This distinct slowing of the heart is one component of the larger diving reflex that diverts blood to essential organs while submerged. If sufficiently chilled, the heart will stop beating, a technique that may be employed during open heart surgery. In this case, the patient’s blood is normally diverted to an artificial heart-lung machine to maintain the body’s blood supply and gas exchange until the surgery is complete, and sinus rhythm can be restored. Excessive hyperthermia and hypothermia will both result in death, as enzymes drive the body systems to cease normal function, beginning with the central nervous system.
Stroke Volume
Many of the same factors that regulate HR also impact cardiac function by altering SV. While a number of variables are involved, SV is ultimately dependent upon the difference between EDV and ESV. The three primary factors to consider are preload, or the stretch on the ventricles prior to contraction; the contractility, or the force or strength of the contraction itself; and afterload, the force the ventricles must generate to pump blood against the resistance in the vessels. These factors are summarized in Table 19.1 and Table 19.2.
Preload
Preload is another way of expressing EDV. Therefore, the greater the EDV is, the greater the preload is. One of the primary factors to consider is filling time, or the duration of ventricular diastole during which filling occurs. The more rapidly the heart contracts, the shorter the filling time becomes, and the lower the EDV and preload are. This effect can be partially overcome by increasing the second variable, contractility, and raising SV, but over time, the heart is unable to compensate for decreased filling time, and preload also decreases.
With increasing ventricular filling, both EDV or preload increase, and the cardiac muscle itself is stretched to a greater degree. At rest, there is little stretch of the ventricular muscle, and the sarcomeres remain short. With increased ventricular filling, the ventricular muscle is increasingly stretched and the sarcomere length increases. As the sarcomeres reach their optimal lengths, they will contract more powerfully, because more of the myosin heads can bind to the actin on the thin filaments, forming cross bridges and increasing the strength of contraction and SV. If this process were to continue and the sarcomeres stretched beyond their optimal lengths, the force of contraction would decrease. However, due to the physical constraints of the location of the heart, this excessive stretch is not a concern.
The relationship between ventricular stretch and contraction has been stated in the well-known Frank-Starling mechanism or simply Starling’s Law of the Heart. This principle states that, within physiological limits, the force of heart contraction is directly proportional to the initial length of the muscle fiber. This means that the greater the stretch of the ventricular muscle (within limits), the more powerful the contraction is, which in turn increases SV. Therefore, by increasing preload, you increase the second variable, contractility.
Otto Frank (1865–1944) was a German physiologist; among his many published works are detailed studies of this important heart relationship. Ernest Starling (1866–1927) was an important English physiologist who also studied the heart. Although they worked largely independently, their combined efforts and similar conclusions have been recognized in the name “Frank-Starling mechanism.”
Any sympathetic stimulation to the venous system will increase venous return to the heart, which contributes to ventricular filling, and EDV and preload. While much of the ventricular filling occurs while both atria and ventricles are in diastole, the contraction of the atria, the atrial kick, plays a crucial role by providing the last 20–30 percent of ventricular filling.
Contractility
It is virtually impossible to consider preload or ESV without including an early mention of the concept of contractility. Indeed, the two parameters are intimately linked. Contractility refers to the force of the contraction of the heart muscle, which controls SV, and is the primary parameter for impacting ESV. The more forceful the contraction is, the greater the SV and smaller the ESV are. Less forceful contractions result in smaller SVs and larger ESVs. Factors that increase contractility are described as positive inotropic factors, and those that decrease contractility are described as negative inotropic factors (ino- = “fiber;” -tropic = “turning toward”).
Not surprisingly, sympathetic stimulation is a positive inotrope, whereas parasympathetic stimulation is a negative inotrope. Sympathetic stimulation triggers the release of NE at the neuromuscular junction from the cardiac nerves and also stimulates the adrenal cortex to secrete epinephrine and NE. In addition to their stimulatory effects on HR, they also bind to both alpha and beta receptors on the cardiac muscle cell membrane to increase metabolic rate and the force of contraction. This combination of actions has the net effect of increasing SV and leaving a smaller residual ESV in the ventricles. In comparison, parasympathetic stimulation releases ACh at the neuromuscular junction from the vagus nerve. The membrane hyperpolarizes and inhibits contraction to decrease the strength of contraction and SV, and to raise ESV. Since parasympathetic fibers are more widespread in the atria than in the ventricles, the primary site of action is in the upper chambers. Parasympathetic stimulation in the atria decreases the atrial kick and reduces EDV, which decreases ventricular stretch and preload, thereby further limiting the force of ventricular contraction. Stronger parasympathetic stimulation also directly decreases the force of contraction of the ventricles.
Several synthetic drugs, including dopamine and isoproterenol, have been developed that mimic the effects of epinephrine and NE by stimulating the influx of calcium ions from the extracellular fluid. Higher concentrations of intracellular calcium ions increase the strength of contraction. Excess calcium (hypercalcemia) also acts as a positive inotropic agent. The drug digitalis lowers HR and increases the strength of the contraction, acting as a positive inotropic agent by blocking the sequestering of calcium ions into the sarcoplasmic reticulum. This leads to higher intracellular calcium levels and greater strength of contraction. In addition to the catecholamines from the adrenal medulla, other hormones also demonstrate positive inotropic effects. These include thyroid hormones and glucagon from the pancreas.
Negative inotropic agents include hypoxia, acidosis, hyperkalemia, and a variety of synthetic drugs. These include numerous beta blockers and calcium channel blockers. Early beta blocker drugs include propranolol and pronethalol, and are credited with revolutionizing treatment of cardiac patients experiencing angina pectoris. There is also a large class of dihydropyridine, phenylalkylamine, and benzothiazepine calcium channel blockers that may be administered decreasing the strength of contraction and SV.
Afterload
Afterload refers to the tension that the ventricles must develop to pump blood effectively against the resistance in the vascular system. Any condition that increases resistance requires a greater afterload to force open the semilunar valves and pump the blood. Damage to the valves, such as stenosis, which makes them harder to open will also increase afterload. Any decrease in resistance decreases the afterload. Figure 19.34 summarizes the major factors influencing SV, Figure 19.35 summarizes the major factors influencing CO, and Table 19.3 and Table 19.4 summarize cardiac responses to increased and decreased blood flow and pressure in order to restore homeostasis.
Figure 19.34 Major Factors Influencing Stroke Volume Multiple factors impact preload, afterload, and contractility, and are the major considerations influencing SV.
Figure 19.35 Summary of Major Factors Influencing Cardiac Output The primary factors influencing HR include autonomic innervation plus endocrine control. Not shown are environmental factors, such as electrolytes, metabolic products, and temperature. The primary factors controlling SV include preload, contractility, and afterload. Other factors such as electrolytes may be classified as either positive or negative inotropic agents.
Cardiac Response to Decreasing Blood Flow and Pressure Due to Decreasing Cardiac Output
| Baroreceptors (aorta, carotid arteries, venae cavae, and atria) | Chemoreceptors (both central nervous system and in proximity to baroreceptors) | |
|---|---|---|
| Sensitive to | Decreasing stretch | Decreasing O2 and increasing CO2, H+, and lactic acid |
| Target | Parasympathetic stimulation suppressed | Sympathetic stimulation increased |
| Response of heart | Increasing heart rate and increasing stroke volume | Increasing heart rate and increasing stroke volume |
| Overall effect | Increasing blood flow and pressure due to increasing cardiac output; homeostasis restored | Increasing blood flow and pressure due to increasing cardiac output; homeostasis restored |
Table 19.3
Cardiac Response to Increasing Blood Flow and Pressure Due to Increasing Cardiac Output
| Baroreceptors (aorta, carotid arteries, venae cavae, and atria) | Chemoreceptors (both central nervous system and in proximity to baroreceptors) | |
|---|---|---|
| Sensitive to | Increasing stretch | Increasing O2 and decreasing CO2, H+, and lactic acid |
| Target | Parasympathetic stimulation increased | Sympathetic stimulation suppressed |
| Response of heart | Decreasing heart rate and decreasing stroke volume | Decreasing heart rate and decreasing stroke volume |
| Overall effect | Decreasing blood flow and pressure due to decreasing cardiac output; homeostasis restored | Decreasing blood flow and pressure due to decreasing cardiac output; homeostasis restored |
Table 19.4
Development of the Heart
- Describe the embryological development of heart structures
- Identify five regions of the fetal heart
- Relate fetal heart structures to adult counterparts
The human heart is the first functional organ to develop. It begins beating and pumping blood around day 21 or 22, a mere three weeks after fertilization. This emphasizes the critical nature of the heart in distributing blood through the vessels and the vital exchange of nutrients, oxygen, and wastes both to and from the developing baby. The critical early development of the heart is reflected by the prominent heart bulge that appears on the anterior surface of the embryo.
The heart forms from an embryonic tissue called mesoderm around 18 to 19 days after fertilization. Mesoderm is one of the three primary germ layers that differentiates early in development that collectively gives rise to all subsequent tissues and organs. The heart begins to develop near the head of the embryo in a region known as the cardiogenic area. Following chemical signals called factors from the underlying endoderm (another of the three primary germ layers), the cardiogenic area begins to form two strands called the cardiogenic cords (Figure 19.36). As the cardiogenic cords develop, a lumen rapidly develops within them. At this point, they are referred to as endocardial tubes. The two tubes migrate together and fuse to form a single primitive heart tube. The primitive heart tube quickly forms five distinct regions. From head to tail, these include the truncus arteriosus, bulbus cordis, primitive ventricle, primitive atrium, and the sinus venosus. Initially, all venous blood flows into the sinus venosus, and contractions propel the blood from tail to head, or from the sinus venosus to the truncus arteriosus. This is a very different pattern from that of an adult.
Figure 19.36 Development of the Human Heart This diagram outlines the embryological development of the human heart during the first eight weeks and the subsequent formation of the four heart chambers.
The five regions of the primitive heart tube develop into recognizable structures in a fully developed heart. The truncus arteriosus will eventually divide and give rise to the ascending aorta and pulmonary trunk. The bulbus cordis develops into the right ventricle. The primitive ventricle forms the left ventricle. The primitive atrium becomes the anterior portions of both the right and left atria, and the two auricles. The sinus venosus develops into the posterior portion of the right atrium, the SA node, and the coronary sinus.
As the primitive heart tube elongates, it begins to fold within the pericardium, eventually forming an S shape, which places the chambers and major vessels into an alignment similar to the adult heart. This process occurs between days 23 and 28. The remainder of the heart development pattern includes development of septa and valves, and remodeling of the actual chambers. Partitioning of the atria and ventricles by the interatrial septum, interventricular septum, and atrioventricular septum is complete by the end of the fifth week, although the fetal blood shunts remain until birth or shortly after. The atrioventricular valves form between weeks five and eight, and the semilunar valves form between weeks five and nine.
Key Terms
- afterload
- force the ventricles must develop to effectively pump blood against the resistance in the vessels
- anastomosis
- (plural = anastomoses) area where vessels unite to allow blood to circulate even if there may be partial blockage in another branch
- anterior cardiac veins
- vessels that parallel the small cardiac arteries and drain the anterior surface of the right ventricle; bypass the coronary sinus and drain directly into the right atrium
- anterior interventricular artery
- (also, left anterior descending artery or LAD) major branch of the left coronary artery that follows the anterior interventricular sulcus
- anterior interventricular sulcus
- sulcus located between the left and right ventricles on the anterior surface of the heart
- aortic valve
- (also, aortic semilunar valve) valve located at the base of the aorta
- artificial pacemaker
- medical device that transmits electrical signals to the heart to ensure that it contracts and pumps blood to the body
- atrial reflex
- (also, called Bainbridge reflex) autonomic reflex that responds to stretch receptors in the atria that send impulses to the cardioaccelerator area to increase HR when venous flow into the atria increases
- atrioventricular (AV) node
- clump of myocardial cells located in the inferior portion of the right atrium within the atrioventricular septum; receives the impulse from the SA node, pauses, and then transmits it into specialized conducting cells within the interventricular septum
- atrioventricular bundle
- (also, bundle of His) group of specialized myocardial conductile cells that transmit the impulse from the AV node through the interventricular septum; form the left and right atrioventricular bundle branches
- atrioventricular bundle branches
- (also, left or right bundle branches) specialized myocardial conductile cells that arise from the bifurcation of the atrioventricular bundle and pass through the interventricular septum; lead to the Purkinje fibers and also to the right papillary muscle via the moderator band
- atrioventricular septum
- cardiac septum located between the atria and ventricles; atrioventricular valves are located here
- atrioventricular valves
- one-way valves located between the atria and ventricles; the valve on the right is called the tricuspid valve, and the one on the left is the mitral or bicuspid valve
- atrium
- (plural = atria) upper or receiving chamber of the heart that pumps blood into the lower chambers just prior to their contraction; the right atrium receives blood from the systemic circuit that flows into the right ventricle; the left atrium receives blood from the pulmonary circuit that flows into the left ventricle
- auricle
- extension of an atrium visible on the superior surface of the heart
- autonomic tone
- contractile state during resting cardiac activity produced by mild sympathetic and parasympathetic stimulation
- autorhythmicity
- ability of cardiac muscle to initiate its own electrical impulse that triggers the mechanical contraction that pumps blood at a fixed pace without nervous or endocrine control
- Bachmann’s bundle
- (also, interatrial band) group of specialized conducting cells that transmit the impulse directly from the SA node in the right atrium to the left atrium
- Bainbridge reflex
- (also, called atrial reflex) autonomic reflex that responds to stretch receptors in the atria that send impulses to the cardioaccelerator area to increase HR when venous flow into the atria increases
- baroreceptor reflex
- autonomic reflex in which the cardiac centers monitor signals from the baroreceptor stretch receptors and regulate heart function based on blood flow
- bicuspid valve
- (also, mitral valve or left atrioventricular valve) valve located between the left atrium and ventricle; consists of two flaps of tissue
- bulbus cordis
- portion of the primitive heart tube that will eventually develop into the right ventricle
- bundle of His
- (also, atrioventricular bundle) group of specialized myocardial conductile cells that transmit the impulse from the AV node through the interventricular septum; form the left and right atrioventricular bundle branches
- cardiac cycle
- period of time between the onset of atrial contraction (atrial systole) and ventricular relaxation (ventricular diastole)
- cardiac notch
- depression in the medial surface of the inferior lobe of the left lung where the apex of the heart is located
- cardiac output (CO)
- amount of blood pumped by each ventricle during one minute; equals HR multiplied by SV
- cardiac plexus
- paired complex network of nerve fibers near the base of the heart that receive sympathetic and parasympathetic stimulations to regulate HR
- cardiac reflexes
- series of autonomic reflexes that enable the cardiovascular centers to regulate heart function based upon sensory information from a variety of visceral sensors
- cardiac reserve
- difference between maximum and resting CO
- cardiac skeleton
- (also, skeleton of the heart) reinforced connective tissue located within the atrioventricular septum; includes four rings that surround the openings between the atria and ventricles, and the openings to the pulmonary trunk and aorta; the point of attachment for the heart valves
- cardiogenic area
- area near the head of the embryo where the heart begins to develop 18–19 days after fertilization
- cardiogenic cords
- two strands of tissue that form within the cardiogenic area
- cardiomyocyte
- muscle cell of the heart
- chordae tendineae
- string-like extensions of tough connective tissue that extend from the flaps of the atrioventricular valves to the papillary muscles
- circumflex artery
- branch of the left coronary artery that follows coronary sulcus
- coronary arteries
- branches of the ascending aorta that supply blood to the heart; the left coronary artery feeds the left side of the heart, the left atrium and ventricle, and the interventricular septum; the right coronary artery feeds the right atrium, portions of both ventricles, and the heart conduction system
- coronary sinus
- large, thin-walled vein on the posterior surface of the heart that lies within the atrioventricular sulcus and drains the heart myocardium directly into the right atrium
- coronary sulcus
- sulcus that marks the boundary between the atria and ventricles
- coronary veins
- vessels that drain the heart and generally parallel the large surface arteries
- diastole
- period of time when the heart muscle is relaxed and the chambers fill with blood
- ejection fraction
- portion of the blood that is pumped or ejected from the heart with each contraction; mathematically represented by SV divided by EDV
- electrocardiogram (ECG)
- surface recording of the electrical activity of the heart that can be used for diagnosis of irregular heart function; also abbreviated as EKG
- end diastolic volume (EDV)
- (also, preload) the amount of blood in the ventricles at the end of atrial systole just prior to ventricular contraction
- end systolic volume (ESV)
- amount of blood remaining in each ventricle following systole
- endocardial tubes
- stage in which lumens form within the expanding cardiogenic cords, forming hollow structures
- endocardium
- innermost layer of the heart lining the heart chambers and heart valves; composed of endothelium reinforced with a thin layer of connective tissue that binds to the myocardium
- endothelium
- layer of smooth, simple squamous epithelium that lines the endocardium and blood vessels
- epicardial coronary arteries
- surface arteries of the heart that generally follow the sulci
- epicardium
- innermost layer of the serous pericardium and the outermost layer of the heart wall
- filling time
- duration of ventricular diastole during which filling occurs
- foramen ovale
- opening in the fetal heart that allows blood to flow directly from the right atrium to the left atrium, bypassing the fetal pulmonary circuit
- fossa ovalis
- oval-shaped depression in the interatrial septum that marks the former location of the foramen ovale
- Frank-Starling mechanism
- relationship between ventricular stretch and contraction in which the force of heart contraction is directly proportional to the initial length of the muscle fiber
- great cardiac vein
- vessel that follows the interventricular sulcus on the anterior surface of the heart and flows along the coronary sulcus into the coronary sinus on the posterior surface; parallels the anterior interventricular artery and drains the areas supplied by this vessel
- heart block
- interruption in the normal conduction pathway
- heart bulge
- prominent feature on the anterior surface of the heart, reflecting early cardiac development
- heart rate (HR)
- number of times the heart contracts (beats) per minute
- heart sounds
- sounds heard via auscultation with a stethoscope of the closing of the atrioventricular valves (“lub”) and semilunar valves (“dub”)
- hypertrophic cardiomyopathy
- pathological enlargement of the heart, generally for no known reason
- inferior vena cava
- large systemic vein that returns blood to the heart from the inferior portion of the body
- interatrial band
- (also, Bachmann’s bundle) group of specialized conducting cells that transmit the impulse directly from the SA node in the right atrium to the left atrium
- interatrial septum
- cardiac septum located between the two atria; contains the fossa ovalis after birth
- intercalated disc
- physical junction between adjacent cardiac muscle cells; consisting of desmosomes, specialized linking proteoglycans, and gap junctions that allow passage of ions between the two cells
- internodal pathways
- specialized conductile cells within the atria that transmit the impulse from the SA node throughout the myocardial cells of the atrium and to the AV node
- interventricular septum
- cardiac septum located between the two ventricles
- isovolumic contraction
- (also, isovolumetric contraction) initial phase of ventricular contraction in which tension and pressure in the ventricle increase, but no blood is pumped or ejected from the heart
- isovolumic ventricular relaxation phase
- initial phase of the ventricular diastole when pressure in the ventricles drops below pressure in the two major arteries, the pulmonary trunk, and the aorta, and blood attempts to flow back into the ventricles, producing the dicrotic notch of the ECG and closing the two semilunar valves
- left atrioventricular valve
- (also, mitral valve or bicuspid valve) valve located between the left atrium and ventricle; consists of two flaps of tissue
- marginal arteries
- branches of the right coronary artery that supply blood to the superficial portions of the right ventricle
- mesoderm
- one of the three primary germ layers that differentiate early in embryonic development
- mesothelium
- simple squamous epithelial portion of serous membranes, such as the superficial portion of the epicardium (the visceral pericardium) and the deepest portion of the pericardium (the parietal pericardium)
- middle cardiac vein
- vessel that parallels and drains the areas supplied by the posterior interventricular artery; drains into the great cardiac vein
- mitral valve
- (also, left atrioventricular valve or bicuspid valve) valve located between the left atrium and ventricle; consists of two flaps of tissue
- moderator band
- band of myocardium covered by endocardium that arises from the inferior portion of the interventricular septum in the right ventricle and crosses to the anterior papillary muscle; contains conductile fibers that carry electrical signals followed by contraction of the heart
- murmur
- unusual heart sound detected by auscultation; typically related to septal or valve defects
- myocardial conducting cells
- specialized cells that transmit electrical impulses throughout the heart and trigger contraction by the myocardial contractile cells
- myocardial contractile cells
- bulk of the cardiac muscle cells in the atria and ventricles that conduct impulses and contract to propel blood
- myocardium
- thickest layer of the heart composed of cardiac muscle cells built upon a framework of primarily collagenous fibers and blood vessels that supply it and the nervous fibers that help to regulate it
- negative inotropic factors
- factors that negatively impact or lower heart contractility
- P wave
- component of the electrocardiogram that represents the depolarization of the atria
- pacemaker
- cluster of specialized myocardial cells known as the SA node that initiates the sinus rhythm
- papillary muscle
- extension of the myocardium in the ventricles to which the chordae tendineae attach
- pectinate muscles
- muscular ridges seen on the anterior surface of the right atrium
- pericardial cavity
- cavity surrounding the heart filled with a lubricating serous fluid that reduces friction as the heart contracts
- pericardial sac
- (also, pericardium) membrane that separates the heart from other mediastinal structures; consists of two distinct, fused sublayers: the fibrous pericardium and the parietal pericardium
- pericardium
- (also, pericardial sac) membrane that separates the heart from other mediastinal structures; consists of two distinct, fused sublayers: the fibrous pericardium and the parietal pericardium
- positive inotropic factors
- factors that positively impact or increase heart contractility
- posterior cardiac vein
- vessel that parallels and drains the areas supplied by the marginal artery branch of the circumflex artery; drains into the great cardiac vein
- posterior interventricular artery
- (also, posterior descending artery) branch of the right coronary artery that runs along the posterior portion of the interventricular sulcus toward the apex of the heart and gives rise to branches that supply the interventricular septum and portions of both ventricles
- posterior interventricular sulcus
- sulcus located between the left and right ventricles on the anterior surface of the heart
- preload
- (also, end diastolic volume) amount of blood in the ventricles at the end of atrial systole just prior to ventricular contraction
- prepotential depolarization
- (also, spontaneous depolarization) mechanism that accounts for the autorhythmic property of cardiac muscle; the membrane potential increases as sodium ions diffuse through the always-open sodium ion channels and causes the electrical potential to rise
- primitive atrium
- portion of the primitive heart tube that eventually becomes the anterior portions of both the right and left atria, and the two auricles
- primitive heart tube
- singular tubular structure that forms from the fusion of the two endocardial tubes
- primitive ventricle
- portion of the primitive heart tube that eventually forms the left ventricle
- pulmonary arteries
- left and right branches of the pulmonary trunk that carry deoxygenated blood from the heart to each of the lungs
- pulmonary capillaries
- capillaries surrounding the alveoli of the lungs where gas exchange occurs: carbon dioxide exits the blood and oxygen enters
- pulmonary circuit
- blood flow to and from the lungs
- pulmonary trunk
- large arterial vessel that carries blood ejected from the right ventricle; divides into the left and right pulmonary arteries
- pulmonary valve
- (also, pulmonary semilunar valve, the pulmonic valve, or the right semilunar valve) valve at the base of the pulmonary trunk that prevents backflow of blood into the right ventricle; consists of three flaps
- pulmonary veins
- veins that carry highly oxygenated blood into the left atrium, which pumps the blood into the left ventricle, which in turn pumps oxygenated blood into the aorta and to the many branches of the systemic circuit
- Purkinje fibers
- specialized myocardial conduction fibers that arise from the bundle branches and spread the impulse to the myocardial contraction fibers of the ventricles
- QRS complex
- component of the electrocardiogram that represents the depolarization of the ventricles and includes, as a component, the repolarization of the atria
- right atrioventricular valve
- (also, tricuspid valve) valve located between the right atrium and ventricle; consists of three flaps of tissue
- semilunar valves
- valves located at the base of the pulmonary trunk and at the base of the aorta
- septum
- (plural = septa) walls or partitions that divide the heart into chambers
- septum primum
- flap of tissue in the fetus that covers the foramen ovale within a few seconds after birth
- sinoatrial (SA) node
- known as the pacemaker, a specialized clump of myocardial conducting cells located in the superior portion of the right atrium that has the highest inherent rate of depolarization that then spreads throughout the heart
- sinus rhythm
- normal contractile pattern of the heart
- sinus venosus
- develops into the posterior portion of the right atrium, the SA node, and the coronary sinus
- small cardiac vein
- parallels the right coronary artery and drains blood from the posterior surfaces of the right atrium and ventricle; drains into the great cardiac vein
- spontaneous depolarization
- (also, prepotential depolarization) the mechanism that accounts for the autorhythmic property of cardiac muscle; the membrane potential increases as sodium ions diffuse through the always-open sodium ion channels and causes the electrical potential to rise
- stroke volume (SV)
- amount of blood pumped by each ventricle per contraction; also, the difference between EDV and ESV
- sulcus
- (plural = sulci) fat-filled groove visible on the surface of the heart; coronary vessels are also located in these areas
- superior vena cava
- large systemic vein that returns blood to the heart from the superior portion of the body
- systemic circuit
- blood flow to and from virtually all of the tissues of the body
- systole
- period of time when the heart muscle is contracting
- T wave
- component of the electrocardiogram that represents the repolarization of the ventricles
- target heart rate
- range in which both the heart and lungs receive the maximum benefit from an aerobic workout
- trabeculae carneae
- ridges of muscle covered by endocardium located in the ventricles
- tricuspid valve
- term used most often in clinical settings for the right atrioventricular valve
- truncus arteriosus
- portion of the primitive heart that will eventually divide and give rise to the ascending aorta and pulmonary trunk
- valve
- in the cardiovascular system, a specialized structure located within the heart or vessels that ensures one-way flow of blood
- ventricle
- one of the primary pumping chambers of the heart located in the lower portion of the heart; the left ventricle is the major pumping chamber on the lower left side of the heart that ejects blood into the systemic circuit via the aorta and receives blood from the left atrium; the right ventricle is the major pumping chamber on the lower right side of the heart that ejects blood into the pulmonary circuit via the pulmonary trunk and receives blood from the right atrium
- ventricular ejection phase
- second phase of ventricular systole during which blood is pumped from the ventricle
Chapter Review
19.1 Heart Anatomy
The heart resides within the pericardial sac and is located in the mediastinal space within the thoracic cavity. The pericardial sac consists of two fused layers: an outer fibrous capsule and an inner parietal pericardium lined with a serous membrane. Between the pericardial sac and the heart is the pericardial cavity, which is filled with lubricating serous fluid. The walls of the heart are composed of an outer epicardium, a thick myocardium, and an inner lining layer of endocardium. The human heart consists of a pair of atria, which receive blood and pump it into a pair of ventricles, which pump blood into the vessels. The right atrium receives systemic blood relatively low in oxygen and pumps it into the right ventricle, which pumps it into the pulmonary circuit. Exchange of oxygen and carbon dioxide occurs in the lungs, and blood high in oxygen returns to the left atrium, which pumps blood into the left ventricle, which in turn pumps blood into the aorta and the remainder of the systemic circuit. The septa are the partitions that separate the chambers of the heart. They include the interatrial septum, the interventricular septum, and the atrioventricular septum. Two of these openings are guarded by the atrioventricular valves, the right tricuspid valve and the left mitral valve, which prevent the backflow of blood. Each is attached to chordae tendineae that extend to the papillary muscles, which are extensions of the myocardium, to prevent the valves from being blown back into the atria. The pulmonary valve is located at the base of the pulmonary trunk, and the left semilunar valve is located at the base of the aorta. The right and left coronary arteries are the first to branch off the aorta and arise from two of the three sinuses located near the base of the aorta and are generally located in the sulci. Cardiac veins parallel the small cardiac arteries and generally drain into the coronary sinus.
19.2 Cardiac Muscle and Electrical Activity
The heart is regulated by both neural and endocrine control, yet it is capable of initiating its own action potential followed by muscular contraction. The conductive cells within the heart establish the heart rate and transmit it through the myocardium. The contractile cells contract and propel the blood. The normal path of transmission for the conductive cells is the sinoatrial (SA) node, internodal pathways, atrioventricular (AV) node, atrioventricular (AV) bundle of His, bundle branches, and Purkinje fibers. The action potential for the conductive cells consists of a prepotential phase with a slow influx of Na+ followed by a rapid influx of Ca2+ and outflux of K+. Contractile cells have an action potential with an extended plateau phase that results in an extended refractory period to allow complete contraction for the heart to pump blood effectively. Recognizable points on the ECG include the P wave that corresponds to atrial depolarization, the QRS complex that corresponds to ventricular depolarization, and the T wave that corresponds to ventricular repolarization.
19.3 Cardiac Cycle
The cardiac cycle comprises a complete relaxation and contraction of both the atria and ventricles, and lasts approximately 0.8 seconds. Beginning with all chambers in diastole, blood flows passively from the veins into the atria and past the atrioventricular valves into the ventricles. The atria begin to contract (atrial systole), following depolarization of the atria, and pump blood into the ventricles. The ventricles begin to contract (ventricular systole), raising pressure within the ventricles. When ventricular pressure rises above the pressure in the atria, blood flows toward the atria, producing the first heart sound, S1 or lub. As pressure in the ventricles rises above two major arteries, blood pushes open the two semilunar valves and moves into the pulmonary trunk and aorta in the ventricular ejection phase. Following ventricular repolarization, the ventricles begin to relax (ventricular diastole), and pressure within the ventricles drops. As ventricular pressure drops, there is a tendency for blood to flow back into the atria from the major arteries, producing the dicrotic notch in the ECG and closing the two semilunar valves. The second heart sound, S2 or dub, occurs when the semilunar valves close. When the pressure falls below that of the atria, blood moves from the atria into the ventricles, opening the atrioventricular valves and marking one complete heart cycle. The valves prevent backflow of blood. Failure of the valves to operate properly produces turbulent blood flow within the heart; the resulting heart murmur can often be heard with a stethoscope.
19.4 Cardiac Physiology
Many factors affect HR and SV, and together, they contribute to cardiac function. HR is largely determined and regulated by autonomic stimulation and hormones. There are several feedback loops that contribute to maintaining homeostasis dependent upon activity levels, such as the atrial reflex, which is determined by venous return.
SV is regulated by autonomic innervation and hormones, but also by filling time and venous return. Venous return is determined by activity of the skeletal muscles, blood volume, and changes in peripheral circulation. Venous return determines preload and the atrial reflex. Filling time directly related to HR also determines preload. Preload then impacts both EDV and ESV. Autonomic innervation and hormones largely regulate contractility. Contractility impacts EDV as does afterload. CO is the product of HR multiplied by SV. SV is the difference between EDV and ESV.
19.5 Development of the Heart
The heart is the first organ to form and become functional, emphasizing the importance of transport of material to and from the developing infant. It originates about day 18 or 19 from the mesoderm and begins beating and pumping blood about day 21 or 22. It forms from the cardiogenic region near the head and is visible as a prominent heart bulge on the surface of the embryo. Originally, it consists of a pair of strands called cardiogenic cords that quickly form a hollow lumen and are referred to as endocardial tubes. These then fuse into a single heart tube and differentiate into the truncus arteriosus, bulbus cordis, primitive ventricle, primitive atrium, and sinus venosus, starting about day 22. The primitive heart begins to form an S shape within the pericardium between days 23 and 28. The internal septa begin to form about day 28, separating the heart into the atria and ventricles, although the foramen ovale persists until shortly after birth. Between weeks five and eight, the atrioventricular valves form. The semilunar valves form between weeks five and nine.
Interactive Link Questions
1.
Visit this site to observe an echocardiogram of actual heart valves opening and closing. Although much of the heart has been “removed” from this gif loop so the chordae tendineae are not visible, why is their presence more critical for the atrioventricular valves (tricuspid and mitral) than the semilunar (aortic and pulmonary) valves?
Review Questions
Which of the following is not important in preventing backflow of blood?
- chordae tendineae
- papillary muscles
- AV valves
- endocardium
Which valve separates the left atrium from the left ventricle?
- mitral
- tricuspid
- pulmonary
- aortic
Which of the following lists the valves in the order through which the blood flows from the vena cava through the heart?
- tricuspid, pulmonary semilunar, bicuspid, aortic semilunar
- mitral, pulmonary semilunar, bicuspid, aortic semilunar
- aortic semilunar, pulmonary semilunar, tricuspid, bicuspid
- bicuspid, aortic semilunar, tricuspid, pulmonary semilunar
Which chamber initially receives blood from the systemic circuit?
- left atrium
- left ventricle
- right atrium
- right ventricle
The ________ layer secretes chemicals that help to regulate ionic environments and strength of contraction and serve as powerful vasoconstrictors.
- pericardial sac
- endocardium
- myocardium
- epicardium
The myocardium would be the thickest in the ________.
- left atrium
- left ventricle
- right atrium
- right ventricle
In which septum is it normal to find openings in the adult?
- interatrial septum
- interventricular septum
- atrioventricular septum
- all of the above
Which of the following is unique to cardiac muscle cells?
- Only cardiac muscle contains a sarcoplasmic reticulum.
- Only cardiac muscle has gap junctions.
- Only cardiac muscle is capable of autorhythmicity
- Only cardiac muscle has a high concentration of mitochondria.
The influx of which ion accounts for the plateau phase?
- sodium
- potassium
- chloride
- calcium
Which portion of the ECG corresponds to repolarization of the atria?
- P wave
- QRS complex
- T wave
- none of the above: atrial repolarization is masked by ventricular depolarization
Which component of the heart conduction system would have the slowest rate of firing?
- atrioventricular node
- atrioventricular bundle
- bundle branches
- Purkinje fibers
The cardiac cycle consists of a distinct relaxation and contraction phase. Which term is typically used to refer ventricular contraction while no blood is being ejected?
- systole
- diastole
- quiescent
- isovolumic contraction
Most blood enters the ventricle during ________.
- atrial systole
- atrial diastole
- ventricular systole
- isovolumic contraction
The first heart sound represents which portion of the cardiac cycle?
- atrial systole
- ventricular systole
- closing of the atrioventricular valves
- closing of the semilunar valves
Ventricular relaxation immediately follows ________.
- atrial depolarization
- ventricular repolarization
- ventricular depolarization
- atrial repolarization
The force the heart must overcome to pump blood is known as ________.
- preload
- afterload
- cardiac output
- stroke volume
The cardiovascular centers are located in which area of the brain?
- medulla oblongata
- pons
- mesencephalon (midbrain)
- cerebrum
In a healthy young adult, what happens to cardiac output when heart rate increases above 160 bpm?
- It increases.
- It decreases.
- It remains constant.
- There is no way to predict.
What happens to preload when there is venous constriction in the veins?
- It increases.
- It decreases.
- It remains constant.
- There is no way to predict.
Which of the following is a positive inotrope?
- Na+
- K+
- Ca2+
- both Na+ and K+
The earliest organ to form and begin function within the developing human is the ________.
- brain
- stomach
- lungs
- heart
Of the three germ layers that give rise to all adult tissues and organs, which gives rise to the heart?
- ectoderm
- endoderm
- mesoderm
- placenta
The two tubes that eventually fuse to form the heart are referred to as the ________.
- primitive heart tubes
- endocardial tubes
- cardiogenic region
- cardiogenic tubes
Which primitive area of the heart will give rise to the right ventricle?
- bulbus cordis
- primitive ventricle
- sinus venosus
- truncus arteriosus
The pulmonary trunk and aorta are derived from which primitive heart structure?
- bulbus cordis
- primitive ventricle
- sinus venosus
- truncus arteriosus
Critical Thinking Questions
Describe how the valves keep the blood moving in one direction.
28.Why is the pressure in the pulmonary circulation lower than in the systemic circulation?
29.Why is the plateau phase so critical to cardiac muscle function?
30.How does the delay of the impulse at the atrioventricular node contribute to cardiac function?
31.How do gap junctions and intercalated disks aid contraction of the heart?
32.Why do the cardiac muscles cells demonstrate autorhythmicity?
33.Describe one cardiac cycle, beginning with both atria and ventricles relaxed.
34.Why does increasing EDV increase contractility?
35.Why is afterload important to cardiac function?
36.Why is it so important for the human heart to develop early and begin functioning within the developing embryo?
37.Describe how the major pumping chambers, the ventricles, form within the developing heart.
|
oercommons
|
2025-03-18T00:39:10.340839
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/58765/overview",
"title": "Anatomy and Physiology, Fluids and Transport",
"author": null
}
|
https://oercommons.org/courseware/lesson/58766/overview
|
The Cardiovascular System: Blood Vessels and Circulation
Introduction
Figure 20.1 Blood Vessels While most blood vessels are located deep from the surface and are not visible, the superficial veins of the upper limb provide an indication of the extent, prominence, and importance of these structures to the body. (credit: Colin Davis)
CHAPTER OBJECTIVES
After studying this chapter, you will be able to:
- Compare and contrast the anatomical structure of arteries, arterioles, capillaries, venules, and veins
- Accurately describe the forces that account for capillary exchange
- List the major factors affecting blood flow, blood pressure, and resistance
- Describe how blood flow, blood pressure, and resistance interrelate
- Discuss how the neural and endocrine mechanisms maintain homeostasis within the blood vessels
- Describe the interaction of the cardiovascular system with other body systems
- Label the major blood vessels of the pulmonary and systemic circulations
- Identify and describe the hepatic portal system
- Describe the development of blood vessels and fetal circulation
- Compare fetal circulation to that of an individual after birth
In this chapter, you will learn about the vascular part of the cardiovascular system, that is, the vessels that transport blood throughout the body and provide the physical site where gases, nutrients, and other substances are exchanged with body cells. When vessel functioning is reduced, blood-borne substances do not circulate effectively throughout the body. As a result, tissue injury occurs, metabolism is impaired, and the functions of every bodily system are threatened.
Structure and Function of Blood Vessels
- Compare and contrast the three tunics that make up the walls of most blood vessels
Distinguish between elastic arteries, muscular arteries, and arterioles on the basis of structure, location, and function
- Describe the basic structure of a capillary bed, from the supplying metarteriole to the venule into which it drains
- Explain the structure and function of venous valves in the large veins of the extremities
Blood is carried through the body via blood vessels. An artery is a blood vessel that carries blood away from the heart, where it branches into ever-smaller vessels. Eventually, the smallest arteries, vessels called arterioles, further branch into tiny capillaries, where nutrients and wastes are exchanged, and then combine with other vessels that exit capillaries to form venules, small blood vessels that carry blood to a vein, a larger blood vessel that returns blood to the heart.
Arteries and veins transport blood in two distinct circuits: the systemic circuit and the pulmonary circuit (Figure 20.2). Systemic arteries provide blood rich in oxygen to the body’s tissues. The blood returned to the heart through systemic veins has less oxygen, since much of the oxygen carried by the arteries has been delivered to the cells. In contrast, in the pulmonary circuit, arteries carry blood low in oxygen exclusively to the lungs for gas exchange. Pulmonary veins then return freshly oxygenated blood from the lungs to the heart to be pumped back out into systemic circulation. Although arteries and veins differ structurally and functionally, they share certain features.
Figure 20.2 Cardiovascular Circulation The pulmonary circuit moves blood from the right side of the heart to the lungs and back to the heart. The systemic circuit moves blood from the left side of the heart to the head and body and returns it to the right side of the heart to repeat the cycle. The arrows indicate the direction of blood flow, and the colors show the relative levels of oxygen concentration.
Shared Structures
Different types of blood vessels vary slightly in their structures, but they share the same general features. Arteries and arterioles have thicker walls than veins and venules because they are closer to the heart and receive blood that is surging at a far greater pressure (Figure 20.3). Each type of vessel has a lumen—a hollow passageway through which blood flows. Arteries have smaller lumens than veins, a characteristic that helps to maintain the pressure of blood moving through the system. Together, their thicker walls and smaller diameters give arterial lumens a more rounded appearance in cross section than the lumens of veins.
Figure 20.3 Structure of Blood Vessels (a) Arteries and (b) veins share the same general features, but the walls of arteries are much thicker because of the higher pressure of the blood that flows through them. (c) A micrograph shows the relative differences in thickness. LM × 160. (Micrograph provided by the Regents of the University of Michigan Medical School © 2012)
By the time blood has passed through capillaries and entered venules, the pressure initially exerted upon it by heart contractions has diminished. In other words, in comparison to arteries, venules and veins withstand a much lower pressure from the blood that flows through them. Their walls are considerably thinner and their lumens are correspondingly larger in diameter, allowing more blood to flow with less vessel resistance. In addition, many veins of the body, particularly those of the limbs, contain valves that assist the unidirectional flow of blood toward the heart. This is critical because blood flow becomes sluggish in the extremities, as a result of the lower pressure and the effects of gravity.
The walls of arteries and veins are largely composed of living cells and their products (including collagenous and elastic fibers); the cells require nourishment and produce waste. Since blood passes through the larger vessels relatively quickly, there is limited opportunity for blood in the lumen of the vessel to provide nourishment to or remove waste from the vessel’s cells. Further, the walls of the larger vessels are too thick for nutrients to diffuse through to all of the cells. Larger arteries and veins contain small blood vessels within their walls known as the vasa vasorum—literally “vessels of the vessel”—to provide them with this critical exchange. Since the pressure within arteries is relatively high, the vasa vasorum must function in the outer layers of the vessel (see Figure 20.3) or the pressure exerted by the blood passing through the vessel would collapse it, preventing any exchange from occurring. The lower pressure within veins allows the vasa vasorum to be located closer to the lumen. The restriction of the vasa vasorum to the outer layers of arteries is thought to be one reason that arterial diseases are more common than venous diseases, since its location makes it more difficult to nourish the cells of the arteries and remove waste products. There are also minute nerves within the walls of both types of vessels that control the contraction and dilation of smooth muscle. These minute nerves are known as the nervi vasorum.
Both arteries and veins have the same three distinct tissue layers, called tunics (from the Latin term tunica), for the garments first worn by ancient Romans; the term tunic is also used for some modern garments. From the most interior layer to the outer, these tunics are the tunica intima, the tunica media, and the tunica externa (see Figure 20.3). Table 20.1 compares and contrasts the tunics of the arteries and veins.
Comparison of Tunics in Arteries and Veins
| Arteries | Veins | |
|---|---|---|
| General appearance | Thick walls with small lumens Generally appear rounded | Thin walls with large lumens Generally appear flattened |
| Tunica intima | Endothelium usually appears wavy due to constriction of smooth muscle Internal elastic membrane present in larger vessels | Endothelium appears smooth Internal elastic membrane absent |
| Tunica media | Normally the thickest layer in arteries Smooth muscle cells and elastic fibers predominate (the proportions of these vary with distance from the heart) External elastic membrane present in larger vessels | Normally thinner than the tunica externa Smooth muscle cells and collagenous fibers predominate Nervi vasorum and vasa vasorum present External elastic membrane absent |
| Tunica externa | Normally thinner than the tunica media in all but the largest arteries Collagenous and elastic fibers Nervi vasorum and vasa vasorum present | Normally the thickest layer in veins Collagenous and smooth fibers predominate Some smooth muscle fibers Nervi vasorum and vasa vasorum present |
Table 20.1
Tunica Intima
The tunica intima (also called the tunica interna) is composed of epithelial and connective tissue layers. Lining the tunica intima is the specialized simple squamous epithelium called the endothelium, which is continuous throughout the entire vascular system, including the lining of the chambers of the heart. Damage to this endothelial lining and exposure of blood to the collagenous fibers beneath is one of the primary causes of clot formation. Until recently, the endothelium was viewed simply as the boundary between the blood in the lumen and the walls of the vessels. Recent studies, however, have shown that it is physiologically critical to such activities as helping to regulate capillary exchange and altering blood flow. The endothelium releases local chemicals called endothelins that can constrict the smooth muscle within the walls of the vessel to increase blood pressure. Uncompensated overproduction of endothelins may contribute to hypertension (high blood pressure) and cardiovascular disease.
Next to the endothelium is the basement membrane, or basal lamina, that effectively binds the endothelium to the connective tissue. The basement membrane provides strength while maintaining flexibility, and it is permeable, allowing materials to pass through it. The thin outer layer of the tunica intima contains a small amount of areolar connective tissue that consists primarily of elastic fibers to provide the vessel with additional flexibility; it also contains some collagenous fibers to provide additional strength.
In larger arteries, there is also a thick, distinct layer of elastic fibers known as the internal elastic membrane (also called the internal elastic lamina) at the boundary with the tunica media. Like the other components of the tunica intima, the internal elastic membrane provides structure while allowing the vessel to stretch. It is permeated with small openings that allow exchange of materials between the tunics. The internal elastic membrane is not apparent in veins. In addition, many veins, particularly in the lower limbs, contain valves formed by sections of thickened endothelium that are reinforced with connective tissue, extending into the lumen.
Under the microscope, the lumen and the entire tunica intima of a vein will appear smooth, whereas those of an artery will normally appear wavy because of the partial constriction of the smooth muscle in the tunica media, the next layer of blood vessel walls.
Tunica Media
The tunica media is the substantial middle layer of the vessel wall (see Figure 20.3). It is generally the thickest layer in arteries, and it is much thicker in arteries than it is in veins. The tunica media consists of layers of smooth muscle supported by connective tissue that is primarily made up of elastic fibers, most of which are arranged in circular sheets. Toward the outer portion of the tunic, there are also layers of longitudinal muscle. Contraction and relaxation of the circular muscles decrease and increase the diameter of the vessel lumen, respectively. Specifically in arteries, vasoconstriction decreases blood flow as the smooth muscle in the walls of the tunica media contracts, making the lumen narrower and increasing blood pressure. Similarly, vasodilation increases blood flow as the smooth muscle relaxes, allowing the lumen to widen and blood pressure to drop. Both vasoconstriction and vasodilation are regulated in part by small vascular nerves, known as nervi vasorum, or “nerves of the vessel,” that run within the walls of blood vessels. These are generally all sympathetic fibers, although some trigger vasodilation and others induce vasoconstriction, depending upon the nature of the neurotransmitter and receptors located on the target cell. Parasympathetic stimulation does trigger vasodilation as well as erection during sexual arousal in the external genitalia of both sexes. Nervous control over vessels tends to be more generalized than the specific targeting of individual blood vessels. Local controls, discussed later, account for this phenomenon. (Seek additional content for more information on these dynamic aspects of the autonomic nervous system.) Hormones and local chemicals also control blood vessels. Together, these neural and chemical mechanisms reduce or increase blood flow in response to changing body conditions, from exercise to hydration. Regulation of both blood flow and blood pressure is discussed in detail later in this chapter.
The smooth muscle layers of the tunica media are supported by a framework of collagenous fibers that also binds the tunica media to the inner and outer tunics. Along with the collagenous fibers are large numbers of elastic fibers that appear as wavy lines in prepared slides. Separating the tunica media from the outer tunica externa in larger arteries is the external elastic membrane (also called the external elastic lamina), which also appears wavy in slides. This structure is not usually seen in smaller arteries, nor is it seen in veins.
Tunica Externa
The outer tunic, the tunica externa (also called the tunica adventitia), is a substantial sheath of connective tissue composed primarily of collagenous fibers. Some bands of elastic fibers are found here as well. The tunica externa in veins also contains groups of smooth muscle fibers. This is normally the thickest tunic in veins and may be thicker than the tunica media in some larger arteries. The outer layers of the tunica externa are not distinct but rather blend with the surrounding connective tissue outside the vessel, helping to hold the vessel in relative position. If you are able to palpate some of the superficial veins on your upper limbs and try to move them, you will find that the tunica externa prevents this. If the tunica externa did not hold the vessel in place, any movement would likely result in disruption of blood flow.
Arteries
An artery is a blood vessel that conducts blood away from the heart. All arteries have relatively thick walls that can withstand the high pressure of blood ejected from the heart. However, those close to the heart have the thickest walls, containing a high percentage of elastic fibers in all three of their tunics. This type of artery is known as an elastic artery (Figure 20.4). Vessels larger than 10 mm in diameter are typically elastic. Their abundant elastic fibers allow them to expand, as blood pumped from the ventricles passes through them, and then to recoil after the surge has passed. If artery walls were rigid and unable to expand and recoil, their resistance to blood flow would greatly increase and blood pressure would rise to even higher levels, which would in turn require the heart to pump harder to increase the volume of blood expelled by each pump (the stroke volume) and maintain adequate pressure and flow. Artery walls would have to become even thicker in response to this increased pressure. The elastic recoil of the vascular wall helps to maintain the pressure gradient that drives the blood through the arterial system. An elastic artery is also known as a conducting artery, because the large diameter of the lumen enables it to accept a large volume of blood from the heart and conduct it to smaller branches.
Figure 20.4 Types of Arteries and Arterioles Comparison of the walls of an elastic artery, a muscular artery, and an arteriole is shown. In terms of scale, the diameter of an arteriole is measured in micrometers compared to millimeters for elastic and muscular arteries.
Farther from the heart, where the surge of blood has dampened, the percentage of elastic fibers in an artery’s tunica intima decreases and the amount of smooth muscle in its tunica media increases. The artery at this point is described as a muscular artery. The diameter of muscular arteries typically ranges from 0.1 mm to 10 mm. Their thick tunica media allows muscular arteries to play a leading role in vasoconstriction. In contrast, their decreased quantity of elastic fibers limits their ability to expand. Fortunately, because the blood pressure has eased by the time it reaches these more distant vessels, elasticity has become less important.
Notice that although the distinctions between elastic and muscular arteries are important, there is no “line of demarcation” where an elastic artery suddenly becomes muscular. Rather, there is a gradual transition as the vascular tree repeatedly branches. In turn, muscular arteries branch to distribute blood to the vast network of arterioles. For this reason, a muscular artery is also known as a distributing artery.
Arterioles
An arteriole is a very small artery that leads to a capillary. Arterioles have the same three tunics as the larger vessels, but the thickness of each is greatly diminished. The critical endothelial lining of the tunica intima is intact. The tunica media is restricted to one or two smooth muscle cell layers in thickness. The tunica externa remains but is very thin (see Figure 20.4).
With a lumen averaging 30 micrometers or less in diameter, arterioles are critical in slowing down—or resisting—blood flow and, thus, causing a substantial drop in blood pressure. Because of this, you may see them referred to as resistance vessels. The muscle fibers in arterioles are normally slightly contracted, causing arterioles to maintain a consistent muscle tone—in this case referred to as vascular tone—in a similar manner to the muscular tone of skeletal muscle. In reality, all blood vessels exhibit vascular tone due to the partial contraction of smooth muscle. The importance of the arterioles is that they will be the primary site of both resistance and regulation of blood pressure. The precise diameter of the lumen of an arteriole at any given moment is determined by neural and chemical controls, and vasoconstriction and vasodilation in the arterioles are the primary mechanisms for distribution of blood flow.
Capillaries
A capillary is a microscopic channel that supplies blood to the tissues themselves, a process called perfusion. Exchange of gases and other substances occurs in the capillaries between the blood and the surrounding cells and their tissue fluid (interstitial fluid). The diameter of a capillary lumen ranges from 5–10 micrometers; the smallest are just barely wide enough for an erythrocyte to squeeze through. Flow through capillaries is often described as microcirculation.
The wall of a capillary consists of the endothelial layer surrounded by a basement membrane with occasional smooth muscle fibers. There is some variation in wall structure: In a large capillary, several endothelial cells bordering each other may line the lumen; in a small capillary, there may be only a single cell layer that wraps around to contact itself.
For capillaries to function, their walls must be leaky, allowing substances to pass through. There are three major types of capillaries, which differ according to their degree of “leakiness:” continuous, fenestrated, and sinusoid capillaries (Figure 20.5).
Continuous Capillaries
The most common type of capillary, the continuous capillary, is found in almost all vascularized tissues. Continuous capillaries are characterized by a complete endothelial lining with tight junctions between endothelial cells. Although a tight junction is usually impermeable and only allows for the passage of water and ions, they are often incomplete in capillaries, leaving intercellular clefts that allow for exchange of water and other very small molecules between the blood plasma and the interstitial fluid. Substances that can pass between cells include metabolic products, such as glucose, water, and small hydrophobic molecules like gases and hormones, as well as various leukocytes. Continuous capillaries not associated with the brain are rich in transport vesicles, contributing to either endocytosis or exocytosis. Those in the brain are part of the blood-brain barrier. Here, there are tight junctions and no intercellular clefts, plus a thick basement membrane and astrocyte extensions called end feet; these structures combine to prevent the movement of nearly all substances.
Figure 20.5 Types of Capillaries The three major types of capillaries: continuous, fenestrated, and sinusoid.
Fenestrated Capillaries
A fenestrated capillary is one that has pores (or fenestrations) in addition to tight junctions in the endothelial lining. These make the capillary permeable to larger molecules. The number of fenestrations and their degree of permeability vary, however, according to their location. Fenestrated capillaries are common in the small intestine, which is the primary site of nutrient absorption, as well as in the kidneys, which filter the blood. They are also found in the choroid plexus of the brain and many endocrine structures, including the hypothalamus, pituitary, pineal, and thyroid glands.
Sinusoid Capillaries
A sinusoid capillary (or sinusoid) is the least common type of capillary. Sinusoid capillaries are flattened, and they have extensive intercellular gaps and incomplete basement membranes, in addition to intercellular clefts and fenestrations. This gives them an appearance not unlike Swiss cheese. These very large openings allow for the passage of the largest molecules, including plasma proteins and even cells. Blood flow through sinusoids is very slow, allowing more time for exchange of gases, nutrients, and wastes. Sinusoids are found in the liver and spleen, bone marrow, lymph nodes (where they carry lymph, not blood), and many endocrine glands including the pituitary and adrenal glands. Without these specialized capillaries, these organs would not be able to provide their myriad of functions. For example, when bone marrow forms new blood cells, the cells must enter the blood supply and can only do so through the large openings of a sinusoid capillary; they cannot pass through the small openings of continuous or fenestrated capillaries. The liver also requires extensive specialized sinusoid capillaries in order to process the materials brought to it by the hepatic portal vein from both the digestive tract and spleen, and to release plasma proteins into circulation.
Metarterioles and Capillary Beds
A metarteriole is a type of vessel that has structural characteristics of both an arteriole and a capillary. Slightly larger than the typical capillary, the smooth muscle of the tunica media of the metarteriole is not continuous but forms rings of smooth muscle (sphincters) prior to the entrance to the capillaries. Each metarteriole arises from a terminal arteriole and branches to supply blood to a capillary bed that may consist of 10–100 capillaries.
The precapillary sphincters, circular smooth muscle cells that surround the capillary at its origin with the metarteriole, tightly regulate the flow of blood from a metarteriole to the capillaries it supplies. Their function is critical: If all of the capillary beds in the body were to open simultaneously, they would collectively hold every drop of blood in the body and there would be none in the arteries, arterioles, venules, veins, or the heart itself. Normally, the precapillary sphincters are closed. When the surrounding tissues need oxygen and have excess waste products, the precapillary sphincters open, allowing blood to flow through and exchange to occur before closing once more (Figure 20.6). If all of the precapillary sphincters in a capillary bed are closed, blood will flow from the metarteriole directly into a thoroughfare channel and then into the venous circulation, bypassing the capillary bed entirely. This creates what is known as a vascular shunt. In addition, an arteriovenous anastomosis may bypass the capillary bed and lead directly to the venous system.
Although you might expect blood flow through a capillary bed to be smooth, in reality, it moves with an irregular, pulsating flow. This pattern is called vasomotion and is regulated by chemical signals that are triggered in response to changes in internal conditions, such as oxygen, carbon dioxide, hydrogen ion, and lactic acid levels. For example, during strenuous exercise when oxygen levels decrease and carbon dioxide, hydrogen ion, and lactic acid levels all increase, the capillary beds in skeletal muscle are open, as they would be in the digestive system when nutrients are present in the digestive tract. During sleep or rest periods, vessels in both areas are largely closed; they open only occasionally to allow oxygen and nutrient supplies to travel to the tissues to maintain basic life processes.
Figure 20.6 Capillary Bed In a capillary bed, arterioles give rise to metarterioles. Precapillary sphincters located at the junction of a metarteriole with a capillary regulate blood flow. A thoroughfare channel connects the metarteriole to a venule. An arteriovenous anastomosis, which directly connects the arteriole with the venule, is shown at the bottom.
Venules
A venule is an extremely small vein, generally 8–100 micrometers in diameter. Postcapillary venules join multiple capillaries exiting from a capillary bed. Multiple venules join to form veins. The walls of venules consist of endothelium, a thin middle layer with a few muscle cells and elastic fibers, plus an outer layer of connective tissue fibers that constitute a very thin tunica externa (Figure 20.7). Venules as well as capillaries are the primary sites of emigration or diapedesis, in which the white blood cells adhere to the endothelial lining of the vessels and then squeeze through adjacent cells to enter the tissue fluid.
Veins
A vein is a blood vessel that conducts blood toward the heart. Compared to arteries, veins are thin-walled vessels with large and irregular lumens (see Figure 20.7). Because they are low-pressure vessels, larger veins are commonly equipped with valves that promote the unidirectional flow of blood toward the heart and prevent backflow toward the capillaries caused by the inherent low blood pressure in veins as well as the pull of gravity. Table 20.2 compares the features of arteries and veins.
Figure 20.7 Comparison of Veins and Venules Many veins have valves to prevent back flow of blood, whereas venules do not. In terms of scale, the diameter of a venule is measured in micrometers compared to millimeters for veins.
Comparison of Arteries and Veins
| Arteries | Veins | |
|---|---|---|
| Direction of blood flow | Conducts blood away from the heart | Conducts blood toward the heart |
| General appearance | Rounded | Irregular, often collapsed |
| Pressure | High | Low |
| Wall thickness | Thick | Thin |
| Relative oxygen concentration | Higher in systemic arteries Lower in pulmonary arteries | Lower in systemic veins Higher in pulmonary veins |
| Valves | Not present | Present most commonly in limbs and in veins inferior to the heart |
Table 20.2
DISORDERS OF THE...
Cardiovascular System: Edema and Varicose Veins
Despite the presence of valves and the contributions of other anatomical and physiological adaptations we will cover shortly, over the course of a day, some blood will inevitably pool, especially in the lower limbs, due to the pull of gravity. Any blood that accumulates in a vein will increase the pressure within it, which can then be reflected back into the smaller veins, venules, and eventually even the capillaries. Increased pressure will promote the flow of fluids out of the capillaries and into the interstitial fluid. The presence of excess tissue fluid around the cells leads to a condition called edema.
Most people experience a daily accumulation of tissue fluid, especially if they spend much of their work life on their feet (like most health professionals). However, clinical edema goes beyond normal swelling and requires medical treatment. Edema has many potential causes, including hypertension and heart failure, severe protein deficiency, renal failure, and many others. In order to treat edema, which is a sign rather than a discrete disorder, the underlying cause must be diagnosed and alleviated.
Figure 20.8 Varicose Veins Varicose veins are commonly found in the lower limbs. (credit: Thomas Kriese)
Edema may be accompanied by varicose veins, especially in the superficial veins of the legs (Figure 20.8). This disorder arises when defective valves allow blood to accumulate within the veins, causing them to distend, twist, and become visible on the surface of the integument. Varicose veins may occur in both sexes, but are more common in women and are often related to pregnancy. More than simple cosmetic blemishes, varicose veins are often painful and sometimes itchy or throbbing. Without treatment, they tend to grow worse over time. The use of support hose, as well as elevating the feet and legs whenever possible, may be helpful in alleviating this condition. Laser surgery and interventional radiologic procedures can reduce the size and severity of varicose veins. Severe cases may require conventional surgery to remove the damaged vessels. As there are typically redundant circulation patterns, that is, anastomoses, for the smaller and more superficial veins, removal does not typically impair the circulation. There is evidence that patients with varicose veins suffer a greater risk of developing a thrombus or clot.
Veins as Blood Reservoirs
In addition to their primary function of returning blood to the heart, veins may be considered blood reservoirs, since systemic veins contain approximately 64 percent of the blood volume at any given time (Figure 20.9). Their ability to hold this much blood is due to their high capacitance, that is, their capacity to distend (expand) readily to store a high volume of blood, even at a low pressure. The large lumens and relatively thin walls of veins make them far more distensible than arteries; thus, they are said to be capacitance vessels.
Figure 20.9 Distribution of Blood Flow
When blood flow needs to be redistributed to other portions of the body, the vasomotor center located in the medulla oblongata sends sympathetic stimulation to the smooth muscles in the walls of the veins, causing constriction—or in this case, venoconstriction. Less dramatic than the vasoconstriction seen in smaller arteries and arterioles, venoconstriction may be likened to a “stiffening” of the vessel wall. This increases pressure on the blood within the veins, speeding its return to the heart. As you will note in Figure 20.9, approximately 21 percent of the venous blood is located in venous networks within the liver, bone marrow, and integument. This volume of blood is referred to as venous reserve. Through venoconstriction, this “reserve” volume of blood can get back to the heart more quickly for redistribution to other parts of the circulation.
CAREER CONNECTION
Vascular Surgeons and Technicians
Vascular surgery is a specialty in which the physician deals primarily with diseases of the vascular portion of the cardiovascular system. This includes repair and replacement of diseased or damaged vessels, removal of plaque from vessels, minimally invasive procedures including the insertion of venous catheters, and traditional surgery. Following completion of medical school, the physician generally completes a 5-year surgical residency followed by an additional 1 to 2 years of vascular specialty training. In the United States, most vascular surgeons are members of the Society of Vascular Surgery.
Vascular technicians are specialists in imaging technologies that provide information on the health of the vascular system. They may also assist physicians in treating disorders involving the arteries and veins. This profession often overlaps with cardiovascular technology, which would also include treatments involving the heart. Although recognized by the American Medical Association, there are currently no licensing requirements for vascular technicians, and licensing is voluntary. Vascular technicians typically have an Associate’s degree or certificate, involving 18 months to 2 years of training. The United States Bureau of Labor projects this profession to grow by 29 percent from 2010 to 2020.
INTERACTIVE LINK
Visit this site to learn more about vascular surgery.
INTERACTIVE LINK
Visit this site to learn more about vascular technicians.
Blood Flow, Blood Pressure, and Resistance
- Distinguish between systolic pressure, diastolic pressure, pulse pressure, and mean arterial pressure
- Describe the clinical measurement of pulse and blood pressure
- Identify and discuss five variables affecting arterial blood flow and blood pressure
- Discuss several factors affecting blood flow in the venous system
Blood flow refers to the movement of blood through a vessel, tissue, or organ, and is usually expressed in terms of volume of blood per unit of time. It is initiated by the contraction of the ventricles of the heart. Ventricular contraction ejects blood into the major arteries, resulting in flow from regions of higher pressure to regions of lower pressure, as blood encounters smaller arteries and arterioles, then capillaries, then the venules and veins of the venous system. This section discusses a number of critical variables that contribute to blood flow throughout the body. It also discusses the factors that impede or slow blood flow, a phenomenon known as resistance.
As noted earlier, hydrostatic pressure is the force exerted by a fluid due to gravitational pull, usually against the wall of the container in which it is located. One form of hydrostatic pressure is blood pressure, the force exerted by blood upon the walls of the blood vessels or the chambers of the heart. Blood pressure may be measured in capillaries and veins, as well as the vessels of the pulmonary circulation; however, the term blood pressure without any specific descriptors typically refers to systemic arterial blood pressure—that is, the pressure of blood flowing in the arteries of the systemic circulation. In clinical practice, this pressure is measured in mm Hg and is usually obtained using the brachial artery of the arm.
Components of Arterial Blood Pressure
Arterial blood pressure in the larger vessels consists of several distinct components (Figure 20.10): systolic and diastolic pressures, pulse pressure, and mean arterial pressure.
Systolic and Diastolic Pressures
When systemic arterial blood pressure is measured, it is recorded as a ratio of two numbers (e.g., 120/80 is a normal adult blood pressure), expressed as systolic pressure over diastolic pressure. The systolic pressure is the higher value (typically around 120 mm Hg) and reflects the arterial pressure resulting from the ejection of blood during ventricular contraction, or systole. The diastolic pressure is the lower value (usually about 80 mm Hg) and represents the arterial pressure of blood during ventricular relaxation, or diastole.
Figure 20.10 Systemic Blood Pressure The graph shows the components of blood pressure throughout the blood vessels, including systolic, diastolic, mean arterial, and pulse pressures.
Pulse Pressure
As shown in Figure 20.10, the difference between the systolic pressure and the diastolic pressure is the pulse pressure. For example, an individual with a systolic pressure of 120 mm Hg and a diastolic pressure of 80 mm Hg would have a pulse pressure of 40 mmHg.
Generally, a pulse pressure should be at least 25 percent of the systolic pressure. A pulse pressure below this level is described as low or narrow. This may occur, for example, in patients with a low stroke volume, which may be seen in congestive heart failure, stenosis of the aortic valve, or significant blood loss following trauma. In contrast, a high or wide pulse pressure is common in healthy people following strenuous exercise, when their resting pulse pressure of 30–40 mm Hg may increase temporarily to 100 mm Hg as stroke volume increases. A persistently high pulse pressure at or above 100 mm Hg may indicate excessive resistance in the arteries and can be caused by a variety of disorders. Chronic high resting pulse pressures can degrade the heart, brain, and kidneys, and warrant medical treatment.
Mean Arterial Pressure
Mean arterial pressure (MAP) represents the “average” pressure of blood in the arteries, that is, the average force driving blood into vessels that serve the tissues. Mean is a statistical concept and is calculated by taking the sum of the values divided by the number of values. Although complicated to measure directly and complicated to calculate, MAP can be approximated by adding the diastolic pressure to one-third of the pulse pressure or systolic pressure minus the diastolic pressure:
MAP = diastolic BP + (systolic-diastolic BP)3MAP = diastolic BP + (systolic-diastolic BP)3
In Figure 20.10, this value is approximately 80 + (120 − 80) / 3, or 93.33. Normally, the MAP falls within the range of 70–110 mm Hg. If the value falls below 60 mm Hg for an extended time, blood pressure will not be high enough to ensure circulation to and through the tissues, which results in ischemia, or insufficient blood flow. A condition called hypoxia, inadequate oxygenation of tissues, commonly accompanies ischemia. The term hypoxemia refers to low levels of oxygen in systemic arterial blood. Neurons are especially sensitive to hypoxia and may die or be damaged if blood flow and oxygen supplies are not quickly restored.
Pulse
After blood is ejected from the heart, elastic fibers in the arteries help maintain a high-pressure gradient as they expand to accommodate the blood, then recoil. This expansion and recoiling effect, known as the pulse, can be palpated manually or measured electronically. Although the effect diminishes over distance from the heart, elements of the systolic and diastolic components of the pulse are still evident down to the level of the arterioles.
Because pulse indicates heart rate, it is measured clinically to provide clues to a patient’s state of health. It is recorded as beats per minute. Both the rate and the strength of the pulse are important clinically. A high or irregular pulse rate can be caused by physical activity or other temporary factors, but it may also indicate a heart condition. The pulse strength indicates the strength of ventricular contraction and cardiac output. If the pulse is strong, then systolic pressure is high. If it is weak, systolic pressure has fallen, and medical intervention may be warranted.
Pulse can be palpated manually by placing the tips of the fingers across an artery that runs close to the body surface and pressing lightly. While this procedure is normally performed using the radial artery in the wrist or the common carotid artery in the neck, any superficial artery that can be palpated may be used (Figure 20.11). Common sites to find a pulse include temporal and facial arteries in the head, brachial arteries in the upper arm, femoral arteries in the thigh, popliteal arteries behind the knees, posterior tibial arteries near the medial tarsal regions, and dorsalis pedis arteries in the feet. A variety of commercial electronic devices are also available to measure pulse.
Figure 20.11 Pulse Sites The pulse is most readily measured at the radial artery, but can be measured at any of the pulse points shown.
Measurement of Blood Pressure
Blood pressure is one of the critical parameters measured on virtually every patient in every healthcare setting. The technique used today was developed more than 100 years ago by a pioneering Russian physician, Dr. Nikolai Korotkoff. Turbulent blood flow through the vessels can be heard as a soft ticking while measuring blood pressure; these sounds are known as Korotkoff sounds. The technique of measuring blood pressure requires the use of a sphygmomanometer (a blood pressure cuff attached to a measuring device) and a stethoscope. The technique is as follows:
- The clinician wraps an inflatable cuff tightly around the patient’s arm at about the level of the heart.
- The clinician squeezes a rubber pump to inject air into the cuff, raising pressure around the artery and temporarily cutting off blood flow into the patient’s arm.
- The clinician places the stethoscope on the patient’s antecubital region and, while gradually allowing air within the cuff to escape, listens for the Korotkoff sounds.
Although there are five recognized Korotkoff sounds, only two are normally recorded. Initially, no sounds are heard since there is no blood flow through the vessels, but as air pressure drops, the cuff relaxes, and blood flow returns to the arm. As shown in Figure 20.12, the first sound heard through the stethoscope—the first Korotkoff sound—indicates systolic pressure. As more air is released from the cuff, blood is able to flow freely through the brachial artery and all sounds disappear. The point at which the last sound is heard is recorded as the patient’s diastolic pressure.
Figure 20.12 Blood Pressure Measurement When pressure in a sphygmomanometer cuff is released, a clinician can hear the Korotkoff sounds. In this graph, a blood pressure tracing is aligned to a measurement of systolic and diastolic pressures.
The majority of hospitals and clinics have automated equipment for measuring blood pressure that work on the same principles. An even more recent innovation is a small instrument that wraps around a patient’s wrist. The patient then holds the wrist over the heart while the device measures blood flow and records pressure.
Variables Affecting Blood Flow and Blood Pressure
Five variables influence blood flow and blood pressure:
- Cardiac output
- Compliance
- Volume of the blood
- Viscosity of the blood
- Blood vessel length and diameter
Recall that blood moves from higher pressure to lower pressure. It is pumped from the heart into the arteries at high pressure. If you increase pressure in the arteries (afterload), and cardiac function does not compensate, blood flow will actually decrease. In the venous system, the opposite relationship is true. Increased pressure in the veins does not decrease flow as it does in arteries, but actually increases flow. Since pressure in the veins is normally relatively low, for blood to flow back into the heart, the pressure in the atria during atrial diastole must be even lower. It normally approaches zero, except when the atria contract (see Figure 20.10).
Cardiac Output
Cardiac output is the measurement of blood flow from the heart through the ventricles, and is usually measured in liters per minute. Any factor that causes cardiac output to increase, by elevating heart rate or stroke volume or both, will elevate blood pressure and promote blood flow. These factors include sympathetic stimulation, the catecholamines epinephrine and norepinephrine, thyroid hormones, and increased calcium ion levels. Conversely, any factor that decreases cardiac output, by decreasing heart rate or stroke volume or both, will decrease arterial pressure and blood flow. These factors include parasympathetic stimulation, elevated or decreased potassium ion levels, decreased calcium levels, anoxia, and acidosis.
Compliance
Compliance is the ability of any compartment to expand to accommodate increased content. A metal pipe, for example, is not compliant, whereas a balloon is. The greater the compliance of an artery, the more effectively it is able to expand to accommodate surges in blood flow without increased resistance or blood pressure. Veins are more compliant than arteries and can expand to hold more blood. When vascular disease causes stiffening of arteries, compliance is reduced and resistance to blood flow is increased. The result is more turbulence, higher pressure within the vessel, and reduced blood flow. This increases the work of the heart.
A Mathematical Approach to Factors Affecting Blood Flow
Jean Louis Marie Poiseuille was a French physician and physiologist who devised a mathematical equation describing blood flow and its relationship to known parameters. The same equation also applies to engineering studies of the flow of fluids. Although understanding the math behind the relationships among the factors affecting blood flow is not necessary to understand blood flow, it can help solidify an understanding of their relationships. Please note that even if the equation looks intimidating, breaking it down into its components and following the relationships will make these relationships clearer, even if you are weak in math. Focus on the three critical variables: radius (r), vessel length (λ), and viscosity (η).
Poiseuille’s equation:
Blood flow = π ΔP r48ηλBlood flow = π ΔP r48ηλ- π is the Greek letter pi, used to represent the mathematical constant that is the ratio of a circle’s circumference to its diameter. It may commonly be represented as 3.14, although the actual number extends to infinity.
- ΔP represents the difference in pressure.
- r4 is the radius (one-half of the diameter) of the vessel to the fourth power.
- η is the Greek letter eta and represents the viscosity of the blood.
- λ is the Greek letter lambda and represents the length of a blood vessel.
One of several things this equation allows us to do is calculate the resistance in the vascular system. Normally this value is extremely difficult to measure, but it can be calculated from this known relationship:
Blood flow = ΔPResistanceBlood flow = ΔPResistanceIf we rearrange this slightly,
Resistance = ΔPBlood flowResistance = ΔPBlood flowThen by substituting Pouseille’s equation for blood flow:
Resistance =8ηλπr4Resistance =8ηλπr4By examining this equation, you can see that there are only three variables: viscosity, vessel length, and radius, since 8 and π are both constants. The important thing to remember is this: Two of these variables, viscosity and vessel length, will change slowly in the body. Only one of these factors, the radius, can be changed rapidly by vasoconstriction and vasodilation, thus dramatically impacting resistance and flow. Further, small changes in the radius will greatly affect flow, since it is raised to the fourth power in the equation.
We have briefly considered how cardiac output and blood volume impact blood flow and pressure; the next step is to see how the other variables (contraction, vessel length, and viscosity) articulate with Pouseille’s equation and what they can teach us about the impact on blood flow.
Blood Volume
The relationship between blood volume, blood pressure, and blood flow is intuitively obvious. Water may merely trickle along a creek bed in a dry season, but rush quickly and under great pressure after a heavy rain. Similarly, as blood volume decreases, pressure and flow decrease. As blood volume increases, pressure and flow increase.
Under normal circumstances, blood volume varies little. Low blood volume, called hypovolemia, may be caused by bleeding, dehydration, vomiting, severe burns, or some medications used to treat hypertension. It is important to recognize that other regulatory mechanisms in the body are so effective at maintaining blood pressure that an individual may be asymptomatic until 10–20 percent of the blood volume has been lost. Treatment typically includes intravenous fluid replacement.
Hypervolemia, excessive fluid volume, may be caused by retention of water and sodium, as seen in patients with heart failure, liver cirrhosis, some forms of kidney disease, hyperaldosteronism, and some glucocorticoid steroid treatments. Restoring homeostasis in these patients depends upon reversing the condition that triggered the hypervolemia.
Blood Viscosity
Viscosity is the thickness of fluids that affects their ability to flow. Clean water, for example, is less viscous than mud. The viscosity of blood is directly proportional to resistance and inversely proportional to flow; therefore, any condition that causes viscosity to increase will also increase resistance and decrease flow. For example, imagine sipping milk, then a milkshake, through the same size straw. You experience more resistance and therefore less flow from the milkshake. Conversely, any condition that causes viscosity to decrease (such as when the milkshake melts) will decrease resistance and increase flow.
Normally the viscosity of blood does not change over short periods of time. The two primary determinants of blood viscosity are the formed elements and plasma proteins. Since the vast majority of formed elements are erythrocytes, any condition affecting erythropoiesis, such as polycythemia or anemia, can alter viscosity. Since most plasma proteins are produced by the liver, any condition affecting liver function can also change the viscosity slightly and therefore alter blood flow. Liver abnormalities such as hepatitis, cirrhosis, alcohol damage, and drug toxicities result in decreased levels of plasma proteins, which decrease blood viscosity. While leukocytes and platelets are normally a small component of the formed elements, there are some rare conditions in which severe overproduction can impact viscosity as well.
Vessel Length and Diameter
The length of a vessel is directly proportional to its resistance: the longer the vessel, the greater the resistance and the lower the flow. As with blood volume, this makes intuitive sense, since the increased surface area of the vessel will impede the flow of blood. Likewise, if the vessel is shortened, the resistance will decrease and flow will increase.
The length of our blood vessels increases throughout childhood as we grow, of course, but is unchanging in adults under normal physiological circumstances. Further, the distribution of vessels is not the same in all tissues. Adipose tissue does not have an extensive vascular supply. One pound of adipose tissue contains approximately 200 miles of vessels, whereas skeletal muscle contains more than twice that. Overall, vessels decrease in length only during loss of mass or amputation. An individual weighing 150 pounds has approximately 60,000 miles of vessels in the body. Gaining about 10 pounds adds from 2000 to 4000 miles of vessels, depending upon the nature of the gained tissue. One of the great benefits of weight reduction is the reduced stress to the heart, which does not have to overcome the resistance of as many miles of vessels.
In contrast to length, the diameter of blood vessels changes throughout the body, according to the type of vessel, as we discussed earlier. The diameter of any given vessel may also change frequently throughout the day in response to neural and chemical signals that trigger vasodilation and vasoconstriction. The vascular tone of the vessel is the contractile state of the smooth muscle and the primary determinant of diameter, and thus of resistance and flow. The effect of vessel diameter on resistance is inverse: Given the same volume of blood, an increased diameter means there is less blood contacting the vessel wall, thus lower friction and lower resistance, subsequently increasing flow. A decreased diameter means more of the blood contacts the vessel wall, and resistance increases, subsequently decreasing flow.
The influence of lumen diameter on resistance is dramatic: A slight increase or decrease in diameter causes a huge decrease or increase in resistance. This is because resistance is inversely proportional to the radius of the blood vessel (one-half of the vessel’s diameter) raised to the fourth power (R = 1/r4). This means, for example, that if an artery or arteriole constricts to one-half of its original radius, the resistance to flow will increase 16 times. And if an artery or arteriole dilates to twice its initial radius, then resistance in the vessel will decrease to 1/16 of its original value and flow will increase 16 times.
The Roles of Vessel Diameter and Total Area in Blood Flow and Blood Pressure
Recall that we classified arterioles as resistance vessels, because given their small lumen, they dramatically slow the flow of blood from arteries. In fact, arterioles are the site of greatest resistance in the entire vascular network. This may seem surprising, given that capillaries have a smaller size. How can this phenomenon be explained?
Figure 20.13 compares vessel diameter, total cross-sectional area, average blood pressure, and blood velocity through the systemic vessels. Notice in parts (a) and (b) that the total cross-sectional area of the body’s capillary beds is far greater than any other type of vessel. Although the diameter of an individual capillary is significantly smaller than the diameter of an arteriole, there are vastly more capillaries in the body than there are other types of blood vessels. Part (c) shows that blood pressure drops unevenly as blood travels from arteries to arterioles, capillaries, venules, and veins, and encounters greater resistance. However, the site of the most precipitous drop, and the site of greatest resistance, is the arterioles. This explains why vasodilation and vasoconstriction of arterioles play more significant roles in regulating blood pressure than do the vasodilation and vasoconstriction of other vessels.
Part (d) shows that the velocity (speed) of blood flow decreases dramatically as the blood moves from arteries to arterioles to capillaries. This slow flow rate allows more time for exchange processes to occur. As blood flows through the veins, the rate of velocity increases, as blood is returned to the heart.
Figure 20.13 Relationships among Vessels in the Systemic Circuit The relationships among blood vessels that can be compared include (a) vessel diameter, (b) total cross-sectional area, (c) average blood pressure, and (d) velocity of blood flow.
DISORDERS OF THE...
Cardiovascular System: Arteriosclerosis
Compliance allows an artery to expand when blood is pumped through it from the heart, and then to recoil after the surge has passed. This helps promote blood flow. In arteriosclerosis, compliance is reduced, and pressure and resistance within the vessel increase. This is a leading cause of hypertension and coronary heart disease, as it causes the heart to work harder to generate a pressure great enough to overcome the resistance.
Arteriosclerosis begins with injury to the endothelium of an artery, which may be caused by irritation from high blood glucose, infection, tobacco use, excessive blood lipids, and other factors. Artery walls that are constantly stressed by blood flowing at high pressure are also more likely to be injured—which means that hypertension can promote arteriosclerosis, as well as result from it.
Recall that tissue injury causes inflammation. As inflammation spreads into the artery wall, it weakens and scars it, leaving it stiff (sclerotic). As a result, compliance is reduced. Moreover, circulating triglycerides and cholesterol can seep between the damaged lining cells and become trapped within the artery wall, where they are frequently joined by leukocytes, calcium, and cellular debris. Eventually, this buildup, called plaque, can narrow arteries enough to impair blood flow. The term for this condition, atherosclerosis (athero- = “porridge”) describes the mealy deposits (Figure 20.14).
Figure 20.14 Atherosclerosis (a) Atherosclerosis can result from plaques formed by the buildup of fatty, calcified deposits in an artery. (b) Plaques can also take other forms, as shown in this micrograph of a coronary artery that has a buildup of connective tissue within the artery wall. LM × 40. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
Sometimes a plaque can rupture, causing microscopic tears in the artery wall that allow blood to leak into the tissue on the other side. When this happens, platelets rush to the site to clot the blood. This clot can further obstruct the artery and—if it occurs in a coronary or cerebral artery—cause a sudden heart attack or stroke. Alternatively, plaque can break off and travel through the bloodstream as an embolus until it blocks a more distant, smaller artery.
Even without total blockage, vessel narrowing leads to ischemia—reduced blood flow—to the tissue region “downstream” of the narrowed vessel. Ischemia in turn leads to hypoxia—decreased supply of oxygen to the tissues. Hypoxia involving cardiac muscle or brain tissue can lead to cell death and severe impairment of brain or heart function.
A major risk factor for both arteriosclerosis and atherosclerosis is advanced age, as the conditions tend to progress over time. Arteriosclerosis is normally defined as the more generalized loss of compliance, “hardening of the arteries,” whereas atherosclerosis is a more specific term for the build-up of plaque in the walls of the vessel and is a specific type of arteriosclerosis. There is also a distinct genetic component, and pre-existing hypertension and/or diabetes also greatly increase the risk. However, obesity, poor nutrition, lack of physical activity, and tobacco use all are major risk factors.
Treatment includes lifestyle changes, such as weight loss, smoking cessation, regular exercise, and adoption of a diet low in sodium and saturated fats. Medications to reduce cholesterol and blood pressure may be prescribed. For blocked coronary arteries, surgery is warranted. In angioplasty, a catheter is inserted into the vessel at the point of narrowing, and a second catheter with a balloon-like tip is inflated to widen the opening. To prevent subsequent collapse of the vessel, a small mesh tube called a stent is often inserted. In an endarterectomy, plaque is surgically removed from the walls of a vessel. This operation is typically performed on the carotid arteries of the neck, which are a prime source of oxygenated blood for the brain. In a coronary bypass procedure, a non-vital superficial vessel from another part of the body (often the great saphenous vein) or a synthetic vessel is inserted to create a path around the blocked area of a coronary artery.
Venous System
The pumping action of the heart propels the blood into the arteries, from an area of higher pressure toward an area of lower pressure. If blood is to flow from the veins back into the heart, the pressure in the veins must be greater than the pressure in the atria of the heart. Two factors help maintain this pressure gradient between the veins and the heart. First, the pressure in the atria during diastole is very low, often approaching zero when the atria are relaxed (atrial diastole). Second, two physiologic “pumps” increase pressure in the venous system. The use of the term “pump” implies a physical device that speeds flow. These physiological pumps are less obvious.
Skeletal Muscle Pump
In many body regions, the pressure within the veins can be increased by the contraction of the surrounding skeletal muscle. This mechanism, known as the skeletal muscle pump (Figure 20.15), helps the lower-pressure veins counteract the force of gravity, increasing pressure to move blood back to the heart. As leg muscles contract, for example during walking or running, they exert pressure on nearby veins with their numerous one-way valves. This increased pressure causes blood to flow upward, opening valves superior to the contracting muscles so blood flows through. Simultaneously, valves inferior to the contracting muscles close; thus, blood should not seep back downward toward the feet. Military recruits are trained to flex their legs slightly while standing at attention for prolonged periods. Failure to do so may allow blood to pool in the lower limbs rather than returning to the heart. Consequently, the brain will not receive enough oxygenated blood, and the individual may lose consciousness.
Figure 20.15 Skeletal Muscle Pump The contraction of skeletal muscles surrounding a vein compresses the blood and increases the pressure in that area. This action forces blood closer to the heart where venous pressure is lower. Note the importance of the one-way valves to assure that blood flows only in the proper direction.
Respiratory Pump
The respiratory pump aids blood flow through the veins of the thorax and abdomen. During inhalation, the volume of the thorax increases, largely through the contraction of the diaphragm, which moves downward and compresses the abdominal cavity. The elevation of the chest caused by the contraction of the external intercostal muscles also contributes to the increased volume of the thorax. The volume increase causes air pressure within the thorax to decrease, allowing us to inhale. Additionally, as air pressure within the thorax drops, blood pressure in the thoracic veins also decreases, falling below the pressure in the abdominal veins. This causes blood to flow along its pressure gradient from veins outside the thorax, where pressure is higher, into the thoracic region, where pressure is now lower. This in turn promotes the return of blood from the thoracic veins to the atria. During exhalation, when air pressure increases within the thoracic cavity, pressure in the thoracic veins increases, speeding blood flow into the heart while valves in the veins prevent blood from flowing backward from the thoracic and abdominal veins.
Pressure Relationships in the Venous System
Although vessel diameter increases from the smaller venules to the larger veins and eventually to the venae cavae (singular = vena cava), the total cross-sectional area actually decreases (see Figure 20.15a and b). The individual veins are larger in diameter than the venules, but their total number is much lower, so their total cross-sectional area is also lower.
Also notice that, as blood moves from venules to veins, the average blood pressure drops (see Figure 20.15c), but the blood velocity actually increases (see Figure 20.15). This pressure gradient drives blood back toward the heart. Again, the presence of one-way valves and the skeletal muscle and respiratory pumps contribute to this increased flow. Since approximately 64 percent of the total blood volume resides in systemic veins, any action that increases the flow of blood through the veins will increase venous return to the heart. Maintaining vascular tone within the veins prevents the veins from merely distending, dampening the flow of blood, and as you will see, vasoconstriction actually enhances the flow.
The Role of Venoconstriction in Resistance, Blood Pressure, and Flow
As previously discussed, vasoconstriction of an artery or arteriole decreases the radius, increasing resistance and pressure, but decreasing flow. Venoconstriction, on the other hand, has a very different outcome. The walls of veins are thin but irregular; thus, when the smooth muscle in those walls constricts, the lumen becomes more rounded. The more rounded the lumen, the less surface area the blood encounters, and the less resistance the vessel offers. Vasoconstriction increases pressure within a vein as it does in an artery, but in veins, the increased pressure increases flow. Recall that the pressure in the atria, into which the venous blood will flow, is very low, approaching zero for at least part of the relaxation phase of the cardiac cycle. Thus, venoconstriction increases the return of blood to the heart. Another way of stating this is that venoconstriction increases the preload or stretch of the cardiac muscle and increases contraction.
Capillary Exchange
- Identify the primary mechanisms of capillary exchange
- Distinguish between capillary hydrostatic pressure and blood colloid osmotic pressure, explaining the contribution of each to net filtration pressure
- Compare filtration and reabsorption
- Explain the fate of fluid that is not reabsorbed from the tissues into the vascular capillaries
The primary purpose of the cardiovascular system is to circulate gases, nutrients, wastes, and other substances to and from the cells of the body. Small molecules, such as gases, lipids, and lipid-soluble molecules, can diffuse directly through the membranes of the endothelial cells of the capillary wall. Glucose, amino acids, and ions—including sodium, potassium, calcium, and chloride—use transporters to move through specific channels in the membrane by facilitated diffusion. Glucose, ions, and larger molecules may also leave the blood through intercellular clefts. Larger molecules can pass through the pores of fenestrated capillaries, and even large plasma proteins can pass through the great gaps in the sinusoids. Some large proteins in blood plasma can move into and out of the endothelial cells packaged within vesicles by endocytosis and exocytosis. Water moves by osmosis.
Bulk Flow
The mass movement of fluids into and out of capillary beds requires a transport mechanism far more efficient than mere diffusion. This movement, often referred to as bulk flow, involves two pressure-driven mechanisms: Volumes of fluid move from an area of higher pressure in a capillary bed to an area of lower pressure in the tissues via filtration. In contrast, the movement of fluid from an area of higher pressure in the tissues into an area of lower pressure in the capillaries is reabsorption. Two types of pressure interact to drive each of these movements: hydrostatic pressure and osmotic pressure.
Hydrostatic Pressure
The primary force driving fluid transport between the capillaries and tissues is hydrostatic pressure, which can be defined as the pressure of any fluid enclosed in a space. Blood hydrostatic pressure is the force exerted by the blood confined within blood vessels or heart chambers. Even more specifically, the pressure exerted by blood against the wall of a capillary is called capillary hydrostatic pressure (CHP), and is the same as capillary blood pressure. CHP is the force that drives fluid out of capillaries and into the tissues.
As fluid exits a capillary and moves into tissues, the hydrostatic pressure in the interstitial fluid correspondingly rises. This opposing hydrostatic pressure is called the interstitial fluid hydrostatic pressure (IFHP). Generally, the CHP originating from the arterial pathways is considerably higher than the IFHP, because lymphatic vessels are continually absorbing excess fluid from the tissues. Thus, fluid generally moves out of the capillary and into the interstitial fluid. This process is called filtration.
Osmotic Pressure
The net pressure that drives reabsorption—the movement of fluid from the interstitial fluid back into the capillaries—is called osmotic pressure (sometimes referred to as oncotic pressure). Whereas hydrostatic pressure forces fluid out of the capillary, osmotic pressure draws fluid back in. Osmotic pressure is determined by osmotic concentration gradients, that is, the difference in the solute-to-water concentrations in the blood and tissue fluid. A region higher in solute concentration (and lower in water concentration) draws water across a semipermeable membrane from a region higher in water concentration (and lower in solute concentration).
As we discuss osmotic pressure in blood and tissue fluid, it is important to recognize that the formed elements of blood do not contribute to osmotic concentration gradients. Rather, it is the plasma proteins that play the key role. Solutes also move across the capillary wall according to their concentration gradient, but overall, the concentrations should be similar and not have a significant impact on osmosis. Because of their large size and chemical structure, plasma proteins are not truly solutes, that is, they do not dissolve but are dispersed or suspended in their fluid medium, forming a colloid rather than a solution.
The pressure created by the concentration of colloidal proteins in the blood is called the blood colloidal osmotic pressure (BCOP). Its effect on capillary exchange accounts for the reabsorption of water. The plasma proteins suspended in blood cannot move across the semipermeable capillary cell membrane, and so they remain in the plasma. As a result, blood has a higher colloidal concentration and lower water concentration than tissue fluid. It therefore attracts water. We can also say that the BCOP is higher than the interstitial fluid colloidal osmotic pressure (IFCOP), which is always very low because interstitial fluid contains few proteins. Thus, water is drawn from the tissue fluid back into the capillary, carrying dissolved molecules with it. This difference in colloidal osmotic pressure accounts for reabsorption.
Interaction of Hydrostatic and Osmotic Pressures
The normal unit used to express pressures within the cardiovascular system is millimeters of mercury (mm Hg). When blood leaving an arteriole first enters a capillary bed, the CHP is quite high—about 35 mm Hg. Gradually, this initial CHP declines as the blood moves through the capillary so that by the time the blood has reached the venous end, the CHP has dropped to approximately 18 mm Hg. In comparison, the plasma proteins remain suspended in the blood, so the BCOP remains fairly constant at about 25 mm Hg throughout the length of the capillary and considerably below the osmotic pressure in the interstitial fluid.
The net filtration pressure (NFP) represents the interaction of the hydrostatic and osmotic pressures, driving fluid out of the capillary. It is equal to the difference between the CHP and the BCOP. Since filtration is, by definition, the movement of fluid out of the capillary, when reabsorption is occurring, the NFP is a negative number.
NFP changes at different points in a capillary bed (Figure 20.16). Close to the arterial end of the capillary, it is approximately 10 mm Hg, because the CHP of 35 mm Hg minus the BCOP of 25 mm Hg equals 10 mm Hg. Recall that the hydrostatic and osmotic pressures of the interstitial fluid are essentially negligible. Thus, the NFP of 10 mm Hg drives a net movement of fluid out of the capillary at the arterial end. At approximately the middle of the capillary, the CHP is about the same as the BCOP of 25 mm Hg, so the NFP drops to zero. At this point, there is no net change of volume: Fluid moves out of the capillary at the same rate as it moves into the capillary. Near the venous end of the capillary, the CHP has dwindled to about 18 mm Hg due to loss of fluid. Because the BCOP remains steady at 25 mm Hg, water is drawn into the capillary, that is, reabsorption occurs. Another way of expressing this is to say that at the venous end of the capillary, there is an NFP of −7 mm Hg.
Figure 20.16 Capillary Exchange Net filtration occurs near the arterial end of the capillary since capillary hydrostatic pressure (CHP) is greater than blood colloidal osmotic pressure (BCOP). There is no net movement of fluid near the midpoint since CHP = BCOP. Net reabsorption occurs near the venous end since BCOP is greater than CHP.
The Role of Lymphatic Capillaries
Since overall CHP is higher than BCOP, it is inevitable that more net fluid will exit the capillary through filtration at the arterial end than enters through reabsorption at the venous end. Considering all capillaries over the course of a day, this can be quite a substantial amount of fluid: Approximately 24 liters per day are filtered, whereas 20.4 liters are reabsorbed. This excess fluid is picked up by capillaries of the lymphatic system. These extremely thin-walled vessels have copious numbers of valves that ensure unidirectional flow through ever-larger lymphatic vessels that eventually drain into the subclavian veins in the neck. An important function of the lymphatic system is to return the fluid (lymph) to the blood. Lymph may be thought of as recycled blood plasma. (Seek additional content for more detail on the lymphatic system.)
INTERACTIVE LINK
Watch this video to explore capillaries and how they function in the body. Capillaries are never more than 100 micrometers away. What is the main component of interstitial fluid?
Homeostatic Regulation of the Vascular System
- Discuss the mechanisms involved in the neural regulation of vascular homeostasis
- Describe the contribution of a variety of hormones to the renal regulation of blood pressure
- Identify the effects of exercise on vascular homeostasis
- Discuss how hypertension, hemorrhage, and circulatory shock affect vascular health
In order to maintain homeostasis in the cardiovascular system and provide adequate blood to the tissues, blood flow must be redirected continually to the tissues as they become more active. In a very real sense, the cardiovascular system engages in resource allocation, because there is not enough blood flow to distribute blood equally to all tissues simultaneously. For example, when an individual is exercising, more blood will be directed to skeletal muscles, the heart, and the lungs. Following a meal, more blood is directed to the digestive system. Only the brain receives a more or less constant supply of blood whether you are active, resting, thinking, or engaged in any other activity.
Table 20.3 provides the distribution of systemic blood at rest and during exercise. Although most of the data appears logical, the values for the distribution of blood to the integument may seem surprising. During exercise, the body distributes more blood to the body surface where it can dissipate the excess heat generated by increased activity into the environment.
Systemic Blood Flow During Rest, Mild Exercise, and Maximal Exercise in a Healthy Young Individual
| Organ | Resting (mL/min) | Mild exercise (mL/min) | Maximal exercise (mL/min) |
|---|---|---|---|
| Skeletal muscle | 1200 | 4500 | 12,500 |
| Heart | 250 | 350 | 750 |
| Brain | 750 | 750 | 750 |
| Integument | 500 | 1500 | 1900 |
| Kidney | 1100 | 900 | 600 |
| Gastrointestinal | 1400 | 1100 | 600 |
| Others (i.e., liver, spleen) | 600 | 400 | 400 |
| Total | 5800 | 9500 | 17,500 |
Table 20.3
Three homeostatic mechanisms ensure adequate blood flow, blood pressure, distribution, and ultimately perfusion: neural, endocrine, and autoregulatory mechanisms. They are summarized in Figure 20.17.
Figure 20.17 Summary of Factors Maintaining Vascular Homeostasis Adequate blood flow, blood pressure, distribution, and perfusion involve autoregulatory, neural, and endocrine mechanisms.
Neural Regulation
The nervous system plays a critical role in the regulation of vascular homeostasis. The primary regulatory sites include the cardiovascular centers in the brain that control both cardiac and vascular functions. In addition, more generalized neural responses from the limbic system and the autonomic nervous system are factors.
The Cardiovascular Centers in the Brain
Neurological regulation of blood pressure and flow depends on the cardiovascular centers located in the medulla oblongata. This cluster of neurons responds to changes in blood pressure as well as blood concentrations of oxygen, carbon dioxide, and hydrogen ions. The cardiovascular center contains three distinct paired components:
- The cardioaccelerator centers stimulate cardiac function by regulating heart rate and stroke volume via sympathetic stimulation from the cardiac accelerator nerve.
- The cardioinhibitor centers slow cardiac function by decreasing heart rate and stroke volume via parasympathetic stimulation from the vagus nerve.
- The vasomotor centers control vessel tone or contraction of the smooth muscle in the tunica media. Changes in diameter affect peripheral resistance, pressure, and flow, which affect cardiac output. The majority of these neurons act via the release of the neurotransmitter norepinephrine from sympathetic neurons.
Although each center functions independently, they are not anatomically distinct.
There is also a small population of neurons that control vasodilation in the vessels of the brain and skeletal muscles by relaxing the smooth muscle fibers in the vessel tunics. Many of these are cholinergic neurons, that is, they release acetylcholine, which in turn stimulates the vessels’ endothelial cells to release nitric oxide (NO), which causes vasodilation. Others release norepinephrine that binds to β2 receptors. A few neurons release NO directly as a neurotransmitter.
Recall that mild stimulation of the skeletal muscles maintains muscle tone. A similar phenomenon occurs with vascular tone in vessels. As noted earlier, arterioles are normally partially constricted: With maximal stimulation, their radius may be reduced to one-half of the resting state. Full dilation of most arterioles requires that this sympathetic stimulation be suppressed. When it is, an arteriole can expand by as much as 150 percent. Such a significant increase can dramatically affect resistance, pressure, and flow.
Baroreceptor Reflexes
Baroreceptors are specialized stretch receptors located within thin areas of blood vessels and heart chambers that respond to the degree of stretch caused by the presence of blood. They send impulses to the cardiovascular center to regulate blood pressure. Vascular baroreceptors are found primarily in sinuses (small cavities) within the aorta and carotid arteries: The aortic sinuses are found in the walls of the ascending aorta just superior to the aortic valve, whereas the carotid sinuses are in the base of the internal carotid arteries. There are also low-pressure baroreceptors located in the walls of the venae cavae and right atrium.
When blood pressure increases, the baroreceptors are stretched more tightly and initiate action potentials at a higher rate. At lower blood pressures, the degree of stretch is lower and the rate of firing is slower. When the cardiovascular center in the medulla oblongata receives this input, it triggers a reflex that maintains homeostasis (Figure 20.18):
- When blood pressure rises too high, the baroreceptors fire at a higher rate and trigger parasympathetic stimulation of the heart. As a result, cardiac output falls. Sympathetic stimulation of the peripheral arterioles will also decrease, resulting in vasodilation. Combined, these activities cause blood pressure to fall.
- When blood pressure drops too low, the rate of baroreceptor firing decreases. This will trigger an increase in sympathetic stimulation of the heart, causing cardiac output to increase. It will also trigger sympathetic stimulation of the peripheral vessels, resulting in vasoconstriction. Combined, these activities cause blood pressure to rise.
Figure 20.18 Baroreceptor Reflexes for Maintaining Vascular Homeostasis Increased blood pressure results in increased rates of baroreceptor firing, whereas decreased blood pressure results in slower rates of fire, both initiating the homeostatic mechanism to restore blood pressure.
The baroreceptors in the venae cavae and right atrium monitor blood pressure as the blood returns to the heart from the systemic circulation. Normally, blood flow into the aorta is the same as blood flow back into the right atrium. If blood is returning to the right atrium more rapidly than it is being ejected from the left ventricle, the atrial receptors will stimulate the cardiovascular centers to increase sympathetic firing and increase cardiac output until homeostasis is achieved. The opposite is also true. This mechanism is referred to as the atrial reflex.
Chemoreceptor Reflexes
In addition to the baroreceptors are chemoreceptors that monitor levels of oxygen, carbon dioxide, and hydrogen ions (pH), and thereby contribute to vascular homeostasis. Chemoreceptors monitoring the blood are located in close proximity to the baroreceptors in the aortic and carotid sinuses. They signal the cardiovascular center as well as the respiratory centers in the medulla oblongata.
Since tissues consume oxygen and produce carbon dioxide and acids as waste products, when the body is more active, oxygen levels fall and carbon dioxide levels rise as cells undergo cellular respiration to meet the energy needs of activities. This causes more hydrogen ions to be produced, causing the blood pH to drop. When the body is resting, oxygen levels are higher, carbon dioxide levels are lower, more hydrogen is bound, and pH rises. (Seek additional content for more detail about pH.)
The chemoreceptors respond to increasing carbon dioxide and hydrogen ion levels (falling pH) by stimulating the cardioaccelerator and vasomotor centers, increasing cardiac output and constricting peripheral vessels. The cardioinhibitor centers are suppressed. With falling carbon dioxide and hydrogen ion levels (increasing pH), the cardioinhibitor centers are stimulated, and the cardioaccelerator and vasomotor centers are suppressed, decreasing cardiac output and causing peripheral vasodilation. In order to maintain adequate supplies of oxygen to the cells and remove waste products such as carbon dioxide, it is essential that the respiratory system respond to changing metabolic demands. In turn, the cardiovascular system will transport these gases to the lungs for exchange, again in accordance with metabolic demands. This interrelationship of cardiovascular and respiratory control cannot be overemphasized.
Other neural mechanisms can also have a significant impact on cardiovascular function. These include the limbic system that links physiological responses to psychological stimuli, as well as generalized sympathetic and parasympathetic stimulation.
Endocrine Regulation
Endocrine control over the cardiovascular system involves the catecholamines, epinephrine and norepinephrine, as well as several hormones that interact with the kidneys in the regulation of blood volume.
Epinephrine and Norepinephrine
The catecholamines epinephrine and norepinephrine are released by the adrenal medulla, and enhance and extend the body’s sympathetic or “fight-or-flight” response (see Figure 20.17). They increase heart rate and force of contraction, while temporarily constricting blood vessels to organs not essential for flight-or-fight responses and redirecting blood flow to the liver, muscles, and heart.
Antidiuretic Hormone
Antidiuretic hormone (ADH), also known as vasopressin, is secreted by the cells in the hypothalamus and transported via the hypothalamic-hypophyseal tracts to the posterior pituitary where it is stored until released upon nervous stimulation. The primary trigger prompting the hypothalamus to release ADH is increasing osmolarity of tissue fluid, usually in response to significant loss of blood volume. ADH signals its target cells in the kidneys to reabsorb more water, thus preventing the loss of additional fluid in the urine. This will increase overall fluid levels and help restore blood volume and pressure. In addition, ADH constricts peripheral vessels.
Renin-Angiotensin-Aldosterone Mechanism
The renin-angiotensin-aldosterone mechanism has a major effect upon the cardiovascular system (Figure 20.19). Renin is an enzyme, although because of its importance in the renin-angiotensin-aldosterone pathway, some sources identify it as a hormone. Specialized cells in the kidneys found in the juxtaglomerular apparatus respond to decreased blood flow by secreting renin into the blood. Renin converts the plasma protein angiotensinogen, which is produced by the liver, into its active form—angiotensin I. Angiotensin I circulates in the blood and is then converted into angiotensin II in the lungs. This reaction is catalyzed by the enzyme angiotensin-converting enzyme (ACE).
Angiotensin II is a powerful vasoconstrictor, greatly increasing blood pressure. It also stimulates the release of ADH and aldosterone, a hormone produced by the adrenal cortex. Aldosterone increases the reabsorption of sodium into the blood by the kidneys. Since water follows sodium, this increases the reabsorption of water. This in turn increases blood volume, raising blood pressure. Angiotensin II also stimulates the thirst center in the hypothalamus, so an individual will likely consume more fluids, again increasing blood volume and pressure.
Figure 20.19 Hormones Involved in Renal Control of Blood Pressure In the renin-angiotensin-aldosterone mechanism, increasing angiotensin II will stimulate the production of antidiuretic hormone and aldosterone. In addition to renin, the kidneys produce erythropoietin, which stimulates the production of red blood cells, further increasing blood volume.
Erythropoietin
Erythropoietin (EPO) is released by the kidneys when blood flow and/or oxygen levels decrease. EPO stimulates the production of erythrocytes within the bone marrow. Erythrocytes are the major formed element of the blood and may contribute 40 percent or more to blood volume, a significant factor of viscosity, resistance, pressure, and flow. In addition, EPO is a vasoconstrictor. Overproduction of EPO or excessive intake of synthetic EPO, often to enhance athletic performance, will increase viscosity, resistance, and pressure, and decrease flow in addition to its contribution as a vasoconstrictor.
Atrial Natriuretic Hormone
Secreted by cells in the atria of the heart, atrial natriuretic hormone (ANH) (also known as atrial natriuretic peptide) is secreted when blood volume is high enough to cause extreme stretching of the cardiac cells. Cells in the ventricle produce a hormone with similar effects, called B-type natriuretic hormone. Natriuretic hormones are antagonists to angiotensin II. They promote loss of sodium and water from the kidneys, and suppress renin, aldosterone, and ADH production and release. All of these actions promote loss of fluid from the body, so blood volume and blood pressure drop.
Autoregulation of Perfusion
As the name would suggest, autoregulation mechanisms require neither specialized nervous stimulation nor endocrine control. Rather, these are local, self-regulatory mechanisms that allow each region of tissue to adjust its blood flow—and thus its perfusion. These local mechanisms include chemical signals and myogenic controls.
Chemical Signals Involved in Autoregulation
Chemical signals work at the level of the precapillary sphincters to trigger either constriction or relaxation. As you know, opening a precapillary sphincter allows blood to flow into that particular capillary, whereas constricting a precapillary sphincter temporarily shuts off blood flow to that region. The factors involved in regulating the precapillary sphincters include the following:
- Opening of the sphincter is triggered in response to decreased oxygen concentrations; increased carbon dioxide concentrations; increasing levels of lactic acid or other byproducts of cellular metabolism; increasing concentrations of potassium ions or hydrogen ions (falling pH); inflammatory chemicals such as histamines; and increased body temperature. These conditions in turn stimulate the release of NO, a powerful vasodilator, from endothelial cells (see Figure 20.17).
- Contraction of the precapillary sphincter is triggered by the opposite levels of the regulators, which prompt the release of endothelins, powerful vasoconstricting peptides secreted by endothelial cells. Platelet secretions and certain prostaglandins may also trigger constriction.
Again, these factors alter tissue perfusion via their effects on the precapillary sphincter mechanism, which regulates blood flow to capillaries. Since the amount of blood is limited, not all capillaries can fill at once, so blood flow is allocated based upon the needs and metabolic state of the tissues as reflected in these parameters. Bear in mind, however, that dilation and constriction of the arterioles feeding the capillary beds is the primary control mechanism.
The Myogenic Response
The myogenic response is a reaction to the stretching of the smooth muscle in the walls of arterioles as changes in blood flow occur through the vessel. This may be viewed as a largely protective function against dramatic fluctuations in blood pressure and blood flow to maintain homeostasis. If perfusion of an organ is too low (ischemia), the tissue will experience low levels of oxygen (hypoxia). In contrast, excessive perfusion could damage the organ’s smaller and more fragile vessels. The myogenic response is a localized process that serves to stabilize blood flow in the capillary network that follows that arteriole.
When blood flow is low, the vessel’s smooth muscle will be only minimally stretched. In response, it relaxes, allowing the vessel to dilate and thereby increase the movement of blood into the tissue. When blood flow is too high, the smooth muscle will contract in response to the increased stretch, prompting vasoconstriction that reduces blood flow.
Figure 20.20 summarizes the effects of nervous, endocrine, and local controls on arterioles.
Figure 20.20 Summary of Mechanisms Regulating Arteriole Smooth Muscle and Veins
Effect of Exercise on Vascular Homeostasis
The heart is a muscle and, like any muscle, it responds dramatically to exercise. For a healthy young adult, cardiac output (heart rate × stroke volume) increases in the nonathlete from approximately 5.0 liters (5.25 quarts) per minute to a maximum of about 20 liters (21 quarts) per minute. Accompanying this will be an increase in blood pressure from about 120/80 to 185/75. However, well-trained aerobic athletes can increase these values substantially. For these individuals, cardiac output soars from approximately 5.3 liters (5.57 quarts) per minute resting to more than 30 liters (31.5 quarts) per minute during maximal exercise. Along with this increase in cardiac output, blood pressure increases from 120/80 at rest to 200/90 at maximum values.
In addition to improved cardiac function, exercise increases the size and mass of the heart. The average weight of the heart for the nonathlete is about 300 g, whereas in an athlete it will increase to 500 g. This increase in size generally makes the heart stronger and more efficient at pumping blood, increasing both stroke volume and cardiac output.
Tissue perfusion also increases as the body transitions from a resting state to light exercise and eventually to heavy exercise (see Figure 20.20). These changes result in selective vasodilation in the skeletal muscles, heart, lungs, liver, and integument. Simultaneously, vasoconstriction occurs in the vessels leading to the kidneys and most of the digestive and reproductive organs. The flow of blood to the brain remains largely unchanged whether at rest or exercising, since the vessels in the brain largely do not respond to regulatory stimuli, in most cases, because they lack the appropriate receptors.
As vasodilation occurs in selected vessels, resistance drops and more blood rushes into the organs they supply. This blood eventually returns to the venous system. Venous return is further enhanced by both the skeletal muscle and respiratory pumps. As blood returns to the heart more quickly, preload rises and the Frank-Starling principle tells us that contraction of the cardiac muscle in the atria and ventricles will be more forceful. Eventually, even the best-trained athletes will fatigue and must undergo a period of rest following exercise. Cardiac output and distribution of blood then return to normal.
Regular exercise promotes cardiovascular health in a variety of ways. Because an athlete’s heart is larger than a nonathlete’s, stroke volume increases, so the athletic heart can deliver the same amount of blood as the nonathletic heart but with a lower heart rate. This increased efficiency allows the athlete to exercise for longer periods of time before muscles fatigue and places less stress on the heart. Exercise also lowers overall cholesterol levels by removing from the circulation a complex form of cholesterol, triglycerides, and proteins known as low-density lipoproteins (LDLs), which are widely associated with increased risk of cardiovascular disease. Although there is no way to remove deposits of plaque from the walls of arteries other than specialized surgery, exercise does promote the health of vessels by decreasing the rate of plaque formation and reducing blood pressure, so the heart does not have to generate as much force to overcome resistance.
Generally as little as 30 minutes of noncontinuous exercise over the course of each day has beneficial effects and has been shown to lower the rate of heart attack by nearly 50 percent. While it is always advisable to follow a healthy diet, stop smoking, and lose weight, studies have clearly shown that fit, overweight people may actually be healthier overall than sedentary slender people. Thus, the benefits of moderate exercise are undeniable.
Clinical Considerations in Vascular Homeostasis
Any disorder that affects blood volume, vascular tone, or any other aspect of vascular functioning is likely to affect vascular homeostasis as well. That includes hypertension, hemorrhage, and shock.
Hypertension and Hypotension
Chronically elevated blood pressure is known clinically as hypertension. It is defined as chronic and persistent blood pressure measurements of 140/90 mm Hg or above. Pressures between 120/80 and 140/90 mm Hg are defined as prehypertension. About 68 million Americans currently suffer from hypertension. Unfortunately, hypertension is typically a silent disorder; therefore, hypertensive patients may fail to recognize the seriousness of their condition and fail to follow their treatment plan. The result is often a heart attack or stroke. Hypertension may also lead to an aneurism (ballooning of a blood vessel caused by a weakening of the wall), peripheral arterial disease (obstruction of vessels in peripheral regions of the body), chronic kidney disease, or heart failure.
INTERACTIVE LINK
Listen to this CDC podcast to learn about hypertension, often described as a “silent killer.” What steps can you take to reduce your risk of a heart attack or stroke?
Hemorrhage
Minor blood loss is managed by hemostasis and repair. Hemorrhage is a loss of blood that cannot be controlled by hemostatic mechanisms. Initially, the body responds to hemorrhage by initiating mechanisms aimed at increasing blood pressure and maintaining blood flow. Ultimately, however, blood volume will need to be restored, either through physiological processes or through medical intervention.
In response to blood loss, stimuli from the baroreceptors trigger the cardiovascular centers to stimulate sympathetic responses to increase cardiac output and vasoconstriction. This typically prompts the heart rate to increase to about 180–200 contractions per minute, restoring cardiac output to normal levels. Vasoconstriction of the arterioles increases vascular resistance, whereas constriction of the veins increases venous return to the heart. Both of these steps will help increase blood pressure. Sympathetic stimulation also triggers the release of epinephrine and norepinephrine, which enhance both cardiac output and vasoconstriction. If blood loss were less than 20 percent of total blood volume, these responses together would usually return blood pressure to normal and redirect the remaining blood to the tissues.
Additional endocrine involvement is necessary, however, to restore the lost blood volume. The angiotensin-renin-aldosterone mechanism stimulates the thirst center in the hypothalamus, which increases fluid consumption to help restore the lost blood. More importantly, it increases renal reabsorption of sodium and water, reducing water loss in urine output. The kidneys also increase the production of EPO, stimulating the formation of erythrocytes that not only deliver oxygen to the tissues but also increase overall blood volume. Figure 20.21 summarizes the responses to loss of blood volume.
Figure 20.21 Homeostatic Responses to Loss of Blood Volume
Circulatory Shock
The loss of too much blood may lead to circulatory shock, a life-threatening condition in which the circulatory system is unable to maintain blood flow to adequately supply sufficient oxygen and other nutrients to the tissues to maintain cellular metabolism. It should not be confused with emotional or psychological shock. Typically, the patient in circulatory shock will demonstrate an increased heart rate but decreased blood pressure, but there are cases in which blood pressure will remain normal. Urine output will fall dramatically, and the patient may appear confused or lose consciousness. Urine output less than 1 mL/kg body weight/hour is cause for concern. Unfortunately, shock is an example of a positive-feedback loop that, if uncorrected, may lead to the death of the patient.
There are several recognized forms of shock:
- Hypovolemic shock in adults is typically caused by hemorrhage, although in children it may be caused by fluid losses related to severe vomiting or diarrhea. Other causes for hypovolemic shock include extensive burns, exposure to some toxins, and excessive urine loss related to diabetes insipidus or ketoacidosis. Typically, patients present with a rapid, almost tachycardic heart rate; a weak pulse often described as “thread;” cool, clammy skin, particularly in the extremities, due to restricted peripheral blood flow; rapid, shallow breathing; hypothermia; thirst; and dry mouth. Treatments generally involve providing intravenous fluids to restore the patient to normal function and various drugs such as dopamine, epinephrine, and norepinephrine to raise blood pressure.
- Cardiogenic shock results from the inability of the heart to maintain cardiac output. Most often, it results from a myocardial infarction (heart attack), but it may also be caused by arrhythmias, valve disorders, cardiomyopathies, cardiac failure, or simply insufficient flow of blood through the cardiac vessels. Treatment involves repairing the damage to the heart or its vessels to resolve the underlying cause, rather than treating cardiogenic shock directly.
- Vascular shock occurs when arterioles lose their normal muscular tone and dilate dramatically. It may arise from a variety of causes, and treatments almost always involve fluid replacement and medications, called inotropic or pressor agents, which restore tone to the muscles of the vessels. In addition, eliminating or at least alleviating the underlying cause of the condition is required. This might include antibiotics and antihistamines, or select steroids, which may aid in the repair of nerve damage. A common cause is sepsis (or septicemia), also called “blood poisoning,” which is a widespread bacterial infection that results in an organismal-level inflammatory response known as septic shock. Neurogenic shock is a form of vascular shock that occurs with cranial or spinal injuries that damage the cardiovascular centers in the medulla oblongata or the nervous fibers originating from this region. Anaphylactic shock is a severe allergic response that causes the widespread release of histamines, triggering vasodilation throughout the body.
- Obstructive shock, as the name would suggest, occurs when a significant portion of the vascular system is blocked. It is not always recognized as a distinct condition and may be grouped with cardiogenic shock, including pulmonary embolism and cardiac tamponade. Treatments depend upon the underlying cause and, in addition to administering fluids intravenously, often include the administration of anticoagulants, removal of fluid from the pericardial cavity, or air from the thoracic cavity, and surgery as required. The most common cause is a pulmonary embolism, a clot that lodges in the pulmonary vessels and interrupts blood flow. Other causes include stenosis of the aortic valve; cardiac tamponade, in which excess fluid in the pericardial cavity interferes with the ability of the heart to fully relax and fill with blood (resulting in decreased preload); and a pneumothorax, in which an excessive amount of air is present in the thoracic cavity, outside of the lungs, which interferes with venous return, pulmonary function, and delivery of oxygen to the tissues.
Circulatory Pathways
- Identify the vessels through which blood travels within the pulmonary circuit, beginning from the right ventricle of the heart and ending at the left atrium
- Create a flow chart showing the major systemic arteries through which blood travels from the aorta and its major branches, to the most significant arteries feeding into the right and left upper and lower limbs
- Create a flow chart showing the major systemic veins through which blood travels from the feet to the right atrium of the heart
Virtually every cell, tissue, organ, and system in the body is impacted by the circulatory system. This includes the generalized and more specialized functions of transport of materials, capillary exchange, maintaining health by transporting white blood cells and various immunoglobulins (antibodies), hemostasis, regulation of body temperature, and helping to maintain acid-base balance. In addition to these shared functions, many systems enjoy a unique relationship with the circulatory system. Figure 20.22 summarizes these relationships.
Figure 20.22 Interaction of the Circulatory System with Other Body Systems
As you learn about the vessels of the systemic and pulmonary circuits, notice that many arteries and veins share the same names, parallel one another throughout the body, and are very similar on the right and left sides of the body. These pairs of vessels will be traced through only one side of the body. Where differences occur in branching patterns or when vessels are singular, this will be indicated. For example, you will find a pair of femoral arteries and a pair of femoral veins, with one vessel on each side of the body. In contrast, some vessels closer to the midline of the body, such as the aorta, are unique. Moreover, some superficial veins, such as the great saphenous vein in the femoral region, have no arterial counterpart. Another phenomenon that can make the study of vessels challenging is that names of vessels can change with location. Like a street that changes name as it passes through an intersection, an artery or vein can change names as it passes an anatomical landmark. For example, the left subclavian artery becomes the axillary artery as it passes through the body wall and into the axillary region, and then becomes the brachial artery as it flows from the axillary region into the upper arm (or brachium). You will also find examples of anastomoses where two blood vessels that previously branched reconnect. Anastomoses are especially common in veins, where they help maintain blood flow even when one vessel is blocked or narrowed, although there are some important ones in the arteries supplying the brain.
As you read about circular pathways, notice that there is an occasional, very large artery referred to as a trunk, a term indicating that the vessel gives rise to several smaller arteries. For example, the celiac trunk gives rise to the left gastric, common hepatic, and splenic arteries.
As you study this section, imagine you are on a “Voyage of Discovery” similar to Lewis and Clark’s expedition in 1804–1806, which followed rivers and streams through unfamiliar territory, seeking a water route from the Atlantic to the Pacific Ocean. You might envision being inside a miniature boat, exploring the various branches of the circulatory system. This simple approach has proven effective for many students in mastering these major circulatory patterns. Another approach that works well for many students is to create simple line drawings similar to the ones provided, labeling each of the major vessels. It is beyond the scope of this text to name every vessel in the body. However, we will attempt to discuss the major pathways for blood and acquaint you with the major named arteries and veins in the body. Also, please keep in mind that individual variations in circulation patterns are not uncommon.
INTERACTIVE LINK
Visit this site for a brief summary of the arteries.
Pulmonary Circulation
Recall that blood returning from the systemic circuit enters the right atrium (Figure 20.23) via the superior and inferior venae cavae and the coronary sinus, which drains the blood supply of the heart muscle. These vessels will be described more fully later in this section. This blood is relatively low in oxygen and relatively high in carbon dioxide, since much of the oxygen has been extracted for use by the tissues and the waste gas carbon dioxide was picked up to be transported to the lungs for elimination. From the right atrium, blood moves into the right ventricle, which pumps it to the lungs for gas exchange. This system of vessels is referred to as the pulmonary circuit.
The single vessel exiting the right ventricle is the pulmonary trunk. At the base of the pulmonary trunk is the pulmonary semilunar valve, which prevents backflow of blood into the right ventricle during ventricular diastole. As the pulmonary trunk reaches the superior surface of the heart, it curves posteriorly and rapidly bifurcates (divides) into two branches, a left and a right pulmonary artery. To prevent confusion between these vessels, it is important to refer to the vessel exiting the heart as the pulmonary trunk, rather than also calling it a pulmonary artery. The pulmonary arteries in turn branch many times within the lung, forming a series of smaller arteries and arterioles that eventually lead to the pulmonary capillaries. The pulmonary capillaries surround lung structures known as alveoli that are the sites of oxygen and carbon dioxide exchange.
Once gas exchange is completed, oxygenated blood flows from the pulmonary capillaries into a series of pulmonary venules that eventually lead to a series of larger pulmonary veins. Four pulmonary veins, two on the left and two on the right, return blood to the left atrium. At this point, the pulmonary circuit is complete. Table 20.4 defines the major arteries and veins of the pulmonary circuit discussed in the text.
Figure 20.23 Pulmonary Circuit Blood exiting from the right ventricle flows into the pulmonary trunk, which bifurcates into the two pulmonary arteries. These vessels branch to supply blood to the pulmonary capillaries, where gas exchange occurs within the lung alveoli. Blood returns via the pulmonary veins to the left atrium.
Pulmonary Arteries and Veins
| Vessel | Description |
|---|---|
| Pulmonary trunk | Single large vessel exiting the right ventricle that divides to form the right and left pulmonary arteries |
| Pulmonary arteries | Left and right vessels that form from the pulmonary trunk and lead to smaller arterioles and eventually to the pulmonary capillaries |
| Pulmonary veins | Two sets of paired vessels—one pair on each side—that are formed from the small venules, leading away from the pulmonary capillaries to flow into the left atrium |
Table 20.4
Overview of Systemic Arteries
Blood relatively high in oxygen concentration is returned from the pulmonary circuit to the left atrium via the four pulmonary veins. From the left atrium, blood moves into the left ventricle, which pumps blood into the aorta. The aorta and its branches—the systemic arteries—send blood to virtually every organ of the body (Figure 20.24).
Figure 20.24 Systemic Arteries The major systemic arteries shown here deliver oxygenated blood throughout the body.
The Aorta
The aorta is the largest artery in the body (Figure 20.25). It arises from the left ventricle and eventually descends to the abdominal region, where it bifurcates at the level of the fourth lumbar vertebra into the two common iliac arteries. The aorta consists of the ascending aorta, the aortic arch, and the descending aorta, which passes through the diaphragm and a landmark that divides into the superior thoracic and inferior abdominal components. Arteries originating from the aorta ultimately distribute blood to virtually all tissues of the body. At the base of the aorta is the aortic semilunar valve that prevents backflow of blood into the left ventricle while the heart is relaxing. After exiting the heart, the ascending aorta moves in a superior direction for approximately 5 cm and ends at the sternal angle. Following this ascent, it reverses direction, forming a graceful arc to the left, called the aortic arch. The aortic arch descends toward the inferior portions of the body and ends at the level of the intervertebral disk between the fourth and fifth thoracic vertebrae. Beyond this point, the descending aorta continues close to the bodies of the vertebrae and passes through an opening in the diaphragm known as the aortic hiatus. Superior to the diaphragm, the aorta is called the thoracic aorta, and inferior to the diaphragm, it is called the abdominal aorta. The abdominal aorta terminates when it bifurcates into the two common iliac arteries at the level of the fourth lumbar vertebra. See Figure 20.25 for an illustration of the ascending aorta, the aortic arch, and the initial segment of the descending aorta plus major branches; Table 20.5 summarizes the structures of the aorta.
Figure 20.25 Aorta The aorta has distinct regions, including the ascending aorta, aortic arch, and the descending aorta, which includes the thoracic and abdominal regions.
Components of the Aorta
| Vessel | Description |
|---|---|
| Aorta | Largest artery in the body, originating from the left ventricle and descending to the abdominal region, where it bifurcates into the common iliac arteries at the level of the fourth lumbar vertebra; arteries originating from the aorta distribute blood to virtually all tissues of the body |
| Ascending aorta | Initial portion of the aorta, rising superiorly from the left ventricle for a distance of approximately 5 cm |
| Aortic arch | Graceful arc to the left that connects the ascending aorta to the descending aorta; ends at the intervertebral disk between the fourth and fifth thoracic vertebrae |
| Descending aorta | Portion of the aorta that continues inferiorly past the end of the aortic arch; subdivided into the thoracic aorta and the abdominal aorta |
| Thoracic aorta | Portion of the descending aorta superior to the aortic hiatus |
| Abdominal aorta | Portion of the aorta inferior to the aortic hiatus and superior to the common iliac arteries |
Table 20.5
Coronary Circulation
The first vessels that branch from the ascending aorta are the paired coronary arteries (see Figure 20.25), which arise from two of the three sinuses in the ascending aorta just superior to the aortic semilunar valve. These sinuses contain the aortic baroreceptors and chemoreceptors critical to maintain cardiac function. The left coronary artery arises from the left posterior aortic sinus. The right coronary artery arises from the anterior aortic sinus. Normally, the right posterior aortic sinus does not give rise to a vessel.
The coronary arteries encircle the heart, forming a ring-like structure that divides into the next level of branches that supplies blood to the heart tissues. (Seek additional content for more detail on cardiac circulation.)
Aortic Arch Branches
There are three major branches of the aortic arch: the brachiocephalic artery, the left common carotid artery, and the left subclavian (literally “under the clavicle”) artery. As you would expect based upon proximity to the heart, each of these vessels is classified as an elastic artery.
The brachiocephalic artery is located only on the right side of the body; there is no corresponding artery on the left. The brachiocephalic artery branches into the right subclavian artery and the right common carotid artery. The left subclavian and left common carotid arteries arise independently from the aortic arch but otherwise follow a similar pattern and distribution to the corresponding arteries on the right side (see Figure 20.23).
Each subclavian artery supplies blood to the arms, chest, shoulders, back, and central nervous system. It then gives rise to three major branches: the internal thoracic artery, the vertebral artery, and the thyrocervical artery. The internal thoracic artery, or mammary artery, supplies blood to the thymus, the pericardium of the heart, and the anterior chest wall. The vertebral arterypasses through the vertebral foramen in the cervical vertebrae and then through the foramen magnum into the cranial cavity to supply blood to the brain and spinal cord. The paired vertebral arteries join together to form the large basilar artery at the base of the medulla oblongata. This is an example of an anastomosis. The subclavian artery also gives rise to the thyrocervical artery that provides blood to the thyroid, the cervical region of the neck, and the upper back and shoulder.
The common carotid artery divides into internal and external carotid arteries. The right common carotid artery arises from the brachiocephalic artery and the left common carotid artery arises directly from the aortic arch. The external carotid artery supplies blood to numerous structures within the face, lower jaw, neck, esophagus, and larynx. These branches include the lingual, facial, occipital, maxillary, and superficial temporal arteries. The internal carotid artery initially forms an expansion known as the carotid sinus, containing the carotid baroreceptors and chemoreceptors. Like their counterparts in the aortic sinuses, the information provided by these receptors is critical to maintaining cardiovascular homeostasis (see Figure 20.23).
The internal carotid arteries along with the vertebral arteries are the two primary suppliers of blood to the human brain. Given the central role and vital importance of the brain to life, it is critical that blood supply to this organ remains uninterrupted. Recall that blood flow to the brain is remarkably constant, with approximately 20 percent of blood flow directed to this organ at any given time. When blood flow is interrupted, even for just a few seconds, a transient ischemic attack (TIA), or mini-stroke, may occur, resulting in loss of consciousness or temporary loss of neurological function. In some cases, the damage may be permanent. Loss of blood flow for longer periods, typically between 3 and 4 minutes, will likely produce irreversible brain damage or a stroke, also called a cerebrovascular accident (CVA). The locations of the arteries in the brain not only provide blood flow to the brain tissue but also prevent interruption in the flow of blood. Both the carotid and vertebral arteries branch once they enter the cranial cavity, and some of these branches form a structure known as the arterial circle (or circle of Willis), an anastomosis that is remarkably like a traffic circle that sends off branches (in this case, arterial branches to the brain). As a rule, branches to the anterior portion of the cerebrum are normally fed by the internal carotid arteries; the remainder of the brain receives blood flow from branches associated with the vertebral arteries.
The internal carotid artery continues through the carotid canal of the temporal bone and enters the base of the brain through the carotid foramen where it gives rise to several branches (Figure 20.26 and Figure 20.27). One of these branches is the anterior cerebral artery that supplies blood to the frontal lobe of the cerebrum. Another branch, the middle cerebral artery, supplies blood to the temporal and parietal lobes, which are the most common sites of CVAs. The ophthalmic artery, the third major branch, provides blood to the eyes.
The right and left anterior cerebral arteries join together to form an anastomosis called the anterior communicating artery. The initial segments of the anterior cerebral arteries and the anterior communicating artery form the anterior portion of the arterial circle. The posterior portion of the arterial circle is formed by a left and a right posterior communicating artery that branches from the posterior cerebral artery, which arises from the basilar artery. It provides blood to the posterior portion of the cerebrum and brain stem. The basilar artery is an anastomosis that begins at the junction of the two vertebral arteries and sends branches to the cerebellum and brain stem. It flows into the posterior cerebral arteries. Table 20.6 summarizes the aortic arch branches, including the major branches supplying the brain.
Figure 20.26 Arteries Supplying the Head and Neck The common carotid artery gives rise to the external and internal carotid arteries. The external carotid artery remains superficial and gives rise to many arteries of the head. The internal carotid artery first forms the carotid sinus and then reaches the brain via the carotid canal and carotid foramen, emerging into the cranium via the foramen lacerum. The vertebral artery branches from the subclavian artery and passes through the transverse foramen in the cervical vertebrae, entering the base of the skull at the vertebral foramen. The subclavian artery continues toward the arm as the axillary artery.
Figure 20.27 Arteries Serving the Brain This inferior view shows the network of arteries serving the brain. The structure is referred to as the arterial circle or circle of Willis.
Aortic Arch Branches and Brain Circulation
| Vessel | Description |
|---|---|
| Brachiocephalic artery | Single vessel located on the right side of the body; the first vessel branching from the aortic arch; gives rise to the right subclavian artery and the right common carotid artery; supplies blood to the head, neck, upper limb, and wall of the thoracic region |
| Subclavian artery | The right subclavian artery arises from the brachiocephalic artery while the left subclavian artery arises from the aortic arch; gives rise to the internal thoracic, vertebral, and thyrocervical arteries; supplies blood to the arms, chest, shoulders, back, and central nervous system |
| Internal thoracic artery | Also called the mammary artery; arises from the subclavian artery; supplies blood to the thymus, pericardium of the heart, and anterior chest wall |
| Vertebral artery | Arises from the subclavian artery and passes through the vertebral foramen through the foramen magnum to the brain; joins with the internal carotid artery to form the arterial circle; supplies blood to the brain and spinal cord |
| Thyrocervical artery | Arises from the subclavian artery; supplies blood to the thyroid, the cervical region, the upper back, and shoulder |
| Common carotid artery | The right common carotid artery arises from the brachiocephalic artery and the left common carotid artery arises from the aortic arch; each gives rise to the external and internal carotid arteries; supplies the respective sides of the head and neck |
| External carotid artery | Arises from the common carotid artery; supplies blood to numerous structures within the face, lower jaw, neck, esophagus, and larynx |
| Internal carotid artery | Arises from the common carotid artery and begins with the carotid sinus; goes through the carotid canal of the temporal bone to the base of the brain; combines with the branches of the vertebral artery, forming the arterial circle; supplies blood to the brain |
| Arterial circle or circle of Willis | An anastomosis located at the base of the brain that ensures continual blood supply; formed from the branches of the internal carotid and vertebral arteries; supplies blood to the brain |
| Anterior cerebral artery | Arises from the internal carotid artery; supplies blood to the frontal lobe of the cerebrum |
| Middle cerebral artery | Another branch of the internal carotid artery; supplies blood to the temporal and parietal lobes of the cerebrum |
| Ophthalmic artery | Branch of the internal carotid artery; supplies blood to the eyes |
| Anterior communicating artery | An anastomosis of the right and left internal carotid arteries; supplies blood to the brain |
| Posterior communicating artery | Branches of the posterior cerebral artery that form part of the posterior portion of the arterial circle; supplies blood to the brain |
| Posterior cerebral artery | Branch of the basilar artery that forms a portion of the posterior segment of the arterial circle of Willis; supplies blood to the posterior portion of the cerebrum and brain stem |
| Basilar artery | Formed from the fusion of the two vertebral arteries; sends branches to the cerebellum, brain stem, and the posterior cerebral arteries; the main blood supply to the brain stem |
Table 20.6
Thoracic Aorta and Major Branches
The thoracic aorta begins at the level of vertebra T5 and continues through to the diaphragm at the level of T12, initially traveling within the mediastinum to the left of the vertebral column. As it passes through the thoracic region, the thoracic aorta gives rise to several branches, which are collectively referred to as visceral branches and parietal branches (Figure 20.28). Those branches that supply blood primarily to visceral organs are known as the visceral branches and include the bronchial arteries, pericardial arteries, esophageal arteries, and the mediastinal arteries, each named after the tissues it supplies. Each bronchial artery (typically two on the left and one on the right) supplies systemic blood to the lungs and visceral pleura, in addition to the blood pumped to the lungs for oxygenation via the pulmonary circuit. The bronchial arteries follow the same path as the respiratory branches, beginning with the bronchi and ending with the bronchioles. There is considerable, but not total, intermingling of the systemic and pulmonary blood at anastomoses in the smaller branches of the lungs. This may sound incongruous—that is, the mixing of systemic arterial blood high in oxygen with the pulmonary arterial blood lower in oxygen—but the systemic vessels also deliver nutrients to the lung tissue just as they do elsewhere in the body. The mixed blood drains into typical pulmonary veins, whereas the bronchial artery branches remain separate and drain into bronchial veins described later. Each pericardial artery supplies blood to the pericardium, the esophageal artery provides blood to the esophagus, and the mediastinal artery provides blood to the mediastinum. The remaining thoracic aorta branches are collectively referred to as parietal branches or somatic branches, and include the intercostal and superior phrenic arteries. Each intercostal artery provides blood to the muscles of the thoracic cavity and vertebral column. The superior phrenic artery provides blood to the superior surface of the diaphragm. Table 20.7 lists the arteries of the thoracic region.
Figure 20.28 Arteries of the Thoracic and Abdominal Regions The thoracic aorta gives rise to the arteries of the visceral and parietal branches.
Arteries of the Thoracic Region
| Vessel | Description |
|---|---|
| Visceral branches | A group of arterial branches of the thoracic aorta; supplies blood to the viscera (i.e., organs) of the thorax |
| Bronchial artery | Systemic branch from the aorta that provides oxygenated blood to the lungs; this blood supply is in addition to the pulmonary circuit that brings blood for oxygenation |
| Pericardial artery | Branch of the thoracic aorta; supplies blood to the pericardium |
| Esophageal artery | Branch of the thoracic aorta; supplies blood to the esophagus |
| Mediastinal artery | Branch of the thoracic aorta; supplies blood to the mediastinum |
| Parietal branches | Also called somatic branches, a group of arterial branches of the thoracic aorta; include those that supply blood to the thoracic wall, vertebral column, and the superior surface of the diaphragm |
| Intercostal artery | Branch of the thoracic aorta; supplies blood to the muscles of the thoracic cavity and vertebral column |
| Superior phrenic artery | Branch of the thoracic aorta; supplies blood to the superior surface of the diaphragm |
Table 20.7
Abdominal Aorta and Major Branches
After crossing through the diaphragm at the aortic hiatus, the thoracic aorta is called the abdominal aorta (see Figure 20.28). This vessel remains to the left of the vertebral column and is embedded in adipose tissue behind the peritoneal cavity. It formally ends at approximately the level of vertebra L4, where it bifurcates to form the common iliac arteries. Before this division, the abdominal aorta gives rise to several important branches. A single celiac trunk (artery) emerges and divides into the left gastric artery to supply blood to the stomach and esophagus, the splenic artery to supply blood to the spleen, and the common hepatic artery, which in turn gives rise to the hepatic artery proper to supply blood to the liver, the right gastric artery to supply blood to the stomach, the cystic artery to supply blood to the gall bladder, and several branches, one to supply blood to the duodenum and another to supply blood to the pancreas. Two additional single vessels arise from the abdominal aorta. These are the superior and inferior mesenteric arteries. The superior mesenteric artery arises approximately 2.5 cm after the celiac trunk and branches into several major vessels that supply blood to the small intestine (duodenum, jejunum, and ileum), the pancreas, and a majority of the large intestine. The inferior mesenteric artery supplies blood to the distal segment of the large intestine, including the rectum. It arises approximately 5 cm superior to the common iliac arteries.
In addition to these single branches, the abdominal aorta gives rise to several significant paired arteries along the way. These include the inferior phrenic arteries, the adrenal arteries, the renal arteries, the gonadal arteries, and the lumbar arteries. Each inferior phrenic artery is a counterpart of a superior phrenic artery and supplies blood to the inferior surface of the diaphragm. The adrenal artery supplies blood to the adrenal (suprarenal) glands and arises near the superior mesenteric artery. Each renal arterybranches approximately 2.5 cm inferior to the superior mesenteric arteries and supplies a kidney. The right renal artery is longer than the left since the aorta lies to the left of the vertebral column and the vessel must travel a greater distance to reach its target. Renal arteries branch repeatedly to supply blood to the kidneys. Each gonadal artery supplies blood to the gonads, or reproductive organs, and is also described as either an ovarian artery or a testicular artery (internal spermatic), depending upon the sex of the individual. An ovarian artery supplies blood to an ovary, uterine (Fallopian) tube, and the uterus, and is located within the suspensory ligament of the uterus. It is considerably shorter than a testicular artery, which ultimately travels outside the body cavity to the testes, forming one component of the spermatic cord. The gonadal arteries arise inferior to the renal arteries and are generally retroperitoneal. The ovarian artery continues to the uterus where it forms an anastomosis with the uterine artery that supplies blood to the uterus. Both the uterine arteries and vaginal arteries, which distribute blood to the vagina, are branches of the internal iliac artery. The four paired lumbar arteries are the counterparts of the intercostal arteries and supply blood to the lumbar region, the abdominal wall, and the spinal cord. In some instances, a fifth pair of lumbar arteries emerges from the median sacral artery.
The aorta divides at approximately the level of vertebra L4 into a left and a right common iliac artery but continues as a small vessel, the median sacral artery, into the sacrum. The common iliac arteries provide blood to the pelvic region and ultimately to the lower limbs. They split into external and internal iliac arteries approximately at the level of the lumbar-sacral articulation. Each internal iliac artery sends branches to the urinary bladder, the walls of the pelvis, the external genitalia, and the medial portion of the femoral region. In females, they also provide blood to the uterus and vagina. The much larger external iliac artery supplies blood to each of the lower limbs. Figure 20.29 shows the distribution of the major branches of the aorta into the thoracic and abdominal regions. Figure 20.30 shows the distribution of the major branches of the common iliac arteries. Table 20.8 summarizes the major branches of the abdominal aorta.
Figure 20.29 Major Branches of the Aorta The flow chart summarizes the distribution of the major branches of the aorta into the thoracic and abdominal regions.
Figure 20.30 Major Branches of the Iliac Arteries The flow chart summarizes the distribution of the major branches of the common iliac arteries into the pelvis and lower limbs. The left side follows a similar pattern to the right.
Vessels of the Abdominal Aorta
| Vessel | Description |
|---|---|
| Celiac trunk | Also called the celiac artery; a major branch of the abdominal aorta; gives rise to the left gastric artery, the splenic artery, and the common hepatic artery that forms the hepatic artery to the liver, the right gastric artery to the stomach, and the cystic artery to the gall bladder |
| Left gastric artery | Branch of the celiac trunk; supplies blood to the stomach |
| Splenic artery | Branch of the celiac trunk; supplies blood to the spleen |
| Common hepatic artery | Branch of the celiac trunk that forms the hepatic artery, the right gastric artery, and the cystic artery |
| Hepatic artery proper | Branch of the common hepatic artery; supplies systemic blood to the liver |
| Right gastric artery | Branch of the common hepatic artery; supplies blood to the stomach |
| Cystic artery | Branch of the common hepatic artery; supplies blood to the gall bladder |
| Superior mesenteric artery | Branch of the abdominal aorta; supplies blood to the small intestine (duodenum, jejunum, and ileum), the pancreas, and a majority of the large intestine |
| Inferior mesenteric artery | Branch of the abdominal aorta; supplies blood to the distal segment of the large intestine and rectum |
| Inferior phrenic arteries | Branches of the abdominal aorta; supply blood to the inferior surface of the diaphragm |
| Adrenal artery | Branch of the abdominal aorta; supplies blood to the adrenal (suprarenal) glands |
| Renal artery | Branch of the abdominal aorta; supplies each kidney |
| Gonadal artery | Branch of the abdominal aorta; supplies blood to the gonads or reproductive organs; also described as ovarian arteries or testicular arteries, depending upon the sex of the individual |
| Ovarian artery | Branch of the abdominal aorta; supplies blood to ovary, uterine (Fallopian) tube, and uterus |
| Testicular artery | Branch of the abdominal aorta; ultimately travels outside the body cavity to the testes and forms one component of the spermatic cord |
| Lumbar arteries | Branches of the abdominal aorta; supply blood to the lumbar region, the abdominal wall, and spinal cord |
| Common iliac artery | Branch of the aorta that leads to the internal and external iliac arteries |
| Median sacral artery | Continuation of the aorta into the sacrum |
| Internal iliac artery | Branch from the common iliac arteries; supplies blood to the urinary bladder, walls of the pelvis, external genitalia, and the medial portion of the femoral region; in females, also provides blood to the uterus and vagina |
| External iliac artery | Branch of the common iliac artery that leaves the body cavity and becomes a femoral artery; supplies blood to the lower limbs |
Table 20.8
Arteries Serving the Upper Limbs
As the subclavian artery exits the thorax into the axillary region, it is renamed the axillary artery. Although it does branch and supply blood to the region near the head of the humerus (via the humeral circumflex arteries), the majority of the vessel continues into the upper arm, or brachium, and becomes the brachial artery (Figure 20.31). The brachial artery supplies blood to much of the brachial region and divides at the elbow into several smaller branches, including the deep brachial arteries, which provide blood to the posterior surface of the arm, and the ulnar collateral arteries, which supply blood to the region of the elbow. As the brachial artery approaches the coronoid fossa, it bifurcates into the radial and ulnar arteries, which continue into the forearm, or antebrachium. The radial artery and ulnar artery parallel their namesake bones, giving off smaller branches until they reach the wrist, or carpal region. At this level, they fuse to form the superficial and deep palmar arches that supply blood to the hand, as well as the digital arteries that supply blood to the digits. Figure 20.32 shows the distribution of systemic arteries from the heart into the upper limb. Table 20.9 summarizes the arteries serving the upper limbs.
Figure 20.31 Major Arteries Serving the Thorax and Upper Limb The arteries that supply blood to the arms and hands are extensions of the subclavian arteries.
Figure 20.32 Major Arteries of the Upper Limb The flow chart summarizes the distribution of the major arteries from the heart into the upper limb.
Arteries Serving the Upper Limbs
| Vessel | Description |
|---|---|
| Axillary artery | Continuation of the subclavian artery as it penetrates the body wall and enters the axillary region; supplies blood to the region near the head of the humerus (humeral circumflex arteries); the majority of the vessel continues into the brachium and becomes the brachial artery |
| Brachial artery | Continuation of the axillary artery in the brachium; supplies blood to much of the brachial region; gives off several smaller branches that provide blood to the posterior surface of the arm in the region of the elbow; bifurcates into the radial and ulnar arteries at the coronoid fossa |
| Radial artery | Formed at the bifurcation of the brachial artery; parallels the radius; gives off smaller branches until it reaches the carpal region where it fuses with the ulnar artery to form the superficial and deep palmar arches; supplies blood to the lower arm and carpal region |
| Ulnar artery | Formed at the bifurcation of the brachial artery; parallels the ulna; gives off smaller branches until it reaches the carpal region where it fuses with the radial artery to form the superficial and deep palmar arches; supplies blood to the lower arm and carpal region |
| Palmar arches (superficial and deep) | Formed from anastomosis of the radial and ulnar arteries; supply blood to the hand and digital arteries |
| Digital arteries | Formed from the superficial and deep palmar arches; supply blood to the digits |
Table 20.9
Arteries Serving the Lower Limbs
The external iliac artery exits the body cavity and enters the femoral region of the lower leg (Figure 20.33). As it passes through the body wall, it is renamed the femoral artery. It gives off several smaller branches as well as the lateral deep femoral artery that in turn gives rise to a lateral circumflex artery. These arteries supply blood to the deep muscles of the thigh as well as ventral and lateral regions of the integument. The femoral artery also gives rise to the genicular artery, which provides blood to the region of the knee. As the femoral artery passes posterior to the knee near the popliteal fossa, it is called the popliteal artery. The popliteal artery branches into the anterior and posterior tibial arteries.
The anterior tibial artery is located between the tibia and fibula, and supplies blood to the muscles and integument of the anterior tibial region. Upon reaching the tarsal region, it becomes the dorsalis pedis artery, which branches repeatedly and provides blood to the tarsal and dorsal regions of the foot. The posterior tibial artery provides blood to the muscles and integument on the posterior surface of the tibial region. The fibular or peroneal artery branches from the posterior tibial artery. It bifurcates and becomes the medial plantar artery and lateral plantar artery, providing blood to the plantar surfaces. There is an anastomosis with the dorsalis pedis artery, and the medial and lateral plantar arteries form two arches called the dorsal arch (also called the arcuate arch) and the plantar arch, which provide blood to the remainder of the foot and toes. Figure 20.34 shows the distribution of the major systemic arteries in the lower limb. Table 20.10 summarizes the major systemic arteries discussed in the text.
Figure 20.33 Major Arteries Serving the Lower Limb Major arteries serving the lower limb are shown in anterior and posterior views.
Figure 20.34 Systemic Arteries of the Lower Limb The flow chart summarizes the distribution of the systemic arteries from the external iliac artery into the lower limb.
Arteries Serving the Lower Limbs
| Vessel | Description |
|---|---|
| Femoral artery | Continuation of the external iliac artery after it passes through the body cavity; divides into several smaller branches, the lateral deep femoral artery, and the genicular artery; becomes the popliteal artery as it passes posterior to the knee |
| Deep femoral artery | Branch of the femoral artery; gives rise to the lateral circumflex arteries |
| Lateral circumflex artery | Branch of the deep femoral artery; supplies blood to the deep muscles of the thigh and the ventral and lateral regions of the integument |
| Genicular artery | Branch of the femoral artery; supplies blood to the region of the knee |
| Popliteal artery | Continuation of the femoral artery posterior to the knee; branches into the anterior and posterior tibial arteries |
| Anterior tibial artery | Branches from the popliteal artery; supplies blood to the anterior tibial region; becomes the dorsalis pedis artery |
| Dorsalis pedis artery | Forms from the anterior tibial artery; branches repeatedly to supply blood to the tarsal and dorsal regions of the foot |
| Posterior tibial artery | Branches from the popliteal artery and gives rise to the fibular or peroneal artery; supplies blood to the posterior tibial region |
| Medial plantar artery | Arises from the bifurcation of the posterior tibial arteries; supplies blood to the medial plantar surfaces of the foot |
| Lateral plantar artery | Arises from the bifurcation of the posterior tibial arteries; supplies blood to the lateral plantar surfaces of the foot |
| Dorsal or arcuate arch | Formed from the anastomosis of the dorsalis pedis artery and the medial and plantar arteries; branches supply the distal portions of the foot and digits |
| Plantar arch | Formed from the anastomosis of the dorsalis pedis artery and the medial and plantar arteries; branches supply the distal portions of the foot and digits |
Table 20.10
Overview of Systemic Veins
Systemic veins return blood to the right atrium. Since the blood has already passed through the systemic capillaries, it will be relatively low in oxygen concentration. In many cases, there will be veins draining organs and regions of the body with the same name as the arteries that supplied these regions and the two often parallel one another. This is often described as a “complementary” pattern. However, there is a great deal more variability in the venous circulation than normally occurs in the arteries. For the sake of brevity and clarity, this text will discuss only the most commonly encountered patterns. However, keep this variation in mind when you move from the classroom to clinical practice.
In both the neck and limb regions, there are often both superficial and deeper levels of veins. The deeper veins generally correspond to the complementary arteries. The superficial veins do not normally have direct arterial counterparts, but in addition to returning blood, they also make contributions to the maintenance of body temperature. When the ambient temperature is warm, more blood is diverted to the superficial veins where heat can be more easily dissipated to the environment. In colder weather, there is more constriction of the superficial veins and blood is diverted deeper where the body can retain more of the heat.
The “Voyage of Discovery” analogy and stick drawings mentioned earlier remain valid techniques for the study of systemic veins, but veins present a more difficult challenge because there are numerous anastomoses and multiple branches. It is like following a river with many tributaries and channels, several of which interconnect. Tracing blood flow through arteries follows the current in the direction of blood flow, so that we move from the heart through the large arteries and into the smaller arteries to the capillaries. From the capillaries, we move into the smallest veins and follow the direction of blood flow into larger veins and back to the heart. Figure 20.35 outlines the path of the major systemic veins.
INTERACTIVE LINK
Visit this site for a brief online summary of the veins.
Figure 20.35 Major Systemic Veins of the Body The major systemic veins of the body are shown here in an anterior view.
The right atrium receives all of the systemic venous return. Most of the blood flows into either the superior vena cava or inferior vena cava. If you draw an imaginary line at the level of the diaphragm, systemic venous circulation from above that line will generally flow into the superior vena cava; this includes blood from the head, neck, chest, shoulders, and upper limbs. The exception to this is that most venous blood flow from the coronary veins flows directly into the coronary sinus and from there directly into the right atrium. Beneath the diaphragm, systemic venous flow enters the inferior vena cava, that is, blood from the abdominal and pelvic regions and the lower limbs.
The Superior Vena Cava
The superior vena cava drains most of the body superior to the diaphragm (Figure 20.36). On both the left and right sides, the subclavian vein forms when the axillary vein passes through the body wall from the axillary region. It fuses with the external and internal jugular veins from the head and neck to form the brachiocephalic vein. Each vertebral vein also flows into the brachiocephalic vein close to this fusion. These veins arise from the base of the brain and the cervical region of the spinal cord, and flow largely through the intervertebral foramina in the cervical vertebrae. They are the counterparts of the vertebral arteries. Each internal thoracic vein, also known as an internal mammary vein, drains the anterior surface of the chest wall and flows into the brachiocephalic vein.
The remainder of the blood supply from the thorax drains into the azygos vein. Each intercostal vein drains muscles of the thoracic wall, each esophageal vein delivers blood from the inferior portions of the esophagus, each bronchial vein drains the systemic circulation from the lungs, and several smaller veins drain the mediastinal region. Bronchial veins carry approximately 13 percent of the blood that flows into the bronchial arteries; the remainder intermingles with the pulmonary circulation and returns to the heart via the pulmonary veins. These veins flow into the azygos vein, and with the smaller hemiazygos vein (hemi- = “half”) on the left of the vertebral column, drain blood from the thoracic region. The hemiazygos vein does not drain directly into the superior vena cava but enters the brachiocephalic vein via the superior intercostal vein.
The azygos vein passes through the diaphragm from the thoracic cavity on the right side of the vertebral column and begins in the lumbar region of the thoracic cavity. It flows into the superior vena cava at approximately the level of T2, making a significant contribution to the flow of blood. It combines with the two large left and right brachiocephalic veins to form the superior vena cava.
Table 20.11 summarizes the veins of the thoracic region that flow into the superior vena cava.
Figure 20.36 Veins of the Thoracic and Abdominal Regions Veins of the thoracic and abdominal regions drain blood from the area above the diaphragm, returning it to the right atrium via the superior vena cava.
Veins of the Thoracic Region
| Vessel | Description |
|---|---|
| Superior vena cava | Large systemic vein; drains blood from most areas superior to the diaphragm; empties into the right atrium |
| Subclavian vein | Located deep in the thoracic cavity; formed by the axillary vein as it enters the thoracic cavity from the axillary region; drains the axillary and smaller local veins near the scapular region and leads to the brachiocephalic vein |
| Brachiocephalic veins | Pair of veins that form from a fusion of the external and internal jugular veins and the subclavian vein; subclavian, external and internal jugulars, vertebral, and internal thoracic veins flow into it; drain the upper thoracic region and lead to the superior vena cava |
| Vertebral vein | Arises from the base of the brain and the cervical region of the spinal cord; passes through the intervertebral foramina in the cervical vertebrae; drains smaller veins from the cranium, spinal cord, and vertebrae, and leads to the brachiocephalic vein; counterpart of the vertebral artery |
| Internal thoracic veins | Also called internal mammary veins; drain the anterior surface of the chest wall and lead to the brachiocephalic vein |
| Intercostal vein | Drains the muscles of the thoracic wall and leads to the azygos vein |
| Esophageal vein | Drains the inferior portions of the esophagus and leads to the azygos vein |
| Bronchial vein | Drains the systemic circulation from the lungs and leads to the azygos vein |
| Azygos vein | Originates in the lumbar region and passes through the diaphragm into the thoracic cavity on the right side of the vertebral column; drains blood from the intercostal veins, esophageal veins, bronchial veins, and other veins draining the mediastinal region, and leads to the superior vena cava |
| Hemiazygos vein | Smaller vein complementary to the azygos vein; drains the esophageal veins from the esophagus and the left intercostal veins, and leads to the brachiocephalic vein via the superior intercostal vein |
Table 20.11
Veins of the Head and Neck
Blood from the brain and the superficial facial vein flow into each internal jugular vein (Figure 20.37). Blood from the more superficial portions of the head, scalp, and cranial regions, including the temporal vein and maxillary vein, flow into each external jugular vein. Although the external and internal jugular veins are separate vessels, there are anastomoses between them close to the thoracic region. Blood from the external jugular vein empties into the subclavian vein. Table 20.12 summarizes the major veins of the head and neck.
Major Veins of the Head and Neck
| Vessel | Description |
|---|---|
| Internal jugular vein | Parallel to the common carotid artery, which is more or less its counterpart, and passes through the jugular foramen and canal; primarily drains blood from the brain, receives the superficial facial vein, and empties into the subclavian vein |
| Temporal vein | Drains blood from the temporal region and flows into the external jugular vein |
| Maxillary vein | Drains blood from the maxillary region and flows into the external jugular vein |
| External jugular vein | Drains blood from the more superficial portions of the head, scalp, and cranial regions, and leads to the subclavian vein |
Table 20.12
Venous Drainage of the Brain
Circulation to the brain is both critical and complex (see Figure 20.37). Many smaller veins of the brain stem and the superficial veins of the cerebrum lead to larger vessels referred to as intracranial sinuses. These include the superior and inferior sagittal sinuses, straight sinus, cavernous sinuses, left and right sinuses, the petrosal sinuses, and the occipital sinuses. Ultimately, sinuses will lead back to either the inferior jugular vein or vertebral vein.
Most of the veins on the superior surface of the cerebrum flow into the largest of the sinuses, the superior sagittal sinus. It is located midsagittally between the meningeal and periosteal layers of the dura mater within the falx cerebri and, at first glance in images or models, can be mistaken for the subarachnoid space. Most reabsorption of cerebrospinal fluid occurs via the chorionic villi (arachnoid granulations) into the superior sagittal sinus. Blood from most of the smaller vessels originating from the inferior cerebral veins flows into the great cerebral vein and into the straight sinus. Other cerebral veins and those from the eye socket flow into the cavernous sinus, which flows into the petrosal sinus and then into the internal jugular vein. The occipital sinus, sagittal sinus, and straight sinuses all flow into the left and right transverse sinuses near the lambdoid suture. The transverse sinuses in turn flow into the sigmoid sinuses that pass through the jugular foramen and into the internal jugular vein. The internal jugular vein flows parallel to the common carotid artery and is more or less its counterpart. It empties into the brachiocephalic vein. The veins draining the cervical vertebrae and the posterior surface of the skull, including some blood from the occipital sinus, flow into the vertebral veins. These parallel the vertebral arteries and travel through the transverse foramina of the cervical vertebrae. The vertebral veins also flow into the brachiocephalic veins. Table 20.13 summarizes the major veins of the brain.
Figure 20.37 Veins of the Head and Neck This left lateral view shows the veins of the head and neck, including the intercranial sinuses.
Major Veins of the Brain
| Vessel | Description |
|---|---|
| Superior sagittal sinus | Enlarged vein located midsagittally between the meningeal and periosteal layers of the dura mater within the falx cerebri; receives most of the blood drained from the superior surface of the cerebrum and leads to the inferior jugular vein and the vertebral vein |
| Great cerebral vein | Receives most of the smaller vessels from the inferior cerebral veins and leads to the straight sinus |
| Straight sinus | Enlarged vein that drains blood from the brain; receives most of the blood from the great cerebral vein and leads to the left or right transverse sinus |
| Cavernous sinus | Enlarged vein that receives blood from most of the other cerebral veins and the eye socket, and leads to the petrosal sinus |
| Petrosal sinus | Enlarged vein that receives blood from the cavernous sinus and leads into the internal jugular veins |
| Occipital sinus | Enlarged vein that drains the occipital region near the falx cerebelli and leads to the left and right transverse sinuses, and also the vertebral veins |
| Transverse sinuses | Pair of enlarged veins near the lambdoid suture that drains the occipital, sagittal, and straight sinuses, and leads to the sigmoid sinuses |
| Sigmoid sinuses | Enlarged vein that receives blood from the transverse sinuses and leads through the jugular foramen to the internal jugular vein |
Table 20.13
Veins Draining the Upper Limbs
The digital veins in the fingers come together in the hand to form the palmar venous arches (Figure 20.38). From here, the veins come together to form the radial vein, the ulnar vein, and the median antebrachial vein. The radial vein and the ulnar vein parallel the bones of the forearm and join together at the antebrachium to form the brachial vein, a deep vein that flows into the axillary vein in the brachium.
The median antebrachial vein parallels the ulnar vein, is more medial in location, and joins the basilic vein in the forearm. As the basilic vein reaches the antecubital region, it gives off a branch called the median cubital vein that crosses at an angle to join the cephalic vein. The median cubital vein is the most common site for drawing venous blood in humans. The basilic vein continues through the arm medially and superficially to the axillary vein.
The cephalic vein begins in the antebrachium and drains blood from the superficial surface of the arm into the axillary vein. It is extremely superficial and easily seen along the surface of the biceps brachii muscle in individuals with good muscle tone and in those without excessive subcutaneous adipose tissue in the arms.
The subscapular vein drains blood from the subscapular region and joins the cephalic vein to form the axillary vein. As it passes through the body wall and enters the thorax, the axillary vein becomes the subclavian vein.
Many of the larger veins of the thoracic and abdominal region and upper limb are further represented in the flow chart in Figure 20.39. Table 20.14 summarizes the veins of the upper limbs.
Figure 20.38 Veins of the Upper Limb This anterior view shows the veins that drain the upper limb.
Figure 20.39 Veins Flowing into the Superior Vena Cava The flow chart summarizes the distribution of the veins flowing into the superior vena cava.
Veins of the Upper Limbs
| Vessel | Description |
|---|---|
| Digital veins | Drain the digits and lead to the palmar arches of the hand and dorsal venous arch of the foot |
| Palmar venous arches | Drain the hand and digits, and lead to the radial vein, ulnar veins, and the median antebrachial vein |
| Radial vein | Vein that parallels the radius and radial artery; arises from the palmar venous arches and leads to the brachial vein |
| Ulnar vein | Vein that parallels the ulna and ulnar artery; arises from the palmar venous arches and leads to the brachial vein |
| Brachial vein | Deeper vein of the arm that forms from the radial and ulnar veins in the lower arm; leads to the axillary vein |
| Median antebrachial vein | Vein that parallels the ulnar vein but is more medial in location; intertwines with the palmar venous arches; leads to the basilic vein |
| Basilic vein | Superficial vein of the arm that arises from the median antebrachial vein, intersects with the median cubital vein, parallels the ulnar vein, and continues into the upper arm; along with the brachial vein, it leads to the axillary vein |
| Median cubital vein | Superficial vessel located in the antecubital region that links the cephalic vein to the basilic vein in the form of a v; a frequent site from which to draw blood |
| Cephalic vein | Superficial vessel in the upper arm; leads to the axillary vein |
| Subscapular vein | Drains blood from the subscapular region and leads to the axillary vein |
| Axillary vein | The major vein in the axillary region; drains the upper limb and becomes the subclavian vein |
Table 20.14
The Inferior Vena Cava
Other than the small amount of blood drained by the azygos and hemiazygos veins, most of the blood inferior to the diaphragm drains into the inferior vena cava before it is returned to the heart (see Figure 20.36). Lying just beneath the parietal peritoneum in the abdominal cavity, the inferior vena cava parallels the abdominal aorta, where it can receive blood from abdominal veins. The lumbar portions of the abdominal wall and spinal cord are drained by a series of lumbar veins, usually four on each side. The ascending lumbar veins drain into either the azygos vein on the right or the hemiazygos vein on the left, and return to the superior vena cava. The remaining lumbar veins drain directly into the inferior vena cava.
Blood supply from the kidneys flows into each renal vein, normally the largest veins entering the inferior vena cava. A number of other, smaller veins empty into the left renal vein. Each adrenal vein drains the adrenal or suprarenal glands located immediately superior to the kidneys. The right adrenal vein enters the inferior vena cava directly, whereas the left adrenal vein enters the left renal vein.
From the male reproductive organs, each testicular vein flows from the scrotum, forming a portion of the spermatic cord. Each ovarian vein drains an ovary in females. Each of these veins is generically called a gonadal vein. The right gonadal vein empties directly into the inferior vena cava, and the left gonadal vein empties into the left renal vein.
Each side of the diaphragm drains into a phrenic vein; the right phrenic vein empties directly into the inferior vena cava, whereas the left phrenic vein empties into the left renal vein. Blood supply from the liver drains into each hepatic vein and directly into the inferior vena cava. Since the inferior vena cava lies primarily to the right of the vertebral column and aorta, the left renal vein is longer, as are the left phrenic, adrenal, and gonadal veins. The longer length of the left renal vein makes the left kidney the primary target of surgeons removing this organ for donation. Figure 20.40 provides a flow chart of the veins flowing into the inferior vena cava. Table 20.15 summarizes the major veins of the abdominal region.
Figure 20.40 Venous Flow into Inferior Vena Cava The flow chart summarizes veins that deliver blood to the inferior vena cava.
Major Veins of the Abdominal Region
| Vessel | Description |
|---|---|
| Inferior vena cava | Large systemic vein that drains blood from areas largely inferior to the diaphragm; empties into the right atrium |
| Lumbar veins | Series of veins that drain the lumbar portion of the abdominal wall and spinal cord; the ascending lumbar veins drain into the azygos vein on the right or the hemiazygos vein on the left; the remaining lumbar veins drain directly into the inferior vena cava |
| Renal vein | Largest vein entering the inferior vena cava; drains the kidneys and flows into the inferior vena cava |
| Adrenal vein | Drains the adrenal or suprarenal; the right adrenal vein enters the inferior vena cava directly and the left adrenal vein enters the left renal vein |
| Testicular vein | Drains the testes and forms part of the spermatic cord; the right testicular vein empties directly into the inferior vena cava and the left testicular vein empties into the left renal vein |
| Ovarian vein | Drains the ovary; the right ovarian vein empties directly into the inferior vena cava and the left ovarian vein empties into the left renal vein |
| Gonadal vein | Generic term for a vein draining a reproductive organ; may be either an ovarian vein or a testicular vein, depending on the sex of the individual |
| Phrenic vein | Drains the diaphragm; the right phrenic vein flows into the inferior vena cava and the left phrenic vein empties into the left renal vein |
| Hepatic vein | Drains systemic blood from the liver and flows into the inferior vena cava |
Table 20.15
Veins Draining the Lower Limbs
The superior surface of the foot drains into the digital veins, and the inferior surface drains into the plantar veins, which flow into a complex series of anastomoses in the feet and ankles, including the dorsal venous arch and the plantar venous arch (Figure 20.41). From the dorsal venous arch, blood supply drains into the anterior and posterior tibial veins. The anterior tibial vein drains the area near the tibialis anterior muscle and combines with the posterior tibial vein and the fibular vein to form the popliteal vein. The posterior tibial vein drains the posterior surface of the tibia and joins the popliteal vein. The fibular vein drains the muscles and integument in proximity to the fibula and also joins the popliteal vein. The small saphenous vein located on the lateral surface of the leg drains blood from the superficial regions of the lower leg and foot, and flows into to the popliteal vein. As the popliteal vein passes behind the knee in the popliteal region, it becomes the femoral vein. It is palpable in patients without excessive adipose tissue.
Close to the body wall, the great saphenous vein, the deep femoral vein, and the femoral circumflex vein drain into the femoral vein. The great saphenous vein is a prominent surface vessel located on the medial surface of the leg and thigh that collects blood from the superficial portions of these areas. The deep femoral vein, as the name suggests, drains blood from the deeper portions of the thigh. The femoral circumflex vein forms a loop around the femur just inferior to the trochanters and drains blood from the areas in proximity to the head and neck of the femur.
As the femoral vein penetrates the body wall from the femoral portion of the upper limb, it becomes the external iliac vein, a large vein that drains blood from the leg to the common iliac vein. The pelvic organs and integument drain into the internal iliac vein, which forms from several smaller veins in the region, including the umbilical veins that run on either side of the bladder. The external and internal iliac veins combine near the inferior portion of the sacroiliac joint to form the common iliac vein. In addition to blood supply from the external and internal iliac veins, the middle sacral vein drains the sacral region into the common iliac vein. Similar to the common iliac arteries, the common iliac veins come together at the level of L5 to form the inferior vena cava.
Figure 20.42 is a flow chart of veins flowing into the lower limb. Table 20.16 summarizes the major veins of the lower limbs.
Figure 20.41 Major Veins Serving the Lower Limbs Anterior and posterior views show the major veins that drain the lower limb into the inferior vena cava.
Figure 20.42 Major Veins of the Lower Limb The flow chart summarizes venous flow from the lower limb.
Veins of the Lower Limbs
| Vessel | Description |
|---|---|
| Plantar veins | Drain the foot and flow into the plantar venous arch |
| Dorsal venous arch | Drains blood from digital veins and vessels on the superior surface of the foot |
| Plantar venous arch | Formed from the plantar veins; flows into the anterior and posterior tibial veins through anastomoses |
| Anterior tibial vein | Formed from the dorsal venous arch; drains the area near the tibialis anterior muscle and flows into the popliteal vein |
| Posterior tibial vein | Formed from the dorsal venous arch; drains the area near the posterior surface of the tibia and flows into the popliteal vein |
| Fibular vein | Drains the muscles and integument near the fibula and flows into the popliteal vein |
| Small saphenous vein | Located on the lateral surface of the leg; drains blood from the superficial regions of the lower leg and foot, and flows into the popliteal vein |
| Popliteal vein | Drains the region behind the knee and forms from the fusion of the fibular, anterior, and posterior tibial veins; flows into the femoral vein |
| Great saphenous vein | Prominent surface vessel located on the medial surface of the leg and thigh; drains the superficial portions of these areas and flows into the femoral vein |
| Deep femoral vein | Drains blood from the deeper portions of the thigh and flows into the femoral vein |
| Femoral circumflex vein | Forms a loop around the femur just inferior to the trochanters; drains blood from the areas around the head and neck of the femur; flows into the femoral vein |
| Femoral vein | Drains the upper leg; receives blood from the great saphenous vein, the deep femoral vein, and the femoral circumflex vein; becomes the external iliac vein when it crosses the body wall |
| External iliac vein | Formed when the femoral vein passes into the body cavity; drains the legs and flows into the common iliac vein |
| Internal iliac vein | Drains the pelvic organs and integument; formed from several smaller veins in the region; flows into the common iliac vein |
| Middle sacral vein | Drains the sacral region and flows into the left common iliac vein |
| Common iliac vein | Flows into the inferior vena cava at the level of L5; the left common iliac vein drains the sacral region; formed from the union of the external and internal iliac veins near the inferior portion of the sacroiliac joint |
Table 20.16
Hepatic Portal System
The liver is a complex biochemical processing plant. It packages nutrients absorbed by the digestive system; produces plasma proteins, clotting factors, and bile; and disposes of worn-out cell components and waste products. Instead of entering the circulation directly, absorbed nutrients and certain wastes (for example, materials produced by the spleen) travel to the liver for processing. They do so via the hepatic portal system (Figure 20.43). Portal systems begin and end in capillaries. In this case, the initial capillaries from the stomach, small intestine, large intestine, and spleen lead to the hepatic portal vein and end in specialized capillaries within the liver, the hepatic sinusoids. You saw the only other portal system with the hypothalamic-hypophyseal portal vessel in the endocrine chapter.
The hepatic portal system consists of the hepatic portal vein and the veins that drain into it. The hepatic portal vein itself is relatively short, beginning at the level of L2 with the confluence of the superior mesenteric and splenic veins. It also receives branches from the inferior mesenteric vein, plus the splenic veins and all their tributaries. The superior mesenteric vein receives blood from the small intestine, two-thirds of the large intestine, and the stomach. The inferior mesenteric vein drains the distal third of the large intestine, including the descending colon, the sigmoid colon, and the rectum. The splenic vein is formed from branches from the spleen, pancreas, and portions of the stomach, and the inferior mesenteric vein. After its formation, the hepatic portal vein also receives branches from the gastric veins of the stomach and cystic veins from the gall bladder. The hepatic portal vein delivers materials from these digestive and circulatory organs directly to the liver for processing.
Because of the hepatic portal system, the liver receives its blood supply from two different sources: from normal systemic circulation via the hepatic artery and from the hepatic portal vein. The liver processes the blood from the portal system to remove certain wastes and excess nutrients, which are stored for later use. This processed blood, as well as the systemic blood that came from the hepatic artery, exits the liver via the right, left, and middle hepatic veins, and flows into the inferior vena cava. Overall systemic blood composition remains relatively stable, since the liver is able to metabolize the absorbed digestive components.
Figure 20.43 Hepatic Portal System The liver receives blood from the normal systemic circulation via the hepatic artery. It also receives and processes blood from other organs, delivered via the veins of the hepatic portal system. All blood exits the liver via the hepatic vein, which delivers the blood to the inferior vena cava. (Different colors are used to help distinguish among the different vessels in the system.)
Development of Blood Vessels and Fetal Circulation
- Describe the development of blood vessels
- Describe the fetal circulation
In a developing embryo,the heart has developed enough by day 21 post-fertilization to begin beating. Circulation patterns are clearly established by the fourth week of embryonic life. It is critical to the survival of the developing human that the circulatory system forms early to supply the growing tissue with nutrients and gases, and to remove waste products. Blood cells and vessel production in structures outside the embryo proper called the yolk sac, chorion, and connecting stalk begin about 15 to 16 days following fertilization. Development of these circulatory elements within the embryo itself begins approximately 2 days later. You will learn more about the formation and function of these early structures when you study the chapter on development. During those first few weeks, blood vessels begin to form from the embryonic mesoderm. The precursor cells are known as hemangioblasts. These in turn differentiate into angioblasts, which give rise to the blood vessels and pluripotent stem cells, which differentiate into the formed elements of blood. (Seek additional content for more detail on fetal development and circulation.) Together, these cells form masses known as blood islands scattered throughout the embryonic disc. Spaces appear on the blood islands that develop into vessel lumens. The endothelial lining of the vessels arise from the angioblasts within these islands. Surrounding mesenchymal cells give rise to the smooth muscle and connective tissue layers of the vessels. While the vessels are developing, the pluripotent stem cells begin to form the blood.
Vascular tubes also develop on the blood islands, and they eventually connect to one another as well as to the developing, tubular heart. Thus, the developmental pattern, rather than beginning from the formation of one central vessel and spreading outward, occurs in many regions simultaneously with vessels later joining together. This angiogenesis—the creation of new blood vessels from existing ones—continues as needed throughout life as we grow and develop.
Blood vessel development often follows the same pattern as nerve development and travels to the same target tissues and organs. This occurs because the many factors directing growth of nerves also stimulate blood vessels to follow a similar pattern. Whether a given vessel develops into an artery or a vein is dependent upon local concentrations of signaling proteins.
As the embryo grows within the mother’s uterus, its requirements for nutrients and gas exchange also grow. The placenta—a circulatory organ unique to pregnancy—develops jointly from the embryo and uterine wall structures to fill this need. Emerging from the placenta is the umbilical vein, which carries oxygen-rich blood from the mother to the fetal inferior vena cava via the ductus venosus to the heart that pumps it into fetal circulation. Two umbilical arteries carry oxygen-depleted fetal blood, including wastes and carbon dioxide, to the placenta. Remnants of the umbilical arteries remain in the adult. (Seek additional content for more information on the role of the placenta in fetal circulation.)
There are three major shunts—alternate paths for blood flow—found in the circulatory system of the fetus. Two of these shunts divert blood from the pulmonary to the systemic circuit, whereas the third connects the umbilical vein to the inferior vena cava. The first two shunts are critical during fetal life, when the lungs are compressed, filled with amniotic fluid, and nonfunctional, and gas exchange is provided by the placenta. These shunts close shortly after birth, however, when the newborn begins to breathe. The third shunt persists a bit longer but becomes nonfunctional once the umbilical cord is severed. The three shunts are as follows (Figure 20.44):
- The foramen ovale is an opening in the interatrial septum that allows blood to flow from the right atrium to the left atrium. A valve associated with this opening prevents backflow of blood during the fetal period. As the newborn begins to breathe and blood pressure in the atria increases, this shunt closes. The fossa ovalis remains in the interatrial septum after birth, marking the location of the former foramen ovale.
- The ductus arteriosus is a short, muscular vessel that connects the pulmonary trunk to the aorta. Most of the blood pumped from the right ventricle into the pulmonary trunk is thereby diverted into the aorta. Only enough blood reaches the fetal lungs to maintain the developing lung tissue. When the newborn takes the first breath, pressure within the lungs drops dramatically, and both the lungs and the pulmonary vessels expand. As the amount of oxygen increases, the smooth muscles in the wall of the ductus arteriosus constrict, sealing off the passage. Eventually, the muscular and endothelial components of the ductus arteriosus degenerate, leaving only the connective tissue component of the ligamentum arteriosum.
- The ductus venosus is a temporary blood vessel that branches from the umbilical vein, allowing much of the freshly oxygenated blood from the placenta—the organ of gas exchange between the mother and fetus—to bypass the fetal liver and go directly to the fetal heart. The ductus venosus closes slowly during the first weeks of infancy and degenerates to become the ligamentum venosum.
Figure 20.44 Fetal Shunts The foramen ovale in the interatrial septum allows blood to flow from the right atrium to the left atrium. The ductus arteriosus is a temporary vessel, connecting the aorta to the pulmonary trunk. The ductus venosus links the umbilical vein to the inferior vena cava largely through the liver.
Key Terms
- abdominal aorta
- portion of the aorta inferior to the aortic hiatus and superior to the common iliac arteries
- adrenal artery
- branch of the abdominal aorta; supplies blood to the adrenal (suprarenal) glands
- adrenal vein
- drains the adrenal or suprarenal glands that are immediately superior to the kidneys; the right adrenal vein enters the inferior vena cava directly and the left adrenal vein enters the left renal vein
- anaphylactic shock
- type of shock that follows a severe allergic reaction and results from massive vasodilation
- angioblasts
- stem cells that give rise to blood vessels
- angiogenesis
- development of new blood vessels from existing vessels
- anterior cerebral artery
- arises from the internal carotid artery; supplies the frontal lobe of the cerebrum
- anterior communicating artery
- anastomosis of the right and left internal carotid arteries; supplies blood to the brain
- anterior tibial artery
- branches from the popliteal artery; supplies blood to the anterior tibial region; becomes the dorsalis pedis artery
- anterior tibial vein
- forms from the dorsal venous arch; drains the area near the tibialis anterior muscle and leads to the popliteal vein
- aorta
- largest artery in the body, originating from the left ventricle and descending to the abdominal region where it bifurcates into the common iliac arteries at the level of the fourth lumbar vertebra; arteries originating from the aorta distribute blood to virtually all tissues of the body
- aortic arch
- arc that connects the ascending aorta to the descending aorta; ends at the intervertebral disk between the fourth and fifth thoracic vertebrae
- aortic hiatus
- opening in the diaphragm that allows passage of the thoracic aorta into the abdominal region where it becomes the abdominal aorta
- aortic sinuses
- small pockets in the ascending aorta near the aortic valve that are the locations of the baroreceptors (stretch receptors) and chemoreceptors that trigger a reflex that aids in the regulation of vascular homeostasis
- arterial circle
- (also, circle of Willis) anastomosis located at the base of the brain that ensures continual blood supply; formed from branches of the internal carotid and vertebral arteries; supplies blood to the brain
- arteriole
- (also, resistance vessel) very small artery that leads to a capillary
- arteriovenous anastomosis
- short vessel connecting an arteriole directly to a venule and bypassing the capillary beds
- artery
- blood vessel that conducts blood away from the heart; may be a conducting or distributing vessel
- ascending aorta
- initial portion of the aorta, rising from the left ventricle for a distance of approximately 5 cm
- atrial reflex
- mechanism for maintaining vascular homeostasis involving atrial baroreceptors: if blood is returning to the right atrium more rapidly than it is being ejected from the left ventricle, the atrial receptors will stimulate the cardiovascular centers to increase sympathetic firing and increase cardiac output until the situation is reversed; the opposite is also true
- axillary artery
- continuation of the subclavian artery as it penetrates the body wall and enters the axillary region; supplies blood to the region near the head of the humerus (humeral circumflex arteries); the majority of the vessel continues into the brachium and becomes the brachial artery
- axillary vein
- major vein in the axillary region; drains the upper limb and becomes the subclavian vein
- azygos vein
- originates in the lumbar region and passes through the diaphragm into the thoracic cavity on the right side of the vertebral column; drains blood from the intercostal veins, esophageal veins, bronchial veins, and other veins draining the mediastinal region; leads to the superior vena cava
- basilar artery
- formed from the fusion of the two vertebral arteries; sends branches to the cerebellum, brain stem, and the posterior cerebral arteries; the main blood supply to the brain stem
- basilic vein
- superficial vein of the arm that arises from the palmar venous arches, intersects with the median cubital vein, parallels the ulnar vein, and continues into the upper arm; along with the brachial vein, it leads to the axillary vein
- blood colloidal osmotic pressure (BCOP)
- pressure exerted by colloids suspended in blood within a vessel; a primary determinant is the presence of plasma proteins
- blood flow
- movement of blood through a vessel, tissue, or organ that is usually expressed in terms of volume per unit of time
- blood hydrostatic pressure
- force blood exerts against the walls of a blood vessel or heart chamber
- blood islands
- masses of developing blood vessels and formed elements from mesodermal cells scattered throughout the embryonic disc
- blood pressure
- force exerted by the blood against the wall of a vessel or heart chamber; can be described with the more generic term hydrostatic pressure
- brachial artery
- continuation of the axillary artery in the brachium; supplies blood to much of the brachial region; gives off several smaller branches that provide blood to the posterior surface of the arm in the region of the elbow; bifurcates into the radial and ulnar arteries at the coronoid fossa
- brachial vein
- deeper vein of the arm that forms from the radial and ulnar veins in the lower arm; leads to the axillary vein
- brachiocephalic artery
- single vessel located on the right side of the body; the first vessel branching from the aortic arch; gives rise to the right subclavian artery and the right common carotid artery; supplies blood to the head, neck, upper limb, and wall of the thoracic region
- brachiocephalic vein
- one of a pair of veins that form from a fusion of the external and internal jugular veins and the subclavian vein; subclavian, external and internal jugulars, vertebral, and internal thoracic veins lead to it; drains the upper thoracic region and flows into the superior vena cava
- bronchial artery
- systemic branch from the aorta that provides oxygenated blood to the lungs in addition to the pulmonary circuit
- bronchial vein
- drains the systemic circulation from the lungs and leads to the azygos vein
- capacitance
- ability of a vein to distend and store blood
- capacitance vessels
- veins
- capillary
- smallest of blood vessels where physical exchange occurs between the blood and tissue cells surrounded by interstitial fluid
- capillary bed
- network of 10–100 capillaries connecting arterioles to venules
- capillary hydrostatic pressure (CHP)
- force blood exerts against a capillary
- cardiogenic shock
- type of shock that results from the inability of the heart to maintain cardiac output
- carotid sinuses
- small pockets near the base of the internal carotid arteries that are the locations of the baroreceptors and chemoreceptors that trigger a reflex that aids in the regulation of vascular homeostasis
- cavernous sinus
- enlarged vein that receives blood from most of the other cerebral veins and the eye socket, and leads to the petrosal sinus
- celiac trunk
- (also, celiac artery) major branch of the abdominal aorta; gives rise to the left gastric artery, the splenic artery, and the common hepatic artery that forms the hepatic artery to the liver, the right gastric artery to the stomach, and the cystic artery to the gall bladder
- cephalic vein
- superficial vessel in the upper arm; leads to the axillary vein
- cerebrovascular accident (CVA)
- blockage of blood flow to the brain; also called a stroke
- circle of Willis
- (also, arterial circle) anastomosis located at the base of the brain that ensures continual blood supply; formed from branches of the internal carotid and vertebral arteries; supplies blood to the brain
- circulatory shock
- also simply called shock; a life-threatening medical condition in which the circulatory system is unable to supply enough blood flow to provide adequate oxygen and other nutrients to the tissues to maintain cellular metabolism
- common carotid artery
- right common carotid artery arises from the brachiocephalic artery, and the left common carotid arises from the aortic arch; gives rise to the external and internal carotid arteries; supplies the respective sides of the head and neck
- common hepatic artery
- branch of the celiac trunk that forms the hepatic artery, the right gastric artery, and the cystic artery
- common iliac artery
- branch of the aorta that leads to the internal and external iliac arteries
- common iliac vein
- one of a pair of veins that flows into the inferior vena cava at the level of L5; the left common iliac vein drains the sacral region; divides into external and internal iliac veins near the inferior portion of the sacroiliac joint
- compliance
- degree to which a blood vessel can stretch as opposed to being rigid
- continuous capillary
- most common type of capillary, found in virtually all tissues except epithelia and cartilage; contains very small gaps in the endothelial lining that permit exchange
- cystic artery
- branch of the common hepatic artery; supplies blood to the gall bladder
- deep femoral artery
- branch of the femoral artery; gives rise to the lateral circumflex arteries
- deep femoral vein
- drains blood from the deeper portions of the thigh and leads to the femoral vein
- descending aorta
- portion of the aorta that continues downward past the end of the aortic arch; subdivided into the thoracic aorta and the abdominal aorta
- diastolic pressure
- lower number recorded when measuring arterial blood pressure; represents the minimal value corresponding to the pressure that remains during ventricular relaxation
- digital arteries
- formed from the superficial and deep palmar arches; supply blood to the digits
- digital veins
- drain the digits and feed into the palmar arches of the hand and dorsal venous arch of the foot
- dorsal arch
- (also, arcuate arch) formed from the anastomosis of the dorsalis pedis artery and medial and plantar arteries; branches supply the distal portions of the foot and digits
- dorsal venous arch
- drains blood from digital veins and vessels on the superior surface of the foot
- dorsalis pedis artery
- forms from the anterior tibial artery; branches repeatedly to supply blood to the tarsal and dorsal regions of the foot
- ductus arteriosus
- shunt in the fetal pulmonary trunk that diverts oxygenated blood back to the aorta
- ductus venosus
- shunt that causes oxygenated blood to bypass the fetal liver on its way to the inferior vena cava
- elastic artery
- (also, conducting artery) artery with abundant elastic fibers located closer to the heart, which maintains the pressure gradient and conducts blood to smaller branches
- esophageal artery
- branch of the thoracic aorta; supplies blood to the esophagus
- esophageal vein
- drains the inferior portions of the esophagus and leads to the azygos vein
- external carotid artery
- arises from the common carotid artery; supplies blood to numerous structures within the face, lower jaw, neck, esophagus, and larynx
- external elastic membrane
- membrane composed of elastic fibers that separates the tunica media from the tunica externa; seen in larger arteries
- external iliac artery
- branch of the common iliac artery that leaves the body cavity and becomes a femoral artery; supplies blood to the lower limbs
- external iliac vein
- formed when the femoral vein passes into the body cavity; drains the legs and leads to the common iliac vein
- external jugular vein
- one of a pair of major veins located in the superficial neck region that drains blood from the more superficial portions of the head, scalp, and cranial regions, and leads to the subclavian vein
- femoral artery
- continuation of the external iliac artery after it passes through the body cavity; divides into several smaller branches, the lateral deep femoral artery, and the genicular artery; becomes the popliteal artery as it passes posterior to the knee
- femoral circumflex vein
- forms a loop around the femur just inferior to the trochanters; drains blood from the areas around the head and neck of the femur; leads to the femoral vein
- femoral vein
- drains the upper leg; receives blood from the great saphenous vein, the deep femoral vein, and the femoral circumflex vein; becomes the external iliac vein when it crosses the body wall
- fenestrated capillary
- type of capillary with pores or fenestrations in the endothelium that allow for rapid passage of certain small materials
- fibular vein
- drains the muscles and integument near the fibula and leads to the popliteal vein
- filtration
- in the cardiovascular system, the movement of material from a capillary into the interstitial fluid, moving from an area of higher pressure to lower pressure
- foramen ovale
- shunt that directly connects the right and left atria and helps to divert oxygenated blood from the fetal pulmonary circuit
- genicular artery
- branch of the femoral artery; supplies blood to the region of the knee
- gonadal artery
- branch of the abdominal aorta; supplies blood to the gonads or reproductive organs; also described as ovarian arteries or testicular arteries, depending upon the sex of the individual
- gonadal vein
- generic term for a vein draining a reproductive organ; may be either an ovarian vein or a testicular vein, depending on the sex of the individual
- great cerebral vein
- receives most of the smaller vessels from the inferior cerebral veins and leads to the straight sinus
- great saphenous vein
- prominent surface vessel located on the medial surface of the leg and thigh; drains the superficial portions of these areas and leads to the femoral vein
- hemangioblasts
- embryonic stem cells that appear in the mesoderm and give rise to both angioblasts and pluripotent stem cells
- hemiazygos vein
- smaller vein complementary to the azygos vein; drains the esophageal veins from the esophagus and the left intercostal veins, and leads to the brachiocephalic vein via the superior intercostal vein
- hepatic artery proper
- branch of the common hepatic artery; supplies systemic blood to the liver
- hepatic portal system
- specialized circulatory pathway that carries blood from digestive organs to the liver for processing before being sent to the systemic circulation
- hepatic vein
- drains systemic blood from the liver and flows into the inferior vena cava
- hypertension
- chronic and persistent blood pressure measurements of 140/90 mm Hg or above
- hypervolemia
- abnormally high levels of fluid and blood within the body
- hypovolemia
- abnormally low levels of fluid and blood within the body
- hypovolemic shock
- type of circulatory shock caused by excessive loss of blood volume due to hemorrhage or possibly dehydration
- hypoxia
- lack of oxygen supply to the tissues
- inferior mesenteric artery
- branch of the abdominal aorta; supplies blood to the distal segment of the large intestine and rectum
- inferior phrenic artery
- branch of the abdominal aorta; supplies blood to the inferior surface of the diaphragm
- inferior vena cava
- large systemic vein that drains blood from areas largely inferior to the diaphragm; empties into the right atrium
- intercostal artery
- branch of the thoracic aorta; supplies blood to the muscles of the thoracic cavity and vertebral column
- intercostal vein
- drains the muscles of the thoracic wall and leads to the azygos vein
- internal carotid artery
- arises from the common carotid artery and begins with the carotid sinus; goes through the carotid canal of the temporal bone to the base of the brain; combines with branches of the vertebral artery forming the arterial circle; supplies blood to the brain
- internal elastic membrane
- membrane composed of elastic fibers that separates the tunica intima from the tunica media; seen in larger arteries
- internal iliac artery
- branch from the common iliac arteries; supplies blood to the urinary bladder, walls of the pelvis, external genitalia, and the medial portion of the femoral region; in females, also provide blood to the uterus and vagina
- internal iliac vein
- drains the pelvic organs and integument; formed from several smaller veins in the region; leads to the common iliac vein
- internal jugular vein
- one of a pair of major veins located in the neck region that passes through the jugular foramen and canal, flows parallel to the common carotid artery that is more or less its counterpart; primarily drains blood from the brain, receives the superficial facial vein, and empties into the subclavian vein
- internal thoracic artery
- (also, mammary artery) arises from the subclavian artery; supplies blood to the thymus, pericardium of the heart, and the anterior chest wall
- internal thoracic vein
- (also, internal mammary vein) drains the anterior surface of the chest wall and leads to the brachiocephalic vein
- interstitial fluid colloidal osmotic pressure (IFCOP)
- pressure exerted by the colloids within the interstitial fluid
- interstitial fluid hydrostatic pressure (IFHP)
- force exerted by the fluid in the tissue spaces
- ischemia
- insufficient blood flow to the tissues
- Korotkoff sounds
- noises created by turbulent blood flow through the vessels
- lateral circumflex artery
- branch of the deep femoral artery; supplies blood to the deep muscles of the thigh and the ventral and lateral regions of the integument
- lateral plantar artery
- arises from the bifurcation of the posterior tibial arteries; supplies blood to the lateral plantar surfaces of the foot
- left gastric artery
- branch of the celiac trunk; supplies blood to the stomach
- lumbar arteries
- branches of the abdominal aorta; supply blood to the lumbar region, the abdominal wall, and spinal cord
- lumbar veins
- drain the lumbar portion of the abdominal wall and spinal cord; the superior lumbar veins drain into the azygos vein on the right or the hemiazygos vein on the left; blood from these vessels is returned to the superior vena cava rather than the inferior vena cava
- lumen
- interior of a tubular structure such as a blood vessel or a portion of the alimentary canal through which blood, chyme, or other substances travel
- maxillary vein
- drains blood from the maxillary region and leads to the external jugular vein
- mean arterial pressure (MAP)
- average driving force of blood to the tissues; approximated by taking diastolic pressure and adding 1/3 of pulse pressure
- medial plantar artery
- arises from the bifurcation of the posterior tibial arteries; supplies blood to the medial plantar surfaces of the foot
- median antebrachial vein
- vein that parallels the ulnar vein but is more medial in location; intertwines with the palmar venous arches
- median cubital vein
- superficial vessel located in the antecubital region that links the cephalic vein to the basilic vein in the form of a v; a frequent site for a blood draw
- median sacral artery
- continuation of the aorta into the sacrum
- mediastinal artery
- branch of the thoracic aorta; supplies blood to the mediastinum
- metarteriole
- short vessel arising from a terminal arteriole that branches to supply a capillary bed
- microcirculation
- blood flow through the capillaries
- middle cerebral artery
- another branch of the internal carotid artery; supplies blood to the temporal and parietal lobes of the cerebrum
- middle sacral vein
- drains the sacral region and leads to the left common iliac vein
- muscular artery
- (also, distributing artery) artery with abundant smooth muscle in the tunica media that branches to distribute blood to the arteriole network
- myogenic response
- constriction or dilation in the walls of arterioles in response to pressures related to blood flow; reduces high blood flow or increases low blood flow to help maintain consistent flow to the capillary network
- nervi vasorum
- small nerve fibers found in arteries and veins that trigger contraction of the smooth muscle in their walls
- net filtration pressure (NFP)
- force driving fluid out of the capillary and into the tissue spaces; equal to the difference of the capillary hydrostatic pressure and the blood colloidal osmotic pressure
- neurogenic shock
- type of shock that occurs with cranial or high spinal injuries that damage the cardiovascular centers in the medulla oblongata or the nervous fibers originating from this region
- obstructive shock
- type of shock that occurs when a significant portion of the vascular system is blocked
- occipital sinus
- enlarged vein that drains the occipital region near the falx cerebelli and flows into the left and right transverse sinuses, and also into the vertebral veins
- ophthalmic artery
- branch of the internal carotid artery; supplies blood to the eyes
- ovarian artery
- branch of the abdominal aorta; supplies blood to the ovary, uterine (Fallopian) tube, and uterus
- ovarian vein
- drains the ovary; the right ovarian vein leads to the inferior vena cava and the left ovarian vein leads to the left renal vein
- palmar arches
- superficial and deep arches formed from anastomoses of the radial and ulnar arteries; supply blood to the hand and digital arteries
- palmar venous arches
- drain the hand and digits, and feed into the radial and ulnar veins
- parietal branches
- (also, somatic branches) group of arterial branches of the thoracic aorta; includes those that supply blood to the thoracic cavity, vertebral column, and the superior surface of the diaphragm
- perfusion
- distribution of blood into the capillaries so the tissues can be supplied
- pericardial artery
- branch of the thoracic aorta; supplies blood to the pericardium
- petrosal sinus
- enlarged vein that receives blood from the cavernous sinus and flows into the internal jugular vein
- phrenic vein
- drains the diaphragm; the right phrenic vein flows into the inferior vena cava and the left phrenic vein leads to the left renal vein
- plantar arch
- formed from the anastomosis of the dorsalis pedis artery and medial and plantar arteries; branches supply the distal portions of the foot and digits
- plantar veins
- drain the foot and lead to the plantar venous arch
- plantar venous arch
- formed from the plantar veins; leads to the anterior and posterior tibial veins through anastomoses
- popliteal artery
- continuation of the femoral artery posterior to the knee; branches into the anterior and posterior tibial arteries
- popliteal vein
- continuation of the femoral vein behind the knee; drains the region behind the knee and forms from the fusion of the fibular and anterior and posterior tibial veins
- posterior cerebral artery
- branch of the basilar artery that forms a portion of the posterior segment of the arterial circle; supplies blood to the posterior portion of the cerebrum and brain stem
- posterior communicating artery
- branch of the posterior cerebral artery that forms part of the posterior portion of the arterial circle; supplies blood to the brain
- posterior tibial artery
- branch from the popliteal artery that gives rise to the fibular or peroneal artery; supplies blood to the posterior tibial region
- posterior tibial vein
- forms from the dorsal venous arch; drains the area near the posterior surface of the tibia and leads to the popliteal vein
- precapillary sphincters
- circular rings of smooth muscle that surround the entrance to a capillary and regulate blood flow into that capillary
- pulmonary artery
- one of two branches, left and right, that divides off from the pulmonary trunk and leads to smaller arterioles and eventually to the pulmonary capillaries
- pulmonary circuit
- system of blood vessels that provide gas exchange via a network of arteries, veins, and capillaries that run from the heart, through the body, and back to the lungs
- pulmonary trunk
- single large vessel exiting the right ventricle that divides to form the right and left pulmonary arteries
- pulmonary veins
- two sets of paired vessels, one pair on each side, that are formed from the small venules leading away from the pulmonary capillaries that flow into the left atrium
- pulse
- alternating expansion and recoil of an artery as blood moves through the vessel; an indicator of heart rate
- pulse pressure
- difference between the systolic and diastolic pressures
- radial artery
- formed at the bifurcation of the brachial artery; parallels the radius; gives off smaller branches until it reaches the carpal region where it fuses with the ulnar artery to form the superficial and deep palmar arches; supplies blood to the lower arm and carpal region
- radial vein
- parallels the radius and radial artery; arises from the palmar venous arches and leads to the brachial vein
- reabsorption
- in the cardiovascular system, the movement of material from the interstitial fluid into the capillaries
- renal artery
- branch of the abdominal aorta; supplies each kidney
- renal vein
- largest vein entering the inferior vena cava; drains the kidneys and leads to the inferior vena cava
- resistance
- any condition or parameter that slows or counteracts the flow of blood
- respiratory pump
- increase in the volume of the thorax during inhalation that decreases air pressure, enabling venous blood to flow into the thoracic region, then exhalation increases pressure, moving blood into the atria
- right gastric artery
- branch of the common hepatic artery; supplies blood to the stomach
- sepsis
- (also, septicemia) organismal-level inflammatory response to a massive infection
- septic shock
- (also, blood poisoning) type of shock that follows a massive infection resulting in organism-wide inflammation
- sigmoid sinuses
- enlarged veins that receive blood from the transverse sinuses; flow through the jugular foramen and into the internal jugular vein
- sinusoid capillary
- rarest type of capillary, which has extremely large intercellular gaps in the basement membrane in addition to clefts and fenestrations; found in areas such as the bone marrow and liver where passage of large molecules occurs
- skeletal muscle pump
- effect on increasing blood pressure within veins by compression of the vessel caused by the contraction of nearby skeletal muscle
- small saphenous vein
- located on the lateral surface of the leg; drains blood from the superficial regions of the lower leg and foot, and leads to the popliteal vein
- sphygmomanometer
- blood pressure cuff attached to a device that measures blood pressure
- splenic artery
- branch of the celiac trunk; supplies blood to the spleen
- straight sinus
- enlarged vein that drains blood from the brain; receives most of the blood from the great cerebral vein and flows into the left or right transverse sinus
- subclavian artery
- right subclavian arises from the brachiocephalic artery, whereas the left subclavian artery arises from the aortic arch; gives rise to the internal thoracic, vertebral, and thyrocervical arteries; supplies blood to the arms, chest, shoulders, back, and central nervous system
- subclavian vein
- located deep in the thoracic cavity; becomes the axillary vein as it enters the axillary region; drains the axillary and smaller local veins near the scapular region; leads to the brachiocephalic vein
- subscapular vein
- drains blood from the subscapular region and leads to the axillary vein
- superior mesenteric artery
- branch of the abdominal aorta; supplies blood to the small intestine (duodenum, jejunum, and ileum), the pancreas, and a majority of the large intestine
- superior phrenic artery
- branch of the thoracic aorta; supplies blood to the superior surface of the diaphragm
- superior sagittal sinus
- enlarged vein located midsagittally between the meningeal and periosteal layers of the dura mater within the falx cerebri; receives most of the blood drained from the superior surface of the cerebrum and leads to the inferior jugular vein and the vertebral vein
- superior vena cava
- large systemic vein; drains blood from most areas superior to the diaphragm; empties into the right atrium
- systolic pressure
- larger number recorded when measuring arterial blood pressure; represents the maximum value following ventricular contraction
- temporal vein
- drains blood from the temporal region and leads to the external jugular vein
- testicular artery
- branch of the abdominal aorta; will ultimately travel outside the body cavity to the testes and form one component of the spermatic cord
- testicular vein
- drains the testes and forms part of the spermatic cord; the right testicular vein empties directly into the inferior vena cava and the left testicular vein empties into the left renal vein
- thoracic aorta
- portion of the descending aorta superior to the aortic hiatus
- thoroughfare channel
- continuation of the metarteriole that enables blood to bypass a capillary bed and flow directly into a venule, creating a vascular shunt
- thyrocervical artery
- arises from the subclavian artery; supplies blood to the thyroid, the cervical region, the upper back, and shoulder
- transient ischemic attack (TIA)
- temporary loss of neurological function caused by a brief interruption in blood flow; also known as a mini-stroke
- transverse sinuses
- pair of enlarged veins near the lambdoid suture that drain the occipital, sagittal, and straight sinuses, and leads to the sigmoid sinuses
- trunk
- large vessel that gives rise to smaller vessels
- tunica externa
- (also, tunica adventitia) outermost layer or tunic of a vessel (except capillaries)
- tunica intima
- (also, tunica interna) innermost lining or tunic of a vessel
- tunica media
- middle layer or tunic of a vessel (except capillaries)
- ulnar artery
- formed at the bifurcation of the brachial artery; parallels the ulna; gives off smaller branches until it reaches the carpal region where it fuses with the radial artery to form the superficial and deep palmar arches; supplies blood to the lower arm and carpal region
- ulnar vein
- parallels the ulna and ulnar artery; arises from the palmar venous arches and leads to the brachial vein
- umbilical arteries
- pair of vessels that runs within the umbilical cord and carries fetal blood low in oxygen and high in waste to the placenta for exchange with maternal blood
- umbilical vein
- single vessel that originates in the placenta and runs within the umbilical cord, carrying oxygen- and nutrient-rich blood to the fetal heart
- vasa vasorum
- small blood vessels located within the walls or tunics of larger vessels that supply nourishment to and remove wastes from the cells of the vessels
- vascular shock
- type of shock that occurs when arterioles lose their normal muscular tone and dilate dramatically
- vascular shunt
- continuation of the metarteriole and thoroughfare channel that allows blood to bypass the capillary beds to flow directly from the arterial to the venous circulation
- vascular tone
- contractile state of smooth muscle in a blood vessel
- vascular tubes
- rudimentary blood vessels in a developing fetus
- vasoconstriction
- constriction of the smooth muscle of a blood vessel, resulting in a decreased vascular diameter
- vasodilation
- relaxation of the smooth muscle in the wall of a blood vessel, resulting in an increased vascular diameter
- vasomotion
- irregular, pulsating flow of blood through capillaries and related structures
- vein
- blood vessel that conducts blood toward the heart
- venous reserve
- volume of blood contained within systemic veins in the integument, bone marrow, and liver that can be returned to the heart for circulation, if needed
- venule
- small vessel leading from the capillaries to veins
- vertebral artery
- arises from the subclavian artery and passes through the vertebral foramen through the foramen magnum to the brain; joins with the internal carotid artery to form the arterial circle; supplies blood to the brain and spinal cord
- vertebral vein
- arises from the base of the brain and the cervical region of the spinal cord; passes through the intervertebral foramina in the cervical vertebrae; drains smaller veins from the cranium, spinal cord, and vertebrae, and leads to the brachiocephalic vein; counterpart of the vertebral artery
- visceral branches
- branches of the descending aorta that supply blood to the viscera
Chapter Review
20.1 Structure and Function of Blood Vessels
Blood pumped by the heart flows through a series of vessels known as arteries, arterioles, capillaries, venules, and veins before returning to the heart. Arteries transport blood away from the heart and branch into smaller vessels, forming arterioles. Arterioles distribute blood to capillary beds, the sites of exchange with the body tissues. Capillaries lead back to small vessels known as venules that flow into the larger veins and eventually back to the heart.
The arterial system is a relatively high-pressure system, so arteries have thick walls that appear round in cross section. The venous system is a lower-pressure system, containing veins that have larger lumens and thinner walls. They often appear flattened. Arteries, arterioles, venules, and veins are composed of three tunics known as the tunica intima, tunica media, and tunica externa. Capillaries have only a tunica intima layer. The tunica intima is a thin layer composed of a simple squamous epithelium known as endothelium and a small amount of connective tissue. The tunica media is a thicker area composed of variable amounts of smooth muscle and connective tissue. It is the thickest layer in all but the largest arteries. The tunica externa is primarily a layer of connective tissue, although in veins, it also contains some smooth muscle. Blood flow through vessels can be dramatically influenced by vasoconstriction and vasodilation in their walls.
20.2 Blood Flow, Blood Pressure, and Resistance
Blood flow is the movement of blood through a vessel, tissue, or organ. The slowing or blocking of blood flow is called resistance. Blood pressure is the force that blood exerts upon the walls of the blood vessels or chambers of the heart. The components of blood pressure include systolic pressure, which results from ventricular contraction, and diastolic pressure, which results from ventricular relaxation. Pulse pressure is the difference between systolic and diastolic measures, and mean arterial pressure is the “average” pressure of blood in the arterial system, driving blood into the tissues. Pulse, the expansion and recoiling of an artery, reflects the heartbeat. The variables affecting blood flow and blood pressure in the systemic circulation are cardiac output, compliance, blood volume, blood viscosity, and the length and diameter of the blood vessels. In the arterial system, vasodilation and vasoconstriction of the arterioles is a significant factor in systemic blood pressure: Slight vasodilation greatly decreases resistance and increases flow, whereas slight vasoconstriction greatly increases resistance and decreases flow. In the arterial system, as resistance increases, blood pressure increases and flow decreases. In the venous system, constriction increases blood pressure as it does in arteries; the increasing pressure helps to return blood to the heart. In addition, constriction causes the vessel lumen to become more rounded, decreasing resistance and increasing blood flow. Venoconstriction, while less important than arterial vasoconstriction, works with the skeletal muscle pump, the respiratory pump, and their valves to promote venous return to the heart.
20.3 Capillary Exchange
Small molecules can cross into and out of capillaries via simple or facilitated diffusion. Some large molecules can cross in vesicles or through clefts, fenestrations, or gaps between cells in capillary walls. However, the bulk flow of capillary and tissue fluid occurs via filtration and reabsorption. Filtration, the movement of fluid out of the capillaries, is driven by the CHP. Reabsorption, the influx of tissue fluid into the capillaries, is driven by the BCOP. Filtration predominates in the arterial end of the capillary; in the middle section, the opposing pressures are virtually identical so there is no net exchange, whereas reabsorption predominates at the venule end of the capillary. The hydrostatic and colloid osmotic pressures in the interstitial fluid are negligible in healthy circumstances.
20.4 Homeostatic Regulation of the Vascular System
Neural, endocrine, and autoregulatory mechanisms affect blood flow, blood pressure, and eventually perfusion of blood to body tissues. Neural mechanisms include the cardiovascular centers in the medulla oblongata, baroreceptors in the aorta and carotid arteries and right atrium, and associated chemoreceptors that monitor blood levels of oxygen, carbon dioxide, and hydrogen ions. Endocrine controls include epinephrine and norepinephrine, as well as ADH, the renin-angiotensin-aldosterone mechanism, ANH, and EPO. Autoregulation is the local control of vasodilation and constriction by chemical signals and the myogenic response. Exercise greatly improves cardiovascular function and reduces the risk of cardiovascular diseases, including hypertension, a leading cause of heart attacks and strokes. Significant hemorrhage can lead to a form of circulatory shock known as hypovolemic shock. Sepsis, obstruction, and widespread inflammation can also cause circulatory shock.
20.5 Circulatory Pathways
The right ventricle pumps oxygen-depleted blood into the pulmonary trunk and right and left pulmonary arteries, which carry it to the right and left lungs for gas exchange. Oxygen-rich blood is transported by pulmonary veins to the left atrium. The left ventricle pumps this blood into the aorta. The main regions of the aorta are the ascending aorta, aortic arch, and descending aorta, which is further divided into the thoracic and abdominal aorta. The coronary arteries branch from the ascending aorta. After oxygenating tissues in the capillaries, systemic blood is returned to the right atrium from the venous system via the superior vena cava, which drains most of the veins superior to the diaphragm, the inferior vena cava, which drains most of the veins inferior to the diaphragm, and the coronary veins via the coronary sinus. The hepatic portal system carries blood to the liver for processing before it enters circulation. Review the figures provided in this section for circulation of blood through the blood vessels.
20.6 Development of Blood Vessels and Fetal Circulation
Blood vessels begin to form from the embryonic mesoderm. The precursor hemangioblasts differentiate into angioblasts, which give rise to the blood vessels and pluripotent stem cells that differentiate into the formed elements of the blood. Together, these cells form blood islands scattered throughout the embryo. Extensions known as vascular tubes eventually connect the vascular network. As the embryo grows within the mother’s womb, the placenta develops to supply blood rich in oxygen and nutrients via the umbilical vein and to remove wastes in oxygen-depleted blood via the umbilical arteries. Three major shunts found in the fetus are the foramen ovale and ductus arteriosus, which divert blood from the pulmonary to the systemic circuit, and the ductus venosus, which carries freshly oxygenated blood high in nutrients to the fetal heart.
Interactive Link Questions
Watch this video to explore capillaries and how they function in the body. Capillaries are never more than 100 micrometers away. What is the main component of interstitial fluid?
2.Listen to this CDC podcast to learn about hypertension, often described as a “silent killer.” What steps can you take to reduce your risk of a heart attack or stroke?
Review Questions
The endothelium is found in the ________.
- tunica intima
- tunica media
- tunica externa
- lumen
Nervi vasorum control ________.
- vasoconstriction
- vasodilation
- capillary permeability
- both vasoconstriction and vasodilation
Closer to the heart, arteries would be expected to have a higher percentage of ________.
- endothelium
- smooth muscle fibers
- elastic fibers
- collagenous fibers
Which of the following best describes veins?
- thick walled, small lumens, low pressure, lack valves
- thin walled, large lumens, low pressure, have valves
- thin walled, small lumens, high pressure, have valves
- thick walled, large lumens, high pressure, lack valves
An especially leaky type of capillary found in the liver and certain other tissues is called a ________.
- capillary bed
- fenestrated capillary
- sinusoid capillary
- metarteriole
In a blood pressure measurement of 110/70, the number 70 is the ________.
- systolic pressure
- diastolic pressure
- pulse pressure
- mean arterial pressure
A healthy elastic artery ________.
- is compliant
- reduces blood flow
- is a resistance artery
- has a thin wall and irregular lumen
Which of the following statements is true?
- The longer the vessel, the lower the resistance and the greater the flow.
- As blood volume decreases, blood pressure and blood flow also decrease.
- Increased viscosity increases blood flow.
- All of the above are true.
Slight vasodilation in an arteriole prompts a ________.
- slight increase in resistance
- huge increase in resistance
- slight decrease in resistance
- huge decrease in resistance
Venoconstriction increases which of the following?
- blood pressure within the vein
- blood flow within the vein
- return of blood to the heart
- all of the above
Hydrostatic pressure is ________.
- greater than colloid osmotic pressure at the venous end of the capillary bed
- the pressure exerted by fluid in an enclosed space
- about zero at the midpoint of a capillary bed
- all of the above
Net filtration pressure is calculated by ________.
- adding the capillary hydrostatic pressure to the interstitial fluid hydrostatic pressure
- subtracting the fluid drained by the lymphatic vessels from the total fluid in the interstitial fluid
- adding the blood colloid osmotic pressure to the capillary hydrostatic pressure
- subtracting the blood colloid osmotic pressure from the capillary hydrostatic pressure
Which of the following statements is true?
- In one day, more fluid exits the capillary through filtration than enters through reabsorption.
- In one day, approximately 35 mm of blood are filtered and 7 mm are reabsorbed.
- In one day, the capillaries of the lymphatic system absorb about 20.4 liters of fluid.
- None of the above are true.
Clusters of neurons in the medulla oblongata that regulate blood pressure are known collectively as ________.
- baroreceptors
- angioreceptors
- the cardiomotor mechanism
- the cardiovascular center
In the renin-angiotensin-aldosterone mechanism, ________.
- decreased blood pressure prompts the release of renin from the liver
- aldosterone prompts increased urine output
- aldosterone prompts the kidneys to reabsorb sodium
- all of the above
In the myogenic response, ________.
- muscle contraction promotes venous return to the heart
- ventricular contraction strength is decreased
- vascular smooth muscle responds to stretch
- endothelins dilate muscular arteries
A form of circulatory shock common in young children with severe diarrhea or vomiting is ________.
- hypovolemic shock
- anaphylactic shock
- obstructive shock
- hemorrhagic shock
The coronary arteries branch off of the ________.
- aortic valve
- ascending aorta
- aortic arch
- thoracic aorta
Which of the following statements is true?
- The left and right common carotid arteries both branch off of the brachiocephalic trunk.
- The brachial artery is the distal branch of the axillary artery.
- The radial and ulnar arteries join to form the palmar arch.
- All of the above are true.
Arteries serving the stomach, pancreas, and liver all branch from the ________.
- superior mesenteric artery
- inferior mesenteric artery
- celiac trunk
- splenic artery
The right and left brachiocephalic veins ________.
- drain blood from the right and left internal jugular veins
- drain blood from the right and left subclavian veins
- drain into the superior vena cava
- all of the above are true
The hepatic portal system delivers blood from the digestive organs to the ________.
- liver
- hypothalamus
- spleen
- left atrium
Blood islands are ________.
- clusters of blood-filtering cells in the placenta
- masses of pluripotent stem cells scattered throughout the fetal bone marrow
- vascular tubes that give rise to the embryonic tubular heart
- masses of developing blood vessels and formed elements scattered throughout the embryonic disc
Which of the following statements is true?
- Two umbilical veins carry oxygen-depleted blood from the fetal circulation to the placenta.
- One umbilical vein carries oxygen-rich blood from the placenta to the fetal heart.
- Two umbilical arteries carry oxygen-depleted blood to the fetal lungs.
- None of the above are true.
The ductus venosus is a shunt that allows ________.
- fetal blood to flow from the right atrium to the left atrium
- fetal blood to flow from the right ventricle to the left ventricle
- most freshly oxygenated blood to flow into the fetal heart
- most oxygen-depleted fetal blood to flow directly into the fetal pulmonary trunk
Critical Thinking Questions
Arterioles are often referred to as resistance vessels. Why?
29.Cocaine use causes vasoconstriction. Is this likely to increase or decrease blood pressure, and why?
30.A blood vessel with a few smooth muscle fibers and connective tissue, and only a very thin tunica externa conducts blood toward the heart. What type of vessel is this?
31.You measure a patient’s blood pressure at 130/85. Calculate the patient’s pulse pressure and mean arterial pressure. Determine whether each pressure is low, normal, or high.
32.An obese patient comes to the clinic complaining of swollen feet and ankles, fatigue, shortness of breath, and often feeling “spaced out.” She is a cashier in a grocery store, a job that requires her to stand all day. Outside of work, she engages in no physical activity. She confesses that, because of her weight, she finds even walking uncomfortable. Explain how the skeletal muscle pump might play a role in this patient’s signs and symptoms.
33.A patient arrives at the emergency department with dangerously low blood pressure. The patient’s blood colloid osmotic pressure is normal. How would you expect this situation to affect the patient’s net filtration pressure?
34.True or false? The plasma proteins suspended in blood cross the capillary cell membrane and enter the tissue fluid via facilitated diffusion. Explain your thinking.
35.A patient arrives in the emergency department with a blood pressure of 70/45 confused and complaining of thirst. Why?
36.Nitric oxide is broken down very quickly after its release. Why?
37.Identify the ventricle of the heart that pumps oxygen-depleted blood and the arteries of the body that carry oxygen-depleted blood.
38.What organs do the gonadal veins drain?
39.What arteries play the leading roles in supplying blood to the brain?
40.All tissues, including malignant tumors, need a blood supply. Explain why drugs called angiogenesis inhibitors would be used in cancer treatment.
41.Explain the location and importance of the ductus arteriosus in fetal circulation.
|
oercommons
|
2025-03-18T00:39:10.554560
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/58766/overview",
"title": "Anatomy and Physiology, Fluids and Transport",
"author": null
}
|
https://oercommons.org/courseware/lesson/58767/overview
|
The Lymphatic and Immune System
Introduction
Figure 21.1 The Worldwide AIDS Epidemic (a) As of 2008, more than 15 percent of adults were infected with HIV in certain African countries. This grim picture had changed little by 2012. (b) In this scanning electron micrograph, HIV virions (green particles) are budding off the surface of a macrophage (pink structure). (credit b: C. Goldsmith)
CHAPTER OBJECTIVES
After studying this chapter, you will be able to:
- Identify the components and anatomy of the lymphatic system
- Discuss the role of the innate immune response against pathogens
- Describe the power of the adaptive immune response to cure disease
- Explain immunological deficiencies and over-reactions of the immune system
- Discuss the role of the immune response in transplantation and cancer
- Describe the interaction of the immune and lymphatic systems with other body systems
In June 1981, the Centers for Disease Control and Prevention (CDC), in Atlanta, Georgia, published a report of an unusual cluster of five patients in Los Angeles, California. All five were diagnosed with a rare pneumonia caused by a fungus called Pneumocystis jirovecii (formerly known as Pneumocystis carinii).
Why was this unusual? Although commonly found in the lungs of healthy individuals, this fungus is an opportunistic pathogen that causes disease in individuals with suppressed or underdeveloped immune systems. The very young, whose immune systems have yet to mature, and the elderly, whose immune systems have declined with age, are particularly susceptible. The five patients from LA, though, were between 29 and 36 years of age and should have been in the prime of their lives, immunologically speaking. What could be going on?
A few days later, a cluster of eight cases was reported in New York City, also involving young patients, this time exhibiting a rare form of skin cancer known as Kaposi’s sarcoma. This cancer of the cells that line the blood and lymphatic vessels was previously observed as a relatively innocuous disease of the elderly. The disease that doctors saw in 1981 was frighteningly more severe, with multiple, fast-growing lesions that spread to all parts of the body, including the trunk and face. Could the immune systems of these young patients have been compromised in some way? Indeed, when they were tested, they exhibited extremely low numbers of a specific type of white blood cell in their bloodstreams, indicating that they had somehow lost a major part of the immune system.
Acquired immune deficiency syndrome, or AIDS, turned out to be a new disease caused by the previously unknown human immunodeficiency virus (HIV). Although nearly 100 percent fatal in those with active HIV infections in the early years, the development of anti-HIV drugs has transformed HIV infection into a chronic, manageable disease and not the certain death sentence it once was. One positive outcome resulting from the emergence of HIV disease was that the public’s attention became focused as never before on the importance of having a functional and healthy immune system.
Anatomy of the Lymphatic and Immune Systems
- Describe the structure and function of the lymphatic tissue (lymph fluid, vessels, ducts, and organs)
- Describe the structure and function of the primary and secondary lymphatic organs
- Discuss the cells of the immune system, how they function, and their relationship with the lymphatic system
The immune system is the complex collection of cells and organs that destroys or neutralizes pathogens that would otherwise cause disease or death. The lymphatic system, for most people, is associated with the immune system to such a degree that the two systems are virtually indistinguishable. The lymphatic system is the system of vessels, cells, and organs that carries excess fluids to the bloodstream and filters pathogens from the blood. The swelling of lymph nodes during an infection and the transport of lymphocytes via the lymphatic vessels are but two examples of the many connections between these critical organ systems.
Functions of the Lymphatic System
A major function of the lymphatic system is to drain body fluids and return them to the bloodstream. Blood pressure causes leakage of fluid from the capillaries, resulting in the accumulation of fluid in the interstitial space—that is, spaces between individual cells in the tissues. In humans, 20 liters of plasma is released into the interstitial space of the tissues each day due to capillary filtration. Once this filtrate is out of the bloodstream and in the tissue spaces, it is referred to as interstitial fluid. Of this, 17 liters is reabsorbed directly by the blood vessels. But what happens to the remaining three liters? This is where the lymphatic system comes into play. It drains the excess fluid and empties it back into the bloodstream via a series of vessels, trunks, and ducts. Lymph is the term used to describe interstitial fluid once it has entered the lymphatic system. When the lymphatic system is damaged in some way, such as by being blocked by cancer cells or destroyed by injury, protein-rich interstitial fluid accumulates (sometimes “backs up” from the lymph vessels) in the tissue spaces. This inappropriate accumulation of fluid referred to as lymphedema may lead to serious medical consequences.
As the vertebrate immune system evolved, the network of lymphatic vessels became convenient avenues for transporting the cells of the immune system. Additionally, the transport of dietary lipids and fat-soluble vitamins absorbed in the gut uses this system.
Cells of the immune system not only use lymphatic vessels to make their way from interstitial spaces back into the circulation, but they also use lymph nodes as major staging areas for the development of critical immune responses. A lymph node is one of the small, bean-shaped organs located throughout the lymphatic system.
INTERACTIVE LINK
Visit this website for an overview of the lymphatic system. What are the three main components of the lymphatic system?
Structure of the Lymphatic System
The lymphatic vessels begin as as blind ending, or closed at one end, capillaries, which feed into larger and larger lymphatic vessels, and eventually empty into the bloodstream by a series of ducts. Along the way, the lymph travels through the lymph nodes, which are commonly found near the groin, armpits, neck, chest, and abdomen. Humans have about 500–600 lymph nodes throughout the body (Figure 21.2).
Figure 21.2 Anatomy of the Lymphatic System Lymphatic vessels in the arms and legs convey lymph to the larger lymphatic vessels in the torso.
A major distinction between the lymphatic and cardiovascular systems in humans is that lymph is not actively pumped by the heart, but is forced through the vessels by the movements of the body, the contraction of skeletal muscles during body movements, and breathing. One-way valves (semi-lunar valves) in lymphatic vessels keep the lymph moving toward the heart. Lymph flows from the lymphatic capillaries, through lymphatic vessels, and then is dumped into the circulatory system via the lymphatic ducts located at the junction of the jugular and subclavian veins in the neck.
Lymphatic Capillaries
Lymphatic capillaries, also called the terminal lymphatics, are vessels where interstitial fluid enters the lymphatic system to become lymph fluid. Located in almost every tissue in the body, these vessels are interlaced among the arterioles and venules of the circulatory system in the soft connective tissues of the body (Figure 21.3). Exceptions are the central nervous system, bone marrow, bones, teeth, and the cornea of the eye, which do not contain lymph vessels.
Figure 21.3 Lymphatic Capillaries Lymphatic capillaries are interlaced with the arterioles and venules of the cardiovascular system. Collagen fibers anchor a lymphatic capillary in the tissue (inset). Interstitial fluid slips through spaces between the overlapping endothelial cells that compose the lymphatic capillary.
Lymphatic capillaries are formed by a one cell-thick layer of endothelial cells and represent the open end of the system, allowing interstitial fluid to flow into them via overlapping cells (see Figure 21.3). When interstitial pressure is low, the endothelial flaps close to prevent “backflow.” As interstitial pressure increases, the spaces between the cells open up, allowing the fluid to enter. Entry of fluid into lymphatic capillaries is also enabled by the collagen filaments that anchor the capillaries to surrounding structures. As interstitial pressure increases, the filaments pull on the endothelial cell flaps, opening up them even further to allow easy entry of fluid.
In the small intestine, lymphatic capillaries called lacteals are critical for the transport of dietary lipids and lipid-soluble vitamins to the bloodstream. In the small intestine, dietary triglycerides combine with other lipids and proteins, and enter the lacteals to form a milky fluid called chyle. The chyle then travels through the lymphatic system, eventually entering the bloodstream.
Larger Lymphatic Vessels, Trunks, and Ducts
The lymphatic capillaries empty into larger lymphatic vessels, which are similar to veins in terms of their three-tunic structure and the presence of valves. These one-way valves are located fairly close to one another, and each one causes a bulge in the lymphatic vessel, giving the vessels a beaded appearance (see Figure 21.3).
The superficial and deep lymphatics eventually merge to form larger lymphatic vessels known as lymphatic trunks. On the right side of the body, the right sides of the head, thorax, and right upper limb drain lymph fluid into the right subclavian vein via the right lymphatic duct (Figure 21.4). On the left side of the body, the remaining portions of the body drain into the larger thoracic duct, which drains into the left subclavian vein. The thoracic duct itself begins just beneath the diaphragm in the cisterna chyli, a sac-like chamber that receives lymph from the lower abdomen, pelvis, and lower limbs by way of the left and right lumbar trunks and the intestinal trunk.
Figure 21.4 Major Trunks and Ducts of the Lymphatic System The thoracic duct drains a much larger portion of the body than does the right lymphatic duct.
The overall drainage system of the body is asymmetrical (see Figure 21.4). The right lymphatic duct receives lymph from only the upper right side of the body. The lymph from the rest of the body enters the bloodstream through the thoracic duct via all the remaining lymphatic trunks. In general, lymphatic vessels of the subcutaneous tissues of the skin, that is, the superficial lymphatics, follow the same routes as veins, whereas the deep lymphatic vessels of the viscera generally follow the paths of arteries.
The Organization of Immune Function
The immune system is a collection of barriers, cells, and soluble proteins that interact and communicate with each other in extraordinarily complex ways. The modern model of immune function is organized into three phases based on the timing of their effects. The three temporal phases consist of the following:
- Barrier defenses such as the skin and mucous membranes, which act instantaneously to prevent pathogenic invasion into the body tissues
- The rapid but nonspecific innate immune response, which consists of a variety of specialized cells and soluble factors
- The slower but more specific and effective adaptive immune response, which involves many cell types and soluble factors, but is primarily controlled by white blood cells (leukocytes) known as lymphocytes, which help control immune responses
The cells of the blood, including all those involved in the immune response, arise in the bone marrow via various differentiation pathways from hematopoietic stem cells (Figure 21.5). In contrast with embryonic stem cells, hematopoietic stem cells are present throughout adulthood and allow for the continuous differentiation of blood cells to replace those lost to age or function. These cells can be divided into three classes based on function:
- Phagocytic cells, which ingest pathogens to destroy them
- Lymphocytes, which specifically coordinate the activities of adaptive immunity
- Cells containing cytoplasmic granules, which help mediate immune responses against parasites and intracellular pathogens such as viruses
Figure 21.5 Hematopoietic System of the Bone Marrow All the cells of the immune response as well as of the blood arise by differentiation from hematopoietic stem cells. Platelets are cell fragments involved in the clotting of blood.
Lymphocytes: B Cells, T Cells, Plasma Cells, and Natural Killer Cells
As stated above, lymphocytes are the primary cells of adaptive immune responses (Table 21.1). The two basic types of lymphocytes, B cells and T cells, are identical morphologically with a large central nucleus surrounded by a thin layer of cytoplasm. They are distinguished from each other by their surface protein markers as well as by the molecules they secrete. While B cells mature in red bone marrow and T cells mature in the thymus, they both initially develop from bone marrow. T cells migrate from bone marrow to the thymus gland where they further mature. B cells and T cells are found in many parts of the body, circulating in the bloodstream and lymph, and residing in secondary lymphoid organs, including the spleen and lymph nodes, which will be described later in this section. The human body contains approximately 1012 lymphocytes.
B Cells
B cells are immune cells that function primarily by producing antibodies. An antibody is any of the group of proteins that binds specifically to pathogen-associated molecules known as antigens. An antigen is a chemical structure on the surface of a pathogen that binds to T or B lymphocyte antigen receptors. Once activated by binding to antigen, B cells differentiate into cells that secrete a soluble form of their surface antibodies. These activated B cells are known as plasma cells.
T Cells
The T cell, on the other hand, does not secrete antibody but performs a variety of functions in the adaptive immune response. Different T cell types have the ability to either secrete soluble factors that communicate with other cells of the adaptive immune response or destroy cells infected with intracellular pathogens. The roles of T and B lymphocytes in the adaptive immune response will be discussed further in this chapter.
Plasma Cells
Another type of lymphocyte of importance is the plasma cell. A plasma cell is a B cell that has differentiated in response to antigen binding, and has thereby gained the ability to secrete soluble antibodies. These cells differ in morphology from standard B and T cells in that they contain a large amount of cytoplasm packed with the protein-synthesizing machinery known as rough endoplasmic reticulum.
Natural Killer Cells
A fourth important lymphocyte is the natural killer cell, a participant in the innate immune response. A natural killer cell (NK) is a circulating blood cell that contains cytotoxic (cell-killing) granules in its extensive cytoplasm. It shares this mechanism with the cytotoxic T cells of the adaptive immune response. NK cells are among the body’s first lines of defense against viruses and certain types of cancer.
Lymphocytes
| Type of lymphocyte | Primary function |
|---|---|
| B lymphocyte | Generates diverse antibodies |
| T lymphocyte | Secretes chemical messengers |
| Plasma cell | Secretes antibodies |
| NK cell | Destroys virally infected cells |
Table 21.1
INTERACTIVE LINK
Visit this website to learn about the many different cell types in the immune system and their very specialized jobs. What is the role of the dendritic cell in an HIV infection?
Primary Lymphoid Organs and Lymphocyte Development
Understanding the differentiation and development of B and T cells is critical to the understanding of the adaptive immune response. It is through this process that the body (ideally) learns to destroy only pathogens and leaves the body’s own cells relatively intact. The primary lymphoid organs are the bone marrow and thymus gland. The lymphoid organs are where lymphocytes mature, proliferate, and are selected, which enables them to attack pathogens without harming the cells of the body.
Bone Marrow
In the embryo, blood cells are made in the yolk sac. As development proceeds, this function is taken over by the spleen, lymph nodes, and liver. Later, the bone marrow takes over most hematopoietic functions, although the final stages of the differentiation of some cells may take place in other organs. The red bone marrow is a loose collection of cells where hematopoiesis occurs, and the yellow bone marrow is a site of energy storage, which consists largely of fat cells (Figure 21.6). The B cell undergoes nearly all of its development in the red bone marrow, whereas the immature T cell, called a thymocyte, leaves the bone marrow and matures largely in the thymus gland.
Figure 21.6 Bone Marrow Red bone marrow fills the head of the femur, and a spot of yellow bone marrow is visible in the center. The white reference bar is 1 cm.
Thymus
The thymus gland is a bilobed organ found in the space between the sternum and the aorta of the heart (Figure 21.7). Connective tissue holds the lobes closely together but also separates them and forms a capsule.
Figure 21.7 Location, Structure, and Histology of the Thymus The thymus lies above the heart. The trabeculae and lobules, including the darkly staining cortex and the lighter staining medulla of each lobule, are clearly visible in the light micrograph of the thymus of a newborn. LM × 100. (Micrograph provided by the Regents of the University of Michigan Medical School © 2012)
INTERACTIVE LINK
View the University of Michigan WebScope to explore the tissue sample in greater detail.
The connective tissue capsule further divides the thymus into lobules via extensions called trabeculae. The outer region of the organ is known as the cortex and contains large numbers of thymocytes with some epithelial cells, macrophages, and dendritic cells (two types of phagocytic cells that are derived from monocytes). The cortex is densely packed so it stains more intensely than the rest of the thymus (see Figure 21.7). The medulla, where thymocytes migrate before leaving the thymus, contains a less dense collection of thymocytes, epithelial cells, and dendritic cells.
AGING AND THE...
Immune System
By the year 2050, 25 percent of the population of the United States will be 60 years of age or older. The CDC estimates that 80 percent of those 60 years and older have one or more chronic disease associated with deficiencies of the immune systems. This loss of immune function with age is called immunosenescence. To treat this growing population, medical professionals must better understand the aging process. One major cause of age-related immune deficiencies is thymic involution, the shrinking of the thymus gland that begins at birth, at a rate of about three percent tissue loss per year, and continues until 35–45 years of age, when the rate declines to about one percent loss per year for the rest of one’s life. At that pace, the total loss of thymic epithelial tissue and thymocytes would occur at about 120 years of age. Thus, this age is a theoretical limit to a healthy human lifespan.
Thymic involution has been observed in all vertebrate species that have a thymus gland. Animal studies have shown that transplanted thymic grafts between inbred strains of mice involuted according to the age of the donor and not of the recipient, implying the process is genetically programmed. There is evidence that the thymic microenvironment, so vital to the development of naïve T cells, loses thymic epithelial cells according to the decreasing expression of the FOXN1 gene with age.
It is also known that thymic involution can be altered by hormone levels. Sex hormones such as estrogen and testosterone enhance involution, and the hormonal changes in pregnant women cause a temporary thymic involution that reverses itself, when the size of the thymus and its hormone levels return to normal, usually after lactation ceases. What does all this tell us? Can we reverse immunosenescence, or at least slow it down? The potential is there for using thymic transplants from younger donors to keep thymic output of naïve T cells high. Gene therapies that target gene expression are also seen as future possibilities. The more we learn through immunosenescence research, the more opportunities there will be to develop therapies, even though these therapies will likely take decades to develop. The ultimate goal is for everyone to live and be healthy longer, but there may be limits to immortality imposed by our genes and hormones.
Secondary Lymphoid Organs and their Roles in Active Immune Responses
Lymphocytes develop and mature in the primary lymphoid organs, but they mount immune responses from the secondary lymphoid organs. A naïve lymphocyte is one that has left the primary organ and entered a secondary lymphoid organ. Naïve lymphocytes are fully functional immunologically, but have yet to encounter an antigen to respond to. In addition to circulating in the blood and lymph, lymphocytes concentrate in secondary lymphoid organs, which include the lymph nodes, spleen, and lymphoid nodules. All of these tissues have many features in common, including the following:
- The presence of lymphoid follicles, the sites of the formation of lymphocytes, with specific B cell-rich and T cell-rich areas
- An internal structure of reticular fibers with associated fixed macrophages
- Germinal centers, which are the sites of rapidly dividing and differentiating B lymphocytes
- Specialized post-capillary vessels known as high endothelial venules; the cells lining these venules are thicker and more columnar than normal endothelial cells, which allow cells from the blood to directly enter these tissues
Lymph Nodes
Lymph nodes function to remove debris and pathogens from the lymph, and are thus sometimes referred to as the “filters of the lymph” (Figure 21.8). Any bacteria that infect the interstitial fluid are taken up by the lymphatic capillaries and transported to a regional lymph node. Dendritic cells and macrophages within this organ internalize and kill many of the pathogens that pass through, thereby removing them from the body. The lymph node is also the site of adaptive immune responses mediated by T cells, B cells, and accessory cells of the adaptive immune system. Like the thymus, the bean-shaped lymph nodes are surrounded by a tough capsule of connective tissue and are separated into compartments by trabeculae, the extensions of the capsule. In addition to the structure provided by the capsule and trabeculae, the structural support of the lymph node is provided by a series of reticular fibers laid down by fibroblasts.
Figure 21.8 Structure and Histology of a Lymph Node Lymph nodes are masses of lymphatic tissue located along the larger lymph vessels. The micrograph of the lymph nodes shows a germinal center, which consists of rapidly dividing B cells surrounded by a layer of T cells and other accessory cells. LM × 128. (Micrograph provided by the Regents of the University of Michigan Medical School © 2012)
INTERACTIVE LINK
View the University of Michigan WebScope to explore the tissue sample in greater detail.
The major routes into the lymph node are via afferent lymphatic vessels (see Figure 21.8). Cells and lymph fluid that leave the lymph node may do so by another set of vessels known as the efferent lymphatic vessels. Lymph enters the lymph node via the subcapsular sinus, which is occupied by dendritic cells, macrophages, and reticular fibers. Within the cortex of the lymph node are lymphoid follicles, which consist of germinal centers of rapidly dividing B cells surrounded by a layer of T cells and other accessory cells. As the lymph continues to flow through the node, it enters the medulla, which consists of medullary cords of B cells and plasma cells, and the medullary sinuses where the lymph collects before leaving the node via the efferent lymphatic vessels.
Spleen
In addition to the lymph nodes, the spleen is a major secondary lymphoid organ (Figure 21.9). It is about 12 cm (5 in) long and is attached to the lateral border of the stomach via the gastrosplenic ligament. The spleen is a fragile organ without a strong capsule, and is dark red due to its extensive vascularization. The spleen is sometimes called the “filter of the blood” because of its extensive vascularization and the presence of macrophages and dendritic cells that remove microbes and other materials from the blood, including dying red blood cells. The spleen also functions as the location of immune responses to blood-borne pathogens.
Figure 21.9 Spleen (a) The spleen is attached to the stomach. (b) A micrograph of spleen tissue shows the germinal center. The marginal zone is the region between the red pulp and white pulp, which sequesters particulate antigens from the circulation and presents these antigens to lymphocytes in the white pulp. EM × 660. (Micrograph provided by the Regents of the University of Michigan Medical School © 2012)
The spleen is also divided by trabeculae of connective tissue, and within each splenic nodule is an area of red pulp, consisting of mostly red blood cells, and white pulp, which resembles the lymphoid follicles of the lymph nodes. Upon entering the spleen, the splenic artery splits into several arterioles (surrounded by white pulp) and eventually into sinusoids. Blood from the capillaries subsequently collects in the venous sinuses and leaves via the splenic vein. The red pulp consists of reticular fibers with fixed macrophages attached, free macrophages, and all of the other cells typical of the blood, including some lymphocytes. The white pulp surrounds a central arteriole and consists of germinal centers of dividing B cells surrounded by T cells and accessory cells, including macrophages and dendritic cells. Thus, the red pulp primarily functions as a filtration system of the blood, using cells of the relatively nonspecific immune response, and white pulp is where adaptive T and B cell responses are mounted.
Lymphoid Nodules
The other lymphoid tissues, the lymphoid nodules, have a simpler architecture than the spleen and lymph nodes in that they consist of a dense cluster of lymphocytes without a surrounding fibrous capsule. These nodules are located in the respiratory and digestive tracts, areas routinely exposed to environmental pathogens.
Tonsils are lymphoid nodules located along the inner surface of the pharynx and are important in developing immunity to oral pathogens (Figure 21.10). The tonsil located at the back of the throat, the pharyngeal tonsil, is sometimes referred to as the adenoid when swollen. Such swelling is an indication of an active immune response to infection. Histologically, tonsils do not contain a complete capsule, and the epithelial layer invaginates deeply into the interior of the tonsil to form tonsillar crypts. These structures, which accumulate all sorts of materials taken into the body through eating and breathing, actually “encourage” pathogens to penetrate deep into the tonsillar tissues where they are acted upon by numerous lymphoid follicles and eliminated. This seems to be the major function of tonsils—to help children’s bodies recognize, destroy, and develop immunity to common environmental pathogens so that they will be protected in their later lives. Tonsils are often removed in those children who have recurring throat infections, especially those involving the palatine tonsils on either side of the throat, whose swelling may interfere with their breathing and/or swallowing.
Figure 21.10 Locations and Histology of the Tonsils (a) The pharyngeal tonsil is located on the roof of the posterior superior wall of the nasopharynx. The palatine tonsils lay on each side of the pharynx. (b) A micrograph shows the palatine tonsil tissue. LM × 40. (Micrograph provided by the Regents of the University of Michigan Medical School © 2012)
INTERACTIVE LINK
View the University of Michigan WebScope to explore the tissue sample in greater detail.
Mucosa-associated lymphoid tissue (MALT) consists of an aggregate of lymphoid follicles directly associated with the mucous membrane epithelia. MALT makes up dome-shaped structures found underlying the mucosa of the gastrointestinal tract, breast tissue, lungs, and eyes. Peyer’s patches, a type of MALT in the small intestine, are especially important for immune responses against ingested substances (Figure 21.11). Peyer’s patches contain specialized endothelial cells called M (or microfold) cells that sample material from the intestinal lumen and transport it to nearby follicles so that adaptive immune responses to potential pathogens can be mounted. A similar process occurs involving MALT in the mucosa and submucosa of the appendix. A blockage of the lumen triggers these cells to elicit an inflammatory response that can lead to appendicitis.
Figure 21.11 Mucosa-associated Lymphoid Tissue (MALT) Nodule LM × 40. (Micrograph provided by the Regents of the University of Michigan Medical School © 2012)
Bronchus-associated lymphoid tissue (BALT) consists of lymphoid follicular structures with an overlying epithelial layer found along the bifurcations of the bronchi, and between bronchi and arteries. They also have the typically less-organized structure of other lymphoid nodules. These tissues, in addition to the tonsils, are effective against inhaled pathogens.
Barrier Defenses and the Innate Immune Response
- Describe the barrier defenses of the body
- Show how the innate immune response is important and how it helps guide and prepare the body for adaptive immune responses
- Describe various soluble factors that are part of the innate immune response
- Explain the steps of inflammation and how they lead to destruction of a pathogen
- Discuss early induced immune responses and their level of effectiveness
The immune system can be divided into two overlapping mechanisms to destroy pathogens: the innate immune response, which is relatively rapid but nonspecific and thus not always effective, and the adaptive immune response, which is slower in its development during an initial infection with a pathogen, but is highly specific and effective at attacking a wide variety of pathogens (Figure 21.12).
Figure 21.12 Cooperation between Innate and Adaptive Immune Responses The innate immune system enhances adaptive immune responses so they can be more effective.
Any discussion of the innate immune response usually begins with the physical barriers that prevent pathogens from entering the body, destroy them after they enter, or flush them out before they can establish themselves in the hospitable environment of the body’s soft tissues. Barrier defenses are part of the body’s most basic defense mechanisms. The barrier defenses are not a response to infections, but they are continuously working to protect against a broad range of pathogens.
The different modes of barrier defenses are associated with the external surfaces of the body, where pathogens may try to enter (Table 21.2). The primary barrier to the entrance of microorganisms into the body is the skin. Not only is the skin covered with a layer of dead, keratinized epithelium that is too dry for bacteria in which to grow, but as these cells are continuously sloughed off from the skin, they carry bacteria and other pathogens with them. Additionally, sweat and other skin secretions may lower pH, contain toxic lipids, and physically wash microbes away.
Barrier Defenses
| Site | Specific defense | Protective aspect |
|---|---|---|
| Skin | Epidermal surface | Keratinized cells of surface, Langerhans cells |
| Skin (sweat/secretions) | Sweat glands, sebaceous glands | Low pH, washing action |
| Oral cavity | Salivary glands | Lysozyme |
| Stomach | Gastrointestinal tract | Low pH |
| Mucosal surfaces | Mucosal epithelium | Nonkeratinized epithelial cells |
| Normal flora (nonpathogenic bacteria) | Mucosal tissues | Prevent pathogens from growing on mucosal surfaces |
Table 21.2
Another barrier is the saliva in the mouth, which is rich in lysozyme—an enzyme that destroys bacteria by digesting their cell walls. The acidic environment of the stomach, which is fatal to many pathogens, is also a barrier. Additionally, the mucus layer of the gastrointestinal tract, respiratory tract, reproductive tract, eyes, ears, and nose traps both microbes and debris, and facilitates their removal. In the case of the upper respiratory tract, ciliated epithelial cells move potentially contaminated mucus upwards to the mouth, where it is then swallowed into the digestive tract, ending up in the harsh acidic environment of the stomach. Considering how often you breathe compared to how often you eat or perform other activities that expose you to pathogens, it is not surprising that multiple barrier mechanisms have evolved to work in concert to protect this vital area.
Cells of the Innate Immune Response
A phagocyte is a cell that is able to surround and engulf a particle or cell, a process called phagocytosis. The phagocytes of the immune system engulf other particles or cells, either to clean an area of debris, old cells, or to kill pathogenic organisms such as bacteria. The phagocytes are the body’s fast acting, first line of immunological defense against organisms that have breached barrier defenses and have entered the vulnerable tissues of the body.
Phagocytes: Macrophages and Neutrophils
Many of the cells of the immune system have a phagocytic ability, at least at some point during their life cycles. Phagocytosis is an important and effective mechanism of destroying pathogens during innate immune responses. The phagocyte takes the organism inside itself as a phagosome, which subsequently fuses with a lysosome and its digestive enzymes, effectively killing many pathogens. On the other hand, some bacteria including Mycobacteria tuberculosis, the cause of tuberculosis, may be resistant to these enzymes and are therefore much more difficult to clear from the body. Macrophages, neutrophils, and dendritic cells are the major phagocytes of the immune system.
A macrophage is an irregularly shaped phagocyte that is amoeboid in nature and is the most versatile of the phagocytes in the body. Macrophages move through tissues and squeeze through capillary walls using pseudopodia. They not only participate in innate immune responses but have also evolved to cooperate with lymphocytes as part of the adaptive immune response. Macrophages exist in many tissues of the body, either freely roaming through connective tissues or fixed to reticular fibers within specific tissues such as lymph nodes. When pathogens breach the body’s barrier defenses, macrophages are the first line of defense (Table 21.3). They are called different names, depending on the tissue: Kupffer cells in the liver, histiocytes in connective tissue, and alveolar macrophages in the lungs.
A neutrophil is a phagocytic cell that is attracted via chemotaxis from the bloodstream to infected tissues. These spherical cells are granulocytes. A granulocyte contains cytoplasmic granules, which in turn contain a variety of vasoactive mediators such as histamine. In contrast, macrophages are agranulocytes. An agranulocyte has few or no cytoplasmic granules. Whereas macrophages act like sentries, always on guard against infection, neutrophils can be thought of as military reinforcements that are called into a battle to hasten the destruction of the enemy. Although, usually thought of as the primary pathogen-killing cell of the inflammatory process of the innate immune response, new research has suggested that neutrophils play a role in the adaptive immune response as well, just as macrophages do.
A monocyte is a circulating precursor cell that differentiates into either a macrophage or dendritic cell, which can be rapidly attracted to areas of infection by signal molecules of inflammation.
Phagocytic Cells of the Innate Immune System
| Cell | Cell type | Primary location | Function in the innate immune response |
|---|---|---|---|
| Macrophage | Agranulocyte | Body cavities/organs | Phagocytosis |
| Neutrophil | Granulocyte | Blood | Phagocytosis |
| Monocyte | Agranulocyte | Blood | Precursor of macrophage/dendritic cell |
Table 21.3
Natural Killer Cells
NK cells are a type of lymphocyte that have the ability to induce apoptosis, that is, programmed cell death, in cells infected with intracellular pathogens such as obligate intracellular bacteria and viruses. NK cells recognize these cells by mechanisms that are still not well understood, but that presumably involve their surface receptors. NK cells can induce apoptosis, in which a cascade of events inside the cell causes its own death by either of two mechanisms:
1) NK cells are able to respond to chemical signals and express the fas ligand. The fas ligand is a surface molecule that binds to the fas molecule on the surface of the infected cell, sending it apoptotic signals, thus killing the cell and the pathogen within it; or
2) The granules of the NK cells release perforins and granzymes. A perforin is a protein that forms pores in the membranes of infected cells. A granzyme is a protein-digesting enzyme that enters the cell via the perforin pores and triggers apoptosis intracellularly.
Both mechanisms are especially effective against virally infected cells. If apoptosis is induced before the virus has the ability to synthesize and assemble all its components, no infectious virus will be released from the cell, thus preventing further infection.
Recognition of Pathogens
Cells of the innate immune response, the phagocytic cells, and the cytotoxic NK cells recognize patterns of pathogen-specific molecules, such as bacterial cell wall components or bacterial flagellar proteins, using pattern recognition receptors. A pattern recognition receptor (PRR) is a membrane-bound receptor that recognizes characteristic features of a pathogen and molecules released by stressed or damaged cells.
These receptors, which are thought to have evolved prior to the adaptive immune response, are present on the cell surface whether they are needed or not. Their variety, however, is limited by two factors. First, the fact that each receptor type must be encoded by a specific gene requires the cell to allocate most or all of its DNA to make receptors able to recognize all pathogens. Secondly, the variety of receptors is limited by the finite surface area of the cell membrane. Thus, the innate immune system must “get by” using only a limited number of receptors that are active against as wide a variety of pathogens as possible. This strategy is in stark contrast to the approach used by the adaptive immune system, which uses large numbers of different receptors, each highly specific to a particular pathogen.
Should the cells of the innate immune system come into contact with a species of pathogen they recognize, the cell will bind to the pathogen and initiate phagocytosis (or cellular apoptosis in the case of an intracellular pathogen) in an effort to destroy the offending microbe. Receptors vary somewhat according to cell type, but they usually include receptors for bacterial components and for complement, discussed below.
Soluble Mediators of the Innate Immune Response
The previous discussions have alluded to chemical signals that can induce cells to change various physiological characteristics, such as the expression of a particular receptor. These soluble factors are secreted during innate or early induced responses, and later during adaptive immune responses.
Cytokines and Chemokines
A cytokine is signaling molecule that allows cells to communicate with each other over short distances. Cytokines are secreted into the intercellular space, and the action of the cytokine induces the receiving cell to change its physiology. A chemokine is a soluble chemical mediator similar to cytokines except that its function is to attract cells (chemotaxis) from longer distances.
INTERACTIVE LINK
Visit this website to learn about phagocyte chemotaxis. Phagocyte chemotaxis is the movement of phagocytes according to the secretion of chemical messengers in the form of interleukins and other chemokines. By what means does a phagocyte destroy a bacterium that it has ingested?
Early induced Proteins
Early induced proteins are those that are not constitutively present in the body, but are made as they are needed early during the innate immune response. Interferons are an example of early induced proteins. Cells infected with viruses secrete interferons that travel to adjacent cells and induce them to make antiviral proteins. Thus, even though the initial cell is sacrificed, the surrounding cells are protected. Other early induced proteins specific for bacterial cell wall components are mannose-binding protein and C-reactive protein, made in the liver, which bind specifically to polysaccharide components of the bacterial cell wall. Phagocytes such as macrophages have receptors for these proteins, and they are thus able to recognize them as they are bound to the bacteria. This brings the phagocyte and bacterium into close proximity and enhances the phagocytosis of the bacterium by the process known as opsonization. Opsonization is the tagging of a pathogen for phagocytosis by the binding of an antibody or an antimicrobial protein.
Complement System
The complement system is a series of proteins constitutively found in the blood plasma. As such, these proteins are not considered part of the early induced immune response, even though they share features with some of the antibacterial proteins of this class. Made in the liver, they have a variety of functions in the innate immune response, using what is known as the “alternate pathway” of complement activation. Additionally, complement functions in the adaptive immune response as well, in what is called the classical pathway. The complement system consists of several proteins that enzymatically alter and fragment later proteins in a series, which is why it is termed cascade. Once activated, the series of reactions is irreversible, and releases fragments that have the following actions:
- Bind to the cell membrane of the pathogen that activates it, labeling it for phagocytosis (opsonization)
- Diffuse away from the pathogen and act as chemotactic agents to attract phagocytic cells to the site of inflammation
- Form damaging pores in the plasma membrane of the pathogen
Figure 21.13 shows the classical pathway, which requires antibodies of the adaptive immune response. The alternate pathway does not require an antibody to become activated.
Figure 21.13 Complement Cascade and Function The classical pathway, used during adaptive immune responses, occurs when C1 reacts with antibodies that have bound an antigen.
The splitting of the C3 protein is the common step to both pathways. In the alternate pathway, C3 is activated spontaneously and, after reacting with the molecules factor P, factor B, and factor D, splits apart. The larger fragment, C3b, binds to the surface of the pathogen and C3a, the smaller fragment, diffuses outward from the site of activation and attracts phagocytes to the site of infection. Surface-bound C3b then activates the rest of the cascade, with the last five proteins, C5–C9, forming the membrane-attack complex (MAC). The MAC can kill certain pathogens by disrupting their osmotic balance. The MAC is especially effective against a broad range of bacteria. The classical pathway is similar, except the early stages of activation require the presence of antibody bound to antigen, and thus is dependent on the adaptive immune response. The earlier fragments of the cascade also have important functions. Phagocytic cells such as macrophages and neutrophils are attracted to an infection site by chemotactic attraction to smaller complement fragments. Additionally, once they arrive, their receptors for surface-bound C3b opsonize the pathogen for phagocytosis and destruction.
Inflammatory Response
The hallmark of the innate immune response is inflammation. Inflammation is something everyone has experienced. Stub a toe, cut a finger, or do any activity that causes tissue damage and inflammation will result, with its four characteristics: heat, redness, pain, and swelling (“loss of function” is sometimes mentioned as a fifth characteristic). It is important to note that inflammation does not have to be initiated by an infection, but can also be caused by tissue injuries. The release of damaged cellular contents into the site of injury is enough to stimulate the response, even in the absence of breaks in physical barriers that would allow pathogens to enter (by hitting your thumb with a hammer, for example). The inflammatory reaction brings in phagocytic cells to the damaged area to clear cellular debris and to set the stage for wound repair (Figure 21.14).
Figure 21.14
This reaction also brings in the cells of the innate immune system, allowing them to get rid of the sources of a possible infection. Inflammation is part of a very basic form of immune response. The process not only brings fluid and cells into the site to destroy the pathogen and remove it and debris from the site, but also helps to isolate the site, limiting the spread of the pathogen. Acute inflammation is a short-term inflammatory response to an insult to the body. If the cause of the inflammation is not resolved, however, it can lead to chronic inflammation, which is associated with major tissue destruction and fibrosis. Chronic inflammationis ongoing inflammation. It can be caused by foreign bodies, persistent pathogens, and autoimmune diseases such as rheumatoid arthritis.
There are four important parts to the inflammatory response:
- Tissue Injury. The released contents of injured cells stimulate the release of mast cell granules and their potent inflammatory mediators such as histamine, leukotrienes, and prostaglandins. Histamine increases the diameter of local blood vessels (vasodilation), causing an increase in blood flow. Histamine also increases the permeability of local capillaries, causing plasma to leak out and form interstitial fluid. This causes the swelling associated with inflammation.
Additionally, injured cells, phagocytes, and basophils are sources of inflammatory mediators, including prostaglandins and leukotrienes. Leukotrienes attract neutrophils from the blood by chemotaxis and increase vascular permeability. Prostaglandins cause vasodilation by relaxing vascular smooth muscle and are a major cause of the pain associated with inflammation. Nonsteroidal anti-inflammatory drugs such as aspirin and ibuprofen relieve pain by inhibiting prostaglandin production. - Vasodilation. Many inflammatory mediators such as histamine are vasodilators that increase the diameters of local capillaries. This causes increased blood flow and is responsible for the heat and redness of inflamed tissue. It allows greater access of the blood to the site of inflammation.
- Increased Vascular Permeability. At the same time, inflammatory mediators increase the permeability of the local vasculature, causing leakage of fluid into the interstitial space, resulting in the swelling, or edema, associated with inflammation.
- Recruitment of Phagocytes. Leukotrienes are particularly good at attracting neutrophils from the blood to the site of infection by chemotaxis. Following an early neutrophil infiltrate stimulated by macrophage cytokines, more macrophages are recruited to clean up the debris left over at the site. When local infections are severe, neutrophils are attracted to the sites of infections in large numbers, and as they phagocytose the pathogens and subsequently die, their accumulated cellular remains are visible as pus at the infection site.
Overall, inflammation is valuable for many reasons. Not only are the pathogens killed and debris removed, but the increase in vascular permeability encourages the entry of clotting factors, the first step towards wound repair. Inflammation also facilitates the transport of antigen to lymph nodes by dendritic cells for the development of the adaptive immune response.
The Adaptive Immune Response: T lymphocytes and Their Functional Types
- Explain the advantages of the adaptive immune response over the innate immune response
- List the various characteristics of an antigen
- Describe the types of T cell antigen receptors
- Outline the steps of T cell development
- Describe the major T cell types and their functions
Innate immune responses (and early induced responses) are in many cases ineffective at completely controlling pathogen growth. However, they slow pathogen growth and allow time for the adaptive immune response to strengthen and either control or eliminate the pathogen. The innate immune system also sends signals to the cells of the adaptive immune system, guiding them in how to attack the pathogen. Thus, these are the two important arms of the immune response.
The Benefits of the Adaptive Immune Response
The specificity of the adaptive immune response—its ability to specifically recognize and make a response against a wide variety of pathogens—is its great strength. Antigens, the small chemical groups often associated with pathogens, are recognized by receptors on the surface of B and T lymphocytes. The adaptive immune response to these antigens is so versatile that it can respond to nearly any pathogen. This increase in specificity comes because the adaptive immune response has a unique way to develop as many as 1011, or 100 trillion, different receptors to recognize nearly every conceivable pathogen. How could so many different types of antibodies be encoded? And what about the many specificities of T cells? There is not nearly enough DNA in a cell to have a separate gene for each specificity. The mechanism was finally worked out in the 1970s and 1980s using the new tools of molecular genetics
Primary Disease and Immunological Memory
The immune system’s first exposure to a pathogen is called a primary adaptive response. Symptoms of a first infection, called primary disease, are always relatively severe because it takes time for an initial adaptive immune response to a pathogen to become effective.
Upon re-exposure to the same pathogen, a secondary adaptive immune response is generated, which is stronger and faster that the primary response. The secondary adaptive response often eliminates a pathogen before it can cause significant tissue damage or any symptoms. Without symptoms, there is no disease, and the individual is not even aware of the infection. This secondary response is the basis of immunological memory, which protects us from getting diseases repeatedly from the same pathogen. By this mechanism, an individual’s exposure to pathogens early in life spares the person from these diseases later in life.
Self Recognition
A third important feature of the adaptive immune response is its ability to distinguish between self-antigens, those that are normally present in the body, and foreign antigens, those that might be on a potential pathogen. As T and B cells mature, there are mechanisms in place that prevent them from recognizing self-antigen, preventing a damaging immune response against the body. These mechanisms are not 100 percent effective, however, and their breakdown leads to autoimmune diseases, which will be discussed later in this chapter.
T Cell-Mediated Immune Responses
The primary cells that control the adaptive immune response are the lymphocytes, the T and B cells. T cells are particularly important, as they not only control a multitude of immune responses directly, but also control B cell immune responses in many cases as well. Thus, many of the decisions about how to attack a pathogen are made at the T cell level, and knowledge of their functional types is crucial to understanding the functioning and regulation of adaptive immune responses as a whole.
T lymphocytes recognize antigens based on a two-chain protein receptor. The most common and important of these are the alpha-beta T cell receptors (Figure 21.15).
Figure 21.15 Alpha-beta T Cell Receptor Notice the constant and variable regions of each chain, anchored by the transmembrane region.
There are two chains in the T cell receptor, and each chain consists of two domains. The variable region domain is furthest away from the T cell membrane and is so named because its amino acid sequence varies between receptors. In contrast, the constant region domain has less variation. The differences in the amino acid sequences of the variable domains are the molecular basis of the diversity of antigens the receptor can recognize. Thus, the antigen-binding site of the receptor consists of the terminal ends of both receptor chains, and the amino acid sequences of those two areas combine to determine its antigenic specificity. Each T cell produces only one type of receptor and thus is specific for a single particular antigen.
Antigens
Antigens on pathogens are usually large and complex, and consist of many antigenic determinants. An antigenic determinant(epitope) is one of the small regions within an antigen to which a receptor can bind, and antigenic determinants are limited by the size of the receptor itself. They usually consist of six or fewer amino acid residues in a protein, or one or two sugar moieties in a carbohydrate antigen. Antigenic determinants on a carbohydrate antigen are usually less diverse than on a protein antigen. Carbohydrate antigens are found on bacterial cell walls and on red blood cells (the ABO blood group antigens). Protein antigens are complex because of the variety of three-dimensional shapes that proteins can assume, and are especially important for the immune responses to viruses and worm parasites. It is the interaction of the shape of the antigen and the complementary shape of the amino acids of the antigen-binding site that accounts for the chemical basis of specificity (Figure 21.16).
Figure 21.16 Antigenic Determinants A typical protein antigen has multiple antigenic determinants, shown by the ability of T cells with three different specificities to bind to different parts of the same antigen.
Antigen Processing and Presentation
Although Figure 21.16 shows T cell receptors interacting with antigenic determinants directly, the mechanism that T cells use to recognize antigens is, in reality, much more complex. T cells do not recognize free-floating or cell-bound antigens as they appear on the surface of the pathogen. They only recognize antigen on the surface of specialized cells called antigen-presenting cells. Antigens are internalized by these cells. Antigen processing is a mechanism that enzymatically cleaves the antigen into smaller pieces. The antigen fragments are then brought to the cell’s surface and associated with a specialized type of antigen-presenting protein known as a major histocompatibility complex (MHC) molecule. The MHC is the cluster of genes that encode these antigen-presenting molecules. The association of the antigen fragments with an MHC molecule on the surface of a cell is known as antigen presentation and results in the recognition of antigen by a T cell. This association of antigen and MHC occurs inside the cell, and it is the complex of the two that is brought to the surface. The peptide-binding cleft is a small indentation at the end of the MHC molecule that is furthest away from the cell membrane; it is here that the processed fragment of antigen sits. MHC molecules are capable of presenting a variety of antigens, depending on the amino acid sequence, in their peptide-binding clefts. It is the combination of the MHC molecule and the fragment of the original peptide or carbohydrate that is actually physically recognized by the T cell receptor (Figure 21.17).
Figure 21.17 Antigen Processing and Presentation
Two distinct types of MHC molecules, MHC class I and MHC class II, play roles in antigen presentation. Although produced from different genes, they both have similar functions. They bring processed antigen to the surface of the cell via a transport vesicle and present the antigen to the T cell and its receptor. Antigens from different classes of pathogens, however, use different MHC classes and take different routes through the cell to get to the surface for presentation. The basic mechanism, though, is the same. Antigens are processed by digestion, are brought into the endomembrane system of the cell, and then are expressed on the surface of the antigen-presenting cell for antigen recognition by a T cell. Intracellular antigens are typical of viruses, which replicate inside the cell, and certain other intracellular parasites and bacteria. These antigens are processed in the cytosol by an enzyme complex known as the proteasome and are then brought into the endoplasmic reticulum by the transporter associated with antigen processing (TAP) system, where they interact with class I MHC molecules and are eventually transported to the cell surface by a transport vesicle.
Extracellular antigens, characteristic of many bacteria, parasites, and fungi that do not replicate inside the cell’s cytoplasm, are brought into the endomembrane system of the cell by receptor-mediated endocytosis. The resulting vesicle fuses with vesicles from the Golgi complex, which contain pre-formed MHC class II molecules. After fusion of these two vesicles and the association of antigen and MHC, the new vesicle makes its way to the cell surface.
Professional Antigen-presenting Cells
Many cell types express class I molecules for the presentation of intracellular antigens. These MHC molecules may then stimulate a cytotoxic T cell immune response, eventually destroying the cell and the pathogen within. This is especially important when it comes to the most common class of intracellular pathogens, the virus. Viruses infect nearly every tissue of the body, so all these tissues must necessarily be able to express class I MHC or no T cell response can be made.
On the other hand, class II MHC molecules are expressed only on the cells of the immune system, specifically cells that affect other arms of the immune response. Thus, these cells are called “professional” antigen-presenting cells to distinguish them from those that bear class I MHC. The three types of professional antigen presenters are macrophages, dendritic cells, and B cells (Table 21.4).
Macrophages stimulate T cells to release cytokines that enhance phagocytosis. Dendritic cells also kill pathogens by phagocytosis (see Figure 21.17), but their major function is to bring antigens to regional draining lymph nodes. The lymph nodes are the locations in which most T cell responses against pathogens of the interstitial tissues are mounted. Macrophages are found in the skin and in the lining of mucosal surfaces, such as the nasopharynx, stomach, lungs, and intestines. B cells may also present antigens to T cells, which are necessary for certain types of antibody responses, to be covered later in this chapter.
Classes of Antigen-presenting Cells
| MHC | Cell type | Phagocytic? | Function |
|---|---|---|---|
| Class I | Many | No | Stimulates cytotoxic T cell immune response |
| Class II | Macrophage | Yes | Stimulates phagocytosis and presentation at primary infection site |
| Class II | Dendritic | Yes, in tissues | Brings antigens to regional lymph nodes |
| Class II | B cell | Yes, internalizes surface Ig and antigen | Stimulates antibody secretion by B cells |
Table 21.4
T Cell Development and Differentiation
The process of eliminating T cells that might attack the cells of one’s own body is referred to as T cell tolerance. While thymocytes are in the cortex of the thymus, they are referred to as “double negatives,” meaning that they do not bear the CD4 or CD8 molecules that you can use to follow their pathways of differentiation (Figure 21.18). In the cortex of the thymus, they are exposed to cortical epithelial cells. In a process known as positive selection, double-negative thymocytes bind to the MHC molecules they observe on the thymic epithelia, and the MHC molecules of “self” are selected. This mechanism kills many thymocytes during T cell differentiation. In fact, only two percent of the thymocytes that enter the thymus leave it as mature, functional T cells.
Figure 21.18 Differentiation of T Cells within the Thymus Thymocytes enter the thymus and go through a series of developmental stages that ensures both function and tolerance before they leave and become functional components of the adaptive immune response.
Later, the cells become double positives that express both CD4 and CD8 markers and move from the cortex to the junction between the cortex and medulla. It is here that negative selection takes place. In negative selection, self-antigens are brought into the thymus from other parts of the body by professional antigen-presenting cells. The T cells that bind to these self-antigens are selected for negatively and are killed by apoptosis. In summary, the only T cells left are those that can bind to MHC molecules of the body with foreign antigens presented on their binding clefts, preventing an attack on one’s own body tissues, at least under normal circumstances. Tolerance can be broken, however, by the development of an autoimmune response, to be discussed later in this chapter.
The cells that leave the thymus become single positives, expressing either CD4 or CD8, but not both (see Figure 21.18). The CD4+ T cells will bind to class II MHC and the CD8+ cells will bind to class I MHC. The discussion that follows explains the functions of these molecules and how they can be used to differentiate between the different T cell functional types.
Mechanisms of T Cell-mediated Immune Responses
Mature T cells become activated by recognizing processed foreign antigen in association with a self-MHC molecule and begin dividing rapidly by mitosis. This proliferation of T cells is called clonal expansion and is necessary to make the immune response strong enough to effectively control a pathogen. How does the body select only those T cells that are needed against a specific pathogen? Again, the specificity of a T cell is based on the amino acid sequence and the three-dimensional shape of the antigen-binding site formed by the variable regions of the two chains of the T cell receptor (Figure 21.19). Clonal selection is the process of antigen binding only to those T cells that have receptors specific to that antigen. Each T cell that is activated has a specific receptor “hard-wired” into its DNA, and all of its progeny will have identical DNA and T cell receptors, forming clones of the original T cell.
Figure 21.19 Clonal Selection and Expansion of T Lymphocytes Stem cells differentiate into T cells with specific receptors, called clones. The clones with receptors specific for antigens on the pathogen are selected for and expanded.
Clonal Selection and Expansion
The clonal selection theory was proposed by Frank Burnet in the 1950s. However, the term clonal selection is not a complete description of the theory, as clonal expansion goes hand in glove with the selection process. The main tenet of the theory is that a typical individual has a multitude (1011) of different types of T cell clones based on their receptors. In this use, a clone is a group of lymphocytes that share the same antigen receptor. Each clone is necessarily present in the body in low numbers. Otherwise, the body would not have room for lymphocytes with so many specificities.
Only those clones of lymphocytes whose receptors are activated by the antigen are stimulated to proliferate. Keep in mind that most antigens have multiple antigenic determinants, so a T cell response to a typical antigen involves a polyclonal response. A polyclonal response is the stimulation of multiple T cell clones. Once activated, the selected clones increase in number and make many copies of each cell type, each clone with its unique receptor. By the time this process is complete, the body will have large numbers of specific lymphocytes available to fight the infection (see Figure 21.19).
The Cellular Basis of Immunological Memory
As already discussed, one of the major features of an adaptive immune response is the development of immunological memory.
During a primary adaptive immune response, both memory T cells and effector T cells are generated. Memory T cells are long-lived and can even persist for a lifetime. Memory cells are primed to act rapidly. Thus, any subsequent exposure to the pathogen will elicit a very rapid T cell response. This rapid, secondary adaptive response generates large numbers of effector T cells so fast that the pathogen is often overwhelmed before it can cause any symptoms of disease. This is what is meant by immunity to a disease. The same pattern of primary and secondary immune responses occurs in B cells and the antibody response, as will be discussed later in the chapter.
T Cell Types and their Functions
In the discussion of T cell development, you saw that mature T cells express either the CD4 marker or the CD8 marker, but not both. These markers are cell adhesion molecules that keep the T cell in close contact with the antigen-presenting cell by directly binding to the MHC molecule (to a different part of the molecule than does the antigen). Thus, T cells and antigen-presenting cells are held together in two ways: by CD4 or CD8 attaching to MHC and by the T cell receptor binding to antigen (Figure 21.20).
Figure 21.20 Pathogen Presentation (a) CD4 is associated with helper and regulatory T cells. An extracellular pathogen is processed and presented in the binding cleft of a class II MHC molecule, and this interaction is strengthened by the CD4 molecule. (b) CD8 is associated with cytotoxic T cells. An intracellular pathogen is presented by a class I MHC molecule, and CD8 interacts with it.
Although the correlation is not 100 percent, CD4-bearing T cells are associated with helper functions and CD8-bearing T cells are associated with cytotoxicity. These functional distinctions based on CD4 and CD8 markers are useful in defining the function of each type.
Helper T Cells and their Cytokines
Helper T cells (Th), bearing the CD4 molecule, function by secreting cytokines that act to enhance other immune responses. There are two classes of Th cells, and they act on different components of the immune response. These cells are not distinguished by their surface molecules but by the characteristic set of cytokines they secrete (Table 21.5).
Th1 cells are a type of helper T cell that secretes cytokines that regulate the immunological activity and development of a variety of cells, including macrophages and other types of T cells.
Th2 cells, on the other hand, are cytokine-secreting cells that act on B cells to drive their differentiation into plasma cells that make antibody. In fact, T cell help is required for antibody responses to most protein antigens, and these are called T cell-dependent antigens.
Cytotoxic T cells
Cytotoxic T cells (Tc) are T cells that kill target cells by inducing apoptosis using the same mechanism as NK cells. They either express Fas ligand, which binds to the fas molecule on the target cell, or act by using perforins and granzymes contained in their cytoplasmic granules. As was discussed earlier with NK cells, killing a virally infected cell before the virus can complete its replication cycle results in the production of no infectious particles. As more Tc cells are developed during an immune response, they overwhelm the ability of the virus to cause disease. In addition, each Tc cell can kill more than one target cell, making them especially effective. Tc cells are so important in the antiviral immune response that some speculate that this was the main reason the adaptive immune response evolved in the first place.
Regulatory T Cells
Regulatory T cells (Treg), or suppressor T cells, are the most recently discovered of the types listed here, so less is understood about them. In addition to CD4, they bear the molecules CD25 and FOXP3. Exactly how they function is still under investigation, but it is known that they suppress other T cell immune responses. This is an important feature of the immune response, because if clonal expansion during immune responses were allowed to continue uncontrolled, these responses could lead to autoimmune diseases and other medical issues.
Not only do T cells directly destroy pathogens, but they regulate nearly all other types of the adaptive immune response as well, as evidenced by the functions of the T cell types, their surface markers, the cells they work on, and the types of pathogens they work against (see Table 21.5).
Functions of T Cell Types and Their Cytokines
| T cell | Main target | Function | Pathogen | Surface marker | MHC | Cytokines or mediators |
|---|---|---|---|---|---|---|
| Tc | Infected cells | Cytotoxicity | Intracellular | CD8 | Class I | Perforins, granzymes, and fas ligand |
| Th1 | Macrophage | Helper inducer | Extracellular | CD4 | Class II | Interferon-γ and TGF-β |
| Th2 | B cell | Helper inducer | Extracellular | CD4 | Class II | IL-4, IL-6, IL-10, and others |
| Treg | Th cell | Suppressor | None | CD4, CD25 | ? | TGF-β and IL-10 |
Table 21.5
The Adaptive Immune Response: B-lymphocytes and Antibodies
- Explain how B cells mature and how B cell tolerance develops
- Discuss how B cells are activated and differentiate into plasma cells
- Describe the structure of the antibody classes and their functions
Antibodies were the first component of the adaptive immune response to be characterized by scientists working on the immune system. It was already known that individuals who survived a bacterial infection were immune to re-infection with the same pathogen. Early microbiologists took serum from an immune patient and mixed it with a fresh culture of the same type of bacteria, then observed the bacteria under a microscope. The bacteria became clumped in a process called agglutination. When a different bacterial species was used, the agglutination did not happen. Thus, there was something in the serum of immune individuals that could specifically bind to and agglutinate bacteria.
Scientists now know the cause of the agglutination is an antibody molecule, also called an immunoglobulin. What is an antibody? An antibody protein is essentially a secreted form of a B cell receptor. (In fact, surface immunoglobulin is another name for the B cell receptor.) Not surprisingly, the same genes encode both the secreted antibodies and the surface immunoglobulins. One minor difference in the way these proteins are synthesized distinguishes a naïve B cell with antibody on its surface from an antibody-secreting plasma cell with no antibodies on its surface. The antibodies of the plasma cell have the exact same antigen-binding site and specificity as their B cell precursors.
There are five different classes of antibody found in humans: IgM, IgD, IgG, IgA, and IgE. Each of these has specific functions in the immune response, so by learning about them, researchers can learn about the great variety of antibody functions critical to many adaptive immune responses.
B cells do not recognize antigen in the complex fashion of T cells. B cells can recognize native, unprocessed antigen and do not require the participation of MHC molecules and antigen-presenting cells.
B Cell Differentiation and Activation
B cells differentiate in the bone marrow. During the process of maturation, up to 100 trillion different clones of B cells are generated, which is similar to the diversity of antigen receptors seen in T cells.
B cell differentiation and the development of tolerance are not quite as well understood as it is in T cells. Central tolerance is the destruction or inactivation of B cells that recognize self-antigens in the bone marrow, and its role is critical and well established. In the process of clonal deletion, immature B cells that bind strongly to self-antigens expressed on tissues are signaled to commit suicide by apoptosis, removing them from the population. In the process of clonal anergy, however, B cells exposed to soluble antigen in the bone marrow are not physically deleted, but become unable to function.
Another mechanism called peripheral tolerance is a direct result of T cell tolerance. In peripheral tolerance, functional, mature B cells leave the bone marrow but have yet to be exposed to self-antigen. Most protein antigens require signals from helper T cells (Th2) to proceed to make antibody. When a B cell binds to a self-antigen but receives no signals from a nearby Th2 cell to produce antibody, the cell is signaled to undergo apoptosis and is destroyed. This is yet another example of the control that T cells have over the adaptive immune response.
After B cells are activated by their binding to antigen, they differentiate into plasma cells. Plasma cells often leave the secondary lymphoid organs, where the response is generated, and migrate back to the bone marrow, where the whole differentiation process started. After secreting antibodies for a specific period, they die, as most of their energy is devoted to making antibodies and not to maintaining themselves. Thus, plasma cells are said to be terminally differentiated.
The final B cell of interest is the memory B cell, which results from the clonal expansion of an activated B cell. Memory B cells function in a way similar to memory T cells. They lead to a stronger and faster secondary response when compared to the primary response, as illustrated below.
Antibody Structure
Antibodies are glycoproteins consisting of two types of polypeptide chains with attached carbohydrates. The heavy chain and the light chain are the two polypeptides that form the antibody. The main differences between the classes of antibodies are in the differences between their heavy chains, but as you shall see, the light chains have an important role, forming part of the antigen-binding site on the antibody molecules.
Four-chain Models of Antibody Structures
All antibody molecules have two identical heavy chains and two identical light chains. (Some antibodies contain multiple units of this four-chain structure.) The Fc region of the antibody is formed by the two heavy chains coming together, usually linked by disulfide bonds (Figure 21.21). The Fc portion of the antibody is important in that many effector cells of the immune system have Fc receptors. Cells having these receptors can then bind to antibody-coated pathogens, greatly increasing the specificity of the effector cells. At the other end of the molecule are two identical antigen-binding sites.
Figure 21.21 Antibody and IgG2 Structures The typical four chain structure of a generic antibody (a) and the corresponding three-dimensional structure of the antibody IgG2 (b). (credit b: modification of work by Tim Vickers)
Five Classes of Antibodies and their Functions
In general, antibodies have two basic functions. They can act as the B cell antigen receptor or they can be secreted, circulate, and bind to a pathogen, often labeling it for identification by other forms of the immune response. Of the five antibody classes, notice that only two can function as the antigen receptor for naïve B cells: IgM and IgD (Figure 21.22). Mature B cells that leave the bone marrow express both IgM and IgD, but both antibodies have the same antigen specificity. Only IgM is secreted, however, and no other nonreceptor function for IgD has been discovered.
Figure 21.22 Five Classes of Antibodies
IgM consists of five four-chain structures (20 total chains with 10 identical antigen-binding sites) and is thus the largest of the antibody molecules. IgM is usually the first antibody made during a primary response. Its 10 antigen-binding sites and large shape allow it to bind well to many bacterial surfaces. It is excellent at binding complement proteins and activating the complement cascade, consistent with its role in promoting chemotaxis, opsonization, and cell lysis. Thus, it is a very effective antibody against bacteria at early stages of a primary antibody response. As the primary response proceeds, the antibody produced in a B cell can change to IgG, IgA, or IgE by the process known as class switching. Class switching is the change of one antibody class to another. While the class of antibody changes, the specificity and the antigen-binding sites do not. Thus, the antibodies made are still specific to the pathogen that stimulated the initial IgM response.
IgG is a major antibody of late primary responses and the main antibody of secondary responses in the blood. This is because class switching occurs during primary responses. IgG is a monomeric antibody that clears pathogens from the blood and can activate complement proteins (although not as well as IgM), taking advantage of its antibacterial activities. Furthermore, this class of antibody is the one that crosses the placenta to protect the developing fetus from disease exits the blood to the interstitial fluid to fight extracellular pathogens.
IgA exists in two forms, a four-chain monomer in the blood and an eight-chain structure, or dimer, in exocrine gland secretions of the mucous membranes, including mucus, saliva, and tears. Thus, dimeric IgA is the only antibody to leave the interior of the body to protect body surfaces. IgA is also of importance to newborns, because this antibody is present in mother’s breast milk (colostrum), which serves to protect the infant from disease.
IgE is usually associated with allergies and anaphylaxis. It is present in the lowest concentration in the blood, because its Fc region binds strongly to an IgE-specific Fc receptor on the surfaces of mast cells. IgE makes mast cell degranulation very specific, such that if a person is allergic to peanuts, there will be peanut-specific IgE bound to his or her mast cells. In this person, eating peanuts will cause the mast cells to degranulate, sometimes causing severe allergic reactions, including anaphylaxis, a severe, systemic allergic response that can cause death.
Clonal Selection of B Cells
Clonal selection and expansion work much the same way in B cells as in T cells. Only B cells with appropriate antigen specificity are selected for and expanded (Figure 21.23). Eventually, the plasma cells secrete antibodies with antigenic specificity identical to those that were on the surfaces of the selected B cells. Notice in the figure that both plasma cells and memory B cells are generated simultaneously.
Figure 21.23 Clonal Selection of B Cells During a primary B cell immune response, both antibody-secreting plasma cells and memory B cells are produced. These memory cells lead to the differentiation of more plasma cells and memory B cells during secondary responses.
Primary versus Secondary B Cell Responses
Primary and secondary responses as they relate to T cells were discussed earlier. This section will look at these responses with B cells and antibody production. Because antibodies are easily obtained from blood samples, they are easy to follow and graph (Figure 21.24). As you will see from the figure, the primary response to an antigen (representing a pathogen) is delayed by several days. This is the time it takes for the B cell clones to expand and differentiate into plasma cells. The level of antibody produced is low, but it is sufficient for immune protection. The second time a person encounters the same antigen, there is no time delay, and the amount of antibody made is much higher. Thus, the secondary antibody response overwhelms the pathogens quickly and, in most situations, no symptoms are felt. When a different antigen is used, another primary response is made with its low antibody levels and time delay.
Figure 21.24 Primary and Secondary Antibody Responses Antigen A is given once to generate a primary response and later to generate a secondary response. When a different antigen is given for the first time, a new primary response is made.
Active versus Passive Immunity
Immunity to pathogens, and the ability to control pathogen growth so that damage to the tissues of the body is limited, can be acquired by (1) the active development of an immune response in the infected individual or (2) the passive transfer of immune components from an immune individual to a nonimmune one. Both active and passive immunity have examples in the natural world and as part of medicine.
Active immunity is the resistance to pathogens acquired during an adaptive immune response within an individual (Table 21.6). Naturally acquired active immunity, the response to a pathogen, is the focus of this chapter. Artificially acquired active immunity involves the use of vaccines. A vaccine is a killed or weakened pathogen or its components that, when administered to a healthy individual, leads to the development of immunological memory (a weakened primary immune response) without causing much in the way of symptoms. Thus, with the use of vaccines, one can avoid the damage from disease that results from the first exposure to the pathogen, yet reap the benefits of protection from immunological memory. The advent of vaccines was one of the major medical advances of the twentieth century and led to the eradication of smallpox and the control of many infectious diseases, including polio, measles, and whooping cough.
Active versus Passive Immunity
| Natural | Artificial | |
|---|---|---|
| Active | Adaptive immune response | Vaccine response |
| Passive | Trans-placental antibodies/breastfeeding | Immune globulin injections |
Table 21.6
Passive immunity arises from the transfer of antibodies to an individual without requiring them to mount their own active immune response. Naturally acquired passive immunity is seen during fetal development. IgG is transferred from the maternal circulation to the fetus via the placenta, protecting the fetus from infection and protecting the newborn for the first few months of its life. As already stated, a newborn benefits from the IgA antibodies it obtains from milk during breastfeeding. The fetus and newborn thus benefit from the immunological memory of the mother to the pathogens to which she has been exposed. In medicine, artificially acquired passive immunity usually involves injections of immunoglobulins, taken from animals previously exposed to a specific pathogen. This treatment is a fast-acting method of temporarily protecting an individual who was possibly exposed to a pathogen. The downside to both types of passive immunity is the lack of the development of immunological memory. Once the antibodies are transferred, they are effective for only a limited time before they degrade.
INTERACTIVE LINK
Immunity can be acquired in an active or passive way, and it can be natural or artificial. Watch this video to see an animated discussion of passive and active immunity. What is an example of natural immunity acquired passively?
T cell-dependent versus T cell-independent Antigens
As discussed previously, Th2 cells secrete cytokines that drive the production of antibodies in a B cell, responding to complex antigens such as those made by proteins. On the other hand, some antigens are T cell independent. A T cell-independent antigenusually is in the form of repeated carbohydrate moieties found on the cell walls of bacteria. Each antibody on the B cell surface has two binding sites, and the repeated nature of T cell-independent antigen leads to crosslinking of the surface antibodies on the B cell. The crosslinking is enough to activate it in the absence of T cell cytokines.
A T cell-dependent antigen, on the other hand, usually is not repeated to the same degree on the pathogen and thus does not crosslink surface antibody with the same efficiency. To elicit a response to such antigens, the B and T cells must come close together (Figure 21.25). The B cell must receive two signals to become activated. Its surface immunoglobulin must recognize native antigen. Some of this antigen is internalized, processed, and presented to the Th2 cells on a class II MHC molecule. The T cell then binds using its antigen receptor and is activated to secrete cytokines that diffuse to the B cell, finally activating it completely. Thus, the B cell receives signals from both its surface antibody and the T cell via its cytokines, and acts as a professional antigen-presenting cell in the process.
Figure 21.25 T and B Cell Binding To elicit a response to a T cell-dependent antigen, the B and T cells must come close together. To become fully activated, the B cell must receive two signals from the native antigen and the T cell’s cytokines.
The Immune Response against Pathogens
- Explain the development of immunological competence
- Describe the mucosal immune response
- Discuss immune responses against bacterial, viral, fungal, and animal pathogens
- Describe different ways pathogens evade immune responses
Now that you understand the development of mature, naïve B cells and T cells, and some of their major functions, how do all of these various cells, proteins, and cytokines come together to actually resolve an infection? Ideally, the immune response will rid the body of a pathogen entirely. The adaptive immune response, with its rapid clonal expansion, is well suited to this purpose. Think of a primary infection as a race between the pathogen and the immune system. The pathogen bypasses barrier defenses and starts multiplying in the host’s body. During the first 4 to 5 days, the innate immune response will partially control, but not stop, pathogen growth. As the adaptive immune response gears up, however, it will begin to clear the pathogen from the body, while at the same time becoming stronger and stronger. When following antibody responses in patients with a particular disease such as a virus, this clearance is referred to as seroconversion (sero- = “serum”). Seroconversion is the reciprocal relationship between virus levels in the blood and antibody levels. As the antibody levels rise, the virus levels decline, and this is a sign that the immune response is being at least partially effective (partially, because in many diseases, seroconversion does not necessarily mean a patient is getting well).
An excellent example of this is seroconversion during HIV disease (Figure 21.26). Notice that antibodies are made early in this disease, and the increase in anti-HIV antibodies correlates with a decrease in detectable virus in the blood. Although these antibodies are an important marker for diagnosing the disease, they are not sufficient to completely clear the virus. Several years later, the vast majority of these individuals, if untreated, will lose their entire adaptive immune response, including the ability to make antibodies, during the final stages of AIDS.
Figure 21.26 HIV Disease Progression Seroconversion, the rise of anti-HIV antibody levels and the concomitant decline in measurable virus levels, happens during the first several months of HIV disease. Unfortunately, this antibody response is ineffective at controlling the disease, as seen by the progression of the disease towards AIDS, in which all adaptive immune responses are compromised.
EVERYDAY CONNECTION
Disinfectants: Fighting the Good Fight?
“Wash your hands!” Parents have been telling their children this for generations. Dirty hands can spread disease. But is it possible to get rid of enough pathogens that children will never get sick? Are children who avoid exposure to pathogens better off? The answers to both these questions appears to be no.
Antibacterial wipes, soaps, gels, and even toys with antibacterial substances embedded in their plastic are ubiquitous in our society. Still, these products do not rid the skin and gastrointestinal tract of bacteria, and it would be harmful to our health if they did. We need these nonpathogenic bacteria on and within our bodies to keep the pathogenic ones from growing. The urge to keep children perfectly clean is thus probably misguided. Children will get sick anyway, and the later benefits of immunological memory far outweigh the minor discomforts of most childhood diseases. In fact, getting diseases such as chickenpox or measles later in life is much harder on the adult and are associated with symptoms significantly worse than those seen in the childhood illnesses. Of course, vaccinations help children avoid some illnesses, but there are so many pathogens, we will never be immune to them all.
Could over-cleanliness be the reason that allergies are increasing in more developed countries? Some scientists think so. Allergies are based on an IgE antibody response. Many scientists think the system evolved to help the body rid itself of worm parasites. The hygiene theory is the idea that the immune system is geared to respond to antigens, and if pathogens are not present, it will respond instead to inappropriate antigens such as allergens and self-antigens. This is one explanation for the rising incidence of allergies in developed countries, where the response to nonpathogens like pollen, shrimp, and cat dander cause allergic responses while not serving any protective function.
The Mucosal Immune Response
Mucosal tissues are major barriers to the entry of pathogens into the body. The IgA (and sometimes IgM) antibodies in mucus and other secretions can bind to the pathogen, and in the cases of many viruses and bacteria, neutralize them. Neutralization is the process of coating a pathogen with antibodies, making it physically impossible for the pathogen to bind to receptors. Neutralization, which occurs in the blood, lymph, and other body fluids and secretions, protects the body constantly. Neutralizing antibodies are the basis for the disease protection offered by vaccines. Vaccinations for diseases that commonly enter the body via mucous membranes, such as influenza, are usually formulated to enhance IgA production.
Immune responses in some mucosal tissues such as the Peyer’s patches (see Figure 21.11) in the small intestine take up particulate antigens by specialized cells known as microfold or M cells (Figure 21.27). These cells allow the body to sample potential pathogens from the intestinal lumen. Dendritic cells then take the antigen to the regional lymph nodes, where an immune response is mounted.
Figure 21.27 IgA Immunity The nasal-associated lymphoid tissue and Peyer’s patches of the small intestine generate IgA immunity. Both use M cells to transport antigen inside the body so that immune responses can be mounted.
Defenses against Bacteria and Fungi
The body fights bacterial pathogens with a wide variety of immunological mechanisms, essentially trying to find one that is effective. Bacteria such as Mycobacterium leprae, the cause of leprosy, are resistant to lysosomal enzymes and can persist in macrophage organelles or escape into the cytosol. In such situations, infected macrophages receiving cytokine signals from Th1 cells turn on special metabolic pathways. Macrophage oxidative metabolism is hostile to intracellular bacteria, often relying on the production of nitric oxide to kill the bacteria inside the macrophage.
Fungal infections, such as those from Aspergillus, Candida, and Pneumocystis, are largely opportunistic infections that take advantage of suppressed immune responses. Most of the same immune mechanisms effective against bacteria have similar effects on fungi, both of which have characteristic cell wall structures that protect their cells.
Defenses against Parasites
Worm parasites such as helminths are seen as the primary reason why the mucosal immune response, IgE-mediated allergy and asthma, and eosinophils evolved. These parasites were at one time very common in human society. When infecting a human, often via contaminated food, some worms take up residence in the gastrointestinal tract. Eosinophils are attracted to the site by T cell cytokines, which release their granule contents upon their arrival. Mast cell degranulation also occurs, and the fluid leakage caused by the increase in local vascular permeability is thought to have a flushing action on the parasite, expelling its larvae from the body. Furthermore, if IgE labels the parasite, the eosinophils can bind to it by its Fc receptor.
Defenses against Viruses
The primary mechanisms against viruses are NK cells, interferons, and cytotoxic T cells. Antibodies are effective against viruses mostly during protection, where an immune individual can neutralize them based on a previous exposure. Antibodies have no effect on viruses or other intracellular pathogens once they enter the cell, since antibodies are not able to penetrate the plasma membrane of the cell. Many cells respond to viral infections by downregulating their expression of MHC class I molecules. This is to the advantage of the virus, because without class I expression, cytotoxic T cells have no activity. NK cells, however, can recognize virally infected class I-negative cells and destroy them. Thus, NK and cytotoxic T cells have complementary activities against virally infected cells.
Interferons have activity in slowing viral replication and are used in the treatment of certain viral diseases, such as hepatitis B and C, but their ability to eliminate the virus completely is limited. The cytotoxic T cell response, though, is key, as it eventually overwhelms the virus and kills infected cells before the virus can complete its replicative cycle. Clonal expansion and the ability of cytotoxic T cells to kill more than one target cell make these cells especially effective against viruses. In fact, without cytotoxic T cells, it is likely that humans would all die at some point from a viral infection (if no vaccine were available).
Evasion of the Immune System by Pathogens
It is important to keep in mind that although the immune system has evolved to be able to control many pathogens, pathogens themselves have evolved ways to evade the immune response. An example already mentioned is in Mycobactrium tuberculosis, which has evolved a complex cell wall that is resistant to the digestive enzymes of the macrophages that ingest them, and thus persists in the host, causing the chronic disease tuberculosis. This section briefly summarizes other ways in which pathogens can “outwit” immune responses. But keep in mind, although it seems as if pathogens have a will of their own, they do not. All of these evasive “strategies” arose strictly by evolution, driven by selection.
Bacteria sometimes evade immune responses because they exist in multiple strains, such as different groups of Staphylococcus aureus. S. aureus is commonly found in minor skin infections, such as boils, and some healthy people harbor it in their nose. One small group of strains of this bacterium, however, called methicillin-resistant Staphylococcus aureus, has become resistant to multiple antibiotics and is essentially untreatable. Different bacterial strains differ in the antigens on their surfaces. The immune response against one strain (antigen) does not affect the other; thus, the species survives.
Another method of immune evasion is mutation. Because viruses’ surface molecules mutate continuously, viruses like influenza change enough each year that the flu vaccine for one year may not protect against the flu common to the next. New vaccine formulations must be derived for each flu season.
Genetic recombination—the combining of gene segments from two different pathogens—is an efficient form of immune evasion. For example, the influenza virus contains gene segments that can recombine when two different viruses infect the same cell. Recombination between human and pig influenza viruses led to the 2010 H1N1 swine flu outbreak.
Pathogens can produce immunosuppressive molecules that impair immune function, and there are several different types. Viruses are especially good at evading the immune response in this way, and many types of viruses have been shown to suppress the host immune response in ways much more subtle than the wholesale destruction caused by HIV.
Diseases Associated with Depressed or Overactive Immune Responses
- Discuss inherited and acquired immunodeficiencies
- Explain the four types of hypersensitivity and how they differ
- Give an example of how autoimmune disease breaks tolerance
This section is about how the immune system goes wrong. When it goes haywire, and becomes too weak or too strong, it leads to a state of disease. The factors that maintain immunological homeostasis are complex and incompletely understood.
Immunodeficiencies
As you have seen, the immune system is quite complex. It has many pathways using many cell types and signals. Because it is so complex, there are many ways for it to go wrong. Inherited immunodeficiencies arise from gene mutations that affect specific components of the immune response. There are also acquired immunodeficiencies with potentially devastating effects on the immune system, such as HIV.
Inherited Immunodeficiencies
A list of all inherited immunodeficiencies is well beyond the scope of this book. The list is almost as long as the list of cells, proteins, and signaling molecules of the immune system itself. Some deficiencies, such as those for complement, cause only a higher susceptibility to some Gram-negative bacteria. Others are more severe in their consequences. Certainly, the most serious of the inherited immunodeficiencies is severe combined immunodeficiency disease (SCID). This disease is complex because it is caused by many different genetic defects. What groups them together is the fact that both the B cell and T cell arms of the adaptive immune response are affected.
Children with this disease usually die of opportunistic infections within their first year of life unless they receive a bone marrow transplant. Such a procedure had not yet been perfected for David Vetter, the “boy in the bubble,” who was treated for SCID by having to live almost his entire life in a sterile plastic cocoon for the 12 years before his death from infection in 1984. One of the features that make bone marrow transplants work as well as they do is the proliferative capability of hematopoietic stem cells of the bone marrow. Only a small amount of bone marrow from a healthy donor is given intravenously to the recipient. It finds its own way to the bone where it populates it, eventually reconstituting the patient’s immune system, which is usually destroyed beforehand by treatment with radiation or chemotherapeutic drugs.
New treatments for SCID using gene therapy, inserting nondefective genes into cells taken from the patient and giving them back, have the advantage of not needing the tissue match required for standard transplants. Although not a standard treatment, this approach holds promise, especially for those in whom standard bone marrow transplantation has failed.
Human Immunodeficiency Virus/AIDS
Although many viruses cause suppression of the immune system, only one wipes it out completely, and that is the previously mentioned HIV. It is worth discussing the biology of this virus, which can lead to the well-known AIDS, so that its full effects on the immune system can be understood. The virus is transmitted through semen, vaginal fluids, and blood, and can be caught by risky sexual behaviors and the sharing of needles by intravenous drug users. There are sometimes, but not always, flu-like symptoms in the first 1 to 2 weeks after infection. This is later followed by seroconversion. The anti-HIV antibodies formed during seroconversion are the basis for most initial HIV screening done in the United States. Because seroconversion takes different lengths of time in different individuals, multiple AIDS tests are given months apart to confirm or eliminate the possibility of infection.
After seroconversion, the amount of virus circulating in the blood drops and stays at a low level for several years. During this time, the levels of CD4+ cells, especially helper T cells, decline steadily, until at some point, the immune response is so weak that opportunistic disease and eventually death result. HIV uses CD4 as the receptor to get inside cells, but it also needs a co-receptor, such as CCR5 or CXCR4. These co-receptors, which usually bind to chemokines, present another target for anti-HIV drug development. Although other antigen-presenting cells are infected with HIV, given that CD4+ helper T cells play an important role in T cell immune responses and antibody responses, it should be no surprise that both types of immune responses are eventually seriously compromised.
Treatment for the disease consists of drugs that target virally encoded proteins that are necessary for viral replication but are absent from normal human cells. By targeting the virus itself and sparing the cells, this approach has been successful in significantly prolonging the lives of HIV-positive individuals. On the other hand, an HIV vaccine has been 30 years in development and is still years away. Because the virus mutates rapidly to evade the immune system, scientists have been looking for parts of the virus that do not change and thus would be good targets for a vaccine candidate.
Hypersensitivities
The word “hypersensitivity” simply means sensitive beyond normal levels of activation. Allergies and inflammatory responses to nonpathogenic environmental substances have been observed since the dawn of history. Hypersensitivity is a medical term describing symptoms that are now known to be caused by unrelated mechanisms of immunity. Still, it is useful for this discussion to use the four types of hypersensitivities as a guide to understand these mechanisms (Figure 21.28).
Figure 21.28 Immune Hypersensitivity Components of the immune system cause four types of hypersensitivity. Notice that types I–III are B cell mediated, whereas type IV hypersensitivity is exclusively a T cell phenomenon.
Immediate (Type I) Hypersensitivity
Antigens that cause allergic responses are often referred to as allergens. The specificity of the immediate hypersensitivityresponse is predicated on the binding of allergen-specific IgE to the mast cell surface. The process of producing allergen-specific IgE is called sensitization, and is a necessary prerequisite for the symptoms of immediate hypersensitivity to occur. Allergies and allergic asthma are mediated by mast cell degranulation that is caused by the crosslinking of the antigen-specific IgE molecules on the mast cell surface. The mediators released have various vasoactive effects already discussed, but the major symptoms of inhaled allergens are the nasal edema and runny nose caused by the increased vascular permeability and increased blood flow of nasal blood vessels. As these mediators are released with mast cell degranulation, type I hypersensitivity reactions are usually rapid and occur within just a few minutes, hence the term immediate hypersensitivity.
Most allergens are in themselves nonpathogenic and therefore innocuous. Some individuals develop mild allergies, which are usually treated with antihistamines. Others develop severe allergies that may cause anaphylactic shock, which can potentially be fatal within 20 to 30 minutes if untreated. This drop in blood pressure (shock) with accompanying contractions of bronchial smooth muscle is caused by systemic mast cell degranulation when an allergen is eaten (for example, shellfish and peanuts), injected (by a bee sting or being administered penicillin), or inhaled (asthma). Because epinephrine raises blood pressure and relaxes bronchial smooth muscle, it is routinely used to counteract the effects of anaphylaxis and can be lifesaving. Patients with known severe allergies are encouraged to keep automatic epinephrine injectors with them at all times, especially when away from easy access to hospitals.
Allergists use skin testing to identify allergens in type I hypersensitivity. In skin testing, allergen extracts are injected into the epidermis, and a positive result of a soft, pale swelling at the site surrounded by a red zone (called the wheal and flare response), caused by the release of histamine and the granule mediators, usually occurs within 30 minutes. The soft center is due to fluid leaking from the blood vessels and the redness is caused by the increased blood flow to the area that results from the dilation of local blood vessels at the site.
Type II and Type III Hypersensitivities
Type II hypersensitivity, which involves IgG-mediated lysis of cells by complement proteins, occurs during mismatched blood transfusions and blood compatibility diseases such as erythroblastosis fetalis (see section on transplantation). Type III hypersensitivity occurs with diseases such as systemic lupus erythematosus, where soluble antigens, mostly DNA and other material from the nucleus, and antibodies accumulate in the blood to the point that the antigen and antibody precipitate along blood vessel linings. These immune complexes often lodge in the kidneys, joints, and other organs where they can activate complement proteins and cause inflammation.
Delayed (Type IV) Hypersensitivity
Delayed hypersensitivity, or type IV hypersensitivity, is basically a standard cellular immune response. In delayed hypersensitivity, the first exposure to an antigen is called sensitization, such that on re-exposure, a secondary cellular response results, secreting cytokines that recruit macrophages and other phagocytes to the site. These sensitized T cells, of the Th1 class, will also activate cytotoxic T cells. The time it takes for this reaction to occur accounts for the 24- to 72-hour delay in development.
The classical test for delayed hypersensitivity is the tuberculin test for tuberculosis, where bacterial proteins from M. tuberculosisare injected into the skin. A couple of days later, a positive test is indicated by a raised red area that is hard to the touch, called an induration, which is a consequence of the cellular infiltrate, an accumulation of activated macrophages. A positive tuberculin test means that the patient has been exposed to the bacteria and exhibits a cellular immune response to it.
Another type of delayed hypersensitivity is contact sensitivity, where substances such as the metal nickel cause a red and swollen area upon contact with the skin. The individual must have been previously sensitized to the metal. A much more severe case of contact sensitivity is poison ivy, but many of the harshest symptoms of the reaction are associated with the toxicity of its oils and are not T cell mediated.
Autoimmune Responses
The worst cases of the immune system over-reacting are autoimmune diseases. Somehow, tolerance breaks down and the immune systems in individuals with these diseases begin to attack their own bodies, causing significant damage. The trigger for these diseases is, more often than not, unknown, and the treatments are usually based on resolving the symptoms using immunosuppressive and anti-inflammatory drugs such as steroids. These diseases can be localized and crippling, as in rheumatoid arthritis, or diffuse in the body with multiple symptoms that differ in different individuals, as is the case with systemic lupus erythematosus (Figure 21.29).
Figure 21.29 Autoimmune Disorders: Rheumatoid Arthritis and Lupus (a) Extensive damage to the right hand of a rheumatoid arthritis sufferer is shown in the x-ray. (b) The diagram shows a variety of possible symptoms of systemic lupus erythematosus.
Environmental triggers seem to play large roles in autoimmune responses. One explanation for the breakdown of tolerance is that, after certain bacterial infections, an immune response to a component of the bacterium cross-reacts with a self-antigen. This mechanism is seen in rheumatic fever, a result of infection with Streptococcus bacteria, which causes strep throat. The antibodies to this pathogen’s M protein cross-react with an antigenic component of heart myosin, a major contractile protein of the heart that is critical to its normal function. The antibody binds to these molecules and activates complement proteins, causing damage to the heart, especially to the heart valves. On the other hand, some theories propose that having multiple common infectious diseases actually prevents autoimmune responses. The fact that autoimmune diseases are rare in countries that have a high incidence of infectious diseases supports this idea, another example of the hygiene hypothesis discussed earlier in this chapter.
There are genetic factors in autoimmune diseases as well. Some diseases are associated with the MHC genes that an individual expresses. The reason for this association is likely because if one’s MHC molecules are not able to present a certain self-antigen, then that particular autoimmune disease cannot occur. Overall, there are more than 80 different autoimmune diseases, which are a significant health problem in the elderly. Table 21.7 lists several of the most common autoimmune diseases, the antigens that are targeted, and the segment of the adaptive immune response that causes the damage.
Autoimmune Diseases
| Disease | Autoantigen | Symptoms |
|---|---|---|
| Celiac disease | Tissue transglutaminase | Damage to small intestine |
| Diabetes mellitus type I | Beta cells of pancreas | Low insulin production; inability to regulate serum glucose |
| Graves’ disease | Thyroid-stimulating hormone receptor (antibody blocks receptor) | Hyperthyroidism |
| Hashimoto’s thyroiditis | Thyroid-stimulating hormone receptor (antibody mimics hormone and stimulates receptor) | Hypothyroidism |
| Lupus erythematosus | Nuclear DNA and proteins | Damage of many body systems |
| Myasthenia gravis | Acetylcholine receptor in neuromuscular junctions | Debilitating muscle weakness |
| Rheumatoid arthritis | Joint capsule antigens | Chronic inflammation of joints |
Table 21.7
Transplantation and Cancer Immunology
- Explain why blood typing is important and what happens when mismatched blood is used in a transfusion
- Describe how tissue typing is done during organ transplantation and the role of transplant anti-rejection drugs
- Show how the immune response is able to control some cancers and how this immune response might be enhanced by cancer vaccines
The immune responses to transplanted organs and to cancer cells are both important medical issues. With the use of tissue typing and anti-rejection drugs, transplantation of organs and the control of the anti-transplant immune response have made huge strides in the past 50 years. Today, these procedures are commonplace. Tissue typing is the determination of MHC molecules in the tissue to be transplanted to better match the donor to the recipient. The immune response to cancer, on the other hand, has been more difficult to understand and control. Although it is clear that the immune system can recognize some cancers and control them, others seem to be resistant to immune mechanisms.
The Rh Factor
Red blood cells can be typed based on their surface antigens. ABO blood type, in which individuals are type A, B, AB, or O according to their genetics, is one example. A separate antigen system seen on red blood cells is the Rh antigen. When someone is “A positive” for example, the positive refers to the presence of the Rh antigen, whereas someone who is “A negative” would lack this molecule.
An interesting consequence of Rh factor expression is seen in erythroblastosis fetalis, a hemolytic disease of the newborn (Figure 21.30). This disease occurs when mothers negative for Rh antigen have multiple Rh-positive children. During the birth of a first Rh-positive child, the mother makes a primary anti-Rh antibody response to the fetal blood cells that enter the maternal bloodstream. If the mother has a second Rh-positive child, IgG antibodies against Rh-positive blood mounted during this secondary response cross the placenta and attack the fetal blood, causing anemia. This is a consequence of the fact that the fetus is not genetically identical to the mother, and thus the mother is capable of mounting an immune response against it. This disease is treated with antibodies specific for Rh factor. These are given to the mother during the first and subsequent births, destroying any fetal blood that might enter her system and preventing the immune response.
Figure 21.30 Erythroblastosis Fetalis Erythroblastosis fetalis (hemolytic disease of the newborn) is the result of an immune response in an Rh-negative mother who has multiple children with an Rh-positive father. During the first birth, fetal blood enters the mother’s circulatory system, and anti-Rh antibodies are made. During the gestation of the second child, these antibodies cross the placenta and attack the blood of the fetus. The treatment for this disease is to give the mother anti-Rh antibodies (RhoGAM) during the first pregnancy to destroy Rh-positive fetal red blood cells from entering her system and causing the anti-Rh antibody response in the first place.
Tissue Transplantation
Tissue transplantation is more complicated than blood transfusions because of two characteristics of MHC molecules. These molecules are the major cause of transplant rejection (hence the name “histocompatibility”). MHC polygeny refers to the multiple MHC proteins on cells, and MHC polymorphism refers to the multiple alleles for each individual MHC locus. Thus, there are many alleles in the human population that can be expressed (Table 21.8 and Table 21.9). When a donor organ expresses MHC molecules that are different from the recipient, the latter will often mount a cytotoxic T cell response to the organ and reject it. Histologically, if a biopsy of a transplanted organ exhibits massive infiltration of T lymphocytes within the first weeks after transplant, it is a sign that the transplant is likely to fail. The response is a classical, and very specific, primary T cell immune response. As far as medicine is concerned, the immune response in this scenario does the patient no good at all and causes significant harm.
Partial Table of Alleles of the Human MHC (Class I)
| Gene | # of alleles | # of possible MHC I protein components |
|---|---|---|
| A | 2132 | 1527 |
| B | 2798 | 2110 |
| C | 1672 | 1200 |
| E | 11 | 3 |
| F | 22 | 4 |
| G | 50 | 16 |
Table 21.8
Partial Table of Alleles of the Human MHC (Class II)
| Gene | # of alleles | # of possible MHC II protein components |
|---|---|---|
| DRA | 7 | 2 |
| DRB | 1297 | 958 |
| DQA1 | 49 | 31 |
| DQB1 | 179 | 128 |
| DPA1 | 36 | 18 |
| DPB1 | 158 | 136 |
| DMA | 7 | 4 |
| DMB | 13 | 7 |
| DOA | 12 | 3 |
| DOB | 13 | 5 |
Table 21.9
Immunosuppressive drugs such as cyclosporine A have made transplants more successful, but matching the MHC molecules is still key. In humans, there are six MHC molecules that show the most polymorphisms, three class I molecules (A, B, and C) and three class II molecules called DP, DQ, and DR. A successful transplant usually requires a match between at least 3–4 of these molecules, with more matches associated with greater success. Family members, since they share a similar genetic background, are much more likely to share MHC molecules than unrelated individuals do. In fact, due to the extensive polymorphisms in these MHC molecules, unrelated donors are found only through a worldwide database. The system is not foolproof however, as there are not enough individuals in the system to provide the organs necessary to treat all patients needing them.
One disease of transplantation occurs with bone marrow transplants, which are used to treat various diseases, including SCID and leukemia. Because the bone marrow cells being transplanted contain lymphocytes capable of mounting an immune response, and because the recipient’s immune response has been destroyed before receiving the transplant, the donor cells may attack the recipient tissues, causing graft-versus-host disease. Symptoms of this disease, which usually include a rash and damage to the liver and mucosa, are variable, and attempts have been made to moderate the disease by first removing mature T cells from the donor bone marrow before transplanting it.
Immune Responses Against Cancer
It is clear that with some cancers, for example Kaposi’s sarcoma, a healthy immune system does a good job at controlling them (Figure 21.31). This disease, which is caused by the human herpesvirus, is almost never observed in individuals with strong immune systems, such as the young and immunocompetent. Other examples of cancers caused by viruses include liver cancer caused by the hepatitis B virus and cervical cancer caused by the human papilloma virus. As these last two viruses have vaccines available for them, getting vaccinated can help prevent these two types of cancer by stimulating the immune response.
Figure 21.31 Karposi’s Sarcoma Lesions (credit: National Cancer Institute)
On the other hand, as cancer cells are often able to divide and mutate rapidly, they may escape the immune response, just as certain pathogens such as HIV do. There are three stages in the immune response to many cancers: elimination, equilibrium, and escape. Elimination occurs when the immune response first develops toward tumor-specific antigens specific to the cancer and actively kills most cancer cells, followed by a period of controlled equilibrium during which the remaining cancer cells are held in check. Unfortunately, many cancers mutate, so they no longer express any specific antigens for the immune system to respond to, and a subpopulation of cancer cells escapes the immune response, continuing the disease process.
This fact has led to extensive research in trying to develop ways to enhance the early immune response to completely eliminate the early cancer and thus prevent a later escape. One method that has shown some success is the use of cancer vaccines, which differ from viral and bacterial vaccines in that they are directed against the cells of one’s own body. Treated cancer cells are injected into cancer patients to enhance their anti-cancer immune response and thereby prolong survival. The immune system has the capability to detect these cancer cells and proliferate faster than the cancer cells do, overwhelming the cancer in a similar way as they do for viruses. Cancer vaccines have been developed for malignant melanoma, a highly fatal skin cancer, and renal (kidney) cell carcinoma. These vaccines are still in the development stages, but some positive and encouraging results have been obtained clinically.
It is tempting to focus on the complexity of the immune system and the problems it causes as a negative. The upside to immunity, however, is so much greater: The benefit of staying alive far outweighs the negatives caused when the system does sometimes go awry. Working on “autopilot,” the immune system helps to maintain your health and kill pathogens. The only time you really miss the immune response is when it is not being effective and illness results, or, as in the extreme case of HIV disease, the immune system is gone completely.
EVERYDAY CONNECTION
How Stress Affects the Immune Response: The Connections between the Immune, Nervous, and Endocrine Systems of the Body
The immune system cannot exist in isolation. After all, it has to protect the entire body from infection. Therefore, the immune system is required to interact with other organ systems, sometimes in complex ways. Thirty years of research focusing on the connections between the immune system, the central nervous system, and the endocrine system have led to a new science with the unwieldy name of called psychoneuroimmunology. The physical connections between these systems have been known for centuries: All primary and secondary organs are connected to sympathetic nerves. What is more complex, though, is the interaction of neurotransmitters, hormones, cytokines, and other soluble signaling molecules, and the mechanism of “crosstalk” between the systems. For example, white blood cells, including lymphocytes and phagocytes, have receptors for various neurotransmitters released by associated neurons. Additionally, hormones such as cortisol (naturally produced by the adrenal cortex) and prednisone (synthetic) are well known for their abilities to suppress T cell immune mechanisms, hence, their prominent use in medicine as long-term, anti-inflammatory drugs.
One well-established interaction of the immune, nervous, and endocrine systems is the effect of stress on immune health. In the human vertebrate evolutionary past, stress was associated with the fight-or-flight response, largely mediated by the central nervous system and the adrenal medulla. This stress was necessary for survival. The physical action of fighting or running, whichever the animal decides, usually resolves the problem in one way or another. On the other hand, there are no physical actions to resolve most modern day stresses, including short-term stressors like taking examinations and long-term stressors such as being unemployed or losing a spouse. The effect of stress can be felt by nearly every organ system, and the immune system is no exception (Table 21.10).
Effects of Stress on Body Systems
| System | Stress-related illness |
|---|---|
| Integumentary system | Acne, skin rashes, irritation |
| Nervous system | Headaches, depression, anxiety, irritability, loss of appetite, lack of motivation, reduced mental performance |
| Muscular and skeletal systems | Muscle and joint pain, neck and shoulder pain |
| Circulatory system | Increased heart rate, hypertension, increased probability of heart attacks |
| Digestive system | Indigestion, heartburn, stomach pain, nausea, diarrhea, constipation, weight gain or loss |
| Immune system | Depressed ability to fight infections |
| Male reproductive system | Lowered sperm production, impotence, reduced sexual desire |
| Female reproductive system | Irregular menstrual cycle, reduced sexual desire |
Table 21.10
At one time, it was assumed that all types of stress reduced all aspects of the immune response, but the last few decades of research have painted a different picture. First, most short-term stress does not impair the immune system in healthy individuals enough to lead to a greater incidence of diseases. However, older individuals and those with suppressed immune responses due to disease or immunosuppressive drugs may respond even to short-term stressors by getting sicker more often. It has been found that short-term stress diverts the body’s resources towards enhancing innate immune responses, which have the ability to act fast and would seem to help the body prepare better for possible infections associated with the trauma that may result from a fight-or-flight exchange. The diverting of resources away from the adaptive immune response, however, causes its own share of problems in fighting disease.
Chronic stress, unlike short-term stress, may inhibit immune responses even in otherwise healthy adults. The suppression of both innate and adaptive immune responses is clearly associated with increases in some diseases, as seen when individuals lose a spouse or have other long-term stresses, such as taking care of a spouse with a fatal disease or dementia. The new science of psychoneuroimmunology, while still in its relative infancy, has great potential to make exciting advances in our understanding of how the nervous, endocrine, and immune systems have evolved together and communicate with each other.
Key Terms
- active immunity
- immunity developed from an individual’s own immune system
- acute inflammation
- inflammation occurring for a limited time period; rapidly developing
- adaptive immune response
- relatively slow but very specific and effective immune response controlled by lymphocytes
- afferent lymphatic vessels
- lead into a lymph node
- antibody
- antigen-specific protein secreted by plasma cells; immunoglobulin
- antigen
- molecule recognized by the receptors of B and T lymphocytes
- antigen presentation
- binding of processed antigen to the protein-binding cleft of a major histocompatibility complex molecule
- antigen processing
- internalization and digestion of antigen in an antigen-presenting cell
- antigen receptor
- two-chain receptor by which lymphocytes recognize antigen
- antigenic determinant
- (also, epitope) one of the chemical groups recognized by a single type of lymphocyte antigen receptor
- B cells
- lymphocytes that act by differentiating into an antibody-secreting plasma cell
- barrier defenses
- antipathogen defenses deriving from a barrier that physically prevents pathogens from entering the body to establish an infection
- bone marrow
- tissue found inside bones; the site of all blood cell differentiation and maturation of B lymphocytes
- bronchus-associated lymphoid tissue (BALT)
- lymphoid nodule associated with the respiratory tract
- central tolerance
- B cell tolerance induced in immature B cells of the bone marrow
- chemokine
- soluble, long-range, cell-to-cell communication molecule
- chronic inflammation
- inflammation occurring for long periods of time
- chyle
- lipid-rich lymph inside the lymphatic capillaries of the small intestine
- cisterna chyli
- bag-like vessel that forms the beginning of the thoracic duct
- class switching
- ability of B cells to change the class of antibody they produce without altering the specificity for antigen
- clonal anergy
- process whereby B cells that react to soluble antigens in bone marrow are made nonfunctional
- clonal deletion
- removal of self-reactive B cells by inducing apoptosis
- clonal expansion
- growth of a clone of selected lymphocytes
- clonal selection
- stimulating growth of lymphocytes that have specific receptors
- clone
- group of lymphocytes sharing the same antigen receptor
- complement
- enzymatic cascade of constitutive blood proteins that have antipathogen effects, including the direct killing of bacteria
- constant region domain
- part of a lymphocyte antigen receptor that does not vary much between different receptor types
- cytokine
- soluble, short-range, cell-to-cell communication molecule
- cytotoxic T cells (Tc)
- T lymphocytes with the ability to induce apoptosis in target cells
- delayed hypersensitivity
- (type IV) T cell-mediated immune response against pathogens infiltrating interstitial tissues, causing cellular infiltrate
- early induced immune response
- includes antimicrobial proteins stimulated during the first several days of an infection
- effector T cells
- immune cells with a direct, adverse effect on a pathogen
- efferent lymphatic vessels
- lead out of a lymph node
- erythroblastosis fetalis
- disease of Rh factor-positive newborns in Rh-negative mothers with multiple Rh-positive children; resulting from the action of maternal antibodies against fetal blood
- fas ligand
- molecule expressed on cytotoxic T cells and NK cells that binds to the fas molecule on a target cell and induces it do undergo apoptosis
- Fc region
- in an antibody molecule, the site where the two termini of the heavy chains come together; many cells have receptors for this portion of the antibody, adding functionality to these molecules
- germinal centers
- clusters of rapidly proliferating B cells found in secondary lymphoid tissues
- graft-versus-host disease
- in bone marrow transplants; occurs when the transplanted cells mount an immune response against the recipient
- granzyme
- apoptosis-inducing substance contained in granules of NK cells and cytotoxic T cells
- heavy chain
- larger protein chain of an antibody
- helper T cells (Th)
- T cells that secrete cytokines to enhance other immune responses, involved in activation of both B and T cell lymphocytes
- high endothelial venules
- vessels containing unique endothelial cells specialized to allow migration of lymphocytes from the blood to the lymph node
- histamine
- vasoactive mediator in granules of mast cells and is the primary cause of allergies and anaphylactic shock
- IgA
- antibody whose dimer is secreted by exocrine glands, is especially effective against digestive and respiratory pathogens, and can pass immunity to an infant through breastfeeding
- IgD
- class of antibody whose only known function is as a receptor on naive B cells; important in B cell activation
- IgE
- antibody that binds to mast cells and causes antigen-specific degranulation during an allergic response
- IgG
- main blood antibody of late primary and early secondary responses; passed from mother to unborn child via placenta
- IgM
- antibody whose monomer is a surface receptor of naive B cells; the pentamer is the first antibody made blood plasma during primary responses
- immediate hypersensitivity
- (type I) IgE-mediated mast cell degranulation caused by crosslinking of surface IgE by antigen
- immune system
- series of barriers, cells, and soluble mediators that combine to response to infections of the body with pathogenic organisms
- immunoglobulin
- protein antibody; occurs as one of five main classes
- immunological memory
- ability of the adaptive immune response to mount a stronger and faster immune response upon re-exposure to a pathogen
- inflammation
- basic innate immune response characterized by heat, redness, pain, and swelling
- innate immune response
- rapid but relatively nonspecific immune response
- interferons
- early induced proteins made in virally infected cells that cause nearby cells to make antiviral proteins
- light chain
- small protein chain of an antibody
- lymph
- fluid contained within the lymphatic system
- lymph node
- one of the bean-shaped organs found associated with the lymphatic vessels
- lymphatic capillaries
- smallest of the lymphatic vessels and the origin of lymph flow
- lymphatic system
- network of lymphatic vessels, lymph nodes, and ducts that carries lymph from the tissues and back to the bloodstream.
- lymphatic trunks
- large lymphatics that collect lymph from smaller lymphatic vessels and empties into the blood via lymphatic ducts
- lymphocytes
- white blood cells characterized by a large nucleus and small rim of cytoplasm
- lymphoid nodules
- unencapsulated patches of lymphoid tissue found throughout the body
- macrophage
- ameboid phagocyte found in several tissues throughout the body
- macrophage oxidative metabolism
- metabolism turned on in macrophages by T cell signals that help destroy intracellular bacteria
- major histocompatibility complex (MHC)
- gene cluster whose proteins present antigens to T cells
- mast cell
- cell found in the skin and the lining of body cells that contains cytoplasmic granules with vasoactive mediators such as histamine
- memory T cells
- long-lived immune cell reserved for future exposure to an pathogen
- MHC class I
- found on most cells of the body, it binds to the CD8 molecule on T cells
- MHC class II
- found on macrophages, dendritic cells, and B cells, it binds to CD4 molecules on T cells
- MHC polygeny
- multiple MHC genes and their proteins found in body cells
- MHC polymorphism
- multiple alleles for each individual MHC locus
- monocyte
- precursor to macrophages and dendritic cells seen in the blood
- mucosa-associated lymphoid tissue (MALT)
- lymphoid nodule associated with the mucosa
- naïve lymphocyte
- mature B or T cell that has not yet encountered antigen for the first time
- natural killer cell (NK)
- cytotoxic lymphocyte of innate immune response
- negative selection
- selection against thymocytes in the thymus that react with self-antigen
- neutralization
- inactivation of a virus by the binding of specific antibody
- neutrophil
- phagocytic white blood cell recruited from the bloodstream to the site of infection via the bloodstream
- opsonization
- enhancement of phagocytosis by the binding of antibody or antimicrobial protein
- passive immunity
- transfer of immunity to a pathogen to an individual that lacks immunity to this pathogen usually by the injection of antibodies
- pattern recognition receptor (PRR)
- leukocyte receptor that binds to specific cell wall components of different bacterial species
- perforin
- molecule in NK cell and cytotoxic T cell granules that form pores in the membrane of a target cell
- peripheral tolerance
- mature B cell made tolerant by lack of T cell help
- phagocytosis
- movement of material from the outside to the inside of the cells via vesicles made from invaginations of the plasma membrane
- plasma cell
- differentiated B cell that is actively secreting antibody
- polyclonal response
- response by multiple clones to a complex antigen with many determinants
- positive selection
- selection of thymocytes within the thymus that interact with self, but not non-self, MHC molecules
- primary adaptive response
- immune system’s response to the first exposure to a pathogen
- primary lymphoid organ
- site where lymphocytes mature and proliferate; red bone marrow and thymus gland
- psychoneuroimmunology
- study of the connections between the immune, nervous, and endocrine systems
- regulatory T cells (Treg)
- (also, suppressor T cells) class of CD4 T cells that regulates other T cell responses
- right lymphatic duct
- drains lymph fluid from the upper right side of body into the right subclavian vein
- secondary adaptive response
- immune response observed upon re-exposure to a pathogen, which is stronger and faster than a primary response
- secondary lymphoid organs
- sites where lymphocytes mount adaptive immune responses; examples include lymph nodes and spleen
- sensitization
- first exposure to an antigen
- seroconversion
- clearance of pathogen in the serum and the simultaneous rise of serum antibody
- severe combined immunodeficiency disease (SCID)
- genetic mutation that affects both T cell and B cell arms of the immune response
- spleen
- secondary lymphoid organ that filters pathogens from the blood (white pulp) and removes degenerating or damaged blood cells (red pulp)
- T cell
- lymphocyte that acts by secreting molecules that regulate the immune system or by causing the destruction of foreign cells, viruses, and cancer cells
- T cell tolerance
- process during T cell differentiation where most T cells that recognize antigens from one’s own body are destroyed
- T cell-dependent antigen
- antigen that binds to B cells, which requires signals from T cells to make antibody
- T cell-independent antigen
- binds to B cells, which do not require signals from T cells to make antibody
- Th1 cells
- cells that secrete cytokines that enhance the activity of macrophages and other cells
- Th2 cells
- cells that secrete cytokines that induce B cells to differentiate into antibody-secreting plasma cells
- thoracic duct
- large duct that drains lymph from the lower limbs, left thorax, left upper limb, and the left side of the head
- thymocyte
- immature T cell found in the thymus
- thymus
- primary lymphoid organ; where T lymphocytes proliferate and mature
- tissue typing
- typing of MHC molecules between a recipient and donor for use in a potential transplantation procedure
- tonsils
- lymphoid nodules associated with the nasopharynx
- type I hypersensitivity
- immediate response mediated by mast cell degranulation caused by the crosslinking of the antigen-specific IgE molecules on the mast cell surface
- type II hypersensitivity
- cell damage caused by the binding of antibody and the activation of complement, usually against red blood cells
- type III hypersensitivity
- damage to tissues caused by the deposition of antibody-antigen (immune) complexes followed by the activation of complement
- variable region domain
- part of a lymphocyte antigen receptor that varies considerably between different receptor types
Chapter Review
21.1 Anatomy of the Lymphatic and Immune Systems
The lymphatic system is a series of vessels, ducts, and trunks that remove interstitial fluid from the tissues and return it the blood. The lymphatics are also used to transport dietary lipids and cells of the immune system. Cells of the immune system all come from the hematopoietic system of the bone marrow. Primary lymphoid organs, the bone marrow and thymus gland, are the locations where lymphocytes of the adaptive immune system proliferate and mature. Secondary lymphoid organs are site in which mature lymphocytes congregate to mount immune responses. Many immune system cells use the lymphatic and circulatory systems for transport throughout the body to search for and then protect against pathogens.
21.2 Barrier Defenses and the Innate Immune Response
Innate immune responses are critical to the early control of infections. Whereas barrier defenses are the body’s first line of physical defense against pathogens, innate immune responses are the first line of physiological defense. Innate responses occur rapidly, but with less specificity and effectiveness than the adaptive immune response. Innate responses can be caused by a variety of cells, mediators, and antibacterial proteins such as complement. Within the first few days of an infection, another series of antibacterial proteins are induced, each with activities against certain bacteria, including opsonization of certain species. Additionally, interferons are induced that protect cells from viruses in their vicinity. Finally, the innate immune response does not stop when the adaptive immune response is developed. In fact, both can cooperate and one can influence the other in their responses against pathogens.
21.3 The Adaptive Immune Response: T lymphocytes and Their Functional Types
T cells recognize antigens with their antigen receptor, a complex of two protein chains on their surface. They do not recognize self-antigens, however, but only processed antigen presented on their surfaces in a binding groove of a major histocompatibility complex molecule. T cells develop in the thymus, where they learn to use self-MHC molecules to recognize only foreign antigens, thus making them tolerant to self-antigens. There are several functional types of T lymphocytes, the major ones being helper, regulatory, and cytotoxic T cells.
21.4 The Adaptive Immune Response: B-lymphocytes and Antibodies
B cells, which develop within the bone marrow, are responsible for making five different classes of antibodies, each with its own functions. B cells have their own mechanisms for tolerance, but in peripheral tolerance, the B cells that leave the bone marrow remain inactive due to T cell tolerance. Some B cells do not need T cell cytokines to make antibody, and they bypass this need by the crosslinking of their surface immunoglobulin by repeated carbohydrate residues found in the cell walls of many bacterial species. Others require T cells to become activated.
21.5 The Immune Response against Pathogens
Early childhood is a time when the body develops much of its immunological memory that protects it from diseases in adulthood. The components of the immune response that have the maximum effectiveness against a pathogen are often associated with the class of pathogen involved. Bacteria and fungi are especially susceptible to damage by complement proteins, whereas viruses are taken care of by interferons and cytotoxic T cells. Worms are attacked by eosinophils. Pathogens have shown the ability, however, to evade the body’s immune responses, some leading to chronic infections or even death. The immune system and pathogens are in a slow, evolutionary race to see who stays on top. Modern medicine, hopefully, will keep the results skewed in humans’ favor.
21.6 Diseases Associated with Depressed or Overactive Immune Responses
The immune response can be under-reactive or over-reactive. Suppressed immunity can result from inherited genetic defects or by acquiring viruses. Over-reactive immune responses include the hypersensitivities: B cell- and T cell-mediated immune responses designed to control pathogens, but that lead to symptoms or medical complications. The worst cases of over-reactive immune responses are autoimmune diseases, where an individual’s immune system attacks his or her own body because of the breakdown of immunological tolerance. These diseases are more common in the aged, so treating them will be a challenge in the future as the aged population in the world increases.
21.7 Transplantation and Cancer Immunology
Blood transfusion and organ transplantation both require an understanding of the immune response to prevent medical complications. Blood needs to be typed so that natural antibodies against mismatched blood will not destroy it, causing more harm than good to the recipient. Transplanted organs must be matched by their MHC molecules and, with the use of immunosuppressive drugs, can be successful even if an exact tissue match cannot be made. Another aspect to the immune response is its ability to control and eradicate cancer. Although this has been shown to occur with some rare cancers and those caused by known viruses, the normal immune response to most cancers is not sufficient to control cancer growth. Thus, cancer vaccines designed to enhance these immune responses show promise for certain types of cancer.
Interactive Link Questions
Visit this website for an overview of the lymphatic system. What are the three main components of the lymphatic system?
2.Visit this website to learn about the many different cell types in the immune system and their very specialized jobs. What is the role of the dendritic cell in infection by HIV?
3.Visit this website to learn about phagocyte chemotaxis. Phagocyte chemotaxis is the movement of phagocytes according to the secretion of chemical messengers in the form of interleukins and other chemokines. By what means does a phagocyte destroy a bacterium that it has ingested?
4.Immunity can be acquired in an active or passive way, and it can be natural or artificial. Watch this video to see an animated discussion of passive and active immunity. What is an example of natural immunity acquired passively?
Review Questions
Which of the following cells is phagocytic?
- plasma cell
- macrophage
- B cell
- NK cell
Which structure allows lymph from the lower right limb to enter the bloodstream?
- thoracic duct
- right lymphatic duct
- right lymphatic trunk
- left lymphatic trunk
Which of the following cells is important in the innate immune response?
- B cells
- T cells
- macrophages
- plasma cells
Which of the following cells would be most active in early, antiviral immune responses the first time one is exposed to pathogen?
- macrophage
- T cell
- neutrophil
- natural killer cell
Which of the lymphoid nodules is most likely to see food antigens first?
- tonsils
- Peyer’s patches
- bronchus-associated lymphoid tissue
- mucosa-associated lymphoid tissue
Which of the following signs is not characteristic of inflammation?
- redness
- pain
- cold
- swelling
Which of the following is not important in the antiviral innate immune response?
- interferons
- natural killer cells
- complement
- microphages
Enhanced phagocytosis of a cell by the binding of a specific protein is called ________.
- endocytosis
- opsonization
- anaphylaxis
- complement activation
Which of the following leads to the redness of inflammation?
- increased vascular permeability
- anaphylactic shock
- increased blood flow
- complement activation
T cells that secrete cytokines that help antibody responses are called ________.
- Th1
- Th2
- regulatory T cells
- thymocytes
The taking in of antigen and digesting it for later presentation is called ________.
- antigen presentation
- antigen processing
- endocytosis
- exocytosis
Why is clonal expansion so important?
- to select for specific cells
- to secrete cytokines
- to kill target cells
- to increase the numbers of specific cells
The elimination of self-reactive thymocytes is called ________.
- positive selection.
- negative selection.
- tolerance.
- clonal selection.
Which type of T cell is most effective against viruses?
- Th1
- Th2
- cytotoxic T cells
- regulatory T cells
Removing functionality from a B cell without killing it is called ________.
- clonal selection
- clonal expansion
- clonal deletion
- clonal anergy
Which class of antibody crosses the placenta in pregnant women?
- IgM
- IgA
- IgE
- IgG
Which class of antibody has no known function other than as an antigen receptor?
- IgM
- IgA
- IgE
- IgD
When does class switching occur?
- primary response
- secondary response
- tolerance
- memory response
Which class of antibody is found in mucus?
- IgM
- IgA
- IgE
- IgD
Which enzymes in macrophages are important for clearing intracellular bacteria?
- metabolic
- mitochondrial
- nuclear
- lysosomal
What type of chronic lung disease is caused by a Mycobacterium?
- asthma
- emphysema
- tuberculosis
- leprosy
Which type of immune response is most directly effective against bacteria?
- natural killer cells
- complement
- cytotoxic T cells
- helper T cells
What is the reason that you have to be immunized with a new influenza vaccine each year?
- the vaccine is only protective for a year
- mutation
- macrophage oxidative metabolism
- memory response
Which type of immune response works in concert with cytotoxic T cells against virally infected cells?
- natural killer cells
- complement
- antibodies
- memory
Which type of hypersensitivity involves soluble antigen-antibody complexes?
- type I
- type II
- type III
- type IV
What causes the delay in delayed hypersensitivity?
- inflammation
- cytokine release
- recruitment of immune cells
- histamine release
Which of the following is a critical feature of immediate hypersensitivity?
- inflammation
- cytotoxic T cells
- recruitment of immune cells
- histamine release
Which of the following is an autoimmune disease of the heart?
- rheumatoid arthritis
- lupus
- rheumatic fever
- Hashimoto’s thyroiditis
What drug is used to counteract the effects of anaphylactic shock?
- epinephrine
- antihistamines
- antibiotics
- aspirin
Which of the following terms means “many genes”?
- polymorphism
- polygeny
- polypeptide
- multiple alleles
Why do we have natural antibodies?
- We don’t know why.
- immunity to environmental bacteria
- immunity to transplants
- from clonal selection
Which type of cancer is associated with HIV disease?
- Kaposi’s sarcoma
- melanoma
- lymphoma
- renal cell carcinoma
How does cyclosporine A work?
- suppresses antibodies
- suppresses T cells
- suppresses macrophages
- suppresses neutrophils
What disease is associated with bone marrow transplants?
- diabetes mellitus type I
- melanoma
- headache
- graft-versus-host disease
Critical Thinking Questions
Describe the flow of lymph from its origins in interstitial fluid to its emptying into the venous bloodstream.
40.Describe the process of inflammation in an area that has been traumatized, but not infected.
41.Describe two early induced responses and what pathogens they affect.
42.Describe the processing and presentation of an intracellular antigen.
43.Describe clonal selection and expansion.
44.Describe how secondary B cell responses are developed.
45.Describe the role of IgM in immunity.
46.Describe how seroconversion works in HIV disease.
47.Describe tuberculosis and the innocent bystander effect.
48.Describe anaphylactic shock in someone sensitive to peanuts?
49.Describe rheumatic fever and how tolerance is broken.
50.Describe how stress affects immune responses.
|
oercommons
|
2025-03-18T00:39:10.715573
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/58767/overview",
"title": "Anatomy and Physiology, Fluids and Transport",
"author": null
}
|
https://oercommons.org/courseware/lesson/56375/overview
|
The Nervous System and Nervous Tissue
Introduction
Figure 12.1 Robotic Arms Playing Foosball As the neural circuitry of the nervous system has become more fully understood and robotics more sophisticated, it is now possible to integrate technology with the body and restore abilities following traumatic events. At some point in the future, will this type of technology lead to the ability to augment our nervous systems? (credit: U.S. Army/Wikimedia Commons)
CHAPTER OBJECTIVES
After studying this chapter, you will be able to:
- Name the major divisions of the nervous system, both anatomical and functional
- Describe the functional and structural differences between gray matter and white matter structures
- Name the parts of the multipolar neuron in order of polarity
- List the types of glial cells and assign each to the proper division of the nervous system, along with their function(s)
- Distinguish the major functions of the nervous system: sensation, integration, and response
- Describe the components of the membrane that establish the resting membrane potential
- Describe the changes that occur to the membrane that result in the action potential
- Explain the differences between types of graded potentials
- Categorize the major neurotransmitters by chemical type and effect
The nervous system is a very complex organ system. In Peter D. Kramer’s book Listening to Prozac, a pharmaceutical researcher is quoted as saying, “If the human brain were simple enough for us to understand, we would be too simple to understand it” (1994). That quote is from the early 1990s; in the two decades since, progress has continued at an amazing rate within the scientific disciplines of neuroscience. It is an interesting conundrum to consider that the complexity of the nervous system may be too complex for it (that is, for us) to completely unravel. But our current level of understanding is probably nowhere close to that limit.
One easy way to begin to understand the structure of the nervous system is to start with the large divisions and work through to a more in-depth understanding. In other chapters, the finer details of the nervous system will be explained, but first looking at an overview of the system will allow you to begin to understand how its parts work together. The focus of this chapter is on nervous (neural) tissue, both its structure and its function. But before you learn about that, you will see a big picture of the system—actually, a few big pictures.
12.1 Basic Structure and Function of the Nervous System
- Identify the anatomical and functional divisions of the nervous system
- Relate the functional and structural differences between gray matter and white matter structures of the nervous system to the structure of neurons
- List the basic functions of the nervous system
The picture you have in your mind of the nervous system probably includes the brain, the nervous tissue contained within the cranium, and the spinal cord, the extension of nervous tissue within the vertebral column. That suggests it is made of two organs—and you may not even think of the spinal cord as an organ—but the nervous system is a very complex structure. Within the brain, many different and separate regions are responsible for many different and separate functions. It is as if the nervous system is composed of many organs that all look similar and can only be differentiated using tools such as the microscope or electrophysiology. In comparison, it is easy to see that the stomach is different than the esophagus or the liver, so you can imagine the digestive system as a collection of specific organs.
The Central and Peripheral Nervous Systems
The nervous system can be divided into two major regions: the central and peripheral nervous systems. The central nervous system (CNS) is the brain and spinal cord, and the peripheral nervous system (PNS) is everything else (Figure 12.2). The brain is contained within the cranial cavity of the skull, and the spinal cord is contained within the vertebral cavity of the vertebral column. It is a bit of an oversimplification to say that the CNS is what is inside these two cavities and the peripheral nervous system is outside of them, but that is one way to start to think about it. In actuality, there are some elements of the peripheral nervous system that are within the cranial or vertebral cavities. The peripheral nervous system is so named because it is on the periphery—meaning beyond the brain and spinal cord. Depending on different aspects of the nervous system, the dividing line between central and peripheral is not necessarily universal.
Figure 12.2 Central and Peripheral Nervous System The structures of the PNS are referred to as ganglia and nerves, which can be seen as distinct structures. The equivalent structures in the CNS are not obvious from this overall perspective and are best examined in prepared tissue under the microscope.
Nervous tissue, present in both the CNS and PNS, contains two basic types of cells: neurons and glial cells. A glial cell is one of a variety of cells that provide a framework of tissue that supports the neurons and their activities. The neuron is the more functionally important of the two, in terms of the communicative function of the nervous system. To describe the functional divisions of the nervous system, it is important to understand the structure of a neuron. Neurons are cells and therefore have a soma, or cell body, but they also have extensions of the cell; each extension is generally referred to as a process. There is one important process that every neuron has called an axon, which is the fiber that connects a neuron with its target. Another type of process that branches off from the soma is the dendrite. Dendrites are responsible for receiving most of the input from other neurons. Looking at nervous tissue, there are regions that predominantly contain cell bodies and regions that are largely composed of just axons. These two regions within nervous system structures are often referred to as gray matter (the regions with many cell bodies and dendrites) or white matter (the regions with many axons). Figure 12.3 demonstrates the appearance of these regions in the brain and spinal cord. The colors ascribed to these regions are what would be seen in “fresh,” or unstained, nervous tissue. Gray matter is not necessarily gray. It can be pinkish because of blood content, or even slightly tan, depending on how long the tissue has been preserved. But white matter is white because axons are insulated by a lipid-rich substance called myelin. Lipids can appear as white (“fatty”) material, much like the fat on a raw piece of chicken or beef. Actually, gray matter may have that color ascribed to it because next to the white matter, it is just darker—hence, gray.
The distinction between gray matter and white matter is most often applied to central nervous tissue, which has large regions that can be seen with the unaided eye. When looking at peripheral structures, often a microscope is used and the tissue is stained with artificial colors. That is not to say that central nervous tissue cannot be stained and viewed under a microscope, but unstained tissue is most likely from the CNS—for example, a frontal section of the brain or cross section of the spinal cord.
Figure 12.3 Gray Matter and White Matter A brain removed during an autopsy, with a partial section removed, shows white matter surrounded by gray matter. Gray matter makes up the outer cortex of the brain. (credit: modification of work by “Suseno”/Wikimedia Commons)
Regardless of the appearance of stained or unstained tissue, the cell bodies of neurons or axons can be located in discrete anatomical structures that need to be named. Those names are specific to whether the structure is central or peripheral. A localized collection of neuron cell bodies in the CNS is referred to as a nucleus. In the PNS, a cluster of neuron cell bodies is referred to as a ganglion. Figure 12.4 indicates how the term nucleus has a few different meanings within anatomy and physiology. It is the center of an atom, where protons and neutrons are found; it is the center of a cell, where the DNA is found; and it is a center of some function in the CNS. There is also a potentially confusing use of the word ganglion (plural = ganglia) that has a historical explanation. In the central nervous system, there is a group of nuclei that are connected together and were once called the basal ganglia before “ganglion” became accepted as a description for a peripheral structure. Some sources refer to this group of nuclei as the “basal nuclei” to avoid confusion.
Figure 12.4 What Is a Nucleus? (a) The nucleus of an atom contains its protons and neutrons. (b) The nucleus of a cell is the organelle that contains DNA. (c) A nucleus in the CNS is a localized center of function with the cell bodies of several neurons, shown here circled in red. (credit c: “Was a bee”/Wikimedia Commons)
Terminology applied to bundles of axons also differs depending on location. A bundle of axons, or fibers, found in the CNS is called a tract whereas the same thing in the PNS would be called a nerve. There is an important point to make about these terms, which is that they can both be used to refer to the same bundle of axons. When those axons are in the PNS, the term is nerve, but if they are CNS, the term is tract. The most obvious example of this is the axons that project from the retina into the brain. Those axons are called the optic nerve as they leave the eye, but when they are inside the cranium, they are referred to as the optic tract. There is a specific place where the name changes, which is the optic chiasm, but they are still the same axons (Figure 12.5). A similar situation outside of science can be described for some roads. Imagine a road called “Broad Street” in a town called “Anyville.” The road leaves Anyville and goes to the next town over, called “Hometown.” When the road crosses the line between the two towns and is in Hometown, its name changes to “Main Street.” That is the idea behind the naming of the retinal axons. In the PNS, they are called the optic nerve, and in the CNS, they are the optic tract. Table 12.1 helps to clarify which of these terms apply to the central or peripheral nervous systems.
Figure 12.5 Optic Nerve Versus Optic Tract This drawing of the connections of the eye to the brain shows the optic nerve extending from the eye to the chiasm, where the structure continues as the optic tract. The same axons extend from the eye to the brain through these two bundles of fibers, but the chiasm represents the border between peripheral and central.
INTERACTIVE LINK
In 2003, the Nobel Prize in Physiology or Medicine was awarded to Paul C. Lauterbur and Sir Peter Mansfield for discoveries related to magnetic resonance imaging (MRI). This is a tool to see the structures of the body (not just the nervous system) that depends on magnetic fields associated with certain atomic nuclei. The utility of this technique in the nervous system is that fat tissue and water appear as different shades between black and white. Because white matter is fatty (from myelin) and gray matter is not, they can be easily distinguished in MRI images. Try this PhET simulation that demonstrates the use of this technology and compares it with other types of imaging technologies. Also, the results from an MRI session are compared with images obtained from X-ray or computed tomography. How do the imaging techniques shown in this game indicate the separation of white and gray matter compared with the freshly dissected tissue shown earlier?
Structures of the CNS and PNS
| CNS | PNS | |
|---|---|---|
| Group of Neuron Cell Bodies (i.e., gray matter) | Nucleus | Ganglion |
| Bundle of Axons (i.e., white matter) | Tract | Nerve |
Table12.1
Functional Divisions of the Nervous System
The nervous system can also be divided on the basis of its functions, but anatomical divisions and functional divisions are different. The CNS and the PNS both contribute to the same functions, but those functions can be attributed to different regions of the brain (such as the cerebral cortex or the hypothalamus) or to different ganglia in the periphery. The problem with trying to fit functional differences into anatomical divisions is that sometimes the same structure can be part of several functions. For example, the optic nerve carries signals from the retina that are either used for the conscious perception of visual stimuli, which takes place in the cerebral cortex, or for the reflexive responses of smooth muscle tissue that are processed through the hypothalamus.
There are two ways to consider how the nervous system is divided functionally. First, the basic functions of the nervous system are sensation, integration, and response. Secondly, control of the body can be somatic or autonomic—divisions that are largely defined by the structures that are involved in the response. There is also a region of the peripheral nervous system that is called the enteric nervous system that is responsible for a specific set of the functions within the realm of autonomic control related to gastrointestinal functions.
Basic Functions
The nervous system is involved in receiving information about the environment around us (sensation) and generating responses to that information (motor responses). The nervous system can be divided into regions that are responsible for sensation (sensory functions) and for the response (motor functions). But there is a third function that needs to be included. Sensory input needs to be integrated with other sensations, as well as with memories, emotional state, or learning (cognition). Some regions of the nervous system are termed integration or association areas. The process of integration combines sensory perceptions and higher cognitive functions such as memories, learning, and emotion to produce a response.
Sensation. The first major function of the nervous system is sensation—receiving information about the environment to gain input about what is happening outside the body (or, sometimes, within the body). The sensory functions of the nervous system register the presence of a change from homeostasis or a particular event in the environment, known as a stimulus. The senses we think of most are the “big five”: taste, smell, touch, sight, and hearing. The stimuli for taste and smell are both chemical substances (molecules, compounds, ions, etc.), touch is physical or mechanical stimuli that interact with the skin, sight is light stimuli, and hearing is the perception of sound, which is a physical stimulus similar to some aspects of touch. There are actually more senses than just those, but that list represents the major senses. Those five are all senses that receive stimuli from the outside world, and of which there is conscious perception. Additional sensory stimuli might be from the internal environment (inside the body), such as the stretch of an organ wall or the concentration of certain ions in the blood.
Response. The nervous system produces a response on the basis of the stimuli perceived by sensory structures. An obvious response would be the movement of muscles, such as withdrawing a hand from a hot stove, but there are broader uses of the term. The nervous system can cause the contraction of all three types of muscle tissue. For example, skeletal muscle contracts to move the skeleton, cardiac muscle is influenced as heart rate increases during exercise, and smooth muscle contracts as the digestive system moves food along the digestive tract. Responses also include the neural control of glands in the body as well, such as the production and secretion of sweat by the eccrine and merocrine sweat glands found in the skin to lower body temperature.
Responses can be divided into those that are voluntary or conscious (contraction of skeletal muscle) and those that are involuntary (contraction of smooth muscles, regulation of cardiac muscle, activation of glands). Voluntary responses are governed by the somatic nervous system and involuntary responses are governed by the autonomic nervous system, which are discussed in the next section.
Integration. Stimuli that are received by sensory structures are communicated to the nervous system where that information is processed. This is called integration. Stimuli are compared with, or integrated with, other stimuli, memories of previous stimuli, or the state of a person at a particular time. This leads to the specific response that will be generated. Seeing a baseball pitched to a batter will not automatically cause the batter to swing. The trajectory of the ball and its speed will need to be considered. Maybe the count is three balls and one strike, and the batter wants to let this pitch go by in the hope of getting a walk to first base. Or maybe the batter’s team is so far ahead, it would be fun to just swing away.
Controlling the Body
The nervous system can be divided into two parts mostly on the basis of a functional difference in responses. The somatic nervous system (SNS) is responsible for conscious perception and voluntary motor responses. Voluntary motor response means the contraction of skeletal muscle, but those contractions are not always voluntary in the sense that you have to want to perform them. Some somatic motor responses are reflexes, and often happen without a conscious decision to perform them. If your friend jumps out from behind a corner and yells “Boo!” you will be startled and you might scream or leap back. You didn’t decide to do that, and you may not have wanted to give your friend a reason to laugh at your expense, but it is a reflex involving skeletal muscle contractions. Other motor responses become automatic (in other words, unconscious) as a person learns motor skills (referred to as “habit learning” or “procedural memory”).
The autonomic nervous system (ANS) is responsible for involuntary control of the body, usually for the sake of homeostasis (regulation of the internal environment). Sensory input for autonomic functions can be from sensory structures tuned to external or internal environmental stimuli. The motor output extends to smooth and cardiac muscle as well as glandular tissue. The role of the autonomic system is to regulate the organ systems of the body, which usually means to control homeostasis. Sweat glands, for example, are controlled by the autonomic system. When you are hot, sweating helps cool your body down. That is a homeostatic mechanism. But when you are nervous, you might start sweating also. That is not homeostatic, it is the physiological response to an emotional state.
There is another division of the nervous system that describes functional responses. The enteric nervous system (ENS) is responsible for controlling the smooth muscle and glandular tissue in your digestive system. It is a large part of the PNS, and is not dependent on the CNS. It is sometimes valid, however, to consider the enteric system to be a part of the autonomic system because the neural structures that make up the enteric system are a component of the autonomic output that regulates digestion. There are some differences between the two, but for our purposes here there will be a good bit of overlap. See Figure 12.6 for examples of where these divisions of the nervous system can be found.
Figure 12.6 Somatic, Autonomic, and Enteric Structures of the Nervous System Somatic structures include the spinal nerves, both motor and sensory fibers, as well as the sensory ganglia (posterior root ganglia and cranial nerve ganglia). Autonomic structures are found in the nerves also, but include the sympathetic and parasympathetic ganglia. The enteric nervous system includes the nervous tissue within the organs of the digestive tract.
INTERACTIVE LINK
Visit this site to read about a woman that notices that her daughter is having trouble walking up the stairs. This leads to the discovery of a hereditary condition that affects the brain and spinal cord. The electromyography and MRI tests indicated deficiencies in the spinal cord and cerebellum, both of which are responsible for controlling coordinated movements. To what functional division of the nervous system would these structures belong?
EVERYDAY CONNECTION
How Much of Your Brain Do You Use?
Have you ever heard the claim that humans only use 10 percent of their brains? Maybe you have seen an advertisement on a website saying that there is a secret to unlocking the full potential of your mind—as if there were 90 percent of your brain sitting idle, just waiting for you to use it. If you see an ad like that, don’t click. It isn’t true.
An easy way to see how much of the brain a person uses is to take measurements of brain activity while performing a task. An example of this kind of measurement is functional magnetic resonance imaging (fMRI), which generates a map of the most active areas and can be generated and presented in three dimensions (Figure 12.7). This procedure is different from the standard MRI technique because it is measuring changes in the tissue in time with an experimental condition or event.
Figure 12.7 fMRI This fMRI shows activation of the visual cortex in response to visual stimuli. (credit: “Superborsuk”/Wikimedia Commons)
The underlying assumption is that active nervous tissue will have greater blood flow. By having the subject perform a visual task, activity all over the brain can be measured. Consider this possible experiment: the subject is told to look at a screen with a black dot in the middle (a fixation point). A photograph of a face is projected on the screen away from the center. The subject has to look at the photograph and decipher what it is. The subject has been instructed to push a button if the photograph is of someone they recognize. The photograph might be of a celebrity, so the subject would press the button, or it might be of a random person unknown to the subject, so the subject would not press the button.
In this task, visual sensory areas would be active, integrating areas would be active, motor areas responsible for moving the eyes would be active, and motor areas for pressing the button with a finger would be active. Those areas are distributed all around the brain and the fMRI images would show activity in more than just 10 percent of the brain (some evidence suggests that about 80 percent of the brain is using energy—based on blood flow to the tissue—during well-defined tasks similar to the one suggested above). This task does not even include all of the functions the brain performs. There is no language response, the body is mostly lying still in the MRI machine, and it does not consider the autonomic functions that would be ongoing in the background.
Nervous Tissue
- Describe the basic structure of a neuron
- Identify the different types of neurons on the basis of polarity
- List the glial cells of the CNS and describe their function
- List the glial cells of the PNS and describe their function
Nervous tissue is composed of two types of cells, neurons and glial cells. Neurons are the primary type of cell that most anyone associates with the nervous system. They are responsible for the computation and communication that the nervous system provides. They are electrically active and release chemical signals to target cells. Glial cells, or glia, are known to play a supporting role for nervous tissue. Ongoing research pursues an expanded role that glial cells might play in signaling, but neurons are still considered the basis of this function. Neurons are important, but without glial support they would not be able to perform their function.
Neurons
Neurons are the cells considered to be the basis of nervous tissue. They are responsible for the electrical signals that communicate information about sensations, and that produce movements in response to those stimuli, along with inducing thought processes within the brain. An important part of the function of neurons is in their structure, or shape. The three-dimensional shape of these cells makes the immense numbers of connections within the nervous system possible.
Parts of a Neuron
As you learned in the first section, the main part of a neuron is the cell body, which is also known as the soma (soma = “body”). The cell body contains the nucleus and most of the major organelles. But what makes neurons special is that they have many extensions of their cell membranes, which are generally referred to as processes. Neurons are usually described as having one, and only one, axon—a fiber that emerges from the cell body and projects to target cells. That single axon can branch repeatedly to communicate with many target cells. It is the axon that propagates the nerve impulse, which is communicated to one or more cells. The other processes of the neuron are dendrites, which receive information from other neurons at specialized areas of contact called synapses. The dendrites are usually highly branched processes, providing locations for other neurons to communicate with the cell body. Information flows through a neuron from the dendrites, across the cell body, and down the axon. This gives the neuron a polarity—meaning that information flows in this one direction. Figure 12.8 shows the relationship of these parts to one another.
Figure 12.8 Parts of a Neuron The major parts of the neuron are labeled on a multipolar neuron from the CNS.
Where the axon emerges from the cell body, there is a special region referred to as the axon hillock. This is a tapering of the cell body toward the axon fiber. Within the axon hillock, the cytoplasm changes to a solution of limited components called axoplasm. Because the axon hillock represents the beginning of the axon, it is also referred to as the initial segment.
Many axons are wrapped by an insulating substance called myelin, which is actually made from glial cells. Myelin acts as insulation much like the plastic or rubber that is used to insulate electrical wires. A key difference between myelin and the insulation on a wire is that there are gaps in the myelin covering of an axon. Each gap is called a node of Ranvier and is important to the way that electrical signals travel down the axon. The length of the axon between each gap, which is wrapped in myelin, is referred to as an axon segment. At the end of the axon is the axon terminal, where there are usually several branches extending toward the target cell, each of which ends in an enlargement called a synaptic end bulb. These bulbs are what make the connection with the target cell at the synapse.
INTERACTIVE LINK
Visit this site to learn about how nervous tissue is composed of neurons and glial cells. Neurons are dynamic cells with the ability to make a vast number of connections, to respond incredibly quickly to stimuli, and to initiate movements on the basis of those stimuli. They are the focus of intense research because failures in physiology can lead to devastating illnesses. Why are neurons only found in animals? Based on what this article says about neuron function, why wouldn't they be helpful for plants or microorganisms?
Types of Neurons
There are many neurons in the nervous system—a number in the trillions. And there are many different types of neurons. They can be classified by many different criteria. The first way to classify them is by the number of processes attached to the cell body. Using the standard model of neurons, one of these processes is the axon, and the rest are dendrites. Because information flows through the neuron from dendrites or cell bodies toward the axon, these names are based on the neuron's polarity (Figure 12.9).
Figure 12.9 Neuron Classification by Shape Unipolar cells have one process that includes both the axon and dendrite. Bipolar cells have two processes, the axon and a dendrite. Multipolar cells have more than two processes, the axon and two or more dendrites.
Unipolar cells have only one process emerging from the cell. True unipolar cells are only found in invertebrate animals, so the unipolar cells in humans are more appropriately called “pseudo-unipolar” cells. Invertebrate unipolar cells do not have dendrites. Human unipolar cells have an axon that emerges from the cell body, but it splits so that the axon can extend along a very long distance. At one end of the axon are dendrites, and at the other end, the axon forms synaptic connections with a target. Unipolar cells are exclusively sensory neurons and have two unique characteristics. First, their dendrites are receiving sensory information, sometimes directly from the stimulus itself. Secondly, the cell bodies of unipolar neurons are always found in ganglia. Sensory reception is a peripheral function (those dendrites are in the periphery, perhaps in the skin) so the cell body is in the periphery, though closer to the CNS in a ganglion. The axon projects from the dendrite endings, past the cell body in a ganglion, and into the central nervous system.
Bipolar cells have two processes, which extend from each end of the cell body, opposite to each other. One is the axon and one the dendrite. Bipolar cells are not very common. They are found mainly in the olfactory epithelium (where smell stimuli are sensed), and as part of the retina.
Multipolar neurons are all of the neurons that are not unipolar or bipolar. They have one axon and two or more dendrites (usually many more). With the exception of the unipolar sensory ganglion cells, and the two specific bipolar cells mentioned above, all other neurons are multipolar. Some cutting edge research suggests that certain neurons in the CNS do not conform to the standard model of “one, and only one” axon. Some sources describe a fourth type of neuron, called an anaxonic neuron. The name suggests that it has no axon (an- = “without”), but this is not accurate. Anaxonic neurons are very small, and if you look through a microscope at the standard resolution used in histology (approximately 400X to 1000X total magnification), you will not be able to distinguish any process specifically as an axon or a dendrite. Any of those processes can function as an axon depending on the conditions at any given time. Nevertheless, even if they cannot be easily seen, and one specific process is definitively the axon, these neurons have multiple processes and are therefore multipolar.
Neurons can also be classified on the basis of where they are found, who found them, what they do, or even what chemicals they use to communicate with each other. Some neurons referred to in this section on the nervous system are named on the basis of those sorts of classifications (Figure 12.10). For example, a multipolar neuron that has a very important role to play in a part of the brain called the cerebellum is known as a Purkinje (commonly pronounced per-KIN-gee) cell. It is named after the anatomist who discovered it (Jan Evangilista Purkinje, 1787–1869).
Figure 12.10 Other Neuron Classifications Three examples of neurons that are classified on the basis of other criteria. (a) The pyramidal cell is a multipolar cell with a cell body that is shaped something like a pyramid. (b) The Purkinje cell in the cerebellum was named after the scientist who originally described it. (c) Olfactory neurons are named for the functional group with which they belong.
Glial Cells
Glial cells, or neuroglia or simply glia, are the other type of cell found in nervous tissue. They are considered to be supporting cells, and many functions are directed at helping neurons complete their function for communication. The name glia comes from the Greek word that means “glue,” and was coined by the German pathologist Rudolph Virchow, who wrote in 1856: “This connective substance, which is in the brain, the spinal cord, and the special sense nerves, is a kind of glue (neuroglia) in which the nervous elements are planted.” Today, research into nervous tissue has shown that there are many deeper roles that these cells play. And research may find much more about them in the future.
There are six types of glial cells. Four of them are found in the CNS and two are found in the PNS. Table 12.2 outlines some common characteristics and functions.
Glial Cell Types by Location and Basic Function
| CNS glia | PNS glia | Basic function |
|---|---|---|
| Astrocyte | Satellite cell | Support |
| Oligodendrocyte | Schwann cell | Insulation, myelination |
| Microglia | - | Immune surveillance and phagocytosis |
| Ependymal cell | - | Creating CSF |
Table12.2
Glial Cells of the CNS
One cell providing support to neurons of the CNS is the astrocyte, so named because it appears to be star-shaped under the microscope (astro- = “star”). Astrocytes have many processes extending from their main cell body (not axons or dendrites like neurons, just cell extensions). Those processes extend to interact with neurons, blood vessels, or the connective tissue covering the CNS that is called the pia mater (Figure 12.11). Generally, they are supporting cells for the neurons in the central nervous system. Some ways in which they support neurons in the central nervous system are by maintaining the concentration of chemicals in the extracellular space, removing excess signaling molecules, reacting to tissue damage, and contributing to the blood-brain barrier (BBB). The blood-brain barrier is a physiological barrier that keeps many substances that circulate in the rest of the body from getting into the central nervous system, restricting what can cross from circulating blood into the CNS. Nutrient molecules, such as glucose or amino acids, can pass through the BBB, but other molecules cannot. This actually causes problems with drug delivery to the CNS. Pharmaceutical companies are challenged to design drugs that can cross the BBB as well as have an effect on the nervous system.
Figure 12.11 Glial Cells of the CNS The CNS has astrocytes, oligodendrocytes, microglia, and ependymal cells that support the neurons of the CNS in several ways.
Like a few other parts of the body, the brain has a privileged blood supply. Very little can pass through by diffusion. Most substances that cross the wall of a blood vessel into the CNS must do so through an active transport process. Because of this, only specific types of molecules can enter the CNS. Glucose—the primary energy source—is allowed, as are amino acids. Water and some other small particles, like gases and ions, can enter. But most everything else cannot, including white blood cells, which are one of the body’s main lines of defense. While this barrier protects the CNS from exposure to toxic or pathogenic substances, it also keeps out the cells that could protect the brain and spinal cord from disease and damage. The BBB also makes it harder for pharmaceuticals to be developed that can affect the nervous system. Aside from finding efficacious substances, the means of delivery is also crucial.
Also found in CNS tissue is the oligodendrocyte, sometimes called just “oligo,” which is the glial cell type that insulates axons in the CNS. The name means “cell of a few branches” (oligo- = “few”; dendro- = “branches”; -cyte = “cell”). There are a few processes that extend from the cell body. Each one reaches out and surrounds an axon to insulate it in myelin. One oligodendrocyte will provide the myelin for multiple axon segments, either for the same axon or for separate axons. The function of myelin will be discussed below.
Microglia are, as the name implies, smaller than most of the other glial cells. Ongoing research into these cells, although not entirely conclusive, suggests that they may originate as white blood cells, called macrophages, that become part of the CNS during early development. While their origin is not conclusively determined, their function is related to what macrophages do in the rest of the body. When macrophages encounter diseased or damaged cells in the rest of the body, they ingest and digest those cells or the pathogens that cause disease. Microglia are the cells in the CNS that can do this in normal, healthy tissue, and they are therefore also referred to as CNS-resident macrophages.
The ependymal cell is a glial cell that filters blood to make cerebrospinal fluid (CSF), the fluid that circulates through the CNS. Because of the privileged blood supply inherent in the BBB, the extracellular space in nervous tissue does not easily exchange components with the blood. Ependymal cells line each ventricle, one of four central cavities that are remnants of the hollow center of the neural tube formed during the embryonic development of the brain. The choroid plexus is a specialized structure in the ventricles where ependymal cells come in contact with blood vessels and filter and absorb components of the blood to produce cerebrospinal fluid. Because of this, ependymal cells can be considered a component of the BBB, or a place where the BBB breaks down. These glial cells appear similar to epithelial cells, making a single layer of cells with little intracellular space and tight connections between adjacent cells. They also have cilia on their apical surface to help move the CSF through the ventricular space. The relationship of these glial cells to the structure of the CNS is seen in Figure 12.11.
Glial Cells of the PNS
One of the two types of glial cells found in the PNS is the satellite cell. Satellite cells are found in sensory and autonomic ganglia, where they surround the cell bodies of neurons. This accounts for the name, based on their appearance under the microscope. They provide support, performing similar functions in the periphery as astrocytes do in the CNS—except, of course, for establishing the BBB.
The second type of glial cell is the Schwann cell, which insulate axons with myelin in the periphery. Schwann cells are different than oligodendrocytes, in that a Schwann cell wraps around a portion of only one axon segment and no others. Oligodendrocytes have processes that reach out to multiple axon segments, whereas the entire Schwann cell surrounds just one axon segment. The nucleus and cytoplasm of the Schwann cell are on the edge of the myelin sheath. The relationship of these two types of glial cells to ganglia and nerves in the PNS is seen in Figure 12.12.
Figure 12.12 Glial Cells of the PNS The PNS has satellite cells and Schwann cells.
Myelin
The insulation for axons in the nervous system is provided by glial cells, oligodendrocytes in the CNS, and Schwann cells in the PNS. Whereas the manner in which either cell is associated with the axon segment, or segments, that it insulates is different, the means of myelinating an axon segment is mostly the same in the two situations. Myelin is a lipid-rich sheath that surrounds the axon and by doing so creates a myelin sheath that facilitates the transmission of electrical signals along the axon. The lipids are essentially the phospholipids of the glial cell membrane. Myelin, however, is more than just the membrane of the glial cell. It also includes important proteins that are integral to that membrane. Some of the proteins help to hold the layers of the glial cell membrane closely together.
The appearance of the myelin sheath can be thought of as similar to the pastry wrapped around a hot dog for “pigs in a blanket” or a similar food. The glial cell is wrapped around the axon several times with little to no cytoplasm between the glial cell layers. For oligodendrocytes, the rest of the cell is separate from the myelin sheath as a cell process extends back toward the cell body. A few other processes provide the same insulation for other axon segments in the area. For Schwann cells, the outermost layer of the cell membrane contains cytoplasm and the nucleus of the cell as a bulge on one side of the myelin sheath. During development, the glial cell is loosely or incompletely wrapped around the axon (Figure 12.13a). The edges of this loose enclosure extend toward each other, and one end tucks under the other. The inner edge wraps around the axon, creating several layers, and the other edge closes around the outside so that the axon is completely enclosed.
INTERACTIVE LINK
View the University of Michigan WebScope to see an electron micrograph of a cross-section of a myelinated nerve fiber. The axon contains microtubules and neurofilaments that are bounded by a plasma membrane known as the axolemma. Outside the plasma membrane of the axon is the myelin sheath, which is composed of the tightly wrapped plasma membrane of a Schwann cell. What aspects of the cells in this image react with the stain to make them a deep, dark, black color, such as the multiple layers that are the myelin sheath?
Myelin sheaths can extend for one or two millimeters, depending on the diameter of the axon. Axon diameters can be as small as 1 to 20 micrometers. Because a micrometer is 1/1000 of a millimeter, this means that the length of a myelin sheath can be 100–1000 times the diameter of the axon. Figure 12.8, Figure 12.11, and Figure 12.12 show the myelin sheath surrounding an axon segment, but are not to scale. If the myelin sheath were drawn to scale, the neuron would have to be immense—possibly covering an entire wall of the room in which you are sitting.
Figure 12.13 The Process of Myelination Myelinating glia wrap several layers of cell membrane around the cell membrane of an axon segment. A single Schwann cell insulates a segment of a peripheral nerve, whereas in the CNS, an oligodendrocyte may provide insulation for a few separate axon segments. EM × 1,460,000. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
DISORDERS OF THE...
Nervous Tissue
Several diseases can result from the demyelination of axons. The causes of these diseases are not the same; some have genetic causes, some are caused by pathogens, and others are the result of autoimmune disorders. Though the causes are varied, the results are largely similar. The myelin insulation of axons is compromised, making electrical signaling slower.
Multiple sclerosis (MS) is one such disease. It is an example of an autoimmune disease. The antibodies produced by lymphocytes (a type of white blood cell) mark myelin as something that should not be in the body. This causes inflammation and the destruction of the myelin in the central nervous system. As the insulation around the axons is destroyed by the disease, scarring becomes obvious. This is where the name of the disease comes from; sclerosis means hardening of tissue, which is what a scar is. Multiple scars are found in the white matter of the brain and spinal cord. The symptoms of MS include both somatic and autonomic deficits. Control of the musculature is compromised, as is control of organs such as the bladder.
Guillain-Barré (pronounced gee-YAN bah-RAY) syndrome is an example of a demyelinating disease of the peripheral nervous system. It is also the result of an autoimmune reaction, but the inflammation is in peripheral nerves. Sensory symptoms or motor deficits are common, and autonomic failures can lead to changes in the heart rhythm or a drop in blood pressure, especially when standing, which causes dizziness.
The Function of Nervous Tissue
- Distinguish the major functions of the nervous system: sensation, integration, and response
- List the sequence of events in a simple sensory receptor–motor response pathway
Having looked at the components of nervous tissue, and the basic anatomy of the nervous system, next comes an understanding of how nervous tissue is capable of communicating within the nervous system. Before getting to the nuts and bolts of how this works, an illustration of how the components come together will be helpful. An example is summarized in Figure 12.14.
Figure 12.14 Testing the Water (1) The sensory neuron has endings in the skin that sense a stimulus such as water temperature. The strength of the signal that starts here is dependent on the strength of the stimulus. (2) The graded potential from the sensory endings, if strong enough, will initiate an action potential at the initial segment of the axon (which is immediately adjacent to the sensory endings in the skin). (3) The axon of the peripheral sensory neuron enters the spinal cord and contacts another neuron in the gray matter. The contact is a synapse where another graded potential is caused by the release of a chemical signal from the axon terminals. (4) An action potential is initiated at the initial segment of this neuron and travels up the sensory pathway to a region of the brain called the thalamus. Another synapse passes the information along to the next neuron. (5) The sensory pathway ends when the signal reaches the cerebral cortex. (6) After integration with neurons in other parts of the cerebral cortex, a motor command is sent from the precentral gyrus of the frontal cortex. (7) The upper motor neuron sends an action potential down to the spinal cord. The target of the upper motor neuron is the dendrites of the lower motor neuron in the gray matter of the spinal cord. (8) The axon of the lower motor neuron emerges from the spinal cord in a nerve and connects to a muscle through a neuromuscular junction to cause contraction of the target muscle.
Imagine you are about to take a shower in the morning before going to school. You have turned on the faucet to start the water as you prepare to get in the shower. After a few minutes, you expect the water to be a temperature that will be comfortable to enter. So you put your hand out into the spray of water. What happens next depends on how your nervous system interacts with the stimulus of the water temperature and what you do in response to that stimulus.
Found in the skin of your fingers or toes is a type of sensory receptor that is sensitive to temperature, called a thermoreceptor. When you place your hand under the shower (Figure 12.15), the cell membrane of the thermoreceptors changes its electrical state (voltage). The amount of change is dependent on the strength of the stimulus (how hot the water is). This is called a graded potential. If the stimulus is strong, the voltage of the cell membrane will change enough to generate an electrical signal that will travel down the axon. You have learned about this type of signaling before, with respect to the interaction of nerves and muscles at the neuromuscular junction. The voltage at which such a signal is generated is called the threshold, and the resulting electrical signal is called an action potential. In this example, the action potential travels—a process known as propagation—along the axon from the axon hillock to the axon terminals and into the synaptic end bulbs. When this signal reaches the end bulbs, it causes the release of a signaling molecule called a neurotransmitter.
Figure 12.15 The Sensory Input Receptors in the skin sense the temperature of the water.
The neurotransmitter diffuses across the short distance of the synapse and binds to a receptor protein of the target neuron. When the molecular signal binds to the receptor, the cell membrane of the target neuron changes its electrical state and a new graded potential begins. If that graded potential is strong enough to reach threshold, the second neuron generates an action potential at its axon hillock. The target of this neuron is another neuron in the thalamus of the brain, the part of the CNS that acts as a relay for sensory information. At another synapse, neurotransmitter is released and binds to its receptor. The thalamus then sends the sensory information to the cerebral cortex, the outermost layer of gray matter in the brain, where conscious perception of that water temperature begins.
Within the cerebral cortex, information is processed among many neurons, integrating the stimulus of the water temperature with other sensory stimuli, with your emotional state (you just aren't ready to wake up; the bed is calling to you), memories (perhaps of the lab notes you have to study before a quiz). Finally, a plan is developed about what to do, whether that is to turn the temperature up, turn the whole shower off and go back to bed, or step into the shower. To do any of these things, the cerebral cortex has to send a command out to your body to move muscles (Figure 12.16).
Figure 12.16 The Motor Response On the basis of the sensory input and the integration in the CNS, a motor response is formulated and executed.
A region of the cortex is specialized for sending signals down to the spinal cord for movement. The upper motor neuron is in this region, called the precentral gyrus of the frontal cortex, which has an axon that extends all the way down the spinal cord. At the level of the spinal cord at which this axon makes a synapse, a graded potential occurs in the cell membrane of a lower motor neuron. This second motor neuron is responsible for causing muscle fibers to contract. In the manner described in the chapter on muscle tissue, an action potential travels along the motor neuron axon into the periphery. The axon terminates on muscle fibers at the neuromuscular junction. Acetylcholine is released at this specialized synapse, which causes the muscle action potential to begin, following a large potential known as an end plate potential. When the lower motor neuron excites the muscle fiber, it contracts. All of this occurs in a fraction of a second, but this story is the basis of how the nervous system functions.
CAREER CONNECTION
Neurophysiologist
Understanding how the nervous system works could be a driving force in your career. Studying neurophysiology is a very rewarding path to follow. It means that there is a lot of work to do, but the rewards are worth the effort.
The career path of a research scientist can be straightforward: college, graduate school, postdoctoral research, academic research position at a university. A Bachelor’s degree in science will get you started, and for neurophysiology that might be in biology, psychology, computer science, engineering, or neuroscience. But the real specialization comes in graduate school. There are many different programs out there to study the nervous system, not just neuroscience itself. Most graduate programs are doctoral, meaning that a Master’s degree is not part of the work. These are usually considered five-year programs, with the first two years dedicated to course work and finding a research mentor, and the last three years dedicated to finding a research topic and pursuing that with a near single-mindedness. The research will usually result in a few publications in scientific journals, which will make up the bulk of a doctoral dissertation. After graduating with a Ph.D., researchers will go on to find specialized work called a postdoctoral fellowship within established labs. In this position, a researcher starts to establish their own research career with the hopes of finding an academic position at a research university.
Other options are available if you are interested in how the nervous system works. Especially for neurophysiology, a medical degree might be more suitable so you can learn about the clinical applications of neurophysiology and possibly work with human subjects. An academic career is not a necessity. Biotechnology firms are eager to find motivated scientists ready to tackle the tough questions about how the nervous system works so that therapeutic chemicals can be tested on some of the most challenging disorders such as Alzheimer’s disease or Parkinson’s disease, or spinal cord injury.
Others with a medical degree and a specialization in neuroscience go on to work directly with patients, diagnosing and treating mental disorders. You can do this as a psychiatrist, a neuropsychologist, a neuroscience nurse, or a neurodiagnostic technician, among other possible career paths.
The Action Potential
- Describe the components of the membrane that establish the resting membrane potential
- Describe the changes that occur to the membrane that result in the action potential
The functions of the nervous system—sensation, integration, and response—depend on the functions of the neurons underlying these pathways. To understand how neurons are able to communicate, it is necessary to describe the role of an excitable membrane in generating these signals. The basis of this communication is the action potential, which demonstrates how changes in the membrane can constitute a signal. Looking at the way these signals work in more variable circumstances involves a look at graded potentials, which will be covered in the next section.
Electrically Active Cell Membranes
Most cells in the body make use of charged particles, ions, to build up a charge across the cell membrane. Previously, this was shown to be a part of how muscle cells work. For skeletal muscles to contract, based on excitation–contraction coupling, requires input from a neuron. Both of the cells make use of the cell membrane to regulate ion movement between the extracellular fluid and cytosol.
As you learned in the chapter on cells, the cell membrane is primarily responsible for regulating what can cross the membrane and what stays on only one side. The cell membrane is a phospholipid bilayer, so only substances that can pass directly through the hydrophobic core can diffuse through unaided. Charged particles, which are hydrophilic by definition, cannot pass through the cell membrane without assistance (Figure 12.17). Transmembrane proteins, specifically channel proteins, make this possible. Several passive transport channels, as well as active transport pumps, are necessary to generate a transmembrane potential and an action potential. Of special interest is the carrier protein referred to as the sodium/potassium pump that moves sodium ions (Na+) out of a cell and potassium ions (K+) into a cell, thus regulating ion concentration on both sides of the cell membrane.
Figure 12.17 Cell Membrane and Transmembrane Proteins The cell membrane is composed of a phospholipid bilayer and has many transmembrane proteins, including different types of channel proteins that serve as ion channels.
The sodium/potassium pump requires energy in the form of adenosine triphosphate (ATP), so it is also referred to as an ATPase. As was explained in the cell chapter, the concentration of Na+ is higher outside the cell than inside, and the concentration of K+is higher inside the cell than outside. That means that this pump is moving the ions against the concentration gradients for sodium and potassium, which is why it requires energy. In fact, the pump basically maintains those concentration gradients.
Ion channels are pores that allow specific charged particles to cross the membrane in response to an existing concentration gradient. Proteins are capable of spanning the cell membrane, including its hydrophobic core, and can interact with the charge of ions because of the varied properties of amino acids found within specific domains or regions of the protein channel. Hydrophobic amino acids are found in the domains that are apposed to the hydrocarbon tails of the phospholipids. Hydrophilic amino acids are exposed to the fluid environments of the extracellular fluid and cytosol. Additionally, the ions will interact with the hydrophilic amino acids, which will be selective for the charge of the ion. Channels for cations (positive ions) will have negatively charged side chains in the pore. Channels for anions (negative ions) will have positively charged side chains in the pore. This is called electrochemical exclusion, meaning that the channel pore is charge-specific.
Ion channels can also be specified by the diameter of the pore. The distance between the amino acids will be specific for the diameter of the ion when it dissociates from the water molecules surrounding it. Because of the surrounding water molecules, larger pores are not ideal for smaller ions because the water molecules will interact, by hydrogen bonds, more readily than the amino acid side chains. This is called size exclusion. Some ion channels are selective for charge but not necessarily for size, and thus are called a nonspecific channel. These nonspecific channels allow cations—particularly Na+, K+, and Ca2+—to cross the membrane, but exclude anions.
Ion channels do not always freely allow ions to diffuse across the membrane. Some are opened by certain events, meaning the channels are gated. So another way that channels can be categorized is on the basis of how they are gated. Although these classes of ion channels are found primarily in the cells of nervous or muscular tissue, they also can be found in the cells of epithelial and connective tissues.
A ligand-gated channel opens because a signaling molecule, a ligand, binds to the extracellular region of the channel. This type of channel is also known as an ionotropic receptor because when the ligand, known as a neurotransmitter in the nervous system, binds to the protein, ions cross the membrane changing its charge (Figure 12.18).
Figure 12.18 Ligand-Gated Channels When the ligand, in this case the neurotransmitter acetylcholine, binds to a specific location on the extracellular surface of the channel protein, the pore opens to allow select ions through. The ions, in this case, are cations of sodium, calcium, and potassium.
A mechanically gated channel opens because of a physical distortion of the cell membrane. Many channels associated with the sense of touch (somatosensation) are mechanically gated. For example, as pressure is applied to the skin, these channels open and allow ions to enter the cell. Similar to this type of channel would be the channel that opens on the basis of temperature changes, as in testing the water in the shower (Figure 12.19).
Figure 12.19 Mechanically Gated Channels When a mechanical change occurs in the surrounding tissue, such as pressure or touch, the channel is physically opened. Thermoreceptors work on a similar principle. When the local tissue temperature changes, the protein reacts by physically opening the channel.
A voltage-gated channel is a channel that responds to changes in the electrical properties of the membrane in which it is embedded. Normally, the inner portion of the membrane is at a negative voltage. When that voltage becomes less negative, the channel begins to allow ions to cross the membrane (Figure 12.20).
Figure 12.20 Voltage-Gated Channels Voltage-gated channels open when the transmembrane voltage changes around them. Amino acids in the structure of the protein are sensitive to charge and cause the pore to open to the selected ion.
A leakage channel is randomly gated, meaning that it opens and closes at random, hence the reference to leaking. There is no actual event that opens the channel; instead, it has an intrinsic rate of switching between the open and closed states. Leakage channels contribute to the resting transmembrane voltage of the excitable membrane (Figure 12.21).
Figure 12.21 Leakage Channels In certain situations, ions need to move across the membrane randomly. The particular electrical properties of certain cells are modified by the presence of this type of channel.
The Membrane Potential
The electrical state of the cell membrane can have several variations. These are all variations in the membrane potential. A potential is a distribution of charge across the cell membrane, measured in millivolts (mV). The standard is to compare the inside of the cell relative to the outside, so the membrane potential is a value representing the charge on the intracellular side of the membrane based on the outside being zero, relatively speaking (Figure 12.22).
Figure 12.22 Measuring Charge across a Membrane with a Voltmeter A recording electrode is inserted into the cell and a reference electrode is outside the cell. By comparing the charge measured by these two electrodes, the transmembrane voltage is determined. It is conventional to express that value for the cytosol relative to the outside.
The concentration of ions in extracellular and intracellular fluids is largely balanced, with a net neutral charge. However, a slight difference in charge occurs right at the membrane surface, both internally and externally. It is the difference in this very limited region that has all the power in neurons (and muscle cells) to generate electrical signals, including action potentials.
Before these electrical signals can be described, the resting state of the membrane must be explained. When the cell is at rest, and the ion channels are closed (except for leakage channels which randomly open), ions are distributed across the membrane in a very predictable way. The concentration of Na+ outside the cell is 10 times greater than the concentration inside. Also, the concentration of K+ inside the cell is greater than outside. The cytosol contains a high concentration of anions, in the form of phosphate ions and negatively charged proteins. Large anions are a component of the inner cell membrane, including specialized phospholipids and proteins associated with the inner leaflet of the membrane (leaflet is a term used for one side of the lipid bilayer membrane). The negative charge is localized in the large anions.
With the ions distributed across the membrane at these concentrations, the difference in charge is measured at -70 mV, the value described as the resting membrane potential. The exact value measured for the resting membrane potential varies between cells, but -70 mV is most commonly used as this value. This voltage would actually be much lower except for the contributions of some important proteins in the membrane. Leakage channels allow Na+ to slowly move into the cell or K+ to slowly move out, and the Na+/K+ pump restores them. This may appear to be a waste of energy, but each has a role in maintaining the membrane potential.
The Action Potential
Resting membrane potential describes the steady state of the cell, which is a dynamic process that is balanced by ion leakage and ion pumping. Without any outside influence, it will not change. To get an electrical signal started, the membrane potential has to change.
This starts with a channel opening for Na+ in the membrane. Because the concentration of Na+ is higher outside the cell than inside the cell by a factor of 10, ions will rush into the cell that are driven largely by the concentration gradient. Because sodium is a positively charged ion, it will change the relative voltage immediately inside the cell relative to immediately outside. The resting potential is the state of the membrane at a voltage of -70 mV, so the sodium cation entering the cell will cause it to become less negative. This is known as depolarization, meaning the membrane potential moves toward zero.
The concentration gradient for Na+ is so strong that it will continue to enter the cell even after the membrane potential has become zero, so that the voltage immediately around the pore begins to become positive. The electrical gradient also plays a role, as negative proteins below the membrane attract the sodium ion. The membrane potential will reach +30 mV by the time sodium has entered the cell.
As the membrane potential reaches +30 mV, other voltage-gated channels are opening in the membrane. These channels are specific for the potassium ion. A concentration gradient acts on K+, as well. As K+ starts to leave the cell, taking a positive charge with it, the membrane potential begins to move back toward its resting voltage. This is called repolarization, meaning that the membrane voltage moves back toward the -70 mV value of the resting membrane potential.
Repolarization returns the membrane potential to the -70 mV value that indicates the resting potential, but it actually overshoots that value. Potassium ions reach equilibrium when the membrane voltage is below -70 mV, so a period of hyperpolarization occurs while the K+ channels are open. Those K+ channels are slightly delayed in closing, accounting for this short overshoot.
What has been described here is the action potential, which is presented as a graph of voltage over time in Figure 12.23. It is the electrical signal that nervous tissue generates for communication. The change in the membrane voltage from -70 mV at rest to +30 mV at the end of depolarization is a 100-mV change. That can also be written as a 0.1-V change. To put that value in perspective, think about a battery. An AA battery that you might find in a television remote has a voltage of 1.5 V, or a 9-V battery (the rectangular battery with two posts on one end) is, obviously, 9 V. The change seen in the action potential is one or two orders of magnitude less than the charge in these batteries. In fact, the membrane potential can be described as a battery. A charge is stored across the membrane that can be released under the correct conditions. A battery in your remote has stored a charge that is “released” when you push a button.
Figure 12.23 Graph of Action Potential Plotting voltage measured across the cell membrane against time, the action potential begins with depolarization, followed by repolarization, which goes past the resting potential into hyperpolarization, and finally the membrane returns to rest.
INTERACTIVE LINK
What happens across the membrane of an electrically active cell is a dynamic process that is hard to visualize with static images or through text descriptions. View this animation to learn more about this process. What is the difference between the driving force for Na+ and K+? And what is similar about the movement of these two ions?
The question is, now, what initiates the action potential? The description above conveniently glosses over that point. But it is vital to understanding what is happening. The membrane potential will stay at the resting voltage until something changes. The description above just says that a Na+ channel opens. Now, to say “a channel opens” does not mean that one individual transmembrane protein changes. Instead, it means that one kind of channel opens. There are a few different types of channels that allow Na+ to cross the membrane. A ligand-gated Na+ channel will open when a neurotransmitter binds to it and a mechanically gated Na+ channel will open when a physical stimulus affects a sensory receptor (like pressure applied to the skin compresses a touch receptor). Whether it is a neurotransmitter binding to its receptor protein or a sensory stimulus activating a sensory receptor cell, some stimulus gets the process started. Sodium starts to enter the cell and the membrane becomes less negative.
A third type of channel that is an important part of depolarization in the action potential is the voltage-gated Na+ channel. The channels that start depolarizing the membrane because of a stimulus help the cell to depolarize from -70 mV to -55 mV. Once the membrane reaches that voltage, the voltage-gated Na+ channels open. This is what is known as the threshold. Any depolarization that does not change the membrane potential to -55 mV or higher will not reach threshold and thus will not result in an action potential. Also, any stimulus that depolarizes the membrane to -55 mV or beyond will cause a large number of channels to open and an action potential will be initiated.
Because of the threshold, the action potential can be likened to a digital event—it either happens or it does not. If the threshold is not reached, then no action potential occurs. If depolarization reaches -55 mV, then the action potential continues and runs all the way to +30 mV, at which K+ causes repolarization, including the hyperpolarizing overshoot. Also, those changes are the same for every action potential, which means that once the threshold is reached, the exact same thing happens. A stronger stimulus, which might depolarize the membrane well past threshold, will not make a “bigger” action potential. Action potentials are “all or none.” Either the membrane reaches the threshold and everything occurs as described above, or the membrane does not reach the threshold and nothing else happens. All action potentials peak at the same voltage (+30 mV), so one action potential is not bigger than another. Stronger stimuli will initiate multiple action potentials more quickly, but the individual signals are not bigger. Thus, for example, you will not feel a greater sensation of pain, or have a stronger muscle contraction, because of the size of the action potential because they are not different sizes.
As we have seen, the depolarization and repolarization of an action potential are dependent on two types of channels (the voltage-gated Na+ channel and the voltage-gated K+ channel). The voltage-gated Na+ channel actually has two gates. One is the activation gate, which opens when the membrane potential crosses -55 mV. The other gate is the inactivation gate, which closes after a specific period of time—on the order of a fraction of a millisecond. When a cell is at rest, the activation gate is closed and the inactivation gate is open. However, when the threshold is reached, the activation gate opens, allowing Na+ to rush into the cell. Timed with the peak of depolarization, the inactivation gate closes. During repolarization, no more sodium can enter the cell. When the membrane potential passes -55 mV again, the activation gate closes. After that, the inactivation gate re-opens, making the channel ready to start the whole process over again.
The voltage-gated K+ channel has only one gate, which is sensitive to a membrane voltage of -50 mV. However, it does not open as quickly as the voltage-gated Na+ channel does. It might take a fraction of a millisecond for the channel to open once that voltage has been reached. The timing of this coincides exactly with when the Na+ flow peaks, so voltage-gated K+ channels open just as the voltage-gated Na+ channels are being inactivated. As the membrane potential repolarizes and the voltage passes -50 mV again, the channel closes—again, with a little delay. Potassium continues to leave the cell for a short while and the membrane potential becomes more negative, resulting in the hyperpolarizing overshoot. Then the channel closes again and the membrane can return to the resting potential because of the ongoing activity of the non-gated channels and the Na+/K+pump.
All of this takes place within approximately 2 milliseconds (Figure 12.24). While an action potential is in progress, another one cannot be initiated. That effect is referred to as the refractory period. There are two phases of the refractory period: the absolute refractory period and the relative refractory period. During the absolute phase, another action potential will not start. This is because of the inactivation gate of the voltage-gated Na+ channel. Once that channel is back to its resting conformation (less than -55 mV), a new action potential could be started, but only by a stronger stimulus than the one that initiated the current action potential. This is because of the flow of K+ out of the cell. Because that ion is rushing out, any Na+that tries to enter will not depolarize the cell, but will only keep the cell from hyperpolarizing.
Figure 12.24 Stages of an Action Potential Plotting voltage measured across the cell membrane against time, the events of the action potential can be related to specific changes in the membrane voltage. (1) At rest, the membrane voltage is -70 mV. (2) The membrane begins to depolarize when an external stimulus is applied. (3) The membrane voltage begins a rapid rise toward +30 mV. (4) The membrane voltage starts to return to a negative value. (5) Repolarization continues past the resting membrane voltage, resulting in hyperpolarization. (6) The membrane voltage returns to the resting value shortly after hyperpolarization.
Propagation of the Action Potential
The action potential is initiated at the beginning of the axon, at what is called the initial segment. There is a high density of voltage-gated Na+ channels so that rapid depolarization can take place here. Going down the length of the axon, the action potential is propagated because more voltage-gated Na+ channels are opened as the depolarization spreads. This spreading occurs because Na+ enters through the channel and moves along the inside of the cell membrane. As the Na+ moves, or flows, a short distance along the cell membrane, its positive charge depolarizes a little more of the cell membrane. As that depolarization spreads, new voltage-gated Na+ channels open and more ions rush into the cell, spreading the depolarization a little farther.
Because voltage-gated Na+ channels are inactivated at the peak of the depolarization, they cannot be opened again for a brief time—the absolute refractory period. Because of this, depolarization spreading back toward previously opened channels has no effect. The action potential must propagate toward the axon terminals; as a result, the polarity of the neuron is maintained, as mentioned above.
Propagation, as described above, applies to unmyelinated axons. When myelination is present, the action potential propagates differently. Sodium ions that enter the cell at the initial segment start to spread along the length of the axon segment, but there are no voltage-gated Na+ channels until the first node of Ranvier. Because there is not constant opening of these channels along the axon segment, the depolarization spreads at an optimal speed. The distance between nodes is the optimal distance to keep the membrane still depolarized above threshold at the next node. As Na+ spreads along the inside of the membrane of the axon segment, the charge starts to dissipate. If the node were any farther down the axon, that depolarization would have fallen off too much for voltage-gated Na+ channels to be activated at the next node of Ranvier. If the nodes were any closer together, the speed of propagation would be slower.
Propagation along an unmyelinated axon is referred to as continuous conduction; along the length of a myelinated axon, it is saltatory conduction. Continuous conduction is slow because there are always voltage-gated Na+ channels opening, and more and more Na+ is rushing into the cell. Saltatory conduction is faster because the action potential basically jumps from one node to the next (saltare = “to leap”), and the new influx of Na+ renews the depolarized membrane. Along with the myelination of the axon, the diameter of the axon can influence the speed of conduction. Much as water runs faster in a wide river than in a narrow creek, Na+-based depolarization spreads faster down a wide axon than down a narrow one. This concept is known as resistance and is generally true for electrical wires or plumbing, just as it is true for axons, although the specific conditions are different at the scales of electrons or ions versus water in a river.
HOMEOSTATIC IMBALANCES
Potassium Concentration
Glial cells, especially astrocytes, are responsible for maintaining the chemical environment of the CNS tissue. The concentrations of ions in the extracellular fluid are the basis for how the membrane potential is established and changes in electrochemical signaling. If the balance of ions is upset, drastic outcomes are possible.
Normally the concentration of K+ is higher inside the neuron than outside. After the repolarizing phase of the action potential, K+ leakage channels and the Na+/K+ pump ensure that the ions return to their original locations. Following a stroke or other ischemic event, extracellular K+ levels are elevated. The astrocytes in the area are equipped to clear excess K+ to aid the pump. But when the level is far out of balance, the effects can be irreversible.
Astrocytes can become reactive in cases such as these, which impairs their ability to maintain the local chemical environment. The glial cells enlarge and their processes swell. They lose their K+ buffering ability and the function of the pump is affected, or even reversed. One of the early signs of cell disease is this "leaking" of sodium ions into the body cells. This sodium/potassium imbalance negatively affects the internal chemistry of cells, preventing them from functioning normally.
INTERACTIVE LINK
Visit this site to see a virtual neurophysiology lab, and to observe electrophysiological processes in the nervous system, where scientists directly measure the electrical signals produced by neurons. Often, the action potentials occur so rapidly that watching a screen to see them occur is not helpful. A speaker is powered by the signals recorded from a neuron and it “pops” each time the neuron fires an action potential. These action potentials are firing so fast that it sounds like static on the radio. Electrophysiologists can recognize the patterns within that static to understand what is happening. Why is the leech model used for measuring the electrical activity of neurons instead of using humans?
Communication Between Neurons
- Explain the differences between the types of graded potentials
- Categorize the major neurotransmitters by chemical type and effect
The electrical changes taking place within a neuron, as described in the previous section, are similar to a light switch being turned on. A stimulus starts the depolarization, but the action potential runs on its own once a threshold has been reached. The question is now, “What flips the light switch on?” Temporary changes to the cell membrane voltage can result from neurons receiving information from the environment, or from the action of one neuron on another. These special types of potentials influence a neuron and determine whether an action potential will occur or not. Many of these transient signals originate at the synapse.
Graded Potentials
Local changes in the membrane potential are called graded potentials and are usually associated with the dendrites of a neuron. The amount of change in the membrane potential is determined by the size of the stimulus that causes it. In the example of testing the temperature of the shower, slightly warm water would only initiate a small change in a thermoreceptor, whereas hot water would cause a large amount of change in the membrane potential.
Graded potentials can be of two sorts, either they are depolarizing or hyperpolarizing (Figure 12.25). For a membrane at the resting potential, a graded potential represents a change in that voltage either above -70 mV or below -70 mV. Depolarizing graded potentials are often the result of Na+ or Ca2+ entering the cell. Both of these ions have higher concentrations outside the cell than inside; because they have a positive charge, they will move into the cell causing it to become less negative relative to the outside. Hyperpolarizing graded potentials can be caused by K+ leaving the cell or Cl- entering the cell. If a positive charge moves out of a cell, the cell becomes more negative; if a negative charge enters the cell, the same thing happens.
Figure 12.25 Graded Potentials Graded potentials are temporary changes in the membrane voltage, the characteristics of which depend on the size of the stimulus. Some types of stimuli cause depolarization of the membrane, whereas others cause hyperpolarization. It depends on the specific ion channels that are activated in the cell membrane.
Types of Graded Potentials
For the unipolar cells of sensory neurons—both those with free nerve endings and those within encapsulations—graded potentials develop in the dendrites that influence the generation of an action potential in the axon of the same cell. This is called a generator potential. For other sensory receptor cells, such as taste cells or photoreceptors of the retina, graded potentials in their membranes result in the release of neurotransmitters at synapses with sensory neurons. This is called a receptor potential.
A postsynaptic potential (PSP) is the graded potential in the dendrites of a neuron that is receiving synapses from other cells. Postsynaptic potentials can be depolarizing or hyperpolarizing. Depolarization in a postsynaptic potential is called an excitatory postsynaptic potential (EPSP) because it causes the membrane potential to move toward threshold. Hyperpolarization in a postsynaptic potential is an inhibitory postsynaptic potential (IPSP) because it causes the membrane potential to move away from threshold.
Summation
All types of graded potentials will result in small changes of either depolarization or hyperpolarization in the voltage of a membrane. These changes can lead to the neuron reaching threshold if the changes add together, or summate. The combined effects of different types of graded potentials are illustrated in Figure 12.26. If the total change in voltage in the membrane is a positive 15 mV, meaning that the membrane depolarizes from -70 mV to -55 mV, then the graded potentials will result in the membrane reaching threshold.
For receptor potentials, threshold is not a factor because the change in membrane potential for receptor cells directly causes neurotransmitter release. However, generator potentials can initiate action potentials in the sensory neuron axon, and postsynaptic potentials can initiate an action potential in the axon of other neurons. Graded potentials summate at a specific location at the beginning of the axon to initiate the action potential, namely the initial segment. For sensory neurons, which do not have a cell body between the dendrites and the axon, the initial segment is directly adjacent to the dendritic endings. For all other neurons, the axon hillock is essentially the initial segment of the axon, and it is where summation takes place. These locations have a high density of voltage-gated Na+ channels that initiate the depolarizing phase of the action potential.
Summation can be spatial or temporal, meaning it can be the result of multiple graded potentials at different locations on the neuron, or all at the same place but separated in time. Spatial summation is related to associating the activity of multiple inputs to a neuron with each other. Temporal summation is the relationship of multiple action potentials from a single cell resulting in a significant change in the membrane potential. Spatial and temporal summation can act together, as well.
Figure 12.26 Postsynaptic Potential Summation The result of summation of postsynaptic potentials is the overall change in the membrane potential. At point A, several different excitatory postsynaptic potentials add up to a large depolarization. At point B, a mix of excitatory and inhibitory postsynaptic potentials result in a different end result for the membrane potential.
INTERACTIVE LINK
Watch this video to learn about summation. The process of converting electrical signals to chemical signals and back requires subtle changes that can result in transient increases or decreases in membrane voltage. To cause a lasting change in the target cell, multiple signals are usually added together, or summated. Does spatial summation have to happen all at once, or can the separate signals arrive on the postsynaptic neuron at slightly different times? Explain your answer.
Synapses
There are two types of connections between electrically active cells, chemical synapses and electrical synapses. In a chemical synapse, a chemical signal—namely, a neurotransmitter—is released from one cell and it affects the other cell. In an electrical synapse, there is a direct connection between the two cells so that ions can pass directly from one cell to the next. If one cell is depolarized in an electrical synapse, the joined cell also depolarizes because the ions pass between the cells. Chemical synapses involve the transmission of chemical information from one cell to the next. This section will concentrate on the chemical type of synapse.
An example of a chemical synapse is the neuromuscular junction (NMJ) described in the chapter on muscle tissue. In the nervous system, there are many more synapses that are essentially the same as the NMJ. All synapses have common characteristics, which can be summarized in this list:
- presynaptic element
- neurotransmitter (packaged in vesicles)
- synaptic cleft
- receptor proteins
- postsynaptic element
- neurotransmitter elimination or re-uptake
For the NMJ, these characteristics are as follows: the presynaptic element is the motor neuron's axon terminals, the neurotransmitter is acetylcholine, the synaptic cleft is the space between the cells where the neurotransmitter diffuses, the receptor protein is the nicotinic acetylcholine receptor, the postsynaptic element is the sarcolemma of the muscle cell, and the neurotransmitter is eliminated by acetylcholinesterase. Other synapses are similar to this, and the specifics are different, but they all contain the same characteristics.
Neurotransmitter Release
When an action potential reaches the axon terminals, voltage-gated Ca2+ channels in the membrane of the synaptic end bulb open. The concentration of Ca2+ increases inside the end bulb, and the Ca2+ ion associates with proteins in the outer surface of neurotransmitter vesicles. The Ca2+ facilitates the merging of the vesicle with the presynaptic membrane so that the neurotransmitter is released through exocytosis into the small gap between the cells, known as the synaptic cleft.
Once in the synaptic cleft, the neurotransmitter diffuses the short distance to the postsynaptic membrane and can interact with neurotransmitter receptors. Receptors are specific for the neurotransmitter, and the two fit together like a key and lock. One neurotransmitter binds to its receptor and will not bind to receptors for other neurotransmitters, making the binding a specific chemical event (Figure 12.27).
Figure 12.27 The Synapse The synapse is a connection between a neuron and its target cell (which is not necessarily a neuron). The presynaptic element is the synaptic end bulb of the axon where Ca2+ enters the bulb to cause vesicle fusion and neurotransmitter release. The neurotransmitter diffuses across the synaptic cleft to bind to its receptor. The neurotransmitter is cleared from the synapse either by enzymatic degradation, neuronal reuptake, or glial reuptake.
Neurotransmitter Systems
There are several systems of neurotransmitters that are found at various synapses in the nervous system. These groups refer to the chemicals that are the neurotransmitters, and within the groups are specific systems.
The first group, which is a neurotransmitter system of its own, is the cholinergic system. It is the system based on acetylcholine. This includes the NMJ as an example of a cholinergic synapse, but cholinergic synapses are found in other parts of the nervous system. They are in the autonomic nervous system, as well as distributed throughout the brain.
The cholinergic system has two types of receptors, the nicotinic receptor is found in the NMJ as well as other synapses. There is also an acetylcholine receptor known as the muscarinic receptor. Both of these receptors are named for drugs that interact with the receptor in addition to acetylcholine. Nicotine will bind to the nicotinic receptor and activate it similar to acetylcholine. Muscarine, a product of certain mushrooms, will bind to the muscarinic receptor. However, nicotine will not bind to the muscarinic receptor and muscarine will not bind to the nicotinic receptor.
Another group of neurotransmitters are amino acids. This includes glutamate (Glu), GABA (gamma-aminobutyric acid, a derivative of glutamate), and glycine (Gly). These amino acids have an amino group and a carboxyl group in their chemical structures. Glutamate is one of the 20 amino acids that are used to make proteins. Each amino acid neurotransmitter would be part of its own system, namely the glutamatergic, GABAergic, and glycinergic systems. They each have their own receptors and do not interact with each other. Amino acid neurotransmitters are eliminated from the synapse by reuptake. A pump in the cell membrane of the presynaptic element, or sometimes a neighboring glial cell, will clear the amino acid from the synaptic cleft so that it can be recycled, repackaged in vesicles, and released again.
Another class of neurotransmitter is the biogenic amine, a group of neurotransmitters that are enzymatically made from amino acids. They have amino groups in them, but no longer have carboxyl groups and are therefore no longer classified as amino acids. Serotonin is made from tryptophan. It is the basis of the serotonergic system, which has its own specific receptors. Serotonin is transported back into the presynaptic cell for repackaging.
Other biogenic amines are made from tyrosine, and include dopamine, norepinephrine, and epinephrine. Dopamine is part of its own system, the dopaminergic system, which has dopamine receptors. Dopamine is removed from the synapse by transport proteins in the presynaptic cell membrane. Norepinephrine and epinephrine belong to the adrenergic neurotransmitter system. The two molecules are very similar and bind to the same receptors, which are referred to as alpha and beta receptors. Norepinephrine and epinephrine are also transported back into the presynaptic cell. The chemical epinephrine (epi- = “on”; “-nephrine” = kidney) is also known as adrenaline (renal = “kidney”), and norepinephrine is sometimes referred to as noradrenaline. The adrenal gland produces epinephrine and norepinephrine to be released into the blood stream as hormones.
A neuropeptide is a neurotransmitter molecule made up of chains of amino acids connected by peptide bonds. This is what a protein is, but the term protein implies a certain length to the molecule. Some neuropeptides are quite short, such as met-enkephalin, which is five amino acids long. Others are long, such as beta-endorphin, which is 31 amino acids long. Neuropeptides are often released at synapses in combination with another neurotransmitter, and they often act as hormones in other systems of the body, such as vasoactive intestinal peptide (VIP) or substance P.
The effect of a neurotransmitter on the postsynaptic element is entirely dependent on the receptor protein. First, if there is no receptor protein in the membrane of the postsynaptic element, then the neurotransmitter has no effect. The depolarizing or hyperpolarizing effect is also dependent on the receptor. When acetylcholine binds to the nicotinic receptor, the postsynaptic cell is depolarized. This is because the receptor is a cation channel and positively charged Na+ will rush into the cell. However, when acetylcholine binds to the muscarinic receptor, of which there are several variants, it might cause depolarization or hyperpolarization of the target cell.
The amino acid neurotransmitters, glutamate, glycine, and GABA, are almost exclusively associated with just one effect. Glutamate is considered an excitatory amino acid, but only because Glu receptors in the adult cause depolarization of the postsynaptic cell. Glycine and GABA are considered inhibitory amino acids, again because their receptors cause hyperpolarization.
The biogenic amines have mixed effects. For example, the dopamine receptors that are classified as D1 receptors are excitatory whereas D2-type receptors are inhibitory. Biogenic amine receptors and neuropeptide receptors can have even more complex effects because some may not directly affect the membrane potential, but rather have an effect on gene transcription or other metabolic processes in the neuron. The characteristics of the various neurotransmitter systems presented in this section are organized in Table 12.3.
The important thing to remember about neurotransmitters, and signaling chemicals in general, is that the effect is entirely dependent on the receptor. Neurotransmitters bind to one of two classes of receptors at the cell surface, ionotropic or metabotropic (Figure 12.28). Ionotropic receptors are ligand-gated ion channels, such as the nicotinic receptor for acetylcholine or the glycine receptor. A metabotropic receptor involves a complex of proteins that result in metabolic changes within the cell. The receptor complex includes the transmembrane receptor protein, a G protein, and an effector protein. The neurotransmitter, referred to as the first messenger, binds to the receptor protein on the extracellular surface of the cell, and the intracellular side of the protein initiates activity of the G protein. The G protein is a guanosine triphosphate (GTP) hydrolase that physically moves from the receptor protein to the effector protein to activate the latter. An effector protein is an enzyme that catalyzes the generation of a new molecule, which acts as the intracellular mediator of the signal that binds to the receptor. This intracellular mediator is called the second messenger.
Different receptors use different second messengers. Two common examples of second messengers are cyclic adenosine monophosphate (cAMP) and inositol triphosphate (IP3). The enzyme adenylate cyclase (an example of an effector protein) makes cAMP, and phospholipase C is the enzyme that makes IP3. Second messengers, after they are produced by the effector protein, cause metabolic changes within the cell. These changes are most likely the activation of other enzymes in the cell. In neurons, they often modify ion channels, either opening or closing them. These enzymes can also cause changes in the cell, such as the activation of genes in the nucleus, and therefore the increased synthesis of proteins. In neurons, these kinds of changes are often the basis of stronger connections between cells at the synapse and may be the basis of learning and memory.
Figure 12.28 Receptor Types (a) An ionotropic receptor is a channel that opens when the neurotransmitter binds to it. (b) A metabotropic receptor is a complex that causes metabolic changes in the cell when the neurotransmitter binds to it (1). After binding, the G protein hydrolyzes GTP and moves to the effector protein (2). When the G protein contacts the effector protein, a second messenger is generated, such as cAMP (3). The second messenger can then go on to cause changes in the neuron, such as opening or closing ion channels, metabolic changes, and changes in gene transcription.
INTERACTIVE LINK
Watch this video to learn about the release of a neurotransmitter. The action potential reaches the end of the axon, called the axon terminal, and a chemical signal is released to tell the target cell to do something—either to initiate a new action potential, or to suppress that activity. In a very short space, the electrical signal of the action potential is changed into the chemical signal of a neurotransmitter and then back to electrical changes in the target cell membrane. What is the importance of voltage-gated calcium channels in the release of neurotransmitters?
Characteristics of Neurotransmitter Systems
| System | Cholinergic | Amino acids | Biogenic amines | Neuropeptides |
|---|---|---|---|---|
| Neurotransmitters | Acetylcholine | Glutamate, glycine, GABA | Serotonin (5-HT), dopamine, norepinephrine, (epinephrine) | Met-enkephalin, beta-endorphin, VIP, Substance P, etc. |
| Receptors | Nicotinic and muscarinic receptors | Glu receptors, gly receptors, GABA receptors | 5-HT receptors, D1 and D2 receptors, α-adrenergic and β-adrenergic receptors | Receptors are too numerous to list, but are specific to the peptides. |
| Elimination | Degradation by acetylcholinesterase | Reuptake by neurons or glia | Reuptake by neurons | Degradation by enzymes called peptidases |
| Postsynaptic effect | Nicotinic receptor causes depolarization. Muscarinic receptors can cause both depolarization or hyperpolarization depending on the subtype. | Glu receptors cause depolarization. Gly and GABA receptors cause hyperpolarization. | Depolarization or hyperpolarization depends on the specific receptor. For example, D1 receptors cause depolarization and D2 receptors cause hyperpolarization. | Depolarization or hyperpolarization depends on the specific receptor. |
Table 12.3
DISORDERS OF THE...
Nervous System
The underlying cause of some neurodegenerative diseases, such as Alzheimer’s and Parkinson’s, appears to be related to proteins—specifically, to proteins behaving badly. One of the strongest theories of what causes Alzheimer’s disease is based on the accumulation of beta-amyloid plaques, dense conglomerations of a protein that is not functioning correctly. Parkinson’s disease is linked to an increase in a protein known as alpha-synuclein that is toxic to the cells of the substantia nigra nucleus in the midbrain.
For proteins to function correctly, they are dependent on their three-dimensional shape. The linear sequence of amino acids folds into a three-dimensional shape that is based on the interactions between and among those amino acids. When the folding is disturbed, and proteins take on a different shape, they stop functioning correctly. But the disease is not necessarily the result of functional loss of these proteins; rather, these altered proteins start to accumulate and may become toxic. For example, in Alzheimer’s, the hallmark of the disease is the accumulation of these amyloid plaques in the cerebral cortex. The term coined to describe this sort of disease is “proteopathy” and it includes other diseases. Creutzfeld-Jacob disease, the human variant of the prion disease known as mad cow disease in the bovine, also involves the accumulation of amyloid plaques, similar to Alzheimer’s. Diseases of other organ systems can fall into this group as well, such as cystic fibrosis or type 2 diabetes. Recognizing the relationship between these diseases has suggested new therapeutic possibilities. Interfering with the accumulation of the proteins, and possibly as early as their original production within the cell, may unlock new ways to alleviate these devastating diseases.
Key Terms
- absolute refractory period
- time during an action period when another action potential cannot be generated because the voltage-gated Na+ channel is inactivated
- action potential
- change in voltage of a cell membrane in response to a stimulus that results in transmission of an electrical signal; unique to neurons and muscle fibers
- activation gate
- part of the voltage-gated Na+ channel that opens when the membrane voltage reaches threshold
- astrocyte
- glial cell type of the CNS that provides support for neurons and maintains the blood-brain barrier
- autonomic nervous system (ANS)
- functional division of the nervous system that is responsible for homeostatic reflexes that coordinate control of cardiac and smooth muscle, as well as glandular tissue
- axon
- single process of the neuron that carries an electrical signal (action potential) away from the cell body toward a target cell
- axon hillock
- tapering of the neuron cell body that gives rise to the axon
- axon segment
- single stretch of the axon insulated by myelin and bounded by nodes of Ranvier at either end (except for the first, which is after the initial segment, and the last, which is followed by the axon terminal)
- axon terminal
- end of the axon, where there are usually several branches extending toward the target cell
- axoplasm
- cytoplasm of an axon, which is different in composition than the cytoplasm of the neuronal cell body
- biogenic amine
- class of neurotransmitters that are enzymatically derived from amino acids but no longer contain a carboxyl group
- bipolar
- shape of a neuron with two processes extending from the neuron cell body—the axon and one dendrite
- blood-brain barrier (BBB)
- physiological barrier between the circulatory system and the central nervous system that establishes a privileged blood supply, restricting the flow of substances into the CNS
- brain
- the large organ of the central nervous system composed of white and gray matter, contained within the cranium and continuous with the spinal cord
- central nervous system (CNS)
- anatomical division of the nervous system located within the cranial and vertebral cavities, namely the brain and spinal cord
- cerebral cortex
- outermost layer of gray matter in the brain, where conscious perception takes place
- cerebrospinal fluid (CSF)
- circulatory medium within the CNS that is produced by ependymal cells in the choroid plexus filtering the blood
- chemical synapse
- connection between two neurons, or between a neuron and its target, where a neurotransmitter diffuses across a very short distance
- cholinergic system
- neurotransmitter system of acetylcholine, which includes its receptors and the enzyme acetylcholinesterase
- choroid plexus
- specialized structure containing ependymal cells that line blood capillaries and filter blood to produce CSF in the four ventricles of the brain
- continuous conduction
- slow propagation of an action potential along an unmyelinated axon owing to voltage-gated Na+ channels located along the entire length of the cell membrane
- dendrite
- one of many branchlike processes that extends from the neuron cell body and functions as a contact for incoming signals (synapses) from other neurons or sensory cells
- depolarization
- change in a cell membrane potential from rest toward zero
- effector protein
- enzyme that catalyzes the generation of a new molecule, which acts as the intracellular mediator of the signal that binds to the receptor
- electrical synapse
- connection between two neurons, or any two electrically active cells, where ions flow directly through channels spanning their adjacent cell membranes
- electrochemical exclusion
- principle of selectively allowing ions through a channel on the basis of their charge
- enteric nervous system (ENS)
- neural tissue associated with the digestive system that is responsible for nervous control through autonomic connections
- ependymal cell
- glial cell type in the CNS responsible for producing cerebrospinal fluid
- excitable membrane
- cell membrane that regulates the movement of ions so that an electrical signal can be generated
- excitatory postsynaptic potential (EPSP)
- graded potential in the postsynaptic membrane that is the result of depolarization and makes an action potential more likely to occur
- G protein
- guanosine triphosphate (GTP) hydrolase that physically moves from the receptor protein to the effector protein to activate the latter
- ganglion
- localized collection of neuron cell bodies in the peripheral nervous system
- gated
- property of a channel that determines how it opens under specific conditions, such as voltage change or physical deformation
- generator potential
- graded potential from dendrites of a unipolar cell which generates the action potential in the initial segment of that cell’s axon
- glial cell
- one of the various types of neural tissue cells responsible for maintenance of the tissue, and largely responsible for supporting neurons
- graded potential
- change in the membrane potential that varies in size, depending on the size of the stimulus that elicits it
- gray matter
- regions of the nervous system containing cell bodies of neurons with few or no myelinated axons; actually may be more pink or tan in color, but called gray in contrast to white matter
- inactivation gate
- part of a voltage-gated Na+ channel that closes when the membrane potential reaches +30 mV
- inhibitory postsynaptic potential (IPSP)
- graded potential in the postsynaptic membrane that is the result of hyperpolarization and makes an action potential less likely to occur
- initial segment
- first part of the axon as it emerges from the axon hillock, where the electrical signals known as action potentials are generated
- integration
- nervous system function that combines sensory perceptions and higher cognitive functions (memories, learning, emotion, etc.) to produce a response
- ionotropic receptor
- neurotransmitter receptor that acts as an ion channel gate, and opens by the binding of the neurotransmitter
- leakage channel
- ion channel that opens randomly and is not gated to a specific event, also known as a non-gated channel
- ligand-gated channels
- another name for an ionotropic receptor for which a neurotransmitter is the ligand
- lower motor neuron
- second neuron in the motor command pathway that is directly connected to the skeletal muscle
- mechanically gated channel
- ion channel that opens when a physical event directly affects the structure of the protein
- membrane potential
- distribution of charge across the cell membrane, based on the charges of ions
- metabotropic receptor
- neurotransmitter receptor that involves a complex of proteins that cause metabolic changes in a cell
- microglia
- glial cell type in the CNS that serves as the resident component of the immune system
- multipolar
- shape of a neuron that has multiple processes—the axon and two or more dendrites
- muscarinic receptor
- type of acetylcholine receptor protein that is characterized by also binding to muscarine and is a metabotropic receptor
- myelin
- lipid-rich insulating substance surrounding the axons of many neurons, allowing for faster transmission of electrical signals
- myelin sheath
- lipid-rich layer of insulation that surrounds an axon, formed by oligodendrocytes in the CNS and Schwann cells in the PNS; facilitates the transmission of electrical signals
- nerve
- cord-like bundle of axons located in the peripheral nervous system that transmits sensory input and response output to and from the central nervous system
- neuron
- neural tissue cell that is primarily responsible for generating and propagating electrical signals into, within, and out of the nervous system
- neuropeptide
- neurotransmitter type that includes protein molecules and shorter chains of amino acids
- neurotransmitter
- chemical signal that is released from the synaptic end bulb of a neuron to cause a change in the target cell
- nicotinic receptor
- type of acetylcholine receptor protein that is characterized by also binding to nicotine and is an ionotropic receptor
- node of Ranvier
- gap between two myelinated regions of an axon, allowing for strengthening of the electrical signal as it propagates down the axon
- nonspecific channel
- channel that is not specific to one ion over another, such as a nonspecific cation channel that allows any positively charged ion across the membrane
- nucleus
- in the nervous system, a localized collection of neuron cell bodies that are functionally related; a “center” of neural function
- oligodendrocyte
- glial cell type in the CNS that provides the myelin insulation for axons in tracts
- peripheral nervous system (PNS)
- anatomical division of the nervous system that is largely outside the cranial and vertebral cavities, namely all parts except the brain and spinal cord
- postsynaptic potential (PSP)
- graded potential in the postsynaptic membrane caused by the binding of neurotransmitter to protein receptors
- precentral gyrus of the frontal cortex
- region of the cerebral cortex responsible for generating motor commands, where the upper motor neuron cell body is located
- process
- in cells, an extension of a cell body; in the case of neurons, this includes the axon and dendrites
- propagation
- movement of an action potential along the length of an axon
- receptor potential
- graded potential in a specialized sensory cell that directly causes the release of neurotransmitter without an intervening action potential
- refractory period
- time after the initiation of an action potential when another action potential cannot be generated
- relative refractory period
- time during the refractory period when a new action potential can only be initiated by a stronger stimulus than the current action potential because voltage-gated K+ channels are not closed
- repolarization
- return of the membrane potential to its normally negative voltage at the end of the action potential
- resistance
- property of an axon that relates to the ability of particles to diffuse through the cytoplasm; this is inversely proportional to the fiber diameter
- response
- nervous system function that causes a target tissue (muscle or gland) to produce an event as a consequence to stimuli
- resting membrane potential
- the difference in voltage measured across a cell membrane under steady-state conditions, typically -70 mV
- saltatory conduction
- quick propagation of the action potential along a myelinated axon owing to voltage-gated Na+ channels being present only at the nodes of Ranvier
- satellite cell
- glial cell type in the PNS that provides support for neurons in the ganglia
- Schwann cell
- glial cell type in the PNS that provides the myelin insulation for axons in nerves
- sensation
- nervous system function that receives information from the environment and translates it into the electrical signals of nervous tissue
- size exclusion
- principle of selectively allowing ions through a channel on the basis of their relative size
- soma
- in neurons, that portion of the cell that contains the nucleus; the cell body, as opposed to the cell processes (axons and dendrites)
- somatic nervous system (SNS)
- functional division of the nervous system that is concerned with conscious perception, voluntary movement, and skeletal muscle reflexes
- spatial summation
- combination of graded potentials across the neuronal cell membrane caused by signals from separate presynaptic elements that add up to initiate an action potential
- spinal cord
- organ of the central nervous system found within the vertebral cavity and connected with the periphery through spinal nerves; mediates reflex behaviors
- stimulus
- an event in the external or internal environment that registers as activity in a sensory neuron
- summate
- to add together, as in the cumulative change in postsynaptic potentials toward reaching threshold in the membrane, either across a span of the membrane or over a certain amount of time
- synapse
- narrow junction across which a chemical signal passes from neuron to the next, initiating a new electrical signal in the target cell
- synaptic cleft
- small gap between cells in a chemical synapse where neurotransmitter diffuses from the presynaptic element to the postsynaptic element
- synaptic end bulb
- swelling at the end of an axon where neurotransmitter molecules are released onto a target cell across a synapse
- temporal summation
- combination of graded potentials at the same location on a neuron resulting in a strong signal from one input
- thalamus
- region of the central nervous system that acts as a relay for sensory pathways
- thermoreceptor
- type of sensory receptor capable of transducing temperature stimuli into neural action potentials
- threshold
- membrane voltage at which an action potential is initiated
- tract
- bundle of axons in the central nervous system having the same function and point of origin
- unipolar
- shape of a neuron which has only one process that includes both the axon and dendrite
- upper motor neuron
- first neuron in the motor command pathway with its cell body in the cerebral cortex that synapses on the lower motor neuron in the spinal cord
- ventricle
- central cavity within the brain where CSF is produced and circulates
- voltage-gated channel
- ion channel that opens because of a change in the charge distributed across the membrane where it is located
- white matter
- regions of the nervous system containing mostly myelinated axons, making the tissue appear white because of the high lipid content of myelin
Chapter Review
12.1 Basic Structure and Function of the Nervous System
The nervous system can be separated into divisions on the basis of anatomy and physiology. The anatomical divisions are the central and peripheral nervous systems. The CNS is the brain and spinal cord. The PNS is everything else. Functionally, the nervous system can be divided into those regions that are responsible for sensation, those that are responsible for integration, and those that are responsible for generating responses. All of these functional areas are found in both the central and peripheral anatomy.
Considering the anatomical regions of the nervous system, there are specific names for the structures within each division. A localized collection of neuron cell bodies is referred to as a nucleus in the CNS and as a ganglion in the PNS. A bundle of axons is referred to as a tract in the CNS and as a nerve in the PNS. Whereas nuclei and ganglia are specifically in the central or peripheral divisions, axons can cross the boundary between the two. A single axon can be part of a nerve and a tract. The name for that specific structure depends on its location.
Nervous tissue can also be described as gray matter and white matter on the basis of its appearance in unstained tissue. These descriptions are more often used in the CNS. Gray matter is where nuclei are found and white matter is where tracts are found. In the PNS, ganglia are basically gray matter and nerves are white matter.
The nervous system can also be divided on the basis of how it controls the body. The somatic nervous system (SNS) is responsible for functions that result in moving skeletal muscles. Any sensory or integrative functions that result in the movement of skeletal muscle would be considered somatic. The autonomic nervous system (ANS) is responsible for functions that affect cardiac or smooth muscle tissue, or that cause glands to produce their secretions. Autonomic functions are distributed between central and peripheral regions of the nervous system. The sensations that lead to autonomic functions can be the same sensations that are part of initiating somatic responses. Somatic and autonomic integrative functions may overlap as well.
A special division of the nervous system is the enteric nervous system, which is responsible for controlling the digestive organs. Parts of the autonomic nervous system overlap with the enteric nervous system. The enteric nervous system is exclusively found in the periphery because it is the nervous tissue in the organs of the digestive system.
12.2 Nervous Tissue
Nervous tissue contains two major cell types, neurons and glial cells. Neurons are the cells responsible for communication through electrical signals. Glial cells are supporting cells, maintaining the environment around the neurons.
Neurons are polarized cells, based on the flow of electrical signals along their membrane. Signals are received at the dendrites, are passed along the cell body, and propagate along the axon towards the target, which may be another neuron, muscle tissue, or a gland. Many axons are insulated by a lipid-rich substance called myelin. Specific types of glial cells provide this insulation.
Several types of glial cells are found in the nervous system, and they can be categorized by the anatomical division in which they are found. In the CNS, astrocytes, oligodendrocytes, microglia, and ependymal cells are found. Astrocytes are important for maintaining the chemical environment around the neuron and are crucial for regulating the blood-brain barrier. Oligodendrocytes are the myelinating glia in the CNS. Microglia act as phagocytes and play a role in immune surveillance. Ependymal cells are responsible for filtering the blood to produce cerebrospinal fluid, which is a circulatory fluid that performs some of the functions of blood in the brain and spinal cord because of the BBB. In the PNS, satellite cells are supporting cells for the neurons, and Schwann cells insulate peripheral axons.
12.3 The Function of Nervous Tissue
Sensation starts with the activation of a sensory ending, such as the thermoreceptor in the skin sensing the temperature of the water. The sensory endings in the skin initiate an electrical signal that travels along the sensory axon within a nerve into the spinal cord, where it synapses with a neuron in the gray matter of the spinal cord. The temperature information represented in that electrical signal is passed to the next neuron by a chemical signal that diffuses across the small gap of the synapse and initiates a new electrical signal in the target cell. That signal travels through the sensory pathway to the brain, passing through the thalamus, where conscious perception of the water temperature is made possible by the cerebral cortex. Following integration of that information with other cognitive processes and sensory information, the brain sends a command back down to the spinal cord to initiate a motor response by controlling a skeletal muscle. The motor pathway is composed of two cells, the upper motor neuron and the lower motor neuron. The upper motor neuron has its cell body in the cerebral cortex and synapses on a cell in the gray matter of the spinal cord. The lower motor neuron is that cell in the gray matter of the spinal cord and its axon extends into the periphery where it synapses with a skeletal muscle in a neuromuscular junction.
12.4 The Action Potential
The nervous system is characterized by electrical signals that are sent from one area to another. Whether those areas are close or very far apart, the signal must travel along an axon. The basis of the electrical signal is the controlled distribution of ions across the membrane. Transmembrane ion channels regulate when ions can move in or out of the cell, so that a precise signal is generated. This signal is the action potential which has a very characteristic shape based on voltage changes across the membrane in a given time period.
The membrane is normally at rest with established Na+ and K+ concentrations on either side. A stimulus will start the depolarization of the membrane, and voltage-gated channels will result in further depolarization followed by repolarization of the membrane. A slight overshoot of hyperpolarization marks the end of the action potential. While an action potential is in progress, another cannot be generated under the same conditions. While the voltage-gated Na+ channel is inactivated, absolutely no action potentials can be generated. Once that channel has returned to its resting state, a new action potential is possible, but it must be started by a relatively stronger stimulus to overcome the K+ leaving the cell.
The action potential travels down the axon as voltage-gated ion channels are opened by the spreading depolarization. In unmyelinated axons, this happens in a continuous fashion because there are voltage-gated channels throughout the membrane. In myelinated axons, propagation is described as saltatory because voltage-gated channels are only found at the nodes of Ranvier and the electrical events seem to “jump” from one node to the next. Saltatory conduction is faster than continuous conduction, meaning that myelinated axons propagate their signals faster. The diameter of the axon also makes a difference as ions diffusing within the cell have less resistance in a wider space.
12.5 Communication Between Neurons
The basis of the electrical signal within a neuron is the action potential that propagates down the axon. For a neuron to generate an action potential, it needs to receive input from another source, either another neuron or a sensory stimulus. That input will result in opening ion channels in the neuron, resulting in a graded potential based on the strength of the stimulus. Graded potentials can be depolarizing or hyperpolarizing and can summate to affect the probability of the neuron reaching threshold.
Graded potentials can be the result of sensory stimuli. If the sensory stimulus is received by the dendrites of a unipolar sensory neuron, such as the sensory neuron ending in the skin, the graded potential is called a generator potential because it can directly generate the action potential in the initial segment of the axon. If the sensory stimulus is received by a specialized sensory receptor cell, the graded potential is called a receptor potential. Graded potentials produced by interactions between neurons at synapses are called postsynaptic potentials (PSPs). A depolarizing graded potential at a synapse is called an excitatory PSP, and a hyperpolarizing graded potential at a synapse is called an inhibitory PSP.
Synapses are the contacts between neurons, which can either be chemical or electrical in nature. Chemical synapses are far more common. At a chemical synapse, neurotransmitter is released from the presynaptic element and diffuses across the synaptic cleft. The neurotransmitter binds to a receptor protein and causes a change in the postsynaptic membrane (the PSP). The neurotransmitter must be inactivated or removed from the synaptic cleft so that the stimulus is limited in time.
The particular characteristics of a synapse vary based on the neurotransmitter system produced by that neuron. The cholinergic system is found at the neuromuscular junction and in certain places within the nervous system. Amino acids, such as glutamate, glycine, and gamma-aminobutyric acid (GABA) are used as neurotransmitters. Other neurotransmitters are the result of amino acids being enzymatically changed, as in the biogenic amines, or being covalently bonded together, as in the neuropeptides.
Interactive Link Questions
In 2003, the Nobel Prize in Physiology or Medicine was awarded to Paul C. Lauterbur and Sir Peter Mansfield for discoveries related to magnetic resonance imaging (MRI). This is a tool to see the structures of the body (not just the nervous system) that depends on magnetic fields associated with certain atomic nuclei. The utility of this technique in the nervous system is that fat tissue and water appear as different shades between black and white. Because white matter is fatty (from myelin) and gray matter is not, they can be easily distinguished in MRI images. Visit the Nobel Prize website to play an interactive game that demonstrates the use of this technology and compares it with other types of imaging technologies. Also, the results from an MRI session are compared with images obtained from x-ray or computed tomography. How do the imaging techniques shown in this game indicate the separation of white and gray matter compared with the freshly dissected tissue shown earlier?
2.Visit this site to read about a woman that notices that her daughter is having trouble walking up the stairs. This leads to the discovery of a hereditary condition that affects the brain and spinal cord. The electromyography and MRI tests indicated deficiencies in the spinal cord and cerebellum, both of which are responsible for controlling coordinated movements. To what functional division of the nervous system would these structures belong?
3.Visit this site to learn about how nervous tissue is composed of neurons and glial cells. The neurons are dynamic cells with the ability to make a vast number of connections and to respond incredibly quickly to stimuli and to initiate movements based on those stimuli. They are the focus of intense research as failures in physiology can lead to devastating illnesses. Why are neurons only found in animals? Based on what this article says about neuron function, why wouldn’t they be helpful for plants or microorganisms?
4.View the University of Michigan Webscope to see an electron micrograph of a cross-section of a myelinated nerve fiber. The axon contains microtubules and neurofilaments, bounded by a plasma membrane known as the axolemma. Outside the plasma membrane of the axon is the myelin sheath, which is composed of the tightly wrapped plasma membrane of a Schwann cell. What aspects of the cells in this image react with the stain that makes them the deep, dark, black color, such as the multiple layers that are the myelin sheath?
5.What happens across the membrane of an electrically active cell is a dynamic process that is hard to visualize with static images or through text descriptions. View this animation to really understand the process. What is the difference between the driving force for Na+ and K+? And what is similar about the movement of these two ions?
6.Visit this site to see a virtual neurophysiology lab, and to observe electrophysiological processes in the nervous system, where scientists directly measure the electrical signals produced by neurons. Often, the action potentials occur so rapidly that watching a screen to see them occur is not helpful. A speaker is powered by the signals recorded from a neuron and it “pops” each time the neuron fires an action potential. These action potentials are firing so fast that it sounds like static on the radio. Electrophysiologists can recognize the patterns within that static to understand what is happening. Why is the leech model used for measuring the electrical activity of neurons instead of using humans?
7.Watch this video to learn about summation. The process of converting electrical signals to chemical signals and back requires subtle changes that can result in transient increases or decreases in membrane voltage. To cause a lasting change in the target cell, multiple signals are usually added together, or summated. Does spatial summation have to happen all at once, or can the separate signals arrive on the postsynaptic neuron at slightly different times? Explain your answer.
8.Watch this video to learn about the release of a neurotransmitter. The action potential reaches the end of the axon, called the axon terminal, and a chemical signal is released to tell the target cell to do something, either initiate a new action potential, or to suppress that activity. In a very short space, the electrical signal of the action potential is changed into the chemical signal of a neurotransmitter, and then back to electrical changes in the target cell membrane. What is the importance of voltage-gated calcium channels in the release of neurotransmitters?
Review Questions
Which of the following cavities contains a component of the central nervous system?
- abdominal
- pelvic
- cranial
- thoracic
Which structure predominates in the white matter of the brain?
- myelinated axons
- neuronal cell bodies
- ganglia of the parasympathetic nerves
- bundles of dendrites from the enteric nervous system
Which part of a neuron transmits an electrical signal to a target cell?
- dendrites
- soma
- cell body
- axon
Which term describes a bundle of axons in the peripheral nervous system?
- nucleus
- ganglion
- tract
- nerve
Which functional division of the nervous system would be responsible for the physiological changes seen during exercise (e.g., increased heart rate and sweating)?
- somatic
- autonomic
- enteric
- central
What type of glial cell provides myelin for the axons in a tract?
- oligodendrocyte
- astrocyte
- Schwann cell
- satellite cell
Which part of a neuron contains the nucleus?
- dendrite
- soma
- axon
- synaptic end bulb
Which of the following substances is least able to cross the blood-brain barrier?
- water
- sodium ions
- glucose
- white blood cells
What type of glial cell is the resident macrophage behind the blood-brain barrier?
- microglia
- astrocyte
- Schwann cell
- satellite cell
What two types of macromolecules are the main components of myelin?
- carbohydrates and lipids
- proteins and nucleic acids
- lipids and proteins
- carbohydrates and nucleic acids
If a thermoreceptor is sensitive to temperature sensations, what would a chemoreceptor be sensitive to?
- light
- sound
- molecules
- vibration
Which of these locations is where the greatest level of integration is taking place in the example of testing the temperature of the shower?
- skeletal muscle
- spinal cord
- thalamus
- cerebral cortex
How long does all the signaling through the sensory pathway, within the central nervous system, and through the motor command pathway take?
- 1 to 2 minutes
- 1 to 2 seconds
- fraction of a second
- varies with graded potential
What is the target of an upper motor neuron?
- cerebral cortex
- lower motor neuron
- skeletal muscle
- thalamus
What ion enters a neuron causing depolarization of the cell membrane?
- sodium
- chloride
- potassium
- phosphate
Voltage-gated Na+ channels open upon reaching what state?
- resting potential
- threshold
- repolarization
- overshoot
What does a ligand-gated channel require in order to open?
- increase in concentration of Na+ ions
- binding of a neurotransmitter
- increase in concentration of K+ ions
- depolarization of the membrane
What does a mechanically gated channel respond to?
- physical stimulus
- chemical stimulus
- increase in resistance
- decrease in resistance
Which of the following voltages would most likely be measured during the relative refractory period?
- +30 mV
- 0 mV
- -45 mV
- -80 mv
Which of the following is probably going to propagate an action potential fastest?
- a thin, unmyelinated axon
- a thin, myelinated axon
- a thick, unmyelinated axon
- a thick, myelinated axon
How much of a change in the membrane potential is necessary for the summation of postsynaptic potentials to result in an action potential being generated?
- +30 mV
- +15 mV
- +10 mV
- -15 mV
A channel opens on a postsynaptic membrane that causes a negative ion to enter the cell. What type of graded potential is this?
- depolarizing
- repolarizing
- hyperpolarizing
- non-polarizing
What neurotransmitter is released at the neuromuscular junction?
- norepinephrine
- serotonin
- dopamine
- acetylcholine
What type of receptor requires an effector protein to initiate a signal?
- biogenic amine
- ionotropic receptor
- cholinergic system
- metabotropic receptor
Which of the following neurotransmitters is associated with inhibition exclusively?
- GABA
- acetylcholine
- glutamate
- norepinephrine
Critical Thinking Questions
What responses are generated by the nervous system when you run on a treadmill? Include an example of each type of tissue that is under nervous system control.
35.When eating food, what anatomical and functional divisions of the nervous system are involved in the perceptual experience?
36.Multiple sclerosis is a demyelinating disease affecting the central nervous system. What type of cell would be the most likely target of this disease? Why?
37.Which type of neuron, based on its shape, is best suited for relaying information directly from one neuron to another? Explain why.
38.Sensory fibers, or pathways, are referred to as “afferent.” Motor fibers, or pathways, are referred to as “efferent.” What can you infer about the meaning of these two terms (afferent and efferent) in a structural or anatomical context?
39.If a person has a motor disorder and cannot move their arm voluntarily, but their muscles have tone, which motor neuron—upper or lower—is probably affected? Explain why.
40.What does it mean for an action potential to be an “all or none” event?
41.The conscious perception of pain is often delayed because of the time it takes for the sensations to reach the cerebral cortex. Why would this be the case based on propagation of the axon potential?
42.If a postsynaptic cell has synapses from five different cells, and three cause EPSPs and two of them cause IPSPs, give an example of a series of depolarizations and hyperpolarizations that would result in the neuron reaching threshold.
43.Why is the receptor the important element determining the effect a neurotransmitter has on a target cell?
|
oercommons
|
2025-03-18T00:39:10.839862
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/56375/overview",
"title": "Anatomy and Physiology, Regulation, Integration, and Control",
"author": null
}
|
https://oercommons.org/courseware/lesson/56376/overview
|
Anatomy of the Nervous System
Introduction
Figure 13.1 Human Nervous System The ability to balance like an acrobat combines functions throughout the nervous system. The central and peripheral divisions coordinate control of the body using the senses of balance, body position, and touch on the soles of the feet. (credit: Rhett Sutphin)
CHAPTER OBJECTIVES
After studying this chapter, you will be able to:
- Relate the developmental processes of the embryonic nervous system to the adult structures
- Name the major regions of the adult nervous system
- Locate regions of the cerebral cortex on the basis of anatomical landmarks common to all human brains
- Describe the regions of the spinal cord in cross-section
- List the cranial nerves in order of anatomical location and provide the central and peripheral connections
- List the spinal nerves by vertebral region and by which nerve plexus each supplies
The nervous system is responsible for controlling much of the body, both through somatic (voluntary) and autonomic (involuntary) functions. The structures of the nervous system must be described in detail to understand how many of these functions are possible. There is a physiological concept known as localization of function that states that certain structures are specifically responsible for prescribed functions. It is an underlying concept in all of anatomy and physiology, but the nervous system illustrates the concept very well.
Fresh, unstained nervous tissue can be described as gray or white matter, and within those two types of tissue it can be very hard to see any detail. However, as specific regions and structures have been described, they were related to specific functions. Understanding these structures and the functions they perform requires a detailed description of the anatomy of the nervous system, delving deep into what the central and peripheral structures are.
The place to start this study of the nervous system is the beginning of the individual human life, within the womb. The embryonic development of the nervous system allows for a simple framework on which progressively more complicated structures can be built. With this framework in place, a thorough investigation of the nervous system is possible.
The Embryologic Perspective
- Describe the growth and differentiation of the neural tube
- Relate the different stages of development to the adult structures of the central nervous system
- Explain the expansion of the ventricular system of the adult brain from the central canal of the neural tube
- Describe the connections of the diencephalon and cerebellum on the basis of patterns of embryonic development
The brain is a complex organ composed of gray parts and white matter, which can be hard to distinguish. Starting from an embryologic perspective allows you to understand more easily how the parts relate to each other. The embryonic nervous system begins as a very simple structure—essentially just a straight line, which then gets increasingly complex. Looking at the development of the nervous system with a couple of early snapshots makes it easier to understand the whole complex system.
Many structures that appear to be adjacent in the adult brain are not connected, and the connections that exist may seem arbitrary. But there is an underlying order to the system that comes from how different parts develop. By following the developmental pattern, it is possible to learn what the major regions of the nervous system are.
The Neural Tube
To begin, a sperm cell and an egg cell fuse to become a fertilized egg. The fertilized egg cell, or zygote, starts dividing to generate the cells that make up an entire organism. Sixteen days after fertilization, the developing embryo’s cells belong to one of three germ layers that give rise to the different tissues in the body. The endoderm, or inner tissue, is responsible for generating the lining tissues of various spaces within the body, such as the mucosae of the digestive and respiratory systems. The mesoderm, or middle tissue, gives rise to most of the muscle and connective tissues. Finally the ectoderm, or outer tissue, develops into the integumentary system (the skin) and the nervous system. It is probably not difficult to see that the outer tissue of the embryo becomes the outer covering of the body. But how is it responsible for the nervous system?
As the embryo develops, a portion of the ectoderm differentiates into a specialized region of neuroectoderm, which is the precursor for the tissue of the nervous system. Molecular signals induce cells in this region to differentiate into the neuroepithelium, forming a neural plate. The cells then begin to change shape, causing the tissue to buckle and fold inward (Figure 13.2). A neural groove forms, visible as a line along the dorsal surface of the embryo. The ridge-like edge on either side of the neural groove is referred as the neural fold. As the neural folds come together and converge, the underlying structure forms into a tube just beneath the ectoderm called the neural tube. Cells from the neural folds then separate from the ectoderm to form a cluster of cells referred to as the neural crest, which runs lateral to the neural tube. The neural crest migrates away from the nascent, or embryonic, central nervous system (CNS) that will form along the neural groove and develops into several parts of the peripheral nervous system (PNS), including the enteric nervous tissue. Many tissues that are not part of the nervous system also arise from the neural crest, such as craniofacial cartilage and bone, and melanocytes.
Figure 13.2 Early Embryonic Development of Nervous System The neuroectoderm begins to fold inward to form the neural groove. As the two sides of the neural groove converge, they form the neural tube, which lies beneath the ectoderm. The anterior end of the neural tube will develop into the brain, and the posterior portion will become the spinal cord. The neural crest develops into peripheral structures.
At this point, the early nervous system is a simple, hollow tube. It runs from the anterior end of the embryo to the posterior end. Beginning at 25 days, the anterior end develops into the brain, and the posterior portion becomes the spinal cord. This is the most basic arrangement of tissue in the nervous system, and it gives rise to the more complex structures by the fourth week of development.
Primary Vesicles
As the anterior end of the neural tube starts to develop into the brain, it undergoes a couple of enlargements; the result is the production of sac-like vesicles. Similar to a child’s balloon animal, the long, straight neural tube begins to take on a new shape. Three vesicles form at the first stage, which are called primary vesicles. These vesicles are given names that are based on Greek words, the main root word being enkephalon, which means “brain” (en- = “inside”; kephalon = “head”). The prefix to each generally corresponds to its position along the length of the developing nervous system.
The prosencephalon (pros- = “in front”) is the forward-most vesicle, and the term can be loosely translated to mean forebrain. The mesencephalon (mes- = “middle”) is the next vesicle, which can be called the midbrain. The third vesicle at this stage is the rhombencephalon. The first part of this word is also the root of the word rhombus, which is a geometrical figure with four sides of equal length (a square is a rhombus with 90° angles). Whereas prosencephalon and mesencephalon translate into the English words forebrain and midbrain, there is not a word for “four-sided-figure-brain.” However, the third vesicle can be called the hindbrain. One way of thinking about how the brain is arranged is to use these three regions—forebrain, midbrain, and hindbrain—which are based on the primary vesicle stage of development (Figure 13.3a).
Secondary Vesicles
The brain continues to develop, and the vesicles differentiate further (see Figure 13.3b). The three primary vesicles become five secondary vesicles. The prosencephalon enlarges into two new vesicles called the telencephalon and the diencephalon. The telecephalon will become the cerebrum. The diencephalon gives rise to several adult structures; two that will be important are the thalamus and the hypothalamus. In the embryonic diencephalon, a structure known as the eye cup develops, which will eventually become the retina, the nervous tissue of the eye called the retina. This is a rare example of nervous tissue developing as part of the CNS structures in the embryo, but becoming a peripheral structure in the fully formed nervous system.
The mesencephalon does not differentiate into any finer divisions. The midbrain is an established region of the brain at the primary vesicle stage of development and remains that way. The rest of the brain develops around it and constitutes a large percentage of the mass of the brain. Dividing the brain into forebrain, midbrain, and hindbrain is useful in considering its developmental pattern, but the midbrain is a small proportion of the entire brain, relatively speaking.
The rhombencephalon develops into the metencephalon and myelencephalon. The metencephalon corresponds to the adult structure known as the pons and also gives rise to the cerebellum. The cerebellum (from the Latin meaning “little brain”) accounts for about 10 percent of the mass of the brain and is an important structure in itself. The most significant connection between the cerebellum and the rest of the brain is at the pons, because the pons and cerebellum develop out of the same vesicle. The myelencephalon corresponds to the adult structure known as the medulla oblongata. The structures that come from the mesencephalon and rhombencephalon, except for the cerebellum, are collectively considered the brain stem, which specifically includes the midbrain, pons, and medulla.
Figure 13.3 Primary and Secondary Vesicle Stages of Development The embryonic brain develops complexity through enlargements of the neural tube called vesicles; (a) The primary vesicle stage has three regions, and (b) the secondary vesicle stage has five regions.
INTERACTIVE LINK
Watch this animation to examine the development of the brain, starting with the neural tube. As the anterior end of the neural tube develops, it enlarges into the primary vesicles that establish the forebrain, midbrain, and hindbrain. Those structures continue to develop throughout the rest of embryonic development and into adolescence. They are the basis of the structure of the fully developed adult brain. How would you describe the difference in the relative sizes of the three regions of the brain when comparing the early (25th embryonic day) brain and the adult brain?
Spinal Cord Development
While the brain is developing from the anterior neural tube, the spinal cord is developing from the posterior neural tube. However, its structure does not differ from the basic layout of the neural tube. It is a long, straight cord with a small, hollow space down the center. The neural tube is defined in terms of its anterior versus posterior portions, but it also has a dorsal–ventral dimension. As the neural tube separates from the rest of the ectoderm, the side closest to the surface is dorsal, and the deeper side is ventral.
As the spinal cord develops, the cells making up the wall of the neural tube proliferate and differentiate into the neurons and glia of the spinal cord. The dorsal tissues will be associated with sensory functions, and the ventral tissues will be associated with motor functions.
Relating Embryonic Development to the Adult Brain
Embryonic development can help in understanding the structure of the adult brain because it establishes a framework on which more complex structures can be built. First, the neural tube establishes the anterior–posterior dimension of the nervous system, which is called the neuraxis. The embryonic nervous system in mammals can be said to have a standard arrangement. Humans (and other primates, to some degree) make this complicated by standing up and walking on two legs. The anterior–posterior dimension of the neuraxis overlays the superior–inferior dimension of the body. However, there is a major curve between the brain stem and forebrain, which is called the cephalic flexure. Because of this, the neuraxis starts in an inferior position—the end of the spinal cord—and ends in an anterior position, the front of the cerebrum. If this is confusing, just imagine a four-legged animal standing up on two legs. Without the flexure in the brain stem, and at the top of the neck, that animal would be looking straight up instead of straight in front (Figure 13.4).
Figure 13.4 Human Neuraxis The mammalian nervous system is arranged with the neural tube running along an anterior to posterior axis, from nose to tail for a four-legged animal like a dog. Humans, as two-legged animals, have a bend in the neuraxis between the brain stem and the diencephalon, along with a bend in the neck, so that the eyes and the face are oriented forward.
In summary, the primary vesicles help to establish the basic regions of the nervous system: forebrain, midbrain, and hindbrain. These divisions are useful in certain situations, but they are not equivalent regions. The midbrain is small compared with the hindbrain and particularly the forebrain. The secondary vesicles go on to establish the major regions of the adult nervous system that will be followed in this text. The telencephalon is the cerebrum, which is the major portion of the human brain. The diencephalon continues to be referred to by this Greek name, because there is no better term for it (dia- = “through”). The diencephalon is between the cerebrum and the rest of the nervous system and can be described as the region through which all projections have to pass between the cerebrum and everything else. The brain stem includes the midbrain, pons, and medulla, which correspond to the mesencephalon, metencephalon, and myelencephalon. The cerebellum, being a large portion of the brain, is considered a separate region. Table 13.1 connects the different stages of development to the adult structures of the CNS.
One other benefit of considering embryonic development is that certain connections are more obvious because of how these adult structures are related. The retina, which began as part of the diencephalon, is primarily connected to the diencephalon. The eyes are just inferior to the anterior-most part of the cerebrum, but the optic nerve extends back to the thalamus as the optic tract, with branches into a region of the hypothalamus. There is also a connection of the optic tract to the midbrain, but the mesencephalon is adjacent to the diencephalon, so that is not difficult to imagine. The cerebellum originates out of the metencephalon, and its largest white matter connection is to the pons, also from the metencephalon. There are connections between the cerebellum and both the medulla and midbrain, which are adjacent structures in the secondary vesicle stage of development. In the adult brain, the cerebellum seems close to the cerebrum, but there is no direct connection between them.
Another aspect of the adult CNS structures that relates to embryonic development is the ventricles—open spaces within the CNS where cerebrospinal fluid circulates. They are the remnant of the hollow center of the neural tube. The four ventricles and the tubular spaces associated with them can be linked back to the hollow center of the embryonic brain (see Table 13.1).
Stages of Embryonic Development
| Neural tube | Primary vesicle stage | Secondary vesicle stage | Adult structures | Ventricles |
|---|---|---|---|---|
| Anterior neural tube | Prosencephalon | Telencephalon | Cerebrum | Lateral ventricles |
| Anterior neural tube | Prosencephalon | Diencephalon | Diencephalon | Third ventricle |
| Anterior neural tube | Mesencephalon | Mesencephalon | Midbrain | Cerebral aqueduct |
| Anterior neural tube | Rhombencephalon | Metencephalon | Pons cerebellum | Fourth ventricle |
| Anterior neural tube | Rhombencephalon | Myelencephalon | Medulla | Fourth ventricle |
| Posterior neural tube | Spinal cord | Central canal |
Table 13.1
DISORDERS OF THE...
Nervous System
Early formation of the nervous system depends on the formation of the neural tube. A groove forms along the dorsal surface of the embryo, which becomes deeper until its edges meet and close off to form the tube. If this fails to happen, especially in the posterior region where the spinal cord forms, a developmental defect called spina bifida occurs. The closing of the neural tube is important for more than just the proper formation of the nervous system. The surrounding tissues are dependent on the correct development of the tube. The connective tissues surrounding the CNS can be involved as well.
There are three classes of this disorder: occulta, meningocele, and myelomeningocele (Figure 13.5). The first type, spina bifida occulta, is the mildest because the vertebral bones do not fully surround the spinal cord, but the spinal cord itself is not affected. No functional differences may be noticed, which is what the word occulta means; it is hidden spina bifida. The other two types both involve the formation of a cyst—a fluid-filled sac of the connective tissues that cover the spinal cord called the meninges. “Meningocele” means that the meninges protrude through the spinal column but nerves may not be involved and few symptoms are present, though complications may arise later in life. “Myelomeningocele” means that the meninges protrude and spinal nerves are involved, and therefore severe neurological symptoms can be present.
Often surgery to close the opening or to remove the cyst is necessary. The earlier that surgery can be performed, the better the chances of controlling or limiting further damage or infection at the opening. For many children with meningocele, surgery will alleviate the pain, although they may experience some functional loss. Because the myelomeningocele form of spina bifida involves more extensive damage to the nervous tissue, neurological damage may persist, but symptoms can often be handled. Complications of the spinal cord may present later in life, but overall life expectancy is not reduced.
Figure 13.5 Spinal Bifida (a) Spina bifida is a birth defect of the spinal cord caused when the neural tube does not completely close, but the rest of development continues. The result is the emergence of meninges and neural tissue through the vertebral column. (b) Fetal myelomeningocele is evident in this ultrasound taken at 21 weeks.
INTERACTIVE LINK
Watch this video to learn about the white matter in the cerebrum that develops during childhood and adolescence. This is a composite of MRI images taken of the brains of people from 5 years of age through 20 years of age, demonstrating how the cerebrum changes. As the color changes to blue, the ratio of gray matter to white matter changes. The caption for the video describes it as “less gray matter,” which is another way of saying “more white matter.” If the brain does not finish developing until approximately 20 years of age, can teenagers be held responsible for behaving badly?
The Central Nervous System
- Name the major regions of the adult brain
- Describe the connections between the cerebrum and brain stem through the diencephalon, and from those regions into the spinal cord
- Recognize the complex connections within the subcortical structures of the basal nuclei
- Explain the arrangement of gray and white matter in the spinal cord
The brain and the spinal cord are the central nervous system, and they represent the main organs of the nervous system. The spinal cord is a single structure, whereas the adult brain is described in terms of four major regions: the cerebrum, the diencephalon, the brain stem, and the cerebellum. A person’s conscious experiences are based on neural activity in the brain. The regulation of homeostasis is governed by a specialized region in the brain. The coordination of reflexes depends on the integration of sensory and motor pathways in the spinal cord.
The Cerebrum
The iconic gray mantle of the human brain, which appears to make up most of the mass of the brain, is the cerebrum (Figure 13.6). The wrinkled portion is the cerebral cortex, and the rest of the structure is beneath that outer covering. There is a large separation between the two sides of the cerebrum called the longitudinal fissure. It separates the cerebrum into two distinct halves, a right and left cerebral hemisphere. Deep within the cerebrum, the white matter of the corpus callosum provides the major pathway for communication between the two hemispheres of the cerebral cortex.
Figure 13.6 The Cerebrum The cerebrum is a large component of the CNS in humans, and the most obvious aspect of it is the folded surface called the cerebral cortex.
Many of the higher neurological functions, such as memory, emotion, and consciousness, are the result of cerebral function. The complexity of the cerebrum is different across vertebrate species. The cerebrum of the most primitive vertebrates is not much more than the connection for the sense of smell. In mammals, the cerebrum comprises the outer gray matter that is the cortex (from the Latin word meaning “bark of a tree”) and several deep nuclei that belong to three important functional groups. The basal nuclei are responsible for cognitive processing, the most important function being that associated with planning movements. The basal forebrain contains nuclei that are important in learning and memory. The limbic cortex is the region of the cerebral cortex that is part of the limbic system, a collection of structures involved in emotion, memory, and behavior.
Cerebral Cortex
The cerebrum is covered by a continuous layer of gray matter that wraps around either side of the forebrain—the cerebral cortex. This thin, extensive region of wrinkled gray matter is responsible for the higher functions of the nervous system. A gyrus(plural = gyri) is the ridge of one of those wrinkles, and a sulcus (plural = sulci) is the groove between two gyri. The pattern of these folds of tissue indicates specific regions of the cerebral cortex.
The head is limited by the size of the birth canal, and the brain must fit inside the cranial cavity of the skull. Extensive folding in the cerebral cortex enables more gray matter to fit into this limited space. If the gray matter of the cortex were peeled off of the cerebrum and laid out flat, its surface area would be roughly equal to one square meter.
The folding of the cortex maximizes the amount of gray matter in the cranial cavity. During embryonic development, as the telencephalon expands within the skull, the brain goes through a regular course of growth that results in everyone’s brain having a similar pattern of folds. The surface of the brain can be mapped on the basis of the locations of large gyri and sulci. Using these landmarks, the cortex can be separated into four major regions, or lobes (Figure 13.7). The lateral sulcus that separates the temporal lobe from the other regions is one such landmark. Superior to the lateral sulcus are the parietal lobe and frontal lobe, which are separated from each other by the central sulcus. The posterior region of the cortex is the occipital lobe, which has no obvious anatomical border between it and the parietal or temporal lobes on the lateral surface of the brain. From the medial surface, an obvious landmark separating the parietal and occipital lobes is called the parieto-occipital sulcus. The fact that there is no obvious anatomical border between these lobes is consistent with the functions of these regions being interrelated.
Figure 13.7 Lobes of the Cerebral Cortex The cerebral cortex is divided into four lobes. Extensive folding increases the surface area available for cerebral functions.
Different regions of the cerebral cortex can be associated with particular functions, a concept known as localization of function. In the early 1900s, a German neuroscientist named Korbinian Brodmann performed an extensive study of the microscopic anatomy—the cytoarchitecture—of the cerebral cortex and divided the cortex into 52 separate regions on the basis of the histology of the cortex. His work resulted in a system of classification known as Brodmann’s areas, which is still used today to describe the anatomical distinctions within the cortex (Figure 13.8). The results from Brodmann’s work on the anatomy align very well with the functional differences within the cortex. Areas 17 and 18 in the occipital lobe are responsible for primary visual perception. That visual information is complex, so it is processed in the temporal and parietal lobes as well.
The temporal lobe is associated with primary auditory sensation, known as Brodmann’s areas 41 and 42 in the superior temporal lobe. Because regions of the temporal lobe are part of the limbic system, memory is an important function associated with that lobe. Memory is essentially a sensory function; memories are recalled sensations such as the smell of Mom’s baking or the sound of a barking dog. Even memories of movement are really the memory of sensory feedback from those movements, such as stretching muscles or the movement of the skin around a joint. Structures in the temporal lobe are responsible for establishing long-term memory, but the ultimate location of those memories is usually in the region in which the sensory perception was processed.
The main sensation associated with the parietal lobe is somatosensation, meaning the general sensations associated with the body. Posterior to the central sulcus is the postcentral gyrus, the primary somatosensory cortex, which is identified as Brodmann’s areas 1, 2, and 3. All of the tactile senses are processed in this area, including touch, pressure, tickle, pain, itch, and vibration, as well as more general senses of the body such as proprioception and kinesthesia, which are the senses of body position and movement, respectively.
Anterior to the central sulcus is the frontal lobe, which is primarily associated with motor functions. The precentral gyrus is the primary motor cortex. Cells from this region of the cerebral cortex are the upper motor neurons that instruct cells in the spinal cord to move skeletal muscles. Anterior to this region are a few areas that are associated with planned movements. The premotor area is responsible for thinking of a movement to be made. The frontal eye fields are important in eliciting eye movements and in attending to visual stimuli. Broca’s area is responsible for the production of language, or controlling movements responsible for speech; in the vast majority of people, it is located only on the left side. Anterior to these regions is the prefrontal lobe, which serves cognitive functions that can be the basis of personality, short-term memory, and consciousness. The prefrontal lobotomy is an outdated mode of treatment for personality disorders (psychiatric conditions) that profoundly affected the personality of the patient.
Figure 13.8 Brodmann's Areas of the Cerebral Cortex Brodmann mapping of functionally distinct regions of the cortex was based on its cytoarchitecture at a microscopic level.
Subcortical structures
Beneath the cerebral cortex are sets of nuclei known as subcortical nuclei that augment cortical processes. The nuclei of the basal forebrain serve as the primary location for acetylcholine production, which modulates the overall activity of the cortex, possibly leading to greater attention to sensory stimuli. Alzheimer’s disease is associated with a loss of neurons in the basal forebrain. The hippocampus and amygdala are medial-lobe structures that, along with the adjacent cortex, are involved in long-term memory formation and emotional responses. The basal nuclei are a set of nuclei in the cerebrum responsible for comparing cortical processing with the general state of activity in the nervous system to influence the likelihood of movement taking place. For example, while a student is sitting in a classroom listening to a lecture, the basal nuclei will keep the urge to jump up and scream from actually happening. (The basal nuclei are also referred to as the basal ganglia, although that is potentially confusing because the term ganglia is typically used for peripheral structures.)
The major structures of the basal nuclei that control movement are the caudate, putamen, and globus pallidus, which are located deep in the cerebrum. The caudate is a long nucleus that follows the basic C-shape of the cerebrum from the frontal lobe, through the parietal and occipital lobes, into the temporal lobe. The putamen is mostly deep in the anterior regions of the frontal and parietal lobes. Together, the caudate and putamen are called the striatum. The globus pallidus is a layered nucleus that lies just medial to the putamen; they are called the lenticular nuclei because they look like curved pieces fitting together like lenses. The globus pallidus has two subdivisions, the external and internal segments, which are lateral and medial, respectively. These nuclei are depicted in a frontal section of the brain in Figure 13.9.
Figure 13.9 Frontal Section of Cerebral Cortex and Basal Nuclei The major components of the basal nuclei, shown in a frontal section of the brain, are the caudate (just lateral to the lateral ventricle), the putamen (inferior to the caudate and separated by the large white-matter structure called the internal capsule), and the globus pallidus (medial to the putamen).
The basal nuclei in the cerebrum are connected with a few more nuclei in the brain stem that together act as a functional group that forms a motor pathway. Two streams of information processing take place in the basal nuclei. All input to the basal nuclei is from the cortex into the striatum (Figure 13.10). The direct pathway is the projection of axons from the striatum to the globus pallidus internal segment (GPi) and the substantia nigra pars reticulata (SNr). The GPi/SNr then projects to the thalamus, which projects back to the cortex. The indirect pathway is the projection of axons from the striatum to the globus pallidus external segment (GPe), then to the subthalamic nucleus (STN), and finally to GPi/SNr. The two streams both target the GPi/SNr, but one has a direct projection and the other goes through a few intervening nuclei. The direct pathway causes the disinhibitionof the thalamus (inhibition of one cell on a target cell that then inhibits the first cell), whereas the indirect pathway causes, or reinforces, the normal inhibition of the thalamus. The thalamus then can either excite the cortex (as a result of the direct pathway) or fail to excite the cortex (as a result of the indirect pathway).
Figure 13.10 Connections of Basal Nuclei Input to the basal nuclei is from the cerebral cortex, which is an excitatory connection releasing glutamate as a neurotransmitter. This input is to the striatum, or the caudate and putamen. In the direct pathway, the striatum projects to the internal segment of the globus pallidus and the substantia nigra pars reticulata (GPi/SNr). This is an inhibitory pathway, in which GABA is released at the synapse, and the target cells are hyperpolarized and less likely to fire. The output from the basal nuclei is to the thalamus, which is an inhibitory projection using GABA.
The switch between the two pathways is the substantia nigra pars compacta, which projects to the striatum and releases the neurotransmitter dopamine. Dopamine receptors are either excitatory (D1-type receptors) or inhibitory (D2-type receptors). The direct pathway is activated by dopamine, and the indirect pathway is inhibited by dopamine. When the substantia nigra pars compacta is firing, it signals to the basal nuclei that the body is in an active state, and movement will be more likely. When the substantia nigra pars compacta is silent, the body is in a passive state, and movement is inhibited. To illustrate this situation, while a student is sitting listening to a lecture, the substantia nigra pars compacta would be silent and the student less likely to get up and walk around. Likewise, while the professor is lecturing, and walking around at the front of the classroom, the professor’s substantia nigra pars compacta would be active, in keeping with his or her activity level.
INTERACTIVE LINK
Watch this video to learn about the basal nuclei (also known as the basal ganglia), which have two pathways that process information within the cerebrum. As shown in this video, the direct pathway is the shorter pathway through the system that results in increased activity in the cerebral cortex and increased motor activity. The direct pathway is described as resulting in “disinhibition” of the thalamus. What does disinhibition mean? What are the two neurons doing individually to cause this?
INTERACTIVE LINK
Watch this video to learn about the basal nuclei (also known as the basal ganglia), which have two pathways that process information within the cerebrum. As shown in this video, the indirect pathway is the longer pathway through the system that results in decreased activity in the cerebral cortex, and therefore less motor activity. The indirect pathway has an extra couple of connections in it, including disinhibition of the subthalamic nucleus. What is the end result on the thalamus, and therefore on movement initiated by the cerebral cortex?
EVERYDAY CONNECTION
The Myth of Left Brain/Right Brain
There is a persistent myth that people are “right-brained” or “left-brained,” which is an oversimplification of an important concept about the cerebral hemispheres. There is some lateralization of function, in which the left side of the brain is devoted to language function and the right side is devoted to spatial and nonverbal reasoning. Whereas these functions are predominantly associated with those sides of the brain, there is no monopoly by either side on these functions. Many pervasive functions, such as language, are distributed globally around the cerebrum.
Some of the support for this misconception has come from studies of split brains. A drastic way to deal with a rare and devastating neurological condition (intractable epilepsy) is to separate the two hemispheres of the brain. After sectioning the corpus callosum, a split-brained patient will have trouble producing verbal responses on the basis of sensory information processed on the right side of the cerebrum, leading to the idea that the left side is responsible for language function.
However, there are well-documented cases of language functions lost from damage to the right side of the brain. The deficits seen in damage to the left side of the brain are classified as aphasia, a loss of speech function; damage on the right side can affect the use of language. Right-side damage can result in a loss of ability to understand figurative aspects of speech, such as jokes, irony, or metaphors. Nonverbal aspects of speech can be affected by damage to the right side, such as facial expression or body language, and right-side damage can lead to a “flat affect” in speech, or a loss of emotional expression in speech—sounding like a robot when talking.
The Diencephalon
The diencephalon is the one region of the adult brain that retains its name from embryologic development. The etymology of the word diencephalon translates to “through brain.” It is the connection between the cerebrum and the rest of the nervous system, with one exception. The rest of the brain, the spinal cord, and the PNS all send information to the cerebrum through the diencephalon. Output from the cerebrum passes through the diencephalon. The single exception is the system associated with olfaction, or the sense of smell, which connects directly with the cerebrum. In the earliest vertebrate species, the cerebrum was not much more than olfactory bulbs that received peripheral information about the chemical environment (to call it smell in these organisms is imprecise because they lived in the ocean).
The diencephalon is deep beneath the cerebrum and constitutes the walls of the third ventricle. The diencephalon can be described as any region of the brain with “thalamus” in its name. The two major regions of the diencephalon are the thalamus itself and the hypothalamus (Figure 13.11). There are other structures, such as the epithalamus, which contains the pineal gland, or the subthalamus, which includes the subthalamic nucleus that is part of the basal nuclei.
Thalamus
The thalamus is a collection of nuclei that relay information between the cerebral cortex and the periphery, spinal cord, or brain stem. All sensory information, except for the sense of smell, passes through the thalamus before processing by the cortex. Axons from the peripheral sensory organs, or intermediate nuclei, synapse in the thalamus, and thalamic neurons project directly to the cerebrum. It is a requisite synapse in any sensory pathway, except for olfaction. The thalamus does not just pass the information on, it also processes that information. For example, the portion of the thalamus that receives visual information will influence what visual stimuli are important, or what receives attention.
The cerebrum also sends information down to the thalamus, which usually communicates motor commands. This involves interactions with the cerebellum and other nuclei in the brain stem. The cerebrum interacts with the basal nuclei, which involves connections with the thalamus. The primary output of the basal nuclei is to the thalamus, which relays that output to the cerebral cortex. The cortex also sends information to the thalamus that will then influence the effects of the basal nuclei.
Hypothalamus
Inferior and slightly anterior to the thalamus is the hypothalamus, the other major region of the diencephalon. The hypothalamus is a collection of nuclei that are largely involved in regulating homeostasis. The hypothalamus is the executive region in charge of the autonomic nervous system and the endocrine system through its regulation of the anterior pituitary gland. Other parts of the hypothalamus are involved in memory and emotion as part of the limbic system.
Figure 13.11 The Diencephalon The diencephalon is composed primarily of the thalamus and hypothalamus, which together define the walls of the third ventricle. The thalami are two elongated, ovoid structures on either side of the midline that make contact in the middle. The hypothalamus is inferior and anterior to the thalamus, culminating in a sharp angle to which the pituitary gland is attached.
Brain Stem
The midbrain and hindbrain (composed of the pons and the medulla) are collectively referred to as the brain stem (Figure 13.12). The structure emerges from the ventral surface of the forebrain as a tapering cone that connects the brain to the spinal cord. Attached to the brain stem, but considered a separate region of the adult brain, is the cerebellum. The midbrain coordinates sensory representations of the visual, auditory, and somatosensory perceptual spaces. The pons is the main connection with the cerebellum. The pons and the medulla regulate several crucial functions, including the cardiovascular and respiratory systems and rates.
The cranial nerves connect through the brain stem and provide the brain with the sensory input and motor output associated with the head and neck, including most of the special senses. The major ascending and descending pathways between the spinal cord and brain, specifically the cerebrum, pass through the brain stem.
Figure 13.12 The Brain Stem The brain stem comprises three regions: the midbrain, the pons, and the medulla.
Midbrain
One of the original regions of the embryonic brain, the midbrain is a small region between the thalamus and pons. It is separated into the tectum and tegmentum, from the Latin words for roof and floor, respectively. The cerebral aqueduct passes through the center of the midbrain, such that these regions are the roof and floor of that canal.
The tectum is composed of four bumps known as the colliculi (singular = colliculus), which means “little hill” in Latin. The inferior colliculus is the inferior pair of these enlargements and is part of the auditory brain stem pathway. Neurons of the inferior colliculus project to the thalamus, which then sends auditory information to the cerebrum for the conscious perception of sound. The superior colliculus is the superior pair and combines sensory information about visual space, auditory space, and somatosensory space. Activity in the superior colliculus is related to orienting the eyes to a sound or touch stimulus. If you are walking along the sidewalk on campus and you hear chirping, the superior colliculus coordinates that information with your awareness of the visual location of the tree right above you. That is the correlation of auditory and visual maps. If you suddenly feel something wet fall on your head, your superior colliculus integrates that with the auditory and visual maps and you know that the chirping bird just relieved itself on you. You want to look up to see the culprit, but do not.
The tegmentum is continuous with the gray matter of the rest of the brain stem. Throughout the midbrain, pons, and medulla, the tegmentum contains the nuclei that receive and send information through the cranial nerves, as well as regions that regulate important functions such as those of the cardiovascular and respiratory systems.
Pons
The word pons comes from the Latin word for bridge. It is visible on the anterior surface of the brain stem as the thick bundle of white matter attached to the cerebellum. The pons is the main connection between the cerebellum and the brain stem. The bridge-like white matter is only the anterior surface of the pons; the gray matter beneath that is a continuation of the tegmentum from the midbrain. Gray matter in the tegmentum region of the pons contains neurons receiving descending input from the forebrain that is sent to the cerebellum.
Medulla
The medulla is the region known as the myelencephalon in the embryonic brain. The initial portion of the name, “myel,” refers to the significant white matter found in this region—especially on its exterior, which is continuous with the white matter of the spinal cord. The tegmentum of the midbrain and pons continues into the medulla because this gray matter is responsible for processing cranial nerve information. A diffuse region of gray matter throughout the brain stem, known as the reticular formation, is related to sleep and wakefulness, such as general brain activity and attention.
The Cerebellum
The cerebellum, as the name suggests, is the “little brain.” It is covered in gyri and sulci like the cerebrum, and looks like a miniature version of that part of the brain (Figure 13.13). The cerebellum is largely responsible for comparing information from the cerebrum with sensory feedback from the periphery through the spinal cord. It accounts for approximately 10 percent of the mass of the brain.
Figure 13.13 The Cerebellum The cerebellum is situated on the posterior surface of the brain stem. Descending input from the cerebellum enters through the large white matter structure of the pons. Ascending input from the periphery and spinal cord enters through the fibers of the inferior olive. Output goes to the midbrain, which sends a descending signal to the spinal cord.
Descending fibers from the cerebrum have branches that connect to neurons in the pons. Those neurons project into the cerebellum, providing a copy of motor commands sent to the spinal cord. Sensory information from the periphery, which enters through spinal or cranial nerves, is copied to a nucleus in the medulla known as the inferior olive. Fibers from this nucleus enter the cerebellum and are compared with the descending commands from the cerebrum. If the primary motor cortex of the frontal lobe sends a command down to the spinal cord to initiate walking, a copy of that instruction is sent to the cerebellum. Sensory feedback from the muscles and joints, proprioceptive information about the movements of walking, and sensations of balance are sent to the cerebellum through the inferior olive and the cerebellum compares them. If walking is not coordinated, perhaps because the ground is uneven or a strong wind is blowing, then the cerebellum sends out a corrective command to compensate for the difference between the original cortical command and the sensory feedback. The output of the cerebellum is into the midbrain, which then sends a descending input to the spinal cord to correct the messages going to skeletal muscles.
The Spinal Cord
The description of the CNS is concentrated on the structures of the brain, but the spinal cord is another major organ of the system. Whereas the brain develops out of expansions of the neural tube into primary and then secondary vesicles, the spinal cord maintains the tube structure and is only specialized into certain regions. As the spinal cord continues to develop in the newborn, anatomical features mark its surface. The anterior midline is marked by the anterior median fissure, and the posterior midline is marked by the posterior median sulcus. Axons enter the posterior side through the dorsal (posterior) nerve root, which marks the posterolateral sulcus on either side. The axons emerging from the anterior side do so through the ventral (anterior) nerve root. Note that it is common to see the terms dorsal (dorsal = “back”) and ventral (ventral = “belly”) used interchangeably with posterior and anterior, particularly in reference to nerves and the structures of the spinal cord. You should learn to be comfortable with both.
On the whole, the posterior regions are responsible for sensory functions and the anterior regions are associated with motor functions. This comes from the initial development of the spinal cord, which is divided into the basal plate and the alar plate. The basal plate is closest to the ventral midline of the neural tube, which will become the anterior face of the spinal cord and gives rise to motor neurons. The alar plate is on the dorsal side of the neural tube and gives rise to neurons that will receive sensory input from the periphery.
The length of the spinal cord is divided into regions that correspond to the regions of the vertebral column. The name of a spinal cord region corresponds to the level at which spinal nerves pass through the intervertebral foramina. Immediately adjacent to the brain stem is the cervical region, followed by the thoracic, then the lumbar, and finally the sacral region. The spinal cord is not the full length of the vertebral column because the spinal cord does not grow significantly longer after the first or second year, but the skeleton continues to grow. The nerves that emerge from the spinal cord pass through the intervertebral formina at the respective levels. As the vertebral column grows, these nerves grow with it and result in a long bundle of nerves that resembles a horse’s tail and is named the cauda equina. The sacral spinal cord is at the level of the upper lumbar vertebral bones. The spinal nerves extend from their various levels to the proper level of the vertebral column.
Gray Horns
In cross-section, the gray matter of the spinal cord has the appearance of an ink-blot test, with the spread of the gray matter on one side replicated on the other—a shape reminiscent of a bulbous capital “H.” As shown in Figure 13.14, the gray matter is subdivided into regions that are referred to as horns. The posterior horn is responsible for sensory processing. The anterior horn sends out motor signals to the skeletal muscles. The lateral horn, which is only found in the thoracic, upper lumbar, and sacral regions, is the central component of the sympathetic division of the autonomic nervous system.
Some of the largest neurons of the spinal cord are the multipolar motor neurons in the anterior horn. The fibers that cause contraction of skeletal muscles are the axons of these neurons. The motor neuron that causes contraction of the big toe, for example, is located in the sacral spinal cord. The axon that has to reach all the way to the belly of that muscle may be a meter in length. The neuronal cell body that maintains that long fiber must be quite large, possibly several hundred micrometers in diameter, making it one of the largest cells in the body.
Figure 13.14 Cross-section of Spinal Cord The cross-section of a thoracic spinal cord segment shows the posterior, anterior, and lateral horns of gray matter, as well as the posterior, anterior, and lateral columns of white matter. LM × 40. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
White Columns
Just as the gray matter is separated into horns, the white matter of the spinal cord is separated into columns. Ascending tractsof nervous system fibers in these columns carry sensory information up to the brain, whereas descending tracts carry motor commands from the brain. Looking at the spinal cord longitudinally, the columns extend along its length as continuous bands of white matter. Between the two posterior horns of gray matter are the posterior columns. Between the two anterior horns, and bounded by the axons of motor neurons emerging from that gray matter area, are the anterior columns. The white matter on either side of the spinal cord, between the posterior horn and the axons of the anterior horn neurons, are the lateral columns. The posterior columns are composed of axons of ascending tracts. The anterior and lateral columns are composed of many different groups of axons of both ascending and descending tracts—the latter carrying motor commands down from the brain to the spinal cord to control output to the periphery.
INTERACTIVE LINK
Watch this video to learn about the gray matter of the spinal cord that receives input from fibers of the dorsal (posterior) root and sends information out through the fibers of the ventral (anterior) root. As discussed in this video, these connections represent the interactions of the CNS with peripheral structures for both sensory and motor functions. The cervical and lumbar spinal cords have enlargements as a result of larger populations of neurons. What are these enlargements responsible for?
DISORDERS OF THE...
Basal Nuclei
Parkinson’s disease is a disorder of the basal nuclei, specifically of the substantia nigra, that demonstrates the effects of the direct and indirect pathways. Parkinson’s disease is the result of neurons in the substantia nigra pars compacta dying. These neurons release dopamine into the striatum. Without that modulatory influence, the basal nuclei are stuck in the indirect pathway, without the direct pathway being activated. The direct pathway is responsible for increasing cortical movement commands. The increased activity of the indirect pathway results in the hypokinetic disorder of Parkinson’s disease.
Parkinson’s disease is neurodegenerative, meaning that neurons die that cannot be replaced, so there is no cure for the disorder. Treatments for Parkinson’s disease are aimed at increasing dopamine levels in the striatum. Currently, the most common way of doing that is by providing the amino acid L-DOPA, which is a precursor to the neurotransmitter dopamine and can cross the blood-brain barrier. With levels of the precursor elevated, the remaining cells of the substantia nigra pars compacta can make more neurotransmitter and have a greater effect. Unfortunately, the patient will become less responsive to L-DOPA treatment as time progresses, and it can cause increased dopamine levels elsewhere in the brain, which are associated with psychosis or schizophrenia.
INTERACTIVE LINK
Visit this site for a thorough explanation of Parkinson’s disease.
INTERACTIVE LINK
Compared with the nearest evolutionary relative, the chimpanzee, the human has a brain that is huge. At a point in the past, a common ancestor gave rise to the two species of humans and chimpanzees. That evolutionary history is long and is still an area of intense study. But something happened to increase the size of the human brain relative to the chimpanzee. Read this article in which the author explores the current understanding of why this happened.
According to one hypothesis about the expansion of brain size, what tissue might have been sacrificed so energy was available to grow our larger brain? Based on what you know about that tissue and nervous tissue, why would there be a trade-off between them in terms of energy use?
Circulation and the Central Nervous System
- Describe the vessels that supply the CNS with blood
- Name the components of the ventricular system and the regions of the brain in which each is located
- Explain the production of cerebrospinal fluid and its flow through the ventricles
- Explain how a disruption in circulation would result in a stroke
The CNS is crucial to the operation of the body, and any compromise in the brain and spinal cord can lead to severe difficulties. The CNS has a privileged blood supply, as suggested by the blood-brain barrier. The function of the tissue in the CNS is crucial to the survival of the organism, so the contents of the blood cannot simply pass into the central nervous tissue. To protect this region from the toxins and pathogens that may be traveling through the blood stream, there is strict control over what can move out of the general systems and into the brain and spinal cord. Because of this privilege, the CNS needs specialized structures for the maintenance of circulation. This begins with a unique arrangement of blood vessels carrying fresh blood into the CNS. Beyond the supply of blood, the CNS filters that blood into cerebrospinal fluid (CSF), which is then circulated through the cavities of the brain and spinal cord called ventricles.
Blood Supply to the Brain
A lack of oxygen to the CNS can be devastating, and the cardiovascular system has specific regulatory reflexes to ensure that the blood supply is not interrupted. There are multiple routes for blood to get into the CNS, with specializations to protect that blood supply and to maximize the ability of the brain to get an uninterrupted perfusion.
Arterial Supply
The major artery carrying recently oxygenated blood away from the heart is the aorta. The very first branches off the aorta supply the heart with nutrients and oxygen. The next branches give rise to the common carotid arteries, which further branch into the internal carotid arteries. The external carotid arteries supply blood to the tissues on the surface of the cranium. The bases of the common carotids contain stretch receptors that immediately respond to the drop in blood pressure upon standing. The orthostatic reflex is a reaction to this change in body position, so that blood pressure is maintained against the increasing effect of gravity (orthostatic means “standing up”). Heart rate increases—a reflex of the sympathetic division of the autonomic nervous system—and this raises blood pressure.
The internal carotid artery enters the cranium through the carotid canal in the temporal bone. A second set of vessels that supply the CNS are the vertebral arteries, which are protected as they pass through the neck region by the transverse foramina of the cervical vertebrae. The vertebral arteries enter the cranium through the foramen magnum of the occipital bone. Branches off the left and right vertebral arteries merge into the anterior spinal artery supplying the anterior aspect of the spinal cord, found along the anterior median fissure. The two vertebral arteries then merge into the basilar artery, which gives rise to branches to the brain stem and cerebellum. The left and right internal carotid arteries and branches of the basilar artery all become the circle of Willis, a confluence of arteries that can maintain perfusion of the brain even if narrowing or a blockage limits flow through one part (Figure 13.15).
Figure 13.15 Circle of Willis The blood supply to the brain enters through the internal carotid arteries and the vertebral arteries, eventually giving rise to the circle of Willis.
INTERACTIVE LINK
Watch this animation to see how blood flows to the brain and passes through the circle of Willis before being distributed through the cerebrum. The circle of Willis is a specialized arrangement of arteries that ensure constant perfusion of the cerebrum even in the event of a blockage of one of the arteries in the circle. The animation shows the normal direction of flow through the circle of Willis to the middle cerebral artery. Where would the blood come from if there were a blockage just posterior to the middle cerebral artery on the left?
Venous Return
After passing through the CNS, blood returns to the circulation through a series of dural sinuses and veins (Figure 13.16). The superior sagittal sinus runs in the groove of the longitudinal fissure, where it absorbs CSF from the meninges. The superior sagittal sinus drains to the confluence of sinuses, along with the occipital sinuses and straight sinus, to then drain into the transverse sinuses. The transverse sinuses connect to the sigmoid sinuses, which then connect to the jugular veins. From there, the blood continues toward the heart to be pumped to the lungs for reoxygenation.
Figure 13.16 Dural Sinuses and Veins Blood drains from the brain through a series of sinuses that connect to the jugular veins.
Protective Coverings of the Brain and Spinal Cord
The outer surface of the CNS is covered by a series of membranes composed of connective tissue called the meninges, which protect the brain. The dura mater is a thick fibrous layer and a strong protective sheath over the entire brain and spinal cord. It is anchored to the inner surface of the cranium and vertebral cavity. The arachnoid mater is a membrane of thin fibrous tissue that forms a loose sac around the CNS. Beneath the arachnoid is a thin, filamentous mesh called the arachnoid trabeculae, which looks like a spider web, giving this layer its name. Directly adjacent to the surface of the CNS is the pia mater, a thin fibrous membrane that follows the convolutions of gyri and sulci in the cerebral cortex and fits into other grooves and indentations (Figure 13.17).
Figure 13.17 Meningeal Layers of Superior Sagittal Sinus The layers of the meninges in the longitudinal fissure of the superior sagittal sinus are shown, with the dura mater adjacent to the inner surface of the cranium, the pia mater adjacent to the surface of the brain, and the arachnoid and subarachnoid space between them. An arachnoid villus is shown emerging into the dural sinus to allow CSF to filter back into the blood for drainage.
Dura Mater
Like a thick cap covering the brain, the dura mater is a tough outer covering. The name comes from the Latin for “tough mother” to represent its physically protective role. It encloses the entire CNS and the major blood vessels that enter the cranium and vertebral cavity. It is directly attached to the inner surface of the bones of the cranium and to the very end of the vertebral cavity.
There are infoldings of the dura that fit into large crevasses of the brain. Two infoldings go through the midline separations of the cerebrum and cerebellum; one forms a shelf-like tent between the occipital lobes of the cerebrum and the cerebellum, and the other surrounds the pituitary gland. The dura also surrounds and supports the venous sinuses.
Arachnoid Mater
The middle layer of the meninges is the arachnoid, named for the spider-web–like trabeculae between it and the pia mater. The arachnoid defines a sac-like enclosure around the CNS. The trabeculae are found in the subarachnoid space, which is filled with circulating CSF. The arachnoid emerges into the dural sinuses as the arachnoid granulations, where the CSF is filtered back into the blood for drainage from the nervous system.
The subarachnoid space is filled with circulating CSF, which also provides a liquid cushion to the brain and spinal cord. Similar to clinical blood work, a sample of CSF can be withdrawn to find chemical evidence of neuropathology or metabolic traces of the biochemical functions of nervous tissue.
Pia Mater
The outer surface of the CNS is covered in the thin fibrous membrane of the pia mater. It is thought to have a continuous layer of cells providing a fluid-impermeable membrane. The name pia mater comes from the Latin for “tender mother,” suggesting the thin membrane is a gentle covering for the brain. The pia extends into every convolution of the CNS, lining the inside of the sulci in the cerebral and cerebellar cortices. At the end of the spinal cord, a thin filament extends from the inferior end of CNS at the upper lumbar region of the vertebral column to the sacral end of the vertebral column. Because the spinal cord does not extend through the lower lumbar region of the vertebral column, a needle can be inserted through the dura and arachnoid layers to withdraw CSF. This procedure is called a lumbar puncture and avoids the risk of damaging the central tissue of the spinal cord. Blood vessels that are nourishing the central nervous tissue are between the pia mater and the nervous tissue.
DISORDERS OF THE...
Meninges
Meningitis is an inflammation of the meninges, the three layers of fibrous membrane that surround the CNS. Meningitis can be caused by infection by bacteria or viruses. The particular pathogens are not special to meningitis; it is just an inflammation of that specific set of tissues from what might be a broader infection. Bacterial meningitis can be caused by Streptococcus, Staphylococcus, or the tuberculosis pathogen, among many others. Viral meningitis is usually the result of common enteroviruses (such as those that cause intestinal disorders), but may be the result of the herpes virus or West Nile virus. Bacterial meningitis tends to be more severe.
The symptoms associated with meningitis can be fever, chills, nausea, vomiting, light sensitivity, soreness of the neck, or severe headache. More important are the neurological symptoms, such as changes in mental state (confusion, memory deficits, and other dementia-type symptoms). A serious risk of meningitis can be damage to peripheral structures because of the nerves that pass through the meninges. Hearing loss is a common result of meningitis.
The primary test for meningitis is a lumbar puncture. A needle inserted into the lumbar region of the spinal column through the dura mater and arachnoid membrane into the subarachnoid space can be used to withdraw the fluid for chemical testing. Fatality occurs in 5 to 40 percent of children and 20 to 50 percent of adults with bacterial meningitis. Treatment of bacterial meningitis is through antibiotics, but viral meningitis cannot be treated with antibiotics because viruses do not respond to that type of drug. Fortunately, the viral forms are milder.
INTERACTIVE LINK
Watch this video that describes the procedure known as the lumbar puncture, a medical procedure used to sample the CSF. Because of the anatomy of the CNS, it is a relative safe location to insert a needle. Why is the lumbar puncture performed in the lower lumbar area of the vertebral column?
The Ventricular System
Cerebrospinal fluid (CSF) circulates throughout and around the CNS. In other tissues, water and small molecules are filtered through capillaries as the major contributor to the interstitial fluid. In the brain, CSF is produced in special structures to perfuse through the nervous tissue of the CNS and is continuous with the interstitial fluid. Specifically, CSF circulates to remove metabolic wastes from the interstitial fluids of nervous tissues and return them to the blood stream. The ventricles are the open spaces within the brain where CSF circulates. In some of these spaces, CSF is produced by filtering of the blood that is performed by a specialized membrane known as a choroid plexus. The CSF circulates through all of the ventricles to eventually emerge into the subarachnoid space where it will be reabsorbed into the blood.
The Ventricles
There are four ventricles within the brain, all of which developed from the original hollow space within the neural tube, the central canal. The first two are named the lateral ventricles and are deep within the cerebrum. These ventricles are connected to the third ventricle by two openings called the interventricular foramina. The third ventricle is the space between the left and right sides of the diencephalon, which opens into the cerebral aqueduct that passes through the midbrain. The aqueduct opens into the fourth ventricle, which is the space between the cerebellum and the pons and upper medulla (Figure 13.18).
Figure 13.18 Cerebrospinal Fluid Circulation The choroid plexus in the four ventricles produce CSF, which is circulated through the ventricular system and then enters the subarachnoid space through the median and lateral apertures. The CSF is then reabsorbed into the blood at the arachnoid granulations, where the arachnoid membrane emerges into the dural sinuses.
As the telencephalon enlarges and grows into the cranial cavity, it is limited by the space within the skull. The telencephalon is the most anterior region of what was the neural tube, but cannot grow past the limit of the frontal bone of the skull. Because the cerebrum fits into this space, it takes on a C-shaped formation, through the frontal, parietal, occipital, and finally temporal regions. The space within the telencephalon is stretched into this same C-shape. The two ventricles are in the left and right sides, and were at one time referred to as the first and second ventricles. The interventricular foramina connect the frontal region of the lateral ventricles with the third ventricle.
The third ventricle is the space bounded by the medial walls of the hypothalamus and thalamus. The two thalami touch in the center in most brains as the massa intermedia, which is surrounded by the third ventricle. The cerebral aqueduct opens just inferior to the epithalamus and passes through the midbrain. The tectum and tegmentum of the midbrain are the roof and floor of the cerebral aqueduct, respectively. The aqueduct opens up into the fourth ventricle. The floor of the fourth ventricle is the dorsal surface of the pons and upper medulla (that gray matter making a continuation of the tegmentum of the midbrain). The fourth ventricle then narrows into the central canal of the spinal cord.
The ventricular system opens up to the subarachnoid space from the fourth ventricle. The single median aperture and the pair of lateral apertures connect to the subarachnoid space so that CSF can flow through the ventricles and around the outside of the CNS. Cerebrospinal fluid is produced within the ventricles by a type of specialized membrane called a choroid plexus. Ependymal cells (one of the types of glial cells described in the introduction to the nervous system) surround blood capillaries and filter the blood to make CSF. The fluid is a clear solution with a limited amount of the constituents of blood. It is essentially water, small molecules, and electrolytes. Oxygen and carbon dioxide are dissolved into the CSF, as they are in blood, and can diffuse between the fluid and the nervous tissue.
Cerebrospinal Fluid Circulation
The choroid plexuses are found in all four ventricles. Observed in dissection, they appear as soft, fuzzy structures that may still be pink, depending on how well the circulatory system is cleared in preparation of the tissue. The CSF is produced from components extracted from the blood, so its flow out of the ventricles is tied to the pulse of cardiovascular circulation.
From the lateral ventricles, the CSF flows into the third ventricle, where more CSF is produced, and then through the cerebral aqueduct into the fourth ventricle where even more CSF is produced. A very small amount of CSF is filtered at any one of the plexuses, for a total of about 500 milliliters daily, but it is continuously made and pulses through the ventricular system, keeping the fluid moving. From the fourth ventricle, CSF can continue down the central canal of the spinal cord, but this is essentially a cul-de-sac, so more of the fluid leaves the ventricular system and moves into the subarachnoid space through the median and lateral apertures.
Within the subarachnoid space, the CSF flows around all of the CNS, providing two important functions. As with elsewhere in its circulation, the CSF picks up metabolic wastes from the nervous tissue and moves it out of the CNS. It also acts as a liquid cushion for the brain and spinal cord. By surrounding the entire system in the subarachnoid space, it provides a thin buffer around the organs within the strong, protective dura mater. The arachnoid granulations are outpocketings of the arachnoid membrane into the dural sinuses so that CSF can be reabsorbed into the blood, along with the metabolic wastes. From the dural sinuses, blood drains out of the head and neck through the jugular veins, along with the rest of the circulation for blood, to be reoxygenated by the lungs and wastes to be filtered out by the kidneys (Table 13.2).
INTERACTIVE LINK
Watch this animation that shows the flow of CSF through the brain and spinal cord, and how it originates from the ventricles and then spreads into the space within the meninges, where the fluids then move into the venous sinuses to return to the cardiovascular circulation. What are the structures that produce CSF and where are they found? How are the structures indicated in this animation?
Components of CSF Circulation
| Lateral ventricles | Third ventricle | Cerebral aqueduct | Fourth ventricle | Central canal | Subarachnoid space | |
|---|---|---|---|---|---|---|
| Location in CNS | Cerebrum | Diencephalon | Midbrain | Between pons/upper medulla and cerebellum | Spinal cord | External to entire CNS |
| Blood vessel structure | Choroid plexus | Choroid plexus | None | Choroid plexus | None | Arachnoid granulations |
Table 13.2
DISORDERS OF THE...
Central Nervous System
The supply of blood to the brain is crucial to its ability to perform many functions. Without a steady supply of oxygen, and to a lesser extent glucose, the nervous tissue in the brain cannot keep up its extensive electrical activity. These nutrients get into the brain through the blood, and if blood flow is interrupted, neurological function is compromised.
The common name for a disruption of blood supply to the brain is a stroke. It is caused by a blockage to an artery in the brain. The blockage is from some type of embolus: a blood clot, a fat embolus, or an air bubble. When the blood cannot travel through the artery, the surrounding tissue that is deprived starves and dies. Strokes will often result in the loss of very specific functions. A stroke in the lateral medulla, for example, can cause a loss in the ability to swallow. Sometimes, seemingly unrelated functions will be lost because they are dependent on structures in the same region. Along with the swallowing in the previous example, a stroke in that region could affect sensory functions from the face or extremities because important white matter pathways also pass through the lateral medulla. Loss of blood flow to specific regions of the cortex can lead to the loss of specific higher functions, from the ability to recognize faces to the ability to move a particular region of the body. Severe or limited memory loss can be the result of a temporal lobe stroke.
Related to strokes are transient ischemic attacks (TIAs), which can also be called “mini-strokes.” These are events in which a physical blockage may be temporary, cutting off the blood supply and oxygen to a region, but not to the extent that it causes cell death in that region. While the neurons in that area are recovering from the event, neurological function may be lost. Function can return if the area is able to recover from the event.
Recovery from a stroke (or TIA) is strongly dependent on the speed of treatment. Often, the person who is present and notices something is wrong must then make a decision. The mnemonic FAST helps people remember what to look for when someone is dealing with sudden losses of neurological function. If someone complains of feeling “funny,” check these things quickly: Look at the person’s face. Does he or she have problems moving Face muscles and making regular facial expressions? Ask the person to raise his or her Arms above the head. Can the person lift one arm but not the other? Has the person’s Speech changed? Is he or she slurring words or having trouble saying things? If any of these things have happened, then it is Time to call for help.
Sometimes, treatment with blood-thinning drugs can alleviate the problem, and recovery is possible. If the tissue is damaged, the amazing thing about the nervous system is that it is adaptable. With physical, occupational, and speech therapy, victims of strokes can recover, or more accurately relearn, functions.
The Peripheral Nervous System
- Describe the structures found in the PNS
- Distinguish between somatic and autonomic structures, including the special peripheral structures of the enteric nervous system
- Name the twelve cranial nerves and explain the functions associated with each
- Describe the sensory and motor components of spinal nerves and the plexuses that they pass through
The PNS is not as contained as the CNS because it is defined as everything that is not the CNS. Some peripheral structures are incorporated into the other organs of the body. In describing the anatomy of the PNS, it is necessary to describe the common structures, the nerves and the ganglia, as they are found in various parts of the body. Many of the neural structures that are incorporated into other organs are features of the digestive system; these structures are known as the enteric nervous systemand are a special subset of the PNS.
Ganglia
A ganglion is a group of neuron cell bodies in the periphery. Ganglia can be categorized, for the most part, as either sensory ganglia or autonomic ganglia, referring to their primary functions. The most common type of sensory ganglion is a dorsal (posterior) root ganglion. These ganglia are the cell bodies of neurons with axons that are sensory endings in the periphery, such as in the skin, and that extend into the CNS through the dorsal nerve root. The ganglion is an enlargement of the nerve root. Under microscopic inspection, it can be seen to include the cell bodies of the neurons, as well as bundles of fibers that are the posterior nerve root (Figure 13.19). The cells of the dorsal root ganglion are unipolar cells, classifying them by shape. Also, the small round nuclei of satellite cells can be seen surrounding—as if they were orbiting—the neuron cell bodies.
Figure 13.19 Dorsal Root Ganglion The cell bodies of sensory neurons, which are unipolar neurons by shape, are seen in this photomicrograph. Also, the fibrous region is composed of the axons of these neurons that are passing through the ganglion to be part of the dorsal nerve root (tissue source: canine). LM × 40. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
Figure 13.20 Spinal Cord and Root Ganglion The slide includes both a cross-section of the lumbar spinal cord and a section of the dorsal root ganglion (see also Figure 13.19) (tissue source: canine). LM × 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
INTERACTIVE LINK
View the University of Michigan WebScope to explore the tissue sample in greater detail. If you zoom in on the dorsal root ganglion, you can see smaller satellite glial cells surrounding the large cell bodies of the sensory neurons. From what structure do satellite cells derive during embryologic development?
Another type of sensory ganglion is a cranial nerve ganglion. This is analogous to the dorsal root ganglion, except that it is associated with a cranial nerve instead of a spinal nerve. The roots of cranial nerves are within the cranium, whereas the ganglia are outside the skull. For example, the trigeminal ganglion is superficial to the temporal bone whereas its associated nerve is attached to the mid-pons region of the brain stem. The neurons of cranial nerve ganglia are also unipolar in shape with associated satellite cells.
The other major category of ganglia are those of the autonomic nervous system, which is divided into the sympathetic and parasympathetic nervous systems. The sympathetic chain ganglia constitute a row of ganglia along the vertebral column that receive central input from the lateral horn of the thoracic and upper lumbar spinal cord. Superior to the chain ganglia are three paravertebral ganglia in the cervical region. Three other autonomic ganglia that are related to the sympathetic chain are the prevertebral ganglia, which are located outside of the chain but have similar functions. They are referred to as prevertebral because they are anterior to the vertebral column. The neurons of these autonomic ganglia are multipolar in shape, with dendrites radiating out around the cell body where synapses from the spinal cord neurons are made. The neurons of the chain, paravertebral, and prevertebral ganglia then project to organs in the head and neck, thoracic, abdominal, and pelvic cavities to regulate the sympathetic aspect of homeostatic mechanisms.
Another group of autonomic ganglia are the terminal ganglia that receive input from cranial nerves or sacral spinal nerves and are responsible for regulating the parasympathetic aspect of homeostatic mechanisms. These two sets of ganglia, sympathetic and parasympathetic, often project to the same organs—one input from the chain ganglia and one input from a terminal ganglion—to regulate the overall function of an organ. For example, the heart receives two inputs such as these; one increases heart rate, and the other decreases it. The terminal ganglia that receive input from cranial nerves are found in the head and neck, as well as the thoracic and upper abdominal cavities, whereas the terminal ganglia that receive sacral input are in the lower abdominal and pelvic cavities.
Terminal ganglia below the head and neck are often incorporated into the wall of the target organ as a plexus. A plexus, in a general sense, is a network of fibers or vessels. This can apply to nervous tissue (as in this instance) or structures containing blood vessels (such as a choroid plexus). For example, the enteric plexus is the extensive network of axons and neurons in the wall of the small and large intestines. The enteric plexus is actually part of the enteric nervous system, along with the gastric plexuses and the esophageal plexus. Though the enteric nervous system receives input originating from central neurons of the autonomic nervous system, it does not require CNS input to function. In fact, it operates independently to regulate the digestive system.
Nerves
Bundles of axons in the PNS are referred to as nerves. These structures in the periphery are different than the central counterpart, called a tract. Nerves are composed of more than just nervous tissue. They have connective tissues invested in their structure, as well as blood vessels supplying the tissues with nourishment. The outer surface of a nerve is a surrounding layer of fibrous connective tissue called the epineurium. Within the nerve, axons are further bundled into fascicles, which are each surrounded by their own layer of fibrous connective tissue called perineurium. Finally, individual axons are surrounded by loose connective tissue called the endoneurium (Figure 13.21). These three layers are similar to the connective tissue sheaths for muscles. Nerves are associated with the region of the CNS to which they are connected, either as cranial nerves connected to the brain or spinal nerves connected to the spinal cord.
Figure 13.21 Nerve Structure The structure of a nerve is organized by the layers of connective tissue on the outside, around each fascicle, and surrounding the individual nerve fibers (tissue source: simian). LM × 40. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
Figure 13.22 Close-Up of Nerve Trunk Zoom in on this slide of a nerve trunk to examine the endoneurium, perineurium, and epineurium in greater detail (tissue source: simian). LM × 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
INTERACTIVE LINK
View the University of Michigan WebScope to explore the tissue sample in greater detail. With what structures in a skeletal muscle are the endoneurium, perineurium, and epineurium comparable?
Cranial Nerves
The nerves attached to the brain are the cranial nerves, which are primarily responsible for the sensory and motor functions of the head and neck (one of these nerves targets organs in the thoracic and abdominal cavities as part of the parasympathetic nervous system). There are twelve cranial nerves, which are designated CNI through CNXII for “Cranial Nerve,” using Roman numerals for 1 through 12. They can be classified as sensory nerves, motor nerves, or a combination of both, meaning that the axons in these nerves originate out of sensory ganglia external to the cranium or motor nuclei within the brain stem. Sensory axons enter the brain to synapse in a nucleus. Motor axons connect to skeletal muscles of the head or neck. Three of the nerves are solely composed of sensory fibers; five are strictly motor; and the remaining four are mixed nerves.
Learning the cranial nerves is a tradition in anatomy courses, and students have always used mnemonic devices to remember the nerve names. A traditional mnemonic is the rhyming couplet, “On Old Olympus’ Towering Tops/A Finn And German Viewed Some Hops,” in which the initial letter of each word corresponds to the initial letter in the name of each nerve. The names of the nerves have changed over the years to reflect current usage and more accurate naming. An exercise to help learn this sort of information is to generate a mnemonic using words that have personal significance. The names of the cranial nerves are listed in Table 13.3 along with a brief description of their function, their source (sensory ganglion or motor nucleus), and their target (sensory nucleus or skeletal muscle). They are listed here with a brief explanation of each nerve (Figure 13.23).
The olfactory nerve and optic nerve are responsible for the sense of smell and vision, respectively. The oculomotor nerve is responsible for eye movements by controlling four of the extraocular muscles. It is also responsible for lifting the upper eyelid when the eyes point up, and for pupillary constriction. The trochlear nerve and the abducens nerve are both responsible for eye movement, but do so by controlling different extraocular muscles. The trigeminal nerve is responsible for cutaneous sensations of the face and controlling the muscles of mastication. The facial nerve is responsible for the muscles involved in facial expressions, as well as part of the sense of taste and the production of saliva. The vestibulocochlear nerve is responsible for the senses of hearing and balance. The glossopharyngeal nerve is responsible for controlling muscles in the oral cavity and upper throat, as well as part of the sense of taste and the production of saliva. The vagus nerve is responsible for contributing to homeostatic control of the organs of the thoracic and upper abdominal cavities. The spinal accessory nerveis responsible for controlling the muscles of the neck, along with cervical spinal nerves. The hypoglossal nerve is responsible for controlling the muscles of the lower throat and tongue.
Figure 13.23 The Cranial Nerves The anatomical arrangement of the roots of the cranial nerves observed from an inferior view of the brain.
Three of the cranial nerves also contain autonomic fibers, and a fourth is almost purely a component of the autonomic system. The oculomotor, facial, and glossopharyngeal nerves contain fibers that contact autonomic ganglia. The oculomotor fibers initiate pupillary constriction, whereas the facial and glossopharyngeal fibers both initiate salivation. The vagus nerve primarily targets autonomic ganglia in the thoracic and upper abdominal cavities.
INTERACTIVE LINK
Visit this site to read about a man who wakes with a headache and a loss of vision. His regular doctor sent him to an ophthalmologist to address the vision loss. The ophthalmologist recognizes a greater problem and immediately sends him to the emergency room. Once there, the patient undergoes a large battery of tests, but a definite cause cannot be found. A specialist recognizes the problem as meningitis, but the question is what caused it originally. How can that be cured? The loss of vision comes from swelling around the optic nerve, which probably presented as a bulge on the inside of the eye. Why is swelling related to meningitis going to push on the optic nerve?
Another important aspect of the cranial nerves that lends itself to a mnemonic is the functional role each nerve plays. The nerves fall into one of three basic groups. They are sensory, motor, or both (see Table 13.3). The sentence, “Some Say Marry Money But My Brother Says Brains Beauty Matter More,” corresponds to the basic function of each nerve. The first, second, and eighth nerves are purely sensory: the olfactory (CNI), optic (CNII), and vestibulocochlear (CNVIII) nerves. The three eye-movement nerves are all motor: the oculomotor (CNIII), trochlear (CNIV), and abducens (CNVI). The spinal accessory (CNXI) and hypoglossal (CNXII) nerves are also strictly motor. The remainder of the nerves contain both sensory and motor fibers. They are the trigeminal (CNV), facial (CNVII), glossopharyngeal (CNIX), and vagus (CNX) nerves. The nerves that convey both are often related to each other. The trigeminal and facial nerves both concern the face; one concerns the sensations and the other concerns the muscle movements. The facial and glossopharyngeal nerves are both responsible for conveying gustatory, or taste, sensations as well as controlling salivary glands. The vagus nerve is involved in visceral responses to taste, namely the gag reflex. This is not an exhaustive list of what these combination nerves do, but there is a thread of relation between them.
Cranial Nerves
| Mnemonic | # | Name | Function (S/M/B) | Central connection (nuclei) | Peripheral connection (ganglion or muscle) |
|---|---|---|---|---|---|
| On | I | Olfactory | Smell (S) | Olfactory bulb | Olfactory epithelium |
| Old | II | Optic | Vision (S) | Hypothalamus/thalamus/midbrain | Retina (retinal ganglion cells) |
| Olympus’ | III | Oculomotor | Eye movements (M) | Oculomotor nucleus | Extraocular muscles (other 4), levator palpebrae superioris, ciliary ganglion (autonomic) |
| Towering | IV | Trochlear | Eye movements (M) | Trochlear nucleus | Superior oblique muscle |
| Tops | V | Trigeminal | Sensory/motor – face (B) | Trigeminal nuclei in the midbrain, pons, and medulla | Trigeminal |
| A | VI | Abducens | Eye movements (M) | Abducens nucleus | Lateral rectus muscle |
| Finn | VII | Facial | Motor – face, Taste (B) | Facial nucleus, solitary nucleus, superior salivatory nucleus | Facial muscles, Geniculate ganglion, Pterygopalatine ganglion (autonomic) |
| And | VIII | Auditory (Vestibulocochlear) | Hearing/balance (S) | Cochlear nucleus, Vestibular nucleus/cerebellum | Spiral ganglion (hearing), Vestibular ganglion (balance) |
| German | IX | Glossopharyngeal | Motor – throat Taste (B) | Solitary nucleus, inferior salivatory nucleus, nucleus ambiguus | Pharyngeal muscles, Geniculate ganglion, Otic ganglion (autonomic) |
| Viewed | X | Vagus | Motor/sensory – viscera (autonomic) (B) | Medulla | Terminal ganglia serving thoracic and upper abdominal organs (heart and small intestines) |
| Some | XI | Spinal Accessory | Motor – head and neck (M) | Spinal accessory nucleus | Neck muscles |
| Hops | XII | Hypoglossal | Motor – lower throat (M) | Hypoglossal nucleus | Muscles of the larynx and lower pharynx |
Table 13.3
Spinal Nerves
The nerves connected to the spinal cord are the spinal nerves. The arrangement of these nerves is much more regular than that of the cranial nerves. All of the spinal nerves are combined sensory and motor axons that separate into two nerve roots. The sensory axons enter the spinal cord as the dorsal nerve root. The motor fibers, both somatic and autonomic, emerge as the ventral nerve root. The dorsal root ganglion for each nerve is an enlargement of the spinal nerve.
There are 31 spinal nerves, named for the level of the spinal cord at which each one emerges. There are eight pairs of cervical nerves designated C1 to C8, twelve thoracic nerves designated T1 to T12, five pairs of lumbar nerves designated L1 to L5, five pairs of sacral nerves designated S1 to S5, and one pair of coccygeal nerves. The nerves are numbered from the superior to inferior positions, and each emerges from the vertebral column through the intervertebral foramen at its level. The first nerve, C1, emerges between the first cervical vertebra and the occipital bone. The second nerve, C2, emerges between the first and second cervical vertebrae. The same occurs for C3 to C7, but C8 emerges between the seventh cervical vertebra and the first thoracic vertebra. For the thoracic and lumbar nerves, each one emerges between the vertebra that has the same designation and the next vertebra in the column. The sacral nerves emerge from the sacral foramina along the length of that unique vertebra.
Spinal nerves extend outward from the vertebral column to enervate the periphery. The nerves in the periphery are not straight continuations of the spinal nerves, but rather the reorganization of the axons in those nerves to follow different courses. Axons from different spinal nerves will come together into a systemic nerve. This occurs at four places along the length of the vertebral column, each identified as a nerve plexus, whereas the other spinal nerves directly correspond to nerves at their respective levels. In this instance, the word plexus is used to describe networks of nerve fibers with no associated cell bodies.
Of the four nerve plexuses, two are found at the cervical level, one at the lumbar level, and one at the sacral level (Figure 13.24). The cervical plexus is composed of axons from spinal nerves C1 through C5 and branches into nerves in the posterior neck and head, as well as the phrenic nerve, which connects to the diaphragm at the base of the thoracic cavity. The other plexus from the cervical level is the brachial plexus. Spinal nerves C4 through T1 reorganize through this plexus to give rise to the nerves of the arms, as the name brachial suggests. A large nerve from this plexus is the radial nerve from which the axillary nerve branches to go to the armpit region. The radial nerve continues through the arm and is paralleled by the ulnar nerve and the median nerve. The lumbar plexus arises from all the lumbar spinal nerves and gives rise to nerves enervating the pelvic region and the anterior leg. The femoral nerve is one of the major nerves from this plexus, which gives rise to the saphenous nerve as a branch that extends through the anterior lower leg. The sacral plexus comes from the lower lumbar nerves L4 and L5 and the sacral nerves S1 to S4. The most significant systemic nerve to come from this plexus is the sciatic nerve, which is a combination of the tibial nerve and the fibular nerve. The sciatic nerve extends across the hip joint and is most commonly associated with the condition sciatica, which is the result of compression or irritation of the nerve or any of the spinal nerves giving rise to it.
These plexuses are described as arising from spinal nerves and giving rise to certain systemic nerves, but they contain fibers that serve sensory functions or fibers that serve motor functions. This means that some fibers extend from cutaneous or other peripheral sensory surfaces and send action potentials into the CNS. Those are axons of sensory neurons in the dorsal root ganglia that enter the spinal cord through the dorsal nerve root. Other fibers are the axons of motor neurons of the anterior horn of the spinal cord, which emerge in the ventral nerve root and send action potentials to cause skeletal muscles to contract in their target regions. For example, the radial nerve contains fibers of cutaneous sensation in the arm, as well as motor fibers that move muscles in the arm.
Spinal nerves of the thoracic region, T2 through T11, are not part of the plexuses but rather emerge and give rise to the intercostal nerves found between the ribs, which articulate with the vertebrae surrounding the spinal nerve.
Figure 13.24 Nerve Plexuses of the Body There are four main nerve plexuses in the human body. The cervical plexus supplies nerves to the posterior head and neck, as well as to the diaphragm. The brachial plexus supplies nerves to the arm. The lumbar plexus supplies nerves to the anterior leg. The sacral plexus supplies nerves to the posterior leg.
AGING AND THE...
Nervous System
Anosmia is the loss of the sense of smell. It is often the result of the olfactory nerve being severed, usually because of blunt force trauma to the head. The sensory neurons of the olfactory epithelium have a limited lifespan of approximately one to four months, and new ones are made on a regular basis. The new neurons extend their axons into the CNS by growing along the existing fibers of the olfactory nerve. The ability of these neurons to be replaced is lost with age. Age-related anosmia is not the result of impact trauma to the head, but rather a slow loss of the sensory neurons with no new neurons born to replace them.
Smell is an important sense, especially for the enjoyment of food. There are only five tastes sensed by the tongue, and two of them are generally thought of as unpleasant tastes (sour and bitter). The rich sensory experience of food is the result of odor molecules associated with the food, both as food is moved into the mouth, and therefore passes under the nose, and when it is chewed and molecules are released to move up the pharynx into the posterior nasal cavity. Anosmia results in a loss of the enjoyment of food.
As the replacement of olfactory neurons declines with age, anosmia can set in. Without the sense of smell, many sufferers complain of food tasting bland. Often, the only way to enjoy food is to add seasoning that can be sensed on the tongue, which usually means adding table salt. The problem with this solution, however, is that this increases sodium intake, which can lead to cardiovascular problems through water retention and the associated increase in blood pressure.
Key Terms
- abducens nerve
- sixth cranial nerve; responsible for contraction of one of the extraocular muscles
- alar plate
- developmental region of the spinal cord that gives rise to the posterior horn of the gray matter
- amygdala
- nucleus deep in the temporal lobe of the cerebrum that is related to memory and emotional behavior
- anterior column
- white matter between the anterior horns of the spinal cord composed of many different groups of axons of both ascending and descending tracts
- anterior horn
- gray matter of the spinal cord containing multipolar motor neurons, sometimes referred to as the ventral horn
- anterior median fissure
- deep midline feature of the anterior spinal cord, marking the separation between the right and left sides of the cord
- anterior spinal artery
- blood vessel from the merged branches of the vertebral arteries that runs along the anterior surface of the spinal cord
- arachnoid granulation
- outpocket of the arachnoid membrane into the dural sinuses that allows for reabsorption of CSF into the blood
- arachnoid mater
- middle layer of the meninges named for the spider-web–like trabeculae that extend between it and the pia mater
- arachnoid trabeculae
- filaments between the arachnoid and pia mater within the subarachnoid space
- ascending tract
- central nervous system fibers carrying sensory information from the spinal cord or periphery to the brain
- axillary nerve
- systemic nerve of the arm that arises from the brachial plexus
- basal forebrain
- nuclei of the cerebrum related to modulation of sensory stimuli and attention through broad projections to the cerebral cortex, loss of which is related to Alzheimer’s disease
- basal nuclei
- nuclei of the cerebrum (with a few components in the upper brain stem and diencephalon) that are responsible for assessing cortical movement commands and comparing them with the general state of the individual through broad modulatory activity of dopamine neurons; largely related to motor functions, as evidenced through the symptoms of Parkinson’s and Huntington’s diseases
- basal plate
- developmental region of the spinal cord that gives rise to the lateral and anterior horns of gray matter
- basilar artery
- blood vessel from the merged vertebral arteries that runs along the dorsal surface of the brain stem
- brachial plexus
- nerve plexus associated with the lower cervical spinal nerves and first thoracic spinal nerve
- brain stem
- region of the adult brain that includes the midbrain, pons, and medulla oblongata and develops from the mesencephalon, metencephalon, and myelencephalon of the embryonic brain
- Broca’s area
- region of the frontal lobe associated with the motor commands necessary for speech production and located only in the cerebral hemisphere responsible for language production, which is the left side in approximately 95 percent of the population
- Brodmann’s areas
- mapping of regions of the cerebral cortex based on microscopic anatomy that relates specific areas to functional differences, as described by Brodmann in the early 1900s
- carotid canal
- opening in the temporal bone through which the internal carotid artery enters the cranium
- cauda equina
- bundle of spinal nerve roots that descend from the lower spinal cord below the first lumbar vertebra and lie within the vertebral cavity; has the appearance of a horse's tail
- caudate
- nucleus deep in the cerebrum that is part of the basal nuclei; along with the putamen, it is part of the striatum
- central canal
- hollow space within the spinal cord that is the remnant of the center of the neural tube
- central sulcus
- surface landmark of the cerebral cortex that marks the boundary between the frontal and parietal lobes
- cephalic flexure
- curve in midbrain of the embryo that positions the forebrain ventrally
- cerebellum
- region of the adult brain connected primarily to the pons that developed from the metencephalon (along with the pons) and is largely responsible for comparing information from the cerebrum with sensory feedback from the periphery through the spinal cord
- cerebral aqueduct
- connection of the ventricular system between the third and fourth ventricles located in the midbrain
- cerebral cortex
- outer gray matter covering the forebrain, marked by wrinkles and folds known as gyri and sulci
- cerebral hemisphere
- one half of the bilaterally symmetrical cerebrum
- cerebrum
- region of the adult brain that develops from the telencephalon and is responsible for higher neurological functions such as memory, emotion, and consciousness
- cervical plexus
- nerve plexus associated with the upper cervical spinal nerves
- choroid plexus
- specialized structures containing ependymal cells lining blood capillaries that filter blood to produce CSF in the four ventricles of the brain
- circle of Willis
- unique anatomical arrangement of blood vessels around the base of the brain that maintains perfusion of blood into the brain even if one component of the structure is blocked or narrowed
- common carotid artery
- blood vessel that branches off the aorta (or the brachiocephalic artery on the right) and supplies blood to the head and neck
- corpus callosum
- large white matter structure that connects the right and left cerebral hemispheres
- cranial nerve
- one of twelve nerves connected to the brain that are responsible for sensory or motor functions of the head and neck
- cranial nerve ganglion
- sensory ganglion of cranial nerves
- descending tract
- central nervous system fibers carrying motor commands from the brain to the spinal cord or periphery
- diencephalon
- region of the adult brain that retains its name from embryonic development and includes the thalamus and hypothalamus
- direct pathway
- connections within the basal nuclei from the striatum to the globus pallidus internal segment and substantia nigra pars reticulata that disinhibit the thalamus to increase cortical control of movement
- disinhibition
- disynaptic connection in which the first synapse inhibits the second cell, which then stops inhibiting the final target
- dorsal (posterior) nerve root
- axons entering the posterior horn of the spinal cord
- dorsal (posterior) root ganglion
- sensory ganglion attached to the posterior nerve root of a spinal nerve
- dura mater
- tough, fibrous, outer layer of the meninges that is attached to the inner surface of the cranium and vertebral column and surrounds the entire CNS
- dural sinus
- any of the venous structures surrounding the brain, enclosed within the dura mater, which drain blood from the CNS to the common venous return of the jugular veins
- endoneurium
- innermost layer of connective tissue that surrounds individual axons within a nerve
- enteric nervous system
- peripheral structures, namely ganglia and nerves, that are incorporated into the digestive system organs
- enteric plexus
- neuronal plexus in the wall of the intestines, which is part of the enteric nervous system
- epineurium
- outermost layer of connective tissue that surrounds an entire nerve
- epithalamus
- region of the diecephalon containing the pineal gland
- esophageal plexus
- neuronal plexus in the wall of the esophagus that is part of the enteric nervous system
- extraocular muscles
- six skeletal muscles that control eye movement within the orbit
- facial nerve
- seventh cranial nerve; responsible for contraction of the facial muscles and for part of the sense of taste, as well as causing saliva production
- fascicle
- small bundles of nerve or muscle fibers enclosed by connective tissue
- femoral nerve
- systemic nerve of the anterior leg that arises from the lumbar plexus
- fibular nerve
- systemic nerve of the posterior leg that begins as part of the sciatic nerve
- foramen magnum
- large opening in the occipital bone of the skull through which the spinal cord emerges and the vertebral arteries enter the cranium
- forebrain
- anterior region of the adult brain that develops from the prosencephalon and includes the cerebrum and diencephalon
- fourth ventricle
- the portion of the ventricular system that is in the region of the brain stem and opens into the subarachnoid space through the median and lateral apertures
- frontal eye field
- region of the frontal lobe associated with motor commands to orient the eyes toward an object of visual attention
- frontal lobe
- region of the cerebral cortex directly beneath the frontal bone of the cranium
- gastric plexuses
- neuronal networks in the wall of the stomach that are part of the enteric nervous system
- globus pallidus
- nuclei deep in the cerebrum that are part of the basal nuclei and can be divided into the internal and external segments
- glossopharyngeal nerve
- ninth cranial nerve; responsible for contraction of muscles in the tongue and throat and for part of the sense of taste, as well as causing saliva production
- gyrus
- ridge formed by convolutions on the surface of the cerebrum or cerebellum
- hindbrain
- posterior region of the adult brain that develops from the rhombencephalon and includes the pons, medulla oblongata, and cerebellum
- hippocampus
- gray matter deep in the temporal lobe that is very important for long-term memory formation
- hypoglossal nerve
- twelfth cranial nerve; responsible for contraction of muscles of the tongue
- hypothalamus
- major region of the diencephalon that is responsible for coordinating autonomic and endocrine control of homeostasis
- indirect pathway
- connections within the basal nuclei from the striatum through the globus pallidus external segment and subthalamic nucleus to the globus pallidus internal segment/substantia nigra pars compacta that result in inhibition of the thalamus to decrease cortical control of movement
- inferior colliculus
- half of the midbrain tectum that is part of the brain stem auditory pathway
- inferior olive
- nucleus in the medulla that is involved in processing information related to motor control
- intercostal nerve
- systemic nerve in the thoracic cavity that is found between two ribs
- internal carotid artery
- branch from the common carotid artery that enters the cranium and supplies blood to the brain
- interventricular foramina
- openings between the lateral ventricles and third ventricle allowing for the passage of CSF
- jugular veins
- blood vessels that return “used” blood from the head and neck
- kinesthesia
- general sensory perception of movement of the body
- lateral apertures
- pair of openings from the fourth ventricle to the subarachnoid space on either side and between the medulla and cerebellum
- lateral column
- white matter of the spinal cord between the posterior horn on one side and the axons from the anterior horn on the same side; composed of many different groups of axons, of both ascending and descending tracts, carrying motor commands to and from the brain
- lateral horn
- region of the spinal cord gray matter in the thoracic, upper lumbar, and sacral regions that is the central component of the sympathetic division of the autonomic nervous system
- lateral sulcus
- surface landmark of the cerebral cortex that marks the boundary between the temporal lobe and the frontal and parietal lobes
- lateral ventricles
- portions of the ventricular system that are in the region of the cerebrum
- limbic cortex
- collection of structures of the cerebral cortex that are involved in emotion, memory, and behavior and are part of the larger limbic system
- limbic system
- structures at the edge (limit) of the boundary between the forebrain and hindbrain that are most associated with emotional behavior and memory formation
- longitudinal fissure
- large separation along the midline between the two cerebral hemispheres
- lumbar plexus
- nerve plexus associated with the lumbar spinal nerves
- lumbar puncture
- procedure used to withdraw CSF from the lower lumbar region of the vertebral column that avoids the risk of damaging CNS tissue because the spinal cord ends at the upper lumbar vertebrae
- median aperture
- singular opening from the fourth ventricle into the subarachnoid space at the midline between the medulla and cerebellum
- median nerve
- systemic nerve of the arm, located between the ulnar and radial nerves
- meninges
- protective outer coverings of the CNS composed of connective tissue
- mesencephalon
- primary vesicle of the embryonic brain that does not significantly change through the rest of embryonic development and becomes the midbrain
- metencephalon
- secondary vesicle of the embryonic brain that develops into the pons and the cerebellum
- midbrain
- middle region of the adult brain that develops from the mesencephalon
- myelencephalon
- secondary vesicle of the embryonic brain that develops into the medulla
- nerve plexus
- network of nerves without neuronal cell bodies included
- neural crest
- tissue that detaches from the edges of the neural groove and migrates through the embryo to develop into peripheral structures of both nervous and non-nervous tissues
- neural fold
- elevated edge of the neural groove
- neural groove
- region of the neural plate that folds into the dorsal surface of the embryo and closes off to become the neural tube
- neural plate
- thickened layer of neuroepithelium that runs longitudinally along the dorsal surface of an embryo and gives rise to nervous system tissue
- neural tube
- precursor to structures of the central nervous system, formed by the invagination and separation of neuroepithelium
- neuraxis
- central axis to the nervous system, from the posterior to anterior ends of the neural tube; the inferior tip of the spinal cord to the anterior surface of the cerebrum
- occipital lobe
- region of the cerebral cortex directly beneath the occipital bone of the cranium
- occipital sinuses
- dural sinuses along the edge of the occipital lobes of the cerebrum
- oculomotor nerve
- third cranial nerve; responsible for contraction of four of the extraocular muscles, the muscle in the upper eyelid, and pupillary constriction
- olfaction
- special sense responsible for smell, which has a unique, direct connection to the cerebrum
- olfactory nerve
- first cranial nerve; responsible for the sense of smell
- optic nerve
- second cranial nerve; responsible for visual sensation
- orthostatic reflex
- sympathetic function that maintains blood pressure when standing to offset the increased effect of gravity
- paravertebral ganglia
- autonomic ganglia superior to the sympathetic chain ganglia
- parietal lobe
- region of the cerebral cortex directly beneath the parietal bone of the cranium
- parieto-occipital sulcus
- groove in the cerebral cortex representing the border between the parietal and occipital cortices
- perineurium
- layer of connective tissue surrounding fascicles within a nerve
- phrenic nerve
- systemic nerve from the cervical plexus that enervates the diaphragm
- pia mater
- thin, innermost membrane of the meninges that directly covers the surface of the CNS
- plexus
- network of nerves or nervous tissue
- postcentral gyrus
- primary motor cortex located in the frontal lobe of the cerebral cortex
- posterior columns
- white matter of the spinal cord that lies between the posterior horns of the gray matter, sometimes referred to as the dorsal column; composed of axons of ascending tracts that carry sensory information up to the brain
- posterior horn
- gray matter region of the spinal cord in which sensory input arrives, sometimes referred to as the dorsal horn
- posterior median sulcus
- midline feature of the posterior spinal cord, marking the separation between right and left sides of the cord
- posterolateral sulcus
- feature of the posterior spinal cord marking the entry of posterior nerve roots and the separation between the posterior and lateral columns of the white matter
- precentral gyrus
- ridge just posterior to the central sulcus, in the parietal lobe, where somatosensory processing initially takes place in the cerebrum
- prefrontal lobe
- specific region of the frontal lobe anterior to the more specific motor function areas, which can be related to the early planning of movements and intentions to the point of being personality-type functions
- premotor area
- region of the frontal lobe responsible for planning movements that will be executed through the primary motor cortex
- prevertebral ganglia
- autonomic ganglia that are anterior to the vertebral column and functionally related to the sympathetic chain ganglia
- primary vesicle
- initial enlargements of the anterior neural tube during embryonic development that develop into the forebrain, midbrain, and hindbrain
- proprioception
- general sensory perceptions providing information about location and movement of body parts; the “sense of the self”
- prosencephalon
- primary vesicle of the embryonic brain that develops into the forebrain, which includes the cerebrum and diencephalon
- putamen
- nucleus deep in the cerebrum that is part of the basal nuclei; along with the caudate, it is part of the striatum
- radial nerve
- systemic nerve of the arm, the distal component of which is located near the radial bone
- reticular formation
- diffuse region of gray matter throughout the brain stem that regulates sleep, wakefulness, and states of consciousness
- rhombencephalon
- primary vesicle of the embryonic brain that develops into the hindbrain, which includes the pons, cerebellum, and medulla
- sacral plexus
- nerve plexus associated with the lower lumbar and sacral spinal nerves
- saphenous nerve
- systemic nerve of the lower anterior leg that is a branch from the femoral nerve
- sciatic nerve
- systemic nerve from the sacral plexus that is a combination of the tibial and fibular nerves and extends across the hip joint and gluteal region into the upper posterior leg
- sciatica
- painful condition resulting from inflammation or compression of the sciatic nerve or any of the spinal nerves that contribute to it
- secondary vesicle
- five vesicles that develop from primary vesicles, continuing the process of differentiation of the embryonic brain
- sigmoid sinuses
- dural sinuses that drain directly into the jugular veins
- somatosensation
- general senses related to the body, usually thought of as the senses of touch, which would include pain, temperature, and proprioception
- spinal accessory nerve
- eleventh cranial nerve; responsible for contraction of neck muscles
- spinal nerve
- one of 31 nerves connected to the spinal cord
- straight sinus
- dural sinus that drains blood from the deep center of the brain to collect with the other sinuses
- striatum
- the caudate and putamen collectively, as part of the basal nuclei, which receive input from the cerebral cortex
- subarachnoid space
- space between the arachnoid mater and pia mater that contains CSF and the fibrous connections of the arachnoid trabeculae
- subcortical nucleus
- all the nuclei beneath the cerebral cortex, including the basal nuclei and the basal forebrain
- substantia nigra pars compacta
- nuclei within the basal nuclei that release dopamine to modulate the function of the striatum; part of the motor pathway
- substantia nigra pars reticulata
- nuclei within the basal nuclei that serve as an output center of the nuclei; part of the motor pathway
- subthalamus
- nucleus within the basal nuclei that is part of the indirect pathway
- sulcus
- groove formed by convolutions in the surface of the cerebral cortex
- superior colliculus
- half of the midbrain tectum that is responsible for aligning visual, auditory, and somatosensory spatial perceptions
- superior sagittal sinus
- dural sinus that runs along the top of the longitudinal fissure and drains blood from the majority of the outer cerebrum
- sympathetic chain ganglia
- autonomic ganglia in a chain along the anterolateral aspect of the vertebral column that are responsible for contributing to homeostatic mechanisms of the autonomic nervous system
- systemic nerve
- nerve in the periphery distal to a nerve plexus or spinal nerve
- tectum
- region of the midbrain, thought of as the roof of the cerebral aqueduct, which is subdivided into the inferior and superior colliculi
- tegmentum
- region of the midbrain, thought of as the floor of the cerebral aqueduct, which continues into the pons and medulla as the floor of the fourth ventricle
- telencephalon
- secondary vesicle of the embryonic brain that develops into the cerebrum
- temporal lobe
- region of the cerebral cortex directly beneath the temporal bone of the cranium
- terminal ganglion
- autonomic ganglia that are near or within the walls of organs that are responsible for contributing to homeostatic mechanisms of the autonomic nervous system
- thalamus
- major region of the diencephalon that is responsible for relaying information between the cerebrum and the hindbrain, spinal cord, and periphery
- third ventricle
- portion of the ventricular system that is in the region of the diencephalon
- tibial nerve
- systemic nerve of the posterior leg that begins as part of the sciatic nerve
- transverse sinuses
- dural sinuses that drain along either side of the occipital–cerebellar space
- trigeminal ganglion
- sensory ganglion that contributes sensory fibers to the trigeminal nerve
- trigeminal nerve
- fifth cranial nerve; responsible for cutaneous sensation of the face and contraction of the muscles of mastication
- trochlear nerve
- fourth cranial nerve; responsible for contraction of one of the extraocular muscles
- ulnar nerve
- systemic nerve of the arm located close to the ulna, a bone of the forearm
- vagus nerve
- tenth cranial nerve; responsible for the autonomic control of organs in the thoracic and upper abdominal cavities
- ventral (anterior) nerve root
- axons emerging from the anterior or lateral horns of the spinal cord
- ventricles
- remnants of the hollow center of the neural tube that are spaces for cerebrospinal fluid to circulate through the brain
- vertebral arteries
- arteries that ascend along either side of the vertebral column through the transverse foramina of the cervical vertebrae and enter the cranium through the foramen magnum
- vestibulocochlear nerve
- eighth cranial nerve; responsible for the sensations of hearing and balance
Chapter Review
13.1 The Embryologic Perspective
The development of the nervous system starts early in embryonic development. The outer layer of the embryo, the ectoderm, gives rise to the skin and the nervous system. A specialized region of this layer, the neuroectoderm, becomes a groove that folds in and becomes the neural tube beneath the dorsal surface of the embryo. The anterior end of the neural tube develops into the brain, and the posterior region becomes the spinal cord. Tissues at the edges of the neural groove, when it closes off, are called the neural crest and migrate through the embryo to give rise to PNS structures as well as some non-nervous tissues.
The brain develops from this early tube structure and gives rise to specific regions of the adult brain. As the neural tube grows and differentiates, it enlarges into three vesicles that correspond to the forebrain, midbrain, and hindbrain regions of the adult brain. Later in development, two of these three vesicles differentiate further, resulting in five vesicles. Those five vesicles can be aligned with the four major regions of the adult brain. The cerebrum is formed directly from the telencephalon. The diencephalon is the only region that keeps its embryonic name. The mesencephalon, metencephalon, and myelencephalon become the brain stem. The cerebellum also develops from the metencephalon and is a separate region of the adult brain.
The spinal cord develops out of the rest of the neural tube and retains the tube structure, with the nervous tissue thickening and the hollow center becoming a very small central canal through the cord. The rest of the hollow center of the neural tube corresponds to open spaces within the brain called the ventricles, where cerebrospinal fluid is found.
13.2 The Central Nervous System
The adult brain is separated into four major regions: the cerebrum, the diencephalon, the brain stem, and the cerebellum. The cerebrum is the largest portion and contains the cerebral cortex and subcortical nuclei. It is divided into two halves by the longitudinal fissure.
The cortex is separated into the frontal, parietal, temporal, and occipital lobes. The frontal lobe is responsible for motor functions, from planning movements through executing commands to be sent to the spinal cord and periphery. The most anterior portion of the frontal lobe is the prefrontal cortex, which is associated with aspects of personality through its influence on motor responses in decision-making.
The other lobes are responsible for sensory functions. The parietal lobe is where somatosensation is processed. The occipital lobe is where visual processing begins, although the other parts of the brain can contribute to visual function. The temporal lobe contains the cortical area for auditory processing, but also has regions crucial for memory formation.
Nuclei beneath the cerebral cortex, known as the subcortical nuclei, are responsible for augmenting cortical functions. The basal nuclei receive input from cortical areas and compare it with the general state of the individual through the activity of a dopamine-releasing nucleus. The output influences the activity of part of the thalamus that can then increase or decrease cortical activity that often results in changes to motor commands. The basal forebrain is responsible for modulating cortical activity in attention and memory. The limbic system includes deep cerebral nuclei that are responsible for emotion and memory.
The diencephalon includes the thalamus and the hypothalamus, along with some other structures. The thalamus is a relay between the cerebrum and the rest of the nervous system. The hypothalamus coordinates homeostatic functions through the autonomic and endocrine systems.
The brain stem is composed of the midbrain, pons, and medulla. It controls the head and neck region of the body through the cranial nerves. There are control centers in the brain stem that regulate the cardiovascular and respiratory systems.
The cerebellum is connected to the brain stem, primarily at the pons, where it receives a copy of the descending input from the cerebrum to the spinal cord. It can compare this with sensory feedback input through the medulla and send output through the midbrain that can correct motor commands for coordination.
13.3 Circulation and the Central Nervous System
The CNS has a privileged blood supply established by the blood-brain barrier. Establishing this barrier are anatomical structures that help to protect and isolate the CNS. The arterial blood to the brain comes from the internal carotid and vertebral arteries, which both contribute to the unique circle of Willis that provides constant perfusion of the brain even if one of the blood vessels is blocked or narrowed. That blood is eventually filtered to make a separate medium, the CSF, that circulates within the spaces of the brain and then into the surrounding space defined by the meninges, the protective covering of the brain and spinal cord.
The blood that nourishes the brain and spinal cord is behind the glial-cell–enforced blood-brain barrier, which limits the exchange of material from blood vessels with the interstitial fluid of the nervous tissue. Thus, metabolic wastes are collected in cerebrospinal fluid that circulates through the CNS. This fluid is produced by filtering blood at the choroid plexuses in the four ventricles of the brain. It then circulates through the ventricles and into the subarachnoid space, between the pia mater and the arachnoid mater. From the arachnoid granulations, CSF is reabsorbed into the blood, removing the waste from the privileged central nervous tissue.
The blood, now with the reabsorbed CSF, drains out of the cranium through the dural sinuses. The dura mater is the tough outer covering of the CNS, which is anchored to the inner surface of the cranial and vertebral cavities. It surrounds the venous space known as the dural sinuses, which connect to the jugular veins, where blood drains from the head and neck.
13.4 The Peripheral Nervous System
The PNS is composed of the groups of neurons (ganglia) and bundles of axons (nerves) that are outside of the brain and spinal cord. Ganglia are of two types, sensory or autonomic. Sensory ganglia contain unipolar sensory neurons and are found on the dorsal root of all spinal nerves as well as associated with many of the cranial nerves. Autonomic ganglia are in the sympathetic chain, the associated paravertebral or prevertebral ganglia, or in terminal ganglia near or within the organs controlled by the autonomic nervous system.
Nerves are classified as cranial nerves or spinal nerves on the basis of their connection to the brain or spinal cord, respectively. The twelve cranial nerves can be strictly sensory in function, strictly motor in function, or a combination of the two functions. Sensory fibers are axons of sensory ganglia that carry sensory information into the brain and target sensory nuclei. Motor fibers are axons of motor neurons in motor nuclei of the brain stem and target skeletal muscles of the head and neck. Spinal nerves are all mixed nerves with both sensory and motor fibers. Spinal nerves emerge from the spinal cord and reorganize through plexuses, which then give rise to systemic nerves. Thoracic spinal nerves are not part of any plexus, but give rise to the intercostal nerves directly.
Interactive Link Questions
Watch this animation to examine the development of the brain, starting with the neural tube. As the anterior end of the neural tube develops, it enlarges into the primary vesicles that establish the forebrain, midbrain, and hindbrain. Those structures continue to develop throughout the rest of embryonic development and into adolescence. They are the basis of the structure of the fully developed adult brain. How would you describe the difference in the relative sizes of the three regions of the brain when comparing the early (25th embryonic day) brain and the adult brain?
2.Watch this video to learn about the white matter in the cerebrum that develops during childhood and adolescence. This is a composite of MRI images taken of the brains of people from 5 years of age through 20 years of age, demonstrating how the cerebrum changes. As the color changes to blue, the ratio of gray matter to white matter changes. The caption for the video describes it as “less gray matter,” which is another way of saying “more white matter.” If the brain does not finish developing until approximately 20 years of age, can teenagers be held responsible for behaving badly?
3.Watch this video to learn about the basal nuclei (also known as the basal ganglia), which have two pathways that process information within the cerebrum. As shown in this video, the direct pathway is the shorter pathway through the system that results in increased activity in the cerebral cortex and increased motor activity. The direct pathway is described as resulting in “disinhibition” of the thalamus. What does disinhibition mean? What are the two neurons doing individually to cause this?
4.Watch this video to learn about the basal nuclei (also known as the basal ganglia), which have two pathways that process information within the cerebrum. As shown in this video, the indirect pathway is the longer pathway through the system that results in decreased activity in the cerebral cortex, and therefore less motor activity. The indirect pathway has an extra couple of connections in it, including disinhibition of the subthalamic nucleus. What is the end result on the thalamus, and therefore on movement initiated by the cerebral cortex?
5.Watch this video to learn about the gray matter of the spinal cord that receives input from fibers of the dorsal (posterior) root and sends information out through the fibers of the ventral (anterior) root. As discussed in this video, these connections represent the interactions of the CNS with peripheral structures for both sensory and motor functions. The cervical and lumbar spinal cords have enlargements as a result of larger populations of neurons. What are these enlargements responsible for?
6.Compared with the nearest evolutionary relative, the chimpanzee, the human has a brain that is huge. At a point in the past, a common ancestor gave rise to the two species of humans and chimpanzees. That evolutionary history is long and is still an area of intense study. But something happened to increase the size of the human brain relative to the chimpanzee. Read this article in which the author explores the current understanding of why this happened.
According to one hypothesis about the expansion of brain size, what tissue might have been sacrificed so energy was available to grow our larger brain? Based on what you know about that tissue and nervous tissue, why would there be a trade-off between them in terms of energy use?
7.Watch this animation to see how blood flows to the brain and passes through the circle of Willis before being distributed through the cerebrum. The circle of Willis is a specialized arrangement of arteries that ensure constant perfusion of the cerebrum even in the event of a blockage of one of the arteries in the circle. The animation shows the normal direction of flow through the circle of Willis to the middle cerebral artery. Where would the blood come from if there were a blockage just posterior to the middle cerebral artery on the left?
8.Watch this video that describes the procedure known as the lumbar puncture, a medical procedure used to sample the CSF. Because of the anatomy of the CNS, it is a relative safe location to insert a needle. Why is the lumbar puncture performed in the lower lumbar area of the vertebral column?
9.Watch this animation that shows the flow of CSF through the brain and spinal cord, and how it originates from the ventricles and then spreads into the space within the meninges, where the fluids then move into the venous sinuses to return to the cardiovascular circulation. What are the structures that produce CSF and where are they found? How are the structures indicated in this animation?
10.Figure 13.20 If you zoom in on the DRG, you can see smaller satellite glial cells surrounding the large cell bodies of the sensory neurons. From what structure do satellite cells derive during embryologic development?
11.Figure 13.22 To what structures in a skeletal muscle are the endoneurium, perineurium, and epineurium comparable?
12.Visit this site to read about a man who wakes with a headache and a loss of vision. His regular doctor sent him to an ophthalmologist to address the vision loss. The ophthalmologist recognizes a greater problem and immediately sends him to the emergency room. Once there, the patient undergoes a large battery of tests, but a definite cause cannot be found. A specialist recognizes the problem as meningitis, but the question is what caused it originally. How can that be cured? The loss of vision comes from swelling around the optic nerve, which probably presented as a bulge on the inside of the eye. Why is swelling related to meningitis going to push on the optic nerve?
Review Questions
Aside from the nervous system, which other organ system develops out of the ectoderm?
- digestive
- respiratory
- integumentary
- urinary
Which primary vesicle of the embryonic nervous system does not differentiate into more vesicles at the secondary stage?
- prosencephalon
- mesencephalon
- diencephalon
- rhombencephalon
Which adult structure(s) arises from the diencephalon?
- thalamus, hypothalamus, retina
- midbrain, pons, medulla
- pons and cerebellum
- cerebrum
Which non-nervous tissue develops from the neuroectoderm?
- respiratory mucosa
- vertebral bone
- digestive lining
- craniofacial bone
Which structure is associated with the embryologic development of the peripheral nervous system?
- neural crest
- neuraxis
- rhombencephalon
- neural tube
Which lobe of the cerebral cortex is responsible for generating motor commands?
- temporal
- parietal
- occipital
- frontal
What region of the diencephalon coordinates homeostasis?
- thalamus
- epithalamus
- hypothalamus
- subthalamus
What level of the brain stem is the major input to the cerebellum?
- midbrain
- pons
- medulla
- spinal cord
What region of the spinal cord contains motor neurons that direct the movement of skeletal muscles?
- anterior horn
- posterior horn
- lateral horn
- alar plate
Brodmann’s areas map different regions of the ________ to particular functions.
- cerebellum
- cerebral cortex
- basal forebrain
- corpus callosum
What blood vessel enters the cranium to supply the brain with fresh, oxygenated blood?
- common carotid artery
- jugular vein
- internal carotid artery
- aorta
Which layer of the meninges surrounds and supports the sinuses that form the route through which blood drains from the CNS?
- dura mater
- arachnoid mater
- subarachnoid
- pia mater
What type of glial cell is responsible for filtering blood to produce CSF at the choroid plexus?
- ependymal cell
- astrocyte
- oligodendrocyte
- Schwann cell
Which portion of the ventricular system is found within the diencephalon?
- lateral ventricles
- third ventricle
- cerebral aqueduct
- fourth ventricle
What condition causes a stroke?
- inflammation of meninges
- lumbar puncture
- infection of cerebral spinal fluid
- disruption of blood to the brain
What type of ganglion contains neurons that control homeostatic mechanisms of the body?
- sensory ganglion
- dorsal root ganglion
- autonomic ganglion
- cranial nerve ganglion
Which ganglion is responsible for cutaneous sensations of the face?
- otic ganglion
- vestibular ganglion
- geniculate ganglion
- trigeminal ganglion
What is the name for a bundle of axons within a nerve?
- fascicle
- tract
- nerve root
- epineurium
Which cranial nerve does not control functions in the head and neck?
- olfactory
- trochlear
- glossopharyngeal
- vagus
Which of these structures is not under direct control of the peripheral nervous system?
- trigeminal ganglion
- gastric plexus
- sympathetic chain ganglia
- cervical plexus
Critical Thinking Questions
Studying the embryonic development of the nervous system makes it easier to understand the complexity of the adult nervous system. Give one example of how development in the embryonic nervous system explains a more complex structure in the adult nervous system.
34.What happens in development that suggests that there is a special relationship between the skeletal structure of the head and the nervous system?
35.Damage to specific regions of the cerebral cortex, such as through a stroke, can result in specific losses of function. What functions would likely be lost by a stroke in the temporal lobe?
36.Why do the anatomical inputs to the cerebellum suggest that it can compare motor commands and sensory feedback?
37.Why can the circle of Willis maintain perfusion of the brain even if there is a blockage in one part of the structure?
38.Meningitis is an inflammation of the meninges that can have severe effects on neurological function. Why is infection of this structure potentially so dangerous?
39.Why are ganglia and nerves not surrounded by protective structures like the meninges of the CNS?
40.Testing for neurological function involves a series of tests of functions associated with the cranial nerves. What functions, and therefore which nerves, are being tested by asking a patient to follow the tip of a pen with their eyes?
|
oercommons
|
2025-03-18T00:39:10.980093
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/56376/overview",
"title": "Anatomy and Physiology, Regulation, Integration, and Control",
"author": null
}
|
https://oercommons.org/courseware/lesson/56377/overview
|
The Somatic Nervous System
Introduction
Figure 14.1 Too Hot to Touch When high temperature is sensed in the skin, a reflexive withdrawal is initiated by the muscles of the arm. Sensory neurons are activated by a stimulus, which is sent to the central nervous system, and a motor response is sent out to the skeletal muscles that control this movement.
CHAPTER OBJECTIVES
After studying this chapter, you will be able to:
- Describe the components of the somatic nervous system
- Name the modalities and submodalities of the sensory systems
- Distinguish between general and special senses
- Describe regions of the central nervous system that contribute to somatic functions
- Explain the stimulus-response motor pathway
The somatic nervous system is traditionally considered a division within the peripheral nervous system. However, this misses an important point: somatic refers to a functional division, whereas peripheral refers to an anatomic division. The somatic nervous system is responsible for our conscious perception of the environment and for our voluntary responses to that perception by means of skeletal muscles. Peripheral sensory neurons receive input from environmental stimuli, but the neurons that produce motor responses originate in the central nervous system. The distinction between the structures (i.e., anatomy) of the peripheral and central nervous systems and functions (i.e., physiology) of the somatic and autonomic systems can most easily be demonstrated through a simple reflex action. When you touch a hot stove, you pull your hand away. Sensory receptors in the skin sense extreme temperature and the early signs of tissue damage. This triggers an action potential, which travels along the sensory fiber from the skin, through the dorsal spinal root to the spinal cord, and directly activates a ventral horn motor neuron. That neuron sends a signal along its axon to excite the biceps brachii, causing contraction of the muscle and flexion of the forearm at the elbow to withdraw the hand from the hot stove. The withdrawal reflex has more components, such as inhibiting the opposing muscle and balancing posture while the arm is forcefully withdrawn, which will be further explored at the end of this chapter.
The basic withdrawal reflex explained above includes sensory input (the painful stimulus), central processing (the synapse in the spinal cord), and motor output (activation of a ventral motor neuron that causes contraction of the biceps brachii). Expanding the explanation of the withdrawal reflex can include inhibition of the opposing muscle, or cross extension, either of which increase the complexity of the example by involving more central neurons. A collateral branch of the sensory axon would inhibit another ventral horn motor neuron so that the triceps brachii do not contract and slow the withdrawal down. The cross extensor reflex provides a counterbalancing movement on the other side of the body, which requires another collateral of the sensory axon to activate contraction of the extensor muscles in the contralateral limb.
A more complex example of somatic function is conscious muscle movement. For example, reading of this text starts with visual sensory input to the retina, which then projects to the thalamus, and on to the cerebral cortex. A sequence of regions of the cerebral cortex process the visual information, starting in the primary visual cortex of the occipital lobe, and resulting in the conscious perception of these letters. Subsequent cognitive processing results in understanding of the content. As you continue reading, regions of the cerebral cortex in the frontal lobe plan how to move the eyes to follow the lines of text. The output from the cortex causes activity in motor neurons in the brain stem that cause movement of the extraocular muscles through the third, fourth, and sixth cranial nerves. This example also includes sensory input (the retinal projection to the thalamus), central processing (the thalamus and subsequent cortical activity), and motor output (activation of neurons in the brain stem that lead to coordinated contraction of extraocular muscles).
Sensory Perception
- Describe different types of sensory receptors
- Describe the structures responsible for the special senses of taste, smell, hearing, balance, and vision
- Distinguish how different tastes are transduced
- Describe the means of mechanoreception for hearing and balance
- List the supporting structures around the eye and describe the structure of the eyeball
- Describe the processes of phototransduction
A major role of sensory receptors is to help us learn about the environment around us, or about the state of our internal environment. Stimuli from varying sources, and of different types, are received and changed into the electrochemical signals of the nervous system. This occurs when a stimulus changes the cell membrane potential of a sensory neuron. The stimulus causes the sensory cell to produce an action potential that is relayed into the central nervous system (CNS), where it is integrated with other sensory information—or sometimes higher cognitive functions—to become a conscious perception of that stimulus. The central integration may then lead to a motor response.
Describing sensory function with the term sensation or perception is a deliberate distinction. Sensation is the activation of sensory receptor cells at the level of the stimulus. Perception is the central processing of sensory stimuli into a meaningful pattern. Perception is dependent on sensation, but not all sensations are perceived. Receptors are the cells or structures that detect sensations. A receptor cell is changed directly by a stimulus. A transmembrane protein receptor is a protein in the cell membrane that mediates a physiological change in a neuron, most often through the opening of ion channels or changes in the cell signaling processes. Transmembrane receptors are activated by chemicals called ligands. For example, a molecule in food can serve as a ligand for taste receptors. Other transmembrane proteins, which are not accurately called receptors, are sensitive to mechanical or thermal changes. Physical changes in these proteins increase ion flow across the membrane, and can generate an action potential or a graded potential in the sensory neurons.
Sensory Receptors
Stimuli in the environment activate specialized receptor cells in the peripheral nervous system. Different types of stimuli are sensed by different types of receptor cells. Receptor cells can be classified into types on the basis of three different criteria: cell type, position, and function. Receptors can be classified structurally on the basis of cell type and their position in relation to stimuli they sense. They can also be classified functionally on the basis of the transduction of stimuli, or how the mechanical stimulus, light, or chemical changed the cell membrane potential.
Structural Receptor Types
The cells that interpret information about the environment can be either (1) a neuron that has a free nerve ending, with dendrites embedded in tissue that would receive a sensation; (2) a neuron that has an encapsulated ending in which the sensory nerve endings are encapsulated in connective tissue that enhances their sensitivity; or (3) a specialized receptor cell, which has distinct structural components that interpret a specific type of stimulus (Figure 14.2). The pain and temperature receptors in the dermis of the skin are examples of neurons that have free nerve endings. Also located in the dermis of the skin are lamellated corpuscles, neurons with encapsulated nerve endings that respond to pressure and touch. The cells in the retina that respond to light stimuli are an example of a specialized receptor, a photoreceptor.
Figure 14.2 Receptor Classification by Cell Type Receptor cell types can be classified on the basis of their structure. Sensory neurons can have either (a) free nerve endings or (b) encapsulated endings. Photoreceptors in the eyes, such as rod cells, are examples of (c) specialized receptor cells. These cells release neurotransmitters onto a bipolar cell, which then synapses with the optic nerve neurons.
Another way that receptors can be classified is based on their location relative to the stimuli. An exteroceptor is a receptor that is located near a stimulus in the external environment, such as the somatosensory receptors that are located in the skin. An interoceptor is one that interprets stimuli from internal organs and tissues, such as the receptors that sense the increase in blood pressure in the aorta or carotid sinus. Finally, a proprioceptor is a receptor located near a moving part of the body, such as a muscle, that interprets the positions of the tissues as they move.
Functional Receptor Types
A third classification of receptors is by how the receptor transduces stimuli into membrane potential changes. Stimuli are of three general types. Some stimuli are ions and macromolecules that affect transmembrane receptor proteins when these chemicals diffuse across the cell membrane. Some stimuli are physical variations in the environment that affect receptor cell membrane potentials. Other stimuli include the electromagnetic radiation from visible light. For humans, the only electromagnetic energy that is perceived by our eyes is visible light. Some other organisms have receptors that humans lack, such as the heat sensors of snakes, the ultraviolet light sensors of bees, or magnetic receptors in migratory birds.
Receptor cells can be further categorized on the basis of the type of stimuli they transduce. Chemical stimuli can be interpreted by a chemoreceptor that interprets chemical stimuli, such as an object’s taste or smell. Osmoreceptors respond to solute concentrations of body fluids. Additionally, pain is primarily a chemical sense that interprets the presence of chemicals from tissue damage, or similar intense stimuli, through a nociceptor. Physical stimuli, such as pressure and vibration, as well as the sensation of sound and body position (balance), are interpreted through a mechanoreceptor. Another physical stimulus that has its own type of receptor is temperature, which is sensed through a thermoreceptor that is either sensitive to temperatures above (heat) or below (cold) normal body temperature.
Sensory Modalities
Ask anyone what the senses are, and they are likely to list the five major senses—taste, smell, touch, hearing, and sight. However, these are not all of the senses. The most obvious omission from this list is balance. Also, what is referred to simply as touch can be further subdivided into pressure, vibration, stretch, and hair-follicle position, on the basis of the type of mechanoreceptors that perceive these touch sensations. Other overlooked senses include temperature perception by thermoreceptors and pain perception by nociceptors.
Within the realm of physiology, senses can be classified as either general or specific. A general sense is one that is distributed throughout the body and has receptor cells within the structures of other organs. Mechanoreceptors in the skin, muscles, or the walls of blood vessels are examples of this type. General senses often contribute to the sense of touch, as described above, or to proprioception (body movement) and kinesthesia (body movement), or to a visceral sense, which is most important to autonomic functions. A special sense is one that has a specific organ devoted to it, namely the eye, inner ear, tongue, or nose.
Each of the senses is referred to as a sensory modality. Modality refers to the way that information is encoded, which is similar to the idea of transduction. The main sensory modalities can be described on the basis of how each is transduced. The chemical senses are taste and smell. The general sense that is usually referred to as touch includes chemical sensation in the form of nociception, or pain. Pressure, vibration, muscle stretch, and the movement of hair by an external stimulus, are all sensed by mechanoreceptors. Hearing and balance are also sensed by mechanoreceptors. Finally, vision involves the activation of photoreceptors.
Listing all the different sensory modalities, which can number as many as 17, involves separating the five major senses into more specific categories, or submodalities, of the larger sense. An individual sensory modality represents the sensation of a specific type of stimulus. For example, the general sense of touch, which is known as somatosensation, can be separated into light pressure, deep pressure, vibration, itch, pain, temperature, or hair movement.
Gustation (Taste)
Only a few recognized submodalities exist within the sense of taste, or gustation. Until recently, only four tastes were recognized: sweet, salty, sour, and bitter. Research at the turn of the 20th century led to recognition of the fifth taste, umami, during the mid-1980s. Umami is a Japanese word that means “delicious taste,” and is often translated to mean savory. Very recent research has suggested that there may also be a sixth taste for fats, or lipids.
Gustation is the special sense associated with the tongue. The surface of the tongue, along with the rest of the oral cavity, is lined by a stratified squamous epithelium. Raised bumps called papillae (singular = papilla) contain the structures for gustatory transduction. There are four types of papillae, based on their appearance (Figure 14.3): circumvallate, foliate, filiform, and fungiform. Within the structure of the papillae are taste buds that contain specialized gustatory receptor cells for the transduction of taste stimuli. These receptor cells are sensitive to the chemicals contained within foods that are ingested, and they release neurotransmitters based on the amount of the chemical in the food. Neurotransmitters from the gustatory cells can activate sensory neurons in the facial, glossopharyngeal, and vagus cranial nerves.
Figure 14.3 The Tongue The tongue is covered with small bumps, called papillae, which contain taste buds that are sensitive to chemicals in ingested food or drink. Different types of papillae are found in different regions of the tongue. The taste buds contain specialized gustatory receptor cells that respond to chemical stimuli dissolved in the saliva. These receptor cells activate sensory neurons that are part of the facial and glossopharyngeal nerves. LM × 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
Salty taste is simply the perception of sodium ions (Na+) in the saliva. When you eat something salty, the salt crystals dissociate into the component ions Na+ and Cl–, which dissolve into the saliva in your mouth. The Na+ concentration becomes high outside the gustatory cells, creating a strong concentration gradient that drives the diffusion of the ion into the cells. The entry of Na+into these cells results in the depolarization of the cell membrane and the generation of a receptor potential.
Sour taste is the perception of H+ concentration. Just as with sodium ions in salty flavors, these hydrogen ions enter the cell and trigger depolarization. Sour flavors are, essentially, the perception of acids in our food. Increasing hydrogen ion concentrations in the saliva (lowering saliva pH) triggers progressively stronger graded potentials in the gustatory cells. For example, orange juice—which contains citric acid—will taste sour because it has a pH value of approximately 3. Of course, it is often sweetened so that the sour taste is masked.
The first two tastes (salty and sour) are triggered by the cations Na+ and H+. The other tastes result from food molecules binding to a G protein–coupled receptor. A G protein signal transduction system ultimately leads to depolarization of the gustatory cell. The sweet taste is the sensitivity of gustatory cells to the presence of glucose dissolved in the saliva. Other monosaccharides such as fructose, or artificial sweeteners such as aspartame (NutraSweet™), saccharine, or sucralose (Splenda™) also activate the sweet receptors. The affinity for each of these molecules varies, and some will taste sweeter than glucose because they bind to the G protein–coupled receptor differently.
Bitter taste is similar to sweet in that food molecules bind to G protein–coupled receptors. However, there are a number of different ways in which this can happen because there are a large diversity of bitter-tasting molecules. Some bitter molecules depolarize gustatory cells, whereas others hyperpolarize gustatory cells. Likewise, some bitter molecules increase G protein activation within the gustatory cells, whereas other bitter molecules decrease G protein activation. The specific response depends on which molecule is binding to the receptor.
One major group of bitter-tasting molecules are alkaloids. Alkaloids are nitrogen containing molecules that are commonly found in bitter-tasting plant products, such as coffee, hops (in beer), tannins (in wine), tea, and aspirin. By containing toxic alkaloids, the plant is less susceptible to microbe infection and less attractive to herbivores.
Therefore, the function of bitter taste may primarily be related to stimulating the gag reflex to avoid ingesting poisons. Because of this, many bitter foods that are normally ingested are often combined with a sweet component to make them more palatable (cream and sugar in coffee, for example). The highest concentration of bitter receptors appear to be in the posterior tongue, where a gag reflex could still spit out poisonous food.
The taste known as umami is often referred to as the savory taste. Like sweet and bitter, it is based on the activation of G protein–coupled receptors by a specific molecule. The molecule that activates this receptor is the amino acid L-glutamate. Therefore, the umami flavor is often perceived while eating protein-rich foods. Not surprisingly, dishes that contain meat are often described as savory.
Once the gustatory cells are activated by the taste molecules, they release neurotransmitters onto the dendrites of sensory neurons. These neurons are part of the facial and glossopharyngeal cranial nerves, as well as a component within the vagus nerve dedicated to the gag reflex. The facial nerve connects to taste buds in the anterior third of the tongue. The glossopharyngeal nerve connects to taste buds in the posterior two thirds of the tongue. The vagus nerve connects to taste buds in the extreme posterior of the tongue, verging on the pharynx, which are more sensitive to noxious stimuli such as bitterness.
INTERACTIVE LINK
Watch this video to learn about Dr. Danielle Reed of the Monell Chemical Senses Center in Philadelphia, Pennsylvania, who became interested in science at an early age because of her sensory experiences. She recognized that her sense of taste was unique compared with other people she knew. Now, she studies the genetic differences between people and their sensitivities to taste stimuli. In the video, there is a brief image of a person sticking out their tongue, which has been covered with a colored dye. This is how Dr. Reed is able to visualize and count papillae on the surface of the tongue. People fall into two groups known as “tasters” and “non-tasters” based on the density of papillae on their tongue, which also indicates the number of taste buds. Non-tasters can taste food, but they are not as sensitive to certain tastes, such as bitterness. Dr. Reed discovered that she is a non-taster, which explains why she perceived bitterness differently than other people she knew. Are you very sensitive to tastes? Can you see any similarities among the members of your family?
Olfaction (Smell)
Like taste, the sense of smell, or olfaction, is also responsive to chemical stimuli. The olfactory receptor neurons are located in a small region within the superior nasal cavity (Figure 14.4). This region is referred to as the olfactory epithelium and contains bipolar sensory neurons. Each olfactory sensory neuron has dendrites that extend from the apical surface of the epithelium into the mucus lining the cavity. As airborne molecules are inhaled through the nose, they pass over the olfactory epithelial region and dissolve into the mucus. These odorant molecules bind to proteins that keep them dissolved in the mucus and help transport them to the olfactory dendrites. The odorant–protein complex binds to a receptor protein within the cell membrane of an olfactory dendrite. These receptors are G protein–coupled, and will produce a graded membrane potential in the olfactory neurons.
The axon of an olfactory neuron extends from the basal surface of the epithelium, through an olfactory foramen in the cribriform plate of the ethmoid bone, and into the brain. The group of axons called the olfactory tract connect to the olfactory bulb on the ventral surface of the frontal lobe. From there, the axons split to travel to several brain regions. Some travel to the cerebrum, specifically to the primary olfactory cortex that is located in the inferior and medial areas of the temporal lobe. Others project to structures within the limbic system and hypothalamus, where smells become associated with long-term memory and emotional responses. This is how certain smells trigger emotional memories, such as the smell of food associated with one’s birthplace. Smell is the one sensory modality that does not synapse in the thalamus before connecting to the cerebral cortex. This intimate connection between the olfactory system and the cerebral cortex is one reason why smell can be a potent trigger of memories and emotion.
The nasal epithelium, including the olfactory cells, can be harmed by airborne toxic chemicals. Therefore, the olfactory neurons are regularly replaced within the nasal epithelium, after which the axons of the new neurons must find their appropriate connections in the olfactory bulb. These new axons grow along the axons that are already in place in the cranial nerve.
Figure 14.4 The Olfactory System (a) The olfactory system begins in the peripheral structures of the nasal cavity. (b) The olfactory receptor neurons are within the olfactory epithelium. (c) Axons of the olfactory receptor neurons project through the cribriform plate of the ethmoid bone and synapse with the neurons of the olfactory bulb (tissue source: simian). LM × 812. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
DISORDERS OF THE...
Olfactory System: Anosmia
Blunt force trauma to the face, such as that common in many car accidents, can lead to the loss of the olfactory nerve, and subsequently, loss of the sense of smell. This condition is known as anosmia. When the frontal lobe of the brain moves relative to the ethmoid bone, the olfactory tract axons may be sheared apart. Professional fighters often experience anosmia because of repeated trauma to face and head. In addition, certain pharmaceuticals, such as antibiotics, can cause anosmia by killing all the olfactory neurons at once. If no axons are in place within the olfactory nerve, then the axons from newly formed olfactory neurons have no guide to lead them to their connections within the olfactory bulb. There are temporary causes of anosmia, as well, such as those caused by inflammatory responses related to respiratory infections or allergies.
Loss of the sense of smell can result in food tasting bland. A person with an impaired sense of smell may require additional spice and seasoning levels for food to be tasted. Anosmia may also be related to some presentations of mild depression, because the loss of enjoyment of food may lead to a general sense of despair.
The ability of olfactory neurons to replace themselves decreases with age, leading to age-related anosmia. This explains why some elderly people salt their food more than younger people do. However, this increased sodium intake can increase blood volume and blood pressure, increasing the risk of cardiovascular diseases in the elderly.
Audition (Hearing)
Hearing, or audition, is the transduction of sound waves into a neural signal that is made possible by the structures of the ear (Figure 14.5). The large, fleshy structure on the lateral aspect of the head is known as the auricle. Some sources will also refer to this structure as the pinna, though that term is more appropriate for a structure that can be moved, such as the external ear of a cat. The C-shaped curves of the auricle direct sound waves toward the auditory canal. The canal enters the skull through the external auditory meatus of the temporal bone. At the end of the auditory canal is the tympanic membrane, or ear drum, which vibrates after it is struck by sound waves. The auricle, ear canal, and tympanic membrane are often referred to as the external ear. The middle ear consists of a space spanned by three small bones called the ossicles. The three ossicles are the malleus, incus, and stapes, which are Latin names that roughly translate to hammer, anvil, and stirrup. The malleus is attached to the tympanic membrane and articulates with the incus. The incus, in turn, articulates with the stapes. The stapes is then attached to the inner ear, where the sound waves will be transduced into a neural signal. The middle ear is connected to the pharynx through the Eustachian tube, which helps equilibrate air pressure across the tympanic membrane. The tube is normally closed but will pop open when the muscles of the pharynx contract during swallowing or yawning.
Figure 14.5 Structures of the Ear The external ear contains the auricle, ear canal, and tympanic membrane. The middle ear contains the ossicles and is connected to the pharynx by the Eustachian tube. The inner ear contains the cochlea and vestibule, which are responsible for audition and equilibrium, respectively.
The inner ear is often described as a bony labyrinth, as it is composed of a series of canals embedded within the temporal bone. It has two separate regions, the cochlea and the vestibule, which are responsible for hearing and balance, respectively. The neural signals from these two regions are relayed to the brain stem through separate fiber bundles. However, these two distinct bundles travel together from the inner ear to the brain stem as the vestibulocochlear nerve. Sound is transduced into neural signals within the cochlear region of the inner ear, which contains the sensory neurons of the spiral ganglia. These ganglia are located within the spiral-shaped cochlea of the inner ear. The cochlea is attached to the stapes through the oval window.
The oval window is located at the beginning of a fluid-filled tube within the cochlea called the scala vestibuli. The scala vestibuli extends from the oval window, travelling above the cochlear duct, which is the central cavity of the cochlea that contains the sound-transducing neurons. At the uppermost tip of the cochlea, the scala vestibuli curves over the top of the cochlear duct. The fluid-filled tube, now called the scala tympani, returns to the base of the cochlea, this time travelling under the cochlear duct. The scala tympani ends at the round window, which is covered by a membrane that contains the fluid within the scala. As vibrations of the ossicles travel through the oval window, the fluid of the scala vestibuli and scala tympani moves in a wave-like motion. The frequency of the fluid waves match the frequencies of the sound waves (Figure 14.6). The membrane covering the round window will bulge out or pucker in with the movement of the fluid within the scala tympani.
Figure 14.6 Transmission of Sound Waves to Cochlea A sound wave causes the tympanic membrane to vibrate. This vibration is amplified as it moves across the malleus, incus, and stapes. The amplified vibration is picked up by the oval window causing pressure waves in the fluid of the scala vestibuli and scala tympani. The complexity of the pressure waves is determined by the changes in amplitude and frequency of the sound waves entering the ear.
A cross-sectional view of the cochlea shows that the scala vestibuli and scala tympani run along both sides of the cochlear duct (Figure 14.7). The cochlear duct contains several organs of Corti, which tranduce the wave motion of the two scala into neural signals. The organs of Corti lie on top of the basilar membrane, which is the side of the cochlear duct located between the organs of Corti and the scala tympani. As the fluid waves move through the scala vestibuli and scala tympani, the basilar membrane moves at a specific spot, depending on the frequency of the waves. Higher frequency waves move the region of the basilar membrane that is close to the base of the cochlea. Lower frequency waves move the region of the basilar membrane that is near the tip of the cochlea.
Figure 14.7 Cross Section of the Cochlea The three major spaces within the cochlea are highlighted. The scala tympani and scala vestibuli lie on either side of the cochlear duct. The organ of Corti, containing the mechanoreceptor hair cells, is adjacent to the scala tympani, where it sits atop the basilar membrane.
The organs of Corti contain hair cells, which are named for the hair-like stereocilia extending from the cell’s apical surfaces (Figure 14.8). The stereocilia are an array of microvilli-like structures arranged from tallest to shortest. Protein fibers tether adjacent hairs together within each array, such that the array will bend in response to movements of the basilar membrane. The stereocilia extend up from the hair cells to the overlying tectorial membrane, which is attached medially to the organ of Corti. When the pressure waves from the scala move the basilar membrane, the tectorial membrane slides across the stereocilia. This bends the stereocilia either toward or away from the tallest member of each array. When the stereocilia bend toward the tallest member of their array, tension in the protein tethers opens ion channels in the hair cell membrane. This will depolarize the hair cell membrane, triggering nerve impulses that travel down the afferent nerve fibers attached to the hair cells. When the stereocilia bend toward the shortest member of their array, the tension on the tethers slackens and the ion channels close. When no sound is present, and the stereocilia are standing straight, a small amount of tension still exists on the tethers, keeping the membrane potential of the hair cell slightly depolarized.
Figure 14.8 Hair Cell The hair cell is a mechanoreceptor with an array of stereocilia emerging from its apical surface. The stereocilia are tethered together by proteins that open ion channels when the array is bent toward the tallest member of their array, and closed when the array is bent toward the shortest member of their array.
Figure 14.9 Cochlea and Organ of Corti LM × 412. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
INTERACTIVE LINK
View the University of Michigan WebScope to explore the tissue sample in greater detail. The basilar membrane is the thin membrane that extends from the central core of the cochlea to the edge. What is anchored to this membrane so that they can be activated by movement of the fluids within the cochlea?
As stated above, a given region of the basilar membrane will only move if the incoming sound is at a specific frequency. Because the tectorial membrane only moves where the basilar membrane moves, the hair cells in this region will also only respond to sounds of this specific frequency. Therefore, as the frequency of a sound changes, different hair cells are activated all along the basilar membrane. The cochlea encodes auditory stimuli for frequencies between 20 and 20,000 Hz, which is the range of sound that human ears can detect. The unit of Hertz measures the frequency of sound waves in terms of cycles produced per second. Frequencies as low as 20 Hz are detected by hair cells at the apex, or tip, of the cochlea. Frequencies in the higher ranges of 20 KHz are encoded by hair cells at the base of the cochlea, close to the round and oval windows (Figure 14.10). Most auditory stimuli contain a mixture of sounds at a variety of frequencies and intensities (represented by the amplitude of the sound wave). The hair cells along the length of the cochlear duct, which are each sensitive to a particular frequency, allow the cochlea to separate auditory stimuli by frequency, just as a prism separates visible light into its component colors.
Figure 14.10 Frequency Coding in the Cochlea The standing sound wave generated in the cochlea by the movement of the oval window deflects the basilar membrane on the basis of the frequency of sound. Therefore, hair cells at the base of the cochlea are activated only by high frequencies, whereas those at the apex of the cochlea are activated only by low frequencies.
INTERACTIVE LINK
Watch this video to learn more about how the structures of the ear convert sound waves into a neural signal by moving the “hairs,” or stereocilia, of the cochlear duct. Specific locations along the length of the duct encode specific frequencies, or pitches. The brain interprets the meaning of the sounds we hear as music, speech, noise, etc. Which ear structures are responsible for the amplification and transfer of sound from the external ear to the inner ear?
INTERACTIVE LINK
Watch this animation to learn more about the inner ear and to see the cochlea unroll, with the base at the back of the image and the apex at the front. Specific wavelengths of sound cause specific regions of the basilar membrane to vibrate, much like the keys of a piano produce sound at different frequencies. Based on the animation, where do frequencies—from high to low pitches—cause activity in the hair cells within the cochlear duct?
Equilibrium (Balance)
Along with audition, the inner ear is responsible for encoding information about equilibrium, the sense of balance. A similar mechanoreceptor—a hair cell with stereocilia—senses head position, head movement, and whether our bodies are in motion. These cells are located within the vestibule of the inner ear. Head position is sensed by the utricle and saccule, whereas head movement is sensed by the semicircular canals. The neural signals generated in the vestibular ganglion are transmitted through the vestibulocochlear nerve to the brain stem and cerebellum.
The utricle and saccule are both largely composed of macula tissue (plural = maculae). The macula is composed of hair cells surrounded by support cells. The stereocilia of the hair cells extend into a viscous gel called the otolithic membrane (Figure 14.11). On top of the otolithic membrane is a layer of calcium carbonate crystals, called otoliths. The otoliths essentially make the otolithic membrane top-heavy. The otolithic membrane moves separately from the macula in response to head movements. Tilting the head causes the otolithic membrane to slide over the macula in the direction of gravity. The moving otolithic membrane, in turn, bends the sterocilia, causing some hair cells to depolarize as others hyperpolarize. The exact position of the head is interpreted by the brain based on the pattern of hair-cell depolarization.
Figure 14.11 Linear Acceleration Coding by Maculae The maculae are specialized for sensing linear acceleration, such as when gravity acts on the tilting head, or if the head starts moving in a straight line. The difference in inertia between the hair cell stereocilia and the otolithic membrane in which they are embedded leads to a shearing force that causes the stereocilia to bend in the direction of that linear acceleration.
The semicircular canals are three ring-like extensions of the vestibule. One is oriented in the horizontal plane, whereas the other two are oriented in the vertical plane. The anterior and posterior vertical canals are oriented at approximately 45 degrees relative to the sagittal plane (Figure 14.12). The base of each semicircular canal, where it meets with the vestibule, connects to an enlarged region known as the ampulla. The ampulla contains the hair cells that respond to rotational movement, such as turning the head while saying “no.” The stereocilia of these hair cells extend into the cupula, a membrane that attaches to the top of the ampulla. As the head rotates in a plane parallel to the semicircular canal, the fluid lags, deflecting the cupula in the direction opposite to the head movement. The semicircular canals contain several ampullae, with some oriented horizontally and others oriented vertically. By comparing the relative movements of both the horizontal and vertical ampullae, the vestibular system can detect the direction of most head movements within three-dimensional (3-D) space.
Figure 14.12 Rotational Coding by Semicircular Canals Rotational movement of the head is encoded by the hair cells in the base of the semicircular canals. As one of the canals moves in an arc with the head, the internal fluid moves in the opposite direction, causing the cupula and stereocilia to bend. The movement of two canals within a plane results in information about the direction in which the head is moving, and activation of all six canals can give a very precise indication of head movement in three dimensions.
Somatosensation (Touch)
Somatosensation is considered a general sense, as opposed to the special senses discussed in this section. Somatosensation is the group of sensory modalities that are associated with touch, proprioception, and interoception. These modalities include pressure, vibration, light touch, tickle, itch, temperature, pain, proprioception, and kinesthesia. This means that its receptors are not associated with a specialized organ, but are instead spread throughout the body in a variety of organs. Many of the somatosensory receptors are located in the skin, but receptors are also found in muscles, tendons, joint capsules, ligaments, and in the walls of visceral organs.
Two types of somatosensory signals that are transduced by free nerve endings are pain and temperature. These two modalities use thermoreceptors and nociceptors to transduce temperature and pain stimuli, respectively. Temperature receptors are stimulated when local temperatures differ from body temperature. Some thermoreceptors are sensitive to just cold and others to just heat. Nociception is the sensation of potentially damaging stimuli. Mechanical, chemical, or thermal stimuli beyond a set threshold will elicit painful sensations. Stressed or damaged tissues release chemicals that activate receptor proteins in the nociceptors. For example, the sensation of heat associated with spicy foods involves capsaicin, the active molecule in hot peppers. Capsaicin molecules bind to a transmembrane ion channel in nociceptors that is sensitive to temperatures above 37°C. The dynamics of capsaicin binding with this transmembrane ion channel is unusual in that the molecule remains bound for a long time. Because of this, it will decrease the ability of other stimuli to elicit pain sensations through the activated nociceptor. For this reason, capsaicin can be used as a topical analgesic, such as in products such as Icy Hot™.
If you drag your finger across a textured surface, the skin of your finger will vibrate. Such low frequency vibrations are sensed by mechanoreceptors called Merkel cells, also known as type I cutaneous mechanoreceptors. Merkel cells are located in the stratum basale of the epidermis. Deep pressure and vibration is transduced by lamellated (Pacinian) corpuscles, which are receptors with encapsulated endings found deep in the dermis, or subcutaneous tissue. Light touch is transduced by the encapsulated endings known as tactile (Meissner) corpuscles. Follicles are also wrapped in a plexus of nerve endings known as the hair follicle plexus. These nerve endings detect the movement of hair at the surface of the skin, such as when an insect may be walking along the skin. Stretching of the skin is transduced by stretch receptors known as bulbous corpuscles. Bulbous corpuscles are also known as Ruffini corpuscles, or type II cutaneous mechanoreceptors.
Other somatosensory receptors are found in the joints and muscles. Stretch receptors monitor the stretching of tendons, muscles, and the components of joints. For example, have you ever stretched your muscles before or after exercise and noticed that you can only stretch so far before your muscles spasm back to a less stretched state? This spasm is a reflex that is initiated by stretch receptors to avoid muscle tearing. Such stretch receptors can also prevent over-contraction of a muscle. In skeletal muscle tissue, these stretch receptors are called muscle spindles. Golgi tendon organs similarly transduce the stretch levels of tendons. Bulbous corpuscles are also present in joint capsules, where they measure stretch in the components of the skeletal system within the joint. The types of nerve endings, their locations, and the stimuli they transduce are presented in Table 14.1.
Mechanoreceptors of Somatosensation
| Name | Historical (eponymous) name | Location(s) | Stimuli |
|---|---|---|---|
| Free nerve endings | * | Dermis, cornea, tongue, joint capsules, visceral organs | Pain, temperature, mechanical deformation |
| Mechanoreceptors | Merkel’s discs | Epidermal–dermal junction, mucosal membranes | Low frequency vibration (5–15 Hz) |
| Bulbous corpuscle | Ruffini’s corpuscle | Dermis, joint capsules | Stretch |
| Tactile corpuscle | Meissner’s corpuscle | Papillary dermis, especially in the fingertips and lips | Light touch, vibrations below 50 Hz |
| Lamellated corpuscle | Pacinian corpuscle | Deep dermis, subcutaneous tissue | Deep pressure, high-frequency vibration (around 250 Hz) |
| Hair follicle plexus | * | Wrapped around hair follicles in the dermis | Movement of hair |
| Muscle spindle | * | In line with skeletal muscle fibers | Muscle contraction and stretch |
| Tendon stretch organ | Golgi tendon organ | In line with tendons | Stretch of tendons |
Table 14.1 *No corresponding eponymous name.
Vision
Vision is the special sense of sight that is based on the transduction of light stimuli received through the eyes. The eyes are located within either orbit in the skull. The bony orbits surround the eyeballs, protecting them and anchoring the soft tissues of the eye (Figure 14.13). The eyelids, with lashes at their leading edges, help to protect the eye from abrasions by blocking particles that may land on the surface of the eye. The inner surface of each lid is a thin membrane known as the palpebral conjunctiva. The conjunctiva extends over the white areas of the eye (the sclera), connecting the eyelids to the eyeball. Tears are produced by the lacrimal gland, located beneath the lateral edges of the nose. Tears produced by this gland flow through the lacrimal duct to the medial corner of the eye, where the tears flow over the conjunctiva, washing away foreign particles.
Figure 14.13 The Eye in the Orbit The eye is located within the orbit and surrounded by soft tissues that protect and support its function. The orbit is surrounded by cranial bones of the skull.
Movement of the eye within the orbit is accomplished by the contraction of six extraocular muscles that originate from the bones of the orbit and insert into the surface of the eyeball (Figure 14.14). Four of the muscles are arranged at the cardinal points around the eye and are named for those locations. They are the superior rectus, medial rectus, inferior rectus, and lateral rectus. When each of these muscles contract, the eye moves toward the contracting muscle. For example, when the superior rectus contracts, the eye rotates to look up. The superior oblique originates at the posterior orbit, near the origin of the four rectus muscles. However, the tendon of the oblique muscles threads through a pulley-like piece of cartilage known as the trochlea. The tendon inserts obliquely into the superior surface of the eye. The angle of the tendon through the trochlea means that contraction of the superior oblique rotates the eye medially. The inferior oblique muscle originates from the floor of the orbit and inserts into the inferolateral surface of the eye. When it contracts, it laterally rotates the eye, in opposition to the superior oblique. Rotation of the eye by the two oblique muscles is necessary because the eye is not perfectly aligned on the sagittal plane. When the eye looks up or down, the eye must also rotate slightly to compensate for the superior rectus pulling at approximately a 20-degree angle, rather than straight up. The same is true for the inferior rectus, which is compensated by contraction of the inferior oblique. A seventh muscle in the orbit is the levator palpebrae superioris, which is responsible for elevating and retracting the upper eyelid, a movement that usually occurs in concert with elevation of the eye by the superior rectus (see Figure 14.13).
The extraocular muscles are innervated by three cranial nerves. The lateral rectus, which causes abduction of the eye, is innervated by the abducens nerve. The superior oblique is innervated by the trochlear nerve. All of the other muscles are innervated by the oculomotor nerve, as is the levator palpebrae superioris. The motor nuclei of these cranial nerves connect to the brain stem, which coordinates eye movements.
Figure 14.14 Extraocular Muscles The extraocular muscles move the eye within the orbit.
The eye itself is a hollow sphere composed of three layers of tissue. The outermost layer is the fibrous tunic, which includes the white sclera and clear cornea. The sclera accounts for five sixths of the surface of the eye, most of which is not visible, though humans are unique compared with many other species in having so much of the “white of the eye” visible (Figure 14.15). The transparent cornea covers the anterior tip of the eye and allows light to enter the eye. The middle layer of the eye is the vascular tunic, which is mostly composed of the choroid, ciliary body, and iris. The choroid is a layer of highly vascularized connective tissue that provides a blood supply to the eyeball. The choroid is posterior to the ciliary body, a muscular structure that is attached to the lens by suspensory ligaments, or zonule fibers. These two structures bend the lens, allowing it to focus light on the back of the eye. Overlaying the ciliary body, and visible in the anterior eye, is the iris—the colored part of the eye. The iris is a smooth muscle that opens or closes the pupil, which is the hole at the center of the eye that allows light to enter. The iris constricts the pupil in response to bright light and dilates the pupil in response to dim light. The innermost layer of the eye is the neural tunic, or retina, which contains the nervous tissue responsible for photoreception.
The eye is also divided into two cavities: the anterior cavity and the posterior cavity. The anterior cavity is the space between the cornea and lens, including the iris and ciliary body. It is filled with a watery fluid called the aqueous humor. The posterior cavity is the space behind the lens that extends to the posterior side of the interior eyeball, where the retina is located. The posterior cavity is filled with a more viscous fluid called the vitreous humor.
The retina is composed of several layers and contains specialized cells for the initial processing of visual stimuli. The photoreceptors (rods and cones) change their membrane potential when stimulated by light energy. The change in membrane potential alters the amount of neurotransmitter that the photoreceptor cells release onto bipolar cells in the outer synaptic layer. It is the bipolar cell in the retina that connects a photoreceptor to a retinal ganglion cell (RGC) in the inner synaptic layer. There, amacrine cells additionally contribute to retinal processing before an action potential is produced by the RGC. The axons of RGCs, which lie at the innermost layer of the retina, collect at the optic disc and leave the eye as the optic nerve(see Figure 14.15). Because these axons pass through the retina, there are no photoreceptors at the very back of the eye, where the optic nerve begins. This creates a “blind spot” in the retina, and a corresponding blind spot in our visual field.
Figure 14.15 Structure of the Eye The sphere of the eye can be divided into anterior and posterior chambers. The wall of the eye is composed of three layers: the fibrous tunic, vascular tunic, and neural tunic. Within the neural tunic is the retina, with three layers of cells and two synaptic layers in between. The center of the retina has a small indentation known as the fovea.
Note that the photoreceptors in the retina (rods and cones) are located behind the axons, RGCs, bipolar cells, and retinal blood vessels. A significant amount of light is absorbed by these structures before the light reaches the photoreceptor cells. However, at the exact center of the retina is a small area known as the fovea. At the fovea, the retina lacks the supporting cells and blood vessels, and only contains photoreceptors. Therefore, visual acuity, or the sharpness of vision, is greatest at the fovea. This is because the fovea is where the least amount of incoming light is absorbed by other retinal structures (see Figure 14.15). As one moves in either direction from this central point of the retina, visual acuity drops significantly. In addition, each photoreceptor cell of the fovea is connected to a single RGC. Therefore, this RGC does not have to integrate inputs from multiple photoreceptors, which reduces the accuracy of visual transduction. Toward the edges of the retina, several photoreceptors converge on RGCs (through the bipolar cells) up to a ratio of 50 to 1. The difference in visual acuity between the fovea and peripheral retina is easily evidenced by looking directly at a word in the middle of this paragraph. The visual stimulus in the middle of the field of view falls on the fovea and is in the sharpest focus. Without moving your eyes off that word, notice that words at the beginning or end of the paragraph are not in focus. The images in your peripheral vision are focused by the peripheral retina, and have vague, blurry edges and words that are not as clearly identified. As a result, a large part of the neural function of the eyes is concerned with moving the eyes and head so that important visual stimuli are centered on the fovea.
Light falling on the retina causes chemical changes to pigment molecules in the photoreceptors, ultimately leading to a change in the activity of the RGCs. Photoreceptor cells have two parts, the inner segment and the outer segment (Figure 14.16). The inner segment contains the nucleus and other common organelles of a cell, whereas the outer segment is a specialized region in which photoreception takes place. There are two types of photoreceptors—rods and cones—which differ in the shape of their outer segment. The rod-shaped outer segments of the rod photoreceptor contain a stack of membrane-bound discs that contain the photosensitive pigment rhodopsin. The cone-shaped outer segments of the cone photoreceptor contain their photosensitive pigments in infoldings of the cell membrane. There are three cone photopigments, called opsins, which are each sensitive to a particular wavelength of light. The wavelength of visible light determines its color. The pigments in human eyes are specialized in perceiving three different primary colors: red, green, and blue.
Figure 14.16 Photoreceptor (a) All photoreceptors have inner segments containing the nucleus and other important organelles and outer segments with membrane arrays containing the photosensitive opsin molecules. Rod outer segments are long columnar shapes with stacks of membrane-bound discs that contain the rhodopsin pigment. Cone outer segments are short, tapered shapes with folds of membrane in place of the discs in the rods. (b) Tissue of the retina shows a dense layer of nuclei of the rods and cones. LM × 800. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
At the molecular level, visual stimuli cause changes in the photopigment molecule that lead to changes in membrane potential of the photoreceptor cell. A single unit of light is called a photon, which is described in physics as a packet of energy with properties of both a particle and a wave. The energy of a photon is represented by its wavelength, with each wavelength of visible light corresponding to a particular color. Visible light is electromagnetic radiation with a wavelength between 380 and 720 nm. Wavelengths of electromagnetic radiation longer than 720 nm fall into the infrared range, whereas wavelengths shorter than 380 nm fall into the ultraviolet range. Light with a wavelength of 380 nm is blue whereas light with a wavelength of 720 nm is dark red. All other colors fall between red and blue at various points along the wavelength scale.
Opsin pigments are actually transmembrane proteins that contain a cofactor known as retinal. Retinal is a hydrocarbon molecule related to vitamin A. When a photon hits retinal, the long hydrocarbon chain of the molecule is biochemically altered. Specifically, photons cause some of the double-bonded carbons within the chain to switch from a cis to a trans conformation. This process is called photoisomerization. Before interacting with a photon, retinal’s flexible double-bonded carbons are in the cis conformation. This molecule is referred to as 11-cis-retinal. A photon interacting with the molecule causes the flexible double-bonded carbons to change to the trans- conformation, forming all-trans-retinal, which has a straight hydrocarbon chain (Figure 14.17).
The shape change of retinal in the photoreceptors initiates visual transduction in the retina. Activation of retinal and the opsin proteins result in activation of a G protein. The G protein changes the membrane potential of the photoreceptor cell, which then releases less neurotransmitter into the outer synaptic layer of the retina. Until the retinal molecule is changed back to the 11-cis-retinal shape, the opsin cannot respond to light energy, which is called bleaching. When a large group of photopigments is bleached, the retina will send information as if opposing visual information is being perceived. After a bright flash of light, afterimages are usually seen in negative. The photoisomerization is reversed by a series of enzymatic changes so that the retinal responds to more light energy.
Figure 14.17 Retinal Isomers The retinal molecule has two isomers, (a) one before a photon interacts with it and (b) one that is altered through photoisomerization.
The opsins are sensitive to limited wavelengths of light. Rhodopsin, the photopigment in rods, is most sensitive to light at a wavelength of 498 nm. The three color opsins have peak sensitivities of 564 nm, 534 nm, and 420 nm corresponding roughly to the primary colors of red, green, and blue (Figure 14.18). The absorbance of rhodopsin in the rods is much more sensitive than in the cone opsins; specifically, rods are sensitive to vision in low light conditions, and cones are sensitive to brighter conditions. In normal sunlight, rhodopsin will be constantly bleached while the cones are active. In a darkened room, there is not enough light to activate cone opsins, and vision is entirely dependent on rods. Rods are so sensitive to light that a single photon can result in an action potential from a rod’s corresponding RGC.
The three types of cone opsins, being sensitive to different wavelengths of light, provide us with color vision. By comparing the activity of the three different cones, the brain can extract color information from visual stimuli. For example, a bright blue light that has a wavelength of approximately 450 nm would activate the “red” cones minimally, the “green” cones marginally, and the “blue” cones predominantly. The relative activation of the three different cones is calculated by the brain, which perceives the color as blue. However, cones cannot react to low-intensity light, and rods do not sense the color of light. Therefore, our low-light vision is—in essence—in grayscale. In other words, in a dark room, everything appears as a shade of gray. If you think that you can see colors in the dark, it is most likely because your brain knows what color something is and is relying on that memory.
Figure 14.18 Comparison of Color Sensitivity of Photopigments Comparing the peak sensitivity and absorbance spectra of the four photopigments suggests that they are most sensitive to particular wavelengths.
INTERACTIVE LINK
Watch this video to learn more about a transverse section through the brain that depicts the visual pathway from the eye to the occipital cortex. The first half of the pathway is the projection from the RGCs through the optic nerve to the lateral geniculate nucleus in the thalamus on either side. This first fiber in the pathway synapses on a thalamic cell that then projects to the visual cortex in the occipital lobe where “seeing,” or visual perception, takes place. This video gives an abbreviated overview of the visual system by concentrating on the pathway from the eyes to the occipital lobe. The video makes the statement (at 0:45) that “specialized cells in the retina called ganglion cells convert the light rays into electrical signals.” What aspect of retinal processing is simplified by that statement? Explain your answer.
Sensory Nerves
Once any sensory cell transduces a stimulus into a nerve impulse, that impulse has to travel along axons to reach the CNS. In many of the special senses, the axons leaving the sensory receptors have a topographical arrangement, meaning that the location of the sensory receptor relates to the location of the axon in the nerve. For example, in the retina, axons from RGCs in the fovea are located at the center of the optic nerve, where they are surrounded by axons from the more peripheral RGCs.
Spinal Nerves
Generally, spinal nerves contain afferent axons from sensory receptors in the periphery, such as from the skin, mixed with efferent axons travelling to the muscles or other effector organs. As the spinal nerve nears the spinal cord, it splits into dorsal and ventral roots. The dorsal root contains only the axons of sensory neurons, whereas the ventral roots contain only the axons of the motor neurons. Some of the branches will synapse with local neurons in the dorsal root ganglion, posterior (dorsal) horn, or even the anterior (ventral) horn, at the level of the spinal cord where they enter. Other branches will travel a short distance up or down the spine to interact with neurons at other levels of the spinal cord. A branch may also turn into the posterior (dorsal) column of the white matter to connect with the brain. For the sake of convenience, we will use the terms ventral and dorsal in reference to structures within the spinal cord that are part of these pathways. This will help to underscore the relationships between the different components. Typically, spinal nerve systems that connect to the brain are contralateral, in that the right side of the body is connected to the left side of the brain and the left side of the body to the right side of the brain.
Cranial Nerves
Cranial nerves convey specific sensory information from the head and neck directly to the brain. For sensations below the neck, the right side of the body is connected to the left side of the brain and the left side of the body to the right side of the brain. Whereas spinal information is contralateral, cranial nerve systems are mostly ipsilateral, meaning that a cranial nerve on the right side of the head is connected to the right side of the brain. Some cranial nerves contain only sensory axons, such as the olfactory, optic, and vestibulocochlear nerves. Other cranial nerves contain both sensory and motor axons, including the trigeminal, facial, glossopharyngeal, and vagus nerves (however, the vagus nerve is not associated with the somatic nervous system). The general senses of somatosensation for the face travel through the trigeminal system.
Central Processing
- Describe the pathways that sensory systems follow into the central nervous system
- Differentiate between the two major ascending pathways in the spinal cord
- Describe the pathway of somatosensory input from the face and compare it to the ascending pathways in the spinal cord
- Explain topographical representations of sensory information in at least two systems
- Describe two pathways of visual processing and the functions associated with each
Sensory Pathways
Specific regions of the CNS coordinate different somatic processes using sensory inputs and motor outputs of peripheral nerves. A simple case is a reflex caused by a synapse between a dorsal sensory neuron axon and a motor neuron in the ventral horn. More complex arrangements are possible to integrate peripheral sensory information with higher processes. The important regions of the CNS that play a role in somatic processes can be separated into the spinal cord brain stem, diencephalon, cerebral cortex, and subcortical structures.
Spinal Cord and Brain Stem
A sensory pathway that carries peripheral sensations to the brain is referred to as an ascending pathway, or ascending tract. The various sensory modalities each follow specific pathways through the CNS. Tactile and other somatosensory stimuli activate receptors in the skin, muscles, tendons, and joints throughout the entire body. However, the somatosensory pathways are divided into two separate systems on the basis of the location of the receptor neurons. Somatosensory stimuli from below the neck pass along the sensory pathways of the spinal cord, whereas somatosensory stimuli from the head and neck travel through the cranial nerves—specifically, the trigeminal system.
The dorsal column system (sometimes referred to as the dorsal column–medial lemniscus) and the spinothalamic tract are two major pathways that bring sensory information to the brain (Figure 14.19). The sensory pathways in each of these systems are composed of three successive neurons.
The dorsal column system begins with the axon of a dorsal root ganglion neuron entering the dorsal root and joining the dorsal column white matter in the spinal cord. As axons of this pathway enter the dorsal column, they take on a positional arrangement so that axons from lower levels of the body position themselves medially, whereas axons from upper levels of the body position themselves laterally. The dorsal column is separated into two component tracts, the fasciculus gracilis that contains axons from the legs and lower body, and the fasciculus cuneatus that contains axons from the upper body and arms.
The axons in the dorsal column terminate in the nuclei of the medulla, where each synapses with the second neuron in their respective pathway. The nucleus gracilis is the target of fibers in the fasciculus gracilis, whereas the nucleus cuneatus is the target of fibers in the fasciculus cuneatus. The second neuron in the system projects from one of the two nuclei and then decussates, or crosses the midline of the medulla. These axons then continue to ascend the brain stem as a bundle called the medial lemniscus. These axons terminate in the thalamus, where each synapses with the third neuron in their respective pathway. The third neuron in the system projects its axons to the postcentral gyrus of the cerebral cortex, where somatosensory stimuli are initially processed and the conscious perception of the stimulus occurs.
The spinothalamic tract also begins with neurons in a dorsal root ganglion. These neurons extend their axons to the dorsal horn, where they synapse with the second neuron in their respective pathway. The name “spinothalamic” comes from this second neuron, which has its cell body in the spinal cord gray matter and connects to the thalamus. Axons from these second neurons then decussate within the spinal cord and ascend to the brain and enter the thalamus, where each synapses with the third neuron in its respective pathway. The neurons in the thalamus then project their axons to the spinothalamic tract, which synapses in the postcentral gyrus of the cerebral cortex.
These two systems are similar in that they both begin with dorsal root ganglion cells, as with most general sensory information. The dorsal column system is primarily responsible for touch sensations and proprioception, whereas the spinothalamic tract pathway is primarily responsible for pain and temperature sensations. Another similarity is that the second neurons in both of these pathways are contralateral, because they project across the midline to the other side of the brain or spinal cord. In the dorsal column system, this decussation takes place in the brain stem; in the spinothalamic pathway, it takes place in the spinal cord at the same spinal cord level at which the information entered. The third neurons in the two pathways are essentially the same. In both, the second neuron synapses in the thalamus, and the thalamic neuron projects to the somatosensory cortex.
Figure 14.19 Ascending Sensory Pathways of the Spinal Cord The dorsal column system and spinothalamic tract are the major ascending pathways that connect the periphery with the brain.
The trigeminal pathway carries somatosensory information from the face, head, mouth, and nasal cavity. As with the previously discussed nerve tracts, the sensory pathways of the trigeminal pathway each involve three successive neurons. First, axons from the trigeminal ganglion enter the brain stem at the level of the pons. These axons project to one of three locations. The spinal trigeminal nucleus of the medulla receives information similar to that carried by spinothalamic tract, such as pain and temperature sensations. Other axons go to either the chief sensory nucleus in the pons or the mesencephalic nuclei in the midbrain. These nuclei receive information like that carried by the dorsal column system, such as touch, pressure, vibration, and proprioception. Axons from the second neuron decussate and ascend to the thalamus along the trigeminothalamic tract. In the thalamus, each axon synapses with the third neuron in its respective pathway. Axons from the third neuron then project from the thalamus to the primary somatosensory cortex of the cerebrum.
The sensory pathway for gustation travels along the facial and glossopharyngeal cranial nerves, which synapse with neurons of the solitary nucleus in the brain stem. Axons from the solitary nucleus then project to the ventral posterior nucleus of the thalamus. Finally, axons from the ventral posterior nucleus project to the gustatory cortex of the cerebral cortex, where taste is processed and consciously perceived.
The sensory pathway for audition travels along the vestibulocochlear nerve, which synapses with neurons in the cochlear nuclei of the superior medulla. Within the brain stem, input from either ear is combined to extract location information from the auditory stimuli. Whereas the initial auditory stimuli received at the cochlea strictly represent the frequency—or pitch—of the stimuli, the locations of sounds can be determined by comparing information arriving at both ears.
Sound localization is a feature of central processing in the auditory nuclei of the brain stem. Sound localization is achieved by the brain calculating the interaural time difference and the interaural intensity difference. A sound originating from a specific location will arrive at each ear at different times, unless the sound is directly in front of the listener. If the sound source is slightly to the left of the listener, the sound will arrive at the left ear microseconds before it arrives at the right ear (Figure 14.20). This time difference is an example of an interaural time difference. Also, the sound will be slightly louder in the left ear than in the right ear because some of the sound waves reaching the opposite ear are blocked by the head. This is an example of an interaural intensity difference.
Figure 14.20 Auditory Brain Stem Mechanisms of Sound Localization Localizing sound in the horizontal plane is achieved by processing in the medullary nuclei of the auditory system. Connections between neurons on either side are able to compare very slight differences in sound stimuli that arrive at either ear and represent interaural time and intensity differences.
Auditory processing continues on to a nucleus in the midbrain called the inferior colliculus. Axons from the inferior colliculus project to two locations, the thalamus and the superior colliculus. The medial geniculate nucleus of the thalamus receives the auditory information and then projects that information to the auditory cortex in the temporal lobe of the cerebral cortex. The superior colliculus receives input from the visual and somatosensory systems, as well as the ears, to initiate stimulation of the muscles that turn the head and neck toward the auditory stimulus.
Balance is coordinated through the vestibular system, the nerves of which are composed of axons from the vestibular ganglion that carries information from the utricle, saccule, and semicircular canals. The system contributes to controlling head and neck movements in response to vestibular signals. An important function of the vestibular system is coordinating eye and head movements to maintain visual attention. Most of the axons terminate in the vestibular nuclei of the medulla. Some axons project from the vestibular ganglion directly to the cerebellum, with no intervening synapse in the vestibular nuclei. The cerebellum is primarily responsible for initiating movements on the basis of equilibrium information.
Neurons in the vestibular nuclei project their axons to targets in the brain stem. One target is the reticular formation, which influences respiratory and cardiovascular functions in relation to body movements. A second target of the axons of neurons in the vestibular nuclei is the spinal cord, which initiates the spinal reflexes involved with posture and balance. To assist the visual system, fibers of the vestibular nuclei project to the oculomotor, trochlear, and abducens nuclei to influence signals sent along the cranial nerves. These connections constitute the pathway of the vestibulo-ocular reflex (VOR), which compensates for head and body movement by stabilizing images on the retina (Figure 14.21). Finally, the vestibular nuclei project to the thalamus to join the proprioceptive pathway of the dorsal column system, allowing conscious perception of equilibrium.
Figure 14.21 Vestibulo-ocular Reflex Connections between the vestibular system and the cranial nerves controlling eye movement keep the eyes centered on a visual stimulus, even though the head is moving. During head movement, the eye muscles move the eyes in the opposite direction as the head movement, keeping the visual stimulus centered in the field of view.
The connections of the optic nerve are more complicated than those of other cranial nerves. Instead of the connections being between each eye and the brain, visual information is segregated between the left and right sides of the visual field. In addition, some of the information from one side of the visual field projects to the opposite side of the brain. Within each eye, the axons projecting from the medial side of the retina decussate at the optic chiasm. For example, the axons from the medial retina of the left eye cross over to the right side of the brain at the optic chiasm. However, within each eye, the axons projecting from the lateral side of the retina do not decussate. For example, the axons from the lateral retina of the right eye project back to the right side of the brain. Therefore the left field of view of each eye is processed on the right side of the brain, whereas the right field of view of each eye is processed on the left side of the brain (Figure 14.22).
Figure 14.22 Segregation of Visual Field Information at the Optic Chiasm Contralateral visual field information from the lateral retina projects to the ipsilateral brain, whereas ipsilateral visual field information has to decussate at the optic chiasm to reach the opposite side of the brain. (Note that this is an inferior view.)
A unique clinical presentation that relates to this anatomic arrangement is the loss of lateral peripheral vision, known as bilateral hemianopia. This is different from “tunnel vision” because the superior and inferior peripheral fields are not lost. Visual field deficits can be disturbing for a patient, but in this case, the cause is not within the visual system itself. A growth of the pituitary gland presses against the optic chiasm and interferes with signal transmission. However, the axons projecting to the same side of the brain are unaffected. Therefore, the patient loses the outermost areas of their field of vision and cannot see objects to their right and left.
Extending from the optic chiasm, the axons of the visual system are referred to as the optic tract instead of the optic nerve. The optic tract has three major targets, two in the diencephalon and one in the midbrain. The connection between the eyes and diencephalon is demonstrated during development, in which the neural tissue of the retina differentiates from that of the diencephalon by the growth of the secondary vesicles. The connections of the retina into the CNS are a holdover from this developmental association. The majority of the connections of the optic tract are to the thalamus—specifically, the lateral geniculate nucleus. Axons from this nucleus then project to the visual cortex of the cerebrum, located in the occipital lobe. Another target of the optic tract is the superior colliculus.
In addition, a very small number of RGC axons project from the optic chiasm to the suprachiasmatic nucleus of the hypothalamus. These RGCs are photosensitive, in that they respond to the presence or absence of light. Unlike the photoreceptors, however, these photosensitive RGCs cannot be used to perceive images. By simply responding to the absence or presence of light, these RGCs can send information about day length. The perceived proportion of sunlight to darkness establishes the circadian rhythm of our bodies, allowing certain physiological events to occur at approximately the same time every day.
Diencephalon
The diencephalon is beneath the cerebrum and includes the thalamus and hypothalamus. In the somatic nervous system, the thalamus is an important relay for communication between the cerebrum and the rest of the nervous system. The hypothalamus has both somatic and autonomic functions. In addition, the hypothalamus communicates with the limbic system, which controls emotions and memory functions.
Sensory input to the thalamus comes from most of the special senses and ascending somatosensory tracts. Each sensory system is relayed through a particular nucleus in the thalamus. The thalamus is a required transfer point for most sensory tracts that reach the cerebral cortex, where conscious sensory perception begins. The one exception to this rule is the olfactory system. The olfactory tract axons from the olfactory bulb project directly to the cerebral cortex, along with the limbic system and hypothalamus.
The thalamus is a collection of several nuclei that can be categorized into three anatomical groups. White matter running through the thalamus defines the three major regions of the thalamus, which are an anterior nucleus, a medial nucleus, and a lateral group of nuclei. The anterior nucleus serves as a relay between the hypothalamus and the emotion and memory-producing limbic system. The medial nuclei serve as a relay for information from the limbic system and basal ganglia to the cerebral cortex. This allows memory creation during learning, but also determines alertness. The special and somatic senses connect to the lateral nuclei, where their information is relayed to the appropriate sensory cortex of the cerebrum.
Cortical Processing
As described earlier, many of the sensory axons are positioned in the same way as their corresponding receptor cells in the body. This allows identification of the position of a stimulus on the basis of which receptor cells are sending information. The cerebral cortex also maintains this sensory topography in the particular areas of the cortex that correspond to the position of the receptor cells. The somatosensory cortex provides an example in which, in essence, the locations of the somatosensory receptors in the body are mapped onto the somatosensory cortex. This mapping is often depicted using a sensory homunculus (Figure 14.23).
The term homunculus comes from the Latin word for “little man” and refers to a map of the human body that is laid across a portion of the cerebral cortex. In the somatosensory cortex, the external genitals, feet, and lower legs are represented on the medial face of the gyrus within the longitudinal fissure. As the gyrus curves out of the fissure and along the surface of the parietal lobe, the body map continues through the thighs, hips, trunk, shoulders, arms, and hands. The head and face are just lateral to the fingers as the gyrus approaches the lateral sulcus. The representation of the body in this topographical map is medial to lateral from the lower to upper body. It is a continuation of the topographical arrangement seen in the dorsal column system, where axons from the lower body are carried in the fasciculus gracilis, whereas axons from the upper body are carried in the fasciculus cuneatus. As the dorsal column system continues into the medial lemniscus, these relationships are maintained. Also, the head and neck axons running from the trigeminal nuclei to the thalamus run adjacent to the upper body fibers. The connections through the thalamus maintain topography such that the anatomic information is preserved. Note that this correspondence does not result in a perfectly miniature scale version of the body, but rather exaggerates the more sensitive areas of the body, such as the fingers and lower face. Less sensitive areas of the body, such as the shoulders and back, are mapped to smaller areas on the cortex.
Figure 14.23 The Sensory Homunculus A cartoon representation of the sensory homunculus arranged adjacent to the cortical region in which the processing takes place.
Likewise, the topographic relationship between the retina and the visual cortex is maintained throughout the visual pathway. The visual field is projected onto the two retinae, as described above, with sorting at the optic chiasm. The right peripheral visual field falls on the medial portion of the right retina and the lateral portion of the left retina. The right medial retina then projects across the midline through the optic chiasm. This results in the right visual field being processed in the left visual cortex. Likewise, the left visual field is processed in the right visual cortex (see Figure 14.22). Though the chiasm is helping to sort right and left visual information, superior and inferior visual information is maintained topographically in the visual pathway. Light from the superior visual field falls on the inferior retina, and light from the inferior visual field falls on the superior retina. This topography is maintained such that the superior region of the visual cortex processes the inferior visual field and vice versa. Therefore, the visual field information is inverted and reversed as it enters the visual cortex—up is down, and left is right. However, the cortex processes the visual information such that the final conscious perception of the visual field is correct. The topographic relationship is evident in that information from the foveal region of the retina is processed in the center of the primary visual cortex. Information from the peripheral regions of the retina are correspondingly processed toward the edges of the visual cortex. Similar to the exaggerations in the sensory homunculus of the somatosensory cortex, the foveal-processing area of the visual cortex is disproportionately larger than the areas processing peripheral vision.
In an experiment performed in the 1960s, subjects wore prism glasses so that the visual field was inverted before reaching the eye. On the first day of the experiment, subjects would duck when walking up to a table, thinking it was suspended from the ceiling. However, after a few days of acclimation, the subjects behaved as if everything were represented correctly. Therefore, the visual cortex is somewhat flexible in adapting to the information it receives from our eyes (Figure 14.24).
Figure 14.24 Topographic Mapping of the Retina onto the Visual Cortex The visual field projects onto the retina through the lenses and falls on the retinae as an inverted, reversed image. The topography of this image is maintained as the visual information travels through the visual pathway to the cortex.
The cortex has been described as having specific regions that are responsible for processing specific information; there is the visual cortex, somatosensory cortex, gustatory cortex, etc. However, our experience of these senses is not divided. Instead, we experience what can be referred to as a seamless percept. Our perceptions of the various sensory modalities—though distinct in their content—are integrated by the brain so that we experience the world as a continuous whole.
In the cerebral cortex, sensory processing begins at the primary sensory cortex, then proceeds to an association area, and finally, into a multimodal integration area. For example, the visual pathway projects from the retinae through the thalamus to the primary visual cortex in the occipital lobe. This area is primarily in the medial wall within the longitudinal fissure. Here, visual stimuli begin to be recognized as basic shapes. Edges of objects are recognized and built into more complex shapes. Also, inputs from both eyes are compared to extract depth information. Because of the overlapping field of view between the two eyes, the brain can begin to estimate the distance of stimuli based on binocular depth cues.
INTERACTIVE LINK
Watch this video to learn more about how the brain perceives 3-D motion. Similar to how retinal disparity offers 3-D moviegoers a way to extract 3-D information from the two-dimensional visual field projected onto the retina, the brain can extract information about movement in space by comparing what the two eyes see. If movement of a visual stimulus is leftward in one eye and rightward in the opposite eye, the brain interprets this as movement toward (or away) from the face along the midline. If both eyes see an object moving in the same direction, but at different rates, what would that mean for spatial movement?
EVERYDAY CONNECTION
Depth Perception, 3-D Movies, and Optical Illusions
The visual field is projected onto the retinal surface, where photoreceptors transduce light energy into neural signals for the brain to interpret. The retina is a two-dimensional surface, so it does not encode three-dimensional information. However, we can perceive depth. How is that accomplished?
Two ways in which we can extract depth information from the two-dimensional retinal signal are based on monocular cues and binocular cues, respectively. Monocular depth cues are those that are the result of information within the two-dimensional visual field. One object that overlaps another object has to be in front. Relative size differences are also a cue. For example, if a basketball appears larger than the basket, then the basket must be further away. On the basis of experience, we can estimate how far away the basket is. Binocular depth cues compare information represented in the two retinae because they do not see the visual field exactly the same.
The centers of the two eyes are separated by a small distance, which is approximately 6 to 6.5 cm in most people. Because of this offset, visual stimuli do not fall on exactly the same spot on both retinae unless we are fixated directly on them and they fall on the fovea of each retina. All other objects in the visual field, either closer or farther away than the fixated object, will fall on different spots on the retina. When vision is fixed on an object in space, closer objects will fall on the lateral retina of each eye, and more distant objects will fall on the medial retina of either eye (Figure 14.25). This is easily observed by holding a finger up in front of your face as you look at a more distant object. You will see two images of your finger that represent the two disparate images that are falling on either retina.
These depth cues, both monocular and binocular, can be exploited to make the brain think there are three dimensions in two-dimensional information. This is the basis of 3-D movies. The projected image on the screen is two dimensional, but it has disparate information embedded in it. The 3-D glasses that are available at the theater filter the information so that only one eye sees one version of what is on the screen, and the other eye sees the other version. If you take the glasses off, the image on the screen will have varying amounts of blur because both eyes are seeing both layers of information, and the third dimension will not be evident. Some optical illusions can take advantage of depth cues as well, though those are more often using monocular cues to fool the brain into seeing different parts of the scene as being at different depths.
Figure 14.25 Retinal Disparity Because of the interocular distance, which results in objects of different distances falling on different spots of the two retinae, the brain can extract depth perception from the two-dimensional information of the visual field.
There are two main regions that surround the primary cortex that are usually referred to as areas V2 and V3 (the primary visual cortex is area V1). These surrounding areas are the visual association cortex. The visual association regions develop more complex visual perceptions by adding color and motion information. The information processed in these areas is then sent to regions of the temporal and parietal lobes. Visual processing has two separate streams of processing: one into the temporal lobe and one into the parietal lobe. These are the ventral and dorsal streams, respectively (Figure 14.26). The ventral streamidentifies visual stimuli and their significance. Because the ventral stream uses temporal lobe structures, it begins to interact with the non-visual cortex and may be important in visual stimuli becoming part of memories. The dorsal stream locates objects in space and helps in guiding movements of the body in response to visual inputs. The dorsal stream enters the parietal lobe, where it interacts with somatosensory cortical areas that are important for our perception of the body and its movements. The dorsal stream can then influence frontal lobe activity where motor functions originate.
Figure 14.26 Ventral and Dorsal Visual Streams From the primary visual cortex in the occipital lobe, visual processing continues in two streams—one into the temporal lobe and one into the parietal lobe.
DISORDERS OF THE...
Brain: Prosopagnosia
The failures of sensory perception can be unusual and debilitating. A particular sensory deficit that inhibits an important social function of humans is prosopagnosia, or face blindness. The word comes from the Greek words prosopa, that means “faces,” and agnosia, that means “not knowing.” Some people may feel that they cannot recognize people easily by their faces. However, a person with prosopagnosia cannot recognize the most recognizable people in their respective cultures. They would not recognize the face of a celebrity, an important historical figure, or even a family member like their mother. They may not even recognize their own face.
Prosopagnosia can be caused by trauma to the brain, or it can be present from birth. The exact cause of proposagnosia and the reason that it happens to some people is unclear. A study of the brains of people born with the deficit found that a specific region of the brain, the anterior fusiform gyrus of the temporal lobe, is often underdeveloped. This region of the brain is concerned with the recognition of visual stimuli and its possible association with memories. Though the evidence is not yet definitive, this region is likely to be where facial recognition occurs.
Though this can be a devastating condition, people who suffer from it can get by—often by using other cues to recognize the people they see. Often, the sound of a person’s voice, or the presence of unique cues such as distinct facial features (a mole, for example) or hair color can help the sufferer recognize a familiar person. In the video on prosopagnosia provided in this section, a woman is shown having trouble recognizing celebrities, family members, and herself. In some situations, she can use other cues to help her recognize faces.
INTERACTIVE LINK
The inability to recognize people by their faces is a troublesome problem. It can be caused by trauma, or it may be inborn. Watch this video to learn more about a person who lost the ability to recognize faces as the result of an injury. She cannot recognize the faces of close family members or herself. What other information can a person suffering from prosopagnosia use to figure out whom they are seeing?
Motor Responses
- List the components of the basic processing stream for the motor system
- Describe the pathway of descending motor commands from the cortex to the skeletal muscles
- Compare different descending pathways, both by structure and function
- Explain the initiation of movement from the neurological connections
- Describe several reflex arcs and their functional roles
The defining characteristic of the somatic nervous system is that it controls skeletal muscles. Somatic senses inform the nervous system about the external environment, but the response to that is through voluntary muscle movement. The term “voluntary” suggests that there is a conscious decision to make a movement. However, some aspects of the somatic system use voluntary muscles without conscious control. One example is the ability of our breathing to switch to unconscious control while we are focused on another task. However, the muscles that are responsible for the basic process of breathing are also utilized for speech, which is entirely voluntary.
Cortical Responses
Let’s start with sensory stimuli that have been registered through receptor cells and the information relayed to the CNS along ascending pathways. In the cerebral cortex, the initial processing of sensory perception progresses to associative processing and then integration in multimodal areas of cortex. These levels of processing can lead to the incorporation of sensory perceptions into memory, but more importantly, they lead to a response. The completion of cortical processing through the primary, associative, and integrative sensory areas initiates a similar progression of motor processing, usually in different cortical areas.
Whereas the sensory cortical areas are located in the occipital, temporal, and parietal lobes, motor functions are largely controlled by the frontal lobe. The most anterior regions of the frontal lobe—the prefrontal areas—are important for executive functions, which are those cognitive functions that lead to goal-directed behaviors. These higher cognitive processes include working memory, which has been called a “mental scratch pad,” that can help organize and represent information that is not in the immediate environment. The prefrontal lobe is responsible for aspects of attention, such as inhibiting distracting thoughts and actions so that a person can focus on a goal and direct behavior toward achieving that goal.
The functions of the prefrontal cortex are integral to the personality of an individual, because it is largely responsible for what a person intends to do and how they accomplish those plans. A famous case of damage to the prefrontal cortex is that of Phineas Gage, dating back to 1848. He was a railroad worker who had a metal spike impale his prefrontal cortex (Figure 14.27). He survived the accident, but according to second-hand accounts, his personality changed drastically. Friends described him as no longer acting like himself. Whereas he was a hardworking, amiable man before the accident, he turned into an irritable, temperamental, and lazy man after the accident. Many of the accounts of his change may have been inflated in the retelling, and some behavior was likely attributable to alcohol used as a pain medication. However, the accounts suggest that some aspects of his personality did change. Also, there is new evidence that though his life changed dramatically, he was able to become a functioning stagecoach driver, suggesting that the brain has the ability to recover even from major trauma such as this.
Figure 14.27 Phineas Gage The victim of an accident while working on a railroad in 1848, Phineas Gage had a large iron rod impaled through the prefrontal cortex of his frontal lobe. After the accident, his personality appeared to change, but he eventually learned to cope with the trauma and lived as a coach driver even after such a traumatic event. (credit b: John M. Harlow, MD)
Secondary Motor Cortices
In generating motor responses, the executive functions of the prefrontal cortex will need to initiate actual movements. One way to define the prefrontal area is any region of the frontal lobe that does not elicit movement when electrically stimulated. These are primarily in the anterior part of the frontal lobe. The regions of the frontal lobe that remain are the regions of the cortex that produce movement. The prefrontal areas project into the secondary motor cortices, which include the premotor cortex and the supplemental motor area.
Two important regions that assist in planning and coordinating movements are located adjacent to the primary motor cortex. The premotor cortex is more lateral, whereas the supplemental motor area is more medial and superior. The premotor area aids in controlling movements of the core muscles to maintain posture during movement, whereas the supplemental motor area is hypothesized to be responsible for planning and coordinating movement. The supplemental motor area also manages sequential movements that are based on prior experience (that is, learned movements). Neurons in these areas are most active leading up to the initiation of movement. For example, these areas might prepare the body for the movements necessary to drive a car in anticipation of a traffic light changing.
Adjacent to these two regions are two specialized motor planning centers. The frontal eye fields are responsible for moving the eyes in response to visual stimuli. There are direct connections between the frontal eye fields and the superior colliculus. Also, anterior to the premotor cortex and primary motor cortex is Broca’s area. This area is responsible for controlling movements of the structures of speech production. The area is named after a French surgeon and anatomist who studied patients who could not produce speech. They did not have impairments to understanding speech, only to producing speech sounds, suggesting a damaged or underdeveloped Broca’s area.
Primary Motor Cortex
The primary motor cortex is located in the precentral gyrus of the frontal lobe. A neurosurgeon, Walter Penfield, described much of the basic understanding of the primary motor cortex by electrically stimulating the surface of the cerebrum. Penfield would probe the surface of the cortex while the patient was only under local anesthesia so that he could observe responses to the stimulation. This led to the belief that the precentral gyrus directly stimulated muscle movement. We now know that the primary motor cortex receives input from several areas that aid in planning movement, and its principle output stimulates spinal cord neurons to stimulate skeletal muscle contraction.
The primary motor cortex is arranged in a similar fashion to the primary somatosensory cortex, in that it has a topographical map of the body, creating a motor homunculus (see Figure 14.23). The neurons responsible for musculature in the feet and lower legs are in the medial wall of the precentral gyrus, with the thighs, trunk, and shoulder at the crest of the longitudinal fissure. The hand and face are in the lateral face of the gyrus. Also, the relative space allotted for the different regions is exaggerated in muscles that have greater enervation. The greatest amount of cortical space is given to muscles that perform fine, agile movements, such as the muscles of the fingers and the lower face. The “power muscles” that perform coarser movements, such as the buttock and back muscles, occupy much less space on the motor cortex.
Descending Pathways
The motor output from the cortex descends into the brain stem and to the spinal cord to control the musculature through motor neurons. Neurons located in the primary motor cortex, named Betz cells, are large cortical neurons that synapse with lower motor neurons in the brain stem or in the spinal cord. The two descending pathways travelled by the axons of Betz cells are the corticobulbar tract and the corticospinal tract, respectively. Both tracts are named for their origin in the cortex and their targets—either the brain stem (the term “bulbar” refers to the brain stem as the bulb, or enlargement, at the top of the spinal cord) or the spinal cord.
These two descending pathways are responsible for the conscious or voluntary movements of skeletal muscles. Any motor command from the primary motor cortex is sent down the axons of the Betz cells to activate upper motor neurons in either the cranial motor nuclei or in the ventral horn of the spinal cord. The axons of the corticobulbar tract are ipsilateral, meaning they project from the cortex to the motor nucleus on the same side of the nervous system. Conversely, the axons of the corticospinal tract are largely contralateral, meaning that they cross the midline of the brain stem or spinal cord and synapse on the opposite side of the body. Therefore, the right motor cortex of the cerebrum controls muscles on the left side of the body, and vice versa.
The corticospinal tract descends from the cortex through the deep white matter of the cerebrum. It then passes between the caudate nucleus and putamen of the basal nuclei as a bundle called the internal capsule. The tract then passes through the midbrain as the cerebral peduncles, after which it burrows through the pons. Upon entering the medulla, the tracts make up the large white matter tract referred to as the pyramids (Figure 14.28). The defining landmark of the medullary-spinal border is the pyramidal decussation, which is where most of the fibers in the corticospinal tract cross over to the opposite side of the brain. At this point, the tract separates into two parts, which have control over different domains of the musculature.
Figure 14.28 Corticospinal Tract The major descending tract that controls skeletal muscle movements is the corticospinal tract. It is composed of two neurons, the upper motor neuron and the lower motor neuron. The upper motor neuron has its cell body in the primary motor cortex of the frontal lobe and synapses on the lower motor neuron, which is in the ventral horn of the spinal cord and projects to the skeletal muscle in the periphery.
Appendicular Control
The lateral corticospinal tract is composed of the fibers that cross the midline at the pyramidal decussation (see Figure 14.28). The axons cross over from the anterior position of the pyramids in the medulla to the lateral column of the spinal cord. These axons are responsible for controlling appendicular muscles.
This influence over the appendicular muscles means that the lateral corticospinal tract is responsible for moving the muscles of the arms and legs. The ventral horn in both the lower cervical spinal cord and the lumbar spinal cord both have wider ventral horns, representing the greater number of muscles controlled by these motor neurons. The cervical enlargement is particularly large because there is greater control over the fine musculature of the upper limbs, particularly of the fingers. The lumbar enlargement is not as significant in appearance because there is less fine motor control of the lower limbs.
Axial Control
The anterior corticospinal tract is responsible for controlling the muscles of the body trunk (see Figure 14.28). These axons do not decussate in the medulla. Instead, they remain in an anterior position as they descend the brain stem and enter the spinal cord. These axons then travel to the spinal cord level at which they synapse with a lower motor neuron. Upon reaching the appropriate level, the axons decussate, entering the ventral horn on the opposite side of the spinal cord from which they entered. In the ventral horn, these axons synapse with their corresponding lower motor neurons. The lower motor neurons are located in the medial regions of the ventral horn, because they control the axial muscles of the trunk.
Because movements of the body trunk involve both sides of the body, the anterior corticospinal tract is not entirely contralateral. Some collateral branches of the tract will project into the ipsilateral ventral horn to control synergistic muscles on that side of the body, or to inhibit antagonistic muscles through interneurons within the ventral horn. Through the influence of both sides of the body, the anterior corticospinal tract can coordinate postural muscles in broad movements of the body. These coordinating axons in the anterior corticospinal tract are often considered bilateral, as they are both ipsilateral and contralateral.
INTERACTIVE LINK
Watch this video to learn more about the descending motor pathway for the somatic nervous system. The autonomic connections are mentioned, which are covered in another chapter. From this brief video, only some of the descending motor pathway of the somatic nervous system is described. Which division of the pathway is described and which division is left out?
Extrapyramidal Controls
Other descending connections between the brain and the spinal cord are called the extrapyramidal system. The name comes from the fact that this system is outside the corticospinal pathway, which includes the pyramids in the medulla. A few pathways originating from the brain stem contribute to this system.
The tectospinal tract projects from the midbrain to the spinal cord and is important for postural movements that are driven by the superior colliculus. The name of the tract comes from an alternate name for the superior colliculus, which is the tectum. The reticulospinal tract connects the reticular system, a diffuse region of gray matter in the brain stem, with the spinal cord. This tract influences trunk and proximal limb muscles related to posture and locomotion. The reticulospinal tract also contributes to muscle tone and influences autonomic functions. The vestibulospinal tract connects the brain stem nuclei of the vestibular system with the spinal cord. This allows posture, movement, and balance to be modulated on the basis of equilibrium information provided by the vestibular system.
The pathways of the extrapyramidal system are influenced by subcortical structures. For example, connections between the secondary motor cortices and the extrapyramidal system modulate spine and cranium movements. The basal nuclei, which are important for regulating movement initiated by the CNS, influence the extrapyramidal system as well as its thalamic feedback to the motor cortex.
The conscious movement of our muscles is more complicated than simply sending a single command from the precentral gyrus down to the proper motor neurons. During the movement of any body part, our muscles relay information back to the brain, and the brain is constantly sending “revised” instructions back to the muscles. The cerebellum is important in contributing to the motor system because it compares cerebral motor commands with proprioceptive feedback. The corticospinal fibers that project to the ventral horn of the spinal cord have branches that also synapse in the pons, which project to the cerebellum. Also, the proprioceptive sensations of the dorsal column system have a collateral projection to the medulla that projects to the cerebellum. These two streams of information are compared in the cerebellar cortex. Conflicts between the motor commands sent by the cerebrum and body position information provided by the proprioceptors cause the cerebellum to stimulate the red nucleus of the midbrain. The red nucleus then sends corrective commands to the spinal cord along the rubrospinal tract. The name of this tract comes from the word for red that is seen in the English word “ruby.”
A good example of how the cerebellum corrects cerebral motor commands can be illustrated by walking in water. An original motor command from the cerebrum to walk will result in a highly coordinated set of learned movements. However, in water, the body cannot actually perform a typical walking movement as instructed. The cerebellum can alter the motor command, stimulating the leg muscles to take larger steps to overcome the water resistance. The cerebellum can make the necessary changes through the rubrospinal tract. Modulating the basic command to walk also relies on spinal reflexes, but the cerebellum is responsible for calculating the appropriate response. When the cerebellum does not work properly, coordination and balance are severely affected. The most dramatic example of this is during the overconsumption of alcohol. Alcohol inhibits the ability of the cerebellum to interpret proprioceptive feedback, making it more difficult to coordinate body movements, such as walking a straight line, or guide the movement of the hand to touch the tip of the nose.
INTERACTIVE LINK
Visit this site to read about an elderly woman who starts to lose the ability to control fine movements, such as speech and the movement of limbs. Many of the usual causes were ruled out. It was not a stroke, Parkinson’s disease, diabetes, or thyroid dysfunction. The next most obvious cause was medication, so her pharmacist had to be consulted. The side effect of a drug meant to help her sleep had resulted in changes in motor control. What regions of the nervous system are likely to be the focus of haloperidol side effects?
Ventral Horn Output
The somatic nervous system provides output strictly to skeletal muscles. The lower motor neurons, which are responsible for the contraction of these muscles, are found in the ventral horn of the spinal cord. These large, multipolar neurons have a corona of dendrites surrounding the cell body and an axon that extends out of the ventral horn. This axon travels through the ventral nerve root to join the emerging spinal nerve. The axon is relatively long because it needs to reach muscles in the periphery of the body. The diameters of cell bodies may be on the order of hundreds of micrometers to support the long axon; some axons are a meter in length, such as the lumbar motor neurons that innervate muscles in the first digits of the feet.
The axons will also branch to innervate multiple muscle fibers. Together, the motor neuron and all the muscle fibers that it controls make up a motor unit. Motor units vary in size. Some may contain up to 1000 muscle fibers, such as in the quadriceps, or they may only have 10 fibers, such as in an extraocular muscle. The number of muscle fibers that are part of a motor unit corresponds to the precision of control of that muscle. Also, muscles that have finer motor control have more motor units connecting to them, and this requires a larger topographical field in the primary motor cortex.
Motor neuron axons connect to muscle fibers at a neuromuscular junction. This is a specialized synaptic structure at which multiple axon terminals synapse with the muscle fiber sarcolemma. The synaptic end bulbs of the motor neurons secrete acetylcholine, which binds to receptors on the sarcolemma. The binding of acetylcholine opens ligand-gated ion channels, increasing the movement of cations across the sarcolemma. This depolarizes the sarcolemma, initiating muscle contraction. Whereas other synapses result in graded potentials that must reach a threshold in the postsynaptic target, activity at the neuromuscular junction reliably leads to muscle fiber contraction with every nerve impulse received from a motor neuron. However, the strength of contraction and the number of fibers that contract can be affected by the frequency of the motor neuron impulses.
Reflexes
This chapter began by introducing reflexes as an example of the basic elements of the somatic nervous system. Simple somatic reflexes do not include the higher centers discussed for conscious or voluntary aspects of movement. Reflexes can be spinal or cranial, depending on the nerves and central components that are involved. The example described at the beginning of the chapter involved heat and pain sensations from a hot stove causing withdrawal of the arm through a connection in the spinal cord that leads to contraction of the biceps brachii. The description of this withdrawal reflex was simplified, for the sake of the introduction, to emphasize the parts of the somatic nervous system. But to consider reflexes fully, more attention needs to be given to this example.
As you withdraw your hand from the stove, you do not want to slow that reflex down. As the biceps brachii contracts, the antagonistic triceps brachii needs to relax. Because the neuromuscular junction is strictly excitatory, the biceps will contract when the motor nerve is active. Skeletal muscles do not actively relax. Instead the motor neuron needs to “quiet down,” or be inhibited. In the hot-stove withdrawal reflex, this occurs through an interneuron in the spinal cord. The interneuron’s cell body is located in the dorsal horn of the spinal cord. The interneuron receives a synapse from the axon of the sensory neuron that detects that the hand is being burned. In response to this stimulation from the sensory neuron, the interneuron then inhibits the motor neuron that controls the triceps brachii. This is done by releasing a neurotransmitter or other signal that hyperpolarizes the motor neuron connected to the triceps brachii, making it less likely to initiate an action potential. With this motor neuron being inhibited, the triceps brachii relaxes. Without the antagonistic contraction, withdrawal from the hot stove is faster and keeps further tissue damage from occurring.
Another example of a withdrawal reflex occurs when you step on a painful stimulus, like a tack or a sharp rock. The nociceptors that are activated by the painful stimulus activate the motor neurons responsible for contraction of the tibialis anterior muscle. This causes dorsiflexion of the foot. An inhibitory interneuron, activated by a collateral branch of the nociceptor fiber, will inhibit the motor neurons of the gastrocnemius and soleus muscles to cancel plantar flexion. An important difference in this reflex is that plantar flexion is most likely in progress as the foot is pressing down onto the tack. Contraction of the tibialis anterior is not the most important aspect of the reflex, as continuation of plantar flexion will result in further damage from stepping onto the tack.
Another type of reflex is a stretch reflex. In this reflex, when a skeletal muscle is stretched, a muscle spindle receptor is activated. The axon from this receptor structure will cause direct contraction of the muscle. A collateral of the muscle spindle fiber will also inhibit the motor neuron of the antagonist muscles. The reflex helps to maintain muscles at a constant length. A common example of this reflex is the knee jerk that is elicited by a rubber hammer struck against the patellar ligament in a physical exam.
A specialized reflex to protect the surface of the eye is the corneal reflex, or the eye blink reflex. When the cornea is stimulated by a tactile stimulus, or even by bright light in a related reflex, blinking is initiated. The sensory component travels through the trigeminal nerve, which carries somatosensory information from the face, or through the optic nerve, if the stimulus is bright light. The motor response travels through the facial nerve and innervates the orbicularis oculi on the same side. This reflex is commonly tested during a physical exam using an air puff or a gentle touch of a cotton-tipped applicator.
INTERACTIVE LINK
Watch this video to learn more about the reflex arc of the corneal reflex. When the right cornea senses a tactile stimulus, what happens to the left eye? Explain your answer.
INTERACTIVE LINK
Watch this video to learn more about newborn reflexes. Newborns have a set of reflexes that are expected to have been crucial to survival before the modern age. These reflexes disappear as the baby grows, as some of them may be unnecessary as they age. The video demonstrates a reflex called the Babinski reflex, in which the foot flexes dorsally and the toes splay out when the sole of the foot is lightly scratched. This is normal for newborns, but it is a sign of reduced myelination of the spinal tract in adults. Why would this reflex be a problem for an adult?
Key Terms
- alkaloid
- substance, usually from a plant source, that is chemically basic with respect to pH and will stimulate bitter receptors
- amacrine cell
- type of cell in the retina that connects to the bipolar cells near the outer synaptic layer and provides the basis for early image processing within the retina
- ampulla
- in the ear, the structure at the base of a semicircular canal that contains the hair cells and cupula for transduction of rotational movement of the head
- anosmia
- loss of the sense of smell; usually the result of physical disruption of the first cranial nerve
- anterior corticospinal tract
- division of the corticospinal pathway that travels through the ventral (anterior) column of the spinal cord and controls axial musculature through the medial motor neurons in the ventral (anterior) horn
- aqueous humor
- watery fluid that fills the anterior chamber containing the cornea, iris, ciliary body, and lens of the eye
- ascending pathway
- fiber structure that relays sensory information from the periphery through the spinal cord and brain stem to other structures of the brain
- association area
- region of cortex connected to a primary sensory cortical area that further processes the information to generate more complex sensory perceptions
- audition
- sense of hearing
- auricle
- fleshy external structure of the ear
- basilar membrane
- in the ear, the floor of the cochlear duct on which the organ of Corti sits
- Betz cells
- output cells of the primary motor cortex that cause musculature to move through synapses on cranial and spinal motor neurons
- binocular depth cues
- indications of the distance of visual stimuli on the basis of slight differences in the images projected onto either retina
- bipolar cell
- cell type in the retina that connects the photoreceptors to the RGCs
- Broca’s area
- region of the frontal lobe associated with the motor commands necessary for speech production
- capsaicin
- molecule that activates nociceptors by interacting with a temperature-sensitive ion channel and is the basis for “hot” sensations in spicy food
- cerebral peduncles
- segments of the descending motor pathway that make up the white matter of the ventral midbrain
- cervical enlargement
- region of the ventral (anterior) horn of the spinal cord that has a larger population of motor neurons for the greater number of and finer control of muscles of the upper limb
- chemoreceptor
- sensory receptor cell that is sensitive to chemical stimuli, such as in taste, smell, or pain
- chief sensory nucleus
- component of the trigeminal nuclei that is found in the pons
- choroid
- highly vascular tissue in the wall of the eye that supplies the outer retina with blood
- ciliary body
- smooth muscle structure on the interior surface of the iris that controls the shape of the lens through the zonule fibers
- circadian rhythm
- internal perception of the daily cycle of light and dark based on retinal activity related to sunlight
- cochlea
- auditory portion of the inner ear containing structures to transduce sound stimuli
- cochlear duct
- space within the auditory portion of the inner ear that contains the organ of Corti and is adjacent to the scala tympani and scala vestibuli on either side
- cone photoreceptor
- one of the two types of retinal receptor cell that is specialized for color vision through the use of three photopigments distributed through three separate populations of cells
- contralateral
- word meaning “on the opposite side,” as in axons that cross the midline in a fiber tract
- cornea
- fibrous covering of the anterior region of the eye that is transparent so that light can pass through it
- corneal reflex
- protective response to stimulation of the cornea causing contraction of the orbicularis oculi muscle resulting in blinking of the eye
- corticobulbar tract
- connection between the cortex and the brain stem responsible for generating movement
- corticospinal tract
- connection between the cortex and the spinal cord responsible for generating movement
- cupula
- specialized structure within the base of a semicircular canal that bends the stereocilia of hair cells when the head rotates by way of the relative movement of the enclosed fluid
- decussate
- to cross the midline, as in fibers that project from one side of the body to the other
- dorsal column system
- ascending tract of the spinal cord associated with fine touch and proprioceptive sensations
- dorsal stream
- connections between cortical areas from the occipital to parietal lobes that are responsible for the perception of visual motion and guiding movement of the body in relation to that motion
- encapsulated ending
- configuration of a sensory receptor neuron with dendrites surrounded by specialized structures to aid in transduction of a particular type of sensation, such as the lamellated corpuscles in the deep dermis and subcutaneous tissue
- equilibrium
- sense of balance that includes sensations of position and movement of the head
- executive functions
- cognitive processes of the prefrontal cortex that lead to directing goal-directed behavior, which is a precursor to executing motor commands
- external ear
- structures on the lateral surface of the head, including the auricle and the ear canal back to the tympanic membrane
- exteroceptor
- sensory receptor that is positioned to interpret stimuli from the external environment, such as photoreceptors in the eye or somatosensory receptors in the skin
- extraocular muscle
- one of six muscles originating out of the bones of the orbit and inserting into the surface of the eye which are responsible for moving the eye
- extrapyramidal system
- pathways between the brain and spinal cord that are separate from the corticospinal tract and are responsible for modulating the movements generated through that primary pathway
- fasciculus cuneatus
- lateral division of the dorsal column system composed of fibers from sensory neurons in the upper body
- fasciculus gracilis
- medial division of the dorsal column system composed of fibers from sensory neurons in the lower body
- fibrous tunic
- outer layer of the eye primarily composed of connective tissue known as the sclera and cornea
- fovea
- exact center of the retina at which visual stimuli are focused for maximal acuity, where the retina is thinnest, at which there is nothing but photoreceptors
- free nerve ending
- configuration of a sensory receptor neuron with dendrites in the connective tissue of the organ, such as in the dermis of the skin, that are most often sensitive to chemical, thermal, and mechanical stimuli
- frontal eye fields
- area of the prefrontal cortex responsible for moving the eyes to attend to visual stimuli
- general sense
- any sensory system that is distributed throughout the body and incorporated into organs of multiple other systems, such as the walls of the digestive organs or the skin
- gustation
- sense of taste
- gustatory receptor cells
- sensory cells in the taste bud that transduce the chemical stimuli of gustation
- hair cells
- mechanoreceptor cells found in the inner ear that transduce stimuli for the senses of hearing and balance
- incus
- (also, anvil) ossicle of the middle ear that connects the malleus to the stapes
- inferior colliculus
- last structure in the auditory brainstem pathway that projects to the thalamus and superior colliculus
- inferior oblique
- extraocular muscle responsible for lateral rotation of the eye
- inferior rectus
- extraocular muscle responsible for looking down
- inner ear
- structure within the temporal bone that contains the sensory apparati of hearing and balance
- inner segment
- in the eye, the section of a photoreceptor that contains the nucleus and other major organelles for normal cellular functions
- inner synaptic layer
- layer in the retina where bipolar cells connect to RGCs
- interaural intensity difference
- cue used to aid sound localization in the horizontal plane that compares the relative loudness of sounds at the two ears, because the ear closer to the sound source will hear a slightly more intense sound
- interaural time difference
- cue used to help with sound localization in the horizontal plane that compares the relative time of arrival of sounds at the two ears, because the ear closer to the sound source will receive the stimulus microseconds before the other ear
- internal capsule
- segment of the descending motor pathway that passes between the caudate nucleus and the putamen
- interoceptor
- sensory receptor that is positioned to interpret stimuli from internal organs, such as stretch receptors in the wall of blood vessels
- ipsilateral
- word meaning on the same side, as in axons that do not cross the midline in a fiber tract
- iris
- colored portion of the anterior eye that surrounds the pupil
- kinesthesia
- sense of body movement based on sensations in skeletal muscles, tendons, joints, and the skin
- lacrimal duct
- duct in the medial corner of the orbit that drains tears into the nasal cavity
- lacrimal gland
- gland lateral to the orbit that produces tears to wash across the surface of the eye
- lateral corticospinal tract
- division of the corticospinal pathway that travels through the lateral column of the spinal cord and controls appendicular musculature through the lateral motor neurons in the ventral (anterior) horn
- lateral geniculate nucleus
- thalamic target of the RGCs that projects to the visual cortex
- lateral rectus
- extraocular muscle responsible for abduction of the eye
- lens
- component of the eye that focuses light on the retina
- levator palpebrae superioris
- muscle that causes elevation of the upper eyelid, controlled by fibers in the oculomotor nerve
- lumbar enlargement
- region of the ventral (anterior) horn of the spinal cord that has a larger population of motor neurons for the greater number of muscles of the lower limb
- macula
- enlargement at the base of a semicircular canal at which transduction of equilibrium stimuli takes place within the ampulla
- malleus
- (also, hammer) ossicle that is directly attached to the tympanic membrane
- mechanoreceptor
- receptor cell that transduces mechanical stimuli into an electrochemical signal
- medial geniculate nucleus
- thalamic target of the auditory brain stem that projects to the auditory cortex
- medial lemniscus
- fiber tract of the dorsal column system that extends from the nuclei gracilis and cuneatus to the thalamus, and decussates
- medial rectus
- extraocular muscle responsible for adduction of the eye
- mesencephalic nucleus
- component of the trigeminal nuclei that is found in the midbrain
- middle ear
- space within the temporal bone between the ear canal and bony labyrinth where the ossicles amplify sound waves from the tympanic membrane to the oval window
- multimodal integration area
- region of the cerebral cortex in which information from more than one sensory modality is processed to arrive at higher level cortical functions such as memory, learning, or cognition
- neural tunic
- layer of the eye that contains nervous tissue, namely the retina
- nociceptor
- receptor cell that senses pain stimuli
- nucleus cuneatus
- medullary nucleus at which first-order neurons of the dorsal column system synapse specifically from the upper body and arms
- nucleus gracilis
- medullary nucleus at which first-order neurons of the dorsal column system synapse specifically from the lower body and legs
- odorant molecules
- volatile chemicals that bind to receptor proteins in olfactory neurons to stimulate the sense of smell
- olfaction
- sense of smell
- olfactory bulb
- central target of the first cranial nerve; located on the ventral surface of the frontal lobe in the cerebrum
- olfactory epithelium
- region of the nasal epithelium where olfactory neurons are located
- olfactory sensory neuron
- receptor cell of the olfactory system, sensitive to the chemical stimuli of smell, the axons of which compose the first cranial nerve
- opsin
- protein that contains the photosensitive cofactor retinal for phototransduction
- optic chiasm
- decussation point in the visual system at which medial retina fibers cross to the other side of the brain
- optic disc
- spot on the retina at which RGC axons leave the eye and blood vessels of the inner retina pass
- optic nerve
- second cranial nerve, which is responsible visual sensation
- optic tract
- name for the fiber structure containing axons from the retina posterior to the optic chiasm representing their CNS location
- organ of Corti
- structure in the cochlea in which hair cells transduce movements from sound waves into electrochemical signals
- osmoreceptor
- receptor cell that senses differences in the concentrations of bodily fluids on the basis of osmotic pressure
- ossicles
- three small bones in the middle ear
- otolith
- layer of calcium carbonate crystals located on top of the otolithic membrane
- otolithic membrane
- gelatinous substance in the utricle and saccule of the inner ear that contains calcium carbonate crystals and into which the stereocilia of hair cells are embedded
- outer segment
- in the eye, the section of a photoreceptor that contains opsin molecules that transduce light stimuli
- outer synaptic layer
- layer in the retina at which photoreceptors connect to bipolar cells
- oval window
- membrane at the base of the cochlea where the stapes attaches, marking the beginning of the scala vestibuli
- palpebral conjunctiva
- membrane attached to the inner surface of the eyelids that covers the anterior surface of the cornea
- papilla
- for gustation, a bump-like projection on the surface of the tongue that contains taste buds
- photoisomerization
- chemical change in the retinal molecule that alters the bonding so that it switches from the 11-cis-retinal isomer to the all-trans-retinal isomer
- photon
- individual “packet” of light
- photoreceptor
- receptor cell specialized to respond to light stimuli
- premotor cortex
- cortical area anterior to the primary motor cortex that is responsible for planning movements
- primary sensory cortex
- region of the cerebral cortex that initially receives sensory input from an ascending pathway from the thalamus and begins the processing that will result in conscious perception of that modality
- proprioception
- sense of position and movement of the body
- proprioceptor
- receptor cell that senses changes in the position and kinesthetic aspects of the body
- pupil
- open hole at the center of the iris that light passes through into the eye
- pyramidal decussation
- location at which corticospinal tract fibers cross the midline and segregate into the anterior and lateral divisions of the pathway
- pyramids
- segment of the descending motor pathway that travels in the anterior position of the medulla
- receptor cell
- cell that transduces environmental stimuli into neural signals
- red nucleus
- midbrain nucleus that sends corrective commands to the spinal cord along the rubrospinal tract, based on disparity between an original command and the sensory feedback from movement
- reticulospinal tract
- extrapyramidal connections between the brain stem and spinal cord that modulate movement, contribute to posture, and regulate muscle tone
- retina
- nervous tissue of the eye at which phototransduction takes place
- retinal
- cofactor in an opsin molecule that undergoes a biochemical change when struck by a photon (pronounced with a stress on the last syllable)
- retinal ganglion cell (RGC)
- neuron of the retina that projects along the second cranial nerve
- rhodopsin
- photopigment molecule found in the rod photoreceptors
- rod photoreceptor
- one of the two types of retinal receptor cell that is specialized for low-light vision
- round window
- membrane that marks the end of the scala tympani
- rubrospinal tract
- descending motor control pathway, originating in the red nucleus, that mediates control of the limbs on the basis of cerebellar processing
- saccule
- structure of the inner ear responsible for transducing linear acceleration in the vertical plane
- scala tympani
- portion of the cochlea that extends from the apex to the round window
- scala vestibuli
- portion of the cochlea that extends from the oval window to the apex
- sclera
- white of the eye
- semicircular canals
- structures within the inner ear responsible for transducing rotational movement information
- sensory homunculus
- topographic representation of the body within the somatosensory cortex demonstrating the correspondence between neurons processing stimuli and sensitivity
- sensory modality
- a particular system for interpreting and perceiving environmental stimuli by the nervous system
- solitary nucleus
- medullar nucleus that receives taste information from the facial and glossopharyngeal nerves
- somatosensation
- general sense associated with modalities lumped together as touch
- special sense
- any sensory system associated with a specific organ structure, namely smell, taste, sight, hearing, and balance
- spinal trigeminal nucleus
- component of the trigeminal nuclei that is found in the medulla
- spinothalamic tract
- ascending tract of the spinal cord associated with pain and temperature sensations
- spiral ganglion
- location of neuronal cell bodies that transmit auditory information along the eighth cranial nerve
- stapes
- (also, stirrup) ossicle of the middle ear that is attached to the inner ear
- stereocilia
- array of apical membrane extensions in a hair cell that transduce movements when they are bent
- stretch reflex
- response to activation of the muscle spindle stretch receptor that causes contraction of the muscle to maintain a constant length
- submodality
- specific sense within a broader major sense such as sweet as a part of the sense of taste, or color as a part of vision
- superior colliculus
- structure in the midbrain that combines visual, auditory, and somatosensory input to coordinate spatial and topographic representations of the three sensory systems
- superior oblique
- extraocular muscle responsible for medial rotation of the eye
- superior rectus
- extraocular muscle responsible for looking up
- supplemental motor area
- cortical area anterior to the primary motor cortex that is responsible for planning movements
- suprachiasmatic nucleus
- hypothalamic target of the retina that helps to establish the circadian rhythm of the body on the basis of the presence or absence of daylight
- taste buds
- structures within a papilla on the tongue that contain gustatory receptor cells
- tectorial membrane
- component of the organ of Corti that lays over the hair cells, into which the stereocilia are embedded
- tectospinal tract
- extrapyramidal connections between the superior colliculus and spinal cord
- thermoreceptor
- sensory receptor specialized for temperature stimuli
- topographical
- relating to positional information
- transduction
- process of changing an environmental stimulus into the electrochemical signals of the nervous system
- trochlea
- cartilaginous structure that acts like a pulley for the superior oblique muscle
- tympanic membrane
- ear drum
- umami
- taste submodality for sensitivity to the concentration of amino acids; also called the savory sense
- utricle
- structure of the inner ear responsible for transducing linear acceleration in the horizontal plane
- vascular tunic
- middle layer of the eye primarily composed of connective tissue with a rich blood supply
- ventral posterior nucleus
- nucleus in the thalamus that is the target of gustatory sensations and projects to the cerebral cortex
- ventral stream
- connections between cortical areas from the occipital lobe to the temporal lobe that are responsible for identification of visual stimuli
- vestibular ganglion
- location of neuronal cell bodies that transmit equilibrium information along the eighth cranial nerve
- vestibular nuclei
- targets of the vestibular component of the eighth cranial nerve
- vestibule
- in the ear, the portion of the inner ear responsible for the sense of equilibrium
- vestibulo-ocular reflex (VOR)
- reflex based on connections between the vestibular system and the cranial nerves of eye movements that ensures images are stabilized on the retina as the head and body move
- vestibulospinal tract
- extrapyramidal connections between the vestibular nuclei in the brain stem and spinal cord that modulate movement and contribute to balance on the basis of the sense of equilibrium
- visceral sense
- sense associated with the internal organs
- vision
- special sense of sight based on transduction of light stimuli
- visual acuity
- property of vision related to the sharpness of focus, which varies in relation to retinal position
- vitreous humor
- viscous fluid that fills the posterior chamber of the eye
- working memory
- function of the prefrontal cortex to maintain a representation of information that is not in the immediate environment
- zonule fibers
- fibrous connections between the ciliary body and the lens
Chapter Review
14.1 Sensory Perception
The senses are olfaction (smell), gustation (taste), somatosensation (sensations associated with the skin and body), audition (hearing), equilibrium (balance), and vision. With the exception of somatosensation, this list represents the special senses, or those systems of the body that are associated with specific organs such as the tongue or eye. Somatosensation belongs to the general senses, which are those sensory structures that are distributed throughout the body and in the walls of various organs. The special senses are all primarily part of the somatic nervous system in that they are consciously perceived through cerebral processes, though some special senses contribute to autonomic function. The general senses can be divided into somatosensation, which is commonly considered touch, but includes tactile, pressure, vibration, temperature, and pain perception. The general senses also include the visceral senses, which are separate from the somatic nervous system function in that they do not normally rise to the level of conscious perception.
The cells that transduce sensory stimuli into the electrochemical signals of the nervous system are classified on the basis of structural or functional aspects of the cells. The structural classifications are either based on the anatomy of the cell that is interacting with the stimulus (free nerve endings, encapsulated endings, or specialized receptor cell), or where the cell is located relative to the stimulus (interoceptor, exteroceptor, proprioceptor). Thirdly, the functional classification is based on how the cell transduces the stimulus into a neural signal. Chemoreceptors respond to chemical stimuli and are the basis for olfaction and gustation. Related to chemoreceptors are osmoreceptors and nociceptors for fluid balance and pain reception, respectively. Mechanoreceptors respond to mechanical stimuli and are the basis for most aspects of somatosensation, as well as being the basis of audition and equilibrium in the inner ear. Thermoreceptors are sensitive to temperature changes, and photoreceptors are sensitive to light energy.
The nerves that convey sensory information from the periphery to the CNS are either spinal nerves, connected to the spinal cord, or cranial nerves, connected to the brain. Spinal nerves have mixed populations of fibers; some are motor fibers and some are sensory. The sensory fibers connect to the spinal cord through the dorsal root, which is attached to the dorsal root ganglion. Sensory information from the body that is conveyed through spinal nerves will project to the opposite side of the brain to be processed by the cerebral cortex. The cranial nerves can be strictly sensory fibers, such as the olfactory, optic, and vestibulocochlear nerves, or mixed sensory and motor nerves, such as the trigeminal, facial, glossopharyngeal, and vagus nerves. The cranial nerves are connected to the same side of the brain from which the sensory information originates.
14.2 Central Processing
Sensory input to the brain enters through pathways that travel through either the spinal cord (for somatosensory input from the body) or the brain stem (for everything else, except the visual and olfactory systems) to reach the diencephalon. In the diencephalon, sensory pathways reach the thalamus. This is necessary for all sensory systems to reach the cerebral cortex, except for the olfactory system that is directly connected to the frontal and temporal lobes.
The two major tracts in the spinal cord, originating from sensory neurons in the dorsal root ganglia, are the dorsal column system and the spinothalamic tract. The major differences between the two are in the type of information that is relayed to the brain and where the tracts decussate. The dorsal column system primarily carries information about touch and proprioception and crosses the midline in the medulla. The spinothalamic tract is primarily responsible for pain and temperature sensation and crosses the midline in the spinal cord at the level at which it enters. The trigeminal nerve adds similar sensation information from the head to these pathways.
The auditory pathway passes through multiple nuclei in the brain stem in which additional information is extracted from the basic frequency stimuli processed by the cochlea. Sound localization is made possible through the activity of these brain stem structures. The vestibular system enters the brain stem and influences activity in the cerebellum, spinal cord, and cerebral cortex.
The visual pathway segregates information from the two eyes so that one half of the visual field projects to the other side of the brain. Within visual cortical areas, the perception of the stimuli and their location is passed along two streams, one ventral and one dorsal. The ventral visual stream connects to structures in the temporal lobe that are important for long-term memory formation. The dorsal visual stream interacts with the somatosensory cortex in the parietal lobe, and together they can influence the activity in the frontal lobe to generate movements of the body in relation to visual information.
14.3 Motor Responses
The motor components of the somatic nervous system begin with the frontal lobe of the brain, where the prefrontal cortex is responsible for higher functions such as working memory. The integrative and associate functions of the prefrontal lobe feed into the secondary motor areas, which help plan movements. The premotor cortex and supplemental motor area then feed into the primary motor cortex that initiates movements. Large Betz cells project through the corticobulbar and corticospinal tracts to synapse on lower motor neurons in the brain stem and ventral horn of the spinal cord, respectively. These connections are responsible for generating movements of skeletal muscles.
The extrapyramidal system includes projections from the brain stem and higher centers that influence movement, mostly to maintain balance and posture, as well as to maintain muscle tone. The superior colliculus and red nucleus in the midbrain, the vestibular nuclei in the medulla, and the reticular formation throughout the brain stem each have tracts projecting to the spinal cord in this system. Descending input from the secondary motor cortices, basal nuclei, and cerebellum connect to the origins of these tracts in the brain stem.
All of these motor pathways project to the spinal cord to synapse with motor neurons in the ventral horn of the spinal cord. These lower motor neurons are the cells that connect to skeletal muscle and cause contractions. These neurons project through the spinal nerves to connect to the muscles at neuromuscular junctions. One motor neuron connects to multiple muscle fibers within a target muscle. The number of fibers that are innervated by a single motor neuron varies on the basis of the precision necessary for that muscle and the amount of force necessary for that motor unit. The quadriceps, for example, have many fibers controlled by single motor neurons for powerful contractions that do not need to be precise. The extraocular muscles have only a small number of fibers controlled by each motor neuron because moving the eyes does not require much force, but needs to be very precise.
Reflexes are the simplest circuits within the somatic nervous system. A withdrawal reflex from a painful stimulus only requires the sensory fiber that enters the spinal cord and the motor neuron that projects to a muscle. Antagonist and postural muscles can be coordinated with the withdrawal, making the connections more complex. The simple, single neuronal connection is the basis of somatic reflexes. The corneal reflex is contraction of the orbicularis oculi muscle to blink the eyelid when something touches the surface of the eye. Stretch reflexes maintain a constant length of muscles by causing a contraction of a muscle to compensate for a stretch that can be sensed by a specialized receptor called a muscle spindle.
Interactive Link Questions
Watch this video to learn about Dr. Danielle Reed of the Monell Chemical Senses Center in Philadelphia, PA, who became interested in science at an early age because of her sensory experiences. She recognized that her sense of taste was unique compared with other people she knew. Now, she studies the genetic differences between people and their sensitivities to taste stimuli. In the video, there is a brief image of a person sticking out their tongue, which has been covered with a colored dye. This is how Dr. Reed is able to visualize and count papillae on the surface of the tongue. People fall into two large groups known as “tasters” and “non-tasters” on the basis of the density of papillae on their tongue, which also indicates the number of taste buds. Non-tasters can taste food, but they are not as sensitive to certain tastes, such as bitterness. Dr. Reed discovered that she is a non-taster, which explains why she perceived bitterness differently than other people she knew. Are you very sensitive to tastes? Can you see any similarities among the members of your family?
2.Figure 14.9 The basilar membrane is the thin membrane that extends from the central core of the cochlea to the edge. What is anchored to this membrane so that they can be activated by movement of the fluids within the cochlea?
3.Watch this video to learn more about how the structures of the ear convert sound waves into a neural signal by moving the “hairs,” or stereocilia, of the cochlear duct. Specific locations along the length of the duct encode specific frequencies, or pitches. The brain interprets the meaning of the sounds we hear as music, speech, noise, etc. Which ear structures are responsible for the amplification and transfer of sound from the external ear to the inner ear?
4.Watch this animation to learn more about the inner ear and to see the cochlea unroll, with the base at the back of the image and the apex at the front. Specific wavelengths of sound cause specific regions of the basilar membrane to vibrate, much like the keys of a piano produce sound at different frequencies. Based on the animation, where do frequencies—from high to low pitches—cause activity in the hair cells within the cochlear duct?
5.Watch this video to learn more about a transverse section through the brain that depicts the visual pathway from the eye to the occipital cortex. The first half of the pathway is the projection from the RGCs through the optic nerve to the lateral geniculate nucleus in the thalamus on either side. This first fiber in the pathway synapses on a thalamic cell that then projects to the visual cortex in the occipital lobe where “seeing,” or visual perception, takes place. This video gives an abbreviated overview of the visual system by concentrating on the pathway from the eyes to the occipital lobe. The video makes the statement (at 0:45) that “specialized cells in the retina called ganglion cells convert the light rays into electrical signals.” What aspect of retinal processing is simplified by that statement? Explain your answer.
6.Watch this video to learn more about how the brain perceives 3-D motion. Similar to how retinal disparity offers 3-D moviegoers a way to extract 3-D information from the two-dimensional visual field projected onto the retina, the brain can extract information about movement in space by comparing what the two eyes see. If movement of a visual stimulus is leftward in one eye and rightward in the opposite eye, the brain interprets this as movement toward (or away) from the face along the midline. If both eyes see an object moving in the same direction, but at different rates, what would that mean for spatial movement?
7.The inability to recognize people by their faces is a troublesome problem. It can be caused by trauma, or it may be inborn. Watch this video to learn more about a person who lost the ability to recognize faces as the result of an injury. She cannot recognize the faces of close family members or herself. What other information can a person suffering from prosopagnosia use to figure out whom they are seeing?
8.Watch this video to learn more about the descending motor pathway for the somatic nervous system. The autonomic connections are mentioned, which are covered in another chapter. From this brief video, only some of the descending motor pathway of the somatic nervous system is described. Which division of the pathway is described and which division is left out?
9.Visit this site to read about an elderly woman who starts to lose the ability to control fine movements, such as speech and the movement of limbs. Many of the usual causes were ruled out. It was not a stroke, Parkinson’s disease, diabetes, or thyroid dysfunction. The next most obvious cause was medication, so her pharmacist had to be consulted. The side effect of a drug meant to help her sleep had resulted in changes in motor control. What regions of the nervous system are likely to be the focus of haloperidol side effects?
10.Watch this video to learn more about the reflex arc of the corneal reflex. When the right cornea senses a tactile stimulus, what happens to the left eye? Explain your answer.
11.Watch this video to learn more about newborn reflexes. Newborns have a set of reflexes that are expected to have been crucial to survival before the modern age. These reflexes disappear as the baby grows, as some of them may be unnecessary as they age. The video demonstrates a reflex called the Babinski reflex, in which the foot flexes dorsally and the toes splay out when the sole of the foot is lightly scratched. This is normal for newborns, but it is a sign of reduced myelination of the spinal tract in adults. Why would this reflex be a problem for an adult?
Review Questions
What type of receptor cell is responsible for transducing pain stimuli?
- mechanoreceptor
- nociceptor
- osmoreceptor
- photoreceptor
Which of these cranial nerves is part of the gustatory system?
- olfactory
- trochlear
- trigeminal
- facial
Which submodality of taste is sensitive to the pH of saliva?
- umami
- sour
- bitter
- sweet
Axons from which neuron in the retina make up the optic nerve?
- amacrine cells
- photoreceptors
- bipolar cells
- retinal ganglion cells
What type of receptor cell is involved in the sensations of sound and balance?
- photoreceptor
- chemoreceptor
- mechanoreceptor
- nociceptor
Which of these sensory modalities does not pass through the ventral posterior thalamus?
- gustatory
- proprioception
- audition
- nociception
Which nucleus in the medulla is connected to the inferior colliculus?
- solitary nucleus
- vestibular nucleus
- chief sensory nucleus
- cochlear nucleus
Visual stimuli in the upper-left visual field will be processed in what region of the primary visual cortex?
- inferior right
- inferior left
- superior right
- superior left
Which location on the body has the largest region of somatosensory cortex representing it, according to the sensory homunculus?
- lips
- thigh
- elbow
- neck
Which of the following is a direct target of the vestibular ganglion?
- superior colliculus
- cerebellum
- thalamus
- optic chiasm
Which region of the frontal lobe is responsible for initiating movement by directly connecting to cranial and spinal motor neurons?
- prefrontal cortex
- supplemental motor area
- premotor cortex
- primary motor cortex
Which extrapyramidal tract incorporates equilibrium sensations with motor commands to aid in posture and movement?
- tectospinal tract
- vestibulospinal tract
- reticulospinal tract
- corticospinal tract
Which region of gray matter in the spinal cord contains motor neurons that innervate skeletal muscles?
- ventral horn
- dorsal horn
- lateral horn
- lateral column
What type of reflex can protect the foot when a painful stimulus is sensed?
- stretch reflex
- gag reflex
- withdrawal reflex
- corneal reflex
What is the name for the topographical representation of the sensory input to the somatosensory cortex?
- homunculus
- homo sapiens
- postcentral gyrus
- primary cortex
Critical Thinking Questions
The sweetener known as stevia can replace glucose in food. What does the molecular similarity of stevia to glucose mean for the gustatory sense?
28.Why does the blind spot from the optic disc in either eye not result in a blind spot in the visual field?
29.Following a motorcycle accident, the victim loses the ability to move the right leg but has normal control over the left one, suggesting a hemisection somewhere in the thoracic region of the spinal cord. What sensory deficits would be expected in terms of touch versus pain? Explain your answer.
30.A pituitary tumor can cause perceptual losses in the lateral visual field. The pituitary gland is located directly inferior to the hypothalamus. Why would this happen?
31.The prefrontal lobotomy is a drastic—and largely out-of-practice—procedure used to disconnect that portion of the cerebral cortex from the rest of the frontal lobe and the diencephalon as a psychiatric therapy. Why would this have been thought necessary for someone with a potentially uncontrollable behavior?
32.If a reflex is a limited circuit within the somatic system, why do physical and neurological exams include them to test the health of an individual?
|
oercommons
|
2025-03-18T00:39:11.118431
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/56377/overview",
"title": "Anatomy and Physiology, Regulation, Integration, and Control",
"author": null
}
|
https://oercommons.org/courseware/lesson/56378/overview
|
The Autonomic Nervous System
Introduction
Figure 15.1 Fight or Flight? Though the threats that modern humans face are not large predators, the autonomic nervous system is adapted to this type of stimulus. The modern world presents stimuli that trigger the same response. (credit: Vernon Swanepoel)
CHAPTER OBJECTIVES
After studying this chapter, you will be able to:
- Describe the components of the autonomic nervous system
- Differentiate between the structures of the sympathetic and parasympathetic divisions in the autonomic nervous system
- Name the components of a visceral reflex specific to the autonomic division to which it belongs
- Predict the response of a target effector to autonomic input on the basis of the released signaling molecule
- Describe how the central nervous system coordinates and contributes to autonomic functions
The autonomic nervous system is often associated with the “fight-or-flight response,” which refers to the preparation of the body to either run away from a threat or to stand and fight in the face of that threat. To suggest what this means, consider the (very unlikely) situation of seeing a lioness hunting out on the savannah. Though this is not a common threat that humans deal with in the modern world, it represents the type of environment in which the human species thrived and adapted. The spread of humans around the world to the present state of the modern age occurred much more quickly than any species would adapt to environmental pressures such as predators. However, the reactions modern humans have in the modern world are based on these prehistoric situations. If your boss is walking down the hallway on Friday afternoon looking for “volunteers” to come in on the weekend, your response is the same as the prehistoric human seeing the lioness running across the savannah: fight or flight.
Most likely, your response to your boss—not to mention the lioness—would be flight. Run away! The autonomic system is responsible for the physiological response to make that possible, and hopefully successful. Adrenaline starts to flood your circulatory system. Your heart rate increases. Sweat glands become active. The bronchi of the lungs dilate to allow more air exchange. Pupils dilate to increase visual information. Blood pressure increases in general, and blood vessels dilate in skeletal muscles. Time to run. Similar physiological responses would occur in preparation for fighting off the threat.
This response should sound a bit familiar. The autonomic nervous system is tied into emotional responses as well, and the fight-or-flight response probably sounds like a panic attack. In the modern world, these sorts of reactions are associated with anxiety as much as with response to a threat. It is engrained in the nervous system to respond like this. In fact, the adaptations of the autonomic nervous system probably predate the human species and are likely to be common to all mammals, and perhaps shared by many animals. That lioness might herself be threatened in some other situation.
However, the autonomic nervous system is not just about responding to threats. Besides the fight-or-flight response, there are the responses referred to as “rest and digest.” If that lioness is successful in her hunting, then she is going to rest from the exertion. Her heart rate will slow. Breathing will return to normal. The digestive system has a big job to do. Much of the function of the autonomic system is based on the connections within an autonomic, or visceral, reflex.
Divisions of the Autonomic Nervous System
- Name the components that generate the sympathetic and parasympathetic responses of the autonomic nervous system
- Explain the differences in output connections within the two divisions of the autonomic nervous system
- Describe the signaling molecules and receptor proteins involved in communication within the two divisions of the autonomic nervous system
The nervous system can be divided into two functional parts: the somatic nervous system and the autonomic nervous system. The major differences between the two systems are evident in the responses that each produces. The somatic nervous system causes contraction of skeletal muscles. The autonomic nervous system controls cardiac and smooth muscle, as well as glandular tissue. The somatic nervous system is associated with voluntary responses (though many can happen without conscious awareness, like breathing), and the autonomic nervous system is associated with involuntary responses, such as those related to homeostasis.
The autonomic nervous system regulates many of the internal organs through a balance of two aspects, or divisions. In addition to the endocrine system, the autonomic nervous system is instrumental in homeostatic mechanisms in the body. The two divisions of the autonomic nervous system are the sympathetic division and the parasympathetic division. The sympathetic system is associated with the fight-or-flight response, and parasympathetic activity is referred to by the epithet of rest and digest. Homeostasis is the balance between the two systems. At each target effector, dual innervation determines activity. For example, the heart receives connections from both the sympathetic and parasympathetic divisions. One causes heart rate to increase, whereas the other causes heart rate to decrease.
INTERACTIVE LINK
Watch this video to learn more about adrenaline and the fight-or-flight response. When someone is said to have a rush of adrenaline, the image of bungee jumpers or skydivers usually comes to mind. But adrenaline, also known as epinephrine, is an important chemical in coordinating the body’s fight-or-flight response. In this video, you look inside the physiology of the fight-or-flight response, as envisioned for a firefighter. His body’s reaction is the result of the sympathetic division of the autonomic nervous system causing system-wide changes as it prepares for extreme responses. What two changes does adrenaline bring about to help the skeletal muscle response?
Sympathetic Division of the Autonomic Nervous System
To respond to a threat—to fight or to run away—the sympathetic system causes divergent effects as many different effector organs are activated together for a common purpose. More oxygen needs to be inhaled and delivered to skeletal muscle. The respiratory, cardiovascular, and musculoskeletal systems are all activated together. Additionally, sweating keeps the excess heat that comes from muscle contraction from causing the body to overheat. The digestive system shuts down so that blood is not absorbing nutrients when it should be delivering oxygen to skeletal muscles. To coordinate all these responses, the connections in the sympathetic system diverge from a limited region of the central nervous system (CNS) to a wide array of ganglia that project to the many effector organs simultaneously. The complex set of structures that compose the output of the sympathetic system make it possible for these disparate effectors to come together in a coordinated, systemic change.
The sympathetic division of the autonomic nervous system influences the various organ systems of the body through connections emerging from the thoracic and upper lumbar spinal cord. It is referred to as the thoracolumbar system to reflect this anatomical basis. A central neuron in the lateral horn of any of these spinal regions projects to ganglia adjacent to the vertebral column through the ventral spinal roots. The majority of ganglia of the sympathetic system belong to a network of sympathetic chain ganglia that runs alongside the vertebral column. The ganglia appear as a series of clusters of neurons linked by axonal bridges. There are typically 23 ganglia in the chain on either side of the spinal column. Three correspond to the cervical region, 12 are in the thoracic region, four are in the lumbar region, and four correspond to the sacral region. The cervical and sacral levels are not connected to the spinal cord directly through the spinal roots, but through ascending or descending connections through the bridges within the chain.
A diagram that shows the connections of the sympathetic system is somewhat like a circuit diagram that shows the electrical connections between different receptacles and devices. In Figure 15.2, the “circuits” of the sympathetic system are intentionally simplified.
Figure 15.2 Connections of Sympathetic Division of the Autonomic Nervous System Neurons from the lateral horn of the spinal cord (preganglionic nerve fibers - solid lines)) project to the chain ganglia on either side of the vertebral column or to collateral (prevertebral) ganglia that are anterior to the vertebral column in the abdominal cavity. Axons from these ganglionic neurons (postganglionic nerve fibers - dotted lines) then project to target effectors throughout the body.
To continue with the analogy of the circuit diagram, there are three different types of “junctions” that operate within the sympathetic system (Figure 15.3). The first type is most direct: the sympathetic nerve projects to the chain ganglion at the same level as the target effector (the organ, tissue, or gland to be innervated). An example of this type is spinal nerve T1 that synapses with the T1 chain ganglion to innervate the trachea. The fibers of this branch are called white rami communicantes(singular = ramus communicans); they are myelinated and therefore referred to as white (see Figure 15.3a). The axon from the central neuron (the preganglionic fiber shown as a solid line) synapses with the ganglionic neuron (with the postganglionic fiber shown as a dashed line). This neuron then projects to a target effector—in this case, the trachea—via gray rami communicantes, which are unmyelinated axons.
In some cases, the target effectors are located superior or inferior to the spinal segment at which the preganglionic fiber emerges. With respect to the “wiring” involved, the synapse with the ganglionic neuron occurs at chain ganglia superior or inferior to the location of the central neuron. An example of this is spinal nerve T1 that innervates the eye. The spinal nerve tracks up through the chain until it reaches the superior cervical ganglion, where it synapses with the postganglionic neuron (see Figure 15.3b). The cervical ganglia are referred to as paravertebral ganglia, given their location adjacent to prevertebral ganglia in the sympathetic chain.
Not all axons from the central neurons terminate in the chain ganglia. Additional branches from the ventral nerve root continue through the chain and on to one of the collateral ganglia as the greater splanchnic nerve or lesser splanchnic nerve. For example, the greater splanchnic nerve at the level of T5 synapses with a collateral ganglion outside the chain before making the connection to the postganglionic nerves that innervate the stomach (see Figure 15.3c).
Collateral ganglia, also called prevertebral ganglia, are situated anterior to the vertebral column and receive inputs from splanchnic nerves as well as central sympathetic neurons. They are associated with controlling organs in the abdominal cavity, and are also considered part of the enteric nervous system. The three collateral ganglia are the celiac ganglion, the superior mesenteric ganglion, and the inferior mesenteric ganglion (see Figure 15.2). The word celiac is derived from the Latin word “coelom,” which refers to a body cavity (in this case, the abdominal cavity), and the word mesenteric refers to the digestive system.
Figure 15.3 Sympathetic Connections and Chain Ganglia The axon from a central sympathetic neuron in the spinal cord can project to the periphery in a number of different ways. (a) The fiber can project out to the ganglion at the same level and synapse on a ganglionic neuron. (b) A branch can project to more superior or inferior ganglion in the chain. (c) A branch can project through the white ramus communicans, but not terminate on a ganglionic neuron in the chain. Instead, it projects through one of the splanchnic nerves to a collateral ganglion or the adrenal medulla (not pictured).
An axon from the central neuron that projects to a sympathetic ganglion is referred to as a preganglionic fiber or neuron, and represents the output from the CNS to the ganglion. Because the sympathetic ganglia are adjacent to the vertebral column, preganglionic sympathetic fibers are relatively short, and they are myelinated. A postganglionic fiber—the axon from a ganglionic neuron that projects to the target effector—represents the output of a ganglion that directly influences the organ. Compared with the preganglionic fibers, postganglionic sympathetic fibers are long because of the relatively greater distance from the ganglion to the target effector. These fibers are unmyelinated. (Note that the term “postganglionic neuron” may be used to describe the projection from a ganglion to the target. The problem with that usage is that the cell body is in the ganglion, and only the fiber is postganglionic. Typically, the term neuron applies to the entire cell.)
One type of preganglionic sympathetic fiber does not terminate in a ganglion. These are the axons from central sympathetic neurons that project to the adrenal medulla, the interior portion of the adrenal gland. These axons are still referred to as preganglionic fibers, but the target is not a ganglion. The adrenal medulla releases signaling molecules into the bloodstream, rather than using axons to communicate with target structures. The cells in the adrenal medulla that are contacted by the preganglionic fibers are called chromaffin cells. These cells are neurosecretory cells that develop from the neural crest along with the sympathetic ganglia, reinforcing the idea that the gland is, functionally, a sympathetic ganglion.
The projections of the sympathetic division of the autonomic nervous system diverge widely, resulting in a broad influence of the system throughout the body. As a response to a threat, the sympathetic system would increase heart rate and breathing rate and cause blood flow to the skeletal muscle to increase and blood flow to the digestive system to decrease. Sweat gland secretion should also increase as part of an integrated response. All of those physiological changes are going to be required to occur together to run away from the hunting lioness, or the modern equivalent. This divergence is seen in the branching patterns of preganglionic sympathetic neurons—a single preganglionic sympathetic neuron may have 10–20 targets. An axon that leaves a central neuron of the lateral horn in the thoracolumbar spinal cord will pass through the white ramus communicans and enter the sympathetic chain, where it will branch toward a variety of targets. At the level of the spinal cord at which the preganglionic sympathetic fiber exits the spinal cord, a branch will synapse on a neuron in the adjacent chain ganglion. Some branches will extend up or down to a different level of the chain ganglia. Other branches will pass through the chain ganglia and project through one of the splanchnic nerves to a collateral ganglion. Finally, some branches may project through the splanchnic nerves to the adrenal medulla. All of these branches mean that one preganglionic neuron can influence different regions of the sympathetic system very broadly, by acting on widely distributed organs.
Parasympathetic Division of the Autonomic Nervous System
The parasympathetic division of the autonomic nervous system is named because its central neurons are located on either side of the thoracolumbar region of the spinal cord (para- = “beside” or “near”). The parasympathetic system can also be referred to as the craniosacral system (or outflow) because the preganglionic neurons are located in nuclei of the brain stem and the lateral horn of the sacral spinal cord.
The connections, or “circuits,” of the parasympathetic division are similar to the general layout of the sympathetic division with a few specific differences (Figure 15.4). The preganglionic fibers from the cranial region travel in cranial nerves, whereas preganglionic fibers from the sacral region travel in spinal nerves. The targets of these fibers are terminal ganglia, which are located near—or even within—the target effector. These ganglia are often referred to as intramural ganglia when they are found within the walls of the target organ. The postganglionic fiber projects from the terminal ganglia a short distance to the target effector, or to the specific target tissue within the organ. Comparing the relative lengths of axons in the parasympathetic system, the preganglionic fibers are long and the postganglionic fibers are short because the ganglia are close to—and sometimes within—the target effectors.
The cranial component of the parasympathetic system is based in particular nuclei of the brain stem. In the midbrain, the Edinger–Westphal nucleus is part of the oculomotor complex, and axons from those neurons travel with the fibers in the oculomotor nerve (cranial nerve III) that innervate the extraocular muscles. The preganglionic parasympathetic fibers within cranial nerve III terminate in the ciliary ganglion, which is located in the posterior orbit. The postganglionic parasympathetic fibers then project to the smooth muscle of the iris to control pupillary size. In the upper medulla, the salivatory nuclei contain neurons with axons that project through the facial and glossopharyngeal nerves to ganglia that control salivary glands. Tear production is influenced by parasympathetic fibers in the facial nerve, which activate a ganglion, and ultimately the lacrimal (tear) gland. Neurons in the dorsal nucleus of the vagus nerve and the nucleus ambiguus project through the vagus nerve (cranial nerve X) to the terminal ganglia of the thoracic and abdominal cavities. Parasympathetic preganglionic fibers primarily influence the heart, bronchi, and esophagus in the thoracic cavity and the stomach, liver, pancreas, gall bladder, and small intestine of the abdominal cavity. The postganglionic fibers from the ganglia activated by the vagus nerve are often incorporated into the structure of the organ, such as the mesenteric plexus of the digestive tract organs and the intramural ganglia.
Figure 15.4 Connections of Parasympathetic Division of the Autonomic Nervous System Neurons from brain-stem nuclei, or from the lateral horn of the sacral spinal cord, project to terminal ganglia near or within the various organs of the body. Axons from these ganglionic neurons then project the short distance to those target effectors.
Chemical Signaling in the Autonomic Nervous System
Where an autonomic neuron connects with a target, there is a synapse. The electrical signal of the action potential causes the release of a signaling molecule, which will bind to receptor proteins on the target cell. Synapses of the autonomic system are classified as either cholinergic, meaning that acetylcholine (ACh) is released, or adrenergic, meaning that norepinephrine is released. The terms cholinergic and adrenergic refer not only to the signaling molecule that is released but also to the class of receptors that each binds.
The cholinergic system includes two classes of receptor: the nicotinic receptor and the muscarinic receptor. Both receptor types bind to ACh and cause changes in the target cell. The nicotinic receptor is a ligand-gated cation channel and the muscarinic receptor is a G protein–coupled receptor. The receptors are named for, and differentiated by, other molecules that bind to them. Whereas nicotine will bind to the nicotinic receptor, and muscarine will bind to the muscarinic receptor, there is no cross-reactivity between the receptors. The situation is similar to locks and keys. Imagine two locks—one for a classroom and the other for an office—that are opened by two separate keys. The classroom key will not open the office door and the office key will not open the classroom door. This is similar to the specificity of nicotine and muscarine for their receptors. However, a master key can open multiple locks, such as a master key for the Biology Department that opens both the classroom and the office doors. This is similar to ACh that binds to both types of receptors. The molecules that define these receptors are not crucial—they are simply tools for researchers to use in the laboratory. These molecules are exogenous, meaning that they are made outside of the human body, so a researcher can use them without any confounding endogenous results (results caused by the molecules produced in the body).
The adrenergic system also has two types of receptors, named the alpha (α)-adrenergic receptor and beta (β)-adrenergic receptor. Unlike cholinergic receptors, these receptor types are not classified by which drugs can bind to them. All of them are G protein–coupled receptors. There are three types of α-adrenergic receptors, termed α1, α2, and α3, and there are two types of β-adrenergic receptors, termed β1 and β2. An additional aspect of the adrenergic system is that there is a second signaling molecule called epinephrine. The chemical difference between norepinephrine and epinephrine is the addition of a methyl group (CH3) in epinephrine. The prefix “nor-” actually refers to this chemical difference, in which a methyl group is missing.
The term adrenergic should remind you of the word adrenaline, which is associated with the fight-or-flight response described at the beginning of the chapter. Adrenaline and epinephrine are two names for the same molecule. The adrenal gland (in Latin, ad- = “on top of”; renal = “kidney”) secretes adrenaline. The ending “-ine” refers to the chemical being derived, or extracted, from the adrenal gland. A similar construction from Greek instead of Latin results in the word epinephrine (epi- = “above”; nephr- = “kidney”). In scientific usage, epinephrine is preferred in the United States, whereas adrenaline is preferred in Great Britain, because “adrenalin” was once a registered, proprietary drug name in the United States. Though the drug is no longer sold, the convention of referring to this molecule by the two different names persists. Similarly, norepinephrine and noradrenaline are two names for the same molecule.
Having understood the cholinergic and adrenergic systems, their role in the autonomic system is relatively simple to understand. All preganglionic fibers, both sympathetic and parasympathetic, release ACh. All ganglionic neurons—the targets of these preganglionic fibers—have nicotinic receptors in their cell membranes. The nicotinic receptor is a ligand-gated cation channel that results in depolarization of the postsynaptic membrane. The postganglionic parasympathetic fibers also release ACh, but the receptors on their targets are muscarinic receptors, which are G protein–coupled receptors and do not exclusively cause depolarization of the postsynaptic membrane. Postganglionic sympathetic fibers release norepinephrine, except for fibers that project to sweat glands and to blood vessels associated with skeletal muscles, which release ACh (Table 15.1).
Autonomic System Signaling Molecules
| Sympathetic | Parasympathetic | |
|---|---|---|
| Preganglionic | Acetylcholine → nicotinic receptor | Acetylcholine → nicotinic receptor |
| Postganglionic | Norepinephrine → α- or β-adrenergic receptors Acetylcholine → muscarinic receptor (associated with sweat glands and the blood vessels associated with skeletal muscles only | Acetylcholine → muscarinic receptor |
Table 15.1
Signaling molecules can belong to two broad groups. Neurotransmitters are released at synapses, whereas hormones are released into the bloodstream. These are simplistic definitions, but they can help to clarify this point. Acetylcholine can be considered a neurotransmitter because it is released by axons at synapses. The adrenergic system, however, presents a challenge. Postganglionic sympathetic fibers release norepinephrine, which can be considered a neurotransmitter. But the adrenal medulla releases epinephrine and norepinephrine into circulation, so they should be considered hormones.
What are referred to here as synapses may not fit the strictest definition of synapse. Some sources will refer to the connection between a postganglionic fiber and a target effector as neuroeffector junctions; neurotransmitters, as defined above, would be called neuromodulators. The structure of postganglionic connections are not the typical synaptic end bulb that is found at the neuromuscular junction, but rather are chains of swellings along the length of a postganglionic fiber called a varicosity (Figure 15.5).
Figure 15.5 Autonomic Varicosities The connection between autonomic fibers and target effectors is not the same as the typical synapse, such as the neuromuscular junction. Instead of a synaptic end bulb, a neurotransmitter is released from swellings along the length of a fiber that makes an extended network of connections in the target effector.
EVERYDAY CONNECTION
Fight or Flight? What About Fright and Freeze?
The original usage of the epithet “fight or flight” comes from a scientist named Walter Cannon who worked at Harvard in 1915. The concept of homeostasis and the functioning of the sympathetic system had been introduced in France in the previous century. Cannon expanded the idea, and introduced the idea that an animal responds to a threat by preparing to stand and fight or run away. The nature of this response was thoroughly explained in a book on the physiology of pain, hunger, fear, and rage.
When students learn about the sympathetic system and the fight-or-flight response, they often stop and wonder about other responses. If you were faced with a lioness running toward you as pictured at the beginning of this chapter, would you run or would you stand your ground? Some people would say that they would freeze and not know what to do. So isn’t there really more to what the autonomic system does than fight, flight, rest, or digest. What about fear and paralysis in the face of a threat?
The common epithet of “fight or flight” is being enlarged to be “fight, flight, or fright” or even “fight, flight, fright, or freeze.” Cannon’s original contribution was a catchy phrase to express some of what the nervous system does in response to a threat, but it is incomplete. The sympathetic system is responsible for the physiological responses to emotional states. The name “sympathetic” can be said to mean that (sym- = “together”; -pathos = “pain,” “suffering,” or “emotion”).
INTERACTIVE LINK
Watch this video to learn more about the nervous system. As described in this video, the nervous system has a way to deal with threats and stress that is separate from the conscious control of the somatic nervous system. The system comes from a time when threats were about survival, but in the modern age, these responses become part of stress and anxiety. This video describes how the autonomic system is only part of the response to threats, or stressors. What other organ system gets involved, and what part of the brain coordinates the two systems for the entire response, including epinephrine (adrenaline) and cortisol?
Autonomic Reflexes and Homeostasis
- Compare the structure of somatic and autonomic reflex arcs
- Explain the differences in sympathetic and parasympathetic reflexes
- Differentiate between short and long reflexes
- Determine the effect of the autonomic nervous system on the regulation of the various organ systems on the basis of the signaling molecules involved
- Describe the effects of drugs that affect autonomic function
The autonomic nervous system regulates organ systems through circuits that resemble the reflexes described in the somatic nervous system. The main difference between the somatic and autonomic systems is in what target tissues are effectors. Somatic responses are solely based on skeletal muscle contraction. The autonomic system, however, targets cardiac and smooth muscle, as well as glandular tissue. Whereas the basic circuit is a reflex arc, there are differences in the structure of those reflexes for the somatic and autonomic systems.
The Structure of Reflexes
One difference between a somatic reflex, such as the withdrawal reflex, and a visceral reflex, which is an autonomic reflex, is in the efferent branch. The output of a somatic reflex is the lower motor neuron in the ventral horn of the spinal cord that projects directly to a skeletal muscle to cause its contraction. The output of a visceral reflex is a two-step pathway starting with the preganglionic fiber emerging from a lateral horn neuron in the spinal cord, or a cranial nucleus neuron in the brain stem, to a ganglion—followed by the postganglionic fiber projecting to a target effector. The other part of a reflex, the afferent branch, is often the same between the two systems. Sensory neurons receiving input from the periphery—with cell bodies in the sensory ganglia, either of a cranial nerve or a dorsal root ganglion adjacent to the spinal cord—project into the CNS to initiate the reflex (Figure 15.6). The Latin root “effere” means “to carry.” Adding the prefix “ef-” suggests the meaning “to carry away,” whereas adding the prefix “af-” suggests “to carry toward or inward.”
Figure 15.6 Comparison of Somatic and Visceral Reflexes The afferent inputs to somatic and visceral reflexes are essentially the same, whereas the efferent branches are different. Somatic reflexes, for instance, involve a direct connection from the ventral horn of the spinal cord to the skeletal muscle. Visceral reflexes involve a projection from the central neuron to a ganglion, followed by a second projection from the ganglion to the target effector.
Afferent Branch
The afferent branch of a reflex arc does differ between somatic and visceral reflexes in some instances. Many of the inputs to visceral reflexes are from special or somatic senses, but particular senses are associated with the viscera that are not part of the conscious perception of the environment through the somatic nervous system. For example, there is a specific type of mechanoreceptor, called a baroreceptor, in the walls of the aorta and carotid sinuses that senses the stretch of those organs when blood volume or pressure increases. You do not have a conscious perception of having high blood pressure, but that is an important afferent branch of the cardiovascular and, particularly, vasomotor reflexes. The sensory neuron is essentially the same as any other general sensory neuron. The baroreceptor apparatus is part of the ending of a unipolar neuron that has a cell body in a sensory ganglion. The baroreceptors from the carotid arteries have axons in the glossopharyngeal nerve, and those from the aorta have axons in the vagus nerve.
Though visceral senses are not primarily a part of conscious perception, those sensations sometimes make it to conscious awareness. If a visceral sense is strong enough, it will be perceived. The sensory homunculus—the representation of the body in the primary somatosensory cortex—only has a small region allotted for the perception of internal stimuli. If you swallow a large bolus of food, for instance, you will probably feel the lump of that food as it pushes through your esophagus, or even if your stomach is distended after a large meal. If you inhale especially cold air, you can feel it as it enters your larynx and trachea. These sensations are not the same as feeling high blood pressure or blood sugar levels.
When particularly strong visceral sensations rise to the level of conscious perception, the sensations are often felt in unexpected places. For example, strong visceral sensations of the heart will be felt as pain in the left shoulder and left arm. This irregular pattern of projection of conscious perception of visceral sensations is called referred pain. Depending on the organ system affected, the referred pain will project to different areas of the body (Figure 15.7). The location of referred pain is not random, but a definitive explanation of the mechanism has not been established. The most broadly accepted theory for this phenomenon is that the visceral sensory fibers enter into the same level of the spinal cord as the somatosensory fibers of the referred pain location. By this explanation, the visceral sensory fibers from the mediastinal region, where the heart is located, would enter the spinal cord at the same level as the spinal nerves from the shoulder and arm, so the brain misinterprets the sensations from the mediastinal region as being from the axillary and brachial regions. Projections from the medial and inferior divisions of the cervical ganglia do enter the spinal cord at the middle to lower cervical levels, which is where the somatosensory fibers enter.
Figure 15.7 Referred Pain Chart Conscious perception of visceral sensations map to specific regions of the body, as shown in this chart. Some sensations are felt locally, whereas others are perceived as affecting areas that are quite distant from the involved organ.
DISORDERS OF THE...
Nervous System: Kehr’s Sign
Kehr’s sign is the presentation of pain in the left shoulder, chest, and neck regions following rupture of the spleen. The spleen is in the upper-left abdominopelvic quadrant, but the pain is more in the shoulder and neck. How can this be? The sympathetic fibers connected to the spleen are from the celiac ganglion, which would be from the mid-thoracic to lower thoracic region whereas parasympathetic fibers are found in the vagus nerve, which connects in the medulla of the brain stem. However, the neck and shoulder would connect to the spinal cord at the mid-cervical level of the spinal cord. These connections do not fit with the expected correspondence of visceral and somatosensory fibers entering at the same level of the spinal cord.
The incorrect assumption would be that the visceral sensations are coming from the spleen directly. In fact, the visceral fibers are coming from the diaphragm. The nerve connecting to the diaphragm takes a special route. The phrenic nerve is connected to the spinal cord at cervical levels 3 to 5. The motor fibers that make up this nerve are responsible for the muscle contractions that drive ventilation. These fibers have left the spinal cord to enter the phrenic nerve, meaning that spinal cord damage below the mid-cervical level is not fatal by making ventilation impossible. Therefore, the visceral fibers from the diaphragm enter the spinal cord at the same level as the somatosensory fibers from the neck and shoulder.
The diaphragm plays a role in Kehr’s sign because the spleen is just inferior to the diaphragm in the upper-left quadrant of the abdominopelvic cavity. When the spleen ruptures, blood spills into this region. The accumulating hemorrhage then puts pressure on the diaphragm. The visceral sensation is actually in the diaphragm, so the referred pain is in a region of the body that corresponds to the diaphragm, not the spleen.
Efferent Branch
The efferent branch of the visceral reflex arc begins with the projection from the central neuron along the preganglionic fiber. This fiber then makes a synapse on the ganglionic neuron that projects to the target effector.
The effector organs that are the targets of the autonomic system range from the iris and ciliary body of the eye to the urinary bladder and reproductive organs. The thoracolumbar output, through the various sympathetic ganglia, reaches all of these organs. The cranial component of the parasympathetic system projects from the eye to part of the intestines. The sacral component picks up with the majority of the large intestine and the pelvic organs of the urinary and reproductive systems.
Short and Long Reflexes
Somatic reflexes involve sensory neurons that connect sensory receptors to the CNS and motor neurons that project back out to the skeletal muscles. Visceral reflexes that involve the thoracolumbar or craniosacral systems share similar connections. However, there are reflexes that do not need to involve any CNS components. A long reflex has afferent branches that enter the spinal cord or brain and involve the efferent branches, as previously explained. A short reflex is completely peripheral and only involves the local integration of sensory input with motor output (Figure 15.8).
Figure 15.8 Short and Long Reflexes Sensory input can stimulate either a short or a long reflex. A sensory neuron can project to the CNS or to an autonomic ganglion. The short reflex involves the direct stimulation of a postganglionic fiber by the sensory neuron, whereas the long reflex involves integration in the spinal cord or brain.
The difference between short and long reflexes is in the involvement of the CNS. Somatic reflexes always involve the CNS, even in a monosynaptic reflex in which the sensory neuron directly activates the motor neuron. That synapse is in the spinal cord or brain stem, so it has to involve the CNS. However, in the autonomic system there is the possibility that the CNS is not involved. Because the efferent branch of a visceral reflex involves two neurons—the central neuron and the ganglionic neuron—a “short circuit” can be possible. If a sensory neuron projects directly to the ganglionic neuron and causes it to activate the effector target, then the CNS is not involved.
A division of the nervous system that is related to the autonomic nervous system is the enteric nervous system. The word enteric refers to the digestive organs, so this represents the nervous tissue that is part of the digestive system. There are a few myenteric plexuses in which the nervous tissue in the wall of the digestive tract organs can directly influence digestive function. If stretch receptors in the stomach are activated by the filling and distension of the stomach, a short reflex will directly activate the smooth muscle fibers of the stomach wall to increase motility to digest the excessive food in the stomach. No CNS involvement is needed because the stretch receptor is directly activating a neuron in the wall of the stomach that causes the smooth muscle to contract. That neuron, connected to the smooth muscle, is a postganglionic parasympathetic neuron that can be controlled by a fiber found in the vagus nerve.
INTERACTIVE LINK
Read this article to learn about a teenager who experiences a series of spells that suggest a stroke. He undergoes endless tests and seeks input from multiple doctors. In the end, one expert, one question, and a simple blood pressure cuff answers the question. Why would the heart have to beat faster when the teenager changes his body position from lying down to sitting, and then to standing?
Balance in Competing Autonomic Reflex Arcs
The autonomic nervous system is important for homeostasis because its two divisions compete at the target effector. The balance of homeostasis is attributable to the competing inputs from the sympathetic and parasympathetic divisions (dual innervation). At the level of the target effector, the signal of which system is sending the message is strictly chemical. A signaling molecule binds to a receptor that causes changes in the target cell, which in turn causes the tissue or organ to respond to the changing conditions of the body.
Competing Neurotransmitters
The postganglionic fibers of the sympathetic and parasympathetic divisions both release neurotransmitters that bind to receptors on their targets. Postganglionic sympathetic fibers release norepinephrine, with a minor exception, whereas postganglionic parasympathetic fibers release ACh. For any given target, the difference in which division of the autonomic nervous system is exerting control is just in what chemical binds to its receptors. The target cells will have adrenergic and muscarinic receptors. If norepinephrine is released, it will bind to the adrenergic receptors present on the target cell, and if ACh is released, it will bind to the muscarinic receptors on the target cell.
In the sympathetic system, there are exceptions to this pattern of dual innervation. The postganglionic sympathetic fibers that contact the blood vessels within skeletal muscle and that contact sweat glands do not release norepinephrine, they release ACh. This does not create any problem because there is no parasympathetic input to the sweat glands. Sweat glands have muscarinic receptors and produce and secrete sweat in response to the presence of ACh.
At most of the other targets of the autonomic system, the effector response is based on which neurotransmitter is released and what receptor is present. For example, regions of the heart that establish heart rate are contacted by postganglionic fibers from both systems. If norepinephrine is released onto those cells, it binds to an adrenergic receptor that causes the cells to depolarize faster, and the heart rate increases. If ACh is released onto those cells, it binds to a muscarinic receptor that causes the cells to hyperpolarize so that they cannot reach threshold as easily, and the heart rate slows. Without this parasympathetic input, the heart would work at a rate of approximately 100 beats per minute (bpm). The sympathetic system speeds that up, as it would during exercise, to 120–140 bpm, for example. The parasympathetic system slows it down to the resting heart rate of 60–80 bpm.
Another example is in the control of pupillary size (Figure 15.9). The afferent branch responds to light hitting the retina. Photoreceptors are activated, and the signal is transferred to the retinal ganglion cells that send an action potential along the optic nerve into the diencephalon. If light levels are low, the sympathetic system sends a signal out through the upper thoracic spinal cord to the superior cervical ganglion of the sympathetic chain. The postganglionic fiber then projects to the iris, where it releases norepinephrine onto the radial fibers of the iris (a smooth muscle). When those fibers contract, the pupil dilates—increasing the amount of light hitting the retina. If light levels are too high, the parasympathetic system sends a signal out from the Eddinger–Westphal nucleus through the oculomotor nerve. This fiber synapses in the ciliary ganglion in the posterior orbit. The postganglionic fiber then projects to the iris, where it releases ACh onto the circular fibers of the iris—another smooth muscle. When those fibers contract, the pupil constricts to limit the amount of light hitting the retina.
Figure 15.9 Autonomic Control of Pupillary Size Activation of the pupillary reflex comes from the amount of light activating the retinal ganglion cells, as sent along the optic nerve. The output of the sympathetic system projects through the superior cervical ganglion, whereas the parasympathetic system originates out of the midbrain and projects through the oculomotor nerve to the ciliary ganglion, which then projects to the iris. The postganglionic fibers of either division release neurotransmitters onto the smooth muscles of the iris to cause changes in the pupillary size. Norepinephrine results in dilation and ACh results in constriction.
In this example, the autonomic system is controlling how much light hits the retina. It is a homeostatic reflex mechanism that keeps the activation of photoreceptors within certain limits. In the context of avoiding a threat like the lioness on the savannah, the sympathetic response for fight or flight will increase pupillary diameter so that more light hits the retina and more visual information is available for running away. Likewise, the parasympathetic response of rest reduces the amount of light reaching the retina, allowing the photoreceptors to cycle through bleaching and be regenerated for further visual perception; this is what the homeostatic process is attempting to maintain.
INTERACTIVE LINK
Watch this video to learn about the pupillary reflexes. The pupillary light reflex involves sensory input through the optic nerve and motor response through the oculomotor nerve to the ciliary ganglion, which projects to the circular fibers of the iris. As shown in this short animation, pupils will constrict to limit the amount of light falling on the retina under bright lighting conditions. What constitutes the afferent and efferent branches of the competing reflex (dilation)?
Autonomic Tone
Organ systems are balanced between the input from the sympathetic and parasympathetic divisions. When something upsets that balance, the homeostatic mechanisms strive to return it to its regular state. For each organ system, there may be more of a sympathetic or parasympathetic tendency to the resting state, which is known as the autonomic tone of the system. For example, the heart rate was described above. Because the resting heart rate is the result of the parasympathetic system slowing the heart down from its intrinsic rate of 100 bpm, the heart can be said to be in parasympathetic tone.
In a similar fashion, another aspect of the cardiovascular system is primarily under sympathetic control. Blood pressure is partially determined by the contraction of smooth muscle in the walls of blood vessels. These tissues have adrenergic receptors that respond to the release of norepinephrine from postganglionic sympathetic fibers by constricting and increasing blood pressure. The hormones released from the adrenal medulla—epinephrine and norepinephrine—will also bind to these receptors. Those hormones travel through the bloodstream where they can easily interact with the receptors in the vessel walls. The parasympathetic system has no significant input to the systemic blood vessels, so the sympathetic system determines their tone.
There are a limited number of blood vessels that respond to sympathetic input in a different fashion. Blood vessels in skeletal muscle, particularly those in the lower limbs, are more likely to dilate. It does not have an overall effect on blood pressure to alter the tone of the vessels, but rather allows for blood flow to increase for those skeletal muscles that will be active in the fight-or-flight response. The blood vessels that have a parasympathetic projection are limited to those in the erectile tissue of the reproductive organs. Acetylcholine released by these postganglionic parasympathetic fibers cause the vessels to dilate, leading to the engorgement of the erectile tissue.
HOMEOSTATIC IMBALANCES
Orthostatic Hypotension
Have you ever stood up quickly and felt dizzy for a moment? This is because, for one reason or another, blood is not getting to your brain so it is briefly deprived of oxygen. When you change position from sitting or lying down to standing, your cardiovascular system has to adjust for a new challenge, keeping blood pumping up into the head while gravity is pulling more and more blood down into the legs.
The reason for this is a sympathetic reflex that maintains the output of the heart in response to postural change. When a person stands up, proprioceptors indicate that the body is changing position. A signal goes to the CNS, which then sends a signal to the upper thoracic spinal cord neurons of the sympathetic division. The sympathetic system then causes the heart to beat faster and the blood vessels to constrict. Both changes will make it possible for the cardiovascular system to maintain the rate of blood delivery to the brain. Blood is being pumped superiorly through the internal branch of the carotid arteries into the brain, against the force of gravity. Gravity is not increasing while standing, but blood is more likely to flow down into the legs as they are extended for standing. This sympathetic reflex keeps the brain well oxygenated so that cognitive and other neural processes are not interrupted.
Sometimes this does not work properly. If the sympathetic system cannot increase cardiac output, then blood pressure into the brain will decrease, and a brief neurological loss can be felt. This can be brief, as a slight “wooziness” when standing up too quickly, or a loss of balance and neurological impairment for a period of time. The name for this is orthostatic hypotension, which means that blood pressure goes below the homeostatic set point when standing. It can be the result of standing up faster than the reflex can occur, which may be referred to as a benign “head rush,” or it may be the result of an underlying cause.
There are two basic reasons that orthostatic hypotension can occur. First, blood volume is too low and the sympathetic reflex is not effective. This hypovolemia may be the result of dehydration or medications that affect fluid balance, such as diuretics or vasodilators. Both of these medications are meant to lower blood pressure, which may be necessary in the case of systemic hypertension, and regulation of the medications may alleviate the problem. Sometimes increasing fluid intake or water retention through salt intake can improve the situation.
The second underlying cause of orthostatic hypotension is autonomic failure. There are several disorders that result in compromised sympathetic functions. The disorders range from diabetes to multiple system atrophy (a loss of control over many systems in the body), and addressing the underlying condition can improve the hypotension. For example, with diabetes, peripheral nerve damage can occur, which would affect the postganglionic sympathetic fibers. Getting blood glucose levels under control can improve neurological deficits associated with diabetes.
Central Control
- Describe the role of higher centers of the brain in autonomic regulation
- Explain the connection of the hypothalamus to homeostasis
- Describe the regions of the CNS that link the autonomic system with emotion
- Describe the pathways important to descending control of the autonomic system
The pupillary light reflex (Figure 15.10) begins when light hits the retina and causes a signal to travel along the optic nerve. This is visual sensation, because the afferent branch of this reflex is simply sharing the special sense pathway. Bright light hitting the retina leads to the parasympathetic response, through the oculomotor nerve, followed by the postganglionic fiber from the ciliary ganglion, which stimulates the circular fibers of the iris to contract and constrict the pupil. When light hits the retina in one eye, both pupils contract. When that light is removed, both pupils dilate again back to the resting position. When the stimulus is unilateral (presented to only one eye), the response is bilateral (both eyes). The same is not true for somatic reflexes. If you touch a hot radiator, you only pull that arm back, not both. Central control of autonomic reflexes is different than for somatic reflexes. The hypothalamus, along with other CNS locations, controls the autonomic system.
Figure 15.10 Pupillary Reflex Pathways The pupil is under competing autonomic control in response to light levels hitting the retina. The sympathetic system will dilate the pupil when the retina is not receiving enough light, and the parasympathetic system will constrict the pupil when too much light hits the retina.
Forebrain Structures
Autonomic control is based on the visceral reflexes, composed of the afferent and efferent branches. These homeostatic mechanisms are based on the balance between the two divisions of the autonomic system, which results in tone for various organs that is based on the predominant input from the sympathetic or parasympathetic systems. Coordinating that balance requires integration that begins with forebrain structures like the hypothalamus and continues into the brain stem and spinal cord.
The Hypothalamus
The hypothalamus is the control center for many homeostatic mechanisms. It regulates both autonomic function and endocrine function. The roles it plays in the pupillary reflexes demonstrates the importance of this control center. The optic nerve projects primarily to the thalamus, which is the necessary relay to the occipital cortex for conscious visual perception. Another projection of the optic nerve, however, goes to the hypothalamus.
The hypothalamus then uses this visual system input to drive the pupillary reflexes. If the retina is activated by high levels of light, the hypothalamus stimulates the parasympathetic response. If the optic nerve message shows that low levels of light are falling on the retina, the hypothalamus activates the sympathetic response. Output from the hypothalamus follows two main tracts, the dorsal longitudinal fasciculus and the medial forebrain bundle (Figure 15.11). Along these two tracts, the hypothalamus can influence the Eddinger–Westphal nucleus of the oculomotor complex or the lateral horns of the thoracic spinal cord.
Figure 15.11 Fiber Tracts of the Central Autonomic System The hypothalamus is the source of most of the central control of autonomic function. It receives input from cerebral structures and projects to brain stem and spinal cord structures to regulate the balance of sympathetic and parasympathetic input to the organ systems of the body. The main pathways for this are the medial forebrain bundle and the dorsal longitudinal fasciculus.
These two tracts connect the hypothalamus with the major parasympathetic nuclei in the brain stem and the preganglionic (central) neurons of the thoracolumbar spinal cord. The hypothalamus also receives input from other areas of the forebrain through the medial forebrain bundle. The olfactory cortex, the septal nuclei of the basal forebrain, and the amygdala project into the hypothalamus through the medial forebrain bundle. These forebrain structures inform the hypothalamus about the state of the nervous system and can influence the regulatory processes of homeostasis. A good example of this is found in the amygdala, which is found beneath the cerebral cortex of the temporal lobe and plays a role in our ability to remember and feel emotions.
The Amygdala
The amygdala is a group of nuclei in the medial region of the temporal lobe that is part of the limbic lobe (Figure 15.12). The limbic lobe includes structures that are involved in emotional responses, as well as structures that contribute to memory function. The limbic lobe has strong connections with the hypothalamus and influences the state of its activity on the basis of emotional state. For example, when you are anxious or scared, the amygdala will send signals to the hypothalamus along the medial forebrain bundle that will stimulate the sympathetic fight-or-flight response. The hypothalamus will also stimulate the release of stress hormones through its control of the endocrine system in response to amygdala input.
Figure 15.12 The Limbic Lobe Structures arranged around the edge of the cerebrum constitute the limbic lobe, which includes the amygdala, hippocampus, and cingulate gyrus, and connects to the hypothalamus.
The Medulla
The medulla contains nuclei referred to as the cardiovascular center, which controls the smooth and cardiac muscle of the cardiovascular system through autonomic connections. When the homeostasis of the cardiovascular system shifts, such as when blood pressure changes, the coordination of the autonomic system can be accomplished within this region. Furthermore, when descending inputs from the hypothalamus stimulate this area, the sympathetic system can increase activity in the cardiovascular system, such as in response to anxiety or stress. The preganglionic sympathetic fibers that are responsible for increasing heart rate are referred to as the cardiac accelerator nerves, whereas the preganglionic sympathetic fibers responsible for constricting blood vessels compose the vasomotor nerves.
Several brain stem nuclei are important for the visceral control of major organ systems. One brain stem nucleus involved in cardiovascular function is the solitary nucleus. It receives sensory input about blood pressure and cardiac function from the glossopharyngeal and vagus nerves, and its output will activate sympathetic stimulation of the heart or blood vessels through the upper thoracic lateral horn. Another brain stem nucleus important for visceral control is the dorsal motor nucleus of the vagus nerve, which is the motor nucleus for the parasympathetic functions ascribed to the vagus nerve, including decreasing the heart rate, relaxing bronchial tubes in the lungs, and activating digestive function through the enteric nervous system. The nucleus ambiguus, which is named for its ambiguous histology, also contributes to the parasympathetic output of the vagus nerve and targets muscles in the pharynx and larynx for swallowing and speech, as well as contributing to the parasympathetic tone of the heart along with the dorsal motor nucleus of the vagus.
EVERYDAY CONNECTION
Exercise and the Autonomic System
In addition to its association with the fight-or-flight response and rest-and-digest functions, the autonomic system is responsible for certain everyday functions. For example, it comes into play when homeostatic mechanisms dynamically change, such as the physiological changes that accompany exercise. Getting on the treadmill and putting in a good workout will cause the heart rate to increase, breathing to be stronger and deeper, sweat glands to activate, and the digestive system to suspend activity. These are the same physiological changes associated with the fight-or-flight response, but there is nothing chasing you on that treadmill.
This is not a simple homeostatic mechanism at work because “maintaining the internal environment” would mean getting all those changes back to their set points. Instead, the sympathetic system has become active during exercise so that your body can cope with what is happening. A homeostatic mechanism is dealing with the conscious decision to push the body away from a resting state. The heart, actually, is moving away from its homeostatic set point. Without any input from the autonomic system, the heart would beat at approximately 100 bpm, and the parasympathetic system slows that down to the resting rate of approximately 70 bpm. But in the middle of a good workout, you should see your heart rate at 120–140 bpm. You could say that the body is stressed because of what you are doing to it. Homeostatic mechanisms are trying to keep blood pH in the normal range, or to keep body temperature under control, but those are in response to the choice to exercise.
INTERACTIVE LINK
Watch this video to learn about physical responses to emotion. The autonomic system, which is important for regulating the homeostasis of the organ systems, is also responsible for our physiological responses to emotions such as fear. The video summarizes the extent of the body’s reactions and describes several effects of the autonomic system in response to fear. On the basis of what you have already studied about autonomic function, which effect would you expect to be associated with parasympathetic, rather than sympathetic, activity?
Drugs that Affect the Autonomic System
- List the classes of pharmaceuticals that interact with the autonomic nervous system
- Differentiate between cholinergic and adrenergic compounds
- Differentiate between sympathomimetic and sympatholytic drugs
- Relate the consequences of nicotine abuse with respect to autonomic control of the cardiovascular system
An important way to understand the effects of native neurochemicals in the autonomic system is in considering the effects of pharmaceutical drugs. This can be considered in terms of how drugs change autonomic function. These effects will primarily be based on how drugs act at the receptors of the autonomic system neurochemistry. The signaling molecules of the nervous system interact with proteins in the cell membranes of various target cells. In fact, no effect can be attributed to just the signaling molecules themselves without considering the receptors. A chemical that the body produces to interact with those receptors is called an endogenous chemical, whereas a chemical introduced to the system from outside is an exogenous chemical. Exogenous chemicals may be of a natural origin, such as a plant extract, or they may be synthetically produced in a pharmaceutical laboratory.
Broad Autonomic Effects
One important drug that affects the autonomic system broadly is not a pharmaceutical therapeutic agent associated with the system. This drug is nicotine. The effects of nicotine on the autonomic nervous system are important in considering the role smoking can play in health.
All ganglionic neurons of the autonomic system, in both sympathetic and parasympathetic ganglia, are activated by ACh released from preganglionic fibers. The ACh receptors on these neurons are of the nicotinic type, meaning that they are ligand-gated ion channels. When the neurotransmitter released from the preganglionic fiber binds to the receptor protein, a channel opens to allow positive ions to cross the cell membrane. The result is depolarization of the ganglia. Nicotine acts as an ACh analog at these synapses, so when someone takes in the drug, it binds to these ACh receptors and activates the ganglionic neurons, causing them to depolarize.
Ganglia of both divisions are activated equally by the drug. For many target organs in the body, this results in no net change. The competing inputs to the system cancel each other out and nothing significant happens. For example, the sympathetic system will cause sphincters in the digestive tract to contract, limiting digestive propulsion, but the parasympathetic system will cause the contraction of other muscles in the digestive tract, which will try to push the contents of the digestive system along. The end result is that the food does not really move along and the digestive system has not appreciably changed.
The system in which this can be problematic is in the cardiovascular system, which is why smoking is a risk factor for cardiovascular disease. First, there is no significant parasympathetic regulation of blood pressure. Only a limited number of blood vessels are affected by parasympathetic input, so nicotine will preferentially cause the vascular tone to become more sympathetic, which means blood pressure will be increased. Second, the autonomic control of the heart is special. Unlike skeletal or smooth muscles, cardiac muscle is intrinsically active, meaning that it generates its own action potentials. The autonomic system does not cause the heart to beat, it just speeds it up (sympathetic) or slows it down (parasympathetic). The mechanisms for this are not mutually exclusive, so the heart receives conflicting signals, and the rhythm of the heart can be affected (Figure 15.13).
Figure 15.13 Autonomic Connections to Heart and Blood Vessels The nicotinic receptor is found on all autonomic ganglia, but the cardiovascular connections are particular, and do not conform to the usual competitive projections that would just cancel each other out when stimulated by nicotine. The opposing signals to the heart would both depolarize and hyperpolarize the heart cells that establish the rhythm of the heartbeat, likely causing arrhythmia. Only the sympathetic system governs systemic blood pressure so nicotine would cause an increase.
Sympathetic Effect
The neurochemistry of the sympathetic system is based on the adrenergic system. Norepinephrine and epinephrine influence target effectors by binding to the α-adrenergic or β-adrenergic receptors. Drugs that affect the sympathetic system affect these chemical systems. The drugs can be classified by whether they enhance the functions of the sympathetic system or interrupt those functions. A drug that enhances adrenergic function is known as a sympathomimetic drug, whereas a drug that interrupts adrenergic function is a sympatholytic drug.
Sympathomimetic Drugs
When the sympathetic system is not functioning correctly or the body is in a state of homeostatic imbalance, these drugs act at postganglionic terminals and synapses in the sympathetic efferent pathway. These drugs either bind to particular adrenergic receptors and mimic norepinephrine at the synapses between sympathetic postganglionic fibers and their targets, or they increase the production and release of norepinephrine from postganglionic fibers. Also, to increase the effectiveness of adrenergic chemicals released from the fibers, some of these drugs may block the removal or reuptake of the neurotransmitter from the synapse.
A common sympathomimetic drug is phenylephrine, which is a common component of decongestants. It can also be used to dilate the pupil and to raise blood pressure. Phenylephrine is known as an α1-adrenergic agonist, meaning that it binds to a specific adrenergic receptor, stimulating a response. In this role, phenylephrine will bind to the adrenergic receptors in bronchioles of the lungs and cause them to dilate. By opening these structures, accumulated mucus can be cleared out of the lower respiratory tract. Phenylephrine is often paired with other pharmaceuticals, such as analgesics, as in the “sinus” version of many over-the-counter drugs, such as Tylenol Sinus® or Excedrin Sinus®, or in expectorants for chest congestion such as in Robitussin CF®.
A related molecule, called pseudoephedrine, was much more commonly used in these applications than was phenylephrine, until the molecule became useful in the illicit production of amphetamines. Phenylephrine is not as effective as a drug because it can be partially broken down in the digestive tract before it is ever absorbed. Like the adrenergic agents, phenylephrine is effective in dilating the pupil, known as mydriasis (Figure 15.14). Phenylephrine is used during an eye exam in an ophthalmologist’s or optometrist’s office for this purpose. It can also be used to increase blood pressure in situations in which cardiac function is compromised, such as under anesthesia or during septic shock.
Figure 15.14 Mydriasis The sympathetic system causes pupillary dilation when norepinephrine binds to an adrenergic receptor in the radial fibers of the iris smooth muscle. Phenylephrine mimics this action by binding to the same receptor when drops are applied onto the surface of the eye in a doctor’s office. (credit: Corey Theiss)
Other drugs that enhance adrenergic function are not associated with therapeutic uses, but affect the functions of the sympathetic system in a similar fashion. Cocaine primarily interferes with the uptake of dopamine at the synapse and can also increase adrenergic function. Caffeine is an antagonist to a different neurotransmitter receptor, called the adenosine receptor. Adenosine will suppress adrenergic activity, specifically the release of norepinephrine at synapses, so caffeine indirectly increases adrenergic activity. There is some evidence that caffeine can aid in the therapeutic use of drugs, perhaps by potentiating (increasing) sympathetic function, as is suggested by the inclusion of caffeine in over-the-counter analgesics such as Excedrin®.
Sympatholytic Drugs
Drugs that interfere with sympathetic function are referred to as sympatholytic, or sympathoplegic, drugs. They primarily work as an antagonist to the adrenergic receptors. They block the ability of norepinephrine or epinephrine to bind to the receptors so that the effect is “cut” or “takes a blow,” to refer to the endings “-lytic” and “-plegic,” respectively. The various drugs of this class will be specific to α-adrenergic or β-adrenergic receptors, or to their receptor subtypes.
Possibly the most familiar type of sympatholytic drug are the β-blockers. These drugs are often used to treat cardiovascular disease because they block the β-receptors associated with vasoconstriction and cardioacceleration. By allowing blood vessels to dilate, or keeping heart rate from increasing, these drugs can improve cardiac function in a compromised system, such as for a person with congestive heart failure or who has previously suffered a heart attack. A couple of common versions of β-blockers are metoprolol, which specifically blocks the β1-receptor, and propanolol, which nonspecifically blocks β-receptors. There are other drugs that are α-blockers and can affect the sympathetic system in a similar way.
Other uses for sympatholytic drugs are as antianxiety medications. A common example of this is clonidine, which is an α-agonist. The sympathetic system is tied to anxiety to the point that the sympathetic response can be referred to as “fight, flight, or fright.” Clonidine is used for other treatments aside from hypertension and anxiety, including pain conditions and attention deficit hyperactivity disorder.
Parasympathetic Effects
Drugs affecting parasympathetic functions can be classified into those that increase or decrease activity at postganglionic terminals. Parasympathetic postganglionic fibers release ACh, and the receptors on the targets are muscarinic receptors. There are several types of muscarinic receptors, M1–M5, but the drugs are not usually specific to the specific types. Parasympathetic drugs can be either muscarinic agonists or antagonists, or have indirect effects on the cholinergic system. Drugs that enhance cholinergic effects are called parasympathomimetic drugs, whereas those that inhibit cholinergic effects are referred to as anticholinergic drugs.
Pilocarpine is a nonspecific muscarinic agonist commonly used to treat disorders of the eye. It reverses mydriasis, such as is caused by phenylephrine, and can be administered after an eye exam. Along with constricting the pupil through the smooth muscle of the iris, pilocarpine will also cause the ciliary muscle to contract. This will open perforations at the base of the cornea, allowing for the drainage of aqueous humor from the anterior compartment of the eye and, therefore, reducing intraocular pressure related to glaucoma.
Atropine and scopolamine are part of a class of muscarinic antagonists that come from the Atropa genus of plants that include belladonna or deadly nightshade (Figure 15.15). The name of one of these plants, belladonna, refers to the fact that extracts from this plant were used cosmetically for dilating the pupil. The active chemicals from this plant block the muscarinic receptors in the iris and allow the pupil to dilate, which is considered attractive because it makes the eyes appear larger. Humans are instinctively attracted to anything with larger eyes, which comes from the fact that the ratio of eye-to-head size is different in infants (or baby animals) and can elicit an emotional response. The cosmetic use of belladonna extract was essentially acting on this response. Atropine is no longer used in this cosmetic capacity for reasons related to the other name for the plant, which is deadly nightshade. Suppression of parasympathetic function, especially when it becomes systemic, can be fatal. Autonomic regulation is disrupted and anticholinergic symptoms develop. The berries of this plant are highly toxic, but can be mistaken for other berries. The antidote for atropine or scopolamine poisoning is pilocarpine.
Figure 15.15 Belladonna Plant The plant from the genus Atropa, which is known as belladonna or deadly nightshade, was used cosmetically to dilate pupils, but can be fatal when ingested. The berries on the plant may seem attractive as a fruit, but they contain the same anticholinergic compounds as the rest of the plant.
Sympathetic and Parasympathetic Effects of Different Drug Types
| Drug type | Example(s) | Sympathetic effect | Parasympathetic effect | Overall result |
|---|---|---|---|---|
| Nicotinic agonists | Nicotine | Mimic ACh at preganglionic synapses, causing activation of postganglionic fibers and the release of norepinephrine onto the target organ | Mimic ACh at preganglionic synapses, causing activation of postganglionic fibers and the release of ACh onto the target organ | Most conflicting signals cancel each other out, but cardiovascular system is susceptible to hypertension and arrhythmias |
| Sympathomimetic drugs | Phenylephrine | Bind to adrenergic receptors or mimics sympathetic action in some other way | No effect | Increase sympathetic tone |
| Sympatholytic drugs | β-blockers such as propanolol or metoprolol; α-agonists such as clonidine | Block binding to adrenergic drug or decrease adrenergic signals | No effect | Increase parasympathetic tone |
| Parasymphatho-mimetics/muscarinic agonists | Pilocarpine | No effect, except on sweat glands | Bind to muscarinic receptor, similar to ACh | Increase parasympathetic tone |
| Anticholinergics/muscarinic antagonists | Atropine, scopolamine, dimenhydrinate | No effect | Block muscarinic receptors and parasympathetic function | Increase sympathetic tone |
Table 15.2
DISORDERS OF THE...
Autonomic Nervous System
Approximately 33 percent of people experience a mild problem with motion sickness, whereas up to 66 percent experience motion sickness under extreme conditions, such as being on a tossing boat with no view of the horizon. Connections between regions in the brain stem and the autonomic system result in the symptoms of nausea, cold sweats, and vomiting.
The part of the brain responsible for vomiting, or emesis, is known as the area postrema. It is located next to the fourth ventricle and is not restricted by the blood–brain barrier, which allows it to respond to chemicals in the bloodstream—namely, toxins that will stimulate emesis. There are significant connections between this area, the solitary nucleus, and the dorsal motor nucleus of the vagus nerve. These autonomic system and nuclei connections are associated with the symptoms of motion sickness.
Motion sickness is the result of conflicting information from the visual and vestibular systems. If motion is perceived by the visual system without the complementary vestibular stimuli, or through vestibular stimuli without visual confirmation, the brain stimulates emesis and the associated symptoms. The area postrema, by itself, appears to be able to stimulate emesis in response to toxins in the blood, but it is also connected to the autonomic system and can trigger a similar response to motion.
Autonomic drugs are used to combat motion sickness. Though it is often described as a dangerous and deadly drug, scopolamine is used to treat motion sickness. A popular treatment for motion sickness is the transdermal scopolamine patch. Scopolamine is one of the substances derived from the Atropa genus along with atropine. At higher doses, those substances are thought to be poisonous and can lead to an extreme sympathetic syndrome. However, the transdermal patch regulates the release of the drug, and the concentration is kept very low so that the dangers are avoided. For those who are concerned about using “The Most Dangerous Drug,” as some websites will call it, antihistamines such as dimenhydrinate (Dramamine®) can be used.
INTERACTIVE LINK
Watch this video to learn about the side effects of 3-D movies. As discussed in this video, movies that are shot in 3-D can cause motion sickness, which elicits the autonomic symptoms of nausea and sweating. The disconnection between the perceived motion on the screen and the lack of any change in equilibrium stimulates these symptoms. Why do you think sitting close to the screen or right in the middle of the theater makes motion sickness during a 3-D movie worse?
Key Terms
- acetylcholine (ACh)
- neurotransmitter that binds at a motor end-plate to trigger depolarization
- adrenal medulla
- interior portion of the adrenal (or suprarenal) gland that releases epinephrine and norepinephrine into the bloodstream as hormones
- adrenergic
- synapse where norepinephrine is released, which binds to α- or β-adrenergic receptors
- afferent branch
- component of a reflex arc that represents the input from a sensory neuron, for either a special or general sense
- agonist
- any exogenous substance that binds to a receptor and produces a similar effect to the endogenous ligand
- alpha (α)-adrenergic receptor
- one of the receptors to which epinephrine and norepinephrine bind, which comes in three subtypes: α1, α2, and α3
- antagonist
- any exogenous substance that binds to a receptor and produces an opposing effect to the endogenous ligand
- anticholinergic drugs
- drugs that interrupt or reduce the function of the parasympathetic system
- autonomic tone
- tendency of an organ system to be governed by one division of the autonomic nervous system over the other, such as heart rate being lowered by parasympathetic input at rest
- baroreceptor
- mechanoreceptor that senses the stretch of blood vessels to indicate changes in blood pressure
- beta (β)-adrenergic receptor
- one of the receptors to which epinephrine and norepinephrine bind, which comes in two subtypes: β1 and β2
- cardiac accelerator nerves
- preganglionic sympathetic fibers that cause the heart rate to increase when the cardiovascular center in the medulla initiates a signal
- cardiovascular center
- region in the medulla that controls the cardiovascular system through cardiac accelerator nerves and vasomotor nerves, which are components of the sympathetic division of the autonomic nervous system
- celiac ganglion
- one of the collateral ganglia of the sympathetic system that projects to the digestive system
- central neuron
- specifically referring to the cell body of a neuron in the autonomic system that is located in the central nervous system, specifically the lateral horn of the spinal cord or a brain stem nucleus
- cholinergic
- synapse at which acetylcholine is released and binds to the nicotinic or muscarinic receptor
- chromaffin cells
- neuroendocrine cells of the adrenal medulla that release epinephrine and norepinephrine into the bloodstream as part of sympathetic system activity
- ciliary ganglion
- one of the terminal ganglia of the parasympathetic system, located in the posterior orbit, axons from which project to the iris
- collateral ganglia
- ganglia outside of the sympathetic chain that are targets of sympathetic preganglionic fibers, which are the celiac, inferior mesenteric, and superior mesenteric ganglia
- craniosacral system
- alternate name for the parasympathetic division of the autonomic nervous system that is based on the anatomical location of central neurons in brain-stem nuclei and the lateral horn of the sacral spinal cord; also referred to as craniosacral outflow
- dorsal longitudinal fasciculus
- major output pathway of the hypothalamus that descends through the gray matter of the brain stem and into the spinal cord
- dorsal nucleus of the vagus nerve
- location of parasympathetic neurons that project through the vagus nerve to terminal ganglia in the thoracic and abdominal cavities
- Eddinger–Westphal nucleus
- location of parasympathetic neurons that project to the ciliary ganglion
- efferent branch
- component of a reflex arc that represents the output, with the target being an effector, such as muscle or glandular tissue
- endogenous
- describes substance made in the human body
- endogenous chemical
- substance produced and released within the body to interact with a receptor protein
- epinephrine
- signaling molecule released from the adrenal medulla into the bloodstream as part of the sympathetic response
- exogenous
- describes substance made outside of the human body
- exogenous chemical
- substance from a source outside the body, whether it be another organism such as a plant or from the synthetic processes of a laboratory, that binds to a transmembrane receptor protein
- fight-or-flight response
- set of responses induced by sympathetic activity that lead to either fleeing a threat or standing up to it, which in the modern world is often associated with anxious feelings
- G protein–coupled receptor
- membrane protein complex that consists of a receptor protein that binds to a signaling molecule—a G protein—that is activated by that binding and in turn activates an effector protein (enzyme) that creates a second-messenger molecule in the cytoplasm of the target cell
- ganglionic neuron
- specifically refers to the cell body of a neuron in the autonomic system that is located in a ganglion
- gray rami communicantes
- (singular = ramus communicans) unmyelinated structures that provide a short connection from a sympathetic chain ganglion to the spinal nerve that contains the postganglionic sympathetic fiber
- greater splanchnic nerve
- nerve that contains fibers of the central sympathetic neurons that do not synapse in the chain ganglia but project onto the celiac ganglion
- inferior mesenteric ganglion
- one of the collateral ganglia of the sympathetic system that projects to the digestive system
- intramural ganglia
- terminal ganglia of the parasympathetic system that are found within the walls of the target effector
- lesser splanchnic nerve
- nerve that contains fibers of the central sympathetic neurons that do not synapse in the chain ganglia but project onto the inferior mesenteric ganglion
- ligand-gated cation channel
- ion channel, such as the nicotinic receptor, that is specific to positively charged ions and opens when a molecule such as a neurotransmitter binds to it
- limbic lobe
- structures arranged around the edges of the cerebrum that are involved in memory and emotion
- long reflex
- reflex arc that includes the central nervous system
- medial forebrain bundle
- fiber pathway that extends anteriorly into the basal forebrain, passes through the hypothalamus, and extends into the brain stem and spinal cord
- mesenteric plexus
- nervous tissue within the wall of the digestive tract that contains neurons that are the targets of autonomic preganglionic fibers and that project to the smooth muscle and glandular tissues in the digestive organ
- muscarinic receptor
- type of acetylcholine receptor protein that is characterized by also binding to muscarine and is a metabotropic receptor
- mydriasis
- dilation of the pupil; typically the result of disease, trauma, or drugs
- nicotinic receptor
- type of acetylcholine receptor protein that is characterized by also binding to nicotine and is an ionotropic receptor
- norepinephrine
- signaling molecule released as a neurotransmitter by most postganglionic sympathetic fibers as part of the sympathetic response, or as a hormone into the bloodstream from the adrenal medulla
- nucleus ambiguus
- brain-stem nucleus that contains neurons that project through the vagus nerve to terminal ganglia in the thoracic cavity; specifically associated with the heart
- parasympathetic division
- division of the autonomic nervous system responsible for restful and digestive functions
- parasympathomimetic drugs
- drugs that enhance or mimic the function of the parasympathetic system
- paravertebral ganglia
- autonomic ganglia superior to the sympathetic chain ganglia
- postganglionic fiber
- axon from a ganglionic neuron in the autonomic nervous system that projects to and synapses with the target effector; sometimes referred to as a postganglionic neuron
- preganglionic fiber
- axon from a central neuron in the autonomic nervous system that projects to and synapses with a ganglionic neuron; sometimes referred to as a preganglionic neuron
- prevertebral ganglia
- autonomic ganglia that are anterior to the vertebral column and functionally related to the sympathetic chain ganglia
- referred pain
- the conscious perception of visceral sensation projected to a different region of the body, such as the left shoulder and arm pain as a sign for a heart attack
- reflex arc
- circuit of a reflex that involves a sensory input and motor output, or an afferent branch and an efferent branch, and an integrating center to connect the two branches
- rest and digest
- set of functions associated with the parasympathetic system that lead to restful actions and digestion
- short reflex
- reflex arc that does not include any components of the central nervous system
- somatic reflex
- reflex involving skeletal muscle as the effector, under the control of the somatic nervous system
- superior cervical ganglion
- one of the paravertebral ganglia of the sympathetic system that projects to the head
- superior mesenteric ganglion
- one of the collateral ganglia of the sympathetic system that projects to the digestive system
- sympathetic chain ganglia
- series of ganglia adjacent to the vertebral column that receive input from central sympathetic neurons
- sympathetic division
- division of the autonomic nervous system associated with the fight-or-flight response
- sympatholytic drug
- drug that interrupts, or “lyses,” the function of the sympathetic system
- sympathomimetic drug
- drug that enhances or mimics the function of the sympathetic system
- target effector
- organ, tissue, or gland that will respond to the control of an autonomic or somatic or endocrine signal
- terminal ganglia
- ganglia of the parasympathetic division of the autonomic system, which are located near or within the target effector, the latter also known as intramural ganglia
- thoracolumbar system
- alternate name for the sympathetic division of the autonomic nervous system that is based on the anatomical location of central neurons in the lateral horn of the thoracic and upper lumbar spinal cord
- varicosity
- structure of some autonomic connections that is not a typical synaptic end bulb, but a string of swellings along the length of a fiber that makes a network of connections with the target effector
- vasomotor nerves
- preganglionic sympathetic fibers that cause the constriction of blood vessels in response to signals from the cardiovascular center
- visceral reflex
- reflex involving an internal organ as the effector, under the control of the autonomic nervous system
- white rami communicantes
- (singular = ramus communicans) myelinated structures that provide a short connection from a sympathetic chain ganglion to the spinal nerve that contains the preganglionic sympathetic fiber
Chapter Review
15.1 Divisions of the Autonomic Nervous System
The primary responsibilities of the autonomic nervous system are to regulate homeostatic mechanisms in the body, which is also part of what the endocrine system does. The key to understanding the autonomic system is to explore the response pathways—the output of the nervous system. The way we respond to the world around us, to manage the internal environment on the basis of the external environment, is divided between two parts of the autonomic nervous system. The sympathetic division responds to threats and produces a readiness to confront the threat or to run away: the fight-or-flight response. The parasympathetic division plays the opposite role. When the external environment does not present any immediate danger, a restful mode descends on the body, and the digestive system is more active.
The sympathetic output of the nervous system originates out of the lateral horn of the thoracolumbar spinal cord. An axon from one of these central neurons projects by way of the ventral spinal nerve root and spinal nerve to a sympathetic ganglion, either in the sympathetic chain ganglia or one of the collateral locations, where it synapses on a ganglionic neuron. These preganglionic fibers release ACh, which excites the ganglionic neuron through the nicotinic receptor. The axon from the ganglionic neuron—the postganglionic fiber—then projects to a target effector where it will release norepinephrine to bind to an adrenergic receptor, causing a change in the physiology of that organ in keeping with the broad, divergent sympathetic response. The postganglionic connections to sweat glands in the skin and blood vessels supplying skeletal muscle are, however, exceptions; those fibers release ACh onto muscarinic receptors. The sympathetic system has a specialized preganglionic connection to the adrenal medulla that causes epinephrine and norepinephrine to be released into the bloodstream rather than exciting a neuron that contacts an organ directly. This hormonal component means that the sympathetic chemical signal can spread throughout the body very quickly and affect many organ systems at once.
The parasympathetic output is based in the brain stem and sacral spinal cord. Neurons from particular nuclei in the brain stem or from the lateral horn of the sacral spinal cord (preganglionic neurons) project to terminal (intramural) ganglia located close to or within the wall of target effectors. These preganglionic fibers also release ACh onto nicotinic receptors to excite the ganglionic neurons. The postganglionic fibers then contact the target tissues within the organ to release ACh, which binds to muscarinic receptors to induce rest-and-digest responses.
Signaling molecules utilized by the autonomic nervous system are released from axons and can be considered as either neurotransmitters (when they directly interact with the effector) or as hormones (when they are released into the bloodstream). The same molecule, such as norepinephrine, could be considered either a neurotransmitter or a hormone on the basis of whether it is released from a postganglionic sympathetic axon or from the adrenal gland. The synapses in the autonomic system are not always the typical type of connection first described in the neuromuscular junction. Instead of having synaptic end bulbs at the very end of an axonal fiber, they may have swellings—called varicosities—along the length of a fiber so that it makes a network of connections within the target tissue.
15.2 Autonomic Reflexes and Homeostasis
Autonomic nervous system function is based on the visceral reflex. This reflex is similar to the somatic reflex, but the efferent branch is composed of two neurons. The central neuron projects from the spinal cord or brain stem to synapse on the ganglionic neuron that projects to the effector. The afferent branch of the somatic and visceral reflexes is very similar, as many somatic and special senses activate autonomic responses. However, there are visceral senses that do not form part of conscious perception. If a visceral sensation, such as cardiac pain, is strong enough, it will rise to the level of consciousness. However, the sensory homunculus does not provide a representation of the internal structures to the same degree as the surface of the body, so visceral sensations are often experienced as referred pain, such as feelings of pain in the left shoulder and arm in connection with a heart attack.
The role of visceral reflexes is to maintain a balance of function in the organ systems of the body. The two divisions of the autonomic system each play a role in effecting change, usually in competing directions. The sympathetic system increases heart rate, whereas the parasympathetic system decreases heart rate. The sympathetic system dilates the pupil of the eye, whereas the parasympathetic system constricts the pupil. The competing inputs can contribute to the resting tone of the organ system. Heart rate is normally under parasympathetic tone, whereas blood pressure is normally under sympathetic tone. The heart rate is slowed by the autonomic system at rest, whereas blood vessels retain a slight constriction at rest.
In a few systems of the body, the competing input from the two divisions is not the norm. The sympathetic tone of blood vessels is caused by the lack of parasympathetic input to the systemic circulatory system. Only certain regions receive parasympathetic input that relaxes the smooth muscle wall of the blood vessels. Sweat glands are another example, which only receive input from the sympathetic system.
15.3 Central Control
The autonomic system integrates sensory information and higher cognitive processes to generate output, which balances homeostatic mechanisms. The central autonomic structure is the hypothalamus, which coordinates sympathetic and parasympathetic efferent pathways to regulate activities of the organ systems of the body. The majority of hypothalamic output travels through the medial forebrain bundle and the dorsal longitudinal fasciculus to influence brain stem and spinal components of the autonomic nervous system. The medial forebrain bundle also connects the hypothalamus with higher centers of the limbic system where emotion can influence visceral responses. The amygdala is a structure within the limbic system that influences the hypothalamus in the regulation of the autonomic system, as well as the endocrine system.
These higher centers have descending control of the autonomic system through brain stem centers, primarily in the medulla, such as the cardiovascular center. This collection of medullary nuclei regulates cardiac function, as well as blood pressure. Sensory input from the heart, aorta, and carotid sinuses project to these regions of the medulla. The solitary nucleus increases sympathetic tone of the cardiovascular system through the cardiac accelerator and vasomotor nerves. The nucleus ambiguus and the dorsal motor nucleus both contribute fibers to the vagus nerve, which exerts parasympathetic control of the heart by decreasing heart rate.
15.4 Drugs that Affect the Autonomic System
The autonomic system is affected by a number of exogenous agents, including some that are therapeutic and some that are illicit. These drugs affect the autonomic system by mimicking or interfering with the endogenous agents or their receptors. A survey of how different drugs affect autonomic function illustrates the role that the neurotransmitters and hormones play in autonomic function. Drugs can be thought of as chemical tools to effect changes in the system with some precision, based on where those drugs are effective.
Nicotine is not a drug that is used therapeutically, except for smoking cessation. When it is introduced into the body via products, it has broad effects on the autonomic system. Nicotine carries a risk for cardiovascular disease because of these broad effects. The drug stimulates both sympathetic and parasympathetic ganglia at the preganglionic fiber synapse. For most organ systems in the body, the competing input from the two postganglionic fibers will essentially cancel each other out. However, for the cardiovascular system, the results are different. Because there is essentially no parasympathetic influence on blood pressure for the entire body, the sympathetic input is increased by nicotine, causing an increase in blood pressure. Also, the influence that the autonomic system has on the heart is not the same as for other systems. Other organs have smooth muscle or glandular tissue that is activated or inhibited by the autonomic system. Cardiac muscle is intrinsically active and is modulated by the autonomic system. The contradictory signals do not just cancel each other out, they alter the regularity of the heart rate and can cause arrhythmias. Both hypertension and arrhythmias are risk factors for heart disease.
Other drugs affect one division of the autonomic system or the other. The sympathetic system is affected by drugs that mimic the actions of adrenergic molecules (norepinephrine and epinephrine) and are called sympathomimetic drugs. Drugs such as phenylephrine bind to the adrenergic receptors and stimulate target organs just as sympathetic activity would. Other drugs are sympatholytic because they block adrenergic activity and cancel the sympathetic influence on the target organ. Drugs that act on the parasympathetic system also work by either enhancing the postganglionic signal or blocking it. A muscarinic agonist (or parasympathomimetic drug) acts just like ACh released by the parasympathetic postganglionic fiber. Anticholinergic drugs block muscarinic receptors, suppressing parasympathetic interaction with the organ.
Interactive Link Questions
Watch this video to learn more about adrenaline and the fight-or-flight response. When someone is said to have a rush of adrenaline, the image of bungee jumpers or skydivers usually comes to mind. But adrenaline, also known as epinephrine, is an important chemical in coordinating the body’s fight-or-flight response. In this video, you look inside the physiology of the fight-or-flight response, as envisioned for a firefighter. His body’s reaction is the result of the sympathetic division of the autonomic nervous system causing system-wide changes as it prepares for extreme responses. What two changes does adrenaline bring about to help the skeletal muscle response?
2.Watch this video to learn more about the nervous system. As described in this video, the nervous system has a way to deal with threats and stress that is separate from the conscious control of the somatic nervous system. The system comes from a time when threats were about survival, but in the modern age, these responses become part of stress and anxiety. This video describes how the autonomic system is only part of the response to threats, or stressors. What other organ system gets involved, and what part of the brain coordinates the two systems for the entire response, including epinephrine (adrenaline) and cortisol?
3.Read this article to learn about a teenager who experiences a series of spells that suggest a stroke. He undergoes endless tests and seeks input from multiple doctors. In the end, one expert, one question, and a simple blood pressure cuff answers the question. Why would the heart have to beat faster when the teenager changes his body position from lying down to sitting, and then to standing?
4.Watch this video to learn about the pupillary reflexes. The pupillary light reflex involves sensory input through the optic nerve and motor response through the oculomotor nerve to the ciliary ganglion, which projects to the circular fibers of the iris. As shown in this short animation, pupils will constrict to limit the amount of light falling on the retina under bright lighting conditions. What constitutes the afferent and efferent branches of the competing reflex (dilation)?
5.Watch this video to learn about physical responses to emotion. The autonomic system, which is important for regulating the homeostasis of the organ systems, is also responsible for our physiological responses to emotions such as fear. The video summarizes the extent of the body’s reactions and describes several effects of the autonomic system in response to fear. On the basis of what you have already studied about autonomic function, which effect would you expect to be associated with parasympathetic, rather than sympathetic, activity?
6.Watch this video to learn about the side effects of 3-D movies. As discussed in this video, movies that are shot in 3-D can cause motion sickness, which elicits the autonomic symptoms of nausea and sweating. The disconnection between the perceived motion on the screen and the lack of any change in equilibrium stimulates these symptoms. Why do you think sitting close to the screen or right in the middle of the theater makes motion sickness during a 3-D movie worse?
Review Questions
Which of these physiological changes would not be considered part of the sympathetic fight-or-flight response?
- increased heart rate
- increased sweating
- dilated pupils
- increased stomach motility
Which type of fiber could be considered the longest?
- preganglionic parasympathetic
- preganglionic sympathetic
- postganglionic parasympathetic
- postganglionic sympathetic
Which signaling molecule is most likely responsible for an increase in digestive activity?
- epinephrine
- norepinephrine
- acetylcholine
- adrenaline
Which of these cranial nerves contains preganglionic parasympathetic fibers?
- optic, CN II
- facial, CN VII
- trigeminal, CN V
- hypoglossal, CN XII
Which of the following is not a target of a sympathetic preganglionic fiber?
- intermural ganglion
- collateral ganglion
- adrenal gland
- chain ganglion
Which of the following represents a sensory input that is not part of both the somatic and autonomic systems?
- vision
- taste
- baroreception
- proprioception
What is the term for a reflex that does not include a CNS component?
- long reflex
- visceral reflex
- somatic reflex
- short reflex
What neurotransmitter will result in constriction of the pupil?
- norepinephrine
- acetylcholine
- epinephrine
- serotonin
What gland produces a secretion that causes fight-or-flight responses in effectors?
- adrenal medulla
- salivatory gland
- reproductive gland
- thymus
Which of the following is an incorrect pairing?
- norepinephrine dilates the pupil
- epinephrine increases blood pressure
- acetylcholine decreases digestion
- norepinephrine increases heart rate
Which of these locations in the forebrain is the master control center for homeostasis through the autonomic and endocrine systems?
- hypothalamus
- thalamus
- amygdala
- cerebral cortex
Which nerve projects to the hypothalamus to indicate the level of light stimuli in the retina?
- glossopharyngeal
- oculomotor
- optic
- vagus
What region of the limbic lobe is responsible for generating stress responses via the hypothalamus?
- hippocampus
- amygdala
- mammillary bodies
- prefrontal cortex
What is another name for the preganglionic sympathetic fibers that project to the heart?
- solitary tract
- vasomotor nerve
- vagus nerve
- cardiac accelerator nerve
What central fiber tract connects forebrain and brain stem structures with the hypothalamus?
- cardiac accelerator nerve
- medial forebrain bundle
- dorsal longitudinal fasciculus
- corticospinal tract
A drug that affects both divisions of the autonomic system is going to bind to, or block, which type of neurotransmitter receptor?
- nicotinic
- muscarinic
- α-adrenergic
- β-adrenergic
A drug is called an agonist if it ________.
- blocks a receptor
- interferes with neurotransmitter reuptake
- acts like the endogenous neurotransmitter by binding to its receptor
- blocks the voltage-gated calcium ion channel
Which type of drug would be an antidote to atropine poisoning?
- nicotinic agonist
- anticholinergic
- muscarinic agonist
- α-blocker
Which kind of drug would have anti-anxiety effects?
- nicotinic agonist
- anticholinergic
- muscarinic agonist
- α-blocker
Which type of drug could be used to treat asthma by opening airways wider?
- sympatholytic drug
- sympathomimetic drug
- anticholinergic drug
- parasympathomimetic drug
Critical Thinking Questions
In the context of a lioness hunting on the savannah, why would the sympathetic system not activate the digestive system?
28.A target effector, such as the heart, receives input from the sympathetic and parasympathetic systems. What is the actual difference between the sympathetic and parasympathetic divisions at the level of those connections (i.e., at the synapse)?
29.Damage to internal organs will present as pain associated with a particular surface area of the body. Why would something like irritation to the diaphragm, which is between the thoracic and abdominal cavities, feel like pain in the shoulder or neck?
30.Medical practice is paying more attention to the autonomic system in considering disease states. Why would autonomic tone be important in considering cardiovascular disease?
31.Horner’s syndrome is a condition that presents with changes in one eye, such as pupillary constriction and dropping of eyelids, as well as decreased sweating in the face. Why could a tumor in the thoracic cavity have an effect on these autonomic functions?
32.The cardiovascular center is responsible for regulating the heart and blood vessels through homeostatic mechanisms. What tone does each component of the cardiovascular system have? What connections does the cardiovascular center invoke to keep these two systems in their resting tone?
33.Why does smoking increase the risk of heart disease? Provide two reasons based on autonomic function.
34.Why might topical, cosmetic application of atropine or scopolamine from the belladonna plant not cause fatal poisoning, as would occur with ingestion of the plant?
|
oercommons
|
2025-03-18T00:39:11.216483
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/56378/overview",
"title": "Anatomy and Physiology, Regulation, Integration, and Control",
"author": null
}
|
https://oercommons.org/courseware/lesson/56379/overview
|
The Neurological Exam
Introduction
Figure 16.1 Neurological Exam Health care professionals, such as this air force nurse, can rapidly assess the neurological functions of a patient using the neurological exam. One part of the exam is the inspection of the oral cavity and pharynx, which enables the doctor to not only inspect the tissues for signs of infection, but also provides a means to test the functions of the cranial nerves associated with the oral cavity. (credit: U.S. Department of Defense)
CHAPTER OBJECTIVES
After studying this chapter, you will be able to:
- Describe the major sections of the neurological exam
- Outline the benefits of rapidly assessing neurological function
- Relate anatomical structures of the nervous system to specific functions
- Diagram the connections of the nervous system to the musculature and integument involved in primary sensorimotor responses
- Compare and contrast the somatic and visceral reflexes with respect to how they are assessed through the neurological exam
A man arrives at the hospital after feeling faint and complaining of a “pins-and-needles” feeling all along one side of his body. The most likely explanation is that he has suffered a stroke, which has caused a loss of oxygen to a particular part of the central nervous system (CNS). The problem is finding where in the entire nervous system the stroke has occurred. By checking reflexes, sensory responses, and motor control, a health care provider can focus on what abilities the patient may have lost as a result of the stroke and can use this information to determine where the injury occurred. In the emergency department of the hospital, this kind of rapid assessment of neurological function is key to treating trauma to the nervous system. In the classroom, the neurological exam is a valuable tool for learning the anatomy and physiology of the nervous system because it allows you to relate the functions of the system to particular locations in the nervous system.
As a student of anatomy and physiology, you may be planning to go into an allied health field, perhaps nursing or physical therapy. You could be in the emergency department treating a patient such as the one just described. An important part of this course is to understand the nervous system. This can be especially challenging because you need to learn about the nervous system using your own nervous system. The first chapter in this unit about the nervous system began with a quote: “If the human brain were simple enough for us to understand, we would be too simple to understand it.” However, you are being asked to understand aspects of it. A healthcare provider can pinpoint problems with the nervous system in minutes by running through the series of tasks to test neurological function that are described in this chapter. You can use the same approach, though not as quickly, to learn about neurological function and its relationship to the structures of the nervous system.
Nervous tissue is different from other tissues in that it is not classified into separate tissue types. It does contain two types of cells, neurons and glia, but it is all just nervous tissue. White matter and gray matter are not types of nervous tissue, but indications of different specializations within the nervous tissue. However, not all nervous tissue performs the same function. Furthermore, specific functions are not wholly localized to individual brain structures in the way that other bodily functions occur strictly within specific organs. In the CNS, we must consider the connections between cells over broad areas, not just the function of cells in one particular nucleus or region. In a broad sense, the nervous system is responsible for the majority of electrochemical signaling in the body, but the use of those signals is different in various regions.
The nervous system is made up of the brain and spinal cord as the central organs, and the ganglia and nerves as organs in the periphery. The brain and spinal cord can be thought of as a collection of smaller organs, most of which would be the nuclei (such as the oculomotor nuclei), but white matter structures play an important role (such as the corpus callosum). Studying the nervous system requires an understanding of the varied physiology of the nervous system. For example, the hypothalamus plays a very different role than the visual cortex. The neurological exam provides a way to elicit behavior that represents those varied functions.
Overview of the Neurological Exam
- List the major sections of the neurological exam
- Explain the connection between location and function in the nervous system
- Explain the benefit of a rapid assessment for neurological function in a clinical setting
- List the causes of neurological deficits
- Describe the different ischemic events in the nervous system
The neurological exam is a clinical assessment tool used to determine what specific parts of the CNS are affected by damage or disease. It can be performed in a short time—sometimes as quickly as 5 minutes—to establish neurological function. In the emergency department, this rapid assessment can make the difference with respect to proper treatment and the extent of recovery that is possible.
The exam is a series of subtests separated into five major sections. The first of these is the mental status exam, which assesses the higher cognitive functions such as memory, orientation, and language. Then there is the cranial nerve exam, which tests the function of the 12 cranial nerves and, therefore, the central and peripheral structures associated with them. The cranial nerve exam tests the sensory and motor functions of each of the nerves, as applicable. Two major sections, the sensory exam and the motor exam, test the sensory and motor functions associated with spinal nerves. Finally, the coordination exam tests the ability to perform complex and coordinated movements. The gait exam, which is often considered a sixth major exam, specifically assesses the motor function of walking and can be considered part of the coordination exam because walking is a coordinated movement.
Neuroanatomy and the Neurological Exam
Localization of function is the concept that circumscribed locations are responsible for specific functions. The neurological exam highlights this relationship. For example, the cognitive functions that are assessed in the mental status exam are based on functions in the cerebrum, mostly in the cerebral cortex. Several of the subtests examine language function. Deficits in neurological function uncovered by these examinations usually point to damage to the left cerebral cortex. In the majority of individuals, language function is localized to the left hemisphere between the superior temporal lobe and the posterior frontal lobe, including the intervening connections through the inferior parietal lobe.
The five major sections of the neurological exam are related to the major regions of the CNS (Figure 16.2). The mental status exam assesses functions related to the cerebrum. The cranial nerve exam is for the nerves that connect to the diencephalon and brain stem (as well as the olfactory connections to the forebrain). The coordination exam and the related gait exam primarily assess the functions of the cerebellum. The motor and sensory exams are associated with the spinal cord and its connections through the spinal nerves.
Figure 16.2 Anatomical Underpinnings of the Neurological Exam The different regions of the CNS relate to the major sections of the neurological exam: the mental status exam, cranial nerve exam, sensory exam, motor exam, and coordination exam (including the gait exam).
Part of the power of the neurological exam is this link between structure and function. Testing the various functions represented in the exam allows an accurate estimation of where the nervous system may be damaged. Consider the patient described in the chapter introduction. In the emergency department, he is given a quick exam to find where the deficit may be localized. Knowledge of where the damage occurred will lead to the most effective therapy.
In rapid succession, he is asked to smile, raise his eyebrows, stick out his tongue, and shrug his shoulders. The doctor tests muscular strength by providing resistance against his arms and legs while he tries to lift them. With his eyes closed, he has to indicate when he feels the tip of a pen touch his legs, arms, fingers, and face. He follows the tip of a pen as the doctor moves it through the visual field and finally toward his face. A formal mental status exam is not needed at this point; the patient will demonstrate any possible deficits in that area during normal interactions with the interviewer. If cognitive or language deficits are apparent, the interviewer can pursue mental status in more depth. All of this takes place in less than 5 minutes. The patient reports that he feels pins and needles in his left arm and leg, and has trouble feeling the tip of the pen when he is touched on those limbs. This suggests a problem with the sensory systems between the spinal cord and the brain. The emergency department has a lead to follow before a CT scan is performed. He is put on aspirin therapy to limit the possibility of blood clots forming, in case the cause is an embolus—an obstruction such as a blood clot that blocks the flow of blood in an artery or vein.
INTERACTIVE LINK
Watch this video to see a demonstration of the neurological exam—a series of tests that can be performed rapidly when a patient is initially brought into an emergency department. The exam can be repeated on a regular basis to keep a record of how and if neurological function changes over time. In what order were the sections of the neurological exam tested in this video, and which section seemed to be left out?
Causes of Neurological Deficits
Damage to the nervous system can be limited to individual structures or can be distributed across broad areas of the brain and spinal cord. Localized, limited injury to the nervous system is most often the result of circulatory problems. Neurons are very sensitive to oxygen deprivation and will start to deteriorate within 1 or 2 minutes, and permanent damage (cell death) could result within a few hours. The loss of blood flow to part of the brain is known as a stroke, or a cerebrovascular accident (CVA).
There are two main types of stroke, depending on how the blood supply is compromised: ischemic and hemorrhagic. An ischemic stroke is the loss of blood flow to an area because vessels are blocked or narrowed. This is often caused by an embolus, which may be a blood clot or fat deposit. Ischemia may also be the result of thickening of the blood vessel wall, or a drop in blood volume in the brain known as hypovolemia.
A related type of CVA is known as a transient ischemic attack (TIA), which is similar to a stroke although it does not last as long. The diagnostic definition of a stroke includes effects that last at least 24 hours. Any stroke symptoms that are resolved within a 24-hour period because of restoration of adequate blood flow are classified as a TIA.
A hemorrhagic stroke is bleeding into the brain because of a damaged blood vessel. Accumulated blood fills a region of the cranial vault and presses against the tissue in the brain (Figure 16.3). Physical pressure on the brain can cause the loss of function, as well as the squeezing of local arteries resulting in compromised blood flow beyond the site of the hemorrhage. As blood pools in the nervous tissue and the vasculature is damaged, the blood-brain barrier can break down and allow additional fluid to accumulate in the region, which is known as edema.
Figure 16.3 Hemorrhagic Stroke (a) A hemorrhage into the tissue of the cerebrum results in a large accumulation of blood with an additional edema in the adjacent tissue. The hemorrhagic area causes the entire brain to be disfigured as suggested here by the lateral ventricles being squeezed into the opposite hemisphere. (b) A CT scan shows an intraparenchymal hemorrhage within the parietal lobe. (credit b: James Heilman)
Whereas hemorrhagic stroke may involve bleeding into a large region of the CNS, such as into the deep white matter of a cerebral hemisphere, other events can cause widespread damage and loss of neurological functions. Infectious diseases can lead to loss of function throughout the CNS as components of nervous tissue, specifically astrocytes and microglia, react to the disease. Blunt force trauma, such as from a motor vehicle accident, can physically damage the CNS.
A class of disorders that affect the nervous system are the neurodegenerative diseases: Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, amyotrophic lateral sclerosis (ALS), Creutzfeld–Jacob disease, multiple sclerosis (MS), and other disorders that are the result of nervous tissue degeneration. In diseases like Alzheimer’s, Parkinson’s, or ALS, neurons die; in diseases like MS, myelin is affected. Some of these disorders affect motor function, and others present with dementia. How patients with these disorders perform in the neurological exam varies, but is often broad in its effects, such as memory deficits that compromise many aspects of the mental status exam, or movement deficits that compromise aspects of the cranial nerve exam, the motor exam, or the coordination exam. The causes of these disorders are also varied. Some are the result of genetics, such as Huntington’s disease, or the result of autoimmunity, such as MS; others are not entirely understood, such as Alzheimer’s and Parkinson’s diseases. Current research suggests that many of these diseases are related in how the degeneration takes place and may be treated by common therapies.
Finally, a common cause of neurological changes is observed in developmental disorders. Whether the result of genetic factors or the environment during development, there are certain situations that result in neurological functions being different from the expected norms. Developmental disorders are difficult to define because they are caused by defects that existed in the past and disrupted the normal development of the CNS. These defects probably involve multiple environmental and genetic factors—most of the time, we don’t know what the cause is other than that it is more complex than just one factor. Furthermore, each defect on its own may not be a problem, but when several are added together, they can disrupt growth processes that are not well understand in the first place. For instance, it is possible for a stroke to damage a specific region of the brain and lead to the loss of the ability to recognize faces (prosopagnosia). The link between cell death in the fusiform gyrus and the symptom is relatively easy to understand. In contrast, similar deficits can be seen in children with the developmental disorder, autism spectrum disorder (ASD). However, these children do not lack a fusiform gyrus, nor is there any damage or defect visible to this brain region. We conclude, rather poorly, that this brain region is not connected properly to other brain regions.
Infection, trauma, and congenital disorders can all lead to significant signs, as identified through the neurological exam. It is important to differentiate between an acute event, such as stroke, and a chronic or global condition such as blunt force trauma. Responses seen in the neurological exam can help. A loss of language function observed in all its aspects is more likely a global event as opposed to a discrete loss of one function, such as not being able to say certain types of words. A concern, however, is that a specific function—such as controlling the muscles of speech—may mask other language functions. The various subtests within the mental status exam can address these finer points and help clarify the underlying cause of the neurological loss.
INTERACTIVE LINK
Watch this video for an introduction to the neurological exam. Studying the neurological exam can give insight into how structure and function in the nervous system are interdependent. This is a tool both in the clinic and in the classroom, but for different reasons. In the clinic, this is a powerful but simple tool to assess a patient’s neurological function. In the classroom, it is a different way to think about the nervous system. Though medical technology provides noninvasive imaging and real-time functional data, the presenter says these cannot replace the history at the core of the medical examination. What does history mean in the context of medical practice?
The Mental Status Exam
- Describe the relationship of mental status exam results to cerebral functions
- Explain the categorization of regions of the cortex based on anatomy and physiology
- Differentiate between primary, association, and integration areas of the cerebral cortex
- Provide examples of localization of function related to the cerebral cortex
In the clinical setting, the set of subtests known as the mental status exam helps us understand the relationship of the brain to the body. Ultimately, this is accomplished by assessing behavior. Tremors related to intentional movements, incoordination, or the neglect of one side of the body can be indicative of failures of the connections of the cerebrum either within the hemispheres, or from the cerebrum to other portions of the nervous system. There is no strict test for what the cerebrum does alone, but rather in what it does through its control of the rest of the CNS, the peripheral nervous system (PNS), and the musculature.
Sometimes eliciting a behavior is as simple as asking a question. Asking a patient to state his or her name is not only to verify that the file folder in a health care provider’s hands is the correct one, but also to be sure that the patient is aware, oriented, and capable of interacting with another person. If the answer to “What is your name?” is “Santa Claus,” the person may have a problem understanding reality. If the person just stares at the examiner with a confused look on their face, the person may have a problem understanding or producing speech.
Functions of the Cerebral Cortex
The cerebrum is the seat of many of the higher mental functions, such as memory and learning, language, and conscious perception, which are the subjects of subtests of the mental status exam. The cerebral cortex is the thin layer of gray matter on the outside of the cerebrum. It is approximately a millimeter thick in most regions and highly folded to fit within the limited space of the cranial vault. These higher functions are distributed across various regions of the cortex, and specific locations can be said to be responsible for particular functions. There is a limited set of regions, for example, that are involved in language function, and they can be subdivided on the basis of the particular part of language function that each governs.
The basis for parceling out areas of the cortex and attributing them to various functions has its root in pure anatomical underpinnings. The German neurologist and histologist Korbinian Brodmann, who made a careful study of the cytoarchitecture of the cerebrum around the turn of the nineteenth century, described approximately 50 regions of the cortex that differed enough from each other to be considered separate areas (Figure 16.4). Brodmann made preparations of many different regions of the cerebral cortex to view with a microscope. He compared the size, shape, and number of neurons to find anatomical differences in the various parts of the cerebral cortex. Continued investigation into these anatomical areas over the subsequent 100 or more years has demonstrated a strong correlation between the structures and the functions attributed to those structures. For example, the first three areas in Brodmann’s list—which are in the postcentral gyrus—compose the primary somatosensory cortex. Within this area, finer separation can be made on the basis of the concept of the sensory homunculus, as well as the different submodalities of somatosensation such as touch, vibration, pain, temperature, or proprioception. Today, we more frequently refer to these regions by their function (i.e., primary sensory cortex) than by the number Brodmann assigned to them, but in some situations the use of Brodmann numbers persists.
Figure 16.4 Brodmann's Areas of the Cerebral Cortex On the basis of cytoarchitecture, the anatomist Korbinian Brodmann described the extensive array of cortical regions, as illustrated in his figure. Subsequent investigations found that these areas corresponded very well to functional differences in the cerebral cortex. (credit: modification of work by “Looie496”/Wikimedia Commons, based on original work by Korvinian Brodmann)
Area 17, as Brodmann described it, is also known as the primary visual cortex. Adjacent to that are areas 18 and 19, which constitute subsequent regions of visual processing. Area 22 is the primary auditory cortex, and it is followed by area 23, which further processes auditory information. Area 4 is the primary motor cortex in the precentral gyrus, whereas area 6 is the premotor cortex. These areas suggest some specialization within the cortex for functional processing, both in sensory and motor regions. The fact that Brodmann’s areas correlate so closely to functional localization in the cerebral cortex demonstrates the strong link between structure and function in these regions.
Areas 1, 2, 3, 4, 17, and 22 are each described as primary cortical areas. The adjoining regions are each referred to as association areas. Primary areas are where sensory information is initially received from the thalamus for conscious perception, or—in the case of the primary motor cortex—where descending commands are sent down to the brain stem or spinal cord to execute movements (Figure 16.5).
Figure 16.5 Types of Cortical Areas The cerebral cortex can be described as containing three types of processing regions: primary, association, and integration areas. The primary cortical areas are where sensory information is initially processed, or where motor commands emerge to go to the brain stem or spinal cord. Association areas are adjacent to primary areas and further process the modality-specific input. Multimodal integration areas are found where the modality-specific regions meet; they can process multiple modalities together or different modalities on the basis of similar functions, such as spatial processing in vision or somatosensation.
A number of other regions, which extend beyond these primary or association areas of the cortex, are referred to as integrative areas. These areas are found in the spaces between the domains for particular sensory or motor functions, and they integrate multisensory information, or process sensory or motor information in more complex ways. Consider, for example, the posterior parietal cortex that lies between the somatosensory cortex and visual cortex regions. This has been ascribed to the coordination of visual and motor functions, such as reaching to pick up a glass. The somatosensory function that would be part of this is the proprioceptive feedback from moving the arm and hand. The weight of the glass, based on what it contains, will influence how those movements are executed.
Cognitive Abilities
Assessment of cerebral functions is directed at cognitive abilities. The abilities assessed through the mental status exam can be separated into four groups: orientation and memory, language and speech, sensorium, and judgment and abstract reasoning.
Orientation and Memory
Orientation is the patient’s awareness of his or her immediate circumstances. It is awareness of time, not in terms of the clock, but of the date and what is occurring around the patient. It is awareness of place, such that a patient should know where he or she is and why. It is also awareness of who the patient is—recognizing personal identity and being able to relate that to the examiner. The initial tests of orientation are based on the questions, “Do you know what the date is?” or “Do you know where you are?” or “What is your name?” Further understanding of a patient’s awareness of orientation can come from questions that address remote memory, such as “Who is the President of the United States?”, or asking what happened on a specific date.
There are also specific tasks to address memory. One is the three-word recall test. The patient is given three words to recall, such as book, clock, and shovel. After a short interval, during which other parts of the interview continue, the patient is asked to recall the three words. Other tasks that assess memory—aside from those related to orientation—have the patient recite the months of the year in reverse order to avoid the overlearned sequence and focus on the memory of the months in an order, or to spell common words backwards, or to recite a list of numbers back.
Memory is largely a function of the temporal lobe, along with structures beneath the cerebral cortex such as the hippocampus and the amygdala. The storage of memory requires these structures of the medial temporal lobe. A famous case of a man who had both medial temporal lobes removed to treat intractable epilepsy provided insight into the relationship between the structures of the brain and the function of memory.
Henry Molaison, who was referred to as patient HM when he was alive, had epilepsy localized to both of his medial temporal lobes. In 1953, a bilateral lobectomy was performed that alleviated the epilepsy but resulted in the inability for HM to form new memories—a condition called anterograde amnesia. HM was able to recall most events from before his surgery, although there was a partial loss of earlier memories, which is referred to as retrograde amnesia. HM became the subject of extensive studies into how memory works. What he was unable to do was form new memories of what happened to him, what are now called episodic memory. Episodic memory is autobiographical in nature, such as remembering riding a bicycle as a child around the neighborhood, as opposed to the procedural memory of how to ride a bike. HM also retained his short-term memory, such as what is tested by the three-word task described above. After a brief period, those memories would dissipate or decay and not be stored in the long-term because the medial temporal lobe structures were removed.
The difference in short-term, procedural, and episodic memory, as evidenced by patient HM, suggests that there are different parts of the brain responsible for those functions. The long-term storage of episodic memory requires the hippocampus and related medial temporal structures, and the location of those memories is in the multimodal integration areas of the cerebral cortex. However, short-term memory—also called working or active memory—is localized to the prefrontal lobe. Because patient HM had only lost his medial temporal lobe—and lost very little of his previous memories, and did not lose the ability to form new short-term memories—it was concluded that the function of the hippocampus, and adjacent structures in the medial temporal lobe, is to move (or consolidate) short-term memories (in the pre-frontal lobe) to long-term memory (in the temporal lobe).
The prefrontal cortex can also be tested for the ability to organize information. In one subtest of the mental status exam called set generation, the patient is asked to generate a list of words that all start with the same letter, but not to include proper nouns or names. The expectation is that a person can generate such a list of at least 10 words within 1 minute. Many people can likely do this much more quickly, but the standard separates the accepted normal from those with compromised prefrontal cortices.
INTERACTIVE LINK
Read this article to learn about a young man who texts his fiancée in a panic as he finds that he is having trouble remembering things. At the hospital, a neurologist administers the mental status exam, which is mostly normal except for the three-word recall test. The young man could not recall them even 30 seconds after hearing them and repeating them back to the doctor. An undiscovered mass in the mediastinum region was found to be Hodgkin’s lymphoma, a type of cancer that affects the immune system and likely caused antibodies to attack the nervous system. The patient eventually regained his ability to remember, though the events in the hospital were always elusive. Considering that the effects on memory were temporary, but resulted in the loss of the specific events of the hospital stay, what regions of the brain were likely to have been affected by the antibodies and what type of memory does that represent?
Language and Speech
Language is, arguably, a very human aspect of neurological function. There are certainly strides being made in understanding communication in other species, but much of what makes the human experience seemingly unique is its basis in language. Any understanding of our species is necessarily reflective, as suggested by the question “What am I?” And the fundamental answer to this question is suggested by the famous quote by René Descartes: “Cogito Ergo Sum” (translated from Latin as “I think, therefore I am”). Formulating an understanding of yourself is largely describing who you are to yourself. It is a confusing topic to delve into, but language is certainly at the core of what it means to be self-aware.
The neurological exam has two specific subtests that address language. One measures the ability of the patient to understand language by asking them to follow a set of instructions to perform an action, such as “touch your right finger to your left elbow and then to your right knee.” Another subtest assesses the fluency and coherency of language by having the patient generate descriptions of objects or scenes depicted in drawings, and by reciting sentences or explaining a written passage. Language, however, is important in so many ways in the neurological exam. The patient needs to know what to do, whether it is as simple as explaining how the knee-jerk reflex is going to be performed, or asking a question such as “What is your name?” Often, language deficits can be determined without specific subtests; if a person cannot reply to a question properly, there may be a problem with the reception of language.
An important example of multimodal integrative areas is associated with language function (Figure 16.6). Adjacent to the auditory association cortex, at the end of the lateral sulcus just anterior to the visual cortex, is Wernicke’s area. In the lateral aspect of the frontal lobe, just anterior to the region of the motor cortex associated with the head and neck, is Broca’s area. Both regions were originally described on the basis of losses of speech and language, which is called aphasia. The aphasia associated with Broca’s area is known as an expressive aphasia, which means that speech production is compromised. This type of aphasia is often described as non-fluency because the ability to say some words leads to broken or halting speech. Grammar can also appear to be lost. The aphasia associated with Wernicke’s area is known as a receptive aphasia, which is not a loss of speech production, but a loss of understanding of content. Patients, after recovering from acute forms of this aphasia, report not being able to understand what is said to them or what they are saying themselves, but they often cannot keep from talking.
The two regions are connected by white matter tracts that run between the posterior temporal lobe and the lateral aspect of the frontal lobe. Conduction aphasia associated with damage to this connection refers to the problem of connecting the understanding of language to the production of speech. This is a very rare condition, but is likely to present as an inability to faithfully repeat spoken language.
Figure 16.6 Broca's and Wernicke's Areas Two important integration areas of the cerebral cortex associated with language function are Broca’s and Wernicke’s areas. The two areas are connected through the deep white matter running from the posterior temporal lobe to the frontal lobe.
Sensorium
Those parts of the brain involved in the reception and interpretation of sensory stimuli are referred to collectively as the sensorium. The cerebral cortex has several regions that are necessary for sensory perception. From the primary cortical areas of the somatosensory, visual, auditory, and gustatory senses to the association areas that process information in these modalities, the cerebral cortex is the seat of conscious sensory perception. In contrast, sensory information can also be processed by deeper brain regions, which we may vaguely describe as subconscious—for instance, we are not constantly aware of the proprioceptive information that the cerebellum uses to maintain balance. Several of the subtests can reveal activity associated with these sensory modalities, such as being able to hear a question or see a picture. Two subtests assess specific functions of these cortical areas.
The first is praxis, a practical exercise in which the patient performs a task completely on the basis of verbal description without any demonstration from the examiner. For example, the patient can be told to take their left hand and place it palm down on their left thigh, then flip it over so the palm is facing up, and then repeat this four times. The examiner describes the activity without any movements on their part to suggest how the movements are to be performed. The patient needs to understand the instructions, transform them into movements, and use sensory feedback, both visual and proprioceptive, to perform the movements correctly.
The second subtest for sensory perception is gnosis, which involves two tasks. The first task, known as stereognosis, involves the naming of objects strictly on the basis of the somatosensory information that comes from manipulating them. The patient keeps their eyes closed and is given a common object, such as a coin, that they have to identify. The patient should be able to indicate the particular type of coin, such as a dime versus a penny, or a nickel versus a quarter, on the basis of the sensory cues involved. For example, the size, thickness, or weight of the coin may be an indication, or to differentiate the pairs of coins suggested here, the smooth or corrugated edge of the coin will correspond to the particular denomination. The second task, graphesthesia, is to recognize numbers or letters written on the palm of the hand with a dull pointer, such as a pen cap.
Praxis and gnosis are related to the conscious perception and cortical processing of sensory information. Being able to transform verbal commands into a sequence of motor responses, or to manipulate and recognize a common object and associate it with a name for that object. Both subtests have language components because language function is integral to these functions. The relationship between the words that describe actions, or the nouns that represent objects, and the cerebral location of these concepts is suggested to be localized to particular cortical areas. Certain aphasias can be characterized by a deficit of verbs or nouns, known as V impairment or N impairment, or may be classified as V–N dissociation. Patients have difficulty using one type of word over the other. To describe what is happening in a photograph as part of the expressive language subtest, a patient will use active- or image-based language. The lack of one or the other of these components of language can relate to the ability to use verbs or nouns. Damage to the region at which the frontal and temporal lobes meet, including the region known as the insula, is associated with V impairment; damage to the middle and inferior temporal lobe is associated with N impairment.
Judgment and Abstract Reasoning
Planning and producing responses requires an ability to make sense of the world around us. Making judgments and reasoning in the abstract are necessary to produce movements as part of larger responses. For example, when your alarm goes off, do you hit the snooze button or jump out of bed? Is 10 extra minutes in bed worth the extra rush to get ready for your day? Will hitting the snooze button multiple times lead to feeling more rested or result in a panic as you run late? How you mentally process these questions can affect your whole day.
The prefrontal cortex is responsible for the functions responsible for planning and making decisions. In the mental status exam, the subtest that assesses judgment and reasoning is directed at three aspects of frontal lobe function. First, the examiner asks questions about problem solving, such as “If you see a house on fire, what would you do?” The patient is also asked to interpret common proverbs, such as “Don’t look a gift horse in the mouth.” Additionally, pairs of words are compared for similarities, such as apple and orange, or lamp and cabinet.
The prefrontal cortex is composed of the regions of the frontal lobe that are not directly related to specific motor functions. The most posterior region of the frontal lobe, the precentral gyrus, is the primary motor cortex. Anterior to that are the premotor cortex, Broca’s area, and the frontal eye fields, which are all related to planning certain types of movements. Anterior to what could be described as motor association areas are the regions of the prefrontal cortex. They are the regions in which judgment, abstract reasoning, and working memory are localized. The antecedents to planning certain movements are judging whether those movements should be made, as in the example of deciding whether to hit the snooze button.
To an extent, the prefrontal cortex may be related to personality. The neurological exam does not necessarily assess personality, but it can be within the realm of neurology or psychiatry. A clinical situation that suggests this link between the prefrontal cortex and personality comes from the story of Phineas Gage, the railroad worker from the mid-1800s who had a metal spike impale his prefrontal cortex. There are suggestions that the steel rod led to changes in his personality. A man who was a quiet, dependable railroad worker became a raucous, irritable drunkard. Later anecdotal evidence from his life suggests that he was able to support himself, although he had to relocate and take on a different career as a stagecoach driver.
A psychiatric practice to deal with various disorders was the prefrontal lobotomy. This procedure was common in the 1940s and early 1950s, until antipsychotic drugs became available. The connections between the prefrontal cortex and other regions of the brain were severed. The disorders associated with this procedure included some aspects of what are now referred to as personality disorders, but also included mood disorders and psychoses. Depictions of lobotomies in popular media suggest a link between cutting the white matter of the prefrontal cortex and changes in a patient’s mood and personality, though this correlation is not well understood.
EVERYDAY CONNECTION
Left Brain, Right Brain
Popular media often refer to right-brained and left-brained people, as if the brain were two independent halves that work differently for different people. This is a popular misinterpretation of an important neurological phenomenon. As an extreme measure to deal with a debilitating condition, the corpus callosum may be sectioned to overcome intractable epilepsy. When the connections between the two cerebral hemispheres are cut, interesting effects can be observed.
If a person with an intact corpus callosum is asked to put their hands in their pockets and describe what is there on the basis of what their hands feel, they might say that they have keys in their right pocket and loose change in the left. They may even be able to count the coins in their pocket and say if they can afford to buy a candy bar from the vending machine. If a person with a sectioned corpus callosum is given the same instructions, they will do something quite peculiar. They will only put their right hand in their pocket and say they have keys there. They will not even move their left hand, much less report that there is loose change in the left pocket.
The reason for this is that the language functions of the cerebral cortex are localized to the left hemisphere in 95 percent of the population. Additionally, the left hemisphere is connected to the right side of the body through the corticospinal tract and the ascending tracts of the spinal cord. Motor commands from the precentral gyrus control the opposite side of the body, whereas sensory information processed by the postcentral gyrus is received from the opposite side of the body. For a verbal command to initiate movement of the right arm and hand, the left side of the brain needs to be connected by the corpus callosum. Language is processed in the left side of the brain and directly influences the left brain and right arm motor functions, but is sent to influence the right brain and left arm motor functions through the corpus callosum. Likewise, the left-handed sensory perception of what is in the left pocket travels across the corpus callosum from the right brain, so no verbal report on those contents would be possible if the hand happened to be in the pocket.
INTERACTIVE LINK
Watch the video titled “The Man With Two Brains” to see the neuroscientist Michael Gazzaniga introduce a patient he has worked with for years who has had his corpus callosum cut, separating his two cerebral hemispheres. A few tests are run to demonstrate how this manifests in tests of cerebral function. Unlike normal people, this patient can perform two independent tasks at the same time because the lines of communication between the right and left sides of his brain have been removed. Whereas a person with an intact corpus callosum cannot overcome the dominance of one hemisphere over the other, this patient can. If the left cerebral hemisphere is dominant in the majority of people, why would right-handedness be most common?
The Mental Status Exam
The cerebrum, particularly the cerebral cortex, is the location of important cognitive functions that are the focus of the mental status exam. The regionalization of the cortex, initially described on the basis of anatomical evidence of cytoarchitecture, reveals the distribution of functionally distinct areas. Cortical regions can be described as primary sensory or motor areas, association areas, or multimodal integration areas. The functions attributed to these regions include attention, memory, language, speech, sensation, judgment, and abstract reasoning.
The mental status exam addresses these cognitive abilities through a series of subtests designed to elicit particular behaviors ascribed to these functions. The loss of neurological function can illustrate the location of damage to the cerebrum. Memory functions are attributed to the temporal lobe, particularly the medial temporal lobe structures known as the hippocampus and amygdala, along with the adjacent cortex. Evidence of the importance of these structures comes from the side effects of a bilateral temporal lobectomy that were studied in detail in patient HM.
Losses of language and speech functions, known as aphasias, are associated with damage to the important integration areas in the left hemisphere known as Broca’s or Wernicke’s areas, as well as the connections in the white matter between them. Different types of aphasia are named for the particular structures that are damaged. Assessment of the functions of the sensorium includes praxis and gnosis. The subtests related to these functions depend on multimodal integration, as well as language-dependent processing.
The prefrontal cortex contains structures important for planning, judgment, reasoning, and working memory. Damage to these areas can result in changes to personality, mood, and behavior. The famous case of Phineas Gage suggests a role for this cortex in personality, as does the outdated practice of prefrontal lobectomy.
The Cranial Nerve Exam
- Describe the functional grouping of cranial nerves
- Match the regions of the forebrain and brain stem that are connected to each cranial nerve
- Suggest diagnoses that would explain certain losses of function in the cranial nerves
- Relate cranial nerve deficits to damage of adjacent, unrelated structures
The twelve cranial nerves are typically covered in introductory anatomy courses, and memorizing their names is facilitated by numerous mnemonics developed by students over the years of this practice. But knowing the names of the nerves in order often leaves much to be desired in understanding what the nerves do. The nerves can be categorized by functions, and subtests of the cranial nerve exam can clarify these functional groupings.
Three of the nerves are strictly responsible for special senses whereas four others contain fibers for special and general senses. Three nerves are connected to the extraocular muscles resulting in the control of gaze. Four nerves connect to muscles of the face, oral cavity, and pharynx, controlling facial expressions, mastication, swallowing, and speech. Four nerves make up the cranial component of the parasympathetic nervous system responsible for pupillary constriction, salivation, and the regulation of the organs of the thoracic and upper abdominal cavities. Finally, one nerve controls the muscles of the neck, assisting with spinal control of the movement of the head and neck.
The cranial nerve exam allows directed tests of forebrain and brain stem structures. The twelve cranial nerves serve the head and neck. The vagus nerve (cranial nerve X) has autonomic functions in the thoracic and superior abdominal cavities. The special senses are served through the cranial nerves, as well as the general senses of the head and neck. The movement of the eyes, face, tongue, throat, and neck are all under the control of cranial nerves. Preganglionic parasympathetic nerve fibers that control pupillary size, salivary glands, and the thoracic and upper abdominal viscera are found in four of the nerves. Tests of these functions can provide insight into damage to specific regions of the brain stem and may uncover deficits in adjacent regions.
Sensory Nerves
The olfactory, optic, and vestibulocochlear nerves (cranial nerves I, II, and VIII) are dedicated to four of the special senses: smell, vision, equilibrium, and hearing, respectively. Taste sensation is relayed to the brain stem through fibers of the facial and glossopharyngeal nerves. The trigeminal nerve is a mixed nerve that carries the general somatic senses from the head, similar to those coming through spinal nerves from the rest of the body.
Testing smell is straightforward, as common smells are presented to one nostril at a time. The patient should be able to recognize the smell of coffee or mint, indicating the proper functioning of the olfactory system. Loss of the sense of smell is called anosmia and can be lost following blunt trauma to the head or through aging. The short axons of the first cranial nerve regenerate on a regular basis. The neurons in the olfactory epithelium have a limited life span, and new cells grow to replace the ones that die off. The axons from these neurons grow back into the CNS by following the existing axons—representing one of the few examples of such growth in the mature nervous system. If all of the fibers are sheared when the brain moves within the cranium, such as in a motor vehicle accident, then no axons can find their way back to the olfactory bulb to re-establish connections. If the nerve is not completely severed, the anosmia may be temporary as new neurons can eventually reconnect.
Olfaction is not the pre-eminent sense, but its loss can be quite detrimental. The enjoyment of food is largely based on our sense of smell. Anosmia means that food will not seem to have the same taste, though the gustatory sense is intact, and food will often be described as being bland. However, the taste of food can be improved by adding ingredients (e.g., salt) that stimulate the gustatory sense.
Testing vision relies on the tests that are common in an optometry office. The Snellen chart (Figure 16.7) demonstrates visual acuity by presenting standard Roman letters in a variety of sizes. The result of this test is a rough generalization of the acuity of a person based on the normal accepted acuity, such that a letter that subtends a visual angle of 5 minutes of an arc at 20 feet can be seen. To have 20/60 vision, for example, means that the smallest letters that a person can see at a 20-foot distance could be seen by a person with normal acuity from 60 feet away. Testing the extent of the visual field means that the examiner can establish the boundaries of peripheral vision as simply as holding their hands out to either side and asking the patient when the fingers are no longer visible without moving the eyes to track them. If it is necessary, further tests can establish the perceptions in the visual fields. Physical inspection of the optic disk, or where the optic nerve emerges from the eye, can be accomplished by looking through the pupil with an ophthalmoscope.
Figure 16.7 The Snellen Chart The Snellen chart for visual acuity presents a limited number of Roman letters in lines of decreasing size. The line with letters that subtend 5 minutes of an arc from 20 feet represents the smallest letters that a person with normal acuity should be able to read at that distance. The different sizes of letters in the other lines represent rough approximations of what a person of normal acuity can read at different distances. For example, the line that represents 20/200 vision would have larger letters so that they are legible to the person with normal acuity at 200 feet.
The optic nerves from both sides enter the cranium through the respective optic canals and meet at the optic chiasm at which fibers sort such that the two halves of the visual field are processed by the opposite sides of the brain. Deficits in visual field perception often suggest damage along the length of the optic pathway between the orbit and the diencephalon. For example, loss of peripheral vision may be the result of a pituitary tumor pressing on the optic chiasm (Figure 16.8). The pituitary, seated in the sella turcica of the sphenoid bone, is directly inferior to the optic chiasm. The axons that decussate in the chiasm are from the medial retinae of either eye, and therefore carry information from the peripheral visual field.
Figure 16.8 Pituitary Tumor The pituitary gland is located in the sella turcica of the sphenoid bone within the cranial floor, placing it immediately inferior to the optic chiasm. If the pituitary gland develops a tumor, it can press against the fibers crossing in the chiasm. Those fibers are conveying peripheral visual information to the opposite side of the brain, so the patient will experience “tunnel vision”—meaning that only the central visual field will be perceived.
The vestibulocochlear nerve (CN VIII) carries both equilibrium and auditory sensations from the inner ear to the medulla. Though the two senses are not directly related, anatomy is mirrored in the two systems. Problems with balance, such as vertigo, and deficits in hearing may both point to problems with the inner ear. Within the petrous region of the temporal bone is the bony labyrinth of the inner ear. The vestibule is the portion for equilibrium, composed of the utricle, saccule, and the three semicircular canals. The cochlea is responsible for transducing sound waves into a neural signal. The sensory nerves from these two structures travel side-by-side as the vestibulocochlear nerve, though they are really separate divisions. They both emerge from the inner ear, pass through the internal auditory meatus, and synapse in nuclei of the superior medulla. Though they are part of distinct sensory systems, the vestibular nuclei and the cochlear nuclei are close neighbors with adjacent inputs. Deficits in one or both systems could occur from damage that encompasses structures close to both. Damage to structures near the two nuclei can result in deficits to one or both systems.
Balance or hearing deficits may be the result of damage to the middle or inner ear structures. Ménière's disease is a disorder that can affect both equilibrium and audition in a variety of ways. The patient can suffer from vertigo, a low-frequency ringing in the ears, or a loss of hearing. From patient to patient, the exact presentation of the disease can be different. Additionally, within a single patient, the symptoms and signs may change as the disease progresses. Use of the neurological exam subtests for the vestibulocochlear nerve illuminates the changes a patient may go through. The disease appears to be the result of accumulation, or over-production, of fluid in the inner ear, in either the vestibule or cochlea.
Tests of equilibrium are important for coordination and gait and are related to other aspects of the neurological exam. The vestibulo-ocular reflex involves the cranial nerves for gaze control. Balance and equilibrium, as tested by the Romberg test, are part of spinal and cerebellar processes and involved in those components of the neurological exam, as discussed later.
Hearing is tested by using a tuning fork in a couple of different ways. The Rinne test involves using a tuning fork to distinguish between conductive hearing and sensorineural hearing. Conductive hearing relies on vibrations being conducted through the ossicles of the middle ear. Sensorineural hearing is the transmission of sound stimuli through the neural components of the inner ear and cranial nerve. A vibrating tuning fork is placed on the mastoid process and the patient indicates when the sound produced from this is no longer present. Then the fork is immediately moved to just next to the ear canal so the sound travels through the air. If the sound is not heard through the ear, meaning the sound is conducted better through the temporal bone than through the ossicles, a conductive hearing deficit is present. The Weber test also uses a tuning fork to differentiate between conductive versus sensorineural hearing loss. In this test, the tuning fork is placed at the top of the skull, and the sound of the tuning fork reaches both inner ears by travelling through bone. In a healthy patient, the sound would appear equally loud in both ears. With unilateral conductive hearing loss, however, the tuning fork sounds louder in the ear with hearing loss. This is because the sound of the tuning fork has to compete with background noise coming from the outer ear, but in conductive hearing loss, the background noise is blocked in the damaged ear, allowing the tuning fork to sound relatively louder in that ear. With unilateral sensorineural hearing loss, however, damage to the cochlea or associated nervous tissue means that the tuning fork sounds quieter in that ear.
The trigeminal system of the head and neck is the equivalent of the ascending spinal cord systems of the dorsal column and the spinothalamic pathways. Somatosensation of the face is conveyed along the nerve to enter the brain stem at the level of the pons. Synapses of those axons, however, are distributed across nuclei found throughout the brain stem. The mesencephalic nucleus processes proprioceptive information of the face, which is the movement and position of facial muscles. It is the sensory component of the jaw-jerk reflex, a stretch reflex of the masseter muscle. The chief nucleus, located in the pons, receives information about light touch as well as proprioceptive information about the mandible, which are both relayed to the thalamus and, ultimately, to the postcentral gyrus of the parietal lobe. The spinal trigeminal nucleus, located in the medulla, receives information about crude touch, pain, and temperature to be relayed to the thalamus and cortex. Essentially, the projection through the chief nucleus is analogous to the dorsal column pathway for the body, and the projection through the spinal trigeminal nucleus is analogous to the spinothalamic pathway.
Subtests for the sensory component of the trigeminal system are the same as those for the sensory exam targeting the spinal nerves. The primary sensory subtest for the trigeminal system is sensory discrimination. A cotton-tipped applicator, which is cotton attached to the end of a thin wooden stick, can be used easily for this. The wood of the applicator can be snapped so that a pointed end is opposite the soft cotton-tipped end. The cotton end provides a touch stimulus, while the pointed end provides a painful, or sharp, stimulus. While the patient’s eyes are closed, the examiner touches the two ends of the applicator to the patient’s face, alternating randomly between them. The patient must identify whether the stimulus is sharp or dull. These stimuli are processed by the trigeminal system separately. Contact with the cotton tip of the applicator is a light touch, relayed by the chief nucleus, but contact with the pointed end of the applicator is a painful stimulus relayed by the spinal trigeminal nucleus. Failure to discriminate these stimuli can localize problems within the brain stem. If a patient cannot recognize a painful stimulus, that might indicate damage to the spinal trigeminal nucleus in the medulla. The medulla also contains important regions that regulate the cardiovascular, respiratory, and digestive systems, as well as being the pathway for ascending and descending tracts between the brain and spinal cord. Damage, such as a stroke, that results in changes in sensory discrimination may indicate these unrelated regions are affected as well.
Gaze Control
The three nerves that control the extraocular muscles are the oculomotor, trochlear, and abducens nerves, which are the third, fourth, and sixth cranial nerves. As the name suggests, the abducens nerve is responsible for abducting the eye, which it controls through contraction of the lateral rectus muscle. The trochlear nerve controls the superior oblique muscle to rotate the eye along its axis in the orbit medially, which is called intorsion, and is a component of focusing the eyes on an object close to the face. The oculomotor nerve controls all the other extraocular muscles, as well as a muscle of the upper eyelid. Movements of the two eyes need to be coordinated to locate and track visual stimuli accurately. When moving the eyes to locate an object in the horizontal plane, or to track movement horizontally in the visual field, the lateral rectus muscle of one eye and medial rectus muscle of the other eye are both active. The lateral rectus is controlled by neurons of the abducens nucleus in the superior medulla, whereas the medial rectus is controlled by neurons in the oculomotor nucleus of the midbrain.
Coordinated movement of both eyes through different nuclei requires integrated processing through the brain stem. In the midbrain, the superior colliculus integrates visual stimuli with motor responses to initiate eye movements. The paramedian pontine reticular formation (PPRF) will initiate a rapid eye movement, or saccade, to bring the eyes to bear on a visual stimulus quickly. These areas are connected to the oculomotor, trochlear, and abducens nuclei by the medial longitudinal fasciculus (MLF) that runs through the majority of the brain stem. The MLF allows for conjugate gaze, or the movement of the eyes in the same direction, during horizontal movements that require the lateral and medial rectus muscles. Control of conjugate gaze strictly in the vertical direction is contained within the oculomotor complex. To elevate the eyes, the oculomotor nerve on either side stimulates the contraction of both superior rectus muscles; to depress the eyes, the oculomotor nerve on either side stimulates the contraction of both inferior rectus muscles.
Purely vertical movements of the eyes are not very common. Movements are often at an angle, so some horizontal components are necessary, adding the medial and lateral rectus muscles to the movement. The rapid movement of the eyes used to locate and direct the fovea onto visual stimuli is called a saccade. Notice that the paths that are traced in Figure 16.9 are not strictly vertical. The movements between the nose and the mouth are closest, but still have a slant to them. Also, the superior and inferior rectus muscles are not perfectly oriented with the line of sight. The origin for both muscles is medial to their insertions, so elevation and depression may require the lateral rectus muscles to compensate for the slight adduction inherent in the contraction of those muscles, requiring MLF activity as well.
Figure 16.9 Saccadic Eye Movements Saccades are rapid, conjugate movements of the eyes to survey a complicated visual stimulus, or to follow a moving visual stimulus. This image represents the shifts in gaze typical of a person studying a face. Notice the concentration of gaze on the major features of the face and the large number of paths traced between the eyes or around the mouth.
Testing eye movement is simply a matter of having the patient track the tip of a pen as it is passed through the visual field. This may appear similar to testing visual field deficits related to the optic nerve, but the difference is that the patient is asked to not move the eyes while the examiner moves a stimulus into the peripheral visual field. Here, the extent of movement is the point of the test. The examiner is watching for conjugate movements representing proper function of the related nuclei and the MLF. Failure of one eye to abduct while the other adducts in a horizontal movement is referred to as internuclear ophthalmoplegia. When this occurs, the patient will experience diplopia, or double vision, as the two eyes are temporarily pointed at different stimuli. Diplopia is not restricted to failure of the lateral rectus, because any of the extraocular muscles may fail to move one eye in perfect conjugation with the other.
The final aspect of testing eye movements is to move the tip of the pen in toward the patient’s face. As visual stimuli move closer to the face, the two medial recti muscles cause the eyes to move in the one nonconjugate movement that is part of gaze control. When the two eyes move to look at something closer to the face, they both adduct, which is referred to as convergence. To keep the stimulus in focus, the eye also needs to change the shape of the lens, which is controlled through the parasympathetic fibers of the oculomotor nerve. The change in focal power of the eye is referred to as accommodation. Accommodation ability changes with age; focusing on nearer objects, such as the written text of a book or on a computer screen, may require corrective lenses later in life. Coordination of the skeletal muscles for convergence and coordination of the smooth muscles of the ciliary body for accommodation are referred to as the accommodation–convergence reflex.
A crucial function of the cranial nerves is to keep visual stimuli centered on the fovea of the retina. The vestibulo-ocular reflex (VOR) coordinates all of the components (Figure 16.10), both sensory and motor, that make this possible. If the head rotates in one direction—for example, to the right—the horizontal pair of semicircular canals in the inner ear indicate the movement by increased activity on the right and decreased activity on the left. The information is sent to the abducens nuclei and oculomotor nuclei on either side to coordinate the lateral and medial rectus muscles. The left lateral rectus and right medial rectus muscles will contract, rotating the eyes in the opposite direction of the head, while nuclei controlling the right lateral rectus and left medial rectus muscles will be inhibited to reduce antagonism of the contracting muscles. These actions stabilize the visual field by compensating for the head rotation with opposite rotation of the eyes in the orbits. Deficits in the VOR may be related to vestibular damage, such as in Ménière’s disease, or from dorsal brain stem damage that would affect the eye movement nuclei or their connections through the MLF.
Figure 16.10 Vestibulo-ocular Reflex If the head is turned in one direction, the coordination of that movement with the fixation of the eyes on a visual stimulus involves a circuit that ties the vestibular sense with the eye movement nuclei through the MLF.
Nerves of the Face and Oral Cavity
An iconic part of a doctor’s visit is the inspection of the oral cavity and pharynx, suggested by the directive to “open your mouth and say ‘ah.’” This is followed by inspection, with the aid of a tongue depressor, of the back of the mouth, or the opening of the oral cavity into the pharynx known as the fauces. Whereas this portion of a medical exam inspects for signs of infection, such as in tonsillitis, it is also the means to test the functions of the cranial nerves that are associated with the oral cavity.
The facial and glossopharyngeal nerves convey gustatory stimulation to the brain. Testing this is as simple as introducing salty, sour, bitter, or sweet stimuli to either side of the tongue. The patient should respond to the taste stimulus before retracting the tongue into the mouth. Stimuli applied to specific locations on the tongue will dissolve into the saliva and may stimulate taste buds connected to either the left or right of the nerves, masking any lateral deficits. Along with taste, the glossopharyngeal nerve relays general sensations from the pharyngeal walls. These sensations, along with certain taste stimuli, can stimulate the gag reflex. If the examiner moves the tongue depressor to contact the lateral wall of the fauces, this should elicit the gag reflex. Stimulation of either side of the fauces should elicit an equivalent response. The motor response, through contraction of the muscles of the pharynx, is mediated through the vagus nerve. Normally, the vagus nerve is considered autonomic in nature. The vagus nerve directly stimulates the contraction of skeletal muscles in the pharynx and larynx to contribute to the swallowing and speech functions. Further testing of vagus motor function has the patient repeating consonant sounds that require movement of the muscles around the fauces. The patient is asked to say “lah-kah-pah” or a similar set of alternating sounds while the examiner observes the movements of the soft palate and arches between the palate and tongue.
The facial and glossopharyngeal nerves are also responsible for the initiation of salivation. Neurons in the salivary nuclei of the medulla project through these two nerves as preganglionic fibers, and synapse in ganglia located in the head. The parasympathetic fibers of the facial nerve synapse in the pterygopalatine ganglion, which projects to the submandibular gland and sublingual gland. The parasympathetic fibers of the glossopharyngeal nerve synapse in the otic ganglion, which projects to the parotid gland. Salivation in response to food in the oral cavity is based on a visceral reflex arc within the facial or glossopharyngeal nerves. Other stimuli that stimulate salivation are coordinated through the hypothalamus, such as the smell and sight of food.
The hypoglossal nerve is the motor nerve that controls the muscles of the tongue, except for the palatoglossus muscle, which is controlled by the vagus nerve. There are two sets of muscles of the tongue. The extrinsic muscles of the tongue are connected to other structures, whereas the intrinsic muscles of the tongue are completely contained within the lingual tissues. While examining the oral cavity, movement of the tongue will indicate whether hypoglossal function is impaired. The test for hypoglossal function is the “stick out your tongue” part of the exam. The genioglossus muscle is responsible for protrusion of the tongue. If the hypoglossal nerves on both sides are working properly, then the tongue will stick straight out. If the nerve on one side has a deficit, the tongue will stick out to that side—pointing to the side with damage. Loss of function of the tongue can interfere with speech and swallowing. Additionally, because the location of the hypoglossal nerve and nucleus is near the cardiovascular center, inspiratory and expiratory areas for respiration, and the vagus nuclei that regulate digestive functions, a tongue that protrudes incorrectly can suggest damage in adjacent structures that have nothing to do with controlling the tongue.
INTERACTIVE LINK
Watch this short video to see an examination of the facial nerve using some simple tests. The facial nerve controls the muscles of facial expression. Severe deficits will be obvious in watching someone use those muscles for normal control. One side of the face might not move like the other side. But directed tests, especially for contraction against resistance, require a formal testing of the muscles. The muscles of the upper and lower face need to be tested. The strength test in this video involves the patient squeezing her eyes shut and the examiner trying to pry her eyes open. Why does the examiner ask her to try a second time?
Motor Nerves of the Neck
The accessory nerve, also referred to as the spinal accessory nerve, innervates the sternocleidomastoid and trapezius muscles (Figure 16.11). When both the sternocleidomastoids contract, the head flexes forward; individually, they cause rotation to the opposite side. The trapezius can act as an antagonist, causing extension and hyperextension of the neck. These two superficial muscles are important for changing the position of the head. Both muscles also receive input from cervical spinal nerves. Along with the spinal accessory nerve, these nerves contribute to elevating the scapula and clavicle through the trapezius, which is tested by asking the patient to shrug both shoulders, and watching for asymmetry. For the sternocleidomastoid, those spinal nerves are primarily sensory projections, whereas the trapezius also has lateral insertions to the clavicle and scapula, and receives motor input from the spinal cord. Calling the nerve the spinal accessory nerve suggests that it is aiding the spinal nerves. Though that is not precisely how the name originated, it does help make the association between the function of this nerve in controlling these muscles and the role these muscles play in movements of the trunk or shoulders.
Figure 16.11 Muscles Controlled by the Accessory Nerve The accessory nerve innervates the sternocleidomastoid and trapezius muscles, both of which attach to the head and to the trunk and shoulders. They can act as antagonists in head flexion and extension, and as synergists in lateral flexion toward the shoulder.
To test these muscles, the patient is asked to flex and extend the neck or shrug the shoulders against resistance, testing the strength of the muscles. Lateral flexion of the neck toward the shoulder tests both at the same time. Any difference on one side versus the other would suggest damage on the weaker side. These strength tests are common for the skeletal muscles controlled by spinal nerves and are a significant component of the motor exam. Deficits associated with the accessory nerve may have an effect on orienting the head, as described with the VOR.
HOMEOSTATIC IMBALANCES
The Pupillary Light Response
The autonomic control of pupillary size in response to a bright light involves the sensory input of the optic nerve and the parasympathetic motor output of the oculomotor nerve. When light hits the retina, specialized photosensitive ganglion cells send a signal along the optic nerve to the pretectal nucleus in the superior midbrain. A neuron from this nucleus projects to the Eddinger–Westphal nuclei in the oculomotor complex in both sides of the midbrain. Neurons in this nucleus give rise to the preganglionic parasympathetic fibers that project through the oculomotor nerve to the ciliary ganglion in the posterior orbit. The postganglionic parasympathetic fibers from the ganglion project to the iris, where they release acetylcholine onto circular fibers that constrict the pupil to reduce the amount of light hitting the retina. The sympathetic nervous system is responsible for dilating the pupil when light levels are low.
Shining light in one eye will elicit constriction of both pupils. The efferent limb of the pupillary light reflex is bilateral. Light shined in one eye causes a constriction of that pupil, as well as constriction of the contralateral pupil. Shining a penlight in the eye of a patient is a very artificial situation, as both eyes are normally exposed to the same light sources. Testing this reflex can illustrate whether the optic nerve or the oculomotor nerve is damaged. If shining the light in one eye results in no changes in pupillary size but shining light in the opposite eye elicits a normal, bilateral response, the damage is associated with the optic nerve on the nonresponsive side. If light in either eye elicits a response in only one eye, the problem is with the oculomotor system.
If light in the right eye only causes the left pupil to constrict, the direct reflex is lost and the consensual reflex is intact, which means that the right oculomotor nerve (or Eddinger–Westphal nucleus) is damaged. Damage to the right oculomotor connections will be evident when light is shined in the left eye. In that case, the direct reflex is intact but the consensual reflex is lost, meaning that the left pupil will constrict while the right does not.
The Cranial Nerve Exam
The cranial nerves can be separated into four major groups associated with the subtests of the cranial nerve exam. First are the sensory nerves, then the nerves that control eye movement, the nerves of the oral cavity and superior pharynx, and the nerve that controls movements of the neck.
The olfactory, optic, and vestibulocochlear nerves are strictly sensory nerves for smell, sight, and balance and hearing, whereas the trigeminal, facial, and glossopharyngeal nerves carry somatosensation of the face, and taste—separated between the anterior two-thirds of the tongue and the posterior one-third. Special senses are tested by presenting the particular stimuli to each receptive organ. General senses can be tested through sensory discrimination of touch versus painful stimuli.
The oculomotor, trochlear, and abducens nerves control the extraocular muscles and are connected by the medial longitudinal fasciculus to coordinate gaze. Testing conjugate gaze is as simple as having the patient follow a visual target, like a pen tip, through the visual field ending with an approach toward the face to test convergence and accommodation. Along with the vestibular functions of the eighth nerve, the vestibulo-ocular reflex stabilizes gaze during head movements by coordinating equilibrium sensations with the eye movement systems.
The trigeminal nerve controls the muscles of chewing, which are tested for stretch reflexes. Motor functions of the facial nerve are usually obvious if facial expressions are compromised, but can be tested by having the patient raise their eyebrows, smile, and frown. Movements of the tongue, soft palate, or superior pharynx can be observed directly while the patient swallows, while the gag reflex is elicited, or while the patient says repetitive consonant sounds. The motor control of the gag reflex is largely controlled by fibers in the vagus nerve and constitutes a test of that nerve because the parasympathetic functions of that nerve are involved in visceral regulation, such as regulating the heartbeat and digestion.
Movement of the head and neck using the sternocleidomastoid and trapezius muscles is controlled by the accessory nerve. Flexing of the neck and strength testing of those muscles reviews the function of that nerve.
The Sensory and Motor Exams
- Describe the arrangement of sensory and motor regions in the spinal cord
- Relate damage in the spinal cord to sensory or motor deficits
- Differentiate between upper motor neuron and lower motor neuron diseases
- Describe the clinical indications of common reflexes
Connections between the body and the CNS occur through the spinal cord. The cranial nerves connect the head and neck directly to the brain, but the spinal cord receives sensory input and sends motor commands out to the body through the spinal nerves. Whereas the brain develops into a complex series of nuclei and fiber tracts, the spinal cord remains relatively simple in its configuration (Figure 16.12). From the initial neural tube early in embryonic development, the spinal cord retains a tube-like structure with gray matter surrounding the small central canal and white matter on the surface in three columns. The dorsal, or posterior, horns of the gray matter are mainly devoted to sensory functions whereas the ventral, or anterior, and lateral horns are associated with motor functions. In the white matter, the dorsal column relays sensory information to the brain, and the anterior column is almost exclusively relaying motor commands to the ventral horn motor neurons. The lateral column, however, conveys both sensory and motor information between the spinal cord and brain.
Figure 16.12 Locations of Spinal Fiber Tracts
Sensory Modalities and Location
The general senses are distributed throughout the body, relying on nervous tissue incorporated into various organs. Somatic senses are incorporated mostly into the skin, muscles, or tendons, whereas the visceral senses come from nervous tissue incorporated into the majority of organs such as the heart or stomach. The somatic senses are those that usually make up the conscious perception of the how the body interacts with the environment. The visceral senses are most often below the limit of conscious perception because they are involved in homeostatic regulation through the autonomic nervous system.
The sensory exam tests the somatic senses, meaning those that are consciously perceived. Testing of the senses begins with examining the regions known as dermatomes that connect to the cortical region where somatosensation is perceived in the postcentral gyrus. To test the sensory fields, a simple stimulus of the light touch of the soft end of a cotton-tipped applicator is applied at various locations on the skin. The spinal nerves, which contain sensory fibers with dendritic endings in the skin, connect with the skin in a topographically organized manner, illustrated as dermatomes (Figure 16.13). For example, the fibers of eighth cervical nerve innervate the medial surface of the forearm and extend out to the fingers. In addition to testing perception at different positions on the skin, it is necessary to test sensory perception within the dermatome from distal to proximal locations in the appendages, or lateral to medial locations in the trunk. In testing the eighth cervical nerve, the patient would be asked if the touch of the cotton to the fingers or the medial forearm was perceptible, and whether there were any differences in the sensations.
Figure 16.13 Dermatomes The surface of the skin can be divided into topographic regions that relate to the location of sensory endings in the skin based on the spinal nerve that contains those fibers. (credit: modification of work by Mikael Häggström)
Other modalities of somatosensation can be tested using a few simple tools. The perception of pain can be tested using the broken end of the cotton-tipped applicator. The perception of vibratory stimuli can be testing using an oscillating tuning fork placed against prominent bone features such as the distal head of the ulna on the medial aspect of the elbow. When the tuning fork is still, the metal against the skin can be perceived as a cold stimulus. Using the cotton tip of the applicator, or even just a fingertip, the perception of tactile movement can be assessed as the stimulus is drawn across the skin for approximately 2–3 cm. The patient would be asked in what direction the stimulus is moving. All of these tests are repeated in distal and proximal locations and for different dermatomes to assess the spatial specificity of perception. The sense of position and motion, proprioception, is tested by moving the fingers or toes and asking the patient if they sense the movement. If the distal locations are not perceived, the test is repeated at increasingly proximal joints.
The various stimuli used to test sensory input assess the function of the major ascending tracts of the spinal cord. The dorsal column pathway conveys fine touch, vibration, and proprioceptive information, whereas the spinothalamic pathway primarily conveys pain and temperature. Testing these stimuli provides information about whether these two major ascending pathways are functioning properly. Within the spinal cord, the two systems are segregated. The dorsal column information ascends ipsilateral to the source of the stimulus and decussates in the medulla, whereas the spinothalamic pathway decussates at the level of entry and ascends contralaterally. The differing sensory stimuli are segregated in the spinal cord so that the various subtests for these stimuli can distinguish which ascending pathway may be damaged in certain situations.
Whereas the basic sensory stimuli are assessed in the subtests directed at each submodality of somatosensation, testing the ability to discriminate sensations is important. Pairing the light touch and pain subtests together makes it possible to compare the two submodalities at the same time, and therefore the two major ascending tracts at the same time. Mistaking painful stimuli for light touch, or vice versa, may point to errors in ascending projections, such as in a hemisection of the spinal cord that might come from a motor vehicle accident.
Another issue of sensory discrimination is not distinguishing between different submodalities, but rather location. The two-point discrimination subtest highlights the density of sensory endings, and therefore receptive fields in the skin. The sensitivity to fine touch, which can give indications of the texture and detailed shape of objects, is highest in the fingertips. To assess the limit of this sensitivity, two-point discrimination is measured by simultaneously touching the skin in two locations, such as could be accomplished with a pair of forceps. Specialized calipers for precisely measuring the distance between points are also available. The patient is asked to indicate whether one or two stimuli are present while keeping their eyes closed. The examiner will switch between using the two points and a single point as the stimulus. Failure to recognize two points may be an indication of a dorsal column pathway deficit.
Similar to two-point discrimination, but assessing laterality of perception, is double simultaneous stimulation. Two stimuli, such as the cotton tips of two applicators, are touched to the same position on both sides of the body. If one side is not perceived, this may indicate damage to the contralateral posterior parietal lobe. Because there is one of each pathway on either side of the spinal cord, they are not likely to interact. If none of the other subtests suggest particular deficits with the pathways, the deficit is likely to be in the cortex where conscious perception is based. The mental status exam contains subtests that assess other functions that are primarily localized to the parietal cortex, such as stereognosis and graphesthesia.
A final subtest of sensory perception that concentrates on the sense of proprioception is known as the Romberg test. The patient is asked to stand straight with feet together. Once the patient has achieved their balance in that position, they are asked to close their eyes. Without visual feedback that the body is in a vertical orientation relative to the surrounding environment, the patient must rely on the proprioceptive stimuli of joint and muscle position, as well as information from the inner ear, to maintain balance. This test can indicate deficits in dorsal column pathway proprioception, as well as problems with proprioceptive projections to the cerebellum through the spinocerebellar tract.
INTERACTIVE LINK
Watch this video to see a quick demonstration of two-point discrimination. Touching a specialized caliper to the surface of the skin will measure the distance between two points that are perceived as distinct stimuli versus a single stimulus. The patient keeps their eyes closed while the examiner switches between using both points of the caliper or just one. The patient then must indicate whether one or two stimuli are in contact with the skin. Why is the distance between the caliper points closer on the fingertips as opposed to the palm of the hand? And what do you think the distance would be on the arm, or the shoulder?
Muscle Strength and Voluntary Movement
The skeletomotor system is largely based on the simple, two-cell projection from the precentral gyrus of the frontal lobe to the skeletal muscles. The corticospinal tract represents the neurons that send output from the primary motor cortex. These fibers travel through the deep white matter of the cerebrum, then through the midbrain and pons, into the medulla where most of them decussate, and finally through the spinal cord white matter in the lateral (crossed fibers) or anterior (uncrossed fibers) columns. These fibers synapse on motor neurons in the ventral horn. The ventral horn motor neurons then project to skeletal muscle and cause contraction. These two cells are termed the upper motor neuron (UMN) and the lower motor neuron (LMN). Voluntary movements require these two cells to be active.
The motor exam tests the function of these neurons and the muscles they control. First, the muscles are inspected and palpated for signs of structural irregularities. Movement disorders may be the result of changes to the muscle tissue, such as scarring, and these possibilities need to be ruled out before testing function. Along with this inspection, muscle tone is assessed by moving the muscles through a passive range of motion. The arm is moved at the elbow and wrist, and the leg is moved at the knee and ankle. Skeletal muscle should have a resting tension representing a slight contraction of the fibers. The lack of muscle tone, known as hypotonicity or flaccidity, may indicate that the LMN is not conducting action potentials that will keep a basal level of acetylcholine in the neuromuscular junction.
If muscle tone is present, muscle strength is tested by having the patient contract muscles against resistance. The examiner will ask the patient to lift the arm, for example, while the examiner is pushing down on it. This is done for both limbs, including shrugging the shoulders. Lateral differences in strength—being able to push against resistance with the right arm but not the left—would indicate a deficit in one corticospinal tract versus the other. An overall loss of strength, without laterality, could indicate a global problem with the motor system. Diseases that result in UMN lesions include cerebral palsy or MS, or it may be the result of a stroke. A sign of UMN lesion is a negative result in the subtest for pronator drift. The patient is asked to extend both arms in front of the body with the palms facing up. While keeping the eyes closed, if the patient unconsciously allows one or the other arm to slowly relax, toward the pronated position, this could indicate a failure of the motor system to maintain the supinated position.
Reflexes
Reflexes combine the spinal sensory and motor components with a sensory input that directly generates a motor response. The reflexes that are tested in the neurological exam are classified into two groups. A deep tendon reflex is commonly known as a stretch reflex, and is elicited by a strong tap to a tendon, such as in the knee-jerk reflex. A superficial reflex is elicited through gentle stimulation of the skin and causes contraction of the associated muscles.
For the arm, the common reflexes to test are of the biceps, brachioradialis, triceps, and flexors for the digits. For the leg, the knee-jerk reflex of the quadriceps is common, as is the ankle reflex for the gastrocnemius and soleus. The tendon at the insertion for each of these muscles is struck with a rubber mallet. The muscle is quickly stretched, resulting in activation of the muscle spindle that sends a signal into the spinal cord through the dorsal root. The fiber synapses directly on the ventral horn motor neuron that activates the muscle, causing contraction. The reflexes are physiologically useful for stability. If a muscle is stretched, it reflexively contracts to return the muscle to compensate for the change in length. In the context of the neurological exam, reflexes indicate that the LMN is functioning properly.
The most common superficial reflex in the neurological exam is the plantar reflex that tests for the Babinski sign on the basis of the extension or flexion of the toes at the plantar surface of the foot. The plantar reflex is commonly tested in newborn infants to establish the presence of neuromuscular function. To elicit this reflex, an examiner brushes a stimulus, usually the examiner’s fingertip, along the plantar surface of the infant’s foot. An infant would present a positive Babinski sign, meaning the foot dorsiflexes and the toes extend and splay out. As a person learns to walk, the plantar reflex changes to cause curling of the toes and a moderate plantar flexion. If superficial stimulation of the sole of the foot caused extension of the foot, keeping one’s balance would be harder. The descending input of the corticospinal tract modifies the response of the plantar reflex, meaning that a negative Babinski sign is the expected response in testing the reflex. Other superficial reflexes are not commonly tested, though a series of abdominal reflexes can target function in the lower thoracic spinal segments.
INTERACTIVE LINK
Watch this video to see how to test reflexes in the abdomen. Testing reflexes of the trunk is not commonly performed in the neurological exam, but if findings suggest a problem with the thoracic segments of the spinal cord, a series of superficial reflexes of the abdomen can localize function to those segments. If contraction is not observed when the skin lateral to the umbilicus (belly button) is stimulated, what level of the spinal cord may be damaged?
Comparison of Upper and Lower Motor Neuron Damage
Many of the tests of motor function can indicate differences that will address whether damage to the motor system is in the upper or lower motor neurons. Signs that suggest a UMN lesion include muscle weakness, strong deep tendon reflexes, decreased control of movement or slowness, pronator drift, a positive Babinski sign, spasticity, and the clasp-knife response. Spasticity is an excess contraction in resistance to stretch. It can result in hyperflexia, which is when joints are overly flexed. The clasp-knife response occurs when the patient initially resists movement, but then releases, and the joint will quickly flex like a pocket knife closing.
A lesion on the LMN would result in paralysis, or at least partial loss of voluntary muscle control, which is known as paresis. The paralysis observed in LMN diseases is referred to as flaccid paralysis, referring to a complete or partial loss of muscle tone, in contrast to the loss of control in UMN lesions in which tone is retained and spasticity is exhibited. Other signs of an LMN lesion are fibrillation, fasciculation, and compromised or lost reflexes resulting from the denervation of the muscle fibers.
DISORDERS OF THE...
Spinal Cord
In certain situations, such as a motorcycle accident, only half of the spinal cord may be damaged in what is known as a hemisection. Forceful trauma to the trunk may cause ribs or vertebrae to fracture, and debris can crush or section through part of the spinal cord. The full section of a spinal cord would result in paraplegia, or loss of voluntary motor control of the lower body, as well as loss of sensations from that point down. A hemisection, however, will leave spinal cord tracts intact on one side. The resulting condition would be hemiplegia on the side of the trauma—one leg would be paralyzed. The sensory results are more complicated.
The ascending tracts in the spinal cord are segregated between the dorsal column and spinothalamic pathways. This means that the sensory deficits will be based on the particular sensory information each pathway conveys. Sensory discrimination between touch and painful stimuli will illustrate the difference in how these pathways divide these functions.
On the paralyzed leg, a patient will acknowledge painful stimuli, but not fine touch or proprioceptive sensations. On the functional leg, the opposite is true. The reason for this is that the dorsal column pathway ascends ipsilateral to the sensation, so it would be damaged the same way as the lateral corticospinal tract. The spinothalamic pathway decussates immediately upon entering the spinal cord and ascends contralateral to the source; it would therefore bypass the hemisection.
The motor system can indicate the loss of input to the ventral horn in the lumbar enlargement where motor neurons to the leg are found, but motor function in the trunk is less clear. The left and right anterior corticospinal tracts are directly adjacent to each other. The likelihood of trauma to the spinal cord resulting in a hemisection that affects one anterior column, but not the other, is very unlikely. Either the axial musculature will not be affected at all, or there will be bilateral losses in the trunk.
Sensory discrimination can pinpoint the level of damage in the spinal cord. Below the hemisection, pain stimuli will be perceived in the damaged side, but not fine touch. The opposite is true on the other side. The pain fibers on the side with motor function cross the midline in the spinal cord and ascend in the contralateral lateral column as far as the hemisection. The dorsal column will be intact ipsilateral to the source on the intact side and reach the brain for conscious perception. The trauma would be at the level just before sensory discrimination returns to normal, helping to pinpoint the trauma. Whereas imaging technology, like magnetic resonance imaging (MRI) or computed tomography (CT) scanning, could localize the injury as well, nothing more complicated than a cotton-tipped applicator can localize the damage. That may be all that is available on the scene when moving the victim requires crucial decisions be made.
The Coordination and Gait Exams
- Explain the relationship between the location of the cerebellum and its function in movement
- Chart the major divisions of the cerebellum
- List the major connections of the cerebellum
- Describe the relationship of the cerebellum to axial and appendicular musculature
- Explain the prevalent causes of cerebellar ataxia
The role of the cerebellum is a subject of debate. There is an obvious connection to motor function based on the clinical implications of cerebellar damage. There is also strong evidence of the cerebellar role in procedural memory. The two are not incompatible; in fact, procedural memory is motor memory, such as learning to ride a bicycle. Significant work has been performed to describe the connections within the cerebellum that result in learning. A model for this learning is classical conditioning, as shown by the famous dogs from the physiologist Ivan Pavlov’s work. This classical conditioning, which can be related to motor learning, fits with the neural connections of the cerebellum. The cerebellum is 10 percent of the mass of the brain and has varied functions that all point to a role in the motor system.
Location and Connections of the Cerebellum
The cerebellum is located in apposition to the dorsal surface of the brain stem, centered on the pons. The name of the pons is derived from its connection to the cerebellum. The word means “bridge” and refers to the thick bundle of myelinated axons that form a bulge on its ventral surface. Those fibers are axons that project from the gray matter of the pons into the contralateral cerebellar cortex. These fibers make up the middle cerebellar peduncle (MCP) and are the major physical connection of the cerebellum to the brain stem (Figure 16.14). Two other white matter bundles connect the cerebellum to the other regions of the brain stem. The superior cerebellar peduncle (SCP) is the connection of the cerebellum to the midbrain and forebrain. The inferior cerebellar peduncle (ICP) is the connection to the medulla.
Figure 16.14 Cerebellar Penduncles The connections to the cerebellum are the three cerebellar peduncles, which are close to each other. The ICP arises from the medulla—specifically from the inferior olive, which is visible as a bulge on the ventral surface of the brain stem. The MCP is the ventral surface of the pons. The SCP projects into the midbrain.
These connections can also be broadly described by their functions. The ICP conveys sensory input to the cerebellum, partially from the spinocerebellar tract, but also through fibers of the inferior olive. The MCP is part of the cortico-ponto-cerebellar pathway that connects the cerebral cortex with the cerebellum and preferentially targets the lateral regions of the cerebellum. It includes a copy of the motor commands sent from the precentral gyrus through the corticospinal tract, arising from collateral branches that synapse in the gray matter of the pons, along with input from other regions such as the visual cortex. The SCP is the major output of the cerebellum, divided between the red nucleus in the midbrain and the thalamus, which will return cerebellar processing to the motor cortex. These connections describe a circuit that compares motor commands and sensory feedback to generate a new output. These comparisons make it possible to coordinate movements. If the cerebral cortex sends a motor command to initiate walking, that command is copied by the pons and sent into the cerebellum through the MCP. Sensory feedback in the form of proprioception from the spinal cord, as well as vestibular sensations from the inner ear, enters through the ICP. If you take a step and begin to slip on the floor because it is wet, the output from the cerebellum—through the SCP—can correct for that and keep you balanced and moving. The red nucleus sends new motor commands to the spinal cord through the rubrospinal tract.
The cerebellum is divided into regions that are based on the particular functions and connections involved. The midline regions of the cerebellum, the vermis and flocculonodular lobe, are involved in comparing visual information, equilibrium, and proprioceptive feedback to maintain balance and coordinate movements such as walking, or gait, through the descending output of the red nucleus (Figure 16.15). The lateral hemispheres are primarily concerned with planning motor functions through frontal lobe inputs that are returned through the thalamic projections back to the premotor and motor cortices. Processing in the midline regions targets movements of the axial musculature, whereas the lateral regions target movements of the appendicular musculature. The vermis is referred to as the spinocerebellum because it primarily receives input from the dorsal columns and spinocerebellar pathways. The flocculonodular lobe is referred to as the vestibulocerebellum because of the vestibular projection into that region. Finally, the lateral cerebellum is referred to as the cerebrocerebellum, reflecting the significant input from the cerebral cortex through the cortico-ponto-cerebellar pathway.
Figure 16.15 Major Regions of the Cerebellum The cerebellum can be divided into two basic regions: the midline and the hemispheres. The midline is composed of the vermis and the flocculonodular lobe, and the hemispheres are the lateral regions.
Coordination and Alternating Movement
Testing for cerebellar function is the basis of the coordination exam. The subtests target appendicular musculature, controlling the limbs, and axial musculature for posture and gait. The assessment of cerebellar function will depend on the normal functioning of other systems addressed in previous sections of the neurological exam. Motor control from the cerebrum, as well as sensory input from somatic, visual, and vestibular senses, are important to cerebellar function.
The subtests that address appendicular musculature, and therefore the lateral regions of the cerebellum, begin with a check for tremor. The patient extends their arms in front of them and holds the position. The examiner watches for the presence of tremors that would not be present if the muscles are relaxed. By pushing down on the arms in this position, the examiner can check for the rebound response, which is when the arms are automatically brought back to the extended position. The extension of the arms is an ongoing motor process, and the tap or push on the arms presents a change in the proprioceptive feedback. The cerebellum compares the cerebral motor command with the proprioceptive feedback and adjusts the descending input to correct. The red nucleus would send an additional signal to the LMN for the arm to increase contraction momentarily to overcome the change and regain the original position.
The check reflex depends on cerebellar input to keep increased contraction from continuing after the removal of resistance. The patient flexes the elbow against resistance from the examiner to extend the elbow. When the examiner releases the arm, the patient should be able to stop the increased contraction and keep the arm from moving. A similar response would be seen if you try to pick up a coffee mug that you believe to be full but turns out to be empty. Without checking the contraction, the mug would be thrown from the overexertion of the muscles expecting to lift a heavier object.
Several subtests of the cerebellum assess the ability to alternate movements, or switch between muscle groups that may be antagonistic to each other. In the finger-to-nose test, the patient touches their finger to the examiner’s finger and then to their nose, and then back to the examiner’s finger, and back to the nose. The examiner moves the target finger to assess a range of movements. A similar test for the lower extremities has the patient touch their toe to a moving target, such as the examiner’s finger. Both of these tests involve flexion and extension around a joint—the elbow or the knee and the shoulder or hip—as well as movements of the wrist and ankle. The patient must switch between the opposing muscles, like the biceps and triceps brachii, to move their finger from the target to their nose. Coordinating these movements involves the motor cortex communicating with the cerebellum through the pons and feedback through the thalamus to plan the movements. Visual cortex information is also part of the processing that occurs in the cerebrocerebellum while it is involved in guiding movements of the finger or toe.
Rapid, alternating movements are tested for the upper and lower extremities. The patient is asked to touch each finger to their thumb, or to pat the palm of one hand on the back of the other, and then flip that hand over and alternate back-and-forth. To test similar function in the lower extremities, the patient touches their heel to their shin near the knee and slides it down toward the ankle, and then back again, repetitively. Rapid, alternating movements are part of speech as well. A patient is asked to repeat the nonsense consonants “lah-kah-pah” to alternate movements of the tongue, lips, and palate. All of these rapid alternations require planning from the cerebrocerebellum to coordinate movement commands that control the coordination.
Posture and Gait
Gait can either be considered a separate part of the neurological exam or a subtest of the coordination exam that addresses walking and balance. Testing posture and gait addresses functions of the spinocerebellum and the vestibulocerebellum because both are part of these activities. A subtest called station begins with the patient standing in a normal position to check for the placement of the feet and balance. The patient is asked to hop on one foot to assess the ability to maintain balance and posture during movement. Though the station subtest appears to be similar to the Romberg test, the difference is that the patient’s eyes are open during station. The Romberg test has the patient stand still with the eyes closed. Any changes in posture would be the result of proprioceptive deficits, and the patient is able to recover when they open their eyes.
Subtests of walking begin with having the patient walk normally for a distance away from the examiner, and then turn and return to the starting position. The examiner watches for abnormal placement of the feet and the movement of the arms relative to the movement. The patient is then asked to walk with a few different variations. Tandem gait is when the patient places the heel of one foot against the toe of the other foot and walks in a straight line in that manner. Walking only on the heels or only on the toes will test additional aspects of balance.
Ataxia
A movement disorder of the cerebellum is referred to as ataxia. It presents as a loss of coordination in voluntary movements. Ataxia can also refer to sensory deficits that cause balance problems, primarily in proprioception and equilibrium. When the problem is observed in movement, it is ascribed to cerebellar damage. Sensory and vestibular ataxia would likely also present with problems in gait and station.
Ataxia is often the result of exposure to exogenous substances, focal lesions, or a genetic disorder. Focal lesions include strokes affecting the cerebellar arteries, tumors that may impinge on the cerebellum, trauma to the back of the head and neck, or MS. Alcohol intoxication or drugs such as ketamine cause ataxia, but it is often reversible. Mercury in fish can cause ataxia as well. Hereditary conditions can lead to degeneration of the cerebellum or spinal cord, as well as malformation of the brain, or the abnormal accumulation of copper seen in Wilson’s disease.
INTERACTIVE LINK
Watch this short video to see a test for station. Station refers to the position a person adopts when they are standing still. The examiner would look for issues with balance, which coordinates proprioceptive, vestibular, and visual information in the cerebellum. To test the ability of a subject to maintain balance, asking them to stand or hop on one foot can be more demanding. The examiner may also push the subject to see if they can maintain balance. An abnormal finding in the test of station is if the feet are placed far apart. Why would a wide stance suggest problems with cerebellar function?
EVERYDAY CONNECTION
The Field Sobriety Test
The neurological exam has been described as a clinical tool throughout this chapter. It is also useful in other ways. A variation of the coordination exam is the Field Sobriety Test (FST) used to assess whether drivers are under the influence of alcohol. The cerebellum is crucial for coordinated movements such as keeping balance while walking, or moving appendicular musculature on the basis of proprioceptive feedback. The cerebellum is also very sensitive to ethanol, the particular type of alcohol found in beer, wine, and liquor.
Walking in a straight line involves comparing the motor command from the primary motor cortex to the proprioceptive and vestibular sensory feedback, as well as following the visual guide of the white line on the side of the road. When the cerebellum is compromised by alcohol, the cerebellum cannot coordinate these movements effectively, and maintaining balance becomes difficult.
Another common aspect of the FST is to have the driver extend their arms out wide and touch their fingertip to their nose, usually with their eyes closed. The point of this is to remove the visual feedback for the movement and force the driver to rely just on proprioceptive information about the movement and position of their fingertip relative to their nose. With eyes open, the corrections to the movement of the arm might be so small as to be hard to see, but proprioceptive feedback is not as immediate and broader movements of the arm will probably be needed, particularly if the cerebellum is affected by alcohol.
Reciting the alphabet backwards is not always a component of the FST, but its relationship to neurological function is interesting. There is a cognitive aspect to remembering how the alphabet goes and how to recite it backwards. That is actually a variation of the mental status subtest of repeating the months backwards. However, the cerebellum is important because speech production is a coordinated activity. The speech rapid alternating movement subtest is specifically using the consonant changes of “lah-kah-pah” to assess coordinated movements of the lips, tongue, pharynx, and palate. But the entire alphabet, especially in the nonrehearsed backwards order, pushes this type of coordinated movement quite far. It is related to the reason that speech becomes slurred when a person is intoxicated.
Key Terms
- accommodation
- in vision, a change in the ability of the eye to focus on objects at different distances
- accommodation–convergence reflex
- coordination of somatic control of the medial rectus muscles of either eye with the parasympathetic control of the ciliary bodies to maintain focus while the eyes converge on visual stimuli near to the face
- anterograde amnesia
- inability to form new memories from a particular time forward
- aphasia
- loss of language function
- ataxia
- movement disorder related to damage of the cerebellum characterized by loss of coordination in voluntary movements
- Babinski sign
- dorsiflexion of the foot with extension and splaying of the toes in response to the plantar reflex, normally suppressed by corticospinal input
- cerebrocerebellum
- lateral regions of the cerebellum; named for the significant input from the cerebral cortex
- check reflex
- response to a release in resistance so that the contractions stop, or check, movement
- clasp-knife response
- sign of UMN disease when a patient initially resists passive movement of a muscle but will quickly release to a lower state of resistance
- conduction aphasia
- loss of language function related to connecting the understanding of speech with the production of speech, without either specific function being lost
- conductive hearing
- hearing dependent on the conduction of vibrations of the tympanic membrane through the ossicles of the middle ear
- conjugate gaze
- coordinated movement of the two eyes simultaneously in the same direction
- convergence
- in vision, the movement of the eyes so that they are both pointed at the same point in space, which increases for stimuli that are closer to the subject
- coordination exam
- major section of the neurological exam that assesses complex, coordinated motor functions of the cerebellum and associated motor pathways
- cortico-ponto-cerebellar pathway
- projection from the cerebral cortex to the cerebellum by way of the gray matter of the pons
- cranial nerve exam
- major section of the neurological exam that assesses sensory and motor functions of the cranial nerves and their associated central and peripheral structures
- cytoarchitecture
- study of a tissue based on the structure and organization of its cellular components; related to the broader term, histology
- deep tendon reflex
- another term for stretch reflex, based on the elicitation through deep stimulation of the tendon at the insertion
- diplopia
- double vision resulting from a failure in conjugate gaze
- edema
- fluid accumulation in tissue; often associated with circulatory deficits
- embolus
- obstruction in a blood vessel such as a blood clot, fatty mass, air bubble, or other foreign matter that interrupts the flow of blood to an organ or some part of the body
- episodic memory
- memory of specific events in an autobiographical sense
- expressive aphasia
- loss of the ability to produce language; usually associated with damage to Broca’s area in the frontal lobe
- extrinsic muscles of the tongue
- muscles that are connected to other structures, such as the hyoid bone or the mandible, and control the position of the tongue
- fasciculation
- small muscle twitch as a result of spontaneous activity from an LMN
- fauces
- opening from the oral cavity into the pharynx
- fibrillation
- in motor responses, a spontaneous muscle action potential that occurs in the absence of neuromuscular input, resulting from LMN lesions
- flaccid paralysis
- loss of voluntary muscle control and muscle tone, as the result of LMN disease
- flaccidity
- presentation of a loss of muscle tone, observed as floppy limbs or a lack of resistance to passive movement
- flocculonodular lobe
- lobe of the cerebellum that receives input from the vestibular system to help with balance and posture
- gait
- rhythmic pattern of alternating movements of the lower limbs during locomotion
- gait exam
- major section of the neurological exam that assesses the cerebellum and descending pathways in the spinal cord through the coordinated motor functions of walking; a portion of the coordination exam
- gnosis
- in a neurological exam, intuitive experiential knowledge tested by interacting with common objects or symbols
- graphesthesia
- perception of symbols, such as letters or numbers, traced in the palm of the hand
- hemisection
- cut through half of a structure, such as the spinal cord
- hemorrhagic stroke
- disruption of blood flow to the brain caused by bleeding within the cranial vault
- hyperflexia
- overly flexed joints
- hypotonicity
- low muscle tone, a sign of LMN disease
- hypovolemia
- decrease in blood volume
- inferior cerebellar peduncle (ICP)
- input to the cerebellum, largely from the inferior olive, that represents sensory feedback from the periphery
- inferior olive
- large nucleus in the medulla that receives input from sensory systems and projects into the cerebellar cortex
- internuclear ophthalmoplegia
- deficit of conjugate lateral gaze because the lateral rectus muscle of one eye does not contract resulting from damage to the abducens nerve or the MLF
- intorsion
- medial rotation of the eye around its axis
- intrinsic muscles of the tongue
- muscles that originate out of, and insert into, other tissues within the tongue and control the shape of the tongue
- ischemic stroke
- disruption of blood flow to the brain because blood cannot flow through blood vessels as a result of a blockage or narrowing of the vessel
- jaw-jerk reflex
- stretch reflex of the masseter muscle
- localization of function
- principle that circumscribed anatomical locations are responsible for specific functions in an organ system
- medial longitudinal fasciculus (MLF)
- fiber pathway that connects structures involved in the control of eye and head position, from the superior colliculus to the vestibular nuclei and cerebellum
- mental status exam
- major section of the neurological exam that assesses cognitive functions of the cerebrum
- middle cerebellar peduncle (MCP)
- large, white-matter bridge from the pons that constitutes the major input to the cerebellar cortex
- motor exam
- major section of the neurological exam that assesses motor functions of the spinal cord and spinal nerves
- neurological exam
- clinical assessment tool that can be used to quickly evaluate neurological function and determine if specific parts of the nervous system have been affected by damage or disease
- paramedian pontine reticular formation (PPRF)
- region of the brain stem adjacent to the motor nuclei for gaze control that coordinates rapid, conjugate eye movements
- paresis
- partial loss of, or impaired, voluntary muscle control
- plantar reflex
- superficial reflex initiated by gentle stimulation of the sole of the foot
- praxis
- in a neurological exam, the act of doing something using ready knowledge or skills in response to verbal instruction
- procedural memory
- memory of how to perform a specific task
- pronator drift
- sign of contralateral corticospinal lesion when the one arm will drift into a pronated position when held straight out with the palms facing upward
- receptive aphasia
- loss of the ability to understand received language, such as what is spoken to the subject or given in written form
- red nucleus
- nucleus in the midbrain that receives output from the cerebellum and projects onto the spinal cord in the rubrospinal tract
- retrograde amnesia
- loss of memories before a particular event
- Rinne test
- use of a tuning fork to test conductive hearing loss versus sensorineural hearing loss
- Romberg test
- test of equilibrium that requires the patient to maintain a straight, upright posture without visual feedback of position
- rubrospinal tract
- descending tract from the red nucleus of the midbrain that results in modification of ongoing motor programs
- saccade
- small, rapid movement of the eyes used to locate and direct the fovea onto visual stimuli
- sensorineural hearing
- hearing dependent on the transduction and propagation of auditory information through the neural components of the peripheral auditory structures
- sensory exam
- major section of the neurological exam that assesses sensory functions of the spinal cord and spinal nerves
- short-term memory
- capacity to retain information actively in the brain for a brief period of time
- Snellen chart
- standardized arrangement of letters in decreasing size presented to a subject at a distance of 20 feet to test visual acuity
- spasticity
- increased contraction of a muscle in response to resistance, often resulting in hyperflexia
- spinocerebellar tract
- ascending fibers that carry proprioceptive input to the cerebellum used in maintaining balance and coordinated movement
- spinocerebellum
- midline region of the cerebellum known as the vermis that receives proprioceptive input from the spinal cord
- stereognosis
- perception of common objects placed in the hand solely on the basis of manipulation of that object in the hand
- stroke
- (also, cerebrovascular accident (CVA)) loss of neurological function caused by an interruption of blood flow to a region of the central nervous system
- superficial reflex
- reflexive contraction initiated by gentle stimulation of the skin
- superior cerebellar peduncle (SCP)
- white-matter tract representing output of the cerebellum to the red nucleus of the midbrain
- transient ischemic attack (TIA)
- temporary disruption of blood flow to the brain in which symptoms occur rapidly but last only a short time
- vermis
- prominent ridge along the midline of the cerebellum that is referred to as the spinocerebellum
- vestibulo-ocular reflex (VOR)
- reflex based on connections between the vestibular system and the cranial nerves of eye movements that ensures that images are stabilized on the retina as the head and body move
- vestibulocerebellum
- flocculonodular lobe of the cerebellum named for the vestibular input from the eighth cranial nerve
- Weber test
- use of a tuning fork to test the laterality of hearing loss by placing it at several locations on the midline of the skull
- Wernicke’s area
- region at the posterior end of the lateral sulcus in which speech comprehension is localized
Chapter Review
16.1 Overview of the Neurological Exam
The neurological exam is a clinical assessment tool to determine the extent of function from the nervous system. It is divided into five major sections that each deal with a specific region of the CNS. The mental status exam is concerned with the cerebrum and assesses higher functions such as memory, language, and emotion. The cranial nerve exam tests the functions of all of the cranial nerves and, therefore, their connections to the CNS through the forebrain and brain stem. The sensory and motor exams assess those functions as they relate to the spinal cord, as well as the combination of the functions in spinal reflexes. The coordination exam targets cerebellar function in coordinated movements, including those functions associated with gait.
Damage to and disease of the nervous system lead to loss of function. The location of the injury will correspond to the functional loss, as suggested by the principle of localization of function. The neurological exam provides the opportunity for a clinician to determine where damage has occurred on the basis of the function that is lost. Damage from acute injuries such as strokes may result in specific functions being lost, whereas broader effects in infection or developmental disorders may result in general losses across an entire section of the neurological exam.
16.4 The Sensory and Motor Exams
The sensory and motor exams assess function related to the spinal cord and the nerves connected to it. Sensory functions are associated with the dorsal regions of the spinal cord, whereas motor function is associated with the ventral side. Localizing damage to the spinal cord is related to assessments of the peripheral projections mapped to dermatomes.
Sensory tests address the various submodalities of the somatic senses: touch, temperature, vibration, pain, and proprioception. Results of the subtests can point to trauma in the spinal cord gray matter, white matter, or even in connections to the cerebral cortex.
Motor tests focus on the function of the muscles and the connections of the descending motor pathway. Muscle tone and strength are tested for upper and lower extremities. Input to the muscles comes from the descending cortical input of upper motor neurons and the direct innervation of lower motor neurons.
Reflexes can either be based on deep stimulation of tendons or superficial stimulation of the skin. The presence of reflexive contractions helps to differentiate motor disorders between the upper and lower motor neurons. The specific signs associated with motor disorders can establish the difference further, based on the type of paralysis, the state of muscle tone, and specific indicators such as pronator drift or the Babinski sign.
16.5 The Coordination and Gait Exams
The cerebellum is an important part of motor function in the nervous system. It apparently plays a role in procedural learning, which would include motor skills such as riding a bike or throwing a football. The basis for these roles is likely to be tied into the role the cerebellum plays as a comparator for voluntary movement.
The motor commands from the cerebral hemispheres travel along the corticospinal pathway, which passes through the pons. Collateral branches of these fibers synapse on neurons in the pons, which then project into the cerebellar cortex through the middle cerebellar peduncles. Ascending sensory feedback, entering through the inferior cerebellar peduncles, provides information about motor performance. The cerebellar cortex compares the command to the actual performance and can adjust the descending input to compensate for any mismatch. The output from deep cerebellar nuclei projects through the superior cerebellar peduncles to initiate descending signals from the red nucleus to the spinal cord.
The primary role of the cerebellum in relation to the spinal cord is through the spinocerebellum; it controls posture and gait with significant input from the vestibular system. Deficits in cerebellar function result in ataxias, or a specific kind of movement disorder. The root cause of the ataxia may be the sensory input—either the proprioceptive input from the spinal cord or the equilibrium input from the vestibular system, or direct damage to the cerebellum by stroke, trauma, hereditary factors, or toxins.
Interactive Link Questions
Watch this video that provides a demonstration of the neurological exam—a series of tests that can be performed rapidly when a patient is initially brought into an emergency department. The exam can be repeated on a regular basis to keep a record of how and if neurological function changes over time. In what order were the sections of the neurological exam tested in this video, and which section seemed to be left out?
2.Watch this video for an introduction to the neurological exam. Studying the neurological exam can give insight into how structure and function in the nervous system are interdependent. This is a tool both in the clinic and in the classroom, but for different reasons. In the clinic, this is a powerful but simple tool to assess a patient’s neurological function. In the classroom, it is a different way to think about the nervous system. Though medical technology provides noninvasive imaging and real-time functional data, the presenter says these cannot replace the history at the core of the medical examination. What does history mean in the context of medical practice?
3.Read this article to learn about a young man who texts his fiancée in a panic as he finds that he is having trouble remembering things. At the hospital, a neurologist administers the mental status exam, which is mostly normal except for the three-word recall test. The young man could not recall them even 30 seconds after hearing them and repeating them back to the doctor. An undiscovered mass in the mediastinum region was found to be Hodgkin’s lymphoma, a type of cancer that affects the immune system and likely caused antibodies to attack the nervous system. The patient eventually regained his ability to remember, though the events in the hospital were always elusive. Considering that the effects on memory were temporary, but resulted in the loss of the specific events of the hospital stay, what regions of the brain were likely to have been affected by the antibodies and what type of memory does that represent?
4.Watch the video titled “The Man With Two Brains” to see the neuroscientist Michael Gazzaniga introduce a patient he has worked with for years who has had his corpus callosum cut, separating his two cerebral hemispheres. A few tests are run to demonstrate how this manifests in tests of cerebral function. Unlike normal people, this patient can perform two independent tasks at the same time because the lines of communication between the right and left sides of his brain have been removed. Whereas a person with an intact corpus callosum cannot overcome the dominance of one hemisphere over the other, this patient can. If the left cerebral hemisphere is dominant in the majority of people, why would right-handedness be most common?
5.Watch this short video to see an examination of the facial nerve using some simple tests. The facial nerve controls the muscles of facial expression. Severe deficits will be obvious in watching someone use those muscles for normal control. One side of the face might not move like the other side. But directed tests, especially for contraction against resistance, require a formal testing of the muscles. The muscles of the upper and lower face need to be tested. The strength test in this video involves the patient squeezing her eyes shut and the examiner trying to pry her eyes open. Why does the examiner ask her to try a second time?
6.Watch this video to see a quick demonstration of two-point discrimination. Touching a specialized caliper to the surface of the skin will measure the distance between two points that are perceived as distinct stimuli versus a single stimulus. The patient keeps their eyes closed while the examiner switches between using both points of the caliper or just one. The patient then must indicate whether one or two stimuli are in contact with the skin. Why is the distance between the caliper points closer on the fingertips as opposed to the palm of the hand? And what do you think the distance would be on the arm, or the shoulder?
7.Watch this video to see how to test reflexes in the abdomen. Testing reflexes of the trunk is not commonly performed in the neurological exam, but if findings suggest a problem with the thoracic segments of the spinal cord, a series of superficial reflexes of the abdomen can localize function to those segments. If contraction is not observed when the skin lateral to the umbilicus (belly button) is stimulated, what level of the spinal cord may be damaged?
8.Watch this short video to see a test for station. Station refers to the position a person adopts when they are standing still. The examiner would look for issues with balance, which coordinates proprioceptive, vestibular, and visual information in the cerebellum. To test the ability of a subject to maintain balance, asking them to stand or hop on one foot can be more demanding. The examiner may also push the subject to see if they can maintain balance. An abnormal finding in the test of station is if the feet are placed far apart. Why would a wide stance suggest problems with cerebellar function?
Review Questions
Which major section of the neurological exam is most likely to reveal damage to the cerebellum?
- cranial nerve exam
- mental status exam
- sensory exam
- coordination exam
What function would most likely be affected by a restriction of a blood vessel in the cerebral cortex?
- language
- gait
- facial expressions
- knee-jerk reflex
Which major section of the neurological exam includes subtests that are sometimes considered a separate set of tests concerned with walking?
- mental status exam
- cranial nerve exam
- coordination exam
- sensory exam
Memory, emotional, language, and sensorimotor deficits together are most likely the result of what kind of damage?
- stroke
- developmental disorder
- whiplash
- gunshot wound
Where is language function localized in the majority of people?
- cerebellum
- right cerebral hemisphere
- hippocampus
- left cerebral hemisphere
Which of the following could be elements of cytoarchitecture, as related to Brodmann’s microscopic studies of the cerebral cortex?
- connections to the cerebellum
- activation by visual stimuli
- number of neurons per square millimeter
- number of gyri or sulci
Which of the following could be a multimodal integrative area?
- primary visual cortex
- premotor cortex
- hippocampus
- Wernicke’s area
Which is an example of episodic memory?
- how to bake a cake
- your last birthday party
- how old you are
- needing to wear an oven mitt to take a cake out of the oven
Which type of aphasia is more like hearing a foreign language spoken?
- receptive aphasia
- expressive aphasia
- conductive aphasia
- Broca’s aphasia
What region of the cerebral cortex is associated with understanding language, both from another person and the language a person generates himself or herself?
- medial temporal lobe
- ventromedial prefrontal cortex
- superior temporal gyrus
- postcentral gyrus
Without olfactory sensation to complement gustatory stimuli, food will taste bland unless it is seasoned with which substance?
- salt
- thyme
- garlic
- olive oil
Which of the following cranial nerves is not part of the VOR?
- optic
- oculomotor
- abducens
- vestibulocochlear
Which nerve is responsible for controlling the muscles that result in the gag reflex?
- trigeminal
- facial
- glossopharyngeal
- vagus
Which nerve is responsible for taste, as well as salivation, in the anterior oral cavity?
- facial
- glossopharyngeal
- vagus
- hypoglossal
Which of the following nerves controls movements of the neck?
- oculomotor
- vestibulocochlear
- spinal accessory
- hypoglossal
Which of the following is not part of the corticospinal pathway?
- cerebellar deep white matter
- midbrain
- medulla
- lateral column
Which subtest is directed at proprioceptive sensation?
- two-point discrimination
- tactile movement
- vibration
- Romberg test
What term describes the inability to lift the arm above the level of the shoulder?
- paralysis
- paresis
- fasciculation
- fibrillation
Which type of reflex is the jaw-jerk reflex that is part of the cranial nerve exam for the vestibulocochlear nerve?
- visceral reflex
- withdrawal reflex
- stretch reflex
- superficial reflex
Which of the following is a feature of both somatic and visceral senses?
- requires cerebral input
- causes skeletal muscle contraction
- projects to a ganglion near the target effector
- involves an axon in the ventral nerve root
Which white matter structure carries information from the cerebral cortex to the cerebellum?
- cerebral peduncle
- superior cerebellar peduncle
- middle cerebellar peduncle
- inferior cerebellar peduncle
Which region of the cerebellum receives proprioceptive input from the spinal cord?
- vermis
- left hemisphere
- flocculonodular lobe
- right hemisphere
Which of the following tests cerebellar function related to gait?
- toe-to-finger
- station
- lah-kah-pah
- finger-to-nose
Which of the following is not a cause of cerebellar ataxia?
- mercury from fish
- drinking alcohol
- antibiotics
- hereditary degeneration of the cerebellum
Which of the following functions cannot be attributed to the cerebellum?
- comparing motor commands and sensory feedback
- associating sensory stimuli with learned behavior
- coordinating complex movements
- processing visual information
Critical Thinking Questions
Why is a rapid assessment of neurological function important in an emergency situation?
35.How is the diagnostic category of TIA different from a stroke?
36.A patient’s performance of the majority of the mental status exam subtests is in line with the expected norms, but the patient cannot repeat a string of numbers given by the examiner. What is a likely explanation?
37.A patient responds to the question “What is your name?” with a look of incomprehension. Which of the two major language areas is most likely affected and what is the name for that type of aphasia?
38.As a person ages, their ability to focus on near objects (accommodation) changes. If a person is already myopic (near-sighted), why would corrective lenses not be necessary to read a book or computer screen?
39.When a patient flexes their neck, the head tips to the right side. Also, their tongue sticks out slightly to the left when they try to stick it straight out. Where is the damage to the brain stem most likely located?
40.The location of somatosensation is based on the topographical map of sensory innervation. What does this mean?
41.Why are upper motor neuron lesions characterized by “spastic paralysis”?
42.Learning to ride a bike is a motor function dependent on the cerebellum. Why are the different regions of the cerebellum involved in this complex motor learning?
43.Alcohol intoxication can produce slurred speech. How is this related to cerebellar function?
|
oercommons
|
2025-03-18T00:39:11.322278
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/56379/overview",
"title": "Anatomy and Physiology, Regulation, Integration, and Control",
"author": null
}
|
https://oercommons.org/courseware/lesson/56380/overview
|
The Endocrine System
Introduction
Figure 17.1 A Child Catches a Falling Leaf Hormones of the endocrine system coordinate and control growth, metabolism, temperature regulation, the stress response, reproduction, and many other functions. (credit: “seenthroughmylense”/flickr.com)
CHAPTER OBJECTIVES
After studying this chapter, you will be able to:
- Identify the contributions of the endocrine system to homeostasis
- Discuss the chemical composition of hormones and the mechanisms of hormone action
- Summarize the site of production, regulation, and effects of the hormones of the pituitary, thyroid, parathyroid, adrenal, and pineal glands
- Discuss the hormonal regulation of the reproductive system
- Explain the role of the pancreatic endocrine cells in the regulation of blood glucose
- Identify the hormones released by the heart, kidneys, and other organs with secondary endocrine functions
- Discuss several common diseases associated with endocrine system dysfunction
- Discuss the embryonic development of, and the effects of aging on, the endocrine system
You may never have thought of it this way, but when you send a text message to two friends to meet you at the dining hall at six, you’re sending digital signals that (you hope) will affect their behavior—even though they are some distance away. Similarly, certain cells send chemical signals to other cells in the body that influence their behavior. This long-distance intercellular communication, coordination, and control is critical for homeostasis, and it is the fundamental function of the endocrine system.
An Overview of the Endocrine System
- Distinguish the types of intercellular communication, their importance, mechanisms, and effects
- Identify the major organs and tissues of the endocrine system and their location in the body
Communication is a process in which a sender transmits signals to one or more receivers to control and coordinate actions. In the human body, two major organ systems participate in relatively “long distance” communication: the nervous system and the endocrine system. Together, these two systems are primarily responsible for maintaining homeostasis in the body.
Neural and Endocrine Signaling
The nervous system uses two types of intercellular communication—electrical and chemical signaling—either by the direct action of an electrical potential, or in the latter case, through the action of chemical neurotransmitters such as serotonin or norepinephrine. Neurotransmitters act locally and rapidly. When an electrical signal in the form of an action potential arrives at the synaptic terminal, they diffuse across the synaptic cleft (the gap between a sending neuron and a receiving neuron or muscle cell). Once the neurotransmitters interact (bind) with receptors on the receiving (post-synaptic) cell, the receptor stimulation is transduced into a response such as continued electrical signaling or modification of cellular response. The target cell responds within milliseconds of receiving the chemical “message”; this response then ceases very quickly once the neural signaling ends. In this way, neural communication enables body functions that involve quick, brief actions, such as movement, sensation, and cognition.In contrast, the endocrine system uses just one method of communication: chemical signaling. These signals are sent by the endocrine organs, which secrete chemicals—the hormone—into the extracellular fluid. Hormones are transported primarily via the bloodstream throughout the body, where they bind to receptors on target cells, inducing a characteristic response. As a result, endocrine signaling requires more time than neural signaling to prompt a response in target cells, though the precise amount of time varies with different hormones. For example, the hormones released when you are confronted with a dangerous or frightening situation, called the fight-or-flight response, occur by the release of adrenal hormones—epinephrine and norepinephrine—within seconds. In contrast, it may take up to 48 hours for target cells to respond to certain reproductive hormones.
INTERACTIVE LINK
Visit this link to watch an animation of the events that occur when a hormone binds to a cell membrane receptor. What is the secondary messenger made by adenylyl cyclase during the activation of liver cells by epinephrine?
In addition, endocrine signaling is typically less specific than neural signaling. The same hormone may play a role in a variety of different physiological processes depending on the target cells involved. For example, the hormone oxytocin promotes uterine contractions in women in labor. It is also important in breastfeeding, and may be involved in the sexual response and in feelings of emotional attachment in both males and females.
In general, the nervous system involves quick responses to rapid changes in the external environment, and the endocrine system is usually slower acting—taking care of the internal environment of the body, maintaining homeostasis, and controlling reproduction (Table 17.1). So how does the fight-or-flight response that was mentioned earlier happen so quickly if hormones are usually slower acting? It is because the two systems are connected. It is the fast action of the nervous system in response to the danger in the environment that stimulates the adrenal glands to secrete their hormones. As a result, the nervous system can cause rapid endocrine responses to keep up with sudden changes in both the external and internal environments when necessary.
Endocrine and Nervous Systems
| Endocrine system | Nervous system | |
|---|---|---|
| Signaling mechanism(s) | Chemical | Chemical/electrical |
| Primary chemical signal | Hormones | Neurotransmitters |
| Distance traveled | Long or short | Always short |
| Response time | Fast or slow | Always fast |
| Environment targeted | Internal | Internal and external |
Table 17.1
Structures of the Endocrine System
The endocrine system consists of cells, tissues, and organs that secrete hormones as a primary or secondary function. The endocrine gland is the major player in this system. The primary function of these ductless glands is to secrete their hormones directly into the surrounding fluid. The interstitial fluid and the blood vessels then transport the hormones throughout the body. The endocrine system includes the pituitary, thyroid, parathyroid, adrenal, and pineal glands (Figure 17.2). Some of these glands have both endocrine and non-endocrine functions. For example, the pancreas contains cells that function in digestion as well as cells that secrete the hormones insulin and glucagon, which regulate blood glucose levels. The hypothalamus, thymus, heart, kidneys, stomach, small intestine, liver, skin, female ovaries, and male testes are other organs that contain cells with endocrine function. Moreover, adipose tissue has long been known to produce hormones, and recent research has revealed that even bone tissue has endocrine functions.
Figure 17.2 Endocrine System Endocrine glands and cells are located throughout the body and play an important role in homeostasis.
The ductless endocrine glands are not to be confused with the body’s exocrine system, whose glands release their secretions through ducts. Examples of exocrine glands include the sebaceous and sweat glands of the skin. As just noted, the pancreas also has an exocrine function: most of its cells secrete pancreatic juice through the pancreatic and accessory ducts to the lumen of the small intestine.
Other Types of Chemical Signaling
In endocrine signaling, hormones secreted into the extracellular fluid diffuse into the blood or lymph, and can then travel great distances throughout the body. In contrast, autocrine signaling takes place within the same cell. An autocrine (auto- = “self”) is a chemical that elicits a response in the same cell that secreted it. Interleukin-1, or IL-1, is a signaling molecule that plays an important role in inflammatory response. The cells that secrete IL-1 have receptors on their cell surface that bind these molecules, resulting in autocrine signaling.
Local intercellular communication is the province of the paracrine, also called a paracrine factor, which is a chemical that induces a response in neighboring cells. Although paracrines may enter the bloodstream, their concentration is generally too low to elicit a response from distant tissues. A familiar example to those with asthma is histamine, a paracrine that is released by immune cells in the bronchial tree. Histamine causes the smooth muscle cells of the bronchi to constrict, narrowing the airways. Another example is the neurotransmitters of the nervous system, which act only locally within the synaptic cleft.
CAREER CONNECTION
Endocrinologist
Endocrinology is a specialty in the field of medicine that focuses on the treatment of endocrine system disorders. Endocrinologists—medical doctors who specialize in this field—are experts in treating diseases associated with hormonal systems, ranging from thyroid disease to diabetes mellitus. Endocrine surgeons treat endocrine disease through the removal, or resection, of the affected endocrine gland.
Patients who are referred to endocrinologists may have signs and symptoms or blood test results that suggest excessive or impaired functioning of an endocrine gland or endocrine cells. The endocrinologist may order additional blood tests to determine whether the patient’s hormonal levels are abnormal, or they may stimulate or suppress the function of the suspect endocrine gland and then have blood taken for analysis. Treatment varies according to the diagnosis. Some endocrine disorders, such as type 2 diabetes, may respond to lifestyle changes such as modest weight loss, adoption of a healthy diet, and regular physical activity. Other disorders may require medication, such as hormone replacement, and routine monitoring by the endocrinologist. These include disorders of the pituitary gland that can affect growth and disorders of the thyroid gland that can result in a variety of metabolic problems.
Some patients experience health problems as a result of the normal decline in hormones that can accompany aging. These patients can consult with an endocrinologist to weigh the risks and benefits of hormone replacement therapy intended to boost their natural levels of reproductive hormones.
In addition to treating patients, endocrinologists may be involved in research to improve the understanding of endocrine system disorders and develop new treatments for these diseases.
Hormones
- Identify the three major classes of hormones on the basis of chemical structure
- Compare and contrast intracellular and cell membrane hormone receptors
- Describe signaling pathways that involve cAMP and IP3
- Identify several factors that influence a target cell’s response
- Discuss the role of feedback loops and humoral, hormonal, and neural stimuli in hormone control
Although a given hormone may travel throughout the body in the bloodstream, it will affect the activity only of its target cells; that is, cells with receptors for that particular hormone. Once the hormone binds to the receptor, a chain of events is initiated that leads to the target cell’s response. Hormones play a critical role in the regulation of physiological processes because of the target cell responses they regulate. These responses contribute to human reproduction, growth and development of body tissues, metabolism, fluid, and electrolyte balance, sleep, and many other body functions. The major hormones of the human body and their effects are identified in Table 17.2.
Endocrine Glands and Their Major Hormones
| Endocrine gland | Associated hormones | Chemical class | Effect |
|---|---|---|---|
| Pituitary (anterior) | Growth hormone (GH) | Protein | Promotes growth of body tissues |
| Pituitary (anterior) | Prolactin (PRL) | Peptide | Promotes milk production |
| Pituitary (anterior) | Thyroid-stimulating hormone (TSH) | Glycoprotein | Stimulates thyroid hormone release |
| Pituitary (anterior) | Adrenocorticotropic hormone (ACTH) | Peptide | Stimulates hormone release by adrenal cortex |
| Pituitary (anterior) | Follicle-stimulating hormone (FSH) | Glycoprotein | Stimulates gamete production |
| Pituitary (anterior) | Luteinizing hormone (LH) | Glycoprotein | Stimulates androgen production by gonads |
| Pituitary (posterior) | Antidiuretic hormone (ADH) | Peptide | Stimulates water reabsorption by kidneys |
| Pituitary (posterior) | Oxytocin | Peptide | Stimulates uterine contractions during childbirth |
| Thyroid | Thyroxine (T4), triiodothyronine (T3) | Amine | Stimulate basal metabolic rate |
| Thyroid | Calcitonin | Peptide | Reduces blood Ca2+ levels |
| Parathyroid | Parathyroid hormone (PTH) | Peptide | Increases blood Ca2+ levels |
| Adrenal (cortex) | Aldosterone | Steroid | Increases blood Na+ levels |
| Adrenal (cortex) | Cortisol, corticosterone, cortisone | Steroid | Increase blood glucose levels |
| Adrenal (medulla) | Epinephrine, norepinephrine | Amine | Stimulate fight-or-flight response |
| Pineal | Melatonin | Amine | Regulates sleep cycles |
| Pancreas | Insulin | Protein | Reduces blood glucose levels |
| Pancreas | Glucagon | Protein | Increases blood glucose levels |
| Testes | Testosterone | Steroid | Stimulates development of male secondary sex characteristics and sperm production |
| Ovaries | Estrogens and progesterone | Steroid | Stimulate development of female secondary sex characteristics and prepare the body for childbirth |
Table 17.2
Types of Hormones
The hormones of the human body can be divided into two major groups on the basis of their chemical structure. Hormones derived from amino acids include amines, peptides, and proteins. Those derived from lipids include steroids (Figure 17.3). These chemical groups affect a hormone’s distribution, the type of receptors it binds to, and other aspects of its function.
Figure 17.3 Amine, Peptide, Protein, and Steroid Hormone Structure
Amine Hormones
Hormones derived from the modification of amino acids are referred to as amine hormones. Typically, the original structure of the amino acid is modified such that a –COOH, or carboxyl, group is removed, whereas the −NH+3−NH3+
Amine hormones are synthesized from the amino acids tryptophan or tyrosine. An example of a hormone derived from tryptophan is melatonin, which is secreted by the pineal gland and helps regulate circadian rhythm. Tyrosine derivatives include the metabolism-regulating thyroid hormones, as well as the catecholamines, such as epinephrine, norepinephrine, and dopamine. Epinephrine and norepinephrine are secreted by the adrenal medulla and play a role in the fight-or-flight response, whereas dopamine is secreted by the hypothalamus and inhibits the release of certain anterior pituitary hormones.
Peptide and Protein Hormones
Whereas the amine hormones are derived from a single amino acid, peptide and protein hormones consist of multiple amino acids that link to form an amino acid chain. Peptide hormones consist of short chains of amino acids, whereas protein hormones are longer polypeptides. Both types are synthesized like other body proteins: DNA is transcribed into mRNA, which is translated into an amino acid chain.
Examples of peptide hormones include antidiuretic hormone (ADH), a pituitary hormone important in fluid balance, and atrial-natriuretic peptide, which is produced by the heart and helps to decrease blood pressure. Some examples of protein hormones include growth hormone, which is produced by the pituitary gland, and follicle-stimulating hormone (FSH), which has an attached carbohydrate group and is thus classified as a glycoprotein. FSH helps stimulate the maturation of eggs in the ovaries and sperm in the testes.
Steroid Hormones
The primary hormones derived from lipids are steroids. Steroid hormones are derived from the lipid cholesterol. For example, the reproductive hormones testosterone and the estrogens—which are produced by the gonads (testes and ovaries)—are steroid hormones. The adrenal glands produce the steroid hormone aldosterone, which is involved in osmoregulation, and cortisol, which plays a role in metabolism.
Like cholesterol, steroid hormones are not soluble in water (they are hydrophobic). Because blood is water-based, lipid-derived hormones must travel to their target cell bound to a transport protein. This more complex structure extends the half-life of steroid hormones much longer than that of hormones derived from amino acids. A hormone’s half-life is the time required for half the concentration of the hormone to be degraded. For example, the lipid-derived hormone cortisol has a half-life of approximately 60 to 90 minutes. In contrast, the amino acid–derived hormone epinephrine has a half-life of approximately one minute.
Pathways of Hormone Action
The message a hormone sends is received by a hormone receptor, a protein located either inside the cell or within the cell membrane. The receptor will process the message by initiating other signaling events or cellular mechanisms that result in the target cell’s response. Hormone receptors recognize molecules with specific shapes and side groups, and respond only to those hormones that are recognized. The same type of receptor may be located on cells in different body tissues, and trigger somewhat different responses. Thus, the response triggered by a hormone depends not only on the hormone, but also on the target cell.
Once the target cell receives the hormone signal, it can respond in a variety of ways. The response may include the stimulation of protein synthesis, activation or deactivation of enzymes, alteration in the permeability of the cell membrane, altered rates of mitosis and cell growth, and stimulation of the secretion of products. Moreover, a single hormone may be capable of inducing different responses in a given cell.
Pathways Involving Intracellular Hormone Receptors
Intracellular hormone receptors are located inside the cell. Hormones that bind to this type of receptor must be able to cross the cell membrane. Steroid hormones are derived from cholesterol and therefore can readily diffuse through the lipid bilayer of the cell membrane to reach the intracellular receptor (Figure 17.4). Thyroid hormones, which contain benzene rings studded with iodine, are also lipid-soluble and can enter the cell.
The location of steroid and thyroid hormone binding differs slightly: a steroid hormone may bind to its receptor within the cytosol or within the nucleus. In either case, this binding generates a hormone-receptor complex that moves toward the chromatin in the cell nucleus and binds to a particular segment of the cell’s DNA. In contrast, thyroid hormones bind to receptors already bound to DNA. For both steroid and thyroid hormones, binding of the hormone-receptor complex with DNA triggers transcription of a target gene to mRNA, which moves to the cytosol and directs protein synthesis by ribosomes.
Figure 17.4 Binding of Lipid-Soluble Hormones A steroid hormone directly initiates the production of proteins within a target cell. Steroid hormones easily diffuse through the cell membrane. The hormone binds to its receptor in the cytosol, forming a receptor–hormone complex. The receptor–hormone complex then enters the nucleus and binds to the target gene on the DNA. Transcription of the gene creates a messenger RNA that is translated into the desired protein within the cytoplasm.
Pathways Involving Cell Membrane Hormone Receptors
Hydrophilic, or water-soluble, hormones are unable to diffuse through the lipid bilayer of the cell membrane and must therefore pass on their message to a receptor located at the surface of the cell. Except for thyroid hormones, which are lipid-soluble, all amino acid–derived hormones bind to cell membrane receptors that are located, at least in part, on the extracellular surface of the cell membrane. Therefore, they do not directly affect the transcription of target genes, but instead initiate a signaling cascade that is carried out by a molecule called a second messenger. In this case, the hormone is called a first messenger.
The second messenger used by most hormones is cyclic adenosine monophosphate (cAMP). In the cAMP second messenger system, a water-soluble hormone binds to its receptor in the cell membrane (Step 1 in Figure 17.5). This receptor is associated with an intracellular component called a G protein, and binding of the hormone activates the G-protein component (Step 2). The activated G protein in turn activates an enzyme called adenylyl cyclase, also known as adenylate cyclase (Step 3), which converts adenosine triphosphate (ATP) to cAMP (Step 4). As the second messenger, cAMP activates a type of enzyme called a protein kinase that is present in the cytosol (Step 5). Activated protein kinases initiate a phosphorylation cascade, in which multiple protein kinases phosphorylate (add a phosphate group to) numerous and various cellular proteins, including other enzymes (Step 6).
Figure 17.5 Binding of Water-Soluble Hormones Water-soluble hormones cannot diffuse through the cell membrane. These hormones must bind to a surface cell-membrane receptor. The receptor then initiates a cell-signaling pathway within the cell involving G proteins, adenylyl cyclase, the secondary messenger cyclic AMP (cAMP), and protein kinases. In the final step, these protein kinases phosphorylate proteins in the cytoplasm. This activates proteins in the cell that carry out the changes specified by the hormone.
The phosphorylation of cellular proteins can trigger a wide variety of effects, from nutrient metabolism to the synthesis of different hormones and other products. The effects vary according to the type of target cell, the G proteins and kinases involved, and the phosphorylation of proteins. Examples of hormones that use cAMP as a second messenger include calcitonin, which is important for bone construction and regulating blood calcium levels; glucagon, which plays a role in blood glucose levels; and thyroid-stimulating hormone, which causes the release of T3 and T4 from the thyroid gland.
Overall, the phosphorylation cascade significantly increases the efficiency, speed, and specificity of the hormonal response, as thousands of signaling events can be initiated simultaneously in response to a very low concentration of hormone in the bloodstream. However, the duration of the hormone signal is short, as cAMP is quickly deactivated by the enzyme phosphodiesterase (PDE), which is located in the cytosol. The action of PDE helps to ensure that a target cell’s response ceases quickly unless new hormones arrive at the cell membrane.
Importantly, there are also G proteins that decrease the levels of cAMP in the cell in response to hormone binding. For example, when growth hormone–inhibiting hormone (GHIH), also known as somatostatin, binds to its receptors in the pituitary gland, the level of cAMP decreases, thereby inhibiting the secretion of human growth hormone.
Not all water-soluble hormones initiate the cAMP second messenger system. One common alternative system uses calcium ions as a second messenger. In this system, G proteins activate the enzyme phospholipase C (PLC), which functions similarly to adenylyl cyclase. Once activated, PLC cleaves a membrane-bound phospholipid into two molecules: diacylglycerol (DAG) and inositol triphosphate (IP3). Like cAMP, DAG activates protein kinases that initiate a phosphorylation cascade. At the same time, IP3 causes calcium ions to be released from storage sites within the cytosol, such as from within the smooth endoplasmic reticulum. The calcium ions then act as second messengers in two ways: they can influence enzymatic and other cellular activities directly, or they can bind to calcium-binding proteins, the most common of which is calmodulin. Upon binding calcium, calmodulin is able to modulate protein kinase within the cell. Examples of hormones that use calcium ions as a second messenger system include angiotensin II, which helps regulate blood pressure through vasoconstriction, and growth hormone–releasing hormone (GHRH), which causes the pituitary gland to release growth hormones.
Factors Affecting Target Cell Response
You will recall that target cells must have receptors specific to a given hormone if that hormone is to trigger a response. But several other factors influence the target cell response. For example, the presence of a significant level of a hormone circulating in the bloodstream can cause its target cells to decrease their number of receptors for that hormone. This process is called downregulation, and it allows cells to become less reactive to the excessive hormone levels. When the level of a hormone is chronically reduced, target cells engage in upregulation to increase their number of receptors. This process allows cells to be more sensitive to the hormone that is present. Cells can also alter the sensitivity of the receptors themselves to various hormones.
Two or more hormones can interact to affect the response of cells in a variety of ways. The three most common types of interaction are as follows:
- The permissive effect, in which the presence of one hormone enables another hormone to act. For example, thyroid hormones have complex permissive relationships with certain reproductive hormones. A dietary deficiency of iodine, a component of thyroid hormones, can therefore affect reproductive system development and functioning.
- The synergistic effect, in which two hormones with similar effects produce an amplified response. In some cases, two hormones are required for an adequate response. For example, two different reproductive hormones—FSH from the pituitary gland and estrogens from the ovaries—are required for the maturation of female ova (egg cells).
- The antagonistic effect, in which two hormones have opposing effects. A familiar example is the effect of two pancreatic hormones, insulin and glucagon. Insulin increases the liver’s storage of glucose as glycogen, decreasing blood glucose, whereas glucagon stimulates the breakdown of glycogen stores, increasing blood glucose.
Regulation of Hormone Secretion
To prevent abnormal hormone levels and a potential disease state, hormone levels must be tightly controlled. The body maintains this control by balancing hormone production and degradation. Feedback loops govern the initiation and maintenance of most hormone secretion in response to various stimuli.
Role of Feedback Loops
The contribution of feedback loops to homeostasis will only be briefly reviewed here. Positive feedback loops are characterized by the release of additional hormone in response to an original hormone release. The release of oxytocin during childbirth is a positive feedback loop. The initial release of oxytocin begins to signal the uterine muscles to contract, which pushes the fetus toward the cervix, causing it to stretch. This, in turn, signals the pituitary gland to release more oxytocin, causing labor contractions to intensify. The release of oxytocin decreases after the birth of the child.
The more common method of hormone regulation is the negative feedback loop. Negative feedback is characterized by the inhibition of further secretion of a hormone in response to adequate levels of that hormone. This allows blood levels of the hormone to be regulated within a narrow range. An example of a negative feedback loop is the release of glucocorticoid hormones from the adrenal glands, as directed by the hypothalamus and pituitary gland. As glucocorticoid concentrations in the blood rise, the hypothalamus and pituitary gland reduce their signaling to the adrenal glands to prevent additional glucocorticoid secretion (Figure 17.6).
Figure 17.6 Negative Feedback Loop The release of adrenal glucocorticoids is stimulated by the release of hormones from the hypothalamus and pituitary gland. This signaling is inhibited when glucocorticoid levels become elevated by causing negative signals to the pituitary gland and hypothalamus.
Role of Endocrine Gland Stimuli
Reflexes triggered by both chemical and neural stimuli control endocrine activity. These reflexes may be simple, involving only one hormone response, or they may be more complex and involve many hormones, as is the case with the hypothalamic control of various anterior pituitary–controlled hormones.
Humoral stimuli are changes in blood levels of non-hormone chemicals, such as nutrients or ions, which cause the release or inhibition of a hormone to, in turn, maintain homeostasis. For example, osmoreceptors in the hypothalamus detect changes in blood osmolarity (the concentration of solutes in the blood plasma). If blood osmolarity is too high, meaning that the blood is not dilute enough, osmoreceptors signal the hypothalamus to release ADH. The hormone causes the kidneys to reabsorb more water and reduce the volume of urine produced. This reabsorption causes a reduction of the osmolarity of the blood, diluting the blood to the appropriate level. The regulation of blood glucose is another example. High blood glucose levels cause the release of insulin from the pancreas, which increases glucose uptake by cells and liver storage of glucose as glycogen.
An endocrine gland may also secrete a hormone in response to the presence of another hormone produced by a different endocrine gland. Such hormonal stimuli often involve the hypothalamus, which produces releasing and inhibiting hormones that control the secretion of a variety of pituitary hormones.
In addition to these chemical signals, hormones can also be released in response to neural stimuli. A common example of neural stimuli is the activation of the fight-or-flight response by the sympathetic nervous system. When an individual perceives danger, sympathetic neurons signal the adrenal glands to secrete norepinephrine and epinephrine. The two hormones dilate blood vessels, increase the heart and respiratory rate, and suppress the digestive and immune systems. These responses boost the body’s transport of oxygen to the brain and muscles, thereby improving the body’s ability to fight or flee.
EVERYDAY CONNECTION
Bisphenol A and Endocrine Disruption
You may have heard news reports about the effects of a chemical called bisphenol A (BPA) in various types of food packaging. BPA is used in the manufacturing of hard plastics and epoxy resins. Common food-related items that may contain BPA include the lining of aluminum cans, plastic food-storage containers, drinking cups, as well as baby bottles and “sippy” cups. Other uses of BPA include medical equipment, dental fillings, and the lining of water pipes.
Research suggests that BPA is an endocrine disruptor, meaning that it negatively interferes with the endocrine system, particularly during the prenatal and postnatal development period. In particular, BPA mimics the hormonal effects of estrogens and has the opposite effect—that of androgens. The U.S. Food and Drug Administration (FDA) notes in their statement about BPA safety that although traditional toxicology studies have supported the safety of low levels of exposure to BPA, recent studies using novel approaches to test for subtle effects have led to some concern about the potential effects of BPA on the brain, behavior, and prostate gland in fetuses, infants, and young children. The FDA is currently facilitating decreased use of BPA in food-related materials. Many US companies have voluntarily removed BPA from baby bottles, “sippy” cups, and the linings of infant formula cans, and most plastic reusable water bottles sold today boast that they are “BPA free.” In contrast, both Canada and the European Union have completely banned the use of BPA in baby products.
The potential harmful effects of BPA have been studied in both animal models and humans and include a large variety of health effects, such as developmental delay and disease. For example, prenatal exposure to BPA during the first trimester of human pregnancy may be associated with wheezing and aggressive behavior during childhood. Adults exposed to high levels of BPA may experience altered thyroid signaling and male sexual dysfunction. BPA exposure during the prenatal or postnatal period of development in animal models has been observed to cause neurological delays, changes in brain structure and function, sexual dysfunction, asthma, and increased risk for multiple cancers. In vitro studies have also shown that BPA exposure causes molecular changes that initiate the development of cancers of the breast, prostate, and brain. Although these studies have implicated BPA in numerous ill health effects, some experts caution that some of these studies may be flawed and that more research needs to be done. In the meantime, the FDA recommends that consumers take precautions to limit their exposure to BPA. In addition to purchasing foods in packaging free of BPA, consumers should avoid carrying or storing foods or liquids in bottles with the recycling code 3 or 7. Foods and liquids should not be microwave-heated in any form of plastic: use paper, glass, or ceramics instead.
The Pituitary Gland and Hypothalamus
- Explain the interrelationships of the anatomy and functions of the hypothalamus and the posterior and anterior lobes of the pituitary gland
- Identify the two hormones released from the posterior pituitary, their target cells, and their principal actions
- Identify the six hormones produced by the anterior lobe of the pituitary gland, their target cells, their principal actions, and their regulation by the hypothalamus
The hypothalamus–pituitary complex can be thought of as the “command center” of the endocrine system. This complex secretes several hormones that directly produce responses in target tissues, as well as hormones that regulate the synthesis and secretion of hormones of other glands. In addition, the hypothalamus–pituitary complex coordinates the messages of the endocrine and nervous systems. In many cases, a stimulus received by the nervous system must pass through the hypothalamus–pituitary complex to be translated into hormones that can initiate a response.
The hypothalamus is a structure of the diencephalon of the brain located anterior and inferior to the thalamus (Figure 17.7). It has both neural and endocrine functions, producing and secreting many hormones. In addition, the hypothalamus is anatomically and functionally related to the pituitary gland (or hypophysis), a bean-sized organ suspended from it by a stem called the infundibulum (or pituitary stalk). The pituitary gland is cradled within the sellaturcica of the sphenoid bone of the skull. It consists of two lobes that arise from distinct parts of embryonic tissue: the posterior pituitary (neurohypophysis) is neural tissue, whereas the anterior pituitary (also known as the adenohypophysis) is glandular tissue that develops from the primitive digestive tract. The hormones secreted by the posterior and anterior pituitary, and the intermediate zone between the lobes are summarized in Table 17.3.
Figure 17.7 Hypothalamus–Pituitary Complex The hypothalamus region lies inferior and anterior to the thalamus. It connects to the pituitary gland by the stalk-like infundibulum. The pituitary gland consists of an anterior and posterior lobe, with each lobe secreting different hormones in response to signals from the hypothalamus.
Pituitary Hormones
| Pituitary lobe | Associated hormones | Chemical class | Effect |
|---|---|---|---|
| Anterior | Growth hormone (GH) | Protein | Promotes growth of body tissues |
| Anterior | Prolactin (PRL) | Peptide | Promotes milk production from mammary glands |
| Anterior | Thyroid-stimulating hormone (TSH) | Glycoprotein | Stimulates thyroid hormone release from thyroid |
| Anterior | Adrenocorticotropic hormone (ACTH) | Peptide | Stimulates hormone release by adrenal cortex |
| Anterior | Follicle-stimulating hormone (FSH) | Glycoprotein | Stimulates gamete production in gonads |
| Anterior | Luteinizing hormone (LH) | Glycoprotein | Stimulates androgen production by gonads |
| Posterior | Antidiuretic hormone (ADH) | Peptide | Stimulates water reabsorption by kidneys |
| Posterior | Oxytocin | Peptide | Stimulates uterine contractions during childbirth |
| Intermediate zone | Melanocyte-stimulating hormone | Peptide | Stimulates melanin formation in melanocytes |
Table 17.3
Posterior Pituitary
The posterior pituitary is actually an extension of the neurons of the paraventricular and supraoptic nuclei of the hypothalamus. The cell bodies of these regions rest in the hypothalamus, but their axons descend as the hypothalamic–hypophyseal tract within the infundibulum, and end in axon terminals that comprise the posterior pituitary (Figure 17.8).
Figure 17.8 Posterior Pituitary Neurosecretory cells in the hypothalamus release oxytocin (OT) or ADH into the posterior lobe of the pituitary gland. These hormones are stored or released into the blood via the capillary plexus.
The posterior pituitary gland does not produce hormones, but rather stores and secretes hormones produced by the hypothalamus. The paraventricular nuclei produce the hormone oxytocin, whereas the supraoptic nuclei produce ADH. These hormones travel along the axons into storage sites in the axon terminals of the posterior pituitary. In response to signals from the same hypothalamic neurons, the hormones are released from the axon terminals into the bloodstream.
Oxytocin
When fetal development is complete, the peptide-derived hormone oxytocin (tocia- = “childbirth”) stimulates uterine contractions and dilation of the cervix. Throughout most of pregnancy, oxytocin hormone receptors are not expressed at high levels in the uterus. Toward the end of pregnancy, the synthesis of oxytocin receptors in the uterus increases, and the smooth muscle cells of the uterus become more sensitive to its effects. Oxytocin is continually released throughout childbirth through a positive feedback mechanism. As noted earlier, oxytocin prompts uterine contractions that push the fetal head toward the cervix. In response, cervical stretching stimulates additional oxytocin to be synthesized by the hypothalamus and released from the pituitary. This increases the intensity and effectiveness of uterine contractions and prompts additional dilation of the cervix. The feedback loop continues until birth.
Although the mother’s high blood levels of oxytocin begin to decrease immediately following birth, oxytocin continues to play a role in maternal and newborn health. First, oxytocin is necessary for the milk ejection reflex (commonly referred to as “let-down”) in breastfeeding women. As the newborn begins suckling, sensory receptors in the nipples transmit signals to the hypothalamus. In response, oxytocin is secreted and released into the bloodstream. Within seconds, cells in the mother’s milk ducts contract, ejecting milk into the infant’s mouth. Secondly, in both males and females, oxytocin is thought to contribute to parent–newborn bonding, known as attachment. Oxytocin is also thought to be involved in feelings of love and closeness, as well as in the sexual response.
Antidiuretic Hormone (ADH)
The solute concentration of the blood, or blood osmolarity, may change in response to the consumption of certain foods and fluids, as well as in response to disease, injury, medications, or other factors. Blood osmolarity is constantly monitored by osmoreceptors—specialized cells within the hypothalamus that are particularly sensitive to the concentration of sodium ions and other solutes.
In response to high blood osmolarity, which can occur during dehydration or following a very salty meal, the osmoreceptors signal the posterior pituitary to release antidiuretic hormone (ADH). The target cells of ADH are located in the tubular cells of the kidneys. Its effect is to increase epithelial permeability to water, allowing increased water reabsorption. The more water reabsorbed from the filtrate, the greater the amount of water that is returned to the blood and the less that is excreted in the urine. A greater concentration of water results in a reduced concentration of solutes. ADH is also known as vasopressin because, in very high concentrations, it causes constriction of blood vessels, which increases blood pressure by increasing peripheral resistance. The release of ADH is controlled by a negative feedback loop. As blood osmolarity decreases, the hypothalamic osmoreceptors sense the change and prompt a corresponding decrease in the secretion of ADH. As a result, less water is reabsorbed from the urine filtrate.
Interestingly, drugs can affect the secretion of ADH. For example, alcohol consumption inhibits the release of ADH, resulting in increased urine production that can eventually lead to dehydration and a hangover. A disease called diabetes insipidus is characterized by chronic underproduction of ADH that causes chronic dehydration. Because little ADH is produced and secreted, not enough water is reabsorbed by the kidneys. Although patients feel thirsty, and increase their fluid consumption, this doesn’t effectively decrease the solute concentration in their blood because ADH levels are not high enough to trigger water reabsorption in the kidneys. Electrolyte imbalances can occur in severe cases of diabetes insipidus.
Anterior Pituitary
The anterior pituitary originates from the digestive tract in the embryo and migrates toward the brain during fetal development. There are three regions: the pars distalis is the most anterior, the pars intermedia is adjacent to the posterior pituitary, and the pars tuberalis is a slender “tube” that wraps the infundibulum.
Recall that the posterior pituitary does not synthesize hormones, but merely stores them. In contrast, the anterior pituitary does manufacture hormones. However, the secretion of hormones from the anterior pituitary is regulated by two classes of hormones. These hormones—secreted by the hypothalamus—are the releasing hormones that stimulate the secretion of hormones from the anterior pituitary and the inhibiting hormones that inhibit secretion.
Hypothalamic hormones are secreted by neurons, but enter the anterior pituitary through blood vessels (Figure 17.9). Within the infundibulum is a bridge of capillaries that connects the hypothalamus to the anterior pituitary. This network, called the hypophyseal portal system, allows hypothalamic hormones to be transported to the anterior pituitary without first entering the systemic circulation. The system originates from the superior hypophyseal artery, which branches off the carotid arteries and transports blood to the hypothalamus. The branches of the superior hypophyseal artery form the hypophyseal portal system (see Figure 17.9). Hypothalamic releasing and inhibiting hormones travel through a primary capillary plexus to the portal veins, which carry them into the anterior pituitary. Hormones produced by the anterior pituitary (in response to releasing hormones) enter a secondary capillary plexus, and from there drain into the circulation.
Figure 17.9 Anterior Pituitary The anterior pituitary manufactures seven hormones. The hypothalamus produces separate hormones that stimulate or inhibit hormone production in the anterior pituitary. Hormones from the hypothalamus reach the anterior pituitary via the hypophyseal portal system.
The anterior pituitary produces seven hormones. These are the growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), beta endorphin, and prolactin. Of the hormones of the anterior pituitary, TSH, ACTH, FSH, and LH are collectively referred to as tropic hormones (trope- = “turning”) because they turn on or off the function of other endocrine glands.
Growth Hormone
The endocrine system regulates the growth of the human body, protein synthesis, and cellular replication. A major hormone involved in this process is growth hormone (GH), also called somatotropin—a protein hormone produced and secreted by the anterior pituitary gland. Its primary function is anabolic; it promotes protein synthesis and tissue building through direct and indirect mechanisms (Figure 17.10). GH levels are controlled by the release of GHRH and GHIH (also known as somatostatin) from the hypothalamus.
Figure 17.10 Hormonal Regulation of Growth Growth hormone (GH) directly accelerates the rate of protein synthesis in skeletal muscle and bones. Insulin-like growth factor 1 (IGF-1) is activated by growth hormone and indirectly supports the formation of new proteins in muscle cells and bone.
A glucose-sparing effect occurs when GH stimulates lipolysis, or the breakdown of adipose tissue, releasing fatty acids into the blood. As a result, many tissues switch from glucose to fatty acids as their main energy source, which means that less glucose is taken up from the bloodstream.
GH also initiates the diabetogenic effect in which GH stimulates the liver to break down glycogen to glucose, which is then deposited into the blood. The name “diabetogenic” is derived from the similarity in elevated blood glucose levels observed between individuals with untreated diabetes mellitus and individuals experiencing GH excess. Blood glucose levels rise as the result of a combination of glucose-sparing and diabetogenic effects.
GH indirectly mediates growth and protein synthesis by triggering the liver and other tissues to produce a group of proteins called insulin-like growth factors (IGFs). These proteins enhance cellular proliferation and inhibit apoptosis, or programmed cell death. IGFs stimulate cells to increase their uptake of amino acids from the blood for protein synthesis. Skeletal muscle and cartilage cells are particularly sensitive to stimulation from IGFs.
Dysfunction of the endocrine system’s control of growth can result in several disorders. For example, gigantism is a disorder in children that is caused by the secretion of abnormally large amounts of GH, resulting in excessive growth. A similar condition in adults is acromegaly, a disorder that results in the growth of bones in the face, hands, and feet in response to excessive levels of GH in individuals who have stopped growing. Abnormally low levels of GH in children can cause growth impairment—a disorder called pituitary dwarfism (also known as growth hormone deficiency).
Thyroid-Stimulating Hormone
The activity of the thyroid gland is regulated by thyroid-stimulating hormone (TSH), also called thyrotropin. TSH is released from the anterior pituitary in response to thyrotropin-releasing hormone (TRH) from the hypothalamus. As discussed shortly, it triggers the secretion of thyroid hormones by the thyroid gland. In a classic negative feedback loop, elevated levels of thyroid hormones in the bloodstream then trigger a drop in production of TRH and subsequently TSH.
Adrenocorticotropic Hormone
The adrenocorticotropic hormone (ACTH), also called corticotropin, stimulates the adrenal cortex (the more superficial “bark” of the adrenal glands) to secrete corticosteroid hormones such as cortisol. ACTH come from a precursor molecule known as pro-opiomelanotropin (POMC) which produces several biologically active molecules when cleaved, including ACTH, melanocyte-stimulating hormone, and the brain opioid peptides known as endorphins.
The release of ACTH is regulated by the corticotropin-releasing hormone (CRH) from the hypothalamus in response to normal physiologic rhythms. A variety of stressors can also influence its release, and the role of ACTH in the stress response is discussed later in this chapter.
Follicle-Stimulating Hormone and Luteinizing Hormone
The endocrine glands secrete a variety of hormones that control the development and regulation of the reproductive system (these glands include the anterior pituitary, the adrenal cortex, and the gonads—the testes in males and the ovaries in females). Much of the development of the reproductive system occurs during puberty and is marked by the development of sex-specific characteristics in both male and female adolescents. Puberty is initiated by gonadotropin-releasing hormone (GnRH), a hormone produced and secreted by the hypothalamus. GnRH stimulates the anterior pituitary to secrete gonadotropins—hormones that regulate the function of the gonads. The levels of GnRH are regulated through a negative feedback loop; high levels of reproductive hormones inhibit the release of GnRH. Throughout life, gonadotropins regulate reproductive function and, in the case of women, the onset and cessation of reproductive capacity.
The gonadotropins include two glycoprotein hormones: follicle-stimulating hormone (FSH) stimulates the production and maturation of sex cells, or gametes, including ova in women and sperm in men. FSH also promotes follicular growth; these follicles then release estrogens in the female ovaries. Luteinizing hormone (LH) triggers ovulation in women, as well as the production of estrogens and progesterone by the ovaries. LH stimulates production of testosterone by the male testes.
Prolactin
As its name implies, prolactin (PRL) promotes lactation (milk production) in women. During pregnancy, it contributes to development of the mammary glands, and after birth, it stimulates the mammary glands to produce breast milk. However, the effects of prolactin depend heavily upon the permissive effects of estrogens, progesterone, and other hormones. And as noted earlier, the let-down of milk occurs in response to stimulation from oxytocin.
In a non-pregnant woman, prolactin secretion is inhibited by prolactin-inhibiting hormone (PIH), which is actually the neurotransmitter dopamine, and is released from neurons in the hypothalamus. Only during pregnancy do prolactin levels rise in response to prolactin-releasing hormone (PRH) from the hypothalamus.
Intermediate Pituitary: Melanocyte-Stimulating Hormone
The cells in the zone between the pituitary lobes secrete a hormone known as melanocyte-stimulating hormone (MSH) that is formed by cleavage of the pro-opiomelanocortin (POMC) precursor protein. Local production of MSH in the skin is responsible for melanin production in response to UV light exposure. The role of MSH made by the pituitary is more complicated. For instance, people with lighter skin generally have the same amount of MSH as people with darker skin. Nevertheless, this hormone is capable of darkening of the skin by inducing melanin production in the skin’s melanocytes. Women also show increased MSH production during pregnancy; in combination with estrogens, it can lead to darker skin pigmentation, especially the skin of the areolas and labia minora. Figure 17.11 is a summary of the pituitary hormones and their principal effects.
Figure 17.11 Major Pituitary Hormones Major pituitary hormones and their target organs.
INTERACTIVE LINK
Visit this link to watch an animation showing the role of the hypothalamus and the pituitary gland. Which hormone is released by the pituitary to stimulate the thyroid gland?
The Thyroid Gland
- Describe the location and anatomy of the thyroid gland
- Discuss the synthesis of triiodothyronine and thyroxine
- Explain the role of thyroid hormones in the regulation of basal metabolism
- Identify the hormone produced by the parafollicular cells of the thyroid
A butterfly-shaped organ, the thyroid gland is located anterior to the trachea, just inferior to the larynx (Figure 17.12). The medial region, called the isthmus, is flanked by wing-shaped left and right lobes. Each of the thyroid lobes are embedded with parathyroid glands, primarily on their posterior surfaces. The tissue of the thyroid gland is composed mostly of thyroid follicles. The follicles are made up of a central cavity filled with a sticky fluid called colloid. Surrounded by a wall of epithelial follicle cells, the colloid is the center of thyroid hormone production, and that production is dependent on the hormones’ essential and unique component: iodine.
Figure 17.12 Thyroid Gland The thyroid gland is located in the neck where it wraps around the trachea. (a) Anterior view of the thyroid gland. (b) Posterior view of the thyroid gland. (c) The glandular tissue is composed primarily of thyroid follicles. The larger parafollicular cells often appear within the matrix of follicle cells. LM × 1332. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
Synthesis and Release of Thyroid Hormones
Hormones are produced in the colloid when atoms of the mineral iodine attach to a glycoprotein, called thyroglobulin, that is secreted into the colloid by the follicle cells. The following steps outline the hormones’ assembly:
- Binding of TSH to its receptors in the follicle cells of the thyroid gland causes the cells to actively transport iodide ions (I–) across their cell membrane, from the bloodstream into the cytosol. As a result, the concentration of iodide ions “trapped” in the follicular cells is many times higher than the concentration in the bloodstream.
- Iodide ions then move to the lumen of the follicle cells that border the colloid. There, the ions undergo oxidation (their negatively charged electrons are removed). The oxidation of two iodide ions (2 I–) results in iodine (I2), which passes through the follicle cell membrane into the colloid.
- In the colloid, peroxidase enzymes link the iodine to the tyrosine amino acids in thyroglobulin to produce two intermediaries: a tyrosine attached to one iodine and a tyrosine attached to two iodines. When one of each of these intermediaries is linked by covalent bonds, the resulting compound is triiodothyronine (T3), a thyroid hormone with three iodines. Much more commonly, two copies of the second intermediary bond, forming tetraiodothyronine, also known as thyroxine (T4), a thyroid hormone with four iodines.
These hormones remain in the colloid center of the thyroid follicles until TSH stimulates endocytosis of colloid back into the follicle cells. There, lysosomal enzymes break apart the thyroglobulin colloid, releasing free T3 and T4, which diffuse across the follicle cell membrane and enter the bloodstream.
In the bloodstream, less than one percent of the circulating T3 and T4 remains unbound. This free T3 and T4 can cross the lipid bilayer of cell membranes and be taken up by cells. The remaining 99 percent of circulating T3 and T4 is bound to specialized transport proteins called thyroxine-binding globulins (TBGs), to albumin, or to other plasma proteins. This “packaging” prevents their free diffusion into body cells. When blood levels of T3 and T4 begin to decline, bound T3 and T4 are released from these plasma proteins and readily cross the membrane of target cells. T3 is more potent than T4, and many cells convert T4 to T3through the removal of an iodine atom.
Regulation of TH Synthesis
The release of T3 and T4 from the thyroid gland is regulated by thyroid-stimulating hormone (TSH). As shown in Figure 17.13, low blood levels of T3 and T4 stimulate the release of thyrotropin-releasing hormone (TRH) from the hypothalamus, which triggers secretion of TSH from the anterior pituitary. In turn, TSH stimulates the thyroid gland to secrete T3 and T4. The levels of TRH, TSH, T3, and T4 are regulated by a negative feedback system in which increasing levels of T3 and T4 decrease the production and secretion of TSH.
Figure 17.13 Classic Negative Feedback Loop A classic negative feedback loop controls the regulation of thyroid hormone levels.
Functions of Thyroid Hormones
The thyroid hormones, T3 and T4, are often referred to as metabolic hormones because their levels influence the body’s basal metabolic rate, the amount of energy used by the body at rest. When T3 and T4 bind to intracellular receptors located on the mitochondria, they cause an increase in nutrient breakdown and the use of oxygen to produce ATP. In addition, T3 and T4 initiate the transcription of genes involved in glucose oxidation. Although these mechanisms prompt cells to produce more ATP, the process is inefficient, and an abnormally increased level of heat is released as a byproduct of these reactions. This so-called calorigenic effect (calor- = “heat”) raises body temperature.
Adequate levels of thyroid hormones are also required for protein synthesis and for fetal and childhood tissue development and growth. They are especially critical for normal development of the nervous system both in utero and in early childhood, and they continue to support neurological function in adults. As noted earlier, these thyroid hormones have a complex interrelationship with reproductive hormones, and deficiencies can influence libido, fertility, and other aspects of reproductive function. Finally, thyroid hormones increase the body’s sensitivity to catecholamines (epinephrine and norepinephrine) from the adrenal medulla by upregulation of receptors in the blood vessels. When levels of T3 and T4 hormones are excessive, this effect accelerates the heart rate, strengthens the heartbeat, and increases blood pressure. Because thyroid hormones regulate metabolism, heat production, protein synthesis, and many other body functions, thyroid disorders can have severe and widespread consequences.
DISORDERS OF THE...
Endocrine System: Iodine Deficiency, Hypothyroidism, and Hyperthyroidism
As discussed above, dietary iodine is required for the synthesis of T3 and T4. But for much of the world’s population, foods do not provide adequate levels of this mineral, because the amount varies according to the level in the soil in which the food was grown, as well as the irrigation and fertilizers used. Marine fish and shrimp tend to have high levels because they concentrate iodine from seawater, but many people in landlocked regions lack access to seafood. Thus, the primary source of dietary iodine in many countries is iodized salt. Fortification of salt with iodine began in the United States in 1924, and international efforts to iodize salt in the world’s poorest nations continue today.
Dietary iodine deficiency can result in the impaired ability to synthesize T3 and T4, leading to a variety of severe disorders. When T3 and T4 cannot be produced, TSH is secreted in increasing amounts. As a result of this hyperstimulation, thyroglobulin accumulates in the thyroid gland follicles, increasing their deposits of colloid. The accumulation of colloid increases the overall size of the thyroid gland, a condition called a goiter (Figure 17.14). A goiter is only a visible indication of the deficiency. Other iodine deficiency disorders include impaired growth and development, decreased fertility, and prenatal and infant death. Moreover, iodine deficiency is the primary cause of preventable mental retardation worldwide. Neonatal hypothyroidism (cretinism) is characterized by cognitive deficits, short stature, and sometimes deafness and muteness in children and adults born to mothers who were iodine-deficient during pregnancy.
Figure 17.14 Goiter (credit: “Almazi”/Wikimedia Commons)
In areas of the world with access to iodized salt, dietary deficiency is rare. Instead, inflammation of the thyroid gland is the more common cause of low blood levels of thyroid hormones. Called hypothyroidism, the condition is characterized by a low metabolic rate, weight gain, cold extremities, constipation, reduced libido, menstrual irregularities, and reduced mental activity. In contrast, hyperthyroidism—an abnormally elevated blood level of thyroid hormones—is often caused by a pituitary or thyroid tumor. In Graves’ disease, the hyperthyroid state results from an autoimmune reaction in which antibodies overstimulate the follicle cells of the thyroid gland. Hyperthyroidism can lead to an increased metabolic rate, excessive body heat and sweating, diarrhea, weight loss, tremors, and increased heart rate. The person’s eyes may bulge (called exophthalmos) as antibodies produce inflammation in the soft tissues of the orbits. The person may also develop a goiter.
Calcitonin
The thyroid gland also secretes a hormone called calcitonin that is produced by the parafollicular cells (also called C cells) that stud the tissue between distinct follicles. Calcitonin is released in response to a rise in blood calcium levels. It appears to have a function in decreasing blood calcium concentrations by:
- Inhibiting the activity of osteoclasts, bone cells that release calcium into the circulation by degrading bone matrix
- Increasing osteoblastic activity
- Decreasing calcium absorption in the intestines
- Increasing calcium loss in the urine
However, these functions are usually not significant in maintaining calcium homeostasis, so the importance of calcitonin is not entirely understood. Pharmaceutical preparations of calcitonin are sometimes prescribed to reduce osteoclast activity in people with osteoporosis and to reduce the degradation of cartilage in people with osteoarthritis. The hormones secreted by thyroid are summarized in Table 17.4.
Thyroid Hormones
| Associated hormones | Chemical class | Effect |
|---|---|---|
| Thyroxine (T4), triiodothyronine (T3) | Amine | Stimulate basal metabolic rate |
| Calcitonin | Peptide | Reduces blood Ca2+ levels |
Table 17.4
Of course, calcium is critical for many other biological processes. It is a second messenger in many signaling pathways, and is essential for muscle contraction, nerve impulse transmission, and blood clotting. Given these roles, it is not surprising that blood calcium levels are tightly regulated by the endocrine system. The organs involved in the regulation are the parathyroid glands.
The Parathyroid Glands
- Describe the location and structure of the parathyroid glands
- Describe the hormonal control of blood calcium levels
- Discuss the physiological response of parathyroid dysfunction
The parathyroid glands are tiny, round structures usually found embedded in the posterior surface of the thyroid gland (Figure 17.15). A thick connective tissue capsule separates the glands from the thyroid tissue. Most people have four parathyroid glands, but occasionally there are more in tissues of the neck or chest. The function of one type of parathyroid cells, the oxyphil cells, is not clear. The primary functional cells of the parathyroid glands are the chief cells. These epithelial cells produce and secrete the parathyroid hormone (PTH), the major hormone involved in the regulation of blood calcium levels.
Figure 17.15 Parathyroid Glands The small parathyroid glands are embedded in the posterior surface of the thyroid gland. LM × 760. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
INTERACTIVE LINK
View the University of Michigan WebScope to explore the tissue sample in greater detail.
The parathyroid glands produce and secrete PTH, a peptide hormone, in response to low blood calcium levels (Figure 17.16). PTH secretion causes the release of calcium from the bones by stimulating osteoclasts, which secrete enzymes that degrade bone and release calcium into the interstitial fluid. PTH also inhibits osteoblasts, the cells involved in bone deposition, thereby sparing blood calcium. PTH causes increased reabsorption of calcium (and magnesium) in the kidney tubules from the urine filtrate. In addition, PTH initiates the production of the steroid hormone calcitriol (also known as 1,25-dihydroxyvitamin D), which is the active form of vitamin D3, in the kidneys. Calcitriol then stimulates increased absorption of dietary calcium by the intestines. A negative feedback loop regulates the levels of PTH, with rising blood calcium levels inhibiting further release of PTH.
Figure 17.16 Parathyroid Hormone in Maintaining Blood Calcium Homeostasis Parathyroid hormone increases blood calcium levels when they drop too low. Conversely, calcitonin, which is released from the thyroid gland, decreases blood calcium levels when they become too high. These two mechanisms constantly maintain blood calcium concentration at homeostasis.
Abnormally high activity of the parathyroid gland can cause hyperparathyroidism, a disorder caused by an overproduction of PTH that results in excessive calcium reabsorption from bone. Hyperparathyroidism can significantly decrease bone density, leading to spontaneous fractures or deformities. As blood calcium levels rise, cell membrane permeability to sodium is decreased, and the responsiveness of the nervous system is reduced. At the same time, calcium deposits may collect in the body’s tissues and organs, impairing their functioning.
In contrast, abnormally low blood calcium levels may be caused by parathyroid hormone deficiency, called hypoparathyroidism, which may develop following injury or surgery involving the thyroid gland. Low blood calcium increases membrane permeability to sodium, resulting in muscle twitching, cramping, spasms, or convulsions. Severe deficits can paralyze muscles, including those involved in breathing, and can be fatal.
When blood calcium levels are high, calcitonin is produced and secreted by the parafollicular cells of the thyroid gland. As discussed earlier, calcitonin inhibits the activity of osteoclasts, reduces the absorption of dietary calcium in the intestine, and signals the kidneys to reabsorb less calcium, resulting in larger amounts of calcium excreted in the urine.
The Adrenal Glands
- Describe the location and structure of the adrenal glands
- Identify the hormones produced by the adrenal cortex and adrenal medulla, and summarize their target cells and effects
The adrenal glands are wedges of glandular and neuroendocrine tissue adhering to the top of the kidneys by a fibrous capsule (Figure 17.17). The adrenal glands have a rich blood supply and experience one of the highest rates of blood flow in the body. They are served by several arteries branching off the aorta, including the suprarenal and renal arteries. Blood flows to each adrenal gland at the adrenal cortex and then drains into the adrenal medulla. Adrenal hormones are released into the circulation via the left and right suprarenal veins.
Figure 17.17 Adrenal Glands Both adrenal glands sit atop the kidneys and are composed of an outer cortex and an inner medulla, all surrounded by a connective tissue capsule. The cortex can be subdivided into additional zones, all of which produce different types of hormones. LM × 204. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
INTERACTIVE LINK
View the University of Michigan WebScope to explore the tissue sample in greater detail.
The adrenal gland consists of an outer cortex of glandular tissue and an inner medulla of nervous tissue. The cortex itself is divided into three zones: the zona glomerulosa, the zona fasciculata, and the zona reticularis. Each region secretes its own set of hormones.
The adrenal cortex, as a component of the hypothalamic-pituitary-adrenal (HPA) axis, secretes steroid hormones important for the regulation of the long-term stress response, blood pressure and blood volume, nutrient uptake and storage, fluid and electrolyte balance, and inflammation. The HPA axis involves the stimulation of hormone release of adrenocorticotropic hormone (ACTH) from the pituitary by the hypothalamus. ACTH then stimulates the adrenal cortex to produce the hormone cortisol. This pathway will be discussed in more detail below.
The adrenal medulla is neuroendocrine tissue composed of postganglionic sympathetic nervous system (SNS) neurons. It is really an extension of the autonomic nervous system, which regulates homeostasis in the body. The sympathomedullary (SAM) pathway involves the stimulation of the medulla by impulses from the hypothalamus via neurons from the thoracic spinal cord. The medulla is stimulated to secrete the amine hormones epinephrine and norepinephrine.
One of the major functions of the adrenal gland is to respond to stress. Stress can be either physical or psychological or both. Physical stresses include exposing the body to injury, walking outside in cold and wet conditions without a coat on, or malnutrition. Psychological stresses include the perception of a physical threat, a fight with a loved one, or just a bad day at school.
The body responds in different ways to short-term stress and long-term stress following a pattern known as the general adaptation syndrome (GAS). Stage one of GAS is called the alarm reaction. This is short-term stress, the fight-or-flight response, mediated by the hormones epinephrine and norepinephrine from the adrenal medulla via the SAM pathway. Their function is to prepare the body for extreme physical exertion. Once this stress is relieved, the body quickly returns to normal. The section on the adrenal medulla covers this response in more detail.
If the stress is not soon relieved, the body adapts to the stress in the second stage called the stage of resistance. If a person is starving for example, the body may send signals to the gastrointestinal tract to maximize the absorption of nutrients from food.
If the stress continues for a longer term however, the body responds with symptoms quite different than the fight-or-flight response. During the stage of exhaustion, individuals may begin to suffer depression, the suppression of their immune response, severe fatigue, or even a fatal heart attack. These symptoms are mediated by the hormones of the adrenal cortex, especially cortisol, released as a result of signals from the HPA axis.
Adrenal hormones also have several non–stress-related functions, including the increase of blood sodium and glucose levels, which will be described in detail below.
Adrenal Cortex
The adrenal cortex consists of multiple layers of lipid-storing cells that occur in three structurally distinct regions. Each of these regions produces different hormones.
INTERACTIVE LINK
Visit this link to view an animation describing the location and function of the adrenal glands. Which hormone produced by the adrenal glands is responsible for the mobilization of energy stores?
Hormones of the Zona Glomerulosa
The most superficial region of the adrenal cortex is the zona glomerulosa, which produces a group of hormones collectively referred to as mineralocorticoids because of their effect on body minerals, especially sodium and potassium. These hormones are essential for fluid and electrolyte balance.
Aldosterone is the major mineralocorticoid. It is important in the regulation of the concentration of sodium and potassium ions in urine, sweat, and saliva. For example, it is released in response to elevated blood K+, low blood Na+, low blood pressure, or low blood volume. In response, aldosterone increases the excretion of K+ and the retention of Na+, which in turn increases blood volume and blood pressure. Its secretion is prompted when CRH from the hypothalamus triggers ACTH release from the anterior pituitary.
Aldosterone is also a key component of the renin-angiotensin-aldosterone system (RAAS) in which specialized cells of the kidneys secrete the enzyme renin in response to low blood volume or low blood pressure. Renin then catalyzes the conversion of the blood protein angiotensinogen, produced by the liver, to the hormone angiotensin I. Angiotensin I is converted in the lungs to angiotensin II by angiotensin-converting enzyme (ACE). Angiotensin II has three major functions:
- Initiating vasoconstriction of the arterioles, decreasing blood flow
- Stimulating kidney tubules to reabsorb NaCl and water, increasing blood volume
- Signaling the adrenal cortex to secrete aldosterone, the effects of which further contribute to fluid retention, restoring blood pressure and blood volume
For individuals with hypertension, or high blood pressure, drugs are available that block the production of angiotensin II. These drugs, known as ACE inhibitors, block the ACE enzyme from converting angiotensin I to angiotensin II, thus mitigating the latter’s ability to increase blood pressure.
Hormones of the Zona Fasciculata
The intermediate region of the adrenal cortex is the zona fasciculata, named as such because the cells form small fascicles (bundles) separated by tiny blood vessels. The cells of the zona fasciculata produce hormones called glucocorticoids because of their role in glucose metabolism. The most important of these is cortisol, some of which the liver converts to cortisone. A glucocorticoid produced in much smaller amounts is corticosterone. In response to long-term stressors, the hypothalamus secretes CRH, which in turn triggers the release of ACTH by the anterior pituitary. ACTH triggers the release of the glucocorticoids. Their overall effect is to inhibit tissue building while stimulating the breakdown of stored nutrients to maintain adequate fuel supplies. In conditions of long-term stress, for example, cortisol promotes the catabolism of glycogen to glucose, the catabolism of stored triglycerides into fatty acids and glycerol, and the catabolism of muscle proteins into amino acids. These raw materials can then be used to synthesize additional glucose and ketones for use as body fuels. The hippocampus, which is part of the temporal lobe of the cerebral cortices and important in memory formation, is highly sensitive to stress levels because of its many glucocorticoid receptors.
You are probably familiar with prescription and over-the-counter medications containing glucocorticoids, such as cortisone injections into inflamed joints, prednisone tablets and steroid-based inhalers used to manage severe asthma, and hydrocortisone creams applied to relieve itchy skin rashes. These drugs reflect another role of cortisol—the downregulation of the immune system, which inhibits the inflammatory response.
Hormones of the Zona Reticularis
The deepest region of the adrenal cortex is the zona reticularis, which produces small amounts of a class of steroid sex hormones called androgens. During puberty and most of adulthood, androgens are produced in the gonads. The androgens produced in the zona reticularis supplement the gonadal androgens. They are produced in response to ACTH from the anterior pituitary and are converted in the tissues to testosterone or estrogens. In adult women, they may contribute to the sex drive, but their function in adult men is not well understood. In post-menopausal women, as the functions of the ovaries decline, the main source of estrogens becomes the androgens produced by the zona reticularis.
Adrenal Medulla
As noted earlier, the adrenal cortex releases glucocorticoids in response to long-term stress such as severe illness. In contrast, the adrenal medulla releases its hormones in response to acute, short-term stress mediated by the sympathetic nervous system (SNS).
The medullary tissue is composed of unique postganglionic SNS neurons called chromaffin cells, which are large and irregularly shaped, and produce the neurotransmitters epinephrine (also called adrenaline) and norepinephrine (or noradrenaline). Epinephrine is produced in greater quantities—approximately a 4 to 1 ratio with norepinephrine—and is the more powerful hormone. Because the chromaffin cells release epinephrine and norepinephrine into the systemic circulation, where they travel widely and exert effects on distant cells, they are considered hormones. Derived from the amino acid tyrosine, they are chemically classified as catecholamines.
The secretion of medullary epinephrine and norepinephrine is controlled by a neural pathway that originates from the hypothalamus in response to danger or stress (the SAM pathway). Both epinephrine and norepinephrine signal the liver and skeletal muscle cells to convert glycogen into glucose, resulting in increased blood glucose levels. These hormones increase the heart rate, pulse, and blood pressure to prepare the body to fight the perceived threat or flee from it. In addition, the pathway dilates the airways, raising blood oxygen levels. It also prompts vasodilation, further increasing the oxygenation of important organs such as the lungs, brain, heart, and skeletal muscle. At the same time, it triggers vasoconstriction to blood vessels serving less essential organs such as the gastrointestinal tract, kidneys, and skin, and downregulates some components of the immune system. Other effects include a dry mouth, loss of appetite, pupil dilation, and a loss of peripheral vision. The major hormones of the adrenal glands are summarized in Table 17.5.
Hormones of the Adrenal Glands
| Adrenal gland | Associated hormones | Chemical class | Effect |
|---|---|---|---|
| Adrenal cortex | Aldosterone | Steroid | Increases blood Na+ levels |
| Adrenal cortex | Cortisol, corticosterone, cortisone | Steroid | Increase blood glucose levels |
| Adrenal medulla | Epinephrine, norepinephrine | Amine | Stimulate fight-or-flight response |
Table 17.5
Disorders Involving the Adrenal Glands
Several disorders are caused by the dysregulation of the hormones produced by the adrenal glands. For example, Cushing’s disease is a disorder characterized by high blood glucose levels and the accumulation of lipid deposits on the face and neck. It is caused by hypersecretion of cortisol. The most common source of Cushing’s disease is a pituitary tumor that secretes cortisol or ACTH in abnormally high amounts. Other common signs of Cushing’s disease include the development of a moon-shaped face, a buffalo hump on the back of the neck, rapid weight gain, and hair loss. Chronically elevated glucose levels are also associated with an elevated risk of developing type 2 diabetes. In addition to hyperglycemia, chronically elevated glucocorticoids compromise immunity, resistance to infection, and memory, and can result in rapid weight gain and hair loss.
In contrast, the hyposecretion of corticosteroids can result in Addison’s disease, a rare disorder that causes low blood glucose levels and low blood sodium levels. The signs and symptoms of Addison’s disease are vague and are typical of other disorders as well, making diagnosis difficult. They may include general weakness, abdominal pain, weight loss, nausea, vomiting, sweating, and cravings for salty food.
The Pineal Gland
- Describe the location and structure of the pineal gland
- Discuss the function of melatonin
Recall that the hypothalamus, part of the diencephalon of the brain, sits inferior and somewhat anterior to the thalamus. Inferior but somewhat posterior to the thalamus is the pineal gland, a tiny endocrine gland whose functions are not entirely clear. The pinealocyte cells that make up the pineal gland are known to produce and secrete the amine hormone melatonin, which is derived from serotonin.
The secretion of melatonin varies according to the level of light received from the environment. When photons of light stimulate the retinas of the eyes, a nerve impulse is sent to a region of the hypothalamus called the suprachiasmatic nucleus (SCN), which is important in regulating biological rhythms. From the SCN, the nerve signal is carried to the spinal cord and eventually to the pineal gland, where the production of melatonin is inhibited. As a result, blood levels of melatonin fall, promoting wakefulness. In contrast, as light levels decline—such as during the evening—melatonin production increases, boosting blood levels and causing drowsiness.
INTERACTIVE LINK
Visit this link to view an animation describing the function of the hormone melatonin. What should you avoid doing in the middle of your sleep cycle that would lower melatonin?
The secretion of melatonin may influence the body’s circadian rhythms, the dark-light fluctuations that affect not only sleepiness and wakefulness, but also appetite and body temperature. Interestingly, children have higher melatonin levels than adults, which may prevent the release of gonadotropins from the anterior pituitary, thereby inhibiting the onset of puberty. Finally, an antioxidant role of melatonin is the subject of current research.
Jet lag occurs when a person travels across several time zones and feels sleepy during the day or wakeful at night. Traveling across multiple time zones significantly disturbs the light-dark cycle regulated by melatonin. It can take up to several days for melatonin synthesis to adjust to the light-dark patterns in the new environment, resulting in jet lag. Some air travelers take melatonin supplements to induce sleep.
Gonadal and Placental Hormones
- Identify the most important hormones produced by the testes and ovaries
- Name the hormones produced by the placenta and state their functions
This section briefly discusses the hormonal role of the gonads—the male testes and female ovaries—which produce the sex cells (sperm and ova) and secrete the gonadal hormones. The roles of the gonadotropins released from the anterior pituitary (FSH and LH) were discussed earlier.
The primary hormone produced by the male testes is testosterone, a steroid hormone important in the development of the male reproductive system, the maturation of sperm cells, and the development of male secondary sex characteristics such as a deepened voice, body hair, and increased muscle mass. Interestingly, testosterone is also produced in the female ovaries, but at a much reduced level. In addition, the testes produce the peptide hormone inhibin, which inhibits the secretion of FSH from the anterior pituitary gland. FSH stimulates spermatogenesis.
The primary hormones produced by the ovaries are estrogens, which include estradiol, estriol, and estrone. Estrogens play an important role in a larger number of physiological processes, including the development of the female reproductive system, regulation of the menstrual cycle, the development of female secondary sex characteristics such as increased adipose tissue and the development of breast tissue, and the maintenance of pregnancy. Another significant ovarian hormone is progesterone, which contributes to regulation of the menstrual cycle and is important in preparing the body for pregnancy as well as maintaining pregnancy. In addition, the granulosa cells of the ovarian follicles produce inhibin, which—as in males—inhibits the secretion of FSH.During the initial stages of pregnancy, an organ called the placenta develops within the uterus. The placenta supplies oxygen and nutrients to the fetus, excretes waste products, and produces and secretes estrogens and progesterone. The placenta produces human chorionic gonadotropin (hCG) as well. The hCG hormone promotes progesterone synthesis and reduces the mother’s immune function to protect the fetus from immune rejection. It also secretes human placental lactogen (hPL), which plays a role in preparing the breasts for lactation, and relaxin, which is thought to help soften and widen the pubic symphysis in preparation for childbirth. The hormones controlling reproduction are summarized in Table 17.6.
Reproductive Hormones
| Gonad | Associated hormones | Chemical class | Effect |
|---|---|---|---|
| Testes | Testosterone | Steroid | Stimulates development of male secondary sex characteristics and sperm production |
| Testes | Inhibin | Protein | Inhibits FSH release from pituitary |
| Ovaries | Estrogens and progesterone | Steroid | Stimulate development of female secondary sex characteristics and prepare the body for childbirth |
| Placenta | Human chorionic gonadotropin | Protein | Promotes progesterone synthesis during pregnancy and inhibits immune response against fetus |
Table 17.6
EVERYDAY CONNECTION
Anabolic Steroids
The endocrine system can be exploited for illegal or unethical purposes. A prominent example of this is the use of steroid drugs by professional athletes.
Commonly used for performance enhancement, anabolic steroids are synthetic versions of the male sex hormone, testosterone. By boosting natural levels of this hormone, athletes experience increased muscle mass. Synthetic versions of human growth hormone are also used to build muscle mass.
The use of performance-enhancing drugs is banned by all major collegiate and professional sports organizations in the United States because they impart an unfair advantage to athletes who take them. In addition, the drugs can cause significant and dangerous side effects. For example, anabolic steroid use can increase cholesterol levels, raise blood pressure, and damage the liver. Altered testosterone levels (both too low or too high) have been implicated in causing structural damage to the heart, and increasing the risk for cardiac arrhythmias, heart attacks, congestive heart failure, and sudden death. Paradoxically, steroids can have a feminizing effect in males, including shriveled testicles and enlarged breast tissue. In females, their use can cause masculinizing effects such as an enlarged clitoris and growth of facial hair. In both sexes, their use can promote increased aggression (commonly known as “roid-rage”), depression, sleep disturbances, severe acne, and infertility.
The Endocrine Pancreas
- Describe the location and structure of the pancreas, and the morphology and function of the pancreatic islets
- Compare and contrast the functions of insulin and glucagon
The pancreas is a long, slender organ, most of which is located posterior to the bottom half of the stomach (Figure 17.18). Although it is primarily an exocrine gland, secreting a variety of digestive enzymes, the pancreas has an endocrine function. Its pancreatic islets—clusters of cells formerly known as the islets of Langerhans—secrete the hormones glucagon, insulin, somatostatin, and pancreatic polypeptide (PP).
Figure 17.18 Pancreas The pancreatic exocrine function involves the acinar cells secreting digestive enzymes that are transported into the small intestine by the pancreatic duct. Its endocrine function involves the secretion of insulin (produced by beta cells) and glucagon (produced by alpha cells) within the pancreatic islets. These two hormones regulate the rate of glucose metabolism in the body. The micrograph reveals pancreatic islets. LM × 760. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
INTERACTIVE LINK
View the University of Michigan WebScope to explore the tissue sample in greater detail.
Cells and Secretions of the Pancreatic Islets
The pancreatic islets each contain four varieties of cells:
- The alpha cell produces the hormone glucagon and makes up approximately 20 percent of each islet. Glucagon plays an important role in blood glucose regulation; low blood glucose levels stimulate its release.
- The beta cell produces the hormone insulin and makes up approximately 75 percent of each islet. Elevated blood glucose levels stimulate the release of insulin.
- The delta cell accounts for four percent of the islet cells and secretes the peptide hormone somatostatin. Recall that somatostatin is also released by the hypothalamus (as GHIH), and the stomach and intestines also secrete it. An inhibiting hormone, pancreatic somatostatin inhibits the release of both glucagon and insulin.
- The PP cell accounts for about one percent of islet cells and secretes the pancreatic polypeptide hormone. It is thought to play a role in appetite, as well as in the regulation of pancreatic exocrine and endocrine secretions. Pancreatic polypeptide released following a meal may reduce further food consumption; however, it is also released in response to fasting.
Regulation of Blood Glucose Levels by Insulin and Glucagon
Glucose is required for cellular respiration and is the preferred fuel for all body cells. The body derives glucose from the breakdown of the carbohydrate-containing foods and drinks we consume. Glucose not immediately taken up by cells for fuel can be stored by the liver and muscles as glycogen, or converted to triglycerides and stored in the adipose tissue. Hormones regulate both the storage and the utilization of glucose as required. Receptors located in the pancreas sense blood glucose levels, and subsequently the pancreatic cells secrete glucagon or insulin to maintain normal levels.
Glucagon
Receptors in the pancreas can sense the decline in blood glucose levels, such as during periods of fasting or during prolonged labor or exercise (Figure 17.19). In response, the alpha cells of the pancreas secrete the hormone glucagon, which has several effects:
- It stimulates the liver to convert its stores of glycogen back into glucose. This response is known as glycogenolysis. The glucose is then released into the circulation for use by body cells.
- It stimulates the liver to take up amino acids from the blood and convert them into glucose. This response is known as gluconeogenesis.
- It stimulates lipolysis, the breakdown of stored triglycerides into free fatty acids and glycerol. Some of the free glycerol released into the bloodstream travels to the liver, which converts it into glucose. This is also a form of gluconeogenesis.
Taken together, these actions increase blood glucose levels. The activity of glucagon is regulated through a negative feedback mechanism; rising blood glucose levels inhibit further glucagon production and secretion.
Figure 17.19 Homeostatic Regulation of Blood Glucose Levels Blood glucose concentration is tightly maintained between 70 mg/dL and 110 mg/dL. If blood glucose concentration rises above this range, insulin is released, which stimulates body cells to remove glucose from the blood. If blood glucose concentration drops below this range, glucagon is released, which stimulates body cells to release glucose into the blood.
Insulin
The primary function of insulin is to facilitate the uptake of glucose into body cells. Red blood cells, as well as cells of the brain, liver, kidneys, and the lining of the small intestine, do not have insulin receptors on their cell membranes and do not require insulin for glucose uptake. Although all other body cells do require insulin if they are to take glucose from the bloodstream, skeletal muscle cells and adipose cells are the primary targets of insulin.
The presence of food in the intestine triggers the release of gastrointestinal tract hormones such as glucose-dependent insulinotropic peptide (previously known as gastric inhibitory peptide). This is in turn the initial trigger for insulin production and secretion by the beta cells of the pancreas. Once nutrient absorption occurs, the resulting surge in blood glucose levels further stimulates insulin secretion.
Precisely how insulin facilitates glucose uptake is not entirely clear. However, insulin appears to activate a tyrosine kinase receptor, triggering the phosphorylation of many substrates within the cell. These multiple biochemical reactions converge to support the movement of intracellular vesicles containing facilitative glucose transporters to the cell membrane. In the absence of insulin, these transport proteins are normally recycled slowly between the cell membrane and cell interior. Insulin triggers the rapid movement of a pool of glucose transporter vesicles to the cell membrane, where they fuse and expose the glucose transporters to the extracellular fluid. The transporters then move glucose by facilitated diffusion into the cell interior.
INTERACTIVE LINK
Visit this link to view an animation describing the location and function of the pancreas. What goes wrong in the function of insulin in type 2 diabetes?
Insulin also reduces blood glucose levels by stimulating glycolysis, the metabolism of glucose for generation of ATP. Moreover, it stimulates the liver to convert excess glucose into glycogen for storage, and it inhibits enzymes involved in glycogenolysis and gluconeogenesis. Finally, insulin promotes triglyceride and protein synthesis. The secretion of insulin is regulated through a negative feedback mechanism. As blood glucose levels decrease, further insulin release is inhibited. The pancreatic hormones are summarized in Table 17.7.
Hormones of the Pancreas
| Associated hormones | Chemical class | Effect |
|---|---|---|
| Insulin (beta cells) | Protein | Reduces blood glucose levels |
| Glucagon (alpha cells) | Protein | Increases blood glucose levels |
| Somatostatin (delta cells) | Protein | Inhibits insulin and glucagon release |
| Pancreatic polypeptide (PP cells) | Protein | Role in appetite |
Table 17.7
DISORDERS OF THE...
Endocrine System: Diabetes Mellitus
Dysfunction of insulin production and secretion, as well as the target cells’ responsiveness to insulin, can lead to a condition called diabetes mellitus. An increasingly common disease, diabetes mellitus has been diagnosed in more than 18 million adults in the United States, and more than 200,000 children. It is estimated that up to 7 million more adults have the condition but have not been diagnosed. In addition, approximately 79 million people in the US are estimated to have pre-diabetes, a condition in which blood glucose levels are abnormally high, but not yet high enough to be classified as diabetes.
There are two main forms of diabetes mellitus. Type 1 diabetes is an autoimmune disease affecting the beta cells of the pancreas. Certain genes are recognized to increase susceptibility. The beta cells of people with type 1 diabetes do not produce insulin; thus, synthetic insulin must be administered by injection or infusion. This form of diabetes accounts for less than five percent of all diabetes cases.
Type 2 diabetes accounts for approximately 95 percent of all cases. It is acquired, and lifestyle factors such as poor diet, inactivity, and the presence of pre-diabetes greatly increase a person’s risk. About 80 to 90 percent of people with type 2 diabetes are overweight or obese. In type 2 diabetes, cells become resistant to the effects of insulin. In response, the pancreas increases its insulin secretion, but over time, the beta cells become exhausted. In many cases, type 2 diabetes can be reversed by moderate weight loss, regular physical activity, and consumption of a healthy diet; however, if blood glucose levels cannot be controlled, the diabetic will eventually require insulin.
Two of the early manifestations of diabetes are excessive urination and excessive thirst. They demonstrate how the out-of-control levels of glucose in the blood affect kidney function. The kidneys are responsible for filtering glucose from the blood. Excessive blood glucose draws water into the urine, and as a result the person eliminates an abnormally large quantity of sweet urine. The use of body water to dilute the urine leaves the body dehydrated, and so the person is unusually and continually thirsty. The person may also experience persistent hunger because the body cells are unable to access the glucose in the bloodstream.
Over time, persistently high levels of glucose in the blood injure tissues throughout the body, especially those of the blood vessels and nerves. Inflammation and injury of the lining of arteries lead to atherosclerosis and an increased risk of heart attack and stroke. Damage to the microscopic blood vessels of the kidney impairs kidney function and can lead to kidney failure. Damage to blood vessels that serve the eyes can lead to blindness. Blood vessel damage also reduces circulation to the limbs, whereas nerve damage leads to a loss of sensation, called neuropathy, particularly in the hands and feet. Together, these changes increase the risk of injury, infection, and tissue death (necrosis), contributing to a high rate of toe, foot, and lower leg amputations in people with diabetes. Uncontrolled diabetes can also lead to a dangerous form of metabolic acidosis called ketoacidosis. Deprived of glucose, cells increasingly rely on fat stores for fuel. However, in a glucose-deficient state, the liver is forced to use an alternative lipid metabolism pathway that results in the increased production of ketone bodies (or ketones), which are acidic. The build-up of ketones in the blood causes ketoacidosis, which—if left untreated—may lead to a life-threatening “diabetic coma.” Together, these complications make diabetes the seventh leading cause of death in the United States.
Diabetes is diagnosed when lab tests reveal that blood glucose levels are higher than normal, a condition called hyperglycemia. The treatment of diabetes depends on the type, the severity of the condition, and the ability of the patient to make lifestyle changes. As noted earlier, moderate weight loss, regular physical activity, and consumption of a healthful diet can reduce blood glucose levels. Some patients with type 2 diabetes may be unable to control their disease with these lifestyle changes, and will require medication. Historically, the first-line treatment of type 2 diabetes was insulin. Research advances have resulted in alternative options, including medications that enhance pancreatic function.
INTERACTIVE LINK
Visit this link to view an animation describing the role of insulin and the pancreas in diabetes.
Organs with Secondary Endocrine Functions
- Identify the organs with a secondary endocrine function, the hormone they produce, and its effects
In your study of anatomy and physiology, you have already encountered a few of the many organs of the body that have secondary endocrine functions. Here, you will learn about the hormone-producing activities of the heart, gastrointestinal tract, kidneys, skeleton, adipose tissue, skin, and thymus.
Heart
When the body experiences an increase in blood volume or pressure, the cells of the heart’s atrial wall stretch. In response, specialized cells in the wall of the atria produce and secrete the peptide hormone atrial natriuretic peptide (ANP). ANP signals the kidneys to reduce sodium reabsorption, thereby decreasing the amount of water reabsorbed from the urine filtrate and reducing blood volume. Other actions of ANP include the inhibition of renin secretion, thus inhibition of the renin-angiotensin-aldosterone system (RAAS) and vasodilation. Therefore, ANP aids in decreasing blood pressure, blood volume, and blood sodium levels.
Gastrointestinal Tract
The endocrine cells of the GI tract are located in the mucosa of the stomach and small intestine. Some of these hormones are secreted in response to eating a meal and aid in digestion. An example of a hormone secreted by the stomach cells is gastrin, a peptide hormone secreted in response to stomach distention that stimulates the release of hydrochloric acid. Secretin is a peptide hormone secreted by the small intestine as acidic chyme (partially digested food and fluid) moves from the stomach. It stimulates the release of bicarbonate from the pancreas, which buffers the acidic chyme, and inhibits the further secretion of hydrochloric acid by the stomach. Cholecystokinin (CCK) is another peptide hormone released from the small intestine. It promotes the secretion of pancreatic enzymes and the release of bile from the gallbladder, both of which facilitate digestion. Other hormones produced by the intestinal cells aid in glucose metabolism, such as by stimulating the pancreatic beta cells to secrete insulin, reducing glucagon secretion from the alpha cells, or enhancing cellular sensitivity to insulin.
Kidneys
The kidneys participate in several complex endocrine pathways and produce certain hormones. A decline in blood flow to the kidneys stimulates them to release the enzyme renin, triggering the renin-angiotensin-aldosterone (RAAS) system, and stimulating the reabsorption of sodium and water. The reabsorption increases blood flow and blood pressure. The kidneys also play a role in regulating blood calcium levels through the production of calcitriol from vitamin D3, which is released in response to the secretion of parathyroid hormone (PTH). In addition, the kidneys produce the hormone erythropoietin (EPO) in response to low oxygen levels. EPO stimulates the production of red blood cells (erythrocytes) in the bone marrow, thereby increasing oxygen delivery to tissues. You may have heard of EPO as a performance-enhancing drug (in a synthetic form).
Skeleton
Although bone has long been recognized as a target for hormones, only recently have researchers recognized that the skeleton itself produces at least two hormones. Fibroblast growth factor 23 (FGF23) is produced by bone cells in response to increased blood levels of vitamin D3 or phosphate. It triggers the kidneys to inhibit the formation of calcitriol from vitamin D3 and to increase phosphorus excretion. Osteocalcin, produced by osteoblasts, stimulates the pancreatic beta cells to increase insulin production. It also acts on peripheral tissues to increase their sensitivity to insulin and their utilization of glucose.
Adipose Tissue
Adipose tissue produces and secretes several hormones involved in lipid metabolism and storage. One important example is leptin, a protein manufactured by adipose cells that circulates in amounts directly proportional to levels of body fat. Leptin is released in response to food consumption and acts by binding to brain neurons involved in energy intake and expenditure. Binding of leptin produces a feeling of satiety after a meal, thereby reducing appetite. It also appears that the binding of leptin to brain receptors triggers the sympathetic nervous system to regulate bone metabolism, increasing deposition of cortical bone. Adiponectin—another hormone synthesized by adipose cells—appears to reduce cellular insulin resistance and to protect blood vessels from inflammation and atherosclerosis. Its levels are lower in people who are obese, and rise following weight loss.
Skin
The skin functions as an endocrine organ in the production of the inactive form of vitamin D3, cholecalciferol. When cholesterol present in the epidermis is exposed to ultraviolet radiation, it is converted to cholecalciferol, which then enters the blood. In the liver, cholecalciferol is converted to an intermediate that travels to the kidneys and is further converted to calcitriol, the active form of vitamin D3. Vitamin D is important in a variety of physiological processes, including intestinal calcium absorption and immune system function. In some studies, low levels of vitamin D have been associated with increased risks of cancer, severe asthma, and multiple sclerosis. Vitamin D deficiency in children causes rickets, and in adults, osteomalacia—both of which are characterized by bone deterioration.
Thymus
The thymus is an organ of the immune system that is larger and more active during infancy and early childhood, and begins to atrophy as we age. Its endocrine function is the production of a group of hormones called thymosins that contribute to the development and differentiation of T lymphocytes, which are immune cells. Although the role of thymosins is not yet well understood, it is clear that they contribute to the immune response. Thymosins have been found in tissues other than the thymus and have a wide variety of functions, so the thymosins cannot be strictly categorized as thymic hormones.
Liver
The liver is responsible for secreting at least four important hormones or hormone precursors: insulin-like growth factor (somatomedin), angiotensinogen, thrombopoetin, and hepcidin. Insulin-like growth factor-1 is the immediate stimulus for growth in the body, especially of the bones. Angiotensinogen is the precursor to angiotensin, mentioned earlier, which increases blood pressure. Thrombopoetin stimulates the production of the blood’s platelets. Hepcidins block the release of iron from cells in the body, helping to regulate iron homeostasis in our body fluids. The major hormones of these other organs are summarized in Table 17.8.
Organs with Secondary Endocrine Functions and Their Major Hormones
| Organ | Major hormones | Effects |
|---|---|---|
| Heart | Atrial natriuretic peptide (ANP) | Reduces blood volume, blood pressure, and Na+concentration |
| Gastrointestinal tract | Gastrin, secretin, and cholecystokinin | Aid digestion of food and buffering of stomach acids |
| Gastrointestinal tract | Glucose-dependent insulinotropic peptide (GIP) and glucagon-like peptide 1 (GLP-1) | Stimulate beta cells of the pancreas to release insulin |
| Kidneys | Renin | Stimulates release of aldosterone |
| Kidneys | Calcitriol | Aids in the absorption of Ca2+ |
| Kidneys | Erythropoietin | Triggers the formation of red blood cells in the bone marrow |
| Skeleton | FGF23 | Inhibits production of calcitriol and increases phosphate excretion |
| Skeleton | Osteocalcin | Increases insulin production |
| Adipose tissue | Leptin | Promotes satiety signals in the brain |
| Adipose tissue | Adiponectin | Reduces insulin resistance |
| Skin | Cholecalciferol | Modified to form vitamin D |
| Thymus (and other organs) | Thymosins | Among other things, aids in the development of T lymphocytes of the immune system |
| Liver | Insulin-like growth factor-1 | Stimulates bodily growth |
| Liver | Angiotensinogen | Raises blood pressure |
| Liver | Thrombopoetin | Causes increase in platelets |
| Liver | Hepcidin | Blocks release of iron into body fluids |
Table 17.8
Development and Aging of the Endocrine System
- Describe the embryonic origins of the endocrine system
- Discuss the effects of aging on the endocrine system
The endocrine system arises from all three embryonic germ layers. The endocrine glands that produce the steroid hormones, such as the gonads and adrenal cortex, arise from the mesoderm. In contrast, endocrine glands that arise from the endoderm and ectoderm produce the amine, peptide, and protein hormones. The pituitary gland arises from two distinct areas of the ectoderm: the anterior pituitary gland arises from the oral ectoderm, whereas the posterior pituitary gland arises from the neural ectoderm at the base of the hypothalamus. The pineal gland also arises from the ectoderm. The two structures of the adrenal glands arise from two different germ layers: the adrenal cortex from the mesoderm and the adrenal medulla from ectoderm neural cells. The endoderm gives rise to the thyroid and parathyroid glands, as well as the pancreas and the thymus.
As the body ages, changes occur that affect the endocrine system, sometimes altering the production, secretion, and catabolism of hormones. For example, the structure of the anterior pituitary gland changes as vascularization decreases and the connective tissue content increases with increasing age. This restructuring affects the gland’s hormone production. For example, the amount of human growth hormone that is produced declines with age, resulting in the reduced muscle mass commonly observed in the elderly.
The adrenal glands also undergo changes as the body ages; as fibrous tissue increases, the production of cortisol and aldosterone decreases. Interestingly, the production and secretion of epinephrine and norepinephrine remain normal throughout the aging process.
A well-known example of the aging process affecting an endocrine gland is menopause and the decline of ovarian function. With increasing age, the ovaries decrease in both size and weight and become progressively less sensitive to gonadotropins. This gradually causes a decrease in estrogen and progesterone levels, leading to menopause and the inability to reproduce. Low levels of estrogens and progesterone are also associated with some disease states, such as osteoporosis, atherosclerosis, and hyperlipidemia, or abnormal blood lipid levels.
Testosterone levels also decline with age, a condition called andropause (or viropause); however, this decline is much less dramatic than the decline of estrogens in women, and much more gradual, rarely affecting sperm production until very old age. Although this means that males maintain their ability to father children for decades longer than females, the quantity, quality, and motility of their sperm is often reduced.
As the body ages, the thyroid gland produces less of the thyroid hormones, causing a gradual decrease in the basal metabolic rate. The lower metabolic rate reduces the production of body heat and increases levels of body fat. Parathyroid hormones, on the other hand, increase with age. This may be because of reduced dietary calcium levels, causing a compensatory increase in parathyroid hormone. However, increased parathyroid hormone levels combined with decreased levels of calcitonin (and estrogens in women) can lead to osteoporosis as PTH stimulates demineralization of bones to increase blood calcium levels. Notice that osteoporosis is common in both elderly males and females.
Increasing age also affects glucose metabolism, as blood glucose levels spike more rapidly and take longer to return to normal in the elderly. In addition, increasing glucose intolerance may occur because of a gradual decline in cellular insulin sensitivity. Almost 27 percent of Americans aged 65 and older have diabetes.
Key Terms
- acromegaly
- disorder in adults caused when abnormally high levels of GH trigger growth of bones in the face, hands, and feet
- adenylyl cyclase
- membrane-bound enzyme that converts ATP to cyclic AMP, creating cAMP, as a result of G-protein activation
- adrenal cortex
- outer region of the adrenal glands consisting of multiple layers of epithelial cells and capillary networks that produces mineralocorticoids and glucocorticoids
- adrenal glands
- endocrine glands located at the top of each kidney that are important for the regulation of the stress response, blood pressure and blood volume, water homeostasis, and electrolyte levels
- adrenal medulla
- inner layer of the adrenal glands that plays an important role in the stress response by producing epinephrine and norepinephrine
- adrenocorticotropic hormone (ACTH)
- anterior pituitary hormone that stimulates the adrenal cortex to secrete corticosteroid hormones (also called corticotropin)
- alarm reaction
- the short-term stress, or the fight-or-flight response, of stage one of the general adaptation syndrome mediated by the hormones epinephrine and norepinephrine
- aldosterone
- hormone produced and secreted by the adrenal cortex that stimulates sodium and fluid retention and increases blood volume and blood pressure
- alpha cell
- pancreatic islet cell type that produces the hormone glucagon
- angiotensin-converting enzyme
- the enzyme that converts angiotensin I to angiotensin II
- antidiuretic hormone (ADH)
- hypothalamic hormone that is stored by the posterior pituitary and that signals the kidneys to reabsorb water
- atrial natriuretic peptide (ANP)
- peptide hormone produced by the walls of the atria in response to high blood pressure, blood volume, or blood sodium that reduces the reabsorption of sodium and water in the kidneys and promotes vasodilation
- autocrine
- chemical signal that elicits a response in the same cell that secreted it
- beta cell
- pancreatic islet cell type that produces the hormone insulin
- calcitonin
- peptide hormone produced and secreted by the parafollicular cells (C cells) of the thyroid gland that functions to decrease blood calcium levels
- chromaffin
- neuroendocrine cells of the adrenal medulla
- colloid
- viscous fluid in the central cavity of thyroid follicles, containing the glycoprotein thyroglobulin
- cortisol
- glucocorticoid important in gluconeogenesis, the catabolism of glycogen, and downregulation of the immune system
- cyclic adenosine monophosphate (cAMP)
- second messenger that, in response to adenylyl cyclase activation, triggers a phosphorylation cascade
- delta cell
- minor cell type in the pancreas that secretes the hormone somatostatin
- diabetes mellitus
- condition caused by destruction or dysfunction of the beta cells of the pancreas or cellular resistance to insulin that results in abnormally high blood glucose levels
- diacylglycerol (DAG)
- molecule that, like cAMP, activates protein kinases, thereby initiating a phosphorylation cascade
- downregulation
- decrease in the number of hormone receptors, typically in response to chronically excessive levels of a hormone
- endocrine gland
- tissue or organ that secretes hormones into the blood and lymph without ducts such that they may be transported to organs distant from the site of secretion
- endocrine system
- cells, tissues, and organs that secrete hormones as a primary or secondary function and play an integral role in normal bodily processes
- epinephrine
- primary and most potent catecholamine hormone secreted by the adrenal medulla in response to short-term stress; also called adrenaline
- erythropoietin (EPO)
- protein hormone secreted in response to low oxygen levels that triggers the bone marrow to produce red blood cells
- estrogens
- class of predominantly female sex hormones important for the development and growth of the female reproductive tract, secondary sex characteristics, the female reproductive cycle, and the maintenance of pregnancy
- exocrine system
- cells, tissues, and organs that secrete substances directly to target tissues via glandular ducts
- first messenger
- hormone that binds to a cell membrane hormone receptor and triggers activation of a second messenger system
- follicle-stimulating hormone (FSH)
- anterior pituitary hormone that stimulates the production and maturation of sex cells
- G protein
- protein associated with a cell membrane hormone receptor that initiates the next step in a second messenger system upon activation by hormone–receptor binding
- general adaptation syndrome (GAS)
- the human body’s three-stage response pattern to short- and long-term stress
- gigantism
- disorder in children caused when abnormally high levels of GH prompt excessive growth
- glucagon
- pancreatic hormone that stimulates the catabolism of glycogen to glucose, thereby increasing blood glucose levels
- glucocorticoids
- hormones produced by the zona fasciculata of the adrenal cortex that influence glucose metabolism
- goiter
- enlargement of the thyroid gland either as a result of iodine deficiency or hyperthyroidism
- gonadotropins
- hormones that regulate the function of the gonads
- growth hormone (GH)
- anterior pituitary hormone that promotes tissue building and influences nutrient metabolism (also called somatotropin)
- hormone
- secretion of an endocrine organ that travels via the bloodstream or lymphatics to induce a response in target cells or tissues in another part of the body
- hormone receptor
- protein within a cell or on the cell membrane that binds a hormone, initiating the target cell response
- hyperglycemia
- abnormally high blood glucose levels
- hyperparathyroidism
- disorder caused by overproduction of PTH that results in abnormally elevated blood calcium
- hyperthyroidism
- clinically abnormal, elevated level of thyroid hormone in the blood; characterized by an increased metabolic rate, excess body heat, sweating, diarrhea, weight loss, and increased heart rate
- hypoparathyroidism
- disorder caused by underproduction of PTH that results in abnormally low blood calcium
- hypophyseal portal system
- network of blood vessels that enables hypothalamic hormones to travel into the anterior lobe of the pituitary without entering the systemic circulation
- hypothalamus
- region of the diencephalon inferior to the thalamus that functions in neural and endocrine signaling
- hypothyroidism
- clinically abnormal, low level of thyroid hormone in the blood; characterized by low metabolic rate, weight gain, cold extremities, constipation, and reduced mental activity
- infundibulum
- stalk containing vasculature and neural tissue that connects the pituitary gland to the hypothalamus (also called the pituitary stalk)
- inhibin
- hormone secreted by the male and female gonads that inhibits FSH production by the anterior pituitary
- inositol triphosphate (IP3)
- molecule that initiates the release of calcium ions from intracellular stores
- insulin
- pancreatic hormone that enhances the cellular uptake and utilization of glucose, thereby decreasing blood glucose levels
- insulin-like growth factors (IGF)
- protein that enhances cellular proliferation, inhibits apoptosis, and stimulates the cellular uptake of amino acids for protein synthesis
- leptin
- protein hormone secreted by adipose tissues in response to food consumption that promotes satiety
- luteinizing hormone (LH)
- anterior pituitary hormone that triggers ovulation and the production of ovarian hormones in females, and the production of testosterone in males
- melatonin
- amino acid–derived hormone that is secreted in response to low light and causes drowsiness
- mineralocorticoids
- hormones produced by the zona glomerulosa cells of the adrenal cortex that influence fluid and electrolyte balance
- neonatal hypothyroidism
- condition characterized by cognitive deficits, short stature, and other signs and symptoms in people born to women who were iodine-deficient during pregnancy
- norepinephrine
- secondary catecholamine hormone secreted by the adrenal medulla in response to short-term stress; also called noradrenaline
- osmoreceptor
- hypothalamic sensory receptor that is stimulated by changes in solute concentration (osmotic pressure) in the blood
- oxytocin
- hypothalamic hormone stored in the posterior pituitary gland and important in stimulating uterine contractions in labor, milk ejection during breastfeeding, and feelings of attachment (also produced in males)
- pancreas
- organ with both exocrine and endocrine functions located posterior to the stomach that is important for digestion and the regulation of blood glucose
- pancreatic islets
- specialized clusters of pancreatic cells that have endocrine functions; also called islets of Langerhans
- paracrine
- chemical signal that elicits a response in neighboring cells; also called paracrine factor
- parathyroid glands
- small, round glands embedded in the posterior thyroid gland that produce parathyroid hormone (PTH)
- parathyroid hormone (PTH)
- peptide hormone produced and secreted by the parathyroid glands in response to low blood calcium levels
- phosphodiesterase (PDE)
- cytosolic enzyme that deactivates and degrades cAMP
- phosphorylation cascade
- signaling event in which multiple protein kinases phosphorylate the next protein substrate by transferring a phosphate group from ATP to the protein
- pineal gland
- endocrine gland that secretes melatonin, which is important in regulating the sleep-wake cycle
- pinealocyte
- cell of the pineal gland that produces and secretes the hormone melatonin
- pituitary dwarfism
- disorder in children caused when abnormally low levels of GH result in growth retardation
- pituitary gland
- bean-sized organ suspended from the hypothalamus that produces, stores, and secretes hormones in response to hypothalamic stimulation (also called hypophysis)
- PP cell
- minor cell type in the pancreas that secretes the hormone pancreatic polypeptide
- progesterone
- predominantly female sex hormone important in regulating the female reproductive cycle and the maintenance of pregnancy
- prolactin (PRL)
- anterior pituitary hormone that promotes development of the mammary glands and the production of breast milk
- protein kinase
- enzyme that initiates a phosphorylation cascade upon activation
- second messenger
- molecule that initiates a signaling cascade in response to hormone binding on a cell membrane receptor and activation of a G protein
- stage of exhaustion
- stage three of the general adaptation syndrome; the body’s long-term response to stress mediated by the hormones of the adrenal cortex
- stage of resistance
- stage two of the general adaptation syndrome; the body’s continued response to stress after stage one diminishes
- testosterone
- steroid hormone secreted by the male testes and important in the maturation of sperm cells, growth and development of the male reproductive system, and the development of male secondary sex characteristics
- thymosins
- hormones produced and secreted by the thymus that play an important role in the development and differentiation of T cells
- thymus
- organ that is involved in the development and maturation of T-cells and is particularly active during infancy and childhood
- thyroid gland
- large endocrine gland responsible for the synthesis of thyroid hormones
- thyroid-stimulating hormone (TSH)
- anterior pituitary hormone that triggers secretion of thyroid hormones by the thyroid gland (also called thyrotropin)
- thyroxine
- (also, tetraiodothyronine, T4) amino acid–derived thyroid hormone that is more abundant but less potent than T3 and often converted to T3 by target cells
- triiodothyronine
- (also, T3) amino acid–derived thyroid hormone that is less abundant but more potent than T4
- upregulation
- increase in the number of hormone receptors, typically in response to chronically reduced levels of a hormone
- zona fasciculata
- intermediate region of the adrenal cortex that produce hormones called glucocorticoids
- zona glomerulosa
- most superficial region of the adrenal cortex, which produces the hormones collectively referred to as mineralocorticoids
- zona reticularis
- deepest region of the adrenal cortex, which produces the steroid sex hormones called androgens
Chapter Review
17.1 An Overview of the Endocrine System
The endocrine system consists of cells, tissues, and organs that secrete hormones critical to homeostasis. The body coordinates its functions through two major types of communication: neural and endocrine. Neural communication includes both electrical and chemical signaling between neurons and target cells. Endocrine communication involves chemical signaling via the release of hormones into the extracellular fluid. From there, hormones diffuse into the bloodstream and may travel to distant body regions, where they elicit a response in target cells. Endocrine glands are ductless glands that secrete hormones. Many organs of the body with other primary functions—such as the heart, stomach, and kidneys—also have hormone-secreting cells.
17.2 Hormones
Hormones are derived from amino acids or lipids. Amine hormones originate from the amino acids tryptophan or tyrosine. Larger amino acid hormones include peptides and protein hormones. Steroid hormones are derived from cholesterol.
Steroid hormones and thyroid hormone are lipid soluble. All other amino acid–derived hormones are water soluble. Hydrophobic hormones are able to diffuse through the membrane and interact with an intracellular receptor. In contrast, hydrophilic hormones must interact with cell membrane receptors. These are typically associated with a G protein, which becomes activated when the hormone binds the receptor. This initiates a signaling cascade that involves a second messenger, such as cyclic adenosine monophosphate (cAMP). Second messenger systems greatly amplify the hormone signal, creating a broader, more efficient, and faster response.
Hormones are released upon stimulation that is of either chemical or neural origin. Regulation of hormone release is primarily achieved through negative feedback. Various stimuli may cause the release of hormones, but there are three major types. Humoral stimuli are changes in ion or nutrient levels in the blood. Hormonal stimuli are changes in hormone levels that initiate or inhibit the secretion of another hormone. Finally, a neural stimulus occurs when a nerve impulse prompts the secretion or inhibition of a hormone.
17.3 The Pituitary Gland and Hypothalamus
The hypothalamus–pituitary complex is located in the diencephalon of the brain. The hypothalamus and the pituitary gland are connected by a structure called the infundibulum, which contains vasculature and nerve axons. The pituitary gland is divided into two distinct structures with different embryonic origins. The posterior lobe houses the axon terminals of hypothalamic neurons. It stores and releases into the bloodstream two hypothalamic hormones: oxytocin and antidiuretic hormone (ADH). The anterior lobe is connected to the hypothalamus by vasculature in the infundibulum and produces and secretes six hormones. Their secretion is regulated, however, by releasing and inhibiting hormones from the hypothalamus. The six anterior pituitary hormones are: growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), and prolactin (PRL).
17.4 The Thyroid Gland
The thyroid gland is a butterfly-shaped organ located in the neck anterior to the trachea. Its hormones regulate basal metabolism, oxygen use, nutrient metabolism, the production of ATP, and calcium homeostasis. They also contribute to protein synthesis and the normal growth and development of body tissues, including maturation of the nervous system, and they increase the body’s sensitivity to catecholamines. The thyroid hormones triiodothyronine (T3) and thyroxine (T4) are produced and secreted by the thyroid gland in response to thyroid-stimulating hormone (TSH) from the anterior pituitary. Synthesis of the amino acid–derived T3 and T4 hormones requires iodine. Insufficient amounts of iodine in the diet can lead to goiter, cretinism, and many other disorders.
17.5 The Parathyroid Glands
Calcium is required for a variety of important physiologic processes, including neuromuscular functioning; thus, blood calcium levels are closely regulated. The parathyroid glands are small structures located on the posterior thyroid gland that produce parathyroid hormone (PTH), which regulates blood calcium levels. Low blood calcium levels cause the production and secretion of PTH. In contrast, elevated blood calcium levels inhibit secretion of PTH and trigger secretion of the thyroid hormone calcitonin. Underproduction of PTH can result in hypoparathyroidism. In contrast, overproduction of PTH can result in hyperparathyroidism.
17.6 The Adrenal Glands
The adrenal glands, located superior to each kidney, consist of two regions: the adrenal cortex and adrenal medulla. The adrenal cortex—the outer layer of the gland—produces mineralocorticoids, glucocorticoids, and androgens. The adrenal medulla at the core of the gland produces epinephrine and norepinephrine.
The adrenal glands mediate a short-term stress response and a long-term stress response. A perceived threat results in the secretion of epinephrine and norepinephrine from the adrenal medulla, which mediate the fight-or-flight response. The long-term stress response is mediated by the secretion of CRH from the hypothalamus, which triggers ACTH, which in turn stimulates the secretion of corticosteroids from the adrenal cortex. The mineralocorticoids, chiefly aldosterone, cause sodium and fluid retention, which increases blood volume and blood pressure.
17.7 The Pineal Gland
The pineal gland is an endocrine structure of the diencephalon of the brain, and is located inferior and posterior to the thalamus. It is made up of pinealocytes. These cells produce and secrete the hormone melatonin in response to low light levels. High blood levels of melatonin induce drowsiness. Jet lag, caused by traveling across several time zones, occurs because melatonin synthesis takes several days to readjust to the light-dark patterns in the new environment.
17.8 Gonadal and Placental Hormones
The male and female reproductive system is regulated by follicle-stimulating hormone (FSH) and luteinizing hormone (LH) produced by the anterior lobe of the pituitary gland in response to gonadotropin-releasing hormone (GnRH) from the hypothalamus. In males, FSH stimulates sperm maturation, which is inhibited by the hormone inhibin. The steroid hormone testosterone, a type of androgen, is released in response to LH and is responsible for the maturation and maintenance of the male reproductive system, as well as the development of male secondary sex characteristics. In females, FSH promotes egg maturation and LH signals the secretion of the female sex hormones, the estrogens and progesterone. Both of these hormones are important in the development and maintenance of the female reproductive system, as well as maintaining pregnancy. The placenta develops during early pregnancy, and secretes several hormones important for maintaining the pregnancy.
17.9 The Endocrine Pancreas
The pancreas has both exocrine and endocrine functions. The pancreatic islet cell types include alpha cells, which produce glucagon; beta cells, which produce insulin; delta cells, which produce somatostatin; and PP cells, which produce pancreatic polypeptide. Insulin and glucagon are involved in the regulation of glucose metabolism. Insulin is produced by the beta cells in response to high blood glucose levels. It enhances glucose uptake and utilization by target cells, as well as the storage of excess glucose for later use. Dysfunction of the production of insulin or target cell resistance to the effects of insulin causes diabetes mellitus, a disorder characterized by high blood glucose levels. The hormone glucagon is produced and secreted by the alpha cells of the pancreas in response to low blood glucose levels. Glucagon stimulates mechanisms that increase blood glucose levels, such as the catabolism of glycogen into glucose.
17.10 Organs with Secondary Endocrine Functions
Some organs have a secondary endocrine function. For example, the walls of the atria of the heart produce the hormone atrial natriuretic peptide (ANP), the gastrointestinal tract produces the hormones gastrin, secretin, and cholecystokinin, which aid in digestion, and the kidneys produce erythropoietin (EPO), which stimulates the formation of red blood cells. Even bone, adipose tissue, and the skin have secondary endocrine functions.
17.11 Development and Aging of the Endocrine System
The endocrine system originates from all three germ layers of the embryo, including the endoderm, ectoderm, and mesoderm. In general, different hormone classes arise from distinct germ layers. Aging affects the endocrine glands, potentially affecting hormone production and secretion, and can cause disease. The production of hormones, such as human growth hormone, cortisol, aldosterone, sex hormones, and the thyroid hormones, decreases with age.
Interactive Link Questions
Visit this link to watch an animation of the events that occur when a hormone binds to a cell membrane receptor. What is the secondary messenger made by adenylyl cyclase during the activation of liver cells by epinephrine?
2.Visit this link to watch an animation showing the role of the hypothalamus and the pituitary gland. Which hormone is released by the pituitary to stimulate the thyroid gland?
3.Visit this link to view an animation describing the location and function of the adrenal glands. Which hormone produced by the adrenal glands is responsible for mobilization of energy stores?
4.Visit this link to view an animation describing the function of the hormone melatonin. What should you avoid doing in the middle of your sleep cycle that would lower melatonin?
5.Visit this link to view an animation describing the location and function of the pancreas. What goes wrong in the function of insulin in type 2 diabetes?
Review Questions
Endocrine glands ________.
- secrete hormones that travel through a duct to the target organs
- release neurotransmitters into the synaptic cleft
- secrete chemical messengers that travel in the bloodstream
- include sebaceous glands and sweat glands
Chemical signaling that affects neighboring cells is called ________.
- autocrine
- paracrine
- endocrine
- neuron
A newly developed pesticide has been observed to bind to an intracellular hormone receptor. If ingested, residue from this pesticide could disrupt levels of ________.
- melatonin
- thyroid hormone
- growth hormone
- insulin
A small molecule binds to a G protein, preventing its activation. What direct effect will this have on signaling that involves cAMP?
- The hormone will not be able to bind to the hormone receptor.
- Adenylyl cyclase will not be activated.
- Excessive quantities of cAMP will be produced.
- The phosphorylation cascade will be initiated.
A student is in a car accident, and although not hurt, immediately experiences pupil dilation, increased heart rate, and rapid breathing. What type of endocrine system stimulus did the student receive?
- humoral
- hormonal
- neural
- positive feedback
The hypothalamus is functionally and anatomically connected to the posterior pituitary lobe by a bridge of ________.
- blood vessels
- nerve axons
- cartilage
- bone
Which of the following is an anterior pituitary hormone?
- ADH
- oxytocin
- TSH
- cortisol
How many hormones are produced by the posterior pituitary?
- 0
- 1
- 2
- 6
Which of the following hormones contributes to the regulation of the body’s fluid and electrolyte balance?
- adrenocorticotropic hormone
- antidiuretic hormone
- luteinizing hormone
- all of the above
Which of the following statements about the thyroid gland is true?
- It is located anterior to the trachea and inferior to the larynx.
- The parathyroid glands are embedded within it.
- It manufactures three hormones.
- all of the above
The secretion of thyroid hormones is controlled by ________.
- TSH from the hypothalamus
- TSH from the anterior pituitary
- thyroxine from the anterior pituitary
- thyroglobulin from the thyroid’s parafollicular cells
The development of a goiter indicates that ________.
- the anterior pituitary is abnormally enlarged
- there is hypertrophy of the thyroid’s follicle cells
- there is an excessive accumulation of colloid in the thyroid follicles
- the anterior pituitary is secreting excessive growth hormone
Iodide ions cross from the bloodstream into follicle cells via ________.
- simple diffusion
- facilitated diffusion
- active transport
- osmosis
When blood calcium levels are low, PTH stimulates ________.
- urinary excretion of calcium by the kidneys
- a reduction in calcium absorption from the intestines
- the activity of osteoblasts
- the activity of osteoclasts
Which of the following can result from hyperparathyroidism?
- increased bone deposition
- fractures
- convulsions
- all of the above
The adrenal glands are attached superiorly to which organ?
- thyroid
- liver
- kidneys
- hypothalamus
What secretory cell type is found in the adrenal medulla?
- chromaffin cells
- neuroglial cells
- follicle cells
- oxyphil cells
Cushing’s disease is a disorder caused by ________.
- abnormally low levels of cortisol
- abnormally high levels of cortisol
- abnormally low levels of aldosterone
- abnormally high levels of aldosterone
Which of the following responses s not part of the fight-or-flight response?
- pupil dilation
- increased oxygen supply to the lungs
- suppressed digestion
- reduced mental activity
What cells secrete melatonin?
- melanocytes
- pinealocytes
- suprachiasmatic nucleus cells
- retinal cells
The production of melatonin is inhibited by ________.
- declining levels of light
- exposure to bright light
- the secretion of serotonin
- the activity of pinealocytes
The gonads produce what class of hormones?
- amine hormones
- peptide hormones
- steroid hormones
- catecholamines
The production of FSH by the anterior pituitary is reduced by which hormone?
- estrogens
- progesterone
- relaxin
- inhibin
The function of the placental hormone human placental lactogen (hPL) is to ________.
- prepare the breasts for lactation
- nourish the placenta
- regulate the menstrual cycle
- all of the above
If an autoimmune disorder targets the alpha cells, production of which hormone would be directly affected?
- somatostatin
- pancreatic polypeptide
- insulin
- glucagon
Which of the following statements about insulin is true?
- Insulin acts as a transport protein, carrying glucose across the cell membrane.
- Insulin facilitates the movement of intracellular glucose transporters to the cell membrane.
- Insulin stimulates the breakdown of stored glycogen into glucose.
- Insulin stimulates the kidneys to reabsorb glucose into the bloodstream.
The walls of the atria produce which hormone?
- cholecystokinin
- atrial natriuretic peptide
- renin
- calcitriol
The end result of the RAAS is to ________.
- reduce blood volume
- increase blood glucose
- reduce blood pressure
- increase blood pressure
Athletes may take synthetic EPO to boost their ________.
- blood calcium levels
- secretion of growth hormone
- blood oxygen levels
- muscle mass
Hormones produced by the thymus play a role in the ________.
- development of T cells
- preparation of the body for childbirth
- regulation of appetite
- release of hydrochloric acid in the stomach
The anterior pituitary gland develops from which embryonic germ layer?
- oral ectoderm
- neural ectoderm
- mesoderm
- endoderm
In the elderly, decreased thyroid function causes ________.
- increased tolerance for cold
- decreased basal metabolic rate
- decreased body fat
- osteoporosis
Critical Thinking Questions
Describe several main differences in the communication methods used by the endocrine system and the nervous system.
39.Compare and contrast endocrine and exocrine glands.
40.True or false: Neurotransmitters are a special class of paracrines. Explain your answer.
41.Compare and contrast the signaling events involved with the second messengers cAMP and IP3.
42.Describe the mechanism of hormone response resulting from the binding of a hormone with an intracellular receptor.
43.Compare and contrast the anatomical relationship of the anterior and posterior lobes of the pituitary gland to the hypothalamus.
44.Name the target tissues for prolactin.
45.Explain why maternal iodine deficiency might lead to neurological impairment in the fetus.
46.Define hyperthyroidism and explain why one of its symptoms is weight loss.
47.Describe the role of negative feedback in the function of the parathyroid gland.
48.Explain why someone with a parathyroid gland tumor might develop kidney stones.
49.What are the three regions of the adrenal cortex and what hormones do they produce?
50.If innervation to the adrenal medulla were disrupted, what would be the physiological outcome?
51.Compare and contrast the short-term and long-term stress response.
52.Seasonal affective disorder (SAD) is a mood disorder characterized by, among other symptoms, increased appetite, sluggishness, and increased sleepiness. It occurs most commonly during the winter months, especially in regions with long winter nights. Propose a role for melatonin in SAD and a possible non-drug therapy.
53.Retinitis pigmentosa (RP) is a disease that causes deterioration of the retinas of the eyes. Describe the impact RP would have on melatonin levels.
54.Compare and contrast the role of estrogens and progesterone.
55.Describe the role of placental secretion of relaxin in preparation for childbirth.
56.What would be the physiological consequence of a disease that destroyed the beta cells of the pancreas?
57.Why is foot care extremely important for people with diabetes mellitus?
58.Summarize the role of GI tract hormones following a meal.
59.Compare and contrast the thymus gland in infancy and adulthood.
60.Distinguish between the effects of menopause and andropause on fertility.
|
oercommons
|
2025-03-18T00:39:11.473001
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/56380/overview",
"title": "Anatomy and Physiology, Regulation, Integration, and Control",
"author": null
}
|
https://oercommons.org/courseware/lesson/72077/overview
|
Chapter 6 Reading Guide
Overview
This reading guide is intended to be used with the Open Stax Anatomy and Physiology textbook.
Open Stax Anatomy and Physiology Chapter 6 Reading Guide
6.1: The Functions of the Skeletal System
- Bone (osseous tissue)
- Hard, dense, connective tissue that makes up most of the adult skeleton.
- The support structure of the body also contains:
- ____________ – semi-rigid connective tissue provides flexibility and smooth surfaces for movement
- The _________________ is the body system composed of bones and cartilage; it performs the following functions:
- Body support – skeleton supports the body’s weight
- Facilitation of movement – serves as points of attachment for muscles.
- Protection of internal organs – skull and ribcage
- Production of blood cells – hematopoiesis occurs in red marrow.
- Storage and release of minerals and fat
- Bones store fat in yellow marrow
6.2 Bone Classification
There are approximately ______ bones that compose the adult skeleton.
These five categories are based on shape and function.
- ________ Bones
- Cylindrical in shape, longer than wide.
- Found in arms, legs, and fingers
- Function as levers; move when muscles contract
- Examples include femur, tibia, fibula, metacarpals, phalanges, and metatarsals.
- ________ Bones
- Cube-like in shape
- Provide stability and support, some limited movement
- Carpals and tarsals
- ________ Bones
- Typically, thin and curved.
- Cranial bones, scapulae, sternum, and ribs
- Serve as muscle points of attachment, protect internal organs
- ________ Bones
- No easily characterized shape fits no other classification.
- More complex in shape.
- Examples include the vertebrae and certain facial bones
- The vertebrae support the spinal cord and protect it against compression
- ________ bone
- Small round bones shaped like a sesame seed.
- Form in tendons where significant amounts of pressure are generated in a joint. Protect tendons against compression forces
- Vary in number from person to person. Found in tendons associated with the feet, hands, and knees
- The patella (kneecap) is the only sesamoid bone found in all people
6.3 Bone Structure
- Gross Anatomy of Bone (using a long bone for the example)
• ________ – the tubular shaft
- ________ cavity – the hollow region in the diaphysis, filled with yellow marrow.
- ________ bone – dense, hard osseous tissue makes up the walls of the diaphysis.
- ________ – wider sections at each end of the diaphysis.
- Composed of ________ bone, which is more porous then compact bone, with red marrow filling the spaces inside the spongy bone.
- ________ plate (growth plate) found where epiphysis meets the diaphysis.
- In children, it consists of a layer of hyaline cartilage in a growing bone.
- In adults, the cartilage is replaced by osseous tissue when growth stops, becomes an ________ line.
- ________ – a thin membrane that lines the medullary cavity
- Contains a layer of bone-forming cells with some connective tissue
- ________ – a touch connective tissue sheath that surrounds the outer surface of the bone, except at the joints
- It contains blood vessels, nerves, and lymphatics to nourish compact bone.
- ________ cartilage – thin layer of hyaline cartilage
- Covers the part of the epiphysis that forms a joint with another bone.
- Reduces shock and friction at freely movable joints, lacks blood vessels, and limited ability to repair the damage.
- ________ – a layer of spongy bone lined with compact bone on both sides, only associated with flat bones, skull, and sternum.
Bone Markings
- Three classes of bone markings: (1) articulations (joints) (2) projections, and (3) holes
- ________ – where two bone surfaces meet
- ________ – an area of bone that projects above the bone’s surface.
- ________ – an opening or groove in the bone that allows blood vessels and nerves to enter the bone.
Bone Cells and Tissue
- Bone consists of small numbers of cells embedded in a matrix of collagen fibers, with inorganic salt crystals.
- Calcium and phosphate ions create hydroxyapatite crystals
- These minerals give bone their hardness, strength, and flexibility
- Types of Bone Cells
- ________ cells – undifferentiated stem cells that divide to form osteoblasts. Found in both periosteum and endosteum.
- ____________ form new bone found in the growing portions,
- Synthesize new matrix will become osteocytes.
- Lack mitotic ability
- ____________ – mature bone cells most common cell type
- Located in spaces called ____________
- Maintain the mineral concentration of the matrix via secretion of enzymes, lack the ability to divide.
- Communicate with other osteocytes via long cytoplasmic extensions through channels called ____________ (singular = canaliculus)
- ____________ – cells derived from monocytes and macrophages.
- Multinucleated, breakdown old bone while osteoblasts form new bone
- This process is called bone remodeling.
Compact and Spongy Bone
- Compact Bone
- Dense, stronger than spongy bone
- Found under the periosteum and in the diaphysis of long bones
- Provides support and protection
- The ____________ or Haversian system
- The microscopic structural unit of compact bone.
- Each osteon is composed of:
- Lamellae (singular = lamella) concentric rings of calcified matrix
- A ____________ canal, or Haversian canal
- Contains blood vessels, nerves and lymphatic vessels.
- ____________ canals, or perforating canals
- Occur at right angles to the central canal, extend to the periosteum and endosteum.
- ____________, spaces in which osteocytes are found
- ____________ – transport nutrients to and wastes from the osteocytes
Spongy (Cancellous) Bone
- ___________ bone matrix is not arranged in concentric circles
- ____________ (singular – trabecula)
- A lattice – like network of matrix spikes, appears to have a spongy appearance to the naked eye.
- Form along the lines of stress, provide strength and balance to dense and heavy compact bone.
- Making bones lighter, so movement is easier.
- Contain red marrow, the site of hematopoiesis’s
Blood and Nerve Supply
- ____________ foramen – small openings in the diaphysis, through which arteries, nerves, and veins pass through.
Figure 6.15
Diagram of Blood and Nerve Supply to Bone
Blood vessels and nerves enter the bone through the nutrient foramen.
6.4 Bone formation and development
_____________ (osteogenesis) begins about the sixth or seventh week of embryonic life.
- Two pathways of osteogenesis
- Intramembranous ossification
- Endochondral ossification
- _______________________ ossification
- Compact and spongy bone directly develop from mesenchymal connective tissue
- Undifferentiated cells begin to specialize in specific tissues and spread out
- ________________ centers (sites of osteoblast formation) appear in clusters throughout the early bone tissue.
- _______________secrete ________ which hardens into a calcified matrix.
- Osteoblasts become osteocytes
- Bones of the skull and clavicles form this way
- Endochondral Ossification
- Bone replaces hyaline cartilage template
- More time consuming than intramembranous ossification
- Long bones, and bones at the base of the skull
- Outline of Endochondral Ossification
- Mesenchymal cells differentiate into chondrocytes forming a cartilaginous precursor of the bone
- __________________ (a membrane) forms that covers the cartilage.
- Chondrocytes grow in the center of the model, matrix calcifies
- Chondrocytes die, and surrounding cartilage disintegrates
- Blood vessels invade the spaces while carrying osteogenic cells with them.
- Enlarging spaces form the medullary cavity
- Growing capillaries penetrate the remaining cartilage; the perichondrium transforms into the periosteum.
- Osteoblasts form a periosteal collar of compact bone around the remaining cartilage
- A __________________________ center forms deep into the periosteal collar
- At the same time, chondrocytes and cartilage grow at the ends of the bone. At the same time bone is replacing the cartilage so that only cartilage that remains is at the:
- Articular surface
- Epiphyseal plate
- After birth, the same sequence continues with each of the new centers of activity are referred to as _____________ ossification center.
How Bones Grow in Length
- Cartilage forms on the epiphyseal side of the plate
- On the diaphyseal side, cartilage is ossified resulting in growth in length
- The ______________ zone does not participate in growth but secures the epiphyseal plate to the osseous tissue of the epiphysis
- ______________ zone – makes new chondrocytes to replace those that die at the diaphyseal end.
- Zone of ______________ and hypertrophy
- Older and larger chondrocytes than those in the proliferative zone.
- Cellular division in the proliferative zone and maturation of cells in the zone of maturation results in longitudinal growth of the bone
- Zone of ______________ matrix
- Chondrocytes die due to the calcification of the matrix around them.
- ______________ line – forms when bone growth stops in early adulthood, the chondrocytes in the epiphyseal plate cease dividing and bone replaces all the cartilage.
- Epiphyseal plates are visible in a growing bone.
- Epiphyseal lines are the remnants of epiphyseal plates in a mature bone.
- Appositional growth – bones grow in diameter, which continues after longitudinal growth stops
- Occurs by the process called modeling
- The old bone along the medullary cavity erodes by the activity of osteoclasts.
- The new bone deposits under the periosteum. This process also increases the diameter of the medullary cavity
Bone Remodeling
- The resorption of old or damaged bone takes place where osteoblasts lay new bone to replace it.
- Injury, exercise, and other activities lead to remodeling
- About ______________ of the skeleton is remodeled annually without injury or exercise.
6.5: Fractures: Bone repair
A fracture is any broken bone
- ______________ reduction – the setting of a broken bone without surgery
- ______________ reduction – requires surgery to expose the fracture and reset the bone
Types of Fractures
- ______________ (or simple) – the skin remains intact
- ______________ (or compound) – the broken bone tears the skin open
- ______________ – at right angle to long axis of the bone
- ______________ – any angle break that is not at 90 degrees
- ______________ – bone segments pulled apart due to a twisting motion
- ______________ – several breaks result in many small pieces between two large segments
- ______________ – a fracture in which at least one end of the broken bone tears, mostly occurs in children
Bone Repair
- Stages in Fracture Repair
- ______________ hematoma – clotting blood results in a disruption of blood flow to the bone, cells around the break die as a result.
- __________ formation (stabilizes the fracture)
- An ______________ callus forms a fibrocartilaginous matrix between the broken ends of the bone.
- Simultaneously, an ______________ callus of hyaline cartilage and bone forms around the outside of the break
- Bone remodeling occurs with bone tissue replacing the damaged areas of the fracture.
6.6: Exercise, Nutrition, Hormones, and Bone Tissue
Exercise and Bone Tissue
- Exercise places mechanical stress on the bones
- Mechanical stress stimulates the deposition of mineral salts and collagen into bone tissue.
- As stress increases or decreases, the internal and external bone structure change to match it.
Nutrition and Bone Tissue
- Calcium and Vitamin D
- The body obtains calcium from food; calcium cannot be absorbed in the small intestine without vitamin D.
- Sources of calcium besides milk and dairy products
- Broccoli, intact salmon, canned sardines, leafy green vegetables
- Sources of vitamin D
- Not found naturally in foods must be added.
- Sunlight upon the skin triggers the body to produce its vitamin D.
Hormones and Bone Tissue
- Several hormones play a key role in controlling bone growth and maintain the bone matrix.
- ______________ hormone (GH) – from anterior pituitary gland
- Triggers chondrocyte proliferation at epiphyseal plates
- Calcium retention which improves bone density
- ______________ – from the thyroid gland
- Promotes osteoblastic activity and synthesis of bone matrix
- ______________ hormone (PTH) (from the parathyroid glands)
- PTH stimulates osteoclast proliferation and activity
- The release from the bones into the bloodstream
- Promotes calcium absorption by the kidneys and small intestine
- ______________ (from thyroid gland)
- Inhibits osteoclast activity reduces the release of calcium into the blood.
Read and pay attention to section 6.7: Calcium Homeostasis: Interactions of the Skeletal System and Other Organ Systems
- You are expected to know the following things from reading this section
- Be able to describe the effects of too much or too little calcium on the body.
- Explain the process of calcium homeostasis.
- Outline the calcium homeostasis process as it occurs in the human body. Include all the facts that pertain to this process.
|
oercommons
|
2025-03-18T00:39:11.522891
|
09/04/2020
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/72077/overview",
"title": "Chapter 6 Reading Guide",
"author": "Bryon Spicci"
}
|
https://oercommons.org/courseware/lesson/56376/overview
|
Anatomy of the Nervous System
Introduction
Figure 13.1 Human Nervous System The ability to balance like an acrobat combines functions throughout the nervous system. The central and peripheral divisions coordinate control of the body using the senses of balance, body position, and touch on the soles of the feet. (credit: Rhett Sutphin)
CHAPTER OBJECTIVES
After studying this chapter, you will be able to:
- Relate the developmental processes of the embryonic nervous system to the adult structures
- Name the major regions of the adult nervous system
- Locate regions of the cerebral cortex on the basis of anatomical landmarks common to all human brains
- Describe the regions of the spinal cord in cross-section
- List the cranial nerves in order of anatomical location and provide the central and peripheral connections
- List the spinal nerves by vertebral region and by which nerve plexus each supplies
The nervous system is responsible for controlling much of the body, both through somatic (voluntary) and autonomic (involuntary) functions. The structures of the nervous system must be described in detail to understand how many of these functions are possible. There is a physiological concept known as localization of function that states that certain structures are specifically responsible for prescribed functions. It is an underlying concept in all of anatomy and physiology, but the nervous system illustrates the concept very well.
Fresh, unstained nervous tissue can be described as gray or white matter, and within those two types of tissue it can be very hard to see any detail. However, as specific regions and structures have been described, they were related to specific functions. Understanding these structures and the functions they perform requires a detailed description of the anatomy of the nervous system, delving deep into what the central and peripheral structures are.
The place to start this study of the nervous system is the beginning of the individual human life, within the womb. The embryonic development of the nervous system allows for a simple framework on which progressively more complicated structures can be built. With this framework in place, a thorough investigation of the nervous system is possible.
The Embryologic Perspective
- Describe the growth and differentiation of the neural tube
- Relate the different stages of development to the adult structures of the central nervous system
- Explain the expansion of the ventricular system of the adult brain from the central canal of the neural tube
- Describe the connections of the diencephalon and cerebellum on the basis of patterns of embryonic development
The brain is a complex organ composed of gray parts and white matter, which can be hard to distinguish. Starting from an embryologic perspective allows you to understand more easily how the parts relate to each other. The embryonic nervous system begins as a very simple structure—essentially just a straight line, which then gets increasingly complex. Looking at the development of the nervous system with a couple of early snapshots makes it easier to understand the whole complex system.
Many structures that appear to be adjacent in the adult brain are not connected, and the connections that exist may seem arbitrary. But there is an underlying order to the system that comes from how different parts develop. By following the developmental pattern, it is possible to learn what the major regions of the nervous system are.
The Neural Tube
To begin, a sperm cell and an egg cell fuse to become a fertilized egg. The fertilized egg cell, or zygote, starts dividing to generate the cells that make up an entire organism. Sixteen days after fertilization, the developing embryo’s cells belong to one of three germ layers that give rise to the different tissues in the body. The endoderm, or inner tissue, is responsible for generating the lining tissues of various spaces within the body, such as the mucosae of the digestive and respiratory systems. The mesoderm, or middle tissue, gives rise to most of the muscle and connective tissues. Finally the ectoderm, or outer tissue, develops into the integumentary system (the skin) and the nervous system. It is probably not difficult to see that the outer tissue of the embryo becomes the outer covering of the body. But how is it responsible for the nervous system?
As the embryo develops, a portion of the ectoderm differentiates into a specialized region of neuroectoderm, which is the precursor for the tissue of the nervous system. Molecular signals induce cells in this region to differentiate into the neuroepithelium, forming a neural plate. The cells then begin to change shape, causing the tissue to buckle and fold inward (Figure 13.2). A neural groove forms, visible as a line along the dorsal surface of the embryo. The ridge-like edge on either side of the neural groove is referred as the neural fold. As the neural folds come together and converge, the underlying structure forms into a tube just beneath the ectoderm called the neural tube. Cells from the neural folds then separate from the ectoderm to form a cluster of cells referred to as the neural crest, which runs lateral to the neural tube. The neural crest migrates away from the nascent, or embryonic, central nervous system (CNS) that will form along the neural groove and develops into several parts of the peripheral nervous system (PNS), including the enteric nervous tissue. Many tissues that are not part of the nervous system also arise from the neural crest, such as craniofacial cartilage and bone, and melanocytes.
Figure 13.2 Early Embryonic Development of Nervous System The neuroectoderm begins to fold inward to form the neural groove. As the two sides of the neural groove converge, they form the neural tube, which lies beneath the ectoderm. The anterior end of the neural tube will develop into the brain, and the posterior portion will become the spinal cord. The neural crest develops into peripheral structures.
At this point, the early nervous system is a simple, hollow tube. It runs from the anterior end of the embryo to the posterior end. Beginning at 25 days, the anterior end develops into the brain, and the posterior portion becomes the spinal cord. This is the most basic arrangement of tissue in the nervous system, and it gives rise to the more complex structures by the fourth week of development.
Primary Vesicles
As the anterior end of the neural tube starts to develop into the brain, it undergoes a couple of enlargements; the result is the production of sac-like vesicles. Similar to a child’s balloon animal, the long, straight neural tube begins to take on a new shape. Three vesicles form at the first stage, which are called primary vesicles. These vesicles are given names that are based on Greek words, the main root word being enkephalon, which means “brain” (en- = “inside”; kephalon = “head”). The prefix to each generally corresponds to its position along the length of the developing nervous system.
The prosencephalon (pros- = “in front”) is the forward-most vesicle, and the term can be loosely translated to mean forebrain. The mesencephalon (mes- = “middle”) is the next vesicle, which can be called the midbrain. The third vesicle at this stage is the rhombencephalon. The first part of this word is also the root of the word rhombus, which is a geometrical figure with four sides of equal length (a square is a rhombus with 90° angles). Whereas prosencephalon and mesencephalon translate into the English words forebrain and midbrain, there is not a word for “four-sided-figure-brain.” However, the third vesicle can be called the hindbrain. One way of thinking about how the brain is arranged is to use these three regions—forebrain, midbrain, and hindbrain—which are based on the primary vesicle stage of development (Figure 13.3a).
Secondary Vesicles
The brain continues to develop, and the vesicles differentiate further (see Figure 13.3b). The three primary vesicles become five secondary vesicles. The prosencephalon enlarges into two new vesicles called the telencephalon and the diencephalon. The telecephalon will become the cerebrum. The diencephalon gives rise to several adult structures; two that will be important are the thalamus and the hypothalamus. In the embryonic diencephalon, a structure known as the eye cup develops, which will eventually become the retina, the nervous tissue of the eye called the retina. This is a rare example of nervous tissue developing as part of the CNS structures in the embryo, but becoming a peripheral structure in the fully formed nervous system.
The mesencephalon does not differentiate into any finer divisions. The midbrain is an established region of the brain at the primary vesicle stage of development and remains that way. The rest of the brain develops around it and constitutes a large percentage of the mass of the brain. Dividing the brain into forebrain, midbrain, and hindbrain is useful in considering its developmental pattern, but the midbrain is a small proportion of the entire brain, relatively speaking.
The rhombencephalon develops into the metencephalon and myelencephalon. The metencephalon corresponds to the adult structure known as the pons and also gives rise to the cerebellum. The cerebellum (from the Latin meaning “little brain”) accounts for about 10 percent of the mass of the brain and is an important structure in itself. The most significant connection between the cerebellum and the rest of the brain is at the pons, because the pons and cerebellum develop out of the same vesicle. The myelencephalon corresponds to the adult structure known as the medulla oblongata. The structures that come from the mesencephalon and rhombencephalon, except for the cerebellum, are collectively considered the brain stem, which specifically includes the midbrain, pons, and medulla.
Figure 13.3 Primary and Secondary Vesicle Stages of Development The embryonic brain develops complexity through enlargements of the neural tube called vesicles; (a) The primary vesicle stage has three regions, and (b) the secondary vesicle stage has five regions.
INTERACTIVE LINK
Watch this animation to examine the development of the brain, starting with the neural tube. As the anterior end of the neural tube develops, it enlarges into the primary vesicles that establish the forebrain, midbrain, and hindbrain. Those structures continue to develop throughout the rest of embryonic development and into adolescence. They are the basis of the structure of the fully developed adult brain. How would you describe the difference in the relative sizes of the three regions of the brain when comparing the early (25th embryonic day) brain and the adult brain?
Spinal Cord Development
While the brain is developing from the anterior neural tube, the spinal cord is developing from the posterior neural tube. However, its structure does not differ from the basic layout of the neural tube. It is a long, straight cord with a small, hollow space down the center. The neural tube is defined in terms of its anterior versus posterior portions, but it also has a dorsal–ventral dimension. As the neural tube separates from the rest of the ectoderm, the side closest to the surface is dorsal, and the deeper side is ventral.
As the spinal cord develops, the cells making up the wall of the neural tube proliferate and differentiate into the neurons and glia of the spinal cord. The dorsal tissues will be associated with sensory functions, and the ventral tissues will be associated with motor functions.
Relating Embryonic Development to the Adult Brain
Embryonic development can help in understanding the structure of the adult brain because it establishes a framework on which more complex structures can be built. First, the neural tube establishes the anterior–posterior dimension of the nervous system, which is called the neuraxis. The embryonic nervous system in mammals can be said to have a standard arrangement. Humans (and other primates, to some degree) make this complicated by standing up and walking on two legs. The anterior–posterior dimension of the neuraxis overlays the superior–inferior dimension of the body. However, there is a major curve between the brain stem and forebrain, which is called the cephalic flexure. Because of this, the neuraxis starts in an inferior position—the end of the spinal cord—and ends in an anterior position, the front of the cerebrum. If this is confusing, just imagine a four-legged animal standing up on two legs. Without the flexure in the brain stem, and at the top of the neck, that animal would be looking straight up instead of straight in front (Figure 13.4).
Figure 13.4 Human Neuraxis The mammalian nervous system is arranged with the neural tube running along an anterior to posterior axis, from nose to tail for a four-legged animal like a dog. Humans, as two-legged animals, have a bend in the neuraxis between the brain stem and the diencephalon, along with a bend in the neck, so that the eyes and the face are oriented forward.
In summary, the primary vesicles help to establish the basic regions of the nervous system: forebrain, midbrain, and hindbrain. These divisions are useful in certain situations, but they are not equivalent regions. The midbrain is small compared with the hindbrain and particularly the forebrain. The secondary vesicles go on to establish the major regions of the adult nervous system that will be followed in this text. The telencephalon is the cerebrum, which is the major portion of the human brain. The diencephalon continues to be referred to by this Greek name, because there is no better term for it (dia- = “through”). The diencephalon is between the cerebrum and the rest of the nervous system and can be described as the region through which all projections have to pass between the cerebrum and everything else. The brain stem includes the midbrain, pons, and medulla, which correspond to the mesencephalon, metencephalon, and myelencephalon. The cerebellum, being a large portion of the brain, is considered a separate region. Table 13.1 connects the different stages of development to the adult structures of the CNS.
One other benefit of considering embryonic development is that certain connections are more obvious because of how these adult structures are related. The retina, which began as part of the diencephalon, is primarily connected to the diencephalon. The eyes are just inferior to the anterior-most part of the cerebrum, but the optic nerve extends back to the thalamus as the optic tract, with branches into a region of the hypothalamus. There is also a connection of the optic tract to the midbrain, but the mesencephalon is adjacent to the diencephalon, so that is not difficult to imagine. The cerebellum originates out of the metencephalon, and its largest white matter connection is to the pons, also from the metencephalon. There are connections between the cerebellum and both the medulla and midbrain, which are adjacent structures in the secondary vesicle stage of development. In the adult brain, the cerebellum seems close to the cerebrum, but there is no direct connection between them.
Another aspect of the adult CNS structures that relates to embryonic development is the ventricles—open spaces within the CNS where cerebrospinal fluid circulates. They are the remnant of the hollow center of the neural tube. The four ventricles and the tubular spaces associated with them can be linked back to the hollow center of the embryonic brain (see Table 13.1).
Stages of Embryonic Development
| Neural tube | Primary vesicle stage | Secondary vesicle stage | Adult structures | Ventricles |
|---|---|---|---|---|
| Anterior neural tube | Prosencephalon | Telencephalon | Cerebrum | Lateral ventricles |
| Anterior neural tube | Prosencephalon | Diencephalon | Diencephalon | Third ventricle |
| Anterior neural tube | Mesencephalon | Mesencephalon | Midbrain | Cerebral aqueduct |
| Anterior neural tube | Rhombencephalon | Metencephalon | Pons cerebellum | Fourth ventricle |
| Anterior neural tube | Rhombencephalon | Myelencephalon | Medulla | Fourth ventricle |
| Posterior neural tube | Spinal cord | Central canal |
Table 13.1
DISORDERS OF THE...
Nervous System
Early formation of the nervous system depends on the formation of the neural tube. A groove forms along the dorsal surface of the embryo, which becomes deeper until its edges meet and close off to form the tube. If this fails to happen, especially in the posterior region where the spinal cord forms, a developmental defect called spina bifida occurs. The closing of the neural tube is important for more than just the proper formation of the nervous system. The surrounding tissues are dependent on the correct development of the tube. The connective tissues surrounding the CNS can be involved as well.
There are three classes of this disorder: occulta, meningocele, and myelomeningocele (Figure 13.5). The first type, spina bifida occulta, is the mildest because the vertebral bones do not fully surround the spinal cord, but the spinal cord itself is not affected. No functional differences may be noticed, which is what the word occulta means; it is hidden spina bifida. The other two types both involve the formation of a cyst—a fluid-filled sac of the connective tissues that cover the spinal cord called the meninges. “Meningocele” means that the meninges protrude through the spinal column but nerves may not be involved and few symptoms are present, though complications may arise later in life. “Myelomeningocele” means that the meninges protrude and spinal nerves are involved, and therefore severe neurological symptoms can be present.
Often surgery to close the opening or to remove the cyst is necessary. The earlier that surgery can be performed, the better the chances of controlling or limiting further damage or infection at the opening. For many children with meningocele, surgery will alleviate the pain, although they may experience some functional loss. Because the myelomeningocele form of spina bifida involves more extensive damage to the nervous tissue, neurological damage may persist, but symptoms can often be handled. Complications of the spinal cord may present later in life, but overall life expectancy is not reduced.
Figure 13.5 Spinal Bifida (a) Spina bifida is a birth defect of the spinal cord caused when the neural tube does not completely close, but the rest of development continues. The result is the emergence of meninges and neural tissue through the vertebral column. (b) Fetal myelomeningocele is evident in this ultrasound taken at 21 weeks.
INTERACTIVE LINK
Watch this video to learn about the white matter in the cerebrum that develops during childhood and adolescence. This is a composite of MRI images taken of the brains of people from 5 years of age through 20 years of age, demonstrating how the cerebrum changes. As the color changes to blue, the ratio of gray matter to white matter changes. The caption for the video describes it as “less gray matter,” which is another way of saying “more white matter.” If the brain does not finish developing until approximately 20 years of age, can teenagers be held responsible for behaving badly?
The Central Nervous System
- Name the major regions of the adult brain
- Describe the connections between the cerebrum and brain stem through the diencephalon, and from those regions into the spinal cord
- Recognize the complex connections within the subcortical structures of the basal nuclei
- Explain the arrangement of gray and white matter in the spinal cord
The brain and the spinal cord are the central nervous system, and they represent the main organs of the nervous system. The spinal cord is a single structure, whereas the adult brain is described in terms of four major regions: the cerebrum, the diencephalon, the brain stem, and the cerebellum. A person’s conscious experiences are based on neural activity in the brain. The regulation of homeostasis is governed by a specialized region in the brain. The coordination of reflexes depends on the integration of sensory and motor pathways in the spinal cord.
The Cerebrum
The iconic gray mantle of the human brain, which appears to make up most of the mass of the brain, is the cerebrum (Figure 13.6). The wrinkled portion is the cerebral cortex, and the rest of the structure is beneath that outer covering. There is a large separation between the two sides of the cerebrum called the longitudinal fissure. It separates the cerebrum into two distinct halves, a right and left cerebral hemisphere. Deep within the cerebrum, the white matter of the corpus callosum provides the major pathway for communication between the two hemispheres of the cerebral cortex.
Figure 13.6 The Cerebrum The cerebrum is a large component of the CNS in humans, and the most obvious aspect of it is the folded surface called the cerebral cortex.
Many of the higher neurological functions, such as memory, emotion, and consciousness, are the result of cerebral function. The complexity of the cerebrum is different across vertebrate species. The cerebrum of the most primitive vertebrates is not much more than the connection for the sense of smell. In mammals, the cerebrum comprises the outer gray matter that is the cortex (from the Latin word meaning “bark of a tree”) and several deep nuclei that belong to three important functional groups. The basal nuclei are responsible for cognitive processing, the most important function being that associated with planning movements. The basal forebrain contains nuclei that are important in learning and memory. The limbic cortex is the region of the cerebral cortex that is part of the limbic system, a collection of structures involved in emotion, memory, and behavior.
Cerebral Cortex
The cerebrum is covered by a continuous layer of gray matter that wraps around either side of the forebrain—the cerebral cortex. This thin, extensive region of wrinkled gray matter is responsible for the higher functions of the nervous system. A gyrus(plural = gyri) is the ridge of one of those wrinkles, and a sulcus (plural = sulci) is the groove between two gyri. The pattern of these folds of tissue indicates specific regions of the cerebral cortex.
The head is limited by the size of the birth canal, and the brain must fit inside the cranial cavity of the skull. Extensive folding in the cerebral cortex enables more gray matter to fit into this limited space. If the gray matter of the cortex were peeled off of the cerebrum and laid out flat, its surface area would be roughly equal to one square meter.
The folding of the cortex maximizes the amount of gray matter in the cranial cavity. During embryonic development, as the telencephalon expands within the skull, the brain goes through a regular course of growth that results in everyone’s brain having a similar pattern of folds. The surface of the brain can be mapped on the basis of the locations of large gyri and sulci. Using these landmarks, the cortex can be separated into four major regions, or lobes (Figure 13.7). The lateral sulcus that separates the temporal lobe from the other regions is one such landmark. Superior to the lateral sulcus are the parietal lobe and frontal lobe, which are separated from each other by the central sulcus. The posterior region of the cortex is the occipital lobe, which has no obvious anatomical border between it and the parietal or temporal lobes on the lateral surface of the brain. From the medial surface, an obvious landmark separating the parietal and occipital lobes is called the parieto-occipital sulcus. The fact that there is no obvious anatomical border between these lobes is consistent with the functions of these regions being interrelated.
Figure 13.7 Lobes of the Cerebral Cortex The cerebral cortex is divided into four lobes. Extensive folding increases the surface area available for cerebral functions.
Different regions of the cerebral cortex can be associated with particular functions, a concept known as localization of function. In the early 1900s, a German neuroscientist named Korbinian Brodmann performed an extensive study of the microscopic anatomy—the cytoarchitecture—of the cerebral cortex and divided the cortex into 52 separate regions on the basis of the histology of the cortex. His work resulted in a system of classification known as Brodmann’s areas, which is still used today to describe the anatomical distinctions within the cortex (Figure 13.8). The results from Brodmann’s work on the anatomy align very well with the functional differences within the cortex. Areas 17 and 18 in the occipital lobe are responsible for primary visual perception. That visual information is complex, so it is processed in the temporal and parietal lobes as well.
The temporal lobe is associated with primary auditory sensation, known as Brodmann’s areas 41 and 42 in the superior temporal lobe. Because regions of the temporal lobe are part of the limbic system, memory is an important function associated with that lobe. Memory is essentially a sensory function; memories are recalled sensations such as the smell of Mom’s baking or the sound of a barking dog. Even memories of movement are really the memory of sensory feedback from those movements, such as stretching muscles or the movement of the skin around a joint. Structures in the temporal lobe are responsible for establishing long-term memory, but the ultimate location of those memories is usually in the region in which the sensory perception was processed.
The main sensation associated with the parietal lobe is somatosensation, meaning the general sensations associated with the body. Posterior to the central sulcus is the postcentral gyrus, the primary somatosensory cortex, which is identified as Brodmann’s areas 1, 2, and 3. All of the tactile senses are processed in this area, including touch, pressure, tickle, pain, itch, and vibration, as well as more general senses of the body such as proprioception and kinesthesia, which are the senses of body position and movement, respectively.
Anterior to the central sulcus is the frontal lobe, which is primarily associated with motor functions. The precentral gyrus is the primary motor cortex. Cells from this region of the cerebral cortex are the upper motor neurons that instruct cells in the spinal cord to move skeletal muscles. Anterior to this region are a few areas that are associated with planned movements. The premotor area is responsible for thinking of a movement to be made. The frontal eye fields are important in eliciting eye movements and in attending to visual stimuli. Broca’s area is responsible for the production of language, or controlling movements responsible for speech; in the vast majority of people, it is located only on the left side. Anterior to these regions is the prefrontal lobe, which serves cognitive functions that can be the basis of personality, short-term memory, and consciousness. The prefrontal lobotomy is an outdated mode of treatment for personality disorders (psychiatric conditions) that profoundly affected the personality of the patient.
Figure 13.8 Brodmann's Areas of the Cerebral Cortex Brodmann mapping of functionally distinct regions of the cortex was based on its cytoarchitecture at a microscopic level.
Subcortical structures
Beneath the cerebral cortex are sets of nuclei known as subcortical nuclei that augment cortical processes. The nuclei of the basal forebrain serve as the primary location for acetylcholine production, which modulates the overall activity of the cortex, possibly leading to greater attention to sensory stimuli. Alzheimer’s disease is associated with a loss of neurons in the basal forebrain. The hippocampus and amygdala are medial-lobe structures that, along with the adjacent cortex, are involved in long-term memory formation and emotional responses. The basal nuclei are a set of nuclei in the cerebrum responsible for comparing cortical processing with the general state of activity in the nervous system to influence the likelihood of movement taking place. For example, while a student is sitting in a classroom listening to a lecture, the basal nuclei will keep the urge to jump up and scream from actually happening. (The basal nuclei are also referred to as the basal ganglia, although that is potentially confusing because the term ganglia is typically used for peripheral structures.)
The major structures of the basal nuclei that control movement are the caudate, putamen, and globus pallidus, which are located deep in the cerebrum. The caudate is a long nucleus that follows the basic C-shape of the cerebrum from the frontal lobe, through the parietal and occipital lobes, into the temporal lobe. The putamen is mostly deep in the anterior regions of the frontal and parietal lobes. Together, the caudate and putamen are called the striatum. The globus pallidus is a layered nucleus that lies just medial to the putamen; they are called the lenticular nuclei because they look like curved pieces fitting together like lenses. The globus pallidus has two subdivisions, the external and internal segments, which are lateral and medial, respectively. These nuclei are depicted in a frontal section of the brain in Figure 13.9.
Figure 13.9 Frontal Section of Cerebral Cortex and Basal Nuclei The major components of the basal nuclei, shown in a frontal section of the brain, are the caudate (just lateral to the lateral ventricle), the putamen (inferior to the caudate and separated by the large white-matter structure called the internal capsule), and the globus pallidus (medial to the putamen).
The basal nuclei in the cerebrum are connected with a few more nuclei in the brain stem that together act as a functional group that forms a motor pathway. Two streams of information processing take place in the basal nuclei. All input to the basal nuclei is from the cortex into the striatum (Figure 13.10). The direct pathway is the projection of axons from the striatum to the globus pallidus internal segment (GPi) and the substantia nigra pars reticulata (SNr). The GPi/SNr then projects to the thalamus, which projects back to the cortex. The indirect pathway is the projection of axons from the striatum to the globus pallidus external segment (GPe), then to the subthalamic nucleus (STN), and finally to GPi/SNr. The two streams both target the GPi/SNr, but one has a direct projection and the other goes through a few intervening nuclei. The direct pathway causes the disinhibitionof the thalamus (inhibition of one cell on a target cell that then inhibits the first cell), whereas the indirect pathway causes, or reinforces, the normal inhibition of the thalamus. The thalamus then can either excite the cortex (as a result of the direct pathway) or fail to excite the cortex (as a result of the indirect pathway).
Figure 13.10 Connections of Basal Nuclei Input to the basal nuclei is from the cerebral cortex, which is an excitatory connection releasing glutamate as a neurotransmitter. This input is to the striatum, or the caudate and putamen. In the direct pathway, the striatum projects to the internal segment of the globus pallidus and the substantia nigra pars reticulata (GPi/SNr). This is an inhibitory pathway, in which GABA is released at the synapse, and the target cells are hyperpolarized and less likely to fire. The output from the basal nuclei is to the thalamus, which is an inhibitory projection using GABA.
The switch between the two pathways is the substantia nigra pars compacta, which projects to the striatum and releases the neurotransmitter dopamine. Dopamine receptors are either excitatory (D1-type receptors) or inhibitory (D2-type receptors). The direct pathway is activated by dopamine, and the indirect pathway is inhibited by dopamine. When the substantia nigra pars compacta is firing, it signals to the basal nuclei that the body is in an active state, and movement will be more likely. When the substantia nigra pars compacta is silent, the body is in a passive state, and movement is inhibited. To illustrate this situation, while a student is sitting listening to a lecture, the substantia nigra pars compacta would be silent and the student less likely to get up and walk around. Likewise, while the professor is lecturing, and walking around at the front of the classroom, the professor’s substantia nigra pars compacta would be active, in keeping with his or her activity level.
INTERACTIVE LINK
Watch this video to learn about the basal nuclei (also known as the basal ganglia), which have two pathways that process information within the cerebrum. As shown in this video, the direct pathway is the shorter pathway through the system that results in increased activity in the cerebral cortex and increased motor activity. The direct pathway is described as resulting in “disinhibition” of the thalamus. What does disinhibition mean? What are the two neurons doing individually to cause this?
INTERACTIVE LINK
Watch this video to learn about the basal nuclei (also known as the basal ganglia), which have two pathways that process information within the cerebrum. As shown in this video, the indirect pathway is the longer pathway through the system that results in decreased activity in the cerebral cortex, and therefore less motor activity. The indirect pathway has an extra couple of connections in it, including disinhibition of the subthalamic nucleus. What is the end result on the thalamus, and therefore on movement initiated by the cerebral cortex?
EVERYDAY CONNECTION
The Myth of Left Brain/Right Brain
There is a persistent myth that people are “right-brained” or “left-brained,” which is an oversimplification of an important concept about the cerebral hemispheres. There is some lateralization of function, in which the left side of the brain is devoted to language function and the right side is devoted to spatial and nonverbal reasoning. Whereas these functions are predominantly associated with those sides of the brain, there is no monopoly by either side on these functions. Many pervasive functions, such as language, are distributed globally around the cerebrum.
Some of the support for this misconception has come from studies of split brains. A drastic way to deal with a rare and devastating neurological condition (intractable epilepsy) is to separate the two hemispheres of the brain. After sectioning the corpus callosum, a split-brained patient will have trouble producing verbal responses on the basis of sensory information processed on the right side of the cerebrum, leading to the idea that the left side is responsible for language function.
However, there are well-documented cases of language functions lost from damage to the right side of the brain. The deficits seen in damage to the left side of the brain are classified as aphasia, a loss of speech function; damage on the right side can affect the use of language. Right-side damage can result in a loss of ability to understand figurative aspects of speech, such as jokes, irony, or metaphors. Nonverbal aspects of speech can be affected by damage to the right side, such as facial expression or body language, and right-side damage can lead to a “flat affect” in speech, or a loss of emotional expression in speech—sounding like a robot when talking.
The Diencephalon
The diencephalon is the one region of the adult brain that retains its name from embryologic development. The etymology of the word diencephalon translates to “through brain.” It is the connection between the cerebrum and the rest of the nervous system, with one exception. The rest of the brain, the spinal cord, and the PNS all send information to the cerebrum through the diencephalon. Output from the cerebrum passes through the diencephalon. The single exception is the system associated with olfaction, or the sense of smell, which connects directly with the cerebrum. In the earliest vertebrate species, the cerebrum was not much more than olfactory bulbs that received peripheral information about the chemical environment (to call it smell in these organisms is imprecise because they lived in the ocean).
The diencephalon is deep beneath the cerebrum and constitutes the walls of the third ventricle. The diencephalon can be described as any region of the brain with “thalamus” in its name. The two major regions of the diencephalon are the thalamus itself and the hypothalamus (Figure 13.11). There are other structures, such as the epithalamus, which contains the pineal gland, or the subthalamus, which includes the subthalamic nucleus that is part of the basal nuclei.
Thalamus
The thalamus is a collection of nuclei that relay information between the cerebral cortex and the periphery, spinal cord, or brain stem. All sensory information, except for the sense of smell, passes through the thalamus before processing by the cortex. Axons from the peripheral sensory organs, or intermediate nuclei, synapse in the thalamus, and thalamic neurons project directly to the cerebrum. It is a requisite synapse in any sensory pathway, except for olfaction. The thalamus does not just pass the information on, it also processes that information. For example, the portion of the thalamus that receives visual information will influence what visual stimuli are important, or what receives attention.
The cerebrum also sends information down to the thalamus, which usually communicates motor commands. This involves interactions with the cerebellum and other nuclei in the brain stem. The cerebrum interacts with the basal nuclei, which involves connections with the thalamus. The primary output of the basal nuclei is to the thalamus, which relays that output to the cerebral cortex. The cortex also sends information to the thalamus that will then influence the effects of the basal nuclei.
Hypothalamus
Inferior and slightly anterior to the thalamus is the hypothalamus, the other major region of the diencephalon. The hypothalamus is a collection of nuclei that are largely involved in regulating homeostasis. The hypothalamus is the executive region in charge of the autonomic nervous system and the endocrine system through its regulation of the anterior pituitary gland. Other parts of the hypothalamus are involved in memory and emotion as part of the limbic system.
Figure 13.11 The Diencephalon The diencephalon is composed primarily of the thalamus and hypothalamus, which together define the walls of the third ventricle. The thalami are two elongated, ovoid structures on either side of the midline that make contact in the middle. The hypothalamus is inferior and anterior to the thalamus, culminating in a sharp angle to which the pituitary gland is attached.
Brain Stem
The midbrain and hindbrain (composed of the pons and the medulla) are collectively referred to as the brain stem (Figure 13.12). The structure emerges from the ventral surface of the forebrain as a tapering cone that connects the brain to the spinal cord. Attached to the brain stem, but considered a separate region of the adult brain, is the cerebellum. The midbrain coordinates sensory representations of the visual, auditory, and somatosensory perceptual spaces. The pons is the main connection with the cerebellum. The pons and the medulla regulate several crucial functions, including the cardiovascular and respiratory systems and rates.
The cranial nerves connect through the brain stem and provide the brain with the sensory input and motor output associated with the head and neck, including most of the special senses. The major ascending and descending pathways between the spinal cord and brain, specifically the cerebrum, pass through the brain stem.
Figure 13.12 The Brain Stem The brain stem comprises three regions: the midbrain, the pons, and the medulla.
Midbrain
One of the original regions of the embryonic brain, the midbrain is a small region between the thalamus and pons. It is separated into the tectum and tegmentum, from the Latin words for roof and floor, respectively. The cerebral aqueduct passes through the center of the midbrain, such that these regions are the roof and floor of that canal.
The tectum is composed of four bumps known as the colliculi (singular = colliculus), which means “little hill” in Latin. The inferior colliculus is the inferior pair of these enlargements and is part of the auditory brain stem pathway. Neurons of the inferior colliculus project to the thalamus, which then sends auditory information to the cerebrum for the conscious perception of sound. The superior colliculus is the superior pair and combines sensory information about visual space, auditory space, and somatosensory space. Activity in the superior colliculus is related to orienting the eyes to a sound or touch stimulus. If you are walking along the sidewalk on campus and you hear chirping, the superior colliculus coordinates that information with your awareness of the visual location of the tree right above you. That is the correlation of auditory and visual maps. If you suddenly feel something wet fall on your head, your superior colliculus integrates that with the auditory and visual maps and you know that the chirping bird just relieved itself on you. You want to look up to see the culprit, but do not.
The tegmentum is continuous with the gray matter of the rest of the brain stem. Throughout the midbrain, pons, and medulla, the tegmentum contains the nuclei that receive and send information through the cranial nerves, as well as regions that regulate important functions such as those of the cardiovascular and respiratory systems.
Pons
The word pons comes from the Latin word for bridge. It is visible on the anterior surface of the brain stem as the thick bundle of white matter attached to the cerebellum. The pons is the main connection between the cerebellum and the brain stem. The bridge-like white matter is only the anterior surface of the pons; the gray matter beneath that is a continuation of the tegmentum from the midbrain. Gray matter in the tegmentum region of the pons contains neurons receiving descending input from the forebrain that is sent to the cerebellum.
Medulla
The medulla is the region known as the myelencephalon in the embryonic brain. The initial portion of the name, “myel,” refers to the significant white matter found in this region—especially on its exterior, which is continuous with the white matter of the spinal cord. The tegmentum of the midbrain and pons continues into the medulla because this gray matter is responsible for processing cranial nerve information. A diffuse region of gray matter throughout the brain stem, known as the reticular formation, is related to sleep and wakefulness, such as general brain activity and attention.
The Cerebellum
The cerebellum, as the name suggests, is the “little brain.” It is covered in gyri and sulci like the cerebrum, and looks like a miniature version of that part of the brain (Figure 13.13). The cerebellum is largely responsible for comparing information from the cerebrum with sensory feedback from the periphery through the spinal cord. It accounts for approximately 10 percent of the mass of the brain.
Figure 13.13 The Cerebellum The cerebellum is situated on the posterior surface of the brain stem. Descending input from the cerebellum enters through the large white matter structure of the pons. Ascending input from the periphery and spinal cord enters through the fibers of the inferior olive. Output goes to the midbrain, which sends a descending signal to the spinal cord.
Descending fibers from the cerebrum have branches that connect to neurons in the pons. Those neurons project into the cerebellum, providing a copy of motor commands sent to the spinal cord. Sensory information from the periphery, which enters through spinal or cranial nerves, is copied to a nucleus in the medulla known as the inferior olive. Fibers from this nucleus enter the cerebellum and are compared with the descending commands from the cerebrum. If the primary motor cortex of the frontal lobe sends a command down to the spinal cord to initiate walking, a copy of that instruction is sent to the cerebellum. Sensory feedback from the muscles and joints, proprioceptive information about the movements of walking, and sensations of balance are sent to the cerebellum through the inferior olive and the cerebellum compares them. If walking is not coordinated, perhaps because the ground is uneven or a strong wind is blowing, then the cerebellum sends out a corrective command to compensate for the difference between the original cortical command and the sensory feedback. The output of the cerebellum is into the midbrain, which then sends a descending input to the spinal cord to correct the messages going to skeletal muscles.
The Spinal Cord
The description of the CNS is concentrated on the structures of the brain, but the spinal cord is another major organ of the system. Whereas the brain develops out of expansions of the neural tube into primary and then secondary vesicles, the spinal cord maintains the tube structure and is only specialized into certain regions. As the spinal cord continues to develop in the newborn, anatomical features mark its surface. The anterior midline is marked by the anterior median fissure, and the posterior midline is marked by the posterior median sulcus. Axons enter the posterior side through the dorsal (posterior) nerve root, which marks the posterolateral sulcus on either side. The axons emerging from the anterior side do so through the ventral (anterior) nerve root. Note that it is common to see the terms dorsal (dorsal = “back”) and ventral (ventral = “belly”) used interchangeably with posterior and anterior, particularly in reference to nerves and the structures of the spinal cord. You should learn to be comfortable with both.
On the whole, the posterior regions are responsible for sensory functions and the anterior regions are associated with motor functions. This comes from the initial development of the spinal cord, which is divided into the basal plate and the alar plate. The basal plate is closest to the ventral midline of the neural tube, which will become the anterior face of the spinal cord and gives rise to motor neurons. The alar plate is on the dorsal side of the neural tube and gives rise to neurons that will receive sensory input from the periphery.
The length of the spinal cord is divided into regions that correspond to the regions of the vertebral column. The name of a spinal cord region corresponds to the level at which spinal nerves pass through the intervertebral foramina. Immediately adjacent to the brain stem is the cervical region, followed by the thoracic, then the lumbar, and finally the sacral region. The spinal cord is not the full length of the vertebral column because the spinal cord does not grow significantly longer after the first or second year, but the skeleton continues to grow. The nerves that emerge from the spinal cord pass through the intervertebral formina at the respective levels. As the vertebral column grows, these nerves grow with it and result in a long bundle of nerves that resembles a horse’s tail and is named the cauda equina. The sacral spinal cord is at the level of the upper lumbar vertebral bones. The spinal nerves extend from their various levels to the proper level of the vertebral column.
Gray Horns
In cross-section, the gray matter of the spinal cord has the appearance of an ink-blot test, with the spread of the gray matter on one side replicated on the other—a shape reminiscent of a bulbous capital “H.” As shown in Figure 13.14, the gray matter is subdivided into regions that are referred to as horns. The posterior horn is responsible for sensory processing. The anterior horn sends out motor signals to the skeletal muscles. The lateral horn, which is only found in the thoracic, upper lumbar, and sacral regions, is the central component of the sympathetic division of the autonomic nervous system.
Some of the largest neurons of the spinal cord are the multipolar motor neurons in the anterior horn. The fibers that cause contraction of skeletal muscles are the axons of these neurons. The motor neuron that causes contraction of the big toe, for example, is located in the sacral spinal cord. The axon that has to reach all the way to the belly of that muscle may be a meter in length. The neuronal cell body that maintains that long fiber must be quite large, possibly several hundred micrometers in diameter, making it one of the largest cells in the body.
Figure 13.14 Cross-section of Spinal Cord The cross-section of a thoracic spinal cord segment shows the posterior, anterior, and lateral horns of gray matter, as well as the posterior, anterior, and lateral columns of white matter. LM × 40. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
White Columns
Just as the gray matter is separated into horns, the white matter of the spinal cord is separated into columns. Ascending tractsof nervous system fibers in these columns carry sensory information up to the brain, whereas descending tracts carry motor commands from the brain. Looking at the spinal cord longitudinally, the columns extend along its length as continuous bands of white matter. Between the two posterior horns of gray matter are the posterior columns. Between the two anterior horns, and bounded by the axons of motor neurons emerging from that gray matter area, are the anterior columns. The white matter on either side of the spinal cord, between the posterior horn and the axons of the anterior horn neurons, are the lateral columns. The posterior columns are composed of axons of ascending tracts. The anterior and lateral columns are composed of many different groups of axons of both ascending and descending tracts—the latter carrying motor commands down from the brain to the spinal cord to control output to the periphery.
INTERACTIVE LINK
Watch this video to learn about the gray matter of the spinal cord that receives input from fibers of the dorsal (posterior) root and sends information out through the fibers of the ventral (anterior) root. As discussed in this video, these connections represent the interactions of the CNS with peripheral structures for both sensory and motor functions. The cervical and lumbar spinal cords have enlargements as a result of larger populations of neurons. What are these enlargements responsible for?
DISORDERS OF THE...
Basal Nuclei
Parkinson’s disease is a disorder of the basal nuclei, specifically of the substantia nigra, that demonstrates the effects of the direct and indirect pathways. Parkinson’s disease is the result of neurons in the substantia nigra pars compacta dying. These neurons release dopamine into the striatum. Without that modulatory influence, the basal nuclei are stuck in the indirect pathway, without the direct pathway being activated. The direct pathway is responsible for increasing cortical movement commands. The increased activity of the indirect pathway results in the hypokinetic disorder of Parkinson’s disease.
Parkinson’s disease is neurodegenerative, meaning that neurons die that cannot be replaced, so there is no cure for the disorder. Treatments for Parkinson’s disease are aimed at increasing dopamine levels in the striatum. Currently, the most common way of doing that is by providing the amino acid L-DOPA, which is a precursor to the neurotransmitter dopamine and can cross the blood-brain barrier. With levels of the precursor elevated, the remaining cells of the substantia nigra pars compacta can make more neurotransmitter and have a greater effect. Unfortunately, the patient will become less responsive to L-DOPA treatment as time progresses, and it can cause increased dopamine levels elsewhere in the brain, which are associated with psychosis or schizophrenia.
INTERACTIVE LINK
Visit this site for a thorough explanation of Parkinson’s disease.
INTERACTIVE LINK
Compared with the nearest evolutionary relative, the chimpanzee, the human has a brain that is huge. At a point in the past, a common ancestor gave rise to the two species of humans and chimpanzees. That evolutionary history is long and is still an area of intense study. But something happened to increase the size of the human brain relative to the chimpanzee. Read this article in which the author explores the current understanding of why this happened.
According to one hypothesis about the expansion of brain size, what tissue might have been sacrificed so energy was available to grow our larger brain? Based on what you know about that tissue and nervous tissue, why would there be a trade-off between them in terms of energy use?
Circulation and the Central Nervous System
- Describe the vessels that supply the CNS with blood
- Name the components of the ventricular system and the regions of the brain in which each is located
- Explain the production of cerebrospinal fluid and its flow through the ventricles
- Explain how a disruption in circulation would result in a stroke
The CNS is crucial to the operation of the body, and any compromise in the brain and spinal cord can lead to severe difficulties. The CNS has a privileged blood supply, as suggested by the blood-brain barrier. The function of the tissue in the CNS is crucial to the survival of the organism, so the contents of the blood cannot simply pass into the central nervous tissue. To protect this region from the toxins and pathogens that may be traveling through the blood stream, there is strict control over what can move out of the general systems and into the brain and spinal cord. Because of this privilege, the CNS needs specialized structures for the maintenance of circulation. This begins with a unique arrangement of blood vessels carrying fresh blood into the CNS. Beyond the supply of blood, the CNS filters that blood into cerebrospinal fluid (CSF), which is then circulated through the cavities of the brain and spinal cord called ventricles.
Blood Supply to the Brain
A lack of oxygen to the CNS can be devastating, and the cardiovascular system has specific regulatory reflexes to ensure that the blood supply is not interrupted. There are multiple routes for blood to get into the CNS, with specializations to protect that blood supply and to maximize the ability of the brain to get an uninterrupted perfusion.
Arterial Supply
The major artery carrying recently oxygenated blood away from the heart is the aorta. The very first branches off the aorta supply the heart with nutrients and oxygen. The next branches give rise to the common carotid arteries, which further branch into the internal carotid arteries. The external carotid arteries supply blood to the tissues on the surface of the cranium. The bases of the common carotids contain stretch receptors that immediately respond to the drop in blood pressure upon standing. The orthostatic reflex is a reaction to this change in body position, so that blood pressure is maintained against the increasing effect of gravity (orthostatic means “standing up”). Heart rate increases—a reflex of the sympathetic division of the autonomic nervous system—and this raises blood pressure.
The internal carotid artery enters the cranium through the carotid canal in the temporal bone. A second set of vessels that supply the CNS are the vertebral arteries, which are protected as they pass through the neck region by the transverse foramina of the cervical vertebrae. The vertebral arteries enter the cranium through the foramen magnum of the occipital bone. Branches off the left and right vertebral arteries merge into the anterior spinal artery supplying the anterior aspect of the spinal cord, found along the anterior median fissure. The two vertebral arteries then merge into the basilar artery, which gives rise to branches to the brain stem and cerebellum. The left and right internal carotid arteries and branches of the basilar artery all become the circle of Willis, a confluence of arteries that can maintain perfusion of the brain even if narrowing or a blockage limits flow through one part (Figure 13.15).
Figure 13.15 Circle of Willis The blood supply to the brain enters through the internal carotid arteries and the vertebral arteries, eventually giving rise to the circle of Willis.
INTERACTIVE LINK
Watch this animation to see how blood flows to the brain and passes through the circle of Willis before being distributed through the cerebrum. The circle of Willis is a specialized arrangement of arteries that ensure constant perfusion of the cerebrum even in the event of a blockage of one of the arteries in the circle. The animation shows the normal direction of flow through the circle of Willis to the middle cerebral artery. Where would the blood come from if there were a blockage just posterior to the middle cerebral artery on the left?
Venous Return
After passing through the CNS, blood returns to the circulation through a series of dural sinuses and veins (Figure 13.16). The superior sagittal sinus runs in the groove of the longitudinal fissure, where it absorbs CSF from the meninges. The superior sagittal sinus drains to the confluence of sinuses, along with the occipital sinuses and straight sinus, to then drain into the transverse sinuses. The transverse sinuses connect to the sigmoid sinuses, which then connect to the jugular veins. From there, the blood continues toward the heart to be pumped to the lungs for reoxygenation.
Figure 13.16 Dural Sinuses and Veins Blood drains from the brain through a series of sinuses that connect to the jugular veins.
Protective Coverings of the Brain and Spinal Cord
The outer surface of the CNS is covered by a series of membranes composed of connective tissue called the meninges, which protect the brain. The dura mater is a thick fibrous layer and a strong protective sheath over the entire brain and spinal cord. It is anchored to the inner surface of the cranium and vertebral cavity. The arachnoid mater is a membrane of thin fibrous tissue that forms a loose sac around the CNS. Beneath the arachnoid is a thin, filamentous mesh called the arachnoid trabeculae, which looks like a spider web, giving this layer its name. Directly adjacent to the surface of the CNS is the pia mater, a thin fibrous membrane that follows the convolutions of gyri and sulci in the cerebral cortex and fits into other grooves and indentations (Figure 13.17).
Figure 13.17 Meningeal Layers of Superior Sagittal Sinus The layers of the meninges in the longitudinal fissure of the superior sagittal sinus are shown, with the dura mater adjacent to the inner surface of the cranium, the pia mater adjacent to the surface of the brain, and the arachnoid and subarachnoid space between them. An arachnoid villus is shown emerging into the dural sinus to allow CSF to filter back into the blood for drainage.
Dura Mater
Like a thick cap covering the brain, the dura mater is a tough outer covering. The name comes from the Latin for “tough mother” to represent its physically protective role. It encloses the entire CNS and the major blood vessels that enter the cranium and vertebral cavity. It is directly attached to the inner surface of the bones of the cranium and to the very end of the vertebral cavity.
There are infoldings of the dura that fit into large crevasses of the brain. Two infoldings go through the midline separations of the cerebrum and cerebellum; one forms a shelf-like tent between the occipital lobes of the cerebrum and the cerebellum, and the other surrounds the pituitary gland. The dura also surrounds and supports the venous sinuses.
Arachnoid Mater
The middle layer of the meninges is the arachnoid, named for the spider-web–like trabeculae between it and the pia mater. The arachnoid defines a sac-like enclosure around the CNS. The trabeculae are found in the subarachnoid space, which is filled with circulating CSF. The arachnoid emerges into the dural sinuses as the arachnoid granulations, where the CSF is filtered back into the blood for drainage from the nervous system.
The subarachnoid space is filled with circulating CSF, which also provides a liquid cushion to the brain and spinal cord. Similar to clinical blood work, a sample of CSF can be withdrawn to find chemical evidence of neuropathology or metabolic traces of the biochemical functions of nervous tissue.
Pia Mater
The outer surface of the CNS is covered in the thin fibrous membrane of the pia mater. It is thought to have a continuous layer of cells providing a fluid-impermeable membrane. The name pia mater comes from the Latin for “tender mother,” suggesting the thin membrane is a gentle covering for the brain. The pia extends into every convolution of the CNS, lining the inside of the sulci in the cerebral and cerebellar cortices. At the end of the spinal cord, a thin filament extends from the inferior end of CNS at the upper lumbar region of the vertebral column to the sacral end of the vertebral column. Because the spinal cord does not extend through the lower lumbar region of the vertebral column, a needle can be inserted through the dura and arachnoid layers to withdraw CSF. This procedure is called a lumbar puncture and avoids the risk of damaging the central tissue of the spinal cord. Blood vessels that are nourishing the central nervous tissue are between the pia mater and the nervous tissue.
DISORDERS OF THE...
Meninges
Meningitis is an inflammation of the meninges, the three layers of fibrous membrane that surround the CNS. Meningitis can be caused by infection by bacteria or viruses. The particular pathogens are not special to meningitis; it is just an inflammation of that specific set of tissues from what might be a broader infection. Bacterial meningitis can be caused by Streptococcus, Staphylococcus, or the tuberculosis pathogen, among many others. Viral meningitis is usually the result of common enteroviruses (such as those that cause intestinal disorders), but may be the result of the herpes virus or West Nile virus. Bacterial meningitis tends to be more severe.
The symptoms associated with meningitis can be fever, chills, nausea, vomiting, light sensitivity, soreness of the neck, or severe headache. More important are the neurological symptoms, such as changes in mental state (confusion, memory deficits, and other dementia-type symptoms). A serious risk of meningitis can be damage to peripheral structures because of the nerves that pass through the meninges. Hearing loss is a common result of meningitis.
The primary test for meningitis is a lumbar puncture. A needle inserted into the lumbar region of the spinal column through the dura mater and arachnoid membrane into the subarachnoid space can be used to withdraw the fluid for chemical testing. Fatality occurs in 5 to 40 percent of children and 20 to 50 percent of adults with bacterial meningitis. Treatment of bacterial meningitis is through antibiotics, but viral meningitis cannot be treated with antibiotics because viruses do not respond to that type of drug. Fortunately, the viral forms are milder.
INTERACTIVE LINK
Watch this video that describes the procedure known as the lumbar puncture, a medical procedure used to sample the CSF. Because of the anatomy of the CNS, it is a relative safe location to insert a needle. Why is the lumbar puncture performed in the lower lumbar area of the vertebral column?
The Ventricular System
Cerebrospinal fluid (CSF) circulates throughout and around the CNS. In other tissues, water and small molecules are filtered through capillaries as the major contributor to the interstitial fluid. In the brain, CSF is produced in special structures to perfuse through the nervous tissue of the CNS and is continuous with the interstitial fluid. Specifically, CSF circulates to remove metabolic wastes from the interstitial fluids of nervous tissues and return them to the blood stream. The ventricles are the open spaces within the brain where CSF circulates. In some of these spaces, CSF is produced by filtering of the blood that is performed by a specialized membrane known as a choroid plexus. The CSF circulates through all of the ventricles to eventually emerge into the subarachnoid space where it will be reabsorbed into the blood.
The Ventricles
There are four ventricles within the brain, all of which developed from the original hollow space within the neural tube, the central canal. The first two are named the lateral ventricles and are deep within the cerebrum. These ventricles are connected to the third ventricle by two openings called the interventricular foramina. The third ventricle is the space between the left and right sides of the diencephalon, which opens into the cerebral aqueduct that passes through the midbrain. The aqueduct opens into the fourth ventricle, which is the space between the cerebellum and the pons and upper medulla (Figure 13.18).
Figure 13.18 Cerebrospinal Fluid Circulation The choroid plexus in the four ventricles produce CSF, which is circulated through the ventricular system and then enters the subarachnoid space through the median and lateral apertures. The CSF is then reabsorbed into the blood at the arachnoid granulations, where the arachnoid membrane emerges into the dural sinuses.
As the telencephalon enlarges and grows into the cranial cavity, it is limited by the space within the skull. The telencephalon is the most anterior region of what was the neural tube, but cannot grow past the limit of the frontal bone of the skull. Because the cerebrum fits into this space, it takes on a C-shaped formation, through the frontal, parietal, occipital, and finally temporal regions. The space within the telencephalon is stretched into this same C-shape. The two ventricles are in the left and right sides, and were at one time referred to as the first and second ventricles. The interventricular foramina connect the frontal region of the lateral ventricles with the third ventricle.
The third ventricle is the space bounded by the medial walls of the hypothalamus and thalamus. The two thalami touch in the center in most brains as the massa intermedia, which is surrounded by the third ventricle. The cerebral aqueduct opens just inferior to the epithalamus and passes through the midbrain. The tectum and tegmentum of the midbrain are the roof and floor of the cerebral aqueduct, respectively. The aqueduct opens up into the fourth ventricle. The floor of the fourth ventricle is the dorsal surface of the pons and upper medulla (that gray matter making a continuation of the tegmentum of the midbrain). The fourth ventricle then narrows into the central canal of the spinal cord.
The ventricular system opens up to the subarachnoid space from the fourth ventricle. The single median aperture and the pair of lateral apertures connect to the subarachnoid space so that CSF can flow through the ventricles and around the outside of the CNS. Cerebrospinal fluid is produced within the ventricles by a type of specialized membrane called a choroid plexus. Ependymal cells (one of the types of glial cells described in the introduction to the nervous system) surround blood capillaries and filter the blood to make CSF. The fluid is a clear solution with a limited amount of the constituents of blood. It is essentially water, small molecules, and electrolytes. Oxygen and carbon dioxide are dissolved into the CSF, as they are in blood, and can diffuse between the fluid and the nervous tissue.
Cerebrospinal Fluid Circulation
The choroid plexuses are found in all four ventricles. Observed in dissection, they appear as soft, fuzzy structures that may still be pink, depending on how well the circulatory system is cleared in preparation of the tissue. The CSF is produced from components extracted from the blood, so its flow out of the ventricles is tied to the pulse of cardiovascular circulation.
From the lateral ventricles, the CSF flows into the third ventricle, where more CSF is produced, and then through the cerebral aqueduct into the fourth ventricle where even more CSF is produced. A very small amount of CSF is filtered at any one of the plexuses, for a total of about 500 milliliters daily, but it is continuously made and pulses through the ventricular system, keeping the fluid moving. From the fourth ventricle, CSF can continue down the central canal of the spinal cord, but this is essentially a cul-de-sac, so more of the fluid leaves the ventricular system and moves into the subarachnoid space through the median and lateral apertures.
Within the subarachnoid space, the CSF flows around all of the CNS, providing two important functions. As with elsewhere in its circulation, the CSF picks up metabolic wastes from the nervous tissue and moves it out of the CNS. It also acts as a liquid cushion for the brain and spinal cord. By surrounding the entire system in the subarachnoid space, it provides a thin buffer around the organs within the strong, protective dura mater. The arachnoid granulations are outpocketings of the arachnoid membrane into the dural sinuses so that CSF can be reabsorbed into the blood, along with the metabolic wastes. From the dural sinuses, blood drains out of the head and neck through the jugular veins, along with the rest of the circulation for blood, to be reoxygenated by the lungs and wastes to be filtered out by the kidneys (Table 13.2).
INTERACTIVE LINK
Watch this animation that shows the flow of CSF through the brain and spinal cord, and how it originates from the ventricles and then spreads into the space within the meninges, where the fluids then move into the venous sinuses to return to the cardiovascular circulation. What are the structures that produce CSF and where are they found? How are the structures indicated in this animation?
Components of CSF Circulation
| Lateral ventricles | Third ventricle | Cerebral aqueduct | Fourth ventricle | Central canal | Subarachnoid space | |
|---|---|---|---|---|---|---|
| Location in CNS | Cerebrum | Diencephalon | Midbrain | Between pons/upper medulla and cerebellum | Spinal cord | External to entire CNS |
| Blood vessel structure | Choroid plexus | Choroid plexus | None | Choroid plexus | None | Arachnoid granulations |
Table 13.2
DISORDERS OF THE...
Central Nervous System
The supply of blood to the brain is crucial to its ability to perform many functions. Without a steady supply of oxygen, and to a lesser extent glucose, the nervous tissue in the brain cannot keep up its extensive electrical activity. These nutrients get into the brain through the blood, and if blood flow is interrupted, neurological function is compromised.
The common name for a disruption of blood supply to the brain is a stroke. It is caused by a blockage to an artery in the brain. The blockage is from some type of embolus: a blood clot, a fat embolus, or an air bubble. When the blood cannot travel through the artery, the surrounding tissue that is deprived starves and dies. Strokes will often result in the loss of very specific functions. A stroke in the lateral medulla, for example, can cause a loss in the ability to swallow. Sometimes, seemingly unrelated functions will be lost because they are dependent on structures in the same region. Along with the swallowing in the previous example, a stroke in that region could affect sensory functions from the face or extremities because important white matter pathways also pass through the lateral medulla. Loss of blood flow to specific regions of the cortex can lead to the loss of specific higher functions, from the ability to recognize faces to the ability to move a particular region of the body. Severe or limited memory loss can be the result of a temporal lobe stroke.
Related to strokes are transient ischemic attacks (TIAs), which can also be called “mini-strokes.” These are events in which a physical blockage may be temporary, cutting off the blood supply and oxygen to a region, but not to the extent that it causes cell death in that region. While the neurons in that area are recovering from the event, neurological function may be lost. Function can return if the area is able to recover from the event.
Recovery from a stroke (or TIA) is strongly dependent on the speed of treatment. Often, the person who is present and notices something is wrong must then make a decision. The mnemonic FAST helps people remember what to look for when someone is dealing with sudden losses of neurological function. If someone complains of feeling “funny,” check these things quickly: Look at the person’s face. Does he or she have problems moving Face muscles and making regular facial expressions? Ask the person to raise his or her Arms above the head. Can the person lift one arm but not the other? Has the person’s Speech changed? Is he or she slurring words or having trouble saying things? If any of these things have happened, then it is Time to call for help.
Sometimes, treatment with blood-thinning drugs can alleviate the problem, and recovery is possible. If the tissue is damaged, the amazing thing about the nervous system is that it is adaptable. With physical, occupational, and speech therapy, victims of strokes can recover, or more accurately relearn, functions.
The Peripheral Nervous System
- Describe the structures found in the PNS
- Distinguish between somatic and autonomic structures, including the special peripheral structures of the enteric nervous system
- Name the twelve cranial nerves and explain the functions associated with each
- Describe the sensory and motor components of spinal nerves and the plexuses that they pass through
The PNS is not as contained as the CNS because it is defined as everything that is not the CNS. Some peripheral structures are incorporated into the other organs of the body. In describing the anatomy of the PNS, it is necessary to describe the common structures, the nerves and the ganglia, as they are found in various parts of the body. Many of the neural structures that are incorporated into other organs are features of the digestive system; these structures are known as the enteric nervous systemand are a special subset of the PNS.
Ganglia
A ganglion is a group of neuron cell bodies in the periphery. Ganglia can be categorized, for the most part, as either sensory ganglia or autonomic ganglia, referring to their primary functions. The most common type of sensory ganglion is a dorsal (posterior) root ganglion. These ganglia are the cell bodies of neurons with axons that are sensory endings in the periphery, such as in the skin, and that extend into the CNS through the dorsal nerve root. The ganglion is an enlargement of the nerve root. Under microscopic inspection, it can be seen to include the cell bodies of the neurons, as well as bundles of fibers that are the posterior nerve root (Figure 13.19). The cells of the dorsal root ganglion are unipolar cells, classifying them by shape. Also, the small round nuclei of satellite cells can be seen surrounding—as if they were orbiting—the neuron cell bodies.
Figure 13.19 Dorsal Root Ganglion The cell bodies of sensory neurons, which are unipolar neurons by shape, are seen in this photomicrograph. Also, the fibrous region is composed of the axons of these neurons that are passing through the ganglion to be part of the dorsal nerve root (tissue source: canine). LM × 40. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
Figure 13.20 Spinal Cord and Root Ganglion The slide includes both a cross-section of the lumbar spinal cord and a section of the dorsal root ganglion (see also Figure 13.19) (tissue source: canine). LM × 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
INTERACTIVE LINK
View the University of Michigan WebScope to explore the tissue sample in greater detail. If you zoom in on the dorsal root ganglion, you can see smaller satellite glial cells surrounding the large cell bodies of the sensory neurons. From what structure do satellite cells derive during embryologic development?
Another type of sensory ganglion is a cranial nerve ganglion. This is analogous to the dorsal root ganglion, except that it is associated with a cranial nerve instead of a spinal nerve. The roots of cranial nerves are within the cranium, whereas the ganglia are outside the skull. For example, the trigeminal ganglion is superficial to the temporal bone whereas its associated nerve is attached to the mid-pons region of the brain stem. The neurons of cranial nerve ganglia are also unipolar in shape with associated satellite cells.
The other major category of ganglia are those of the autonomic nervous system, which is divided into the sympathetic and parasympathetic nervous systems. The sympathetic chain ganglia constitute a row of ganglia along the vertebral column that receive central input from the lateral horn of the thoracic and upper lumbar spinal cord. Superior to the chain ganglia are three paravertebral ganglia in the cervical region. Three other autonomic ganglia that are related to the sympathetic chain are the prevertebral ganglia, which are located outside of the chain but have similar functions. They are referred to as prevertebral because they are anterior to the vertebral column. The neurons of these autonomic ganglia are multipolar in shape, with dendrites radiating out around the cell body where synapses from the spinal cord neurons are made. The neurons of the chain, paravertebral, and prevertebral ganglia then project to organs in the head and neck, thoracic, abdominal, and pelvic cavities to regulate the sympathetic aspect of homeostatic mechanisms.
Another group of autonomic ganglia are the terminal ganglia that receive input from cranial nerves or sacral spinal nerves and are responsible for regulating the parasympathetic aspect of homeostatic mechanisms. These two sets of ganglia, sympathetic and parasympathetic, often project to the same organs—one input from the chain ganglia and one input from a terminal ganglion—to regulate the overall function of an organ. For example, the heart receives two inputs such as these; one increases heart rate, and the other decreases it. The terminal ganglia that receive input from cranial nerves are found in the head and neck, as well as the thoracic and upper abdominal cavities, whereas the terminal ganglia that receive sacral input are in the lower abdominal and pelvic cavities.
Terminal ganglia below the head and neck are often incorporated into the wall of the target organ as a plexus. A plexus, in a general sense, is a network of fibers or vessels. This can apply to nervous tissue (as in this instance) or structures containing blood vessels (such as a choroid plexus). For example, the enteric plexus is the extensive network of axons and neurons in the wall of the small and large intestines. The enteric plexus is actually part of the enteric nervous system, along with the gastric plexuses and the esophageal plexus. Though the enteric nervous system receives input originating from central neurons of the autonomic nervous system, it does not require CNS input to function. In fact, it operates independently to regulate the digestive system.
Nerves
Bundles of axons in the PNS are referred to as nerves. These structures in the periphery are different than the central counterpart, called a tract. Nerves are composed of more than just nervous tissue. They have connective tissues invested in their structure, as well as blood vessels supplying the tissues with nourishment. The outer surface of a nerve is a surrounding layer of fibrous connective tissue called the epineurium. Within the nerve, axons are further bundled into fascicles, which are each surrounded by their own layer of fibrous connective tissue called perineurium. Finally, individual axons are surrounded by loose connective tissue called the endoneurium (Figure 13.21). These three layers are similar to the connective tissue sheaths for muscles. Nerves are associated with the region of the CNS to which they are connected, either as cranial nerves connected to the brain or spinal nerves connected to the spinal cord.
Figure 13.21 Nerve Structure The structure of a nerve is organized by the layers of connective tissue on the outside, around each fascicle, and surrounding the individual nerve fibers (tissue source: simian). LM × 40. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
Figure 13.22 Close-Up of Nerve Trunk Zoom in on this slide of a nerve trunk to examine the endoneurium, perineurium, and epineurium in greater detail (tissue source: simian). LM × 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
INTERACTIVE LINK
View the University of Michigan WebScope to explore the tissue sample in greater detail. With what structures in a skeletal muscle are the endoneurium, perineurium, and epineurium comparable?
Cranial Nerves
The nerves attached to the brain are the cranial nerves, which are primarily responsible for the sensory and motor functions of the head and neck (one of these nerves targets organs in the thoracic and abdominal cavities as part of the parasympathetic nervous system). There are twelve cranial nerves, which are designated CNI through CNXII for “Cranial Nerve,” using Roman numerals for 1 through 12. They can be classified as sensory nerves, motor nerves, or a combination of both, meaning that the axons in these nerves originate out of sensory ganglia external to the cranium or motor nuclei within the brain stem. Sensory axons enter the brain to synapse in a nucleus. Motor axons connect to skeletal muscles of the head or neck. Three of the nerves are solely composed of sensory fibers; five are strictly motor; and the remaining four are mixed nerves.
Learning the cranial nerves is a tradition in anatomy courses, and students have always used mnemonic devices to remember the nerve names. A traditional mnemonic is the rhyming couplet, “On Old Olympus’ Towering Tops/A Finn And German Viewed Some Hops,” in which the initial letter of each word corresponds to the initial letter in the name of each nerve. The names of the nerves have changed over the years to reflect current usage and more accurate naming. An exercise to help learn this sort of information is to generate a mnemonic using words that have personal significance. The names of the cranial nerves are listed in Table 13.3 along with a brief description of their function, their source (sensory ganglion or motor nucleus), and their target (sensory nucleus or skeletal muscle). They are listed here with a brief explanation of each nerve (Figure 13.23).
The olfactory nerve and optic nerve are responsible for the sense of smell and vision, respectively. The oculomotor nerve is responsible for eye movements by controlling four of the extraocular muscles. It is also responsible for lifting the upper eyelid when the eyes point up, and for pupillary constriction. The trochlear nerve and the abducens nerve are both responsible for eye movement, but do so by controlling different extraocular muscles. The trigeminal nerve is responsible for cutaneous sensations of the face and controlling the muscles of mastication. The facial nerve is responsible for the muscles involved in facial expressions, as well as part of the sense of taste and the production of saliva. The vestibulocochlear nerve is responsible for the senses of hearing and balance. The glossopharyngeal nerve is responsible for controlling muscles in the oral cavity and upper throat, as well as part of the sense of taste and the production of saliva. The vagus nerve is responsible for contributing to homeostatic control of the organs of the thoracic and upper abdominal cavities. The spinal accessory nerveis responsible for controlling the muscles of the neck, along with cervical spinal nerves. The hypoglossal nerve is responsible for controlling the muscles of the lower throat and tongue.
Figure 13.23 The Cranial Nerves The anatomical arrangement of the roots of the cranial nerves observed from an inferior view of the brain.
Three of the cranial nerves also contain autonomic fibers, and a fourth is almost purely a component of the autonomic system. The oculomotor, facial, and glossopharyngeal nerves contain fibers that contact autonomic ganglia. The oculomotor fibers initiate pupillary constriction, whereas the facial and glossopharyngeal fibers both initiate salivation. The vagus nerve primarily targets autonomic ganglia in the thoracic and upper abdominal cavities.
INTERACTIVE LINK
Visit this site to read about a man who wakes with a headache and a loss of vision. His regular doctor sent him to an ophthalmologist to address the vision loss. The ophthalmologist recognizes a greater problem and immediately sends him to the emergency room. Once there, the patient undergoes a large battery of tests, but a definite cause cannot be found. A specialist recognizes the problem as meningitis, but the question is what caused it originally. How can that be cured? The loss of vision comes from swelling around the optic nerve, which probably presented as a bulge on the inside of the eye. Why is swelling related to meningitis going to push on the optic nerve?
Another important aspect of the cranial nerves that lends itself to a mnemonic is the functional role each nerve plays. The nerves fall into one of three basic groups. They are sensory, motor, or both (see Table 13.3). The sentence, “Some Say Marry Money But My Brother Says Brains Beauty Matter More,” corresponds to the basic function of each nerve. The first, second, and eighth nerves are purely sensory: the olfactory (CNI), optic (CNII), and vestibulocochlear (CNVIII) nerves. The three eye-movement nerves are all motor: the oculomotor (CNIII), trochlear (CNIV), and abducens (CNVI). The spinal accessory (CNXI) and hypoglossal (CNXII) nerves are also strictly motor. The remainder of the nerves contain both sensory and motor fibers. They are the trigeminal (CNV), facial (CNVII), glossopharyngeal (CNIX), and vagus (CNX) nerves. The nerves that convey both are often related to each other. The trigeminal and facial nerves both concern the face; one concerns the sensations and the other concerns the muscle movements. The facial and glossopharyngeal nerves are both responsible for conveying gustatory, or taste, sensations as well as controlling salivary glands. The vagus nerve is involved in visceral responses to taste, namely the gag reflex. This is not an exhaustive list of what these combination nerves do, but there is a thread of relation between them.
Cranial Nerves
| Mnemonic | # | Name | Function (S/M/B) | Central connection (nuclei) | Peripheral connection (ganglion or muscle) |
|---|---|---|---|---|---|
| On | I | Olfactory | Smell (S) | Olfactory bulb | Olfactory epithelium |
| Old | II | Optic | Vision (S) | Hypothalamus/thalamus/midbrain | Retina (retinal ganglion cells) |
| Olympus’ | III | Oculomotor | Eye movements (M) | Oculomotor nucleus | Extraocular muscles (other 4), levator palpebrae superioris, ciliary ganglion (autonomic) |
| Towering | IV | Trochlear | Eye movements (M) | Trochlear nucleus | Superior oblique muscle |
| Tops | V | Trigeminal | Sensory/motor – face (B) | Trigeminal nuclei in the midbrain, pons, and medulla | Trigeminal |
| A | VI | Abducens | Eye movements (M) | Abducens nucleus | Lateral rectus muscle |
| Finn | VII | Facial | Motor – face, Taste (B) | Facial nucleus, solitary nucleus, superior salivatory nucleus | Facial muscles, Geniculate ganglion, Pterygopalatine ganglion (autonomic) |
| And | VIII | Auditory (Vestibulocochlear) | Hearing/balance (S) | Cochlear nucleus, Vestibular nucleus/cerebellum | Spiral ganglion (hearing), Vestibular ganglion (balance) |
| German | IX | Glossopharyngeal | Motor – throat Taste (B) | Solitary nucleus, inferior salivatory nucleus, nucleus ambiguus | Pharyngeal muscles, Geniculate ganglion, Otic ganglion (autonomic) |
| Viewed | X | Vagus | Motor/sensory – viscera (autonomic) (B) | Medulla | Terminal ganglia serving thoracic and upper abdominal organs (heart and small intestines) |
| Some | XI | Spinal Accessory | Motor – head and neck (M) | Spinal accessory nucleus | Neck muscles |
| Hops | XII | Hypoglossal | Motor – lower throat (M) | Hypoglossal nucleus | Muscles of the larynx and lower pharynx |
Table 13.3
Spinal Nerves
The nerves connected to the spinal cord are the spinal nerves. The arrangement of these nerves is much more regular than that of the cranial nerves. All of the spinal nerves are combined sensory and motor axons that separate into two nerve roots. The sensory axons enter the spinal cord as the dorsal nerve root. The motor fibers, both somatic and autonomic, emerge as the ventral nerve root. The dorsal root ganglion for each nerve is an enlargement of the spinal nerve.
There are 31 spinal nerves, named for the level of the spinal cord at which each one emerges. There are eight pairs of cervical nerves designated C1 to C8, twelve thoracic nerves designated T1 to T12, five pairs of lumbar nerves designated L1 to L5, five pairs of sacral nerves designated S1 to S5, and one pair of coccygeal nerves. The nerves are numbered from the superior to inferior positions, and each emerges from the vertebral column through the intervertebral foramen at its level. The first nerve, C1, emerges between the first cervical vertebra and the occipital bone. The second nerve, C2, emerges between the first and second cervical vertebrae. The same occurs for C3 to C7, but C8 emerges between the seventh cervical vertebra and the first thoracic vertebra. For the thoracic and lumbar nerves, each one emerges between the vertebra that has the same designation and the next vertebra in the column. The sacral nerves emerge from the sacral foramina along the length of that unique vertebra.
Spinal nerves extend outward from the vertebral column to enervate the periphery. The nerves in the periphery are not straight continuations of the spinal nerves, but rather the reorganization of the axons in those nerves to follow different courses. Axons from different spinal nerves will come together into a systemic nerve. This occurs at four places along the length of the vertebral column, each identified as a nerve plexus, whereas the other spinal nerves directly correspond to nerves at their respective levels. In this instance, the word plexus is used to describe networks of nerve fibers with no associated cell bodies.
Of the four nerve plexuses, two are found at the cervical level, one at the lumbar level, and one at the sacral level (Figure 13.24). The cervical plexus is composed of axons from spinal nerves C1 through C5 and branches into nerves in the posterior neck and head, as well as the phrenic nerve, which connects to the diaphragm at the base of the thoracic cavity. The other plexus from the cervical level is the brachial plexus. Spinal nerves C4 through T1 reorganize through this plexus to give rise to the nerves of the arms, as the name brachial suggests. A large nerve from this plexus is the radial nerve from which the axillary nerve branches to go to the armpit region. The radial nerve continues through the arm and is paralleled by the ulnar nerve and the median nerve. The lumbar plexus arises from all the lumbar spinal nerves and gives rise to nerves enervating the pelvic region and the anterior leg. The femoral nerve is one of the major nerves from this plexus, which gives rise to the saphenous nerve as a branch that extends through the anterior lower leg. The sacral plexus comes from the lower lumbar nerves L4 and L5 and the sacral nerves S1 to S4. The most significant systemic nerve to come from this plexus is the sciatic nerve, which is a combination of the tibial nerve and the fibular nerve. The sciatic nerve extends across the hip joint and is most commonly associated with the condition sciatica, which is the result of compression or irritation of the nerve or any of the spinal nerves giving rise to it.
These plexuses are described as arising from spinal nerves and giving rise to certain systemic nerves, but they contain fibers that serve sensory functions or fibers that serve motor functions. This means that some fibers extend from cutaneous or other peripheral sensory surfaces and send action potentials into the CNS. Those are axons of sensory neurons in the dorsal root ganglia that enter the spinal cord through the dorsal nerve root. Other fibers are the axons of motor neurons of the anterior horn of the spinal cord, which emerge in the ventral nerve root and send action potentials to cause skeletal muscles to contract in their target regions. For example, the radial nerve contains fibers of cutaneous sensation in the arm, as well as motor fibers that move muscles in the arm.
Spinal nerves of the thoracic region, T2 through T11, are not part of the plexuses but rather emerge and give rise to the intercostal nerves found between the ribs, which articulate with the vertebrae surrounding the spinal nerve.
Figure 13.24 Nerve Plexuses of the Body There are four main nerve plexuses in the human body. The cervical plexus supplies nerves to the posterior head and neck, as well as to the diaphragm. The brachial plexus supplies nerves to the arm. The lumbar plexus supplies nerves to the anterior leg. The sacral plexus supplies nerves to the posterior leg.
AGING AND THE...
Nervous System
Anosmia is the loss of the sense of smell. It is often the result of the olfactory nerve being severed, usually because of blunt force trauma to the head. The sensory neurons of the olfactory epithelium have a limited lifespan of approximately one to four months, and new ones are made on a regular basis. The new neurons extend their axons into the CNS by growing along the existing fibers of the olfactory nerve. The ability of these neurons to be replaced is lost with age. Age-related anosmia is not the result of impact trauma to the head, but rather a slow loss of the sensory neurons with no new neurons born to replace them.
Smell is an important sense, especially for the enjoyment of food. There are only five tastes sensed by the tongue, and two of them are generally thought of as unpleasant tastes (sour and bitter). The rich sensory experience of food is the result of odor molecules associated with the food, both as food is moved into the mouth, and therefore passes under the nose, and when it is chewed and molecules are released to move up the pharynx into the posterior nasal cavity. Anosmia results in a loss of the enjoyment of food.
As the replacement of olfactory neurons declines with age, anosmia can set in. Without the sense of smell, many sufferers complain of food tasting bland. Often, the only way to enjoy food is to add seasoning that can be sensed on the tongue, which usually means adding table salt. The problem with this solution, however, is that this increases sodium intake, which can lead to cardiovascular problems through water retention and the associated increase in blood pressure.
Key Terms
- abducens nerve
- sixth cranial nerve; responsible for contraction of one of the extraocular muscles
- alar plate
- developmental region of the spinal cord that gives rise to the posterior horn of the gray matter
- amygdala
- nucleus deep in the temporal lobe of the cerebrum that is related to memory and emotional behavior
- anterior column
- white matter between the anterior horns of the spinal cord composed of many different groups of axons of both ascending and descending tracts
- anterior horn
- gray matter of the spinal cord containing multipolar motor neurons, sometimes referred to as the ventral horn
- anterior median fissure
- deep midline feature of the anterior spinal cord, marking the separation between the right and left sides of the cord
- anterior spinal artery
- blood vessel from the merged branches of the vertebral arteries that runs along the anterior surface of the spinal cord
- arachnoid granulation
- outpocket of the arachnoid membrane into the dural sinuses that allows for reabsorption of CSF into the blood
- arachnoid mater
- middle layer of the meninges named for the spider-web–like trabeculae that extend between it and the pia mater
- arachnoid trabeculae
- filaments between the arachnoid and pia mater within the subarachnoid space
- ascending tract
- central nervous system fibers carrying sensory information from the spinal cord or periphery to the brain
- axillary nerve
- systemic nerve of the arm that arises from the brachial plexus
- basal forebrain
- nuclei of the cerebrum related to modulation of sensory stimuli and attention through broad projections to the cerebral cortex, loss of which is related to Alzheimer’s disease
- basal nuclei
- nuclei of the cerebrum (with a few components in the upper brain stem and diencephalon) that are responsible for assessing cortical movement commands and comparing them with the general state of the individual through broad modulatory activity of dopamine neurons; largely related to motor functions, as evidenced through the symptoms of Parkinson’s and Huntington’s diseases
- basal plate
- developmental region of the spinal cord that gives rise to the lateral and anterior horns of gray matter
- basilar artery
- blood vessel from the merged vertebral arteries that runs along the dorsal surface of the brain stem
- brachial plexus
- nerve plexus associated with the lower cervical spinal nerves and first thoracic spinal nerve
- brain stem
- region of the adult brain that includes the midbrain, pons, and medulla oblongata and develops from the mesencephalon, metencephalon, and myelencephalon of the embryonic brain
- Broca’s area
- region of the frontal lobe associated with the motor commands necessary for speech production and located only in the cerebral hemisphere responsible for language production, which is the left side in approximately 95 percent of the population
- Brodmann’s areas
- mapping of regions of the cerebral cortex based on microscopic anatomy that relates specific areas to functional differences, as described by Brodmann in the early 1900s
- carotid canal
- opening in the temporal bone through which the internal carotid artery enters the cranium
- cauda equina
- bundle of spinal nerve roots that descend from the lower spinal cord below the first lumbar vertebra and lie within the vertebral cavity; has the appearance of a horse's tail
- caudate
- nucleus deep in the cerebrum that is part of the basal nuclei; along with the putamen, it is part of the striatum
- central canal
- hollow space within the spinal cord that is the remnant of the center of the neural tube
- central sulcus
- surface landmark of the cerebral cortex that marks the boundary between the frontal and parietal lobes
- cephalic flexure
- curve in midbrain of the embryo that positions the forebrain ventrally
- cerebellum
- region of the adult brain connected primarily to the pons that developed from the metencephalon (along with the pons) and is largely responsible for comparing information from the cerebrum with sensory feedback from the periphery through the spinal cord
- cerebral aqueduct
- connection of the ventricular system between the third and fourth ventricles located in the midbrain
- cerebral cortex
- outer gray matter covering the forebrain, marked by wrinkles and folds known as gyri and sulci
- cerebral hemisphere
- one half of the bilaterally symmetrical cerebrum
- cerebrum
- region of the adult brain that develops from the telencephalon and is responsible for higher neurological functions such as memory, emotion, and consciousness
- cervical plexus
- nerve plexus associated with the upper cervical spinal nerves
- choroid plexus
- specialized structures containing ependymal cells lining blood capillaries that filter blood to produce CSF in the four ventricles of the brain
- circle of Willis
- unique anatomical arrangement of blood vessels around the base of the brain that maintains perfusion of blood into the brain even if one component of the structure is blocked or narrowed
- common carotid artery
- blood vessel that branches off the aorta (or the brachiocephalic artery on the right) and supplies blood to the head and neck
- corpus callosum
- large white matter structure that connects the right and left cerebral hemispheres
- cranial nerve
- one of twelve nerves connected to the brain that are responsible for sensory or motor functions of the head and neck
- cranial nerve ganglion
- sensory ganglion of cranial nerves
- descending tract
- central nervous system fibers carrying motor commands from the brain to the spinal cord or periphery
- diencephalon
- region of the adult brain that retains its name from embryonic development and includes the thalamus and hypothalamus
- direct pathway
- connections within the basal nuclei from the striatum to the globus pallidus internal segment and substantia nigra pars reticulata that disinhibit the thalamus to increase cortical control of movement
- disinhibition
- disynaptic connection in which the first synapse inhibits the second cell, which then stops inhibiting the final target
- dorsal (posterior) nerve root
- axons entering the posterior horn of the spinal cord
- dorsal (posterior) root ganglion
- sensory ganglion attached to the posterior nerve root of a spinal nerve
- dura mater
- tough, fibrous, outer layer of the meninges that is attached to the inner surface of the cranium and vertebral column and surrounds the entire CNS
- dural sinus
- any of the venous structures surrounding the brain, enclosed within the dura mater, which drain blood from the CNS to the common venous return of the jugular veins
- endoneurium
- innermost layer of connective tissue that surrounds individual axons within a nerve
- enteric nervous system
- peripheral structures, namely ganglia and nerves, that are incorporated into the digestive system organs
- enteric plexus
- neuronal plexus in the wall of the intestines, which is part of the enteric nervous system
- epineurium
- outermost layer of connective tissue that surrounds an entire nerve
- epithalamus
- region of the diecephalon containing the pineal gland
- esophageal plexus
- neuronal plexus in the wall of the esophagus that is part of the enteric nervous system
- extraocular muscles
- six skeletal muscles that control eye movement within the orbit
- facial nerve
- seventh cranial nerve; responsible for contraction of the facial muscles and for part of the sense of taste, as well as causing saliva production
- fascicle
- small bundles of nerve or muscle fibers enclosed by connective tissue
- femoral nerve
- systemic nerve of the anterior leg that arises from the lumbar plexus
- fibular nerve
- systemic nerve of the posterior leg that begins as part of the sciatic nerve
- foramen magnum
- large opening in the occipital bone of the skull through which the spinal cord emerges and the vertebral arteries enter the cranium
- forebrain
- anterior region of the adult brain that develops from the prosencephalon and includes the cerebrum and diencephalon
- fourth ventricle
- the portion of the ventricular system that is in the region of the brain stem and opens into the subarachnoid space through the median and lateral apertures
- frontal eye field
- region of the frontal lobe associated with motor commands to orient the eyes toward an object of visual attention
- frontal lobe
- region of the cerebral cortex directly beneath the frontal bone of the cranium
- gastric plexuses
- neuronal networks in the wall of the stomach that are part of the enteric nervous system
- globus pallidus
- nuclei deep in the cerebrum that are part of the basal nuclei and can be divided into the internal and external segments
- glossopharyngeal nerve
- ninth cranial nerve; responsible for contraction of muscles in the tongue and throat and for part of the sense of taste, as well as causing saliva production
- gyrus
- ridge formed by convolutions on the surface of the cerebrum or cerebellum
- hindbrain
- posterior region of the adult brain that develops from the rhombencephalon and includes the pons, medulla oblongata, and cerebellum
- hippocampus
- gray matter deep in the temporal lobe that is very important for long-term memory formation
- hypoglossal nerve
- twelfth cranial nerve; responsible for contraction of muscles of the tongue
- hypothalamus
- major region of the diencephalon that is responsible for coordinating autonomic and endocrine control of homeostasis
- indirect pathway
- connections within the basal nuclei from the striatum through the globus pallidus external segment and subthalamic nucleus to the globus pallidus internal segment/substantia nigra pars compacta that result in inhibition of the thalamus to decrease cortical control of movement
- inferior colliculus
- half of the midbrain tectum that is part of the brain stem auditory pathway
- inferior olive
- nucleus in the medulla that is involved in processing information related to motor control
- intercostal nerve
- systemic nerve in the thoracic cavity that is found between two ribs
- internal carotid artery
- branch from the common carotid artery that enters the cranium and supplies blood to the brain
- interventricular foramina
- openings between the lateral ventricles and third ventricle allowing for the passage of CSF
- jugular veins
- blood vessels that return “used” blood from the head and neck
- kinesthesia
- general sensory perception of movement of the body
- lateral apertures
- pair of openings from the fourth ventricle to the subarachnoid space on either side and between the medulla and cerebellum
- lateral column
- white matter of the spinal cord between the posterior horn on one side and the axons from the anterior horn on the same side; composed of many different groups of axons, of both ascending and descending tracts, carrying motor commands to and from the brain
- lateral horn
- region of the spinal cord gray matter in the thoracic, upper lumbar, and sacral regions that is the central component of the sympathetic division of the autonomic nervous system
- lateral sulcus
- surface landmark of the cerebral cortex that marks the boundary between the temporal lobe and the frontal and parietal lobes
- lateral ventricles
- portions of the ventricular system that are in the region of the cerebrum
- limbic cortex
- collection of structures of the cerebral cortex that are involved in emotion, memory, and behavior and are part of the larger limbic system
- limbic system
- structures at the edge (limit) of the boundary between the forebrain and hindbrain that are most associated with emotional behavior and memory formation
- longitudinal fissure
- large separation along the midline between the two cerebral hemispheres
- lumbar plexus
- nerve plexus associated with the lumbar spinal nerves
- lumbar puncture
- procedure used to withdraw CSF from the lower lumbar region of the vertebral column that avoids the risk of damaging CNS tissue because the spinal cord ends at the upper lumbar vertebrae
- median aperture
- singular opening from the fourth ventricle into the subarachnoid space at the midline between the medulla and cerebellum
- median nerve
- systemic nerve of the arm, located between the ulnar and radial nerves
- meninges
- protective outer coverings of the CNS composed of connective tissue
- mesencephalon
- primary vesicle of the embryonic brain that does not significantly change through the rest of embryonic development and becomes the midbrain
- metencephalon
- secondary vesicle of the embryonic brain that develops into the pons and the cerebellum
- midbrain
- middle region of the adult brain that develops from the mesencephalon
- myelencephalon
- secondary vesicle of the embryonic brain that develops into the medulla
- nerve plexus
- network of nerves without neuronal cell bodies included
- neural crest
- tissue that detaches from the edges of the neural groove and migrates through the embryo to develop into peripheral structures of both nervous and non-nervous tissues
- neural fold
- elevated edge of the neural groove
- neural groove
- region of the neural plate that folds into the dorsal surface of the embryo and closes off to become the neural tube
- neural plate
- thickened layer of neuroepithelium that runs longitudinally along the dorsal surface of an embryo and gives rise to nervous system tissue
- neural tube
- precursor to structures of the central nervous system, formed by the invagination and separation of neuroepithelium
- neuraxis
- central axis to the nervous system, from the posterior to anterior ends of the neural tube; the inferior tip of the spinal cord to the anterior surface of the cerebrum
- occipital lobe
- region of the cerebral cortex directly beneath the occipital bone of the cranium
- occipital sinuses
- dural sinuses along the edge of the occipital lobes of the cerebrum
- oculomotor nerve
- third cranial nerve; responsible for contraction of four of the extraocular muscles, the muscle in the upper eyelid, and pupillary constriction
- olfaction
- special sense responsible for smell, which has a unique, direct connection to the cerebrum
- olfactory nerve
- first cranial nerve; responsible for the sense of smell
- optic nerve
- second cranial nerve; responsible for visual sensation
- orthostatic reflex
- sympathetic function that maintains blood pressure when standing to offset the increased effect of gravity
- paravertebral ganglia
- autonomic ganglia superior to the sympathetic chain ganglia
- parietal lobe
- region of the cerebral cortex directly beneath the parietal bone of the cranium
- parieto-occipital sulcus
- groove in the cerebral cortex representing the border between the parietal and occipital cortices
- perineurium
- layer of connective tissue surrounding fascicles within a nerve
- phrenic nerve
- systemic nerve from the cervical plexus that enervates the diaphragm
- pia mater
- thin, innermost membrane of the meninges that directly covers the surface of the CNS
- plexus
- network of nerves or nervous tissue
- postcentral gyrus
- primary motor cortex located in the frontal lobe of the cerebral cortex
- posterior columns
- white matter of the spinal cord that lies between the posterior horns of the gray matter, sometimes referred to as the dorsal column; composed of axons of ascending tracts that carry sensory information up to the brain
- posterior horn
- gray matter region of the spinal cord in which sensory input arrives, sometimes referred to as the dorsal horn
- posterior median sulcus
- midline feature of the posterior spinal cord, marking the separation between right and left sides of the cord
- posterolateral sulcus
- feature of the posterior spinal cord marking the entry of posterior nerve roots and the separation between the posterior and lateral columns of the white matter
- precentral gyrus
- ridge just posterior to the central sulcus, in the parietal lobe, where somatosensory processing initially takes place in the cerebrum
- prefrontal lobe
- specific region of the frontal lobe anterior to the more specific motor function areas, which can be related to the early planning of movements and intentions to the point of being personality-type functions
- premotor area
- region of the frontal lobe responsible for planning movements that will be executed through the primary motor cortex
- prevertebral ganglia
- autonomic ganglia that are anterior to the vertebral column and functionally related to the sympathetic chain ganglia
- primary vesicle
- initial enlargements of the anterior neural tube during embryonic development that develop into the forebrain, midbrain, and hindbrain
- proprioception
- general sensory perceptions providing information about location and movement of body parts; the “sense of the self”
- prosencephalon
- primary vesicle of the embryonic brain that develops into the forebrain, which includes the cerebrum and diencephalon
- putamen
- nucleus deep in the cerebrum that is part of the basal nuclei; along with the caudate, it is part of the striatum
- radial nerve
- systemic nerve of the arm, the distal component of which is located near the radial bone
- reticular formation
- diffuse region of gray matter throughout the brain stem that regulates sleep, wakefulness, and states of consciousness
- rhombencephalon
- primary vesicle of the embryonic brain that develops into the hindbrain, which includes the pons, cerebellum, and medulla
- sacral plexus
- nerve plexus associated with the lower lumbar and sacral spinal nerves
- saphenous nerve
- systemic nerve of the lower anterior leg that is a branch from the femoral nerve
- sciatic nerve
- systemic nerve from the sacral plexus that is a combination of the tibial and fibular nerves and extends across the hip joint and gluteal region into the upper posterior leg
- sciatica
- painful condition resulting from inflammation or compression of the sciatic nerve or any of the spinal nerves that contribute to it
- secondary vesicle
- five vesicles that develop from primary vesicles, continuing the process of differentiation of the embryonic brain
- sigmoid sinuses
- dural sinuses that drain directly into the jugular veins
- somatosensation
- general senses related to the body, usually thought of as the senses of touch, which would include pain, temperature, and proprioception
- spinal accessory nerve
- eleventh cranial nerve; responsible for contraction of neck muscles
- spinal nerve
- one of 31 nerves connected to the spinal cord
- straight sinus
- dural sinus that drains blood from the deep center of the brain to collect with the other sinuses
- striatum
- the caudate and putamen collectively, as part of the basal nuclei, which receive input from the cerebral cortex
- subarachnoid space
- space between the arachnoid mater and pia mater that contains CSF and the fibrous connections of the arachnoid trabeculae
- subcortical nucleus
- all the nuclei beneath the cerebral cortex, including the basal nuclei and the basal forebrain
- substantia nigra pars compacta
- nuclei within the basal nuclei that release dopamine to modulate the function of the striatum; part of the motor pathway
- substantia nigra pars reticulata
- nuclei within the basal nuclei that serve as an output center of the nuclei; part of the motor pathway
- subthalamus
- nucleus within the basal nuclei that is part of the indirect pathway
- sulcus
- groove formed by convolutions in the surface of the cerebral cortex
- superior colliculus
- half of the midbrain tectum that is responsible for aligning visual, auditory, and somatosensory spatial perceptions
- superior sagittal sinus
- dural sinus that runs along the top of the longitudinal fissure and drains blood from the majority of the outer cerebrum
- sympathetic chain ganglia
- autonomic ganglia in a chain along the anterolateral aspect of the vertebral column that are responsible for contributing to homeostatic mechanisms of the autonomic nervous system
- systemic nerve
- nerve in the periphery distal to a nerve plexus or spinal nerve
- tectum
- region of the midbrain, thought of as the roof of the cerebral aqueduct, which is subdivided into the inferior and superior colliculi
- tegmentum
- region of the midbrain, thought of as the floor of the cerebral aqueduct, which continues into the pons and medulla as the floor of the fourth ventricle
- telencephalon
- secondary vesicle of the embryonic brain that develops into the cerebrum
- temporal lobe
- region of the cerebral cortex directly beneath the temporal bone of the cranium
- terminal ganglion
- autonomic ganglia that are near or within the walls of organs that are responsible for contributing to homeostatic mechanisms of the autonomic nervous system
- thalamus
- major region of the diencephalon that is responsible for relaying information between the cerebrum and the hindbrain, spinal cord, and periphery
- third ventricle
- portion of the ventricular system that is in the region of the diencephalon
- tibial nerve
- systemic nerve of the posterior leg that begins as part of the sciatic nerve
- transverse sinuses
- dural sinuses that drain along either side of the occipital–cerebellar space
- trigeminal ganglion
- sensory ganglion that contributes sensory fibers to the trigeminal nerve
- trigeminal nerve
- fifth cranial nerve; responsible for cutaneous sensation of the face and contraction of the muscles of mastication
- trochlear nerve
- fourth cranial nerve; responsible for contraction of one of the extraocular muscles
- ulnar nerve
- systemic nerve of the arm located close to the ulna, a bone of the forearm
- vagus nerve
- tenth cranial nerve; responsible for the autonomic control of organs in the thoracic and upper abdominal cavities
- ventral (anterior) nerve root
- axons emerging from the anterior or lateral horns of the spinal cord
- ventricles
- remnants of the hollow center of the neural tube that are spaces for cerebrospinal fluid to circulate through the brain
- vertebral arteries
- arteries that ascend along either side of the vertebral column through the transverse foramina of the cervical vertebrae and enter the cranium through the foramen magnum
- vestibulocochlear nerve
- eighth cranial nerve; responsible for the sensations of hearing and balance
Chapter Review
13.1 The Embryologic Perspective
The development of the nervous system starts early in embryonic development. The outer layer of the embryo, the ectoderm, gives rise to the skin and the nervous system. A specialized region of this layer, the neuroectoderm, becomes a groove that folds in and becomes the neural tube beneath the dorsal surface of the embryo. The anterior end of the neural tube develops into the brain, and the posterior region becomes the spinal cord. Tissues at the edges of the neural groove, when it closes off, are called the neural crest and migrate through the embryo to give rise to PNS structures as well as some non-nervous tissues.
The brain develops from this early tube structure and gives rise to specific regions of the adult brain. As the neural tube grows and differentiates, it enlarges into three vesicles that correspond to the forebrain, midbrain, and hindbrain regions of the adult brain. Later in development, two of these three vesicles differentiate further, resulting in five vesicles. Those five vesicles can be aligned with the four major regions of the adult brain. The cerebrum is formed directly from the telencephalon. The diencephalon is the only region that keeps its embryonic name. The mesencephalon, metencephalon, and myelencephalon become the brain stem. The cerebellum also develops from the metencephalon and is a separate region of the adult brain.
The spinal cord develops out of the rest of the neural tube and retains the tube structure, with the nervous tissue thickening and the hollow center becoming a very small central canal through the cord. The rest of the hollow center of the neural tube corresponds to open spaces within the brain called the ventricles, where cerebrospinal fluid is found.
13.2 The Central Nervous System
The adult brain is separated into four major regions: the cerebrum, the diencephalon, the brain stem, and the cerebellum. The cerebrum is the largest portion and contains the cerebral cortex and subcortical nuclei. It is divided into two halves by the longitudinal fissure.
The cortex is separated into the frontal, parietal, temporal, and occipital lobes. The frontal lobe is responsible for motor functions, from planning movements through executing commands to be sent to the spinal cord and periphery. The most anterior portion of the frontal lobe is the prefrontal cortex, which is associated with aspects of personality through its influence on motor responses in decision-making.
The other lobes are responsible for sensory functions. The parietal lobe is where somatosensation is processed. The occipital lobe is where visual processing begins, although the other parts of the brain can contribute to visual function. The temporal lobe contains the cortical area for auditory processing, but also has regions crucial for memory formation.
Nuclei beneath the cerebral cortex, known as the subcortical nuclei, are responsible for augmenting cortical functions. The basal nuclei receive input from cortical areas and compare it with the general state of the individual through the activity of a dopamine-releasing nucleus. The output influences the activity of part of the thalamus that can then increase or decrease cortical activity that often results in changes to motor commands. The basal forebrain is responsible for modulating cortical activity in attention and memory. The limbic system includes deep cerebral nuclei that are responsible for emotion and memory.
The diencephalon includes the thalamus and the hypothalamus, along with some other structures. The thalamus is a relay between the cerebrum and the rest of the nervous system. The hypothalamus coordinates homeostatic functions through the autonomic and endocrine systems.
The brain stem is composed of the midbrain, pons, and medulla. It controls the head and neck region of the body through the cranial nerves. There are control centers in the brain stem that regulate the cardiovascular and respiratory systems.
The cerebellum is connected to the brain stem, primarily at the pons, where it receives a copy of the descending input from the cerebrum to the spinal cord. It can compare this with sensory feedback input through the medulla and send output through the midbrain that can correct motor commands for coordination.
13.3 Circulation and the Central Nervous System
The CNS has a privileged blood supply established by the blood-brain barrier. Establishing this barrier are anatomical structures that help to protect and isolate the CNS. The arterial blood to the brain comes from the internal carotid and vertebral arteries, which both contribute to the unique circle of Willis that provides constant perfusion of the brain even if one of the blood vessels is blocked or narrowed. That blood is eventually filtered to make a separate medium, the CSF, that circulates within the spaces of the brain and then into the surrounding space defined by the meninges, the protective covering of the brain and spinal cord.
The blood that nourishes the brain and spinal cord is behind the glial-cell–enforced blood-brain barrier, which limits the exchange of material from blood vessels with the interstitial fluid of the nervous tissue. Thus, metabolic wastes are collected in cerebrospinal fluid that circulates through the CNS. This fluid is produced by filtering blood at the choroid plexuses in the four ventricles of the brain. It then circulates through the ventricles and into the subarachnoid space, between the pia mater and the arachnoid mater. From the arachnoid granulations, CSF is reabsorbed into the blood, removing the waste from the privileged central nervous tissue.
The blood, now with the reabsorbed CSF, drains out of the cranium through the dural sinuses. The dura mater is the tough outer covering of the CNS, which is anchored to the inner surface of the cranial and vertebral cavities. It surrounds the venous space known as the dural sinuses, which connect to the jugular veins, where blood drains from the head and neck.
13.4 The Peripheral Nervous System
The PNS is composed of the groups of neurons (ganglia) and bundles of axons (nerves) that are outside of the brain and spinal cord. Ganglia are of two types, sensory or autonomic. Sensory ganglia contain unipolar sensory neurons and are found on the dorsal root of all spinal nerves as well as associated with many of the cranial nerves. Autonomic ganglia are in the sympathetic chain, the associated paravertebral or prevertebral ganglia, or in terminal ganglia near or within the organs controlled by the autonomic nervous system.
Nerves are classified as cranial nerves or spinal nerves on the basis of their connection to the brain or spinal cord, respectively. The twelve cranial nerves can be strictly sensory in function, strictly motor in function, or a combination of the two functions. Sensory fibers are axons of sensory ganglia that carry sensory information into the brain and target sensory nuclei. Motor fibers are axons of motor neurons in motor nuclei of the brain stem and target skeletal muscles of the head and neck. Spinal nerves are all mixed nerves with both sensory and motor fibers. Spinal nerves emerge from the spinal cord and reorganize through plexuses, which then give rise to systemic nerves. Thoracic spinal nerves are not part of any plexus, but give rise to the intercostal nerves directly.
Interactive Link Questions
Watch this animation to examine the development of the brain, starting with the neural tube. As the anterior end of the neural tube develops, it enlarges into the primary vesicles that establish the forebrain, midbrain, and hindbrain. Those structures continue to develop throughout the rest of embryonic development and into adolescence. They are the basis of the structure of the fully developed adult brain. How would you describe the difference in the relative sizes of the three regions of the brain when comparing the early (25th embryonic day) brain and the adult brain?
2.Watch this video to learn about the white matter in the cerebrum that develops during childhood and adolescence. This is a composite of MRI images taken of the brains of people from 5 years of age through 20 years of age, demonstrating how the cerebrum changes. As the color changes to blue, the ratio of gray matter to white matter changes. The caption for the video describes it as “less gray matter,” which is another way of saying “more white matter.” If the brain does not finish developing until approximately 20 years of age, can teenagers be held responsible for behaving badly?
3.Watch this video to learn about the basal nuclei (also known as the basal ganglia), which have two pathways that process information within the cerebrum. As shown in this video, the direct pathway is the shorter pathway through the system that results in increased activity in the cerebral cortex and increased motor activity. The direct pathway is described as resulting in “disinhibition” of the thalamus. What does disinhibition mean? What are the two neurons doing individually to cause this?
4.Watch this video to learn about the basal nuclei (also known as the basal ganglia), which have two pathways that process information within the cerebrum. As shown in this video, the indirect pathway is the longer pathway through the system that results in decreased activity in the cerebral cortex, and therefore less motor activity. The indirect pathway has an extra couple of connections in it, including disinhibition of the subthalamic nucleus. What is the end result on the thalamus, and therefore on movement initiated by the cerebral cortex?
5.Watch this video to learn about the gray matter of the spinal cord that receives input from fibers of the dorsal (posterior) root and sends information out through the fibers of the ventral (anterior) root. As discussed in this video, these connections represent the interactions of the CNS with peripheral structures for both sensory and motor functions. The cervical and lumbar spinal cords have enlargements as a result of larger populations of neurons. What are these enlargements responsible for?
6.Compared with the nearest evolutionary relative, the chimpanzee, the human has a brain that is huge. At a point in the past, a common ancestor gave rise to the two species of humans and chimpanzees. That evolutionary history is long and is still an area of intense study. But something happened to increase the size of the human brain relative to the chimpanzee. Read this article in which the author explores the current understanding of why this happened.
According to one hypothesis about the expansion of brain size, what tissue might have been sacrificed so energy was available to grow our larger brain? Based on what you know about that tissue and nervous tissue, why would there be a trade-off between them in terms of energy use?
7.Watch this animation to see how blood flows to the brain and passes through the circle of Willis before being distributed through the cerebrum. The circle of Willis is a specialized arrangement of arteries that ensure constant perfusion of the cerebrum even in the event of a blockage of one of the arteries in the circle. The animation shows the normal direction of flow through the circle of Willis to the middle cerebral artery. Where would the blood come from if there were a blockage just posterior to the middle cerebral artery on the left?
8.Watch this video that describes the procedure known as the lumbar puncture, a medical procedure used to sample the CSF. Because of the anatomy of the CNS, it is a relative safe location to insert a needle. Why is the lumbar puncture performed in the lower lumbar area of the vertebral column?
9.Watch this animation that shows the flow of CSF through the brain and spinal cord, and how it originates from the ventricles and then spreads into the space within the meninges, where the fluids then move into the venous sinuses to return to the cardiovascular circulation. What are the structures that produce CSF and where are they found? How are the structures indicated in this animation?
10.Figure 13.20 If you zoom in on the DRG, you can see smaller satellite glial cells surrounding the large cell bodies of the sensory neurons. From what structure do satellite cells derive during embryologic development?
11.Figure 13.22 To what structures in a skeletal muscle are the endoneurium, perineurium, and epineurium comparable?
12.Visit this site to read about a man who wakes with a headache and a loss of vision. His regular doctor sent him to an ophthalmologist to address the vision loss. The ophthalmologist recognizes a greater problem and immediately sends him to the emergency room. Once there, the patient undergoes a large battery of tests, but a definite cause cannot be found. A specialist recognizes the problem as meningitis, but the question is what caused it originally. How can that be cured? The loss of vision comes from swelling around the optic nerve, which probably presented as a bulge on the inside of the eye. Why is swelling related to meningitis going to push on the optic nerve?
Review Questions
Aside from the nervous system, which other organ system develops out of the ectoderm?
- digestive
- respiratory
- integumentary
- urinary
Which primary vesicle of the embryonic nervous system does not differentiate into more vesicles at the secondary stage?
- prosencephalon
- mesencephalon
- diencephalon
- rhombencephalon
Which adult structure(s) arises from the diencephalon?
- thalamus, hypothalamus, retina
- midbrain, pons, medulla
- pons and cerebellum
- cerebrum
Which non-nervous tissue develops from the neuroectoderm?
- respiratory mucosa
- vertebral bone
- digestive lining
- craniofacial bone
Which structure is associated with the embryologic development of the peripheral nervous system?
- neural crest
- neuraxis
- rhombencephalon
- neural tube
Which lobe of the cerebral cortex is responsible for generating motor commands?
- temporal
- parietal
- occipital
- frontal
What region of the diencephalon coordinates homeostasis?
- thalamus
- epithalamus
- hypothalamus
- subthalamus
What level of the brain stem is the major input to the cerebellum?
- midbrain
- pons
- medulla
- spinal cord
What region of the spinal cord contains motor neurons that direct the movement of skeletal muscles?
- anterior horn
- posterior horn
- lateral horn
- alar plate
Brodmann’s areas map different regions of the ________ to particular functions.
- cerebellum
- cerebral cortex
- basal forebrain
- corpus callosum
What blood vessel enters the cranium to supply the brain with fresh, oxygenated blood?
- common carotid artery
- jugular vein
- internal carotid artery
- aorta
Which layer of the meninges surrounds and supports the sinuses that form the route through which blood drains from the CNS?
- dura mater
- arachnoid mater
- subarachnoid
- pia mater
What type of glial cell is responsible for filtering blood to produce CSF at the choroid plexus?
- ependymal cell
- astrocyte
- oligodendrocyte
- Schwann cell
Which portion of the ventricular system is found within the diencephalon?
- lateral ventricles
- third ventricle
- cerebral aqueduct
- fourth ventricle
What condition causes a stroke?
- inflammation of meninges
- lumbar puncture
- infection of cerebral spinal fluid
- disruption of blood to the brain
What type of ganglion contains neurons that control homeostatic mechanisms of the body?
- sensory ganglion
- dorsal root ganglion
- autonomic ganglion
- cranial nerve ganglion
Which ganglion is responsible for cutaneous sensations of the face?
- otic ganglion
- vestibular ganglion
- geniculate ganglion
- trigeminal ganglion
What is the name for a bundle of axons within a nerve?
- fascicle
- tract
- nerve root
- epineurium
Which cranial nerve does not control functions in the head and neck?
- olfactory
- trochlear
- glossopharyngeal
- vagus
Which of these structures is not under direct control of the peripheral nervous system?
- trigeminal ganglion
- gastric plexus
- sympathetic chain ganglia
- cervical plexus
Critical Thinking Questions
Studying the embryonic development of the nervous system makes it easier to understand the complexity of the adult nervous system. Give one example of how development in the embryonic nervous system explains a more complex structure in the adult nervous system.
34.What happens in development that suggests that there is a special relationship between the skeletal structure of the head and the nervous system?
35.Damage to specific regions of the cerebral cortex, such as through a stroke, can result in specific losses of function. What functions would likely be lost by a stroke in the temporal lobe?
36.Why do the anatomical inputs to the cerebellum suggest that it can compare motor commands and sensory feedback?
37.Why can the circle of Willis maintain perfusion of the brain even if there is a blockage in one part of the structure?
38.Meningitis is an inflammation of the meninges that can have severe effects on neurological function. Why is infection of this structure potentially so dangerous?
39.Why are ganglia and nerves not surrounded by protective structures like the meninges of the CNS?
40.Testing for neurological function involves a series of tests of functions associated with the cranial nerves. What functions, and therefore which nerves, are being tested by asking a patient to follow the tip of a pen with their eyes?
|
oercommons
|
2025-03-18T00:39:11.652190
|
07/23/2019
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/56376/overview",
"title": "Anatomy and Physiology, Regulation, Integration, and Control, Anatomy of the Nervous System",
"author": null
}
|
https://oercommons.org/courseware/lesson/56369/overview
|
Muscle Tissue
Introduction
Figure 10.1 Tennis Player Athletes rely on toned skeletal muscles to supply the force required for movement. (credit: Emmanuel Huybrechts/flickr)
CHAPTER OBJECTIVES
After studying this chapter, you will be able to:
- Explain the organization of muscle tissue
- Describe the function and structure of skeletal, cardiac muscle, and smooth muscle
- Explain how muscles work with tendons to move the body
- Describe how muscles contract and relax
- Define the process of muscle metabolism
- Explain how the nervous system controls muscle tension
- Relate the connections between exercise and muscle performance
- Explain the development and regeneration of muscle tissue
When most people think of muscles, they think of the muscles that are visible just under the skin, particularly of the limbs. These are skeletal muscles, so-named because most of them move the skeleton. But there are two other types of muscle in the body, with distinctly different jobs. Cardiac muscle, found in the heart, is concerned with pumping blood through the circulatory system. Smooth muscle is concerned with various involuntary movements, such as having one’s hair stand on end when cold or frightened, or moving food through the digestive system. This chapter will examine the structure and function of these three types of muscles.
Overview of Muscle Tissues
- Describe the different types of muscle
- Explain contractibility and extensibility
Muscle is one of the four primary tissue types of the body, and the body contains three types of muscle tissue: skeletal muscle, cardiac muscle, and smooth muscle (Figure 10.2). All three muscle tissues have some properties in common; they all exhibit a quality called excitability as their plasma membranes can change their electrical states (from polarized to depolarized) and send an electrical wave called an action potential along the entire length of the membrane. While the nervous system can influence the excitability of cardiac and smooth muscle to some degree, skeletal muscle completely depends on signaling from the nervous system to work properly. On the other hand, both cardiac muscle and smooth muscle can respond to other stimuli, such as hormones and local stimuli.
Figure 10.2 The Three Types of Muscle Tissue The body contains three types of muscle tissue: (a) skeletal muscle, (b) smooth muscle, and (c) cardiac muscle. From top, LM × 1600, LM × 1600, LM × 1600. (Micrographs provided by the Regents of University of Michigan Medical School © 2012)
The muscles all begin the actual process of contracting (shortening) when a protein called actin is pulled by a protein called myosin. This occurs in striated muscle (skeletal and cardiac) after specific binding sites on the actin have been exposed in response to the interaction between calcium ions (Ca++) and proteins (troponin and tropomyosin) that “shield” the actin-binding sites. Ca++ also is required for the contraction of smooth muscle, although its role is different: here Ca++ activates enzymes, which in turn activate myosin heads. All muscles require adenosine triphosphate (ATP) to continue the process of contracting, and they all relax when the Ca++ is removed and the actin-binding sites are re-shielded.
A muscle can return to its original length when relaxed due to a quality of muscle tissue called elasticity. It can recoil back to its original length due to elastic fibers. Muscle tissue also has the quality of extensibility; it can stretch or extend. Contractilityallows muscle tissue to pull on its attachment points and shorten with force.
Differences among the three muscle types include the microscopic organization of their contractile proteins—actin and myosin. The actin and myosin proteins are arranged very regularly in the cytoplasm of individual muscle cells (referred to as fibers) in both skeletal muscle and cardiac muscle, which creates a pattern, or stripes, called striations. The striations are visible with a light microscope under high magnification (see Figure 10.2). Skeletal muscle fibers are multinucleated structures that compose the skeletal muscle. Cardiac muscle fibers each have one to two nuclei and are physically and electrically connected to each other so that the entire heart contracts as one unit (called a syncytium).
Because the actin and myosin are not arranged in such regular fashion in smooth muscle, the cytoplasm of a smooth muscle fiber (which has only a single nucleus) has a uniform, nonstriated appearance (resulting in the name smooth muscle). However, the less organized appearance of smooth muscle should not be interpreted as less efficient. Smooth muscle in the walls of arteries is a critical component that regulates blood pressure necessary to push blood through the circulatory system; and smooth muscle in the skin, visceral organs, and internal passageways is essential for moving all materials through the body.
Skeletal Muscle
- Describe the layers of connective tissues packaging skeletal muscle
- Explain how muscles work with tendons to move the body
- Identify areas of the skeletal muscle fibers
- Describe excitation-contraction coupling
The best-known feature of skeletal muscle is its ability to contract and cause movement. Skeletal muscles act not only to produce movement but also to stop movement, such as resisting gravity to maintain posture. Small, constant adjustments of the skeletal muscles are needed to hold a body upright or balanced in any position. Muscles also prevent excess movement of the bones and joints, maintaining skeletal stability and preventing skeletal structure damage or deformation. Joints can become misaligned or dislocated entirely by pulling on the associated bones; muscles work to keep joints stable. Skeletal muscles are located throughout the body at the openings of internal tracts to control the movement of various substances. These muscles allow functions, such as swallowing, urination, and defecation, to be under voluntary control. Skeletal muscles also protect internal organs (particularly abdominal and pelvic organs) by acting as an external barrier or shield to external trauma and by supporting the weight of the organs.
Skeletal muscles contribute to the maintenance of homeostasis in the body by generating heat. Muscle contraction requires energy, and when ATP is broken down, heat is produced. This heat is very noticeable during exercise, when sustained muscle movement causes body temperature to rise, and in cases of extreme cold, when shivering produces random skeletal muscle contractions to generate heat.
Each skeletal muscle is an organ that consists of various integrated tissues. These tissues include the skeletal muscle fibers, blood vessels, nerve fibers, and connective tissue. Each skeletal muscle has three layers of connective tissue (called “mysia”) that enclose it and provide structure to the muscle as a whole, and also compartmentalize the muscle fibers within the muscle (Figure 10.3). Each muscle is wrapped in a sheath of dense, irregular connective tissue called the epimysium, which allows a muscle to contract and move powerfully while maintaining its structural integrity. The epimysium also separates muscle from other tissues and organs in the area, allowing the muscle to move independently.
Figure 10.3 The Three Connective Tissue Layers Bundles of muscle fibers, called fascicles, are covered by the perimysium. Muscle fibers are covered by the endomysium.
Inside each skeletal muscle, muscle fibers are organized into individual bundles, each called a fascicle, by a middle layer of connective tissue called the perimysium. This fascicular organization is common in muscles of the limbs; it allows the nervous system to trigger a specific movement of a muscle by activating a subset of muscle fibers within a bundle, or fascicle of the muscle. Inside each fascicle, each muscle fiber is encased in a thin connective tissue layer of collagen and reticular fibers called the endomysium. The endomysium contains the extracellular fluid and nutrients to support the muscle fiber. These nutrients are supplied via blood to the muscle tissue.
In skeletal muscles that work with tendons to pull on bones, the collagen in the three tissue layers (the mysia) intertwines with the collagen of a tendon. At the other end of the tendon, it fuses with the periosteum coating the bone. The tension created by contraction of the muscle fibers is then transferred though the mysia, to the tendon, and then to the periosteum to pull on the bone for movement of the skeleton. In other places, the mysia may fuse with a broad, tendon-like sheet called an aponeurosis, or to fascia, the connective tissue between skin and bones. The broad sheet of connective tissue in the lower back that the latissimus dorsi muscles (the “lats”) fuse into is an example of an aponeurosis.
Every skeletal muscle is also richly supplied by blood vessels for nourishment, oxygen delivery, and waste removal. In addition, every muscle fiber in a skeletal muscle is supplied by the axon branch of a somatic motor neuron, which signals the fiber to contract. Unlike cardiac and smooth muscle, the only way to functionally contract a skeletal muscle is through signaling from the nervous system.
Skeletal Muscle Fibers
Because skeletal muscle cells are long and cylindrical, they are commonly referred to as muscle fibers. Skeletal muscle fibers can be quite large for human cells, with diameters up to 100 μm and lengths up to 30 cm (11.8 in) in the Sartorius of the upper leg. During early development, embryonic myoblasts, each with its own nucleus, fuse with up to hundreds of other myoblasts to form the multinucleated skeletal muscle fibers. Multiple nuclei mean multiple copies of genes, permitting the production of the large amounts of proteins and enzymes needed for muscle contraction.
Some other terminology associated with muscle fibers is rooted in the Greek sarco, which means “flesh.” The plasma membrane of muscle fibers is called the sarcolemma, the cytoplasm is referred to as sarcoplasm, and the specialized smooth endoplasmic reticulum, which stores, releases, and retrieves calcium ions (Ca++) is called the sarcoplasmic reticulum (SR)(Figure 10.4). As will soon be described, the functional unit of a skeletal muscle fiber is the sarcomere, a highly organized arrangement of the contractile myofilaments actin (thin filament) and myosin (thick filament), along with other support proteins.
Figure 10.4 Muscle Fiber A skeletal muscle fiber is surrounded by a plasma membrane called the sarcolemma, which contains sarcoplasm, the cytoplasm of muscle cells. A muscle fiber is composed of many fibrils, which give the cell its striated appearance.
The Sarcomere
The striated appearance of skeletal muscle fibers is due to the arrangement of the myofilaments of actin and myosin in sequential order from one end of the muscle fiber to the other. Each packet of these microfilaments and their regulatory proteins, troponin and tropomyosin (along with other proteins) is called a sarcomere.
INTERACTIVE LINK
Watch this video to learn more about macro- and microstructures of skeletal muscles. (a) What are the names of the “junction points” between sarcomeres? (b) What are the names of the “subunits” within the myofibrils that run the length of skeletal muscle fibers? (c) What is the “double strand of pearls” described in the video? (d) What gives a skeletal muscle fiber its striated appearance?
The sarcomere is the functional unit of the muscle fiber. The sarcomere itself is bundled within the myofibril that runs the entire length of the muscle fiber and attaches to the sarcolemma at its end. As myofibrils contract, the entire muscle cell contracts. Because myofibrils are only approximately 1.2 μm in diameter, hundreds to thousands (each with thousands of sarcomeres) can be found inside one muscle fiber. Each sarcomere is approximately 2 μm in length with a three-dimensional cylinder-like arrangement and is bordered by structures called Z-discs (also called Z-lines, because pictures are two-dimensional), to which the actin myofilaments are anchored (Figure 10.5). Because the actin and its troponin-tropomyosin complex (projecting from the Z-discs toward the center of the sarcomere) form strands that are thinner than the myosin, it is called the thin filament of the sarcomere. Likewise, because the myosin strands and their multiple heads (projecting from the center of the sarcomere, toward but not all to way to, the Z-discs) have more mass and are thicker, they are called the thick filament of the sarcomere.
Figure 10.5 The Sarcomere The sarcomere, the region from one Z-line to the next Z-line, is the functional unit of a skeletal muscle fiber.
The Neuromuscular Junction
Another specialization of the skeletal muscle is the site where a motor neuron’s terminal meets the muscle fiber—called the neuromuscular junction (NMJ). This is where the muscle fiber first responds to signaling by the motor neuron. Every skeletal muscle fiber in every skeletal muscle is innervated by a motor neuron at the NMJ. Excitation signals from the neuron are the only way to functionally activate the fiber to contract.
INTERACTIVE LINK
Every skeletal muscle fiber is supplied by a motor neuron at the NMJ. Watch this video to learn more about what happens at the NMJ. (a) What is the definition of a motor unit? (b) What is the structural and functional difference between a large motor unit and a small motor unit? (c) Can you give an example of each? (d) Why is the neurotransmitter acetylcholine degraded after binding to its receptor?
Excitation-Contraction Coupling
All living cells have membrane potentials, or electrical gradients across their membranes. The inside of the membrane is usually around -60 to -90 mV, relative to the outside. This is referred to as a cell’s membrane potential. Neurons and muscle cells can use their membrane potentials to generate electrical signals. They do this by controlling the movement of charged particles, called ions, across their membranes to create electrical currents. This is achieved by opening and closing specialized proteins in the membrane called ion channels. Although the currents generated by ions moving through these channel proteins are very small, they form the basis of both neural signaling and muscle contraction.
Both neurons and skeletal muscle cells are electrically excitable, meaning that they are able to generate action potentials. An action potential is a special type of electrical signal that can travel along a cell membrane as a wave. This allows a signal to be transmitted quickly and faithfully over long distances.
Although the term excitation-contraction coupling confuses or scares some students, it comes down to this: for a skeletal muscle fiber to contract, its membrane must first be “excited”—in other words, it must be stimulated to fire an action potential. The muscle fiber action potential, which sweeps along the sarcolemma as a wave, is “coupled” to the actual contraction through the release of calcium ions (Ca++) from the SR. Once released, the Ca++ interacts with the shielding proteins, forcing them to move aside so that the actin-binding sites are available for attachment by myosin heads. The myosin then pulls the actin filaments toward the center, shortening the muscle fiber.
In skeletal muscle, this sequence begins with signals from the somatic motor division of the nervous system. In other words, the “excitation” step in skeletal muscles is always triggered by signaling from the nervous system (Figure 10.6).
Figure 10.6 Motor End-Plate and Innervation At the NMJ, the axon terminal releases ACh. The motor end-plate is the location of the ACh-receptors in the muscle fiber sarcolemma. When ACh molecules are released, they diffuse across a minute space called the synaptic cleft and bind to the receptors.
The motor neurons that tell the skeletal muscle fibers to contract originate in the spinal cord, with a smaller number located in the brainstem for activation of skeletal muscles of the face, head, and neck. These neurons have long processes, called axons, which are specialized to transmit action potentials long distances— in this case, all the way from the spinal cord to the muscle itself (which may be up to three feet away). The axons of multiple neurons bundle together to form nerves, like wires bundled together in a cable.
Signaling begins when a neuronal action potential travels along the axon of a motor neuron, and then along the individual branches to terminate at the NMJ. At the NMJ, the axon terminal releases a chemical messenger, or neurotransmitter, called acetylcholine (ACh). The ACh molecules diffuse across a minute space called the synaptic cleft and bind to ACh receptors located within the motor end-plate of the sarcolemma on the other side of the synapse. Once ACh binds, a channel in the ACh receptor opens and positively charged ions can pass through into the muscle fiber, causing it to depolarize, meaning that the membrane potential of the muscle fiber becomes less negative (closer to zero.)
As the membrane depolarizes, another set of ion channels called voltage-gated sodium channels are triggered to open. Sodium ions enter the muscle fiber, and an action potential rapidly spreads (or “fires”) along the entire membrane to initiate excitation-contraction coupling.
Things happen very quickly in the world of excitable membranes (just think about how quickly you can snap your fingers as soon as you decide to do it). Immediately following depolarization of the membrane, it repolarizes, re-establishing the negative membrane potential. Meanwhile, the ACh in the synaptic cleft is degraded by the enzyme acetylcholinesterase (AChE) so that the ACh cannot rebind to a receptor and reopen its channel, which would cause unwanted extended muscle excitation and contraction.
Propagation of an action potential along the sarcolemma is the excitation portion of excitation-contraction coupling. Recall that this excitation actually triggers the release of calcium ions (Ca++) from its storage in the cell’s SR. For the action potential to reach the membrane of the SR, there are periodic invaginations in the sarcolemma, called T-tubules (“T” stands for “transverse”). You will recall that the diameter of a muscle fiber can be up to 100 μm, so these T-tubules ensure that the membrane can get close to the SR in the sarcoplasm. The arrangement of a T-tubule with the membranes of SR on either side is called a triad (Figure 10.7). The triad surrounds the cylindrical structure called a myofibril, which contains actin and myosin.
Figure 10.7 The T-tubule Narrow T-tubules permit the conduction of electrical impulses. The SR functions to regulate intracellular levels of calcium. Two terminal cisternae (where enlarged SR connects to the T-tubule) and one T-tubule comprise a triad—a “threesome” of membranes, with those of SR on two sides and the T-tubule sandwiched between them.
The T-tubules carry the action potential into the interior of the cell, which triggers the opening of calcium channels in the membrane of the adjacent SR, causing Ca++ to diffuse out of the SR and into the sarcoplasm. It is the arrival of Ca++ in the sarcoplasm that initiates contraction of the muscle fiber by its contractile units, or sarcomeres.
Muscle Fiber Contraction and Relaxation
- Describe the components involved in a muscle contraction
- Explain how muscles contract and relax
- Describe the sliding filament model of muscle contraction
The sequence of events that result in the contraction of an individual muscle fiber begins with a signal—the neurotransmitter, ACh—from the motor neuron innervating that fiber. The local membrane of the fiber will depolarize as positively charged sodium ions (Na+) enter, triggering an action potential that spreads to the rest of the membrane which will depolarize, including the T-tubules. This triggers the release of calcium ions (Ca++) from storage in the sarcoplasmic reticulum (SR). The Ca++ then initiates contraction, which is sustained by ATP (Figure 10.8). As long as Ca++ ions remain in the sarcoplasm to bind to troponin, which keeps the actin-binding sites “unshielded,” and as long as ATP is available to drive the cross-bridge cycling and the pulling of actin strands by myosin, the muscle fiber will continue to shorten to an anatomical limit.
Figure 10.8 Contraction of a Muscle Fiber A cross-bridge forms between actin and the myosin heads triggering contraction. As long as Ca++ ions remain in the sarcoplasm to bind to troponin, and as long as ATP is available, the muscle fiber will continue to shorten.
Muscle contraction usually stops when signaling from the motor neuron ends, which repolarizes the sarcolemma and T-tubules, and closes the voltage-gated calcium channels in the SR. Ca++ ions are then pumped back into the SR, which causes the tropomyosin to reshield (or re-cover) the binding sites on the actin strands. A muscle also can stop contracting when it runs out of ATP and becomes fatigued (Figure 10.9).
Figure 10.9 Relaxation of a Muscle Fiber Ca++ ions are pumped back into the SR, which causes the tropomyosin to reshield the binding sites on the actin strands. A muscle may also stop contracting when it runs out of ATP and becomes fatigued.
INTERACTIVE LINK
The release of calcium ions initiates muscle contractions. Watch this video to learn more about the role of calcium. (a) What are “T-tubules” and what is their role? (b) Please describe how actin-binding sites are made available for cross-bridging with myosin heads during contraction.
The molecular events of muscle fiber shortening occur within the fiber’s sarcomeres (see Figure 10.10). The contraction of a striated muscle fiber occurs as the sarcomeres, linearly arranged within myofibrils, shorten as myosin heads pull on the actin filaments.
The region where thick and thin filaments overlap has a dense appearance, as there is little space between the filaments. This zone where thin and thick filaments overlap is very important to muscle contraction, as it is the site where filament movement starts. Thin filaments, anchored at their ends by the Z-discs, do not extend completely into the central region that only contains thick filaments, anchored at their bases at a spot called the M-line. A myofibril is composed of many sarcomeres running along its length; thus, myofibrils and muscle cells contract as the sarcomeres contract.
The Sliding Filament Model of Contraction
When signaled by a motor neuron, a skeletal muscle fiber contracts as the thin filaments are pulled and then slide past the thick filaments within the fiber’s sarcomeres. This process is known as the sliding filament model of muscle contraction (Figure 10.10). The sliding can only occur when myosin-binding sites on the actin filaments are exposed by a series of steps that begins with Ca++ entry into the sarcoplasm.
Figure 10.10 The Sliding Filament Model of Muscle Contraction When a sarcomere contracts, the Z lines move closer together, and the I band becomes smaller. The A band stays the same width. At full contraction, the thin and thick filaments overlap completely.
Tropomyosin is a protein that winds around the chains of the actin filament and covers the myosin-binding sites to prevent actin from binding to myosin. Tropomyosin binds to troponin to form a troponin-tropomyosin complex. The troponin-tropomyosin complex prevents the myosin “heads” from binding to the active sites on the actin microfilaments. Troponin also has a binding site for Ca++ ions.
To initiate muscle contraction, tropomyosin has to expose the myosin-binding site on an actin filament to allow cross-bridge formation between the actin and myosin microfilaments. The first step in the process of contraction is for Ca++ to bind to troponin so that tropomyosin can slide away from the binding sites on the actin strands. This allows the myosin heads to bind to these exposed binding sites and form cross-bridges. The thin filaments are then pulled by the myosin heads to slide past the thick filaments toward the center of the sarcomere. But each head can only pull a very short distance before it has reached its limit and must be “re-cocked” before it can pull again, a step that requires ATP.
ATP and Muscle Contraction
For thin filaments to continue to slide past thick filaments during muscle contraction, myosin heads must pull the actin at the binding sites, detach, re-cock, attach to more binding sites, pull, detach, re-cock, etc. This repeated movement is known as the cross-bridge cycle. This motion of the myosin heads is similar to the oars when an individual rows a boat: The paddle of the oars (the myosin heads) pull, are lifted from the water (detach), repositioned (re-cocked) and then immersed again to pull (Figure 10.11). Each cycle requires energy, and the action of the myosin heads in the sarcomeres repetitively pulling on the thin filaments also requires energy, which is provided by ATP.
Figure 10.11 Skeletal Muscle Contraction (a) The active site on actin is exposed as calcium binds to troponin. (b) The myosin head is attracted to actin, and myosin binds actin at its actin-binding site, forming the cross-bridge. (c) During the power stroke, the phosphate generated in the previous contraction cycle is released. This results in the myosin head pivoting toward the center of the sarcomere, after which the attached ADP and phosphate group are released. (d) A new molecule of ATP attaches to the myosin head, causing the cross-bridge to detach. (e) The myosin head hydrolyzes ATP to ADP and phosphate, which returns the myosin to the cocked position.
Cross-bridge formation occurs when the myosin head attaches to the actin while adenosine diphosphate (ADP) and inorganic phosphate (Pi) are still bound to myosin (Figure 10.11a,b). Pi is then released, causing myosin to form a stronger attachment to the actin, after which the myosin head moves toward the M-line, pulling the actin along with it. As actin is pulled, the filaments move approximately 10 nm toward the M-line. This movement is called the power stroke, as movement of the thin filament occurs at this step (Figure 10.11c). In the absence of ATP, the myosin head will not detach from actin.
One part of the myosin head attaches to the binding site on the actin, but the head has another binding site for ATP. ATP binding causes the myosin head to detach from the actin (Figure 10.11d). After this occurs, ATP is converted to ADP and Pi by the intrinsic ATPase activity of myosin. The energy released during ATP hydrolysis changes the angle of the myosin head into a cocked position (Figure 10.11e). The myosin head is now in position for further movement.
When the myosin head is cocked, myosin is in a high-energy configuration. This energy is expended as the myosin head moves through the power stroke, and at the end of the power stroke, the myosin head is in a low-energy position. After the power stroke, ADP is released; however, the formed cross-bridge is still in place, and actin and myosin are bound together. As long as ATP is available, it readily attaches to myosin, the cross-bridge cycle can recur, and muscle contraction can continue.
Note that each thick filament of roughly 300 myosin molecules has multiple myosin heads, and many cross-bridges form and break continuously during muscle contraction. Multiply this by all of the sarcomeres in one myofibril, all the myofibrils in one muscle fiber, and all of the muscle fibers in one skeletal muscle, and you can understand why so much energy (ATP) is needed to keep skeletal muscles working. In fact, it is the loss of ATP that results in the rigor mortis observed soon after someone dies. With no further ATP production possible, there is no ATP available for myosin heads to detach from the actin-binding sites, so the cross-bridges stay in place, causing the rigidity in the skeletal muscles.
Sources of ATP
ATP supplies the energy for muscle contraction to take place. In addition to its direct role in the cross-bridge cycle, ATP also provides the energy for the active-transport Ca++ pumps in the SR. Muscle contraction does not occur without sufficient amounts of ATP. The amount of ATP stored in muscle is very low, only sufficient to power a few seconds worth of contractions. As it is broken down, ATP must therefore be regenerated and replaced quickly to allow for sustained contraction. There are three mechanisms by which ATP can be regenerated: creatine phosphate metabolism, anaerobic glycolysis, and fermentation and aerobic respiration.
Creatine phosphate is a molecule that can store energy in its phosphate bonds. In a resting muscle, excess ATP transfers its energy to creatine, producing ADP and creatine phosphate. This acts as an energy reserve that can be used to quickly create more ATP. When the muscle starts to contract and needs energy, creatine phosphate transfers its phosphate back to ADP to form ATP and creatine. This reaction is catalyzed by the enzyme creatine kinase and occurs very quickly; thus, creatine phosphate-derived ATP powers the first few seconds of muscle contraction. However, creatine phosphate can only provide approximately 15 seconds worth of energy, at which point another energy source has to be used (Figure 10.12).
Figure 10.12 Muscle Metabolism (a) Some ATP is stored in a resting muscle. As contraction starts, it is used up in seconds. More ATP is generated from creatine phosphate for about 15 seconds. (b) Each glucose molecule produces two ATP and two molecules of pyruvic acid, which can be used in aerobic respiration or converted to lactic acid. If oxygen is not available, pyruvic acid is converted to lactic acid, which may contribute to muscle fatigue. This occurs during strenuous exercise when high amounts of energy are needed but oxygen cannot be sufficiently delivered to muscle. (c) Aerobic respiration is the breakdown of glucose in the presence of oxygen (O2) to produce carbon dioxide, water, and ATP. Approximately 95 percent of the ATP required for resting or moderately active muscles is provided by aerobic respiration, which takes place in mitochondria.
As the ATP produced by creatine phosphate is depleted, muscles turn to glycolysis as an ATP source. Glycolysis is an anaerobic (non-oxygen-dependent) process that breaks down glucose (sugar) to produce ATP; however, glycolysis cannot generate ATP as quickly as creatine phosphate. Thus, the switch to glycolysis results in a slower rate of ATP availability to the muscle. The sugar used in glycolysis can be provided by blood glucose or by metabolizing glycogen that is stored in the muscle. The breakdown of one glucose molecule produces two ATP and two molecules of pyruvic acid, which can be used in aerobic respiration or when oxygen levels are low, converted to lactic acid (Figure 10.12b).
If oxygen is available, pyruvic acid is used in aerobic respiration. However, if oxygen is not available, pyruvic acid is converted to lactic acid, which may contribute to muscle fatigue. This conversion allows the recycling of the enzyme NAD+ from NADH, which is needed for glycolysis to continue. This occurs during strenuous exercise when high amounts of energy are needed but oxygen cannot be sufficiently delivered to muscle. Glycolysis itself cannot be sustained for very long (approximately 1 minute of muscle activity), but it is useful in facilitating short bursts of high-intensity output. This is because glycolysis does not utilize glucose very efficiently, producing a net gain of two ATPs per molecule of glucose, and the end product of lactic acid, which may contribute to muscle fatigue as it accumulates.
Aerobic respiration is the breakdown of glucose or other nutrients in the presence of oxygen (O2) to produce carbon dioxide, water, and ATP. Approximately 95 percent of the ATP required for resting or moderately active muscles is provided by aerobic respiration, which takes place in mitochondria. The inputs for aerobic respiration include glucose circulating in the bloodstream, pyruvic acid, and fatty acids. Aerobic respiration is much more efficient than anaerobic glycolysis, producing approximately 36 ATPs per molecule of glucose versus four from glycolysis. However, aerobic respiration cannot be sustained without a steady supply of O2 to the skeletal muscle and is much slower (Figure 10.12c). To compensate, muscles store small amount of excess oxygen in proteins call myoglobin, allowing for more efficient muscle contractions and less fatigue. Aerobic training also increases the efficiency of the circulatory system so that O2 can be supplied to the muscles for longer periods of time.
Muscle fatigue occurs when a muscle can no longer contract in response to signals from the nervous system. The exact causes of muscle fatigue are not fully known, although certain factors have been correlated with the decreased muscle contraction that occurs during fatigue. ATP is needed for normal muscle contraction, and as ATP reserves are reduced, muscle function may decline. This may be more of a factor in brief, intense muscle output rather than sustained, lower intensity efforts. Lactic acid buildup may lower intracellular pH, affecting enzyme and protein activity. Imbalances in Na+ and K+ levels as a result of membrane depolarization may disrupt Ca++ flow out of the SR. Long periods of sustained exercise may damage the SR and the sarcolemma, resulting in impaired Ca++ regulation.
Intense muscle activity results in an oxygen debt, which is the amount of oxygen needed to compensate for ATP produced without oxygen during muscle contraction. Oxygen is required to restore ATP and creatine phosphate levels, convert lactic acid to pyruvic acid, and, in the liver, to convert lactic acid into glucose or glycogen. Other systems used during exercise also require oxygen, and all of these combined processes result in the increased breathing rate that occurs after exercise. Until the oxygen debt has been met, oxygen intake is elevated, even after exercise has stopped.
Relaxation of a Skeletal Muscle
Relaxing skeletal muscle fibers, and ultimately, the skeletal muscle, begins with the motor neuron, which stops releasing its chemical signal, ACh, into the synapse at the NMJ. The muscle fiber will repolarize, which closes the gates in the SR where Ca++ was being released. ATP-driven pumps will move Ca++ out of the sarcoplasm back into the SR. This results in the “reshielding” of the actin-binding sites on the thin filaments. Without the ability to form cross-bridges between the thin and thick filaments, the muscle fiber loses its tension and relaxes.
Muscle Strength
The number of skeletal muscle fibers in a given muscle is genetically determined and does not change. Muscle strength is directly related to the amount of myofibrils and sarcomeres within each fiber. Factors, such as hormones and stress (and artificial anabolic steroids), acting on the muscle can increase the production of sarcomeres and myofibrils within the muscle fibers, a change called hypertrophy, which results in the increased mass and bulk in a skeletal muscle. Likewise, decreased use of a skeletal muscle results in atrophy, where the number of sarcomeres and myofibrils disappear (but not the number of muscle fibers). It is common for a limb in a cast to show atrophied muscles when the cast is removed, and certain diseases, such as polio, show atrophied muscles.
DISORDERS OF THE...
Muscular System
Duchenne muscular dystrophy (DMD) is a progressive weakening of the skeletal muscles. It is one of several diseases collectively referred to as “muscular dystrophy.” DMD is caused by a lack of the protein dystrophin, which helps the thin filaments of myofibrils bind to the sarcolemma. Without sufficient dystrophin, muscle contractions cause the sarcolemma to tear, causing an influx of Ca++, leading to cellular damage and muscle fiber degradation. Over time, as muscle damage accumulates, muscle mass is lost, and greater functional impairments develop.
DMD is an inherited disorder caused by an abnormal X chromosome. It primarily affects males, and it is usually diagnosed in early childhood. DMD usually first appears as difficulty with balance and motion, and then progresses to an inability to walk. It continues progressing upward in the body from the lower extremities to the upper body, where it affects the muscles responsible for breathing and circulation. It ultimately causes death due to respiratory failure, and those afflicted do not usually live past their 20s.
Because DMD is caused by a mutation in the gene that codes for dystrophin, it was thought that introducing healthy myoblasts into patients might be an effective treatment. Myoblasts are the embryonic cells responsible for muscle development, and ideally, they would carry healthy genes that could produce the dystrophin needed for normal muscle contraction. This approach has been largely unsuccessful in humans. A recent approach has involved attempting to boost the muscle’s production of utrophin, a protein similar to dystrophin that may be able to assume the role of dystrophin and prevent cellular damage from occurring.
Nervous System Control of Muscle Tension
- Explain concentric, isotonic, and eccentric contractions
- Describe the length-tension relationship
- Describe the three phases of a muscle twitch
- Define wave summation, tetanus, and treppe
To move an object, referred to as load, the sarcomeres in the muscle fibers of the skeletal muscle must shorten. The force generated by the contraction of the muscle (or shortening of the sarcomeres) is called muscle tension. However, muscle tension also is generated when the muscle is contracting against a load that does not move, resulting in two main types of skeletal muscle contractions: isotonic contractions and isometric contractions.
In isotonic contractions, where the tension in the muscle stays constant, a load is moved as the length of the muscle changes (shortens). There are two types of isotonic contractions: concentric and eccentric. A concentric contraction involves the muscle shortening to move a load. An example of this is the biceps brachii muscle contracting when a hand weight is brought upward with increasing muscle tension. As the biceps brachii contract, the angle of the elbow joint decreases as the forearm is brought toward the body. Here, the biceps brachii contracts as sarcomeres in its muscle fibers are shortening and cross-bridges form; the myosin heads pull the actin. An eccentric contraction occurs as the muscle tension diminishes and the muscle lengthens. In this case, the hand weight is lowered in a slow and controlled manner as the amount of cross-bridges being activated by nervous system stimulation decreases. In this case, as tension is released from the biceps brachii, the angle of the elbow joint increases. Eccentric contractions are also used for movement and balance of the body.
An isometric contraction occurs as the muscle produces tension without changing the angle of a skeletal joint. Isometric contractions involve sarcomere shortening and increasing muscle tension, but do not move a load, as the force produced cannot overcome the resistance provided by the load. For example, if one attempts to lift a hand weight that is too heavy, there will be sarcomere activation and shortening to a point, and ever-increasing muscle tension, but no change in the angle of the elbow joint. In everyday living, isometric contractions are active in maintaining posture and maintaining bone and joint stability. However, holding your head in an upright position occurs not because the muscles cannot move the head, but because the goal is to remain stationary and not produce movement. Most actions of the body are the result of a combination of isotonic and isometric contractions working together to produce a wide range of outcomes (Figure 10.13).
Figure 10.13 Types of Muscle Contractions During isotonic contractions, muscle length changes to move a load. During isometric contractions, muscle length does not change because the load exceeds the tension the muscle can generate.
All of these muscle activities are under the exquisite control of the nervous system. Neural control regulates concentric, eccentric and isometric contractions, muscle fiber recruitment, and muscle tone. A crucial aspect of nervous system control of skeletal muscles is the role of motor units.
Motor Units
As you have learned, every skeletal muscle fiber must be innervated by the axon terminal of a motor neuron in order to contract. Each muscle fiber is innervated by only one motor neuron. The actual group of muscle fibers in a muscle innervated by a single motor neuron is called a motor unit. The size of a motor unit is variable depending on the nature of the muscle.
A small motor unit is an arrangement where a single motor neuron supplies a small number of muscle fibers in a muscle. Small motor units permit very fine motor control of the muscle. The best example in humans is the small motor units of the extraocular eye muscles that move the eyeballs. There are thousands of muscle fibers in each muscle, but every six or so fibers are supplied by a single motor neuron, as the axons branch to form synaptic connections at their individual NMJs. This allows for exquisite control of eye movements so that both eyes can quickly focus on the same object. Small motor units are also involved in the many fine movements of the fingers and thumb of the hand for grasping, texting, etc.
A large motor unit is an arrangement where a single motor neuron supplies a large number of muscle fibers in a muscle. Large motor units are concerned with simple, or “gross,” movements, such as powerfully extending the knee joint. The best example is the large motor units of the thigh muscles or back muscles, where a single motor neuron will supply thousands of muscle fibers in a muscle, as its axon splits into thousands of branches.
There is a wide range of motor units within many skeletal muscles, which gives the nervous system a wide range of control over the muscle. The small motor units in the muscle will have smaller, lower-threshold motor neurons that are more excitable, firing first to their skeletal muscle fibers, which also tend to be the smallest. Activation of these smaller motor units, results in a relatively small degree of contractile strength (tension) generated in the muscle. As more strength is needed, larger motor units, with bigger, higher-threshold motor neurons are enlisted to activate larger muscle fibers. This increasing activation of motor units produces an increase in muscle contraction known as recruitment. As more motor units are recruited, the muscle contraction grows progressively stronger. In some muscles, the largest motor units may generate a contractile force of 50 times more than the smallest motor units in the muscle. This allows a feather to be picked up using the biceps brachii arm muscle with minimal force, and a heavy weight to be lifted by the same muscle by recruiting the largest motor units.
When necessary, the maximal number of motor units in a muscle can be recruited simultaneously, producing the maximum force of contraction for that muscle, but this cannot last for very long because of the energy requirements to sustain the contraction. To prevent complete muscle fatigue, motor units are generally not all simultaneously active, but instead some motor units rest while others are active, which allows for longer muscle contractions. The nervous system uses recruitment as a mechanism to efficiently utilize a skeletal muscle.
The Length-Tension Range of a Sarcomere
When a skeletal muscle fiber contracts, myosin heads attach to actin to form cross-bridges followed by the thin filaments sliding over the thick filaments as the heads pull the actin, and this results in sarcomere shortening, creating the tension of the muscle contraction. The cross-bridges can only form where thin and thick filaments already overlap, so that the length of the sarcomere has a direct influence on the force generated when the sarcomere shortens. This is called the length-tension relationship.
The ideal length of a sarcomere to produce maximal tension occurs at 80 percent to 120 percent of its resting length, with 100 percent being the state where the medial edges of the thin filaments are just at the most-medial myosin heads of the thick filaments (Figure 10.14). This length maximizes the overlap of actin-binding sites and myosin heads. If a sarcomere is stretched past this ideal length (beyond 120 percent), thick and thin filaments do not overlap sufficiently, which results in less tension produced. If a sarcomere is shortened beyond 80 percent, the zone of overlap is reduced with the thin filaments jutting beyond the last of the myosin heads and shrinks the H zone, which is normally composed of myosin tails. Eventually, there is nowhere else for the thin filaments to go and the amount of tension is diminished. If the muscle is stretched to the point where thick and thin filaments do not overlap at all, no cross-bridges can be formed, and no tension is produced in that sarcomere. This amount of stretching does not usually occur, as accessory proteins and connective tissue oppose extreme stretching.
Figure 10.14 The Ideal Length of a Sarcomere Sarcomeres produce maximal tension when thick and thin filaments overlap between about 80 percent to 120 percent.
The Frequency of Motor Neuron Stimulation
A single action potential from a motor neuron will produce a single contraction in the muscle fibers of its motor unit. This isolated contraction is called a twitch. A twitch can last for a few milliseconds or 100 milliseconds, depending on the muscle type. The tension produced by a single twitch can be measured by a myogram, an instrument that measures the amount of tension produced over time (Figure 10.15). Each twitch undergoes three phases. The first phase is the latent period, during which the action potential is being propagated along the sarcolemma and Ca++ ions are released from the SR. This is the phase during which excitation and contraction are being coupled but contraction has yet to occur. The contraction phase occurs next. The Ca++ ions in the sarcoplasm have bound to troponin, tropomyosin has shifted away from actin-binding sites, cross-bridges formed, and sarcomeres are actively shortening to the point of peak tension. The last phase is the relaxation phase, when tension decreases as contraction stops. Ca++ ions are pumped out of the sarcoplasm into the SR, and cross-bridge cycling stops, returning the muscle fibers to their resting state.
Figure 10.15 A Myogram of a Muscle Twitch A single muscle twitch has a latent period, a contraction phase when tension increases, and a relaxation phase when tension decreases. During the latent period, the action potential is being propagated along the sarcolemma. During the contraction phase, Ca++ ions in the sarcoplasm bind to troponin, tropomyosin moves from actin-binding sites, cross-bridges form, and sarcomeres shorten. During the relaxation phase, tension decreases as Ca++ ions are pumped out of the sarcoplasm and cross-bridge cycling stops.
Although a person can experience a muscle “twitch,” a single twitch does not produce any significant muscle activity in a living body. A series of action potentials to the muscle fibers is necessary to produce a muscle contraction that can produce work. Normal muscle contraction is more sustained, and it can be modified by input from the nervous system to produce varying amounts of force; this is called a graded muscle response. The frequency of action potentials (nerve impulses) from a motor neuron and the number of motor neurons transmitting action potentials both affect the tension produced in skeletal muscle.
The rate at which a motor neuron fires action potentials affects the tension produced in the skeletal muscle. If the fibers are stimulated while a previous twitch is still occurring, the second twitch will be stronger. This response is called wave summation, because the excitation-contraction coupling effects of successive motor neuron signaling is summed, or added together (Figure 10.16a). At the molecular level, summation occurs because the second stimulus triggers the release of more Ca++ ions, which become available to activate additional sarcomeres while the muscle is still contracting from the first stimulus. Summation results in greater contraction of the motor unit.
Figure 10.16 Wave Summation and Tetanus (a) The excitation-contraction coupling effects of successive motor neuron signaling is added together which is referred to as wave summation. The bottom of each wave, the end of the relaxation phase, represents the point of stimulus. (b) When the stimulus frequency is so high that the relaxation phase disappears completely, the contractions become continuous; this is called tetanus.
If the frequency of motor neuron signaling increases, summation and subsequent muscle tension in the motor unit continues to rise until it reaches a peak point. The tension at this point is about three to four times greater than the tension of a single twitch, a state referred to as incomplete tetanus. During incomplete tetanus, the muscle goes through quick cycles of contraction with a short relaxation phase for each. If the stimulus frequency is so high that the relaxation phase disappears completely, contractions become continuous in a process called complete tetanus (Figure 10.16b).
During tetanus, the concentration of Ca++ ions in the sarcoplasm allows virtually all of the sarcomeres to form cross-bridges and shorten, so that a contraction can continue uninterrupted (until the muscle fatigues and can no longer produce tension).
Treppe
When a skeletal muscle has been dormant for an extended period and then activated to contract, with all other things being equal, the initial contractions generate about one-half the force of later contractions. The muscle tension increases in a graded manner that to some looks like a set of stairs. This tension increase is called treppe, a condition where muscle contractions become more efficient. It’s also known as the “staircase effect” (Figure 10.17).
Figure 10.17 Treppe When muscle tension increases in a graded manner that looks like a set of stairs, it is called treppe. The bottom of each wave represents the point of stimulus.
It is believed that treppe results from a higher concentration of Ca++ in the sarcoplasm resulting from the steady stream of signals from the motor neuron. It can only be maintained with adequate ATP.
Muscle Tone
Skeletal muscles are rarely completely relaxed, or flaccid. Even if a muscle is not producing movement, it is contracted a small amount to maintain its contractile proteins and produce muscle tone. The tension produced by muscle tone allows muscles to continually stabilize joints and maintain posture.
Muscle tone is accomplished by a complex interaction between the nervous system and skeletal muscles that results in the activation of a few motor units at a time, most likely in a cyclical manner. In this manner, muscles never fatigue completely, as some motor units can recover while others are active.
The absence of the low-level contractions that lead to muscle tone is referred to as hypotonia, and can result from damage to parts of the central nervous system (CNS), such as the cerebellum, or from loss of innervations to a skeletal muscle, as in poliomyelitis. Hypotonic muscles have a flaccid appearance and display functional impairments, such as weak reflexes. Conversely, excessive muscle tone is referred to as hypertonia, accompanied by hyperreflexia (excessive reflex responses), often the result of damage to upper motor neurons in the CNS. Hypertonia can present with muscle rigidity (as seen in Parkinson’s disease) or spasticity, a phasic change in muscle tone, where a limb will “snap” back from passive stretching (as seen in some strokes).
Types of Muscle Fibers
- Describe the types of skeletal muscle fibers
- Explain fast and slow muscle fibers
Two criteria to consider when classifying the types of muscle fibers are how fast some fibers contract relative to others, and how fibers produce ATP. Using these criteria, there are three main types of skeletal muscle fibers. Slow oxidative (SO) fibers contract relatively slowly and use aerobic respiration (oxygen and glucose) to produce ATP. Fast oxidative (FO) fibers have fast contractions and primarily use aerobic respiration, but because they may switch to anaerobic respiration (glycolysis), can fatigue more quickly than SO fibers. Lastly, fast glycolytic (FG) fibers have fast contractions and primarily use anaerobic glycolysis. The FG fibers fatigue more quickly than the others. Most skeletal muscles in a human contain(s) all three types, although in varying proportions.
The speed of contraction is dependent on how quickly myosin’s ATPase hydrolyzes ATP to produce cross-bridge action. Fast fibers hydrolyze ATP approximately twice as quickly as slow fibers, resulting in much quicker cross-bridge cycling (which pulls the thin filaments toward the center of the sarcomeres at a faster rate). The primary metabolic pathway used by a muscle fiber determines whether the fiber is classified as oxidative or glycolytic. If a fiber primarily produces ATP through aerobic pathways it is oxidative. More ATP can be produced during each metabolic cycle, making the fiber more resistant to fatigue. Glycolytic fibers primarily create ATP through anaerobic glycolysis, which produces less ATP per cycle. As a result, glycolytic fibers fatigue at a quicker rate.
The oxidative fibers contain many more mitochondria than the glycolytic fibers, because aerobic metabolism, which uses oxygen (O2) in the metabolic pathway, occurs in the mitochondria. The SO fibers possess a large number of mitochondria and are capable of contracting for longer periods because of the large amount of ATP they can produce, but they have a relatively small diameter and do not produce a large amount of tension. SO fibers are extensively supplied with blood capillaries to supply O2 from the red blood cells in the bloodstream. The SO fibers also possess myoglobin, an O2-carrying molecule similar to O2-carrying hemoglobin in the red blood cells. The myoglobin stores some of the needed O2 within the fibers themselves (and gives SO fibers their red color). All of these features allow SO fibers to produce large quantities of ATP, which can sustain muscle activity without fatiguing for long periods of time.
The fact that SO fibers can function for long periods without fatiguing makes them useful in maintaining posture, producing isometric contractions, stabilizing bones and joints, and making small movements that happen often but do not require large amounts of energy. They do not produce high tension, and thus they are not used for powerful, fast movements that require high amounts of energy and rapid cross-bridge cycling.
FO fibers are sometimes called intermediate fibers because they possess characteristics that are intermediate between fast fibers and slow fibers. They produce ATP relatively quickly, more quickly than SO fibers, and thus can produce relatively high amounts of tension. They are oxidative because they produce ATP aerobically, possess high amounts of mitochondria, and do not fatigue quickly. However, FO fibers do not possess significant myoglobin, giving them a lighter color than the red SO fibers. FO fibers are used primarily for movements, such as walking, that require more energy than postural control but less energy than an explosive movement, such as sprinting. FO fibers are useful for this type of movement because they produce more tension than SO fibers but they are more fatigue-resistant than FG fibers.
FG fibers primarily use anaerobic glycolysis as their ATP source. They have a large diameter and possess high amounts of glycogen, which is used in glycolysis to generate ATP quickly to produce high levels of tension. Because they do not primarily use aerobic metabolism, they do not possess substantial numbers of mitochondria or significant amounts of myoglobin and therefore have a white color. FG fibers are used to produce rapid, forceful contractions to make quick, powerful movements. These fibers fatigue quickly, permitting them to only be used for short periods. Most muscles possess a mixture of each fiber type. The predominant fiber type in a muscle is determined by the primary function of the muscle.
Exercise and Muscle Performance
- Describe hypertrophy and atrophy
- Explain how resistance exercise builds muscle
- Explain how performance-enhancing substances affect muscle
Physical training alters the appearance of skeletal muscles and can produce changes in muscle performance. Conversely, a lack of use can result in decreased performance and muscle appearance. Although muscle cells can change in size, new cells are not formed when muscles grow. Instead, structural proteins are added to muscle fibers in a process called hypertrophy, so cell diameter increases. The reverse, when structural proteins are lost and muscle mass decreases, is called atrophy. Age-related muscle atrophy is called sarcopenia. Cellular components of muscles can also undergo changes in response to changes in muscle use.
Endurance Exercise
Slow fibers are predominantly used in endurance exercises that require little force but involve numerous repetitions. The aerobic metabolism used by slow-twitch fibers allows them to maintain contractions over long periods. Endurance training modifies these slow fibers to make them even more efficient by producing more mitochondria to enable more aerobic metabolism and more ATP production. Endurance exercise can also increase the amount of myoglobin in a cell, as increased aerobic respiration increases the need for oxygen. Myoglobin is found in the sarcoplasm and acts as an oxygen storage supply for the mitochondria.
The training can trigger the formation of more extensive capillary networks around the fiber, a process called angiogenesis, to supply oxygen and remove metabolic waste. To allow these capillary networks to supply the deep portions of the muscle, muscle mass does not greatly increase in order to maintain a smaller area for the diffusion of nutrients and gases. All of these cellular changes result in the ability to sustain low levels of muscle contractions for greater periods without fatiguing.
The proportion of SO muscle fibers in muscle determines the suitability of that muscle for endurance, and may benefit those participating in endurance activities. Postural muscles have a large number of SO fibers and relatively few FO and FG fibers, to keep the back straight (Figure 10.18). Endurance athletes, like marathon-runners also would benefit from a larger proportion of SO fibers, but it is unclear if the most-successful marathoners are those with naturally high numbers of SO fibers, or whether the most successful marathon runners develop high numbers of SO fibers with repetitive training. Endurance training can result in overuse injuries such as stress fractures and joint and tendon inflammation.
Figure 10.18 Marathoners Long-distance runners have a large number of SO fibers and relatively few FO and FG fibers. (credit: “Tseo2”/Wikimedia Commons)
Resistance Exercise
Resistance exercises, as opposed to endurance exercise, require large amounts of FG fibers to produce short, powerful movements that are not repeated over long periods. The high rates of ATP hydrolysis and cross-bridge formation in FG fibers result in powerful muscle contractions. Muscles used for power have a higher ratio of FG to SO/FO fibers, and trained athletes possess even higher levels of FG fibers in their muscles. Resistance exercise affects muscles by increasing the formation of myofibrils, thereby increasing the thickness of muscle fibers. This added structure causes hypertrophy, or the enlargement of muscles, exemplified by the large skeletal muscles seen in body builders and other athletes (Figure 10.19). Because this muscular enlargement is achieved by the addition of structural proteins, athletes trying to build muscle mass often ingest large amounts of protein.
Figure 10.19 Hypertrophy Body builders have a large number of FG fibers and relatively few FO and SO fibers. (credit: Lin Mei/flickr)
Except for the hypertrophy that follows an increase in the number of sarcomeres and myofibrils in a skeletal muscle, the cellular changes observed during endurance training do not usually occur with resistance training. There is usually no significant increase in mitochondria or capillary density. However, resistance training does increase the development of connective tissue, which adds to the overall mass of the muscle and helps to contain muscles as they produce increasingly powerful contractions. Tendons also become stronger to prevent tendon damage, as the force produced by muscles is transferred to tendons that attach the muscle to bone.
For effective strength training, the intensity of the exercise must continually be increased. For instance, continued weight lifting without increasing the weight of the load does not increase muscle size. To produce ever-greater results, the weights lifted must become increasingly heavier, making it more difficult for muscles to move the load. The muscle then adapts to this heavier load, and an even heavier load must be used if even greater muscle mass is desired.
If done improperly, resistance training can lead to overuse injuries of the muscle, tendon, or bone. These injuries can occur if the load is too heavy or if the muscles are not given sufficient time between workouts to recover or if joints are not aligned properly during the exercises. Cellular damage to muscle fibers that occurs after intense exercise includes damage to the sarcolemma and myofibrils. This muscle damage contributes to the feeling of soreness after strenuous exercise, but muscles gain mass as this damage is repaired, and additional structural proteins are added to replace the damaged ones. Overworking skeletal muscles can also lead to tendon damage and even skeletal damage if the load is too great for the muscles to bear.
Performance-Enhancing Substances
Some athletes attempt to boost their performance by using various agents that may enhance muscle performance. Anabolic steroids are one of the more widely known agents used to boost muscle mass and increase power output. Anabolic steroids are a form of testosterone, a male sex hormone that stimulates muscle formation, leading to increased muscle mass.
Endurance athletes may also try to boost the availability of oxygen to muscles to increase aerobic respiration by using substances such as erythropoietin (EPO), a hormone normally produced in the kidneys, which triggers the production of red blood cells. The extra oxygen carried by these blood cells can then be used by muscles for aerobic respiration. Human growth hormone (hGH) is another supplement, and although it can facilitate building muscle mass, its main role is to promote the healing of muscle and other tissues after strenuous exercise. Increased hGH may allow for faster recovery after muscle damage, reducing the rest required after exercise, and allowing for more sustained high-level performance.
Although performance-enhancing substances often do improve performance, most are banned by governing bodies in sports and are illegal for nonmedical purposes. Their use to enhance performance raises ethical issues of cheating because they give users an unfair advantage over nonusers. A greater concern, however, is that their use carries serious health risks. The side effects of these substances are often significant, nonreversible, and in some cases fatal. The physiological strain caused by these substances is often greater than what the body can handle, leading to effects that are unpredictable and dangerous. Anabolic steroid use has been linked to infertility, aggressive behavior, cardiovascular disease, and brain cancer.
Similarly, some athletes have used creatine to increase power output. Creatine phosphate provides quick bursts of ATP to muscles in the initial stages of contraction. Increasing the amount of creatine available to cells is thought to produce more ATP and therefore increase explosive power output, although its effectiveness as a supplement has been questioned.
EVERYDAY CONNECTION
Aging and Muscle Tissue
Although atrophy due to disuse can often be reversed with exercise, muscle atrophy with age, referred to as sarcopenia, is irreversible. This is a primary reason why even highly trained athletes succumb to declining performance with age. This decline is noticeable in athletes whose sports require strength and powerful movements, such as sprinting, whereas the effects of age are less noticeable in endurance athletes such as marathon runners or long-distance cyclists. As muscles age, muscle fibers die, and they are replaced by connective tissue and adipose tissue (Figure 10.20). Because those tissues cannot contract and generate force as muscle can, muscles lose the ability to produce powerful contractions. The decline in muscle mass causes a loss of strength, including the strength required for posture and mobility. This may be caused by a reduction in FG fibers that hydrolyze ATP quickly to produce short, powerful contractions. Muscles in older people sometimes possess greater numbers of SO fibers, which are responsible for longer contractions and do not produce powerful movements. There may also be a reduction in the size of motor units, resulting in fewer fibers being stimulated and less muscle tension being produced.
Figure 10.20 Atrophy Muscle mass is reduced as muscles atrophy with disuse.
Sarcopenia can be delayed to some extent by exercise, as training adds structural proteins and causes cellular changes that can offset the effects of atrophy. Increased exercise can produce greater numbers of cellular mitochondria, increase capillary density, and increase the mass and strength of connective tissue. The effects of age-related atrophy are especially pronounced in people who are sedentary, as the loss of muscle cells is displayed as functional impairments such as trouble with locomotion, balance, and posture. This can lead to a decrease in quality of life and medical problems, such as joint problems because the muscles that stabilize bones and joints are weakened. Problems with locomotion and balance can also cause various injuries due to falls.
Cardiac Muscle Tissue
- Describe intercalated discs and gap junctions
- Describe a desmosome
Cardiac muscle tissue is only found in the heart. Highly coordinated contractions of cardiac muscle pump blood into the vessels of the circulatory system. Similar to skeletal muscle, cardiac muscle is striated and organized into sarcomeres, possessing the same banding organization as skeletal muscle (Figure 10.21). However, cardiac muscle fibers are shorter than skeletal muscle fibers and usually contain only one nucleus, which is located in the central region of the cell. Cardiac muscle fibers also possess many mitochondria and myoglobin, as ATP is produced primarily through aerobic metabolism. Cardiac muscle fibers cells also are extensively branched and are connected to one another at their ends by intercalated discs. An intercalated disc allows the cardiac muscle cells to contract in a wave-like pattern so that the heart can work as a pump.
Figure 10.21 Cardiac Muscle Tissue Cardiac muscle tissue is only found in the heart. LM × 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
INTERACTIVE LINK
View the University of Michigan WebScope to explore the tissue sample in greater detail.
Intercalated discs are part of the sarcolemma and contain two structures important in cardiac muscle contraction: gap junctions and desmosomes. A gap junction forms channels between adjacent cardiac muscle fibers that allow the depolarizing current produced by cations to flow from one cardiac muscle cell to the next. This joining is called electric coupling, and in cardiac muscle it allows the quick transmission of action potentials and the coordinated contraction of the entire heart. This network of electrically connected cardiac muscle cells creates a functional unit of contraction called a syncytium. The remainder of the intercalated disc is composed of desmosomes. A desmosome is a cell structure that anchors the ends of cardiac muscle fibers together so the cells do not pull apart during the stress of individual fibers contracting (Figure 10.22).
Figure 10.22 Cardiac Muscle Intercalated discs are part of the cardiac muscle sarcolemma and they contain gap junctions and desmosomes.
Contractions of the heart (heartbeats) are controlled by specialized cardiac muscle cells called pacemaker cells that directly control heart rate. Although cardiac muscle cannot be consciously controlled, the pacemaker cells respond to signals from the autonomic nervous system (ANS) to speed up or slow down the heart rate. The pacemaker cells can also respond to various hormones that modulate heart rate to control blood pressure.
The wave of contraction that allows the heart to work as a unit, called a functional syncytium, begins with the pacemaker cells. This group of cells is self-excitable and able to depolarize to threshold and fire action potentials on their own, a feature called autorhythmicity; they do this at set intervals which determine heart rate. Because they are connected with gap junctions to surrounding muscle fibers and the specialized fibers of the heart’s conduction system, the pacemaker cells are able to transfer the depolarization to the other cardiac muscle fibers in a manner that allows the heart to contract in a coordinated manner.
Another feature of cardiac muscle is its relatively long action potentials in its fibers, having a sustained depolarization “plateau.” The plateau is produced by Ca++ entry though voltage-gated calcium channels in the sarcolemma of cardiac muscle fibers. This sustained depolarization (and Ca++ entry) provides for a longer contraction than is produced by an action potential in skeletal muscle. Unlike skeletal muscle, a large percentage of the Ca++ that initiates contraction in cardiac muscles comes from outside the cell rather than from the SR.
Smooth Muscle
- Describe a dense body
- Explain how smooth muscle works with internal organs and passageways through the body
- Explain how smooth muscles differ from skeletal and cardiac muscles
- Explain the difference between single-unit and multi-unit smooth muscle
Smooth muscle (so-named because the cells do not have striations) is present in the walls of hollow organs like the urinary bladder, uterus, stomach, intestines, and in the walls of passageways, such as the arteries and veins of the circulatory system, and the tracts of the respiratory, urinary, and reproductive systems (Figure 10.23ab). Smooth muscle is also present in the eyes, where it functions to change the size of the iris and alter the shape of the lens; and in the skin where it causes hair to stand erect in response to cold temperature or fear.
Figure 10.23 Smooth Muscle Tissue Smooth muscle tissue is found around organs in the digestive, respiratory, reproductive tracts and the iris of the eye. LM × 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
INTERACTIVE LINK
View the University of Michigan WebScope to explore the tissue sample in greater detail.
Smooth muscle fibers are spindle-shaped (wide in the middle and tapered at both ends, somewhat like a football) and have a single nucleus; they range from about 30 to 200 μm (thousands of times shorter than skeletal muscle fibers), and they produce their own connective tissue, endomysium. Although they do not have striations and sarcomeres, smooth muscle fibers do have actin and myosin contractile proteins, and thick and thin filaments. These thin filaments are anchored by dense bodies. A dense body is analogous to the Z-discs of skeletal and cardiac muscle fibers and is fastened to the sarcolemma. Calcium ions are supplied by the SR in the fibers and by sequestration from the extracellular fluid through membrane indentations called calveoli.
Because smooth muscle cells do not contain troponin, cross-bridge formation is not regulated by the troponin-tropomyosin complex but instead by the regulatory protein calmodulin. In a smooth muscle fiber, external Ca++ ions passing through opened calcium channels in the sarcolemma, and additional Ca++ released from SR, bind to calmodulin. The Ca++-calmodulin complex then activates an enzyme called myosin (light chain) kinase, which, in turn, activates the myosin heads by phosphorylating them (converting ATP to ADP and Pi, with the Pi attaching to the head). The heads can then attach to actin-binding sites and pull on the thin filaments. The thin filaments also are anchored to the dense bodies; the structures invested in the inner membrane of the sarcolemma (at adherens junctions) that also have cord-like intermediate filaments attached to them. When the thin filaments slide past the thick filaments, they pull on the dense bodies, structures tethered to the sarcolemma, which then pull on the intermediate filaments networks throughout the sarcoplasm. This arrangement causes the entire muscle fiber to contract in a manner whereby the ends are pulled toward the center, causing the midsection to bulge in a corkscrew motion (Figure 10.24).
Figure 10.24 Muscle Contraction The dense bodies and intermediate filaments are networked through the sarcoplasm, which cause the muscle fiber to contract.
Although smooth muscle contraction relies on the presence of Ca++ ions, smooth muscle fibers have a much smaller diameter than skeletal muscle cells. T-tubules are not required to reach the interior of the cell and therefore not necessary to transmit an action potential deep into the fiber. Smooth muscle fibers have a limited calcium-storing SR but have calcium channels in the sarcolemma (similar to cardiac muscle fibers) that open during the action potential along the sarcolemma. The influx of extracellular Ca++ ions, which diffuse into the sarcoplasm to reach the calmodulin, accounts for most of the Ca++ that triggers contraction of a smooth muscle cell.
Muscle contraction continues until ATP-dependent calcium pumps actively transport Ca++ ions back into the SR and out of the cell. However, a low concentration of calcium remains in the sarcoplasm to maintain muscle tone. This remaining calcium keeps the muscle slightly contracted, which is important in certain tracts and around blood vessels.
Because most smooth muscles must function for long periods without rest, their power output is relatively low, but contractions can continue without using large amounts of energy. Some smooth muscle can also maintain contractions even as Ca++ is removed and myosin kinase is inactivated/dephosphorylated. This can happen as a subset of cross-bridges between myosin heads and actin, called latch-bridges, keep the thick and thin filaments linked together for a prolonged period, and without the need for ATP. This allows for the maintaining of muscle “tone” in smooth muscle that lines arterioles and other visceral organs with very little energy expenditure.
Smooth muscle is not under voluntary control; thus, it is called involuntary muscle. The triggers for smooth muscle contraction include hormones, neural stimulation by the ANS, and local factors. In certain locations, such as the walls of visceral organs, stretching the muscle can trigger its contraction (the stress-relaxation response).
Axons of neurons in the ANS do not form the highly organized NMJs with smooth muscle, as seen between motor neurons and skeletal muscle fibers. Instead, there is a series of neurotransmitter-filled bulges called varicosities as an axon courses through smooth muscle, loosely forming motor units (Figure 10.25). A varicosity releases neurotransmitters into the synaptic cleft. Also, visceral muscle in the walls of the hollow organs (except the heart) contains pacesetter cells. A pacesetter cell can spontaneously trigger action potentials and contractions in the muscle.
Figure 10.25 Motor Units A series of axon-like swelling, called varicosities or “boutons,” from autonomic neurons form motor units through the smooth muscle.
Smooth muscle is organized in two ways: as single-unit smooth muscle, which is much more common; and as multiunit smooth muscle. The two types have different locations in the body and have different characteristics. Single-unit muscle has its muscle fibers joined by gap junctions so that the muscle contracts as a single unit. This type of smooth muscle is found in the walls of all visceral organs except the heart (which has cardiac muscle in its walls), and so it is commonly called visceral muscle. Because the muscle fibers are not constrained by the organization and stretchability limits of sarcomeres, visceral smooth muscle has a stress-relaxation response. This means that as the muscle of a hollow organ is stretched when it fills, the mechanical stress of the stretching will trigger contraction, but this is immediately followed by relaxation so that the organ does not empty its contents prematurely. This is important for hollow organs, such as the stomach or urinary bladder, which continuously expand as they fill. The smooth muscle around these organs also can maintain a muscle tone when the organ empties and shrinks, a feature that prevents “flabbiness” in the empty organ. In general, visceral smooth muscle produces slow, steady contractions that allow substances, such as food in the digestive tract, to move through the body.
Multiunit smooth muscle cells rarely possess gap junctions, and thus are not electrically coupled. As a result, contraction does not spread from one cell to the next, but is instead confined to the cell that was originally stimulated. Stimuli for multiunit smooth muscles come from autonomic nerves or hormones but not from stretching. This type of tissue is found around large blood vessels, in the respiratory airways, and in the eyes.
Hyperplasia in Smooth Muscle
Similar to skeletal and cardiac muscle cells, smooth muscle can undergo hypertrophy to increase in size. Unlike other muscle, smooth muscle can also divide to produce more cells, a process called hyperplasia. This can most evidently be observed in the uterus at puberty, which responds to increased estrogen levels by producing more uterine smooth muscle fibers, and greatly increases the size of the myometrium.
Development and Regeneration of Muscle Tissue
- Describe the function of satellite cells
- Define fibrosis
- Explain which muscle has the greatest regeneration ability
Most muscle tissue of the body arises from embryonic mesoderm. Paraxial mesodermal cells adjacent to the neural tube form blocks of cells called somites. Skeletal muscles, excluding those of the head and limbs, develop from mesodermal somites, whereas skeletal muscle in the head and limbs develop from general mesoderm. Somites give rise to myoblasts. A myoblast is a muscle-forming stem cell that migrates to different regions in the body and then fuse(s) to form a syncytium, or myotube. As a myotube is formed from many different myoblast cells, it contains many nuclei, but has a continuous cytoplasm. This is why skeletal muscle cells are multinucleate, as the nucleus of each contributing myoblast remains intact in the mature skeletal muscle cell. However, cardiac and smooth muscle cells are not multinucleate because the myoblasts that form their cells do not fuse.
Gap junctions develop in the cardiac and single-unit smooth muscle in the early stages of development. In skeletal muscles, ACh receptors are initially present along most of the surface of the myoblasts, but spinal nerve innervation causes the release of growth factors that stimulate the formation of motor end-plates and NMJs. As neurons become active, electrical signals that are sent through the muscle influence the distribution of slow and fast fibers in the muscle.
Although the number of muscle cells is set during development, satellite cells help to repair skeletal muscle cells. A satellite cellis similar to a myoblast because it is a type of stem cell; however, satellite cells are incorporated into muscle cells and facilitate the protein synthesis required for repair and growth. These cells are located outside the sarcolemma and are stimulated to grow and fuse with muscle cells by growth factors that are released by muscle fibers under certain forms of stress. Satellite cells can regenerate muscle fibers to a very limited extent, but they primarily help to repair damage in living cells. If a cell is damaged to a greater extent than can be repaired by satellite cells, the muscle fibers are replaced by scar tissue in a process called fibrosis. Because scar tissue cannot contract, muscle that has sustained significant damage loses strength and cannot produce the same amount of power or endurance as it could before being damaged.
Smooth muscle tissue can regenerate from a type of stem cell called a pericyte, which is found in some small blood vessels. Pericytes allow smooth muscle cells to regenerate and repair much more readily than skeletal and cardiac muscle tissue. Similar to skeletal muscle tissue, cardiac muscle does not regenerate to a great extent. Dead cardiac muscle tissue is replaced by scar tissue, which cannot contract. As scar tissue accumulates, the heart loses its ability to pump because of the loss of contractile power. However, some minor regeneration may occur due to stem cells found in the blood that occasionally enter cardiac tissue.
CAREER CONNECTION
Physical Therapist
As muscle cells die, they are not regenerated but instead are replaced by connective tissue and adipose tissue, which do not possess the contractile abilities of muscle tissue. Muscles atrophy when they are not used, and over time if atrophy is prolonged, muscle cells die. It is therefore important that those who are susceptible to muscle atrophy exercise to maintain muscle function and prevent the complete loss of muscle tissue. In extreme cases, when movement is not possible, electrical stimulation can be introduced to a muscle from an external source. This acts as a substitute for endogenous neural stimulation, stimulating the muscle to contract and preventing the loss of proteins that occurs with a lack of use.
Physiotherapists work with patients to maintain muscles. They are trained to target muscles susceptible to atrophy, and to prescribe and monitor exercises designed to stimulate those muscles. There are various causes of atrophy, including mechanical injury, disease, and age. After breaking a limb or undergoing surgery, muscle use is impaired and can lead to disuse atrophy. If the muscles are not exercised, this atrophy can lead to long-term muscle weakness. A stroke can also cause muscle impairment by interrupting neural stimulation to certain muscles. Without neural inputs, these muscles do not contract and thus begin to lose structural proteins. Exercising these muscles can help to restore muscle function and minimize functional impairments. Age-related muscle loss is also a target of physical therapy, as exercise can reduce the effects of age-related atrophy and improve muscle function.
The goal of a physiotherapist is to improve physical functioning and reduce functional impairments; this is achieved by understanding the cause of muscle impairment and assessing the capabilities of a patient, after which a program to enhance these capabilities is designed. Some factors that are assessed include strength, balance, and endurance, which are continually monitored as exercises are introduced to track improvements in muscle function. Physiotherapists can also instruct patients on the proper use of equipment, such as crutches, and assess whether someone has sufficient strength to use the equipment and when they can function without it.
Key Terms
- acetylcholine (ACh)
- neurotransmitter that binds at a motor end-plate to trigger depolarization
- actin
- protein that makes up most of the thin myofilaments in a sarcomere muscle fiber
- action potential
- change in voltage of a cell membrane in response to a stimulus that results in transmission of an electrical signal; unique to neurons and muscle fibers
- aerobic respiration
- production of ATP in the presence of oxygen
- angiogenesis
- formation of blood capillary networks
- aponeurosis
- broad, tendon-like sheet of connective tissue that attaches a skeletal muscle to another skeletal muscle or to a bone
- ATPase
- enzyme that hydrolyzes ATP to ADP
- atrophy
- loss of structural proteins from muscle fibers
- autorhythmicity
- heart’s ability to control its own contractions
- calmodulin
- regulatory protein that facilitates contraction in smooth muscles
- cardiac muscle
- striated muscle found in the heart; joined to one another at intercalated discs and under the regulation of pacemaker cells, which contract as one unit to pump blood through the circulatory system. Cardiac muscle is under involuntary control.
- concentric contraction
- muscle contraction that shortens the muscle to move a load
- contractility
- ability to shorten (contract) forcibly
- contraction phase
- twitch contraction phase when tension increases
- creatine phosphate
- phosphagen used to store energy from ATP and transfer it to muscle
- dense body
- sarcoplasmic structure that attaches to the sarcolemma and shortens the muscle as thin filaments slide past thick filaments
- depolarize
- to reduce the voltage difference between the inside and outside of a cell’s plasma membrane (the sarcolemma for a muscle fiber), making the inside less negative than at rest
- desmosome
- cell structure that anchors the ends of cardiac muscle fibers to allow contraction to occur
- eccentric contraction
- muscle contraction that lengthens the muscle as the tension is diminished
- elasticity
- ability to stretch and rebound
- endomysium
- loose, and well-hydrated connective tissue covering each muscle fiber in a skeletal muscle
- epimysium
- outer layer of connective tissue around a skeletal muscle
- excitability
- ability to undergo neural stimulation
- excitation-contraction coupling
- sequence of events from motor neuron signaling to a skeletal muscle fiber to contraction of the fiber’s sarcomeres
- extensibility
- ability to lengthen (extend)
- fascicle
- bundle of muscle fibers within a skeletal muscle
- fast glycolytic (FG)
- muscle fiber that primarily uses anaerobic glycolysis
- fast oxidative (FO)
- intermediate muscle fiber that is between slow oxidative and fast glycolytic fibers
- fibrosis
- replacement of muscle fibers by scar tissue
- glycolysis
- anaerobic breakdown of glucose to ATP
- graded muscle response
- modification of contraction strength
- hyperplasia
- process in which one cell splits to produce new cells
- hypertonia
- abnormally high muscle tone
- hypertrophy
- addition of structural proteins to muscle fibers
- hypotonia
- abnormally low muscle tone caused by the absence of low-level contractions
- intercalated disc
- part of the sarcolemma that connects cardiac tissue, and contains gap junctions and desmosomes
- isometric contraction
- muscle contraction that occurs with no change in muscle length
- isotonic contraction
- muscle contraction that involves changes in muscle length
- lactic acid
- product of anaerobic glycolysis
- latch-bridges
- subset of a cross-bridge in which actin and myosin remain locked together
- latent period
- the time when a twitch does not produce contraction
- motor end-plate
- sarcolemma of muscle fiber at the neuromuscular junction, with receptors for the neurotransmitter acetylcholine
- motor unit
- motor neuron and the group of muscle fibers it innervates
- muscle tension
- force generated by the contraction of the muscle; tension generated during isotonic contractions and isometric contractions
- muscle tone
- low levels of muscle contraction that occur when a muscle is not producing movement
- myoblast
- muscle-forming stem cell
- myofibril
- long, cylindrical organelle that runs parallel within the muscle fiber and contains the sarcomeres
- myogram
- instrument used to measure twitch tension
- myosin
- protein that makes up most of the thick cylindrical myofilament within a sarcomere muscle fiber
- myotube
- fusion of many myoblast cells
- neuromuscular junction (NMJ)
- synapse between the axon terminal of a motor neuron and the section of the membrane of a muscle fiber with receptors for the acetylcholine released by the terminal
- neurotransmitter
- signaling chemical released by nerve terminals that bind to and activate receptors on target cells
- oxygen debt
- amount of oxygen needed to compensate for ATP produced without oxygen during muscle contraction
- pacesetter cell
- cell that triggers action potentials in smooth muscle
- pericyte
- stem cell that regenerates smooth muscle cells
- perimysium
- connective tissue that bundles skeletal muscle fibers into fascicles within a skeletal muscle
- power stroke
- action of myosin pulling actin inward (toward the M line)
- pyruvic acid
- product of glycolysis that can be used in aerobic respiration or converted to lactic acid
- recruitment
- increase in the number of motor units involved in contraction
- relaxation phase
- period after twitch contraction when tension decreases
- sarcolemma
- plasma membrane of a skeletal muscle fiber
- sarcomere
- longitudinally, repeating functional unit of skeletal muscle, with all of the contractile and associated proteins involved in contraction
- sarcopenia
- age-related muscle atrophy
- sarcoplasm
- cytoplasm of a muscle cell
- sarcoplasmic reticulum (SR)
- specialized smooth endoplasmic reticulum, which stores, releases, and retrieves Ca++
- satellite cell
- stem cell that helps to repair muscle cells
- skeletal muscle
- striated, multinucleated muscle that requires signaling from the nervous system to trigger contraction; most skeletal muscles are referred to as voluntary muscles that move bones and produce movement
- slow oxidative (SO)
- muscle fiber that primarily uses aerobic respiration
- smooth muscle
- nonstriated, mononucleated muscle in the skin that is associated with hair follicles; assists in moving materials in the walls of internal organs, blood vessels, and internal passageways
- somites
- blocks of paraxial mesoderm cells
- stress-relaxation response
- relaxation of smooth muscle tissue after being stretched
- synaptic cleft
- space between a nerve (axon) terminal and a motor end-plate
- T-tubule
- projection of the sarcolemma into the interior of the cell
- tetanus
- a continuous fused contraction
- thick filament
- the thick myosin strands and their multiple heads projecting from the center of the sarcomere toward, but not all to way to, the Z-discs
- thin filament
- thin strands of actin and its troponin-tropomyosin complex projecting from the Z-discs toward the center of the sarcomere
- treppe
- stepwise increase in contraction tension
- triad
- the grouping of one T-tubule and two terminal cisternae
- tropomyosin
- regulatory protein that covers myosin-binding sites to prevent actin from binding to myosin
- troponin
- regulatory protein that binds to actin, tropomyosin, and calcium
- twitch
- single contraction produced by one action potential
- varicosity
- enlargement of neurons that release neurotransmitters into synaptic clefts
- visceral muscle
- smooth muscle found in the walls of visceral organs
- voltage-gated sodium channels
- membrane proteins that open sodium channels in response to a sufficient voltage change, and initiate and transmit the action potential as Na+ enters through the channel
- wave summation
- addition of successive neural stimuli to produce greater contraction
Chapter Review
10.1 Overview of Muscle Tissues
Muscle is the tissue in animals that allows for active movement of the body or materials within the body. There are three types of muscle tissue: skeletal muscle, cardiac muscle, and smooth muscle. Most of the body’s skeletal muscle produces movement by acting on the skeleton. Cardiac muscle is found in the wall of the heart and pumps blood through the circulatory system.
Smooth muscle is found in the skin, where it is associated with hair follicles; it also is found in the walls of internal organs, blood vessels, and internal passageways, where it assists in moving materials.
10.2 Skeletal Muscle
Skeletal muscles contain connective tissue, blood vessels, and nerves. There are three layers of connective tissue: epimysium, perimysium, and endomysium. Skeletal muscle fibers are organized into groups called fascicles. Blood vessels and nerves enter the connective tissue and branch in the cell. Muscles attach to bones directly or through tendons or aponeuroses. Skeletal muscles maintain posture, stabilize bones and joints, control internal movement, and generate heat.
Skeletal muscle fibers are long, multinucleated cells. The membrane of the cell is the sarcolemma; the cytoplasm of the cell is the sarcoplasm. The sarcoplasmic reticulum (SR) is a form of endoplasmic reticulum. Muscle fibers are composed of myofibrils. The striations are created by the organization of actin and myosin resulting in the banding pattern of myofibrils.
10.3 Muscle Fiber Contraction and Relaxation
A sarcomere is the smallest contractile portion of a muscle. Myofibrils are composed of thick and thin filaments. Thick filaments are composed of the protein myosin; thin filaments are composed of the protein actin. Troponin and tropomyosin are regulatory proteins.
Muscle contraction is described by the sliding filament model of contraction. ACh is the neurotransmitter that binds at the neuromuscular junction (NMJ) to trigger depolarization, and an action potential travels along the sarcolemma to trigger calcium release from SR. The actin sites are exposed after Ca++ enters the sarcoplasm from its SR storage to activate the troponin-tropomyosin complex so that the tropomyosin shifts away from the sites. The cross-bridging of myposin heads docking into actin-binding sites is followed by the “power stroke”—the sliding of the thin filaments by thick filaments. The power strokes are powered by ATP. Ultimately, the sarcomeres, myofibrils, and muscle fibers shorten to produce movement.
10.4 Nervous System Control of Muscle Tension
The number of cross-bridges formed between actin and myosin determines the amount of tension produced by a muscle. The length of a sarcomere is optimal when the zone of overlap between thin and thick filaments is greatest. Muscles that are stretched or compressed too greatly do not produce maximal amounts of power. A motor unit is formed by a motor neuron and all of the muscle fibers that are innervated by that same motor neuron. A single contraction is called a twitch. A muscle twitch has a latent period, a contraction phase, and a relaxation phase. A graded muscle response allows variation in muscle tension. Summation occurs as successive stimuli are added together to produce a stronger muscle contraction. Tetanus is the fusion of contractions to produce a continuous contraction. Increasing the number of motor neurons involved increases the amount of motor units activated in a muscle, which is called recruitment. Muscle tone is the constant low-level contractions that allow for posture and stability.
10.5 Types of Muscle Fibers
ATP provides the energy for muscle contraction. The three mechanisms for ATP regeneration are creatine phosphate, anaerobic glycolysis, and aerobic metabolism. Creatine phosphate provides about the first 15 seconds of ATP at the beginning of muscle contraction. Anaerobic glycolysis produces small amounts of ATP in the absence of oxygen for a short period. Aerobic metabolism utilizes oxygen to produce much more ATP, allowing a muscle to work for longer periods. Muscle fatigue, which has many contributing factors, occurs when muscle can no longer contract. An oxygen debt is created as a result of muscle use. The three types of muscle fiber are slow oxidative (SO), fast oxidative (FO) and fast glycolytic (FG). SO fibers use aerobic metabolism to produce low power contractions over long periods and are slow to fatigue. FO fibers use aerobic metabolism to produce ATP but produce higher tension contractions than SO fibers. FG fibers use anaerobic metabolism to produce powerful, high-tension contractions but fatigue quickly.
10.6 Exercise and Muscle Performance
Hypertrophy is an increase in muscle mass due to the addition of structural proteins. The opposite of hypertrophy is atrophy, the loss of muscle mass due to the breakdown of structural proteins. Endurance exercise causes an increase in cellular mitochondria, myoglobin, and capillary networks in SO fibers. Endurance athletes have a high level of SO fibers relative to the other fiber types. Resistance exercise causes hypertrophy. Power-producing muscles have a higher number of FG fibers than of slow fibers. Strenuous exercise causes muscle cell damage that requires time to heal. Some athletes use performance-enhancing substances to enhance muscle performance. Muscle atrophy due to age is called sarcopenia and occurs as muscle fibers die and are replaced by connective and adipose tissue.
10.7 Cardiac Muscle Tissue
Cardiac muscle is striated muscle that is present only in the heart. Cardiac muscle fibers have a single nucleus, are branched, and joined to one another by intercalated discs that contain gap junctions for depolarization between cells and desmosomes to hold the fibers together when the heart contracts. Contraction in each cardiac muscle fiber is triggered by Ca++ ions in a similar manner as skeletal muscle, but here the Ca++ ions come from SR and through voltage-gated calcium channels in the sarcolemma. Pacemaker cells stimulate the spontaneous contraction of cardiac muscle as a functional unit, called a syncytium.
10.8 Smooth Muscle
Smooth muscle is found throughout the body around various organs and tracts. Smooth muscle cells have a single nucleus, and are spindle-shaped. Smooth muscle cells can undergo hyperplasia, mitotically dividing to produce new cells. The smooth cells are nonstriated, but their sarcoplasm is filled with actin and myosin, along with dense bodies in the sarcolemma to anchor the thin filaments and a network of intermediate filaments involved in pulling the sarcolemma toward the fiber’s middle, shortening it in the process. Ca++ ions trigger contraction when they are released from SR and enter through opened voltage-gated calcium channels. Smooth muscle contraction is initiated when the Ca++ binds to intracellular calmodulin, which then activates an enzyme called myosin kinase that phosphorylates myosin heads so they can form the cross-bridges with actin and then pull on the thin filaments. Smooth muscle can be stimulated by pacesetter cells, by the autonomic nervous system, by hormones, spontaneously, or by stretching. The fibers in some smooth muscle have latch-bridges, cross-bridges that cycle slowly without the need for ATP; these muscles can maintain low-level contractions for long periods. Single-unit smooth muscle tissue contains gap junctions to synchronize membrane depolarization and contractions so that the muscle contracts as a single unit. Single-unit smooth muscle in the walls of the viscera, called visceral muscle, has a stress-relaxation response that permits muscle to stretch, contract, and relax as the organ expands. Multiunit smooth muscle cells do not possess gap junctions, and contraction does not spread from one cell to the next.
10.9 Development and Regeneration of Muscle Tissue
Muscle tissue arises from embryonic mesoderm. Somites give rise to myoblasts and fuse to form a myotube. The nucleus of each contributing myoblast remains intact in the mature skeletal muscle cell, resulting in a mature, multinucleate cell. Satellite cells help to repair skeletal muscle cells. Smooth muscle tissue can regenerate from stem cells called pericytes, whereas dead cardiac muscle tissue is replaced by scar tissue. Aging causes muscle mass to decrease and be replaced by noncontractile connective tissue and adipose tissue.
Interactive Link Questions
Watch this video to learn more about macro- and microstructures of skeletal muscles. (a) What are the names of the “junction points” between sarcomeres? (b) What are the names of the “subunits” within the myofibrils that run the length of skeletal muscle fibers? (c) What is the “double strand of pearls” described in the video? (d) What gives a skeletal muscle fiber its striated appearance?
2.Every skeletal muscle fiber is supplied by a motor neuron at the NMJ. Watch this video to learn more about what happens at the neuromuscular junction. (a) What is the definition of a motor unit? (b) What is the structural and functional difference between a large motor unit and a small motor unit? Can you give an example of each? (c) Why is the neurotransmitter acetylcholine degraded after binding to its receptor?
3.The release of calcium ions initiates muscle contractions. Watch this video to learn more about the role of calcium. (a) What are “T-tubules” and what is their role? (b) Please also describe how actin-binding sites are made available for cross-bridging with myosin heads during contraction.
Review Questions
Muscle that has a striped appearance is described as being ________.
- elastic
- nonstriated
- excitable
- striated
Which element is important in directly triggering contraction?
- sodium (Na+)
- calcium (Ca++)
- potassium (K+)
- chloride (Cl-)
Which of the following properties is not common to all three muscle tissues?
- excitability
- the need for ATP
- at rest, uses shielding proteins to cover actin-binding sites
- elasticity
The correct order for the smallest to the largest unit of organization in muscle tissue is ________.
- fascicle, filament, muscle fiber, myofibril
- filament, myofibril, muscle fiber, fascicle
- muscle fiber, fascicle, filament, myofibril
- myofibril, muscle fiber, filament, fascicle
Depolarization of the sarcolemma means ________.
- the inside of the membrane has become less negative as sodium ions accumulate
- the outside of the membrane has become less negative as sodium ions accumulate
- the inside of the membrane has become more negative as sodium ions accumulate
- the sarcolemma has completely lost any electrical charge
In relaxed muscle, the myosin-binding site on actin is blocked by ________.
- titin
- troponin
- myoglobin
- tropomyosin
According to the sliding filament model, binding sites on actin open when ________.
- creatine phosphate levels rise
- ATP levels rise
- acetylcholine levels rise
- calcium ion levels rise
The cell membrane of a muscle fiber is called ________.
- myofibril
- sarcolemma
- sarcoplasm
- myofilament
Muscle relaxation occurs when ________.
- calcium ions are actively transported out of the sarcoplasmic reticulum
- calcium ions diffuse out of the sarcoplasmic reticulum
- calcium ions are actively transported into the sarcoplasmic reticulum
- calcium ions diffuse into the sarcoplasmic reticulum
During muscle contraction, the cross-bridge detaches when ________.
- the myosin head binds to an ADP molecule
- the myosin head binds to an ATP molecule
- calcium ions bind to troponin
- calcium ions bind to actin
Thin and thick filaments are organized into functional units called ________.
- myofibrils
- myofilaments
- T-tubules
- sarcomeres
During which phase of a twitch in a muscle fiber is tension the greatest?
- resting phase
- repolarization phase
- contraction phase
- relaxation phase
Muscle fatigue is caused by ________.
- buildup of ATP and lactic acid levels
- exhaustion of energy reserves and buildup of lactic acid levels
- buildup of ATP and pyruvic acid levels
- exhaustion of energy reserves and buildup of pyruvic acid levels
A sprinter would experience muscle fatigue sooner than a marathon runner due to ________.
- anaerobic metabolism in the muscles of the sprinter
- anaerobic metabolism in the muscles of the marathon runner
- aerobic metabolism in the muscles of the sprinter
- glycolysis in the muscles of the marathon runner
What aspect of creatine phosphate allows it to supply energy to muscles?
- ATPase activity
- phosphate bonds
- carbon bonds
- hydrogen bonds
Drug X blocks ATP regeneration from ADP and phosphate. How will muscle cells respond to this drug?
- by absorbing ATP from the bloodstream
- by using ADP as an energy source
- by using glycogen as an energy source
- none of the above
The muscles of a professional sprinter are most likely to have ________.
- 80 percent fast-twitch muscle fibers and 20 percent slow-twitch muscle fibers
- 20 percent fast-twitch muscle fibers and 80 percent slow-twitch muscle fibers
- 50 percent fast-twitch muscle fibers and 50 percent slow-twitch muscle fibers
- 40 percent fast-twitch muscle fibers and 60 percent slow-twitch muscle fibers
The muscles of a professional marathon runner are most likely to have ________.
- 80 percent fast-twitch muscle fibers and 20 percent slow-twitch muscle fibers
- 20 percent fast-twitch muscle fibers and 80 percent slow-twitch muscle fibers
- 50 percent fast-twitch muscle fibers and 50 percent slow-twitch muscle fibers
- 40 percent fast-twitch muscle fibers and 60 percent slow-twitch muscle fibers
Which of the following statements is true?
- Fast fibers have a small diameter.
- Fast fibers contain loosely packed myofibrils.
- Fast fibers have large glycogen reserves.
- Fast fibers have many mitochondria.
Which of the following statements is false?
- Slow fibers have a small network of capillaries.
- Slow fibers contain the pigment myoglobin.
- Slow fibers contain a large number of mitochondria.
- Slow fibers contract for extended periods.
Cardiac muscles differ from skeletal muscles in that they ________.
- are striated
- utilize aerobic metabolism
- contain myofibrils
- contain intercalated discs
If cardiac muscle cells were prevented from undergoing aerobic metabolism, they ultimately would ________.
- undergo glycolysis
- synthesize ATP
- stop contracting
- start contracting
Smooth muscles differ from skeletal and cardiac muscles in that they ________.
- lack myofibrils
- are under voluntary control
- lack myosin
- lack actin
Which of the following statements describes smooth muscle cells?
- They are resistant to fatigue.
- They have a rapid onset of contractions.
- They cannot exhibit tetanus.
- They primarily use anaerobic metabolism.
From which embryonic cell type does muscle tissue develop?
- ganglion cells
- myotube cells
- myoblast cells
- satellite cells
Which cell type helps to repair injured muscle fibers?
- ganglion cells
- myotube cells
- myoblast cells
- satellite cells
Critical Thinking Questions
Why is elasticity an important quality of muscle tissue?
31.What would happen to skeletal muscle if the epimysium were destroyed?
32.Describe how tendons facilitate body movement.
33.What are the five primary functions of skeletal muscle?
34.What are the opposite roles of voltage-gated sodium channels and voltage-gated potassium channels?
35.How would muscle contractions be affected if skeletal muscle fibers did not have T-tubules?
36.What causes the striated appearance of skeletal muscle tissue?
37.How would muscle contractions be affected if ATP was completely depleted in a muscle fiber?
38.Why does a motor unit of the eye have few muscle fibers compared to a motor unit of the leg?
39.What factors contribute to the amount of tension produced in an individual muscle fiber?
40.Why do muscle cells use creatine phosphate instead of glycolysis to supply ATP for the first few seconds of muscle contraction?
41.Is aerobic respiration more or less efficient than glycolysis? Explain your answer.
42.What changes occur at the cellular level in response to endurance training?
43.What changes occur at the cellular level in response to resistance training?
44.What would be the drawback of cardiac contractions being the same duration as skeletal muscle contractions?
45.How are cardiac muscle cells similar to and different from skeletal muscle cells?
46.Why can smooth muscles contract over a wider range of resting lengths than skeletal and cardiac muscle?
47.Describe the differences between single-unit smooth muscle and multiunit smooth muscle.
48.Why is muscle that has sustained significant damage unable to produce the same amount of power as it could before being damaged?
49.Which muscle type(s) (skeletal, smooth, or cardiac) can regenerate new muscle cells/fibers? Explain your answer.
|
oercommons
|
2025-03-18T00:39:11.765274
|
07/23/2019
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/56369/overview",
"title": "Anatomy and Physiology, Support and Movement, Muscle Tissue",
"author": null
}
|
https://oercommons.org/courseware/lesson/56367/overview
|
The Appendicular Skeleton
Introduction
Figure 8.1 Dancer The appendicular skeleton consists of the upper and lower limb bones, the bones of the hands and feet, and the bones that anchor the limbs to the axial skeleton. (credit: Melissa Dooley/flickr)
CHAPTER OBJECTIVES
After studying this chapter, you will be able to:
- Discuss the bones of the pectoral and pelvic girdles, and describe how these unite the limbs with the axial skeleton
- Describe the bones of the upper limb, including the bones of the arm, forearm, wrist, and hand
- Identify the features of the pelvis and explain how these differ between the adult male and female pelvis
- Describe the bones of the lower limb, including the bones of the thigh, leg, ankle, and foot
- Describe the embryonic formation and growth of the limb bones
Your skeleton provides the internal supporting structure of the body. The adult axial skeleton consists of 80 bones that form the head and body trunk. Attached to this are the limbs, whose 126 bones constitute the appendicular skeleton. These bones are divided into two groups: the bones that are located within the limbs themselves, and the girdle bones that attach the limbs to the axial skeleton. The bones of the shoulder region form the pectoral girdle, which anchors the upper limb to the thoracic cage of the axial skeleton. The lower limb is attached to the vertebral column by the pelvic girdle.
Because of our upright stance, different functional demands are placed upon the upper and lower limbs. Thus, the bones of the lower limbs are adapted for weight-bearing support and stability, as well as for body locomotion via walking or running. In contrast, our upper limbs are not required for these functions. Instead, our upper limbs are highly mobile and can be utilized for a wide variety of activities. The large range of upper limb movements, coupled with the ability to easily manipulate objects with our hands and opposable thumbs, has allowed humans to construct the modern world in which we live.
The Pectoral Girdle
- Describe the bones that form the pectoral girdle
- List the functions of the pectoral girdle
The appendicular skeleton includes all of the limb bones, plus the bones that unite each limb with the axial skeleton (Figure 8.2). The bones that attach each upper limb to the axial skeleton form the pectoral girdle (shoulder girdle). This consists of two bones, the scapula and clavicle (Figure 8.3). The clavicle (collarbone) is an S-shaped bone located on the anterior side of the shoulder. It is attached on its medial end to the sternum of the thoracic cage, which is part of the axial skeleton. The lateral end of the clavicle articulates (joins) with the scapula just above the shoulder joint. You can easily palpate, or feel with your fingers, the entire length of your clavicle.
Figure 8.2 Axial and Appendicular Skeletons The axial skeleton forms the central axis of the body and consists of the skull, vertebral column, and thoracic cage. The appendicular skeleton consists of the pectoral and pelvic girdles, the limb bones, and the bones of the hands and feet.
Figure 8.3 Pectoral Girdle The pectoral girdle consists of the clavicle and the scapula, which serve to attach the upper limb to the sternum of the axial skeleton.
The scapula (shoulder blade) lies on the posterior aspect of the shoulder. It is supported by the clavicle and articulates with the humerus (arm bone) to form the shoulder joint. The scapula is a flat, triangular-shaped bone with a prominent ridge running across its posterior surface. This ridge extends out laterally, where it forms the bony tip of the shoulder and joins with the lateral end of the clavicle. By following along the clavicle, you can palpate out to the bony tip of the shoulder, and from there, you can move back across your posterior shoulder to follow the ridge of the scapula. Move your shoulder around and feel how the clavicle and scapula move together as a unit. Both of these bones serve as important attachment sites for muscles that aid with movements of the shoulder and arm.
The right and left pectoral girdles are not joined to each other, allowing each to operate independently. In addition, the clavicle of each pectoral girdle is anchored to the axial skeleton by a single, highly mobile joint. This allows for the extensive mobility of the entire pectoral girdle, which in turn enhances movements of the shoulder and upper limb.
Clavicle
The clavicle is the only long bone that lies in a horizontal position in the body (see Figure 8.3). The clavicle has several important functions. First, anchored by muscles from above, it serves as a strut that extends laterally to support the scapula. This in turn holds the shoulder joint superiorly and laterally from the body trunk, allowing for maximal freedom of motion for the upper limb. The clavicle also transmits forces acting on the upper limb to the sternum and axial skeleton. Finally, it serves to protect the underlying nerves and blood vessels as they pass between the trunk of the body and the upper limb.
The clavicle has three regions: the medial end, the lateral end, and the shaft. The medial end, known as the sternal end of the clavicle, has a triangular shape and articulates with the manubrium portion of the sternum. This forms the sternoclavicular joint, which is the only bony articulation between the pectoral girdle of the upper limb and the axial skeleton. This joint allows considerable mobility, enabling the clavicle and scapula to move in upward/downward and anterior/posterior directions during shoulder movements. The sternoclavicular joint is indirectly supported by the costoclavicular ligament (costo- = “rib”), which spans the sternal end of the clavicle and the underlying first rib. The lateral or acromial end of the clavicle articulates with the acromion of the scapula, the portion of the scapula that forms the bony tip of the shoulder. There are some sex differences in the morphology of the clavicle. In women, the clavicle tends to be shorter, thinner, and less curved. In men, the clavicle is heavier and longer, and has a greater curvature and rougher surfaces where muscles attach, features that are more pronounced in manual workers.
The clavicle is the most commonly fractured bone in the body. Such breaks often occur because of the force exerted on the clavicle when a person falls onto his or her outstretched arms, or when the lateral shoulder receives a strong blow. Because the sternoclavicular joint is strong and rarely dislocated, excessive force results in the breaking of the clavicle, usually between the middle and lateral portions of the bone. If the fracture is complete, the shoulder and lateral clavicle fragment will drop due to the weight of the upper limb, causing the person to support the sagging limb with their other hand. Muscles acting across the shoulder will also pull the shoulder and lateral clavicle anteriorly and medially, causing the clavicle fragments to override. The clavicle overlies many important blood vessels and nerves for the upper limb, but fortunately, due to the anterior displacement of a broken clavicle, these structures are rarely affected when the clavicle is fractured.
Scapula
The scapula is also part of the pectoral girdle and thus plays an important role in anchoring the upper limb to the body. The scapula is located on the posterior side of the shoulder. It is surrounded by muscles on both its anterior (deep) and posterior (superficial) sides, and thus does not articulate with the ribs of the thoracic cage.
The scapula has several important landmarks (Figure 8.4). The three margins or borders of the scapula, named for their positions within the body, are the superior border of the scapula, the medial border of the scapula, and the lateral border of the scapula. The suprascapular notch is located lateral to the midpoint of the superior border. The corners of the triangular scapula, at either end of the medial border, are the superior angle of the scapula, located between the medial and superior borders, and the inferior angle of the scapula, located between the medial and lateral borders. The inferior angle is the most inferior portion of the scapula, and is particularly important because it serves as the attachment point for several powerful muscles involved in shoulder and upper limb movements. The remaining corner of the scapula, between the superior and lateral borders, is the location of the glenoid cavity (glenoid fossa). This shallow depression articulates with the humerus bone of the arm to form the glenohumeral joint (shoulder joint). The small bony bumps located immediately above and below the glenoid cavity are the supraglenoid tubercle and the infraglenoid tubercle, respectively. These provide attachments for muscles of the arm.
Figure 8.4 Scapula The isolated scapula is shown here from its anterior (deep) side and its posterior (superficial) side.
The scapula also has two prominent projections. Toward the lateral end of the superior border, between the suprascapular notch and glenoid cavity, is the hook-like coracoid process (coracoid = “shaped like a crow’s beak”). This process projects anteriorly and curves laterally. At the shoulder, the coracoid process is located inferior to the lateral end of the clavicle. It is anchored to the clavicle by a strong ligament, and serves as the attachment site for muscles of the anterior chest and arm. On the posterior aspect, the spine of the scapula is a long and prominent ridge that runs across its upper portion. Extending laterally from the spine is a flattened and expanded region called the acromion or acromial process. The acromion forms the bony tip of the superior shoulder region and articulates with the lateral end of the clavicle, forming the acromioclavicular joint (see Figure 8.3). Together, the clavicle, acromion, and spine of the scapula form a V-shaped bony line that provides for the attachment of neck and back muscles that act on the shoulder, as well as muscles that pass across the shoulder joint to act on the arm.
The scapula has three depressions, each of which is called a fossa (plural = fossae). Two of these are found on the posterior scapula, above and below the scapular spine. Superior to the spine is the narrow supraspinous fossa, and inferior to the spine is the broad infraspinous fossa. The anterior (deep) surface of the scapula forms the broad subscapular fossa. All of these fossae provide large surface areas for the attachment of muscles that cross the shoulder joint to act on the humerus.
The acromioclavicular joint transmits forces from the upper limb to the clavicle. The ligaments around this joint are relatively weak. A hard fall onto the elbow or outstretched hand can stretch or tear the acromioclavicular ligaments, resulting in a moderate injury to the joint. However, the primary support for the acromioclavicular joint comes from a very strong ligament called the coracoclavicular ligament (see Figure 8.3). This connective tissue band anchors the coracoid process of the scapula to the inferior surface of the acromial end of the clavicle and thus provides important indirect support for the acromioclavicular joint. Following a strong blow to the lateral shoulder, such as when a hockey player is driven into the boards, a complete dislocation of the acromioclavicular joint can result. In this case, the acromion is thrust under the acromial end of the clavicle, resulting in ruptures of both the acromioclavicular and coracoclavicular ligaments. The scapula then separates from the clavicle, with the weight of the upper limb pulling the shoulder downward. This dislocation injury of the acromioclavicular joint is known as a “shoulder separation” and is common in contact sports such as hockey, football, or martial arts.
Bones of the Upper Limb
- Identify the divisions of the upper limb and describe the bones in each region
- List the bones and bony landmarks that articulate at each joint of the upper limb
The upper limb is divided into three regions. These consist of the arm, located between the shoulder and elbow joints; the forearm, which is between the elbow and wrist joints; and the hand, which is located distal to the wrist. There are 30 bones in each upper limb (see Figure 8.2). The humerus is the single bone of the upper arm, and the ulna (medially) and the radius(laterally) are the paired bones of the forearm. The base of the hand contains eight bones, each called a carpal bone, and the palm of the hand is formed by five bones, each called a metacarpal bone. The fingers and thumb contain a total of 14 bones, each of which is a phalanx bone of the hand.
Humerus
The humerus is the single bone of the upper arm region (Figure 8.5). At its proximal end is the head of the humerus. This is the large, round, smooth region that faces medially. The head articulates with the glenoid cavity of the scapula to form the glenohumeral (shoulder) joint. The margin of the smooth area of the head is the anatomical neck of the humerus. Located on the lateral side of the proximal humerus is an expanded bony area called the greater tubercle. The smaller lesser tubercle of the humerus is found on the anterior aspect of the humerus. Both the greater and lesser tubercles serve as attachment sites for muscles that act across the shoulder joint. Passing between the greater and lesser tubercles is the narrow intertubercular groove (sulcus), which is also known as the bicipital groove because it provides passage for a tendon of the biceps brachii muscle. The surgical neck is located at the base of the expanded, proximal end of the humerus, where it joins the narrow shaft of the humerus. The surgical neck is a common site of arm fractures. The deltoid tuberosity is a roughened, V-shaped region located on the lateral side in the middle of the humerus shaft. As its name indicates, it is the site of attachment for the deltoid muscle.
Figure 8.5 Humerus and Elbow Joint The humerus is the single bone of the upper arm region. It articulates with the radius and ulna bones of the forearm to form the elbow joint.
Distally, the humerus becomes flattened. The prominent bony projection on the medial side is the medial epicondyle of the humerus. The much smaller lateral epicondyle of the humerus is found on the lateral side of the distal humerus. The roughened ridge of bone above the lateral epicondyle is the lateral supracondylar ridge. All of these areas are attachment points for muscles that act on the forearm, wrist, and hand. The powerful grasping muscles of the anterior forearm arise from the medial epicondyle, which is thus larger and more robust than the lateral epicondyle that gives rise to the weaker posterior forearm muscles.
The distal end of the humerus has two articulation areas, which join the ulna and radius bones of the forearm to form the elbow joint. The more medial of these areas is the trochlea, a spindle- or pulley-shaped region (trochlea = “pulley”), which articulates with the ulna bone. Immediately lateral to the trochlea is the capitulum (“small head”), a knob-like structure located on the anterior surface of the distal humerus. The capitulum articulates with the radius bone of the forearm. Just above these bony areas are two small depressions. These spaces accommodate the forearm bones when the elbow is fully bent (flexed). Superior to the trochlea is the coronoid fossa, which receives the coronoid process of the ulna, and above the capitulum is the radial fossa, which receives the head of the radius when the elbow is flexed. Similarly, the posterior humerus has the olecranon fossa, a larger depression that receives the olecranon process of the ulna when the forearm is fully extended.
Ulna
The ulna is the medial bone of the forearm. It runs parallel to the radius, which is the lateral bone of the forearm (Figure 8.6). The proximal end of the ulna resembles a crescent wrench with its large, C-shaped trochlear notch. This region articulates with the trochlea of the humerus as part of the elbow joint. The inferior margin of the trochlear notch is formed by a prominent lip of bone called the coronoid process of the ulna. Just below this on the anterior ulna is a roughened area called the ulnar tuberosity. To the lateral side and slightly inferior to the trochlear notch is a small, smooth area called the radial notch of the ulna. This area is the site of articulation between the proximal radius and the ulna, forming the proximal radioulnar joint. The posterior and superior portions of the proximal ulna make up the olecranon process, which forms the bony tip of the elbow.
Figure 8.6 Ulna and Radius The ulna is located on the medial side of the forearm, and the radius is on the lateral side. These bones are attached to each other by an interosseous membrane.
More distal is the shaft of the ulna. The lateral side of the shaft forms a ridge called the interosseous border of the ulna. This is the line of attachment for the interosseous membrane of the forearm, a sheet of dense connective tissue that unites the ulna and radius bones. The small, rounded area that forms the distal end is the head of the ulna. Projecting from the posterior side of the ulnar head is the styloid process of the ulna, a short bony projection. This serves as an attachment point for a connective tissue structure that unites the distal ends of the ulna and radius.
In the anatomical position, with the elbow fully extended and the palms facing forward, the arm and forearm do not form a straight line. Instead, the forearm deviates laterally by 5–15 degrees from the line of the arm. This deviation is called the carrying angle. It allows the forearm and hand to swing freely or to carry an object without hitting the hip. The carrying angle is larger in females to accommodate their wider pelvis.
Radius
The radius runs parallel to the ulna, on the lateral (thumb) side of the forearm (see Figure 8.6). The head of the radius is a disc-shaped structure that forms the proximal end. The small depression on the surface of the head articulates with the capitulum of the humerus as part of the elbow joint, whereas the smooth, outer margin of the head articulates with the radial notch of the ulna at the proximal radioulnar joint. The neck of the radius is the narrowed region immediately below the expanded head. Inferior to this point on the medial side is the radial tuberosity, an oval-shaped, bony protuberance that serves as a muscle attachment point. The shaft of the radius is slightly curved and has a small ridge along its medial side. This ridge forms the interosseous border of the radius, which, like the similar border of the ulna, is the line of attachment for the interosseous membrane that unites the two forearm bones. The distal end of the radius has a smooth surface for articulation with two carpal bones to form the radiocarpal joint or wrist joint (Figure 8.7 and Figure 8.8). On the medial side of the distal radius is the ulnar notch of the radius. This shallow depression articulates with the head of the ulna, which together form the distal radioulnar joint. The lateral end of the radius has a pointed projection called the styloid process of the radius. This provides attachment for ligaments that support the lateral side of the wrist joint. Compared to the styloid process of the ulna, the styloid process of the radius projects more distally, thereby limiting the range of movement for lateral deviations of the hand at the wrist joint.
INTERACTIVE LINK
Watch this video to see how fractures of the distal radius bone can affect the wrist joint. Explain the problems that may occur if a fracture of the distal radius involves the joint surface of the radiocarpal joint of the wrist.
Carpal Bones
The wrist and base of the hand are formed by a series of eight small carpal bones (see Figure 8.7). The carpal bones are arranged in two rows, forming a proximal row of four carpal bones and a distal row of four carpal bones. The bones in the proximal row, running from the lateral (thumb) side to the medial side, are the scaphoid (“boat-shaped”), lunate (“moon-shaped”), triquetrum (“three-cornered”), and pisiform (“pea-shaped”) bones. The small, rounded pisiform bone articulates with the anterior surface of the triquetrum bone. The pisiform thus projects anteriorly, where it forms the bony bump that can be felt at the medial base of your hand. The distal bones (lateral to medial) are the trapezium (“table”), trapezoid (“resembles a table”), capitate (“head-shaped”), and hamate (“hooked bone”) bones. The hamate bone is characterized by a prominent bony extension on its anterior side called the hook of the hamate bone.
A helpful mnemonic for remembering the arrangement of the carpal bones is “So Long To Pinky, Here Comes The Thumb.” This mnemonic starts on the lateral side and names the proximal bones from lateral to medial (scaphoid, lunate, triquetrum, pisiform), then makes a U-turn to name the distal bones from medial to lateral (hamate, capitate, trapezoid, trapezium). Thus, it starts and finishes on the lateral side.
Figure 8.7 Bones of the Wrist and Hand The eight carpal bones form the base of the hand. These are arranged into proximal and distal rows of four bones each. The metacarpal bones form the palm of the hand. The thumb and fingers consist of the phalanx bones.
The carpal bones form the base of the hand. This can be seen in the radiograph (X-ray image) of the hand that shows the relationships of the hand bones to the skin creases of the hand (see Figure 8.8). Within the carpal bones, the four proximal bones are united to each other by ligaments to form a unit. Only three of these bones, the scaphoid, lunate, and triquetrum, contribute to the radiocarpal joint. The scaphoid and lunate bones articulate directly with the distal end of the radius, whereas the triquetrum bone articulates with a fibrocartilaginous pad that spans the radius and styloid process of the ulna. The distal end of the ulna thus does not directly articulate with any of the carpal bones.
The four distal carpal bones are also held together as a group by ligaments. The proximal and distal rows of carpal bones articulate with each other to form the midcarpal joint (see Figure 8.8). Together, the radiocarpal and midcarpal joints are responsible for all movements of the hand at the wrist. The distal carpal bones also articulate with the metacarpal bones of the hand.
Figure 8.8 Bones of the Hand This radiograph shows the position of the bones within the hand. Note the carpal bones that form the base of the hand. (credit: modification of work by Trace Meek)
In the articulated hand, the carpal bones form a U-shaped grouping. A strong ligament called the flexor retinaculum spans the top of this U-shaped area to maintain this grouping of the carpal bones. The flexor retinaculum is attached laterally to the trapezium and scaphoid bones, and medially to the hamate and pisiform bones. Together, the carpal bones and the flexor retinaculum form a passageway called the carpal tunnel, with the carpal bones forming the walls and floor, and the flexor retinaculum forming the roof of this space (Figure 8.9). The tendons of nine muscles of the anterior forearm and an important nerve pass through this narrow tunnel to enter the hand. Overuse of the muscle tendons or wrist injury can produce inflammation and swelling within this space. This produces compression of the nerve, resulting in carpal tunnel syndrome, which is characterized by pain or numbness, and muscle weakness in those areas of the hand supplied by this nerve.
Figure 8.9 Carpal Tunnel The carpal tunnel is the passageway by which nine muscle tendons and a major nerve enter the hand from the anterior forearm. The walls and floor of the carpal tunnel are formed by the U-shaped grouping of the carpal bones, and the roof is formed by the flexor retinaculum, a strong ligament that anteriorly unites the bones.
Metacarpal Bones
The palm of the hand contains five elongated metacarpal bones. These bones lie between the carpal bones of the wrist and the bones of the fingers and thumb (see Figure 8.7). The proximal end of each metacarpal bone articulates with one of the distal carpal bones. Each of these articulations is a carpometacarpal joint (see Figure 8.8). The expanded distal end of each metacarpal bone articulates at the metacarpophalangeal joint with the proximal phalanx bone of the thumb or one of the fingers. The distal end also forms the knuckles of the hand, at the base of the fingers. The metacarpal bones are numbered 1–5, beginning at the thumb.
The first metacarpal bone, at the base of the thumb, is separated from the other metacarpal bones. This allows it a freedom of motion that is independent of the other metacarpal bones, which is very important for thumb mobility. The remaining metacarpal bones are united together to form the palm of the hand. The second and third metacarpal bones are firmly anchored in place and are immobile. However, the fourth and fifth metacarpal bones have limited anterior-posterior mobility, a motion that is greater for the fifth bone. This mobility is important during power gripping with the hand (Figure 8.10). The anterior movement of these bones, particularly the fifth metacarpal bone, increases the strength of contact for the medial hand during gripping actions.
Figure 8.10 Hand During Gripping During tight gripping—compare (b) to (a)—the fourth and, particularly, the fifth metatarsal bones are pulled anteriorly. This increases the contact between the object and the medial side of the hand, thus improving the firmness of the grip.
Phalanx Bones
The fingers and thumb contain 14 bones, each of which is called a phalanx bone (plural = phalanges), named after the ancient Greek phalanx (a rectangular block of soldiers). The thumb (pollex) is digit number 1 and has two phalanges, a proximal phalanx, and a distal phalanx bone (see Figure 8.7). Digits 2 (index finger) through 5 (little finger) have three phalanges each, called the proximal, middle, and distal phalanx bones. An interphalangeal joint is one of the articulations between adjacent phalanges of the digits (see Figure 8.8).
INTERACTIVE LINK
Visit this site to explore the bones and joints of the hand. What are the three arches of the hand, and what is the importance of these during the gripping of an object?
DISORDERS OF THE...
Appendicular System: Fractures of Upper Limb Bones
Due to our constant use of the hands and the rest of our upper limbs, an injury to any of these areas will cause a significant loss of functional ability. Many fractures result from a hard fall onto an outstretched hand. The resulting transmission of force up the limb may result in a fracture of the humerus, radius, or scaphoid bones. These injuries are especially common in elderly people whose bones are weakened due to osteoporosis.
Falls onto the hand or elbow, or direct blows to the arm, can result in fractures of the humerus (Figure 8.11). Following a fall, fractures at the surgical neck, the region at which the expanded proximal end of the humerus joins with the shaft, can result in an impacted fracture, in which the distal portion of the humerus is driven into the proximal portion. Falls or blows to the arm can also produce transverse or spiral fractures of the humeral shaft.
In children, a fall onto the tip of the elbow frequently results in a distal humerus fracture. In these, the olecranon of the ulna is driven upward, resulting in a fracture across the distal humerus, above both epicondyles (supracondylar fracture), or a fracture between the epicondyles, thus separating one or both of the epicondyles from the body of the humerus (intercondylar fracture). With these injuries, the immediate concern is possible compression of the artery to the forearm due to swelling of the surrounding tissues. If compression occurs, the resulting ischemia (lack of oxygen) due to reduced blood flow can quickly produce irreparable damage to the forearm muscles. In addition, four major nerves for shoulder and upper limb muscles are closely associated with different regions of the humerus, and thus, humeral fractures may also damage these nerves.
Another frequent injury following a fall onto an outstretched hand is a Colles fracture (“col-lees”) of the distal radius (see Figure 8.11). This involves a complete transverse fracture across the distal radius that drives the separated distal fragment of the radius posteriorly and superiorly. This injury results in a characteristic “dinner fork” bend of the forearm just above the wrist due to the posterior displacement of the hand. This is the most frequent forearm fracture and is a common injury in persons over the age of 50, particularly in older women with osteoporosis. It also commonly occurs following a high-speed fall onto the hand during activities such as snowboarding or skating.
The most commonly fractured carpal bone is the scaphoid, often resulting from a fall onto the hand. Deep pain at the lateral wrist may yield an initial diagnosis of a wrist sprain, but a radiograph taken several weeks after the injury, after tissue swelling has subsided, will reveal the fracture. Due to the poor blood supply to the scaphoid bone, healing will be slow and there is the danger of bone necrosis and subsequent degenerative joint disease of the wrist.
Figure 8.11 Fractures of the Humerus and Radius Falls or direct blows can result in fractures of the surgical neck or shaft of the humerus. Falls onto the elbow can fracture the distal humerus. A Colles fracture of the distal radius is the most common forearm fracture.
INTERACTIVE LINK
Watch this video to learn about a Colles fracture, a break of the distal radius, usually caused by falling onto an outstretched hand. When would surgery be required and how would the fracture be repaired in this case?
The Pelvic Girdle and Pelvis
- Define the pelvic girdle and describe the bones and ligaments of the pelvis
- Explain the three regions of the hip bone and identify their bony landmarks
- Describe the openings of the pelvis and the boundaries of the greater and lesser pelvis
The pelvic girdle (hip girdle) is formed by a single bone, the hip bone or coxal bone (coxal = “hip”), which serves as the attachment point for each lower limb. Each hip bone, in turn, is firmly joined to the axial skeleton via its attachment to the sacrum of the vertebral column. The right and left hip bones also converge anteriorly to attach to each other. The bony pelvis is the entire structure formed by the two hip bones, the sacrum, and, attached inferiorly to the sacrum, the coccyx (Figure 8.12).
Unlike the bones of the pectoral girdle, which are highly mobile to enhance the range of upper limb movements, the bones of the pelvis are strongly united to each other to form a largely immobile, weight-bearing structure. This is important for stability because it enables the weight of the body to be easily transferred laterally from the vertebral column, through the pelvic girdle and hip joints, and into either lower limb whenever the other limb is not bearing weight. Thus, the immobility of the pelvis provides a strong foundation for the upper body as it rests on top of the mobile lower limbs.
Figure 8.12 Pelvis The pelvic girdle is formed by a single hip bone. The hip bone attaches the lower limb to the axial skeleton through its articulation with the sacrum. The right and left hip bones, plus the sacrum and the coccyx, together form the pelvis.
Hip Bone
The hip bone, or coxal bone, forms the pelvic girdle portion of the pelvis. The paired hip bones are the large, curved bones that form the lateral and anterior aspects of the pelvis. Each adult hip bone is formed by three separate bones that fuse together during the late teenage years. These bony components are the ilium, ischium, and pubis (Figure 8.13). These names are retained and used to define the three regions of the adult hip bone.
Figure 8.13 The Hip Bone The adult hip bone consists of three regions. The ilium forms the large, fan-shaped superior portion, the ischium forms the posteroinferior portion, and the pubis forms the anteromedial portion.
The ilium is the fan-like, superior region that forms the largest part of the hip bone. It is firmly united to the sacrum at the largely immobile sacroiliac joint (see Figure 8.12). The ischium forms the posteroinferior region of each hip bone. It supports the body when sitting. The pubis forms the anterior portion of the hip bone. The pubis curves medially, where it joins to the pubis of the opposite hip bone at a specialized joint called the pubic symphysis.
Ilium
When you place your hands on your waist, you can feel the arching, superior margin of the ilium along your waistline (see Figure 8.13). This curved, superior margin of the ilium is the iliac crest. The rounded, anterior termination of the iliac crest is the anterior superior iliac spine. This important bony landmark can be felt at your anterolateral hip. Inferior to the anterior superior iliac spine is a rounded protuberance called the anterior inferior iliac spine. Both of these iliac spines serve as attachment points for muscles of the thigh. Posteriorly, the iliac crest curves downward to terminate as the posterior superior iliac spine. Muscles and ligaments surround but do not cover this bony landmark, thus sometimes producing a depression seen as a “dimple” located on the lower back. More inferiorly is the posterior inferior iliac spine. This is located at the inferior end of a large, roughened area called the auricular surface of the ilium. The auricular surface articulates with the auricular surface of the sacrum to form the sacroiliac joint. Both the posterior superior and posterior inferior iliac spines serve as attachment points for the muscles and very strong ligaments that support the sacroiliac joint.
The shallow depression located on the anteromedial (internal) surface of the upper ilium is called the iliac fossa. The inferior margin of this space is formed by the arcuate line of the ilium, the ridge formed by the pronounced change in curvature between the upper and lower portions of the ilium. The large, inverted U-shaped indentation located on the posterior margin of the lower ilium is called the greater sciatic notch.
Ischium
The ischium forms the posterolateral portion of the hip bone (see Figure 8.13). The large, roughened area of the inferior ischium is the ischial tuberosity. This serves as the attachment for the posterior thigh muscles and also carries the weight of the body when sitting. You can feel the ischial tuberosity if you wiggle your pelvis against the seat of a chair. Projecting superiorly and anteriorly from the ischial tuberosity is a narrow segment of bone called the ischial ramus. The slightly curved posterior margin of the ischium above the ischial tuberosity is the lesser sciatic notch. The bony projection separating the lesser sciatic notch and greater sciatic notch is the ischial spine.
Pubis
The pubis forms the anterior portion of the hip bone (see Figure 8.13). The enlarged medial portion of the pubis is the pubic body. Located superiorly on the pubic body is a small bump called the pubic tubercle. The superior pubic ramus is the segment of bone that passes laterally from the pubic body to join the ilium. The narrow ridge running along the superior margin of the superior pubic ramus is the pectineal line of the pubis.
The pubic body is joined to the pubic body of the opposite hip bone by the pubic symphysis. Extending downward and laterally from the body is the inferior pubic ramus. The pubic arch is the bony structure formed by the pubic symphysis, and the bodies and inferior pubic rami of the adjacent pubic bones. The inferior pubic ramus extends downward to join the ischial ramus. Together, these form the single ischiopubic ramus, which extends from the pubic body to the ischial tuberosity. The inverted V-shape formed as the ischiopubic rami from both sides come together at the pubic symphysis is called the subpubic angle.
Pelvis
The pelvis consists of four bones: the right and left hip bones, the sacrum, and the coccyx (see Figure 8.12). The pelvis has several important functions. Its primary role is to support the weight of the upper body when sitting and to transfer this weight to the lower limbs when standing. It serves as an attachment point for trunk and lower limb muscles, and also protects the internal pelvic organs. When standing in the anatomical position, the pelvis is tilted anteriorly. In this position, the anterior superior iliac spines and the pubic tubercles lie in the same vertical plane, and the anterior (internal) surface of the sacrum faces forward and downward.
The three areas of each hip bone, the ilium, pubis, and ischium, converge centrally to form a deep, cup-shaped cavity called the acetabulum. This is located on the lateral side of the hip bone and is part of the hip joint. The large opening in the anteroinferior hip bone between the ischium and pubis is the obturator foramen. This space is largely filled in by a layer of connective tissue and serves for the attachment of muscles on both its internal and external surfaces.
Several ligaments unite the bones of the pelvis (Figure 8.14). The largely immobile sacroiliac joint is supported by a pair of strong ligaments that are attached between the sacrum and ilium portions of the hip bone. These are the anterior sacroiliac ligamenton the anterior side of the joint and the posterior sacroiliac ligament on the posterior side. Also spanning the sacrum and hip bone are two additional ligaments. The sacrospinous ligament runs from the sacrum to the ischial spine, and the sacrotuberous ligament runs from the sacrum to the ischial tuberosity. These ligaments help to support and immobilize the sacrum as it carries the weight of the body.
Figure 8.14 Ligaments of the Pelvis The posterior sacroiliac ligament supports the sacroiliac joint. The sacrospinous ligament spans the sacrum to the ischial spine, and the sacrotuberous ligament spans the sacrum to the ischial tuberosity. The sacrospinous and sacrotuberous ligaments contribute to the formation of the greater and lesser sciatic foramens.
INTERACTIVE LINK
Watch this video for a 3-D view of the pelvis and its associated ligaments. What is the large opening in the bony pelvis, located between the ischium and pubic regions, and what two parts of the pubis contribute to the formation of this opening?
The sacrospinous and sacrotuberous ligaments also help to define two openings on the posterolateral sides of the pelvis through which muscles, nerves, and blood vessels for the lower limb exit. The superior opening is the greater sciatic foramen. This large opening is formed by the greater sciatic notch of the hip bone, the sacrum, and the sacrospinous ligament. The smaller, more inferior lesser sciatic foramen is formed by the lesser sciatic notch of the hip bone, together with the sacrospinous and sacrotuberous ligaments.
The space enclosed by the bony pelvis is divided into two regions (Figure 8.15). The broad, superior region, defined laterally by the large, fan-like portion of the upper hip bone, is called the greater pelvis (greater pelvic cavity; false pelvis). This broad area is occupied by portions of the small and large intestines, and because it is more closely associated with the abdominal cavity, it is sometimes referred to as the false pelvis. More inferiorly, the narrow, rounded space of the lesser pelvis (lesser pelvic cavity; true pelvis) contains the bladder and other pelvic organs, and thus is also known as the true pelvis. The pelvic brim (also known as the pelvic inlet) forms the superior margin of the lesser pelvis, separating it from the greater pelvis. The pelvic brim is defined by a line formed by the upper margin of the pubic symphysis anteriorly, and the pectineal line of the pubis, the arcuate line of the ilium, and the sacral promontory (the anterior margin of the superior sacrum) posteriorly. The inferior limit of the lesser pelvic cavity is called the pelvic outlet. This large opening is defined by the inferior margin of the pubic symphysis anteriorly, and the ischiopubic ramus, the ischial tuberosity, the sacrotuberous ligament, and the inferior tip of the coccyx posteriorly. Because of the anterior tilt of the pelvis, the lesser pelvis is also angled, giving it an anterosuperior (pelvic inlet) to posteroinferior (pelvic outlet) orientation.
Figure 8.15 Male and Female Pelvis The female pelvis is adapted for childbirth and is broader, with a larger subpubic angle, a rounder pelvic brim, and a wider and more shallow lesser pelvic cavity than the male pelvis.
Comparison of the Female and Male Pelvis
The differences between the adult female and male pelvis relate to function and body size. In general, the bones of the male pelvis are thicker and heavier, adapted for support of the male’s heavier physical build and stronger muscles. The greater sciatic notch of the male hip bone is narrower and deeper than the broader notch of females. Because the female pelvis is adapted for childbirth, it is wider than the male pelvis, as evidenced by the distance between the anterior superior iliac spines (see Figure 8.15). The ischial tuberosities of females are also farther apart, which increases the size of the pelvic outlet. Because of this increased pelvic width, the subpubic angle is larger in females (greater than 80 degrees) than it is in males (less than 70 degrees). The female sacrum is wider, shorter, and less curved, and the sacral promontory projects less into the pelvic cavity, thus giving the female pelvic inlet (pelvic brim) a more rounded or oval shape compared to males. The lesser pelvic cavity of females is also wider and more shallow than the narrower, deeper, and tapering lesser pelvis of males. Because of the obvious differences between female and male hip bones, this is the one bone of the body that allows for the most accurate sex determination. Table 8.1 provides an overview of the general differences between the female and male pelvis.
Overview of Differences between the Female and Male Pelvis
| Female pelvis | Male pelvis | |
|---|---|---|
| Pelvic weight | Bones of the pelvis are lighter and thinner | Bones of the pelvis are thicker and heavier |
| Pelvic inlet shape | Pelvic inlet has a round or oval shape | Pelvic inlet is heart-shaped |
| Lesser pelvic cavity shape | Lesser pelvic cavity is shorter and wider | Lesser pelvic cavity is longer and narrower |
| Subpubic angle | Subpubic angle is greater than 80 degrees | Subpubic angle is less than 70 degrees |
| Pelvic outlet shape | Pelvic outlet is rounded and larger | Pelvic outlet is smaller |
Table 8.1
CAREER CONNECTION
Forensic Pathology and Forensic Anthropology
A forensic pathologist (also known as a medical examiner) is a medically trained physician who has been specifically trained in pathology to examine the bodies of the deceased to determine the cause of death. A forensic pathologist applies his or her understanding of disease as well as toxins, blood and DNA analysis, firearms and ballistics, and other factors to assess the cause and manner of death. At times, a forensic pathologist will be called to testify under oath in situations that involve a possible crime. Forensic pathology is a field that has received much media attention on television shows or following a high-profile death.
While forensic pathologists are responsible for determining whether the cause of someone’s death was natural, a suicide, accidental, or a homicide, there are times when uncovering the cause of death is more complex, and other skills are needed. Forensic anthropology brings the tools and knowledge of physical anthropology and human osteology (the study of the skeleton) to the task of investigating a death. A forensic anthropologist assists medical and legal professionals in identifying human remains. The science behind forensic anthropology involves the study of archaeological excavation; the examination of hair; an understanding of plants, insects, and footprints; the ability to determine how much time has elapsed since the person died; the analysis of past medical history and toxicology; the ability to determine whether there are any postmortem injuries or alterations of the skeleton; and the identification of the decedent (deceased person) using skeletal and dental evidence.
Due to the extensive knowledge and understanding of excavation techniques, a forensic anthropologist is an integral and invaluable team member to have on-site when investigating a crime scene, especially when the recovery of human skeletal remains is involved. When remains are bought to a forensic anthropologist for examination, he or she must first determine whether the remains are in fact human. Once the remains have been identified as belonging to a person and not to an animal, the next step is to approximate the individual’s age, sex, race, and height. The forensic anthropologist does not determine the cause of death, but rather provides information to the forensic pathologist, who will use all of the data collected to make a final determination regarding the cause of death.
Bones of the Lower Limb
- Identify the divisions of the lower limb and describe the bones of each region
- Describe the bones and bony landmarks that articulate at each joint of the lower limb
Like the upper limb, the lower limb is divided into three regions. The thigh is that portion of the lower limb located between the hip joint and knee joint. The leg is specifically the region between the knee joint and the ankle joint. Distal to the ankle is the foot. The lower limb contains 30 bones. These are the femur, patella, tibia, fibula, tarsal bones, metatarsal bones, and phalanges (see Figure 8.2). The femur is the single bone of the thigh. The patella is the kneecap and articulates with the distal femur. The tibia is the larger, weight-bearing bone located on the medial side of the leg, and the fibula is the thin bone of the lateral leg. The bones of the foot are divided into three groups. The posterior portion of the foot is formed by a group of seven bones, each of which is known as a tarsal bone, whereas the mid-foot contains five elongated bones, each of which is a metatarsal bone. The toes contain 14 small bones, each of which is a phalanx bone of the foot.
Femur
The femur, or thigh bone, is the single bone of the thigh region (Figure 8.16). It is the longest and strongest bone of the body, and accounts for approximately one-quarter of a person’s total height. The rounded, proximal end is the head of the femur, which articulates with the acetabulum of the hip bone to form the hip joint. The fovea capitis is a minor indentation on the medial side of the femoral head that serves as the site of attachment for the ligament of the head of the femur. This ligament spans the femur and acetabulum, but is weak and provides little support for the hip joint. It does, however, carry an important artery that supplies the head of the femur.
Figure 8.16 Femur and Patella The femur is the single bone of the thigh region. It articulates superiorly with the hip bone at the hip joint, and inferiorly with the tibia at the knee joint. The patella only articulates with the distal end of the femur.
The narrowed region below the head is the neck of the femur. This is a common area for fractures of the femur. The greater trochanter is the large, upward, bony projection located above the base of the neck. Multiple muscles that act across the hip joint attach to the greater trochanter, which, because of its projection from the femur, gives additional leverage to these muscles. The greater trochanter can be felt just under the skin on the lateral side of your upper thigh. The lesser trochanter is a small, bony prominence that lies on the medial aspect of the femur, just below the neck. A single, powerful muscle attaches to the lesser trochanter. Running between the greater and lesser trochanters on the anterior side of the femur is the roughened intertrochanteric line. The trochanters are also connected on the posterior side of the femur by the larger intertrochanteric crest.
The elongated shaft of the femur has a slight anterior bowing or curvature. At its proximal end, the posterior shaft has the gluteal tuberosity, a roughened area extending inferiorly from the greater trochanter. More inferiorly, the gluteal tuberosity becomes continuous with the linea aspera (“rough line”). This is the roughened ridge that passes distally along the posterior side of the mid-femur. Multiple muscles of the hip and thigh regions make long, thin attachments to the femur along the linea aspera.
The distal end of the femur has medial and lateral bony expansions. On the lateral side, the smooth portion that covers the distal and posterior aspects of the lateral expansion is the lateral condyle of the femur. The roughened area on the outer, lateral side of the condyle is the lateral epicondyle of the femur. Similarly, the smooth region of the distal and posterior medial femur is the medial condyle of the femur, and the irregular outer, medial side of this is the medial epicondyle of the femur. The lateral and medial condyles articulate with the tibia to form the knee joint. The epicondyles provide attachment for muscles and supporting ligaments of the knee. The adductor tubercle is a small bump located at the superior margin of the medial epicondyle. Posteriorly, the medial and lateral condyles are separated by a deep depression called the intercondylar fossa. Anteriorly, the smooth surfaces of the condyles join together to form a wide groove called the patellar surface, which provides for articulation with the patella bone. The combination of the medial and lateral condyles with the patellar surface gives the distal end of the femur a horseshoe (U) shape.
INTERACTIVE LINK
Watch this video to view how a fracture of the mid-femur is surgically repaired. How are the two portions of the broken femur stabilized during surgical repair of a fractured femur?
Patella
The patella (kneecap) is largest sesamoid bone of the body (see Figure 8.16). A sesamoid bone is a bone that is incorporated into the tendon of a muscle where that tendon crosses a joint. The sesamoid bone articulates with the underlying bones to prevent damage to the muscle tendon due to rubbing against the bones during movements of the joint. The patella is found in the tendon of the quadriceps femoris muscle, the large muscle of the anterior thigh that passes across the anterior knee to attach to the tibia. The patella articulates with the patellar surface of the femur and thus prevents rubbing of the muscle tendon against the distal femur. The patella also lifts the tendon away from the knee joint, which increases the leverage power of the quadriceps femoris muscle as it acts across the knee. The patella does not articulate with the tibia.
INTERACTIVE LINK
Visit this site to perform a virtual knee replacement surgery. The prosthetic knee components must be properly aligned to function properly. How is this alignment ensured?
HOMEOSTATIC IMBALANCES
Runner’s Knee
Runner’s knee, also known as patellofemoral syndrome, is the most common overuse injury among runners. It is most frequent in adolescents and young adults, and is more common in females. It often results from excessive running, particularly downhill, but may also occur in athletes who do a lot of knee bending, such as jumpers, skiers, cyclists, weight lifters, and soccer players. It is felt as a dull, aching pain around the front of the knee and deep to the patella. The pain may be felt when walking or running, going up or down stairs, kneeling or squatting, or after sitting with the knee bent for an extended period.
Patellofemoral syndrome may be initiated by a variety of causes, including individual variations in the shape and movement of the patella, a direct blow to the patella, or flat feet or improper shoes that cause excessive turning in or out of the feet or leg. These factors may cause in an imbalance in the muscle pull that acts on the patella, resulting in an abnormal tracking of the patella that allows it to deviate too far toward the lateral side of the patellar surface on the distal femur.
Because the hips are wider than the knee region, the femur has a diagonal orientation within the thigh, in contrast to the vertically oriented tibia of the leg (Figure 8.17). The Q-angle is a measure of how far the femur is angled laterally away from vertical. The Q-angle is normally 10–15 degrees, with females typically having a larger Q-angle due to their wider pelvis. During extension of the knee, the quadriceps femoris muscle pulls the patella both superiorly and laterally, with the lateral pull greater in women due to their large Q-angle. This makes women more vulnerable to developing patellofemoral syndrome than men. Normally, the large lip on the lateral side of the patellar surface of the femur compensates for the lateral pull on the patella, and thus helps to maintain its proper tracking.
However, if the pull produced by the medial and lateral sides of the quadriceps femoris muscle is not properly balanced, abnormal tracking of the patella toward the lateral side may occur. With continued use, this produces pain and could result in damage to the articulating surfaces of the patella and femur, and the possible future development of arthritis. Treatment generally involves stopping the activity that produces knee pain for a period of time, followed by a gradual resumption of activity. Proper strengthening of the quadriceps femoris muscle to correct for imbalances is also important to help prevent reoccurrence.
Figure 8.17 The Q-Angle The Q-angle is a measure of the amount of lateral deviation of the femur from the vertical line of the tibia. Adult females have a larger Q-angle due to their wider pelvis than adult males.
Tibia
The tibia (shin bone) is the medial bone of the leg and is larger than the fibula, with which it is paired (Figure 8.18). The tibia is the main weight-bearing bone of the lower leg and the second longest bone of the body, after the femur. The medial side of the tibia is located immediately under the skin, allowing it to be easily palpated down the entire length of the medial leg.
Figure 8.18 Tibia and Fibula The tibia is the larger, weight-bearing bone located on the medial side of the leg. The fibula is the slender bone of the lateral side of the leg and does not bear weight.
The proximal end of the tibia is greatly expanded. The two sides of this expansion form the medial condyle of the tibia and the lateral condyle of the tibia. The tibia does not have epicondyles. The top surface of each condyle is smooth and flattened. These areas articulate with the medial and lateral condyles of the femur to form the knee joint. Between the articulating surfaces of the tibial condyles is the intercondylar eminence, an irregular, elevated area that serves as the inferior attachment point for two supporting ligaments of the knee.
The tibial tuberosity is an elevated area on the anterior side of the tibia, near its proximal end. It is the final site of attachment for the muscle tendon associated with the patella. More inferiorly, the shaft of the tibia becomes triangular in shape. The anterior apex of
MH this triangle forms the anterior border of the tibia, which begins at the tibial tuberosity and runs inferiorly along the length of the tibia. Both the anterior border and the medial side of the triangular shaft are located immediately under the skin and can be easily palpated along the entire length of the tibia. A small ridge running down the lateral side of the tibial shaft is the interosseous border of the tibia. This is for the attachment of the interosseous membrane of the leg, the sheet of dense connective tissue that unites the tibia and fibula bones. Located on the posterior side of the tibia is the soleal line, a diagonally running, roughened ridge that begins below the base of the lateral condyle, and runs down and medially across the proximal third of the posterior tibia. Muscles of the posterior leg attach to this line.
The large expansion found on the medial side of the distal tibia is the medial malleolus (“little hammer”). This forms the large bony bump found on the medial side of the ankle region. Both the smooth surface on the inside of the medial malleolus and the smooth area at the distal end of the tibia articulate with the talus bone of the foot as part of the ankle joint. On the lateral side of the distal tibia is a wide groove called the fibular notch. This area articulates with the distal end of the fibula, forming the distal tibiofibular joint.
Fibula
The fibula is the slender bone located on the lateral side of the leg (see Figure 8.18). The fibula does not bear weight. It serves primarily for muscle attachments and thus is largely surrounded by muscles. Only the proximal and distal ends of the fibula can be palpated.
The head of the fibula is the small, knob-like, proximal end of the fibula. It articulates with the inferior aspect of the lateral tibial condyle, forming the proximal tibiofibular joint. The thin shaft of the fibula has the interosseous border of the fibula, a narrow ridge running down its medial side for the attachment of the interosseous membrane that spans the fibula and tibia. The distal end of the fibula forms the lateral malleolus, which forms the easily palpated bony bump on the lateral side of the ankle. The deep (medial) side of the lateral malleolus articulates with the talus bone of the foot as part of the ankle joint. The distal fibula also articulates with the fibular notch of the tibia.
Tarsal Bones
The posterior half of the foot is formed by seven tarsal bones (Figure 8.19). The most superior bone is the talus. This has a relatively square-shaped, upper surface that articulates with the tibia and fibula to form the ankle joint. Three areas of articulation form the ankle joint: The superomedial surface of the talus bone articulates with the medial malleolus of the tibia, the top of the talus articulates with the distal end of the tibia, and the lateral side of the talus articulates with the lateral malleolus of the fibula. Inferiorly, the talus articulates with the calcaneus (heel bone), the largest bone of the foot, which forms the heel. Body weight is transferred from the tibia to the talus to the calcaneus, which rests on the ground. The medial calcaneus has a prominent bony extension called the sustentaculum tali (“support for the talus”) that supports the medial side of the talus bone.
Figure 8.19 Bones of the Foot The bones of the foot are divided into three groups. The posterior foot is formed by the seven tarsal bones. The mid-foot has the five metatarsal bones. The toes contain the phalanges.
The cuboid bone articulates with the anterior end of the calcaneus bone. The cuboid has a deep groove running across its inferior surface, which provides passage for a muscle tendon. The talus bone articulates anteriorly with the navicular bone, which in turn articulates anteriorly with the three cuneiform (“wedge-shaped”) bones. These bones are the medial cuneiform, the intermediate cuneiform, and the lateral cuneiform. Each of these bones has a broad superior surface and a narrow inferior surface, which together produce the transverse (medial-lateral) curvature of the foot. The navicular and lateral cuneiform bones also articulate with the medial side of the cuboid bone.
INTERACTIVE LINK
Use this tutorial to review the bones of the foot. Which tarsal bones are in the proximal, intermediate, and distal groups?
Metatarsal Bones
The anterior half of the foot is formed by the five metatarsal bones, which are located between the tarsal bones of the posterior foot and the phalanges of the toes (see Figure 8.19). These elongated bones are numbered 1–5, starting with the medial side of the foot. The first metatarsal bone is shorter and thicker than the others. The second metatarsal is the longest. The base of the metatarsal bone is the proximal end of each metatarsal bone. These articulate with the cuboid or cuneiform bones. The base of the fifth metatarsal has a large, lateral expansion that provides for muscle attachments. This expanded base of the fifth metatarsal can be felt as a bony bump at the midpoint along the lateral border of the foot. The expanded distal end of each metatarsal is the head of the metatarsal bone. Each metatarsal bone articulates with the proximal phalanx of a toe to form a metatarsophalangeal joint. The heads of the metatarsal bones also rest on the ground and form the ball (anterior end) of the foot.
Phalanges
The toes contain a total of 14 phalanx bones (phalanges), arranged in a similar manner as the phalanges of the fingers (see Figure 8.19). The toes are numbered 1–5, starting with the big toe (hallux). The big toe has two phalanx bones, the proximal and distal phalanges. The remaining toes all have proximal, middle, and distal phalanges. A joint between adjacent phalanx bones is called an interphalangeal joint.
INTERACTIVE LINK
View this link to learn about a bunion, a localized swelling on the medial side of the foot, next to the first metatarsophalangeal joint, at the base of the big toe. What is a bunion and what type of shoe is most likely to cause this to develop?
Arches of the Foot
When the foot comes into contact with the ground during walking, running, or jumping activities, the impact of the body weight puts a tremendous amount of pressure and force on the foot. During running, the force applied to each foot as it contacts the ground can be up to 2.5 times your body weight. The bones, joints, ligaments, and muscles of the foot absorb this force, thus greatly reducing the amount of shock that is passed superiorly into the lower limb and body. The arches of the foot play an important role in this shock-absorbing ability. When weight is applied to the foot, these arches will flatten somewhat, thus absorbing energy. When the weight is removed, the arch rebounds, giving “spring” to the step. The arches also serve to distribute body weight side to side and to either end of the foot.
The foot has a transverse arch, a medial longitudinal arch, and a lateral longitudinal arch (see Figure 8.19). The transverse arch forms the medial-lateral curvature of the mid-foot. It is formed by the wedge shapes of the cuneiform bones and bases (proximal ends) of the first to fourth metatarsal bones. This arch helps to distribute body weight from side to side within the foot, thus allowing the foot to accommodate uneven terrain.
The longitudinal arches run down the length of the foot. The lateral longitudinal arch is relatively flat, whereas the medial longitudinal arch is larger (taller). The longitudinal arches are formed by the tarsal bones posteriorly and the metatarsal bones anteriorly. These arches are supported at either end, where they contact the ground. Posteriorly, this support is provided by the calcaneus bone and anteriorly by the heads (distal ends) of the metatarsal bones. The talus bone, which receives the weight of the body, is located at the top of the longitudinal arches. Body weight is then conveyed from the talus to the ground by the anterior and posterior ends of these arches. Strong ligaments unite the adjacent foot bones to prevent disruption of the arches during weight bearing. On the bottom of the foot, additional ligaments tie together the anterior and posterior ends of the arches. These ligaments have elasticity, which allows them to stretch somewhat during weight bearing, thus allowing the longitudinal arches to spread. The stretching of these ligaments stores energy within the foot, rather than passing these forces into the leg. Contraction of the foot muscles also plays an important role in this energy absorption. When the weight is removed, the elastic ligaments recoil and pull the ends of the arches closer together. This recovery of the arches releases the stored energy and improves the energy efficiency of walking.
Stretching of the ligaments that support the longitudinal arches can lead to pain. This can occur in overweight individuals, with people who have jobs that involve standing for long periods of time (such as a waitress), or walking or running long distances. If stretching of the ligaments is prolonged, excessive, or repeated, it can result in a gradual lengthening of the supporting ligaments, with subsequent depression or collapse of the longitudinal arches, particularly on the medial side of the foot. This condition is called pes planus (“flat foot” or “fallen arches”).
Development of the Appendicular Skeleton
- Describe the growth and development of the embryonic limb buds
- Discuss the appearance of primary and secondary ossification centers
Embryologically, the appendicular skeleton arises from mesenchyme, a type of embryonic tissue that can differentiate into many types of tissues, including bone or muscle tissue. Mesenchyme gives rise to the bones of the upper and lower limbs, as well as to the pectoral and pelvic girdles. Development of the limbs begins near the end of the fourth embryonic week, with the upper limbs appearing first. Thereafter, the development of the upper and lower limbs follows similar patterns, with the lower limbs lagging behind the upper limbs by a few days.
Limb Growth
Each upper and lower limb initially develops as a small bulge called a limb bud, which appears on the lateral side of the early embryo. The upper limb bud appears near the end of the fourth week of development, with the lower limb bud appearing shortly after (Figure 8.20).
Figure 8.20 Embryo at Seven Weeks Limb buds are visible in an embryo at the end of the seventh week of development (embryo derived from an ectopic pregnancy). (credit: Ed Uthman/flickr)
Initially, the limb buds consist of a core of mesenchyme covered by a layer of ectoderm. The ectoderm at the end of the limb bud thickens to form a narrow crest called the apical ectodermal ridge. This ridge stimulates the underlying mesenchyme to rapidly proliferate, producing the outgrowth of the developing limb. As the limb bud elongates, cells located farther from the apical ectodermal ridge slow their rates of cell division and begin to differentiate. In this way, the limb develops along a proximal-to-distal axis.
During the sixth week of development, the distal ends of the upper and lower limb buds expand and flatten into a paddle shape. This region will become the hand or foot. The wrist or ankle areas then appear as a constriction that develops at the base of the paddle. Shortly after this, a second constriction on the limb bud appears at the future site of the elbow or knee. Within the paddle, areas of tissue undergo cell death, producing separations between the growing fingers and toes. Also during the sixth week of development, mesenchyme within the limb buds begins to differentiate into hyaline cartilage that will form models of the future limb bones.
The early outgrowth of the upper and lower limb buds initially has the limbs positioned so that the regions that will become the palm of the hand or the bottom of the foot are facing medially toward the body, with the future thumb or big toe both oriented toward the head. During the seventh week of development, the upper limb rotates laterally by 90 degrees, so that the palm of the hand faces anteriorly and the thumb points laterally. In contrast, the lower limb undergoes a 90-degree medial rotation, thus bringing the big toe to the medial side of the foot.
INTERACTIVE LINK
Watch this animation to follow the development and growth of the upper and lower limb buds. On what days of embryonic development do these events occur: (a) first appearance of the upper limb bud (limb ridge); (b) the flattening of the distal limb to form the handplate or footplate; and (c) the beginning of limb rotation?
Ossification of Appendicular Bones
All of the girdle and limb bones, except for the clavicle, develop by the process of endochondral ossification. This process begins as the mesenchyme within the limb bud differentiates into hyaline cartilage to form cartilage models for future bones. By the twelfth week, a primary ossification center will have appeared in the diaphysis (shaft) region of the long bones, initiating the process that converts the cartilage model into bone. A secondary ossification center will appear in each epiphysis (expanded end) of these bones at a later time, usually after birth. The primary and secondary ossification centers are separated by the epiphyseal plate, a layer of growing hyaline cartilage. This plate is located between the diaphysis and each epiphysis. It continues to grow and is responsible for the lengthening of the bone. The epiphyseal plate is retained for many years, until the bone reaches its final, adult size, at which time the epiphyseal plate disappears and the epiphysis fuses to the diaphysis. (Seek additional content on ossification in the chapter on bone tissue.)
Small bones, such as the phalanges, will develop only one secondary ossification center and will thus have only a single epiphyseal plate. Large bones, such as the femur, will develop several secondary ossification centers, with an epiphyseal plate associated with each secondary center. Thus, ossification of the femur begins at the end of the seventh week with the appearance of the primary ossification center in the diaphysis, which rapidly expands to ossify the shaft of the bone prior to birth. Secondary ossification centers develop at later times. Ossification of the distal end of the femur, to form the condyles and epicondyles, begins shortly before birth. Secondary ossification centers also appear in the femoral head late in the first year after birth, in the greater trochanter during the fourth year, and in the lesser trochanter between the ages of 9 and 10 years. Once these areas have ossified, their fusion to the diaphysis and the disappearance of each epiphyseal plate follow a reversed sequence. Thus, the lesser trochanter is the first to fuse, doing so at the onset of puberty (around 11 years of age), followed by the greater trochanter approximately 1 year later. The femoral head fuses between the ages of 14–17 years, whereas the distal condyles of the femur are the last to fuse, between the ages of 16–19 years. Knowledge of the age at which different epiphyseal plates disappear is important when interpreting radiographs taken of children. Since the cartilage of an epiphyseal plate is less dense than bone, the plate will appear dark in a radiograph image. Thus, a normal epiphyseal plate may be mistaken for a bone fracture.
The clavicle is the one appendicular skeleton bone that does not develop via endochondral ossification. Instead, the clavicle develops through the process of intramembranous ossification. During this process, mesenchymal cells differentiate directly into bone-producing cells, which produce the clavicle directly, without first making a cartilage model. Because of this early production of bone, the clavicle is the first bone of the body to begin ossification, with ossification centers appearing during the fifth week of development. However, ossification of the clavicle is not complete until age 25.
DISORDERS OF THE...
Appendicular System: Congenital Clubfoot
Clubfoot, also known as talipes, is a congenital (present at birth) disorder of unknown cause and is the most common deformity of the lower limb. It affects the foot and ankle, causing the foot to be twisted inward at a sharp angle, like the head of a golf club (Figure 8.21). Clubfoot has a frequency of about 1 out of every 1,000 births, and is twice as likely to occur in a male child as in a female child. In 50 percent of cases, both feet are affected.
Figure 8.21 Clubfoot Clubfoot is a common deformity of the ankle and foot that is present at birth. Most cases are corrected without surgery, and affected individuals will grow up to lead normal, active lives. (credit: James W. Hanson)
At birth, children with a clubfoot have the heel turned inward and the anterior foot twisted so that the lateral side of the foot is facing inferiorly, commonly due to ligaments or leg muscles attached to the foot that are shortened or abnormally tight. These pull the foot into an abnormal position, resulting in bone deformities. Other symptoms may include bending of the ankle that lifts the heel of the foot and an extremely high foot arch. Due to the limited range of motion in the affected foot, it is difficult to place the foot into the correct position. Additionally, the affected foot may be shorter than normal, and the calf muscles are usually underdeveloped on the affected side. Despite the appearance, this is not a painful condition for newborns. However, it must be treated early to avoid future pain and impaired walking ability.
Although the cause of clubfoot is idiopathic (unknown), evidence indicates that fetal position within the uterus is not a contributing factor. Genetic factors are involved, because clubfoot tends to run within families. Cigarette smoking during pregnancy has been linked to the development of clubfoot, particularly in families with a history of clubfoot.
Previously, clubfoot required extensive surgery. Today, 90 percent of cases are successfully treated without surgery using new corrective casting techniques. The best chance for a full recovery requires that clubfoot treatment begin during the first 2 weeks after birth. Corrective casting gently stretches the foot, which is followed by the application of a holding cast to keep the foot in the proper position. This stretching and casting is repeated weekly for several weeks. In severe cases, surgery may also be required, after which the foot typically remains in a cast for 6 to 8 weeks. After the cast is removed following either surgical or nonsurgical treatment, the child will be required to wear a brace part-time (at night) for up to 4 years. In addition, special exercises will be prescribed, and the child must also wear special shoes. Close monitoring by the parents and adherence to postoperative instructions are imperative in minimizing the risk of relapse.
Despite these difficulties, treatment for clubfoot is usually successful, and the child will grow up to lead a normal, active life. Numerous examples of individuals born with a clubfoot who went on to successful careers include Dudley Moore (comedian and actor), Damon Wayans (comedian and actor), Troy Aikman (three-time Super Bowl-winning quarterback), Kristi Yamaguchi (Olympic gold medalist in figure skating), Mia Hamm (two-time Olympic gold medalist in soccer), and Charles Woodson (Heisman trophy and Super Bowl winner).
Key Terms
- acetabulum
- large, cup-shaped cavity located on the lateral side of the hip bone; formed by the junction of the ilium, pubis, and ischium portions of the hip bone
- acromial end of the clavicle
- lateral end of the clavicle that articulates with the acromion of the scapula
- acromial process
- acromion of the scapula
- acromioclavicular joint
- articulation between the acromion of the scapula and the acromial end of the clavicle
- acromion
- flattened bony process that extends laterally from the scapular spine to form the bony tip of the shoulder
- adductor tubercle
- small, bony bump located on the superior aspect of the medial epicondyle of the femur
- anatomical neck
- line on the humerus located around the outside margin of the humeral head
- ankle joint
- joint that separates the leg and foot portions of the lower limb; formed by the articulations between the talus bone of the foot inferiorly, and the distal end of the tibia, medial malleolus of the tibia, and lateral malleolus of the fibula superiorly
- anterior border of the tibia
- narrow, anterior margin of the tibia that extends inferiorly from the tibial tuberosity
- anterior inferior iliac spine
- small, bony projection located on the anterior margin of the ilium, below the anterior superior iliac spine
- anterior sacroiliac ligament
- strong ligament between the sacrum and the ilium portions of the hip bone that supports the anterior side of the sacroiliac joint
- anterior superior iliac spine
- rounded, anterior end of the iliac crest
- apical ectodermal ridge
- enlarged ridge of ectoderm at the distal end of a limb bud that stimulates growth and elongation of the limb
- arcuate line of the ilium
- smooth ridge located at the inferior margin of the iliac fossa; forms the lateral portion of the pelvic brim
- arm
- region of the upper limb located between the shoulder and elbow joints; contains the humerus bone
- auricular surface of the ilium
- roughened area located on the posterior, medial side of the ilium of the hip bone; articulates with the auricular surface of the sacrum to form the sacroiliac joint
- base of the metatarsal bone
- expanded, proximal end of each metatarsal bone
- bicipital groove
- intertubercular groove; narrow groove located between the greater and lesser tubercles of the humerus
- calcaneus
- heel bone; posterior, inferior tarsal bone that forms the heel of the foot
- capitate
- from the lateral side, the third of the four distal carpal bones; articulates with the scaphoid and lunate proximally, the trapezoid laterally, the hamate medially, and primarily with the third metacarpal distally
- capitulum
- knob-like bony structure located anteriorly on the lateral, distal end of the humerus
- carpal bone
- one of the eight small bones that form the wrist and base of the hand; these are grouped as a proximal row consisting of (from lateral to medial) the scaphoid, lunate, triquetrum, and pisiform bones, and a distal row containing (from lateral to medial) the trapezium, trapezoid, capitate, and hamate bones
- carpal tunnel
- passageway between the anterior forearm and hand formed by the carpal bones and flexor retinaculum
- carpometacarpal joint
- articulation between one of the carpal bones in the distal row and a metacarpal bone of the hand
- clavicle
- collarbone; elongated bone that articulates with the manubrium of the sternum medially and the acromion of the scapula laterally
- coracoclavicular ligament
- strong band of connective tissue that anchors the coracoid process of the scapula to the lateral clavicle; provides important indirect support for the acromioclavicular joint
- coracoid process
- short, hook-like process that projects anteriorly and laterally from the superior margin of the scapula
- coronoid fossa
- depression on the anterior surface of the humerus above the trochlea; this space receives the coronoid process of the ulna when the elbow is maximally flexed
- coronoid process of the ulna
- projecting bony lip located on the anterior, proximal ulna; forms the inferior margin of the trochlear notch
- costoclavicular ligament
- band of connective tissue that unites the medial clavicle with the first rib
- coxal bone
- hip bone
- cuboid
- tarsal bone that articulates posteriorly with the calcaneus bone, medially with the lateral cuneiform bone, and anteriorly with the fourth and fifth metatarsal bones
- deltoid tuberosity
- roughened, V-shaped region located laterally on the mid-shaft of the humerus
- distal radioulnar joint
- articulation between the head of the ulna and the ulnar notch of the radius
- distal tibiofibular joint
- articulation between the distal fibula and the fibular notch of the tibia
- elbow joint
- joint located between the upper arm and forearm regions of the upper limb; formed by the articulations between the trochlea of the humerus and the trochlear notch of the ulna, and the capitulum of the humerus and the head of the radius
- femur
- thigh bone; the single bone of the thigh
- fibula
- thin, non-weight-bearing bone found on the lateral side of the leg
- fibular notch
- wide groove on the lateral side of the distal tibia for articulation with the fibula at the distal tibiofibular joint
- flexor retinaculum
- strong band of connective tissue at the anterior wrist that spans the top of the U-shaped grouping of the carpal bones to form the roof of the carpal tunnel
- foot
- portion of the lower limb located distal to the ankle joint
- forearm
- region of the upper limb located between the elbow and wrist joints; contains the radius and ulna bones
- fossa
- (plural = fossae) shallow depression on the surface of a bone
- fovea capitis
- minor indentation on the head of the femur that serves as the site of attachment for the ligament to the head of the femur
- glenohumeral joint
- shoulder joint; formed by the articulation between the glenoid cavity of the scapula and the head of the humerus
- glenoid cavity
- (also, glenoid fossa) shallow depression located on the lateral scapula, between the superior and lateral borders
- gluteal tuberosity
- roughened area on the posterior side of the proximal femur, extending inferiorly from the base of the greater trochanter
- greater pelvis
- (also, greater pelvic cavity or false pelvis) broad space above the pelvic brim defined laterally by the fan-like portion of the upper ilium
- greater sciatic foramen
- pelvic opening formed by the greater sciatic notch of the hip bone, the sacrum, and the sacrospinous ligament
- greater sciatic notch
- large, U-shaped indentation located on the posterior margin of the ilium, superior to the ischial spine
- greater trochanter
- large, bony expansion of the femur that projects superiorly from the base of the femoral neck
- greater tubercle
- enlarged prominence located on the lateral side of the proximal humerus
- hallux
- big toe; digit 1 of the foot
- hamate
- from the lateral side, the fourth of the four distal carpal bones; articulates with the lunate and triquetrum proximally, the fourth and fifth metacarpals distally, and the capitate laterally
- hand
- region of the upper limb distal to the wrist joint
- head of the femur
- rounded, proximal end of the femur that articulates with the acetabulum of the hip bone to form the hip joint
- head of the fibula
- small, knob-like, proximal end of the fibula; articulates with the inferior aspect of the lateral condyle of the tibia
- head of the humerus
- smooth, rounded region on the medial side of the proximal humerus; articulates with the glenoid fossa of the scapula to form the glenohumeral (shoulder) joint
- head of the metatarsal bone
- expanded, distal end of each metatarsal bone
- head of the radius
- disc-shaped structure that forms the proximal end of the radius; articulates with the capitulum of the humerus as part of the elbow joint, and with the radial notch of the ulna as part of the proximal radioulnar joint
- head of the ulna
- small, rounded distal end of the ulna; articulates with the ulnar notch of the distal radius, forming the distal radioulnar joint
- hip bone
- coxal bone; single bone that forms the pelvic girdle; consists of three areas, the ilium, ischium, and pubis
- hip joint
- joint located at the proximal end of the lower limb; formed by the articulation between the acetabulum of the hip bone and the head of the femur
- hook of the hamate bone
- bony extension located on the anterior side of the hamate carpal bone
- humerus
- single bone of the upper arm
- iliac crest
- curved, superior margin of the ilium
- iliac fossa
- shallow depression found on the anterior and medial surfaces of the upper ilium
- ilium
- superior portion of the hip bone
- inferior angle of the scapula
- inferior corner of the scapula located where the medial and lateral borders meet
- inferior pubic ramus
- narrow segment of bone that passes inferiorly and laterally from the pubic body; joins with the ischial ramus to form the ischiopubic ramus
- infraglenoid tubercle
- small bump or roughened area located on the lateral border of the scapula, near the inferior margin of the glenoid cavity
- infraspinous fossa
- broad depression located on the posterior scapula, inferior to the spine
- intercondylar eminence
- irregular elevation on the superior end of the tibia, between the articulating surfaces of the medial and lateral condyles
- intercondylar fossa
- deep depression on the posterior side of the distal femur that separates the medial and lateral condyles
- intermediate cuneiform
- middle of the three cuneiform tarsal bones; articulates posteriorly with the navicular bone, medially with the medial cuneiform bone, laterally with the lateral cuneiform bone, and anteriorly with the second metatarsal bone
- interosseous border of the fibula
- small ridge running down the medial side of the fibular shaft; for attachment of the interosseous membrane between the fibula and tibia
- interosseous border of the radius
- narrow ridge located on the medial side of the radial shaft; for attachment of the interosseous membrane between the ulna and radius bones
- interosseous border of the tibia
- small ridge running down the lateral side of the tibial shaft; for attachment of the interosseous membrane between the tibia and fibula
- interosseous border of the ulna
- narrow ridge located on the lateral side of the ulnar shaft; for attachment of the interosseous membrane between the ulna and radius
- interosseous membrane of the forearm
- sheet of dense connective tissue that unites the radius and ulna bones
- interosseous membrane of the leg
- sheet of dense connective tissue that unites the shafts of the tibia and fibula bones
- interphalangeal joint
- articulation between adjacent phalanx bones of the hand or foot digits
- intertrochanteric crest
- short, prominent ridge running between the greater and lesser trochanters on the posterior side of the proximal femur
- intertrochanteric line
- small ridge running between the greater and lesser trochanters on the anterior side of the proximal femur
- intertubercular groove (sulcus)
- bicipital groove; narrow groove located between the greater and lesser tubercles of the humerus
- ischial ramus
- bony extension projecting anteriorly and superiorly from the ischial tuberosity; joins with the inferior pubic ramus to form the ischiopubic ramus
- ischial spine
- pointed, bony projection from the posterior margin of the ischium that separates the greater sciatic notch and lesser sciatic notch
- ischial tuberosity
- large, roughened protuberance that forms the posteroinferior portion of the hip bone; weight-bearing region of the pelvis when sitting
- ischiopubic ramus
- narrow extension of bone that connects the ischial tuberosity to the pubic body; formed by the junction of the ischial ramus and inferior pubic ramus
- ischium
- posteroinferior portion of the hip bone
- knee joint
- joint that separates the thigh and leg portions of the lower limb; formed by the articulations between the medial and lateral condyles of the femur, and the medial and lateral condyles of the tibia
- lateral border of the scapula
- diagonally oriented lateral margin of the scapula
- lateral condyle of the femur
- smooth, articulating surface that forms the distal and posterior sides of the lateral expansion of the distal femur
- lateral condyle of the tibia
- lateral, expanded region of the proximal tibia that includes the smooth surface that articulates with the lateral condyle of the femur as part of the knee joint
- lateral cuneiform
- most lateral of the three cuneiform tarsal bones; articulates posteriorly with the navicular bone, medially with the intermediate cuneiform bone, laterally with the cuboid bone, and anteriorly with the third metatarsal bone
- lateral epicondyle of the femur
- roughened area of the femur located on the lateral side of the lateral condyle
- lateral epicondyle of the humerus
- small projection located on the lateral side of the distal humerus
- lateral malleolus
- expanded distal end of the fibula
- lateral supracondylar ridge
- narrow, bony ridge located along the lateral side of the distal humerus, superior to the lateral epicondyle
- leg
- portion of the lower limb located between the knee and ankle joints
- lesser pelvis
- (also, lesser pelvic cavity or true pelvis) narrow space located within the pelvis, defined superiorly by the pelvic brim (pelvic inlet) and inferiorly by the pelvic outlet
- lesser sciatic foramen
- pelvic opening formed by the lesser sciatic notch of the hip bone, the sacrospinous ligament, and the sacrotuberous ligament
- lesser sciatic notch
- shallow indentation along the posterior margin of the ischium, inferior to the ischial spine
- lesser trochanter
- small, bony projection on the medial side of the proximal femur, at the base of the femoral neck
- lesser tubercle
- small, bony prominence located on anterior side of the proximal humerus
- ligament of the head of the femur
- ligament that spans the acetabulum of the hip bone and the fovea capitis of the femoral head
- limb bud
- small elevation that appears on the lateral side of the embryo during the fourth or fifth week of development, which gives rise to an upper or lower limb
- linea aspera
- longitudinally running bony ridge located in the middle third of the posterior femur
- lunate
- from the lateral side, the second of the four proximal carpal bones; articulates with the radius proximally, the capitate and hamate distally, the scaphoid laterally, and the triquetrum medially
- medial border of the scapula
- elongated, medial margin of the scapula
- medial condyle of the femur
- smooth, articulating surface that forms the distal and posterior sides of the medial expansion of the distal femur
- medial condyle of the tibia
- medial, expanded region of the proximal tibia that includes the smooth surface that articulates with the medial condyle of the femur as part of the knee joint
- medial cuneiform
- most medial of the three cuneiform tarsal bones; articulates posteriorly with the navicular bone, laterally with the intermediate cuneiform bone, and anteriorly with the first and second metatarsal bones
- medial epicondyle of the femur
- roughened area of the distal femur located on the medial side of the medial condyle
- medial epicondyle of the humerus
- enlarged projection located on the medial side of the distal humerus
- medial malleolus
- bony expansion located on the medial side of the distal tibia
- metacarpal bone
- one of the five long bones that form the palm of the hand; numbered 1–5, starting on the lateral (thumb) side of the hand
- metacarpophalangeal joint
- articulation between the distal end of a metacarpal bone of the hand and a proximal phalanx bone of the thumb or a finger
- metatarsal bone
- one of the five elongated bones that forms the anterior half of the foot; numbered 1–5, starting on the medial side of the foot
- metatarsophalangeal joint
- articulation between a metatarsal bone of the foot and the proximal phalanx bone of a toe
- midcarpal joint
- articulation between the proximal and distal rows of the carpal bones; contributes to movements of the hand at the wrist
- navicular
- tarsal bone that articulates posteriorly with the talus bone, laterally with the cuboid bone, and anteriorly with the medial, intermediate, and lateral cuneiform bones
- neck of the femur
- narrowed region located inferior to the head of the femur
- neck of the radius
- narrowed region immediately distal to the head of the radius
- obturator foramen
- large opening located in the anterior hip bone, between the pubis and ischium regions
- olecranon fossa
- large depression located on the posterior side of the distal humerus; this space receives the olecranon process of the ulna when the elbow is fully extended
- olecranon process
- expanded posterior and superior portions of the proximal ulna; forms the bony tip of the elbow
- patella
- kneecap; the largest sesamoid bone of the body; articulates with the distal femur
- patellar surface
- smooth groove located on the anterior side of the distal femur, between the medial and lateral condyles; site of articulation for the patella
- pectineal line
- narrow ridge located on the superior surface of the superior pubic ramus
- pectoral girdle
- shoulder girdle; the set of bones, consisting of the scapula and clavicle, which attaches each upper limb to the axial skeleton
- pelvic brim
- pelvic inlet; the dividing line between the greater and lesser pelvic regions; formed by the superior margin of the pubic symphysis, the pectineal lines of each pubis, the arcuate lines of each ilium, and the sacral promontory
- pelvic girdle
- hip girdle; consists of a single hip bone, which attaches a lower limb to the sacrum of the axial skeleton
- pelvic inlet
- pelvic brim
- pelvic outlet
- inferior opening of the lesser pelvis; formed by the inferior margin of the pubic symphysis, right and left ischiopubic rami and sacrotuberous ligaments, and the tip of the coccyx
- pelvis
- ring of bone consisting of the right and left hip bones, the sacrum, and the coccyx
- phalanx bone of the foot
- (plural = phalanges) one of the 14 bones that form the toes; these include the proximal and distal phalanges of the big toe, and the proximal, middle, and distal phalanx bones of toes two through five
- phalanx bone of the hand
- (plural = phalanges) one of the 14 bones that form the thumb and fingers; these include the proximal and distal phalanges of the thumb, and the proximal, middle, and distal phalanx bones of the fingers two through five
- pisiform
- from the lateral side, the fourth of the four proximal carpal bones; articulates with the anterior surface of the triquetrum
- pollex
- (also, thumb) digit 1 of the hand
- posterior inferior iliac spine
- small, bony projection located at the inferior margin of the auricular surface on the posterior ilium
- posterior sacroiliac ligament
- strong ligament spanning the sacrum and ilium of the hip bone that supports the posterior side of the sacroiliac joint
- posterior superior iliac spine
- rounded, posterior end of the iliac crest
- proximal radioulnar joint
- articulation formed by the radial notch of the ulna and the head of the radius
- proximal tibiofibular joint
- articulation between the head of the fibula and the inferior aspect of the lateral condyle of the tibia
- pubic arch
- bony structure formed by the pubic symphysis, and the bodies and inferior pubic rami of the right and left pubic bones
- pubic body
- enlarged, medial portion of the pubis region of the hip bone
- pubic symphysis
- joint formed by the articulation between the pubic bodies of the right and left hip bones
- pubic tubercle
- small bump located on the superior aspect of the pubic body
- pubis
- anterior portion of the hip bone
- radial fossa
- small depression located on the anterior humerus above the capitulum; this space receives the head of the radius when the elbow is maximally flexed
- radial notch of the ulna
- small, smooth area on the lateral side of the proximal ulna; articulates with the head of the radius as part of the proximal radioulnar joint
- radial tuberosity
- oval-shaped, roughened protuberance located on the medial side of the proximal radius
- radiocarpal joint
- wrist joint, located between the forearm and hand regions of the upper limb; articulation formed proximally by the distal end of the radius and the fibrocartilaginous pad that unites the distal radius and ulna bone, and distally by the scaphoid, lunate, and triquetrum carpal bones
- radius
- bone located on the lateral side of the forearm
- sacroiliac joint
- joint formed by the articulation between the auricular surfaces of the sacrum and ilium
- sacrospinous ligament
- ligament that spans the sacrum to the ischial spine of the hip bone
- sacrotuberous ligament
- ligament that spans the sacrum to the ischial tuberosity of the hip bone
- scaphoid
- from the lateral side, the first of the four proximal carpal bones; articulates with the radius proximally, the trapezoid, trapezium, and capitate distally, and the lunate medially
- scapula
- shoulder blade bone located on the posterior side of the shoulder
- shaft of the femur
- cylindrically shaped region that forms the central portion of the femur
- shaft of the fibula
- elongated, slender portion located between the expanded ends of the fibula
- shaft of the humerus
- narrow, elongated, central region of the humerus
- shaft of the radius
- narrow, elongated, central region of the radius
- shaft of the tibia
- triangular-shaped, central portion of the tibia
- shaft of the ulna
- narrow, elongated, central region of the ulna
- soleal line
- small, diagonally running ridge located on the posterior side of the proximal tibia
- spine of the scapula
- prominent ridge passing mediolaterally across the upper portion of the posterior scapular surface
- sternal end of the clavicle
- medial end of the clavicle that articulates with the manubrium of the sternum
- sternoclavicular joint
- articulation between the manubrium of the sternum and the sternal end of the clavicle; forms the only bony attachment between the pectoral girdle of the upper limb and the axial skeleton
- styloid process of the radius
- pointed projection located on the lateral end of the distal radius
- styloid process of the ulna
- short, bony projection located on the medial end of the distal ulna
- subpubic angle
- inverted V-shape formed by the convergence of the right and left ischiopubic rami; this angle is greater than 80 degrees in females and less than 70 degrees in males
- subscapular fossa
- broad depression located on the anterior (deep) surface of the scapula
- superior angle of the scapula
- corner of the scapula between the superior and medial borders of the scapula
- superior border of the scapula
- superior margin of the scapula
- superior pubic ramus
- narrow segment of bone that passes laterally from the pubic body to join the ilium
- supraglenoid tubercle
- small bump located at the superior margin of the glenoid cavity
- suprascapular notch
- small notch located along the superior border of the scapula, medial to the coracoid process
- supraspinous fossa
- narrow depression located on the posterior scapula, superior to the spine
- surgical neck
- region of the humerus where the expanded, proximal end joins with the narrower shaft
- sustentaculum tali
- bony ledge extending from the medial side of the calcaneus bone
- talus
- tarsal bone that articulates superiorly with the tibia and fibula at the ankle joint; also articulates inferiorly with the calcaneus bone and anteriorly with the navicular bone
- tarsal bone
- one of the seven bones that make up the posterior foot; includes the calcaneus, talus, navicular, cuboid, medial cuneiform, intermediate cuneiform, and lateral cuneiform bones
- thigh
- portion of the lower limb located between the hip and knee joints
- tibia
- shin bone; the large, weight-bearing bone located on the medial side of the leg
- tibial tuberosity
- elevated area on the anterior surface of the proximal tibia
- trapezium
- from the lateral side, the first of the four distal carpal bones; articulates with the scaphoid proximally, the first and second metacarpals distally, and the trapezoid medially
- trapezoid
- from the lateral side, the second of the four distal carpal bones; articulates with the scaphoid proximally, the second metacarpal distally, the trapezium laterally, and the capitate medially
- triquetrum
- from the lateral side, the third of the four proximal carpal bones; articulates with the lunate laterally, the hamate distally, and has a facet for the pisiform
- trochlea
- pulley-shaped region located medially at the distal end of the humerus; articulates at the elbow with the trochlear notch of the ulna
- trochlear notch
- large, C-shaped depression located on the anterior side of the proximal ulna; articulates at the elbow with the trochlea of the humerus
- ulna
- bone located on the medial side of the forearm
- ulnar notch of the radius
- shallow, smooth area located on the medial side of the distal radius; articulates with the head of the ulna at the distal radioulnar joint
- ulnar tuberosity
- roughened area located on the anterior, proximal ulna inferior to the coronoid process
Chapter Review
8.1 The Pectoral Girdle
The pectoral girdle, consisting of the clavicle and the scapula, attaches each upper limb to the axial skeleton. The clavicle is an anterior bone whose sternal end articulates with the manubrium of the sternum at the sternoclavicular joint. The sternal end is also anchored to the first rib by the costoclavicular ligament. The acromial end of the clavicle articulates with the acromion of the scapula at the acromioclavicular joint. This end is also anchored to the coracoid process of the scapula by the coracoclavicular ligament, which provides indirect support for the acromioclavicular joint. The clavicle supports the scapula, transmits the weight and forces from the upper limb to the body trunk, and protects the underlying nerves and blood vessels.
The scapula lies on the posterior aspect of the pectoral girdle. It mediates the attachment of the upper limb to the clavicle, and contributes to the formation of the glenohumeral (shoulder) joint. This triangular bone has three sides called the medial, lateral, and superior borders. The suprascapular notch is located on the superior border. The scapula also has three corners, two of which are the superior and inferior angles. The third corner is occupied by the glenoid cavity. Posteriorly, the spine separates the supraspinous and infraspinous fossae, and then extends laterally as the acromion. The subscapular fossa is located on the anterior surface of the scapula. The coracoid process projects anteriorly, passing inferior to the lateral end of the clavicle.
8.2 Bones of the Upper Limb
Each upper limb is divided into three regions and contains a total of 30 bones. The upper arm is the region located between the shoulder and elbow joints. This area contains the humerus. The proximal humerus consists of the head, which articulates with the scapula at the glenohumeral joint, the greater and lesser tubercles separated by the intertubercular (bicipital) groove, and the anatomical and surgical necks. The humeral shaft has the roughened area of the deltoid tuberosity on its lateral side. The distal humerus is flattened, forming a lateral supracondylar ridge that terminates at the small lateral epicondyle. The medial side of the distal humerus has the large, medial epicondyle. The articulating surfaces of the distal humerus consist of the trochlea medially and the capitulum laterally. Depressions on the humerus that accommodate the forearm bones during bending (flexing) and straightening (extending) of the elbow include the coronoid fossa, the radial fossa, and the olecranon fossa.
The forearm is the region of the upper limb located between the elbow and wrist joints. This region contains two bones, the ulna medially and the radius on the lateral (thumb) side. The elbow joint is formed by the articulation between the trochlea of the humerus and the trochlear notch of the ulna, plus the articulation between the capitulum of the humerus and the head of the radius. The proximal radioulnar joint is the articulation between the head of the radius and the radial notch of the ulna. The proximal ulna also has the olecranon process, forming an expanded posterior region, and the coronoid process and ulnar tuberosity on its anterior aspect. On the proximal radius, the narrowed region below the head is the neck; distal to this is the radial tuberosity. The shaft portions of both the ulna and radius have an interosseous border, whereas the distal ends of each bone have a pointed styloid process. The distal radioulnar joint is found between the head of the ulna and the ulnar notch of the radius. The distal end of the radius articulates with the proximal carpal bones, but the ulna does not.
The base of the hand is formed by eight carpal bones. The carpal bones are united into two rows of bones. The proximal row contains (from lateral to medial) the scaphoid, lunate, triquetrum, and pisiform bones. The scaphoid, lunate, and triquetrum bones contribute to the formation of the radiocarpal joint. The distal row of carpal bones contains (from medial to lateral) the hamate, capitate, trapezoid, and trapezium bones (“So Long To Pinky, Here Comes The Thumb”). The anterior hamate has a prominent bony hook. The proximal and distal carpal rows articulate with each other at the midcarpal joint. The carpal bones, together with the flexor retinaculum, also form the carpal tunnel of the wrist.
The five metacarpal bones form the palm of the hand. The metacarpal bones are numbered 1–5, starting with the thumb side. The first metacarpal bone is freely mobile, but the other bones are united as a group. The digits are also numbered 1–5, with the thumb being number 1. The fingers and thumb contain a total of 14 phalanges (phalanx bones). The thumb contains a proximal and a distal phalanx, whereas the remaining digits each contain proximal, middle, and distal phalanges.
8.3 The Pelvic Girdle and Pelvis
The pelvic girdle, consisting of a hip bone, serves to attach a lower limb to the axial skeleton. The hip bone articulates posteriorly at the sacroiliac joint with the sacrum, which is part of the axial skeleton. The right and left hip bones converge anteriorly and articulate with each other at the pubic symphysis. The combination of the hip bone, the sacrum, and the coccyx forms the pelvis. The pelvis has a pronounced anterior tilt. The primary function of the pelvis is to support the upper body and transfer body weight to the lower limbs. It also serves as the site of attachment for multiple muscles.
The hip bone consists of three regions: the ilium, ischium, and pubis. The ilium forms the large, fan-like region of the hip bone. The superior margin of this area is the iliac crest. Located at either end of the iliac crest are the anterior superior and posterior superior iliac spines. Inferior to these are the anterior inferior and posterior inferior iliac spines. The auricular surface of the ilium articulates with the sacrum to form the sacroiliac joint. The medial surface of the upper ilium forms the iliac fossa, with the arcuate line marking the inferior limit of this area. The posterior margin of the ilium has the large greater sciatic notch.
The posterolateral portion of the hip bone is the ischium. It has the expanded ischial tuberosity, which supports body weight when sitting. The ischial ramus projects anteriorly and superiorly. The posterior margin of the ischium has the shallow lesser sciatic notch and the ischial spine, which separates the greater and lesser sciatic notches.
The pubis forms the anterior portion of the hip bone. The body of the pubis articulates with the pubis of the opposite hip bone at the pubic symphysis. The superior margin of the pubic body has the pubic tubercle. The pubis is joined to the ilium by the superior pubic ramus, the superior surface of which forms the pectineal line. The inferior pubic ramus projects inferiorly and laterally. The pubic arch is formed by the pubic symphysis, the bodies of the adjacent pubic bones, and the two inferior pubic rami. The inferior pubic ramus joins the ischial ramus to form the ischiopubic ramus. The subpubic angle is formed by the medial convergence of the right and left ischiopubic rami.
The lateral side of the hip bone has the cup-like acetabulum, which is part of the hip joint. The large anterior opening is the obturator foramen. The sacroiliac joint is supported by the anterior and posterior sacroiliac ligaments. The sacrum is also joined to the hip bone by the sacrospinous ligament, which attaches to the ischial spine, and the sacrotuberous ligament, which attaches to the ischial tuberosity. The sacrospinous and sacrotuberous ligaments contribute to the formation of the greater and lesser sciatic foramina.
The broad space of the upper pelvis is the greater pelvis, and the narrow, inferior space is the lesser pelvis. These areas are separated by the pelvic brim (pelvic inlet). The inferior opening of the pelvis is the pelvic outlet. Compared to the male, the female pelvis is wider to accommodate childbirth, has a larger subpubic angle, and a broader greater sciatic notch.
8.4 Bones of the Lower Limb
The lower limb is divided into three regions. These are the thigh, located between the hip and knee joints; the leg, located between the knee and ankle joints; and distal to the ankle, the foot. There are 30 bones in each lower limb. These are the femur, patella, tibia, fibula, seven tarsal bones, five metatarsal bones, and 14 phalanges.
The femur is the single bone of the thigh. Its rounded head articulates with the acetabulum of the hip bone to form the hip joint. The head has the fovea capitis for attachment of the ligament of the head of the femur. The narrow neck joins inferiorly with the greater and lesser trochanters. Passing between these bony expansions are the intertrochanteric line on the anterior femur and the larger intertrochanteric crest on the posterior femur. On the posterior shaft of the femur is the gluteal tuberosity proximally and the linea aspera in the mid-shaft region. The expanded distal end consists of three articulating surfaces: the medial and lateral condyles, and the patellar surface. The outside margins of the condyles are the medial and lateral epicondyles. The adductor tubercle is on the superior aspect of the medial epicondyle.
The patella is a sesamoid bone located within a muscle tendon. It articulates with the patellar surface on the anterior side of the distal femur, thereby protecting the muscle tendon from rubbing against the femur.
The leg contains the large tibia on the medial side and the slender fibula on the lateral side. The tibia bears the weight of the body, whereas the fibula does not bear weight. The interosseous border of each bone is the attachment site for the interosseous membrane of the leg, the connective tissue sheet that unites the tibia and fibula.
The proximal tibia consists of the expanded medial and lateral condyles, which articulate with the medial and lateral condyles of the femur to form the knee joint. Between the tibial condyles is the intercondylar eminence. On the anterior side of the proximal tibia is the tibial tuberosity, which is continuous inferiorly with the anterior border of the tibia. On the posterior side, the proximal tibia has the curved soleal line. The bony expansion on the medial side of the distal tibia is the medial malleolus. The groove on the lateral side of the distal tibia is the fibular notch.
The head of the fibula forms the proximal end and articulates with the underside of the lateral condyle of the tibia. The distal fibula articulates with the fibular notch of the tibia. The expanded distal end of the fibula is the lateral malleolus.
The posterior foot is formed by the seven tarsal bones. The talus articulates superiorly with the distal tibia, the medial malleolus of the tibia, and the lateral malleolus of the fibula to form the ankle joint. The talus articulates inferiorly with the calcaneus bone. The sustentaculum tali of the calcaneus helps to support the talus. Anterior to the talus is the navicular bone, and anterior to this are the medial, intermediate, and lateral cuneiform bones. The cuboid bone is anterior to the calcaneus.
The five metatarsal bones form the anterior foot. The base of these bones articulate with the cuboid or cuneiform bones. The metatarsal heads, at their distal ends, articulate with the proximal phalanges of the toes. The big toe (toe number 1) has proximal and distal phalanx bones. The remaining toes have proximal, middle, and distal phalanges.
8.5 Development of the Appendicular Skeleton
The bones of the appendicular skeleton arise from embryonic mesenchyme. Limb buds appear at the end of the fourth week. The apical ectodermal ridge, located at the end of the limb bud, stimulates growth and elongation of the limb. During the sixth week, the distal end of the limb bud becomes paddle-shaped, and selective cell death separates the developing fingers and toes. At the same time, mesenchyme within the limb bud begins to differentiate into hyaline cartilage, forming models for future bones. During the seventh week, the upper limbs rotate laterally and the lower limbs rotate medially, bringing the limbs into their final positions.
Endochondral ossification, the process that converts the hyaline cartilage model into bone, begins in most appendicular bones by the twelfth fetal week. This begins as a primary ossification center in the diaphysis, followed by the later appearance of one or more secondary ossifications centers in the regions of the epiphyses. Each secondary ossification center is separated from the primary ossification center by an epiphyseal plate. Continued growth of the epiphyseal plate cartilage provides for bone lengthening. Disappearance of the epiphyseal plate is followed by fusion of the bony components to form a single, adult bone.
The clavicle develops via intramembranous ossification, in which mesenchyme is converted directly into bone tissue. Ossification within the clavicle begins during the fifth week of development and continues until 25 years of age.
Interactive Link Questions
Watch this video to see how fractures of the distal radius bone can affect the wrist joint. Explain the problems that may occur if a fracture of the distal radius involves the joint surface of the radiocarpal joint of the wrist.
2.Visit this site to explore the bones and joints of the hand. What are the three arches of the hand, and what is the importance of these during the gripping of an object?
3.Watch this video to learn about a Colles fracture, a break of the distal radius, usually caused by falling onto an outstretched hand. When would surgery be required and how would the fracture be repaired in this case?
4.Watch this video for a 3-D view of the pelvis and its associated ligaments. What is the large opening in the bony pelvis, located between the ischium and pubic regions, and what two parts of the pubis contribute to the formation of this opening?
5.Watch this video to view how a fracture of the mid-femur is surgically repaired. How are the two portions of the broken femur stabilized during surgical repair of a fractured femur?
6.Visit this site to perform a virtual knee replacement surgery. The prosthetic knee components must be properly aligned to function properly. How is this alignment ensured?
7.Use this tutorial to review the bones of the foot. Which tarsal bones are in the proximal, intermediate, and distal groups?
8.View this link to learn about a bunion, a localized swelling on the medial side of the foot, next to the first metatarsophalangeal joint, at the base of the big toe. What is a bunion and what type of shoe is most likely to cause this to develop?
9.Watch this animation to follow the development and growth of the upper and lower limb buds. On what days of embryonic development do these events occur: (a) first appearance of the upper limb bud (limb ridge); (b) the flattening of the distal limb to form the handplate or footplate; and (c) the beginning of limb rotation?
Review Questions
Which part of the clavicle articulates with the manubrium?
- shaft
- sternal end
- acromial end
- coracoid process
A shoulder separation results from injury to the ________.
- glenohumeral joint
- costoclavicular joint
- acromioclavicular joint
- sternoclavicular joint
Which feature lies between the spine and superior border of the scapula?
- suprascapular notch
- glenoid cavity
- superior angle
- supraspinous fossa
What structure is an extension of the spine of the scapula?
- acromion
- coracoid process
- supraglenoid tubercle
- glenoid cavity
Name the short, hook-like bony process of the scapula that projects anteriorly.
- acromial process
- clavicle
- coracoid process
- glenoid fossa
How many bones are there in the upper limbs combined?
- 20
- 30
- 40
- 60
Which bony landmark is located on the lateral side of the proximal humerus?
- greater tubercle
- trochlea
- lateral epicondyle
- lesser tubercle
Which region of the humerus articulates with the radius as part of the elbow joint?
- trochlea
- styloid process
- capitulum
- olecranon process
Which is the lateral-most carpal bone of the proximal row?
- trapezium
- hamate
- pisiform
- scaphoid
The radius bone ________.
- is found on the medial side of the forearm
- has a head that articulates with the radial notch of the ulna
- does not articulate with any of the carpal bones
- has the radial tuberosity located near its distal end
How many bones fuse in adulthood to form the hip bone?
- 2
- 3
- 4
- 5
Which component forms the superior part of the hip bone?
- ilium
- pubis
- ischium
- sacrum
Which of the following supports body weight when sitting?
- iliac crest
- ischial tuberosity
- ischiopubic ramus
- pubic body
The ischial spine is found between which of the following structures?
- inferior pubic ramus and ischial ramus
- pectineal line and arcuate line
- lesser sciatic notch and greater sciatic notch
- anterior superior iliac spine and posterior superior iliac spine
The pelvis ________.
- has a subpubic angle that is larger in females
- consists of the two hip bones, but does not include the sacrum or coccyx
- has an obturator foramen, an opening that is defined in part by the sacrospinous and sacrotuberous ligaments
- has a space located inferior to the pelvic brim called the greater pelvis
Which bony landmark of the femur serves as a site for muscle attachments?
- fovea capitis
- lesser trochanter
- head
- medial condyle
What structure contributes to the knee joint?
- lateral malleolus of the fibula
- tibial tuberosity
- medial condyle of the tibia
- lateral epicondyle of the femur
Which tarsal bone articulates with the tibia and fibula?
- calcaneus
- cuboid
- navicular
- talus
What is the total number of bones found in the foot and toes?
- 7
- 14
- 26
- 30
The tibia ________.
- has an expanded distal end called the lateral malleolus
- is not a weight-bearing bone
- is firmly anchored to the fibula by an interosseous membrane
- can be palpated (felt) under the skin only at its proximal and distal ends
Which event takes place during the seventh week of development?
- appearance of the upper and lower limb buds
- flattening of the distal limb bud into a paddle shape
- the first appearance of hyaline cartilage models of future bones
- the rotation of the limbs
During endochondral ossification of a long bone, ________.
- a primary ossification center will develop within the epiphysis
- mesenchyme will differentiate directly into bone tissue
- growth of the epiphyseal plate will produce bone lengthening
- all epiphyseal plates will disappear before birth
The clavicle ________.
- develops via intramembranous ossification
- develops via endochondral ossification
- is the last bone of the body to begin ossification
- is fully ossified at the time of birth
Critical Thinking Questions
Describe the shape and palpable line formed by the clavicle and scapula.
34.Discuss two possible injuries of the pectoral girdle that may occur following a strong blow to the shoulder or a hard fall onto an outstretched hand.
35.Your friend runs out of gas and you have to help push his car. Discuss the sequence of bones and joints that convey the forces passing from your hand, through your upper limb and your pectoral girdle, and to your axial skeleton.
36.Name the bones in the wrist and hand, and describe or sketch out their locations and articulations.
37.Describe the articulations and ligaments that unite the four bones of the pelvis to each other.
38.Discuss the ways in which the female pelvis is adapted for childbirth.
39.Define the regions of the lower limb, name the bones found in each region, and describe the bony landmarks that articulate together to form the hip, knee, and ankle joints.
40.The talus bone of the foot receives the weight of the body from the tibia. The talus bone then distributes this weight toward the ground in two directions: one-half of the body weight is passed in a posterior direction and one-half of the weight is passed in an anterior direction. Describe the arrangement of the tarsal and metatarsal bones that are involved in both the posterior and anterior distribution of body weight.
41.How can a radiograph of a child’s femur be used to determine the approximate age of that child?
42.How does the development of the clavicle differ from the development of other appendicular skeleton bones?
|
oercommons
|
2025-03-18T00:39:11.896113
|
07/23/2019
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/56367/overview",
"title": "Anatomy and Physiology, Support and Movement, The Appendicular Skeleton",
"author": null
}
|
https://oercommons.org/courseware/lesson/58767/overview
|
The Lymphatic and Immune System
Introduction
Figure 21.1 The Worldwide AIDS Epidemic (a) As of 2008, more than 15 percent of adults were infected with HIV in certain African countries. This grim picture had changed little by 2012. (b) In this scanning electron micrograph, HIV virions (green particles) are budding off the surface of a macrophage (pink structure). (credit b: C. Goldsmith)
CHAPTER OBJECTIVES
After studying this chapter, you will be able to:
- Identify the components and anatomy of the lymphatic system
- Discuss the role of the innate immune response against pathogens
- Describe the power of the adaptive immune response to cure disease
- Explain immunological deficiencies and over-reactions of the immune system
- Discuss the role of the immune response in transplantation and cancer
- Describe the interaction of the immune and lymphatic systems with other body systems
In June 1981, the Centers for Disease Control and Prevention (CDC), in Atlanta, Georgia, published a report of an unusual cluster of five patients in Los Angeles, California. All five were diagnosed with a rare pneumonia caused by a fungus called Pneumocystis jirovecii (formerly known as Pneumocystis carinii).
Why was this unusual? Although commonly found in the lungs of healthy individuals, this fungus is an opportunistic pathogen that causes disease in individuals with suppressed or underdeveloped immune systems. The very young, whose immune systems have yet to mature, and the elderly, whose immune systems have declined with age, are particularly susceptible. The five patients from LA, though, were between 29 and 36 years of age and should have been in the prime of their lives, immunologically speaking. What could be going on?
A few days later, a cluster of eight cases was reported in New York City, also involving young patients, this time exhibiting a rare form of skin cancer known as Kaposi’s sarcoma. This cancer of the cells that line the blood and lymphatic vessels was previously observed as a relatively innocuous disease of the elderly. The disease that doctors saw in 1981 was frighteningly more severe, with multiple, fast-growing lesions that spread to all parts of the body, including the trunk and face. Could the immune systems of these young patients have been compromised in some way? Indeed, when they were tested, they exhibited extremely low numbers of a specific type of white blood cell in their bloodstreams, indicating that they had somehow lost a major part of the immune system.
Acquired immune deficiency syndrome, or AIDS, turned out to be a new disease caused by the previously unknown human immunodeficiency virus (HIV). Although nearly 100 percent fatal in those with active HIV infections in the early years, the development of anti-HIV drugs has transformed HIV infection into a chronic, manageable disease and not the certain death sentence it once was. One positive outcome resulting from the emergence of HIV disease was that the public’s attention became focused as never before on the importance of having a functional and healthy immune system.
Anatomy of the Lymphatic and Immune Systems
- Describe the structure and function of the lymphatic tissue (lymph fluid, vessels, ducts, and organs)
- Describe the structure and function of the primary and secondary lymphatic organs
- Discuss the cells of the immune system, how they function, and their relationship with the lymphatic system
The immune system is the complex collection of cells and organs that destroys or neutralizes pathogens that would otherwise cause disease or death. The lymphatic system, for most people, is associated with the immune system to such a degree that the two systems are virtually indistinguishable. The lymphatic system is the system of vessels, cells, and organs that carries excess fluids to the bloodstream and filters pathogens from the blood. The swelling of lymph nodes during an infection and the transport of lymphocytes via the lymphatic vessels are but two examples of the many connections between these critical organ systems.
Functions of the Lymphatic System
A major function of the lymphatic system is to drain body fluids and return them to the bloodstream. Blood pressure causes leakage of fluid from the capillaries, resulting in the accumulation of fluid in the interstitial space—that is, spaces between individual cells in the tissues. In humans, 20 liters of plasma is released into the interstitial space of the tissues each day due to capillary filtration. Once this filtrate is out of the bloodstream and in the tissue spaces, it is referred to as interstitial fluid. Of this, 17 liters is reabsorbed directly by the blood vessels. But what happens to the remaining three liters? This is where the lymphatic system comes into play. It drains the excess fluid and empties it back into the bloodstream via a series of vessels, trunks, and ducts. Lymph is the term used to describe interstitial fluid once it has entered the lymphatic system. When the lymphatic system is damaged in some way, such as by being blocked by cancer cells or destroyed by injury, protein-rich interstitial fluid accumulates (sometimes “backs up” from the lymph vessels) in the tissue spaces. This inappropriate accumulation of fluid referred to as lymphedema may lead to serious medical consequences.
As the vertebrate immune system evolved, the network of lymphatic vessels became convenient avenues for transporting the cells of the immune system. Additionally, the transport of dietary lipids and fat-soluble vitamins absorbed in the gut uses this system.
Cells of the immune system not only use lymphatic vessels to make their way from interstitial spaces back into the circulation, but they also use lymph nodes as major staging areas for the development of critical immune responses. A lymph node is one of the small, bean-shaped organs located throughout the lymphatic system.
INTERACTIVE LINK
Visit this website for an overview of the lymphatic system. What are the three main components of the lymphatic system?
Structure of the Lymphatic System
The lymphatic vessels begin as as blind ending, or closed at one end, capillaries, which feed into larger and larger lymphatic vessels, and eventually empty into the bloodstream by a series of ducts. Along the way, the lymph travels through the lymph nodes, which are commonly found near the groin, armpits, neck, chest, and abdomen. Humans have about 500–600 lymph nodes throughout the body (Figure 21.2).
Figure 21.2 Anatomy of the Lymphatic System Lymphatic vessels in the arms and legs convey lymph to the larger lymphatic vessels in the torso.
A major distinction between the lymphatic and cardiovascular systems in humans is that lymph is not actively pumped by the heart, but is forced through the vessels by the movements of the body, the contraction of skeletal muscles during body movements, and breathing. One-way valves (semi-lunar valves) in lymphatic vessels keep the lymph moving toward the heart. Lymph flows from the lymphatic capillaries, through lymphatic vessels, and then is dumped into the circulatory system via the lymphatic ducts located at the junction of the jugular and subclavian veins in the neck.
Lymphatic Capillaries
Lymphatic capillaries, also called the terminal lymphatics, are vessels where interstitial fluid enters the lymphatic system to become lymph fluid. Located in almost every tissue in the body, these vessels are interlaced among the arterioles and venules of the circulatory system in the soft connective tissues of the body (Figure 21.3). Exceptions are the central nervous system, bone marrow, bones, teeth, and the cornea of the eye, which do not contain lymph vessels.
Figure 21.3 Lymphatic Capillaries Lymphatic capillaries are interlaced with the arterioles and venules of the cardiovascular system. Collagen fibers anchor a lymphatic capillary in the tissue (inset). Interstitial fluid slips through spaces between the overlapping endothelial cells that compose the lymphatic capillary.
Lymphatic capillaries are formed by a one cell-thick layer of endothelial cells and represent the open end of the system, allowing interstitial fluid to flow into them via overlapping cells (see Figure 21.3). When interstitial pressure is low, the endothelial flaps close to prevent “backflow.” As interstitial pressure increases, the spaces between the cells open up, allowing the fluid to enter. Entry of fluid into lymphatic capillaries is also enabled by the collagen filaments that anchor the capillaries to surrounding structures. As interstitial pressure increases, the filaments pull on the endothelial cell flaps, opening up them even further to allow easy entry of fluid.
In the small intestine, lymphatic capillaries called lacteals are critical for the transport of dietary lipids and lipid-soluble vitamins to the bloodstream. In the small intestine, dietary triglycerides combine with other lipids and proteins, and enter the lacteals to form a milky fluid called chyle. The chyle then travels through the lymphatic system, eventually entering the bloodstream.
Larger Lymphatic Vessels, Trunks, and Ducts
The lymphatic capillaries empty into larger lymphatic vessels, which are similar to veins in terms of their three-tunic structure and the presence of valves. These one-way valves are located fairly close to one another, and each one causes a bulge in the lymphatic vessel, giving the vessels a beaded appearance (see Figure 21.3).
The superficial and deep lymphatics eventually merge to form larger lymphatic vessels known as lymphatic trunks. On the right side of the body, the right sides of the head, thorax, and right upper limb drain lymph fluid into the right subclavian vein via the right lymphatic duct (Figure 21.4). On the left side of the body, the remaining portions of the body drain into the larger thoracic duct, which drains into the left subclavian vein. The thoracic duct itself begins just beneath the diaphragm in the cisterna chyli, a sac-like chamber that receives lymph from the lower abdomen, pelvis, and lower limbs by way of the left and right lumbar trunks and the intestinal trunk.
Figure 21.4 Major Trunks and Ducts of the Lymphatic System The thoracic duct drains a much larger portion of the body than does the right lymphatic duct.
The overall drainage system of the body is asymmetrical (see Figure 21.4). The right lymphatic duct receives lymph from only the upper right side of the body. The lymph from the rest of the body enters the bloodstream through the thoracic duct via all the remaining lymphatic trunks. In general, lymphatic vessels of the subcutaneous tissues of the skin, that is, the superficial lymphatics, follow the same routes as veins, whereas the deep lymphatic vessels of the viscera generally follow the paths of arteries.
The Organization of Immune Function
The immune system is a collection of barriers, cells, and soluble proteins that interact and communicate with each other in extraordinarily complex ways. The modern model of immune function is organized into three phases based on the timing of their effects. The three temporal phases consist of the following:
- Barrier defenses such as the skin and mucous membranes, which act instantaneously to prevent pathogenic invasion into the body tissues
- The rapid but nonspecific innate immune response, which consists of a variety of specialized cells and soluble factors
- The slower but more specific and effective adaptive immune response, which involves many cell types and soluble factors, but is primarily controlled by white blood cells (leukocytes) known as lymphocytes, which help control immune responses
The cells of the blood, including all those involved in the immune response, arise in the bone marrow via various differentiation pathways from hematopoietic stem cells (Figure 21.5). In contrast with embryonic stem cells, hematopoietic stem cells are present throughout adulthood and allow for the continuous differentiation of blood cells to replace those lost to age or function. These cells can be divided into three classes based on function:
- Phagocytic cells, which ingest pathogens to destroy them
- Lymphocytes, which specifically coordinate the activities of adaptive immunity
- Cells containing cytoplasmic granules, which help mediate immune responses against parasites and intracellular pathogens such as viruses
Figure 21.5 Hematopoietic System of the Bone Marrow All the cells of the immune response as well as of the blood arise by differentiation from hematopoietic stem cells. Platelets are cell fragments involved in the clotting of blood.
Lymphocytes: B Cells, T Cells, Plasma Cells, and Natural Killer Cells
As stated above, lymphocytes are the primary cells of adaptive immune responses (Table 21.1). The two basic types of lymphocytes, B cells and T cells, are identical morphologically with a large central nucleus surrounded by a thin layer of cytoplasm. They are distinguished from each other by their surface protein markers as well as by the molecules they secrete. While B cells mature in red bone marrow and T cells mature in the thymus, they both initially develop from bone marrow. T cells migrate from bone marrow to the thymus gland where they further mature. B cells and T cells are found in many parts of the body, circulating in the bloodstream and lymph, and residing in secondary lymphoid organs, including the spleen and lymph nodes, which will be described later in this section. The human body contains approximately 1012 lymphocytes.
B Cells
B cells are immune cells that function primarily by producing antibodies. An antibody is any of the group of proteins that binds specifically to pathogen-associated molecules known as antigens. An antigen is a chemical structure on the surface of a pathogen that binds to T or B lymphocyte antigen receptors. Once activated by binding to antigen, B cells differentiate into cells that secrete a soluble form of their surface antibodies. These activated B cells are known as plasma cells.
T Cells
The T cell, on the other hand, does not secrete antibody but performs a variety of functions in the adaptive immune response. Different T cell types have the ability to either secrete soluble factors that communicate with other cells of the adaptive immune response or destroy cells infected with intracellular pathogens. The roles of T and B lymphocytes in the adaptive immune response will be discussed further in this chapter.
Plasma Cells
Another type of lymphocyte of importance is the plasma cell. A plasma cell is a B cell that has differentiated in response to antigen binding, and has thereby gained the ability to secrete soluble antibodies. These cells differ in morphology from standard B and T cells in that they contain a large amount of cytoplasm packed with the protein-synthesizing machinery known as rough endoplasmic reticulum.
Natural Killer Cells
A fourth important lymphocyte is the natural killer cell, a participant in the innate immune response. A natural killer cell (NK) is a circulating blood cell that contains cytotoxic (cell-killing) granules in its extensive cytoplasm. It shares this mechanism with the cytotoxic T cells of the adaptive immune response. NK cells are among the body’s first lines of defense against viruses and certain types of cancer.
Lymphocytes
| Type of lymphocyte | Primary function |
|---|---|
| B lymphocyte | Generates diverse antibodies |
| T lymphocyte | Secretes chemical messengers |
| Plasma cell | Secretes antibodies |
| NK cell | Destroys virally infected cells |
Table 21.1
INTERACTIVE LINK
Visit this website to learn about the many different cell types in the immune system and their very specialized jobs. What is the role of the dendritic cell in an HIV infection?
Primary Lymphoid Organs and Lymphocyte Development
Understanding the differentiation and development of B and T cells is critical to the understanding of the adaptive immune response. It is through this process that the body (ideally) learns to destroy only pathogens and leaves the body’s own cells relatively intact. The primary lymphoid organs are the bone marrow and thymus gland. The lymphoid organs are where lymphocytes mature, proliferate, and are selected, which enables them to attack pathogens without harming the cells of the body.
Bone Marrow
In the embryo, blood cells are made in the yolk sac. As development proceeds, this function is taken over by the spleen, lymph nodes, and liver. Later, the bone marrow takes over most hematopoietic functions, although the final stages of the differentiation of some cells may take place in other organs. The red bone marrow is a loose collection of cells where hematopoiesis occurs, and the yellow bone marrow is a site of energy storage, which consists largely of fat cells (Figure 21.6). The B cell undergoes nearly all of its development in the red bone marrow, whereas the immature T cell, called a thymocyte, leaves the bone marrow and matures largely in the thymus gland.
Figure 21.6 Bone Marrow Red bone marrow fills the head of the femur, and a spot of yellow bone marrow is visible in the center. The white reference bar is 1 cm.
Thymus
The thymus gland is a bilobed organ found in the space between the sternum and the aorta of the heart (Figure 21.7). Connective tissue holds the lobes closely together but also separates them and forms a capsule.
Figure 21.7 Location, Structure, and Histology of the Thymus The thymus lies above the heart. The trabeculae and lobules, including the darkly staining cortex and the lighter staining medulla of each lobule, are clearly visible in the light micrograph of the thymus of a newborn. LM × 100. (Micrograph provided by the Regents of the University of Michigan Medical School © 2012)
INTERACTIVE LINK
View the University of Michigan WebScope to explore the tissue sample in greater detail.
The connective tissue capsule further divides the thymus into lobules via extensions called trabeculae. The outer region of the organ is known as the cortex and contains large numbers of thymocytes with some epithelial cells, macrophages, and dendritic cells (two types of phagocytic cells that are derived from monocytes). The cortex is densely packed so it stains more intensely than the rest of the thymus (see Figure 21.7). The medulla, where thymocytes migrate before leaving the thymus, contains a less dense collection of thymocytes, epithelial cells, and dendritic cells.
AGING AND THE...
Immune System
By the year 2050, 25 percent of the population of the United States will be 60 years of age or older. The CDC estimates that 80 percent of those 60 years and older have one or more chronic disease associated with deficiencies of the immune systems. This loss of immune function with age is called immunosenescence. To treat this growing population, medical professionals must better understand the aging process. One major cause of age-related immune deficiencies is thymic involution, the shrinking of the thymus gland that begins at birth, at a rate of about three percent tissue loss per year, and continues until 35–45 years of age, when the rate declines to about one percent loss per year for the rest of one’s life. At that pace, the total loss of thymic epithelial tissue and thymocytes would occur at about 120 years of age. Thus, this age is a theoretical limit to a healthy human lifespan.
Thymic involution has been observed in all vertebrate species that have a thymus gland. Animal studies have shown that transplanted thymic grafts between inbred strains of mice involuted according to the age of the donor and not of the recipient, implying the process is genetically programmed. There is evidence that the thymic microenvironment, so vital to the development of naïve T cells, loses thymic epithelial cells according to the decreasing expression of the FOXN1 gene with age.
It is also known that thymic involution can be altered by hormone levels. Sex hormones such as estrogen and testosterone enhance involution, and the hormonal changes in pregnant women cause a temporary thymic involution that reverses itself, when the size of the thymus and its hormone levels return to normal, usually after lactation ceases. What does all this tell us? Can we reverse immunosenescence, or at least slow it down? The potential is there for using thymic transplants from younger donors to keep thymic output of naïve T cells high. Gene therapies that target gene expression are also seen as future possibilities. The more we learn through immunosenescence research, the more opportunities there will be to develop therapies, even though these therapies will likely take decades to develop. The ultimate goal is for everyone to live and be healthy longer, but there may be limits to immortality imposed by our genes and hormones.
Secondary Lymphoid Organs and their Roles in Active Immune Responses
Lymphocytes develop and mature in the primary lymphoid organs, but they mount immune responses from the secondary lymphoid organs. A naïve lymphocyte is one that has left the primary organ and entered a secondary lymphoid organ. Naïve lymphocytes are fully functional immunologically, but have yet to encounter an antigen to respond to. In addition to circulating in the blood and lymph, lymphocytes concentrate in secondary lymphoid organs, which include the lymph nodes, spleen, and lymphoid nodules. All of these tissues have many features in common, including the following:
- The presence of lymphoid follicles, the sites of the formation of lymphocytes, with specific B cell-rich and T cell-rich areas
- An internal structure of reticular fibers with associated fixed macrophages
- Germinal centers, which are the sites of rapidly dividing and differentiating B lymphocytes
- Specialized post-capillary vessels known as high endothelial venules; the cells lining these venules are thicker and more columnar than normal endothelial cells, which allow cells from the blood to directly enter these tissues
Lymph Nodes
Lymph nodes function to remove debris and pathogens from the lymph, and are thus sometimes referred to as the “filters of the lymph” (Figure 21.8). Any bacteria that infect the interstitial fluid are taken up by the lymphatic capillaries and transported to a regional lymph node. Dendritic cells and macrophages within this organ internalize and kill many of the pathogens that pass through, thereby removing them from the body. The lymph node is also the site of adaptive immune responses mediated by T cells, B cells, and accessory cells of the adaptive immune system. Like the thymus, the bean-shaped lymph nodes are surrounded by a tough capsule of connective tissue and are separated into compartments by trabeculae, the extensions of the capsule. In addition to the structure provided by the capsule and trabeculae, the structural support of the lymph node is provided by a series of reticular fibers laid down by fibroblasts.
Figure 21.8 Structure and Histology of a Lymph Node Lymph nodes are masses of lymphatic tissue located along the larger lymph vessels. The micrograph of the lymph nodes shows a germinal center, which consists of rapidly dividing B cells surrounded by a layer of T cells and other accessory cells. LM × 128. (Micrograph provided by the Regents of the University of Michigan Medical School © 2012)
INTERACTIVE LINK
View the University of Michigan WebScope to explore the tissue sample in greater detail.
The major routes into the lymph node are via afferent lymphatic vessels (see Figure 21.8). Cells and lymph fluid that leave the lymph node may do so by another set of vessels known as the efferent lymphatic vessels. Lymph enters the lymph node via the subcapsular sinus, which is occupied by dendritic cells, macrophages, and reticular fibers. Within the cortex of the lymph node are lymphoid follicles, which consist of germinal centers of rapidly dividing B cells surrounded by a layer of T cells and other accessory cells. As the lymph continues to flow through the node, it enters the medulla, which consists of medullary cords of B cells and plasma cells, and the medullary sinuses where the lymph collects before leaving the node via the efferent lymphatic vessels.
Spleen
In addition to the lymph nodes, the spleen is a major secondary lymphoid organ (Figure 21.9). It is about 12 cm (5 in) long and is attached to the lateral border of the stomach via the gastrosplenic ligament. The spleen is a fragile organ without a strong capsule, and is dark red due to its extensive vascularization. The spleen is sometimes called the “filter of the blood” because of its extensive vascularization and the presence of macrophages and dendritic cells that remove microbes and other materials from the blood, including dying red blood cells. The spleen also functions as the location of immune responses to blood-borne pathogens.
Figure 21.9 Spleen (a) The spleen is attached to the stomach. (b) A micrograph of spleen tissue shows the germinal center. The marginal zone is the region between the red pulp and white pulp, which sequesters particulate antigens from the circulation and presents these antigens to lymphocytes in the white pulp. EM × 660. (Micrograph provided by the Regents of the University of Michigan Medical School © 2012)
The spleen is also divided by trabeculae of connective tissue, and within each splenic nodule is an area of red pulp, consisting of mostly red blood cells, and white pulp, which resembles the lymphoid follicles of the lymph nodes. Upon entering the spleen, the splenic artery splits into several arterioles (surrounded by white pulp) and eventually into sinusoids. Blood from the capillaries subsequently collects in the venous sinuses and leaves via the splenic vein. The red pulp consists of reticular fibers with fixed macrophages attached, free macrophages, and all of the other cells typical of the blood, including some lymphocytes. The white pulp surrounds a central arteriole and consists of germinal centers of dividing B cells surrounded by T cells and accessory cells, including macrophages and dendritic cells. Thus, the red pulp primarily functions as a filtration system of the blood, using cells of the relatively nonspecific immune response, and white pulp is where adaptive T and B cell responses are mounted.
Lymphoid Nodules
The other lymphoid tissues, the lymphoid nodules, have a simpler architecture than the spleen and lymph nodes in that they consist of a dense cluster of lymphocytes without a surrounding fibrous capsule. These nodules are located in the respiratory and digestive tracts, areas routinely exposed to environmental pathogens.
Tonsils are lymphoid nodules located along the inner surface of the pharynx and are important in developing immunity to oral pathogens (Figure 21.10). The tonsil located at the back of the throat, the pharyngeal tonsil, is sometimes referred to as the adenoid when swollen. Such swelling is an indication of an active immune response to infection. Histologically, tonsils do not contain a complete capsule, and the epithelial layer invaginates deeply into the interior of the tonsil to form tonsillar crypts. These structures, which accumulate all sorts of materials taken into the body through eating and breathing, actually “encourage” pathogens to penetrate deep into the tonsillar tissues where they are acted upon by numerous lymphoid follicles and eliminated. This seems to be the major function of tonsils—to help children’s bodies recognize, destroy, and develop immunity to common environmental pathogens so that they will be protected in their later lives. Tonsils are often removed in those children who have recurring throat infections, especially those involving the palatine tonsils on either side of the throat, whose swelling may interfere with their breathing and/or swallowing.
Figure 21.10 Locations and Histology of the Tonsils (a) The pharyngeal tonsil is located on the roof of the posterior superior wall of the nasopharynx. The palatine tonsils lay on each side of the pharynx. (b) A micrograph shows the palatine tonsil tissue. LM × 40. (Micrograph provided by the Regents of the University of Michigan Medical School © 2012)
INTERACTIVE LINK
View the University of Michigan WebScope to explore the tissue sample in greater detail.
Mucosa-associated lymphoid tissue (MALT) consists of an aggregate of lymphoid follicles directly associated with the mucous membrane epithelia. MALT makes up dome-shaped structures found underlying the mucosa of the gastrointestinal tract, breast tissue, lungs, and eyes. Peyer’s patches, a type of MALT in the small intestine, are especially important for immune responses against ingested substances (Figure 21.11). Peyer’s patches contain specialized endothelial cells called M (or microfold) cells that sample material from the intestinal lumen and transport it to nearby follicles so that adaptive immune responses to potential pathogens can be mounted. A similar process occurs involving MALT in the mucosa and submucosa of the appendix. A blockage of the lumen triggers these cells to elicit an inflammatory response that can lead to appendicitis.
Figure 21.11 Mucosa-associated Lymphoid Tissue (MALT) Nodule LM × 40. (Micrograph provided by the Regents of the University of Michigan Medical School © 2012)
Bronchus-associated lymphoid tissue (BALT) consists of lymphoid follicular structures with an overlying epithelial layer found along the bifurcations of the bronchi, and between bronchi and arteries. They also have the typically less-organized structure of other lymphoid nodules. These tissues, in addition to the tonsils, are effective against inhaled pathogens.
Barrier Defenses and the Innate Immune Response
- Describe the barrier defenses of the body
- Show how the innate immune response is important and how it helps guide and prepare the body for adaptive immune responses
- Describe various soluble factors that are part of the innate immune response
- Explain the steps of inflammation and how they lead to destruction of a pathogen
- Discuss early induced immune responses and their level of effectiveness
The immune system can be divided into two overlapping mechanisms to destroy pathogens: the innate immune response, which is relatively rapid but nonspecific and thus not always effective, and the adaptive immune response, which is slower in its development during an initial infection with a pathogen, but is highly specific and effective at attacking a wide variety of pathogens (Figure 21.12).
Figure 21.12 Cooperation between Innate and Adaptive Immune Responses The innate immune system enhances adaptive immune responses so they can be more effective.
Any discussion of the innate immune response usually begins with the physical barriers that prevent pathogens from entering the body, destroy them after they enter, or flush them out before they can establish themselves in the hospitable environment of the body’s soft tissues. Barrier defenses are part of the body’s most basic defense mechanisms. The barrier defenses are not a response to infections, but they are continuously working to protect against a broad range of pathogens.
The different modes of barrier defenses are associated with the external surfaces of the body, where pathogens may try to enter (Table 21.2). The primary barrier to the entrance of microorganisms into the body is the skin. Not only is the skin covered with a layer of dead, keratinized epithelium that is too dry for bacteria in which to grow, but as these cells are continuously sloughed off from the skin, they carry bacteria and other pathogens with them. Additionally, sweat and other skin secretions may lower pH, contain toxic lipids, and physically wash microbes away.
Barrier Defenses
| Site | Specific defense | Protective aspect |
|---|---|---|
| Skin | Epidermal surface | Keratinized cells of surface, Langerhans cells |
| Skin (sweat/secretions) | Sweat glands, sebaceous glands | Low pH, washing action |
| Oral cavity | Salivary glands | Lysozyme |
| Stomach | Gastrointestinal tract | Low pH |
| Mucosal surfaces | Mucosal epithelium | Nonkeratinized epithelial cells |
| Normal flora (nonpathogenic bacteria) | Mucosal tissues | Prevent pathogens from growing on mucosal surfaces |
Table 21.2
Another barrier is the saliva in the mouth, which is rich in lysozyme—an enzyme that destroys bacteria by digesting their cell walls. The acidic environment of the stomach, which is fatal to many pathogens, is also a barrier. Additionally, the mucus layer of the gastrointestinal tract, respiratory tract, reproductive tract, eyes, ears, and nose traps both microbes and debris, and facilitates their removal. In the case of the upper respiratory tract, ciliated epithelial cells move potentially contaminated mucus upwards to the mouth, where it is then swallowed into the digestive tract, ending up in the harsh acidic environment of the stomach. Considering how often you breathe compared to how often you eat or perform other activities that expose you to pathogens, it is not surprising that multiple barrier mechanisms have evolved to work in concert to protect this vital area.
Cells of the Innate Immune Response
A phagocyte is a cell that is able to surround and engulf a particle or cell, a process called phagocytosis. The phagocytes of the immune system engulf other particles or cells, either to clean an area of debris, old cells, or to kill pathogenic organisms such as bacteria. The phagocytes are the body’s fast acting, first line of immunological defense against organisms that have breached barrier defenses and have entered the vulnerable tissues of the body.
Phagocytes: Macrophages and Neutrophils
Many of the cells of the immune system have a phagocytic ability, at least at some point during their life cycles. Phagocytosis is an important and effective mechanism of destroying pathogens during innate immune responses. The phagocyte takes the organism inside itself as a phagosome, which subsequently fuses with a lysosome and its digestive enzymes, effectively killing many pathogens. On the other hand, some bacteria including Mycobacteria tuberculosis, the cause of tuberculosis, may be resistant to these enzymes and are therefore much more difficult to clear from the body. Macrophages, neutrophils, and dendritic cells are the major phagocytes of the immune system.
A macrophage is an irregularly shaped phagocyte that is amoeboid in nature and is the most versatile of the phagocytes in the body. Macrophages move through tissues and squeeze through capillary walls using pseudopodia. They not only participate in innate immune responses but have also evolved to cooperate with lymphocytes as part of the adaptive immune response. Macrophages exist in many tissues of the body, either freely roaming through connective tissues or fixed to reticular fibers within specific tissues such as lymph nodes. When pathogens breach the body’s barrier defenses, macrophages are the first line of defense (Table 21.3). They are called different names, depending on the tissue: Kupffer cells in the liver, histiocytes in connective tissue, and alveolar macrophages in the lungs.
A neutrophil is a phagocytic cell that is attracted via chemotaxis from the bloodstream to infected tissues. These spherical cells are granulocytes. A granulocyte contains cytoplasmic granules, which in turn contain a variety of vasoactive mediators such as histamine. In contrast, macrophages are agranulocytes. An agranulocyte has few or no cytoplasmic granules. Whereas macrophages act like sentries, always on guard against infection, neutrophils can be thought of as military reinforcements that are called into a battle to hasten the destruction of the enemy. Although, usually thought of as the primary pathogen-killing cell of the inflammatory process of the innate immune response, new research has suggested that neutrophils play a role in the adaptive immune response as well, just as macrophages do.
A monocyte is a circulating precursor cell that differentiates into either a macrophage or dendritic cell, which can be rapidly attracted to areas of infection by signal molecules of inflammation.
Phagocytic Cells of the Innate Immune System
| Cell | Cell type | Primary location | Function in the innate immune response |
|---|---|---|---|
| Macrophage | Agranulocyte | Body cavities/organs | Phagocytosis |
| Neutrophil | Granulocyte | Blood | Phagocytosis |
| Monocyte | Agranulocyte | Blood | Precursor of macrophage/dendritic cell |
Table 21.3
Natural Killer Cells
NK cells are a type of lymphocyte that have the ability to induce apoptosis, that is, programmed cell death, in cells infected with intracellular pathogens such as obligate intracellular bacteria and viruses. NK cells recognize these cells by mechanisms that are still not well understood, but that presumably involve their surface receptors. NK cells can induce apoptosis, in which a cascade of events inside the cell causes its own death by either of two mechanisms:
1) NK cells are able to respond to chemical signals and express the fas ligand. The fas ligand is a surface molecule that binds to the fas molecule on the surface of the infected cell, sending it apoptotic signals, thus killing the cell and the pathogen within it; or
2) The granules of the NK cells release perforins and granzymes. A perforin is a protein that forms pores in the membranes of infected cells. A granzyme is a protein-digesting enzyme that enters the cell via the perforin pores and triggers apoptosis intracellularly.
Both mechanisms are especially effective against virally infected cells. If apoptosis is induced before the virus has the ability to synthesize and assemble all its components, no infectious virus will be released from the cell, thus preventing further infection.
Recognition of Pathogens
Cells of the innate immune response, the phagocytic cells, and the cytotoxic NK cells recognize patterns of pathogen-specific molecules, such as bacterial cell wall components or bacterial flagellar proteins, using pattern recognition receptors. A pattern recognition receptor (PRR) is a membrane-bound receptor that recognizes characteristic features of a pathogen and molecules released by stressed or damaged cells.
These receptors, which are thought to have evolved prior to the adaptive immune response, are present on the cell surface whether they are needed or not. Their variety, however, is limited by two factors. First, the fact that each receptor type must be encoded by a specific gene requires the cell to allocate most or all of its DNA to make receptors able to recognize all pathogens. Secondly, the variety of receptors is limited by the finite surface area of the cell membrane. Thus, the innate immune system must “get by” using only a limited number of receptors that are active against as wide a variety of pathogens as possible. This strategy is in stark contrast to the approach used by the adaptive immune system, which uses large numbers of different receptors, each highly specific to a particular pathogen.
Should the cells of the innate immune system come into contact with a species of pathogen they recognize, the cell will bind to the pathogen and initiate phagocytosis (or cellular apoptosis in the case of an intracellular pathogen) in an effort to destroy the offending microbe. Receptors vary somewhat according to cell type, but they usually include receptors for bacterial components and for complement, discussed below.
Soluble Mediators of the Innate Immune Response
The previous discussions have alluded to chemical signals that can induce cells to change various physiological characteristics, such as the expression of a particular receptor. These soluble factors are secreted during innate or early induced responses, and later during adaptive immune responses.
Cytokines and Chemokines
A cytokine is signaling molecule that allows cells to communicate with each other over short distances. Cytokines are secreted into the intercellular space, and the action of the cytokine induces the receiving cell to change its physiology. A chemokine is a soluble chemical mediator similar to cytokines except that its function is to attract cells (chemotaxis) from longer distances.
INTERACTIVE LINK
Visit this website to learn about phagocyte chemotaxis. Phagocyte chemotaxis is the movement of phagocytes according to the secretion of chemical messengers in the form of interleukins and other chemokines. By what means does a phagocyte destroy a bacterium that it has ingested?
Early induced Proteins
Early induced proteins are those that are not constitutively present in the body, but are made as they are needed early during the innate immune response. Interferons are an example of early induced proteins. Cells infected with viruses secrete interferons that travel to adjacent cells and induce them to make antiviral proteins. Thus, even though the initial cell is sacrificed, the surrounding cells are protected. Other early induced proteins specific for bacterial cell wall components are mannose-binding protein and C-reactive protein, made in the liver, which bind specifically to polysaccharide components of the bacterial cell wall. Phagocytes such as macrophages have receptors for these proteins, and they are thus able to recognize them as they are bound to the bacteria. This brings the phagocyte and bacterium into close proximity and enhances the phagocytosis of the bacterium by the process known as opsonization. Opsonization is the tagging of a pathogen for phagocytosis by the binding of an antibody or an antimicrobial protein.
Complement System
The complement system is a series of proteins constitutively found in the blood plasma. As such, these proteins are not considered part of the early induced immune response, even though they share features with some of the antibacterial proteins of this class. Made in the liver, they have a variety of functions in the innate immune response, using what is known as the “alternate pathway” of complement activation. Additionally, complement functions in the adaptive immune response as well, in what is called the classical pathway. The complement system consists of several proteins that enzymatically alter and fragment later proteins in a series, which is why it is termed cascade. Once activated, the series of reactions is irreversible, and releases fragments that have the following actions:
- Bind to the cell membrane of the pathogen that activates it, labeling it for phagocytosis (opsonization)
- Diffuse away from the pathogen and act as chemotactic agents to attract phagocytic cells to the site of inflammation
- Form damaging pores in the plasma membrane of the pathogen
Figure 21.13 shows the classical pathway, which requires antibodies of the adaptive immune response. The alternate pathway does not require an antibody to become activated.
Figure 21.13 Complement Cascade and Function The classical pathway, used during adaptive immune responses, occurs when C1 reacts with antibodies that have bound an antigen.
The splitting of the C3 protein is the common step to both pathways. In the alternate pathway, C3 is activated spontaneously and, after reacting with the molecules factor P, factor B, and factor D, splits apart. The larger fragment, C3b, binds to the surface of the pathogen and C3a, the smaller fragment, diffuses outward from the site of activation and attracts phagocytes to the site of infection. Surface-bound C3b then activates the rest of the cascade, with the last five proteins, C5–C9, forming the membrane-attack complex (MAC). The MAC can kill certain pathogens by disrupting their osmotic balance. The MAC is especially effective against a broad range of bacteria. The classical pathway is similar, except the early stages of activation require the presence of antibody bound to antigen, and thus is dependent on the adaptive immune response. The earlier fragments of the cascade also have important functions. Phagocytic cells such as macrophages and neutrophils are attracted to an infection site by chemotactic attraction to smaller complement fragments. Additionally, once they arrive, their receptors for surface-bound C3b opsonize the pathogen for phagocytosis and destruction.
Inflammatory Response
The hallmark of the innate immune response is inflammation. Inflammation is something everyone has experienced. Stub a toe, cut a finger, or do any activity that causes tissue damage and inflammation will result, with its four characteristics: heat, redness, pain, and swelling (“loss of function” is sometimes mentioned as a fifth characteristic). It is important to note that inflammation does not have to be initiated by an infection, but can also be caused by tissue injuries. The release of damaged cellular contents into the site of injury is enough to stimulate the response, even in the absence of breaks in physical barriers that would allow pathogens to enter (by hitting your thumb with a hammer, for example). The inflammatory reaction brings in phagocytic cells to the damaged area to clear cellular debris and to set the stage for wound repair (Figure 21.14).
Figure 21.14
This reaction also brings in the cells of the innate immune system, allowing them to get rid of the sources of a possible infection. Inflammation is part of a very basic form of immune response. The process not only brings fluid and cells into the site to destroy the pathogen and remove it and debris from the site, but also helps to isolate the site, limiting the spread of the pathogen. Acute inflammation is a short-term inflammatory response to an insult to the body. If the cause of the inflammation is not resolved, however, it can lead to chronic inflammation, which is associated with major tissue destruction and fibrosis. Chronic inflammationis ongoing inflammation. It can be caused by foreign bodies, persistent pathogens, and autoimmune diseases such as rheumatoid arthritis.
There are four important parts to the inflammatory response:
- Tissue Injury. The released contents of injured cells stimulate the release of mast cell granules and their potent inflammatory mediators such as histamine, leukotrienes, and prostaglandins. Histamine increases the diameter of local blood vessels (vasodilation), causing an increase in blood flow. Histamine also increases the permeability of local capillaries, causing plasma to leak out and form interstitial fluid. This causes the swelling associated with inflammation.
Additionally, injured cells, phagocytes, and basophils are sources of inflammatory mediators, including prostaglandins and leukotrienes. Leukotrienes attract neutrophils from the blood by chemotaxis and increase vascular permeability. Prostaglandins cause vasodilation by relaxing vascular smooth muscle and are a major cause of the pain associated with inflammation. Nonsteroidal anti-inflammatory drugs such as aspirin and ibuprofen relieve pain by inhibiting prostaglandin production. - Vasodilation. Many inflammatory mediators such as histamine are vasodilators that increase the diameters of local capillaries. This causes increased blood flow and is responsible for the heat and redness of inflamed tissue. It allows greater access of the blood to the site of inflammation.
- Increased Vascular Permeability. At the same time, inflammatory mediators increase the permeability of the local vasculature, causing leakage of fluid into the interstitial space, resulting in the swelling, or edema, associated with inflammation.
- Recruitment of Phagocytes. Leukotrienes are particularly good at attracting neutrophils from the blood to the site of infection by chemotaxis. Following an early neutrophil infiltrate stimulated by macrophage cytokines, more macrophages are recruited to clean up the debris left over at the site. When local infections are severe, neutrophils are attracted to the sites of infections in large numbers, and as they phagocytose the pathogens and subsequently die, their accumulated cellular remains are visible as pus at the infection site.
Overall, inflammation is valuable for many reasons. Not only are the pathogens killed and debris removed, but the increase in vascular permeability encourages the entry of clotting factors, the first step towards wound repair. Inflammation also facilitates the transport of antigen to lymph nodes by dendritic cells for the development of the adaptive immune response.
The Adaptive Immune Response: T lymphocytes and Their Functional Types
- Explain the advantages of the adaptive immune response over the innate immune response
- List the various characteristics of an antigen
- Describe the types of T cell antigen receptors
- Outline the steps of T cell development
- Describe the major T cell types and their functions
Innate immune responses (and early induced responses) are in many cases ineffective at completely controlling pathogen growth. However, they slow pathogen growth and allow time for the adaptive immune response to strengthen and either control or eliminate the pathogen. The innate immune system also sends signals to the cells of the adaptive immune system, guiding them in how to attack the pathogen. Thus, these are the two important arms of the immune response.
The Benefits of the Adaptive Immune Response
The specificity of the adaptive immune response—its ability to specifically recognize and make a response against a wide variety of pathogens—is its great strength. Antigens, the small chemical groups often associated with pathogens, are recognized by receptors on the surface of B and T lymphocytes. The adaptive immune response to these antigens is so versatile that it can respond to nearly any pathogen. This increase in specificity comes because the adaptive immune response has a unique way to develop as many as 1011, or 100 trillion, different receptors to recognize nearly every conceivable pathogen. How could so many different types of antibodies be encoded? And what about the many specificities of T cells? There is not nearly enough DNA in a cell to have a separate gene for each specificity. The mechanism was finally worked out in the 1970s and 1980s using the new tools of molecular genetics
Primary Disease and Immunological Memory
The immune system’s first exposure to a pathogen is called a primary adaptive response. Symptoms of a first infection, called primary disease, are always relatively severe because it takes time for an initial adaptive immune response to a pathogen to become effective.
Upon re-exposure to the same pathogen, a secondary adaptive immune response is generated, which is stronger and faster that the primary response. The secondary adaptive response often eliminates a pathogen before it can cause significant tissue damage or any symptoms. Without symptoms, there is no disease, and the individual is not even aware of the infection. This secondary response is the basis of immunological memory, which protects us from getting diseases repeatedly from the same pathogen. By this mechanism, an individual’s exposure to pathogens early in life spares the person from these diseases later in life.
Self Recognition
A third important feature of the adaptive immune response is its ability to distinguish between self-antigens, those that are normally present in the body, and foreign antigens, those that might be on a potential pathogen. As T and B cells mature, there are mechanisms in place that prevent them from recognizing self-antigen, preventing a damaging immune response against the body. These mechanisms are not 100 percent effective, however, and their breakdown leads to autoimmune diseases, which will be discussed later in this chapter.
T Cell-Mediated Immune Responses
The primary cells that control the adaptive immune response are the lymphocytes, the T and B cells. T cells are particularly important, as they not only control a multitude of immune responses directly, but also control B cell immune responses in many cases as well. Thus, many of the decisions about how to attack a pathogen are made at the T cell level, and knowledge of their functional types is crucial to understanding the functioning and regulation of adaptive immune responses as a whole.
T lymphocytes recognize antigens based on a two-chain protein receptor. The most common and important of these are the alpha-beta T cell receptors (Figure 21.15).
Figure 21.15 Alpha-beta T Cell Receptor Notice the constant and variable regions of each chain, anchored by the transmembrane region.
There are two chains in the T cell receptor, and each chain consists of two domains. The variable region domain is furthest away from the T cell membrane and is so named because its amino acid sequence varies between receptors. In contrast, the constant region domain has less variation. The differences in the amino acid sequences of the variable domains are the molecular basis of the diversity of antigens the receptor can recognize. Thus, the antigen-binding site of the receptor consists of the terminal ends of both receptor chains, and the amino acid sequences of those two areas combine to determine its antigenic specificity. Each T cell produces only one type of receptor and thus is specific for a single particular antigen.
Antigens
Antigens on pathogens are usually large and complex, and consist of many antigenic determinants. An antigenic determinant(epitope) is one of the small regions within an antigen to which a receptor can bind, and antigenic determinants are limited by the size of the receptor itself. They usually consist of six or fewer amino acid residues in a protein, or one or two sugar moieties in a carbohydrate antigen. Antigenic determinants on a carbohydrate antigen are usually less diverse than on a protein antigen. Carbohydrate antigens are found on bacterial cell walls and on red blood cells (the ABO blood group antigens). Protein antigens are complex because of the variety of three-dimensional shapes that proteins can assume, and are especially important for the immune responses to viruses and worm parasites. It is the interaction of the shape of the antigen and the complementary shape of the amino acids of the antigen-binding site that accounts for the chemical basis of specificity (Figure 21.16).
Figure 21.16 Antigenic Determinants A typical protein antigen has multiple antigenic determinants, shown by the ability of T cells with three different specificities to bind to different parts of the same antigen.
Antigen Processing and Presentation
Although Figure 21.16 shows T cell receptors interacting with antigenic determinants directly, the mechanism that T cells use to recognize antigens is, in reality, much more complex. T cells do not recognize free-floating or cell-bound antigens as they appear on the surface of the pathogen. They only recognize antigen on the surface of specialized cells called antigen-presenting cells. Antigens are internalized by these cells. Antigen processing is a mechanism that enzymatically cleaves the antigen into smaller pieces. The antigen fragments are then brought to the cell’s surface and associated with a specialized type of antigen-presenting protein known as a major histocompatibility complex (MHC) molecule. The MHC is the cluster of genes that encode these antigen-presenting molecules. The association of the antigen fragments with an MHC molecule on the surface of a cell is known as antigen presentation and results in the recognition of antigen by a T cell. This association of antigen and MHC occurs inside the cell, and it is the complex of the two that is brought to the surface. The peptide-binding cleft is a small indentation at the end of the MHC molecule that is furthest away from the cell membrane; it is here that the processed fragment of antigen sits. MHC molecules are capable of presenting a variety of antigens, depending on the amino acid sequence, in their peptide-binding clefts. It is the combination of the MHC molecule and the fragment of the original peptide or carbohydrate that is actually physically recognized by the T cell receptor (Figure 21.17).
Figure 21.17 Antigen Processing and Presentation
Two distinct types of MHC molecules, MHC class I and MHC class II, play roles in antigen presentation. Although produced from different genes, they both have similar functions. They bring processed antigen to the surface of the cell via a transport vesicle and present the antigen to the T cell and its receptor. Antigens from different classes of pathogens, however, use different MHC classes and take different routes through the cell to get to the surface for presentation. The basic mechanism, though, is the same. Antigens are processed by digestion, are brought into the endomembrane system of the cell, and then are expressed on the surface of the antigen-presenting cell for antigen recognition by a T cell. Intracellular antigens are typical of viruses, which replicate inside the cell, and certain other intracellular parasites and bacteria. These antigens are processed in the cytosol by an enzyme complex known as the proteasome and are then brought into the endoplasmic reticulum by the transporter associated with antigen processing (TAP) system, where they interact with class I MHC molecules and are eventually transported to the cell surface by a transport vesicle.
Extracellular antigens, characteristic of many bacteria, parasites, and fungi that do not replicate inside the cell’s cytoplasm, are brought into the endomembrane system of the cell by receptor-mediated endocytosis. The resulting vesicle fuses with vesicles from the Golgi complex, which contain pre-formed MHC class II molecules. After fusion of these two vesicles and the association of antigen and MHC, the new vesicle makes its way to the cell surface.
Professional Antigen-presenting Cells
Many cell types express class I molecules for the presentation of intracellular antigens. These MHC molecules may then stimulate a cytotoxic T cell immune response, eventually destroying the cell and the pathogen within. This is especially important when it comes to the most common class of intracellular pathogens, the virus. Viruses infect nearly every tissue of the body, so all these tissues must necessarily be able to express class I MHC or no T cell response can be made.
On the other hand, class II MHC molecules are expressed only on the cells of the immune system, specifically cells that affect other arms of the immune response. Thus, these cells are called “professional” antigen-presenting cells to distinguish them from those that bear class I MHC. The three types of professional antigen presenters are macrophages, dendritic cells, and B cells (Table 21.4).
Macrophages stimulate T cells to release cytokines that enhance phagocytosis. Dendritic cells also kill pathogens by phagocytosis (see Figure 21.17), but their major function is to bring antigens to regional draining lymph nodes. The lymph nodes are the locations in which most T cell responses against pathogens of the interstitial tissues are mounted. Macrophages are found in the skin and in the lining of mucosal surfaces, such as the nasopharynx, stomach, lungs, and intestines. B cells may also present antigens to T cells, which are necessary for certain types of antibody responses, to be covered later in this chapter.
Classes of Antigen-presenting Cells
| MHC | Cell type | Phagocytic? | Function |
|---|---|---|---|
| Class I | Many | No | Stimulates cytotoxic T cell immune response |
| Class II | Macrophage | Yes | Stimulates phagocytosis and presentation at primary infection site |
| Class II | Dendritic | Yes, in tissues | Brings antigens to regional lymph nodes |
| Class II | B cell | Yes, internalizes surface Ig and antigen | Stimulates antibody secretion by B cells |
Table 21.4
T Cell Development and Differentiation
The process of eliminating T cells that might attack the cells of one’s own body is referred to as T cell tolerance. While thymocytes are in the cortex of the thymus, they are referred to as “double negatives,” meaning that they do not bear the CD4 or CD8 molecules that you can use to follow their pathways of differentiation (Figure 21.18). In the cortex of the thymus, they are exposed to cortical epithelial cells. In a process known as positive selection, double-negative thymocytes bind to the MHC molecules they observe on the thymic epithelia, and the MHC molecules of “self” are selected. This mechanism kills many thymocytes during T cell differentiation. In fact, only two percent of the thymocytes that enter the thymus leave it as mature, functional T cells.
Figure 21.18 Differentiation of T Cells within the Thymus Thymocytes enter the thymus and go through a series of developmental stages that ensures both function and tolerance before they leave and become functional components of the adaptive immune response.
Later, the cells become double positives that express both CD4 and CD8 markers and move from the cortex to the junction between the cortex and medulla. It is here that negative selection takes place. In negative selection, self-antigens are brought into the thymus from other parts of the body by professional antigen-presenting cells. The T cells that bind to these self-antigens are selected for negatively and are killed by apoptosis. In summary, the only T cells left are those that can bind to MHC molecules of the body with foreign antigens presented on their binding clefts, preventing an attack on one’s own body tissues, at least under normal circumstances. Tolerance can be broken, however, by the development of an autoimmune response, to be discussed later in this chapter.
The cells that leave the thymus become single positives, expressing either CD4 or CD8, but not both (see Figure 21.18). The CD4+ T cells will bind to class II MHC and the CD8+ cells will bind to class I MHC. The discussion that follows explains the functions of these molecules and how they can be used to differentiate between the different T cell functional types.
Mechanisms of T Cell-mediated Immune Responses
Mature T cells become activated by recognizing processed foreign antigen in association with a self-MHC molecule and begin dividing rapidly by mitosis. This proliferation of T cells is called clonal expansion and is necessary to make the immune response strong enough to effectively control a pathogen. How does the body select only those T cells that are needed against a specific pathogen? Again, the specificity of a T cell is based on the amino acid sequence and the three-dimensional shape of the antigen-binding site formed by the variable regions of the two chains of the T cell receptor (Figure 21.19). Clonal selection is the process of antigen binding only to those T cells that have receptors specific to that antigen. Each T cell that is activated has a specific receptor “hard-wired” into its DNA, and all of its progeny will have identical DNA and T cell receptors, forming clones of the original T cell.
Figure 21.19 Clonal Selection and Expansion of T Lymphocytes Stem cells differentiate into T cells with specific receptors, called clones. The clones with receptors specific for antigens on the pathogen are selected for and expanded.
Clonal Selection and Expansion
The clonal selection theory was proposed by Frank Burnet in the 1950s. However, the term clonal selection is not a complete description of the theory, as clonal expansion goes hand in glove with the selection process. The main tenet of the theory is that a typical individual has a multitude (1011) of different types of T cell clones based on their receptors. In this use, a clone is a group of lymphocytes that share the same antigen receptor. Each clone is necessarily present in the body in low numbers. Otherwise, the body would not have room for lymphocytes with so many specificities.
Only those clones of lymphocytes whose receptors are activated by the antigen are stimulated to proliferate. Keep in mind that most antigens have multiple antigenic determinants, so a T cell response to a typical antigen involves a polyclonal response. A polyclonal response is the stimulation of multiple T cell clones. Once activated, the selected clones increase in number and make many copies of each cell type, each clone with its unique receptor. By the time this process is complete, the body will have large numbers of specific lymphocytes available to fight the infection (see Figure 21.19).
The Cellular Basis of Immunological Memory
As already discussed, one of the major features of an adaptive immune response is the development of immunological memory.
During a primary adaptive immune response, both memory T cells and effector T cells are generated. Memory T cells are long-lived and can even persist for a lifetime. Memory cells are primed to act rapidly. Thus, any subsequent exposure to the pathogen will elicit a very rapid T cell response. This rapid, secondary adaptive response generates large numbers of effector T cells so fast that the pathogen is often overwhelmed before it can cause any symptoms of disease. This is what is meant by immunity to a disease. The same pattern of primary and secondary immune responses occurs in B cells and the antibody response, as will be discussed later in the chapter.
T Cell Types and their Functions
In the discussion of T cell development, you saw that mature T cells express either the CD4 marker or the CD8 marker, but not both. These markers are cell adhesion molecules that keep the T cell in close contact with the antigen-presenting cell by directly binding to the MHC molecule (to a different part of the molecule than does the antigen). Thus, T cells and antigen-presenting cells are held together in two ways: by CD4 or CD8 attaching to MHC and by the T cell receptor binding to antigen (Figure 21.20).
Figure 21.20 Pathogen Presentation (a) CD4 is associated with helper and regulatory T cells. An extracellular pathogen is processed and presented in the binding cleft of a class II MHC molecule, and this interaction is strengthened by the CD4 molecule. (b) CD8 is associated with cytotoxic T cells. An intracellular pathogen is presented by a class I MHC molecule, and CD8 interacts with it.
Although the correlation is not 100 percent, CD4-bearing T cells are associated with helper functions and CD8-bearing T cells are associated with cytotoxicity. These functional distinctions based on CD4 and CD8 markers are useful in defining the function of each type.
Helper T Cells and their Cytokines
Helper T cells (Th), bearing the CD4 molecule, function by secreting cytokines that act to enhance other immune responses. There are two classes of Th cells, and they act on different components of the immune response. These cells are not distinguished by their surface molecules but by the characteristic set of cytokines they secrete (Table 21.5).
Th1 cells are a type of helper T cell that secretes cytokines that regulate the immunological activity and development of a variety of cells, including macrophages and other types of T cells.
Th2 cells, on the other hand, are cytokine-secreting cells that act on B cells to drive their differentiation into plasma cells that make antibody. In fact, T cell help is required for antibody responses to most protein antigens, and these are called T cell-dependent antigens.
Cytotoxic T cells
Cytotoxic T cells (Tc) are T cells that kill target cells by inducing apoptosis using the same mechanism as NK cells. They either express Fas ligand, which binds to the fas molecule on the target cell, or act by using perforins and granzymes contained in their cytoplasmic granules. As was discussed earlier with NK cells, killing a virally infected cell before the virus can complete its replication cycle results in the production of no infectious particles. As more Tc cells are developed during an immune response, they overwhelm the ability of the virus to cause disease. In addition, each Tc cell can kill more than one target cell, making them especially effective. Tc cells are so important in the antiviral immune response that some speculate that this was the main reason the adaptive immune response evolved in the first place.
Regulatory T Cells
Regulatory T cells (Treg), or suppressor T cells, are the most recently discovered of the types listed here, so less is understood about them. In addition to CD4, they bear the molecules CD25 and FOXP3. Exactly how they function is still under investigation, but it is known that they suppress other T cell immune responses. This is an important feature of the immune response, because if clonal expansion during immune responses were allowed to continue uncontrolled, these responses could lead to autoimmune diseases and other medical issues.
Not only do T cells directly destroy pathogens, but they regulate nearly all other types of the adaptive immune response as well, as evidenced by the functions of the T cell types, their surface markers, the cells they work on, and the types of pathogens they work against (see Table 21.5).
Functions of T Cell Types and Their Cytokines
| T cell | Main target | Function | Pathogen | Surface marker | MHC | Cytokines or mediators |
|---|---|---|---|---|---|---|
| Tc | Infected cells | Cytotoxicity | Intracellular | CD8 | Class I | Perforins, granzymes, and fas ligand |
| Th1 | Macrophage | Helper inducer | Extracellular | CD4 | Class II | Interferon-γ and TGF-β |
| Th2 | B cell | Helper inducer | Extracellular | CD4 | Class II | IL-4, IL-6, IL-10, and others |
| Treg | Th cell | Suppressor | None | CD4, CD25 | ? | TGF-β and IL-10 |
Table 21.5
The Adaptive Immune Response: B-lymphocytes and Antibodies
- Explain how B cells mature and how B cell tolerance develops
- Discuss how B cells are activated and differentiate into plasma cells
- Describe the structure of the antibody classes and their functions
Antibodies were the first component of the adaptive immune response to be characterized by scientists working on the immune system. It was already known that individuals who survived a bacterial infection were immune to re-infection with the same pathogen. Early microbiologists took serum from an immune patient and mixed it with a fresh culture of the same type of bacteria, then observed the bacteria under a microscope. The bacteria became clumped in a process called agglutination. When a different bacterial species was used, the agglutination did not happen. Thus, there was something in the serum of immune individuals that could specifically bind to and agglutinate bacteria.
Scientists now know the cause of the agglutination is an antibody molecule, also called an immunoglobulin. What is an antibody? An antibody protein is essentially a secreted form of a B cell receptor. (In fact, surface immunoglobulin is another name for the B cell receptor.) Not surprisingly, the same genes encode both the secreted antibodies and the surface immunoglobulins. One minor difference in the way these proteins are synthesized distinguishes a naïve B cell with antibody on its surface from an antibody-secreting plasma cell with no antibodies on its surface. The antibodies of the plasma cell have the exact same antigen-binding site and specificity as their B cell precursors.
There are five different classes of antibody found in humans: IgM, IgD, IgG, IgA, and IgE. Each of these has specific functions in the immune response, so by learning about them, researchers can learn about the great variety of antibody functions critical to many adaptive immune responses.
B cells do not recognize antigen in the complex fashion of T cells. B cells can recognize native, unprocessed antigen and do not require the participation of MHC molecules and antigen-presenting cells.
B Cell Differentiation and Activation
B cells differentiate in the bone marrow. During the process of maturation, up to 100 trillion different clones of B cells are generated, which is similar to the diversity of antigen receptors seen in T cells.
B cell differentiation and the development of tolerance are not quite as well understood as it is in T cells. Central tolerance is the destruction or inactivation of B cells that recognize self-antigens in the bone marrow, and its role is critical and well established. In the process of clonal deletion, immature B cells that bind strongly to self-antigens expressed on tissues are signaled to commit suicide by apoptosis, removing them from the population. In the process of clonal anergy, however, B cells exposed to soluble antigen in the bone marrow are not physically deleted, but become unable to function.
Another mechanism called peripheral tolerance is a direct result of T cell tolerance. In peripheral tolerance, functional, mature B cells leave the bone marrow but have yet to be exposed to self-antigen. Most protein antigens require signals from helper T cells (Th2) to proceed to make antibody. When a B cell binds to a self-antigen but receives no signals from a nearby Th2 cell to produce antibody, the cell is signaled to undergo apoptosis and is destroyed. This is yet another example of the control that T cells have over the adaptive immune response.
After B cells are activated by their binding to antigen, they differentiate into plasma cells. Plasma cells often leave the secondary lymphoid organs, where the response is generated, and migrate back to the bone marrow, where the whole differentiation process started. After secreting antibodies for a specific period, they die, as most of their energy is devoted to making antibodies and not to maintaining themselves. Thus, plasma cells are said to be terminally differentiated.
The final B cell of interest is the memory B cell, which results from the clonal expansion of an activated B cell. Memory B cells function in a way similar to memory T cells. They lead to a stronger and faster secondary response when compared to the primary response, as illustrated below.
Antibody Structure
Antibodies are glycoproteins consisting of two types of polypeptide chains with attached carbohydrates. The heavy chain and the light chain are the two polypeptides that form the antibody. The main differences between the classes of antibodies are in the differences between their heavy chains, but as you shall see, the light chains have an important role, forming part of the antigen-binding site on the antibody molecules.
Four-chain Models of Antibody Structures
All antibody molecules have two identical heavy chains and two identical light chains. (Some antibodies contain multiple units of this four-chain structure.) The Fc region of the antibody is formed by the two heavy chains coming together, usually linked by disulfide bonds (Figure 21.21). The Fc portion of the antibody is important in that many effector cells of the immune system have Fc receptors. Cells having these receptors can then bind to antibody-coated pathogens, greatly increasing the specificity of the effector cells. At the other end of the molecule are two identical antigen-binding sites.
Figure 21.21 Antibody and IgG2 Structures The typical four chain structure of a generic antibody (a) and the corresponding three-dimensional structure of the antibody IgG2 (b). (credit b: modification of work by Tim Vickers)
Five Classes of Antibodies and their Functions
In general, antibodies have two basic functions. They can act as the B cell antigen receptor or they can be secreted, circulate, and bind to a pathogen, often labeling it for identification by other forms of the immune response. Of the five antibody classes, notice that only two can function as the antigen receptor for naïve B cells: IgM and IgD (Figure 21.22). Mature B cells that leave the bone marrow express both IgM and IgD, but both antibodies have the same antigen specificity. Only IgM is secreted, however, and no other nonreceptor function for IgD has been discovered.
Figure 21.22 Five Classes of Antibodies
IgM consists of five four-chain structures (20 total chains with 10 identical antigen-binding sites) and is thus the largest of the antibody molecules. IgM is usually the first antibody made during a primary response. Its 10 antigen-binding sites and large shape allow it to bind well to many bacterial surfaces. It is excellent at binding complement proteins and activating the complement cascade, consistent with its role in promoting chemotaxis, opsonization, and cell lysis. Thus, it is a very effective antibody against bacteria at early stages of a primary antibody response. As the primary response proceeds, the antibody produced in a B cell can change to IgG, IgA, or IgE by the process known as class switching. Class switching is the change of one antibody class to another. While the class of antibody changes, the specificity and the antigen-binding sites do not. Thus, the antibodies made are still specific to the pathogen that stimulated the initial IgM response.
IgG is a major antibody of late primary responses and the main antibody of secondary responses in the blood. This is because class switching occurs during primary responses. IgG is a monomeric antibody that clears pathogens from the blood and can activate complement proteins (although not as well as IgM), taking advantage of its antibacterial activities. Furthermore, this class of antibody is the one that crosses the placenta to protect the developing fetus from disease exits the blood to the interstitial fluid to fight extracellular pathogens.
IgA exists in two forms, a four-chain monomer in the blood and an eight-chain structure, or dimer, in exocrine gland secretions of the mucous membranes, including mucus, saliva, and tears. Thus, dimeric IgA is the only antibody to leave the interior of the body to protect body surfaces. IgA is also of importance to newborns, because this antibody is present in mother’s breast milk (colostrum), which serves to protect the infant from disease.
IgE is usually associated with allergies and anaphylaxis. It is present in the lowest concentration in the blood, because its Fc region binds strongly to an IgE-specific Fc receptor on the surfaces of mast cells. IgE makes mast cell degranulation very specific, such that if a person is allergic to peanuts, there will be peanut-specific IgE bound to his or her mast cells. In this person, eating peanuts will cause the mast cells to degranulate, sometimes causing severe allergic reactions, including anaphylaxis, a severe, systemic allergic response that can cause death.
Clonal Selection of B Cells
Clonal selection and expansion work much the same way in B cells as in T cells. Only B cells with appropriate antigen specificity are selected for and expanded (Figure 21.23). Eventually, the plasma cells secrete antibodies with antigenic specificity identical to those that were on the surfaces of the selected B cells. Notice in the figure that both plasma cells and memory B cells are generated simultaneously.
Figure 21.23 Clonal Selection of B Cells During a primary B cell immune response, both antibody-secreting plasma cells and memory B cells are produced. These memory cells lead to the differentiation of more plasma cells and memory B cells during secondary responses.
Primary versus Secondary B Cell Responses
Primary and secondary responses as they relate to T cells were discussed earlier. This section will look at these responses with B cells and antibody production. Because antibodies are easily obtained from blood samples, they are easy to follow and graph (Figure 21.24). As you will see from the figure, the primary response to an antigen (representing a pathogen) is delayed by several days. This is the time it takes for the B cell clones to expand and differentiate into plasma cells. The level of antibody produced is low, but it is sufficient for immune protection. The second time a person encounters the same antigen, there is no time delay, and the amount of antibody made is much higher. Thus, the secondary antibody response overwhelms the pathogens quickly and, in most situations, no symptoms are felt. When a different antigen is used, another primary response is made with its low antibody levels and time delay.
Figure 21.24 Primary and Secondary Antibody Responses Antigen A is given once to generate a primary response and later to generate a secondary response. When a different antigen is given for the first time, a new primary response is made.
Active versus Passive Immunity
Immunity to pathogens, and the ability to control pathogen growth so that damage to the tissues of the body is limited, can be acquired by (1) the active development of an immune response in the infected individual or (2) the passive transfer of immune components from an immune individual to a nonimmune one. Both active and passive immunity have examples in the natural world and as part of medicine.
Active immunity is the resistance to pathogens acquired during an adaptive immune response within an individual (Table 21.6). Naturally acquired active immunity, the response to a pathogen, is the focus of this chapter. Artificially acquired active immunity involves the use of vaccines. A vaccine is a killed or weakened pathogen or its components that, when administered to a healthy individual, leads to the development of immunological memory (a weakened primary immune response) without causing much in the way of symptoms. Thus, with the use of vaccines, one can avoid the damage from disease that results from the first exposure to the pathogen, yet reap the benefits of protection from immunological memory. The advent of vaccines was one of the major medical advances of the twentieth century and led to the eradication of smallpox and the control of many infectious diseases, including polio, measles, and whooping cough.
Active versus Passive Immunity
| Natural | Artificial | |
|---|---|---|
| Active | Adaptive immune response | Vaccine response |
| Passive | Trans-placental antibodies/breastfeeding | Immune globulin injections |
Table 21.6
Passive immunity arises from the transfer of antibodies to an individual without requiring them to mount their own active immune response. Naturally acquired passive immunity is seen during fetal development. IgG is transferred from the maternal circulation to the fetus via the placenta, protecting the fetus from infection and protecting the newborn for the first few months of its life. As already stated, a newborn benefits from the IgA antibodies it obtains from milk during breastfeeding. The fetus and newborn thus benefit from the immunological memory of the mother to the pathogens to which she has been exposed. In medicine, artificially acquired passive immunity usually involves injections of immunoglobulins, taken from animals previously exposed to a specific pathogen. This treatment is a fast-acting method of temporarily protecting an individual who was possibly exposed to a pathogen. The downside to both types of passive immunity is the lack of the development of immunological memory. Once the antibodies are transferred, they are effective for only a limited time before they degrade.
INTERACTIVE LINK
Immunity can be acquired in an active or passive way, and it can be natural or artificial. Watch this video to see an animated discussion of passive and active immunity. What is an example of natural immunity acquired passively?
T cell-dependent versus T cell-independent Antigens
As discussed previously, Th2 cells secrete cytokines that drive the production of antibodies in a B cell, responding to complex antigens such as those made by proteins. On the other hand, some antigens are T cell independent. A T cell-independent antigenusually is in the form of repeated carbohydrate moieties found on the cell walls of bacteria. Each antibody on the B cell surface has two binding sites, and the repeated nature of T cell-independent antigen leads to crosslinking of the surface antibodies on the B cell. The crosslinking is enough to activate it in the absence of T cell cytokines.
A T cell-dependent antigen, on the other hand, usually is not repeated to the same degree on the pathogen and thus does not crosslink surface antibody with the same efficiency. To elicit a response to such antigens, the B and T cells must come close together (Figure 21.25). The B cell must receive two signals to become activated. Its surface immunoglobulin must recognize native antigen. Some of this antigen is internalized, processed, and presented to the Th2 cells on a class II MHC molecule. The T cell then binds using its antigen receptor and is activated to secrete cytokines that diffuse to the B cell, finally activating it completely. Thus, the B cell receives signals from both its surface antibody and the T cell via its cytokines, and acts as a professional antigen-presenting cell in the process.
Figure 21.25 T and B Cell Binding To elicit a response to a T cell-dependent antigen, the B and T cells must come close together. To become fully activated, the B cell must receive two signals from the native antigen and the T cell’s cytokines.
The Immune Response against Pathogens
- Explain the development of immunological competence
- Describe the mucosal immune response
- Discuss immune responses against bacterial, viral, fungal, and animal pathogens
- Describe different ways pathogens evade immune responses
Now that you understand the development of mature, naïve B cells and T cells, and some of their major functions, how do all of these various cells, proteins, and cytokines come together to actually resolve an infection? Ideally, the immune response will rid the body of a pathogen entirely. The adaptive immune response, with its rapid clonal expansion, is well suited to this purpose. Think of a primary infection as a race between the pathogen and the immune system. The pathogen bypasses barrier defenses and starts multiplying in the host’s body. During the first 4 to 5 days, the innate immune response will partially control, but not stop, pathogen growth. As the adaptive immune response gears up, however, it will begin to clear the pathogen from the body, while at the same time becoming stronger and stronger. When following antibody responses in patients with a particular disease such as a virus, this clearance is referred to as seroconversion (sero- = “serum”). Seroconversion is the reciprocal relationship between virus levels in the blood and antibody levels. As the antibody levels rise, the virus levels decline, and this is a sign that the immune response is being at least partially effective (partially, because in many diseases, seroconversion does not necessarily mean a patient is getting well).
An excellent example of this is seroconversion during HIV disease (Figure 21.26). Notice that antibodies are made early in this disease, and the increase in anti-HIV antibodies correlates with a decrease in detectable virus in the blood. Although these antibodies are an important marker for diagnosing the disease, they are not sufficient to completely clear the virus. Several years later, the vast majority of these individuals, if untreated, will lose their entire adaptive immune response, including the ability to make antibodies, during the final stages of AIDS.
Figure 21.26 HIV Disease Progression Seroconversion, the rise of anti-HIV antibody levels and the concomitant decline in measurable virus levels, happens during the first several months of HIV disease. Unfortunately, this antibody response is ineffective at controlling the disease, as seen by the progression of the disease towards AIDS, in which all adaptive immune responses are compromised.
EVERYDAY CONNECTION
Disinfectants: Fighting the Good Fight?
“Wash your hands!” Parents have been telling their children this for generations. Dirty hands can spread disease. But is it possible to get rid of enough pathogens that children will never get sick? Are children who avoid exposure to pathogens better off? The answers to both these questions appears to be no.
Antibacterial wipes, soaps, gels, and even toys with antibacterial substances embedded in their plastic are ubiquitous in our society. Still, these products do not rid the skin and gastrointestinal tract of bacteria, and it would be harmful to our health if they did. We need these nonpathogenic bacteria on and within our bodies to keep the pathogenic ones from growing. The urge to keep children perfectly clean is thus probably misguided. Children will get sick anyway, and the later benefits of immunological memory far outweigh the minor discomforts of most childhood diseases. In fact, getting diseases such as chickenpox or measles later in life is much harder on the adult and are associated with symptoms significantly worse than those seen in the childhood illnesses. Of course, vaccinations help children avoid some illnesses, but there are so many pathogens, we will never be immune to them all.
Could over-cleanliness be the reason that allergies are increasing in more developed countries? Some scientists think so. Allergies are based on an IgE antibody response. Many scientists think the system evolved to help the body rid itself of worm parasites. The hygiene theory is the idea that the immune system is geared to respond to antigens, and if pathogens are not present, it will respond instead to inappropriate antigens such as allergens and self-antigens. This is one explanation for the rising incidence of allergies in developed countries, where the response to nonpathogens like pollen, shrimp, and cat dander cause allergic responses while not serving any protective function.
The Mucosal Immune Response
Mucosal tissues are major barriers to the entry of pathogens into the body. The IgA (and sometimes IgM) antibodies in mucus and other secretions can bind to the pathogen, and in the cases of many viruses and bacteria, neutralize them. Neutralization is the process of coating a pathogen with antibodies, making it physically impossible for the pathogen to bind to receptors. Neutralization, which occurs in the blood, lymph, and other body fluids and secretions, protects the body constantly. Neutralizing antibodies are the basis for the disease protection offered by vaccines. Vaccinations for diseases that commonly enter the body via mucous membranes, such as influenza, are usually formulated to enhance IgA production.
Immune responses in some mucosal tissues such as the Peyer’s patches (see Figure 21.11) in the small intestine take up particulate antigens by specialized cells known as microfold or M cells (Figure 21.27). These cells allow the body to sample potential pathogens from the intestinal lumen. Dendritic cells then take the antigen to the regional lymph nodes, where an immune response is mounted.
Figure 21.27 IgA Immunity The nasal-associated lymphoid tissue and Peyer’s patches of the small intestine generate IgA immunity. Both use M cells to transport antigen inside the body so that immune responses can be mounted.
Defenses against Bacteria and Fungi
The body fights bacterial pathogens with a wide variety of immunological mechanisms, essentially trying to find one that is effective. Bacteria such as Mycobacterium leprae, the cause of leprosy, are resistant to lysosomal enzymes and can persist in macrophage organelles or escape into the cytosol. In such situations, infected macrophages receiving cytokine signals from Th1 cells turn on special metabolic pathways. Macrophage oxidative metabolism is hostile to intracellular bacteria, often relying on the production of nitric oxide to kill the bacteria inside the macrophage.
Fungal infections, such as those from Aspergillus, Candida, and Pneumocystis, are largely opportunistic infections that take advantage of suppressed immune responses. Most of the same immune mechanisms effective against bacteria have similar effects on fungi, both of which have characteristic cell wall structures that protect their cells.
Defenses against Parasites
Worm parasites such as helminths are seen as the primary reason why the mucosal immune response, IgE-mediated allergy and asthma, and eosinophils evolved. These parasites were at one time very common in human society. When infecting a human, often via contaminated food, some worms take up residence in the gastrointestinal tract. Eosinophils are attracted to the site by T cell cytokines, which release their granule contents upon their arrival. Mast cell degranulation also occurs, and the fluid leakage caused by the increase in local vascular permeability is thought to have a flushing action on the parasite, expelling its larvae from the body. Furthermore, if IgE labels the parasite, the eosinophils can bind to it by its Fc receptor.
Defenses against Viruses
The primary mechanisms against viruses are NK cells, interferons, and cytotoxic T cells. Antibodies are effective against viruses mostly during protection, where an immune individual can neutralize them based on a previous exposure. Antibodies have no effect on viruses or other intracellular pathogens once they enter the cell, since antibodies are not able to penetrate the plasma membrane of the cell. Many cells respond to viral infections by downregulating their expression of MHC class I molecules. This is to the advantage of the virus, because without class I expression, cytotoxic T cells have no activity. NK cells, however, can recognize virally infected class I-negative cells and destroy them. Thus, NK and cytotoxic T cells have complementary activities against virally infected cells.
Interferons have activity in slowing viral replication and are used in the treatment of certain viral diseases, such as hepatitis B and C, but their ability to eliminate the virus completely is limited. The cytotoxic T cell response, though, is key, as it eventually overwhelms the virus and kills infected cells before the virus can complete its replicative cycle. Clonal expansion and the ability of cytotoxic T cells to kill more than one target cell make these cells especially effective against viruses. In fact, without cytotoxic T cells, it is likely that humans would all die at some point from a viral infection (if no vaccine were available).
Evasion of the Immune System by Pathogens
It is important to keep in mind that although the immune system has evolved to be able to control many pathogens, pathogens themselves have evolved ways to evade the immune response. An example already mentioned is in Mycobactrium tuberculosis, which has evolved a complex cell wall that is resistant to the digestive enzymes of the macrophages that ingest them, and thus persists in the host, causing the chronic disease tuberculosis. This section briefly summarizes other ways in which pathogens can “outwit” immune responses. But keep in mind, although it seems as if pathogens have a will of their own, they do not. All of these evasive “strategies” arose strictly by evolution, driven by selection.
Bacteria sometimes evade immune responses because they exist in multiple strains, such as different groups of Staphylococcus aureus. S. aureus is commonly found in minor skin infections, such as boils, and some healthy people harbor it in their nose. One small group of strains of this bacterium, however, called methicillin-resistant Staphylococcus aureus, has become resistant to multiple antibiotics and is essentially untreatable. Different bacterial strains differ in the antigens on their surfaces. The immune response against one strain (antigen) does not affect the other; thus, the species survives.
Another method of immune evasion is mutation. Because viruses’ surface molecules mutate continuously, viruses like influenza change enough each year that the flu vaccine for one year may not protect against the flu common to the next. New vaccine formulations must be derived for each flu season.
Genetic recombination—the combining of gene segments from two different pathogens—is an efficient form of immune evasion. For example, the influenza virus contains gene segments that can recombine when two different viruses infect the same cell. Recombination between human and pig influenza viruses led to the 2010 H1N1 swine flu outbreak.
Pathogens can produce immunosuppressive molecules that impair immune function, and there are several different types. Viruses are especially good at evading the immune response in this way, and many types of viruses have been shown to suppress the host immune response in ways much more subtle than the wholesale destruction caused by HIV.
Diseases Associated with Depressed or Overactive Immune Responses
- Discuss inherited and acquired immunodeficiencies
- Explain the four types of hypersensitivity and how they differ
- Give an example of how autoimmune disease breaks tolerance
This section is about how the immune system goes wrong. When it goes haywire, and becomes too weak or too strong, it leads to a state of disease. The factors that maintain immunological homeostasis are complex and incompletely understood.
Immunodeficiencies
As you have seen, the immune system is quite complex. It has many pathways using many cell types and signals. Because it is so complex, there are many ways for it to go wrong. Inherited immunodeficiencies arise from gene mutations that affect specific components of the immune response. There are also acquired immunodeficiencies with potentially devastating effects on the immune system, such as HIV.
Inherited Immunodeficiencies
A list of all inherited immunodeficiencies is well beyond the scope of this book. The list is almost as long as the list of cells, proteins, and signaling molecules of the immune system itself. Some deficiencies, such as those for complement, cause only a higher susceptibility to some Gram-negative bacteria. Others are more severe in their consequences. Certainly, the most serious of the inherited immunodeficiencies is severe combined immunodeficiency disease (SCID). This disease is complex because it is caused by many different genetic defects. What groups them together is the fact that both the B cell and T cell arms of the adaptive immune response are affected.
Children with this disease usually die of opportunistic infections within their first year of life unless they receive a bone marrow transplant. Such a procedure had not yet been perfected for David Vetter, the “boy in the bubble,” who was treated for SCID by having to live almost his entire life in a sterile plastic cocoon for the 12 years before his death from infection in 1984. One of the features that make bone marrow transplants work as well as they do is the proliferative capability of hematopoietic stem cells of the bone marrow. Only a small amount of bone marrow from a healthy donor is given intravenously to the recipient. It finds its own way to the bone where it populates it, eventually reconstituting the patient’s immune system, which is usually destroyed beforehand by treatment with radiation or chemotherapeutic drugs.
New treatments for SCID using gene therapy, inserting nondefective genes into cells taken from the patient and giving them back, have the advantage of not needing the tissue match required for standard transplants. Although not a standard treatment, this approach holds promise, especially for those in whom standard bone marrow transplantation has failed.
Human Immunodeficiency Virus/AIDS
Although many viruses cause suppression of the immune system, only one wipes it out completely, and that is the previously mentioned HIV. It is worth discussing the biology of this virus, which can lead to the well-known AIDS, so that its full effects on the immune system can be understood. The virus is transmitted through semen, vaginal fluids, and blood, and can be caught by risky sexual behaviors and the sharing of needles by intravenous drug users. There are sometimes, but not always, flu-like symptoms in the first 1 to 2 weeks after infection. This is later followed by seroconversion. The anti-HIV antibodies formed during seroconversion are the basis for most initial HIV screening done in the United States. Because seroconversion takes different lengths of time in different individuals, multiple AIDS tests are given months apart to confirm or eliminate the possibility of infection.
After seroconversion, the amount of virus circulating in the blood drops and stays at a low level for several years. During this time, the levels of CD4+ cells, especially helper T cells, decline steadily, until at some point, the immune response is so weak that opportunistic disease and eventually death result. HIV uses CD4 as the receptor to get inside cells, but it also needs a co-receptor, such as CCR5 or CXCR4. These co-receptors, which usually bind to chemokines, present another target for anti-HIV drug development. Although other antigen-presenting cells are infected with HIV, given that CD4+ helper T cells play an important role in T cell immune responses and antibody responses, it should be no surprise that both types of immune responses are eventually seriously compromised.
Treatment for the disease consists of drugs that target virally encoded proteins that are necessary for viral replication but are absent from normal human cells. By targeting the virus itself and sparing the cells, this approach has been successful in significantly prolonging the lives of HIV-positive individuals. On the other hand, an HIV vaccine has been 30 years in development and is still years away. Because the virus mutates rapidly to evade the immune system, scientists have been looking for parts of the virus that do not change and thus would be good targets for a vaccine candidate.
Hypersensitivities
The word “hypersensitivity” simply means sensitive beyond normal levels of activation. Allergies and inflammatory responses to nonpathogenic environmental substances have been observed since the dawn of history. Hypersensitivity is a medical term describing symptoms that are now known to be caused by unrelated mechanisms of immunity. Still, it is useful for this discussion to use the four types of hypersensitivities as a guide to understand these mechanisms (Figure 21.28).
Figure 21.28 Immune Hypersensitivity Components of the immune system cause four types of hypersensitivity. Notice that types I–III are B cell mediated, whereas type IV hypersensitivity is exclusively a T cell phenomenon.
Immediate (Type I) Hypersensitivity
Antigens that cause allergic responses are often referred to as allergens. The specificity of the immediate hypersensitivityresponse is predicated on the binding of allergen-specific IgE to the mast cell surface. The process of producing allergen-specific IgE is called sensitization, and is a necessary prerequisite for the symptoms of immediate hypersensitivity to occur. Allergies and allergic asthma are mediated by mast cell degranulation that is caused by the crosslinking of the antigen-specific IgE molecules on the mast cell surface. The mediators released have various vasoactive effects already discussed, but the major symptoms of inhaled allergens are the nasal edema and runny nose caused by the increased vascular permeability and increased blood flow of nasal blood vessels. As these mediators are released with mast cell degranulation, type I hypersensitivity reactions are usually rapid and occur within just a few minutes, hence the term immediate hypersensitivity.
Most allergens are in themselves nonpathogenic and therefore innocuous. Some individuals develop mild allergies, which are usually treated with antihistamines. Others develop severe allergies that may cause anaphylactic shock, which can potentially be fatal within 20 to 30 minutes if untreated. This drop in blood pressure (shock) with accompanying contractions of bronchial smooth muscle is caused by systemic mast cell degranulation when an allergen is eaten (for example, shellfish and peanuts), injected (by a bee sting or being administered penicillin), or inhaled (asthma). Because epinephrine raises blood pressure and relaxes bronchial smooth muscle, it is routinely used to counteract the effects of anaphylaxis and can be lifesaving. Patients with known severe allergies are encouraged to keep automatic epinephrine injectors with them at all times, especially when away from easy access to hospitals.
Allergists use skin testing to identify allergens in type I hypersensitivity. In skin testing, allergen extracts are injected into the epidermis, and a positive result of a soft, pale swelling at the site surrounded by a red zone (called the wheal and flare response), caused by the release of histamine and the granule mediators, usually occurs within 30 minutes. The soft center is due to fluid leaking from the blood vessels and the redness is caused by the increased blood flow to the area that results from the dilation of local blood vessels at the site.
Type II and Type III Hypersensitivities
Type II hypersensitivity, which involves IgG-mediated lysis of cells by complement proteins, occurs during mismatched blood transfusions and blood compatibility diseases such as erythroblastosis fetalis (see section on transplantation). Type III hypersensitivity occurs with diseases such as systemic lupus erythematosus, where soluble antigens, mostly DNA and other material from the nucleus, and antibodies accumulate in the blood to the point that the antigen and antibody precipitate along blood vessel linings. These immune complexes often lodge in the kidneys, joints, and other organs where they can activate complement proteins and cause inflammation.
Delayed (Type IV) Hypersensitivity
Delayed hypersensitivity, or type IV hypersensitivity, is basically a standard cellular immune response. In delayed hypersensitivity, the first exposure to an antigen is called sensitization, such that on re-exposure, a secondary cellular response results, secreting cytokines that recruit macrophages and other phagocytes to the site. These sensitized T cells, of the Th1 class, will also activate cytotoxic T cells. The time it takes for this reaction to occur accounts for the 24- to 72-hour delay in development.
The classical test for delayed hypersensitivity is the tuberculin test for tuberculosis, where bacterial proteins from M. tuberculosisare injected into the skin. A couple of days later, a positive test is indicated by a raised red area that is hard to the touch, called an induration, which is a consequence of the cellular infiltrate, an accumulation of activated macrophages. A positive tuberculin test means that the patient has been exposed to the bacteria and exhibits a cellular immune response to it.
Another type of delayed hypersensitivity is contact sensitivity, where substances such as the metal nickel cause a red and swollen area upon contact with the skin. The individual must have been previously sensitized to the metal. A much more severe case of contact sensitivity is poison ivy, but many of the harshest symptoms of the reaction are associated with the toxicity of its oils and are not T cell mediated.
Autoimmune Responses
The worst cases of the immune system over-reacting are autoimmune diseases. Somehow, tolerance breaks down and the immune systems in individuals with these diseases begin to attack their own bodies, causing significant damage. The trigger for these diseases is, more often than not, unknown, and the treatments are usually based on resolving the symptoms using immunosuppressive and anti-inflammatory drugs such as steroids. These diseases can be localized and crippling, as in rheumatoid arthritis, or diffuse in the body with multiple symptoms that differ in different individuals, as is the case with systemic lupus erythematosus (Figure 21.29).
Figure 21.29 Autoimmune Disorders: Rheumatoid Arthritis and Lupus (a) Extensive damage to the right hand of a rheumatoid arthritis sufferer is shown in the x-ray. (b) The diagram shows a variety of possible symptoms of systemic lupus erythematosus.
Environmental triggers seem to play large roles in autoimmune responses. One explanation for the breakdown of tolerance is that, after certain bacterial infections, an immune response to a component of the bacterium cross-reacts with a self-antigen. This mechanism is seen in rheumatic fever, a result of infection with Streptococcus bacteria, which causes strep throat. The antibodies to this pathogen’s M protein cross-react with an antigenic component of heart myosin, a major contractile protein of the heart that is critical to its normal function. The antibody binds to these molecules and activates complement proteins, causing damage to the heart, especially to the heart valves. On the other hand, some theories propose that having multiple common infectious diseases actually prevents autoimmune responses. The fact that autoimmune diseases are rare in countries that have a high incidence of infectious diseases supports this idea, another example of the hygiene hypothesis discussed earlier in this chapter.
There are genetic factors in autoimmune diseases as well. Some diseases are associated with the MHC genes that an individual expresses. The reason for this association is likely because if one’s MHC molecules are not able to present a certain self-antigen, then that particular autoimmune disease cannot occur. Overall, there are more than 80 different autoimmune diseases, which are a significant health problem in the elderly. Table 21.7 lists several of the most common autoimmune diseases, the antigens that are targeted, and the segment of the adaptive immune response that causes the damage.
Autoimmune Diseases
| Disease | Autoantigen | Symptoms |
|---|---|---|
| Celiac disease | Tissue transglutaminase | Damage to small intestine |
| Diabetes mellitus type I | Beta cells of pancreas | Low insulin production; inability to regulate serum glucose |
| Graves’ disease | Thyroid-stimulating hormone receptor (antibody blocks receptor) | Hyperthyroidism |
| Hashimoto’s thyroiditis | Thyroid-stimulating hormone receptor (antibody mimics hormone and stimulates receptor) | Hypothyroidism |
| Lupus erythematosus | Nuclear DNA and proteins | Damage of many body systems |
| Myasthenia gravis | Acetylcholine receptor in neuromuscular junctions | Debilitating muscle weakness |
| Rheumatoid arthritis | Joint capsule antigens | Chronic inflammation of joints |
Table 21.7
Transplantation and Cancer Immunology
- Explain why blood typing is important and what happens when mismatched blood is used in a transfusion
- Describe how tissue typing is done during organ transplantation and the role of transplant anti-rejection drugs
- Show how the immune response is able to control some cancers and how this immune response might be enhanced by cancer vaccines
The immune responses to transplanted organs and to cancer cells are both important medical issues. With the use of tissue typing and anti-rejection drugs, transplantation of organs and the control of the anti-transplant immune response have made huge strides in the past 50 years. Today, these procedures are commonplace. Tissue typing is the determination of MHC molecules in the tissue to be transplanted to better match the donor to the recipient. The immune response to cancer, on the other hand, has been more difficult to understand and control. Although it is clear that the immune system can recognize some cancers and control them, others seem to be resistant to immune mechanisms.
The Rh Factor
Red blood cells can be typed based on their surface antigens. ABO blood type, in which individuals are type A, B, AB, or O according to their genetics, is one example. A separate antigen system seen on red blood cells is the Rh antigen. When someone is “A positive” for example, the positive refers to the presence of the Rh antigen, whereas someone who is “A negative” would lack this molecule.
An interesting consequence of Rh factor expression is seen in erythroblastosis fetalis, a hemolytic disease of the newborn (Figure 21.30). This disease occurs when mothers negative for Rh antigen have multiple Rh-positive children. During the birth of a first Rh-positive child, the mother makes a primary anti-Rh antibody response to the fetal blood cells that enter the maternal bloodstream. If the mother has a second Rh-positive child, IgG antibodies against Rh-positive blood mounted during this secondary response cross the placenta and attack the fetal blood, causing anemia. This is a consequence of the fact that the fetus is not genetically identical to the mother, and thus the mother is capable of mounting an immune response against it. This disease is treated with antibodies specific for Rh factor. These are given to the mother during the first and subsequent births, destroying any fetal blood that might enter her system and preventing the immune response.
Figure 21.30 Erythroblastosis Fetalis Erythroblastosis fetalis (hemolytic disease of the newborn) is the result of an immune response in an Rh-negative mother who has multiple children with an Rh-positive father. During the first birth, fetal blood enters the mother’s circulatory system, and anti-Rh antibodies are made. During the gestation of the second child, these antibodies cross the placenta and attack the blood of the fetus. The treatment for this disease is to give the mother anti-Rh antibodies (RhoGAM) during the first pregnancy to destroy Rh-positive fetal red blood cells from entering her system and causing the anti-Rh antibody response in the first place.
Tissue Transplantation
Tissue transplantation is more complicated than blood transfusions because of two characteristics of MHC molecules. These molecules are the major cause of transplant rejection (hence the name “histocompatibility”). MHC polygeny refers to the multiple MHC proteins on cells, and MHC polymorphism refers to the multiple alleles for each individual MHC locus. Thus, there are many alleles in the human population that can be expressed (Table 21.8 and Table 21.9). When a donor organ expresses MHC molecules that are different from the recipient, the latter will often mount a cytotoxic T cell response to the organ and reject it. Histologically, if a biopsy of a transplanted organ exhibits massive infiltration of T lymphocytes within the first weeks after transplant, it is a sign that the transplant is likely to fail. The response is a classical, and very specific, primary T cell immune response. As far as medicine is concerned, the immune response in this scenario does the patient no good at all and causes significant harm.
Partial Table of Alleles of the Human MHC (Class I)
| Gene | # of alleles | # of possible MHC I protein components |
|---|---|---|
| A | 2132 | 1527 |
| B | 2798 | 2110 |
| C | 1672 | 1200 |
| E | 11 | 3 |
| F | 22 | 4 |
| G | 50 | 16 |
Table 21.8
Partial Table of Alleles of the Human MHC (Class II)
| Gene | # of alleles | # of possible MHC II protein components |
|---|---|---|
| DRA | 7 | 2 |
| DRB | 1297 | 958 |
| DQA1 | 49 | 31 |
| DQB1 | 179 | 128 |
| DPA1 | 36 | 18 |
| DPB1 | 158 | 136 |
| DMA | 7 | 4 |
| DMB | 13 | 7 |
| DOA | 12 | 3 |
| DOB | 13 | 5 |
Table 21.9
Immunosuppressive drugs such as cyclosporine A have made transplants more successful, but matching the MHC molecules is still key. In humans, there are six MHC molecules that show the most polymorphisms, three class I molecules (A, B, and C) and three class II molecules called DP, DQ, and DR. A successful transplant usually requires a match between at least 3–4 of these molecules, with more matches associated with greater success. Family members, since they share a similar genetic background, are much more likely to share MHC molecules than unrelated individuals do. In fact, due to the extensive polymorphisms in these MHC molecules, unrelated donors are found only through a worldwide database. The system is not foolproof however, as there are not enough individuals in the system to provide the organs necessary to treat all patients needing them.
One disease of transplantation occurs with bone marrow transplants, which are used to treat various diseases, including SCID and leukemia. Because the bone marrow cells being transplanted contain lymphocytes capable of mounting an immune response, and because the recipient’s immune response has been destroyed before receiving the transplant, the donor cells may attack the recipient tissues, causing graft-versus-host disease. Symptoms of this disease, which usually include a rash and damage to the liver and mucosa, are variable, and attempts have been made to moderate the disease by first removing mature T cells from the donor bone marrow before transplanting it.
Immune Responses Against Cancer
It is clear that with some cancers, for example Kaposi’s sarcoma, a healthy immune system does a good job at controlling them (Figure 21.31). This disease, which is caused by the human herpesvirus, is almost never observed in individuals with strong immune systems, such as the young and immunocompetent. Other examples of cancers caused by viruses include liver cancer caused by the hepatitis B virus and cervical cancer caused by the human papilloma virus. As these last two viruses have vaccines available for them, getting vaccinated can help prevent these two types of cancer by stimulating the immune response.
Figure 21.31 Karposi’s Sarcoma Lesions (credit: National Cancer Institute)
On the other hand, as cancer cells are often able to divide and mutate rapidly, they may escape the immune response, just as certain pathogens such as HIV do. There are three stages in the immune response to many cancers: elimination, equilibrium, and escape. Elimination occurs when the immune response first develops toward tumor-specific antigens specific to the cancer and actively kills most cancer cells, followed by a period of controlled equilibrium during which the remaining cancer cells are held in check. Unfortunately, many cancers mutate, so they no longer express any specific antigens for the immune system to respond to, and a subpopulation of cancer cells escapes the immune response, continuing the disease process.
This fact has led to extensive research in trying to develop ways to enhance the early immune response to completely eliminate the early cancer and thus prevent a later escape. One method that has shown some success is the use of cancer vaccines, which differ from viral and bacterial vaccines in that they are directed against the cells of one’s own body. Treated cancer cells are injected into cancer patients to enhance their anti-cancer immune response and thereby prolong survival. The immune system has the capability to detect these cancer cells and proliferate faster than the cancer cells do, overwhelming the cancer in a similar way as they do for viruses. Cancer vaccines have been developed for malignant melanoma, a highly fatal skin cancer, and renal (kidney) cell carcinoma. These vaccines are still in the development stages, but some positive and encouraging results have been obtained clinically.
It is tempting to focus on the complexity of the immune system and the problems it causes as a negative. The upside to immunity, however, is so much greater: The benefit of staying alive far outweighs the negatives caused when the system does sometimes go awry. Working on “autopilot,” the immune system helps to maintain your health and kill pathogens. The only time you really miss the immune response is when it is not being effective and illness results, or, as in the extreme case of HIV disease, the immune system is gone completely.
EVERYDAY CONNECTION
How Stress Affects the Immune Response: The Connections between the Immune, Nervous, and Endocrine Systems of the Body
The immune system cannot exist in isolation. After all, it has to protect the entire body from infection. Therefore, the immune system is required to interact with other organ systems, sometimes in complex ways. Thirty years of research focusing on the connections between the immune system, the central nervous system, and the endocrine system have led to a new science with the unwieldy name of called psychoneuroimmunology. The physical connections between these systems have been known for centuries: All primary and secondary organs are connected to sympathetic nerves. What is more complex, though, is the interaction of neurotransmitters, hormones, cytokines, and other soluble signaling molecules, and the mechanism of “crosstalk” between the systems. For example, white blood cells, including lymphocytes and phagocytes, have receptors for various neurotransmitters released by associated neurons. Additionally, hormones such as cortisol (naturally produced by the adrenal cortex) and prednisone (synthetic) are well known for their abilities to suppress T cell immune mechanisms, hence, their prominent use in medicine as long-term, anti-inflammatory drugs.
One well-established interaction of the immune, nervous, and endocrine systems is the effect of stress on immune health. In the human vertebrate evolutionary past, stress was associated with the fight-or-flight response, largely mediated by the central nervous system and the adrenal medulla. This stress was necessary for survival. The physical action of fighting or running, whichever the animal decides, usually resolves the problem in one way or another. On the other hand, there are no physical actions to resolve most modern day stresses, including short-term stressors like taking examinations and long-term stressors such as being unemployed or losing a spouse. The effect of stress can be felt by nearly every organ system, and the immune system is no exception (Table 21.10).
Effects of Stress on Body Systems
| System | Stress-related illness |
|---|---|
| Integumentary system | Acne, skin rashes, irritation |
| Nervous system | Headaches, depression, anxiety, irritability, loss of appetite, lack of motivation, reduced mental performance |
| Muscular and skeletal systems | Muscle and joint pain, neck and shoulder pain |
| Circulatory system | Increased heart rate, hypertension, increased probability of heart attacks |
| Digestive system | Indigestion, heartburn, stomach pain, nausea, diarrhea, constipation, weight gain or loss |
| Immune system | Depressed ability to fight infections |
| Male reproductive system | Lowered sperm production, impotence, reduced sexual desire |
| Female reproductive system | Irregular menstrual cycle, reduced sexual desire |
Table 21.10
At one time, it was assumed that all types of stress reduced all aspects of the immune response, but the last few decades of research have painted a different picture. First, most short-term stress does not impair the immune system in healthy individuals enough to lead to a greater incidence of diseases. However, older individuals and those with suppressed immune responses due to disease or immunosuppressive drugs may respond even to short-term stressors by getting sicker more often. It has been found that short-term stress diverts the body’s resources towards enhancing innate immune responses, which have the ability to act fast and would seem to help the body prepare better for possible infections associated with the trauma that may result from a fight-or-flight exchange. The diverting of resources away from the adaptive immune response, however, causes its own share of problems in fighting disease.
Chronic stress, unlike short-term stress, may inhibit immune responses even in otherwise healthy adults. The suppression of both innate and adaptive immune responses is clearly associated with increases in some diseases, as seen when individuals lose a spouse or have other long-term stresses, such as taking care of a spouse with a fatal disease or dementia. The new science of psychoneuroimmunology, while still in its relative infancy, has great potential to make exciting advances in our understanding of how the nervous, endocrine, and immune systems have evolved together and communicate with each other.
Key Terms
- active immunity
- immunity developed from an individual’s own immune system
- acute inflammation
- inflammation occurring for a limited time period; rapidly developing
- adaptive immune response
- relatively slow but very specific and effective immune response controlled by lymphocytes
- afferent lymphatic vessels
- lead into a lymph node
- antibody
- antigen-specific protein secreted by plasma cells; immunoglobulin
- antigen
- molecule recognized by the receptors of B and T lymphocytes
- antigen presentation
- binding of processed antigen to the protein-binding cleft of a major histocompatibility complex molecule
- antigen processing
- internalization and digestion of antigen in an antigen-presenting cell
- antigen receptor
- two-chain receptor by which lymphocytes recognize antigen
- antigenic determinant
- (also, epitope) one of the chemical groups recognized by a single type of lymphocyte antigen receptor
- B cells
- lymphocytes that act by differentiating into an antibody-secreting plasma cell
- barrier defenses
- antipathogen defenses deriving from a barrier that physically prevents pathogens from entering the body to establish an infection
- bone marrow
- tissue found inside bones; the site of all blood cell differentiation and maturation of B lymphocytes
- bronchus-associated lymphoid tissue (BALT)
- lymphoid nodule associated with the respiratory tract
- central tolerance
- B cell tolerance induced in immature B cells of the bone marrow
- chemokine
- soluble, long-range, cell-to-cell communication molecule
- chronic inflammation
- inflammation occurring for long periods of time
- chyle
- lipid-rich lymph inside the lymphatic capillaries of the small intestine
- cisterna chyli
- bag-like vessel that forms the beginning of the thoracic duct
- class switching
- ability of B cells to change the class of antibody they produce without altering the specificity for antigen
- clonal anergy
- process whereby B cells that react to soluble antigens in bone marrow are made nonfunctional
- clonal deletion
- removal of self-reactive B cells by inducing apoptosis
- clonal expansion
- growth of a clone of selected lymphocytes
- clonal selection
- stimulating growth of lymphocytes that have specific receptors
- clone
- group of lymphocytes sharing the same antigen receptor
- complement
- enzymatic cascade of constitutive blood proteins that have antipathogen effects, including the direct killing of bacteria
- constant region domain
- part of a lymphocyte antigen receptor that does not vary much between different receptor types
- cytokine
- soluble, short-range, cell-to-cell communication molecule
- cytotoxic T cells (Tc)
- T lymphocytes with the ability to induce apoptosis in target cells
- delayed hypersensitivity
- (type IV) T cell-mediated immune response against pathogens infiltrating interstitial tissues, causing cellular infiltrate
- early induced immune response
- includes antimicrobial proteins stimulated during the first several days of an infection
- effector T cells
- immune cells with a direct, adverse effect on a pathogen
- efferent lymphatic vessels
- lead out of a lymph node
- erythroblastosis fetalis
- disease of Rh factor-positive newborns in Rh-negative mothers with multiple Rh-positive children; resulting from the action of maternal antibodies against fetal blood
- fas ligand
- molecule expressed on cytotoxic T cells and NK cells that binds to the fas molecule on a target cell and induces it do undergo apoptosis
- Fc region
- in an antibody molecule, the site where the two termini of the heavy chains come together; many cells have receptors for this portion of the antibody, adding functionality to these molecules
- germinal centers
- clusters of rapidly proliferating B cells found in secondary lymphoid tissues
- graft-versus-host disease
- in bone marrow transplants; occurs when the transplanted cells mount an immune response against the recipient
- granzyme
- apoptosis-inducing substance contained in granules of NK cells and cytotoxic T cells
- heavy chain
- larger protein chain of an antibody
- helper T cells (Th)
- T cells that secrete cytokines to enhance other immune responses, involved in activation of both B and T cell lymphocytes
- high endothelial venules
- vessels containing unique endothelial cells specialized to allow migration of lymphocytes from the blood to the lymph node
- histamine
- vasoactive mediator in granules of mast cells and is the primary cause of allergies and anaphylactic shock
- IgA
- antibody whose dimer is secreted by exocrine glands, is especially effective against digestive and respiratory pathogens, and can pass immunity to an infant through breastfeeding
- IgD
- class of antibody whose only known function is as a receptor on naive B cells; important in B cell activation
- IgE
- antibody that binds to mast cells and causes antigen-specific degranulation during an allergic response
- IgG
- main blood antibody of late primary and early secondary responses; passed from mother to unborn child via placenta
- IgM
- antibody whose monomer is a surface receptor of naive B cells; the pentamer is the first antibody made blood plasma during primary responses
- immediate hypersensitivity
- (type I) IgE-mediated mast cell degranulation caused by crosslinking of surface IgE by antigen
- immune system
- series of barriers, cells, and soluble mediators that combine to response to infections of the body with pathogenic organisms
- immunoglobulin
- protein antibody; occurs as one of five main classes
- immunological memory
- ability of the adaptive immune response to mount a stronger and faster immune response upon re-exposure to a pathogen
- inflammation
- basic innate immune response characterized by heat, redness, pain, and swelling
- innate immune response
- rapid but relatively nonspecific immune response
- interferons
- early induced proteins made in virally infected cells that cause nearby cells to make antiviral proteins
- light chain
- small protein chain of an antibody
- lymph
- fluid contained within the lymphatic system
- lymph node
- one of the bean-shaped organs found associated with the lymphatic vessels
- lymphatic capillaries
- smallest of the lymphatic vessels and the origin of lymph flow
- lymphatic system
- network of lymphatic vessels, lymph nodes, and ducts that carries lymph from the tissues and back to the bloodstream.
- lymphatic trunks
- large lymphatics that collect lymph from smaller lymphatic vessels and empties into the blood via lymphatic ducts
- lymphocytes
- white blood cells characterized by a large nucleus and small rim of cytoplasm
- lymphoid nodules
- unencapsulated patches of lymphoid tissue found throughout the body
- macrophage
- ameboid phagocyte found in several tissues throughout the body
- macrophage oxidative metabolism
- metabolism turned on in macrophages by T cell signals that help destroy intracellular bacteria
- major histocompatibility complex (MHC)
- gene cluster whose proteins present antigens to T cells
- mast cell
- cell found in the skin and the lining of body cells that contains cytoplasmic granules with vasoactive mediators such as histamine
- memory T cells
- long-lived immune cell reserved for future exposure to an pathogen
- MHC class I
- found on most cells of the body, it binds to the CD8 molecule on T cells
- MHC class II
- found on macrophages, dendritic cells, and B cells, it binds to CD4 molecules on T cells
- MHC polygeny
- multiple MHC genes and their proteins found in body cells
- MHC polymorphism
- multiple alleles for each individual MHC locus
- monocyte
- precursor to macrophages and dendritic cells seen in the blood
- mucosa-associated lymphoid tissue (MALT)
- lymphoid nodule associated with the mucosa
- naïve lymphocyte
- mature B or T cell that has not yet encountered antigen for the first time
- natural killer cell (NK)
- cytotoxic lymphocyte of innate immune response
- negative selection
- selection against thymocytes in the thymus that react with self-antigen
- neutralization
- inactivation of a virus by the binding of specific antibody
- neutrophil
- phagocytic white blood cell recruited from the bloodstream to the site of infection via the bloodstream
- opsonization
- enhancement of phagocytosis by the binding of antibody or antimicrobial protein
- passive immunity
- transfer of immunity to a pathogen to an individual that lacks immunity to this pathogen usually by the injection of antibodies
- pattern recognition receptor (PRR)
- leukocyte receptor that binds to specific cell wall components of different bacterial species
- perforin
- molecule in NK cell and cytotoxic T cell granules that form pores in the membrane of a target cell
- peripheral tolerance
- mature B cell made tolerant by lack of T cell help
- phagocytosis
- movement of material from the outside to the inside of the cells via vesicles made from invaginations of the plasma membrane
- plasma cell
- differentiated B cell that is actively secreting antibody
- polyclonal response
- response by multiple clones to a complex antigen with many determinants
- positive selection
- selection of thymocytes within the thymus that interact with self, but not non-self, MHC molecules
- primary adaptive response
- immune system’s response to the first exposure to a pathogen
- primary lymphoid organ
- site where lymphocytes mature and proliferate; red bone marrow and thymus gland
- psychoneuroimmunology
- study of the connections between the immune, nervous, and endocrine systems
- regulatory T cells (Treg)
- (also, suppressor T cells) class of CD4 T cells that regulates other T cell responses
- right lymphatic duct
- drains lymph fluid from the upper right side of body into the right subclavian vein
- secondary adaptive response
- immune response observed upon re-exposure to a pathogen, which is stronger and faster than a primary response
- secondary lymphoid organs
- sites where lymphocytes mount adaptive immune responses; examples include lymph nodes and spleen
- sensitization
- first exposure to an antigen
- seroconversion
- clearance of pathogen in the serum and the simultaneous rise of serum antibody
- severe combined immunodeficiency disease (SCID)
- genetic mutation that affects both T cell and B cell arms of the immune response
- spleen
- secondary lymphoid organ that filters pathogens from the blood (white pulp) and removes degenerating or damaged blood cells (red pulp)
- T cell
- lymphocyte that acts by secreting molecules that regulate the immune system or by causing the destruction of foreign cells, viruses, and cancer cells
- T cell tolerance
- process during T cell differentiation where most T cells that recognize antigens from one’s own body are destroyed
- T cell-dependent antigen
- antigen that binds to B cells, which requires signals from T cells to make antibody
- T cell-independent antigen
- binds to B cells, which do not require signals from T cells to make antibody
- Th1 cells
- cells that secrete cytokines that enhance the activity of macrophages and other cells
- Th2 cells
- cells that secrete cytokines that induce B cells to differentiate into antibody-secreting plasma cells
- thoracic duct
- large duct that drains lymph from the lower limbs, left thorax, left upper limb, and the left side of the head
- thymocyte
- immature T cell found in the thymus
- thymus
- primary lymphoid organ; where T lymphocytes proliferate and mature
- tissue typing
- typing of MHC molecules between a recipient and donor for use in a potential transplantation procedure
- tonsils
- lymphoid nodules associated with the nasopharynx
- type I hypersensitivity
- immediate response mediated by mast cell degranulation caused by the crosslinking of the antigen-specific IgE molecules on the mast cell surface
- type II hypersensitivity
- cell damage caused by the binding of antibody and the activation of complement, usually against red blood cells
- type III hypersensitivity
- damage to tissues caused by the deposition of antibody-antigen (immune) complexes followed by the activation of complement
- variable region domain
- part of a lymphocyte antigen receptor that varies considerably between different receptor types
Chapter Review
21.1 Anatomy of the Lymphatic and Immune Systems
The lymphatic system is a series of vessels, ducts, and trunks that remove interstitial fluid from the tissues and return it the blood. The lymphatics are also used to transport dietary lipids and cells of the immune system. Cells of the immune system all come from the hematopoietic system of the bone marrow. Primary lymphoid organs, the bone marrow and thymus gland, are the locations where lymphocytes of the adaptive immune system proliferate and mature. Secondary lymphoid organs are site in which mature lymphocytes congregate to mount immune responses. Many immune system cells use the lymphatic and circulatory systems for transport throughout the body to search for and then protect against pathogens.
21.2 Barrier Defenses and the Innate Immune Response
Innate immune responses are critical to the early control of infections. Whereas barrier defenses are the body’s first line of physical defense against pathogens, innate immune responses are the first line of physiological defense. Innate responses occur rapidly, but with less specificity and effectiveness than the adaptive immune response. Innate responses can be caused by a variety of cells, mediators, and antibacterial proteins such as complement. Within the first few days of an infection, another series of antibacterial proteins are induced, each with activities against certain bacteria, including opsonization of certain species. Additionally, interferons are induced that protect cells from viruses in their vicinity. Finally, the innate immune response does not stop when the adaptive immune response is developed. In fact, both can cooperate and one can influence the other in their responses against pathogens.
21.3 The Adaptive Immune Response: T lymphocytes and Their Functional Types
T cells recognize antigens with their antigen receptor, a complex of two protein chains on their surface. They do not recognize self-antigens, however, but only processed antigen presented on their surfaces in a binding groove of a major histocompatibility complex molecule. T cells develop in the thymus, where they learn to use self-MHC molecules to recognize only foreign antigens, thus making them tolerant to self-antigens. There are several functional types of T lymphocytes, the major ones being helper, regulatory, and cytotoxic T cells.
21.4 The Adaptive Immune Response: B-lymphocytes and Antibodies
B cells, which develop within the bone marrow, are responsible for making five different classes of antibodies, each with its own functions. B cells have their own mechanisms for tolerance, but in peripheral tolerance, the B cells that leave the bone marrow remain inactive due to T cell tolerance. Some B cells do not need T cell cytokines to make antibody, and they bypass this need by the crosslinking of their surface immunoglobulin by repeated carbohydrate residues found in the cell walls of many bacterial species. Others require T cells to become activated.
21.5 The Immune Response against Pathogens
Early childhood is a time when the body develops much of its immunological memory that protects it from diseases in adulthood. The components of the immune response that have the maximum effectiveness against a pathogen are often associated with the class of pathogen involved. Bacteria and fungi are especially susceptible to damage by complement proteins, whereas viruses are taken care of by interferons and cytotoxic T cells. Worms are attacked by eosinophils. Pathogens have shown the ability, however, to evade the body’s immune responses, some leading to chronic infections or even death. The immune system and pathogens are in a slow, evolutionary race to see who stays on top. Modern medicine, hopefully, will keep the results skewed in humans’ favor.
21.6 Diseases Associated with Depressed or Overactive Immune Responses
The immune response can be under-reactive or over-reactive. Suppressed immunity can result from inherited genetic defects or by acquiring viruses. Over-reactive immune responses include the hypersensitivities: B cell- and T cell-mediated immune responses designed to control pathogens, but that lead to symptoms or medical complications. The worst cases of over-reactive immune responses are autoimmune diseases, where an individual’s immune system attacks his or her own body because of the breakdown of immunological tolerance. These diseases are more common in the aged, so treating them will be a challenge in the future as the aged population in the world increases.
21.7 Transplantation and Cancer Immunology
Blood transfusion and organ transplantation both require an understanding of the immune response to prevent medical complications. Blood needs to be typed so that natural antibodies against mismatched blood will not destroy it, causing more harm than good to the recipient. Transplanted organs must be matched by their MHC molecules and, with the use of immunosuppressive drugs, can be successful even if an exact tissue match cannot be made. Another aspect to the immune response is its ability to control and eradicate cancer. Although this has been shown to occur with some rare cancers and those caused by known viruses, the normal immune response to most cancers is not sufficient to control cancer growth. Thus, cancer vaccines designed to enhance these immune responses show promise for certain types of cancer.
Interactive Link Questions
Visit this website for an overview of the lymphatic system. What are the three main components of the lymphatic system?
2.Visit this website to learn about the many different cell types in the immune system and their very specialized jobs. What is the role of the dendritic cell in infection by HIV?
3.Visit this website to learn about phagocyte chemotaxis. Phagocyte chemotaxis is the movement of phagocytes according to the secretion of chemical messengers in the form of interleukins and other chemokines. By what means does a phagocyte destroy a bacterium that it has ingested?
4.Immunity can be acquired in an active or passive way, and it can be natural or artificial. Watch this video to see an animated discussion of passive and active immunity. What is an example of natural immunity acquired passively?
Review Questions
Which of the following cells is phagocytic?
- plasma cell
- macrophage
- B cell
- NK cell
Which structure allows lymph from the lower right limb to enter the bloodstream?
- thoracic duct
- right lymphatic duct
- right lymphatic trunk
- left lymphatic trunk
Which of the following cells is important in the innate immune response?
- B cells
- T cells
- macrophages
- plasma cells
Which of the following cells would be most active in early, antiviral immune responses the first time one is exposed to pathogen?
- macrophage
- T cell
- neutrophil
- natural killer cell
Which of the lymphoid nodules is most likely to see food antigens first?
- tonsils
- Peyer’s patches
- bronchus-associated lymphoid tissue
- mucosa-associated lymphoid tissue
Which of the following signs is not characteristic of inflammation?
- redness
- pain
- cold
- swelling
Which of the following is not important in the antiviral innate immune response?
- interferons
- natural killer cells
- complement
- microphages
Enhanced phagocytosis of a cell by the binding of a specific protein is called ________.
- endocytosis
- opsonization
- anaphylaxis
- complement activation
Which of the following leads to the redness of inflammation?
- increased vascular permeability
- anaphylactic shock
- increased blood flow
- complement activation
T cells that secrete cytokines that help antibody responses are called ________.
- Th1
- Th2
- regulatory T cells
- thymocytes
The taking in of antigen and digesting it for later presentation is called ________.
- antigen presentation
- antigen processing
- endocytosis
- exocytosis
Why is clonal expansion so important?
- to select for specific cells
- to secrete cytokines
- to kill target cells
- to increase the numbers of specific cells
The elimination of self-reactive thymocytes is called ________.
- positive selection.
- negative selection.
- tolerance.
- clonal selection.
Which type of T cell is most effective against viruses?
- Th1
- Th2
- cytotoxic T cells
- regulatory T cells
Removing functionality from a B cell without killing it is called ________.
- clonal selection
- clonal expansion
- clonal deletion
- clonal anergy
Which class of antibody crosses the placenta in pregnant women?
- IgM
- IgA
- IgE
- IgG
Which class of antibody has no known function other than as an antigen receptor?
- IgM
- IgA
- IgE
- IgD
When does class switching occur?
- primary response
- secondary response
- tolerance
- memory response
Which class of antibody is found in mucus?
- IgM
- IgA
- IgE
- IgD
Which enzymes in macrophages are important for clearing intracellular bacteria?
- metabolic
- mitochondrial
- nuclear
- lysosomal
What type of chronic lung disease is caused by a Mycobacterium?
- asthma
- emphysema
- tuberculosis
- leprosy
Which type of immune response is most directly effective against bacteria?
- natural killer cells
- complement
- cytotoxic T cells
- helper T cells
What is the reason that you have to be immunized with a new influenza vaccine each year?
- the vaccine is only protective for a year
- mutation
- macrophage oxidative metabolism
- memory response
Which type of immune response works in concert with cytotoxic T cells against virally infected cells?
- natural killer cells
- complement
- antibodies
- memory
Which type of hypersensitivity involves soluble antigen-antibody complexes?
- type I
- type II
- type III
- type IV
What causes the delay in delayed hypersensitivity?
- inflammation
- cytokine release
- recruitment of immune cells
- histamine release
Which of the following is a critical feature of immediate hypersensitivity?
- inflammation
- cytotoxic T cells
- recruitment of immune cells
- histamine release
Which of the following is an autoimmune disease of the heart?
- rheumatoid arthritis
- lupus
- rheumatic fever
- Hashimoto’s thyroiditis
What drug is used to counteract the effects of anaphylactic shock?
- epinephrine
- antihistamines
- antibiotics
- aspirin
Which of the following terms means “many genes”?
- polymorphism
- polygeny
- polypeptide
- multiple alleles
Why do we have natural antibodies?
- We don’t know why.
- immunity to environmental bacteria
- immunity to transplants
- from clonal selection
Which type of cancer is associated with HIV disease?
- Kaposi’s sarcoma
- melanoma
- lymphoma
- renal cell carcinoma
How does cyclosporine A work?
- suppresses antibodies
- suppresses T cells
- suppresses macrophages
- suppresses neutrophils
What disease is associated with bone marrow transplants?
- diabetes mellitus type I
- melanoma
- headache
- graft-versus-host disease
Critical Thinking Questions
Describe the flow of lymph from its origins in interstitial fluid to its emptying into the venous bloodstream.
40.Describe the process of inflammation in an area that has been traumatized, but not infected.
41.Describe two early induced responses and what pathogens they affect.
42.Describe the processing and presentation of an intracellular antigen.
43.Describe clonal selection and expansion.
44.Describe how secondary B cell responses are developed.
45.Describe the role of IgM in immunity.
46.Describe how seroconversion works in HIV disease.
47.Describe tuberculosis and the innocent bystander effect.
48.Describe anaphylactic shock in someone sensitive to peanuts?
49.Describe rheumatic fever and how tolerance is broken.
50.Describe how stress affects immune responses.
|
oercommons
|
2025-03-18T00:39:12.049807
|
10/14/2019
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/58767/overview",
"title": "Anatomy and Physiology, Fluids and Transport, The Lymphatic and Immune System",
"author": null
}
|
https://oercommons.org/courseware/lesson/58773/overview
|
The Urinary System
Introduction
Figure 25.1 Sewage Treatment Plant (credit: “eutrophication&hypoxia”/flickr.com)
CHAPTER OBJECTIVES
After studying this chapter, you will be able to:
- Describe the composition of urine
- Label structures of the urinary system
- Characterize the roles of each of the parts of the urinary system
- Illustrate the macroscopic and microscopic structures of the kidney
- Trace the flow of blood through the kidney
- Outline how blood is filtered in the kidney nephron
- Provide symptoms of kidney failure
- List some of the solutes filtered, secreted, and reabsorbed in different parts of the nephron
- Describe the role of a portal system in the kidney
- Explain how urine osmolarity is hormonally regulated
- Describe the regulation of major ions by the kidney
- Summarize the role of the kidneys in maintaining acid–base balance
The urinary system has roles you may be well aware of: cleansing the blood and ridding the body of wastes probably come to mind. However, there are additional, equally important functions played by the system. Take for example, regulation of pH, a function shared with the lungs and the buffers in the blood. Additionally, the regulation of blood pressure is a role shared with the heart and blood vessels. What about regulating the concentration of solutes in the blood? Did you know that the kidney is important in determining the concentration of red blood cells? Eighty-five percent of the erythropoietin (EPO) produced to stimulate red blood cell production is produced in the kidneys. The kidneys also perform the final synthesis step of vitamin D production, converting calcidiol to calcitriol, the active form of vitamin D.
If the kidneys fail, these functions are compromised or lost altogether, with devastating effects on homeostasis. The affected individual might experience weakness, lethargy, shortness of breath, anemia, widespread edema (swelling), metabolic acidosis, rising potassium levels, heart arrhythmias, and more. Each of these functions is vital to your well-being and survival. The urinary system, controlled by the nervous system, also stores urine until a convenient time for disposal and then provides the anatomical structures to transport this waste liquid to the outside of the body. Failure of nervous control or the anatomical structures leading to a loss of control of urination results in a condition called incontinence.
This chapter will help you to understand the anatomy of the urinary system and how it enables the physiologic functions critical to homeostasis. It is best to think of the kidney as a regulator of plasma makeup rather than simply a urine producer. As you read each section, ask yourself this question: “What happens if this does not work?” This question will help you to understand how the urinary system maintains homeostasis and affects all the other systems of the body and the quality of one’s life.
INTERACTIVE LINK
Watch this video from the Howard Hughes Medical Institute for an introduction to the urinary system.
Physical Characteristics of Urine
- Compare and contrast blood plasma, glomerular filtrate, and urine characteristics
- Describe the characteristics of a normal urine sample, including normal range of pH, osmolarity, and volume
The urinary system’s ability to filter the blood resides in about 2 to 3 million tufts of specialized capillaries—the glomeruli—distributed more or less equally between the two kidneys. Because the glomeruli filter the blood based mostly on particle size, large elements like blood cells, platelets, antibodies, and albumen are excluded. The glomerulus is the first part of the nephron, which then continues as a highly specialized tubular structure responsible for creating the final urine composition. All other solutes, such as ions, amino acids, vitamins, and wastes, are filtered to create a filtrate composition very similar to plasma. The glomeruli create about 200 liters (189 quarts) of this filtrate every day, yet you excrete less than two liters of waste you call urine.
Characteristics of the urine change, depending on influences such as water intake, exercise, environmental temperature, nutrient intake, and other factors (Table 25.1). Some of the characteristics such as color and odor are rough descriptors of your state of hydration. For example, if you exercise or work outside, and sweat a great deal, your urine will turn darker and produce a slight odor, even if you drink plenty of water. Athletes are often advised to consume water until their urine is clear. This is good advice; however, it takes time for the kidneys to process body fluids and store it in the bladder. Another way of looking at this is that the quality of the urine produced is an average over the time it takes to make that urine. Producing clear urine may take only a few minutes if you are drinking a lot of water or several hours if you are working outside and not drinking much.
Normal Urine Characteristics
| Characteristic | Normal values |
|---|---|
| Color | Pale yellow to deep amber |
| Odor | Odorless |
| Volume | 750–2000 mL/24 hour |
| pH | 4.5–8.0 |
| Specific gravity | 1.003–1.032 |
| Osmolarity | 40–1350 mOsmol/kg |
| Urobilinogen | 0.2–1.0 mg/100 mL |
| White blood cells | 0–2 HPF (per high-power field of microscope) |
| Leukocyte esterase | None |
| Protein | None or trace |
| Bilirubin | <0.3 mg/100 mL |
| Ketones | None |
| Nitrites | None |
| Blood | None |
| Glucose | None |
Table 25.1
Urinalysis (urine analysis) often provides clues to renal disease. Normally, only traces of protein are found in urine, and when higher amounts are found, damage to the glomeruli is the likely basis. Unusually large quantities of urine may point to diseases like diabetes mellitus or hypothalamic tumors that cause diabetes insipidus. The color of urine is determined mostly by the breakdown products of red blood cell destruction (Figure 25.2). The “heme” of hemoglobin is converted by the liver into water-soluble forms that can be excreted into the bile and indirectly into the urine. This yellow pigment is urochrome. Urine color may also be affected by certain foods like beets, berries, and fava beans. A kidney stone or a cancer of the urinary system may produce sufficient bleeding to manifest as pink or even bright red urine. Diseases of the liver or obstructions of bile drainage from the liver impart a dark “tea” or “cola” hue to the urine. Dehydration produces darker, concentrated urine that may also possess the slight odor of ammonia. Most of the ammonia produced from protein breakdown is converted into urea by the liver, so ammonia is rarely detected in fresh urine. The strong ammonia odor you may detect in bathrooms or alleys is due to the breakdown of urea into ammonia by bacteria in the environment. About one in five people detect a distinctive odor in their urine after consuming asparagus; other foods such as onions, garlic, and fish can impart their own aromas! These food-caused odors are harmless.
Figure 25.2 Urine Color
Urine volume varies considerably. The normal range is one to two liters per day (Table 25.2). The kidneys must produce a minimum urine volume of about 500 mL/day to rid the body of wastes. Output below this level may be caused by severe dehydration or renal disease and is termed oliguria. The virtual absence of urine production is termed anuria. Excessive urine production is polyuria, which may be due to diabetes mellitus or diabetes insipidus. In diabetes mellitus, blood glucose levels exceed the number of available sodium-glucose transporters in the kidney, and glucose appears in the urine. The osmotic nature of glucose attracts water, leading to its loss in the urine. In the case of diabetes insipidus, insufficient pituitary antidiuretic hormone (ADH) release or insufficient numbers of ADH receptors in the collecting ducts means that too few water channels are inserted into the cell membranes that line the collecting ducts of the kidney. Insufficient numbers of water channels (aquaporins) reduce water absorption, resulting in high volumes of very dilute urine.
Urine Volumes
| Volume condition | Volume | Causes |
|---|---|---|
| Normal | 1–2 L/day | |
| Polyuria | >2.5 L/day | Diabetes mellitus; diabetes insipidus; excess caffeine or alcohol; kidney disease; certain drugs, such as diuretics; sickle cell anemia; excessive water intake |
| Oliguria | 300–500 mL/day | Dehydration; blood loss; diarrhea; cardiogenic shock; kidney disease; enlarged prostate |
| Anuria | <50 mL/day | Kidney failure; obstruction, such as kidney stone or tumor; enlarged prostate |
Table 25.2
The pH (hydrogen ion concentration) of the urine can vary more than 1000-fold, from a normal low of 4.5 to a maximum of 8.0. Diet can influence pH; meats lower the pH, whereas citrus fruits, vegetables, and dairy products raise the pH. Chronically high or low pH can lead to disorders, such as the development of kidney stones or osteomalacia.
Specific gravity is a measure of the quantity of solutes per unit volume of a solution and is traditionally easier to measure than osmolarity. Urine will always have a specific gravity greater than pure water (water = 1.0) due to the presence of solutes. Laboratories can now measure urine osmolarity directly, which is a more accurate indicator of urinary solutes than specific gravity. Remember that osmolarity is the number of osmoles or milliosmoles per liter of fluid (mOsmol/L). Urine osmolarity ranges from a low of 50–100 mOsmol/L to as high as 1200 mOsmol/L H2O.
Cells are not normally found in the urine. The presence of leukocytes may indicate a urinary tract infection. Leukocyte esteraseis released by leukocytes; if detected in the urine, it can be taken as indirect evidence of a urinary tract infection (UTI).
Protein does not normally leave the glomerular capillaries, so only trace amounts of protein should be found in the urine, approximately 10 mg/100 mL in a random sample. If excessive protein is detected in the urine, it usually means that the glomerulus is damaged and is allowing protein to “leak” into the filtrate.
Ketones are byproducts of fat metabolism. Finding ketones in the urine suggests that the body is using fat as an energy source in preference to glucose. In diabetes mellitus when there is not enough insulin (type I diabetes mellitus) or because of insulin resistance (type II diabetes mellitus), there is plenty of glucose, but without the action of insulin, the cells cannot take it up, so it remains in the bloodstream. Instead, the cells are forced to use fat as their energy source, and fat consumed at such a level produces excessive ketones as byproducts. These excess ketones will appear in the urine. Ketones may also appear if there is a severe deficiency of proteins or carbohydrates in the diet.
Nitrates (NO3–) occur normally in the urine. Gram-negative bacteria metabolize nitrate into nitrite (NO2–), and its presence in the urine is indirect evidence of infection.
There should be no blood found in the urine. It may sometimes appear in urine samples as a result of menstrual contamination, but this is not an abnormal condition. Now that you understand what the normal characteristics of urine are, the next section will introduce you to how you store and dispose of this waste product and how you make it.
Gross Anatomy of Urine Transport
- Identify the ureters, urinary bladder, and urethra, as well as their location, structure, histology, and function
- Compare and contrast male and female urethras
- Describe the micturition reflex
- Describe voluntary and involuntary neural control of micturition
Rather than start with urine formation, this section will start with urine excretion. Urine is a fluid of variable composition that requires specialized structures to remove it from the body safely and efficiently. Blood is filtered, and the filtrate is transformed into urine at a relatively constant rate throughout the day. This processed liquid is stored until a convenient time for excretion. All structures involved in the transport and storage of the urine are large enough to be visible to the naked eye. This transport and storage system not only stores the waste, but it protects the tissues from damage due to the wide range of pH and osmolarity of the urine, prevents infection by foreign organisms, and for the male, provides reproductive functions.
Urethra
The urethra transports urine from the bladder to the outside of the body for disposal. The urethra is the only urologic organ that shows any significant anatomic difference between males and females; all other urine transport structures are identical (Figure 25.3).
Figure 25.3 Female and Male Urethras The urethra transports urine from the bladder to the outside of the body. This image shows (a) a female urethra and (b) a male urethra.
The urethra in both males and females begins inferior and central to the two ureteral openings forming the three points of a triangular-shaped area at the base of the bladder called the trigone (Greek tri- = “triangle” and the root of the word “trigonometry”). The urethra tracks posterior and inferior to the pubic symphysis (see Figure 25.3a). In both males and females, the proximal urethra is lined by transitional epithelium, whereas the terminal portion is a nonkeratinized, stratified squamous epithelium. In the male, pseudostratified columnar epithelium lines the urethra between these two cell types. Voiding is regulated by an involuntary autonomic nervous system-controlled internal urinary sphincter, consisting of smooth muscle and voluntary skeletal muscle that forms the external urinary sphincter below it.
Female Urethra
The external urethral orifice is embedded in the anterior vaginal wall inferior to the clitoris, superior to the vaginal opening (introitus), and medial to the labia minora. Its short length, about 4 cm, is less of a barrier to fecal bacteria than the longer male urethra and the best explanation for the greater incidence of UTI in women. Voluntary control of the external urethral sphincter is a function of the pudendal nerve. It arises in the sacral region of the spinal cord, traveling via the S2–S4 nerves of the sacral plexus.
Male Urethra
The male urethra passes through the prostate gland immediately inferior to the bladder before passing below the pubic symphysis (see Figure 25.3b). The length of the male urethra varies between men but averages 20 cm in length. It is divided into four regions: the preprostatic urethra, the prostatic urethra, the membranous urethra, and the spongy or penile urethra. The preprostatic urethra is very short and incorporated into the bladder wall. The prostatic urethra passes through the prostate gland. During sexual intercourse, it receives sperm via the ejaculatory ducts and secretions from the seminal vesicles. Paired Cowper’s glands (bulbourethral glands) produce and secrete mucus into the urethra to buffer urethral pH during sexual stimulation. The mucus neutralizes the usually acidic environment and lubricates the urethra, decreasing the resistance to ejaculation. The membranous urethra passes through the deep muscles of the perineum, where it is invested by the overlying urethral sphincters. The spongy urethra exits at the tip (external urethral orifice) of the penis after passing through the corpus spongiosum. Mucous glands are found along much of the length of the urethra and protect the urethra from extremes of urine pH. Innervation is the same in both males and females.
Bladder
The urinary bladder collects urine from both ureters (Figure 25.4). The bladder lies anterior to the uterus in females, posterior to the pubic bone and anterior to the rectum. During late pregnancy, its capacity is reduced due to compression by the enlarging uterus, resulting in increased frequency of urination. In males, the anatomy is similar, minus the uterus, and with the addition of the prostate inferior to the bladder. The bladder is partially retroperitoneal (outside the peritoneal cavity) with its peritoneal-covered “dome” projecting into the abdomen when the bladder is distended with urine.
Figure 25.4 Bladder (a) Anterior cross section of the bladder. (b) The detrusor muscle of the bladder (source: monkey tissue) LM × 448. (Micrograph provided by the Regents of the University of Michigan Medical School © 2012)
INTERACTIVE LINK
View the University of Michigan WebScope to explore the tissue sample in greater detail.
The bladder is a highly distensible organ comprised of irregular crisscrossing bands of smooth muscle collectively called the detrusor muscle. The interior surface is made of transitional cellular epithelium that is structurally suited for the large volume fluctuations of the bladder. When empty, it resembles columnar epithelia, but when stretched, it “transitions” (hence the name) to a squamous appearance (see Figure 25.4). Volumes in adults can range from nearly zero to 500–600 mL.
The detrusor muscle contracts with significant force in the young. The bladder’s strength diminishes with age, but voluntary contractions of abdominal skeletal muscles can increase intra-abdominal pressure to promote more forceful bladder emptying. Such voluntary contraction is also used in forceful defecation and childbirth.
Micturition Reflex
Micturition is a less-often used, but proper term for urination or voiding. It results from an interplay of involuntary and voluntary actions by the internal and external urethral sphincters. When bladder volume reaches about 150 mL, an urge to void is sensed but is easily overridden. Voluntary control of urination relies on consciously preventing relaxation of the external urethral sphincter to maintain urinary continence. As the bladder fills, subsequent urges become harder to ignore. Ultimately, voluntary constraint fails with resulting incontinence, which will occur as bladder volume approaches 300 to 400 mL.
Normal micturition is a result of stretch receptors in the bladder wall that transmit nerve impulses to the sacral region of the spinal cord to generate a spinal reflex. The resulting parasympathetic neural outflow causes contraction of the detrusor muscle and relaxation of the involuntary internal urethral sphincter. At the same time, the spinal cord inhibits somatic motor neurons, resulting in the relaxation of the skeletal muscle of the external urethral sphincter. The micturition reflex is active in infants but with maturity, children learn to override the reflex by asserting external sphincter control, thereby delaying voiding (potty training). This reflex may be preserved even in the face of spinal cord injury that results in paraplegia or quadriplegia. However, relaxation of the external sphincter may not be possible in all cases, and therefore, periodic catheterization may be necessary for bladder emptying.
Nerves involved in the control of urination include the hypogastric, pelvic, and pudendal (Figure 25.5). Voluntary micturition requires an intact spinal cord and functional pudendal nerve arising from the sacral micturition center. Since the external urinary sphincter is voluntary skeletal muscle, actions by cholinergic neurons maintain contraction (and thereby continence) during filling of the bladder. At the same time, sympathetic nervous activity via the hypogastric nerves suppresses contraction of the detrusor muscle. With further bladder stretch, afferent signals traveling over sacral pelvic nerves activate parasympathetic neurons. This activates efferent neurons to release acetylcholine at the neuromuscular junctions, producing detrusor contraction and bladder emptying.
Figure 25.5 Nerves Innervating the Urinary System
Ureters
The kidneys and ureters are completely retroperitoneal, and the bladder has a peritoneal covering only over the dome. As urine is formed, it drains into the calyces of the kidney, which merge to form the funnel-shaped renal pelvis in the hilum of each kidney. The renal pelvis narrows to become the ureter of each kidney. As urine passes through the ureter, it does not passively drain into the bladder but rather is propelled by waves of peristalsis. As the ureters enter the pelvis, they sweep laterally, hugging the pelvic walls. As they approach the bladder, they turn medially and pierce the bladder wall obliquely. This is important because it creates an one-way valve (a physiological sphincter rather than an anatomical sphincter) that allows urine into the bladder but prevents reflux of urine from the bladder back into the ureter. Children born lacking this oblique course of the ureter through the bladder wall are susceptible to “vesicoureteral reflux,” which dramatically increases their risk of serious UTI. Pregnancy also increases the likelihood of reflux and UTI.
The ureters are approximately 30 cm long. The inner mucosa is lined with transitional epithelium (Figure 25.6) and scattered goblet cells that secrete protective mucus. The muscular layer of the ureter consists of longitudinal and circular smooth muscles that create the peristaltic contractions to move the urine into the bladder without the aid of gravity. Finally, a loose adventitial layer composed of collagen and fat anchors the ureters between the parietal peritoneum and the posterior abdominal wall.
Figure 25.6 Ureter Peristaltic contractions help to move urine through the lumen with contributions from fluid pressure and gravity. LM × 128. (Micrograph provided by the Regents of the University of Michigan Medical School © 2012)
Gross Anatomy of the Kidney
- Describe the external structure of the kidney, including its location, support structures, and covering
- Identify the major internal divisions and structures of the kidney
- Identify the major blood vessels associated with the kidney and trace the path of blood through the kidney
- Compare and contrast the cortical and juxtamedullary nephrons
- Name structures found in the cortex and medulla
- Describe the physiological characteristics of the cortex and medulla
The kidneys lie on either side of the spine in the retroperitoneal space between the parietal peritoneum and the posterior abdominal wall, well protected by muscle, fat, and ribs. They are roughly the size of your fist, and the male kidney is typically a bit larger than the female kidney. The kidneys are well vascularized, receiving about 25 percent of the cardiac output at rest.
INTERACTIVE LINK
There have never been sufficient kidney donations to provide a kidney to each person needing one. Watch this videoto learn about the TED (Technology, Entertainment, Design) Conference held in March 2011. In this video, Dr. Anthony Atala discusses a cutting-edge technique in which a new kidney is “printed.” The successful utilization of this technology is still several years in the future, but imagine a time when you can print a replacement organ or tissue on demand.
External Anatomy
The left kidney is located at about the T12 to L3 vertebrae, whereas the right is lower due to slight displacement by the liver. Upper portions of the kidneys are somewhat protected by the eleventh and twelfth ribs (Figure 25.7). Each kidney weighs about 125–175 g in males and 115–155 g in females. They are about 11–14 cm in length, 6 cm wide, and 4 cm thick, and are directly covered by a fibrous capsule composed of dense, irregular connective tissue that helps to hold their shape and protect them. This capsule is covered by a shock-absorbing layer of adipose tissue called the renal fat pad, which in turn is encompassed by a tough renal fascia. The fascia and, to a lesser extent, the overlying peritoneum serve to firmly anchor the kidneys to the posterior abdominal wall in a retroperitoneal position.
Figure 25.7 Kidneys The kidneys are slightly protected by the ribs and are surrounded by fat for protection (not shown).
On the superior aspect of each kidney is the adrenal gland. The adrenal cortex directly influences renal function through the production of the hormone aldosterone to stimulate sodium reabsorption.
Internal Anatomy
A frontal section through the kidney reveals an outer region called the renal cortex and an inner region called the medulla(Figure 25.8). The renal columns are connective tissue extensions that radiate downward from the cortex through the medulla to separate the most characteristic features of the medulla, the renal pyramids and renal papillae. The papillae are bundles of collecting ducts that transport urine made by nephrons to the calyces of the kidney for excretion. The renal columns also serve to divide the kidney into 6–8 lobes and provide a supportive framework for vessels that enter and exit the cortex. The pyramids and renal columns taken together constitute the kidney lobes.
Figure 25.8 Left Kidney
Renal Hilum
The renal hilum is the entry and exit site for structures servicing the kidneys: vessels, nerves, lymphatics, and ureters. The medial-facing hila are tucked into the sweeping convex outline of the cortex. Emerging from the hilum is the renal pelvis, which is formed from the major and minor calyxes in the kidney. The smooth muscle in the renal pelvis funnels urine via peristalsis into the ureter. The renal arteries form directly from the descending aorta, whereas the renal veins return cleansed blood directly to the inferior vena cava. The artery, vein, and renal pelvis are arranged in an anterior-to-posterior order.
Nephrons and Vessels
The renal artery first divides into segmental arteries, followed by further branching to form interlobar arteries that pass through the renal columns to reach the cortex (Figure 25.9). The interlobar arteries, in turn, branch into arcuate arteries, cortical radiate arteries, and then into afferent arterioles. The afferent arterioles service about 1.3 million nephrons in each kidney.
Figure 25.9 Blood Flow in the Kidney
Nephrons are the “functional units” of the kidney; they cleanse the blood and balance the constituents of the circulation. The afferent arterioles form a tuft of high-pressure capillaries about 200 µm in diameter, the glomerulus. The rest of the nephron consists of a continuous sophisticated tubule whose proximal end surrounds the glomerulus in an intimate embrace—this is Bowman’s capsule. The glomerulus and Bowman’s capsule together form the renal corpuscle. As mentioned earlier, these glomerular capillaries filter the blood based on particle size. After passing through the renal corpuscle, the capillaries form a second arteriole, the efferent arteriole (Figure 25.10). These will next form a capillary network around the more distal portions of the nephron tubule, the peritubular capillaries and vasa recta, before returning to the venous system. As the glomerular filtrate progresses through the nephron, these capillary networks recover most of the solutes and water, and return them to the circulation. Since a capillary bed (the glomerulus) drains into a vessel that in turn forms a second capillary bed, the definition of a portal system is met. This is the only portal system in which an arteriole is found between the first and second capillary beds. (Portal systems also link the hypothalamus to the anterior pituitary, and the blood vessels of the digestive viscera to the liver.)
Figure 25.10 Blood Flow in the Nephron The two capillary beds are clearly shown in this figure. The efferent arteriole is the connecting vessel between the glomerulus and the peritubular capillaries and vasa recta.
INTERACTIVE LINK
Visit this link to view an interactive tutorial of the flow of blood through the kidney.
Cortex
In a dissected kidney, it is easy to identify the cortex; it appears lighter in color compared to the rest of the kidney. All of the renal corpuscles as well as both the proximal convoluted tubules (PCTs) and distal convoluted tubules are found here. Some nephrons have a short loop of Henle that does not dip beyond the cortex. These nephrons are called cortical nephrons. About 15 percent of nephrons have long loops of Henle that extend deep into the medulla and are called juxtamedullary nephrons.
Microscopic Anatomy of the Kidney
- Distinguish the histological differences between the renal cortex and medulla
- Describe the structure of the filtration membrane
- Identify the major structures and subdivisions of the renal corpuscles, renal tubules, and renal capillaries
- Discuss the function of the peritubular capillaries and vasa recta
- Identify the location of the juxtaglomerular apparatus and describe the cells that line it
- Describe the histology of the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting ducts
The renal structures that conduct the essential work of the kidney cannot be seen by the naked eye. Only a light or electron microscope can reveal these structures. Even then, serial sections and computer reconstruction are necessary to give us a comprehensive view of the functional anatomy of the nephron and its associated blood vessels.
Nephrons: The Functional Unit
Nephrons take a simple filtrate of the blood and modify it into urine. Many changes take place in the different parts of the nephron before urine is created for disposal. The term forming urine will be used hereafter to describe the filtrate as it is modified into true urine. The principle task of the nephron population is to balance the plasma to homeostatic set points and excrete potential toxins in the urine. They do this by accomplishing three principle functions—filtration, reabsorption, and secretion. They also have additional secondary functions that exert control in three areas: blood pressure (via production of renin), red blood cell production (via the hormone EPO), and calcium absorption (via conversion of calcidiol into calcitriol, the active form of vitamin D).
Renal Corpuscle
As discussed earlier, the renal corpuscle consists of a tuft of capillaries called the glomerulus that is largely surrounded by Bowman’s (glomerular) capsule. The glomerulus is a high-pressure capillary bed between afferent and efferent arterioles. Bowman’s capsule surrounds the glomerulus to form a lumen, and captures and directs this filtrate to the PCT. The outermost part of Bowman’s capsule, the parietal layer, is a simple squamous epithelium. It transitions onto the glomerular capillaries in an intimate embrace to form the visceral layer of the capsule. Here, the cells are not squamous, but uniquely shaped cells (podocytes) extending finger-like arms (pedicels) to cover the glomerular capillaries (Figure 25.11). These projections interdigitate to form filtration slits, leaving small gaps between the digits to form a sieve. As blood passes through the glomerulus, 10 to 20 percent of the plasma filters between these sieve-like fingers to be captured by Bowman’s capsule and funneled to the PCT. Where the fenestrae (windows) in the glomerular capillaries match the spaces between the podocyte “fingers,” the only thing separating the capillary lumen and the lumen of Bowman’s capsule is their shared basement membrane (Figure 25.12). These three features comprise what is known as the filtration membrane. This membrane permits very rapid movement of filtrate from capillary to capsule though pores that are only 70 nm in diameter.
Figure 25.11 Podocytes Podocytes interdigitate with structures called pedicels and filter substances in a way similar to fenestrations. In (a), the large cell body can be seen at the top right corner, with branches extending from the cell body. The smallest finger-like extensions are the pedicels. Pedicels on one podocyte always interdigitate with the pedicels of another podocyte. (b) This capillary has three podocytes wrapped around it.
Figure 25.12 Fenestrated Capillary Fenestrations allow many substances to diffuse from the blood based primarily on size.
The fenestrations prevent filtration of blood cells or large proteins, but allow most other constituents through. These substances cross readily if they are less than 4 nm in size and most pass freely up to 8 nm in size. An additional factor affecting the ability of substances to cross this barrier is their electric charge. The proteins associated with these pores are negatively charged, so they tend to repel negatively charged substances and allow positively charged substances to pass more readily. The basement membrane prevents filtration of medium-to-large proteins such as globulins. There are also mesangial cells in the filtration membrane that can contract to help regulate the rate of filtration of the glomerulus. Overall, filtration is regulated by fenestrations in capillary endothelial cells, podocytes with filtration slits, membrane charge, and the basement membrane between capillary cells. The result is the creation of a filtrate that does not contain cells or large proteins, and has a slight predominance of positively charged substances.
Lying just outside Bowman’s capsule and the glomerulus is the juxtaglomerular apparatus (JGA) (Figure 25.13). At the juncture where the afferent and efferent arterioles enter and leave Bowman’s capsule, the initial part of the distal convoluted tubule (DCT) comes into direct contact with the arterioles. The wall of the DCT at that point forms a part of the JGA known as the macula densa. This cluster of cuboidal epithelial cells monitors the fluid composition of fluid flowing through the DCT. In response to the concentration of Na+ in the fluid flowing past them, these cells release paracrine signals. They also have a single, nonmotile cilium that responds to the rate of fluid movement in the tubule. The paracrine signals released in response to changes in flow rate and Na+ concentration are adenosine triphosphate (ATP) and adenosine.
Figure 25.13 Juxtaglomerular Apparatus and Glomerulus (a) The JGA allows specialized cells to monitor the composition of the fluid in the DCT and adjust the glomerular filtration rate. (b) This micrograph shows the glomerulus and surrounding structures. LM × 1540. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
A second cell type in this apparatus is the juxtaglomerular cell. This is a modified, smooth muscle cell lining the afferent arteriole that can contract or relax in response to ATP or adenosine released by the macula densa. Such contraction and relaxation regulate blood flow to the glomerulus. If the osmolarity of the filtrate is too high (hyperosmotic), the juxtaglomerular cells will contract, decreasing the glomerular filtration rate (GFR) so less plasma is filtered, leading to less urine formation and greater retention of fluid. This will ultimately decrease blood osmolarity toward the physiologic norm. If the osmolarity of the filtrate is too low, the juxtaglomerular cells will relax, increasing the GFR and enhancing the loss of water to the urine, causing blood osmolarity to rise. In other words, when osmolarity goes up, filtration and urine formation decrease and water is retained. When osmolarity goes down, filtration and urine formation increase and water is lost by way of the urine. The net result of these opposing actions is to keep the rate of filtration relatively constant. A second function of the macula densa cells is to regulate renin release from the juxtaglomerular cells of the afferent arteriole (Figure 25.14). Active renin is a protein comprised of 304 amino acids that cleaves several amino acids from angiotensinogen to produce angiotensin I. Angiotensin I is not biologically active until converted to angiotensin II by angiotensin-converting enzyme (ACE) from the lungs. Angiotensin II is a systemic vasoconstrictor that helps to regulate blood pressure by increasing it. Angiotensin II also stimulates the release of the steroid hormone aldosterone from the adrenal cortex. Aldosterone stimulates Na+ reabsorption by the kidney, which also results in water retention and increased blood pressure.
Figure 25.14 Conversion of Angiotensin I to Angiotensin II The enzyme renin converts the pro-enzyme angiotensin I; the lung-derived enzyme ACE converts angiotensin I into active angiotensin II.
Proximal Convoluted Tubule (PCT)
Filtered fluid collected by Bowman’s capsule enters into the PCT. It is called convoluted due to its tortuous path. Simple cuboidal cells form this tubule with prominent microvilli on the luminal surface, forming a brush border. These microvilli create a large surface area to maximize the absorption and secretion of solutes (Na+, Cl–, glucose, etc.), the most essential function of this portion of the nephron. These cells actively transport ions across their membranes, so they possess a high concentration of mitochondria in order to produce sufficient ATP.
Loop of Henle
The descending and ascending portions of the loop of Henle (sometimes referred to as the nephron loop) are, of course, just continuations of the same tubule. They run adjacent and parallel to each other after having made a hairpin turn at the deepest point of their descent. The descending loop of Henle consists of an initial short, thick portion and long, thin portion, whereas the ascending loop consists of an initial short, thin portion followed by a long, thick portion. The descending thick portion consists of simple cuboidal epithelium similar to that of the PCT. The descending and ascending thin portions consists of simple squamous epithelium. As you will see later, these are important differences, since different portions of the loop have different permeabilities for solutes and water. The ascending thick portion consists of simple cuboidal epithelium similar to the DCT.
Distal Convoluted Tubule (DCT)
The DCT, like the PCT, is very tortuous and formed by simple cuboidal epithelium, but it is shorter than the PCT. These cells are not as active as those in the PCT; thus, there are fewer microvilli on the apical surface. However, these cells must also pump ions against their concentration gradient, so you will find of large numbers of mitochondria, although fewer than in the PCT.
Collecting Ducts
The collecting ducts are continuous with the nephron but not technically part of it. In fact, each duct collects filtrate from several nephrons for final modification. Collecting ducts merge as they descend deeper in the medulla to form about 30 terminal ducts, which empty at a papilla. They are lined with simple squamous epithelium with receptors for ADH. When stimulated by ADH, these cells will insert aquaporin channel proteins into their membranes, which as their name suggests, allow water to pass from the duct lumen through the cells and into the interstitial spaces to be recovered by the vasa recta. This process allows for the recovery of large amounts of water from the filtrate back into the blood. In the absence of ADH, these channels are not inserted, resulting in the excretion of water in the form of dilute urine. Most, if not all, cells of the body contain aquaporin molecules, whose channels are so small that only water can pass. At least 10 types of aquaporins are known in humans, and six of those are found in the kidney. The function of all aquaporins is to allow the movement of water across the lipid-rich, hydrophobic cell membrane (Figure 25.15).
Figure 25.15 Aquaporin Water Channel Positive charges inside the channel prevent the leakage of electrolytes across the cell membrane, while allowing water to move due to osmosis.
Physiology of Urine Formation
- Describe the hydrostatic and colloid osmotic forces that favor and oppose filtration
- Describe glomerular filtration rate (GFR), state the average value of GFR, and explain how clearance rate can be used to measure GFR
- Predict specific factors that will increase or decrease GFR
- State the percent of the filtrate that is normally reabsorbed and explain why the process of reabsorption is so important
- Calculate daily urine production
- List common symptoms of kidney failure
Having reviewed the anatomy and microanatomy of the urinary system, now is the time to focus on the physiology. You will discover that different parts of the nephron utilize specific processes to produce urine: filtration, reabsorption, and secretion. You will learn how each of these processes works and where they occur along the nephron and collecting ducts. The physiologic goal is to modify the composition of the plasma and, in doing so, produce the waste product urine.
Failure of the renal anatomy and/or physiology can lead suddenly or gradually to renal failure. In this event, a number of symptoms, signs, or laboratory findings point to the diagnosis (Table 25.3).
Symptoms of Kidney Failure
| Weakness |
| Lethargy |
| Shortness of breath |
| Widespread edema |
| Anemia |
| Metabolic acidosis |
| Metabolic alkalosis |
| Heart arrhythmias |
| Uremia (high urea level in the blood) |
| Loss of appetite |
| Fatigue |
| Excessive urination |
| Oliguria (too little urine output) |
Table 25.3
Glomerular Filtration Rate (GFR)
The volume of filtrate formed by both kidneys per minute is termed the glomerular filtration rate (GFR). The heart pumps about 5 L blood per min under resting conditions. Approximately 20 percent or one liter enters the kidneys to be filtered. On average, this liter results in the production of about 125 mL/min filtrate produced in men (range of 90 to 140 mL/min) and 105 mL/min filtrate produced in women (range of 80 to 125 mL/min). This amount equates to a volume of about 180 L/day in men and 150 L/day in women. Ninety-nine percent of this filtrate is returned to the circulation by reabsorption so that only about 1–2 liters of urine are produced per day (Table 25.4).
Calculating Urine Formation per Day
| Flow per minute (mL) | Calculation | |
|---|---|---|
| Renal blood flow | 1050 | Cardiac output is about 5000 mL/minute, of which 21 percent flows through the kidney. 5000*0.21 = 1050 mL blood/min |
| Renal plasma flow | 578 | Renal plasma flow equals the blood flow per minute times the hematocrit. If a person has a hematocrit of 45, then the renal plasma flow is 55 percent. 1050*0.55 = 578 mL plasma/min |
| Glomerular filtration rate | 110 | The GFR is the amount of plasma entering Bowman’s capsule per minute. It is the renal plasma flow times the fraction that enters the renal capsule (19 percent). 578*0.19 = 110 mL filtrate/min |
| Urine | 1296 ml/day | The filtrate not recovered by the kidney is the urine that will be eliminated. It is the GFR times the fraction of the filtrate that is not reabsorbed (0.8 percent). 110*.008 = 0.9 mL urine /min Multiply urine/min times 60 minutes times 24 hours to get daily urine production. 0.9*60*24 = 1296 mL/day urine |
Table 25.4
GFR is influenced by the hydrostatic pressure and colloid osmotic pressure on either side of the capillary membrane of the glomerulus. Recall that filtration occurs as pressure forces fluid and solutes through a semipermeable barrier with the solute movement constrained by particle size. Hydrostatic pressure is the pressure produced by a fluid against a surface. If you have a fluid on both sides of a barrier, both fluids exert a pressure in opposing directions. Net fluid movement will be in the direction of the lower pressure. Osmosis is the movement of solvent (water) across a membrane that is impermeable to a solute in the solution. This creates a pressure, osmotic pressure, which will exist until the solute concentration is the same on both sides of a semipermeable membrane. As long as the concentration differs, water will move. Glomerular filtration occurs when glomerular hydrostatic pressure exceeds the luminal hydrostatic pressure of Bowman’s capsule. There is also an opposing force, the osmotic pressure, which is typically higher in the glomerular capillary.
To understand why this is so, look more closely at the microenvironment on either side of the filtration membrane. You will find osmotic pressure exerted by the solutes inside the lumen of the capillary as well as inside of Bowman’s capsule. Since the filtration membrane limits the size of particles crossing the membrane, the osmotic pressure inside the glomerular capillary is higher than the osmotic pressure in Bowman’s capsule. Recall that cells and the medium-to-large proteins cannot pass between the podocyte processes or through the fenestrations of the capillary endothelial cells. This means that red and white blood cells, platelets, albumins, and other proteins too large to pass through the filter remain in the capillary, creating an average colloid osmotic pressure of 30 mm Hg within the capillary. The absence of proteins in Bowman’s space (the lumen within Bowman’s capsule) results in an osmotic pressure near zero. Thus, the only pressure moving fluid across the capillary wall into the lumen of Bowman’s space is hydrostatic pressure. Hydrostatic (fluid) pressure is sufficient to push water through the membrane despite the osmotic pressure working against it. The sum of all of the influences, both osmotic and hydrostatic, results in a net filtration pressure (NFP) of about 10 mm Hg (Figure 25.16).
Figure 25.16 Net Filtration Pressure The NFP is the sum of osmotic and hydrostatic pressures.
A proper concentration of solutes in the blood is important in maintaining osmotic pressure both in the glomerulus and systemically. There are disorders in which too much protein passes through the filtration slits into the kidney filtrate. This excess protein in the filtrate leads to a deficiency of circulating plasma proteins. In turn, the presence of protein in the urine increases its osmolarity; this holds more water in the filtrate and results in an increase in urine volume. Because there is less circulating protein, principally albumin, the osmotic pressure of the blood falls. Less osmotic pressure pulling water into the capillaries tips the balance towards hydrostatic pressure, which tends to push it out of the capillaries. The net effect is that water is lost from the circulation to interstitial tissues and cells. This “plumps up” the tissues and cells, a condition termed systemic edema.
Net Filtration Pressure (NFP)
NFP determines filtration rates through the kidney. It is determined as follows:
NFP = Glomerular blood hydrostatic pressure (GBHP) – [capsular hydrostatic pressure (CHP) + blood colloid osmotic pressure (BCOP)] = 10 mm Hg
That is:
NFP = GBHP – [CHP + BCOP] = 10 mm Hg
Or:
NFP = 55 – [15 + 30] = 10 mm Hg
As you can see, there is a low net pressure across the filtration membrane. Intuitively, you should realize that minor changes in osmolarity of the blood or changes in capillary blood pressure result in major changes in the amount of filtrate formed at any given point in time. The kidney is able to cope with a wide range of blood pressures. In large part, this is due to the autoregulatory nature of smooth muscle. When you stretch it, it contracts. Thus, when blood pressure goes up, smooth muscle in the afferent capillaries contracts to limit any increase in blood flow and filtration rate. When blood pressure drops, the same capillaries relax to maintain blood flow and filtration rate. The net result is a relatively steady flow of blood into the glomerulus and a relatively steady filtration rate in spite of significant systemic blood pressure changes. Mean arterial blood pressure is calculated by adding 1/3 of the difference between the systolic and diastolic pressures to the diastolic pressure. Therefore, if the blood pressure is 110/80, the difference between systolic and diastolic pressure is 30. One third of this is 10, and when you add this to the diastolic pressure of 80, you arrive at a calculated mean arterial pressure of 90 mm Hg. Therefore, if you use mean arterial pressure for the GBHP in the formula for calculating NFP, you can determine that as long as mean arterial pressure is above approximately 60 mm Hg, the pressure will be adequate to maintain glomerular filtration. Blood pressures below this level will impair renal function and cause systemic disorders that are severe enough to threaten survival. This condition is called shock.
Determination of the GFR is one of the tools used to assess the kidney’s excretory function. This is more than just an academic exercise. Since many drugs are excreted in the urine, a decline in renal function can lead to toxic accumulations. Additionally, administration of appropriate drug dosages for those drugs primarily excreted by the kidney requires an accurate assessment of GFR. GFR can be estimated closely by intravenous administration of inulin. Inulin is a plant polysaccharide that is neither reabsorbed nor secreted by the kidney. Its appearance in the urine is directly proportional to the rate at which it is filtered by the renal corpuscle. However, since measuring inulin clearance is cumbersome in the clinical setting, most often, the GFR is estimated by measuring naturally occurring creatinine, a protein-derived molecule produced by muscle metabolism that is not reabsorbed and only slightly secreted by the nephron.
Tubular Reabsorption
- List specific transport mechanisms occurring in different parts of the nephron, including active transport, osmosis, facilitated diffusion, and passive electrochemical gradients
- List the different membrane proteins of the nephron, including channels, transporters, and ATPase pumps
- Compare and contrast passive and active tubular reabsorption
- Explain why the differential permeability or impermeability of specific sections of the nephron tubules is necessary for urine formation
- Describe how and where water, organic compounds, and ions are reabsorbed in the nephron
- Explain the role of the loop of Henle, the vasa recta, and the countercurrent multiplication mechanisms in the concentration of urine
- List the locations in the nephron where tubular secretion occurs
With up to 180 liters per day passing through the nephrons of the kidney, it is quite obvious that most of that fluid and its contents must be reabsorbed. That recovery occurs in the PCT, loop of Henle, DCT, and the collecting ducts (Table 25.5 and Figure 25.17). Various portions of the nephron differ in their capacity to reabsorb water and specific solutes. While much of the reabsorption and secretion occur passively based on concentration gradients, the amount of water that is reabsorbed or lost is tightly regulated. This control is exerted directly by ADH and aldosterone, and indirectly by renin. Most water is recovered in the PCT, loop of Henle, and DCT. About 10 percent (about 18 L) reaches the collecting ducts. The collecting ducts, under the influence of ADH, can recover almost all of the water passing through them, in cases of dehydration, or almost none of the water, in cases of over-hydration.
Figure 25.17 Locations of Secretion and Reabsorption in the Nephron
Substances Secreted or Reabsorbed in the Nephron and Their Locations
| Substance | PCT | Loop of Henle | DCT | Collecting ducts |
|---|---|---|---|---|
| Glucose | Almost 100 percent reabsorbed; secondary active transport with Na+ | |||
| Oligopeptides, proteins, amino acids | Almost 100 percent reabsorbed; symport with Na+ | |||
| Vitamins | Reabsorbed | |||
| Lactate | Reabsorbed | |||
| Creatinine | Secreted | |||
| Urea | 50 percent reabsorbed by diffusion; also secreted | Secretion, diffusion in descending limb | Reabsorption in medullary collecting ducts; diffusion | |
| Sodium | 65 percent actively reabsorbed | 25 percent reabsorbed in thick ascending limb; active transport | 5 percent reabsorbed; active | 5 percent reabsorbed, stimulated by aldosterone; active |
| Chloride | Reabsorbed, symport with Na+, diffusion | Reabsorbed in thin and thick ascending limb; diffusion in ascending limb | Reabsorbed; diffusion | Reabsorbed; symport |
| Water | 67 percent reabsorbed osmotically with solutes | 15 percent reabsorbed in descending limb; osmosis | 8 percent reabsorbed if ADH; osmosis | Variable amounts reabsorbed, controlled by ADH, osmosis |
| Bicarbonate | 80–90 percent symport reabsorption with Na+ | Reabsorbed, symport with Na+ and antiport with Cl–; in ascending limb | Reabsorbed antiport with Cl– | |
| H+ | Secreted; diffusion | Secreted; active | Secreted; active | |
| NH4+ | Secreted; diffusion | Secreted; diffusion | Secreted; diffusion | |
| HCO3– | Reabsorbed; diffusion | Reabsorbed; diffusion in ascending limb | Reabsorbed; diffusion | Reabsorbed; antiport with Na+ |
| Some drugs | Secreted | Secreted; active | Secreted; active | |
| Potassium | 65 percent reabsorbed; diffusion | 20 percent reabsorbed in thick ascending limb; symport | Secreted; active | Secretion controlled by aldosterone; active |
| Calcium | Reabsorbed; diffusion | Reabsorbed in thick ascending limb; diffusion | Reabsorbed if parathyroid hormone present; active | |
| Magnesium | Reabsorbed; diffusion | Reabsorbed in thick ascending limb; diffusion | Reabsorbed | |
| Phosphate | 85 percent reabsorbed, inhibited by parathyroid hormone, diffusion | Reabsorbed; diffusion |
Table 25.5
Mechanisms of Recovery
Mechanisms by which substances move across membranes for reabsorption or secretion include active transport, diffusion, facilitated diffusion, secondary active transport, and osmosis. These were discussed in an earlier chapter, and you may wish to review them.
Active transport utilizes energy, usually the energy found in a phosphate bond of ATP, to move a substance across a membrane from a low to a high concentration. It is very specific and must have an appropriately shaped receptor for the substance to be transported. An example would be the active transport of Na+ out of a cell and K+ into a cell by the Na+/K+ pump. Both ions are moved in opposite directions from a lower to a higher concentration.
Simple diffusion moves a substance from a higher to a lower concentration down its concentration gradient. It requires no energy and only needs to be soluble.
Facilitated diffusion is similar to diffusion in that it moves a substance down its concentration gradient. The difference is that it requires specific membrane receptors or channel proteins for movement. The movement of glucose and, in certain situations, Na+ ions, is an example of facilitated diffusion. In some cases of mediated transport, two different substances share the same channel protein port; these mechanisms are described by the terms symport and antiport.
Symport mechanisms move two or more substances in the same direction at the same time, whereas antiport mechanisms move two or more substances in opposite directions across the cell membrane. Both mechanisms may utilize concentration gradients maintained by ATP pumps. As described previously, when active transport powers the transport of another substance in this way, it is called “secondary active transport.” Glucose reabsorption in the kidneys is by secondary active transport. Na+/K+ ATPases on the basal membrane of a tubular cell constantly pump Na+ out of the cell, maintaining a strong electrochemical gradient for Na+ to move into the cell from the tubular lumen. On the luminal (apical) surface, a Na+/glucose symport protein assists both Na+ and glucose movement into the cell. The cotransporter moves glucose into the cell against its concentration gradient as Na+ moves down the electrochemical gradient created by the basal membranes Na+/K+ ATPases. The glucose molecule then diffuses across the basal membrane by facilitated diffusion into the interstitial space and from there into peritubular capillaries.
Most of the Ca++, Na+, glucose, and amino acids must be reabsorbed by the nephron to maintain homeostatic plasma concentrations. Other substances, such as urea, K+, ammonia (NH3), creatinine, and some drugs are secreted into the filtrate as waste products. Acid–base balance is maintained through actions of the lungs and kidneys: The lungs rid the body of H+, whereas the kidneys secrete or reabsorb H+ and HCO3– (Table 25.6). In the case of urea, about 50 percent is passively reabsorbed by the PCT. More is recovered by in the collecting ducts as needed. ADH induces the insertion of urea transporters and aquaporin channel proteins.
Substances Filtered and Reabsorbed by the Kidney per 24 Hours
| Substance | Amount filtered (grams) | Amount reabsorbed (grams) | Amount in urine (grams) |
|---|---|---|---|
| Water | 180 L | 179 L | 1 L |
| Proteins | 10–20 | 10–20 | 0 |
| Chlorine | 630 | 625 | 5 |
| Sodium | 540 | 537 | 3 |
| Bicarbonate | 300 | 299.7 | 0.3 |
| Glucose | 180 | 180 | 0 |
| Urea | 53 | 28 | 25 |
| Potassium | 28 | 24 | 4 |
| Uric acid | 8.5 | 7.7 | 0.8 |
| Creatinine | 1.4 | 0 | 1.4 |
Table 25.6
Reabsorption and Secretion in the PCT
The renal corpuscle filters the blood to create a filtrate that differs from blood mainly in the absence of cells and large proteins. From this point to the ends of the collecting ducts, the filtrate or forming urine is undergoing modification through secretion and reabsorption before true urine is produced. The first point at which the forming urine is modified is in the PCT. Here, some substances are reabsorbed, whereas others are secreted. Note the use of the term “reabsorbed.” All of these substances were “absorbed” in the digestive tract—99 percent of the water and most of the solutes filtered by the nephron must be reabsorbed. Water and substances that are reabsorbed are returned to the circulation by the peritubular and vasa recta capillaries. It is important to understand the difference between the glomerulus and the peritubular and vasa recta capillaries. The glomerulus has a relatively high pressure inside its capillaries and can sustain this by dilating the afferent arteriole while constricting the efferent arteriole. This assures adequate filtration pressure even as the systemic blood pressure varies. Movement of water into the peritubular capillaries and vasa recta will be influenced primarily by osmolarity and concentration gradients. Sodium is actively pumped out of the PCT into the interstitial spaces between cells and diffuses down its concentration gradient into the peritubular capillary. As it does so, water will follow passively to maintain an isotonic fluid environment inside the capillary. This is called obligatory water reabsorption, because water is “obliged” to follow the Na+ (Figure 25.18).
Figure 25.18 Substances Reabsorbed and Secreted by the PCT
More substances move across the membranes of the PCT than any other portion of the nephron. Many of these substances (amino acids and glucose) use symport mechanisms for transport along with Na+. Antiport, active transport, diffusion, and facilitated diffusion are additional mechanisms by which substances are moved from one side of a membrane to the other. Recall that cells have two surfaces: apical and basal. The apical surface is the one facing the lumen or open space of a cavity or tube, in this case, the inside of the PCT. The basal surface of the cell faces the connective tissue base to which the cell attaches (basement membrane) or the cell membrane closer to the basement membrane if there is a stratified layer of cells. In the PCT, there is a single layer of simple cuboidal endothelial cells against the basement membrane. The numbers and particular types of pumps and channels vary between the apical and basilar surfaces. A few of the substances that are transported with Na+(symport mechanism) on the apical membrane include Cl–, Ca++, amino acids, glucose, and PO3−4PO43−+ using ATP on the basal membrane. Most of the substances transported by a symport mechanism on the apical membrane are transported by facilitated diffusion on the basal membrane. At least three ions, K+, Ca++, and Mg++, diffuse laterally between adjacent cell membranes (transcellular).
About 67 percent of the water, Na+, and K+ entering the nephron is reabsorbed in the PCT and returned to the circulation. Almost 100 percent of glucose, amino acids, and other organic substances such as vitamins are normally recovered here. Some glucose may appear in the urine if circulating glucose levels are high enough that all the glucose transporters in the PCT are saturated, so that their capacity to move glucose is exceeded (transport maximum, or Tm). In men, the maximum amount of glucose that can be recovered is about 375 mg/min, whereas in women, it is about 300 mg/min. This recovery rate translates to an arterial concentration of about 200 mg/dL. Though an exceptionally high sugar intake might cause sugar to appear briefly in the urine, the appearance of glycosuria usually points to type I or II diabetes mellitus. The transport of glucose from the lumen of the PCT to the interstitial space is similar to the way it is absorbed by the small intestine. Both glucose and Na+ bind simultaneously to the same symport proteins on the apical surface of the cell to be transported in the same direction, toward the interstitial space. Sodium moves down its electrochemical and concentration gradient into the cell and takes glucose with it. Na+ is then actively pumped out of the cell at the basal surface of the cell into the interstitial space. Glucose leaves the cell to enter the interstitial space by facilitated diffusion. The energy to move glucose comes from the Na+/K+ ATPase that pumps Na+out of the cell on the basal surface. Fifty percent of Cl– and variable quantities of Ca++, Mg++, and HPO2−4HPO42− are also recovered in the PCT.
Recovery of bicarbonate (HCO3–) is vital to the maintenance of acid–base balance, since it is a very powerful and fast-acting buffer. An important enzyme is used to catalyze this mechanism: carbonic anhydrase (CA). This same enzyme and reaction is used in red blood cells in the transportation of CO2, in the stomach to produce hydrochloric acid, and in the pancreas to produce HCO3– to buffer acidic chyme from the stomach. In the kidney, most of the CA is located within the cell, but a small amount is bound to the brush border of the membrane on the apical surface of the cell. In the lumen of the PCT, HCO3–combines with hydrogen ions to form carbonic acid (H2CO3). This is enzymatically catalyzed into CO2 and water, which diffuse across the apical membrane into the cell. Water can move osmotically across the lipid bilayer membrane due to the presence of aquaporin water channels. Inside the cell, the reverse reaction occurs to produce bicarbonate ions (HCO3–). These bicarbonate ions are cotransported with Na+ across the basal membrane to the interstitial space around the PCT (Figure 25.19). At the same time this is occurring, a Na+/H+ antiporter excretes H+ into the lumen, while it recovers Na+. Note how the hydrogen ion is recycled so that bicarbonate can be recovered. Also, note that a Na+ gradient is created by the Na+/K+ pump.
HCO3−+ H+↔H2CO3↔CO2 + H2OHCO3−+ H+↔H2CO3↔CO2 + H2O
The significant recovery of solutes from the PCT lumen to the interstitial space creates an osmotic gradient that promotes water recovery. As noted before, water moves through channels created by the aquaporin proteins. These proteins are found in all cells in varying amounts and help regulate water movement across membranes and through cells by creating a passageway across the hydrophobic lipid bilayer membrane. Changing the number of aquaporin proteins in membranes of the collecting ducts also helps to regulate the osmolarity of the blood. The movement of many positively charged ions also creates an electrochemical gradient. This charge promotes the movement of negative ions toward the interstitial spaces and the movement of positive ions toward the lumen.
Figure 25.19 Reabsorption of Bicarbonate from the PCT
Reabsorption and Secretion in the Loop of Henle
The loop of Henle consists of two sections: thick and thin descending and thin and thick ascending sections. The loops of cortical nephrons do not extend into the renal medulla very far, if at all. Juxtamedullary nephrons have loops that extend variable distances, some very deep into the medulla. The descending and ascending portions of the loop are highly specialized to enable recovery of much of the Na+ and water that were filtered by the glomerulus. As the forming urine moves through the loop, the osmolarity will change from isosmotic with blood (about 278–300 mOsmol/kg) to both a very hypertonic solution of about 1200 mOsmol/kg and a very hypotonic solution of about 100 mOsmol/kg. These changes are accomplished by osmosis in the descending limb and active transport in the ascending limb. Solutes and water recovered from these loops are returned to the circulation by way of the vasa recta.
Descending Loop
The majority of the descending loop is comprised of simple squamous epithelial cells; to simplify the function of the loop, this discussion focuses on these cells. These membranes have permanent aquaporin channel proteins that allow unrestricted movement of water from the descending loop into the surrounding interstitium as osmolarity increases from about 300 mOsmol/kg to about 1200 mOsmol/kg. This increase results in reabsorption of up to 15 percent of the water entering the nephron. Modest amounts of urea, Na+, and other ions are also recovered here.
Most of the solutes that were filtered in the glomerulus have now been recovered along with a majority of water, about 82 percent. As the forming urine enters the ascending loop, major adjustments will be made to the concentration of solutes to create what you perceive as urine.
Ascending Loop
The ascending loop is made of very short thin and longer thick portions. Once again, to simplify the function, this section only considers the thick portion. The thick portion is lined with simple cuboidal epithelium without a brush border. It is completely impermeable to water due to the absence of aquaporin proteins, but ions, mainly Na+ and CL–, are actively reabsorbed by a cotransport system. This has two significant effects: Removal of NaCl while retaining water leads to a hypoosomotic filtrate by the time it reaches the DCT; pumping NaCl into the interstitial space contributes to the hyperosmotic environment in the kidney medulla.
The Na+/K+ ATPase pumps in the basal membrane create an electrochemical gradient, allowing reabsorption of Cl– by Na+/Cl–symporters in the apical membrane. At the same time that Na+ is actively pumped from the basal side of the cell into the interstitial fluid, Cl– follows the Na+ from the lumen into the interstitial fluid by a paracellular route between cells through leaky tight junctions. These are found between cells of the ascending loop, where they allow certain solutes to move according to their concentration gradient. Most of the K+ that enters the cell via symporters returns to the lumen (down its concentration gradient) through leaky channels in the apical membrane. Note the environment now created in the interstitial space: With the “back door exiting” K+, there is one Na+ and two Cl– ions left in the interstitium surrounding the ascending loop. Therefore, in comparison to the lumen of the loop, the interstitial space is now a negatively charged environment. This negative charge attracts cations (Na+, K+, Ca++, and Mg++) from the lumen via a paracellular route to the interstitial space and vasa recta.
Countercurrent Multiplier System
The structure of the loop of Henle and associated vasa recta create a countercurrent multiplier system (Figure 25.20). The countercurrent term comes from the fact that the descending and ascending loops are next to each other and their fluid flows in opposite directions (countercurrent). The multiplier term is due to the action of solute pumps that increase (multiply) the concentrations of urea and Na+ deep in the medulla.
Figure 25.20 Countercurrent Multiplier System
As discussed above, the ascending loop actively reabsorbs NaCl out of the forming urine into the interstitial spaces. In addition, collecting ducts have urea pumps that actively pump urea into the interstitial spaces. This results in the recovery of NaCl to the circulation via the vasa recta and creates a high osmolar environment in the depths of the medulla.
Ammonia (NH3) is a toxic byproduct of protein metabolism. It is formed as amino acids are deaminated by liver hepatocytes. That means that the amine group, NH2, is removed from amino acids as they are broken down. Most of the resulting ammonia is converted into urea by liver hepatocytes. Urea is not only less toxic but is utilized to aid in the recovery of water by the loop of Henle and collecting ducts. At the same time that water is freely diffusing out of the descending loop through aquaporin channels into the interstitial spaces of the medulla, urea freely diffuses into the lumen of the descending loop as it descends deeper into the medulla, much of it to be reabsorbed from the forming urine when it reaches the collecting duct. Thus, the movement of Na+ and urea into the interstitial spaces by these mechanisms creates the hyperosmotic environment of the medulla. The net result of this countercurrent multiplier system is to recover both water and Na+ in the circulation.
The amino acid glutamine can be deaminated by the kidney. As NH2 from the amino acid is converted into NH3 and pumped into the lumen of the PCT, Na+ and HCO3– are excreted into the interstitial fluid of the renal pyramid via a symport mechanism. When this process occurs in the cells of the PCT, the added benefit is a net loss of a hydrogen ion (complexed to ammonia to form the weak acid NH4+) in the urine and a gain of a bicarbonate ion (HCO3–) in the blood. Ammonia and bicarbonate are exchanged in a one-to-one ratio. This exchange is yet another means by which the body can buffer and excrete acid. The presence of aquaporin channels in the descending loop allows prodigious quantities of water to leave the loop and enter the hyperosmolar interstitium of the pyramid, where it is returned to the circulation by the vasa recta. As the loop turns to become the ascending loop, there is an absence of aquaporin channels, so water cannot leave the loop. However, in the basal membrane of cells of the thick ascending loop, ATPase pumps actively remove Na+ from the cell. A Na+/K+/2Cl– symporter in the apical membrane passively allows these ions to enter the cell cytoplasm from the lumen of the loop down a concentration gradient created by the pump. This mechanism works to dilute the fluid of the ascending loop ultimately to approximately 50–100 mOsmol/L.
At the transition from the DCT to the collecting duct, about 20 percent of the original water is still present and about 10 percent of the sodium. If no other mechanism for water reabsorption existed, about 20–25 liters of urine would be produced. Now consider what is happening in the adjacent capillaries, the vasa recta. They are recovering both solutes and water at a rate that preserves the countercurrent multiplier system. In general, blood flows slowly in capillaries to allow time for exchange of nutrients and wastes. In the vasa recta particularly, this rate of flow is important for two additional reasons. The flow must be slow to allow blood cells to lose and regain water without either crenating or bursting. Second, a rapid flow would remove too much Na+ and urea, destroying the osmolar gradient that is necessary for the recovery of solutes and water. Thus, by flowing slowly to preserve the countercurrent mechanism, as the vasa recta descend, Na+ and urea are freely able to enter the capillary, while water freely leaves; as they ascend, Na+ and urea are secreted into the surrounding medulla, while water reenters and is removed.
INTERACTIVE LINK
Watch this video to learn about the countercurrent multiplier system.
Reabsorption and Secretion in the Distal Convoluted Tubule
Approximately 80 percent of filtered water has been recovered by the time the dilute forming urine enters the DCT. The DCT will recover another 10–15 percent before the forming urine enters the collecting ducts. Aldosterone increases the amount of Na+/K+ATPase in the basal membrane of the DCT and collecting duct. The movement of Na+ out of the lumen of the collecting duct creates a negative charge that promotes the movement of Cl– out of the lumen into the interstitial space by a paracellular route across tight junctions. Peritubular capillaries receive the solutes and water, returning them to the circulation.
Cells of the DCT also recover Ca++ from the filtrate. Receptors for parathyroid hormone (PTH) are found in DCT cells and when bound to PTH, induce the insertion of calcium channels on their luminal surface. The channels enhance Ca++ recovery from the forming urine. In addition, as Na+ is pumped out of the cell, the resulting electrochemical gradient attracts Ca++ into the cell. Finally, calcitriol (1,25 dihydroxyvitamin D, the active form of vitamin D) is very important for calcium recovery. It induces the production of calcium-binding proteins that transport Ca++ into the cell. These binding proteins are also important for the movement of calcium inside the cell and aid in exocytosis of calcium across the basolateral membrane. Any Ca++ not reabsorbed at this point is lost in the urine.
Collecting Ducts and Recovery of Water
Solutes move across the membranes of the collecting ducts, which contain two distinct cell types, principal cells and intercalated cells. A principal cell possesses channels for the recovery or loss of sodium and potassium. An intercalated cellsecretes or absorbs acid or bicarbonate. As in other portions of the nephron, there is an array of micromachines (pumps and channels) on display in the membranes of these cells.
Regulation of urine volume and osmolarity are major functions of the collecting ducts. By varying the amount of water that is recovered, the collecting ducts play a major role in maintaining the body’s normal osmolarity. If the blood becomes hyperosmotic, the collecting ducts recover more water to dilute the blood; if the blood becomes hyposmotic, the collecting ducts recover less of the water, leading to concentration of the blood. Another way of saying this is: If plasma osmolarity rises, more water is recovered and urine volume decreases; if plasma osmolarity decreases, less water is recovered and urine volume increases. This function is regulated by the posterior pituitary hormone ADH (vasopressin). With mild dehydration, plasma osmolarity rises slightly. This increase is detected by osmoreceptors in the hypothalamus, which stimulates the release of ADH from the posterior pituitary. If plasma osmolarity decreases slightly, the opposite occurs.
When stimulated by ADH, aquaporin channels are inserted into the apical membrane of principal cells, which line the collecting ducts. As the ducts descend through the medulla, the osmolarity surrounding them increases (due to the countercurrent mechanisms described above). If aquaporin water channels are present, water will be osmotically pulled from the collecting duct into the surrounding interstitial space and into the peritubular capillaries. Therefore, the final urine will be more concentrated. If less ADH is secreted, fewer aquaporin channels are inserted and less water is recovered, resulting in dilute urine. By altering the number of aquaporin channels, the volume of water recovered or lost is altered. This, in turn, regulates the blood osmolarity, blood pressure, and osmolarity of the urine.
As Na+ is pumped from the forming urine, water is passively recaptured for the circulation; this preservation of vascular volume is critically important for the maintenance of a normal blood pressure. Aldosterone is secreted by the adrenal cortex in response to angiotensin II stimulation. As an extremely potent vasoconstrictor, angiotensin II functions immediately to increase blood pressure. By also stimulating aldosterone production, it provides a longer-lasting mechanism to support blood pressure by maintaining vascular volume (water recovery).
In addition to receptors for ADH, principal cells have receptors for the steroid hormone aldosterone. While ADH is primarily involved in the regulation of water recovery, aldosterone regulates Na+ recovery. Aldosterone stimulates principal cells to manufacture luminal Na+ and K+ channels as well as Na+/K+ ATPase pumps on the basal membrane of the cells. When aldosterone output increases, more Na+ is recovered from the forming urine and water follows the Na+ passively. As the pump recovers Na+ for the body, it is also pumping K+ into the forming urine, since the pump moves K+ in the opposite direction. When aldosterone decreases, more Na+ remains in the forming urine and more K+ is recovered in the circulation. Symport channels move Na+ and Cl– together. Still other channels in the principal cells secrete K+ into the collecting duct in direct proportion to the recovery of Na+.
Intercalated cells play significant roles in regulating blood pH. Intercalated cells reabsorb K+ and HCO3– while secreting H+. This function lowers the acidity of the plasma while increasing the acidity of the urine.
Regulation of Renal Blood Flow
- Describe the myogenic and tubuloglomerular feedback mechanisms and explain how they affect urine volume and composition
- Describe the function of the juxtaglomerular apparatus
It is vital that the flow of blood through the kidney be at a suitable rate to allow for filtration. This rate determines how much solute is retained or discarded, how much water is retained or discarded, and ultimately, the osmolarity of blood and the blood pressure of the body.
Sympathetic Nerves
The kidneys are innervated by the sympathetic neurons of the autonomic nervous system via the celiac plexus and splanchnic nerves. Reduction of sympathetic stimulation results in vasodilation and increased blood flow through the kidneys during resting conditions. When the frequency of action potentials increases, the arteriolar smooth muscle constricts (vasoconstriction), resulting in diminished glomerular flow, so less filtration occurs. Under conditions of stress, sympathetic nervous activity increases, resulting in the direct vasoconstriction of afferent arterioles (norepinephrine effect) as well as stimulation of the adrenal medulla. The adrenal medulla, in turn, produces a generalized vasoconstriction through the release of epinephrine. This includes vasoconstriction of the afferent arterioles, further reducing the volume of blood flowing through the kidneys. This process redirects blood to other organs with more immediate needs. If blood pressure falls, the sympathetic nerves will also stimulate the release of renin. Additional renin increases production of the powerful vasoconstrictor angiotensin II. Angiotensin II, as discussed above, will also stimulate aldosterone production to augment blood volume through retention of more Na+ and water. Only a 10 mm Hg pressure differential across the glomerulus is required for normal GFR, so very small changes in afferent arterial pressure significantly increase or decrease GFR.
Autoregulation
The kidneys are very effective at regulating the rate of blood flow over a wide range of blood pressures. Your blood pressure will decrease when you are relaxed or sleeping. It will increase when exercising. Yet, despite these changes, the filtration rate through the kidney will change very little. This is due to two internal autoregulatory mechanisms that operate without outside influence: the myogenic mechanism and the tubuloglomerular feedback mechanism.
Arteriole Myogenic Mechanism
The myogenic mechanism regulating blood flow within the kidney depends upon a characteristic shared by most smooth muscle cells of the body. When you stretch a smooth muscle cell, it contracts; when you stop, it relaxes, restoring its resting length. This mechanism works in the afferent arteriole that supplies the glomerulus. When blood pressure increases, smooth muscle cells in the wall of the arteriole are stretched and respond by contracting to resist the pressure, resulting in little change in flow. When blood pressure drops, the same smooth muscle cells relax to lower resistance, allowing a continued even flow of blood.
Tubuloglomerular Feedback
The tubuloglomerular feedback mechanism involves the JGA and a paracrine signaling mechanism utilizing ATP, adenosine, and nitric oxide (NO). This mechanism stimulates either contraction or relaxation of afferent arteriolar smooth muscle cells (Table 25.7). Recall that the DCT is in intimate contact with the afferent and efferent arterioles of the glomerulus. Specialized macula densa cells in this segment of the tubule respond to changes in the fluid flow rate and Na+ concentration. As GFR increases, there is less time for NaCl to be reabsorbed in the PCT, resulting in higher osmolarity in the filtrate. The increased fluid movement more strongly deflects single nonmotile cilia on macula densa cells. This increased osmolarity of the forming urine, and the greater flow rate within the DCT, activates macula densa cells to respond by releasing ATP and adenosine (a metabolite of ATP). ATP and adenosine act locally as paracrine factors to stimulate the myogenic juxtaglomerular cells of the afferent arteriole to constrict, slowing blood flow and reducing GFR. Conversely, when GFR decreases, less Na+ is in the forming urine, and most will be reabsorbed before reaching the macula densa, which will result in decreased ATP and adenosine, allowing the afferent arteriole to dilate and increase GFR. NO has the opposite effect, relaxing the afferent arteriole at the same time ATP and adenosine are stimulating it to contract. Thus, NO fine-tunes the effects of adenosine and ATP on GFR.
Paracrine Mechanisms Controlling Glomerular Filtration Rate
| Change in GFR | NaCl Absorption | Role of ATP and adenosine/Role of NO | Effect on GFR |
|---|---|---|---|
| Increased GFR | Tubular NaCl increases | ATP and adenosine increase, causing vasoconstriction | Vasoconstriction slows GFR |
| Decreased GFR | Tubular NaCl decreases | ATP and adenosine decrease, causing vasodilation | Vasodilation increases GFR |
| Increased GFR | Tubular NaCl increases | NO increases, causing vasodilation | Vasodilation increases GFR |
| Decreased GFR | Tubular NaCl decreases | NO decreases, causing vasoconstricton | Vasoconstriction decreases GFR |
Table 25.7
Endocrine Regulation of Kidney Function
- Describe how each of the following functions in the extrinsic control of GFR: renin–angiotensin mechanism, natriuretic peptides, and sympathetic adrenergic activity
- Describe how each of the following works to regulate reabsorption and secretion, so as to affect urine volume and composition: renin–angiotensin system, aldosterone, antidiuretic hormone, and natriuretic peptides
- Name and define the roles of other hormones that regulate kidney control
Several hormones have specific, important roles in regulating kidney function. They act to stimulate or inhibit blood flow. Some of these are endocrine, acting from a distance, whereas others are paracrine, acting locally.
Renin–Angiotensin–Aldosterone
Renin is an enzyme that is produced by the granular cells of the afferent arteriole at the JGA. It enzymatically converts angiotensinogen (made by the liver, freely circulating) into angiotensin I. Its release is stimulated by prostaglandins and NO from the JGA in response to decreased extracellular fluid volume.
ACE is not a hormone but it is functionally important in regulating systemic blood pressure and kidney function. It is produced in the lungs but binds to the surfaces of endothelial cells in the afferent arterioles and glomerulus. It enzymatically converts inactive angiotensin I into active angiotensin II. ACE is important in raising blood pressure. People with high blood pressure are sometimes prescribed ACE inhibitors to lower their blood pressure.
Angiotensin II is a potent vasoconstrictor that plays an immediate role in the regulation of blood pressure. It acts systemically to cause vasoconstriction as well as constriction of both the afferent and efferent arterioles of the glomerulus. In instances of blood loss or dehydration, it reduces both GFR and renal blood flow, thereby limiting fluid loss and preserving blood volume. Its release is usually stimulated by decreases in blood pressure, and so the preservation of adequate blood pressure is its primary role.
Aldosterone, often called the “salt-retaining hormone,” is released from the adrenal cortex in response to angiotensin II or directly in response to increased plasma K+. It promotes Na+ reabsorption by the nephron, promoting the retention of water. It is also important in regulating K+, promoting its excretion. (This dual effect on two minerals and its origin in the adrenal cortex explains its designation as a mineralocorticoid.) As a result, renin has an immediate effect on blood pressure due to angiotensin II–stimulated vasoconstriction and a prolonged effect through Na+ recovery due to aldosterone. At the same time that aldosterone causes increased recovery of Na+, it also causes greater loss of K+. Progesterone is a steroid that is structurally similar to aldosterone. It binds to the aldosterone receptor and weakly stimulates Na+ reabsorption and increased water recovery. This process is unimportant in men due to low levels of circulating progesterone. It may cause increased retention of water during some periods of the menstrual cycle in women when progesterone levels increase.
Antidiuretic Hormone (ADH)
Diuretics are drugs that can increase water loss by interfering with the recapture of solutes and water from the forming urine. They are often prescribed to lower blood pressure. Coffee, tea, and alcoholic beverages are familiar diuretics. ADH, a 9-amino acid peptide released by the posterior pituitary, works to do the exact opposite. It promotes the recovery of water, decreases urine volume, and maintains plasma osmolarity and blood pressure. It does so by stimulating the movement of aquaporin proteins into the apical cell membrane of principal cells of the collecting ducts to form water channels, allowing the transcellular movement of water from the lumen of the collecting duct into the interstitial space in the medulla of the kidney by osmosis. From there, it enters the vasa recta capillaries to return to the circulation. Water is attracted by the high osmotic environment of the deep kidney medulla.
Endothelin
Endothelins, 21-amino acid peptides, are extremely powerful vasoconstrictors. They are produced by endothelial cells of the renal blood vessels, mesangial cells, and cells of the DCT. Hormones stimulating endothelin release include angiotensin II, bradykinin, and epinephrine. They do not typically influence blood pressure in healthy people. On the other hand, in people with diabetic kidney disease, endothelin is chronically elevated, resulting in sodium retention. They also diminish GFR by damaging the podocytes and by potently vasoconstricting both the afferent and efferent arterioles.
Natriuretic Hormones
Natriuretic hormones are peptides that stimulate the kidneys to excrete sodium—an effect opposite that of aldosterone. Natriuretic hormones act by inhibiting aldosterone release and therefore inhibiting Na+ recovery in the collecting ducts. If Na+remains in the forming urine, its osmotic force will cause a concurrent loss of water. Natriuretic hormones also inhibit ADH release, which of course will result in less water recovery. Therefore, natriuretic peptides inhibit both Na+ and water recovery. One example from this family of hormones is atrial natriuretic hormone (ANH), a 28-amino acid peptide produced by heart atria in response to over-stretching of the atrial wall. The over-stretching occurs in persons with elevated blood pressure or heart failure. It increases GFR through concurrent vasodilation of the afferent arteriole and vasoconstriction of the efferent arteriole. These events lead to an increased loss of water and sodium in the forming urine. It also decreases sodium reabsorption in the DCT. There is also B-type natriuretic peptide (BNP) of 32 amino acids produced in the ventricles of the heart. It has a 10-fold lower affinity for its receptor, so its effects are less than those of ANH. Its role may be to provide “fine tuning” for the regulation of blood pressure. BNP’s longer biologic half-life makes it a good diagnostic marker of congestive heart failure (Figure 25.21).
Parathyroid Hormone
Parathyroid hormone (PTH) is an 84-amino acid peptide produced by the parathyroid glands in response to decreased circulating Ca++ levels. Among its targets is the PCT, where it stimulates the hydroxylation of calcidiol to calcitriol (1,25-hydroxycholecalciferol, the active form of vitamin D). It also blocks reabsorption of phosphate (PO3–), causing its loss in the urine. The retention of phosphate would result in the formation of calcium phosphate in the plasma, reducing circulating Ca++levels. By ridding the blood of phosphate, higher circulating Ca++ levels are permitted.
Figure 25.21 Major Hormones That Influence GFR and RFB
Regulation of Fluid Volume and Composition
- Explain the mechanism of action of diuretics
- Explain why the differential permeability or impermeability of specific sections of the nephron tubules is necessary for urine formation
The major hormones influencing total body water are ADH, aldosterone, and ANH. Circumstances that lead to fluid depletion in the body include blood loss and dehydration. Homeostasis requires that volume and osmolarity be preserved. Blood volume is important in maintaining sufficient blood pressure, and there are nonrenal mechanisms involved in its preservation, including vasoconstriction, which can act within seconds of a drop in pressure. Thirst mechanisms are also activated to promote the consumption of water lost through respiration, evaporation, or urination. Hormonal mechanisms are activated to recover volume while maintaining a normal osmotic environment. These mechanisms act principally on the kidney.
Volume-sensing Mechanisms
The body cannot directly measure blood volume, but blood pressure can be measured. Blood pressure often reflects blood volume and is measured by baroreceptors in the aorta and carotid sinuses. When blood pressure increases, baroreceptors send more frequent action potentials to the central nervous system, leading to widespread vasodilation. Included in this vasodilation are the afferent arterioles supplying the glomerulus, resulting in increased GFR, and water loss by the kidneys. If pressure decreases, fewer action potentials travel to the central nervous system, resulting in more sympathetic stimulation-producing vasoconstriction, which will result in decreased filtration and GFR, and water loss.
Decreased blood pressure is also sensed by the granular cells in the afferent arteriole of the JGA. In response, the enzyme renin is released. You saw earlier in the chapter that renin activity leads to an almost immediate rise in blood pressure as activated angiotensin II produces vasoconstriction. The rise in pressure is sustained by the aldosterone effects initiated by angiotensin II; this includes an increase in Na+ retention and water volume. As an aside, late in the menstrual cycle, progesterone has a modest influence on water retention. Due to its structural similarity to aldosterone, progesterone binds to the aldosterone receptor in the collecting duct of the kidney, causing the same, albeit weaker, effect on Na+ and water retention.
Cardiomyocytes of the atria also respond to greater stretch (as blood pressure rises) by secreting ANH. ANH opposes the action of aldosterone by inhibiting the recovery of Na+ by the DCT and collecting ducts. More Na+ is lost, and as water follows, total blood volume and pressure decline. In low-pressure states, ANH does not seem to have much effect.
ADH is also called vasopressin. Early researchers found that in cases of unusually high secretion of ADH, the hormone caused vasoconstriction (vasopressor activity, hence the name). Only later were its antidiuretic properties identified. Synthetic ADH is still used occasionally to stem life-threatening esophagus bleeding in alcoholics.
When blood volume drops 5–10 percent, causing a decrease in blood pressure, there is a rapid and significant increase in ADH release from the posterior pituitary. Immediate vasoconstriction to increase blood pressure is the result. ADH also causes activation of aquaporin channels in the collecting ducts to affect the recovery of water to help restore vascular volume.
Diuretics and Fluid Volume
A diuretic is a compound that increases urine volume. Three familiar drinks contain diuretic compounds: coffee, tea, and alcohol. The caffeine in coffee and tea works by promoting vasodilation in the nephron, which increases GFR. Alcohol increases GFR by inhibiting ADH release from the posterior pituitary, resulting in less water recovery by the collecting duct. In cases of high blood pressure, diuretics may be prescribed to reduce blood volume and, thereby, reduce blood pressure. The most frequently prescribed anti-hypertensive diuretic is hydrochlorothiazide. It inhibits the Na+/ Cl– symporter in the DCT and collecting duct. The result is a loss of Na+ with water following passively by osmosis.
Osmotic diuretics promote water loss by osmosis. An example is the indigestible sugar mannitol, which is most often administered to reduce brain swelling after head injury. However, it is not the only sugar that can produce a diuretic effect. In cases of poorly controlled diabetes mellitus, glucose levels exceed the capacity of the tubular glucose symporters, resulting in glucose in the urine. The unrecovered glucose becomes a powerful osmotic diuretic. Classically, in the days before glucose could be detected in the blood and urine, clinicians identified diabetes mellitus by the three Ps: polyuria (diuresis), polydipsia (increased thirst), and polyphagia (increased hunger).
Regulation of Extracellular Na+
Sodium has a very strong osmotic effect and attracts water. It plays a larger role in the osmolarity of the plasma than any other circulating component of the blood. If there is too much Na+ present, either due to poor control or excess dietary consumption, a series of metabolic problems ensue. There is an increase in total volume of water, which leads to hypertension (high blood pressure). Over a long period, this increases the risk of serious complications such as heart attacks, strokes, and aneurysms. It can also contribute to system-wide edema (swelling).
Mechanisms for regulating Na+ concentration include the renin–angiotensin–aldosterone system and ADH (see Figure 25.14). Aldosterone stimulates the uptake of Na+ on the apical cell membrane of cells in the DCT and collecting ducts, whereas ADH helps to regulate Na+ concentration indirectly by regulating the reabsorption of water.
Regulation of Extracellular K+
Potassium is present in a 30-fold greater concentration inside the cell than outside the cell. A generalization can be made that K+ and Na+ concentrations will move in opposite directions. When more Na+ is reabsorbed, more K+ is secreted; when less Na+is reabsorbed (leading to excretion by the kidney), more K+ is retained. When aldosterone causes a recovery of Na+ in the nephron, a negative electrical gradient is created that promotes the secretion of K+ and Cl– into the lumen.
Regulation of Cl–
Chloride is important in acid–base balance in the extracellular space and has other functions, such as in the stomach, where it combines with hydrogen ions in the stomach lumen to form hydrochloric acid, aiding digestion. Its close association with Na+ in the extracellular environment makes it the dominant anion of this compartment, and its regulation closely mirrors that of Na+.
Regulation of Ca++ and Phosphate
The parathyroid glands monitor and respond to circulating levels of Ca++ in the blood. When levels drop too low, PTH is released to stimulate the DCT to reabsorb Ca++ from the forming urine. When levels are adequate or high, less PTH is released and more Ca++ remains in the forming urine to be lost. Phosphate levels move in the opposite direction. When Ca++ levels are low, PTH inhibits reabsorption of HPO2−4HPO42− so that its blood level drops, allowing Ca++ levels to rise. PTH also stimulates the renal conversion of calcidiol into calcitriol, the active form of vitamin D. Calcitriol then stimulates the intestines to absorb more Ca++from the diet.
Regulation of H+, Bicarbonate, and pH
The acid–base homeostasis of the body is a function of chemical buffers and physiologic buffering provided by the lungs and kidneys. Buffers, especially proteins, HCO2−3HCO32−+ as needed to resist a change in pH. They can act within fractions of a second. The lungs can rid the body of excess acid very rapidly (seconds to minutes) through the conversion of HCO3– into CO2, which is then exhaled. It is rapid but has limited capacity in the face of a significant acid challenge. The kidneys can rid the body of both acid and base. The renal capacity is large but slow (minutes to hours). The cells of the PCT actively secrete H+ into the forming urine as Na+ is reabsorbed. The body rids itself of excess H+and raises blood pH. In the collecting ducts, the apical surfaces of intercalated cells have proton pumps that actively secrete H+into the luminal, forming urine to remove it from the body.
As hydrogen ions are pumped into the forming urine, it is buffered by bicarbonate (HCO3–), H2PO4– (dihydrogen phosphate ion), or ammonia (forming NH4+, ammonium ion). Urine pH typically varies in a normal range from 4.5 to 8.0.
Regulation of Nitrogen Wastes
Nitrogen wastes are produced by the breakdown of proteins during normal metabolism. Proteins are broken down into amino acids, which in turn are deaminated by having their nitrogen groups removed. Deamination converts the amino (NH2) groups into ammonia (NH3), ammonium ion (NH4+), urea, or uric acid (Figure 25.22). Ammonia is extremely toxic, so most of it is very rapidly converted into urea in the liver. Human urinary wastes typically contain primarily urea with small amounts of ammonium and very little uric acid.
Figure 25.22 Nitrogen Wastes
Elimination of Drugs and Hormones
Water-soluble drugs may be excreted in the urine and are influenced by one or all of the following processes: glomerular filtration, tubular secretion, or tubular reabsorption. Drugs that are structurally small can be filtered by the glomerulus with the filtrate. Large drug molecules such as heparin or those that are bound to plasma proteins cannot be filtered and are not readily eliminated. Some drugs can be eliminated by carrier proteins that enable secretion of the drug into the tubule lumen. There are specific carriers that eliminate basic (such as dopamine or histamine) or acidic drugs (such as penicillin or indomethacin). As is the case with other substances, drugs may be both filtered and reabsorbed passively along a concentration gradient.
The Urinary System and Homeostasis
- Describe the role of the kidneys in vitamin D activation
- Describe the role of the kidneys in regulating erythropoiesis
- Provide specific examples to demonstrate how the urinary system responds to maintain homeostasis in the body
- Explain how the urinary system relates to other body systems in maintaining homeostasis
- Predict factors or situations affecting the urinary system that could disrupt homeostasis
- Predict the types of problems that would occur in the body if the urinary system could not maintain homeostasis
All systems of the body are interrelated. A change in one system may affect all other systems in the body, with mild to devastating effects. A failure of urinary continence can be embarrassing and inconvenient, but is not life threatening. The loss of other urinary functions may prove fatal. A failure to synthesize vitamin D is one such example.
Vitamin D Synthesis
In order for vitamin D to become active, it must undergo a hydroxylation reaction in the kidney, that is, an –OH group must be added to calcidiol to make calcitriol (1,25-dihydroxycholecalciferol). Activated vitamin D is important for absorption of Ca++ in the digestive tract, its reabsorption in the kidney, and the maintenance of normal serum concentrations of Ca++ and phosphate. Calcium is vitally important in bone health, muscle contraction, hormone secretion, and neurotransmitter release. Inadequate Ca++ leads to disorders like osteoporosis and osteomalacia in adults and rickets in children. Deficits may also result in problems with cell proliferation, neuromuscular function, blood clotting, and the inflammatory response. Recent research has confirmed that vitamin D receptors are present in most, if not all, cells of the body, reflecting the systemic importance of vitamin D. Many scientists have suggested it be referred to as a hormone rather than a vitamin.
Erythropoiesis
EPO is a 193-amino acid protein that stimulates the formation of red blood cells in the bone marrow. The kidney produces 85 percent of circulating EPO; the liver, the remainder. If you move to a higher altitude, the partial pressure of oxygen is lower, meaning there is less pressure to push oxygen across the alveolar membrane and into the red blood cell. One way the body compensates is to manufacture more red blood cells by increasing EPO production. If you start an aerobic exercise program, your tissues will need more oxygen to cope, and the kidney will respond with more EPO. If erythrocytes are lost due to severe or prolonged bleeding, or under produced due to disease or severe malnutrition, the kidneys come to the rescue by producing more EPO. Renal failure (loss of EPO production) is associated with anemia, which makes it difficult for the body to cope with increased oxygen demands or to supply oxygen adequately even under normal conditions. Anemia diminishes performance and can be life threatening.
Blood Pressure Regulation
Due to osmosis, water follows where Na+ leads. Much of the water the kidneys recover from the forming urine follows the reabsorption of Na+. ADH stimulation of aquaporin channels allows for regulation of water recovery in the collecting ducts. Normally, all of the glucose is recovered, but loss of glucose control (diabetes mellitus) may result in an osmotic dieresis severe enough to produce severe dehydration and death. A loss of renal function means a loss of effective vascular volume control, leading to hypotension (low blood pressure) or hypertension (high blood pressure), which can lead to stroke, heart attack, and aneurysm formation.
The kidneys cooperate with the lungs, liver, and adrenal cortex through the renin–angiotensin–aldosterone system (see Figure 25.14). The liver synthesizes and secretes the inactive precursor angiotensinogen. When the blood pressure is low, the kidney synthesizes and releases renin. Renin converts angiotensinogen into angiotensin I, and ACE produced in the lung converts angiotensin I into biologically active angiotensin II (Figure 25.23). The immediate and short-term effect of angiotensin II is to raise blood pressure by causing widespread vasoconstriction. angiotensin II also stimulates the adrenal cortex to release the steroid hormone aldosterone, which results in renal reabsorption of Na+ and its associated osmotic recovery of water. The reabsorption of Na+ helps to raise and maintain blood pressure over a longer term.
Figure 25.23 The Enzyme Renin Converts the Pro-enzyme Angiotensin
Regulation of Osmolarity
Blood pressure and osmolarity are regulated in a similar fashion. Severe hypo-osmolarity can cause problems like lysis (rupture) of blood cells or widespread edema, which is due to a solute imbalance. Inadequate solute concentration (such as protein) in the plasma results in water moving toward an area of greater solute concentration, in this case, the interstitial space and cell cytoplasm. If the kidney glomeruli are damaged by an autoimmune illness, large quantities of protein may be lost in the urine. The resultant drop in serum osmolarity leads to widespread edema that, if severe, may lead to damaging or fatal brain swelling. Severe hypertonic conditions may arise with severe dehydration from lack of water intake, severe vomiting, or uncontrolled diarrhea. When the kidney is unable to recover sufficient water from the forming urine, the consequences may be severe (lethargy, confusion, muscle cramps, and finally, death) .
Recovery of Electrolytes
Sodium, calcium, and potassium must be closely regulated. The role of Na+ and Ca++ homeostasis has been discussed at length. Failure of K+ regulation can have serious consequences on nerve conduction, skeletal muscle function, and most significantly, on cardiac muscle contraction and rhythm.
pH Regulation
Recall that enzymes lose their three-dimensional conformation and, therefore, their function if the pH is too acidic or basic. This loss of conformation may be a consequence of the breaking of hydrogen bonds. Move the pH away from the optimum for a specific enzyme and you may severely hamper its function throughout the body, including hormone binding, central nervous system signaling, or myocardial contraction. Proper kidney function is essential for pH homeostasis.
EVERYDAY CONNECTION
Stem Cells and Repair of Kidney Damage
Stem cells are unspecialized cells that can reproduce themselves via cell division, sometimes after years of inactivity. Under certain conditions, they may differentiate into tissue-specific or organ-specific cells with special functions. In some cases, stem cells may continually divide to produce a mature cell and to replace themselves. Stem cell therapy has an enormous potential to improve the quality of life or save the lives of people suffering from debilitating or life-threatening diseases. There have been several studies in animals, but since stem cell therapy is still in its infancy, there have been limited experiments in humans.
Acute kidney injury can be caused by a number of factors, including transplants and other surgeries. It affects 7–10 percent of all hospitalized patients, resulting in the deaths of 35–40 percent of inpatients. In limited studies using mesenchymal stem cells, there have been fewer instances of kidney damage after surgery, the length of hospital stays has been reduced, and there have been fewer readmissions after release.
How do these stem cells work to protect or repair the kidney? Scientists are unsure at this point, but some evidence has shown that these stem cells release several growth factors in endocrine and paracrine ways. As further studies are conducted to assess the safety and effectiveness of stem cell therapy, we will move closer to a day when kidney injury is rare, and curative treatments are routine.
Key Terms
- anatomical sphincter
- smooth or skeletal muscle surrounding the lumen of a vessel or hollow organ that can restrict flow when contracted
- angiotensin I
- protein produced by the enzymatic action of renin on angiotensinogen; inactive precursor of angiotensin II
- angiotensin II
- protein produced by the enzymatic action of ACE on inactive angiotensin I; actively causes vasoconstriction and stimulates aldosterone release by the adrenal cortex
- angiotensin-converting enzyme (ACE)
- enzyme produced by the lungs that catalyzes the reaction of inactive angiotensin I into active angiotensin II
- angiotensinogen
- inactive protein in the circulation produced by the liver; precursor of angiotensin I; must be modified by the enzymes renin and ACE to be activated
- anuria
- absence of urine produced; production of 50 mL or less per day
- aquaporin
- protein-forming water channels through the lipid bilayer of the cell; allows water to cross; activation in the collecting ducts is under the control of ADH
- Bowman’s capsule
- cup-shaped sack lined by a simple squamous epithelium (parietal surface) and specialized cells called podocytes (visceral surface) that participate in the filtration process; receives the filtrate which then passes on to the PCTs
- brush border
- formed by microvilli on the surface of certain cuboidal cells; in the kidney it is found in the PCT; increases surface area for absorption in the kidney
- calyces
- cup-like structures receiving urine from the collecting ducts where it passes on to the renal pelvis and ureter
- cortical nephrons
- nephrons with loops of Henle that do not extend into the renal medulla
- countercurrent multiplier system
- involves the descending and ascending loops of Henle directing forming urine in opposing directions to create a concentration gradient when combined with variable permeability and sodium pumping
- detrusor muscle
- smooth muscle in the bladder wall; fibers run in all directions to reduce the size of the organ when emptying it of urine
- distal convoluted tubules
- portions of the nephron distal to the loop of Henle that receive hyposmotic filtrate from the loop of Henle and empty into collecting ducts
- diuretic
- compound that increases urine output, leading to decreased water conservation
- efferent arteriole
- arteriole carrying blood from the glomerulus to the capillary beds around the convoluted tubules and loop of Henle; portion of the portal system
- endothelins
- group of vasoconstrictive, 21-amino acid peptides; produced by endothelial cells of the renal blood vessels, mesangial cells, and cells of the DCT
- external urinary sphincter
- skeletal muscle; must be relaxed consciously to void urine
- fenestrations
- small windows through a cell, allowing rapid filtration based on size; formed in such a way as to allow substances to cross through a cell without mixing with cell contents
- filtration slits
- formed by pedicels of podocytes; substances filter between the pedicels based on size
- forming urine
- filtrate undergoing modifications through secretion and reabsorption before true urine is produced
- glomerular filtration rate (GFR)
- rate of renal filtration
- glomerulus
- tuft of capillaries surrounded by Bowman’s capsule; filters the blood based on size
- glycosuria
- presence of glucose in the urine; caused by high blood glucose levels that exceed the ability of the kidneys to reabsorb the glucose; usually the result of untreated or poorly controlled diabetes mellitus
- incontinence
- loss of ability to control micturition
- intercalated cell
- specialized cell of the collecting ducts that secrete or absorb acid or bicarbonate; important in acid–base balance
- internal urinary sphincter
- smooth muscle at the juncture of the bladder and urethra; relaxes as the bladder fills to allow urine into the urethra
- inulin
- plant polysaccharide injected to determine GFR; is neither secreted nor absorbed by the kidney, so its appearance in the urine is directly proportional to its filtration rate
- juxtaglomerular apparatus (JGA)
- located at the juncture of the DCT and the afferent and efferent arterioles of the glomerulus; plays a role in the regulation of renal blood flow and GFR
- juxtaglomerular cell
- modified smooth muscle cells of the afferent arteriole; secretes renin in response to a drop in blood pressure
- juxtamedullary nephrons
- nephrons adjacent to the border of the cortex and medulla with loops of Henle that extend into the renal medulla
- leaky tight junctions
- tight junctions in which the sealing strands of proteins between the membranes of adjacent cells are fewer in number and incomplete; allows limited intercellular movement of solvent and solutes
- leukocyte esterase
- enzyme produced by leukocytes that can be detected in the urine and that serves as an indirect indicator of urinary tract infection
- loop of Henle
- descending and ascending portions between the proximal and distal convoluted tubules; those of cortical nephrons do not extend into the medulla, whereas those of juxtamedullary nephrons do extend into the medulla
- macula densa
- cells found in the part of the DCT forming the JGA; sense Na+ concentration in the forming urine
- medulla
- inner region of kidney containing the renal pyramids
- mesangial
- contractile cells found in the glomerulus; can contract or relax to regulate filtration rate
- micturition
- also called urination or voiding
- myogenic mechanism
- mechanism by which smooth muscle responds to stretch by contracting; an increase in blood pressure causes vasoconstriction and a decrease in blood pressure causes vasodilation so that blood flow downstream remains steady
- nephrons
- functional units of the kidney that carry out all filtration and modification to produce urine; consist of renal corpuscles, proximal and distal convoluted tubules, and descending and ascending loops of Henle; drain into collecting ducts
- net filtration pressure (NFP)
- pressure of fluid across the glomerulus; calculated by taking the hydrostatic pressure of the capillary and subtracting the colloid osmotic pressure of the blood and the hydrostatic pressure of Bowman’s capsule
- oliguria
- below normal urine production of 400–500 mL/day
- osteomalacia
- softening of bones due to a lack of mineralization with calcium and phosphate; most often due to lack of vitamin D; in children, osteomalacia is termed rickets; not to be confused with osteoporosis
- pedicels
- finger-like projections of podocytes surrounding glomerular capillaries; interdigitate to form a filtration membrane
- peritubular capillaries
- second capillary bed of the renal portal system; surround the proximal and distal convoluted tubules; associated with the vasa recta
- physiological sphincter
- sphincter consisting of circular smooth muscle indistinguishable from adjacent muscle but possessing differential innervations, permitting its function as a sphincter; structurally weak
- podocytes
- cells forming finger-like processes; form the visceral layer of Bowman’s capsule; pedicels of the podocytes interdigitate to form a filtration membrane
- polyuria
- urine production in excess of 2.5 L/day; may be caused by diabetes insipidus, diabetes mellitus, or excessive use of diuretics
- principal cell
- found in collecting ducts and possess channels for the recovery or loss of sodium and potassium; under the control of aldosterone; also have aquaporin channels under ADH control to regulate recovery of water
- proximal convoluted tubules (PCTs)
- tortuous tubules receiving filtrate from Bowman’s capsule; most active part of the nephron in reabsorption and secretion
- renal columns
- extensions of the renal cortex into the renal medulla; separates the renal pyramids; contains blood vessels and connective tissues
- renal corpuscle
- consists of the glomerulus and Bowman’s capsule
- renal cortex
- outer part of kidney containing all of the nephrons; some nephrons have loops of Henle extending into the medulla
- renal fat pad
- adipose tissue between the renal fascia and the renal capsule that provides protective cushioning to the kidney
- renal hilum
- recessed medial area of the kidney through which the renal artery, renal vein, ureters, lymphatics, and nerves pass
- renal papillae
- medullary area of the renal pyramids where collecting ducts empty urine into the minor calyces
- renal pyramids
- six to eight cone-shaped tissues in the medulla of the kidney containing collecting ducts and the loops of Henle of juxtamedullary nephrons
- renin
- enzyme produced by juxtaglomerular cells in response to decreased blood pressure or sympathetic nervous activity; catalyzes the conversion of angiotensinogen into angiotensin I
- retroperitoneal
- outside the peritoneal cavity; in the case of the kidney and ureters, between the parietal peritoneum and the abdominal wall
- sacral micturition center
- group of neurons in the sacral region of the spinal cord that controls urination; acts reflexively unless its action is modified by higher brain centers to allow voluntary urination
- specific gravity
- weight of a liquid compared to pure water, which has a specific gravity of 1.0; any solute added to water will increase its specific gravity
- systemic edema
- increased fluid retention in the interstitial spaces and cells of the body; can be seen as swelling over large areas of the body, particularly the lower extremities
- trigone
- area at the base of the bladder marked by the two ureters in the posterior–lateral aspect and the urethral orifice in the anterior aspect oriented like points on a triangle
- tubuloglomerular feedback
- feedback mechanism involving the JGA; macula densa cells monitor Na+ concentration in the terminal portion of the ascending loop of Henle and act to cause vasoconstriction or vasodilation of afferent and efferent arterioles to alter GFR
- urethra
- transports urine from the bladder to the outside environment
- urinalysis
- analysis of urine to diagnose disease
- urochrome
- heme-derived pigment that imparts the typical yellow color of urine
- vasa recta
- branches of the efferent arterioles that parallel the course of the loops of Henle and are continuous with the peritubular capillaries; with the glomerulus, form a portal system
Chapter Review
25.1 Physical Characteristics of Urine
The kidney glomerulus filters blood mainly based on particle size to produce a filtrate lacking cells or large proteins. Most of the ions and molecules in the filtrate are needed by the body and must be reabsorbed farther down the nephron tubules, resulting in the formation of urine. Urine characteristics change depending on water intake, exercise, environmental temperature, and nutrient intake. Urinalysis analyzes characteristics of the urine and is used to diagnose diseases. A minimum of 400 to 500 mL urine must be produced daily to rid the body of wastes. Excessive quantities of urine may indicate diabetes insipidus or diabetes mellitus. The pH range of urine is 4.5 to 8.0, and is affected by diet. Osmolarity ranges from 50 to 1200 milliosmoles, and is a reflection of the amount of water being recovered or lost by renal nephrons.
25.2 Gross Anatomy of Urine Transport
The urethra is the only urinary structure that differs significantly between males and females. This is due to the dual role of the male urethra in transporting both urine and semen. The urethra arises from the trigone area at the base of the bladder. Urination is controlled by an involuntary internal sphincter of smooth muscle and a voluntary external sphincter of skeletal muscle. The shorter female urethra contributes to the higher incidence of bladder infections in females. The male urethra receives secretions from the prostate gland, Cowper’s gland, and seminal vesicles as well as sperm. The bladder is largely retroperitoneal and can hold up to 500–600 mL urine. Micturition is the process of voiding the urine and involves both involuntary and voluntary actions. Voluntary control of micturition requires a mature and intact sacral micturition center. It also requires an intact spinal cord. Loss of control of micturition is called incontinence and results in voiding when the bladder contains about 250 mL urine. The ureters are retroperitoneal and lead from the renal pelvis of the kidney to the trigone area at the base of the bladder. A thick muscular wall consisting of longitudinal and circular smooth muscle helps move urine toward the bladder by way of peristaltic contractions.
25.3 Gross Anatomy of the Kidney
As noted previously, the structure of the kidney is divided into two principle regions—the peripheral rim of cortex and the central medulla. The two kidneys receive about 25 percent of cardiac output. They are protected in the retroperitoneal space by the renal fat pad and overlying ribs and muscle. Ureters, blood vessels, lymph vessels, and nerves enter and leave at the renal hilum. The renal arteries arise directly from the aorta, and the renal veins drain directly into the inferior vena cava. Kidney function is derived from the actions of about 1.3 million nephrons per kidney; these are the “functional units.” A capillary bed, the glomerulus, filters blood and the filtrate is captured by Bowman’s capsule. A portal system is formed when the blood flows through a second capillary bed surrounding the proximal and distal convoluted tubules and the loop of Henle. Most water and solutes are recovered by this second capillary bed. This filtrate is processed and finally gathered by collecting ducts that drain into the minor calyces, which merge to form major calyces; the filtrate then proceeds to the renal pelvis and finally the ureters.
25.4 Microscopic Anatomy of the Kidney
The functional unit of the kidney, the nephron, consists of the renal corpuscle, PCT, loop of Henle, and DCT. Cortical nephrons have short loops of Henle, whereas juxtamedullary nephrons have long loops of Henle extending into the medulla. About 15 percent of nephrons are juxtamedullary. The glomerulus is a capillary bed that filters blood principally based on particle size. The filtrate is captured by Bowman’s capsule and directed to the PCT. A filtration membrane is formed by the fused basement membranes of the podocytes and the capillary endothelial cells that they embrace. Contractile mesangial cells further perform a role in regulating the rate at which the blood is filtered. Specialized cells in the JGA produce paracrine signals to regulate blood flow and filtration rates of the glomerulus. Other JGA cells produce the enzyme renin, which plays a central role in blood pressure regulation. The filtrate enters the PCT where absorption and secretion of several substances occur. The descending and ascending limbs of the loop of Henle consist of thick and thin segments. Absorption and secretion continue in the DCT but to a lesser extent than in the PCT. Each collecting duct collects forming urine from several nephrons and responds to the posterior pituitary hormone ADH by inserting aquaporin water channels into the cell membrane to fine tune water recovery.
25.5 Physiology of Urine Formation
The entire volume of the blood is filtered through the kidneys about 300 times per day, and 99 percent of the water filtered is recovered. The GFR is influenced by hydrostatic pressure and colloid osmotic pressure. Under normal circumstances, hydrostatic pressure is significantly greater and filtration occurs. The hydrostatic pressure of the glomerulus depends on systemic blood pressure, autoregulatory mechanisms, sympathetic nervous activity, and paracrine hormones. The kidney can function normally under a wide range of blood pressures due to the autoregulatory nature of smooth muscle.
25.6 Tubular Reabsorption
The kidney regulates water recovery and blood pressure by producing the enzyme renin. It is renin that starts a series of reactions, leading to the production of the vasoconstrictor angiotensin II and the salt-retaining steroid aldosterone. Water recovery is also powerfully and directly influenced by the hormone ADH. Even so, it only influences the last 10 percent of water available for recovery after filtration at the glomerulus, because 90 percent of water is recovered before reaching the collecting ducts. Depending on the body’s fluid status at any given time, the collecting ducts can recover none or almost all of the water reaching them.
Mechanisms of solute recovery include active transport, simple diffusion, and facilitated diffusion. Most filtered substances are reabsorbed. Urea, NH3, creatinine, and some drugs are filtered or secreted as wastes. H+ and HCO3– are secreted or reabsorbed as needed to maintain acid–base balance. Movement of water from the glomerulus is primarily due to pressure, whereas that of peritubular capillaries and vasa recta is due to osmolarity and concentration gradients. The PCT is the most metabolically active part of the nephron and uses a wide array of protein micromachines to maintain homeostasis—symporters, antiporters, and ATPase active transporters—in conjunction with diffusion, both simple and facilitated. Almost 100 percent of glucose, amino acids, and vitamins are recovered in the PCT. Bicarbonate (HCO3–) is recovered using the same enzyme, carbonic anhydrase (CA), found in erythrocytes. The recovery of solutes creates an osmotic gradient to promote the recovery of water. The descending loop of the juxtaglomerular nephrons reaches an osmolarity of up to 1200 mOsmol/kg, promoting the recovery of water. The ascending loop is impervious to water but actively recovers Na+, reducing filtrate osmolarity to 50–100 mOsmol/kg. The descending and ascending loop and vasa recta form a countercurrent multiplier system to increase Na+concentration in the kidney medulla. The collecting ducts actively pump urea into the medulla, further contributing to the high osmotic environment. The vasa recta recover the solute and water in the medulla, returning them to the circulation. Nearly 90 percent of water is recovered before the forming urine reaches the DCT, which will recover another 10 percent. Calcium recovery in the DCT is influenced by PTH and active vitamin D. In the collecting ducts, ADH stimulates aquaporin channel insertion to increase water recovery and thereby regulate osmolarity of the blood. Aldosterone stimulates Na+ recovery by the collecting duct.
25.7 Regulation of Renal Blood Flow
The kidneys are innervated by sympathetic nerves of the autonomic nervous system. Sympathetic nervous activity decreases blood flow to the kidney, making more blood available to other areas of the body during times of stress. The arteriolar myogenic mechanism maintains a steady blood flow by causing arteriolar smooth muscle to contract when blood pressure increases and causing it to relax when blood pressure decreases. Tubuloglomerular feedback involves paracrine signaling at the JGA to cause vasoconstriction or vasodilation to maintain a steady rate of blood flow.
25.8 Endocrine Regulation of Kidney Function
Endocrine hormones act from a distance and paracrine hormones act locally. The renal enzyme renin converts angiotensinogen into angiotensin I. The lung enzyme, ACE, converts angiotensin I into active angiotensin II. Angiotensin II is an active vasoconstrictor that increases blood pressure. Angiotensin II also stimulates aldosterone release from the adrenal cortex, causing the collecting duct to retain Na+, which promotes water retention and a longer-term rise in blood pressure. ADH promotes water recovery by the collecting ducts by stimulating the insertion of aquaporin water channels into cell membranes. Endothelins are elevated in cases of diabetic kidney disease, increasing Na+ retention and decreasing GFR. Natriuretic hormones, released primarily from the atria of the heart in response to stretching of the atrial walls, stimulate Na+ excretion and thereby decrease blood pressure. PTH stimulates the final step in the formation of active vitamin D3 and reduces phosphate reabsorption, resulting in higher circulating Ca++ levels.
25.9 Regulation of Fluid Volume and Composition
The major hormones regulating body fluids are ADH, aldosterone and ANH. Progesterone is similar in structure to aldosterone and can bind to and weakly stimulate aldosterone receptors, providing a similar but diminished response. Blood pressure is a reflection of blood volume and is monitored by baroreceptors in the aortic arch and carotid sinuses. When blood pressure increases, more action potentials are sent to the central nervous system, resulting in greater vasodilation, greater GFR, and more water lost in the urine. ANH is released by the cardiomyocytes when blood pressure increases, causing Na+ and water loss. ADH at high levels causes vasoconstriction in addition to its action on the collecting ducts to recover more water. Diuretics increase urine volume. Mechanisms for controlling Na+ concentration in the blood include the renin–angiotensin–aldosterone system and ADH. When Na+ is retained, K+ is excreted; when Na+ is lost, K+ is retained. When circulating Ca++ decreases, PTH stimulates the reabsorption of Ca++ and inhibits reabsorption of HPO2−4HPO42−2, and excretion of acid or base by the kidneys. The breakdown of amino acids produces ammonia. Most ammonia is converted into less-toxic urea in the liver and excreted in the urine. Regulation of drugs is by glomerular filtration, tubular secretion, and tubular reabsorption.
25.10 The Urinary System and Homeostasis
The effects of failure of parts of the urinary system may range from inconvenient (incontinence) to fatal (loss of filtration and many others). The kidneys catalyze the final reaction in the synthesis of active vitamin D that in turn helps regulate Ca++. The kidney hormone EPO stimulates erythrocyte development and promotes adequate O2 transport. The kidneys help regulate blood pressure through Na+ and water retention and loss. The kidneys work with the adrenal cortex, lungs, and liver in the renin–angiotensin–aldosterone system to regulate blood pressure. They regulate osmolarity of the blood by regulating both solutes and water. Three electrolytes are more closely regulated than others: Na+, Ca++, and K+. The kidneys share pH regulation with the lungs and plasma buffers, so that proteins can preserve their three-dimensional conformation and thus their function.
Review Questions
Diabetes insipidus or diabetes mellitus would most likely be indicated by ________.
- anuria
- polyuria
- oliguria
- none of the above
The color of urine is determined mainly by ________.
- diet
- filtration rate
- byproducts of red blood cell breakdown
- filtration efficiency
Production of less than 50 mL/day of urine is called ________.
- normal
- polyuria
- oliguria
- anuria
Peristaltic contractions occur in the ________.
- urethra
- bladder
- ureters
- urethra, bladder, and ureters
Somatic motor neurons must be ________ to relax the external urethral sphincter to allow urination.
- stimulated
- inhibited
Which part of the urinary system is not completely retroperitoneal?
- kidneys
- ureters
- bladder
- nephrons
The renal pyramids are separated from each other by extensions of the renal cortex called ________.
- renal medulla
- minor calyces
- medullary cortices
- renal columns
The primary structure found within the medulla is the ________.
- loop of Henle
- minor calyces
- portal system
- ureter
The right kidney is slightly lower because ________.
- it is displaced by the liver
- it is displace by the heart
- it is slightly smaller
- it needs protection of the lower ribs
Blood filtrate is captured in the lumen of the ________.
- glomerulus
- Bowman’s capsule
- calyces
- renal papillae
What are the names of the capillaries following the efferent arteriole?
- arcuate and medullary
- interlobar and interlobular
- peritubular and vasa recta
- peritubular and medullary
The functional unit of the kidney is called ________.
- the renal hilus
- the renal corpuscle
- the nephron
- Bowman’s capsule
________ pressure must be greater on the capillary side of the filtration membrane to achieve filtration.
- Osmotic
- Hydrostatic
Production of urine to modify plasma makeup is the result of ________.
- filtration
- absorption
- secretion
- filtration, absorption, and secretion
Systemic blood pressure must stay above 60 so that the proper amount of filtration occurs.
- true
- false
Aquaporin channels are only found in the collecting duct.
- true
- false
Most absorption and secretion occurs in this part of the nephron.
- proximal convoluted tubule
- descending loop of Henle
- ascending loop of Henle
- distal convoluted tubule
- collecting ducts
The fine tuning of water recovery or disposal occurs in ________.
- the proximal convoluted tubule
- the collecting ducts
- the ascending loop of Henle
- the distal convoluted tubule
Vasodilation of blood vessels to the kidneys is due to ________.
- more frequent action potentials
- less frequent action potentials
When blood pressure increases, blood vessels supplying the kidney will ________ to mount a steady rate of filtration.
- contract
- relax
Which of these three paracrine chemicals cause vasodilation?
- ATP
- adenosine
- nitric oxide
What hormone directly opposes the actions of natriuretic hormones?
- renin
- nitric oxide
- dopamine
- aldosterone
Which of these is a vasoconstrictor?
- nitric oxide
- natriuretic hormone
- bradykinin
- angiotensin II
What signal causes the heart to secrete atrial natriuretic hormone?
- increased blood pressure
- decreased blood pressure
- increased Na+ levels
- decreased Na+ levels
Which of these beverages does not have a diuretic effect?
- tea
- coffee
- alcohol
- milk
Progesterone can bind to receptors for which hormone that, when released, activates water retention?
- aldosterone
- ADH
- PTH
- ANH
Renin is released in response to ________.
- increased blood pressure
- decreased blood pressure
- ACE
- diuretics
Which step in vitamin D production does the kidney perform?
- converts cholecalciferol into calcidiol
- converts calcidiol into calcitriol
- stores vitamin D
- none of these
Which hormone does the kidney produce that stimulates red blood cell production?
- thrombopoeitin
- vitamin D
- EPO
- renin
If there were no aquaporin channels in the collecting duct, ________.
- you would develop systemic edema
- you would retain excess Na+
- you would lose vitamins and electrolytes
- you would suffer severe dehydration
Critical Thinking Questions
What is suggested by the presence of white blood cells found in the urine?
32.Both diabetes mellitus and diabetes insipidus produce large urine volumes, but how would other characteristics of the urine differ between the two diseases?
33.Why are females more likely to contract bladder infections than males?
34.Describe how forceful urination is accomplished.
35.What anatomical structures provide protection to the kidney?
36.How does the renal portal system differ from the hypothalamo–hypophyseal and digestive portal systems?
37.Name the structures found in the renal hilum.
38.Which structures make up the renal corpuscle?
39.What are the major structures comprising the filtration membrane?
40.Give the formula for net filtration pressure.
41.Name at least five symptoms of kidney failure.
42.Which vessels and what part of the nephron are involved in countercurrent multiplication?
43.Give the approximate osmolarity of fluid in the proximal convoluted tubule, deepest part of the loop of Henle, distal convoluted tubule, and the collecting ducts.
44.Explain what happens to Na+ concentration in the nephron when GFR increases.
45.If you want the kidney to excrete more Na+ in the urine, what do you want the blood flow to do?
46.What organs produce which hormones or enzymes in the renin–angiotensin system?
47.PTH affects absorption and reabsorption of what?
48.Why is ADH also called vasopressin?
49.How can glucose be a diuretic?
50.How does lack of protein in the blood cause edema?
51.Which three electrolytes are most closely regulated by the kidney?
|
oercommons
|
2025-03-18T00:39:12.189957
|
10/14/2019
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/58773/overview",
"title": "Anatomy and Physiology, Energy, Maintenance, and Environmental Exchange, The Urinary System",
"author": null
}
|
https://oercommons.org/courseware/lesson/56353/overview
|
The Cellular Level of Organization
Introduction
Figure 3.1 Fluorescence-stained Cell Undergoing Mitosis A lung cell from a newt, commonly studied for its similarity to human lung cells, is stained with fluorescent dyes. The green stain reveals mitotic spindles, red is the cell membrane and part of the cytoplasm, and the structures that appear light blue are chromosomes. This cell is in anaphase of mitosis. (credit: “Mortadelo2005”/Wikimedia Commons)
CHAPTER OBJECTIVES
After studying this chapter, you will be able to:
- Describe the structure and function of the cell membrane, including its regulation of materials into and out of the cell
- Describe the functions of the various cytoplasmic organelles
- Explain the structure and contents of the nucleus, as well as the process of DNA replication
- Explain the process by which a cell builds proteins using the DNA code
- List the stages of the cell cycle in order, including the steps of cell division in somatic cells
- Discuss how a cell differentiates and becomes more specialized
- List the morphological and physiological characteristics of some representative cell types in the human body
You developed from a single fertilized egg cell into the complex organism containing trillions of cells that you see when you look in a mirror. During this developmental process, early, undifferentiated cells differentiate and become specialized in their structure and function. These different cell types form specialized tissues that work in concert to perform all of the functions necessary for the living organism. Cellular and developmental biologists study how the continued division of a single cell leads to such complexity and differentiation.
Consider the difference between a structural cell in the skin and a nerve cell. A structural skin cell may be shaped like a flat plate (squamous) and live only for a short time before it is shed and replaced. Packed tightly into rows and sheets, the squamous skin cells provide a protective barrier for the cells and tissues that lie beneath. A nerve cell, on the other hand, may be shaped something like a star, sending out long processes up to a meter in length and may live for the entire lifetime of the organism. With their long winding appendages, nerve cells can communicate with one another and with other types of body cells and send rapid signals that inform the organism about its environment and allow it to interact with that environment. These differences illustrate one very important theme that is consistent at all organizational levels of biology: the form of a structure is optimally suited to perform particular functions assigned to that structure. Keep this theme in mind as you tour the inside of a cell and are introduced to the various types of cells in the body.
A primary responsibility of each cell is to contribute to homeostasis. Homeostasis is a term used in biology that refers to a dynamic state of balance within parameters that are compatible with life. For example, living cells require a water-based environment to survive in, and there are various physical (anatomical) and physiological mechanisms that keep all of the trillions of living cells in the human body moist. This is one aspect of homeostasis. When a particular parameter, such as blood pressure or blood oxygen content, moves far enough out of homeostasis (generally becoming too high or too low), illness or disease—and sometimes death—inevitably results.
The concept of a cell started with microscopic observations of dead cork tissue by scientist Robert Hooke in 1665. Without realizing their function or importance, Hook coined the term “cell” based on the resemblance of the small subdivisions in the cork to the rooms that monks inhabited, called cells. About ten years later, Antonie van Leeuwenhoek became the first person to observe living and moving cells under a microscope. In the century that followed, the theory that cells represented the basic unit of life would develop. These tiny fluid-filled sacs house components responsible for the thousands of biochemical reactions necessary for an organism to grow and survive. In this chapter, you will learn about the major components and functions of a prototypical, generalized cell and discover some of the different types of cells in the human body.
The Cell Membrane
- Describe the molecular components that make up the cell membrane
- Explain the major features and properties of the cell membrane
- Differentiate between materials that can and cannot diffuse through the lipid bilayer
- Compare and contrast different types of passive transport with active transport, providing examples of each
Despite differences in structure and function, all living cells in multicellular organisms have a surrounding cell membrane. As the outer layer of your skin separates your body from its environment, the cell membrane (also known as the plasma membrane) separates the inner contents of a cell from its exterior environment. This cell membrane provides a protective barrier around the cell and regulates which materials can pass in or out.
Structure and Composition of the Cell Membrane
The cell membrane is an extremely pliable structure composed primarily of back-to-back phospholipids (a “bilayer”). Cholesterol is also present, which contributes to the fluidity of the membrane, and there are various proteins embedded within the membrane that have a variety of functions.
A single phospholipid molecule has a phosphate group on one end, called the “head,” and two side-by-side chains of fatty acids that make up the lipid tails (Figure 3.2). The phosphate group is negatively charged, making the head polar and hydrophilic—or “water loving.” A hydrophilic molecule (or region of a molecule) is one that is attracted to water. The phosphate heads are thus attracted to the water molecules of both the extracellular and intracellular environments. The lipid tails, on the other hand, are uncharged, or nonpolar, and are hydrophobic—or “water fearing.” A hydrophobic molecule (or region of a molecule) repels and is repelled by water. Some lipid tails consist of saturated fatty acids and some contain unsaturated fatty acids. This combination adds to the fluidity of the tails that are constantly in motion. Phospholipids are thus amphipathic molecules. An amphipathic molecule is one that contains both a hydrophilic and a hydrophobic region. In fact, soap works to remove oil and grease stains because it has amphipathic properties. The hydrophilic portion can dissolve in water while the hydrophobic portion can trap grease in micelles that then can be washed away.
Figure 3.2 Phospholipid Structure A phospholipid molecule consists of a polar phosphate “head,” which is hydrophilic and a non-polar lipid “tail,” which is hydrophobic. Unsaturated fatty acids result in kinks in the hydrophobic tails.
The cell membrane consists of two adjacent layers of phospholipids. The lipid tails of one layer face the lipid tails of the other layer, meeting at the interface of the two layers. The phospholipid heads face outward, one layer exposed to the interior of the cell and one layer exposed to the exterior (Figure 3.3). Because the phosphate groups are polar and hydrophilic, they are attracted to water in the intracellular fluid. Intracellular fluid (ICF) is the fluid interior of the cell. The phosphate groups are also attracted to the extracellular fluid. Extracellular fluid (ECF) is the fluid environment outside the enclosure of the cell membrane. Interstitial fluid (IF) is the term given to extracellular fluid not contained within blood vessels. Because the lipid tails are hydrophobic, they meet in the inner region of the membrane, excluding watery intracellular and extracellular fluid from this space. The cell membrane has many proteins, as well as other lipids (such as cholesterol), that are associated with the phospholipid bilayer. An important feature of the membrane is that it remains fluid; the lipids and proteins in the cell membrane are not rigidly locked in place.
Figure 3.3 Phospolipid Bilayer The phospholipid bilayer consists of two adjacent sheets of phospholipids, arranged tail to tail. The hydrophobic tails associate with one another, forming the interior of the membrane. The polar heads contact the fluid inside and outside of the cell.
Membrane Proteins
The lipid bilayer forms the basis of the cell membrane, but it is peppered throughout with various proteins. Two different types of proteins that are commonly associated with the cell membrane are the integral proteins and peripheral protein (Figure 3.4). As its name suggests, an integral protein is a protein that is embedded in the membrane. A channel protein is an example of an integral protein that selectively allows particular materials, such as certain ions, to pass into or out of the cell.
Figure 3.4 Cell Membrane The cell membrane of the cell is a phospholipid bilayer containing many different molecular components, including proteins and cholesterol, some with carbohydrate groups attached.
Another important group of integral proteins are cell recognition proteins, which serve to mark a cell’s identity so that it can be recognized by other cells. A receptor is a type of recognition protein that can selectively bind a specific molecule outside the cell, and this binding induces a chemical reaction within the cell. A ligand is the specific molecule that binds to and activates a receptor. Some integral proteins serve dual roles as both a receptor and an ion channel. One example of a receptor-ligand interaction is the receptors on nerve cells that bind neurotransmitters, such as dopamine. When a dopamine molecule binds to a dopamine receptor protein, a channel within the transmembrane protein opens to allow certain ions to flow into the cell.
Some integral membrane proteins are glycoproteins. A glycoprotein is a protein that has carbohydrate molecules attached, which extend into the extracellular matrix. The attached carbohydrate tags on glycoproteins aid in cell recognition. The carbohydrates that extend from membrane proteins and even from some membrane lipids collectively form the glycocalyx. The glycocalyx is a fuzzy-appearing coating around the cell formed from glycoproteins and other carbohydrates attached to the cell membrane. The glycocalyx can have various roles. For example, it may have molecules that allow the cell to bind to another cell, it may contain receptors for hormones, or it might have enzymes to break down nutrients. The glycocalyces found in a person’s body are products of that person’s genetic makeup. They give each of the individual’s trillions of cells the “identity” of belonging in the person’s body. This identity is the primary way that a person’s immune defense cells “know” not to attack the person’s own body cells, but it also is the reason organs donated by another person might be rejected.
Peripheral proteins are typically found on the inner or outer surface of the lipid bilayer but can also be attached to the internal or external surface of an integral protein. These proteins typically perform a specific function for the cell. Some peripheral proteins on the surface of intestinal cells, for example, act as digestive enzymes to break down nutrients to sizes that can pass through the cells and into the bloodstream.
Transport across the Cell Membrane
One of the great wonders of the cell membrane is its ability to regulate the concentration of substances inside the cell. These substances include ions such as Ca++, Na+, K+, and Cl–; nutrients including sugars, fatty acids, and amino acids; and waste products, particularly carbon dioxide (CO2), which must leave the cell.
The membrane’s lipid bilayer structure provides the first level of control. The phospholipids are tightly packed together, and the membrane has a hydrophobic interior. This structure causes the membrane to be selectively permeable. A membrane that has selective permeability allows only substances meeting certain criteria to pass through it unaided. In the case of the cell membrane, only relatively small, nonpolar materials can move through the lipid bilayer (remember, the lipid tails of the membrane are nonpolar). Some examples of these are other lipids, oxygen and carbon dioxide gases, and alcohol. However, water-soluble materials—like glucose, amino acids, and electrolytes—need some assistance to cross the membrane because they are repelled by the hydrophobic tails of the phospholipid bilayer. All substances that move through the membrane do so by one of two general methods, which are categorized based on whether or not energy is required. Passive transport is the movement of substances across the membrane without the expenditure of cellular energy. In contrast, active transport is the movement of substances across the membrane using energy from adenosine triphosphate (ATP).
Passive Transport
In order to understand how substances move passively across a cell membrane, it is necessary to understand concentration gradients and diffusion. A concentration gradient is the difference in concentration of a substance across a space. Molecules (or ions) will spread/diffuse from where they are more concentrated to where they are less concentrated until they are equally distributed in that space. (When molecules move in this way, they are said to move down their concentration gradient.) Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration. A couple of common examples will help to illustrate this concept. Imagine being inside a closed bathroom. If a bottle of perfume were sprayed, the scent molecules would naturally diffuse from the spot where they left the bottle to all corners of the bathroom, and this diffusion would go on until no more concentration gradient remains. Another example is a spoonful of sugar placed in a cup of tea. Eventually the sugar will diffuse throughout the tea until no concentration gradient remains. In both cases, if the room is warmer or the tea hotter, diffusion occurs even faster as the molecules are bumping into each other and spreading out faster than at cooler temperatures. Having an internal body temperature around 98.6° F thus also aids in diffusion of particles within the body.
INTERACTIVE LINK
Visit this link to see diffusion and how it is propelled by the kinetic energy of molecules in solution. How does temperature affect diffusion rate, and why?
Whenever a substance exists in greater concentration on one side of a semipermeable membrane, such as the cell membranes, any substance that can move down its concentration gradient across the membrane will do so. Consider substances that can easily diffuse through the lipid bilayer of the cell membrane, such as the gases oxygen (O2) and CO2. O2generally diffuses into cells because it is more concentrated outside of them, and CO2 typically diffuses out of cells because it is more concentrated inside of them. Neither of these examples requires any energy on the part of the cell, and therefore they use passive transport to move across the membrane.
Before moving on, you need to review the gases that can diffuse across a cell membrane. Because cells rapidly use up oxygen during metabolism, there is typically a lower concentration of O2 inside the cell than outside. As a result, oxygen will diffuse from the interstitial fluid directly through the lipid bilayer of the membrane and into the cytoplasm within the cell. On the other hand, because cells produce CO2 as a byproduct of metabolism, CO2 concentrations rise within the cytoplasm; therefore, CO2will move from the cell through the lipid bilayer and into the interstitial fluid, where its concentration is lower. This mechanism of molecules moving across a cell membrane from the side where they are more concentrated to the side where they are less concentrated is a form of passive transport called simple diffusion (Figure 3.5).
Figure 3.5 Simple Diffusion across the Cell (Plasma) MembraneThe structure of the lipid bilayer allows small, uncharged substances such as oxygen and carbon dioxide, and hydrophobic molecules such as lipids, to pass through the cell membrane, down their concentration gradient, by simple diffusion.
Large polar or ionic molecules, which are hydrophilic, cannot easily cross the phospholipid bilayer. Very small polar molecules, such as water, can cross via simple diffusion due to their small size. Charged atoms or molecules of any size cannot cross the cell membrane via simple diffusion as the charges are repelled by the hydrophobic tails in the interior of the phospholipid bilayer. Solutes dissolved in water on either side of the cell membrane will tend to diffuse down their concentration gradients, but because most substances cannot pass freely through the lipid bilayer of the cell membrane, their movement is restricted to protein channels and specialized transport mechanisms in the membrane. Facilitated diffusion is the diffusion process used for those substances that cannot cross the lipid bilayer due to their size, charge, and/or polarity (Figure 3.6). A common example of facilitated diffusion is the movement of glucose into the cell, where it is used to make ATP. Although glucose can be more concentrated outside of a cell, it cannot cross the lipid bilayer via simple diffusion because it is both large and polar. To resolve this, a specialized carrier protein called the glucose transporter will transfer glucose molecules into the cell to facilitate its inward diffusion.
Figure 3.6 Facilitated Diffusion (a) Facilitated diffusion of substances crossing the cell (plasma) membrane takes place with the help of proteins such as channel proteins and carrier proteins. Channel proteins are less selective than carrier proteins, and usually mildly discriminate between their cargo based on size and charge. (b) Carrier proteins are more selective, often only allowing one particular type of molecule to cross.
As an example, even though sodium ions (Na+) are highly concentrated outside of cells, these electrolytes are charged and cannot pass through the nonpolar lipid bilayer of the membrane. Their diffusion is facilitated by membrane proteins that form sodium channels (or “pores”), so that Na+ ions can move down their concentration gradient from outside the cells to inside the cells. There are many other solutes that must undergo facilitated diffusion to move into a cell, such as amino acids, or to move out of a cell, such as wastes. Because facilitated diffusion is a passive process, it does not require energy expenditure by the cell.
Water also can move freely across the cell membrane of all cells, either through protein channels or by slipping between the lipid tails of the membrane itself. Osmosis is the diffusion of water through a semipermeable membrane (Figure 3.7).
Figure 3.7 Osmosis Osmosis is the diffusion of water through a semipermeable membrane down its concentration gradient. If a membrane is permeable to water, though not to a solute, water will equalize its own concentration by diffusing to the side of lower water concentration (and thus the side of higher solute concentration). In the beaker on the left, the solution on the right side of the membrane is hypertonic.
The movement of water molecules is not itself regulated by cells, so it is important that cells are exposed to an environment in which the concentration of solutes outside of the cells (in the extracellular fluid) is equal to the concentration of solutes inside the cells (in the cytoplasm). Two solutions that have the same concentration of solutes are said to be isotonic (equal tension). When cells and their extracellular environments are isotonic, the concentration of water molecules is the same outside and inside the cells, and the cells maintain their normal shape (and function).
Osmosis occurs when there is an imbalance of solutes outside of a cell versus inside the cell. A solution that has a higher concentration of solutes than another solution is said to be hypertonic, and water molecules tend to diffuse into a hypertonic solution (Figure 3.8). Cells in a hypertonic solution will shrivel as water leaves the cell via osmosis. In contrast, a solution that has a lower concentration of solutes than another solution is said to be hypotonic, and water molecules tend to diffuse out of a hypotonic solution. Cells in a hypotonic solution will take on too much water and swell, with the risk of eventually bursting. A critical aspect of homeostasis in living things is to create an internal environment in which all of the body’s cells are in an isotonic solution. Various organ systems, particularly the kidneys, work to maintain this homeostasis.
Figure 3.8 Concentration of Solutions A hypertonic solution has a solute concentration higher than another solution. An isotonic solution has a solute concentration equal to another solution. A hypotonic solution has a solute concentration lower than another solution.
Another mechanism besides diffusion to passively transport materials between compartments is filtration. Unlike diffusion of a substance from where it is more concentrated to less concentrated, filtration uses a hydrostatic pressure gradient that pushes the fluid—and the solutes within it—from a higher pressure area to a lower pressure area. Filtration is an extremely important process in the body. For example, the circulatory system uses filtration to move plasma and substances across the endothelial lining of capillaries and into surrounding tissues, supplying cells with the nutrients. Filtration pressure in the kidneys provides the mechanism to remove wastes from the bloodstream.
Active Transport
For all of the transport methods described above, the cell expends no energy. Membrane proteins that aid in the passive transport of substances do so without the use of ATP. During active transport, ATP is required to move a substance across a membrane, often with the help of protein carriers, and usually against its concentration gradient.
One of the most common types of active transport involves proteins that serve as pumps. The word “pump” probably conjures up thoughts of using energy to pump up the tire of a bicycle or a basketball. Similarly, energy from ATP is required for these membrane proteins to transport substances—molecules or ions—across the membrane, usually against their concentration gradients (from an area of low concentration to an area of high concentration).
The sodium-potassium pump, which is also called Na+/K+ ATPase, transports sodium out of a cell while moving potassium into the cell. The Na+/K+ pump is an important ion pump found in the membranes of many types of cells. These pumps are particularly abundant in nerve cells, which are constantly pumping out sodium ions and pulling in potassium ions to maintain an electrical gradient across their cell membranes. An electrical gradient is a difference in electrical charge across a space. In the case of nerve cells, for example, the electrical gradient exists between the inside and outside of the cell, with the inside being negatively-charged (at around -70 mV) relative to the outside. The negative electrical gradient is maintained because each Na+/K+ pump moves three Na+ ions out of the cell and two K+ ions into the cell for each ATP molecule that is used (Figure 3.9). This process is so important for nerve cells that it accounts for the majority of their ATP usage.
Figure 3.9 Sodium-Potassium Pump The sodium-potassium pump is found in many cell (plasma) membranes. Powered by ATP, the pump moves sodium and potassium ions in opposite directions, each against its concentration gradient. In a single cycle of the pump, three sodium ions are extruded from and two potassium ions are imported into the cell.
Active transport pumps can also work together with other active or passive transport systems to move substances across the membrane. For example, the sodium-potassium pump maintains a high concentration of sodium ions outside of the cell. Therefore, if the cell needs sodium ions, all it has to do is open a passive sodium channel, as the concentration gradient of the sodium ions will drive them to diffuse into the cell. In this way, the action of an active transport pump (the sodium-potassium pump) powers the passive transport of sodium ions by creating a concentration gradient. When active transport powers the transport of another substance in this way, it is called secondary active transport.
Symporters are secondary active transporters that move two substances in the same direction. For example, the sodium-glucose symporter uses sodium ions to “pull” glucose molecules into the cell. Because cells store glucose for energy, glucose is typically at a higher concentration inside of the cell than outside. However, due to the action of the sodium-potassium pump, sodium ions will easily diffuse into the cell when the symporter is opened. The flood of sodium ions through the symporter provides the energy that allows glucose to move through the symporter and into the cell, against its concentration gradient.
Conversely, antiporters are secondary active transport systems that transport substances in opposite directions. For example, the sodium-hydrogen ion antiporter uses the energy from the inward flood of sodium ions to move hydrogen ions (H+) out of the cell. The sodium-hydrogen antiporter is used to maintain the pH of the cell's interior.
Other forms of active transport do not involve membrane carriers. Endocytosis (bringing “into the cell”) is the process of a cell ingesting material by enveloping it in a portion of its cell membrane, and then pinching off that portion of membrane (Figure 3.10). Once pinched off, the portion of membrane and its contents becomes an independent, intracellular vesicle. A vesicle is a membranous sac—a spherical and hollow organelle bounded by a lipid bilayer membrane. Endocytosis often brings materials into the cell that must to be broken down or digested. Phagocytosis (“cell eating”) is the endocytosis of large particles. Many immune cells engage in phagocytosis of invading pathogens. Like little Pac-men, their job is to patrol body tissues for unwanted matter, such as invading bacterial cells, phagocytize them, and digest them. In contrast to phagocytosis, pinocytosis (“cell drinking”) brings fluid containing dissolved substances into a cell through membrane vesicles.
Figure 3.10 Three Forms of Endocytosis Endocytosis is a form of active transport in which a cell envelopes extracellular materials using its cell membrane. (a) In phagocytosis, which is relatively nonselective, the cell takes in a large particle. (b) In pinocytosis, the cell takes in small particles in fluid. (c) In contrast, receptor-mediated endocytosis is quite selective. When external receptors bind a specific ligand, the cell responds by endocytosing the ligand.
Phagocytosis and pinocytosis take in large portions of extracellular material, and they are typically not highly selective in the substances they bring in. Cells regulate the endocytosis of specific substances via receptor-mediated endocytosis. Receptor-mediated endocytosis is endocytosis by a portion of the cell membrane that contains many receptors that are specific for a certain substance. Once the surface receptors have bound sufficient amounts of the specific substance (the receptor’s ligand), the cell will endocytose the part of the cell membrane containing the receptor-ligand complexes. Iron, a required component of hemoglobin, is endocytosed by red blood cells in this way. Iron is bound to a protein called transferrin in the blood. Specific transferrin receptors on red blood cell surfaces bind the iron-transferrin molecules, and the cell endocytoses the receptor-ligand complexes.
In contrast with endocytosis, exocytosis (taking “out of the cell”) is the process of a cell exporting material using vesicular transport (Figure 3.11). Many cells manufacture substances that must be secreted, like a factory manufacturing a product for export. These substances are typically packaged into membrane-bound vesicles within the cell. When the vesicle membrane fuses with the cell membrane, the vesicle releases it contents into the interstitial fluid. The vesicle membrane then becomes part of the cell membrane. Cells of the stomach and pancreas produce and secrete digestive enzymes through exocytosis (Figure 3.12). Endocrine cells produce and secrete hormones that are sent throughout the body, and certain immune cells produce and secrete large amounts of histamine, a chemical important for immune responses.
Figure 3.11 Exocytosis Exocytosis is much like endocytosis in reverse. Material destined for export is packaged into a vesicle inside the cell. The membrane of the vesicle fuses with the cell membrane, and the contents are released into the extracellular space.
Figure 3.12 Pancreatic Cells' Enzyme Products The pancreatic acinar cells produce and secrete many enzymes that digest food. The tiny black granules in this electron micrograph are secretory vesicles filled with enzymes that will be exported from the cells via exocytosis. LM × 2900. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
INTERACTIVE LINK
View the University of Michigan WebScope to explore the tissue sample in greater detail.
DISEASES OF THE...
Cell: Cystic Fibrosis
Cystic fibrosis (CF) affects approximately 30,000 people in the United States, with about 1,000 new cases reported each year. The genetic disease is most well known for its damage to the lungs, causing breathing difficulties and chronic lung infections, but it also affects the liver, pancreas, and intestines. Only about 50 years ago, the prognosis for children born with CF was very grim—a life expectancy rarely over 10 years. Today, with advances in medical treatment, many CF patients live into their 30s.
The symptoms of CF result from a malfunctioning membrane ion channel called the cystic fibrosis transmembrane conductance regulator, or CFTR. In healthy people, the CFTR protein is an integral membrane protein that transports Cl– ions out of the cell. In a person who has CF, the gene for the CFTR is mutated, thus, the cell manufactures a defective channel protein that typically is not incorporated into the membrane, but is instead degraded by the cell.
The CFTR requires ATP in order to function, making its Cl– transport a form of active transport. This characteristic puzzled researchers for a long time because the Cl– ions are actually flowing down their concentration gradient when transported out of cells. Active transport generally pumps ions against their concentration gradient, but the CFTR presents an exception to this rule.
In normal lung tissue, the movement of Cl– out of the cell maintains a Cl–-rich, negatively charged environment immediately outside of the cell. This is particularly important in the epithelial lining of the respiratory system. Respiratory epithelial cells secrete mucus, which serves to trap dust, bacteria, and other debris. A cilium (plural = cilia) is one of the hair-like appendages found on certain cells. Cilia on the epithelial cells move the mucus and its trapped particles up the airways away from the lungs and toward the outside. In order to be effectively moved upward, the mucus cannot be too viscous; rather it must have a thin, watery consistency. The transport of Cl– and the maintenance of an electronegative environment outside of the cell attract positive ions such as Na+ to the extracellular space. The accumulation of both Cl– and Na+ ions in the extracellular space creates solute-rich mucus, which has a low concentration of water molecules. As a result, through osmosis, water moves from cells and extracellular matrix into the mucus, “thinning” it out. This is how, in a normal respiratory system, the mucus is kept sufficiently watered-down to be propelled out of the respiratory system.
If the CFTR channel is absent, Cl– ions are not transported out of the cell in adequate numbers, thus preventing them from drawing positive ions. The absence of ions in the secreted mucus results in the lack of a normal water concentration gradient. Thus, there is no osmotic pressure pulling water into the mucus. The resulting mucus is thick and sticky, and the ciliated epithelia cannot effectively remove it from the respiratory system. Passageways in the lungs become blocked with mucus, along with the debris it carries. Bacterial infections occur more easily because bacterial cells are not effectively carried away from the lungs.
The Cytoplasm and Cellular Organelles
- Describe the structure and function of the cellular organelles associated with the endomembrane system, including the endoplasmic reticulum, Golgi apparatus, and lysosomes
- Describe the structure and function of mitochondria and peroxisomes
- Explain the three components of the cytoskeleton, including their composition and functions
Now that you have learned that the cell membrane surrounds all cells, you can dive inside of a prototypical human cell to learn about its internal components and their functions. All living cells in multicellular organisms contain an internal cytoplasmic compartment, and a nucleus within the cytoplasm. Cytosol, the jelly-like substance within the cell, provides the fluid medium necessary for biochemical reactions. Eukaryotic cells, including all animal cells, also contain various cellular organelles. An organelle (“little organ”) is one of several different types of membrane-enclosed bodies in the cell, each performing a unique function. Just as the various bodily organs work together in harmony to perform all of a human’s functions, the many different cellular organelles work together to keep the cell healthy and performing all of its important functions. The organelles and cytosol, taken together, compose the cell’s cytoplasm. The nucleus is a cell’s central organelle, which contains the cell’s DNA (Figure 3.13).
Figure 3.13 Prototypical Human Cell While this image is not indicative of any one particular human cell, it is a prototypical example of a cell containing the primary organelles and internal structures.
Organelles of the Endomembrane System
A set of three major organelles together form a system within the cell called the endomembrane system. These organelles work together to perform various cellular jobs, including the task of producing, packaging, and exporting certain cellular products. The organelles of the endomembrane system include the endoplasmic reticulum, Golgi apparatus, and vesicles.
Endoplasmic Reticulum
The endoplasmic reticulum (ER) is a system of channels that is continuous with the nuclear membrane (or “envelope”) covering the nucleus and composed of the same lipid bilayer material. The ER can be thought of as a series of winding thoroughfares similar to the waterway canals in Venice. The ER provides passages throughout much of the cell that function in transporting, synthesizing, and storing materials. The winding structure of the ER results in a large membranous surface area that supports its many functions (Figure 3.14).
Figure 3.14 Endoplasmic Reticulum (ER) (a) The ER is a winding network of thin membranous sacs found in close association with the cell nucleus. The smooth and rough endoplasmic reticula are very different in appearance and function (source: mouse tissue). (b) Rough ER is studded with numerous ribosomes, which are sites of protein synthesis (source: mouse tissue). EM × 110,000. (c) Smooth ER synthesizes phospholipids, steroid hormones, regulates the concentration of cellular Ca++, metabolizes some carbohydrates, and breaks down certain toxins (source: mouse tissue). EM × 110,510. (Micrographs provided by the Regents of University of Michigan Medical School © 2012)
Endoplasmic reticulum can exist in two forms: rough ER and smooth ER. These two types of ER perform some very different functions and can be found in very different amounts depending on the type of cell. Rough ER (RER) is so-called because its membrane is dotted with embedded granules—organelles called ribosomes, giving the RER a bumpy appearance. A ribosomeis an organelle that serves as the site of protein synthesis. It is composed of two ribosomal RNA subunits that wrap around mRNA to start the process of translation, followed by protein synthesis. Smooth ER (SER) lacks these ribosomes.
One of the main functions of the smooth ER is in the synthesis of lipids. The smooth ER synthesizes phospholipids, the main component of biological membranes, as well as steroid hormones. For this reason, cells that produce large quantities of such hormones, such as those of the female ovaries and male testes, contain large amounts of smooth ER. In addition to lipid synthesis, the smooth ER also sequesters (i.e., stores) and regulates the concentration of cellular Ca++, a function extremely important in cells of the nervous system where Ca++ is the trigger for neurotransmitter release. The smooth ER additionally metabolizes some carbohydrates and performs a detoxification role, breaking down certain toxins.
In contrast with the smooth ER, the primary job of the rough ER is the synthesis and modification of proteins destined for the cell membrane or for export from the cell. For this protein synthesis, many ribosomes attach to the ER (giving it the studded appearance of rough ER). Typically, a protein is synthesized within the ribosome and released inside the channel of the rough ER, where sugars can be added to it (by a process called glycosylation) before it is transported within a vesicle to the next stage in the packaging and shipping process: the Golgi apparatus.
The Golgi Apparatus
The Golgi apparatus is responsible for sorting, modifying, and shipping off the products that come from the rough ER, much like a post-office. The Golgi apparatus looks like stacked flattened discs, almost like stacks of oddly shaped pancakes. Like the ER, these discs are membranous. The Golgi apparatus has two distinct sides, each with a different role. One side of the apparatus receives products in vesicles. These products are sorted through the apparatus, and then they are released from the opposite side after being repackaged into new vesicles. If the product is to be exported from the cell, the vesicle migrates to the cell surface and fuses to the cell membrane, and the cargo is secreted (Figure 3.15).
Figure 3.15 Golgi Apparatus (a) The Golgi apparatus manipulates products from the rough ER, and also produces new organelles called lysosomes. Proteins and other products of the ER are sent to the Golgi apparatus, which organizes, modifies, packages, and tags them. Some of these products are transported to other areas of the cell and some are exported from the cell through exocytosis. Enzymatic proteins are packaged as new lysosomes (or packaged and sent for fusion with existing lysosomes). (b) An electron micrograph of the Golgi apparatus.
Lysosomes
Some of the protein products packaged by the Golgi include digestive enzymes that are meant to remain inside the cell for use in breaking down certain materials. The enzyme-containing vesicles released by the Golgi may form new lysosomes, or fuse with existing, lysosomes. A lysosome is an organelle that contains enzymes that break down and digest unneeded cellular components, such as a damaged organelle. (A lysosome is similar to a wrecking crew that takes down old and unsound buildings in a neighborhood.) Autophagy (“self-eating”) is the process of a cell digesting its own structures. Lysosomes are also important for breaking down foreign material. For example, when certain immune defense cells (white blood cells) phagocytize bacteria, the bacterial cell is transported into a lysosome and digested by the enzymes inside. As one might imagine, such phagocytic defense cells contain large numbers of lysosomes.
Under certain circumstances, lysosomes perform a more grand and dire function. In the case of damaged or unhealthy cells, lysosomes can be triggered to open up and release their digestive enzymes into the cytoplasm of the cell, killing the cell. This “self-destruct” mechanism is called autolysis, and makes the process of cell death controlled (a mechanism called “apoptosis”).
INTERACTIVE LINK
Watch this video to learn about the endomembrane system, which includes the rough and smooth ER and the Golgi body as well as lysosomes and vesicles. What is the primary role of the endomembrane system?
Organelles for Energy Production and Detoxification
In addition to the jobs performed by the endomembrane system, the cell has many other important functions. Just as you must consume nutrients to provide yourself with energy, so must each of your cells take in nutrients, some of which convert to chemical energy that can be used to power biochemical reactions. Another important function of the cell is detoxification. Humans take in all sorts of toxins from the environment and also produce harmful chemicals as byproducts of cellular processes. Cells called hepatocytes in the liver detoxify many of these toxins.
Mitochondria
A mitochondrion (plural = mitochondria) is a membranous, bean-shaped organelle that is the “energy transformer” of the cell. Mitochondria consist of an outer lipid bilayer membrane as well as an additional inner lipid bilayer membrane (Figure 3.16). The inner membrane is highly folded into winding structures with a great deal of surface area, called cristae. It is along this inner membrane that a series of proteins, enzymes, and other molecules perform the biochemical reactions of cellular respiration. These reactions convert energy stored in nutrient molecules (such as glucose) into adenosine triphosphate (ATP), which provides usable cellular energy to the cell. Cells use ATP constantly, and so the mitochondria are constantly at work. Oxygen molecules are required during cellular respiration, which is why you must constantly breathe it in. One of the organ systems in the body that uses huge amounts of ATP is the muscular system because ATP is required to sustain muscle contraction. As a result, muscle cells are packed full of mitochondria. Nerve cells also need large quantities of ATP to run their sodium-potassium pumps. Therefore, an individual neuron will be loaded with over a thousand mitochondria. On the other hand, a bone cell, which is not nearly as metabolically-active, might only have a couple hundred mitochondria.
Figure 3.16 Mitochondrion The mitochondria are the energy-conversion factories of the cell. (a) A mitochondrion is composed of two separate lipid bilayer membranes. Along the inner membrane are various molecules that work together to produce ATP, the cell’s major energy currency. (b) An electron micrograph of mitochondria. EM × 236,000. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
Peroxisomes
Like lysosomes, a peroxisome is a membrane-bound cellular organelle that contains mostly enzymes (Figure 3.17). Peroxisomes perform a couple of different functions, including lipid metabolism and chemical detoxification. In contrast to the digestive enzymes found in lysosomes, the enzymes within peroxisomes serve to transfer hydrogen atoms from various molecules to oxygen, producing hydrogen peroxide (H2O2). In this way, peroxisomes neutralize poisons such as alcohol. In order to appreciate the importance of peroxisomes, it is necessary to understand the concept of reactive oxygen species.
Figure 3.17 Peroxisome Peroxisomes are membrane-bound organelles that contain an abundance of enzymes for detoxifying harmful substances and lipid metabolism.
Reactive oxygen species (ROS) such as peroxides and free radicals are the highly reactive products of many normal cellular processes, including the mitochondrial reactions that produce ATP and oxygen metabolism. Examples of ROS include the hydroxyl radical OH, H2O2, and superoxide (O−2O2−
Peroxisomes, on the other hand, oversee reactions that neutralize free radicals. Peroxisomes produce large amounts of the toxic H2O2 in the process, but peroxisomes contain enzymes that convert H2O2 into water and oxygen. These byproducts are safely released into the cytoplasm. Like miniature sewage treatment plants, peroxisomes neutralize harmful toxins so that they do not wreak havoc in the cells. The liver is the organ primarily responsible for detoxifying the blood before it travels throughout the body, and liver cells contain an exceptionally high number of peroxisomes.
Defense mechanisms such as detoxification within the peroxisome and certain cellular antioxidants serve to neutralize many of these molecules. Some vitamins and other substances, found primarily in fruits and vegetables, have antioxidant properties. Antioxidants work by being oxidized themselves, halting the destructive reaction cascades initiated by the free radicals. Sometimes though, ROS accumulate beyond the capacity of such defenses.
Oxidative stress is the term used to describe damage to cellular components caused by ROS. Due to their characteristic unpaired electrons, ROS can set off chain reactions where they remove electrons from other molecules, which then become oxidized and reactive, and do the same to other molecules, causing a chain reaction. ROS can cause permanent damage to cellular lipids, proteins, carbohydrates, and nucleic acids. Damaged DNA can lead to genetic mutations and even cancer. A mutation is a change in the nucleotide sequence in a gene within a cell’s DNA, potentially altering the protein coded by that gene. Other diseases believed to be triggered or exacerbated by ROS include Alzheimer’s disease, cardiovascular diseases, diabetes, Parkinson’s disease, arthritis, Huntington’s disease, and schizophrenia, among many others. It is noteworthy that these diseases are largely age-related. Many scientists believe that oxidative stress is a major contributor to the aging process.
AGING AND THE...
Cell: The Free Radical Theory
The free radical theory on aging was originally proposed in the 1950s, and still remains under debate. Generally speaking, the free radical theory of aging suggests that accumulated cellular damage from oxidative stress contributes to the physiological and anatomical effects of aging. There are two significantly different versions of this theory: one states that the aging process itself is a result of oxidative damage, and the other states that oxidative damage causes age-related disease and disorders. The latter version of the theory is more widely accepted than the former. However, many lines of evidence suggest that oxidative damage does contribute to the aging process. Research has shown that reducing oxidative damage can result in a longer lifespan in certain organisms such as yeast, worms, and fruit flies. Conversely, increasing oxidative damage can shorten the lifespan of mice and worms. Interestingly, a manipulation called calorie-restriction (moderately restricting the caloric intake) has been shown to increase life span in some laboratory animals. It is believed that this increase is at least in part due to a reduction of oxidative stress. However, a long-term study of primates with calorie-restriction showed no increase in their lifespan. A great deal of additional research will be required to better understand the link between reactive oxygen species and aging.
The Cytoskeleton
Much like the bony skeleton structurally supports the human body, the cytoskeleton helps the cells to maintain their structural integrity. The cytoskeleton is a group of fibrous proteins that provide structural support for cells, but this is only one of the functions of the cytoskeleton. Cytoskeletal components are also critical for cell motility, cell reproduction, and transportation of substances within the cell.
The cytoskeleton forms a complex thread-like network throughout the cell consisting of three different kinds of protein-based filaments: microfilaments, intermediate filaments, and microtubules (Figure 3.18). The thickest of the three is the microtubule, a structural filament composed of subunits of a protein called tubulin. Microtubules maintain cell shape and structure, help resist compression of the cell, and play a role in positioning the organelles within the cell. Microtubules also make up two types of cellular appendages important for motion: cilia and flagella. Cilia are found on many cells of the body, including the epithelial cells that line the airways of the respiratory system. Cilia move rhythmically; they beat constantly, moving waste materials such as dust, mucus, and bacteria upward through the airways, away from the lungs and toward the mouth. Beating cilia on cells in the female fallopian tubes move egg cells from the ovary towards the uterus. A flagellum (plural = flagella) is an appendage larger than a cilium and specialized for cell locomotion. The only flagellated cell in humans is the sperm cell that must propel itself towards female egg cells.
Figure 3.18 The Three Components of the Cytoskeleton The cytoskeleton consists of (a) microtubules, (b) microfilaments, and (c) intermediate filaments. The cytoskeleton plays an important role in maintaining cell shape and structure, promoting cellular movement, and aiding cell division.
A very important function of microtubules is to set the paths (somewhat like railroad tracks) along which the genetic material can be pulled (a process requiring ATP) during cell division, so that each new daughter cell receives the appropriate set of chromosomes. Two short, identical microtubule structures called centrioles are found near the nucleus of cells. A centriole can serve as the cellular origin point for microtubules extending outward as cilia or flagella or can assist with the separation of DNA during cell division. Microtubules grow out from the centrioles by adding more tubulin subunits, like adding additional links to a chain.
In contrast with microtubules, the microfilament is a thinner type of cytoskeletal filament (see Figure 3.18b). Actin, a protein that forms chains, is the primary component of these microfilaments. Actin fibers, twisted chains of actin filaments, constitute a large component of muscle tissue and, along with the protein myosin, are responsible for muscle contraction. Like microtubules, actin filaments are long chains of single subunits (called actin subunits). In muscle cells, these long actin strands, called thin filaments, are “pulled” by thick filaments of the myosin protein to contract the cell.
Actin also has an important role during cell division. When a cell is about to split in half during cell division, actin filaments work with myosin to create a cleavage furrow that eventually splits the cell down the middle, forming two new cells from the original cell.
The final cytoskeletal filament is the intermediate filament. As its name would suggest, an intermediate filament is a filament intermediate in thickness between the microtubules and microfilaments (see Figure 3.18c). Intermediate filaments are made up of long fibrous subunits of a protein called keratin that are wound together like the threads that compose a rope. Intermediate filaments, in concert with the microtubules, are important for maintaining cell shape and structure. Unlike the microtubules, which resist compression, intermediate filaments resist tension—the forces that pull apart cells. There are many cases in which cells are prone to tension, such as when epithelial cells of the skin are compressed, tugging them in different directions. Intermediate filaments help anchor organelles together within a cell and also link cells to other cells by forming special cell-to-cell junctions.
The Nucleus and DNA Replication
- Describe the structure and features of the nuclear membrane
- List the contents of the nucleus
- Explain the organization of the DNA molecule within the nucleus
- Describe the process of DNA replication
The nucleus is the largest and most prominent of a cell’s organelles (Figure 3.19). The nucleus is generally considered the control center of the cell because it stores all of the genetic instructions for manufacturing proteins. Interestingly, some cells in the body, such as muscle cells, contain more than one nucleus (Figure 3.20), which is known as multinucleated. Other cells, such as mammalian red blood cells (RBCs), do not contain nuclei at all. RBCs eject their nuclei as they mature, making space for the large numbers of hemoglobin molecules that carry oxygen throughout the body (Figure 3.21). Without nuclei, the life span of RBCs is short, and so the body must produce new ones constantly.
Figure 3.19 The Nucleus The nucleus is the control center of the cell. The nucleus of living cells contains the genetic material that determines the entire structure and function of that cell.
Figure 3.20 Multinucleate Muscle Cell Unlike cardiac muscle cells and smooth muscle cells, which have a single nucleus, a skeletal muscle cell contains many nuclei, and is referred to as “multinucleated.” These muscle cells are long and fibrous (often referred to as muscle fibers). During development, many smaller cells fuse to form a mature muscle fiber. The nuclei of the fused cells are conserved in the mature cell, thus imparting a multinucleate characteristic to mature muscle cells. LM × 104.3. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
INTERACTIVE LINK
View the University of Michigan WebScope to explore the tissue sample in greater detail.
Figure 3.21 Red Blood Cell Extruding Its Nucleus Mature red blood cells lack a nucleus. As they mature, erythroblasts extrude their nucleus, making room for more hemoglobin. The two panels here show an erythroblast before and after ejecting its nucleus, respectively. (credit: modification of micrograph provided by the Regents of University of Michigan Medical School © 2012)
INTERACTIVE LINK
View the University of Michigan WebScope to explore the tissue sample in greater detail.
Inside the nucleus lies the blueprint that dictates everything a cell will do and all of the products it will make. This information is stored within DNA. The nucleus sends “commands” to the cell via molecular messengers that translate the information from DNA. Each cell in your body (with the exception of germ cells) contains the complete set of your DNA. When a cell divides, the DNA must be duplicated so that the each new cell receives a full complement of DNA. The following section will explore the structure of the nucleus and its contents, as well as the process of DNA replication.
Organization of the Nucleus and Its DNA
Like most other cellular organelles, the nucleus is surrounded by a membrane called the nuclear envelope. This membranous covering consists of two adjacent lipid bilayers with a thin fluid space in between them. Spanning these two bilayers are nuclear pores. A nuclear pore is a tiny passageway for the passage of proteins, RNA, and solutes between the nucleus and the cytoplasm. Proteins called pore complexes lining the nuclear pores regulate the passage of materials into and out of the nucleus.
Inside the nuclear envelope is a gel-like nucleoplasm with solutes that include the building blocks of nucleic acids. There also can be a dark-staining mass often visible under a simple light microscope, called a nucleolus (plural = nucleoli). The nucleolus is a region of the nucleus that is responsible for manufacturing the RNA necessary for construction of ribosomes. Once synthesized, newly made ribosomal subunits exit the cell’s nucleus through the nuclear pores.
The genetic instructions that are used to build and maintain an organism are arranged in an orderly manner in strands of DNA. Within the nucleus are threads of chromatin composed of DNA and associated proteins (Figure 3.22). Along the chromatin threads, the DNA is wrapped around a set of histone proteins. A nucleosome is a single, wrapped DNA-histone complex. Multiple nucleosomes along the entire molecule of DNA appear like a beaded necklace, in which the string is the DNA and the beads are the associated histones. When a cell is in the process of division, the chromatin condenses into chromosomes, so that the DNA can be safely transported to the “daughter cells.” The chromosome is composed of DNA and proteins; it is the condensed form of chromatin. It is estimated that humans have almost 22,000 genes distributed on 46 chromosomes.
Figure 3.22 DNA Macrostructure Strands of DNA are wrapped around supporting histones. These proteins are increasingly bundled and condensed into chromatin, which is packed tightly into chromosomes when the cell is ready to divide.
DNA Replication
In order for an organism to grow, develop, and maintain its health, cells must reproduce themselves by dividing to produce two new daughter cells, each with the full complement of DNA as found in the original cell. Billions of new cells are produced in an adult human every day. Only very few cell types in the body do not divide, including nerve cells, skeletal muscle fibers, and cardiac muscle cells. The division time of different cell types varies. Epithelial cells of the skin and gastrointestinal lining, for instance, divide very frequently to replace those that are constantly being rubbed off of the surface by friction.
A DNA molecule is made of two strands that “complement” each other in the sense that the molecules that compose the strands fit together and bind to each other, creating a double-stranded molecule that looks much like a long, twisted ladder. Each side rail of the DNA ladder is composed of alternating sugar and phosphate groups (Figure 3.23). The two sides of the ladder are not identical, but are complementary. These two backbones are bonded to each other across pairs of protruding bases, each bonded pair forming one “rung,” or cross member. The four DNA bases are adenine (A), thymine (T), cytosine (C), and guanine (G). Because of their shape and charge, the two bases that compose a pair always bond together. Adenine always binds with thymine, and cytosine always binds with guanine. The particular sequence of bases along the DNA molecule determines the genetic code. Therefore, if the two complementary strands of DNA were pulled apart, you could infer the order of the bases in one strand from the bases in the other, complementary strand. For example, if one strand has a region with the sequence AGTGCCT, then the sequence of the complementary strand would be TCACGGA.
Figure 3.23 Molecular Structure of DNA The DNA double helix is composed of two complementary strands. The strands are bonded together via their nitrogenous base pairs using hydrogen bonds.
DNA replication is the copying of DNA that occurs before cell division can take place. After a great deal of debate and experimentation, the general method of DNA replication was deduced in 1958 by two scientists in California, Matthew Meselson and Franklin Stahl. This method is illustrated in Figure 3.24 and described below.
Figure 3.24 DNA Replication DNA replication faithfully duplicates the entire genome of the cell. During DNA replication, a number of different enzymes work together to pull apart the two strands so each strand can be used as a template to synthesize new complementary strands. The two new daughter DNA molecules each contain one pre-existing strand and one newly synthesized strand. Thus, DNA replication is said to be “semiconservative.”
Stage 1: Initiation. The two complementary strands are separated, much like unzipping a zipper. Special enzymes, including helicase, untwist and separate the two strands of DNA.
Stage 2: Elongation. Each strand becomes a template along which a new complementary strand is built. DNA polymerasebrings in the correct bases to complement the template strand, synthesizing a new strand base by base. A DNA polymerase is an enzyme that adds free nucleotides to the end of a chain of DNA, making a new double strand. This growing strand continues to be built until it has fully complemented the template strand.
Stage 3: Termination. Once the two original strands are bound to their own, finished, complementary strands, DNA replication is stopped and the two new identical DNA molecules are complete.
Each new DNA molecule contains one strand from the original molecule and one newly synthesized strand. The term for this mode of replication is “semiconservative,” because half of the original DNA molecule is conserved in each new DNA molecule. This process continues until the cell’s entire genome, the entire complement of an organism’s DNA, is replicated. As you might imagine, it is very important that DNA replication take place precisely so that new cells in the body contain the exact same genetic material as their parent cells. Mistakes made during DNA replication, such as the accidental addition of an inappropriate nucleotide, have the potential to render a gene dysfunctional or useless. Fortunately, there are mechanisms in place to minimize such mistakes. A DNA proofreading process enlists the help of special enzymes that scan the newly synthesized molecule for mistakes and corrects them. Once the process of DNA replication is complete, the cell is ready to divide. You will explore the process of cell division later in the chapter.
INTERACTIVE LINK
Watch this video to learn about DNA replication. DNA replication proceeds simultaneously at several sites on the same molecule. What separates the base pair at the start of DNA replication?
Protein Synthesis
- Explain how the genetic code stored within DNA determines the protein that will form
- Describe the process of transcription
- Describe the process of translation
- Discuss the function of ribosomes
It was mentioned earlier that DNA provides a “blueprint” for the cell structure and physiology. This refers to the fact that DNA contains the information necessary for the cell to build one very important type of molecule: the protein. Most structural components of the cell are made up, at least in part, by proteins and virtually all the functions that a cell carries out are completed with the help of proteins. One of the most important classes of proteins is enzymes, which help speed up necessary biochemical reactions that take place inside the cell. Some of these critical biochemical reactions include building larger molecules from smaller components (such as occurs during DNA replication or synthesis of microtubules) and breaking down larger molecules into smaller components (such as when harvesting chemical energy from nutrient molecules). Whatever the cellular process may be, it is almost sure to involve proteins. Just as the cell’s genome describes its full complement of DNA, a cell’s proteome is its full complement of proteins. Protein synthesis begins with genes. A gene is a functional segment of DNA that provides the genetic information necessary to build a protein. Each particular gene provides the code necessary to construct a particular protein. Gene expression, which transforms the information coded in a gene to a final gene product, ultimately dictates the structure and function of a cell by determining which proteins are made.
The interpretation of genes works in the following way. Recall that proteins are polymers, or chains, of many amino acid building blocks. The sequence of bases in a gene (that is, its sequence of A, T, C, G nucleotides) translates to an amino acid sequence. A triplet is a section of three DNA bases in a row that codes for a specific amino acid. Similar to the way in which the three-letter code d-o-g signals the image of a dog, the three-letter DNA base code signals the use of a particular amino acid. For example, the DNA triplet CAC (cytosine, adenine, and cytosine) specifies the amino acid valine. Therefore, a gene, which is composed of multiple triplets in a unique sequence, provides the code to build an entire protein, with multiple amino acids in the proper sequence (Figure 3.25). The mechanism by which cells turn the DNA code into a protein product is a two-step process, with an RNA molecule as the intermediate.
Figure 3.25 The Genetic Code DNA holds all of the genetic information necessary to build a cell’s proteins. The nucleotide sequence of a gene is ultimately translated into an amino acid sequence of the gene’s corresponding protein.
From DNA to RNA: Transcription
DNA is housed within the nucleus, and protein synthesis takes place in the cytoplasm, thus there must be some sort of intermediate messenger that leaves the nucleus and manages protein synthesis. This intermediate messenger is messenger RNA (mRNA), a single-stranded nucleic acid that carries a copy of the genetic code for a single gene out of the nucleus and into the cytoplasm where it is used to produce proteins.
There are several different types of RNA, each having different functions in the cell. The structure of RNA is similar to DNA with a few small exceptions. For one thing, unlike DNA, most types of RNA, including mRNA, are single-stranded and contain no complementary strand. Second, the ribose sugar in RNA contains an additional oxygen atom compared with DNA. Finally, instead of the base thymine, RNA contains the base uracil. This means that adenine will always pair up with uracil during the protein synthesis process.
Gene expression begins with the process called transcription, which is the synthesis of a strand of mRNA that is complementary to the gene of interest. This process is called transcription because the mRNA is like a transcript, or copy, of the gene’s DNA code. Transcription begins in a fashion somewhat like DNA replication, in that a region of DNA unwinds and the two strands separate, however, only that small portion of the DNA will be split apart. The triplets within the gene on this section of the DNA molecule are used as the template to transcribe the complementary strand of RNA (Figure 3.26). A codonis a three-base sequence of mRNA, so-called because they directly encode amino acids. Like DNA replication, there are three stages to transcription: initiation, elongation, and termination.
Figure 3.26 Transcription: from DNA to mRNA In the first of the two stages of making protein from DNA, a gene on the DNA molecule is transcribed into a complementary mRNA molecule.
Stage 1: Initiation. A region at the beginning of the gene called a promoter—a particular sequence of nucleotides—triggers the start of transcription.
Stage 2: Elongation. Transcription starts when RNA polymerase unwinds the DNA segment. One strand, referred to as the coding strand, becomes the template with the genes to be coded. The polymerase then aligns the correct nucleic acid (A, C, G, or U) with its complementary base on the coding strand of DNA. RNA polymerase is an enzyme that adds new nucleotides to a growing strand of RNA. This process builds a strand of mRNA.
Stage 3: Termination. When the polymerase has reached the end of the gene, one of three specific triplets (UAA, UAG, or UGA) codes a “stop” signal, which triggers the enzymes to terminate transcription and release the mRNA transcript.
Before the mRNA molecule leaves the nucleus and proceeds to protein synthesis, it is modified in a number of ways. For this reason, it is often called a pre-mRNA at this stage. For example, your DNA, and thus complementary mRNA, contains long regions called non-coding regions that do not code for amino acids. Their function is still a mystery, but the process called splicing removes these non-coding regions from the pre-mRNA transcript (Figure 3.27). A spliceosome—a structure composed of various proteins and other molecules—attaches to the mRNA and “splices” or cuts out the non-coding regions. The removed segment of the transcript is called an intron. The remaining exons are pasted together. An exon is a segment of RNA that remains after splicing. Interestingly, some introns that are removed from mRNA are not always non-coding. When different coding regions of mRNA are spliced out, different variations of the protein will eventually result, with differences in structure and function. This process results in a much larger variety of possible proteins and protein functions. When the mRNA transcript is ready, it travels out of the nucleus and into the cytoplasm.
Figure 3.27 Splicing DNA In the nucleus, a structure called a spliceosome cuts out introns (noncoding regions) within a pre-mRNA transcript and reconnects the exons.
From RNA to Protein: Translation
Like translating a book from one language into another, the codons on a strand of mRNA must be translated into the amino acid alphabet of proteins. Translation is the process of synthesizing a chain of amino acids called a polypeptide. Translation requires two major aids: first, a “translator,” the molecule that will conduct the translation, and second, a substrate on which the mRNA strand is translated into a new protein, like the translator’s “desk.” Both of these requirements are fulfilled by other types of RNA. The substrate on which translation takes place is the ribosome.
Remember that many of a cell’s ribosomes are found associated with the rough ER, and carry out the synthesis of proteins destined for the Golgi apparatus. Ribosomal RNA (rRNA) is a type of RNA that, together with proteins, composes the structure of the ribosome. Ribosomes exist in the cytoplasm as two distinct components, a small and a large subunit. When an mRNA molecule is ready to be translated, the two subunits come together and attach to the mRNA. The ribosome provides a substrate for translation, bringing together and aligning the mRNA molecule with the molecular “translators” that must decipher its code.
The other major requirement for protein synthesis is the translator molecules that physically “read” the mRNA codons. Transfer RNA (tRNA) is a type of RNA that ferries the appropriate corresponding amino acids to the ribosome, and attaches each new amino acid to the last, building the polypeptide chain one-by-one. Thus tRNA transfers specific amino acids from the cytoplasm to a growing polypeptide. The tRNA molecules must be able to recognize the codons on mRNA and match them with the correct amino acid. The tRNA is modified for this function. On one end of its structure is a binding site for a specific amino acid. On the other end is a base sequence that matches the codon specifying its particular amino acid. This sequence of three bases on the tRNA molecule is called an anticodon. For example, a tRNA responsible for shuttling the amino acid glycine contains a binding site for glycine on one end. On the other end it contains an anticodon that complements the glycine codon (GGA is a codon for glycine, and so the tRNAs anticodon would read CCU). Equipped with its particular cargo and matching anticodon, a tRNA molecule can read its recognized mRNA codon and bring the corresponding amino acid to the growing chain (Figure 3.28).
Figure 3.28 Translation from RNA to ProteinDuring translation, the mRNA transcript is “read” by a functional complex consisting of the ribosome and tRNA molecules. tRNAs bring the appropriate amino acids in sequence to the growing polypeptide chain by matching their anti-codons with codons on the mRNA strand.
Much like the processes of DNA replication and transcription, translation consists of three main stages: initiation, elongation, and termination. Initiation takes place with the binding of a ribosome to an mRNA transcript. The elongation stage involves the recognition of a tRNA anticodon with the next mRNA codon in the sequence. Once the anticodon and codon sequences are bound (remember, they are complementary base pairs), the tRNA presents its amino acid cargo and the growing polypeptide strand is attached to this next amino acid. This attachment takes place with the assistance of various enzymes and requires energy. The tRNA molecule then releases the mRNA strand, the mRNA strand shifts one codon over in the ribosome, and the next appropriate tRNA arrives with its matching anticodon. This process continues until the final codon on the mRNA is reached which provides a “stop” message that signals termination of translation and triggers the release of the complete, newly synthesized protein. Thus, a gene within the DNA molecule is transcribed into mRNA, which is then translated into a protein product (Figure 3.29).
Figure 3.29 From DNA to Protein: Transcription through Translation Transcription within the cell nucleus produces an mRNA molecule, which is modified and then sent into the cytoplasm for translation. The transcript is decoded into a protein with the help of a ribosome and tRNA molecules.
Commonly, an mRNA transcription will be translated simultaneously by several adjacent ribosomes. This increases the efficiency of protein synthesis. A single ribosome might translate an mRNA molecule in approximately one minute; so multiple ribosomes aboard a single transcript could produce multiple times the number of the same protein in the same minute. A polyribosome is a string of ribosomes translating a single mRNA strand.
INTERACTIVE LINK
Watch this video to learn about ribosomes. The ribosome binds to the mRNA molecule to start translation of its code into a protein. What happens to the small and large ribosomal subunits at the end of translation?
Cell Growth and Division
- Describe the stages of the cell cycle
- Discuss how the cell cycle is regulated
- Describe the implications of losing control over the cell cycle
- Describe the stages of mitosis and cytokinesis, in order
So far in this chapter, you have read numerous times of the importance and prevalence of cell division. While there are a few cells in the body that do not undergo cell division (such as gametes, red blood cells, most neurons, and some muscle cells), most somatic cells divide regularly. A somatic cell is a general term for a body cell, and all human cells, except for the cells that produce eggs and sperm (which are referred to as germ cells), are somatic cells. Somatic cells contain two copies of each of their chromosomes (one copy received from each parent). A homologous pair of chromosomes is the two copies of a single chromosome found in each somatic cell. The human is a diploid organism, having 23 homologous pairs of chromosomes in each of the somatic cells. The condition of having pairs of chromosomes is known as diploidy.
Cells in the body replace themselves over the lifetime of a person. For example, the cells lining the gastrointestinal tract must be frequently replaced when constantly “worn off” by the movement of food through the gut. But what triggers a cell to divide, and how does it prepare for and complete cell division? The cell cycle is the sequence of events in the life of the cell from the moment it is created at the end of a previous cycle of cell division until it then divides itself, generating two new cells.
The Cell Cycle
One “turn” or cycle of the cell cycle consists of two general phases: interphase, followed by mitosis and cytokinesis. Interphase is the period of the cell cycle during which the cell is not dividing. The majority of cells are in interphase most of the time. Mitosis is the division of genetic material, during which the cell nucleus breaks down and two new, fully functional, nuclei are formed. Cytokinesis divides the cytoplasm into two distinctive cells.
Interphase
A cell grows and carries out all normal metabolic functions and processes in a period called G1 (Figure 3.30). G1 phase (gap 1 phase) is the first gap, or growth phase in the cell cycle. For cells that will divide again, G1 is followed by replication of the DNA, during the S phase. The S phase (synthesis phase) is period during which a cell replicates its DNA.
Figure 3.30 Cell Cycle The two major phases of the cell cycle include mitosis (cell division), and interphase, when the cell grows and performs all of its normal functions. Interphase is further subdivided into G1, S, and G2 phases.
After the synthesis phase, the cell proceeds through the G2 phase. The G2 phase is a second gap phase, during which the cell continues to grow and makes the necessary preparations for mitosis. Between G1, S, and G2 phases, cells will vary the most in their duration of the G1 phase. It is here that a cell might spend a couple of hours, or many days. The S phase typically lasts between 8-10 hours and the G2 phase approximately 5 hours. In contrast to these phases, the G0 phase is a resting phase of the cell cycle. Cells that have temporarily stopped dividing and are resting (a common condition) and cells that have permanently ceased dividing (like nerve cells) are said to be in G0.
The Structure of Chromosomes
Billions of cells in the human body divide every day. During the synthesis phase (S, for DNA synthesis) of interphase, the amount of DNA within the cell precisely doubles. Therefore, after DNA replication but before cell division, each cell actually contains two copies of each chromosome. Each copy of the chromosome is referred to as a sister chromatid and is physically bound to the other copy. The centromere is the structure that attaches one sister chromatid to another. Because a human cell has 46 chromosomes, during this phase, there are 92 chromatids (46 × 2) in the cell. Make sure not to confuse the concept of a pair of chromatids (one chromosome and its exact copy attached during mitosis) and a homologous pair of chromosomes (two paired chromosomes which were inherited separately, one from each parent) (Figure 3.31).
Figure 3.31 A Homologous Pair of Chromosomes with their Attached Sister Chromatids The red and blue colors correspond to a homologous pair of chromosomes. Each member of the pair was separately inherited from one parent. Each chromosome in the homologous pair is also bound to an identical sister chromatid, which is produced by DNA replication, and results in the familiar “X” shape.
Mitosis and Cytokinesis
The mitotic phase of the cell typically takes between 1 and 2 hours. During this phase, a cell undergoes two major processes. First, it completes mitosis, during which the contents of the nucleus are equitably pulled apart and distributed between its two halves. Cytokinesis then occurs, dividing the cytoplasm and cell body into two new cells. Mitosis is divided into four major stages that take place after interphase (Figure 3.32) and in the following order: prophase, metaphase, anaphase, and telophase. The process is then followed by cytokinesis.
Figure 3.32 Cell Division: Mitosis Followed by Cytokinesis The stages of cell division oversee the separation of identical genetic material into two new nuclei, followed by the division of the cytoplasm.
Prophase is the first phase of mitosis, during which the loosely packed chromatin coils and condenses into visible chromosomes. During prophase, each chromosome becomes visible with its identical partner attached, forming the familiar X-shape of sister chromatids. The nucleolus disappears early during this phase, and the nuclear envelope also disintegrates.
A major occurrence during prophase concerns a very important structure that contains the origin site for microtubule growth. Recall the cellular structures called centrioles that serve as origin points from which microtubules extend. These tiny structures also play a very important role during mitosis. A centrosome is a pair of centrioles together. The cell contains two centrosomes side-by-side, which begin to move apart during prophase. As the centrosomes migrate to two different sides of the cell, microtubules begin to extend from each like long fingers from two hands extending toward each other. The mitotic spindle is the structure composed of the centrosomes and their emerging microtubules.
Near the end of prophase there is an invasion of the nuclear area by microtubules from the mitotic spindle. The nuclear membrane has disintegrated, and the microtubules attach themselves to the centromeres that adjoin pairs of sister chromatids. The kinetochore is a protein structure on the centromere that is the point of attachment between the mitotic spindle and the sister chromatids. This stage is referred to as late prophase or “prometaphase” to indicate the transition between prophase and metaphase.
Metaphase is the second stage of mitosis. During this stage, the sister chromatids, with their attached microtubules, line up along a linear plane in the middle of the cell. A metaphase plate forms between the centrosomes that are now located at either end of the cell. The metaphase plate is the name for the plane through the center of the spindle on which the sister chromatids are positioned. The microtubules are now poised to pull apart the sister chromatids and bring one from each pair to each side of the cell.
Anaphase is the third stage of mitosis. Anaphase takes place over a few minutes, when the pairs of sister chromatids are separated from one another, forming individual chromosomes once again. These chromosomes are pulled to opposite ends of the cell by their kinetochores, as the microtubules shorten. Each end of the cell receives one partner from each pair of sister chromatids, ensuring that the two new daughter cells will contain identical genetic material.
Telophase is the final stage of mitosis. Telophase is characterized by the formation of two new daughter nuclei at either end of the dividing cell. These newly formed nuclei surround the genetic material, which uncoils such that the chromosomes return to loosely packed chromatin. Nucleoli also reappear within the new nuclei, and the mitotic spindle breaks apart, each new cell receiving its own complement of DNA, organelles, membranes, and centrioles. At this point, the cell is already beginning to split in half as cytokinesis begins.
The cleavage furrow is a contractile band made up of microfilaments that forms around the midline of the cell during cytokinesis. (Recall that microfilaments consist of actin.) This contractile band squeezes the two cells apart until they finally separate. Two new cells are now formed. One of these cells (the “stem cell”) enters its own cell cycle; able to grow and divide again at some future time. The other cell transforms into the functional cell of the tissue, typically replacing an “old” cell there.
Imagine a cell that completed mitosis but never underwent cytokinesis. In some cases, a cell may divide its genetic material and grow in size, but fail to undergo cytokinesis. This results in larger cells with more than one nucleus. Usually this is an unwanted aberration and can be a sign of cancerous cells.
Cell Cycle Control
A very elaborate and precise system of regulation controls direct the way cells proceed from one phase to the next in the cell cycle and begin mitosis. The control system involves molecules within the cell as well as external triggers. These internal and external control triggers provide “stop” and “advance” signals for the cell. Precise regulation of the cell cycle is critical for maintaining the health of an organism, and loss of cell cycle control can lead to cancer.
Mechanisms of Cell Cycle Control
As the cell proceeds through its cycle, each phase involves certain processes that must be completed before the cell should advance to the next phase. A checkpoint is a point in the cell cycle at which the cycle can be signaled to move forward or stopped. At each of these checkpoints, different varieties of molecules provide the stop or go signals, depending on certain conditions within the cell. A cyclin is one of the primary classes of cell cycle control molecules (Figure 3.33). A cyclin-dependent kinase (CDK) is one of a group of molecules that work together with cyclins to determine progression past cell checkpoints. By interacting with many additional molecules, these triggers push the cell cycle forward unless prevented from doing so by “stop” signals, if for some reason the cell is not ready. At the G1 checkpoint, the cell must be ready for DNA synthesis to occur. At the G2 checkpoint the cell must be fully prepared for mitosis. Even during mitosis, a crucial stop and go checkpoint in metaphase ensures that the cell is fully prepared to complete cell division. The metaphase checkpoint ensures that all sister chromatids are properly attached to their respective microtubules and lined up at the metaphase plate before the signal is given to separate them during anaphase.
Figure 3.33 Control of the Cell Cycle Cells proceed through the cell cycle under the control of a variety of molecules, such as cyclins and cyclin-dependent kinases. These control molecules determine whether or not the cell is prepared to move into the following stage.
The Cell Cycle Out of Control: Implications
Most people understand that cancer or tumors are caused by abnormal cells that multiply continuously. If the abnormal cells continue to divide unstopped, they can damage the tissues around them, spread to other parts of the body, and eventually result in death. In healthy cells, the tight regulation mechanisms of the cell cycle prevent this from happening, while failures of cell cycle control can cause unwanted and excessive cell division. Failures of control may be caused by inherited genetic abnormalities that compromise the function of certain “stop” and “go” signals. Environmental insult that damages DNA can also cause dysfunction in those signals. Often, a combination of both genetic predisposition and environmental factors lead to cancer.
The process of a cell escaping its normal control system and becoming cancerous may actually happen throughout the body quite frequently. Fortunately, certain cells of the immune system are capable of recognizing cells that have become cancerous and destroying them. However, in certain cases the cancerous cells remain undetected and continue to proliferate. If the resulting tumor does not pose a threat to surrounding tissues, it is said to be benign and can usually be easily removed. If capable of damage, the tumor is considered malignant and the patient is diagnosed with cancer.
HOMEOSTATIC IMBALANCES
Cancer Arises from Homeostatic Imbalances
Cancer is an extremely complex condition, capable of arising from a wide variety of genetic and environmental causes. Typically, mutations or aberrations in a cell’s DNA that compromise normal cell cycle control systems lead to cancerous tumors. Cell cycle control is an example of a homeostatic mechanism that maintains proper cell function and health. While progressing through the phases of the cell cycle, a large variety of intracellular molecules provide stop and go signals to regulate movement forward to the next phase. These signals are maintained in an intricate balance so that the cell only proceeds to the next phase when it is ready. This homeostatic control of the cell cycle can be thought of like a car’s cruise control. Cruise control will continually apply just the right amount of acceleration to maintain a desired speed, unless the driver hits the brakes, in which case the car will slow down. Similarly, the cell includes molecular messengers, such as cyclins, that push the cell forward in its cycle.
In addition to cyclins, a class of proteins that are encoded by genes called proto-oncogenes provide important signals that regulate the cell cycle and move it forward. Examples of proto-oncogene products include cell-surface receptors for growth factors, or cell-signaling molecules, two classes of molecules that can promote DNA replication and cell division. In contrast, a second class of genes known as tumor suppressor genes sends stop signals during a cell cycle. For example, certain protein products of tumor suppressor genes signal potential problems with the DNA and thus stop the cell from dividing, while other proteins signal the cell to die if it is damaged beyond repair. Some tumor suppressor proteins also signal a sufficient surrounding cellular density, which indicates that the cell need not presently divide. The latter function is uniquely important in preventing tumor growth: normal cells exhibit a phenomenon called “contact inhibition;” thus, extensive cellular contact with neighboring cells causes a signal that stops further cell division.
These two contrasting classes of genes, proto-oncogenes and tumor suppressor genes, are like the accelerator and brake pedal of the cell’s own “cruise control system,” respectively. Under normal conditions, these stop and go signals are maintained in a homeostatic balance. Generally speaking, there are two ways that the cell’s cruise control can lose control: a malfunctioning (overactive) accelerator, or a malfunctioning (underactive) brake. When compromised through a mutation, or otherwise altered, proto-oncogenes can be converted to oncogenes, which produce oncoproteins that push a cell forward in its cycle and stimulate cell division even when it is undesirable to do so. For example, a cell that should be programmed to self-destruct (a process called apoptosis) due to extensive DNA damage might instead be triggered to proliferate by an oncoprotein. On the other hand, a dysfunctional tumor suppressor gene may fail to provide the cell with a necessary stop signal, also resulting in unwanted cell division and proliferation.
A delicate homeostatic balance between the many proto-oncogenes and tumor suppressor genes delicately controls the cell cycle and ensures that only healthy cells replicate. Therefore, a disruption of this homeostatic balance can cause aberrant cell division and cancerous growths.
INTERACTIVE LINK
Visit this link to learn about mitosis. Mitosis results in two identical diploid cells. What structures forms during prophase?
Cellular Differentiation
- Discuss how the generalized cells of a developing embryo or the stem cells of an adult organism become differentiated into specialized cells
- Distinguish between the categories of stem cells
How does a complex organism such as a human develop from a single cell—a fertilized egg—into the vast array of cell types such as nerve cells, muscle cells, and epithelial cells that characterize the adult? Throughout development and adulthood, the process of cellular differentiation leads cells to assume their final morphology and physiology. Differentiation is the process by which unspecialized cells become specialized to carry out distinct functions.
Stem Cells
A stem cell is an unspecialized cell that can divide without limit as needed and can, under specific conditions, differentiate into specialized cells. Stem cells are divided into several categories according to their potential to differentiate.
The first embryonic cells that arise from the division of the zygote are the ultimate stem cells; these stems cells are described as totipotent because they have the potential to differentiate into any of the cells needed to enable an organism to grow and develop.
The embryonic cells that develop from totipotent stem cells and are precursors to the fundamental tissue layers of the embryo are classified as pluripotent. A pluripotent stem cell is one that has the potential to differentiate into any type of human tissue but cannot support the full development of an organism. These cells then become slightly more specialized, and are referred to as multipotent cells.
A multipotent stem cell has the potential to differentiate into different types of cells within a given cell lineage or small number of lineages, such as a red blood cell or white blood cell.
Finally, multipotent cells can become further specialized oligopotent cells. An oligopotent stem cell is limited to becoming one of a few different cell types. In contrast, a unipotent cell is fully specialized and can only reproduce to generate more of its own specific cell type.
Stem cells are unique in that they can also continually divide and regenerate new stem cells instead of further specializing. There are different stem cells present at different stages of a human’s life. They include the embryonic stem cells of the embryo, fetal stem cells of the fetus, and adult stem cells in the adult. One type of adult stem cell is the epithelial stem cell, which gives rise to the keratinocytes in the multiple layers of epithelial cells in the epidermis of skin. Adult bone marrow has three distinct types of stem cells: hematopoietic stem cells, which give rise to red blood cells, white blood cells, and platelets (Figure 3.34); endothelial stem cells, which give rise to the endothelial cell types that line blood and lymph vessels; and mesenchymal stem cells, which give rise to the different types of muscle cells.
Figure 3.34 Hematopoiesis The process of hematopoiesis involves the differentiation of multipotent cells into blood and immune cells. The multipotent hematopoietic stem cells give rise to many different cell types, including the cells of the immune system and red blood cells.
Differentiation
When a cell differentiates (becomes more specialized), it may undertake major changes in its size, shape, metabolic activity, and overall function. Because all cells in the body, beginning with the fertilized egg, contain the same DNA, how do the different cell types come to be so different? The answer is analogous to a movie script. The different actors in a movie all read from the same script, however, they are each only reading their own part of the script. Similarly, all cells contain the same full complement of DNA, but each type of cell only “reads” the portions of DNA that are relevant to its own function. In biology, this is referred to as the unique genetic expression of each cell.
In order for a cell to differentiate into its specialized form and function, it need only manipulate those genes (and thus those proteins) that will be expressed, and not those that will remain silent. The primary mechanism by which genes are turned “on” or “off” is through transcription factors. A transcription factor is one of a class of proteins that bind to specific genes on the DNA molecule and either promote or inhibit their transcription (Figure 3.35).
Figure 3.35 Transcription Factors Regulate Gene Expression While each body cell contains the organism’s entire genome, different cells regulate gene expression with the use of various transcription factors. Transcription factors are proteins that affect the binding of RNA polymerase to a particular gene on the DNA molecule.
EVERYDAY CONNECTION
Stem Cell Research
Stem cell research aims to find ways to use stem cells to regenerate and repair cellular damage. Over time, most adult cells undergo the wear and tear of aging and lose their ability to divide and repair themselves. Stem cells do not display a particular morphology or function. Adult stem cells, which exist as a small subset of cells in most tissues, keep dividing and can differentiate into a number of specialized cells generally formed by that tissue. These cells enable the body to renew and repair body tissues.
The mechanisms that induce a non-differentiated cell to become a specialized cell are poorly understood. In a laboratory setting, it is possible to induce stem cells to differentiate into specialized cells by changing the physical and chemical conditions of growth. Several sources of stem cells are used experimentally and are classified according to their origin and potential for differentiation. Human embryonic stem cells (hESCs) are extracted from embryos and are pluripotent. The adult stem cells that are present in many organs and differentiated tissues, such as bone marrow and skin, are multipotent, being limited in differentiation to the types of cells found in those tissues. The stem cells isolated from umbilical cord blood are also multipotent, as are cells from deciduous teeth (baby teeth). Researchers have recently developed induced pluripotent stem cells (iPSCs) from mouse and human adult stem cells. These cells are genetically reprogrammed multipotent adult cells that function like embryonic stem cells; they are capable of generating cells characteristic of all three germ layers.
Because of their capacity to divide and differentiate into specialized cells, stem cells offer a potential treatment for diseases such as diabetes and heart disease (Figure 3.36). Cell-based therapy refers to treatment in which stem cells induced to differentiate in a growth dish are injected into a patient to repair damaged or destroyed cells or tissues. Many obstacles must be overcome for the application of cell-based therapy. Although embryonic stem cells have a nearly unlimited range of differentiation potential, they are seen as foreign by the patient’s immune system and may trigger rejection. Also, the destruction of embryos to isolate embryonic stem cells raises considerable ethical and legal questions.
Figure 3.36 Stem Cells The capacity of stem cells to differentiate into specialized cells make them potentially valuable in therapeutic applications designed to replace damaged cells of different body tissues.
In contrast, adult stem cells isolated from a patient are not seen as foreign by the body, but they have a limited range of differentiation. Some individuals bank the cord blood or deciduous teeth of their child, storing away those sources of stem cells for future use, should their child need it. Induced pluripotent stem cells are considered a promising advance in the field because using them avoids the legal, ethical, and immunological pitfalls of embryonic stem cells.
Key Terms
- active transport
- form of transport across the cell membrane that requires input of cellular energy
- amphipathic
- describes a molecule that exhibits a difference in polarity between its two ends, resulting in a difference in water solubility
- anaphase
- third stage of mitosis (and meiosis), during which sister chromatids separate into two new nuclear regions of a dividing cell
- anticodon
- consecutive sequence of three nucleotides on a tRNA molecule that is complementary to a specific codon on an mRNA molecule
- autolysis
- breakdown of cells by their own enzymatic action
- autophagy
- lysosomal breakdown of a cell’s own components
- cell cycle
- life cycle of a single cell, from its birth until its division into two new daughter cells
- cell membrane
- membrane surrounding all animal cells, composed of a lipid bilayer interspersed with various molecules; also known as plasma membrane
- centriole
- small, self-replicating organelle that provides the origin for microtubule growth and moves DNA during cell division
- centromere
- region of attachment for two sister chromatids
- centrosome
- cellular structure that organizes microtubules during cell division
- channel protein
- membrane-spanning protein that has an inner pore which allows the passage of one or more substances
- checkpoint
- progress point in the cell cycle during which certain conditions must be met in order for the cell to proceed to a subsequence phase
- chromatin
- substance consisting of DNA and associated proteins
- chromosome
- condensed version of chromatin
- cilia
- small appendage on certain cells formed by microtubules and modified for movement of materials across the cellular surface
- cleavage furrow
- contractile ring that forms around a cell during cytokinesis that pinches the cell into two halves
- codon
- consecutive sequence of three nucleotides on an mRNA molecule that corresponds to a specific amino acid
- concentration gradient
- difference in the concentration of a substance between two regions
- cyclin
- one of a group of proteins that function in the progression of the cell cycle
- cyclin-dependent kinase (CDK)
- one of a group of enzymes associated with cyclins that help them perform their functions
- cytokinesis
- final stage in cell division, where the cytoplasm divides to form two separate daughter cells
- cytoplasm
- internal material between the cell membrane and nucleus of a cell, mainly consisting of a water-based fluid called cytosol, within which are all the other organelles and cellular solute and suspended materials
- cytoskeleton
- “skeleton” of a cell; formed by rod-like proteins that support the cell’s shape and provide, among other functions, locomotive abilities
- cytosol
- clear, semi-fluid medium of the cytoplasm, made up mostly of water
- diffusion
- movement of a substance from an area of higher concentration to one of lower concentration
- diploid
- condition marked by the presence of a double complement of genetic material (two sets of chromosomes, one set inherited from each of two parents)
- DNA polymerase
- enzyme that functions in adding new nucleotides to a growing strand of DNA during DNA replication
- DNA replication
- process of duplicating a molecule of DNA
- electrical gradient
- difference in the electrical charge (potential) between two regions
- endocytosis
- import of material into the cell by formation of a membrane-bound vesicle
- endoplasmic reticulum (ER)
- cellular organelle that consists of interconnected membrane-bound tubules, which may or may not be associated with ribosomes (rough type or smooth type, respectively)
- exocytosis
- export of a substance out of a cell by formation of a membrane-bound vesicle
- exon
- one of the coding regions of an mRNA molecule that remain after splicing
- extracellular fluid (ECF)
- fluid exterior to cells; includes the interstitial fluid, blood plasma, and fluid found in other reservoirs in the body
- facilitated diffusion
- diffusion of a substance with the aid of a membrane protein
- flagellum
- appendage on certain cells formed by microtubules and modified for movement
- G0 phase
- phase of the cell cycle, usually entered from the G1 phase; characterized by long or permanent periods where the cell does not move forward into the DNA synthesis phase
- G1 phase
- first phase of the cell cycle, after a new cell is born
- G2 phase
- third phase of the cell cycle, after the DNA synthesis phase
- gene
- functional length of DNA that provides the genetic information necessary to build a protein
- gene expression
- active interpretation of the information coded in a gene to produce a functional gene product
- genome
- entire complement of an organism’s DNA; found within virtually every cell
- glycocalyx
- coating of sugar molecules that surrounds the cell membrane
- glycoprotein
- protein that has one or more carbohydrates attached
- Golgi apparatus
- cellular organelle formed by a series of flattened, membrane-bound sacs that functions in protein modification, tagging, packaging, and transport
- helicase
- enzyme that functions to separate the two DNA strands of a double helix during DNA replication
- histone
- family of proteins that associate with DNA in the nucleus to form chromatin
- homologous
- describes two copies of the same chromosome (not identical), one inherited from each parent
- hydrophilic
- describes a substance or structure attracted to water
- hydrophobic
- describes a substance or structure repelled by water
- hypertonic
- describes a solution concentration that is higher than a reference concentration
- hypotonic
- describes a solution concentration that is lower than a reference concentration
- integral protein
- membrane-associated protein that spans the entire width of the lipid bilayer
- intermediate filament
- type of cytoskeletal filament made of keratin, characterized by an intermediate thickness, and playing a role in resisting cellular tension
- interphase
- entire life cycle of a cell, excluding mitosis
- interstitial fluid (IF)
- fluid in the small spaces between cells not contained within blood vessels
- intracellular fluid (ICF)
- fluid in the cytosol of cells
- intron
- non-coding regions of a pre-mRNA transcript that may be removed during splicing
- isotonic
- describes a solution concentration that is the same as a reference concentration
- kinetochore
- region of a centromere where microtubules attach to a pair of sister chromatids
- ligand
- molecule that binds with specificity to a specific receptor molecule
- lysosome
- membrane-bound cellular organelle originating from the Golgi apparatus and containing digestive enzymes
- messenger RNA (mRNA)
- nucleotide molecule that serves as an intermediate in the genetic code between DNA and protein
- metaphase
- second stage of mitosis (and meiosis), characterized by the linear alignment of sister chromatids in the center of the cell
- metaphase plate
- linear alignment of sister chromatids in the center of the cell, which takes place during metaphase
- microfilament
- the thinnest of the cytoskeletal filaments; composed of actin subunits that function in muscle contraction and cellular structural support
- microtubule
- the thickest of the cytoskeletal filaments, composed of tubulin subunits that function in cellular movement and structural support
- mitochondrion
- one of the cellular organelles bound by a double lipid bilayer that function primarily in the production of cellular energy (ATP)
- mitosis
- division of genetic material, during which the cell nucleus breaks down and two new, fully functional, nuclei are formed
- mitotic phase
- phase of the cell cycle in which a cell undergoes mitosis
- mitotic spindle
- network of microtubules, originating from centrioles, that arranges and pulls apart chromosomes during mitosis
- multipotent
- describes the condition of being able to differentiate into different types of cells within a given cell lineage or small number of lineages, such as a red blood cell or white blood cell
- mutation
- change in the nucleotide sequence in a gene within a cell’s DNA
- nuclear envelope
- membrane that surrounds the nucleus; consisting of a double lipid-bilayer
- nuclear pore
- one of the small, protein-lined openings found scattered throughout the nuclear envelope
- nucleolus
- small region of the nucleus that functions in ribosome synthesis
- nucleosome
- unit of chromatin consisting of a DNA strand wrapped around histone proteins
- nucleus
- cell’s central organelle; contains the cell’s DNA
- oligopotent
- describes the condition of being more specialized than multipotency; the condition of being able to differentiate into one of a few possible cell types
- organelle
- any of several different types of membrane-enclosed specialized structures in the cell that perform specific functions for the cell
- osmosis
- diffusion of water molecules down their concentration gradient across a selectively permeable membrane
- passive transport
- form of transport across the cell membrane that does not require input of cellular energy
- peripheral protein
- membrane-associated protein that does not span the width of the lipid bilayer, but is attached peripherally to integral proteins, membrane lipids, or other components of the membrane
- peroxisome
- membrane-bound organelle that contains enzymes primarily responsible for detoxifying harmful substances
- phagocytosis
- endocytosis of large particles
- pinocytosis
- endocytosis of fluid
- pluripotent
- describes the condition of being able to differentiate into a large variety of cell types
- polypeptide
- chain of amino acids linked by peptide bonds
- polyribosome
- simultaneous translation of a single mRNA transcript by multiple ribosomes
- promoter
- region of DNA that signals transcription to begin at that site within the gene
- prophase
- first stage of mitosis (and meiosis), characterized by breakdown of the nuclear envelope and condensing of the chromatin to form chromosomes
- proteome
- full complement of proteins produced by a cell (determined by the cell’s specific gene expression)
- reactive oxygen species (ROS)
- a group of extremely reactive peroxides and oxygen-containing radicals that may contribute to cellular damage
- receptor
- protein molecule that contains a binding site for another specific molecule (called a ligand)
- receptor-mediated endocytosis
- endocytosis of ligands attached to membrane-bound receptors
- ribosomal RNA (rRNA)
- RNA that makes up the subunits of a ribosome
- ribosome
- cellular organelle that functions in protein synthesis
- RNA polymerase
- enzyme that unwinds DNA and then adds new nucleotides to a growing strand of RNA for the transcription phase of protein synthesis
- S phase
- stage of the cell cycle during which DNA replication occurs
- selective permeability
- feature of any barrier that allows certain substances to cross but excludes others
- sister chromatid
- one of a pair of identical chromosomes, formed during DNA replication
- sodium-potassium pump
- (also, Na+/K+ ATP-ase) membrane-embedded protein pump that uses ATP to move Na+ out of a cell and K+ into the cell
- somatic cell
- all cells of the body excluding gamete cells
- spliceosome
- complex of enzymes that serves to splice out the introns of a pre-mRNA transcript
- splicing
- the process of modifying a pre-mRNA transcript by removing certain, typically non-coding, regions
- stem cell
- cell that is oligo-, multi-, or pleuripotent that has the ability to produce additional stem cells rather than becoming further specialized
- telophase
- final stage of mitosis (and meiosis), preceding cytokinesis, characterized by the formation of two new daughter nuclei
- totipotent
- embryonic cells that have the ability to differentiate into any type of cell and organ in the body
- transcription
- process of producing an mRNA molecule that is complementary to a particular gene of DNA
- transcription factor
- one of the proteins that regulate the transcription of genes
- transfer RNA (tRNA)
- molecules of RNA that serve to bring amino acids to a growing polypeptide strand and properly place them into the sequence
- translation
- process of producing a protein from the nucleotide sequence code of an mRNA transcript
- triplet
- consecutive sequence of three nucleotides on a DNA molecule that, when transcribed into an mRNA codon, corresponds to a particular amino acid
- unipotent
- describes the condition of being committed to a single specialized cell type
- vesicle
- membrane-bound structure that contains materials within or outside of the cell
Chapter Review
3.1 The Cell Membrane
The cell membrane provides a barrier around the cell, separating its internal components from the extracellular environment. It is composed of a phospholipid bilayer, with hydrophobic internal lipid “tails” and hydrophilic external phosphate “heads.” Various membrane proteins are scattered throughout the bilayer, both inserted within it and attached to it peripherally. The cell membrane is selectively permeable, allowing only a limited number of materials to diffuse through its lipid bilayer. All materials that cross the membrane do so using passive (non energy-requiring) or active (energy-requiring) transport processes. During passive transport, materials move by simple diffusion or by facilitated diffusion through the membrane, down their concentration gradient. Water passes through the membrane in a diffusion process called osmosis. During active transport, energy is expended to assist material movement across the membrane in a direction against their concentration gradient. Active transport may take place with the help of protein pumps or through the use of vesicles.
3.2 The Cytoplasm and Cellular Organelles
The internal environmental of a living cell is made up of a fluid, jelly-like substance called cytosol, which consists mainly of water, but also contains various dissolved nutrients and other molecules. The cell contains an array of cellular organelles, each one performing a unique function and helping to maintain the health and activity of the cell. The cytosol and organelles together compose the cell’s cytoplasm. Most organelles are surrounded by a lipid membrane similar to the cell membrane of the cell. The endoplasmic reticulum (ER), Golgi apparatus, and lysosomes share a functional connectivity and are collectively referred to as the endomembrane system. There are two types of ER: smooth and rough. While the smooth ER performs many functions, including lipid synthesis and ion storage, the rough ER is mainly responsible for protein synthesis using its associated ribosomes. The rough ER sends newly made proteins to the Golgi apparatus where they are modified and packaged for delivery to various locations within or outside of the cell. Some of these protein products are enzymes destined to break down unwanted material and are packaged as lysosomes for use inside the cell.
Cells also contain mitochondria and peroxisomes, which are the organelles responsible for producing the cell’s energy supply and detoxifying certain chemicals, respectively. Biochemical reactions within mitochondria transform energy-carrying molecules into the usable form of cellular energy known as ATP. Peroxisomes contain enzymes that transform harmful substances such as free radicals into oxygen and water. Cells also contain a miniaturized “skeleton” of protein filaments that extend throughout its interior. Three different kinds of filaments compose this cytoskeleton (in order of increasing thickness): microfilaments, intermediate filaments, and microtubules. Each cytoskeletal component performs unique functions as well as provides a supportive framework for the cell.
3.3 The Nucleus and DNA Replication
The nucleus is the command center of the cell, containing the genetic instructions for all of the materials a cell will make (and thus all of its functions it can perform). The nucleus is encased within a membrane of two interconnected lipid bilayers, side-by-side. This nuclear envelope is studded with protein-lined pores that allow materials to be trafficked into and out of the nucleus. The nucleus contains one or more nucleoli, which serve as sites for ribosome synthesis. The nucleus houses the genetic material of the cell: DNA. DNA is normally found as a loosely contained structure called chromatin within the nucleus, where it is wound up and associated with a variety of histone proteins. When a cell is about to divide, the chromatin coils tightly and condenses to form chromosomes.
There is a pool of cells constantly dividing within your body. The result is billions of new cells being created each day. Before any cell is ready to divide, it must replicate its DNA so that each new daughter cell will receive an exact copy of the organism’s genome. A variety of enzymes are enlisted during DNA replication. These enzymes unwind the DNA molecule, separate the two strands, and assist with the building of complementary strands along each parent strand. The original DNA strands serve as templates from which the nucleotide sequence of the new strands are determined and synthesized. When replication is completed, two identical DNA molecules exist. Each one contains one original strand and one newly synthesized complementary strand.
3.4 Protein Synthesis
DNA stores the information necessary for instructing the cell to perform all of its functions. Cells use the genetic code stored within DNA to build proteins, which ultimately determine the structure and function of the cell. This genetic code lies in the particular sequence of nucleotides that make up each gene along the DNA molecule. To “read” this code, the cell must perform two sequential steps. In the first step, transcription, the DNA code is converted into a RNA code. A molecule of messenger RNA that is complementary to a specific gene is synthesized in a process similar to DNA replication. The molecule of mRNA provides the code to synthesize a protein. In the process of translation, the mRNA attaches to a ribosome. Next, tRNA molecules shuttle the appropriate amino acids to the ribosome, one-by-one, coded by sequential triplet codons on the mRNA, until the protein is fully synthesized. When completed, the mRNA detaches from the ribosome, and the protein is released. Typically, multiple ribosomes attach to a single mRNA molecule at once such that multiple proteins can be manufactured from the mRNA concurrently.
3.5 Cell Growth and Division
The life of cell consists of stages that make up the cell cycle. After a cell is born, it passes through an interphase before it is ready to replicate itself and produce daughter cells. This interphase includes two gap phases (G1 and G2), as well as an S phase, during which its DNA is replicated in preparation for cell division. The cell cycle is under precise regulation by chemical messengers both inside and outside the cell that provide “stop” and “go” signals for movement from one phase to the next. Failures of these signals can result in cells that continue to divide uncontrollably, which can lead to cancer.
Once a cell has completed interphase and is ready for cell division, it proceeds through four separate stages of mitosis (prophase, metaphase, anaphase, and telophase). Telophase is followed by the division of the cytoplasm (cytokinesis), which generates two daughter cells. This process takes place in all normally dividing cells of the body except for the germ cells that produce eggs and sperm.
3.6 Cellular Differentiation
One of the major areas of research in biology is that of how cells specialize to assume their unique structures and functions, since all cells essentially originate from a single fertilized egg. Cell differentiation is the process of cells becoming specialized as they body develops. A stem cell is an unspecialized cell that can divide without limit as needed and can, under specific conditions, differentiate into specialized cells. Stem cells are divided into several categories according to their potential to differentiate. While all somatic cells contain the exact same genome, different cell types only express some of those genes at any given time. These differences in gene expression ultimately dictate a cell’s unique morphological and physiological characteristics. The primary mechanism that determines which genes will be expressed and which ones will not is through the use of different transcription factor proteins, which bind to DNA and promote or hinder the transcription of different genes. Through the action of these transcription factors, cells specialize into one of hundreds of different cell types in the human body.
Interactive Link Questions
1.
Visit this link to see diffusion and how it is propelled by the kinetic energy of molecules in solution. How does temperature affect diffusion rate, and why?
2.Watch this video to learn about the endomembrane system, which includes the rough and smooth ER and the Golgi body as well as lysosomes and vesicles. What is the primary role of the endomembrane system?
3.Watch this video to learn about DNA replication. DNA replication proceeds simultaneously at several sites on the same molecule. What separates the base pair at the start of DNA replication?
4.Watch this video to learn about ribosomes. The ribosome binds to the mRNA molecule to start translation of its code into a protein. What happens to the small and large ribosomal subunits at the end of translation?
5.Visit this link to learn about mitosis. Mitosis results in two identical diploid cells. What structures form during prophase?
Review Questions
Because they are embedded within the membrane, ion channels are examples of ________.
- receptor proteins
- integral proteins
- peripheral proteins
- glycoproteins
The diffusion of substances within a solution tends to move those substances ________ their ________ gradient.
- up; electrical
- up; electrochemical
- down; pressure
- down; concentration
Ion pumps and phagocytosis are both examples of ________.
- endocytosis
- passive transport
- active transport
- facilitated diffusion
Choose the answer that best completes the following analogy: Diffusion is to ________ as endocytosis is to ________.
- filtration; phagocytosis
- osmosis; pinocytosis
- solutes; fluid
- gradient; chemical energy
Choose the term that best completes the following analogy: Cytoplasm is to cytosol as a swimming pool containing chlorine and flotation toys is to ________.
- the walls of the pool
- the chlorine
- the flotation toys
- the water
The rough ER has its name due to what associated structures?
- Golgi apparatus
- ribosomes
- lysosomes
- proteins
Which of the following is a function of the rough ER?
- production of proteins
- detoxification of certain substances
- synthesis of steroid hormones
- regulation of intracellular calcium concentration
Which of the following is a feature common to all three components of the cytoskeleton?
- They all serve to scaffold the organelles within the cell.
- They are all characterized by roughly the same diameter.
- They are all polymers of protein subunits.
- They all help the cell resist compression and tension.
Which of the following organelles produces large quantities of ATP when both glucose and oxygen are available to the cell?
- mitochondria
- peroxisomes
- lysosomes
- ER
The nucleus and mitochondria share which of the following features?
- protein-lined membrane pores
- a double cell membrane
- the synthesis of ribosomes
- the production of cellular energy
Which of the following structures could be found within the nucleolus?
- chromatin
- histones
- ribosomes
- nucleosomes
Which of the following sequences on a DNA molecule would be complementary to GCTTATAT?
- TAGGCGCG
- ATCCGCGC
- CGAATATA
- TGCCTCTC
Place the following structures in order from least to most complex organization: chromatin, nucleosome, DNA, chromosome
- DNA, nucleosome, chromatin, chromosome
- nucleosome, DNA, chromosome, chromatin
- DNA, chromatin, nucleosome, chromosome
- nucleosome, chromatin, DNA, chromosome
Which of the following is part of the elongation step of DNA synthesis?
- pulling apart the two DNA strands
- attaching complementary nucleotides to the template strand
- untwisting the DNA helix
- none of the above
Which of the following is not a difference between DNA and RNA?
- DNA contains thymine whereas RNA contains uracil
- DNA contains deoxyribose and RNA contains ribose
- DNA contains alternating sugar-phosphate molecules whereas RNA does not contain sugars
- RNA is single stranded and DNA is double stranded
Transcription and translation take place in the ________ and ________, respectively.
- nucleus; cytoplasm
- nucleolus; nucleus
- nucleolus; cytoplasm
- cytoplasm; nucleus
How many “letters” of an RNA molecule, in sequence, does it take to provide the code for a single amino acid?
- 1
- 2
- 3
- 4
Which of the following is not made out of RNA?
- the carriers that shuffle amino acids to a growing polypeptide strand
- the ribosome
- the messenger molecule that provides the code for protein synthesis
- the intron
Which of the following phases is characterized by preparation for DNA synthesis?
- G0
- G1
- G2
- S
A mutation in the gene for a cyclin protein might result in which of the following?
- a cell with additional genetic material than normal
- cancer
- a cell with less genetic material than normal
- any of the above
What is a primary function of tumor suppressor genes?
- stop all cells from dividing
- stop certain cells from dividing
- help oncogenes produce oncoproteins
- allow the cell to skip certain phases of the cell cycle
Arrange the following terms in order of increasing specialization: oligopotency, pleuripotency, unipotency, multipotency.
- multipotency, pleuripotency, oligopotency, unipotency
- pleuripotency, oligopotency, multipotency unipotency
- oligopotency, pleuripotency, unipotency, multipotency
- pleuripotency, multipotency, oligopotency, unipotency
Which type of stem cell gives rise to red and white blood cells?
- endothelial
- epithelial
- hematopoietic
- mesenchymal
What multipotent stem cells from children sometimes banked by parents?
- fetal stem cells
- embryonic stem cells
- cells from the umbilical cord and from baby teeth
- hematopoietic stem cells from red and white blood cells
Critical Thinking Questions
What materials can easily diffuse through the lipid bilayer, and why?
31.Why is receptor-mediated endocytosis said to be more selective than phagocytosis or pinocytosis?
32.What do osmosis, diffusion, filtration, and the movement of ions away from like charge all have in common? In what way do they differ?
33.Explain why the structure of the ER, mitochondria, and Golgi apparatus assist their respective functions.
34.Compare and contrast lysosomes with peroxisomes: name at least two similarities and one difference.
35.Explain in your own words why DNA replication is said to be “semiconservative”?
36.Why is it important that DNA replication take place before cell division? What would happen if cell division of a body cell took place without DNA replication, or when DNA replication was incomplete?
37.Briefly explain the similarities between transcription and DNA replication.
38.Contrast transcription and translation. Name at least three differences between the two processes.
39.What would happen if anaphase proceeded even though the sister chromatids were not properly attached to their respective microtubules and lined up at the metaphase plate?
40.What are cyclins and cyclin-dependent kinases, and how do they interact?
41.Explain how a transcription factor ultimately determines whether or not a protein will be present in a given cell?
42.Discuss two reasons why the therapeutic use of embryonic stem cells can present a problem.
|
oercommons
|
2025-03-18T00:39:12.311710
|
07/23/2019
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/56353/overview",
"title": "Anatomy and Physiology, Levels of Organization, The Cellular Level of Organization",
"author": null
}
|
https://oercommons.org/courseware/lesson/56378/overview
|
The Autonomic Nervous System
Introduction
Figure 15.1 Fight or Flight? Though the threats that modern humans face are not large predators, the autonomic nervous system is adapted to this type of stimulus. The modern world presents stimuli that trigger the same response. (credit: Vernon Swanepoel)
CHAPTER OBJECTIVES
After studying this chapter, you will be able to:
- Describe the components of the autonomic nervous system
- Differentiate between the structures of the sympathetic and parasympathetic divisions in the autonomic nervous system
- Name the components of a visceral reflex specific to the autonomic division to which it belongs
- Predict the response of a target effector to autonomic input on the basis of the released signaling molecule
- Describe how the central nervous system coordinates and contributes to autonomic functions
The autonomic nervous system is often associated with the “fight-or-flight response,” which refers to the preparation of the body to either run away from a threat or to stand and fight in the face of that threat. To suggest what this means, consider the (very unlikely) situation of seeing a lioness hunting out on the savannah. Though this is not a common threat that humans deal with in the modern world, it represents the type of environment in which the human species thrived and adapted. The spread of humans around the world to the present state of the modern age occurred much more quickly than any species would adapt to environmental pressures such as predators. However, the reactions modern humans have in the modern world are based on these prehistoric situations. If your boss is walking down the hallway on Friday afternoon looking for “volunteers” to come in on the weekend, your response is the same as the prehistoric human seeing the lioness running across the savannah: fight or flight.
Most likely, your response to your boss—not to mention the lioness—would be flight. Run away! The autonomic system is responsible for the physiological response to make that possible, and hopefully successful. Adrenaline starts to flood your circulatory system. Your heart rate increases. Sweat glands become active. The bronchi of the lungs dilate to allow more air exchange. Pupils dilate to increase visual information. Blood pressure increases in general, and blood vessels dilate in skeletal muscles. Time to run. Similar physiological responses would occur in preparation for fighting off the threat.
This response should sound a bit familiar. The autonomic nervous system is tied into emotional responses as well, and the fight-or-flight response probably sounds like a panic attack. In the modern world, these sorts of reactions are associated with anxiety as much as with response to a threat. It is engrained in the nervous system to respond like this. In fact, the adaptations of the autonomic nervous system probably predate the human species and are likely to be common to all mammals, and perhaps shared by many animals. That lioness might herself be threatened in some other situation.
However, the autonomic nervous system is not just about responding to threats. Besides the fight-or-flight response, there are the responses referred to as “rest and digest.” If that lioness is successful in her hunting, then she is going to rest from the exertion. Her heart rate will slow. Breathing will return to normal. The digestive system has a big job to do. Much of the function of the autonomic system is based on the connections within an autonomic, or visceral, reflex.
Divisions of the Autonomic Nervous System
- Name the components that generate the sympathetic and parasympathetic responses of the autonomic nervous system
- Explain the differences in output connections within the two divisions of the autonomic nervous system
- Describe the signaling molecules and receptor proteins involved in communication within the two divisions of the autonomic nervous system
The nervous system can be divided into two functional parts: the somatic nervous system and the autonomic nervous system. The major differences between the two systems are evident in the responses that each produces. The somatic nervous system causes contraction of skeletal muscles. The autonomic nervous system controls cardiac and smooth muscle, as well as glandular tissue. The somatic nervous system is associated with voluntary responses (though many can happen without conscious awareness, like breathing), and the autonomic nervous system is associated with involuntary responses, such as those related to homeostasis.
The autonomic nervous system regulates many of the internal organs through a balance of two aspects, or divisions. In addition to the endocrine system, the autonomic nervous system is instrumental in homeostatic mechanisms in the body. The two divisions of the autonomic nervous system are the sympathetic division and the parasympathetic division. The sympathetic system is associated with the fight-or-flight response, and parasympathetic activity is referred to by the epithet of rest and digest. Homeostasis is the balance between the two systems. At each target effector, dual innervation determines activity. For example, the heart receives connections from both the sympathetic and parasympathetic divisions. One causes heart rate to increase, whereas the other causes heart rate to decrease.
INTERACTIVE LINK
Watch this video to learn more about adrenaline and the fight-or-flight response. When someone is said to have a rush of adrenaline, the image of bungee jumpers or skydivers usually comes to mind. But adrenaline, also known as epinephrine, is an important chemical in coordinating the body’s fight-or-flight response. In this video, you look inside the physiology of the fight-or-flight response, as envisioned for a firefighter. His body’s reaction is the result of the sympathetic division of the autonomic nervous system causing system-wide changes as it prepares for extreme responses. What two changes does adrenaline bring about to help the skeletal muscle response?
Sympathetic Division of the Autonomic Nervous System
To respond to a threat—to fight or to run away—the sympathetic system causes divergent effects as many different effector organs are activated together for a common purpose. More oxygen needs to be inhaled and delivered to skeletal muscle. The respiratory, cardiovascular, and musculoskeletal systems are all activated together. Additionally, sweating keeps the excess heat that comes from muscle contraction from causing the body to overheat. The digestive system shuts down so that blood is not absorbing nutrients when it should be delivering oxygen to skeletal muscles. To coordinate all these responses, the connections in the sympathetic system diverge from a limited region of the central nervous system (CNS) to a wide array of ganglia that project to the many effector organs simultaneously. The complex set of structures that compose the output of the sympathetic system make it possible for these disparate effectors to come together in a coordinated, systemic change.
The sympathetic division of the autonomic nervous system influences the various organ systems of the body through connections emerging from the thoracic and upper lumbar spinal cord. It is referred to as the thoracolumbar system to reflect this anatomical basis. A central neuron in the lateral horn of any of these spinal regions projects to ganglia adjacent to the vertebral column through the ventral spinal roots. The majority of ganglia of the sympathetic system belong to a network of sympathetic chain ganglia that runs alongside the vertebral column. The ganglia appear as a series of clusters of neurons linked by axonal bridges. There are typically 23 ganglia in the chain on either side of the spinal column. Three correspond to the cervical region, 12 are in the thoracic region, four are in the lumbar region, and four correspond to the sacral region. The cervical and sacral levels are not connected to the spinal cord directly through the spinal roots, but through ascending or descending connections through the bridges within the chain.
A diagram that shows the connections of the sympathetic system is somewhat like a circuit diagram that shows the electrical connections between different receptacles and devices. In Figure 15.2, the “circuits” of the sympathetic system are intentionally simplified.
Figure 15.2 Connections of Sympathetic Division of the Autonomic Nervous System Neurons from the lateral horn of the spinal cord (preganglionic nerve fibers - solid lines)) project to the chain ganglia on either side of the vertebral column or to collateral (prevertebral) ganglia that are anterior to the vertebral column in the abdominal cavity. Axons from these ganglionic neurons (postganglionic nerve fibers - dotted lines) then project to target effectors throughout the body.
To continue with the analogy of the circuit diagram, there are three different types of “junctions” that operate within the sympathetic system (Figure 15.3). The first type is most direct: the sympathetic nerve projects to the chain ganglion at the same level as the target effector (the organ, tissue, or gland to be innervated). An example of this type is spinal nerve T1 that synapses with the T1 chain ganglion to innervate the trachea. The fibers of this branch are called white rami communicantes(singular = ramus communicans); they are myelinated and therefore referred to as white (see Figure 15.3a). The axon from the central neuron (the preganglionic fiber shown as a solid line) synapses with the ganglionic neuron (with the postganglionic fiber shown as a dashed line). This neuron then projects to a target effector—in this case, the trachea—via gray rami communicantes, which are unmyelinated axons.
In some cases, the target effectors are located superior or inferior to the spinal segment at which the preganglionic fiber emerges. With respect to the “wiring” involved, the synapse with the ganglionic neuron occurs at chain ganglia superior or inferior to the location of the central neuron. An example of this is spinal nerve T1 that innervates the eye. The spinal nerve tracks up through the chain until it reaches the superior cervical ganglion, where it synapses with the postganglionic neuron (see Figure 15.3b). The cervical ganglia are referred to as paravertebral ganglia, given their location adjacent to prevertebral ganglia in the sympathetic chain.
Not all axons from the central neurons terminate in the chain ganglia. Additional branches from the ventral nerve root continue through the chain and on to one of the collateral ganglia as the greater splanchnic nerve or lesser splanchnic nerve. For example, the greater splanchnic nerve at the level of T5 synapses with a collateral ganglion outside the chain before making the connection to the postganglionic nerves that innervate the stomach (see Figure 15.3c).
Collateral ganglia, also called prevertebral ganglia, are situated anterior to the vertebral column and receive inputs from splanchnic nerves as well as central sympathetic neurons. They are associated with controlling organs in the abdominal cavity, and are also considered part of the enteric nervous system. The three collateral ganglia are the celiac ganglion, the superior mesenteric ganglion, and the inferior mesenteric ganglion (see Figure 15.2). The word celiac is derived from the Latin word “coelom,” which refers to a body cavity (in this case, the abdominal cavity), and the word mesenteric refers to the digestive system.
Figure 15.3 Sympathetic Connections and Chain Ganglia The axon from a central sympathetic neuron in the spinal cord can project to the periphery in a number of different ways. (a) The fiber can project out to the ganglion at the same level and synapse on a ganglionic neuron. (b) A branch can project to more superior or inferior ganglion in the chain. (c) A branch can project through the white ramus communicans, but not terminate on a ganglionic neuron in the chain. Instead, it projects through one of the splanchnic nerves to a collateral ganglion or the adrenal medulla (not pictured).
An axon from the central neuron that projects to a sympathetic ganglion is referred to as a preganglionic fiber or neuron, and represents the output from the CNS to the ganglion. Because the sympathetic ganglia are adjacent to the vertebral column, preganglionic sympathetic fibers are relatively short, and they are myelinated. A postganglionic fiber—the axon from a ganglionic neuron that projects to the target effector—represents the output of a ganglion that directly influences the organ. Compared with the preganglionic fibers, postganglionic sympathetic fibers are long because of the relatively greater distance from the ganglion to the target effector. These fibers are unmyelinated. (Note that the term “postganglionic neuron” may be used to describe the projection from a ganglion to the target. The problem with that usage is that the cell body is in the ganglion, and only the fiber is postganglionic. Typically, the term neuron applies to the entire cell.)
One type of preganglionic sympathetic fiber does not terminate in a ganglion. These are the axons from central sympathetic neurons that project to the adrenal medulla, the interior portion of the adrenal gland. These axons are still referred to as preganglionic fibers, but the target is not a ganglion. The adrenal medulla releases signaling molecules into the bloodstream, rather than using axons to communicate with target structures. The cells in the adrenal medulla that are contacted by the preganglionic fibers are called chromaffin cells. These cells are neurosecretory cells that develop from the neural crest along with the sympathetic ganglia, reinforcing the idea that the gland is, functionally, a sympathetic ganglion.
The projections of the sympathetic division of the autonomic nervous system diverge widely, resulting in a broad influence of the system throughout the body. As a response to a threat, the sympathetic system would increase heart rate and breathing rate and cause blood flow to the skeletal muscle to increase and blood flow to the digestive system to decrease. Sweat gland secretion should also increase as part of an integrated response. All of those physiological changes are going to be required to occur together to run away from the hunting lioness, or the modern equivalent. This divergence is seen in the branching patterns of preganglionic sympathetic neurons—a single preganglionic sympathetic neuron may have 10–20 targets. An axon that leaves a central neuron of the lateral horn in the thoracolumbar spinal cord will pass through the white ramus communicans and enter the sympathetic chain, where it will branch toward a variety of targets. At the level of the spinal cord at which the preganglionic sympathetic fiber exits the spinal cord, a branch will synapse on a neuron in the adjacent chain ganglion. Some branches will extend up or down to a different level of the chain ganglia. Other branches will pass through the chain ganglia and project through one of the splanchnic nerves to a collateral ganglion. Finally, some branches may project through the splanchnic nerves to the adrenal medulla. All of these branches mean that one preganglionic neuron can influence different regions of the sympathetic system very broadly, by acting on widely distributed organs.
Parasympathetic Division of the Autonomic Nervous System
The parasympathetic division of the autonomic nervous system is named because its central neurons are located on either side of the thoracolumbar region of the spinal cord (para- = “beside” or “near”). The parasympathetic system can also be referred to as the craniosacral system (or outflow) because the preganglionic neurons are located in nuclei of the brain stem and the lateral horn of the sacral spinal cord.
The connections, or “circuits,” of the parasympathetic division are similar to the general layout of the sympathetic division with a few specific differences (Figure 15.4). The preganglionic fibers from the cranial region travel in cranial nerves, whereas preganglionic fibers from the sacral region travel in spinal nerves. The targets of these fibers are terminal ganglia, which are located near—or even within—the target effector. These ganglia are often referred to as intramural ganglia when they are found within the walls of the target organ. The postganglionic fiber projects from the terminal ganglia a short distance to the target effector, or to the specific target tissue within the organ. Comparing the relative lengths of axons in the parasympathetic system, the preganglionic fibers are long and the postganglionic fibers are short because the ganglia are close to—and sometimes within—the target effectors.
The cranial component of the parasympathetic system is based in particular nuclei of the brain stem. In the midbrain, the Edinger–Westphal nucleus is part of the oculomotor complex, and axons from those neurons travel with the fibers in the oculomotor nerve (cranial nerve III) that innervate the extraocular muscles. The preganglionic parasympathetic fibers within cranial nerve III terminate in the ciliary ganglion, which is located in the posterior orbit. The postganglionic parasympathetic fibers then project to the smooth muscle of the iris to control pupillary size. In the upper medulla, the salivatory nuclei contain neurons with axons that project through the facial and glossopharyngeal nerves to ganglia that control salivary glands. Tear production is influenced by parasympathetic fibers in the facial nerve, which activate a ganglion, and ultimately the lacrimal (tear) gland. Neurons in the dorsal nucleus of the vagus nerve and the nucleus ambiguus project through the vagus nerve (cranial nerve X) to the terminal ganglia of the thoracic and abdominal cavities. Parasympathetic preganglionic fibers primarily influence the heart, bronchi, and esophagus in the thoracic cavity and the stomach, liver, pancreas, gall bladder, and small intestine of the abdominal cavity. The postganglionic fibers from the ganglia activated by the vagus nerve are often incorporated into the structure of the organ, such as the mesenteric plexus of the digestive tract organs and the intramural ganglia.
Figure 15.4 Connections of Parasympathetic Division of the Autonomic Nervous System Neurons from brain-stem nuclei, or from the lateral horn of the sacral spinal cord, project to terminal ganglia near or within the various organs of the body. Axons from these ganglionic neurons then project the short distance to those target effectors.
Chemical Signaling in the Autonomic Nervous System
Where an autonomic neuron connects with a target, there is a synapse. The electrical signal of the action potential causes the release of a signaling molecule, which will bind to receptor proteins on the target cell. Synapses of the autonomic system are classified as either cholinergic, meaning that acetylcholine (ACh) is released, or adrenergic, meaning that norepinephrine is released. The terms cholinergic and adrenergic refer not only to the signaling molecule that is released but also to the class of receptors that each binds.
The cholinergic system includes two classes of receptor: the nicotinic receptor and the muscarinic receptor. Both receptor types bind to ACh and cause changes in the target cell. The nicotinic receptor is a ligand-gated cation channel and the muscarinic receptor is a G protein–coupled receptor. The receptors are named for, and differentiated by, other molecules that bind to them. Whereas nicotine will bind to the nicotinic receptor, and muscarine will bind to the muscarinic receptor, there is no cross-reactivity between the receptors. The situation is similar to locks and keys. Imagine two locks—one for a classroom and the other for an office—that are opened by two separate keys. The classroom key will not open the office door and the office key will not open the classroom door. This is similar to the specificity of nicotine and muscarine for their receptors. However, a master key can open multiple locks, such as a master key for the Biology Department that opens both the classroom and the office doors. This is similar to ACh that binds to both types of receptors. The molecules that define these receptors are not crucial—they are simply tools for researchers to use in the laboratory. These molecules are exogenous, meaning that they are made outside of the human body, so a researcher can use them without any confounding endogenous results (results caused by the molecules produced in the body).
The adrenergic system also has two types of receptors, named the alpha (α)-adrenergic receptor and beta (β)-adrenergic receptor. Unlike cholinergic receptors, these receptor types are not classified by which drugs can bind to them. All of them are G protein–coupled receptors. There are three types of α-adrenergic receptors, termed α1, α2, and α3, and there are two types of β-adrenergic receptors, termed β1 and β2. An additional aspect of the adrenergic system is that there is a second signaling molecule called epinephrine. The chemical difference between norepinephrine and epinephrine is the addition of a methyl group (CH3) in epinephrine. The prefix “nor-” actually refers to this chemical difference, in which a methyl group is missing.
The term adrenergic should remind you of the word adrenaline, which is associated with the fight-or-flight response described at the beginning of the chapter. Adrenaline and epinephrine are two names for the same molecule. The adrenal gland (in Latin, ad- = “on top of”; renal = “kidney”) secretes adrenaline. The ending “-ine” refers to the chemical being derived, or extracted, from the adrenal gland. A similar construction from Greek instead of Latin results in the word epinephrine (epi- = “above”; nephr- = “kidney”). In scientific usage, epinephrine is preferred in the United States, whereas adrenaline is preferred in Great Britain, because “adrenalin” was once a registered, proprietary drug name in the United States. Though the drug is no longer sold, the convention of referring to this molecule by the two different names persists. Similarly, norepinephrine and noradrenaline are two names for the same molecule.
Having understood the cholinergic and adrenergic systems, their role in the autonomic system is relatively simple to understand. All preganglionic fibers, both sympathetic and parasympathetic, release ACh. All ganglionic neurons—the targets of these preganglionic fibers—have nicotinic receptors in their cell membranes. The nicotinic receptor is a ligand-gated cation channel that results in depolarization of the postsynaptic membrane. The postganglionic parasympathetic fibers also release ACh, but the receptors on their targets are muscarinic receptors, which are G protein–coupled receptors and do not exclusively cause depolarization of the postsynaptic membrane. Postganglionic sympathetic fibers release norepinephrine, except for fibers that project to sweat glands and to blood vessels associated with skeletal muscles, which release ACh (Table 15.1).
Autonomic System Signaling Molecules
| Sympathetic | Parasympathetic | |
|---|---|---|
| Preganglionic | Acetylcholine → nicotinic receptor | Acetylcholine → nicotinic receptor |
| Postganglionic | Norepinephrine → α- or β-adrenergic receptors Acetylcholine → muscarinic receptor (associated with sweat glands and the blood vessels associated with skeletal muscles only | Acetylcholine → muscarinic receptor |
Table 15.1
Signaling molecules can belong to two broad groups. Neurotransmitters are released at synapses, whereas hormones are released into the bloodstream. These are simplistic definitions, but they can help to clarify this point. Acetylcholine can be considered a neurotransmitter because it is released by axons at synapses. The adrenergic system, however, presents a challenge. Postganglionic sympathetic fibers release norepinephrine, which can be considered a neurotransmitter. But the adrenal medulla releases epinephrine and norepinephrine into circulation, so they should be considered hormones.
What are referred to here as synapses may not fit the strictest definition of synapse. Some sources will refer to the connection between a postganglionic fiber and a target effector as neuroeffector junctions; neurotransmitters, as defined above, would be called neuromodulators. The structure of postganglionic connections are not the typical synaptic end bulb that is found at the neuromuscular junction, but rather are chains of swellings along the length of a postganglionic fiber called a varicosity (Figure 15.5).
Figure 15.5 Autonomic Varicosities The connection between autonomic fibers and target effectors is not the same as the typical synapse, such as the neuromuscular junction. Instead of a synaptic end bulb, a neurotransmitter is released from swellings along the length of a fiber that makes an extended network of connections in the target effector.
EVERYDAY CONNECTION
Fight or Flight? What About Fright and Freeze?
The original usage of the epithet “fight or flight” comes from a scientist named Walter Cannon who worked at Harvard in 1915. The concept of homeostasis and the functioning of the sympathetic system had been introduced in France in the previous century. Cannon expanded the idea, and introduced the idea that an animal responds to a threat by preparing to stand and fight or run away. The nature of this response was thoroughly explained in a book on the physiology of pain, hunger, fear, and rage.
When students learn about the sympathetic system and the fight-or-flight response, they often stop and wonder about other responses. If you were faced with a lioness running toward you as pictured at the beginning of this chapter, would you run or would you stand your ground? Some people would say that they would freeze and not know what to do. So isn’t there really more to what the autonomic system does than fight, flight, rest, or digest. What about fear and paralysis in the face of a threat?
The common epithet of “fight or flight” is being enlarged to be “fight, flight, or fright” or even “fight, flight, fright, or freeze.” Cannon’s original contribution was a catchy phrase to express some of what the nervous system does in response to a threat, but it is incomplete. The sympathetic system is responsible for the physiological responses to emotional states. The name “sympathetic” can be said to mean that (sym- = “together”; -pathos = “pain,” “suffering,” or “emotion”).
INTERACTIVE LINK
Watch this video to learn more about the nervous system. As described in this video, the nervous system has a way to deal with threats and stress that is separate from the conscious control of the somatic nervous system. The system comes from a time when threats were about survival, but in the modern age, these responses become part of stress and anxiety. This video describes how the autonomic system is only part of the response to threats, or stressors. What other organ system gets involved, and what part of the brain coordinates the two systems for the entire response, including epinephrine (adrenaline) and cortisol?
Autonomic Reflexes and Homeostasis
- Compare the structure of somatic and autonomic reflex arcs
- Explain the differences in sympathetic and parasympathetic reflexes
- Differentiate between short and long reflexes
- Determine the effect of the autonomic nervous system on the regulation of the various organ systems on the basis of the signaling molecules involved
- Describe the effects of drugs that affect autonomic function
The autonomic nervous system regulates organ systems through circuits that resemble the reflexes described in the somatic nervous system. The main difference between the somatic and autonomic systems is in what target tissues are effectors. Somatic responses are solely based on skeletal muscle contraction. The autonomic system, however, targets cardiac and smooth muscle, as well as glandular tissue. Whereas the basic circuit is a reflex arc, there are differences in the structure of those reflexes for the somatic and autonomic systems.
The Structure of Reflexes
One difference between a somatic reflex, such as the withdrawal reflex, and a visceral reflex, which is an autonomic reflex, is in the efferent branch. The output of a somatic reflex is the lower motor neuron in the ventral horn of the spinal cord that projects directly to a skeletal muscle to cause its contraction. The output of a visceral reflex is a two-step pathway starting with the preganglionic fiber emerging from a lateral horn neuron in the spinal cord, or a cranial nucleus neuron in the brain stem, to a ganglion—followed by the postganglionic fiber projecting to a target effector. The other part of a reflex, the afferent branch, is often the same between the two systems. Sensory neurons receiving input from the periphery—with cell bodies in the sensory ganglia, either of a cranial nerve or a dorsal root ganglion adjacent to the spinal cord—project into the CNS to initiate the reflex (Figure 15.6). The Latin root “effere” means “to carry.” Adding the prefix “ef-” suggests the meaning “to carry away,” whereas adding the prefix “af-” suggests “to carry toward or inward.”
Figure 15.6 Comparison of Somatic and Visceral Reflexes The afferent inputs to somatic and visceral reflexes are essentially the same, whereas the efferent branches are different. Somatic reflexes, for instance, involve a direct connection from the ventral horn of the spinal cord to the skeletal muscle. Visceral reflexes involve a projection from the central neuron to a ganglion, followed by a second projection from the ganglion to the target effector.
Afferent Branch
The afferent branch of a reflex arc does differ between somatic and visceral reflexes in some instances. Many of the inputs to visceral reflexes are from special or somatic senses, but particular senses are associated with the viscera that are not part of the conscious perception of the environment through the somatic nervous system. For example, there is a specific type of mechanoreceptor, called a baroreceptor, in the walls of the aorta and carotid sinuses that senses the stretch of those organs when blood volume or pressure increases. You do not have a conscious perception of having high blood pressure, but that is an important afferent branch of the cardiovascular and, particularly, vasomotor reflexes. The sensory neuron is essentially the same as any other general sensory neuron. The baroreceptor apparatus is part of the ending of a unipolar neuron that has a cell body in a sensory ganglion. The baroreceptors from the carotid arteries have axons in the glossopharyngeal nerve, and those from the aorta have axons in the vagus nerve.
Though visceral senses are not primarily a part of conscious perception, those sensations sometimes make it to conscious awareness. If a visceral sense is strong enough, it will be perceived. The sensory homunculus—the representation of the body in the primary somatosensory cortex—only has a small region allotted for the perception of internal stimuli. If you swallow a large bolus of food, for instance, you will probably feel the lump of that food as it pushes through your esophagus, or even if your stomach is distended after a large meal. If you inhale especially cold air, you can feel it as it enters your larynx and trachea. These sensations are not the same as feeling high blood pressure or blood sugar levels.
When particularly strong visceral sensations rise to the level of conscious perception, the sensations are often felt in unexpected places. For example, strong visceral sensations of the heart will be felt as pain in the left shoulder and left arm. This irregular pattern of projection of conscious perception of visceral sensations is called referred pain. Depending on the organ system affected, the referred pain will project to different areas of the body (Figure 15.7). The location of referred pain is not random, but a definitive explanation of the mechanism has not been established. The most broadly accepted theory for this phenomenon is that the visceral sensory fibers enter into the same level of the spinal cord as the somatosensory fibers of the referred pain location. By this explanation, the visceral sensory fibers from the mediastinal region, where the heart is located, would enter the spinal cord at the same level as the spinal nerves from the shoulder and arm, so the brain misinterprets the sensations from the mediastinal region as being from the axillary and brachial regions. Projections from the medial and inferior divisions of the cervical ganglia do enter the spinal cord at the middle to lower cervical levels, which is where the somatosensory fibers enter.
Figure 15.7 Referred Pain Chart Conscious perception of visceral sensations map to specific regions of the body, as shown in this chart. Some sensations are felt locally, whereas others are perceived as affecting areas that are quite distant from the involved organ.
DISORDERS OF THE...
Nervous System: Kehr’s Sign
Kehr’s sign is the presentation of pain in the left shoulder, chest, and neck regions following rupture of the spleen. The spleen is in the upper-left abdominopelvic quadrant, but the pain is more in the shoulder and neck. How can this be? The sympathetic fibers connected to the spleen are from the celiac ganglion, which would be from the mid-thoracic to lower thoracic region whereas parasympathetic fibers are found in the vagus nerve, which connects in the medulla of the brain stem. However, the neck and shoulder would connect to the spinal cord at the mid-cervical level of the spinal cord. These connections do not fit with the expected correspondence of visceral and somatosensory fibers entering at the same level of the spinal cord.
The incorrect assumption would be that the visceral sensations are coming from the spleen directly. In fact, the visceral fibers are coming from the diaphragm. The nerve connecting to the diaphragm takes a special route. The phrenic nerve is connected to the spinal cord at cervical levels 3 to 5. The motor fibers that make up this nerve are responsible for the muscle contractions that drive ventilation. These fibers have left the spinal cord to enter the phrenic nerve, meaning that spinal cord damage below the mid-cervical level is not fatal by making ventilation impossible. Therefore, the visceral fibers from the diaphragm enter the spinal cord at the same level as the somatosensory fibers from the neck and shoulder.
The diaphragm plays a role in Kehr’s sign because the spleen is just inferior to the diaphragm in the upper-left quadrant of the abdominopelvic cavity. When the spleen ruptures, blood spills into this region. The accumulating hemorrhage then puts pressure on the diaphragm. The visceral sensation is actually in the diaphragm, so the referred pain is in a region of the body that corresponds to the diaphragm, not the spleen.
Efferent Branch
The efferent branch of the visceral reflex arc begins with the projection from the central neuron along the preganglionic fiber. This fiber then makes a synapse on the ganglionic neuron that projects to the target effector.
The effector organs that are the targets of the autonomic system range from the iris and ciliary body of the eye to the urinary bladder and reproductive organs. The thoracolumbar output, through the various sympathetic ganglia, reaches all of these organs. The cranial component of the parasympathetic system projects from the eye to part of the intestines. The sacral component picks up with the majority of the large intestine and the pelvic organs of the urinary and reproductive systems.
Short and Long Reflexes
Somatic reflexes involve sensory neurons that connect sensory receptors to the CNS and motor neurons that project back out to the skeletal muscles. Visceral reflexes that involve the thoracolumbar or craniosacral systems share similar connections. However, there are reflexes that do not need to involve any CNS components. A long reflex has afferent branches that enter the spinal cord or brain and involve the efferent branches, as previously explained. A short reflex is completely peripheral and only involves the local integration of sensory input with motor output (Figure 15.8).
Figure 15.8 Short and Long Reflexes Sensory input can stimulate either a short or a long reflex. A sensory neuron can project to the CNS or to an autonomic ganglion. The short reflex involves the direct stimulation of a postganglionic fiber by the sensory neuron, whereas the long reflex involves integration in the spinal cord or brain.
The difference between short and long reflexes is in the involvement of the CNS. Somatic reflexes always involve the CNS, even in a monosynaptic reflex in which the sensory neuron directly activates the motor neuron. That synapse is in the spinal cord or brain stem, so it has to involve the CNS. However, in the autonomic system there is the possibility that the CNS is not involved. Because the efferent branch of a visceral reflex involves two neurons—the central neuron and the ganglionic neuron—a “short circuit” can be possible. If a sensory neuron projects directly to the ganglionic neuron and causes it to activate the effector target, then the CNS is not involved.
A division of the nervous system that is related to the autonomic nervous system is the enteric nervous system. The word enteric refers to the digestive organs, so this represents the nervous tissue that is part of the digestive system. There are a few myenteric plexuses in which the nervous tissue in the wall of the digestive tract organs can directly influence digestive function. If stretch receptors in the stomach are activated by the filling and distension of the stomach, a short reflex will directly activate the smooth muscle fibers of the stomach wall to increase motility to digest the excessive food in the stomach. No CNS involvement is needed because the stretch receptor is directly activating a neuron in the wall of the stomach that causes the smooth muscle to contract. That neuron, connected to the smooth muscle, is a postganglionic parasympathetic neuron that can be controlled by a fiber found in the vagus nerve.
INTERACTIVE LINK
Read this article to learn about a teenager who experiences a series of spells that suggest a stroke. He undergoes endless tests and seeks input from multiple doctors. In the end, one expert, one question, and a simple blood pressure cuff answers the question. Why would the heart have to beat faster when the teenager changes his body position from lying down to sitting, and then to standing?
Balance in Competing Autonomic Reflex Arcs
The autonomic nervous system is important for homeostasis because its two divisions compete at the target effector. The balance of homeostasis is attributable to the competing inputs from the sympathetic and parasympathetic divisions (dual innervation). At the level of the target effector, the signal of which system is sending the message is strictly chemical. A signaling molecule binds to a receptor that causes changes in the target cell, which in turn causes the tissue or organ to respond to the changing conditions of the body.
Competing Neurotransmitters
The postganglionic fibers of the sympathetic and parasympathetic divisions both release neurotransmitters that bind to receptors on their targets. Postganglionic sympathetic fibers release norepinephrine, with a minor exception, whereas postganglionic parasympathetic fibers release ACh. For any given target, the difference in which division of the autonomic nervous system is exerting control is just in what chemical binds to its receptors. The target cells will have adrenergic and muscarinic receptors. If norepinephrine is released, it will bind to the adrenergic receptors present on the target cell, and if ACh is released, it will bind to the muscarinic receptors on the target cell.
In the sympathetic system, there are exceptions to this pattern of dual innervation. The postganglionic sympathetic fibers that contact the blood vessels within skeletal muscle and that contact sweat glands do not release norepinephrine, they release ACh. This does not create any problem because there is no parasympathetic input to the sweat glands. Sweat glands have muscarinic receptors and produce and secrete sweat in response to the presence of ACh.
At most of the other targets of the autonomic system, the effector response is based on which neurotransmitter is released and what receptor is present. For example, regions of the heart that establish heart rate are contacted by postganglionic fibers from both systems. If norepinephrine is released onto those cells, it binds to an adrenergic receptor that causes the cells to depolarize faster, and the heart rate increases. If ACh is released onto those cells, it binds to a muscarinic receptor that causes the cells to hyperpolarize so that they cannot reach threshold as easily, and the heart rate slows. Without this parasympathetic input, the heart would work at a rate of approximately 100 beats per minute (bpm). The sympathetic system speeds that up, as it would during exercise, to 120–140 bpm, for example. The parasympathetic system slows it down to the resting heart rate of 60–80 bpm.
Another example is in the control of pupillary size (Figure 15.9). The afferent branch responds to light hitting the retina. Photoreceptors are activated, and the signal is transferred to the retinal ganglion cells that send an action potential along the optic nerve into the diencephalon. If light levels are low, the sympathetic system sends a signal out through the upper thoracic spinal cord to the superior cervical ganglion of the sympathetic chain. The postganglionic fiber then projects to the iris, where it releases norepinephrine onto the radial fibers of the iris (a smooth muscle). When those fibers contract, the pupil dilates—increasing the amount of light hitting the retina. If light levels are too high, the parasympathetic system sends a signal out from the Eddinger–Westphal nucleus through the oculomotor nerve. This fiber synapses in the ciliary ganglion in the posterior orbit. The postganglionic fiber then projects to the iris, where it releases ACh onto the circular fibers of the iris—another smooth muscle. When those fibers contract, the pupil constricts to limit the amount of light hitting the retina.
Figure 15.9 Autonomic Control of Pupillary Size Activation of the pupillary reflex comes from the amount of light activating the retinal ganglion cells, as sent along the optic nerve. The output of the sympathetic system projects through the superior cervical ganglion, whereas the parasympathetic system originates out of the midbrain and projects through the oculomotor nerve to the ciliary ganglion, which then projects to the iris. The postganglionic fibers of either division release neurotransmitters onto the smooth muscles of the iris to cause changes in the pupillary size. Norepinephrine results in dilation and ACh results in constriction.
In this example, the autonomic system is controlling how much light hits the retina. It is a homeostatic reflex mechanism that keeps the activation of photoreceptors within certain limits. In the context of avoiding a threat like the lioness on the savannah, the sympathetic response for fight or flight will increase pupillary diameter so that more light hits the retina and more visual information is available for running away. Likewise, the parasympathetic response of rest reduces the amount of light reaching the retina, allowing the photoreceptors to cycle through bleaching and be regenerated for further visual perception; this is what the homeostatic process is attempting to maintain.
INTERACTIVE LINK
Watch this video to learn about the pupillary reflexes. The pupillary light reflex involves sensory input through the optic nerve and motor response through the oculomotor nerve to the ciliary ganglion, which projects to the circular fibers of the iris. As shown in this short animation, pupils will constrict to limit the amount of light falling on the retina under bright lighting conditions. What constitutes the afferent and efferent branches of the competing reflex (dilation)?
Autonomic Tone
Organ systems are balanced between the input from the sympathetic and parasympathetic divisions. When something upsets that balance, the homeostatic mechanisms strive to return it to its regular state. For each organ system, there may be more of a sympathetic or parasympathetic tendency to the resting state, which is known as the autonomic tone of the system. For example, the heart rate was described above. Because the resting heart rate is the result of the parasympathetic system slowing the heart down from its intrinsic rate of 100 bpm, the heart can be said to be in parasympathetic tone.
In a similar fashion, another aspect of the cardiovascular system is primarily under sympathetic control. Blood pressure is partially determined by the contraction of smooth muscle in the walls of blood vessels. These tissues have adrenergic receptors that respond to the release of norepinephrine from postganglionic sympathetic fibers by constricting and increasing blood pressure. The hormones released from the adrenal medulla—epinephrine and norepinephrine—will also bind to these receptors. Those hormones travel through the bloodstream where they can easily interact with the receptors in the vessel walls. The parasympathetic system has no significant input to the systemic blood vessels, so the sympathetic system determines their tone.
There are a limited number of blood vessels that respond to sympathetic input in a different fashion. Blood vessels in skeletal muscle, particularly those in the lower limbs, are more likely to dilate. It does not have an overall effect on blood pressure to alter the tone of the vessels, but rather allows for blood flow to increase for those skeletal muscles that will be active in the fight-or-flight response. The blood vessels that have a parasympathetic projection are limited to those in the erectile tissue of the reproductive organs. Acetylcholine released by these postganglionic parasympathetic fibers cause the vessels to dilate, leading to the engorgement of the erectile tissue.
HOMEOSTATIC IMBALANCES
Orthostatic Hypotension
Have you ever stood up quickly and felt dizzy for a moment? This is because, for one reason or another, blood is not getting to your brain so it is briefly deprived of oxygen. When you change position from sitting or lying down to standing, your cardiovascular system has to adjust for a new challenge, keeping blood pumping up into the head while gravity is pulling more and more blood down into the legs.
The reason for this is a sympathetic reflex that maintains the output of the heart in response to postural change. When a person stands up, proprioceptors indicate that the body is changing position. A signal goes to the CNS, which then sends a signal to the upper thoracic spinal cord neurons of the sympathetic division. The sympathetic system then causes the heart to beat faster and the blood vessels to constrict. Both changes will make it possible for the cardiovascular system to maintain the rate of blood delivery to the brain. Blood is being pumped superiorly through the internal branch of the carotid arteries into the brain, against the force of gravity. Gravity is not increasing while standing, but blood is more likely to flow down into the legs as they are extended for standing. This sympathetic reflex keeps the brain well oxygenated so that cognitive and other neural processes are not interrupted.
Sometimes this does not work properly. If the sympathetic system cannot increase cardiac output, then blood pressure into the brain will decrease, and a brief neurological loss can be felt. This can be brief, as a slight “wooziness” when standing up too quickly, or a loss of balance and neurological impairment for a period of time. The name for this is orthostatic hypotension, which means that blood pressure goes below the homeostatic set point when standing. It can be the result of standing up faster than the reflex can occur, which may be referred to as a benign “head rush,” or it may be the result of an underlying cause.
There are two basic reasons that orthostatic hypotension can occur. First, blood volume is too low and the sympathetic reflex is not effective. This hypovolemia may be the result of dehydration or medications that affect fluid balance, such as diuretics or vasodilators. Both of these medications are meant to lower blood pressure, which may be necessary in the case of systemic hypertension, and regulation of the medications may alleviate the problem. Sometimes increasing fluid intake or water retention through salt intake can improve the situation.
The second underlying cause of orthostatic hypotension is autonomic failure. There are several disorders that result in compromised sympathetic functions. The disorders range from diabetes to multiple system atrophy (a loss of control over many systems in the body), and addressing the underlying condition can improve the hypotension. For example, with diabetes, peripheral nerve damage can occur, which would affect the postganglionic sympathetic fibers. Getting blood glucose levels under control can improve neurological deficits associated with diabetes.
Central Control
- Describe the role of higher centers of the brain in autonomic regulation
- Explain the connection of the hypothalamus to homeostasis
- Describe the regions of the CNS that link the autonomic system with emotion
- Describe the pathways important to descending control of the autonomic system
The pupillary light reflex (Figure 15.10) begins when light hits the retina and causes a signal to travel along the optic nerve. This is visual sensation, because the afferent branch of this reflex is simply sharing the special sense pathway. Bright light hitting the retina leads to the parasympathetic response, through the oculomotor nerve, followed by the postganglionic fiber from the ciliary ganglion, which stimulates the circular fibers of the iris to contract and constrict the pupil. When light hits the retina in one eye, both pupils contract. When that light is removed, both pupils dilate again back to the resting position. When the stimulus is unilateral (presented to only one eye), the response is bilateral (both eyes). The same is not true for somatic reflexes. If you touch a hot radiator, you only pull that arm back, not both. Central control of autonomic reflexes is different than for somatic reflexes. The hypothalamus, along with other CNS locations, controls the autonomic system.
Figure 15.10 Pupillary Reflex Pathways The pupil is under competing autonomic control in response to light levels hitting the retina. The sympathetic system will dilate the pupil when the retina is not receiving enough light, and the parasympathetic system will constrict the pupil when too much light hits the retina.
Forebrain Structures
Autonomic control is based on the visceral reflexes, composed of the afferent and efferent branches. These homeostatic mechanisms are based on the balance between the two divisions of the autonomic system, which results in tone for various organs that is based on the predominant input from the sympathetic or parasympathetic systems. Coordinating that balance requires integration that begins with forebrain structures like the hypothalamus and continues into the brain stem and spinal cord.
The Hypothalamus
The hypothalamus is the control center for many homeostatic mechanisms. It regulates both autonomic function and endocrine function. The roles it plays in the pupillary reflexes demonstrates the importance of this control center. The optic nerve projects primarily to the thalamus, which is the necessary relay to the occipital cortex for conscious visual perception. Another projection of the optic nerve, however, goes to the hypothalamus.
The hypothalamus then uses this visual system input to drive the pupillary reflexes. If the retina is activated by high levels of light, the hypothalamus stimulates the parasympathetic response. If the optic nerve message shows that low levels of light are falling on the retina, the hypothalamus activates the sympathetic response. Output from the hypothalamus follows two main tracts, the dorsal longitudinal fasciculus and the medial forebrain bundle (Figure 15.11). Along these two tracts, the hypothalamus can influence the Eddinger–Westphal nucleus of the oculomotor complex or the lateral horns of the thoracic spinal cord.
Figure 15.11 Fiber Tracts of the Central Autonomic System The hypothalamus is the source of most of the central control of autonomic function. It receives input from cerebral structures and projects to brain stem and spinal cord structures to regulate the balance of sympathetic and parasympathetic input to the organ systems of the body. The main pathways for this are the medial forebrain bundle and the dorsal longitudinal fasciculus.
These two tracts connect the hypothalamus with the major parasympathetic nuclei in the brain stem and the preganglionic (central) neurons of the thoracolumbar spinal cord. The hypothalamus also receives input from other areas of the forebrain through the medial forebrain bundle. The olfactory cortex, the septal nuclei of the basal forebrain, and the amygdala project into the hypothalamus through the medial forebrain bundle. These forebrain structures inform the hypothalamus about the state of the nervous system and can influence the regulatory processes of homeostasis. A good example of this is found in the amygdala, which is found beneath the cerebral cortex of the temporal lobe and plays a role in our ability to remember and feel emotions.
The Amygdala
The amygdala is a group of nuclei in the medial region of the temporal lobe that is part of the limbic lobe (Figure 15.12). The limbic lobe includes structures that are involved in emotional responses, as well as structures that contribute to memory function. The limbic lobe has strong connections with the hypothalamus and influences the state of its activity on the basis of emotional state. For example, when you are anxious or scared, the amygdala will send signals to the hypothalamus along the medial forebrain bundle that will stimulate the sympathetic fight-or-flight response. The hypothalamus will also stimulate the release of stress hormones through its control of the endocrine system in response to amygdala input.
Figure 15.12 The Limbic Lobe Structures arranged around the edge of the cerebrum constitute the limbic lobe, which includes the amygdala, hippocampus, and cingulate gyrus, and connects to the hypothalamus.
The Medulla
The medulla contains nuclei referred to as the cardiovascular center, which controls the smooth and cardiac muscle of the cardiovascular system through autonomic connections. When the homeostasis of the cardiovascular system shifts, such as when blood pressure changes, the coordination of the autonomic system can be accomplished within this region. Furthermore, when descending inputs from the hypothalamus stimulate this area, the sympathetic system can increase activity in the cardiovascular system, such as in response to anxiety or stress. The preganglionic sympathetic fibers that are responsible for increasing heart rate are referred to as the cardiac accelerator nerves, whereas the preganglionic sympathetic fibers responsible for constricting blood vessels compose the vasomotor nerves.
Several brain stem nuclei are important for the visceral control of major organ systems. One brain stem nucleus involved in cardiovascular function is the solitary nucleus. It receives sensory input about blood pressure and cardiac function from the glossopharyngeal and vagus nerves, and its output will activate sympathetic stimulation of the heart or blood vessels through the upper thoracic lateral horn. Another brain stem nucleus important for visceral control is the dorsal motor nucleus of the vagus nerve, which is the motor nucleus for the parasympathetic functions ascribed to the vagus nerve, including decreasing the heart rate, relaxing bronchial tubes in the lungs, and activating digestive function through the enteric nervous system. The nucleus ambiguus, which is named for its ambiguous histology, also contributes to the parasympathetic output of the vagus nerve and targets muscles in the pharynx and larynx for swallowing and speech, as well as contributing to the parasympathetic tone of the heart along with the dorsal motor nucleus of the vagus.
EVERYDAY CONNECTION
Exercise and the Autonomic System
In addition to its association with the fight-or-flight response and rest-and-digest functions, the autonomic system is responsible for certain everyday functions. For example, it comes into play when homeostatic mechanisms dynamically change, such as the physiological changes that accompany exercise. Getting on the treadmill and putting in a good workout will cause the heart rate to increase, breathing to be stronger and deeper, sweat glands to activate, and the digestive system to suspend activity. These are the same physiological changes associated with the fight-or-flight response, but there is nothing chasing you on that treadmill.
This is not a simple homeostatic mechanism at work because “maintaining the internal environment” would mean getting all those changes back to their set points. Instead, the sympathetic system has become active during exercise so that your body can cope with what is happening. A homeostatic mechanism is dealing with the conscious decision to push the body away from a resting state. The heart, actually, is moving away from its homeostatic set point. Without any input from the autonomic system, the heart would beat at approximately 100 bpm, and the parasympathetic system slows that down to the resting rate of approximately 70 bpm. But in the middle of a good workout, you should see your heart rate at 120–140 bpm. You could say that the body is stressed because of what you are doing to it. Homeostatic mechanisms are trying to keep blood pH in the normal range, or to keep body temperature under control, but those are in response to the choice to exercise.
INTERACTIVE LINK
Watch this video to learn about physical responses to emotion. The autonomic system, which is important for regulating the homeostasis of the organ systems, is also responsible for our physiological responses to emotions such as fear. The video summarizes the extent of the body’s reactions and describes several effects of the autonomic system in response to fear. On the basis of what you have already studied about autonomic function, which effect would you expect to be associated with parasympathetic, rather than sympathetic, activity?
Drugs that Affect the Autonomic System
- List the classes of pharmaceuticals that interact with the autonomic nervous system
- Differentiate between cholinergic and adrenergic compounds
- Differentiate between sympathomimetic and sympatholytic drugs
- Relate the consequences of nicotine abuse with respect to autonomic control of the cardiovascular system
An important way to understand the effects of native neurochemicals in the autonomic system is in considering the effects of pharmaceutical drugs. This can be considered in terms of how drugs change autonomic function. These effects will primarily be based on how drugs act at the receptors of the autonomic system neurochemistry. The signaling molecules of the nervous system interact with proteins in the cell membranes of various target cells. In fact, no effect can be attributed to just the signaling molecules themselves without considering the receptors. A chemical that the body produces to interact with those receptors is called an endogenous chemical, whereas a chemical introduced to the system from outside is an exogenous chemical. Exogenous chemicals may be of a natural origin, such as a plant extract, or they may be synthetically produced in a pharmaceutical laboratory.
Broad Autonomic Effects
One important drug that affects the autonomic system broadly is not a pharmaceutical therapeutic agent associated with the system. This drug is nicotine. The effects of nicotine on the autonomic nervous system are important in considering the role smoking can play in health.
All ganglionic neurons of the autonomic system, in both sympathetic and parasympathetic ganglia, are activated by ACh released from preganglionic fibers. The ACh receptors on these neurons are of the nicotinic type, meaning that they are ligand-gated ion channels. When the neurotransmitter released from the preganglionic fiber binds to the receptor protein, a channel opens to allow positive ions to cross the cell membrane. The result is depolarization of the ganglia. Nicotine acts as an ACh analog at these synapses, so when someone takes in the drug, it binds to these ACh receptors and activates the ganglionic neurons, causing them to depolarize.
Ganglia of both divisions are activated equally by the drug. For many target organs in the body, this results in no net change. The competing inputs to the system cancel each other out and nothing significant happens. For example, the sympathetic system will cause sphincters in the digestive tract to contract, limiting digestive propulsion, but the parasympathetic system will cause the contraction of other muscles in the digestive tract, which will try to push the contents of the digestive system along. The end result is that the food does not really move along and the digestive system has not appreciably changed.
The system in which this can be problematic is in the cardiovascular system, which is why smoking is a risk factor for cardiovascular disease. First, there is no significant parasympathetic regulation of blood pressure. Only a limited number of blood vessels are affected by parasympathetic input, so nicotine will preferentially cause the vascular tone to become more sympathetic, which means blood pressure will be increased. Second, the autonomic control of the heart is special. Unlike skeletal or smooth muscles, cardiac muscle is intrinsically active, meaning that it generates its own action potentials. The autonomic system does not cause the heart to beat, it just speeds it up (sympathetic) or slows it down (parasympathetic). The mechanisms for this are not mutually exclusive, so the heart receives conflicting signals, and the rhythm of the heart can be affected (Figure 15.13).
Figure 15.13 Autonomic Connections to Heart and Blood Vessels The nicotinic receptor is found on all autonomic ganglia, but the cardiovascular connections are particular, and do not conform to the usual competitive projections that would just cancel each other out when stimulated by nicotine. The opposing signals to the heart would both depolarize and hyperpolarize the heart cells that establish the rhythm of the heartbeat, likely causing arrhythmia. Only the sympathetic system governs systemic blood pressure so nicotine would cause an increase.
Sympathetic Effect
The neurochemistry of the sympathetic system is based on the adrenergic system. Norepinephrine and epinephrine influence target effectors by binding to the α-adrenergic or β-adrenergic receptors. Drugs that affect the sympathetic system affect these chemical systems. The drugs can be classified by whether they enhance the functions of the sympathetic system or interrupt those functions. A drug that enhances adrenergic function is known as a sympathomimetic drug, whereas a drug that interrupts adrenergic function is a sympatholytic drug.
Sympathomimetic Drugs
When the sympathetic system is not functioning correctly or the body is in a state of homeostatic imbalance, these drugs act at postganglionic terminals and synapses in the sympathetic efferent pathway. These drugs either bind to particular adrenergic receptors and mimic norepinephrine at the synapses between sympathetic postganglionic fibers and their targets, or they increase the production and release of norepinephrine from postganglionic fibers. Also, to increase the effectiveness of adrenergic chemicals released from the fibers, some of these drugs may block the removal or reuptake of the neurotransmitter from the synapse.
A common sympathomimetic drug is phenylephrine, which is a common component of decongestants. It can also be used to dilate the pupil and to raise blood pressure. Phenylephrine is known as an α1-adrenergic agonist, meaning that it binds to a specific adrenergic receptor, stimulating a response. In this role, phenylephrine will bind to the adrenergic receptors in bronchioles of the lungs and cause them to dilate. By opening these structures, accumulated mucus can be cleared out of the lower respiratory tract. Phenylephrine is often paired with other pharmaceuticals, such as analgesics, as in the “sinus” version of many over-the-counter drugs, such as Tylenol Sinus® or Excedrin Sinus®, or in expectorants for chest congestion such as in Robitussin CF®.
A related molecule, called pseudoephedrine, was much more commonly used in these applications than was phenylephrine, until the molecule became useful in the illicit production of amphetamines. Phenylephrine is not as effective as a drug because it can be partially broken down in the digestive tract before it is ever absorbed. Like the adrenergic agents, phenylephrine is effective in dilating the pupil, known as mydriasis (Figure 15.14). Phenylephrine is used during an eye exam in an ophthalmologist’s or optometrist’s office for this purpose. It can also be used to increase blood pressure in situations in which cardiac function is compromised, such as under anesthesia or during septic shock.
Figure 15.14 Mydriasis The sympathetic system causes pupillary dilation when norepinephrine binds to an adrenergic receptor in the radial fibers of the iris smooth muscle. Phenylephrine mimics this action by binding to the same receptor when drops are applied onto the surface of the eye in a doctor’s office. (credit: Corey Theiss)
Other drugs that enhance adrenergic function are not associated with therapeutic uses, but affect the functions of the sympathetic system in a similar fashion. Cocaine primarily interferes with the uptake of dopamine at the synapse and can also increase adrenergic function. Caffeine is an antagonist to a different neurotransmitter receptor, called the adenosine receptor. Adenosine will suppress adrenergic activity, specifically the release of norepinephrine at synapses, so caffeine indirectly increases adrenergic activity. There is some evidence that caffeine can aid in the therapeutic use of drugs, perhaps by potentiating (increasing) sympathetic function, as is suggested by the inclusion of caffeine in over-the-counter analgesics such as Excedrin®.
Sympatholytic Drugs
Drugs that interfere with sympathetic function are referred to as sympatholytic, or sympathoplegic, drugs. They primarily work as an antagonist to the adrenergic receptors. They block the ability of norepinephrine or epinephrine to bind to the receptors so that the effect is “cut” or “takes a blow,” to refer to the endings “-lytic” and “-plegic,” respectively. The various drugs of this class will be specific to α-adrenergic or β-adrenergic receptors, or to their receptor subtypes.
Possibly the most familiar type of sympatholytic drug are the β-blockers. These drugs are often used to treat cardiovascular disease because they block the β-receptors associated with vasoconstriction and cardioacceleration. By allowing blood vessels to dilate, or keeping heart rate from increasing, these drugs can improve cardiac function in a compromised system, such as for a person with congestive heart failure or who has previously suffered a heart attack. A couple of common versions of β-blockers are metoprolol, which specifically blocks the β1-receptor, and propanolol, which nonspecifically blocks β-receptors. There are other drugs that are α-blockers and can affect the sympathetic system in a similar way.
Other uses for sympatholytic drugs are as antianxiety medications. A common example of this is clonidine, which is an α-agonist. The sympathetic system is tied to anxiety to the point that the sympathetic response can be referred to as “fight, flight, or fright.” Clonidine is used for other treatments aside from hypertension and anxiety, including pain conditions and attention deficit hyperactivity disorder.
Parasympathetic Effects
Drugs affecting parasympathetic functions can be classified into those that increase or decrease activity at postganglionic terminals. Parasympathetic postganglionic fibers release ACh, and the receptors on the targets are muscarinic receptors. There are several types of muscarinic receptors, M1–M5, but the drugs are not usually specific to the specific types. Parasympathetic drugs can be either muscarinic agonists or antagonists, or have indirect effects on the cholinergic system. Drugs that enhance cholinergic effects are called parasympathomimetic drugs, whereas those that inhibit cholinergic effects are referred to as anticholinergic drugs.
Pilocarpine is a nonspecific muscarinic agonist commonly used to treat disorders of the eye. It reverses mydriasis, such as is caused by phenylephrine, and can be administered after an eye exam. Along with constricting the pupil through the smooth muscle of the iris, pilocarpine will also cause the ciliary muscle to contract. This will open perforations at the base of the cornea, allowing for the drainage of aqueous humor from the anterior compartment of the eye and, therefore, reducing intraocular pressure related to glaucoma.
Atropine and scopolamine are part of a class of muscarinic antagonists that come from the Atropa genus of plants that include belladonna or deadly nightshade (Figure 15.15). The name of one of these plants, belladonna, refers to the fact that extracts from this plant were used cosmetically for dilating the pupil. The active chemicals from this plant block the muscarinic receptors in the iris and allow the pupil to dilate, which is considered attractive because it makes the eyes appear larger. Humans are instinctively attracted to anything with larger eyes, which comes from the fact that the ratio of eye-to-head size is different in infants (or baby animals) and can elicit an emotional response. The cosmetic use of belladonna extract was essentially acting on this response. Atropine is no longer used in this cosmetic capacity for reasons related to the other name for the plant, which is deadly nightshade. Suppression of parasympathetic function, especially when it becomes systemic, can be fatal. Autonomic regulation is disrupted and anticholinergic symptoms develop. The berries of this plant are highly toxic, but can be mistaken for other berries. The antidote for atropine or scopolamine poisoning is pilocarpine.
Figure 15.15 Belladonna Plant The plant from the genus Atropa, which is known as belladonna or deadly nightshade, was used cosmetically to dilate pupils, but can be fatal when ingested. The berries on the plant may seem attractive as a fruit, but they contain the same anticholinergic compounds as the rest of the plant.
Sympathetic and Parasympathetic Effects of Different Drug Types
| Drug type | Example(s) | Sympathetic effect | Parasympathetic effect | Overall result |
|---|---|---|---|---|
| Nicotinic agonists | Nicotine | Mimic ACh at preganglionic synapses, causing activation of postganglionic fibers and the release of norepinephrine onto the target organ | Mimic ACh at preganglionic synapses, causing activation of postganglionic fibers and the release of ACh onto the target organ | Most conflicting signals cancel each other out, but cardiovascular system is susceptible to hypertension and arrhythmias |
| Sympathomimetic drugs | Phenylephrine | Bind to adrenergic receptors or mimics sympathetic action in some other way | No effect | Increase sympathetic tone |
| Sympatholytic drugs | β-blockers such as propanolol or metoprolol; α-agonists such as clonidine | Block binding to adrenergic drug or decrease adrenergic signals | No effect | Increase parasympathetic tone |
| Parasymphatho-mimetics/muscarinic agonists | Pilocarpine | No effect, except on sweat glands | Bind to muscarinic receptor, similar to ACh | Increase parasympathetic tone |
| Anticholinergics/muscarinic antagonists | Atropine, scopolamine, dimenhydrinate | No effect | Block muscarinic receptors and parasympathetic function | Increase sympathetic tone |
Table 15.2
DISORDERS OF THE...
Autonomic Nervous System
Approximately 33 percent of people experience a mild problem with motion sickness, whereas up to 66 percent experience motion sickness under extreme conditions, such as being on a tossing boat with no view of the horizon. Connections between regions in the brain stem and the autonomic system result in the symptoms of nausea, cold sweats, and vomiting.
The part of the brain responsible for vomiting, or emesis, is known as the area postrema. It is located next to the fourth ventricle and is not restricted by the blood–brain barrier, which allows it to respond to chemicals in the bloodstream—namely, toxins that will stimulate emesis. There are significant connections between this area, the solitary nucleus, and the dorsal motor nucleus of the vagus nerve. These autonomic system and nuclei connections are associated with the symptoms of motion sickness.
Motion sickness is the result of conflicting information from the visual and vestibular systems. If motion is perceived by the visual system without the complementary vestibular stimuli, or through vestibular stimuli without visual confirmation, the brain stimulates emesis and the associated symptoms. The area postrema, by itself, appears to be able to stimulate emesis in response to toxins in the blood, but it is also connected to the autonomic system and can trigger a similar response to motion.
Autonomic drugs are used to combat motion sickness. Though it is often described as a dangerous and deadly drug, scopolamine is used to treat motion sickness. A popular treatment for motion sickness is the transdermal scopolamine patch. Scopolamine is one of the substances derived from the Atropa genus along with atropine. At higher doses, those substances are thought to be poisonous and can lead to an extreme sympathetic syndrome. However, the transdermal patch regulates the release of the drug, and the concentration is kept very low so that the dangers are avoided. For those who are concerned about using “The Most Dangerous Drug,” as some websites will call it, antihistamines such as dimenhydrinate (Dramamine®) can be used.
INTERACTIVE LINK
Watch this video to learn about the side effects of 3-D movies. As discussed in this video, movies that are shot in 3-D can cause motion sickness, which elicits the autonomic symptoms of nausea and sweating. The disconnection between the perceived motion on the screen and the lack of any change in equilibrium stimulates these symptoms. Why do you think sitting close to the screen or right in the middle of the theater makes motion sickness during a 3-D movie worse?
Key Terms
- acetylcholine (ACh)
- neurotransmitter that binds at a motor end-plate to trigger depolarization
- adrenal medulla
- interior portion of the adrenal (or suprarenal) gland that releases epinephrine and norepinephrine into the bloodstream as hormones
- adrenergic
- synapse where norepinephrine is released, which binds to α- or β-adrenergic receptors
- afferent branch
- component of a reflex arc that represents the input from a sensory neuron, for either a special or general sense
- agonist
- any exogenous substance that binds to a receptor and produces a similar effect to the endogenous ligand
- alpha (α)-adrenergic receptor
- one of the receptors to which epinephrine and norepinephrine bind, which comes in three subtypes: α1, α2, and α3
- antagonist
- any exogenous substance that binds to a receptor and produces an opposing effect to the endogenous ligand
- anticholinergic drugs
- drugs that interrupt or reduce the function of the parasympathetic system
- autonomic tone
- tendency of an organ system to be governed by one division of the autonomic nervous system over the other, such as heart rate being lowered by parasympathetic input at rest
- baroreceptor
- mechanoreceptor that senses the stretch of blood vessels to indicate changes in blood pressure
- beta (β)-adrenergic receptor
- one of the receptors to which epinephrine and norepinephrine bind, which comes in two subtypes: β1 and β2
- cardiac accelerator nerves
- preganglionic sympathetic fibers that cause the heart rate to increase when the cardiovascular center in the medulla initiates a signal
- cardiovascular center
- region in the medulla that controls the cardiovascular system through cardiac accelerator nerves and vasomotor nerves, which are components of the sympathetic division of the autonomic nervous system
- celiac ganglion
- one of the collateral ganglia of the sympathetic system that projects to the digestive system
- central neuron
- specifically referring to the cell body of a neuron in the autonomic system that is located in the central nervous system, specifically the lateral horn of the spinal cord or a brain stem nucleus
- cholinergic
- synapse at which acetylcholine is released and binds to the nicotinic or muscarinic receptor
- chromaffin cells
- neuroendocrine cells of the adrenal medulla that release epinephrine and norepinephrine into the bloodstream as part of sympathetic system activity
- ciliary ganglion
- one of the terminal ganglia of the parasympathetic system, located in the posterior orbit, axons from which project to the iris
- collateral ganglia
- ganglia outside of the sympathetic chain that are targets of sympathetic preganglionic fibers, which are the celiac, inferior mesenteric, and superior mesenteric ganglia
- craniosacral system
- alternate name for the parasympathetic division of the autonomic nervous system that is based on the anatomical location of central neurons in brain-stem nuclei and the lateral horn of the sacral spinal cord; also referred to as craniosacral outflow
- dorsal longitudinal fasciculus
- major output pathway of the hypothalamus that descends through the gray matter of the brain stem and into the spinal cord
- dorsal nucleus of the vagus nerve
- location of parasympathetic neurons that project through the vagus nerve to terminal ganglia in the thoracic and abdominal cavities
- Eddinger–Westphal nucleus
- location of parasympathetic neurons that project to the ciliary ganglion
- efferent branch
- component of a reflex arc that represents the output, with the target being an effector, such as muscle or glandular tissue
- endogenous
- describes substance made in the human body
- endogenous chemical
- substance produced and released within the body to interact with a receptor protein
- epinephrine
- signaling molecule released from the adrenal medulla into the bloodstream as part of the sympathetic response
- exogenous
- describes substance made outside of the human body
- exogenous chemical
- substance from a source outside the body, whether it be another organism such as a plant or from the synthetic processes of a laboratory, that binds to a transmembrane receptor protein
- fight-or-flight response
- set of responses induced by sympathetic activity that lead to either fleeing a threat or standing up to it, which in the modern world is often associated with anxious feelings
- G protein–coupled receptor
- membrane protein complex that consists of a receptor protein that binds to a signaling molecule—a G protein—that is activated by that binding and in turn activates an effector protein (enzyme) that creates a second-messenger molecule in the cytoplasm of the target cell
- ganglionic neuron
- specifically refers to the cell body of a neuron in the autonomic system that is located in a ganglion
- gray rami communicantes
- (singular = ramus communicans) unmyelinated structures that provide a short connection from a sympathetic chain ganglion to the spinal nerve that contains the postganglionic sympathetic fiber
- greater splanchnic nerve
- nerve that contains fibers of the central sympathetic neurons that do not synapse in the chain ganglia but project onto the celiac ganglion
- inferior mesenteric ganglion
- one of the collateral ganglia of the sympathetic system that projects to the digestive system
- intramural ganglia
- terminal ganglia of the parasympathetic system that are found within the walls of the target effector
- lesser splanchnic nerve
- nerve that contains fibers of the central sympathetic neurons that do not synapse in the chain ganglia but project onto the inferior mesenteric ganglion
- ligand-gated cation channel
- ion channel, such as the nicotinic receptor, that is specific to positively charged ions and opens when a molecule such as a neurotransmitter binds to it
- limbic lobe
- structures arranged around the edges of the cerebrum that are involved in memory and emotion
- long reflex
- reflex arc that includes the central nervous system
- medial forebrain bundle
- fiber pathway that extends anteriorly into the basal forebrain, passes through the hypothalamus, and extends into the brain stem and spinal cord
- mesenteric plexus
- nervous tissue within the wall of the digestive tract that contains neurons that are the targets of autonomic preganglionic fibers and that project to the smooth muscle and glandular tissues in the digestive organ
- muscarinic receptor
- type of acetylcholine receptor protein that is characterized by also binding to muscarine and is a metabotropic receptor
- mydriasis
- dilation of the pupil; typically the result of disease, trauma, or drugs
- nicotinic receptor
- type of acetylcholine receptor protein that is characterized by also binding to nicotine and is an ionotropic receptor
- norepinephrine
- signaling molecule released as a neurotransmitter by most postganglionic sympathetic fibers as part of the sympathetic response, or as a hormone into the bloodstream from the adrenal medulla
- nucleus ambiguus
- brain-stem nucleus that contains neurons that project through the vagus nerve to terminal ganglia in the thoracic cavity; specifically associated with the heart
- parasympathetic division
- division of the autonomic nervous system responsible for restful and digestive functions
- parasympathomimetic drugs
- drugs that enhance or mimic the function of the parasympathetic system
- paravertebral ganglia
- autonomic ganglia superior to the sympathetic chain ganglia
- postganglionic fiber
- axon from a ganglionic neuron in the autonomic nervous system that projects to and synapses with the target effector; sometimes referred to as a postganglionic neuron
- preganglionic fiber
- axon from a central neuron in the autonomic nervous system that projects to and synapses with a ganglionic neuron; sometimes referred to as a preganglionic neuron
- prevertebral ganglia
- autonomic ganglia that are anterior to the vertebral column and functionally related to the sympathetic chain ganglia
- referred pain
- the conscious perception of visceral sensation projected to a different region of the body, such as the left shoulder and arm pain as a sign for a heart attack
- reflex arc
- circuit of a reflex that involves a sensory input and motor output, or an afferent branch and an efferent branch, and an integrating center to connect the two branches
- rest and digest
- set of functions associated with the parasympathetic system that lead to restful actions and digestion
- short reflex
- reflex arc that does not include any components of the central nervous system
- somatic reflex
- reflex involving skeletal muscle as the effector, under the control of the somatic nervous system
- superior cervical ganglion
- one of the paravertebral ganglia of the sympathetic system that projects to the head
- superior mesenteric ganglion
- one of the collateral ganglia of the sympathetic system that projects to the digestive system
- sympathetic chain ganglia
- series of ganglia adjacent to the vertebral column that receive input from central sympathetic neurons
- sympathetic division
- division of the autonomic nervous system associated with the fight-or-flight response
- sympatholytic drug
- drug that interrupts, or “lyses,” the function of the sympathetic system
- sympathomimetic drug
- drug that enhances or mimics the function of the sympathetic system
- target effector
- organ, tissue, or gland that will respond to the control of an autonomic or somatic or endocrine signal
- terminal ganglia
- ganglia of the parasympathetic division of the autonomic system, which are located near or within the target effector, the latter also known as intramural ganglia
- thoracolumbar system
- alternate name for the sympathetic division of the autonomic nervous system that is based on the anatomical location of central neurons in the lateral horn of the thoracic and upper lumbar spinal cord
- varicosity
- structure of some autonomic connections that is not a typical synaptic end bulb, but a string of swellings along the length of a fiber that makes a network of connections with the target effector
- vasomotor nerves
- preganglionic sympathetic fibers that cause the constriction of blood vessels in response to signals from the cardiovascular center
- visceral reflex
- reflex involving an internal organ as the effector, under the control of the autonomic nervous system
- white rami communicantes
- (singular = ramus communicans) myelinated structures that provide a short connection from a sympathetic chain ganglion to the spinal nerve that contains the preganglionic sympathetic fiber
Chapter Review
15.1 Divisions of the Autonomic Nervous System
The primary responsibilities of the autonomic nervous system are to regulate homeostatic mechanisms in the body, which is also part of what the endocrine system does. The key to understanding the autonomic system is to explore the response pathways—the output of the nervous system. The way we respond to the world around us, to manage the internal environment on the basis of the external environment, is divided between two parts of the autonomic nervous system. The sympathetic division responds to threats and produces a readiness to confront the threat or to run away: the fight-or-flight response. The parasympathetic division plays the opposite role. When the external environment does not present any immediate danger, a restful mode descends on the body, and the digestive system is more active.
The sympathetic output of the nervous system originates out of the lateral horn of the thoracolumbar spinal cord. An axon from one of these central neurons projects by way of the ventral spinal nerve root and spinal nerve to a sympathetic ganglion, either in the sympathetic chain ganglia or one of the collateral locations, where it synapses on a ganglionic neuron. These preganglionic fibers release ACh, which excites the ganglionic neuron through the nicotinic receptor. The axon from the ganglionic neuron—the postganglionic fiber—then projects to a target effector where it will release norepinephrine to bind to an adrenergic receptor, causing a change in the physiology of that organ in keeping with the broad, divergent sympathetic response. The postganglionic connections to sweat glands in the skin and blood vessels supplying skeletal muscle are, however, exceptions; those fibers release ACh onto muscarinic receptors. The sympathetic system has a specialized preganglionic connection to the adrenal medulla that causes epinephrine and norepinephrine to be released into the bloodstream rather than exciting a neuron that contacts an organ directly. This hormonal component means that the sympathetic chemical signal can spread throughout the body very quickly and affect many organ systems at once.
The parasympathetic output is based in the brain stem and sacral spinal cord. Neurons from particular nuclei in the brain stem or from the lateral horn of the sacral spinal cord (preganglionic neurons) project to terminal (intramural) ganglia located close to or within the wall of target effectors. These preganglionic fibers also release ACh onto nicotinic receptors to excite the ganglionic neurons. The postganglionic fibers then contact the target tissues within the organ to release ACh, which binds to muscarinic receptors to induce rest-and-digest responses.
Signaling molecules utilized by the autonomic nervous system are released from axons and can be considered as either neurotransmitters (when they directly interact with the effector) or as hormones (when they are released into the bloodstream). The same molecule, such as norepinephrine, could be considered either a neurotransmitter or a hormone on the basis of whether it is released from a postganglionic sympathetic axon or from the adrenal gland. The synapses in the autonomic system are not always the typical type of connection first described in the neuromuscular junction. Instead of having synaptic end bulbs at the very end of an axonal fiber, they may have swellings—called varicosities—along the length of a fiber so that it makes a network of connections within the target tissue.
15.2 Autonomic Reflexes and Homeostasis
Autonomic nervous system function is based on the visceral reflex. This reflex is similar to the somatic reflex, but the efferent branch is composed of two neurons. The central neuron projects from the spinal cord or brain stem to synapse on the ganglionic neuron that projects to the effector. The afferent branch of the somatic and visceral reflexes is very similar, as many somatic and special senses activate autonomic responses. However, there are visceral senses that do not form part of conscious perception. If a visceral sensation, such as cardiac pain, is strong enough, it will rise to the level of consciousness. However, the sensory homunculus does not provide a representation of the internal structures to the same degree as the surface of the body, so visceral sensations are often experienced as referred pain, such as feelings of pain in the left shoulder and arm in connection with a heart attack.
The role of visceral reflexes is to maintain a balance of function in the organ systems of the body. The two divisions of the autonomic system each play a role in effecting change, usually in competing directions. The sympathetic system increases heart rate, whereas the parasympathetic system decreases heart rate. The sympathetic system dilates the pupil of the eye, whereas the parasympathetic system constricts the pupil. The competing inputs can contribute to the resting tone of the organ system. Heart rate is normally under parasympathetic tone, whereas blood pressure is normally under sympathetic tone. The heart rate is slowed by the autonomic system at rest, whereas blood vessels retain a slight constriction at rest.
In a few systems of the body, the competing input from the two divisions is not the norm. The sympathetic tone of blood vessels is caused by the lack of parasympathetic input to the systemic circulatory system. Only certain regions receive parasympathetic input that relaxes the smooth muscle wall of the blood vessels. Sweat glands are another example, which only receive input from the sympathetic system.
15.3 Central Control
The autonomic system integrates sensory information and higher cognitive processes to generate output, which balances homeostatic mechanisms. The central autonomic structure is the hypothalamus, which coordinates sympathetic and parasympathetic efferent pathways to regulate activities of the organ systems of the body. The majority of hypothalamic output travels through the medial forebrain bundle and the dorsal longitudinal fasciculus to influence brain stem and spinal components of the autonomic nervous system. The medial forebrain bundle also connects the hypothalamus with higher centers of the limbic system where emotion can influence visceral responses. The amygdala is a structure within the limbic system that influences the hypothalamus in the regulation of the autonomic system, as well as the endocrine system.
These higher centers have descending control of the autonomic system through brain stem centers, primarily in the medulla, such as the cardiovascular center. This collection of medullary nuclei regulates cardiac function, as well as blood pressure. Sensory input from the heart, aorta, and carotid sinuses project to these regions of the medulla. The solitary nucleus increases sympathetic tone of the cardiovascular system through the cardiac accelerator and vasomotor nerves. The nucleus ambiguus and the dorsal motor nucleus both contribute fibers to the vagus nerve, which exerts parasympathetic control of the heart by decreasing heart rate.
15.4 Drugs that Affect the Autonomic System
The autonomic system is affected by a number of exogenous agents, including some that are therapeutic and some that are illicit. These drugs affect the autonomic system by mimicking or interfering with the endogenous agents or their receptors. A survey of how different drugs affect autonomic function illustrates the role that the neurotransmitters and hormones play in autonomic function. Drugs can be thought of as chemical tools to effect changes in the system with some precision, based on where those drugs are effective.
Nicotine is not a drug that is used therapeutically, except for smoking cessation. When it is introduced into the body via products, it has broad effects on the autonomic system. Nicotine carries a risk for cardiovascular disease because of these broad effects. The drug stimulates both sympathetic and parasympathetic ganglia at the preganglionic fiber synapse. For most organ systems in the body, the competing input from the two postganglionic fibers will essentially cancel each other out. However, for the cardiovascular system, the results are different. Because there is essentially no parasympathetic influence on blood pressure for the entire body, the sympathetic input is increased by nicotine, causing an increase in blood pressure. Also, the influence that the autonomic system has on the heart is not the same as for other systems. Other organs have smooth muscle or glandular tissue that is activated or inhibited by the autonomic system. Cardiac muscle is intrinsically active and is modulated by the autonomic system. The contradictory signals do not just cancel each other out, they alter the regularity of the heart rate and can cause arrhythmias. Both hypertension and arrhythmias are risk factors for heart disease.
Other drugs affect one division of the autonomic system or the other. The sympathetic system is affected by drugs that mimic the actions of adrenergic molecules (norepinephrine and epinephrine) and are called sympathomimetic drugs. Drugs such as phenylephrine bind to the adrenergic receptors and stimulate target organs just as sympathetic activity would. Other drugs are sympatholytic because they block adrenergic activity and cancel the sympathetic influence on the target organ. Drugs that act on the parasympathetic system also work by either enhancing the postganglionic signal or blocking it. A muscarinic agonist (or parasympathomimetic drug) acts just like ACh released by the parasympathetic postganglionic fiber. Anticholinergic drugs block muscarinic receptors, suppressing parasympathetic interaction with the organ.
Interactive Link Questions
Watch this video to learn more about adrenaline and the fight-or-flight response. When someone is said to have a rush of adrenaline, the image of bungee jumpers or skydivers usually comes to mind. But adrenaline, also known as epinephrine, is an important chemical in coordinating the body’s fight-or-flight response. In this video, you look inside the physiology of the fight-or-flight response, as envisioned for a firefighter. His body’s reaction is the result of the sympathetic division of the autonomic nervous system causing system-wide changes as it prepares for extreme responses. What two changes does adrenaline bring about to help the skeletal muscle response?
2.Watch this video to learn more about the nervous system. As described in this video, the nervous system has a way to deal with threats and stress that is separate from the conscious control of the somatic nervous system. The system comes from a time when threats were about survival, but in the modern age, these responses become part of stress and anxiety. This video describes how the autonomic system is only part of the response to threats, or stressors. What other organ system gets involved, and what part of the brain coordinates the two systems for the entire response, including epinephrine (adrenaline) and cortisol?
3.Read this article to learn about a teenager who experiences a series of spells that suggest a stroke. He undergoes endless tests and seeks input from multiple doctors. In the end, one expert, one question, and a simple blood pressure cuff answers the question. Why would the heart have to beat faster when the teenager changes his body position from lying down to sitting, and then to standing?
4.Watch this video to learn about the pupillary reflexes. The pupillary light reflex involves sensory input through the optic nerve and motor response through the oculomotor nerve to the ciliary ganglion, which projects to the circular fibers of the iris. As shown in this short animation, pupils will constrict to limit the amount of light falling on the retina under bright lighting conditions. What constitutes the afferent and efferent branches of the competing reflex (dilation)?
5.Watch this video to learn about physical responses to emotion. The autonomic system, which is important for regulating the homeostasis of the organ systems, is also responsible for our physiological responses to emotions such as fear. The video summarizes the extent of the body’s reactions and describes several effects of the autonomic system in response to fear. On the basis of what you have already studied about autonomic function, which effect would you expect to be associated with parasympathetic, rather than sympathetic, activity?
6.Watch this video to learn about the side effects of 3-D movies. As discussed in this video, movies that are shot in 3-D can cause motion sickness, which elicits the autonomic symptoms of nausea and sweating. The disconnection between the perceived motion on the screen and the lack of any change in equilibrium stimulates these symptoms. Why do you think sitting close to the screen or right in the middle of the theater makes motion sickness during a 3-D movie worse?
Review Questions
Which of these physiological changes would not be considered part of the sympathetic fight-or-flight response?
- increased heart rate
- increased sweating
- dilated pupils
- increased stomach motility
Which type of fiber could be considered the longest?
- preganglionic parasympathetic
- preganglionic sympathetic
- postganglionic parasympathetic
- postganglionic sympathetic
Which signaling molecule is most likely responsible for an increase in digestive activity?
- epinephrine
- norepinephrine
- acetylcholine
- adrenaline
Which of these cranial nerves contains preganglionic parasympathetic fibers?
- optic, CN II
- facial, CN VII
- trigeminal, CN V
- hypoglossal, CN XII
Which of the following is not a target of a sympathetic preganglionic fiber?
- intermural ganglion
- collateral ganglion
- adrenal gland
- chain ganglion
Which of the following represents a sensory input that is not part of both the somatic and autonomic systems?
- vision
- taste
- baroreception
- proprioception
What is the term for a reflex that does not include a CNS component?
- long reflex
- visceral reflex
- somatic reflex
- short reflex
What neurotransmitter will result in constriction of the pupil?
- norepinephrine
- acetylcholine
- epinephrine
- serotonin
What gland produces a secretion that causes fight-or-flight responses in effectors?
- adrenal medulla
- salivatory gland
- reproductive gland
- thymus
Which of the following is an incorrect pairing?
- norepinephrine dilates the pupil
- epinephrine increases blood pressure
- acetylcholine decreases digestion
- norepinephrine increases heart rate
Which of these locations in the forebrain is the master control center for homeostasis through the autonomic and endocrine systems?
- hypothalamus
- thalamus
- amygdala
- cerebral cortex
Which nerve projects to the hypothalamus to indicate the level of light stimuli in the retina?
- glossopharyngeal
- oculomotor
- optic
- vagus
What region of the limbic lobe is responsible for generating stress responses via the hypothalamus?
- hippocampus
- amygdala
- mammillary bodies
- prefrontal cortex
What is another name for the preganglionic sympathetic fibers that project to the heart?
- solitary tract
- vasomotor nerve
- vagus nerve
- cardiac accelerator nerve
What central fiber tract connects forebrain and brain stem structures with the hypothalamus?
- cardiac accelerator nerve
- medial forebrain bundle
- dorsal longitudinal fasciculus
- corticospinal tract
A drug that affects both divisions of the autonomic system is going to bind to, or block, which type of neurotransmitter receptor?
- nicotinic
- muscarinic
- α-adrenergic
- β-adrenergic
A drug is called an agonist if it ________.
- blocks a receptor
- interferes with neurotransmitter reuptake
- acts like the endogenous neurotransmitter by binding to its receptor
- blocks the voltage-gated calcium ion channel
Which type of drug would be an antidote to atropine poisoning?
- nicotinic agonist
- anticholinergic
- muscarinic agonist
- α-blocker
Which kind of drug would have anti-anxiety effects?
- nicotinic agonist
- anticholinergic
- muscarinic agonist
- α-blocker
Which type of drug could be used to treat asthma by opening airways wider?
- sympatholytic drug
- sympathomimetic drug
- anticholinergic drug
- parasympathomimetic drug
Critical Thinking Questions
In the context of a lioness hunting on the savannah, why would the sympathetic system not activate the digestive system?
28.A target effector, such as the heart, receives input from the sympathetic and parasympathetic systems. What is the actual difference between the sympathetic and parasympathetic divisions at the level of those connections (i.e., at the synapse)?
29.Damage to internal organs will present as pain associated with a particular surface area of the body. Why would something like irritation to the diaphragm, which is between the thoracic and abdominal cavities, feel like pain in the shoulder or neck?
30.Medical practice is paying more attention to the autonomic system in considering disease states. Why would autonomic tone be important in considering cardiovascular disease?
31.Horner’s syndrome is a condition that presents with changes in one eye, such as pupillary constriction and dropping of eyelids, as well as decreased sweating in the face. Why could a tumor in the thoracic cavity have an effect on these autonomic functions?
32.The cardiovascular center is responsible for regulating the heart and blood vessels through homeostatic mechanisms. What tone does each component of the cardiovascular system have? What connections does the cardiovascular center invoke to keep these two systems in their resting tone?
33.Why does smoking increase the risk of heart disease? Provide two reasons based on autonomic function.
34.Why might topical, cosmetic application of atropine or scopolamine from the belladonna plant not cause fatal poisoning, as would occur with ingestion of the plant?
|
oercommons
|
2025-03-18T00:39:12.406389
|
07/23/2019
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/56378/overview",
"title": "Anatomy and Physiology, Regulation, Integration, and Control, The Autonomic Nervous System",
"author": null
}
|
https://oercommons.org/courseware/lesson/56377/overview
|
The Somatic Nervous System
Introduction
Figure 14.1 Too Hot to Touch When high temperature is sensed in the skin, a reflexive withdrawal is initiated by the muscles of the arm. Sensory neurons are activated by a stimulus, which is sent to the central nervous system, and a motor response is sent out to the skeletal muscles that control this movement.
CHAPTER OBJECTIVES
After studying this chapter, you will be able to:
- Describe the components of the somatic nervous system
- Name the modalities and submodalities of the sensory systems
- Distinguish between general and special senses
- Describe regions of the central nervous system that contribute to somatic functions
- Explain the stimulus-response motor pathway
The somatic nervous system is traditionally considered a division within the peripheral nervous system. However, this misses an important point: somatic refers to a functional division, whereas peripheral refers to an anatomic division. The somatic nervous system is responsible for our conscious perception of the environment and for our voluntary responses to that perception by means of skeletal muscles. Peripheral sensory neurons receive input from environmental stimuli, but the neurons that produce motor responses originate in the central nervous system. The distinction between the structures (i.e., anatomy) of the peripheral and central nervous systems and functions (i.e., physiology) of the somatic and autonomic systems can most easily be demonstrated through a simple reflex action. When you touch a hot stove, you pull your hand away. Sensory receptors in the skin sense extreme temperature and the early signs of tissue damage. This triggers an action potential, which travels along the sensory fiber from the skin, through the dorsal spinal root to the spinal cord, and directly activates a ventral horn motor neuron. That neuron sends a signal along its axon to excite the biceps brachii, causing contraction of the muscle and flexion of the forearm at the elbow to withdraw the hand from the hot stove. The withdrawal reflex has more components, such as inhibiting the opposing muscle and balancing posture while the arm is forcefully withdrawn, which will be further explored at the end of this chapter.
The basic withdrawal reflex explained above includes sensory input (the painful stimulus), central processing (the synapse in the spinal cord), and motor output (activation of a ventral motor neuron that causes contraction of the biceps brachii). Expanding the explanation of the withdrawal reflex can include inhibition of the opposing muscle, or cross extension, either of which increase the complexity of the example by involving more central neurons. A collateral branch of the sensory axon would inhibit another ventral horn motor neuron so that the triceps brachii do not contract and slow the withdrawal down. The cross extensor reflex provides a counterbalancing movement on the other side of the body, which requires another collateral of the sensory axon to activate contraction of the extensor muscles in the contralateral limb.
A more complex example of somatic function is conscious muscle movement. For example, reading of this text starts with visual sensory input to the retina, which then projects to the thalamus, and on to the cerebral cortex. A sequence of regions of the cerebral cortex process the visual information, starting in the primary visual cortex of the occipital lobe, and resulting in the conscious perception of these letters. Subsequent cognitive processing results in understanding of the content. As you continue reading, regions of the cerebral cortex in the frontal lobe plan how to move the eyes to follow the lines of text. The output from the cortex causes activity in motor neurons in the brain stem that cause movement of the extraocular muscles through the third, fourth, and sixth cranial nerves. This example also includes sensory input (the retinal projection to the thalamus), central processing (the thalamus and subsequent cortical activity), and motor output (activation of neurons in the brain stem that lead to coordinated contraction of extraocular muscles).
Sensory Perception
- Describe different types of sensory receptors
- Describe the structures responsible for the special senses of taste, smell, hearing, balance, and vision
- Distinguish how different tastes are transduced
- Describe the means of mechanoreception for hearing and balance
- List the supporting structures around the eye and describe the structure of the eyeball
- Describe the processes of phototransduction
A major role of sensory receptors is to help us learn about the environment around us, or about the state of our internal environment. Stimuli from varying sources, and of different types, are received and changed into the electrochemical signals of the nervous system. This occurs when a stimulus changes the cell membrane potential of a sensory neuron. The stimulus causes the sensory cell to produce an action potential that is relayed into the central nervous system (CNS), where it is integrated with other sensory information—or sometimes higher cognitive functions—to become a conscious perception of that stimulus. The central integration may then lead to a motor response.
Describing sensory function with the term sensation or perception is a deliberate distinction. Sensation is the activation of sensory receptor cells at the level of the stimulus. Perception is the central processing of sensory stimuli into a meaningful pattern. Perception is dependent on sensation, but not all sensations are perceived. Receptors are the cells or structures that detect sensations. A receptor cell is changed directly by a stimulus. A transmembrane protein receptor is a protein in the cell membrane that mediates a physiological change in a neuron, most often through the opening of ion channels or changes in the cell signaling processes. Transmembrane receptors are activated by chemicals called ligands. For example, a molecule in food can serve as a ligand for taste receptors. Other transmembrane proteins, which are not accurately called receptors, are sensitive to mechanical or thermal changes. Physical changes in these proteins increase ion flow across the membrane, and can generate an action potential or a graded potential in the sensory neurons.
Sensory Receptors
Stimuli in the environment activate specialized receptor cells in the peripheral nervous system. Different types of stimuli are sensed by different types of receptor cells. Receptor cells can be classified into types on the basis of three different criteria: cell type, position, and function. Receptors can be classified structurally on the basis of cell type and their position in relation to stimuli they sense. They can also be classified functionally on the basis of the transduction of stimuli, or how the mechanical stimulus, light, or chemical changed the cell membrane potential.
Structural Receptor Types
The cells that interpret information about the environment can be either (1) a neuron that has a free nerve ending, with dendrites embedded in tissue that would receive a sensation; (2) a neuron that has an encapsulated ending in which the sensory nerve endings are encapsulated in connective tissue that enhances their sensitivity; or (3) a specialized receptor cell, which has distinct structural components that interpret a specific type of stimulus (Figure 14.2). The pain and temperature receptors in the dermis of the skin are examples of neurons that have free nerve endings. Also located in the dermis of the skin are lamellated corpuscles, neurons with encapsulated nerve endings that respond to pressure and touch. The cells in the retina that respond to light stimuli are an example of a specialized receptor, a photoreceptor.
Figure 14.2 Receptor Classification by Cell Type Receptor cell types can be classified on the basis of their structure. Sensory neurons can have either (a) free nerve endings or (b) encapsulated endings. Photoreceptors in the eyes, such as rod cells, are examples of (c) specialized receptor cells. These cells release neurotransmitters onto a bipolar cell, which then synapses with the optic nerve neurons.
Another way that receptors can be classified is based on their location relative to the stimuli. An exteroceptor is a receptor that is located near a stimulus in the external environment, such as the somatosensory receptors that are located in the skin. An interoceptor is one that interprets stimuli from internal organs and tissues, such as the receptors that sense the increase in blood pressure in the aorta or carotid sinus. Finally, a proprioceptor is a receptor located near a moving part of the body, such as a muscle, that interprets the positions of the tissues as they move.
Functional Receptor Types
A third classification of receptors is by how the receptor transduces stimuli into membrane potential changes. Stimuli are of three general types. Some stimuli are ions and macromolecules that affect transmembrane receptor proteins when these chemicals diffuse across the cell membrane. Some stimuli are physical variations in the environment that affect receptor cell membrane potentials. Other stimuli include the electromagnetic radiation from visible light. For humans, the only electromagnetic energy that is perceived by our eyes is visible light. Some other organisms have receptors that humans lack, such as the heat sensors of snakes, the ultraviolet light sensors of bees, or magnetic receptors in migratory birds.
Receptor cells can be further categorized on the basis of the type of stimuli they transduce. Chemical stimuli can be interpreted by a chemoreceptor that interprets chemical stimuli, such as an object’s taste or smell. Osmoreceptors respond to solute concentrations of body fluids. Additionally, pain is primarily a chemical sense that interprets the presence of chemicals from tissue damage, or similar intense stimuli, through a nociceptor. Physical stimuli, such as pressure and vibration, as well as the sensation of sound and body position (balance), are interpreted through a mechanoreceptor. Another physical stimulus that has its own type of receptor is temperature, which is sensed through a thermoreceptor that is either sensitive to temperatures above (heat) or below (cold) normal body temperature.
Sensory Modalities
Ask anyone what the senses are, and they are likely to list the five major senses—taste, smell, touch, hearing, and sight. However, these are not all of the senses. The most obvious omission from this list is balance. Also, what is referred to simply as touch can be further subdivided into pressure, vibration, stretch, and hair-follicle position, on the basis of the type of mechanoreceptors that perceive these touch sensations. Other overlooked senses include temperature perception by thermoreceptors and pain perception by nociceptors.
Within the realm of physiology, senses can be classified as either general or specific. A general sense is one that is distributed throughout the body and has receptor cells within the structures of other organs. Mechanoreceptors in the skin, muscles, or the walls of blood vessels are examples of this type. General senses often contribute to the sense of touch, as described above, or to proprioception (body movement) and kinesthesia (body movement), or to a visceral sense, which is most important to autonomic functions. A special sense is one that has a specific organ devoted to it, namely the eye, inner ear, tongue, or nose.
Each of the senses is referred to as a sensory modality. Modality refers to the way that information is encoded, which is similar to the idea of transduction. The main sensory modalities can be described on the basis of how each is transduced. The chemical senses are taste and smell. The general sense that is usually referred to as touch includes chemical sensation in the form of nociception, or pain. Pressure, vibration, muscle stretch, and the movement of hair by an external stimulus, are all sensed by mechanoreceptors. Hearing and balance are also sensed by mechanoreceptors. Finally, vision involves the activation of photoreceptors.
Listing all the different sensory modalities, which can number as many as 17, involves separating the five major senses into more specific categories, or submodalities, of the larger sense. An individual sensory modality represents the sensation of a specific type of stimulus. For example, the general sense of touch, which is known as somatosensation, can be separated into light pressure, deep pressure, vibration, itch, pain, temperature, or hair movement.
Gustation (Taste)
Only a few recognized submodalities exist within the sense of taste, or gustation. Until recently, only four tastes were recognized: sweet, salty, sour, and bitter. Research at the turn of the 20th century led to recognition of the fifth taste, umami, during the mid-1980s. Umami is a Japanese word that means “delicious taste,” and is often translated to mean savory. Very recent research has suggested that there may also be a sixth taste for fats, or lipids.
Gustation is the special sense associated with the tongue. The surface of the tongue, along with the rest of the oral cavity, is lined by a stratified squamous epithelium. Raised bumps called papillae (singular = papilla) contain the structures for gustatory transduction. There are four types of papillae, based on their appearance (Figure 14.3): circumvallate, foliate, filiform, and fungiform. Within the structure of the papillae are taste buds that contain specialized gustatory receptor cells for the transduction of taste stimuli. These receptor cells are sensitive to the chemicals contained within foods that are ingested, and they release neurotransmitters based on the amount of the chemical in the food. Neurotransmitters from the gustatory cells can activate sensory neurons in the facial, glossopharyngeal, and vagus cranial nerves.
Figure 14.3 The Tongue The tongue is covered with small bumps, called papillae, which contain taste buds that are sensitive to chemicals in ingested food or drink. Different types of papillae are found in different regions of the tongue. The taste buds contain specialized gustatory receptor cells that respond to chemical stimuli dissolved in the saliva. These receptor cells activate sensory neurons that are part of the facial and glossopharyngeal nerves. LM × 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
Salty taste is simply the perception of sodium ions (Na+) in the saliva. When you eat something salty, the salt crystals dissociate into the component ions Na+ and Cl–, which dissolve into the saliva in your mouth. The Na+ concentration becomes high outside the gustatory cells, creating a strong concentration gradient that drives the diffusion of the ion into the cells. The entry of Na+into these cells results in the depolarization of the cell membrane and the generation of a receptor potential.
Sour taste is the perception of H+ concentration. Just as with sodium ions in salty flavors, these hydrogen ions enter the cell and trigger depolarization. Sour flavors are, essentially, the perception of acids in our food. Increasing hydrogen ion concentrations in the saliva (lowering saliva pH) triggers progressively stronger graded potentials in the gustatory cells. For example, orange juice—which contains citric acid—will taste sour because it has a pH value of approximately 3. Of course, it is often sweetened so that the sour taste is masked.
The first two tastes (salty and sour) are triggered by the cations Na+ and H+. The other tastes result from food molecules binding to a G protein–coupled receptor. A G protein signal transduction system ultimately leads to depolarization of the gustatory cell. The sweet taste is the sensitivity of gustatory cells to the presence of glucose dissolved in the saliva. Other monosaccharides such as fructose, or artificial sweeteners such as aspartame (NutraSweet™), saccharine, or sucralose (Splenda™) also activate the sweet receptors. The affinity for each of these molecules varies, and some will taste sweeter than glucose because they bind to the G protein–coupled receptor differently.
Bitter taste is similar to sweet in that food molecules bind to G protein–coupled receptors. However, there are a number of different ways in which this can happen because there are a large diversity of bitter-tasting molecules. Some bitter molecules depolarize gustatory cells, whereas others hyperpolarize gustatory cells. Likewise, some bitter molecules increase G protein activation within the gustatory cells, whereas other bitter molecules decrease G protein activation. The specific response depends on which molecule is binding to the receptor.
One major group of bitter-tasting molecules are alkaloids. Alkaloids are nitrogen containing molecules that are commonly found in bitter-tasting plant products, such as coffee, hops (in beer), tannins (in wine), tea, and aspirin. By containing toxic alkaloids, the plant is less susceptible to microbe infection and less attractive to herbivores.
Therefore, the function of bitter taste may primarily be related to stimulating the gag reflex to avoid ingesting poisons. Because of this, many bitter foods that are normally ingested are often combined with a sweet component to make them more palatable (cream and sugar in coffee, for example). The highest concentration of bitter receptors appear to be in the posterior tongue, where a gag reflex could still spit out poisonous food.
The taste known as umami is often referred to as the savory taste. Like sweet and bitter, it is based on the activation of G protein–coupled receptors by a specific molecule. The molecule that activates this receptor is the amino acid L-glutamate. Therefore, the umami flavor is often perceived while eating protein-rich foods. Not surprisingly, dishes that contain meat are often described as savory.
Once the gustatory cells are activated by the taste molecules, they release neurotransmitters onto the dendrites of sensory neurons. These neurons are part of the facial and glossopharyngeal cranial nerves, as well as a component within the vagus nerve dedicated to the gag reflex. The facial nerve connects to taste buds in the anterior third of the tongue. The glossopharyngeal nerve connects to taste buds in the posterior two thirds of the tongue. The vagus nerve connects to taste buds in the extreme posterior of the tongue, verging on the pharynx, which are more sensitive to noxious stimuli such as bitterness.
INTERACTIVE LINK
Watch this video to learn about Dr. Danielle Reed of the Monell Chemical Senses Center in Philadelphia, Pennsylvania, who became interested in science at an early age because of her sensory experiences. She recognized that her sense of taste was unique compared with other people she knew. Now, she studies the genetic differences between people and their sensitivities to taste stimuli. In the video, there is a brief image of a person sticking out their tongue, which has been covered with a colored dye. This is how Dr. Reed is able to visualize and count papillae on the surface of the tongue. People fall into two groups known as “tasters” and “non-tasters” based on the density of papillae on their tongue, which also indicates the number of taste buds. Non-tasters can taste food, but they are not as sensitive to certain tastes, such as bitterness. Dr. Reed discovered that she is a non-taster, which explains why she perceived bitterness differently than other people she knew. Are you very sensitive to tastes? Can you see any similarities among the members of your family?
Olfaction (Smell)
Like taste, the sense of smell, or olfaction, is also responsive to chemical stimuli. The olfactory receptor neurons are located in a small region within the superior nasal cavity (Figure 14.4). This region is referred to as the olfactory epithelium and contains bipolar sensory neurons. Each olfactory sensory neuron has dendrites that extend from the apical surface of the epithelium into the mucus lining the cavity. As airborne molecules are inhaled through the nose, they pass over the olfactory epithelial region and dissolve into the mucus. These odorant molecules bind to proteins that keep them dissolved in the mucus and help transport them to the olfactory dendrites. The odorant–protein complex binds to a receptor protein within the cell membrane of an olfactory dendrite. These receptors are G protein–coupled, and will produce a graded membrane potential in the olfactory neurons.
The axon of an olfactory neuron extends from the basal surface of the epithelium, through an olfactory foramen in the cribriform plate of the ethmoid bone, and into the brain. The group of axons called the olfactory tract connect to the olfactory bulb on the ventral surface of the frontal lobe. From there, the axons split to travel to several brain regions. Some travel to the cerebrum, specifically to the primary olfactory cortex that is located in the inferior and medial areas of the temporal lobe. Others project to structures within the limbic system and hypothalamus, where smells become associated with long-term memory and emotional responses. This is how certain smells trigger emotional memories, such as the smell of food associated with one’s birthplace. Smell is the one sensory modality that does not synapse in the thalamus before connecting to the cerebral cortex. This intimate connection between the olfactory system and the cerebral cortex is one reason why smell can be a potent trigger of memories and emotion.
The nasal epithelium, including the olfactory cells, can be harmed by airborne toxic chemicals. Therefore, the olfactory neurons are regularly replaced within the nasal epithelium, after which the axons of the new neurons must find their appropriate connections in the olfactory bulb. These new axons grow along the axons that are already in place in the cranial nerve.
Figure 14.4 The Olfactory System (a) The olfactory system begins in the peripheral structures of the nasal cavity. (b) The olfactory receptor neurons are within the olfactory epithelium. (c) Axons of the olfactory receptor neurons project through the cribriform plate of the ethmoid bone and synapse with the neurons of the olfactory bulb (tissue source: simian). LM × 812. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
DISORDERS OF THE...
Olfactory System: Anosmia
Blunt force trauma to the face, such as that common in many car accidents, can lead to the loss of the olfactory nerve, and subsequently, loss of the sense of smell. This condition is known as anosmia. When the frontal lobe of the brain moves relative to the ethmoid bone, the olfactory tract axons may be sheared apart. Professional fighters often experience anosmia because of repeated trauma to face and head. In addition, certain pharmaceuticals, such as antibiotics, can cause anosmia by killing all the olfactory neurons at once. If no axons are in place within the olfactory nerve, then the axons from newly formed olfactory neurons have no guide to lead them to their connections within the olfactory bulb. There are temporary causes of anosmia, as well, such as those caused by inflammatory responses related to respiratory infections or allergies.
Loss of the sense of smell can result in food tasting bland. A person with an impaired sense of smell may require additional spice and seasoning levels for food to be tasted. Anosmia may also be related to some presentations of mild depression, because the loss of enjoyment of food may lead to a general sense of despair.
The ability of olfactory neurons to replace themselves decreases with age, leading to age-related anosmia. This explains why some elderly people salt their food more than younger people do. However, this increased sodium intake can increase blood volume and blood pressure, increasing the risk of cardiovascular diseases in the elderly.
Audition (Hearing)
Hearing, or audition, is the transduction of sound waves into a neural signal that is made possible by the structures of the ear (Figure 14.5). The large, fleshy structure on the lateral aspect of the head is known as the auricle. Some sources will also refer to this structure as the pinna, though that term is more appropriate for a structure that can be moved, such as the external ear of a cat. The C-shaped curves of the auricle direct sound waves toward the auditory canal. The canal enters the skull through the external auditory meatus of the temporal bone. At the end of the auditory canal is the tympanic membrane, or ear drum, which vibrates after it is struck by sound waves. The auricle, ear canal, and tympanic membrane are often referred to as the external ear. The middle ear consists of a space spanned by three small bones called the ossicles. The three ossicles are the malleus, incus, and stapes, which are Latin names that roughly translate to hammer, anvil, and stirrup. The malleus is attached to the tympanic membrane and articulates with the incus. The incus, in turn, articulates with the stapes. The stapes is then attached to the inner ear, where the sound waves will be transduced into a neural signal. The middle ear is connected to the pharynx through the Eustachian tube, which helps equilibrate air pressure across the tympanic membrane. The tube is normally closed but will pop open when the muscles of the pharynx contract during swallowing or yawning.
Figure 14.5 Structures of the Ear The external ear contains the auricle, ear canal, and tympanic membrane. The middle ear contains the ossicles and is connected to the pharynx by the Eustachian tube. The inner ear contains the cochlea and vestibule, which are responsible for audition and equilibrium, respectively.
The inner ear is often described as a bony labyrinth, as it is composed of a series of canals embedded within the temporal bone. It has two separate regions, the cochlea and the vestibule, which are responsible for hearing and balance, respectively. The neural signals from these two regions are relayed to the brain stem through separate fiber bundles. However, these two distinct bundles travel together from the inner ear to the brain stem as the vestibulocochlear nerve. Sound is transduced into neural signals within the cochlear region of the inner ear, which contains the sensory neurons of the spiral ganglia. These ganglia are located within the spiral-shaped cochlea of the inner ear. The cochlea is attached to the stapes through the oval window.
The oval window is located at the beginning of a fluid-filled tube within the cochlea called the scala vestibuli. The scala vestibuli extends from the oval window, travelling above the cochlear duct, which is the central cavity of the cochlea that contains the sound-transducing neurons. At the uppermost tip of the cochlea, the scala vestibuli curves over the top of the cochlear duct. The fluid-filled tube, now called the scala tympani, returns to the base of the cochlea, this time travelling under the cochlear duct. The scala tympani ends at the round window, which is covered by a membrane that contains the fluid within the scala. As vibrations of the ossicles travel through the oval window, the fluid of the scala vestibuli and scala tympani moves in a wave-like motion. The frequency of the fluid waves match the frequencies of the sound waves (Figure 14.6). The membrane covering the round window will bulge out or pucker in with the movement of the fluid within the scala tympani.
Figure 14.6 Transmission of Sound Waves to Cochlea A sound wave causes the tympanic membrane to vibrate. This vibration is amplified as it moves across the malleus, incus, and stapes. The amplified vibration is picked up by the oval window causing pressure waves in the fluid of the scala vestibuli and scala tympani. The complexity of the pressure waves is determined by the changes in amplitude and frequency of the sound waves entering the ear.
A cross-sectional view of the cochlea shows that the scala vestibuli and scala tympani run along both sides of the cochlear duct (Figure 14.7). The cochlear duct contains several organs of Corti, which tranduce the wave motion of the two scala into neural signals. The organs of Corti lie on top of the basilar membrane, which is the side of the cochlear duct located between the organs of Corti and the scala tympani. As the fluid waves move through the scala vestibuli and scala tympani, the basilar membrane moves at a specific spot, depending on the frequency of the waves. Higher frequency waves move the region of the basilar membrane that is close to the base of the cochlea. Lower frequency waves move the region of the basilar membrane that is near the tip of the cochlea.
Figure 14.7 Cross Section of the Cochlea The three major spaces within the cochlea are highlighted. The scala tympani and scala vestibuli lie on either side of the cochlear duct. The organ of Corti, containing the mechanoreceptor hair cells, is adjacent to the scala tympani, where it sits atop the basilar membrane.
The organs of Corti contain hair cells, which are named for the hair-like stereocilia extending from the cell’s apical surfaces (Figure 14.8). The stereocilia are an array of microvilli-like structures arranged from tallest to shortest. Protein fibers tether adjacent hairs together within each array, such that the array will bend in response to movements of the basilar membrane. The stereocilia extend up from the hair cells to the overlying tectorial membrane, which is attached medially to the organ of Corti. When the pressure waves from the scala move the basilar membrane, the tectorial membrane slides across the stereocilia. This bends the stereocilia either toward or away from the tallest member of each array. When the stereocilia bend toward the tallest member of their array, tension in the protein tethers opens ion channels in the hair cell membrane. This will depolarize the hair cell membrane, triggering nerve impulses that travel down the afferent nerve fibers attached to the hair cells. When the stereocilia bend toward the shortest member of their array, the tension on the tethers slackens and the ion channels close. When no sound is present, and the stereocilia are standing straight, a small amount of tension still exists on the tethers, keeping the membrane potential of the hair cell slightly depolarized.
Figure 14.8 Hair Cell The hair cell is a mechanoreceptor with an array of stereocilia emerging from its apical surface. The stereocilia are tethered together by proteins that open ion channels when the array is bent toward the tallest member of their array, and closed when the array is bent toward the shortest member of their array.
Figure 14.9 Cochlea and Organ of Corti LM × 412. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
INTERACTIVE LINK
View the University of Michigan WebScope to explore the tissue sample in greater detail. The basilar membrane is the thin membrane that extends from the central core of the cochlea to the edge. What is anchored to this membrane so that they can be activated by movement of the fluids within the cochlea?
As stated above, a given region of the basilar membrane will only move if the incoming sound is at a specific frequency. Because the tectorial membrane only moves where the basilar membrane moves, the hair cells in this region will also only respond to sounds of this specific frequency. Therefore, as the frequency of a sound changes, different hair cells are activated all along the basilar membrane. The cochlea encodes auditory stimuli for frequencies between 20 and 20,000 Hz, which is the range of sound that human ears can detect. The unit of Hertz measures the frequency of sound waves in terms of cycles produced per second. Frequencies as low as 20 Hz are detected by hair cells at the apex, or tip, of the cochlea. Frequencies in the higher ranges of 20 KHz are encoded by hair cells at the base of the cochlea, close to the round and oval windows (Figure 14.10). Most auditory stimuli contain a mixture of sounds at a variety of frequencies and intensities (represented by the amplitude of the sound wave). The hair cells along the length of the cochlear duct, which are each sensitive to a particular frequency, allow the cochlea to separate auditory stimuli by frequency, just as a prism separates visible light into its component colors.
Figure 14.10 Frequency Coding in the Cochlea The standing sound wave generated in the cochlea by the movement of the oval window deflects the basilar membrane on the basis of the frequency of sound. Therefore, hair cells at the base of the cochlea are activated only by high frequencies, whereas those at the apex of the cochlea are activated only by low frequencies.
INTERACTIVE LINK
Watch this video to learn more about how the structures of the ear convert sound waves into a neural signal by moving the “hairs,” or stereocilia, of the cochlear duct. Specific locations along the length of the duct encode specific frequencies, or pitches. The brain interprets the meaning of the sounds we hear as music, speech, noise, etc. Which ear structures are responsible for the amplification and transfer of sound from the external ear to the inner ear?
INTERACTIVE LINK
Watch this animation to learn more about the inner ear and to see the cochlea unroll, with the base at the back of the image and the apex at the front. Specific wavelengths of sound cause specific regions of the basilar membrane to vibrate, much like the keys of a piano produce sound at different frequencies. Based on the animation, where do frequencies—from high to low pitches—cause activity in the hair cells within the cochlear duct?
Equilibrium (Balance)
Along with audition, the inner ear is responsible for encoding information about equilibrium, the sense of balance. A similar mechanoreceptor—a hair cell with stereocilia—senses head position, head movement, and whether our bodies are in motion. These cells are located within the vestibule of the inner ear. Head position is sensed by the utricle and saccule, whereas head movement is sensed by the semicircular canals. The neural signals generated in the vestibular ganglion are transmitted through the vestibulocochlear nerve to the brain stem and cerebellum.
The utricle and saccule are both largely composed of macula tissue (plural = maculae). The macula is composed of hair cells surrounded by support cells. The stereocilia of the hair cells extend into a viscous gel called the otolithic membrane (Figure 14.11). On top of the otolithic membrane is a layer of calcium carbonate crystals, called otoliths. The otoliths essentially make the otolithic membrane top-heavy. The otolithic membrane moves separately from the macula in response to head movements. Tilting the head causes the otolithic membrane to slide over the macula in the direction of gravity. The moving otolithic membrane, in turn, bends the sterocilia, causing some hair cells to depolarize as others hyperpolarize. The exact position of the head is interpreted by the brain based on the pattern of hair-cell depolarization.
Figure 14.11 Linear Acceleration Coding by Maculae The maculae are specialized for sensing linear acceleration, such as when gravity acts on the tilting head, or if the head starts moving in a straight line. The difference in inertia between the hair cell stereocilia and the otolithic membrane in which they are embedded leads to a shearing force that causes the stereocilia to bend in the direction of that linear acceleration.
The semicircular canals are three ring-like extensions of the vestibule. One is oriented in the horizontal plane, whereas the other two are oriented in the vertical plane. The anterior and posterior vertical canals are oriented at approximately 45 degrees relative to the sagittal plane (Figure 14.12). The base of each semicircular canal, where it meets with the vestibule, connects to an enlarged region known as the ampulla. The ampulla contains the hair cells that respond to rotational movement, such as turning the head while saying “no.” The stereocilia of these hair cells extend into the cupula, a membrane that attaches to the top of the ampulla. As the head rotates in a plane parallel to the semicircular canal, the fluid lags, deflecting the cupula in the direction opposite to the head movement. The semicircular canals contain several ampullae, with some oriented horizontally and others oriented vertically. By comparing the relative movements of both the horizontal and vertical ampullae, the vestibular system can detect the direction of most head movements within three-dimensional (3-D) space.
Figure 14.12 Rotational Coding by Semicircular Canals Rotational movement of the head is encoded by the hair cells in the base of the semicircular canals. As one of the canals moves in an arc with the head, the internal fluid moves in the opposite direction, causing the cupula and stereocilia to bend. The movement of two canals within a plane results in information about the direction in which the head is moving, and activation of all six canals can give a very precise indication of head movement in three dimensions.
Somatosensation (Touch)
Somatosensation is considered a general sense, as opposed to the special senses discussed in this section. Somatosensation is the group of sensory modalities that are associated with touch, proprioception, and interoception. These modalities include pressure, vibration, light touch, tickle, itch, temperature, pain, proprioception, and kinesthesia. This means that its receptors are not associated with a specialized organ, but are instead spread throughout the body in a variety of organs. Many of the somatosensory receptors are located in the skin, but receptors are also found in muscles, tendons, joint capsules, ligaments, and in the walls of visceral organs.
Two types of somatosensory signals that are transduced by free nerve endings are pain and temperature. These two modalities use thermoreceptors and nociceptors to transduce temperature and pain stimuli, respectively. Temperature receptors are stimulated when local temperatures differ from body temperature. Some thermoreceptors are sensitive to just cold and others to just heat. Nociception is the sensation of potentially damaging stimuli. Mechanical, chemical, or thermal stimuli beyond a set threshold will elicit painful sensations. Stressed or damaged tissues release chemicals that activate receptor proteins in the nociceptors. For example, the sensation of heat associated with spicy foods involves capsaicin, the active molecule in hot peppers. Capsaicin molecules bind to a transmembrane ion channel in nociceptors that is sensitive to temperatures above 37°C. The dynamics of capsaicin binding with this transmembrane ion channel is unusual in that the molecule remains bound for a long time. Because of this, it will decrease the ability of other stimuli to elicit pain sensations through the activated nociceptor. For this reason, capsaicin can be used as a topical analgesic, such as in products such as Icy Hot™.
If you drag your finger across a textured surface, the skin of your finger will vibrate. Such low frequency vibrations are sensed by mechanoreceptors called Merkel cells, also known as type I cutaneous mechanoreceptors. Merkel cells are located in the stratum basale of the epidermis. Deep pressure and vibration is transduced by lamellated (Pacinian) corpuscles, which are receptors with encapsulated endings found deep in the dermis, or subcutaneous tissue. Light touch is transduced by the encapsulated endings known as tactile (Meissner) corpuscles. Follicles are also wrapped in a plexus of nerve endings known as the hair follicle plexus. These nerve endings detect the movement of hair at the surface of the skin, such as when an insect may be walking along the skin. Stretching of the skin is transduced by stretch receptors known as bulbous corpuscles. Bulbous corpuscles are also known as Ruffini corpuscles, or type II cutaneous mechanoreceptors.
Other somatosensory receptors are found in the joints and muscles. Stretch receptors monitor the stretching of tendons, muscles, and the components of joints. For example, have you ever stretched your muscles before or after exercise and noticed that you can only stretch so far before your muscles spasm back to a less stretched state? This spasm is a reflex that is initiated by stretch receptors to avoid muscle tearing. Such stretch receptors can also prevent over-contraction of a muscle. In skeletal muscle tissue, these stretch receptors are called muscle spindles. Golgi tendon organs similarly transduce the stretch levels of tendons. Bulbous corpuscles are also present in joint capsules, where they measure stretch in the components of the skeletal system within the joint. The types of nerve endings, their locations, and the stimuli they transduce are presented in Table 14.1.
Mechanoreceptors of Somatosensation
| Name | Historical (eponymous) name | Location(s) | Stimuli |
|---|---|---|---|
| Free nerve endings | * | Dermis, cornea, tongue, joint capsules, visceral organs | Pain, temperature, mechanical deformation |
| Mechanoreceptors | Merkel’s discs | Epidermal–dermal junction, mucosal membranes | Low frequency vibration (5–15 Hz) |
| Bulbous corpuscle | Ruffini’s corpuscle | Dermis, joint capsules | Stretch |
| Tactile corpuscle | Meissner’s corpuscle | Papillary dermis, especially in the fingertips and lips | Light touch, vibrations below 50 Hz |
| Lamellated corpuscle | Pacinian corpuscle | Deep dermis, subcutaneous tissue | Deep pressure, high-frequency vibration (around 250 Hz) |
| Hair follicle plexus | * | Wrapped around hair follicles in the dermis | Movement of hair |
| Muscle spindle | * | In line with skeletal muscle fibers | Muscle contraction and stretch |
| Tendon stretch organ | Golgi tendon organ | In line with tendons | Stretch of tendons |
Table 14.1 *No corresponding eponymous name.
Vision
Vision is the special sense of sight that is based on the transduction of light stimuli received through the eyes. The eyes are located within either orbit in the skull. The bony orbits surround the eyeballs, protecting them and anchoring the soft tissues of the eye (Figure 14.13). The eyelids, with lashes at their leading edges, help to protect the eye from abrasions by blocking particles that may land on the surface of the eye. The inner surface of each lid is a thin membrane known as the palpebral conjunctiva. The conjunctiva extends over the white areas of the eye (the sclera), connecting the eyelids to the eyeball. Tears are produced by the lacrimal gland, located beneath the lateral edges of the nose. Tears produced by this gland flow through the lacrimal duct to the medial corner of the eye, where the tears flow over the conjunctiva, washing away foreign particles.
Figure 14.13 The Eye in the Orbit The eye is located within the orbit and surrounded by soft tissues that protect and support its function. The orbit is surrounded by cranial bones of the skull.
Movement of the eye within the orbit is accomplished by the contraction of six extraocular muscles that originate from the bones of the orbit and insert into the surface of the eyeball (Figure 14.14). Four of the muscles are arranged at the cardinal points around the eye and are named for those locations. They are the superior rectus, medial rectus, inferior rectus, and lateral rectus. When each of these muscles contract, the eye moves toward the contracting muscle. For example, when the superior rectus contracts, the eye rotates to look up. The superior oblique originates at the posterior orbit, near the origin of the four rectus muscles. However, the tendon of the oblique muscles threads through a pulley-like piece of cartilage known as the trochlea. The tendon inserts obliquely into the superior surface of the eye. The angle of the tendon through the trochlea means that contraction of the superior oblique rotates the eye medially. The inferior oblique muscle originates from the floor of the orbit and inserts into the inferolateral surface of the eye. When it contracts, it laterally rotates the eye, in opposition to the superior oblique. Rotation of the eye by the two oblique muscles is necessary because the eye is not perfectly aligned on the sagittal plane. When the eye looks up or down, the eye must also rotate slightly to compensate for the superior rectus pulling at approximately a 20-degree angle, rather than straight up. The same is true for the inferior rectus, which is compensated by contraction of the inferior oblique. A seventh muscle in the orbit is the levator palpebrae superioris, which is responsible for elevating and retracting the upper eyelid, a movement that usually occurs in concert with elevation of the eye by the superior rectus (see Figure 14.13).
The extraocular muscles are innervated by three cranial nerves. The lateral rectus, which causes abduction of the eye, is innervated by the abducens nerve. The superior oblique is innervated by the trochlear nerve. All of the other muscles are innervated by the oculomotor nerve, as is the levator palpebrae superioris. The motor nuclei of these cranial nerves connect to the brain stem, which coordinates eye movements.
Figure 14.14 Extraocular Muscles The extraocular muscles move the eye within the orbit.
The eye itself is a hollow sphere composed of three layers of tissue. The outermost layer is the fibrous tunic, which includes the white sclera and clear cornea. The sclera accounts for five sixths of the surface of the eye, most of which is not visible, though humans are unique compared with many other species in having so much of the “white of the eye” visible (Figure 14.15). The transparent cornea covers the anterior tip of the eye and allows light to enter the eye. The middle layer of the eye is the vascular tunic, which is mostly composed of the choroid, ciliary body, and iris. The choroid is a layer of highly vascularized connective tissue that provides a blood supply to the eyeball. The choroid is posterior to the ciliary body, a muscular structure that is attached to the lens by suspensory ligaments, or zonule fibers. These two structures bend the lens, allowing it to focus light on the back of the eye. Overlaying the ciliary body, and visible in the anterior eye, is the iris—the colored part of the eye. The iris is a smooth muscle that opens or closes the pupil, which is the hole at the center of the eye that allows light to enter. The iris constricts the pupil in response to bright light and dilates the pupil in response to dim light. The innermost layer of the eye is the neural tunic, or retina, which contains the nervous tissue responsible for photoreception.
The eye is also divided into two cavities: the anterior cavity and the posterior cavity. The anterior cavity is the space between the cornea and lens, including the iris and ciliary body. It is filled with a watery fluid called the aqueous humor. The posterior cavity is the space behind the lens that extends to the posterior side of the interior eyeball, where the retina is located. The posterior cavity is filled with a more viscous fluid called the vitreous humor.
The retina is composed of several layers and contains specialized cells for the initial processing of visual stimuli. The photoreceptors (rods and cones) change their membrane potential when stimulated by light energy. The change in membrane potential alters the amount of neurotransmitter that the photoreceptor cells release onto bipolar cells in the outer synaptic layer. It is the bipolar cell in the retina that connects a photoreceptor to a retinal ganglion cell (RGC) in the inner synaptic layer. There, amacrine cells additionally contribute to retinal processing before an action potential is produced by the RGC. The axons of RGCs, which lie at the innermost layer of the retina, collect at the optic disc and leave the eye as the optic nerve(see Figure 14.15). Because these axons pass through the retina, there are no photoreceptors at the very back of the eye, where the optic nerve begins. This creates a “blind spot” in the retina, and a corresponding blind spot in our visual field.
Figure 14.15 Structure of the Eye The sphere of the eye can be divided into anterior and posterior chambers. The wall of the eye is composed of three layers: the fibrous tunic, vascular tunic, and neural tunic. Within the neural tunic is the retina, with three layers of cells and two synaptic layers in between. The center of the retina has a small indentation known as the fovea.
Note that the photoreceptors in the retina (rods and cones) are located behind the axons, RGCs, bipolar cells, and retinal blood vessels. A significant amount of light is absorbed by these structures before the light reaches the photoreceptor cells. However, at the exact center of the retina is a small area known as the fovea. At the fovea, the retina lacks the supporting cells and blood vessels, and only contains photoreceptors. Therefore, visual acuity, or the sharpness of vision, is greatest at the fovea. This is because the fovea is where the least amount of incoming light is absorbed by other retinal structures (see Figure 14.15). As one moves in either direction from this central point of the retina, visual acuity drops significantly. In addition, each photoreceptor cell of the fovea is connected to a single RGC. Therefore, this RGC does not have to integrate inputs from multiple photoreceptors, which reduces the accuracy of visual transduction. Toward the edges of the retina, several photoreceptors converge on RGCs (through the bipolar cells) up to a ratio of 50 to 1. The difference in visual acuity between the fovea and peripheral retina is easily evidenced by looking directly at a word in the middle of this paragraph. The visual stimulus in the middle of the field of view falls on the fovea and is in the sharpest focus. Without moving your eyes off that word, notice that words at the beginning or end of the paragraph are not in focus. The images in your peripheral vision are focused by the peripheral retina, and have vague, blurry edges and words that are not as clearly identified. As a result, a large part of the neural function of the eyes is concerned with moving the eyes and head so that important visual stimuli are centered on the fovea.
Light falling on the retina causes chemical changes to pigment molecules in the photoreceptors, ultimately leading to a change in the activity of the RGCs. Photoreceptor cells have two parts, the inner segment and the outer segment (Figure 14.16). The inner segment contains the nucleus and other common organelles of a cell, whereas the outer segment is a specialized region in which photoreception takes place. There are two types of photoreceptors—rods and cones—which differ in the shape of their outer segment. The rod-shaped outer segments of the rod photoreceptor contain a stack of membrane-bound discs that contain the photosensitive pigment rhodopsin. The cone-shaped outer segments of the cone photoreceptor contain their photosensitive pigments in infoldings of the cell membrane. There are three cone photopigments, called opsins, which are each sensitive to a particular wavelength of light. The wavelength of visible light determines its color. The pigments in human eyes are specialized in perceiving three different primary colors: red, green, and blue.
Figure 14.16 Photoreceptor (a) All photoreceptors have inner segments containing the nucleus and other important organelles and outer segments with membrane arrays containing the photosensitive opsin molecules. Rod outer segments are long columnar shapes with stacks of membrane-bound discs that contain the rhodopsin pigment. Cone outer segments are short, tapered shapes with folds of membrane in place of the discs in the rods. (b) Tissue of the retina shows a dense layer of nuclei of the rods and cones. LM × 800. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
At the molecular level, visual stimuli cause changes in the photopigment molecule that lead to changes in membrane potential of the photoreceptor cell. A single unit of light is called a photon, which is described in physics as a packet of energy with properties of both a particle and a wave. The energy of a photon is represented by its wavelength, with each wavelength of visible light corresponding to a particular color. Visible light is electromagnetic radiation with a wavelength between 380 and 720 nm. Wavelengths of electromagnetic radiation longer than 720 nm fall into the infrared range, whereas wavelengths shorter than 380 nm fall into the ultraviolet range. Light with a wavelength of 380 nm is blue whereas light with a wavelength of 720 nm is dark red. All other colors fall between red and blue at various points along the wavelength scale.
Opsin pigments are actually transmembrane proteins that contain a cofactor known as retinal. Retinal is a hydrocarbon molecule related to vitamin A. When a photon hits retinal, the long hydrocarbon chain of the molecule is biochemically altered. Specifically, photons cause some of the double-bonded carbons within the chain to switch from a cis to a trans conformation. This process is called photoisomerization. Before interacting with a photon, retinal’s flexible double-bonded carbons are in the cis conformation. This molecule is referred to as 11-cis-retinal. A photon interacting with the molecule causes the flexible double-bonded carbons to change to the trans- conformation, forming all-trans-retinal, which has a straight hydrocarbon chain (Figure 14.17).
The shape change of retinal in the photoreceptors initiates visual transduction in the retina. Activation of retinal and the opsin proteins result in activation of a G protein. The G protein changes the membrane potential of the photoreceptor cell, which then releases less neurotransmitter into the outer synaptic layer of the retina. Until the retinal molecule is changed back to the 11-cis-retinal shape, the opsin cannot respond to light energy, which is called bleaching. When a large group of photopigments is bleached, the retina will send information as if opposing visual information is being perceived. After a bright flash of light, afterimages are usually seen in negative. The photoisomerization is reversed by a series of enzymatic changes so that the retinal responds to more light energy.
Figure 14.17 Retinal Isomers The retinal molecule has two isomers, (a) one before a photon interacts with it and (b) one that is altered through photoisomerization.
The opsins are sensitive to limited wavelengths of light. Rhodopsin, the photopigment in rods, is most sensitive to light at a wavelength of 498 nm. The three color opsins have peak sensitivities of 564 nm, 534 nm, and 420 nm corresponding roughly to the primary colors of red, green, and blue (Figure 14.18). The absorbance of rhodopsin in the rods is much more sensitive than in the cone opsins; specifically, rods are sensitive to vision in low light conditions, and cones are sensitive to brighter conditions. In normal sunlight, rhodopsin will be constantly bleached while the cones are active. In a darkened room, there is not enough light to activate cone opsins, and vision is entirely dependent on rods. Rods are so sensitive to light that a single photon can result in an action potential from a rod’s corresponding RGC.
The three types of cone opsins, being sensitive to different wavelengths of light, provide us with color vision. By comparing the activity of the three different cones, the brain can extract color information from visual stimuli. For example, a bright blue light that has a wavelength of approximately 450 nm would activate the “red” cones minimally, the “green” cones marginally, and the “blue” cones predominantly. The relative activation of the three different cones is calculated by the brain, which perceives the color as blue. However, cones cannot react to low-intensity light, and rods do not sense the color of light. Therefore, our low-light vision is—in essence—in grayscale. In other words, in a dark room, everything appears as a shade of gray. If you think that you can see colors in the dark, it is most likely because your brain knows what color something is and is relying on that memory.
Figure 14.18 Comparison of Color Sensitivity of Photopigments Comparing the peak sensitivity and absorbance spectra of the four photopigments suggests that they are most sensitive to particular wavelengths.
INTERACTIVE LINK
Watch this video to learn more about a transverse section through the brain that depicts the visual pathway from the eye to the occipital cortex. The first half of the pathway is the projection from the RGCs through the optic nerve to the lateral geniculate nucleus in the thalamus on either side. This first fiber in the pathway synapses on a thalamic cell that then projects to the visual cortex in the occipital lobe where “seeing,” or visual perception, takes place. This video gives an abbreviated overview of the visual system by concentrating on the pathway from the eyes to the occipital lobe. The video makes the statement (at 0:45) that “specialized cells in the retina called ganglion cells convert the light rays into electrical signals.” What aspect of retinal processing is simplified by that statement? Explain your answer.
Sensory Nerves
Once any sensory cell transduces a stimulus into a nerve impulse, that impulse has to travel along axons to reach the CNS. In many of the special senses, the axons leaving the sensory receptors have a topographical arrangement, meaning that the location of the sensory receptor relates to the location of the axon in the nerve. For example, in the retina, axons from RGCs in the fovea are located at the center of the optic nerve, where they are surrounded by axons from the more peripheral RGCs.
Spinal Nerves
Generally, spinal nerves contain afferent axons from sensory receptors in the periphery, such as from the skin, mixed with efferent axons travelling to the muscles or other effector organs. As the spinal nerve nears the spinal cord, it splits into dorsal and ventral roots. The dorsal root contains only the axons of sensory neurons, whereas the ventral roots contain only the axons of the motor neurons. Some of the branches will synapse with local neurons in the dorsal root ganglion, posterior (dorsal) horn, or even the anterior (ventral) horn, at the level of the spinal cord where they enter. Other branches will travel a short distance up or down the spine to interact with neurons at other levels of the spinal cord. A branch may also turn into the posterior (dorsal) column of the white matter to connect with the brain. For the sake of convenience, we will use the terms ventral and dorsal in reference to structures within the spinal cord that are part of these pathways. This will help to underscore the relationships between the different components. Typically, spinal nerve systems that connect to the brain are contralateral, in that the right side of the body is connected to the left side of the brain and the left side of the body to the right side of the brain.
Cranial Nerves
Cranial nerves convey specific sensory information from the head and neck directly to the brain. For sensations below the neck, the right side of the body is connected to the left side of the brain and the left side of the body to the right side of the brain. Whereas spinal information is contralateral, cranial nerve systems are mostly ipsilateral, meaning that a cranial nerve on the right side of the head is connected to the right side of the brain. Some cranial nerves contain only sensory axons, such as the olfactory, optic, and vestibulocochlear nerves. Other cranial nerves contain both sensory and motor axons, including the trigeminal, facial, glossopharyngeal, and vagus nerves (however, the vagus nerve is not associated with the somatic nervous system). The general senses of somatosensation for the face travel through the trigeminal system.
Central Processing
- Describe the pathways that sensory systems follow into the central nervous system
- Differentiate between the two major ascending pathways in the spinal cord
- Describe the pathway of somatosensory input from the face and compare it to the ascending pathways in the spinal cord
- Explain topographical representations of sensory information in at least two systems
- Describe two pathways of visual processing and the functions associated with each
Sensory Pathways
Specific regions of the CNS coordinate different somatic processes using sensory inputs and motor outputs of peripheral nerves. A simple case is a reflex caused by a synapse between a dorsal sensory neuron axon and a motor neuron in the ventral horn. More complex arrangements are possible to integrate peripheral sensory information with higher processes. The important regions of the CNS that play a role in somatic processes can be separated into the spinal cord brain stem, diencephalon, cerebral cortex, and subcortical structures.
Spinal Cord and Brain Stem
A sensory pathway that carries peripheral sensations to the brain is referred to as an ascending pathway, or ascending tract. The various sensory modalities each follow specific pathways through the CNS. Tactile and other somatosensory stimuli activate receptors in the skin, muscles, tendons, and joints throughout the entire body. However, the somatosensory pathways are divided into two separate systems on the basis of the location of the receptor neurons. Somatosensory stimuli from below the neck pass along the sensory pathways of the spinal cord, whereas somatosensory stimuli from the head and neck travel through the cranial nerves—specifically, the trigeminal system.
The dorsal column system (sometimes referred to as the dorsal column–medial lemniscus) and the spinothalamic tract are two major pathways that bring sensory information to the brain (Figure 14.19). The sensory pathways in each of these systems are composed of three successive neurons.
The dorsal column system begins with the axon of a dorsal root ganglion neuron entering the dorsal root and joining the dorsal column white matter in the spinal cord. As axons of this pathway enter the dorsal column, they take on a positional arrangement so that axons from lower levels of the body position themselves medially, whereas axons from upper levels of the body position themselves laterally. The dorsal column is separated into two component tracts, the fasciculus gracilis that contains axons from the legs and lower body, and the fasciculus cuneatus that contains axons from the upper body and arms.
The axons in the dorsal column terminate in the nuclei of the medulla, where each synapses with the second neuron in their respective pathway. The nucleus gracilis is the target of fibers in the fasciculus gracilis, whereas the nucleus cuneatus is the target of fibers in the fasciculus cuneatus. The second neuron in the system projects from one of the two nuclei and then decussates, or crosses the midline of the medulla. These axons then continue to ascend the brain stem as a bundle called the medial lemniscus. These axons terminate in the thalamus, where each synapses with the third neuron in their respective pathway. The third neuron in the system projects its axons to the postcentral gyrus of the cerebral cortex, where somatosensory stimuli are initially processed and the conscious perception of the stimulus occurs.
The spinothalamic tract also begins with neurons in a dorsal root ganglion. These neurons extend their axons to the dorsal horn, where they synapse with the second neuron in their respective pathway. The name “spinothalamic” comes from this second neuron, which has its cell body in the spinal cord gray matter and connects to the thalamus. Axons from these second neurons then decussate within the spinal cord and ascend to the brain and enter the thalamus, where each synapses with the third neuron in its respective pathway. The neurons in the thalamus then project their axons to the spinothalamic tract, which synapses in the postcentral gyrus of the cerebral cortex.
These two systems are similar in that they both begin with dorsal root ganglion cells, as with most general sensory information. The dorsal column system is primarily responsible for touch sensations and proprioception, whereas the spinothalamic tract pathway is primarily responsible for pain and temperature sensations. Another similarity is that the second neurons in both of these pathways are contralateral, because they project across the midline to the other side of the brain or spinal cord. In the dorsal column system, this decussation takes place in the brain stem; in the spinothalamic pathway, it takes place in the spinal cord at the same spinal cord level at which the information entered. The third neurons in the two pathways are essentially the same. In both, the second neuron synapses in the thalamus, and the thalamic neuron projects to the somatosensory cortex.
Figure 14.19 Ascending Sensory Pathways of the Spinal Cord The dorsal column system and spinothalamic tract are the major ascending pathways that connect the periphery with the brain.
The trigeminal pathway carries somatosensory information from the face, head, mouth, and nasal cavity. As with the previously discussed nerve tracts, the sensory pathways of the trigeminal pathway each involve three successive neurons. First, axons from the trigeminal ganglion enter the brain stem at the level of the pons. These axons project to one of three locations. The spinal trigeminal nucleus of the medulla receives information similar to that carried by spinothalamic tract, such as pain and temperature sensations. Other axons go to either the chief sensory nucleus in the pons or the mesencephalic nuclei in the midbrain. These nuclei receive information like that carried by the dorsal column system, such as touch, pressure, vibration, and proprioception. Axons from the second neuron decussate and ascend to the thalamus along the trigeminothalamic tract. In the thalamus, each axon synapses with the third neuron in its respective pathway. Axons from the third neuron then project from the thalamus to the primary somatosensory cortex of the cerebrum.
The sensory pathway for gustation travels along the facial and glossopharyngeal cranial nerves, which synapse with neurons of the solitary nucleus in the brain stem. Axons from the solitary nucleus then project to the ventral posterior nucleus of the thalamus. Finally, axons from the ventral posterior nucleus project to the gustatory cortex of the cerebral cortex, where taste is processed and consciously perceived.
The sensory pathway for audition travels along the vestibulocochlear nerve, which synapses with neurons in the cochlear nuclei of the superior medulla. Within the brain stem, input from either ear is combined to extract location information from the auditory stimuli. Whereas the initial auditory stimuli received at the cochlea strictly represent the frequency—or pitch—of the stimuli, the locations of sounds can be determined by comparing information arriving at both ears.
Sound localization is a feature of central processing in the auditory nuclei of the brain stem. Sound localization is achieved by the brain calculating the interaural time difference and the interaural intensity difference. A sound originating from a specific location will arrive at each ear at different times, unless the sound is directly in front of the listener. If the sound source is slightly to the left of the listener, the sound will arrive at the left ear microseconds before it arrives at the right ear (Figure 14.20). This time difference is an example of an interaural time difference. Also, the sound will be slightly louder in the left ear than in the right ear because some of the sound waves reaching the opposite ear are blocked by the head. This is an example of an interaural intensity difference.
Figure 14.20 Auditory Brain Stem Mechanisms of Sound Localization Localizing sound in the horizontal plane is achieved by processing in the medullary nuclei of the auditory system. Connections between neurons on either side are able to compare very slight differences in sound stimuli that arrive at either ear and represent interaural time and intensity differences.
Auditory processing continues on to a nucleus in the midbrain called the inferior colliculus. Axons from the inferior colliculus project to two locations, the thalamus and the superior colliculus. The medial geniculate nucleus of the thalamus receives the auditory information and then projects that information to the auditory cortex in the temporal lobe of the cerebral cortex. The superior colliculus receives input from the visual and somatosensory systems, as well as the ears, to initiate stimulation of the muscles that turn the head and neck toward the auditory stimulus.
Balance is coordinated through the vestibular system, the nerves of which are composed of axons from the vestibular ganglion that carries information from the utricle, saccule, and semicircular canals. The system contributes to controlling head and neck movements in response to vestibular signals. An important function of the vestibular system is coordinating eye and head movements to maintain visual attention. Most of the axons terminate in the vestibular nuclei of the medulla. Some axons project from the vestibular ganglion directly to the cerebellum, with no intervening synapse in the vestibular nuclei. The cerebellum is primarily responsible for initiating movements on the basis of equilibrium information.
Neurons in the vestibular nuclei project their axons to targets in the brain stem. One target is the reticular formation, which influences respiratory and cardiovascular functions in relation to body movements. A second target of the axons of neurons in the vestibular nuclei is the spinal cord, which initiates the spinal reflexes involved with posture and balance. To assist the visual system, fibers of the vestibular nuclei project to the oculomotor, trochlear, and abducens nuclei to influence signals sent along the cranial nerves. These connections constitute the pathway of the vestibulo-ocular reflex (VOR), which compensates for head and body movement by stabilizing images on the retina (Figure 14.21). Finally, the vestibular nuclei project to the thalamus to join the proprioceptive pathway of the dorsal column system, allowing conscious perception of equilibrium.
Figure 14.21 Vestibulo-ocular Reflex Connections between the vestibular system and the cranial nerves controlling eye movement keep the eyes centered on a visual stimulus, even though the head is moving. During head movement, the eye muscles move the eyes in the opposite direction as the head movement, keeping the visual stimulus centered in the field of view.
The connections of the optic nerve are more complicated than those of other cranial nerves. Instead of the connections being between each eye and the brain, visual information is segregated between the left and right sides of the visual field. In addition, some of the information from one side of the visual field projects to the opposite side of the brain. Within each eye, the axons projecting from the medial side of the retina decussate at the optic chiasm. For example, the axons from the medial retina of the left eye cross over to the right side of the brain at the optic chiasm. However, within each eye, the axons projecting from the lateral side of the retina do not decussate. For example, the axons from the lateral retina of the right eye project back to the right side of the brain. Therefore the left field of view of each eye is processed on the right side of the brain, whereas the right field of view of each eye is processed on the left side of the brain (Figure 14.22).
Figure 14.22 Segregation of Visual Field Information at the Optic Chiasm Contralateral visual field information from the lateral retina projects to the ipsilateral brain, whereas ipsilateral visual field information has to decussate at the optic chiasm to reach the opposite side of the brain. (Note that this is an inferior view.)
A unique clinical presentation that relates to this anatomic arrangement is the loss of lateral peripheral vision, known as bilateral hemianopia. This is different from “tunnel vision” because the superior and inferior peripheral fields are not lost. Visual field deficits can be disturbing for a patient, but in this case, the cause is not within the visual system itself. A growth of the pituitary gland presses against the optic chiasm and interferes with signal transmission. However, the axons projecting to the same side of the brain are unaffected. Therefore, the patient loses the outermost areas of their field of vision and cannot see objects to their right and left.
Extending from the optic chiasm, the axons of the visual system are referred to as the optic tract instead of the optic nerve. The optic tract has three major targets, two in the diencephalon and one in the midbrain. The connection between the eyes and diencephalon is demonstrated during development, in which the neural tissue of the retina differentiates from that of the diencephalon by the growth of the secondary vesicles. The connections of the retina into the CNS are a holdover from this developmental association. The majority of the connections of the optic tract are to the thalamus—specifically, the lateral geniculate nucleus. Axons from this nucleus then project to the visual cortex of the cerebrum, located in the occipital lobe. Another target of the optic tract is the superior colliculus.
In addition, a very small number of RGC axons project from the optic chiasm to the suprachiasmatic nucleus of the hypothalamus. These RGCs are photosensitive, in that they respond to the presence or absence of light. Unlike the photoreceptors, however, these photosensitive RGCs cannot be used to perceive images. By simply responding to the absence or presence of light, these RGCs can send information about day length. The perceived proportion of sunlight to darkness establishes the circadian rhythm of our bodies, allowing certain physiological events to occur at approximately the same time every day.
Diencephalon
The diencephalon is beneath the cerebrum and includes the thalamus and hypothalamus. In the somatic nervous system, the thalamus is an important relay for communication between the cerebrum and the rest of the nervous system. The hypothalamus has both somatic and autonomic functions. In addition, the hypothalamus communicates with the limbic system, which controls emotions and memory functions.
Sensory input to the thalamus comes from most of the special senses and ascending somatosensory tracts. Each sensory system is relayed through a particular nucleus in the thalamus. The thalamus is a required transfer point for most sensory tracts that reach the cerebral cortex, where conscious sensory perception begins. The one exception to this rule is the olfactory system. The olfactory tract axons from the olfactory bulb project directly to the cerebral cortex, along with the limbic system and hypothalamus.
The thalamus is a collection of several nuclei that can be categorized into three anatomical groups. White matter running through the thalamus defines the three major regions of the thalamus, which are an anterior nucleus, a medial nucleus, and a lateral group of nuclei. The anterior nucleus serves as a relay between the hypothalamus and the emotion and memory-producing limbic system. The medial nuclei serve as a relay for information from the limbic system and basal ganglia to the cerebral cortex. This allows memory creation during learning, but also determines alertness. The special and somatic senses connect to the lateral nuclei, where their information is relayed to the appropriate sensory cortex of the cerebrum.
Cortical Processing
As described earlier, many of the sensory axons are positioned in the same way as their corresponding receptor cells in the body. This allows identification of the position of a stimulus on the basis of which receptor cells are sending information. The cerebral cortex also maintains this sensory topography in the particular areas of the cortex that correspond to the position of the receptor cells. The somatosensory cortex provides an example in which, in essence, the locations of the somatosensory receptors in the body are mapped onto the somatosensory cortex. This mapping is often depicted using a sensory homunculus (Figure 14.23).
The term homunculus comes from the Latin word for “little man” and refers to a map of the human body that is laid across a portion of the cerebral cortex. In the somatosensory cortex, the external genitals, feet, and lower legs are represented on the medial face of the gyrus within the longitudinal fissure. As the gyrus curves out of the fissure and along the surface of the parietal lobe, the body map continues through the thighs, hips, trunk, shoulders, arms, and hands. The head and face are just lateral to the fingers as the gyrus approaches the lateral sulcus. The representation of the body in this topographical map is medial to lateral from the lower to upper body. It is a continuation of the topographical arrangement seen in the dorsal column system, where axons from the lower body are carried in the fasciculus gracilis, whereas axons from the upper body are carried in the fasciculus cuneatus. As the dorsal column system continues into the medial lemniscus, these relationships are maintained. Also, the head and neck axons running from the trigeminal nuclei to the thalamus run adjacent to the upper body fibers. The connections through the thalamus maintain topography such that the anatomic information is preserved. Note that this correspondence does not result in a perfectly miniature scale version of the body, but rather exaggerates the more sensitive areas of the body, such as the fingers and lower face. Less sensitive areas of the body, such as the shoulders and back, are mapped to smaller areas on the cortex.
Figure 14.23 The Sensory Homunculus A cartoon representation of the sensory homunculus arranged adjacent to the cortical region in which the processing takes place.
Likewise, the topographic relationship between the retina and the visual cortex is maintained throughout the visual pathway. The visual field is projected onto the two retinae, as described above, with sorting at the optic chiasm. The right peripheral visual field falls on the medial portion of the right retina and the lateral portion of the left retina. The right medial retina then projects across the midline through the optic chiasm. This results in the right visual field being processed in the left visual cortex. Likewise, the left visual field is processed in the right visual cortex (see Figure 14.22). Though the chiasm is helping to sort right and left visual information, superior and inferior visual information is maintained topographically in the visual pathway. Light from the superior visual field falls on the inferior retina, and light from the inferior visual field falls on the superior retina. This topography is maintained such that the superior region of the visual cortex processes the inferior visual field and vice versa. Therefore, the visual field information is inverted and reversed as it enters the visual cortex—up is down, and left is right. However, the cortex processes the visual information such that the final conscious perception of the visual field is correct. The topographic relationship is evident in that information from the foveal region of the retina is processed in the center of the primary visual cortex. Information from the peripheral regions of the retina are correspondingly processed toward the edges of the visual cortex. Similar to the exaggerations in the sensory homunculus of the somatosensory cortex, the foveal-processing area of the visual cortex is disproportionately larger than the areas processing peripheral vision.
In an experiment performed in the 1960s, subjects wore prism glasses so that the visual field was inverted before reaching the eye. On the first day of the experiment, subjects would duck when walking up to a table, thinking it was suspended from the ceiling. However, after a few days of acclimation, the subjects behaved as if everything were represented correctly. Therefore, the visual cortex is somewhat flexible in adapting to the information it receives from our eyes (Figure 14.24).
Figure 14.24 Topographic Mapping of the Retina onto the Visual Cortex The visual field projects onto the retina through the lenses and falls on the retinae as an inverted, reversed image. The topography of this image is maintained as the visual information travels through the visual pathway to the cortex.
The cortex has been described as having specific regions that are responsible for processing specific information; there is the visual cortex, somatosensory cortex, gustatory cortex, etc. However, our experience of these senses is not divided. Instead, we experience what can be referred to as a seamless percept. Our perceptions of the various sensory modalities—though distinct in their content—are integrated by the brain so that we experience the world as a continuous whole.
In the cerebral cortex, sensory processing begins at the primary sensory cortex, then proceeds to an association area, and finally, into a multimodal integration area. For example, the visual pathway projects from the retinae through the thalamus to the primary visual cortex in the occipital lobe. This area is primarily in the medial wall within the longitudinal fissure. Here, visual stimuli begin to be recognized as basic shapes. Edges of objects are recognized and built into more complex shapes. Also, inputs from both eyes are compared to extract depth information. Because of the overlapping field of view between the two eyes, the brain can begin to estimate the distance of stimuli based on binocular depth cues.
INTERACTIVE LINK
Watch this video to learn more about how the brain perceives 3-D motion. Similar to how retinal disparity offers 3-D moviegoers a way to extract 3-D information from the two-dimensional visual field projected onto the retina, the brain can extract information about movement in space by comparing what the two eyes see. If movement of a visual stimulus is leftward in one eye and rightward in the opposite eye, the brain interprets this as movement toward (or away) from the face along the midline. If both eyes see an object moving in the same direction, but at different rates, what would that mean for spatial movement?
EVERYDAY CONNECTION
Depth Perception, 3-D Movies, and Optical Illusions
The visual field is projected onto the retinal surface, where photoreceptors transduce light energy into neural signals for the brain to interpret. The retina is a two-dimensional surface, so it does not encode three-dimensional information. However, we can perceive depth. How is that accomplished?
Two ways in which we can extract depth information from the two-dimensional retinal signal are based on monocular cues and binocular cues, respectively. Monocular depth cues are those that are the result of information within the two-dimensional visual field. One object that overlaps another object has to be in front. Relative size differences are also a cue. For example, if a basketball appears larger than the basket, then the basket must be further away. On the basis of experience, we can estimate how far away the basket is. Binocular depth cues compare information represented in the two retinae because they do not see the visual field exactly the same.
The centers of the two eyes are separated by a small distance, which is approximately 6 to 6.5 cm in most people. Because of this offset, visual stimuli do not fall on exactly the same spot on both retinae unless we are fixated directly on them and they fall on the fovea of each retina. All other objects in the visual field, either closer or farther away than the fixated object, will fall on different spots on the retina. When vision is fixed on an object in space, closer objects will fall on the lateral retina of each eye, and more distant objects will fall on the medial retina of either eye (Figure 14.25). This is easily observed by holding a finger up in front of your face as you look at a more distant object. You will see two images of your finger that represent the two disparate images that are falling on either retina.
These depth cues, both monocular and binocular, can be exploited to make the brain think there are three dimensions in two-dimensional information. This is the basis of 3-D movies. The projected image on the screen is two dimensional, but it has disparate information embedded in it. The 3-D glasses that are available at the theater filter the information so that only one eye sees one version of what is on the screen, and the other eye sees the other version. If you take the glasses off, the image on the screen will have varying amounts of blur because both eyes are seeing both layers of information, and the third dimension will not be evident. Some optical illusions can take advantage of depth cues as well, though those are more often using monocular cues to fool the brain into seeing different parts of the scene as being at different depths.
Figure 14.25 Retinal Disparity Because of the interocular distance, which results in objects of different distances falling on different spots of the two retinae, the brain can extract depth perception from the two-dimensional information of the visual field.
There are two main regions that surround the primary cortex that are usually referred to as areas V2 and V3 (the primary visual cortex is area V1). These surrounding areas are the visual association cortex. The visual association regions develop more complex visual perceptions by adding color and motion information. The information processed in these areas is then sent to regions of the temporal and parietal lobes. Visual processing has two separate streams of processing: one into the temporal lobe and one into the parietal lobe. These are the ventral and dorsal streams, respectively (Figure 14.26). The ventral streamidentifies visual stimuli and their significance. Because the ventral stream uses temporal lobe structures, it begins to interact with the non-visual cortex and may be important in visual stimuli becoming part of memories. The dorsal stream locates objects in space and helps in guiding movements of the body in response to visual inputs. The dorsal stream enters the parietal lobe, where it interacts with somatosensory cortical areas that are important for our perception of the body and its movements. The dorsal stream can then influence frontal lobe activity where motor functions originate.
Figure 14.26 Ventral and Dorsal Visual Streams From the primary visual cortex in the occipital lobe, visual processing continues in two streams—one into the temporal lobe and one into the parietal lobe.
DISORDERS OF THE...
Brain: Prosopagnosia
The failures of sensory perception can be unusual and debilitating. A particular sensory deficit that inhibits an important social function of humans is prosopagnosia, or face blindness. The word comes from the Greek words prosopa, that means “faces,” and agnosia, that means “not knowing.” Some people may feel that they cannot recognize people easily by their faces. However, a person with prosopagnosia cannot recognize the most recognizable people in their respective cultures. They would not recognize the face of a celebrity, an important historical figure, or even a family member like their mother. They may not even recognize their own face.
Prosopagnosia can be caused by trauma to the brain, or it can be present from birth. The exact cause of proposagnosia and the reason that it happens to some people is unclear. A study of the brains of people born with the deficit found that a specific region of the brain, the anterior fusiform gyrus of the temporal lobe, is often underdeveloped. This region of the brain is concerned with the recognition of visual stimuli and its possible association with memories. Though the evidence is not yet definitive, this region is likely to be where facial recognition occurs.
Though this can be a devastating condition, people who suffer from it can get by—often by using other cues to recognize the people they see. Often, the sound of a person’s voice, or the presence of unique cues such as distinct facial features (a mole, for example) or hair color can help the sufferer recognize a familiar person. In the video on prosopagnosia provided in this section, a woman is shown having trouble recognizing celebrities, family members, and herself. In some situations, she can use other cues to help her recognize faces.
INTERACTIVE LINK
The inability to recognize people by their faces is a troublesome problem. It can be caused by trauma, or it may be inborn. Watch this video to learn more about a person who lost the ability to recognize faces as the result of an injury. She cannot recognize the faces of close family members or herself. What other information can a person suffering from prosopagnosia use to figure out whom they are seeing?
Motor Responses
- List the components of the basic processing stream for the motor system
- Describe the pathway of descending motor commands from the cortex to the skeletal muscles
- Compare different descending pathways, both by structure and function
- Explain the initiation of movement from the neurological connections
- Describe several reflex arcs and their functional roles
The defining characteristic of the somatic nervous system is that it controls skeletal muscles. Somatic senses inform the nervous system about the external environment, but the response to that is through voluntary muscle movement. The term “voluntary” suggests that there is a conscious decision to make a movement. However, some aspects of the somatic system use voluntary muscles without conscious control. One example is the ability of our breathing to switch to unconscious control while we are focused on another task. However, the muscles that are responsible for the basic process of breathing are also utilized for speech, which is entirely voluntary.
Cortical Responses
Let’s start with sensory stimuli that have been registered through receptor cells and the information relayed to the CNS along ascending pathways. In the cerebral cortex, the initial processing of sensory perception progresses to associative processing and then integration in multimodal areas of cortex. These levels of processing can lead to the incorporation of sensory perceptions into memory, but more importantly, they lead to a response. The completion of cortical processing through the primary, associative, and integrative sensory areas initiates a similar progression of motor processing, usually in different cortical areas.
Whereas the sensory cortical areas are located in the occipital, temporal, and parietal lobes, motor functions are largely controlled by the frontal lobe. The most anterior regions of the frontal lobe—the prefrontal areas—are important for executive functions, which are those cognitive functions that lead to goal-directed behaviors. These higher cognitive processes include working memory, which has been called a “mental scratch pad,” that can help organize and represent information that is not in the immediate environment. The prefrontal lobe is responsible for aspects of attention, such as inhibiting distracting thoughts and actions so that a person can focus on a goal and direct behavior toward achieving that goal.
The functions of the prefrontal cortex are integral to the personality of an individual, because it is largely responsible for what a person intends to do and how they accomplish those plans. A famous case of damage to the prefrontal cortex is that of Phineas Gage, dating back to 1848. He was a railroad worker who had a metal spike impale his prefrontal cortex (Figure 14.27). He survived the accident, but according to second-hand accounts, his personality changed drastically. Friends described him as no longer acting like himself. Whereas he was a hardworking, amiable man before the accident, he turned into an irritable, temperamental, and lazy man after the accident. Many of the accounts of his change may have been inflated in the retelling, and some behavior was likely attributable to alcohol used as a pain medication. However, the accounts suggest that some aspects of his personality did change. Also, there is new evidence that though his life changed dramatically, he was able to become a functioning stagecoach driver, suggesting that the brain has the ability to recover even from major trauma such as this.
Figure 14.27 Phineas Gage The victim of an accident while working on a railroad in 1848, Phineas Gage had a large iron rod impaled through the prefrontal cortex of his frontal lobe. After the accident, his personality appeared to change, but he eventually learned to cope with the trauma and lived as a coach driver even after such a traumatic event. (credit b: John M. Harlow, MD)
Secondary Motor Cortices
In generating motor responses, the executive functions of the prefrontal cortex will need to initiate actual movements. One way to define the prefrontal area is any region of the frontal lobe that does not elicit movement when electrically stimulated. These are primarily in the anterior part of the frontal lobe. The regions of the frontal lobe that remain are the regions of the cortex that produce movement. The prefrontal areas project into the secondary motor cortices, which include the premotor cortex and the supplemental motor area.
Two important regions that assist in planning and coordinating movements are located adjacent to the primary motor cortex. The premotor cortex is more lateral, whereas the supplemental motor area is more medial and superior. The premotor area aids in controlling movements of the core muscles to maintain posture during movement, whereas the supplemental motor area is hypothesized to be responsible for planning and coordinating movement. The supplemental motor area also manages sequential movements that are based on prior experience (that is, learned movements). Neurons in these areas are most active leading up to the initiation of movement. For example, these areas might prepare the body for the movements necessary to drive a car in anticipation of a traffic light changing.
Adjacent to these two regions are two specialized motor planning centers. The frontal eye fields are responsible for moving the eyes in response to visual stimuli. There are direct connections between the frontal eye fields and the superior colliculus. Also, anterior to the premotor cortex and primary motor cortex is Broca’s area. This area is responsible for controlling movements of the structures of speech production. The area is named after a French surgeon and anatomist who studied patients who could not produce speech. They did not have impairments to understanding speech, only to producing speech sounds, suggesting a damaged or underdeveloped Broca’s area.
Primary Motor Cortex
The primary motor cortex is located in the precentral gyrus of the frontal lobe. A neurosurgeon, Walter Penfield, described much of the basic understanding of the primary motor cortex by electrically stimulating the surface of the cerebrum. Penfield would probe the surface of the cortex while the patient was only under local anesthesia so that he could observe responses to the stimulation. This led to the belief that the precentral gyrus directly stimulated muscle movement. We now know that the primary motor cortex receives input from several areas that aid in planning movement, and its principle output stimulates spinal cord neurons to stimulate skeletal muscle contraction.
The primary motor cortex is arranged in a similar fashion to the primary somatosensory cortex, in that it has a topographical map of the body, creating a motor homunculus (see Figure 14.23). The neurons responsible for musculature in the feet and lower legs are in the medial wall of the precentral gyrus, with the thighs, trunk, and shoulder at the crest of the longitudinal fissure. The hand and face are in the lateral face of the gyrus. Also, the relative space allotted for the different regions is exaggerated in muscles that have greater enervation. The greatest amount of cortical space is given to muscles that perform fine, agile movements, such as the muscles of the fingers and the lower face. The “power muscles” that perform coarser movements, such as the buttock and back muscles, occupy much less space on the motor cortex.
Descending Pathways
The motor output from the cortex descends into the brain stem and to the spinal cord to control the musculature through motor neurons. Neurons located in the primary motor cortex, named Betz cells, are large cortical neurons that synapse with lower motor neurons in the brain stem or in the spinal cord. The two descending pathways travelled by the axons of Betz cells are the corticobulbar tract and the corticospinal tract, respectively. Both tracts are named for their origin in the cortex and their targets—either the brain stem (the term “bulbar” refers to the brain stem as the bulb, or enlargement, at the top of the spinal cord) or the spinal cord.
These two descending pathways are responsible for the conscious or voluntary movements of skeletal muscles. Any motor command from the primary motor cortex is sent down the axons of the Betz cells to activate upper motor neurons in either the cranial motor nuclei or in the ventral horn of the spinal cord. The axons of the corticobulbar tract are ipsilateral, meaning they project from the cortex to the motor nucleus on the same side of the nervous system. Conversely, the axons of the corticospinal tract are largely contralateral, meaning that they cross the midline of the brain stem or spinal cord and synapse on the opposite side of the body. Therefore, the right motor cortex of the cerebrum controls muscles on the left side of the body, and vice versa.
The corticospinal tract descends from the cortex through the deep white matter of the cerebrum. It then passes between the caudate nucleus and putamen of the basal nuclei as a bundle called the internal capsule. The tract then passes through the midbrain as the cerebral peduncles, after which it burrows through the pons. Upon entering the medulla, the tracts make up the large white matter tract referred to as the pyramids (Figure 14.28). The defining landmark of the medullary-spinal border is the pyramidal decussation, which is where most of the fibers in the corticospinal tract cross over to the opposite side of the brain. At this point, the tract separates into two parts, which have control over different domains of the musculature.
Figure 14.28 Corticospinal Tract The major descending tract that controls skeletal muscle movements is the corticospinal tract. It is composed of two neurons, the upper motor neuron and the lower motor neuron. The upper motor neuron has its cell body in the primary motor cortex of the frontal lobe and synapses on the lower motor neuron, which is in the ventral horn of the spinal cord and projects to the skeletal muscle in the periphery.
Appendicular Control
The lateral corticospinal tract is composed of the fibers that cross the midline at the pyramidal decussation (see Figure 14.28). The axons cross over from the anterior position of the pyramids in the medulla to the lateral column of the spinal cord. These axons are responsible for controlling appendicular muscles.
This influence over the appendicular muscles means that the lateral corticospinal tract is responsible for moving the muscles of the arms and legs. The ventral horn in both the lower cervical spinal cord and the lumbar spinal cord both have wider ventral horns, representing the greater number of muscles controlled by these motor neurons. The cervical enlargement is particularly large because there is greater control over the fine musculature of the upper limbs, particularly of the fingers. The lumbar enlargement is not as significant in appearance because there is less fine motor control of the lower limbs.
Axial Control
The anterior corticospinal tract is responsible for controlling the muscles of the body trunk (see Figure 14.28). These axons do not decussate in the medulla. Instead, they remain in an anterior position as they descend the brain stem and enter the spinal cord. These axons then travel to the spinal cord level at which they synapse with a lower motor neuron. Upon reaching the appropriate level, the axons decussate, entering the ventral horn on the opposite side of the spinal cord from which they entered. In the ventral horn, these axons synapse with their corresponding lower motor neurons. The lower motor neurons are located in the medial regions of the ventral horn, because they control the axial muscles of the trunk.
Because movements of the body trunk involve both sides of the body, the anterior corticospinal tract is not entirely contralateral. Some collateral branches of the tract will project into the ipsilateral ventral horn to control synergistic muscles on that side of the body, or to inhibit antagonistic muscles through interneurons within the ventral horn. Through the influence of both sides of the body, the anterior corticospinal tract can coordinate postural muscles in broad movements of the body. These coordinating axons in the anterior corticospinal tract are often considered bilateral, as they are both ipsilateral and contralateral.
INTERACTIVE LINK
Watch this video to learn more about the descending motor pathway for the somatic nervous system. The autonomic connections are mentioned, which are covered in another chapter. From this brief video, only some of the descending motor pathway of the somatic nervous system is described. Which division of the pathway is described and which division is left out?
Extrapyramidal Controls
Other descending connections between the brain and the spinal cord are called the extrapyramidal system. The name comes from the fact that this system is outside the corticospinal pathway, which includes the pyramids in the medulla. A few pathways originating from the brain stem contribute to this system.
The tectospinal tract projects from the midbrain to the spinal cord and is important for postural movements that are driven by the superior colliculus. The name of the tract comes from an alternate name for the superior colliculus, which is the tectum. The reticulospinal tract connects the reticular system, a diffuse region of gray matter in the brain stem, with the spinal cord. This tract influences trunk and proximal limb muscles related to posture and locomotion. The reticulospinal tract also contributes to muscle tone and influences autonomic functions. The vestibulospinal tract connects the brain stem nuclei of the vestibular system with the spinal cord. This allows posture, movement, and balance to be modulated on the basis of equilibrium information provided by the vestibular system.
The pathways of the extrapyramidal system are influenced by subcortical structures. For example, connections between the secondary motor cortices and the extrapyramidal system modulate spine and cranium movements. The basal nuclei, which are important for regulating movement initiated by the CNS, influence the extrapyramidal system as well as its thalamic feedback to the motor cortex.
The conscious movement of our muscles is more complicated than simply sending a single command from the precentral gyrus down to the proper motor neurons. During the movement of any body part, our muscles relay information back to the brain, and the brain is constantly sending “revised” instructions back to the muscles. The cerebellum is important in contributing to the motor system because it compares cerebral motor commands with proprioceptive feedback. The corticospinal fibers that project to the ventral horn of the spinal cord have branches that also synapse in the pons, which project to the cerebellum. Also, the proprioceptive sensations of the dorsal column system have a collateral projection to the medulla that projects to the cerebellum. These two streams of information are compared in the cerebellar cortex. Conflicts between the motor commands sent by the cerebrum and body position information provided by the proprioceptors cause the cerebellum to stimulate the red nucleus of the midbrain. The red nucleus then sends corrective commands to the spinal cord along the rubrospinal tract. The name of this tract comes from the word for red that is seen in the English word “ruby.”
A good example of how the cerebellum corrects cerebral motor commands can be illustrated by walking in water. An original motor command from the cerebrum to walk will result in a highly coordinated set of learned movements. However, in water, the body cannot actually perform a typical walking movement as instructed. The cerebellum can alter the motor command, stimulating the leg muscles to take larger steps to overcome the water resistance. The cerebellum can make the necessary changes through the rubrospinal tract. Modulating the basic command to walk also relies on spinal reflexes, but the cerebellum is responsible for calculating the appropriate response. When the cerebellum does not work properly, coordination and balance are severely affected. The most dramatic example of this is during the overconsumption of alcohol. Alcohol inhibits the ability of the cerebellum to interpret proprioceptive feedback, making it more difficult to coordinate body movements, such as walking a straight line, or guide the movement of the hand to touch the tip of the nose.
INTERACTIVE LINK
Visit this site to read about an elderly woman who starts to lose the ability to control fine movements, such as speech and the movement of limbs. Many of the usual causes were ruled out. It was not a stroke, Parkinson’s disease, diabetes, or thyroid dysfunction. The next most obvious cause was medication, so her pharmacist had to be consulted. The side effect of a drug meant to help her sleep had resulted in changes in motor control. What regions of the nervous system are likely to be the focus of haloperidol side effects?
Ventral Horn Output
The somatic nervous system provides output strictly to skeletal muscles. The lower motor neurons, which are responsible for the contraction of these muscles, are found in the ventral horn of the spinal cord. These large, multipolar neurons have a corona of dendrites surrounding the cell body and an axon that extends out of the ventral horn. This axon travels through the ventral nerve root to join the emerging spinal nerve. The axon is relatively long because it needs to reach muscles in the periphery of the body. The diameters of cell bodies may be on the order of hundreds of micrometers to support the long axon; some axons are a meter in length, such as the lumbar motor neurons that innervate muscles in the first digits of the feet.
The axons will also branch to innervate multiple muscle fibers. Together, the motor neuron and all the muscle fibers that it controls make up a motor unit. Motor units vary in size. Some may contain up to 1000 muscle fibers, such as in the quadriceps, or they may only have 10 fibers, such as in an extraocular muscle. The number of muscle fibers that are part of a motor unit corresponds to the precision of control of that muscle. Also, muscles that have finer motor control have more motor units connecting to them, and this requires a larger topographical field in the primary motor cortex.
Motor neuron axons connect to muscle fibers at a neuromuscular junction. This is a specialized synaptic structure at which multiple axon terminals synapse with the muscle fiber sarcolemma. The synaptic end bulbs of the motor neurons secrete acetylcholine, which binds to receptors on the sarcolemma. The binding of acetylcholine opens ligand-gated ion channels, increasing the movement of cations across the sarcolemma. This depolarizes the sarcolemma, initiating muscle contraction. Whereas other synapses result in graded potentials that must reach a threshold in the postsynaptic target, activity at the neuromuscular junction reliably leads to muscle fiber contraction with every nerve impulse received from a motor neuron. However, the strength of contraction and the number of fibers that contract can be affected by the frequency of the motor neuron impulses.
Reflexes
This chapter began by introducing reflexes as an example of the basic elements of the somatic nervous system. Simple somatic reflexes do not include the higher centers discussed for conscious or voluntary aspects of movement. Reflexes can be spinal or cranial, depending on the nerves and central components that are involved. The example described at the beginning of the chapter involved heat and pain sensations from a hot stove causing withdrawal of the arm through a connection in the spinal cord that leads to contraction of the biceps brachii. The description of this withdrawal reflex was simplified, for the sake of the introduction, to emphasize the parts of the somatic nervous system. But to consider reflexes fully, more attention needs to be given to this example.
As you withdraw your hand from the stove, you do not want to slow that reflex down. As the biceps brachii contracts, the antagonistic triceps brachii needs to relax. Because the neuromuscular junction is strictly excitatory, the biceps will contract when the motor nerve is active. Skeletal muscles do not actively relax. Instead the motor neuron needs to “quiet down,” or be inhibited. In the hot-stove withdrawal reflex, this occurs through an interneuron in the spinal cord. The interneuron’s cell body is located in the dorsal horn of the spinal cord. The interneuron receives a synapse from the axon of the sensory neuron that detects that the hand is being burned. In response to this stimulation from the sensory neuron, the interneuron then inhibits the motor neuron that controls the triceps brachii. This is done by releasing a neurotransmitter or other signal that hyperpolarizes the motor neuron connected to the triceps brachii, making it less likely to initiate an action potential. With this motor neuron being inhibited, the triceps brachii relaxes. Without the antagonistic contraction, withdrawal from the hot stove is faster and keeps further tissue damage from occurring.
Another example of a withdrawal reflex occurs when you step on a painful stimulus, like a tack or a sharp rock. The nociceptors that are activated by the painful stimulus activate the motor neurons responsible for contraction of the tibialis anterior muscle. This causes dorsiflexion of the foot. An inhibitory interneuron, activated by a collateral branch of the nociceptor fiber, will inhibit the motor neurons of the gastrocnemius and soleus muscles to cancel plantar flexion. An important difference in this reflex is that plantar flexion is most likely in progress as the foot is pressing down onto the tack. Contraction of the tibialis anterior is not the most important aspect of the reflex, as continuation of plantar flexion will result in further damage from stepping onto the tack.
Another type of reflex is a stretch reflex. In this reflex, when a skeletal muscle is stretched, a muscle spindle receptor is activated. The axon from this receptor structure will cause direct contraction of the muscle. A collateral of the muscle spindle fiber will also inhibit the motor neuron of the antagonist muscles. The reflex helps to maintain muscles at a constant length. A common example of this reflex is the knee jerk that is elicited by a rubber hammer struck against the patellar ligament in a physical exam.
A specialized reflex to protect the surface of the eye is the corneal reflex, or the eye blink reflex. When the cornea is stimulated by a tactile stimulus, or even by bright light in a related reflex, blinking is initiated. The sensory component travels through the trigeminal nerve, which carries somatosensory information from the face, or through the optic nerve, if the stimulus is bright light. The motor response travels through the facial nerve and innervates the orbicularis oculi on the same side. This reflex is commonly tested during a physical exam using an air puff or a gentle touch of a cotton-tipped applicator.
INTERACTIVE LINK
Watch this video to learn more about the reflex arc of the corneal reflex. When the right cornea senses a tactile stimulus, what happens to the left eye? Explain your answer.
INTERACTIVE LINK
Watch this video to learn more about newborn reflexes. Newborns have a set of reflexes that are expected to have been crucial to survival before the modern age. These reflexes disappear as the baby grows, as some of them may be unnecessary as they age. The video demonstrates a reflex called the Babinski reflex, in which the foot flexes dorsally and the toes splay out when the sole of the foot is lightly scratched. This is normal for newborns, but it is a sign of reduced myelination of the spinal tract in adults. Why would this reflex be a problem for an adult?
Key Terms
- alkaloid
- substance, usually from a plant source, that is chemically basic with respect to pH and will stimulate bitter receptors
- amacrine cell
- type of cell in the retina that connects to the bipolar cells near the outer synaptic layer and provides the basis for early image processing within the retina
- ampulla
- in the ear, the structure at the base of a semicircular canal that contains the hair cells and cupula for transduction of rotational movement of the head
- anosmia
- loss of the sense of smell; usually the result of physical disruption of the first cranial nerve
- anterior corticospinal tract
- division of the corticospinal pathway that travels through the ventral (anterior) column of the spinal cord and controls axial musculature through the medial motor neurons in the ventral (anterior) horn
- aqueous humor
- watery fluid that fills the anterior chamber containing the cornea, iris, ciliary body, and lens of the eye
- ascending pathway
- fiber structure that relays sensory information from the periphery through the spinal cord and brain stem to other structures of the brain
- association area
- region of cortex connected to a primary sensory cortical area that further processes the information to generate more complex sensory perceptions
- audition
- sense of hearing
- auricle
- fleshy external structure of the ear
- basilar membrane
- in the ear, the floor of the cochlear duct on which the organ of Corti sits
- Betz cells
- output cells of the primary motor cortex that cause musculature to move through synapses on cranial and spinal motor neurons
- binocular depth cues
- indications of the distance of visual stimuli on the basis of slight differences in the images projected onto either retina
- bipolar cell
- cell type in the retina that connects the photoreceptors to the RGCs
- Broca’s area
- region of the frontal lobe associated with the motor commands necessary for speech production
- capsaicin
- molecule that activates nociceptors by interacting with a temperature-sensitive ion channel and is the basis for “hot” sensations in spicy food
- cerebral peduncles
- segments of the descending motor pathway that make up the white matter of the ventral midbrain
- cervical enlargement
- region of the ventral (anterior) horn of the spinal cord that has a larger population of motor neurons for the greater number of and finer control of muscles of the upper limb
- chemoreceptor
- sensory receptor cell that is sensitive to chemical stimuli, such as in taste, smell, or pain
- chief sensory nucleus
- component of the trigeminal nuclei that is found in the pons
- choroid
- highly vascular tissue in the wall of the eye that supplies the outer retina with blood
- ciliary body
- smooth muscle structure on the interior surface of the iris that controls the shape of the lens through the zonule fibers
- circadian rhythm
- internal perception of the daily cycle of light and dark based on retinal activity related to sunlight
- cochlea
- auditory portion of the inner ear containing structures to transduce sound stimuli
- cochlear duct
- space within the auditory portion of the inner ear that contains the organ of Corti and is adjacent to the scala tympani and scala vestibuli on either side
- cone photoreceptor
- one of the two types of retinal receptor cell that is specialized for color vision through the use of three photopigments distributed through three separate populations of cells
- contralateral
- word meaning “on the opposite side,” as in axons that cross the midline in a fiber tract
- cornea
- fibrous covering of the anterior region of the eye that is transparent so that light can pass through it
- corneal reflex
- protective response to stimulation of the cornea causing contraction of the orbicularis oculi muscle resulting in blinking of the eye
- corticobulbar tract
- connection between the cortex and the brain stem responsible for generating movement
- corticospinal tract
- connection between the cortex and the spinal cord responsible for generating movement
- cupula
- specialized structure within the base of a semicircular canal that bends the stereocilia of hair cells when the head rotates by way of the relative movement of the enclosed fluid
- decussate
- to cross the midline, as in fibers that project from one side of the body to the other
- dorsal column system
- ascending tract of the spinal cord associated with fine touch and proprioceptive sensations
- dorsal stream
- connections between cortical areas from the occipital to parietal lobes that are responsible for the perception of visual motion and guiding movement of the body in relation to that motion
- encapsulated ending
- configuration of a sensory receptor neuron with dendrites surrounded by specialized structures to aid in transduction of a particular type of sensation, such as the lamellated corpuscles in the deep dermis and subcutaneous tissue
- equilibrium
- sense of balance that includes sensations of position and movement of the head
- executive functions
- cognitive processes of the prefrontal cortex that lead to directing goal-directed behavior, which is a precursor to executing motor commands
- external ear
- structures on the lateral surface of the head, including the auricle and the ear canal back to the tympanic membrane
- exteroceptor
- sensory receptor that is positioned to interpret stimuli from the external environment, such as photoreceptors in the eye or somatosensory receptors in the skin
- extraocular muscle
- one of six muscles originating out of the bones of the orbit and inserting into the surface of the eye which are responsible for moving the eye
- extrapyramidal system
- pathways between the brain and spinal cord that are separate from the corticospinal tract and are responsible for modulating the movements generated through that primary pathway
- fasciculus cuneatus
- lateral division of the dorsal column system composed of fibers from sensory neurons in the upper body
- fasciculus gracilis
- medial division of the dorsal column system composed of fibers from sensory neurons in the lower body
- fibrous tunic
- outer layer of the eye primarily composed of connective tissue known as the sclera and cornea
- fovea
- exact center of the retina at which visual stimuli are focused for maximal acuity, where the retina is thinnest, at which there is nothing but photoreceptors
- free nerve ending
- configuration of a sensory receptor neuron with dendrites in the connective tissue of the organ, such as in the dermis of the skin, that are most often sensitive to chemical, thermal, and mechanical stimuli
- frontal eye fields
- area of the prefrontal cortex responsible for moving the eyes to attend to visual stimuli
- general sense
- any sensory system that is distributed throughout the body and incorporated into organs of multiple other systems, such as the walls of the digestive organs or the skin
- gustation
- sense of taste
- gustatory receptor cells
- sensory cells in the taste bud that transduce the chemical stimuli of gustation
- hair cells
- mechanoreceptor cells found in the inner ear that transduce stimuli for the senses of hearing and balance
- incus
- (also, anvil) ossicle of the middle ear that connects the malleus to the stapes
- inferior colliculus
- last structure in the auditory brainstem pathway that projects to the thalamus and superior colliculus
- inferior oblique
- extraocular muscle responsible for lateral rotation of the eye
- inferior rectus
- extraocular muscle responsible for looking down
- inner ear
- structure within the temporal bone that contains the sensory apparati of hearing and balance
- inner segment
- in the eye, the section of a photoreceptor that contains the nucleus and other major organelles for normal cellular functions
- inner synaptic layer
- layer in the retina where bipolar cells connect to RGCs
- interaural intensity difference
- cue used to aid sound localization in the horizontal plane that compares the relative loudness of sounds at the two ears, because the ear closer to the sound source will hear a slightly more intense sound
- interaural time difference
- cue used to help with sound localization in the horizontal plane that compares the relative time of arrival of sounds at the two ears, because the ear closer to the sound source will receive the stimulus microseconds before the other ear
- internal capsule
- segment of the descending motor pathway that passes between the caudate nucleus and the putamen
- interoceptor
- sensory receptor that is positioned to interpret stimuli from internal organs, such as stretch receptors in the wall of blood vessels
- ipsilateral
- word meaning on the same side, as in axons that do not cross the midline in a fiber tract
- iris
- colored portion of the anterior eye that surrounds the pupil
- kinesthesia
- sense of body movement based on sensations in skeletal muscles, tendons, joints, and the skin
- lacrimal duct
- duct in the medial corner of the orbit that drains tears into the nasal cavity
- lacrimal gland
- gland lateral to the orbit that produces tears to wash across the surface of the eye
- lateral corticospinal tract
- division of the corticospinal pathway that travels through the lateral column of the spinal cord and controls appendicular musculature through the lateral motor neurons in the ventral (anterior) horn
- lateral geniculate nucleus
- thalamic target of the RGCs that projects to the visual cortex
- lateral rectus
- extraocular muscle responsible for abduction of the eye
- lens
- component of the eye that focuses light on the retina
- levator palpebrae superioris
- muscle that causes elevation of the upper eyelid, controlled by fibers in the oculomotor nerve
- lumbar enlargement
- region of the ventral (anterior) horn of the spinal cord that has a larger population of motor neurons for the greater number of muscles of the lower limb
- macula
- enlargement at the base of a semicircular canal at which transduction of equilibrium stimuli takes place within the ampulla
- malleus
- (also, hammer) ossicle that is directly attached to the tympanic membrane
- mechanoreceptor
- receptor cell that transduces mechanical stimuli into an electrochemical signal
- medial geniculate nucleus
- thalamic target of the auditory brain stem that projects to the auditory cortex
- medial lemniscus
- fiber tract of the dorsal column system that extends from the nuclei gracilis and cuneatus to the thalamus, and decussates
- medial rectus
- extraocular muscle responsible for adduction of the eye
- mesencephalic nucleus
- component of the trigeminal nuclei that is found in the midbrain
- middle ear
- space within the temporal bone between the ear canal and bony labyrinth where the ossicles amplify sound waves from the tympanic membrane to the oval window
- multimodal integration area
- region of the cerebral cortex in which information from more than one sensory modality is processed to arrive at higher level cortical functions such as memory, learning, or cognition
- neural tunic
- layer of the eye that contains nervous tissue, namely the retina
- nociceptor
- receptor cell that senses pain stimuli
- nucleus cuneatus
- medullary nucleus at which first-order neurons of the dorsal column system synapse specifically from the upper body and arms
- nucleus gracilis
- medullary nucleus at which first-order neurons of the dorsal column system synapse specifically from the lower body and legs
- odorant molecules
- volatile chemicals that bind to receptor proteins in olfactory neurons to stimulate the sense of smell
- olfaction
- sense of smell
- olfactory bulb
- central target of the first cranial nerve; located on the ventral surface of the frontal lobe in the cerebrum
- olfactory epithelium
- region of the nasal epithelium where olfactory neurons are located
- olfactory sensory neuron
- receptor cell of the olfactory system, sensitive to the chemical stimuli of smell, the axons of which compose the first cranial nerve
- opsin
- protein that contains the photosensitive cofactor retinal for phototransduction
- optic chiasm
- decussation point in the visual system at which medial retina fibers cross to the other side of the brain
- optic disc
- spot on the retina at which RGC axons leave the eye and blood vessels of the inner retina pass
- optic nerve
- second cranial nerve, which is responsible visual sensation
- optic tract
- name for the fiber structure containing axons from the retina posterior to the optic chiasm representing their CNS location
- organ of Corti
- structure in the cochlea in which hair cells transduce movements from sound waves into electrochemical signals
- osmoreceptor
- receptor cell that senses differences in the concentrations of bodily fluids on the basis of osmotic pressure
- ossicles
- three small bones in the middle ear
- otolith
- layer of calcium carbonate crystals located on top of the otolithic membrane
- otolithic membrane
- gelatinous substance in the utricle and saccule of the inner ear that contains calcium carbonate crystals and into which the stereocilia of hair cells are embedded
- outer segment
- in the eye, the section of a photoreceptor that contains opsin molecules that transduce light stimuli
- outer synaptic layer
- layer in the retina at which photoreceptors connect to bipolar cells
- oval window
- membrane at the base of the cochlea where the stapes attaches, marking the beginning of the scala vestibuli
- palpebral conjunctiva
- membrane attached to the inner surface of the eyelids that covers the anterior surface of the cornea
- papilla
- for gustation, a bump-like projection on the surface of the tongue that contains taste buds
- photoisomerization
- chemical change in the retinal molecule that alters the bonding so that it switches from the 11-cis-retinal isomer to the all-trans-retinal isomer
- photon
- individual “packet” of light
- photoreceptor
- receptor cell specialized to respond to light stimuli
- premotor cortex
- cortical area anterior to the primary motor cortex that is responsible for planning movements
- primary sensory cortex
- region of the cerebral cortex that initially receives sensory input from an ascending pathway from the thalamus and begins the processing that will result in conscious perception of that modality
- proprioception
- sense of position and movement of the body
- proprioceptor
- receptor cell that senses changes in the position and kinesthetic aspects of the body
- pupil
- open hole at the center of the iris that light passes through into the eye
- pyramidal decussation
- location at which corticospinal tract fibers cross the midline and segregate into the anterior and lateral divisions of the pathway
- pyramids
- segment of the descending motor pathway that travels in the anterior position of the medulla
- receptor cell
- cell that transduces environmental stimuli into neural signals
- red nucleus
- midbrain nucleus that sends corrective commands to the spinal cord along the rubrospinal tract, based on disparity between an original command and the sensory feedback from movement
- reticulospinal tract
- extrapyramidal connections between the brain stem and spinal cord that modulate movement, contribute to posture, and regulate muscle tone
- retina
- nervous tissue of the eye at which phototransduction takes place
- retinal
- cofactor in an opsin molecule that undergoes a biochemical change when struck by a photon (pronounced with a stress on the last syllable)
- retinal ganglion cell (RGC)
- neuron of the retina that projects along the second cranial nerve
- rhodopsin
- photopigment molecule found in the rod photoreceptors
- rod photoreceptor
- one of the two types of retinal receptor cell that is specialized for low-light vision
- round window
- membrane that marks the end of the scala tympani
- rubrospinal tract
- descending motor control pathway, originating in the red nucleus, that mediates control of the limbs on the basis of cerebellar processing
- saccule
- structure of the inner ear responsible for transducing linear acceleration in the vertical plane
- scala tympani
- portion of the cochlea that extends from the apex to the round window
- scala vestibuli
- portion of the cochlea that extends from the oval window to the apex
- sclera
- white of the eye
- semicircular canals
- structures within the inner ear responsible for transducing rotational movement information
- sensory homunculus
- topographic representation of the body within the somatosensory cortex demonstrating the correspondence between neurons processing stimuli and sensitivity
- sensory modality
- a particular system for interpreting and perceiving environmental stimuli by the nervous system
- solitary nucleus
- medullar nucleus that receives taste information from the facial and glossopharyngeal nerves
- somatosensation
- general sense associated with modalities lumped together as touch
- special sense
- any sensory system associated with a specific organ structure, namely smell, taste, sight, hearing, and balance
- spinal trigeminal nucleus
- component of the trigeminal nuclei that is found in the medulla
- spinothalamic tract
- ascending tract of the spinal cord associated with pain and temperature sensations
- spiral ganglion
- location of neuronal cell bodies that transmit auditory information along the eighth cranial nerve
- stapes
- (also, stirrup) ossicle of the middle ear that is attached to the inner ear
- stereocilia
- array of apical membrane extensions in a hair cell that transduce movements when they are bent
- stretch reflex
- response to activation of the muscle spindle stretch receptor that causes contraction of the muscle to maintain a constant length
- submodality
- specific sense within a broader major sense such as sweet as a part of the sense of taste, or color as a part of vision
- superior colliculus
- structure in the midbrain that combines visual, auditory, and somatosensory input to coordinate spatial and topographic representations of the three sensory systems
- superior oblique
- extraocular muscle responsible for medial rotation of the eye
- superior rectus
- extraocular muscle responsible for looking up
- supplemental motor area
- cortical area anterior to the primary motor cortex that is responsible for planning movements
- suprachiasmatic nucleus
- hypothalamic target of the retina that helps to establish the circadian rhythm of the body on the basis of the presence or absence of daylight
- taste buds
- structures within a papilla on the tongue that contain gustatory receptor cells
- tectorial membrane
- component of the organ of Corti that lays over the hair cells, into which the stereocilia are embedded
- tectospinal tract
- extrapyramidal connections between the superior colliculus and spinal cord
- thermoreceptor
- sensory receptor specialized for temperature stimuli
- topographical
- relating to positional information
- transduction
- process of changing an environmental stimulus into the electrochemical signals of the nervous system
- trochlea
- cartilaginous structure that acts like a pulley for the superior oblique muscle
- tympanic membrane
- ear drum
- umami
- taste submodality for sensitivity to the concentration of amino acids; also called the savory sense
- utricle
- structure of the inner ear responsible for transducing linear acceleration in the horizontal plane
- vascular tunic
- middle layer of the eye primarily composed of connective tissue with a rich blood supply
- ventral posterior nucleus
- nucleus in the thalamus that is the target of gustatory sensations and projects to the cerebral cortex
- ventral stream
- connections between cortical areas from the occipital lobe to the temporal lobe that are responsible for identification of visual stimuli
- vestibular ganglion
- location of neuronal cell bodies that transmit equilibrium information along the eighth cranial nerve
- vestibular nuclei
- targets of the vestibular component of the eighth cranial nerve
- vestibule
- in the ear, the portion of the inner ear responsible for the sense of equilibrium
- vestibulo-ocular reflex (VOR)
- reflex based on connections between the vestibular system and the cranial nerves of eye movements that ensures images are stabilized on the retina as the head and body move
- vestibulospinal tract
- extrapyramidal connections between the vestibular nuclei in the brain stem and spinal cord that modulate movement and contribute to balance on the basis of the sense of equilibrium
- visceral sense
- sense associated with the internal organs
- vision
- special sense of sight based on transduction of light stimuli
- visual acuity
- property of vision related to the sharpness of focus, which varies in relation to retinal position
- vitreous humor
- viscous fluid that fills the posterior chamber of the eye
- working memory
- function of the prefrontal cortex to maintain a representation of information that is not in the immediate environment
- zonule fibers
- fibrous connections between the ciliary body and the lens
Chapter Review
14.1 Sensory Perception
The senses are olfaction (smell), gustation (taste), somatosensation (sensations associated with the skin and body), audition (hearing), equilibrium (balance), and vision. With the exception of somatosensation, this list represents the special senses, or those systems of the body that are associated with specific organs such as the tongue or eye. Somatosensation belongs to the general senses, which are those sensory structures that are distributed throughout the body and in the walls of various organs. The special senses are all primarily part of the somatic nervous system in that they are consciously perceived through cerebral processes, though some special senses contribute to autonomic function. The general senses can be divided into somatosensation, which is commonly considered touch, but includes tactile, pressure, vibration, temperature, and pain perception. The general senses also include the visceral senses, which are separate from the somatic nervous system function in that they do not normally rise to the level of conscious perception.
The cells that transduce sensory stimuli into the electrochemical signals of the nervous system are classified on the basis of structural or functional aspects of the cells. The structural classifications are either based on the anatomy of the cell that is interacting with the stimulus (free nerve endings, encapsulated endings, or specialized receptor cell), or where the cell is located relative to the stimulus (interoceptor, exteroceptor, proprioceptor). Thirdly, the functional classification is based on how the cell transduces the stimulus into a neural signal. Chemoreceptors respond to chemical stimuli and are the basis for olfaction and gustation. Related to chemoreceptors are osmoreceptors and nociceptors for fluid balance and pain reception, respectively. Mechanoreceptors respond to mechanical stimuli and are the basis for most aspects of somatosensation, as well as being the basis of audition and equilibrium in the inner ear. Thermoreceptors are sensitive to temperature changes, and photoreceptors are sensitive to light energy.
The nerves that convey sensory information from the periphery to the CNS are either spinal nerves, connected to the spinal cord, or cranial nerves, connected to the brain. Spinal nerves have mixed populations of fibers; some are motor fibers and some are sensory. The sensory fibers connect to the spinal cord through the dorsal root, which is attached to the dorsal root ganglion. Sensory information from the body that is conveyed through spinal nerves will project to the opposite side of the brain to be processed by the cerebral cortex. The cranial nerves can be strictly sensory fibers, such as the olfactory, optic, and vestibulocochlear nerves, or mixed sensory and motor nerves, such as the trigeminal, facial, glossopharyngeal, and vagus nerves. The cranial nerves are connected to the same side of the brain from which the sensory information originates.
14.2 Central Processing
Sensory input to the brain enters through pathways that travel through either the spinal cord (for somatosensory input from the body) or the brain stem (for everything else, except the visual and olfactory systems) to reach the diencephalon. In the diencephalon, sensory pathways reach the thalamus. This is necessary for all sensory systems to reach the cerebral cortex, except for the olfactory system that is directly connected to the frontal and temporal lobes.
The two major tracts in the spinal cord, originating from sensory neurons in the dorsal root ganglia, are the dorsal column system and the spinothalamic tract. The major differences between the two are in the type of information that is relayed to the brain and where the tracts decussate. The dorsal column system primarily carries information about touch and proprioception and crosses the midline in the medulla. The spinothalamic tract is primarily responsible for pain and temperature sensation and crosses the midline in the spinal cord at the level at which it enters. The trigeminal nerve adds similar sensation information from the head to these pathways.
The auditory pathway passes through multiple nuclei in the brain stem in which additional information is extracted from the basic frequency stimuli processed by the cochlea. Sound localization is made possible through the activity of these brain stem structures. The vestibular system enters the brain stem and influences activity in the cerebellum, spinal cord, and cerebral cortex.
The visual pathway segregates information from the two eyes so that one half of the visual field projects to the other side of the brain. Within visual cortical areas, the perception of the stimuli and their location is passed along two streams, one ventral and one dorsal. The ventral visual stream connects to structures in the temporal lobe that are important for long-term memory formation. The dorsal visual stream interacts with the somatosensory cortex in the parietal lobe, and together they can influence the activity in the frontal lobe to generate movements of the body in relation to visual information.
14.3 Motor Responses
The motor components of the somatic nervous system begin with the frontal lobe of the brain, where the prefrontal cortex is responsible for higher functions such as working memory. The integrative and associate functions of the prefrontal lobe feed into the secondary motor areas, which help plan movements. The premotor cortex and supplemental motor area then feed into the primary motor cortex that initiates movements. Large Betz cells project through the corticobulbar and corticospinal tracts to synapse on lower motor neurons in the brain stem and ventral horn of the spinal cord, respectively. These connections are responsible for generating movements of skeletal muscles.
The extrapyramidal system includes projections from the brain stem and higher centers that influence movement, mostly to maintain balance and posture, as well as to maintain muscle tone. The superior colliculus and red nucleus in the midbrain, the vestibular nuclei in the medulla, and the reticular formation throughout the brain stem each have tracts projecting to the spinal cord in this system. Descending input from the secondary motor cortices, basal nuclei, and cerebellum connect to the origins of these tracts in the brain stem.
All of these motor pathways project to the spinal cord to synapse with motor neurons in the ventral horn of the spinal cord. These lower motor neurons are the cells that connect to skeletal muscle and cause contractions. These neurons project through the spinal nerves to connect to the muscles at neuromuscular junctions. One motor neuron connects to multiple muscle fibers within a target muscle. The number of fibers that are innervated by a single motor neuron varies on the basis of the precision necessary for that muscle and the amount of force necessary for that motor unit. The quadriceps, for example, have many fibers controlled by single motor neurons for powerful contractions that do not need to be precise. The extraocular muscles have only a small number of fibers controlled by each motor neuron because moving the eyes does not require much force, but needs to be very precise.
Reflexes are the simplest circuits within the somatic nervous system. A withdrawal reflex from a painful stimulus only requires the sensory fiber that enters the spinal cord and the motor neuron that projects to a muscle. Antagonist and postural muscles can be coordinated with the withdrawal, making the connections more complex. The simple, single neuronal connection is the basis of somatic reflexes. The corneal reflex is contraction of the orbicularis oculi muscle to blink the eyelid when something touches the surface of the eye. Stretch reflexes maintain a constant length of muscles by causing a contraction of a muscle to compensate for a stretch that can be sensed by a specialized receptor called a muscle spindle.
Interactive Link Questions
Watch this video to learn about Dr. Danielle Reed of the Monell Chemical Senses Center in Philadelphia, PA, who became interested in science at an early age because of her sensory experiences. She recognized that her sense of taste was unique compared with other people she knew. Now, she studies the genetic differences between people and their sensitivities to taste stimuli. In the video, there is a brief image of a person sticking out their tongue, which has been covered with a colored dye. This is how Dr. Reed is able to visualize and count papillae on the surface of the tongue. People fall into two large groups known as “tasters” and “non-tasters” on the basis of the density of papillae on their tongue, which also indicates the number of taste buds. Non-tasters can taste food, but they are not as sensitive to certain tastes, such as bitterness. Dr. Reed discovered that she is a non-taster, which explains why she perceived bitterness differently than other people she knew. Are you very sensitive to tastes? Can you see any similarities among the members of your family?
2.Figure 14.9 The basilar membrane is the thin membrane that extends from the central core of the cochlea to the edge. What is anchored to this membrane so that they can be activated by movement of the fluids within the cochlea?
3.Watch this video to learn more about how the structures of the ear convert sound waves into a neural signal by moving the “hairs,” or stereocilia, of the cochlear duct. Specific locations along the length of the duct encode specific frequencies, or pitches. The brain interprets the meaning of the sounds we hear as music, speech, noise, etc. Which ear structures are responsible for the amplification and transfer of sound from the external ear to the inner ear?
4.Watch this animation to learn more about the inner ear and to see the cochlea unroll, with the base at the back of the image and the apex at the front. Specific wavelengths of sound cause specific regions of the basilar membrane to vibrate, much like the keys of a piano produce sound at different frequencies. Based on the animation, where do frequencies—from high to low pitches—cause activity in the hair cells within the cochlear duct?
5.Watch this video to learn more about a transverse section through the brain that depicts the visual pathway from the eye to the occipital cortex. The first half of the pathway is the projection from the RGCs through the optic nerve to the lateral geniculate nucleus in the thalamus on either side. This first fiber in the pathway synapses on a thalamic cell that then projects to the visual cortex in the occipital lobe where “seeing,” or visual perception, takes place. This video gives an abbreviated overview of the visual system by concentrating on the pathway from the eyes to the occipital lobe. The video makes the statement (at 0:45) that “specialized cells in the retina called ganglion cells convert the light rays into electrical signals.” What aspect of retinal processing is simplified by that statement? Explain your answer.
6.Watch this video to learn more about how the brain perceives 3-D motion. Similar to how retinal disparity offers 3-D moviegoers a way to extract 3-D information from the two-dimensional visual field projected onto the retina, the brain can extract information about movement in space by comparing what the two eyes see. If movement of a visual stimulus is leftward in one eye and rightward in the opposite eye, the brain interprets this as movement toward (or away) from the face along the midline. If both eyes see an object moving in the same direction, but at different rates, what would that mean for spatial movement?
7.The inability to recognize people by their faces is a troublesome problem. It can be caused by trauma, or it may be inborn. Watch this video to learn more about a person who lost the ability to recognize faces as the result of an injury. She cannot recognize the faces of close family members or herself. What other information can a person suffering from prosopagnosia use to figure out whom they are seeing?
8.Watch this video to learn more about the descending motor pathway for the somatic nervous system. The autonomic connections are mentioned, which are covered in another chapter. From this brief video, only some of the descending motor pathway of the somatic nervous system is described. Which division of the pathway is described and which division is left out?
9.Visit this site to read about an elderly woman who starts to lose the ability to control fine movements, such as speech and the movement of limbs. Many of the usual causes were ruled out. It was not a stroke, Parkinson’s disease, diabetes, or thyroid dysfunction. The next most obvious cause was medication, so her pharmacist had to be consulted. The side effect of a drug meant to help her sleep had resulted in changes in motor control. What regions of the nervous system are likely to be the focus of haloperidol side effects?
10.Watch this video to learn more about the reflex arc of the corneal reflex. When the right cornea senses a tactile stimulus, what happens to the left eye? Explain your answer.
11.Watch this video to learn more about newborn reflexes. Newborns have a set of reflexes that are expected to have been crucial to survival before the modern age. These reflexes disappear as the baby grows, as some of them may be unnecessary as they age. The video demonstrates a reflex called the Babinski reflex, in which the foot flexes dorsally and the toes splay out when the sole of the foot is lightly scratched. This is normal for newborns, but it is a sign of reduced myelination of the spinal tract in adults. Why would this reflex be a problem for an adult?
Review Questions
What type of receptor cell is responsible for transducing pain stimuli?
- mechanoreceptor
- nociceptor
- osmoreceptor
- photoreceptor
Which of these cranial nerves is part of the gustatory system?
- olfactory
- trochlear
- trigeminal
- facial
Which submodality of taste is sensitive to the pH of saliva?
- umami
- sour
- bitter
- sweet
Axons from which neuron in the retina make up the optic nerve?
- amacrine cells
- photoreceptors
- bipolar cells
- retinal ganglion cells
What type of receptor cell is involved in the sensations of sound and balance?
- photoreceptor
- chemoreceptor
- mechanoreceptor
- nociceptor
Which of these sensory modalities does not pass through the ventral posterior thalamus?
- gustatory
- proprioception
- audition
- nociception
Which nucleus in the medulla is connected to the inferior colliculus?
- solitary nucleus
- vestibular nucleus
- chief sensory nucleus
- cochlear nucleus
Visual stimuli in the upper-left visual field will be processed in what region of the primary visual cortex?
- inferior right
- inferior left
- superior right
- superior left
Which location on the body has the largest region of somatosensory cortex representing it, according to the sensory homunculus?
- lips
- thigh
- elbow
- neck
Which of the following is a direct target of the vestibular ganglion?
- superior colliculus
- cerebellum
- thalamus
- optic chiasm
Which region of the frontal lobe is responsible for initiating movement by directly connecting to cranial and spinal motor neurons?
- prefrontal cortex
- supplemental motor area
- premotor cortex
- primary motor cortex
Which extrapyramidal tract incorporates equilibrium sensations with motor commands to aid in posture and movement?
- tectospinal tract
- vestibulospinal tract
- reticulospinal tract
- corticospinal tract
Which region of gray matter in the spinal cord contains motor neurons that innervate skeletal muscles?
- ventral horn
- dorsal horn
- lateral horn
- lateral column
What type of reflex can protect the foot when a painful stimulus is sensed?
- stretch reflex
- gag reflex
- withdrawal reflex
- corneal reflex
What is the name for the topographical representation of the sensory input to the somatosensory cortex?
- homunculus
- homo sapiens
- postcentral gyrus
- primary cortex
Critical Thinking Questions
The sweetener known as stevia can replace glucose in food. What does the molecular similarity of stevia to glucose mean for the gustatory sense?
28.Why does the blind spot from the optic disc in either eye not result in a blind spot in the visual field?
29.Following a motorcycle accident, the victim loses the ability to move the right leg but has normal control over the left one, suggesting a hemisection somewhere in the thoracic region of the spinal cord. What sensory deficits would be expected in terms of touch versus pain? Explain your answer.
30.A pituitary tumor can cause perceptual losses in the lateral visual field. The pituitary gland is located directly inferior to the hypothalamus. Why would this happen?
31.The prefrontal lobotomy is a drastic—and largely out-of-practice—procedure used to disconnect that portion of the cerebral cortex from the rest of the frontal lobe and the diencephalon as a psychiatric therapy. Why would this have been thought necessary for someone with a potentially uncontrollable behavior?
32.If a reflex is a limited circuit within the somatic system, why do physical and neurological exams include them to test the health of an individual?
|
oercommons
|
2025-03-18T00:39:12.540226
|
07/23/2019
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/56377/overview",
"title": "Anatomy and Physiology, Regulation, Integration, and Control, The Somatic Nervous System",
"author": null
}
|
https://oercommons.org/courseware/lesson/64164/overview
|
WaKIDS Family Activities for Home
Overview
We would like to remind you of the Resources Library located within the Family area of the online platform, GOLD®. If you have added your student's family members and their emails to the online platform (if not see instructions below), you will be able to directly email families those activities that are based on the areas of development for your students. These activities are intended to be shared and are therefore, family friendly. Take some time to explore the Family area in GOLD® and consider using these lessons as a way to support your remote learning plan.
WaKIDS Family Activities
We would like to remind you of the Resources Library located within the Family area of the online platform, GOLD®. If you have added your student's family members and their emails to the online platform (if not see instructions below), you will be able to directly email families those activities that are based on the areas of development for your students. These activities are intended to be shared and are therefore, family friendly. Take some time to explore the Family area in GOLD® and consider using these lessons as a way to support your remote learning plan.
|
oercommons
|
2025-03-18T00:39:12.567388
|
Amber havens
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/64164/overview",
"title": "WaKIDS Family Activities for Home",
"author": "Teaching/Learning Strategy"
}
|
https://oercommons.org/courseware/lesson/87886/overview
|
Spanish Colonization
Overview
Spanish Colonization
Portugal and Spain, the first two nations to explore the Atlantic Ocean, the Americas, and Africa, took different approaches. The Portuguese, the first of the two, focused on Africa and Asia, constructing trading settlements, while the Spanish established settlements oriented toward the exploitation of natural resources in the Americas and the Caribbean Sea. After Christopher Columbus set sail in 1492 CE and “discovered” the New World for the Spanish, a wave of many other Spaniards sailed westward in attempts to find their own riches and lands. Individuals like Cortes and Pizzaro blazed the trail that other Spaniards would use to build the Spanish colonial system. The Spanish found the Aztec and the Incan civilizations and tried to integrate the indigenous populations into their colonial settlements. Furthermore, the central role of the Catholic Church and the economic model employed were key differences between the Spanish and the Portuguese. The separate continents and oceans that the Portuguese and the Spanish explored, settled, and exploited mitigated any conflict between them.
Learning Objectives
- Evaluate the differences between the Spanish, Portuguese, English, Dutch, and French colonization.
- Analyze how the Spanish colonization was different between the center and periphery regions.
- Evaluate the impact of Potosi on global economics.
- Analyze the differences in how the Spanish integrated different groups into their colonial world.
Key Terms / Key Concepts
Treaty of Tordesillas: a 1494 treaty that divided the newly discovered lands outside Europe between Portugal and the Crown of Castile, along a meridian 370 leagues west of the Cape Verde islands, off the west coast of Africa (This line of demarcation was about halfway between the Cape Verde islands, which was already Portuguese, and the islands entered by Christopher Columbus on his first voyage, which he claimed for Castile and León.)
Christopher Columbus: an Italian explorer, navigator, and colonizer who completed four voyages across the Atlantic Ocean under the monarchy of Spain, which led to general European awareness of the American continents
Bartolomé de las Casas: sixteenth-century Spanish historian, social reformer, and Dominican friar, who arrived as one of the first European settlers in the Americas and participated in the atrocities committed against the Native Americans by the Spanish colonists (In 1515, de las Casas reformed his views and advocated before King Charles V, Holy Roman Emperor, on behalf of rights for the natives.)
Mita: a form of labor tax that required one person from each family to work in the mines, which was enforced by the Spanish once they gained control of the region
Spanish
The colonial activity in the Iberian Peninsula meant that the two major states of Portugal and Spain were deep rivals. The proximity of the two states meant that they were natural rivals. When the Spanish started to explore, the Portuguese began to push back, and tensions rose between these two. In the 15th century, one of the only ways to resolve international tensions was to turn to the Pope to solve these types of conflicts. In the Middle Ages, the Pope had more political power than kings, due to the fact that the Pope could choose who could become the king. As the Spanish and Portuguese tensions rose the Pope became involved.
The Pope in 1492 was Pope Alexander VI helped to formulate the treaty between the Spanish and the Portuguese known as the Treaty of Tordesillas. TheTreaty of Tordesillas was a treaty that divided the newly discovered lands outside Europe along the meridian 370 leagues west of the Cape Verde islands, off the west coast of Africa. The treaty gave all territory outside of Europe to the East to the Portuguese, while the Spanish got everything to the West. This treaty was seen at the time as a completely fair and equal treaty between both the Spanish and the Portuguese. However, there were several underlying problems with this treaty. First, the Pope did not ask other peoples of the world, such as Africa, Asia, and Latin America, if they were okay with being owned by either the Spanish or Portuguese. The second problem was that this divided the world between the two European powers, but other groups, such as the French, Dutch, and English, were left out of colonization. But it did leave the Spanish with many new territories to expand and explore in the North and South American worlds.
Columbus was the first to sail for the Spanish and he helped to create several of the ways that the Spanish lived with the indigenous people. The island of Hispaniola had many indigenous groups, such as the Arawak. The Arawak were friendly to the Spanish and helped to establish the colonies. The Spanish, on the other hand, treated the Arawak very badly. The Spanish friar and historian Bartolomé de las Casas wrote about the treatment of the Arawak, which included enslavement, starvation, and even crucifixion. This shocking and horrible treatment of indigenous people was at odds with the laws of Spain. When Columbus left the Americas after his first voyage, he brought an indigenous ambassador to meet with Isabel and Ferdinand, the king and queen of Spain. Queen Isabel found the indigenous people very interesting and said that it was illegal to enslave the indigenous people because they had “souls.” However, the colonists, needing labor and looking down on the indigenous people, would continue a long history of mistreatment of indigenous populations.
Historians question the role of Christopher Columbus in establishing rules for the Spanish and whether or not he wanted the mistreatment of indigenous peoples or if he was simply acting from human greed. Either way, the lawlessness of the Spanish towards the indigenous people would become a key feature of the Spanish colonization. This becomes one of the biggest differences between the Spanish and the Portuguese. The Spanish developed a system of mistreatment and brutality, building their colonial empire on brutality and conquest; whereas, the Portuguese built their colonial model on a “I mapped it, I owned it” mentality.
Center vs Surrounding Regions
Another big difference between the Spanish and almost all of the other colonial states in the 15th and 16th centuries was that the Spanish found indigenous empires. This meant that they had a big advantage by taking over both the Aztec and Inca empires. These empires gave the Spanish colonial world a source of wealth and materials that they would continue to expand upon throughout the 15th to 18th centuries. The Spanish colonial system had many problems, but despite these, had a firm basis of power in Latin America.
Many Spaniards heard dreams and tales of the conquest and wanted to take as much as they could of the New World; this led to a wash of many Spaniards, each looking for another empire to conquer. While this was a dream of many conquistadors, the problem was that there were few imperial centers to conquer. Most of the conquistadors traveled throughout the Americas, searching for gold and riches, only to leave empty handed. Looking for treasures meant that they were not interested in establishing long term holdings or staying in the regions, and that created a unique opportunity for the Catholic Church to establish a center in these regions. This led to a periphery area, one that was outside of the colonial imperial centers and would have a unique role in Spanish colonization. The center of power in these periphery areas was the Catholic Church, and the areas had limited relationships with the established centers of power, which meant lack of guidance from the crown.
After Christopher Columbus landed in the Americas, the Spanish quickly established the Caribbean as a major area of colonization. The island of Hispaniola, in particular, was the center of the Spanish for exploration and conquest. Many of the conquistadors were eager to go and explore the Americas because they were lower class individuals who dreamt of having riches and treasures. The Spanish crown gave political rights to the governor of Hispaniola and Cuba specifically to allow conquistadors to travel. This meant that ventures had to be approved by the Hispaniola and Cuban governors.
Conquest of Mexico
The model of conquest that the Spanish followed was to move into a society, quickly remove the head of the government, and destroy the native religion. Then replace the local government and religion with that of the Spanish and Catholic church. This model was key for the Spanish in the conquest of the Aztec and Inca, and it would be the goal of many Spaniards following the conquest.
Learning Objectives
- Analyze how the Spanish conquered the Aztec Empire.
- Evaluate how the Spanish established a colony in the Aztec population.
Conquest of the Aztec Empire
One such conquistador that wanted to test their fortunes in the unexplored Americas was Hernán Cortés. Cortés was born to a lesser nobility family and saw exploring the Americas as his way to earn fame and fortune. He first settled in Hispaniola and found that he was not happy with the lands there before moving to Cuba. In Cuba, he earned a small plot of land and laborers. He also worked closely with the Spanish governor and became part of the colonial administration, helping to conquer the island. However, he gave up this life when he heard stories of riches elsewhere and dreamed of gaining them. The Spanish were telling tales of cities of gold, riches beyond their wildest dreams, and lands that were almost infinite. Cortés wanted to leave Cuba and gain those riches. The problem was the governor of Cuba heard about Cortés’s ambitions and the relationship between the two men became difficult. The governor had heard tales of how Cortés gaining followers and decided to revoke Cortés’s approval of exploration. Cortés, hearing that the expedition that he planned was declared illegal, decided to take the band of men that followed him and leave before the Spanish governor could arrest him. The expedition that Cortés first made to Mexico was technically illegal and was against the Spanish crown’s own wishes. But that was not the only problem that Cortés faced at that time; he was also headed into the lands of Central America, where there were existing powerful empires of indigenous people.
The Aztec were known throughout Central America as a warrior tribe that had vast riches in the capital city. The Aztecs built their empire on trade and conquest. The center of the empire was the capital city of Tenochtitlan, a city that was built on the ancient lake and had many great resources that would prove difficult for the Spanish to overtake. The Aztec emperor Montezuma II was a good emperor who expanded trade, extended the empire throughout the central valley of Mexico, and made sure that the general peace and prosperity of the Aztec empire grew during the late 15th and early 16th centuries.
Cortés arrived in Central America at the Yucatan Peninsula in 1519. Cortés and his men left the Yucatan Peninsula and moved further north, to what is today Veracruz in Mexico, before deciding where to land and untimately “discover” the Aztecs. Many of Cortés’s men had heard about the large number of people in the Aztec empire, and they knew that they would be outnumbered. To prevent mutiny of his own men, Cortés had all of his ships burned but one; this was meant to send a message to his men that they were not returning back to Cuba. Cortés was determined to defeat any opposition. Marching with few men and limited supplies towards Tenochtitlan, he wanted a chance to meet with Montezuma—the leader of the Aztecs. As he marched forward, Cortés met with and found alliances with other indigenous populations. These alliances were important because many of the indigenous people were not friendly with the Aztec and would become key alliances of the Spanish during the attack on Tenochtitlan.
In Tenochtitlan, Montezuma held a different feeling. After hearing about strangers from the east landing and looking for gold, Montezuma thought that this was an angry god that needed to be appeased. Montezuma began sending messengers with money to Cortés and his men, with messages that this was tribute to the god. Cortés, on the other hand, looking for gold, received these tribute packages and realized that there was actually much money to be made in Tenochtitlan. When Cortés did not turn around, Montezuma became worried that maybe that amount of gold was not enough. Montezuma began sending more gold and riches, as a way to appease Cortés. This only further encouraged Cortéd and his men to march toward the city.
When Cortés arrived in Tenochtitlan, he was greeted by Montezuma. After months of traveling and gaining indigenous alliances, Cortés had built an army of indigenous people that supported his overthrow of the Aztec. Montezuma peacefully received Cortés and his massive army and treated them well. Many historians believe that Montezuma thought that Cortés was a representative of the Aztec god Quetzalcoatl. But the situation changed when Cortés heard of an Aztec attack on his Spanish men near the coast of Veracruz. Meanwhile in Cuba, the Spanish governor sent other Spaniards to defeat Cortés, who left Tenochtitlan to stop the Spanish attack in Veracruz. Cortés was successful against the Cuban governor’s men, and he banded them together with his own forces.
Back in Tenochtitlan, the situation changed quickly. Cortés left Pedro Alvalrado as one of a few leaders of the Spanish in Tenochtitlan. Montezuma asked Alvarado for permission to celebrate the Feast of Toxcatl on May 22, 1520. This was a festival during which the Aztecs celebrated a popular god by sacrificing humans. While Alvalrado at first approved the celebration, once he realized that there would be human sacrifice, he attempted to stop it. When the Spanish went to the Aztec temple and attempted to stop the event, the Aztec pushed back, upset that the Spanish were intervening. A fight ensued, known as the Massacre in the Great Temple. This was not good for the Spanish conquistadors, who were vastly outnumbered in the city of Tenochtitlan and saw that the Aztec population began to turn on them. These tensions were not helped by the plagues that the Aztec suffered during this time. The Aztecs became very sick with European diseases, such as smallpox, measles, mumps, and flu. This meant that the city of close to one million people had a rampant plague attacking the population. The Aztec upper class became very upset with Montezuma because he was engaging with the Spanish. Montezuma was killed on July 1, 1520, but the history is unclear who killed Montezuma. The Spanish report that it was the Aztec that killed Montezuma because of his betrayal. The Aztec claim that it was the Spanish that killed Montezuma for fear of another attack. The death of Montezuma and the attack at the Feast of Toxcatl were two events that meant that Cortés had to return quickly to Tenochtitlan.
The death of Montezuma put the city of Tenochtitlan on edge and the people were upset at the Spanish. On the night of June 30th – July 1, 1520, the Spanish were barely able to escape from Tenochtitlan; this became known as Noche Triste. Cortés ordered his men to retreat to the nearby city of Tlaxcala. Much of the treasure looted by Cortés and his men was lost during their escape.
The city of Tenochtitlan became an epicenter of disease, and over the next few months the city’s population fell drastically ill. The population suffered greatly, and the defenses of the city were weakened. Cortés, on the other hand, began to put together an army to attack the city. Cortes was a master at finding weaknesses in the Aztec empire. One of the key problems that the Aztec had in the building of their empire was they fought many other indigenous groups in the region surrounding Tenochtitlan. Cortes brought together these groups between July to August to practice sieging and taking down the capital. By August, Cortes marched on Tenochtitlan. The yearlong attacks on the city worked, and on August 13, 1521, Cortés and the Spanish captured the Aztec Empire and claimed it for Spain. Cortés, after almost three years of fighting and conquest, was the sole leader of the largest empire in the Americas.
Cortés’s new position as leader of a large New World empire was problematic. When he left for Mexico in 1518, he did so illegally. The entire conquest of Mexico was not sanctioned by the Spanish crown. Cortés, wanting to ensure that he had the support of Charles V, began writing letters of heavy apologies. Cortés, also began sending larger than the required amounts of gold, to ensure that Charles would accept his apologies. Charles V in return granted Cortés the governorship of Mexico.
Cortés began creating the government of New Spain, one of the two centers of government in the Americas. The establishment of New Spain meant that the Spanish military was centered in the newly named Mexico City, as were the royal courts and justice buildings, which means it also became the center of Spanish bureaucracy. This meant that the Aztec population became subject to the Spanish laws and customs.
It is important to note, that most of the actions taken by the Spanish were meant to remove the indigenous ways of living and replace those with the Spanish culture. For example, many of the Aztec priests were killed and those in training, that were young enough, were sent to Catholic schools for training in Christianity. The Spanish killed all of the upper class and removed their positions of power so that the people would stop paying tribute to the Aztec upper class and instead pay that tribute to their new Spanish.
The Spanish did not want to revise many of the methods that made the Aztecs successful. Instead, the Spanish integrated many of the Aztec’s ways of government and society into the newly forming colonial culture. Some of the key differences between the Spanish and Aztec government was that the Spanish used a system of labor and tribute known as the encomienda. This was a system of rewarding Spaniards who were loyal to the conquest by giving them lands in the New World. The size of the lands that were granted were meant to be proportional to the risk each grantee undertook during the campaigns. This helped to inspire and get many lower-class Spaniards to go fight in the New World. The goal of landowning for the Spanish was not just to have land for the sake of owning more land but to produce goods. This meant that the Spanish wanted to turn many of these new territories into vast farms. But that came with another problem. The indigenous populations were forced to work for the owner of the lands that they lived on; they were not paid for their work, nor were they able to complain that this system was unfair. If the indigenous people felt that they could not live or work under the conditions of the encomienda, they were able to leave and move to a different plot of land. Unfortunately, all the areas of Mexico were given as encomiendas to loyal Spaniards. This meant that the indigenous population was forced to work for a Spaniard no manor where they went and were never able to escape Spanish control.
The conquest of Mexico demonstrates one of the two ways that the Spanish conquered a center. Cortés’s followers became rich because of the encomienda system. News spread in Spain of the wealth and power in the New World. This helped to fuel a new generation of explorers who would travel to the Americas, searching for their own riches. The conquistadors had filled their heads full of tales of riches and exotic lands, and the prize for any of their followers was vast tracks of land that could make them wealthy. This method allowed the Spanish to more easily take over an established empire and turn it into a Spanish territory.
Conquest of the Inca
Learning Objectives
- Evaluate the differences in the Spanish colonization between the Aztec and the Incan populations.
- Analyze the Incan population's impact on the colonial society.
Conquest of the Incan Empire
With the Conquest of Peru, the second center that the Spanish created was in the Andes. The Incan empire was built from a trade federation that spanned the majority of South America. The Inca empire was the largest of the civilizations in the Americas before Columbus. Formed in the Peruvian highlands in the 13th century, the Inca spread southward throughout the Andes by the 15th century. One of the keys to the Incan success was their use of tools to create central roads, terrace farming, and federation of labor and tribute from local tribes. The federation saw great successes throughout the South American continent through trade, and there was very little conflict about political leadership. The son of the Inca ruler was usually the leader of the army, this gave the leadership key understanding and insights to how the military worked.
The Incan leadership remained stable throughout the 13th to 15th centuries. In 1524, the Inca leader died of a high fever, probably due to the diseases that were appearing in South America. His death was a very big problem because he had two sons that would begin to fight for the throne of the Inca. For five years, the two brothers ruled peacefully, Atahualpa in the north and Huascar in the south. But Huascar wanted to have power in the Incan capital of Cuzco. He marched to Cuzco and arrested Atahualpa. This started a great fight between the Incan nobles, as there were some who supported Huascar as the legitimate leader of the Inca and others who supported Atahualpa. After a very bloody civil war, Atahualpa was victorious. Even though Atahualpa won, it did not mean that the Inca were not hurt, the deep division would be a key reason why the Spaniard Pizarro would be victorious.
Francisco Pizarro was a unique conquistador. He was born in Spain in 1478 CE to pig farmers. Being poor, Pizarro never learned to read or write. He left for the New World, in search of fortune and fame, in 1509 CE. Pizarro made a name for himself by accompanying Balbo as he crossed the Isthmus of Panama in 1513 CE, when he became one of the early Europeans to see the Pacific Ocean. But when there was division between Balboa and other conquistadors, Pizarro arrested Balboa and put him on trial. Balboa was ultimately beheaded in 1519 CE. Pizarro, on the other hand, was rewarded with leadership positions in the newly forming city of Panama City. While the leader of Panama City, Pizarro began to hear tales of the city of gold, which the Spanish called El Dorado. Tales began to grow throughout Panama, and Pizarro found he was interested in exploring this famed city. The conquest of Mexico in 1521 also fueled rumors and pushed Pizarro to begin looking at South America. New stories of a large empire in South America began to circulate, centering around a civilization in the mountains, that was divided. Pizarro put together an expedition in 1524 CE, but this failed due to bad weather and negative relationships with the indigenous peoples.
In 1526, Pizarro attempted his second expedition with his long-time trade partner with whom Pizarro agreed to divide the spoils of the conquest equally. After sailing south, the Spanish expedition ran into troubles with bad weather and fighting indigenous populations. Pizarro and his partner were constantly fighting about who should lead and how the expedition should be ran; this led them to dividing their men. On an island off the coast of Columbia, Pizarro divided the party by sending Almagro northward to Panama for more resources and men, while Pizarro decided to move south into Peru with only thirteen men. In 1528 CE, after several months at sea, Pizarro landed in Peru. He and his men were welcomed by indigenous people, who had numerous gold and silver decorations. Upon landing, Pizarro heard tales of a powerful king who ruled the area. He was afraid to attack with his sall number of men and returned to Panama for more resources. After much thought, Pizarro decided it best to ask the Spanish king for a request to formally conquer this new territory. This was to secure his position as the only ruler, if successful, and to ensure that he would be the most powerful man in the South American continent. After King Charles granted Pizarro his request, he began to plan for his expedition set for 1530 CE.
Pizarro’s third expedition was successful in landing in Peru. He arrived near Caxas on the Peruvian coast and sent his commander Hernando de Soto to establish relationships with the local population. It was here that Pizarro learned that the Incan leader was very close in a city called Cajamarca. Pizarro marched a small number of men south to the city of Cajamarca to meet with Atahualpa. The meeting between the two leaders was disastrous. Following the Conquest of Mexico, the Spanish crown made new laws that said before war could be declared on a population a priest had to deliver a message that any indigenous peoples who converted to Christianity and swore allegiance to the Spanish king, as well as agreed to pay tribute, would be spared and war would be averted. During the meeting between Atahualpa and Pizarro, the priest told Atahualpa this command from the king of Spain. It was reported by the Spanish that Atahualpa said that he was no man’s tributary, and war then ensued. The Battle of Cajamarca on November 16, 1532 CE ended with the defeat and capture of Atahualpa.
From legends of cities of gold, to the conquest of Mexico, Pizarro and his men were interested in getting the riches of the Inca. When Pizarro landed, seeing the gold and silver, he knew that there were vast riches in South America. With Atahualpa as a captive, Pizarro began demanding payment from the Inca for their leader. These ransom notes requested rooms full of gold and silver. At first, the Inca complied, giving the Spanish one room of gold and two of silver. Pizarro had made promises that he would release the leader when this was accomplished. Yet, when the Inca satisfied these conditions, Pizarro increased his demands. The divisions of the Inca started to show at this point, where the supporters of Huascar began to call for Atahualpa’s death, while supporters of Atahualpa wanted to continue to pay the Spanish for his release. It was clear by the middle of 1533 CE that Pizarro and the Spanish had no intention of releasing Atahualpa, after Pizarro drew twelve charges against Atahualpa. Pizarro convicted Atahualpa, and Almagro sentenced him to death in August 1533 CE.
It is interesting to note, that there was division between the Spanish on what to do with Atahualpa. Pizarro and de Soto wanted Atahualpa to remain alive, while Almagro sentenced his death. The consequences of Atahualpa’s death were immediate; the division of the Inca became unified against the Spanish. The majority of the Incan leaders began to fight against the Spanish. It would take another 200 years and the death of another Incan leader named Túpac Amaru before the Spanish were able to peacefully integrate all of the Incan society into their reign.
The integration of Peru was an important step for the Spanish Conquistadors, as they were able to successfully bring a second major empire in the Americas into their own growing political organization. The biggest difference between the Spanish conquest of the Inca was the system of trade and tribute that the Spanish gained from the Inca. The Spanish were highly interested in silver, and the Incan people brought tribute from the southern reaches of their territory. Additionally, the Spanish developed a system of forced labor called the Mita, which had originated from the Inca; the Mita used temporary forced labor to help finish projects, such as roads and bridges. However, the Spanish Mita was a bit different; the Spanish required each indigenous person to work in the mines of Potosí for a short period every several years without pay from the crown. The goal was to get as much silver from the region as possible. The problem was the Spanish used very harsh working conditions in the mining of silver and the population was not well taken care of. Silver mining was very dangerous in itself, but the other part of the Mita was the purification of the silver. After the rock was removed from the mountain, everything that was not silver had to be removed from the ore. To do this, the preferred method of the time was to boil mercury and put the silver ore in the mercury for purification. This was a very dangerous process and was very unhealthy due to the side effects of mercury poisoning. The process was first introduced by Viceroy Francisco de Toledo in the 1570s and became the backbone of the Spanish labor system in the Andes throughout the colonial period. It was estimated that 11,000 workers were forced into labor. One of the biggest effects of the Mita was the significant drop in the indigenous populations, due to harsh working conditions and unhealthy environments. The Incan empire became an important part of the economics of the Spanish in South America, the mining of silver was key to the Spanish empire and finding trade goods to send to China.
Other Spanish Conquests
Learning Objectives
- Evaluate the differences between the colonization of empires versus the other regions of Latin America.
- Evaluate the role of government and society in Latin American colonies.
Other Conquests
The Spanish conquistadors Cortés and Pizarro established a colonial stronghold that was the center of political, military, economic, and cultural life in the Americas. These centers of power were unique to the Spanish because other European powers did not find existing empires. The Spanish being early in the colonization of the Americas also meant that the extensive trade network that these centers provided allowed goods and diseases to travel quickly throughout the Americas. The quick spread of diseases is an important component of why later European explorers, most notably the English, pointed out the lack of indigenous populations in North America.
The Spanish center approach meant that political and economic power was concentrated in either Mexico City or Lima, which meant the Spanish militaries were centered in these two cities. That caused a great deal of problems. For example, as the English were starting to gain more naval experience at the end of the 16th century, they were attacking the Spanish outskirts and robbing the Spanish of their treasures. The Spanish had a very difficult time with stopping the English buccaneers' forces because of the distance from the center to the outskirts. By the time the Spanish could react the English would have been gone for months. While there were both positives and negatives of the Center model of colonization, not all the conquistadors were happy with this arrangement and several wanted to explore to find new centers of their own.
The lure of wealth and power swept through Spain as stories of the Inca and Aztec colonization became well known. This was the fuel for a new generation of conquistadors, who were eager to make it to the New World with a dream of finding that next indigenous empire to conquer. The problem with this mentality was that there were only the two major civilization centers in the Americas, and many of these newly energized conquistadors came to the New World with limited prospects. The Spanish explorers started traveling north from the Caribbean region into Florida and the American Southeast. Ponce de Leon traveled throughout Florida looking for a mythological fountain of youth, but what he found instead was that the land was very difficult to maintain agriculture and the indigenous population was very hostile to the Spanish. Hernando de Soto traveled throughout the American Southeast, establishing forts as far north as North Carolina. De Soto’s relationship with the indigenous population was very good, mainly because the Cherokee became one of the first indigenous groups to immediately adopt Spanish weapons and farming techniques. But there were no large indigenous civilizations to be found in the American Southeast, and de Soto turned southward to the Caribbean. Other conquistadors, such as Álvar Núñez Cabeza de Vaca went north from Mexico City to the American Southwest, near what is now Albuquerque, New Mexico. Finding no large civilization, Cabeza de Vaca returned back to Mexico City. Others, such as Almargo, went south from Lima in search of the next large empire in the Andes. Almagro found the expanse of the Bolivian desert to be too much and stayed closer to the Pacific Coastline, creating a tiny strip that later became Chile. These conquistadors never found the riches in the Americas that they longed for. It is important to note that the conquistadors, once they moved into a region, would often become upset at the lack of resources, interrogate the indigenous population, then move on towards a new goal. Often, these conquistadors would then leave behind priests and other Spaniards that would help to establish the region as a Spanish stronghold.
The Periphery
The conquistadors often would move quickly from place to place and leave behind other Spaniards who would do the majority of the work of colonization, especially in the periphery. The majority of the Spanish holdings were in what is considered periphery regions, which included what would later be known as California, New Mexico, Florida, Colombia, Venezuela, Chile, and the Rio de la Plata region. There was little government or need for large bureaucracy. This meant that the central power in many of these periphery areas was typically the Catholic Church. The Catholic Church became a major power in the periphery because the central mission of the Catholic Church was the indoctrination of Christianity to the indigenous populations. There were few questions and little care about the methods that the Catholic Church used to ensure that the indigenous peoples became Christians. For example, in the Rio de la Plata region of South America, the Jesuit priests created agricultural, shop craft workers and soldiers out of the Guaraní population indigenous to the region. In New Mexico, the Catholic Church used indigenous laborers and farmers to enrich itself. This led, of course, to revolts. In 1680 CE the Pueblo, of latter-day New Mexico, revolted and were successful in removing from the region the Catholic Church and the Spanish government for another 100 years.
Life in the periphery was very different than in the center. Similar to the divisions of the urban and rural today, the periphery had limitations on how strong the colonial government could be. The laws that were to provide health and safety that were created in the cities, were often times not enforced in the periphery regions. This meant that many of the indigenous people suffered and were put in unsafe and unhealthy conditions. This demanding and dangerous work of indigenous people meant that there was a significant decrease in the indigenous populations throughout the 15th to 18th centuries.
Spanish Colonial Culture
Learning Objectives
- Evaluate the differences in the colonial Latin American structures.
Latin American colonial culture rested upon the mixture of African, indigenous, and European cultures. While most people think that the conquistadors were individuals who conquered large territories, it is important to note that these were usually single men from Spain. The conquistadors traditionally came from the lower class and were single males. This is important to note, because Spanish colonization affected by these men who found themselves surrounded by women from indigenous and African heritages.
Very soon, it was clear to the Spanish administration that they needed to keep track and provided a chart to help understand and organize the different racial categories in the Spanish world. These were known as the Casta Charts, the name came from the Indian Caste system. The organization was to help colonial and bureaucratic leaders understand and know the populations that they served. While it appears that the Spanish system was very structured and individuals had only one option in life, this is not quite true. The Catholic Church kept records of births within the colonial system. An individual could go to a priest where their records were held, and ask the priest, for a fee, to remove the racial category that they were at and move them to a higher one. This type of bribery demonstrates that individuals in the Spanish system could purchase whiteness and move higher in the racial hierarchy. Being higher in the racial hierarchy meant better access to jobs and social circles. The division of ethnicity was one of the complicated measures that would divide the colonial Spanish Americas, another was birthplace.
The Spanish used places of birth to assign political and economic powers. Spaniards born on the Iberian Peninsula were called Peninsulares. The Peninsulares were individuals who could rise to the level of governor; they had the ability to go to Latin America and had limited restraints on their power. People born in the Americas were called Creoles, they were individuals who had less power, usually they were not able to rise to middle to upper level of government. This division created deep resentment in populations in the Americas because these were quality jobs with political and economic powers attached. The division between Creole and Peninsulares created a long-term division that would help to push the Spanish colonial society to the brink of revolution in the end of the 18th century.
Social circles and classes were very important to the colonial Spanish America. The encomiendas was where large estates and vast amounts of material wealth were centered. These newly forming estates were critical for the upper class and developed what historians have termed the plantocracy, a hierarchy based on plantation ownership. In plantocracies plantation owners are at the top, and their families are in the tier below, enjoying less power. Usually the plantation owner’s wife, known as the plantation mistress, would have been the second most powerful person on the plantation, followed closely by the plantation owner’s children. Because these farms were so big and needed so much help to manage, the plantation owners often times hired lower class whites to help manage farms and resources. This third tier is important because the community that came from outside the family life was central to the political and economic status of the plantation owner. The lowest rung was the slaves and indigenous populations that were forced to do the work; they were often brutalized and treated very terribly by those ranked above them. The class system in Spanish America demonstrated the key problems of class and race in the colonial world.
The other way that Spanish Latin American culture was divided was along gender lines. The Spanish colonial system included rigid gender roles for both men and women. Women were expected to support the males and provide children. There were few jobs for women and limited educational opportunities. In popular culture, women inhabited one of two roles: either the Madonna or the prostitute. Men, on the other hand, were not held to the same standards and the role of masculinity was defined by domination. It is during this period that the development of the hypermasculine became the traditional role of men. The stark differences between men and women provides a unique lens when viewing such amazing women like Sor Juanita. Juana Inés de la Cruz was a Mexican writer, philosopher, composer, poet, and nun. She was a central figure during the Spanish Golden Age of literature. She was taught herself to read from a library that she inherited from her grandfather and began to write poems after becoming a nun. Sor Juanita became a voice for women and spoke out against the corruption of the church and the men of Mexico City.
The Spanish system demonstrates how different they were from the English, French, or Dutch with their colonial worlds. The Spanish social division between creole and peninsulares was the critical division that other Europeans did not create. The role of African and indigenous in the Spanish system was another key difference from those of the English and the French. The centers of power meant that the Spanish integrated the indigenous populations quickly into their world as laborers, which led to their constant mistreatment by those at upper levels of the colonial society. One of the most significant points for the Spanish colonization was the economic resources that were extracted in the colonial peripheries that would have a critical role on the world stage. The Spanish colonial system had two significant components, the center and the periphery. The conquest of Mexico and the Inca were important because they were the empires that the Spanish built most of their own political power upon. Life in the periphery was dominated by the Catholic Church and centered on the relationship between the indigenous and farm life. The Spanish were different than their Portuguese counterparts in that the Portuguese had a very hands-on mentality. The “I mapped it, I own it,” provided a good starting point. The Spanish, on the other hand, used brutality to repress indigenous and African populations. While the Spanish had incredible amounts of resource wealth with the empire systems, other colonizers did not have such good luck, and were forced to focus their empires on trade relationships.
Economics: Potosí
The original goal of Europeans sailing westward was to find new ways to get to China and get more trade goods. The discovery of America was a serendipitous event that created new opportunities for Europeans. But while Latin America was growing economically profitable, Europeans were still wanting to gain a bigger footprint in China. However, the Chinese were not interested in any of the new products that the Spanish brought from the New World. The Spanish goods would go to China and would languish with little to no interest from Chinese buyers. The turning point for Spanish goods was the trade of silver from Latin America. The Spanish discovered the mountain of Potosí in the South American Andes mountains that was almost a pure silver vein. This mountain, in modern Bolivia, provided the majority of the silver Spain sent to China.
During the Middle Ages, the printing of flying cash meant that the Chinese economy was heavily hit by rampant inflation. The government began demanding taxes to be paid in silver. With the Spanish importing silver in massive quantities, this meant that silver value began to decrease in comparison to other metals and the everyday person saw relief from their government debts. The importing of silver was a significant benefit to the average Chinese person, and this opened China for the Spanish. On the flip side, this caused significant problems for the Chinese, because the massive amounts of silver that was imported from Latin America caused rampant deflation of silver, and the value crashed. This caused a ripple effect that helped to destabilize the Chinese economy.
In the Spanish empire, the rampant inflation caused ripple effects for the colonizer. The colonization of Latin America gave the Spanish access to large territories and many trade goods. But, on the Iberian side of the Atlantic, there were significant political and economic problems. The reign of Charles V was the high-water mark for the Spanish crown. Charles’s administration requested that all the silver that went to China first pass through the Iberian Peninsula. This meant that the Spanish added an extra leg of the journey for the silver and added vast amounts of cost for transport of that silver. This was at the same time of the Protestant Reformation and when Charles V, as Holy Roman Emperor, was attempting to squash the Protestants in the German territories. To leverage more war materials, Charles took loans against the silver coming out of Latin America. This put Spain in a weaker state because the silver was a key resource in the global trade with China, and European bankers understood that value. For many years, Charles took loans against the silver investments of Latin America, but eventually Spain became too indebted to bankers. This meant that the Spanish could no longer use the silver to finance wars, such as the Thirty Years War, as well as that they lost political and economic power in Europe. This weakened state had a dramatic effect on the colonial world. Throughout the 15th to 18th centuries, the Spanish had power over their colonies, but through unusual laws, cultural practices like the creole and government positions, and the growth of the British in the Americas, the Spanish empire lost significant political and economic power in the New World. This weakening of the Spanish led to a significant opening for other European colonizers, such as the French, Dutch and British.
The Spanish in the Pacific
Middle-aged but bold, Ferdinand Magellan sought to strengthen Portuguese claims in the Pacific. Specifically, he sought a westward route to the Spice Islands. This precarious, uncharted route would give the Portuguese uncontested access to the Spice Islands. However, unimpressed by the proposal, the Portuguese king quickly dismissed Magellan. [b]Not dissuaded, Magellan immediately turned his attention to Portugal’s direct rival, Spain. Unlike his Portuguese counterpart, King Charles I was quick to support Magellan’s endeavor. In 1519, under Spain’s banner, Magellan’s fleet set forth on the new, westward route across the Atlantic and Pacific to the Spice Islands.
Magellan’s voyage was fraught with trouble for months. Disease, malnutrition, starvation and mutiny all plagued his fleet. Harsh seas battled their ships for eighteen months, as the crews navigated the fierce waters around the horn of South America, now famously known as the Straights of Magellan. In spring 1521, the crews spotted Guam. A month later, they landed in the present-day Philippines.
Reception of Magellan by the indigenous peoples in the Philippines was mixed. At times, the Europeans were treated as guests. Other encounters proved hostile. Hostility arose over Magellan’s attempt to convert local inhabitants to Christianity. The chieftain of the Mactan tribe in the Philippines considered the new arrivals a serious threat. In April 1521, conflict exploded between the Mactan peoples and Magellan’s forces. The Spanish were overwhelmed, and Magellan was speared and killed in the battle. The surviving Spanish retreated to Spain, bedraggled and defeated.
Fifty years after the defeat by the Mactan, the Spanish returned to the Philippines under the leadership of Miguel Lopez de Legazpi. After that they remained a dominant presence in the Philippines, establishing a stronghold at Manila: “The Pearl of the Orient.”
With the Spanish domination of Manila came the spread of Catholicism. Augustinians, Jesuits, Franciscans, and Dominican friars and missionaries established themselves in the Philippines. And conversion spread throughout the Philippines.
Manila grew into a cosmopolitan city that outshone Seville in its brilliance and diversity. A unique, blended culture of Spanish, Chinese, Malay, Tagalong, and Muslim peoples and customs [4]emerged. But like its Portuguese rivals, the Spanish capital in the Philippines remained under threat of internal and external attack. The trade network which the Spanish had worked so hard to establish, flourished. However, it would not be long before new rivals threatened to destroy everything the Spanish had worked to create.
Primary Source: Letter from Christopher Columbus
Letter from Christopher Columbus [Abridged]
Christopher Columbus (1493)
On the thirty-third day after leaving Cadiz I came into the Indian Sea, where I discovered many islands inhabited by numerous people. I took possession of all of them for our most fortunate King by making public proclamation and unfurling his standard, no one making any resistance. To the first of them I have given the name of our blessed Saviour, whose aid I have reached this and all the rest; but the Indians call it Guanahani. To each of the others also I gave a new name, ordering one to be called Sancta Maria de Concepcion, another Fernandina, another Isabella, another Juana; and so with all the rest. As soon as we reached the island which I have just said was called Juana, I sailed along its coast some considerable distance towards the West, and found it to be so large, without any apparent end, that I believed it was not an island, but a continent, a province of Cathay. But I saw neither towns nor cities lying on the seaboard, only some villages and country farms, with whose inhabitants I could not get speech, because they fled as soon as they beheld us. I continued on, supposing I should come upon some city, or country-houses. At last, finding that no discoveries rewarded our further progress, and that this course was leading us towards the North, which I was desirous of avoiding, as it was now winter in these regions, and it had always been my intention to proceed Southwards, and the winds also were favorable to such desires, I concluded not to attempt any other adventures; so, turning back, I came again to a certain harbor, which I had remarked. From there I sent two of our men into the country to learn whether there was any king or cities in that land. They journeyed for three days, and found innumerable people and habitations, but small and having no fixed government; on which account they returned. Meanwhile I had learned from some Indians, whom I had seized at this place, that this country was really an island. Consequently I continued along towards the East, as much as 322 miles, always hugging the shore. Where was the very extremity of the island, from there I saw another island to the Eastwards, distant 54 miles from this Juana, which I named Hispana; and proceeded to it, and directed my course for 564 miles East by North as it were, just as I had done at Juana…
…The inhabitants of both sexes of this and of all the other island I have seen, or of which I have any knowledge, always go as 2 naked as they came into the world, except that some of the women cover their private parts with leaves or branches, or a veil of cotton, which they prepare themselves for this purpose. They are all, as I said before, unprovided with any sort of iron, and they are destitute of arms, which are entirely unknown to them, and for which they are not adapted; not on account of any bodily deformity, for they are well made, but because they are timid and full of terror. They carry, however, canes dried in the sun in place of weapons, upon whose roots they fix a wooded shaft, dried and sharpened to a point. But they never dare to make use of these; for it has often happened, when I have sent two or three of my men to some of their villages to speak with the inhabitants, that a crowd of Indians has sallied forth; but when they saw our men approaching, they speedily took to flight, parents abandoning children, and children their parents. This happened not because any loss or injury had been inflicted upon any of them. On the contrary I gave whatever I had, cloth and many other things, to whomsoever I approached, or with whom I could get speech, without any return being made to me; but they are by nature fearful and timid. But when they see that they are safe, and all fear is banished, they are very guileless and honest, and very liberal of all they have. No one refuses the asker anything that he possesses; on the contrary they themselves invite us to ask for it. They manifest the greatest affection towards all of us, exchanging valuable things for trifles, content with the very least thing or nothing at all. But I forbade giving them a very trifling thing and of no value, such as bits of plates, dishes, or glass; also nails and straps; although it seemed to them, if they could get such, that they had acquired the most beautiful jewels in the world.
From The University of Texas at Austin, Thomas Jefferson Center for the Study of Core Texts & Ideas
Primary Source: Aztec Account of Spanish Colonization
In 1519 Hernan Cortés sailed from Cuba, landed in Mexico and made his way to the Aztec capital. Miguel LeonPortilla, a Mexican anthropologist, gathered accounts by the Aztecs, some of which were written shortly after the conquest.
Speeches of Motecuhzoma and Cortés
When Motecuhzoma [Montezuma] had given necklaces to each one, Cortés asked him: "Are you Motecuhzoma? Are you the king? Is it true that you are the king Motecuhzoma?"
And the king said: "Yes, I am Motecuhzoma." Then he stood up to welcome Cortés; he came forward, bowed his head low and addressed him in these words: "Our lord, you are weary. The journey has tired you, but now you have arrived on the earth. You have come to your city, Mexico. You have come here to sit on your throne, to sit under its canopy.
"The kings who have gone before, your representatives, guarded it and preserved it for your coming. The kings Itzcoatl, Motecuhzoma the Elder, Axayacatl, Tizoc and Ahuitzol ruled for you in the City of Mexico. The people were protected by their swords and sheltered by their shields.
"Do the kings know the destiny of those they left behind, their posterity? If only they are watching! If only they can see what I see!
"No, it is not a dream. I am not walking in my sleep. I am not seeing you in my dreams.... I have seen you at last! I have met you face to face! I was in agony for five days, for ten days, with my eyes fixed on the Region of the Mystery. And now you have come out of the clouds and mists to sit on your throne again.
"This was foretold by the kings who governed your city, and now it has taken place. You have come back to us; you have come down from the sky. Rest now, and take possession of your royal houses. Welcome to your land, my lords! "
When Motecuhzoma had finished, La Malinche translated his address into Spanish so that the Captain could understand it. Cortés replied in his strange and savage tongue, speaking first to La Malinche: "Tell Motecuhzoma that we are his friends. There is nothing to fear. We have wanted to see him for a long time, and now we have seen his face and heard his words. Tell him that we love him well and that our hearts are contented."
Then he said to Motecuhzoma: "We have come to your house in Mexico as friends. There is nothing to fear."
La Malinche translated this speech and the Spaniards grasped Motecuhzoma's hands and patted his back to show their affection for him....
Massacre in the Main Temple
During this time, the people asked Motecuhzoma how they should celebrate their god's fiesta. He said: "Dress him in all his finery, in all his sacred ornaments."
During this same time, The Sun commanded that Motecuhzoma and Itzcohuatzin, the military chief of Tlatelolco, be made prisoners. The Spaniards hanged a chief from Acolhuacan named Nezahualquentzin. They also murdered the king of Nauhtla, Cohualpopocatzin, by wounding him with arrows and then burning him alive.
For this reason, our warriors were on guard at the Eagle Gate. The sentries from Tenochtitlan stood at one side of the gate, and the sentries from Tlatelolco at the other. But messengers came to tell them to dress the figure of Huitzilopochtli. They left their posts and went to dress him in his sacred finery: his ornaments and his paper clothing.
When this had been done, the celebrants began to sing their songs. That is how they celebrated the first day of the fiesta. On the second day they began to sing again, but without warning they were all put to death. The dancers and singers were completely unarmed. They brought only their embroidered cloaks, their turquoises, their lip plugs, their necklaces, their clusters of heron feathers, their trinkets made of deer hooves. Those who played the drums, the old men, had brought their gourds of snuff and their timbrels.
The Spaniards attacked the musicians first, slashing at their hands and faces until they had killed all of them. The singers-and even the spectators- were also killed. This slaughter in the Sacred Patio went on for three hours. Then the Spaniards burst into the rooms of the temple to kill the others: those who were carrying water, or bringing fodder for the horses, or grinding meal, or sweeping, or standing watch over this work.
The king Motecuhzoma, who was accompanied by Itzcohuatzin and by those who had brought food for the Spaniards, protested: "Our lords, that is enough! What are you doing? These people are not carrying shields or macanas. Our lords, they are completely unarmed!"
The Sun had treacherously murdered our people on the twentieth day after the captain left for the coast. We allowed the Captain to return to the city in peace. But on the following day we attacked him with all our might, and that was the beginning of the war
From Miguel LeonPortilla, ed., The Brohen Spears: The Aztec Account of the Conquest of Mexico (Boston: Beacon Press, 1962), pp. 6466, 129131.
Primary Source: Las Casas Destruction of the West Indies
A Short Account of the Destruction of the Indies
Bartolome de las Casas (1542)
The Americas were discovered in 1492, and the first Christian settlements established by the Spanish the following year. It is accordingly forty-nine years now since Spaniards began arriving in numbers in this part of the world. They first settled the large and fertile island of Hispaniola, which boasts six hundred leagues of coastline and is surrounded by a great many other large islands, all of them, as I saw for myself, with as high a native population as anywhere on earth. Of the coast of the mainland, which, at its nearest point, is a little over two hundred and fifty leagues from Hispaniola, more than ten thousand leagues had been explored by 1541, and more are being discovered every day. This coastline, too, was swarming with people and it would seem, if we are to judge by those areas so far explored, that the Almighty selected this part of the world as home to the greater part of the human race.
God made all the peoples of this area, many and varied as they are, as open and as innocent as can be imagined. The simplest people in the world - unassuming, long-suffering, unassertive, and submissive - they are without malice or guile, and are utterly faithful and obedient both to their own native lords and to the Spaniards in whose service they now find themselves. Never quarrelsome or belligerent or boisterous, they harbour no grudges and do not seek to settle old scores; indeed, the notions of revenge, rancour, and hatred are quite foreign to them. At the same time, they are among the least robust of human beings: their delicate constitutions make them unable to withstand hard work or suffering and render them liable to succumb to almost any illness, no matter how mild. Even the common people are no tougher than princes or than other Europeans born with a silver spoon in their mouths and who spend their lives shielded from the rigours of the outside world. They are also among the poorest people on the face of the earth; they own next to nothing and have no urge to acquire material possessions. As a result they are neither ambitious nor greedy, and are totally uninterested in worldly power. Their diet is every bit as poor and as monotonous, in quantity and in kind, as that enjoyed by the Desert Fathers. Most of them go naked, save for a loincloth to cover their modesty; at best they may wrap themselves in a piece of cotton material a yard or two square. Most sleep on matting, although a few possess a kind of hanging net, known in the language of Hispaniola as a hammock. They are innocent and pure in mind and have a lively intelligence, all of which makes them particularly receptive to learning and understanding the truths of our Catholic faith and to being instructed in virtue; indeed, God has invested them with fewer impediments in this regard than any other people on earth. Once they begin to learn of the Christian faith they become so keen to know more, to receive the Sacraments, and to worship God, that the missionaries who instruct them do truly have to be men of exceptional patience and forbearance; and over the years I have time and again met Spanish laymen who have been so struck by the natural goodness that shines through these people that they frequently can be heard to exclaim: 'These would be the most blessed people on earth if only they were given the chance to convert to Christianity.'
It was upon these gentle lambs, imbued by the Creator with all the qualities we have mentioned, that from the very first day they clapped eyes on them the Spanish fell like ravening wolves upon the fold, or like tigers and savage lions who have not eaten meat for days. The pattern established at the outset has remained unchanged to this day, and the Spaniards still do nothing save tear the natives to shreds, murder them and inflict upon them untold misery, suffering and distress, tormenting, harrying and persecuting them mercilessly. We shall in due course describe some of the many ingenious methods of torture they have invented and refined for this purpose, but one can get some idea of the effectiveness of their methods from the figures alone. When the Spanish first journeyed there, the indigenous population of the island of Hispaniola stood at some three million; today only two hundred survive. The island of Cuba, which extends for a distance almost as great as that separating Valladolid from Rome, is now to all intents and purposes uninhabited;" and two other large, beautiful and fertile islands, Puerto Rico and Jamaica, have been similarly devastated. Not a living soul remains today on any of the islands of the Bahamas, which lie to the north of Hispaniola and Cuba, even though every single one of the sixty or so islands in the group, as well as those known as the Isles of Giants and others in the area, both large and small, is more fertile and more beautiful than the Royal Gardens in Seville and the climate is as healthy as anywhere on earth. The native population, which once numbered some five hundred thousand, was wiped out by forcible expatriation to the island of Hispaniola, a policy adopted by the Spaniards in an endeavour to make up losses among the indigenous population of that island. One God-fearing individual was moved to mount an expedition to seek out those who had escaped the Spanish trawl and were still living in the Bahamas and to save their souls by converting them to Christianity, but, by the end of a search lasting three whole years, they had found only the eleven survivors I saw with my own eyes. A further thirty or so islands in the region of Puerto Rico are also now uninhabited and left to go to rack and ruin as a direct result of the same practices. All these islands, which together must run to over two thousand leagues, are now abandoned and desolate.
From Modern History Sourcebook, Fordham University
Attributions
Attributions
Images courtesy of Wikimedia Commons: https://en.wikipedia.org/wiki/Luis_de_Mena#/media/File:Casta_Painting_by_Luis_de_Mena.jpg
Boundless World History
https://www.coursehero.com/study-guides/boundless-worldhistory/the-age-of-discovery/
https://www.coursehero.com/study-guides/boundless-worldhistory/spain-and-catholicism/
Work based around the ideas of Patricia Seed: Ceremonies of Possession in Europe's Conquest of the New World, 1492–1640
|
oercommons
|
2025-03-18T00:39:12.614715
|
Neil Greenwood
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/87886/overview",
"title": "Statewide Dual Credit World History, The Making of Early Modern World 1450-1700 CE, Chapter 6: Exploration, Spanish Colonization",
"author": "Anna McCollum"
}
|
https://oercommons.org/courseware/lesson/113295/overview
|
OER Storytelling Template
Overview
OER Fellows are invited to remix this OER Storytelling Template to share their stories of impact with Open Educational Resources (OER).
My OER Story of Impact
OER Fellows are invited to remix this OER Storytelling Template to share their stories of impact. Once logged in, click the remix button on this resource to make your own version of this template. Change the title to describe your video and add any relevant text, links, and/or attachments to this section. Delete these instructions before publishing. When you are ready to publish, click the green next button, then update the overview, license, and description of your resource, and then click publish. For Core Story and Story Spine & Structure Activities, and OER in Arizona Videos, check out the OER Fellowship Storytelling Activities.
You are welcome to record and share your stories, using the video recording devices and software that you prefer and have access to, or digital tools such as zoom, youtube, or Pecha Kucha (for image sharing and voice recording). Be sure to give your video a clear title and description, highlight what’s happening with OER and the impact it is having on your campus in your story, and make sure that you have permission to share it. We recommend that your video is less than 10 minutes long for optimal engagement.
Describe your video in this section. You have various options for how you can share your video, as a link, upload and embed the video in this section, or add it as an attachment.
- To add your video as a link, click the link button in the editor above. Add the display text and the url, then click save. Here is a video link example: Why OER? Video
- To upload your video from your computer, click on the video button in the editor above. Click the upload tab, then Browse Server to select your video and click upload. Then add your title to the info tab and click save. Here is an uploaded video example:
- To add your video as an attachment. Click the "Attach Section 1 Resources" paperclip image below, then choose the correct file from your computer, name your video, and save. An example of a video attachment is displayed below and in the resource library.
|
oercommons
|
2025-03-18T00:39:12.642706
|
02/21/2024
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/113295/overview",
"title": "OER Storytelling Template",
"author": "Megan Simmons"
}
|
https://oercommons.org/courseware/lesson/69820/overview
|
Safely Reopening Schools
Overview
This training intends to inform educators about the various roles and responsibilities that stakeholders must fulfill in order for school districts to safely reopen schools.
Workshop Description
The Safely Reopening Schools workshop invites Belchertown High School educators to learn school district, family, student, and educator roles and responsibilities regarding how to safely reopen schools during the COVID-19 pandemic, preferably during the Fall 2020 term.
Intended Audience
All Belchertown High School educators must successfully complete this training prior to students' first day of in person classes.
Learning Objectives
Educators will:
- Explain and model school district, familiy, student, and educators roles and responsibilities regarding how to maintain a safe and healthy teaching and learning environment at Belchertown High School.
- Develop ways to educate students about all stakeholders' roles and responsibilities.
- Recommend educational strategies that ensure that all stakeholders are fullfilling their roles and responsibilities to the entire faculty and staff.
Tools
The Safely Reopening Schools training will employ readings, videos, and Google documents to ensure that educators learn and can articulate all stakeholders' roles and responsibilities.
Delivery Format
The Safely Reopening Schools training will include both asynchronous and synchronous components. Educators will participate in a majority of the workshop asynchronously to allow them to access and digest the content at their own pace. Educators will then meet synchronously at a designated date and time to ask questions and share thoughts, concerns, and recommendations. The synchronous portion of the training will also provide opportunities for educators to reach consensus regarding how to effectively ensure that all stakeholders fulfill their roles and responsibilities.
Agenda
- Welcome!
- Module 1: Current COVID-19 Research
- Module 2: School District Roles and Responsibilities
- Module 3: Family and Student Roles and Responsibilities
- Module 4: Educator Roles and Responsibilities
- Module 5: Anticipated Daily School Routine and Schedule
- Module 6: Encouraging Stakeholders to Fulfill Their Roles and Responsibilities
Module 1: Current COVID-19 Research
Learning Objectives:
Educators will:
- Summarize current COVID-19 research.
- Explain why and how the research indicates that school districts can safely reopen.
Module 2: School District Roles and Responsibilities
Learning Objectives:
Educators will:
- Summarize and explain school districts' roles and responsibilities respective to safely reopening schools.
Module 3: Family and Student Roles and Responsibilities
Learning Objectives:
Educators will:
- Summarize and explain families' and students' roles and responsibilities respective to safely reopening schools.
Module 4: Educator Roles and Responsibilities
Learning Objectives:
Educators will:
- Summarize and explain educators' roles and responsibilities respective to safely reopening schools.
Module 5: Anticipated Daily School Routine and Schedule
Learning Objectives:
Educators will:
- Summarize and explain the steps educators and students must take prior to leaving their home to go to school.
- Summarize and explain the steps educators and students must take upon entering the school building.
- Summarize and explain procedures that allow students and staff to transition from one space in the building to another space in the building.
- Summarize and explain the new lunch routine.
- Summarize and explain the new dismissal routine.
Module 6: Encouraging Stakeholders to Fulfill Their Roles and Responsibilities
Learning Objectives:
Educators will:
- Develop and recommend practices that will encourage stakeholders to fulfill their roles and responsibilities.
Workshop Evaluation
Please complete the short feedback survey to help improve future training sessions.
|
oercommons
|
2025-03-18T00:39:12.665487
|
07/19/2020
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/69820/overview",
"title": "Safely Reopening Schools",
"author": "Robert Marchewka"
}
|
https://oercommons.org/courseware/lesson/82018/overview
|
Education Standards
B. WA OER Third Grade EFSIS-Weather
C. WA OER Third Grade EFSIS-Weather PDF
D. NASA weather climate poster
E. National Geographic Neil DeGrasse Tyson Climate vs. Weather
F. Comparative Input Chart
G. Google Doc Scout Cards
H. PDF Scout Cards
I. WA OER Learning Log Questions- Weather
J. History Channel National Parks
K. WA OER Printable-List-of-US-National-Parks-in-Alphabetical-Order-PDF
L. Data Collection Pages
M. Travel Brochure Template
N. Opinion Essay
O. NOAA Hurricane Satelite Imagery page
P. NOAA Hurricane Names List
Q. Extreme Weather Partner Cards
R. Extreme Weather Graphic Organizer
S. Process Grid
T. Weather Related Hazard Design Solution (Student Version)
Third Grade Elementary Science and Integrated Subjects-Weather
Overview
The Third Grade Elementary Framework for Science and Integrated Subjects, Weather, uses the phenomena of extreme weather events. It is part of Elementary Framework for Science and Integrated Subjects project, a statewide Clime Time collaboration among ESD 123, ESD 105, North Central ESD, and the Office of Superintendent of Public Instruction. Development of the resources is in response to a need for research- based science lessons for elementary teachers that are integrated with English language arts, mathematics and other subjects such as social studies. The template for Elementary Science and Integrated Subjects can serve as an organized, coherent and research-based roadmap for teachers in the development of their own NGSS aligned science lessons. Lessons can also be useful for classrooms that have no adopted curriculum as well as to serve as enhancements for current science curriculum. The EFSIS project brings together grade level teams of teachers to develop lessons or suites of lessons that are 1) pnenomena based, focused on grade level Performance Expectations, and 2) leverage ELA and Mathematics Washington State Learning Standards.
Standards, Phenomena Big Ideas and Routines
Development Team:
Jamie Whitmire, Kennewick School District
Sarah Winakur, Pasco School District
Katy Cavanaugh, Ellensburg School District
Third Grade
Weather
Frameworks for Elementary Science and Integrated Subjects are designed to be an example of how to develop a coherent lesson or suite of lessons that integrate other content areas such as English Language Arts, Mathematics and other subjects into science learning for students. The examples provide teachers with ways to think about all standards, identify anchoring phenomena, and plan for coherence in science and integrated subjects learning
Third Grade Disciplinary Core Ideas include PS2, LS1, LS2, LS3, LS4, ESS2, and ESS3
For LS4, ESS2, and ESS3 Third Grade students are expected to develop an understanding of:
- How to organize and use data to describe typical weather conditions expected during a particular season.
- How to apply their understanding of weather-related hazards to make a claim about the merit of a design solution that reduces the impacts of such hazards.
The Crosscutting Concepts are called out as organizing concepts for these disciplinary core ideas.
Crosscutting Concepts:
- Patterns
- Cause and effect
Students are expected to use the practices to demonstrate understanding of the core ideas.
Science and Engineering Practices:
- Analyzing and interpreting data
- Engaging in argument from evidence
- Obtaining, evaluating, and communicating information
Performance Expectation(s)
Identify Performance Expectation(s) from Next Generation Science Standards that will be your focus (Climate Science related PEs preferred but not mandatory). Copy and paste below all the possible disciplinary core ideas and performance expectations that relate to your topic.
3-ESS2-1. Represent data in tables and graphical displays to describe typical weather conditions expected during a
particular season. [Clarification Statement: Examples of data could include average temperature, precipitation, and wind direction.] [Assessment Boundary:
Assessment of graphical displays is limited to pictographs and bar graphs. Assessment does not include climate change.]
3-ESS2-2. Obtain and combine information to describe climates in different regions of the world.
3-ESS3-1. Make a claim about the merit of a design solution that reduces the impacts of a weather-related hazard.*
[Clarification Statement: Examples of design solutions to weather-related hazards could include barriers to prevent flooding, wind resistant roofs, and lightning rods.]
Science and Engineering Practices
Which SEPs will be a focus for investigating this topic/phenomenon?
Analyzing data in 3–5 builds on K–2 experiences and progresses to introducing quantitative approaches to collecting data and conducting multiple trials of qualitative observations. When possible and feasible, digital tools should be used.
(3-LS4-1) Analyze and interpret data to make sense of phenomena using logical reasoning.
(3-ESS2-1) Represent data in tables and various graphical displays (bar graphs and pictographs) to reveal patterns that indicate relationships
Obtaining, evaluating, and communicating information in 3–5 builds on K–2 experiences and progresses to evaluating the merit and accuracy of ideas and methods.
(3-ESS2-2) Obtain and combine information from books and other reliable media to explain phenomena.
Engaging in argument from evidence in 3–5 builds on K–2 experiences and progresses to critiquing the scientific explanations or solutions proposed by peers by citing relevant evidence about the natural and designed world(s).
(3-ESS3-1) Make a claim about the merit of a solution to a problem by citing relevant evidence about how it meets the criteria and constraints of the problem. (3-ESS3-1)
Crosscutting Concepts
Which Crosscutting Concepts will be a focus for investigating this topic/phenomenon?
(3-ESS2-1) (3-ESS2-2) Patterns – Patterns of change can be used to make predictions.
(3-ESS3-1) Cause and Effect - Cause and effect relationships are routinely identified, tested, and used to explain change.
English Language Arts (ELA) Standards
How will I Integrate ELA Standards (which standard, what strategy…?)
RI.3.1 Ask and answer questions to demonstrate understanding of a text, referring explicitly to the text as the basis for the answers.
(3-LS4-1) (3-ESS2-2
RI.3.2 Determine the main idea of a text; recount the key details and explain how they support the main idea. (3-LS4-1)
RI.3.3 Describe the relationship between a series of historical events, scientific ideas or concepts, or steps in technical procedures in a text, using language that pertains to time, sequence, and cause/effect. (3-LS4-1)
RI.3.9 Compare and contrast the most important points and key details presented in two texts on the same topic. (3-ESS2-2)
W.3.1 Write opinion pieces on topics or texts, supporting a point of view with reasons. (3-LS4-1) (3-ESS3-1)
W.3.2 Write informative/explanatory texts to examine a topic and convey ideas and information clearly. (3-LS4-1)
W.3.7 Conduct short research projects that build knowledge about a topic. (3-ESS3-1)
W.3.8 Recall information from experiences or gather information from print and digital sources; take brief notes on sources and sort evidence into provided categories. (3-LS4-1) (3-ESS2-2)
Mathematics Standards
How will I Integrate Mathematics Standards?
MP.2 Reason abstractly and quantitatively. (3-LS4-1) (3-ESS2-1) (3-ESS2-2) (3-ESS3-1) (3-ESS3-1):
MP.4 Model with mathematics. (3-LS4-1) (3-ESS2-1). (3-ESS2-2)
MP.5 Use appropriate tools strategically. (3-LS4-1) (3-ESS2-1)
3.MD.B.3 Draw a scaled picture graph and a scaled bar graph to represent a data set with several categories. Solve one- and two-step “how many more” and “how many less” problems using information presented in bar graphs. (3-ESS2-1)
Phenomena
National Geographic Extreme Weather on our Planet https://www.nationalgeographic.org/activity/extreme-weather-on-our-planet/
Students view examples of various types of extreme weather in still picture images and record observations for each weather type.
They record which ones they have experienced and write their wonders about extreme weather.
- Why do some places get certain types of weather and others don’t?
Phenomena Resources:
Communicating in Scientific Ways | OpenSciEd
Big Ideas
Which one of the ideas from the curriculum and the Standards now seems the most central - meaning they might help explain other ideas you’ve listed and explain a wide range of natural phenomena? You must use more than a name to express your idea, express it as a set of relationships. Explain your choice clearly enough so a colleague could understand why you made the choice you did.
- Some types of extreme weather
- Why it is important to know about weather
- Weather and climate are different things
- How meteorologists predict weather
- Staying safe during extreme weather
Open Sci Ed Routines
Lesson 1: Local Weather
Materials
- Weather and Climate Information
- Weather Whiz Kids https://www.weatherwizkids.com/weather-climate.htm
- NASA Climate Kids https://climatekids.nasa.gov/menu/weather-and-climate/
- NASA Weather and Climate Poster https://drive.google.com/drive/u/0/folders/1Qul-edEHrPZb-9hcbttwYuSUqc3IPbWi
- Learning Log Questions https://drive.google.com/drive/u/0/folders/1Qul-edEHrPZb-9hcbttwYuSUqc3IPbWi
- Comparative Input Chart/ Weather Vs. Climate
- Climate Map picture for Comparative Weather Vs. Climate Input Chart https://drive.google.com/drive/u/0/folders/1Qul-edEHrPZb-9hcbttwYuSUqc3IPbWi
- Weather Tools Scout Cards https://docs.google.com/document/d/1wcrex5Wz9GdqIngGE3vNsN0Afm-U7lcWdN0oT93TFOc/copy
- National Geographic Neil DeGrasse Tyson Weather Vs. Climate video “https://www.youtube.com/watch?v=cBdxDFpDp_k”
Preparation
- Prepare weather vs climate input chart sketched out in light lead pencil.
- Identify Weather and Climate Resources that you will use with students to project or use on the computer.
- If using the NASA Poster make copies.
Vocabulary
- Climate: Weather trends typical of a place over a long period of time
- Weather: The condition of the atmosphere at a certain specific place and time
- Meteorologist: Scientists who study the Earth’s atmosphere
Procedures
Procedure 1: Read Aloud
Integrated content
- Pass out the Learning Log of the questions. Use any of the resources in the Materials list to read to or have students read. As your reading stop at pages
- talk with students about weather and climate. ask them, “What do you know about weather? what do you know about climate?”
- discuss with students the job of a meteorologists
- talk about the different clouds
- When you are done reading, look at the learning log with questions and have students answer any of the questions.
- Ask students to share answers, passing out the SCOUT awards to any students who share out ( RI.3.3) (W.3.8)
Procedure 2 Input Chart
- Let’s learn about Climate Vs. Weather
- Make sure to make the poster beforehand. To do this lightly sketch in the key ideas you will discuss with students. As you talk together use markers or bright colors to trace over the ideas you lightly sketched as they come up. This makes the poster more memorable for students and focuses them on the words and ideas.
- Go over the definition of climate, then the definition of weather. Then glue a cut out copy of the climate map on the poster and go over the climate map and weather examples that you draw in. It's crucial for kids to understand the difference between climate and weather.
- Have students turn back to their Learning Log questions and fill in any more of the answers to questions from the previous procedure after learning some more.
Procedure 3
Integrated content
- Play this video, “https://www.youtube.com/watch?v=cBdxDFpDp_k”
- Ask students to share with a partner discussing what weather, is and what climate, is (SL..3.1)
Formative Assessment:
- In Learning logs and in student discussion look for students to express these ideas about weather and climate:
- Weather and Climate are different
- Weather can change from minute-to-minute, hour-to-hour, day-to-day, and season-to-season
- Climate is the average of weather over time and space. For example, you can expect snow in the Northeast in January or for it to be hot and humid in the Southeast in July. This is climate. The climate record also includes extreme values such as record high temperatures or record amounts of rainfall. If you’ve ever heard your local weather person say “today we hit a record high for this day,” she is talking about climate records.
- In Learning logs and in student discussion look for students to express these ideas about weather and climate:
Lesson 2: Vacation Data Collection
This Lesson takes 3-5 days.
Materials
- computer for accessing websites & for students to do research
- History Channel - Explore the history of how the National Park Service came to be https://www.youtube.com/watch?v=ipUdTv_fHgM&list=PLXXwMuzjkuHqQzNk_ArVyW14dM9E5ot67
- Touropia - Take a tour of 25 of the most popular National Parks
https://www.youtube.com/watch?v=zTBmv-Gzf2w&list=PLXXwMuzjkuHqQzNk_ArVyW14dM9E5ot67&index=2
- National Park books or Brochures if you have them - AAA is a good place to collect brochures.
- Crayons or colored pencils
- Data Collection pages and Graphs https://docs.google.com/presentation/d/1O6V7uGq-kP4QxU0W_zxxEipslRlC-IXSHUV7kAlzlN4/copy
- Travel Brochure https://docs.google.com/presentation/d/1m2kjhYrhkndcxV3z9bb0A5LRSEm_N8s0Wu1ht-ihO3c/copy
- National Park List
Preparation
- Print Class Set of Brochures and Data Collection/Graphing pages
- Print and lists of National Parks https://drive.google.com/drive/u/0/folders/1-7d7USoOjx0rdoWZGOywao1UlNIK5DKJ OR Top 25 National Parks https://docs.google.com/document/d/1Fvdlwa12Z92GKtA_1soMdsjzv3my4mgqDvX73WSq7qg/copy
Vocabulary
- Average
- Prediction
- Claim
Procedures
Procedure 1 - National Parks Introduction
- Do you know what a National Park is? Have you ever traveled to one? Do you know one that you would like to travel to? Here’s a video introducing the National Park Service https://www.youtube.com/watch?v=ipUdTv_fHgM&list=PLXXwMuzjkuHqQzNk_ArVyW14dM9E5ot67
- Students need to pick a National Park they would like to learn more about. (Here’s a video that gives a quick overview of the top 25 parks https://www.youtube.com/watch?v=zTBmv-Gzf2w&list=PLXXwMuzjkuHqQzNk_ArVyW14dM9E5ot67&index=2)
- While students watch the video they can highlight the ones that they find most interesting on the Top 25 National Parks list.
Procedure 2 - Graphing Weather Data
- Students will collect weather data for their location.
- Look up weather history online for location. There are many weather sites to use, I recommend Weather Underground
- On Weather Underground click MORE and Click HISTORICAL WEATHER
- In this page enter the name of the national park, It will give you a nearby weather station
- Enter the prior month from one year ago (ex. If it is now July 1, 2021, enter June 1 2020)
- Scroll through the various graphs for temperature, windspeed, etc. and find the data at the bottom
Integrated content
- Collect the High and Low temperature averages for each month for the previous year. On the graph in this link
- Use the data collected to complete the High and Low temperature bar graphs. (3MD.B3)
Procedure 3 - Creating a Travel Brochure
Integrated content
- Students will research their National Park so that they create a travel brochure. They will need to find….
- Interesting Facts
- Where it is located
- Activities to do there
- Write a Claim to express the best time to travel
- Websites for research
- Complete the information in the brochure using their research. (W.3.7)
- Complete the line plot on the back of the brochure using the weather data from previous activity. (3MD.B3)
Procedure 4 (Check for Understanding) - Opinion Essay or Exit Ticket
Integrated content
- Opinion Essay
- Write an opinion essay https://docs.google.com/document/d/1D853o-pE80cKveI7L2Qrt968suoEw3rD-dU8Cpss5Tc/copy to persuade people to visit the National Park that you chose.
- Use the graphic organizer to get started
- Two Reasons/paragraphs could be: Things to see, things to do, Interesting facts, park history.
- One reason/paragraph should be about the weather. Tell us about what we could expect the weather to be. When would be the best time to visit (Claim with evidence)? Why?
- Use your graphic organizer to write your essay. (student samples are in resources) (W.3.1)
Summative Assessment:
- Science and Math:
- Look for students to include information about the weather. They should mention the typical weather for various months.
- Writing:
- Look for students to have a topic sentence or statement, 3 paragraphs with multiple sentences and reasons to back up their claims about the national park, and a conclusion sentence. Spelling and punctuation should not be major factors in the grade unless that is the focus of their writing study at this point.
- Exit TIcket
- Write a claim supported by evidence and reasoning.
- The best time to travel to _______________ is _____________because_____________________________.
- Write a claim supported by evidence and reasoning.
Summative or Formative Assessment:
- Science and Math:
- Look for students to include information about the weather. They should mention the typical weather for various months.
- Look for evidence from their research that supports the claim
Lesson 3: Extreme Weather
This Lesson takes 3-5 days.
Materials
- computer for accessing websites & for students to do research
- NOAA Hurricane Imagery page https://www.nesdis.noaa.gov/hurricane-imagery
- NOAA Hurricane Names page https://www.nhc.noaa.gov/aboutnames_history.shtml
- map of the world (could reuse climate zones map)
- chart paper
- Extreme Weather Graphic Organizer Frayer Model https://drive.google.com/drive/u/0/folders/19QWlW8vQDYKiCE2KUHDKgYd7RCcqdKIq
- summary paper/computer to type “script”
- optional: iphone/ipad with the iMovie app for recording
- optional: green or blue paper or bedsheet to drape for greenscreen taping
- Make a class wondering chart
- Make a process grid for recording information after students present.
- Graphic Organizer: Weather Hazard Solutions
student version https://drive.google.com/drive/u/0/folders/19QWlW8vQDYKiCE2KUHDKgYd7RCcqdKIq
teacher version https://drive.google.com/drive/u/0/folders/19QWlW8vQDYKiCE2KUHDKgYd7RCcqdKIq
Vocabulary
- extreme
- tropical
- hazard
- eye (relating a hurricane)
- reduce
- impacts
Procedures
Procedure 1: Tropical Storms
Phenomena: Ask the kids “Did you know the National Weather Service says there are 1,800 thunderstorms occurring worldwide every minute?” Challenge them to do the math to calculate how many storms there are every day, week, or every year.
- Visit the National Hurricane Center NOAA page Hurricane Imagery https://www.nesdis.noaa.gov/hurricane-imagery Scroll down to choose examples. Most have video if you explore that hurricane’s page.
- Click around to explore different areas and different images created. (Images below are 2020 Hurricanes
Typhoon Masak Soaks Southern Japan -NOAA Hurricane Delta -NOAA
- Talk about what makes an ideal setting for a tropical storm. Show a picture of a hurricane. Point out the directions of the wind circling and the center “eye”. Let kids know air can become unstable when it warms and rises rapidly. Point out that warm and cold air masses, sea breezes, mountains, and heat from the sun can lift the air. If you add moisture, it can be the perfect combination for a storm.
- Talk about where some of the latest major tropical storms have happened. (Find a list of Atlantic hurricanes here https://www.nhc.noaa.gov/aboutnames_history.shtml scroll down to the bottom)
- Pinpoint on a map where some of these locations are. Ask students: “What patterns do you see? Where are you likely to see a tropical storm? Where are you least likely to see them? How do you know?”
Procedure 2: Extreme Weather Research Report
Integrated content
- Show this video without sound (video 4 in appendix) https://www.nationalgeographic.org/video/extreme-weather-interconnections-in-extreme-weather/
- Ask the students what it they notice and wonder about the extreme weather on the video and write their ideas on a class chart.
- Tell them that they’re going to be doing a research project. They will pick an extreme weather type to research. Choose how you want them to pair up (they can choose partners and topics openly or you can use the partner pair cards to randomly assign partners and/or extreme weather types).
- Introduce the Extreme Weather Graphic Organizer Frayer Model https://drive.google.com/drive/u/0/folders/19QWlW8vQDYKiCE2KUHDKgYd7RCcqdKIq they should use to collect information on their hazard.
- Some good places to research are:
- https://www.sciencekids.co.nz/weather.html
- https://kids.nationalgeographic.com/ (search for extreme weather in the Search Box)
- http://www.weatherwizkids.com/ (Links to extreme weather types at the bottom)
- http://eo.ucar.edu/webweather/
- Allow time for students to select appropriate sources and complete their graphic organizer. While students conduct their research, walk around and offer assistance as needed.
- Once pairs have finished gathering information have them write up a summary/weather script.
- Extensions Ideas: zoom or invite a local weatherman to talk to the class. Possibly tour the news station and talk about the green screen they use. 2. Watch clips of weather forecasts and talk about the use of a teleprompt. This is what they are writing “a script/prompt” to read. (W.3.1)
Procedure 3: Presenting your report
- In this part of the lesson students will present their reports by either reading their summary/essay aloud or becoming weather anchors and videoing on a green screen.
- Whichever choice you choose, be sure to stop after each final presentation/viewing and fill in the process grid of each hazard studied. (the tool it’s measured with, one interesting fact, how to stay safe, how it’s formed, and what other hazards their extreme weather is linked to).
- Written papers read aloud
- allow students to read their reports aloud to the class.
- Green Screen
- Once you have had ample time to practice reading your script with your partner, it’s time to video on a green screen.
- Show students the screen. Point out that they need to use their outside voices to project so it comes through on the video. Let them know it’s ok if they mess up, just push through and that you can always do another take it needed.
- Pull up their “script” on a computer. Make it as large as possible on the screen. You will need one person to tape and one person to scroll the computer to act as a teleprompter (like a real news broadcast!)
- iMovie (on an ipad or iphone) works wonderfully. (see Video 5 in appendix for help)
- Have a day to celebrate the students and watch all of the weather videos together! (SL.3.4) (SL.3.5)
Summative or Formative Assessment:
- Look for evidence from their research that answers each of the 4 questions
- See samples below for further guidance
Student samples:
Procedure 4: Reducing Impacts of Weather Hazards
- Pair up the students. Ask them to choose a weather hazard they want to continue on. Could be the same topic they researched in Procedure 2 or they could choose a new one after learning about it from their classmates.
- Introduce the graphic organizer they should use to collect information on their hazard. There’s a blank student one to use and there’s a teacher one as an example. You can use this link to get an editable copy of the student one in google draw https://docs.google.com/drawings/d/1fHTYdIaJHni6zvXWyfkSr4BWka7lR5dlkK8QqkZlw44/copy (editable)
PDF version: https://drive.google.com/drive/u/0/folders/19QWlW8vQDYKiCE2KUHDKgYd7RCcqdKIq
- Some good places to research are:
- American Meteorological Society
- Natural Disasters Association
- National Severe Storms Laboratory
- National Weather Service – Weather Ready Nation
- Ready.gov
- UCAR Center for Science Education
- Weather Channel
- Weather Underground
Formative Assessment:
Use the teacher version https://drive.google.com/drive/u/0/folders/19QWlW8vQDYKiCE2KUHDKgYd7RCcqdKIq for guidance in how to evaluate the formative assessment
Appendix: Lesson Resources
Lesson 1:
Video 1
What are weather and climate?
National Geographic Neil DeGrasse Tyson Weather Vs. Climate video https://www.youtube.com/watch?v=cBdxDFpDp_k
Resource 1 Weather Vs Climate Information Resources
- Weather and Climate Information
- Weather Whiz Kids https://www.weatherwizkids.com/weather-climate.htm
- NASA Climate Kids https://climatekids.nasa.gov/menu/weather-and-climate/
- NASA Weather and Climate Poster https://drive.google.com/drive/u/0/folders/1Qul-edEHrPZb-9hcbttwYuSUqc3IPbWi
Resource 2 Learning Log Questions https://drive.google.com/drive/u/0/folders/1Qul-edEHrPZb-9hcbttwYuSUqc3IPbWi
Resource 3 Scout Cards
https://docs.google.com/document/d/1wcrex5Wz9GdqIngGE3vNsN0Afm-U7lcWdN0oT93TFOc/copy
Resource 4 Comparative Climate VS Input Chart
https://drive.google.com/drive/u/0/folders/1Qul-edEHrPZb-9hcbttwYuSUqc3IPbWi
Climate Map picture for Comparative Weather Vs. Climate Input Chart https://drive.google.com/drive/u/0/folders/1Qul-edEHrPZb-9hcbttwYuSUqc3IPbWi
Lesson 2:
Video 2
History Channel - Explore the history of how the National Park Service came to be https://www.youtube.com/watch?v=ipUdTv_fHgM&list=PLXXwMuzjkuHqQzNk_ArVyW14dM9E5ot67
Video 3
Touropia - Take a tour of 25 of the most popular National Parks
https://www.youtube.com/watch?v=zTBmv-Gzf2w&list=PLXXwMuzjkuHqQzNk_ArVyW14dM9E5ot67&index=2
Resource 5
Data Collection pages and Graphs
https://docs.google.com/presentation/d/1O6V7uGq-kP4QxU0W_zxxEipslRlC-IXSHUV7kAlzlN4/copy
Resource 6
Travel Brochure
https://docs.google.com/presentation/d/1m2kjhYrhkndcxV3z9bb0A5LRSEm_N8s0Wu1ht-ihO3c/copy
Opinion Essay Graphic Organizer and Student Examples
https://docs.google.com/document/d/1D853o-pE80cKveI7L2Qrt968suoEw3rD-dU8Cpss5Tc/copy
Lesson 3:
Video 4
Extreme Weather kick off video: National Geographic https://www.nationalgeographic.org/video/extreme-weather-interconnections-in-extreme-weather/
Video 5
How To video for editing iMovie https://www.youtube.com/watch?v=RKDAba4nsh0
Resource 8
Extreme Weather Graphic Organizer Frayer Model https://drive.google.com/drive/u/0/folders/19QWlW8vQDYKiCE2KUHDKgYd7RCcqdKIq
Resource 9
Weather Hazard cards Partner pair up Google Doc https://docs.google.com/document/d/1dUYwE6agb_kBUIAMkXOkBNDfQjBfKs3cmPyubO3UiGI/copy
PDF https://drive.google.com/drive/u/0/folders/19QWlW8vQDYKiCE2KUHDKgYd7RCcqdKIq
Resource 10
Process Grid https://docs.google.com/document/d/1WoXcDbFTig90_mAszFbwI__gDc0hvu0mIX5XrHWPES4/copy
Resource 11
Graphic Organizer: Weather Hazard Solutions
student version https://drive.google.com/drive/u/0/folders/19QWlW8vQDYKiCE2KUHDKgYd7RCcqdKIq
teacher version https://drive.google.com/drive/u/0/folders/19QWlW8vQDYKiCE2KUHDKgYd7RCcqdKIq
Attribution
Cover photo Image by Umut AVCI from Pixabay
NGSS Lead States. 2013. Next Generation Science Standards: For States, By States. Washington, DC: The National Academies Press | Public License
Common Core State Standards © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved | Public License
License
Except where otherwise noted, this work developed for ClimeTime is licensed under a Creative Commons Attribution License. All logos and trademarks are the property of their respective owners. Sections used under fair use doctrine (17 U.S.C. § 107) are marked.
This resource may contain links to websites operated by third parties. These links are provided for your convenience only and do not constitute or imply any endorsement or monitoring.
If this work is adapted, note the substantive changes and re-title, removing any ClimeTime logos. Provide the following attribution:
This resource was adapted from {Resource Name} by ClimeTime and licensed under a Creative Commons Attribution 4.0 International License. Access the original work for free in the ClimeTime group on the OER Commons Washington Hub.
This resource was made possible by funding from the ClimeTime initiative, a state-led network for climate science learning that helps teachers and their students understand climate science issues affecting Washington communities.
|
oercommons
|
2025-03-18T00:39:12.855647
|
Measurement and Data
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/82018/overview",
"title": "Third Grade Elementary Science and Integrated Subjects-Weather",
"author": "Elementary Education"
}
|
https://oercommons.org/courseware/lesson/82469/overview
|
Permissions Guide for Educators
Overview
This guide provides a primer on copyright and use permissions. It is intended to support teachers, librarians, curriculum experts and others in identifying the terms of use for digital resources, so that the resources may be appropriately (and legally) used as part of lessons and instruction. The guide also helps educators and curriculum experts in approaching the task of securing permission to use copyrighted materials in their classrooms, collections, libraries or elsewhere in new ways and with fewer restrictions than fair use potentially offers. The guide was created as part of ISKME's Primary Source Project, and is the result of collaboration with copyright holders, intellectual property experts, and educators.
* "Copyright license choice" by opensource.com is licensed under CC BY-SA 2.0.
About this Guide
This guide is intended to support curriculum developers--including educators, curriculum experts, librarians, and others--in determining the legal ways that they can use digital resources created by others in their own lessons and collections.
The guide also serves as a primer on how to seek permission to use resources that are currently under copyright. It includes considerations around whether to ask for permission, as well as resources to aid in conversations and negotiations with rights holders.
Notes: The context of your particular use matters greatly, and this guide does not identify all legal issues that may arise from your use. This guide only concerns U.S. copyright law. This guide provides an introduction to some of the copyright issues you may face, but is not intended to be comprehensive and is not a substitute for legal advice. We do not guarantee the accuracy or completeness of this guide, or of any third party resources it links to. Consult an attorney for advice regarding your specific case if needed. There may be important legal issues beyond copyright – for example, trademark infringement, defamation, and violation of the privacy or publicity rights of others – that may be implicated by your use of OER Commons or other resources.
Understanding Copyright
What is Copyright?
Copyright is a form of legal protection that affords the copyright owner the exclusive rights to, among other things:
- Reproduce (i.e., copy)
- Distribute
- Publicly perform
- Publicly display
- Create “derivative works”
A “derivative work” is a work based upon one or more preexisting works, such as a translation, musical arrangement, dramatization, fictionalization, motion picture version, sound recording, art reproduction, abridgment, condensation, or any other form in which a work may be recast, transformed, or adapted. A work consisting of editorial revisions, annotations, elaborations, or other modifications which, as a whole, represent an original work of authorship, is a “derivative work”.
Without permission from the copyright owner, or an exception to the exclusive rights (such as fair use), it is a violation of copyright law to exercise any of the copyright owner's exclusive rights.
In general, copyright is secured automatically as soon as a work is created, regardless of whether or not the work is published. No registration or other action in the Copyright Office is required. There is no longer a requirement that a copyright notice be present on a work in order for copyright to be secured.
What is a Copyright License?
A copyright license is a grant of permission to use certain copyright rights. Copyright licenses often have specific limitations that are outlined; for example they may:
- Be limited in time
- Contain geographical restrictions
- Only allow for educational uses
- Only grant permission to use some of the copyright rights (for example, a license may grant permission to download and distribute a work, but not the right to create derivative works)
Creative Commons licenses, discussed later in this toolkit, are copyright licenses. Many works in OER Commons are Creative Commons licensed. There are six main Creative Commons licenses. All require that any uses include attribution to the original author; some permit only noncommercial uses; some do not allow the creation of derivative works. When evaluating the permitted scope of uses, read all copyright licenses closely. Using a work in a manner that exceeds the scope of permissions granted in a license is copyright infringement.
Reference Materials
- The U.S. Copyright Office's informational materials on U.S. copyright.
What is Fair Use?
Fair use is a limitation on a copyright owner’s exclusive rights, set forth in the Copyright Act. See 17 U.S.C. §107. If a use is a legitimate fair use, permission from the copyright owner is not needed.
It can be difficult to determine whether a given use is a fair use. Fair use evaluations are highly fact-specific, and depend greatly on the facts of your particular situation. Claiming fair use involves risks, and fair use law can be very complex. Exercising fair use is a right, not an obligation. In evaluating whether a given use is a fair use, the Copyright Act sets forth the following factors:
- The purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
- the nature of the copyrighted work;
- the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
- the effect of the use upon the potential market for or value of the copyrighted work.
Create a Fair Use Evaluation
You can use the ALA Fair Use Evaluator Tool to create collect, organize & archive the information you might need to support a fair use evaluation. This wizard will walk you through a step by step form, and provide you with a time-stamped, PDF document for your records. You can see an example evaluation document here: http://librarycopyright.net/resources/fairuse/example.pdf
Reference Materials
- Stanford University Libraries, Measuring Fair Use: The Four Factors
- Center for Media & Social Impact, Resources on fair use
What is the “Classroom Use Exemption”?
The Classroom Use Exemption (17 U.S.C. §110(1)) applies in a narrow range of situations. To qualify for this exemption, you must be:
- in a classroom ("or similar place devoted to instruction"),
- there in person, engaged in face-to-face teaching activities, and
- at a nonprofit educational institution.
If you qualify for the exemption, you may perform or display copyrighted works – but not exercise any other exclusive rights of the copyright owner (e.g., this exemption does not entitle you to copy, distribute or create derivative works). In the case of a motion picture or other audiovisual work, you may only perform or display lawfully-made copies.
By way of example, if you qualify for the classroom use exemption, you can, without seeking permission, without giving anyone payment, and without having to deal with the complications of fair use:
- Play movies and music for your students, at any length (only from legitimate copies).
- Show students images, or original artworks.
- Lead students in performances of musical compositions, scenes from plays, and the like.
Even if you qualify for the classroom use exemption, it does not apply to:
- Online activities of any kind (e.g., a class website, any activities that are not face-to-face and in-person)
- Making or distributing copies of any kind (e.g., handing out readings in class)
What is the TEACH Act?
The TEACH Act is codified at 17 U.S.C. 110(2). It provides that it is not copyright infringement for teachers and students at an accredited, nonprofit educational institution to transmit certain performances and displays of copyrighted works as part of a course if certain conditions are met. If these conditions are not or cannot be met, use of the material will have to qualify as fair use or permission from the copyright holder(s) must be obtained.
The requirements of the TEACH Act are onerous, and require the cooperation of your educational institution, including the IT department. Implementing TEACH can be very difficult because of its complexity and the many detailed requirements for instructors, technologists, and institutions.
Some of the requirements you and your institution must meet before being able to take advantage of the TEACH Act include:
- You must be an accredited nonprofit education institution or governmental body;
- You can perform a nondramatic literary work, a nondramatic musical work, or reasonable portions of any other work;
- You can display any other work in an amount comparable to that typically displayed in a live classroom setting;
- You cannot (under TEACH) use works produced or marketed primarily for performance/display as part of mediated instructional activities transmitted via digital networks or unlawfully made copies;
- The works used must be under the actual supervision of an instructor as part of a class session;
- The works must be used as part of systematic mediated instructional activities and directly related and of material assistance to the teaching content;
- You may digitize an analog work if no digital version is available to the institution or the digital version is locked to prevent TEACH uses.
In addition, there are a number of technical requirements:
- The transmission (of the performance or display) must be made solely for and reception limited to students enrolled in the course, i.e., access controls
- You must institute technological measures that reasonably prevent retention in accessible form for longer than a class session (this means to prevent printing, saving, downloading, etc.)
- You must not interfere with technological measures that prevent retention and dissemination put there by the copyright holder
And, your institution must:
- Promulgate copyright policies
- Provide accurate information about copyright
- Promote copyright compliance
- Provide notice to students that course materials may be copyrighted
Reference Materials
- Copyright Law andDistance Education: Overview of the TEACH Act
- Copyright Checklist: Compliance with the TEACH Act
- TEACH Act Toolkit
Are There Other Exceptions and Limitations to Copyright?
Absolutely. A full list is beyond the scope of this guide, but some other exceptions include:
- The first sale doctrine: Once a physical copy of a copyrighted work has been sold, that particular copy may be redistributed by resale, lending or donations. In general, the doctrine does not apply to digital works, and other exceptions apply.
- Idea/expression dichotomy: ideas are not copyrightable; the expression of those ideas is copyrightable. The rule is simply stated, but can be complicated to apply.
- 17 U.S.C. 108 provides certain exceptions for libraries and archives.
- 17 U.S.C. 117 sets forth a number of limitations on exclusive rights related to computer software.
Are There Other Rules Imposed By Copyright Law That Are Not Covered Here?
Yes. This guide is not intended to be comprehensive. By way of example, 17 U.S.C. 1201 makes it illegal to circumvent a technological protection measure (such as copy protection, or digital rights management software). In other words, you may not descramble a scrambled work, decrypt an encrypted work, or otherwise avoid, bypass, remove, deactivate, or impair a technological measure, without the authority of the copyright owner. There are some limited exemptions available.
What is the Public Domain?
Public domain works are not restricted by copyright, and do not require a license or fee to use. Public domain works may be used without any restrictions--they may be downloaded, shared, edited, remixed, repurposed, etc.
Works in the public domain are those whose copyright rights have expired, have been forfeited, or are inapplicable. There are three main categories of public domain works:
- Works that automatically enter the public domain upon creation, because they are not copyrightable. For example, book titles, short phrases and slogans, ideas and facts, processes and systems, and certain government documents;
- Works that have been assigned to the public domain by their creators; and
- Works that have entered the public domain because the copyright on them has expired.
Determining Permissions
Look carefully at the resource you want to use and any information surrounding the resource; also review the "about" and "terms of use" pages of the resource's website for permissions information.
Can you find a copyright license or other form of permission that applies to the resource?
Can you determine if the resource falls into one of the categories below?
Determine if the Resource is in the Public Domain
The mere fact that a resource is made available in a collection of public domain materials, or is on a list of public domain materials is insufficient; you should independently determine whether or not a resource is in the public domain by conducting an appropriate analysis of its copyright status.
Examine the resource and determine whether it is in the public domain. It can often be difficult to determine with certainty whether a given work is in the public domain. Public domain status is likely (for copyrighted works other than sound recordings) for the following situations:
- The resource was published in the United States before 1923
- The resource was published before 1964 and the copyright registration was not renewed
- The resource was published without a valid copyright notice prior to 1977
Reference Materials
- UC Berkeley's Public Domain Handbook for determining whether a work is in the public domain.
Determine if the Resource is a U.S. Government Work
Examine the resource to determine if it is a U.S. government work prepared by an officer or employee as part of that person's official duties. These works are free to use without restrictions. This does not apply to works created by U.S. state governments, or the governments of other jurisdictions – such works may be protected by copyright.
U.S. government records and works often contain or include within them copyrighted works, so examine the resource and read the terms of use carefully. For example:
- A U.S. federal court opinion written by a U.S. federal judge may contain copyrighted materials, such as photographs. The fact that such a photograph appears in a judicial opinion does not make it a U.S. government work; it is still subject to copyright protection.
- The Congressional Record is a U.S. government work, but it may contain documents and other materials that remain subject to copyright protection.
- A sound recording or photograph that appears in the collection of the Library of Congress or National Archives is not necessarily a U.S. government work. The photograph may not have been prepared by an officer or employee of the United States government as part of that person’s official duties.
Determine if the Resource is Creative Commons Licensed
Resources that are Creative Commons (CC) licensed are free to use with certain restrictions depending on the specific license.
Look for the following licensing terms:
- CC0 - In general, you may treat the resource as if it were in the public domain.
- CC BY - Attribution to the author/creator required.
- CC BY-SA - Attribution required, and you agree to license new derivative versions of the resource that you create under CC BY-SA as well.
- CC BY-NC - Attribution required; non-commercial use only; commercial use requires a separate, negotiated license.
- CC BY-ND - Attribution required; no derivative works permitted; creation of derivative works requires a separate, negotiated license.
Read the license carefully and follow its terms. Failure to abide by the restrictions in a Creative Commons or other license is copyright infringement. Outside of CC0, all CC licenses require attribution--meaning, if you use the resource as part of your teaching, you must give credit to the original author or creator. For guidance on how to attribute properly, see: http://wiki.creativecommons.org/Best_practices_for_attribution.
Determine if the Resource Has Another License
Look carefully to identify terms of the license and comply with its restrictions.
If your intended use of the material is not permitted by the license, and does not fall within a copyright exception or limitation (such as fair use), contact the rights owner(s) for permission.
Seeking Permission to Use
1. Determine Whether You Need to Ask for Permission.
You do not need to ask permission if:
- The resource is in the public domain. See UC Berkeley's Public Domain Handbook to help determine if the work is in the public domain.
- It is a U.S. government work that was prepared by an officer or employee as part of that person's official duties.
- Your intended use falls within a copyright exception or limitation (such as fair use).
- The way that you want to use the resource is in compliance with the terms a copyright license that applies to you (i.e., you already have permission in this case).
You do need to ask permission if:
- You wish to use a resource that is protected by copyright, and your intended use would be infringing copyright law if you were absent permission from the rights owner.
- You wish to use a resource in a way that is beyond the scope of the permission granted to users in an applicable copyright license.
You should consider asking for permission if:
- You are uncertain about whether your intended use is permitted by an applicable copyright license.
- You are uncertain about whether a work is protected by copyright.
- You are uncertain about whether your intended use falls within a copyright exception or limitation (such as fair use).
2. Ask for Permission.
Work through the following steps when asking a rights owner for permission:
a. Identify the owner.
Sometimes identifying the owner is easy – if the work contains a copyright notice that identifies the owner, you can simply contact the owner and ask for permission. Sometimes, identifying the owner can be more challenging – for example, companies change ownership, owners are deceased, and the like. Sometimes a diligent search of the copyright records can reveal who owns rights in a work.
Many copyrighted works have more than one owner, each with separate rights in the works. You may or may not need permission from each owner, depending on which rights you need to license. For example, if you want to use recorded music, you will generally need to obtain permission from:
- The record company, which owns the sound recording copyright
- The music publisher, who owns a copyright in the musical composition
- The artists themselves (or their estates)
On the other hand, if you merely want to publicly perform the musical composition, you would not need permission for the sound recording. Each industry has its own customs and practices regarding copyright licensing and permissions; be sure to research carefully to determine that you have identified all owners of the rights you need, and that you have identified them correctly.
b. Identify the rights you need.
Be clear about the uses you intend to make, and negotiate with the rights owner(s) for those rights.
- Do you need only the right to reproduce?
- Do you want to modify the work?
- Do you want to publicly perform or publicly display the work?
- Do you need exclusive or nonexclusive rights?
- Will you need the rights forever, or just for a limited term of years?
- Will you need the right to post the work on the internet, adapt the work into a film?
- Will you need worldwide rights, or just U.S. rights?
c. Compile your message.
Craft a message to the owner of the resource, or to the person responsible for the permissions agreements related to the resource. Include any information about the rights you would like to secure, how you would like to use and share the resource, and what parties the agreement is between (e.g., between you and the publisher, the publisher and the public at large, etc.). A sample letter to rights holders is provided below.
Dear [insert name if you can get it],
I am writing on behalf of the Primary Source Project. We are an initiative comprised of teacher leaders to create K-12 lessons that embed informational texts, or “primary sources,” in alignment with new education standards known as the Common Core State Standards.
The overarching aim of our project is to offer lessons and primary sources that support students in developing the skills called for in the Common Core State Standards, including close reading of primary source texts, critical thinking, and problem solving. The lessons developed as part of the project will be freely available for access online through a digital library.
In order for your work to be included in our online collection, we would like to ask you to license your work under a Creative Commons license. Such a license will make it possible for other teachers to make important uses of your work, and to further share those uses with others. Creative Commons offers six licensing options (https://creativecommons.org/licenses/), which allow licensors to choose (1) whether or not to allow derivative works; (2) whether or not to allow commercial uses; and (3) whether or not derivative works, if allowed, must be licensed pursuant to the same terms as the original work (“share-alike”). All Creative Commons licenses require attribution.
Our project encourages licensors to choose a Creative Commons Attribution (CC BY) license from among those options presented. It is our position that this option provides an educational user with the greatest degree of freedom while at the same time preserving and extending an author’s original authorship in derivative works.
Thank you very much for your time and attention to this matter. We look forward to discussing with you how the Work can be included in our exciting new educational project.
3. Negotiate Terms and Payment if Needed.
Rights owners may respond to your request with their own licensing terms, either formally in the form of an agreement that they've constructed, or informally in the form of an email text. They may, for example, indicate that you or your users are welcome to include the resource in your collection, but that no derivative works may be made. Many for-profit rights owners may be uncomfortable licensing the right to create derivative works of their material to the public. Thus, you may have better luck negotiating such rights for a specific project. Alternatively, if derivative work rights are important to you, it may be most efficient to start with nonprofit rights owners, particularly those in the education industry, as they are most likely to be familiar with, and supportive of, open access principles.
Rights owners may or may not require a fee for permission. Fees often vary based on the size of the audience your work will reach, whether the use is commercial or noncommercial, the number and kind of rights you wish to license, and the like.
4. Get Your Permission Agreement in Writing.
Obtain a written copyright license that clearly describes the scope of permission.
Sometimes, in addition to getting permission for yourself, you may want to get permission for the public generally. Below is an example of a permissions agreement to rights owners asking them to grant a Creative Commons license to the public.
Owner of the Work:
Owner Address and Contact:
Title of the Work:
Description of the Work:
Year of first publication of the Work:
Creator(s) of the Work, and any others designated to receive attribution:
URI or hyperlink to the Work:
Preferred form of copyright notice/marking:
I, the rights owner for the Work(s) set forth above, hereby license the Work(s) pursuant to the following Creative Commons License (please initial only one option):
__ Creative Commons Attribution-ShareAlike (CC BY-SA), the full text of which is available online at https://creativecommons.org/licenses/by-sa/4.0/legalcode.
__Creative Commons Attribution-NonCommercial-ShareAlike (CC BY-NC-SA), the full text of which is available online at https://creativecommons.org/licenses/by-nc-sa/4.0/legalcode
__Other [fill in the blank]:
I further authorize the [your name or name of your institution] to create, publish and distribute marked copies of the Work(s) reflecting the CC license terms I have selected in accordance with Creative Commons marking guidelines (http://wiki.creativecommons.org/Marking_your_work_with_a_CC_license), or in any other reasonable manner.
I represent and warrant that I am the copyright owner of the Work(s), and/or that I am authorized to act on behalf of the copyright owner of the Work(s), and that the Work(s) are hereby licensed pursuant to the above-noted Creative Commons license.
Signature:
Name:
Date:
Please complete, sign, and date this form and return original signed copy to:
Institution/Individual Name:
Address:
Phone/Fax:
Email:
Reference Materials
- Stanford University's introduction to getting permission.
|
oercommons
|
2025-03-18T00:39:12.896634
|
06/16/2021
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/82469/overview",
"title": "Permissions Guide for Educators",
"author": "Melinda Newfarmer"
}
|
https://oercommons.org/courseware/lesson/122940/overview
|
SEARCH TECHNIQUES
Overview
Search techniques are strategies, methodologies, or tools used in locating and retrieving information, especially in large volumes of data. These techniques are used in many contexts, from search engines to academic research, data retrieval, and even physical archives. They aim at maximizing efficiency, relevance, and accuracy in finding the information you are looking for.
The search technique is largely depends on the understanding of users to execute these techniques and the sophistication of the search engine or tool in interpreting queries and giving relevant results.
INTRODUCTION
Searching is the activity of looking carefully in order to find something. In library and information science, searching refers to looking through records carefully in order to find desired information. You have already studied retrieval tools like catalogues, indexes, etc., for retrieving information. In this lesson, you will learn need and ways of searching organized information for retrieval purposes. You will also be exposed to basic concepts of search techniques for information retrieval from electronic sources.
OBJECTIVES
Upon successful completion of this lesson, you should be able to:
- Define search techniques;
- Explain organization of words in a dictionary;
- Use dictionary, numeric and numeric/classified techniques for arranging and retrieving library material;
- Define search engines;
- Identify search process and design a search query;
- Know the role of search operators;
- Define Boolean logic; • understand types of searches;
- Define, explain and differentiate the field based and full text search with examples.
Search Techniques
The search technique acts as a mechanism through which relevant information can be recovered from information systems. One will find that the information system may either be inhouse or online. In house, information system is such whereby information is stored with scope to an organization as used for retrieval purposes. Then one finds the online information system whereby electronic information sources happen to be stored centrally yet accessed through a given mechanism of communication. Most of the online information systems are compatible with World Wide Web (WWW) and are accessible through internet. The in-house information system may have information sources in both printed and electronic form. Thus, storing mechanism and search techniques are two different aspects. We will discuss these two aspects of storage and retrieval of information.
Storage Mechanism
In-house information systems and online information systems are developed to store specific information or information on a specific theme or subject. Such systems have their search mechanisms and a set of instructions to locate particular information. In library and information centres, information is available both in print and electronic format. Some of the storage mechanisms and their role in information retrieval are as follows:
- Dictionary Arrangement
- Numeric Arrangement
- Classified Arrangement
Dictionary Arrangement
Dictionary arrangement means an arrangement where words are organized in alphabetical order of the language. The alphabetical order is the sequence based on the position of a particular alphabet in the script of the language. For example, the English language uses roman alphabets and the order is A, B, C, D,……….Z. Here, the alphabet "A" is in first position, "B" at second and similarly "Z" at twenty sixth position in the sequence. Hence the words in dictionary arrangement are arranged according to the sequential position of alphabets. For instance, Action Ante Apple Art Catalogue Classification Search…. Here, the first four words are starting with "A" but their positions are fixed according to the position of second, third or fourth letter.
This is followed by another set of two words starting with "C". Thus, the words starting with "C" have been given position after the words starting with "A". Following this process, the words are organized in this arrangement. This mechanism of arrangement is followed for arranging entries in catalogues, which have words as access point. For example, author, title, subject, etc.
Numeric Arrangement
The numeric arrangement is the order where numbers are arranged in ascending or descending order. For instance,
123.45
234.15
234.51
435.21
541.23
……………………………
Here, you find that all the numbers have the same set of five digits, i.e., 1 to 5, but according to their numeric value, these are organized in ascending order and sequence has been made. In libraries, that follow Dewey Decimal Classification system, you will find that the books are kept on the shelves in numerical order.
Classified Arrangement
Most of the libraries organize their books on the shelves according to the call number of books. The call numbers are the combination of class number, book number and collection number. These three numbers may be numeric or alphanumeric as per the scheme, followed by the library. Hence, retrieving books from the shelves becomes easy when we understand the numeric, alphanumeric or classified arrangement. For example, a few call numbers based on DDC scheme have been arranged below as they are arranged on shelves.
321.4 RAM
370.1150954 DEM
370.1523 DES
371 ILLS
371.3078 KEM
371.32 NIS
371.397 GRE
371.926 BRA
Another example of information retrieval following these arrangements is taken from a book index. You might have noticed that almost all books have an index at the end. The book index is a list of words/terms along with page numbers on which those appear in the text. Depending upon the size and nature of the book, the terms in the book index are organized either in dictionary or classified order. After understanding these arrangements, you can find information on a topic from the book easily.
Search Engine
Searching information from the electronic or digital media is different from the print media. When information is stored in electronic or digital form, user interface is provided to find relevant information from the system. This user interface is a software, which has provisions to accept keywords or terms, representing required information to conduct the search. It brings the result of the search in the format defined in the software. The software meant for searching information from the information system is referred as search engine. Therefore, we can define a search engine as 'a software, meant for searching information from electronic or digital information domain, on the basis of input given by a searcher that displays the result in user friendly format'.
The input to the search engine is known as search string or query. The query may be a single term or a set of terms representing the information one is looking for. The search engine searches information based on the query and provides a list of sources which match the query. The list is displayed in a format, designed by the search engines. Depending upon the nature of the search engine, the list may contain brief description of information sources, on the basis of which, the searcher may decide to acquire or refer to full record or not. You might have searched Online Public Access Catalogue (OPAC) of your library or Library of Congress Online Catalog (LCOC) or PubMed as well as Google or Yahoo on internet. All are the search engines.
Search Process
The search process is a set of functions which are performed for searching the relevant information effectively. The process follows some basic steps to conduct search and get desired results. These steps are as follows:
(i) Recognise and State the Need
(ii) Development of Search Strategy
(iii) Execution of the Search Strategy
(iv) Review Search Results
(v) Edit Search Results
(vi) Evaluation and Feedback
Recognise and State the Need
It is important for an information professional or searcher to know why the search is being done and what purpose the search will serve.
A topic may be searched on a need for general knowledge, research and development, and so forth. With knowledge of the need and purpose of the search, a query statement should then be formulated. There needs to be an agreement between the seeker of information and the searcher of information as to the search requirements. This agreement leads to formulation of effective search strategy for relevant and effective result.
Development of Search Strategy
The development of the search strategy includes conceptual formulation of query, translation of conceptual formulation into the language of keywords, descriptors or facets, identification of synonym and associated terms, etc.
The concept of facet analysis (PMEST), given by Ranganathan as well as the concept of specific subject can be used as an effective tool for designing a query.
After this, it is important to select the information domain to be searched like, the OPAC of a library, database or likewise, depending upon requirements. The search string or query, is the combining of terms, keywords or descriptors which represent information. Since search strings contain language, the linguistic features, and their consequences for searching and retrieving have to be examined. In this place, three areas, which are Syntactic Value, Semantic Value and Boolean Operators are to be defined.
- Syntactic Value
The syntactic value of a search string deals with the kind of formula or connecting symbols through which keywords or terms are connected to represent the concept to be searched by the search engines. We will try to understand the syntactic value of a query by this example. There are two terms, say "poetry" and "Indians" connected by two different connectors, "among" and "by'. Each gives a different meaning, as follows:
- 'Poetry among Indians 'means 'What is the status of poetry among Indians?" Or 'What is the approach of Indians towards poetry?"
- 'Poetry by Indians 'means, poetry composed by Indians.
B. Semantic Value
The semantic value of a search string is the meaning of the string in the context of the required information and the interpretation by the search engine. To establish the meaning of the concept to be searched and understood by the search engines, we use operators as connectors of keywords as permitted by the search engines. We can understand the semantic value of a query through two examples given below:
- The question 'contribution of Indian society in mathematics' means the contribution of Indian society in the field of Mathematics.
- The question 'contribution of mathematics in Indian society 'means contribution of Mathematics in shaping Indian society
These two examples give us clear perception of the semantic value of a question. The same set of keywords and connectors give different meaning when written in different order.
C. Boolean Operators
Boolean Operators are simple words, such as AND, OR, and NOT, used like conjunctions to combine keywords or exclude them in searching.
They are used in order to connect and identify the relationship between the searching terms.
Hence resulting in more focused and fruitful results. These three terms have been accepted by the designers of the search engines. Their meaning is well defined while they used as operators in information search. The three operators of Boolean logic are logical sum (+) OR, logical product (x) AND, and logical difference (-) NOT. All information retrieval systems permit the use of these operators by allowing users to express their queries. Let us now examine the meaning of these three operators.
OR Operator:
The OR operator lets the user specify alternatives among the terms in the search. When a string is constructed with the help of OR operator, then search engines obtain all those resources where at least one of the terms or keywords linked with 'OR' exist.
If we construct a search string like, 'student OR education' and search for it, then the output of the search will be a list of references of all those resources, available in the system where either student or education exists.
AND Operator: The AND operator is used to combine two or more terms. When we create a string using AND operator, the search engine retrieves all those resources in which all the terms or keyword connected with 'AND 'exist. For example, if we design a search string like, 'student AND education 'and search, then the output of the search will be a list of references of all those resources where student and education, both the terms exist.
NOT Operator:
The NOT operator is used to eliminate the term from a collection of resources. For instance, if we formulate a search string like 'student NOT education' and search then the result of the search will be a list of references of all those resources, available in the system, where term student exists but not education.
Implementation of the Search Strategy
The searcher should know the data structure adopted by the information system that stores data before executing a search. The system-based search engines are designed to search information in a database according to its architecture. Like in OPAC, if we put a query as 'Tagore, Rabindra Nath' and search in author field, then only those records will be retrieved and displayed from the database which have been authored by him.
But, if we put the same query into the title field, then all those records will appear which have 'Tagore, Rabindra Nath 'in title or a portion of title. So, in result references of all such materials which are written about 'Tagore, Rabindra Nath 'will be included. Depending upon the need and purpose of the search and expertise of the searcher, the search may be conducted using the features of the search engines. Hence a searcher should know the types of search and implications to get effective output. The types of searches are:
a) Field Based Search
b) Full Text Search
c) Truncation Search
d) Proximity Search
e) Limiting Search
f) Range Search
g) Simple Search
h) Advanced Search
(a) Field Based Search
The search conducted on a particular field of the database to get required information is termed as field-based search. As you are aware, the complete information of catalogue is stored in different fields in a bibliographic database. If you wish to search an author, direct the search engine to author field or if you wish to search through title or subject, direct the search engine to title or subject field.
(b) Full Text Search
Full text search is a searching mechanism, which performs the search on each and every field of the database and retrieves all those records which match the query. For instance, the same search (Amartya Sen) when performed on LCOC with keyword option, which acts as full text search, retrieved a list of 193 records.
This indicates that, in full text search the number of hits increased as it extracted all those records which had 'Sen, Amartya 'in any fields.
(c) Truncation Search
Truncation search, is a search technique, in which, the search is conducted for different forms of a word having the same common root. It is one of the most widely adopted methods in information retrieval system. In this method, the root word is taken along with truncation mark and then search is done. Suppose we are searching for 'India*' then all the records will be fetched where term 'India 'is appearing full or partial of any word.
All the records of the domain containing, India, Indian, Indiana, Indianization or similar will be listed.
(d) Proximity Search
The proximity search, is a search technique, which allows the searcher to define the distance of two terms from each other. Whether, the two search terms, should occur adjacent to each other, or, one or more words occur in between the search terms; or the search terms should occur in the same paragraph, irrespective of the intervening words, etc. Different search engines use different set of operators for this purpose.
(e) Limiting Search
In limiting search technique, a searcher limits the string as per the architecture of database and searches different terms of the same string in different fields. For example, if a searcher is searching 'Development as freedom by Amartya Sen' then the string will be broken into two sub-strings, viz. 'Development as freedom 'and 'Amartya Sen'.
The sub-string 'Development as freedom' will be put in title field and sub string 'Amartya Sen' will be put in author filed and then search will be conducted.
Range Search
Range search technique is a technique, which allows searchers to select records within certain data ranges. This technique is more suitable for numeric data search. The operators and their meaning differ from search engine to search engine. A few commonly used operators are:
Greater than (>)
Less than (<>)
Greater than or equal to (>=)
Less than or equal to (<=) For example, if we place publication year 2000 >=, then the result will list all those resources which have been published 2000 AD onwards.
Simple Search
Simple search is such a technique where a searcher puts keywords in a simple format without understanding the behaviour of the search engine or the architecture of the database or the impact of the operators and connectors. Almost all the search engines provide the facility of using simple search technique. The simple search works on the model of Full text search discussed above.
Advanced Search
Advanced search technique is the method through which a searcher looks up information with various tools and mechanisms in order to attain exact and relevant results. In this technique, the search string is constructed using the operators and parameters supplied by the search engine by the searcher. Looking up information, by merging all of the methods mentioned above also falls under this category. Here, the scope of each and every term of the string may be defined according to facility available in the search engine.
Review Search Results
The best reviewer of the search results is the user. But the information professionals should also review the search results on the basis of criteria given for evaluating information retrieval systems.
Edit Search Results
The editing of search results is a transformation of the search results into a user-friendly format. This can involve the arrangement of the results into a well-organised package, highlighting important entities, adding more information to the entities and reformatted of information to suit the user's requirements.
Evaluation and Feedback
The evolution of search results involves participation of both, the users and the searchers. The quality and quantity of the results are assessed and if needed, the process may be redefined and restarted if the final result does not satisfy the users' needs
LEARNT ABOUT SEARCH TECHNIQUES
- The standard mechanism, called information search techniques is used for retrieving information from any information system.
- The search technology is a tool by which, one can retrieve relevant data from information systems. The information system may be inhouse or online.
- Storage technology can be dictionary, numeric and classified arrangement of data.
- Search operation is performed by a set of functions as:
- Determination of user's needs of information search;
- Designing search strategy;
- Selecting the information system to be searched and accordingly the search engine;
- Creation of search query or string using keywords and operators that, expresses the semantic value of the user's requirements and the syntactic format that the engine interprets;
- Performing the search;
- Evaluation of the result. If necessary, again filter or redefine or restart the search process; and
- Presentation of the search results in a user-friendly format.
- For getting relevant and effective search results, a searcher should have knowledge of the types of searches and skills of conducting them.
Conclusion
Search techniques are foundational to various fields, such as computer science, information retrieval, and problem-solving. They play a critical role in efficiently finding solutions, data, or information from large datasets, whether it's for navigating through the internet, exploring databases, or optimizing algorithms in problem-solving.
|
oercommons
|
2025-03-18T00:39:12.970436
|
12/12/2024
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/122940/overview",
"title": "SEARCH TECHNIQUES",
"author": "FIROZ ALI LASKAR"
}
|
https://oercommons.org/courseware/lesson/103900/overview
|
WHAT EXACTLY DOES THE U.S. CONSTITUTION DO?
Overview
The "Future Ready" content focuses on civics and the United States Constitution. It explains the Constitution's purpose and how it was ratified. The objectives are to teach about power distribution between national and state governments, principles of the American constitutional federal republic, the role of law in the political system, government institutions created during the Revolution, and different levels of government in the US.
Key terms related to the Constitution are highlighted, such as amendments, bicameral legislature, Bill of Rights, checks and balances, Declaration of Independence, federalism, Preamble, separation of powers, and unalienable rights.
The content emphasizes the Constitution as the highest law and the relationship between the federal government and states. The Bill of Rights, the first ten amendments, is explained along with specific rights and protections. Important amendments like the Fourth, Fifth, Sixth, Ninth, and Tenth are described.
The passage also covers the separation of powers and checks and balances in the American government to prevent any one branch from becoming too powerful. It concludes with questions to test understanding, including the purpose of the Preamble, the meaning of domestic tranquility, the provision for common defense, and the importance of the separation of powers.
Future Ready
CIVICS
The Constitution
What exactly does the Constitution do?
Photo by Dave Sherrill on Unsplash
OBJECTIVES
Explain how the United States Constitution grants and distributes power to national and state governments and how it seeks to prevent the abuse of power.
Explain the central ideas and principles of the American constitutional federal republic.
Evaluate, take, and defend positions on the role and importance of law in the American political system; on issues regarding the distribution of powers and responsibilities within the federal system.
Evaluate the institutions and practices of government created during the Revolution and how they were revised between 1787 to create the foundation of the American political system based on the U.S. Constitution and the Bill of Rights.
Recognize the structure and functions of different levels of government in the United States, including concepts of power and authority
RATIFYING THE CONSTITUTION
What is the greatest and longest-living of all the constitutions in history? The Constitution of the United States of America! It is the document that embodies the fundamental laws and principles by which the U.S. is governed. The Constitution was written just after the Revolutionary War, the war for American independence from Britain. In 1787, the Founders - most notably, Ben Franklin, James Madison, Alexander Hamilton, and George Washington - gathered in Philadelphia and framed the Constitution. Their purpose was to create a document that provided a stronger central government than that was provided by the Articles of Confederation. All states were invited to send delegates to the Constitutional Convention that designed a government with separate legislative, executive, and judicial branches. Most of the debates at the Constitutional Convention centered on ideas about where power should lie. It established Congress as a lawmaking body with two houses: each state is given two representatives in the Senate, whereas representation in the House of Representatives is based on population. Only nine of the 13 states needed to ratify the Constitution for it to take effect in 1789.
Photo by Anthony Garand on Unsplash
KEY TERMS
Amendment a change to the Constitution or an addition to the Constitution
bicameral legislature a legislature with two houses or chambers. The British parliament is a bicameral legislature, made up of the House of Commons and the House of Lords. Likewise, the United States Congress is made up of the House of Representatives and the Senate.
Bill of Rights the first ten amendments to the Constitution
checks and balances a fundamental principle of American government, guaranteed by the Constitution, whereby each branch of government was given specific and unique powers that could not be exercised by the other branches, and no one branch of government would overpower another
Photo by Joshua Sukoff on Unsplash
Declaration of Independence the document that announced our independence from England
federalism the relationship between the state and national government
Preamble the introduction to the Constitution, declaring the purpose of the document
separation of powers between branches of government. writers of the Constitution built this principle into one of the key features of the American government.
unalienable an inalienable right is impossible to take away or give up
THE SUPREME LAW OF THE LAND
The founders of the United States defeated the world’s strongest military and financial power and won their independence. They then faced the task of forming a country that would honor and implement the principles upon which they had declared their independence.
The bedrock upon which the American political system is built is the rule of law. The vast difference between tyranny and the rule of law is a central theme of political thinkers back to classical antiquity. The idea that the law is superior to rulers is the cornerstone of English constitutional thought as it developed over the centuries. The concept was transferred to the American colonies and can be seen expressed throughout colonial pamphlets and political writings.
To assure such a government, Americans demanded a written legal document that would create both a structure and a process for securing their rights and liberties and spell out the divisions and limits of the powers of government. That legal document must be above ordinary legislation and day-to-day politics. That is what the founders meant by “constitution,” and why our Constitution is “the supreme Law of the Land.”
Article VI of the Constitution explicitly states that the Constitution is the “supreme Law of the Land.” The Constitution and the laws created by Congress take priority over state and local laws. This is called the “supremacy clause.” However, the Tenth Amendment, which was added to the Constitution as part of the Bill of Rights, protects the states and citizens from a too-powerful federal government. This amendment establishes that powers not given to the federal government and not denied to the states in the Constitution belong to the states or to the people.
Which of the following would be considered a “check,” or limit, on the federal government’s power?
The "supremacy clause"
The amendment process
The Tenth Amendment
The limiting of state powers
WHAT IS THE BILL OF RIGHTS?
Walk through the thinking process for understanding the Constitution.
When the Constitution was ratified it did not contain a Bill of Rights. In order to gain Anti-Federalist support, Federalists—including James Madison—promised that a Bill of Rights would be proposed to the states for their ratification by the first Congress under the new Constitution. The Bill of Rights is the first 10 of 27 Amendments to the Constitution. It spells out Americans’ rights in relation to their government. It guarantees civil rights and liberties to the individual—like freedom of speech, press, and religion. It sets rules for due process of law and reserves all powers not delegated to the federal government to the people or the States. And it specifies that “the enumeration in the Constitution, of certain rights, shall not be construed to deny or disparage others retained by the people.”
The First Amendment guarantees several rights to U.S. citizens: the right to express ideas through speech and the press, to assemble or gather with a group to protest or for other reasons, and to ask the government to fix problems. It also protects the right to religious beliefs and practices. It prevents the government from creating or favoring a religion.
The Second Amendment protects the right to keep and bear arms.
The Third Amendment prevents the government from forcing homeowners to allow soldiers to use their homes. Before the Revolutionary War, laws gave British soldiers the right to take over private homes.
Photo by Unseen Histories on Unsplash
The Fourth Amendment bars the government from unreasonable search and seizure of an individual or their private property.
The Fifth Amendment provides several protections for people accused of crimes. It states that serious criminal charges must be started by a grand jury. A person cannot be tried twice for the same offense (double jeopardy) or have property taken away without just compensation. People have the right against self-incrimination and cannot be imprisoned without due process of law (fair procedures and trials.)
The Sixth Amendment provides additional protections to people accused of crimes, such as the right to a speedy and public trial, trial by an impartial jury in criminal cases, and to be informed of criminal charges. Witnesses must face the accused, and the accused is allowed his or her own witnesses and to be represented by a lawyer.
The Seventh Amendment extends the right to a jury trial in federal civil cases.
The Eighth Amendment bans excessive bail and fines and cruel and unusual punishment.
The Ninth Amendment states that listing specific rights in the Constitution does not mean that people do not have other rights that have not been spelled out.
The Tenth Amendment says that the federal government only has those powers delegated in the Constitution. If it isn’t listed, it belongs to the states or to the people.
REMEMBER: Just because a right is not in the Bill of Rights does not mean that it is not a citizen’s right!
Photo by NASA on Unsplash
NOW IT’S YOUR TURN.
Use what you have learned to answer the question.
Almost before the ink was dry on the original document, several of the Founders set about drafting amendments to the Constitution. The first ten of these, collectively known as the Bill of Rights, were ratified on December 15, 1791. There have been an additional 17 amendments ratified since then. Go deeper here for great resources on the Amendments to the Constitution.
Based on the description in the passage “What is the Bill of Rights?”, which amendment contains the following text?
“...no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized…” - The Constitution
Fourth
Fifth
Sixth
Ninth
In 1962, in Gideon v. Wainwright, the U.S. Supreme Court ruled that the constitutional right to be represented by a lawyer extended to trials in state court. On which amendment would the Court base that right?
First
Fourth
Fifth
Sixth
The Fifteenth, Nineteenth, Twenty-third, Twenty-fourth, and Twenty-sixth Amendments are voting rights amendments. The Fifteenth Amendment made it clear that former slaves would be allowed to vote. The Nineteenth Amendment secured the right for women to vote. The Twenty-third Amendment grants citizens of Washington D.C. the right to vote for president. The Twenty-fourth Amendment says that a person cannot be prevented from voting in any election just because he or she has not paid a poll tax or a fee that a person had to pay before 1964 in order to vote. According to the Twenty-sixth Amendment, any citizen of the United States who is eighteen years of age or older has the right to vote.
Which of these voting amendments affected the largest share of Americans?
Fifteenth
Nineteenth - The Nineteenth Amendment, allowing women to vote, impacted roughly half of the population.
Twenty-fourth
Twenty-sixth
CHOOSE THE RIGHT ANSWER.
Use the following to answer the question below. Then read why each answer is correct or incorrect.
This excerpt is from the Constitution.
The President shall be Commander in Chief of the Army and Navy of the United States, and of the Militia of the several States, when called into the actual Service of the United States…. He shall have Power, by and with the Advice and Consent of the Senate, to make Treaties, provided two thirds of the Senators present concur.
In this portion of the Constitution, which branch of the government checks the power of another branch of government by a two-thirds agreement?
The executive checks the power of the legislative.
The judicial system checks the power of the executive.
The legislative checks the power of the executive.
The legislative checks the power of the judicial.
Check to see if you chose the right answer.
The legislative branch involves the Senate and the House of Representatives, and the executive branch consists of the president and the president's administration. The fact that the Senate must have a two-thirds agreement to allow the president to make a treaty means that the legislative branch is checking the power of the executive branch so that the president does not have full and unopposed power to make treaties.
So, Choice C is the correct answer!
Why are the other answer choices incorrect?!
Choice A is incorrect.
The president is part of the executive branch, and that branch is not checking the powers of any other branches but actually having its own powers checked.
Choice B is incorrect.
The judicial branch involves the courts, and this branch is not even mentioned in this section of the Constitution.
Choice D is incorrect.
The judicial branch does not factor into this section of the constitution.
NOW IT’S YOUR TURN.
Answer the questions below. Use the following hints to avoid mistakes.
Read all passages carefully and think about what the author is saying, the conclusions he or she is drawing, the arguments that are being presented, and the details used to support these arguments.
Go back and find specific examples in the passage that are related to the question.
The intent of the Founders of the Constitution was to construct a government that would be sufficiently strong to perform those essential tasks that only a government can perform (such as establishing justice, ensuring domestic tranquility, providing for the common defense, and promoting the general welfare—the main tasks named in the document’s Preamble), but not so strong as to jeopardize the people’s liberties. In other words, the new government needed to be strong enough to have the power to secure rights without having so much power as to enable or encourage it to infringe rights.
They also believed that the role of the federal government should be limited to performing those tasks that only a national government can do, such as providing for the nation’s security or regulating commerce between the states, and that most tasks were properly the responsibility of the states. And they believed that strong states, as competing power centers, would act as counterweights against a potentially overweening central government, in the same way, that the separation of powers checks and balances the branches of the federal government. For example, Congress can impeach and remove officers of the executive and judicial branches, the executive branch has law enforcement power and commands the military, and the judicial branch has the power to interpret the law and the Constitution.
One important feature of our Constitution is the careful way it limits the powers of each branch of government— that is, states what those branches may do, and by implication what they may not do. This is the real meaning of “limited government”: not that the government’s size or funding levels remain small, but that government’s powers and activities must remain limited to certain carefully defined areas and responsibilities as guarded by bicameralism, federalism, and the separation of powers. The idea behind the separation of powers is to use the structure of government to control the government and the officials within the government.
Why is the word more included in the phrase "a more perfect union"?
The Founders thought it sounded better.
The Preamble was too short without it.
There was already a union in place that was to be made better by the Constitution.
It was Benjamin Franklin’s favorite word.
Domestic tranquility most closely means
peace at home
peace abroad
war at home
war abroad
One of the reasons the Constitution was created was to provide for the common defense.
true
False
Why did the Founders think it was important to have a separation of powers in the government?
so that they would win the Revolutionary War
so that George Washington would be the first president
so no single branch of government becomes too powerful
so they could punish the British
Photo by Joshua Hoehne on Unsplash
WRITING THE BEST ANSWER POSSIBLE
Study the model below. It’s a good example of a written answer.
The Constitution serves as the framework of the national, or federal, government. The Founders created a strong federal government, but they did not deprive states of all authority. In ratifying the Constitution, the states granted powers to the federal government but retained others. This sharing of powers and responsibilities is called federalism. States have some authority to handle their specific needs but also must work with the federal government and the other states in matters of national importance.
American conceptions of a republic as the proper form of government can be traced back to classical antiquity, but the American Founders put forth a somewhat new understanding of republican government both at the Constitutional Convention debates and in the ratification debates. In their definition, a republic is a nation in which the people hold supreme power, and designate representatives to carry out their will. After a decade of being taxed without being represented in Parliament, Americans officially declared their independence in 1776, proclaiming that governments “deriv[e] their just powers from the consent of the governed.” The Framers decided that members of the House of Representatives would be elected directly by the people and that Senators would be elected by state legislatures. The new federal republic created a bicameral Congress in which the interests of the people and the states were balanced.
What principles of government are reflected in the debates at the Constitutional Convention? In the ratification debates?
Principles of government reflected in the debates at the Constitutional Convention and in the ratification debates include limited government, federalism, republicanism, consent, bicameral legislature, and representation.
Photo by Joey Csunyo on Unsplash
NOW IT’S YOUR TURN.
Answer the question. Use what you have learned from the model.
During the Constitutional Convention and while states were ratifying the Constitution, Federalists, like Alexander Hamilton and James Madison, believed that a large government protects liberties. They believed that the sheer size of the government protected the rights of the people. Madison made the argument that the large diversity of opinions protects any one group from becoming too powerful. The Federalists also argued that the division of powers between the federal government and the states, the supreme nature of the federal government, the voting rights nature of the government, and the checks and balances within the federal government were enough to protect people’s liberties.
An Anti-Federalist was a person who felt the federal government had been given too much power. They were concerned that the Constitution as it was did not protect the basic rights of the citizens and needed to include a Bill of Rights. Anti-Federalists charged that it was impossible to provide fair and true representation in such a large republic. They believed that a large government would gain too much power over the people, and therefore, a smaller government was better.
Summarize the main arguments that the Anti-Federalists made against the method of representation provided under the Constitution. How did the Federalists answer those concerns? Which side do you favor, and why? Give evidence supporting your argument, while anticipating and responding to the views of the opposing side as well.
GET READY FOR YOUR FUTURE!
As you answer the questions below, remember to:
Read each question carefully and consider your answer options.
Check your instincts: In most cases when taking any type of test, your first instinct is correct. But before you submit it, check your answer by looking back at the text to find details that support it.
Answer each question.
Which of the following best explains the insertion of the Supremacy Clause in Article VI of the Constitution?
To prevent the states and citizens from challenging federal laws
To prevent conflicts between federal, state, and local laws
To prevent states from becoming too powerful
To prevent states from writing laws similar to federal laws
Based on information in the passage, which of the following conclusions is most plausible concerning the Tenth Amendment?
Proponents of a strong federal government wanted the Tenth Amendment included in the Bill of Rights.
The Tenth Amendment was included to negate the powers granted to Congress in Article VI.
The Framers added the Tenth Amendment to limit the voices of common citizens.
The Framers added the Tenth Amendment to appease those who supported states' rights and individual liberties.
Photo by Samuel Ferrara on Unsplash
Use the following to answer the question below.
The Constitution of the United States of America, 1789
Amendment XIX
The right of citizens of the United States to vote shall not be denied or abridged by the United States or by any State on account of sex.
Congress shall have power to enforce this article by appropriate legislation.
The Nineteenth Amendment de facto gives the right to vote, or suffrage, to
African Americans.
immigrants.
all citizens 16 years of age and older.
Women.
The Founders were only creating the Constitution for themselves, leaving future generations in charge of securing their own liberty.
true
false
Photo by Sebastian Pichler on Unsplash
The Preamble explains how the President will be elected.
true
false
The Preamble has nothing to do with the rest of the Constitution.
true
false
Under our Constitution, some powers belong to the federal government. What is one power of the federal government?
Under our Constitution, some powers belong to the states. What is one power of the states?
Use this excerpt from James Madison’s Federalist #10 to answer the questions that follow.
Complaints are everywhere heard from our most considerate and virtuous citizens, equally the friends of public and private faith, and of public and personal liberty, that our governments are too unstable, that the public good is disregarded in the conflicts of rival parties, and that measures are too often decided, not according to the rules of justice and the rights of the minor
party, but by the superior force of an interested and overbearing majority. However anxiously we may wish that these complaints had no foundation, the evidence, of known facts will not permit us to deny that they are in some degree true.
According to Madison’s argument in Federalist No. 10, how would the new Constitution protect against the fears that political factions would corrupt the new republic?
THE CONSTITUTION ALLOWS YOU TO SAY WHATEVER YOU WANT. TRUE OR FALSE?
How often do you think about the Constitution in your daily life? Probably not very much. Even though, for instance, the First Amendment guarantees freedom of speech, there are still exceptions, like saying something with the knowledge that it could cause physical harm to people. But just because many of us are not thinking about the Constitution when we raise our hand in class, watch election results come in, or gather together with friends, the fact is that the Constitution affects almost every part of our lives. Our entire government, the laws it makes, the judges who interpret them, and the rights and liberties we have are all determined by the Constitution.
Photo by Swapnil Bhagwat on Unsplash
REFERENCES, ATTRIBUTION, AND LICENSE
References
“The Bill of Rights: What Does It Say?” by National Archives and Records Administration
“1776 Unites: Uplifting Everyday Americans.” by 1776 Unites
“Constitutional Amendments Playlist.” by Bill of Rights Institute
Attribution
Lesson by
Benjamin Troutman and Jennifer O’Neil, Griffin Bay School, San Juan Island School District
Portions of content adapted from
American Government by OpenStax, Rice University | CC BY
American Government (2E-Second Edition) by University of Central Florida
The Constitution by Library of Congress
Structure of the United States Constitution by Michelle Huebel | CC BY
The Preamble to The United States Constitution by Michelle Huebel | CC BY
Introduction to the U.S. Constitution by Ronald Stump | CC BY
License
Except where otherwise noted, Future Ready Civics The Constitution by San Juan Island School District is available under a Creative Commons Attribution 4.0 International License. All logos and trademarks are the property of their respective owners. Sections used under the fair use doctrine (17 U.S.C. § 107) are marked.
|
oercommons
|
2025-03-18T00:39:13.000879
|
Political Science
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/103900/overview",
"title": "WHAT EXACTLY DOES THE U.S. CONSTITUTION DO?",
"author": "History, Law, Politics"
}
|
https://oercommons.org/courseware/lesson/80402/overview
|
Mechanical Test gr11
Mechanical test Memo
ENGINEERING GRAPHIS AND DESIGN
Overview
According to (Douglas SotsakaI; Asheena Singh-PillayII, n.d.) students need to develop a skill which allows them to apply visual reasoning and interpret graphical text. The aim of this Lesson on Mechanical assembly is to broaden students graphical and visual reasoning skills.
individual should be in a postion to interpret every mechanical assembly drawing and all sectioning that is involved in this chapter.
INTRODUCTION
TABLE OF CONTENT
1. INTRODUCTION: https://www.oercommons.org/courseware/lesson/80402/student/315219
2. LESSON 1: https://www.oercommons.org/courseware/lesson/80402/student/315219
3. POWERPOINT PRESENTATION: https://www.oercommons.org/courseware/lesson/80402/student/315219?task=3
4. INFORMAL TASK: https://www.oercommons.org/courseware/lesson/80402/student/315219?task=4
5. IFORMAL TASK MEMORANDUM: https://www.oercommons.org/courseware/lesson/80402/student/315219?task=5
Engineering graphics and design is offered from grade 10-12, its aim is to develop student's cognitive skill and enable them to communicate graphically, apply spatial visual reasoming through reading and interpreting graphical text (Douglas Sotsakal; Asheena Signh-Pillayll, n.d). Element of this subjecr are introdeced in grade 8 through an elementary subject known as technology and the focus then is to teach students basic inductory skills to drawing.
The link below gives an overwiew and a brief introduction of what Engineering graphics and design is.
https://www.youtube.com/watch?v=NJmEGLcUiWk
MECHANICAL ASSEMBLY DRAWING
Lesson Objective:
by they end of this lesson student need to know and understand different sectional views of mechanical assembly and understand how each component is hatched.
CONTENT.
The image below demonstrates a half sectoned mechanical assembly
Figure two demonstrate important mechanical parts. i will label each part and its function.
- KEY is used to prevent a shaft slipping inside a pulley
- KEYWAY is a groove for a key
- SHAFT is a long cylindrical component used to transmit turning force
- TAPER is a reducing diameter, from thin to thicker or thicker to thinner
- EXTERNAL TREAD has helical grooves cut around a shaft to attach a nut or other internal thread
- SHAFT
- THROUGH HOLE passes from one side of the component through to the other
- FLANGE is a disk at the end of a pipes or shafts to join them together
- KEYWAY is a groove that is used with a key to prevent rotation of components
- FIXED COLLAR is a raised part of a shaft used to position components
- SQUARE on shaft can be used to rotate another component instead of a key
- SQUARE SLOT is a shape to fit another feature
- RIB (also called a web) strengthens or supports another feature
- RECESS is a shallow hole
- SLOT is an elongated hole usually with round ends 16. TEE SLOT or TEE GROOVE allows another shaped component to slide along it
- CHAMFER is when a corner is removed, usually at 45 degrees
- GROOVE is a long narrow channel
- ROUND is when an outside corner is rounded
- BLIND HOLE is a hole that does not go all the way through a part
- MOVABLE COLLAR is a ring used to locate components on a shaft
- LUG is part of a casting that sticks out used for securing or adjusting the position
- THROUGH HOLE passes from one side to the other 24. INTERNAL THREAD has helical grooves cut inside a hole to attach a bolt or other external thread
- WEB (rib) is a thinner, strengthening piece on a casting
- FILLET is a rounded corner to prevent casting cracking
- SQUARE HOLE saves material
- BRACKET is a supporting device
- BORE is a cylindrical hole inside a casting
- BUSH is a sleeve placed inside a casting that will wear our and can be replaced easier and cheaper than the casting
the video below is an example of how to use calculations when drawing nuts and bolts.
https://www.youtube.com/watch?v=JEIy30EQVEQ
Grade 11 JPEGD solution for the Activiy.
POWERPOINT PRESENTATION
Powerpoint slide presentation explain in detail, the importance of hatching and calculations of nuts and bolts
ASSESSMENT TASK
Below there is an attached informal assessment you need to complete to evaluate your understanding regarding Mechanical assessmbly drawing.
After completing the task you can use the memo to evaluate your work.
MEMO FOR ASSESSMENT 1
Students below is an attchement for the previous informal you complete. and the video will enhance you spatial visual reasoning skills;
|
oercommons
|
2025-03-18T00:39:13.048305
|
Homework/Assignment
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/80402/overview",
"title": "ENGINEERING GRAPHIS AND DESIGN",
"author": "Assessment"
}
|
https://oercommons.org/courseware/lesson/80441/overview
|
Worksheet 1
Worksheet 1 MEMO
Worksheet 2
Worksheet 2 MEMO
NATURAL SCIENCES AND TECHNOLOGY GRADE 5
Overview
This resource shares information about some Natural Sciences and Technology Grade 5, term 1 topics.
INTRODUCTION TO NATURAL SCIENCES AND TECHNOLOGY
Science is a systematic way of looking for explanations and connecting the ideas we have. In Science certain methods of inquiry and investigation are generally used. These methods lend themselves to replication and a systematic approach to scientific inquiry that attempts at objectivity. The methods include formulating hypotheses, designing and carrying out experiments to test the hypotheses. Repeated investigations are undertaken, and the resulting methods and results are carefully examined and debated before they are accepted as valid.
Technology has also existed throughout history. People use the combination of knowledge, skills and available
resources to develop solutions that meet their daily needs and wants. Economic and environmental factors and a wide range of attitudes and values need to be taken into account when developing technological solutions. Technology also advances as our knowledge and needs expand.
Science and Technology have made a major impact, both positive and negative, on our world. Knowledge grows out
of a view of how the world works. Watch the first two minutes of this video to give you an idea of how Science and Technology are related: https://study.com/academy/lesson/how-science-technology-are-related.html
Some of the term 1 topics that we will cover include:
- Animal skeletons (Section 2) - Skeletons of vertebrates, functions of the skeleton, and joints in the human body https://www.oercommons.org/courseware/lesson/80441/student/315299?task=2
- Skeletons as structures (Section 3) - Frame and shell structures https://www.oercommons.org/courseware/lesson/80441/student/315299?task=3
- Food chains (Section 4) - https://www.oercommons.org/courseware/lesson/80441/student/315299?task=4
In section 5 you will find all the answers to the activities from the previous sections. https://www.oercommons.org/courseware/lesson/80441/student/315299?task=5
TOPIC 2 - ANIMAL SKELETONS
In this topic we will look at the skeletons of vertebrates.
A skeleton is a hard framework inside or outside an animal's body. A vertebrate skeleton consists of bones and joints, and is inside the body.
Biologists divide all the animals in the world into two main groups according to the type of skeleton that they have.
- Animals with ENDOSKELETONS: Some animals have a skeleton inside their bodies, which is called an endoskeleton.
- Animals with EXOSKELETONS: Some animals have a skeleton on the outside of their bodies, which is called an exoskeleton.
Now that you have knowledge about endoskeletons and exoskeletons, complete "Worksheet 1" which can be found under the attachments. The memo to the worksheet can be found under section 5 https://www.oercommons.org/courseware/lesson/80441/student/315299?task=5
FUNCTIONS OF THE SKELETON
The skeleton is made up of many different bones. Each bone supports the animal's body and protects its organs.
Click on the link below to watch a fun video about the skeletal system and while you watch the video think about the function of the skull, rib cage and backbone.
The skull protects the brain.
The backbone protects the spinal cord.
The rib cage protects internal organs like the heart and the lungs.
Shoulder blades, arms, legs and hip bones are for movement.
A skeleton also gives an animal shape and support.
Now complete "Worksheet 2" under attachments. The memo to the worksheet can be found under section 5 https://www.oercommons.org/courseware/lesson/80441/student/315299?task=5
JOINTS IN THE HUMAN BODY
Your bones can move because they are connected by joints. A joint is the place where two or more bones are attached to each other.
Examples of hinge joints: elbow and knee joint.
Examples of ball and socket joints: hip and shoulder joint.
Example of a chain joint: backbone.
TOPIC 3 - SKELETONS AS STRUCTURES
In this section we will look at frame and shell structures.
Before we look at frame and shell structures, let's watch a video to remind us about the function of structures, natural structures, and man-made structures.
FRAME STRUCTURES
Frame structures are structures that are made up of different parts. These parts are put together to form a frame.
Look at some examples of frame structures and answer Activity 1.
Activity 1
1. Why do we classify these structures under frame structures?
2. Which of these structures are made by humans and which of these are natural?
SHELL STRUCTURES
Shell structures are made from one solid part.
Look at some examples of shell structures and answer Activity 2.
Activity 2
1. Why do we classify these structures under shell structures?
2. Which of these structures are made by humans and which of these are natural?
(ANSWERS TO ACTIVITY 1 AND 2 CAN BE FOUND UNDER SECTION 5 -https://www.oercommons.org/courseware/lesson/80441/student/315299?task=5 )
TOPIC 4 - FOOD CHAINS
In this section we will look at food chains.
All food chains in nature start with green plants . Green plants use the energy from the sun to make their own food. This process is called photosynthesis. Plants are therefore called PRODUCERS. Watch the video below which explains more about photosynthesis.
Living things that get energy by eating either a plant or animals are called CONSUMERS.
Many animals eat plants to get energy. We call these animals herbivores. Examples: Giraffe, sheep, cow, springbuck.
Some animals eat other animals to get energy. We call these animals carnivores. Examples: Lion, frog, snake, vulture.
Other animals can eat plants and animals. We call these animals omnivores. Examples: pig, baboon, chicken.
We get special animals called scavengers (hyenas, vultures) and decomposers (fungi, bacteria). They eat dead animals and break their bodies into tiny pieces that can go into the soil as compost.
Listen to the narrated PowerPoint on FOOD CHAINS attached below and try to answer the activity on slide 3 (Answers to the activity can be found under section 5 https://www.oercommons.org/courseware/lesson/80441/student/315299?task=5 ).
ANSWERS TO ACTIVITIES
In this section you will find all the answers to the activities from the previous sections.
SECTION 2 https://www.oercommons.org/courseware/lesson/80441/student/315299?task=2
Worksheet memo 1 - find attachment below
Worksheet memo 2 - find attachment below
SECTION 3 https://www.oercommons.org/courseware/lesson/80441/student/315299?task=3
Activity 1
1. Why do we classify these structures under frame structures?
Because they are made from different parts that are put together to form a frame.
2. Which of these structures are made by humans and which of these are natural?
Made by humans: bird cage and ladder
Natural: human skeleton and spider's web
Activity 2
1. Why do we classify these structures under shell structures?
Because they are made from one solid part.
2. Which of these structures are made by humans and which of these are natural?
Made by humans: bird bath
Natural: crab shell and snail shell
SECTION 4 https://www.oercommons.org/courseware/lesson/80441/student/315299?task=4
PowerPoint slide 3 activity
GRASS -----------> MOUSE ------------> SNAKE ----------> EAGLE
|
oercommons
|
2025-03-18T00:39:13.082382
|
05/16/2021
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/80441/overview",
"title": "NATURAL SCIENCES AND TECHNOLOGY GRADE 5",
"author": "Yvette Uzziel Pillay"
}
|
https://oercommons.org/courseware/lesson/15293/overview
|
Introduction
Overview
Opening image caption:
Psychology is the scientific study of mind and behavior. (credit "background": modification of work by Nattachai Noogure; credit "top left": modification of work by U.S. Navy; credit "top middle-left": modification of work by Peter Shanks; credit "top middle-right": modification of work by "devinf"/Flickr; credit "top right": modification of work by Alejandra Quintero Sinisterra; credit "bottom left": modification of work by Gabriel Rocha; credit "bottom middle-left": modification of work by Caleb Roenigk; credit "bottom middle-right": modification of work by Staffan Scherz; credit "bottom right": modification of work by Czech Provincial Reconstruction Team)
Psychology is designed to meet scope and sequence requirements for the single-semester introduction to psychology course. The book offers a comprehensive treatment of core concepts, grounded in both classic studies and current and emerging research. The text also includes coverage of the DSM-5 in examinations of psychological disorders. Psychology incorporates discussions that reflect the diversity within the discipline, as well as the diversity of cultures and communities across the globe.
Introduction
Introduction References:
References
American Board of Forensic Psychology. (2014). Brochure. Retrieved from http://www.abfp.com/brochure.asp
American Psychological Association. (2014). Retrieved from www.apa.org
American Psychological Association. (2014). Graduate training and career possibilities in exercise and sport psychology. Retrieved from http://www.apadivisions.org/division-47/about/resources/training.aspx?item=1
American Psychological Association. (2011). Psychology as a career. Retrieved from http://www.apa.org/education/undergrad/psych-career.aspx
Ashliman, D. L. (2001). Cupid and Psyche. In Folktexts: A library of folktales, folklore, fairy tales, and mythology. Retrieved from http://www.pitt.edu/~dash/cupid.html
Betancourt, H., & López, S. R. (1993). The study of culture, ethnicity, and race in American psychology. American Psychologist, 48, 629–637.
Black, S. R., Spence, S. A., & Omari, S. R. (2004). Contributions of African Americans to the field of psychology. Journal of Black Studies, 35, 40–64.
Bulfinch, T. (1855). The age of fable: Or, stories of gods and heroes. Boston, MA: Chase, Nichols and Hill.
Buss, D. M. (1989). Sex differences in human mate preferences: Evolutionary hypotheses tested in 37 cultures. Behavioral and Brain Sciences, 12, 1–49.
Carlson, N. R. (2013). Physiology of Behavior (11th ed.). Boston, MA: Pearson.
Confer, J. C., Easton, J. A., Fleischman, D. S., Goetz, C. D., Lewis, D. M. G., Perilloux, C., & Buss, D. M. (2010). Evolutionary psychology. Controversies, questions, prospects, and limitations. American Psychologist, 65, 100–126.
Crawford, M., & Marecek, J. (1989). Psychology reconstructs the female 1968–1988. Psychology of Women Quarterly, 13, 147–165.
Danziger, K. (1980). The history of introspection reconsidered. Journal of the History of the Behavioral Sciences, 16, 241–262.
Darwin, C. (1871). The descent of man and selection in relation to sex. London: John Murray.
Darwin, C. (1872). The expression of the emotions in man and animals. London: John Murray.
DeAngelis, T. (2010). Fear not. gradPSYCH Magazine, 8, 38.
Department of Health and Human Services. (n.d.). Projected future growth of the older population. Retrieved from http://www.aoa.gov/Aging_Statistics/future_growth/future_growth.aspx#age
Endler, J. A. (1986). Natural Selection in the Wild. Princeton, NJ: Princeton University Press.
Fogg, N. P., Harrington, P. E., Harrington, T. F., & Shatkin, L. (2012). College majors handbook with real career paths and payoffs (3rd ed.). St. Paul, MN: JIST Publishing.
Franko, D. L., et al. (2012). Racial/ethnic differences in adults in randomized clinical trials of binge eating disorder. Journal of Consulting and Clinical Psychology, 80, 186–195.
Friedman, H. (2008), Humanistic and positive psychology: The methodological and epistemological divide. The Humanistic Psychologist, 36, 113–126.
Gordon, O. E. (1995). A brief history of psychology. Retrieved from http://www.psych.utah.edu/gordon/Classes/Psy4905Docs/PsychHistory/index.html#maptop
Greek Myths & Greek Mythology. (2014). The myth of Psyche and Eros. Retrieved from http://www.greekmyths-greekmythology.com/psyche-and-eros-myth/
Green, C. D. (2001). Classics in the history of psychology. Retrieved from http://psychclassics.yorku.ca/Krstic/marulic.htm
Greengrass, M. (2004). 100 years of B.F. Skinner. Monitor on Psychology, 35, 80.
Halonen, J. S. (2011). White paper: Are there too many psychology majors? Prepared for the Staff of the State University System of Florida Board of Governors. Retrieved from http://www.cogdop.org/page_attachments/0000/0200/FLA_White_Paper_for_cogop_posting.pdf
Hock, R. R. (2009). Social psychology. Forty studies that changed psychology: Explorations into the history of psychological research(pp. 308–317). Upper Saddle River, NJ: Pearson.
Hoffman, C. (2012). Careers in clinical, counseling, or school psychology; mental health counseling; clinical social work; marriage & family therapy and related professions. Retrieved from http://www.indiana.edu/~psyugrad/advising/docs/Careers%20in%20Mental%20Health%20Counseling.pdf
Jang, K. L., Livesly, W. J., & Vernon, P. A. (1996). Heritability of the Big Five personality dimensions and their facets: A twin study. Journal of Personality, 64, 577–591.
Johnson, R., & Lubin, G. (2011). College exposed: What majors are most popular, highest paying and most likely to get you a job. Business Insider.com. Retrieved from http://www.businessinsider.com/best-college-majors-highest-income-most-employed-georgetwon-study-2011-6?op=1
Knekt, P. P., et al. (2008). Randomized trial on the effectiveness of long- and short-term psychodynamic psychotherapy and solution-focused therapy on psychiatric symptoms during a 3-year follow-up. Psychological Medicine: A Journal of Research In Psychiatry And The Allied Sciences, 38, 689–703.
Landers, R. N. (2011, June 14). Grad school: Should I get a PhD or Master’s in I/O psychology? [Web log post]. Retrieved from http://neoacademic.com/2011/06/14/grad-school-should-i-get-a-ph-d-or-masters-in-io-psychology/#.UuKKLftOnGg
Macdonald, C. (2013). Health psychology center presents: What is health psychology? Retrieved from http://healthpsychology.org/what-is-health-psychology/
McCrae, R. R. & Costa, P. T. (2008). Empirical and theoretical status of the five-factor model of personality traits. In G. J. Boyle, G. Matthews, & D. H. Saklofske (Eds.), The Sage handbook of personality theory and assessment. Vol. 1 Personality theories and models. London: Sage.
Michalski, D., Kohout, J., Wicherski, M., & Hart, B. (2011). 2009 Doctorate Employment Survey. APA Center for Workforce Studies. Retrieved from http://www.apa.org/workforce/publications/09-doc-empl/index.aspx
Miller, G. A. (2003). The cognitive revolution: A historical perspective. Trends in Cognitive Sciences, 7, 141–144.
Munakata, Y., McClelland, J. L., Johnson, M. H., & Siegler, R. S. (1997). Rethinking infant knowledge: Toward an adaptive process account of successes and failures in object permanence tasks. Psychological Review, 104, 689–713.
Mundasad, S. (2013). Word-taste synaesthesia: Tasting names, places, and Anne Boleyn. Retrieved from http://www.bbc.co.uk/news/health-21060207
Munsey, C. (2009). More states forgo a postdoc requirement. Monitor on Psychology, 40, 10.
National Association of School Psychologists. (n.d.). Becoming a nationally certified school psychologist (NCSP). Retrieved from http://www.nasponline.org/CERTIFICATION/becomeNCSP.aspx
Nicolas, S., & Ferrand, L. (1999). Wundt’s laboratory at Leipzig in 1891. History of Psychology, 2, 194–203.
Norcross, J. C. (n.d.) Clinical versus counseling psychology: What’s the diff? Available at http://www.csun.edu/~hcpsy002/Clinical%20Versus%20Counseling%20Psychology.pdf
Norcross, J. C., & Castle, P. H. (2002). Appreciating the PsyD: The facts. Eye on Psi Chi, 7, 22–26.
O’Connor, J. J., & Robertson, E. F. (2002). John Forbes Nash. Retrieved from http://www-groups.dcs.st-and.ac.uk/~history/Biographies/Nash.html
O’Hara, M. (n.d.). Historic review of humanistic psychology. Retrieved from http://www.ahpweb.org/index.php?option=com_k2&view=item&layout=item&id=14&Itemid=24
Person, E. S. (1980). Sexuality as the mainstay of identity: Psychoanalytic perspectives. Signs, 5, 605–630.
Rantanen, J., Metsäpelto, R. L., Feldt, T., Pulkkinen, L., & Kokko, K. (2007). Long-term stability in the Big Five personality traits in adulthood. Scandinavian Journal of Psychology, 48, 511–518.
Riggio, R. E. (2013). What is industrial/organizational psychology? Psychology Today. Retrieved from http://www.psychologytoday.com/blog/cutting-edge-leadership/201303/what-is-industrialorganizational-psychology
Sacks, O. (2007). A neurologists notebook: The abyss, music and amnesia. The New Yorker. Retrieved from http://www.newyorker.com/reporting/2007/09/24/070924fa_fact_sacks?currentPage=all
Shedler, J. (2010). The efficacy of psychodynamic psychotherapy. American Psychologist, 65(2), 98–109.
Soldz, S., & Vaillant, G. E. (1999). The Big Five personality traits and the life course: A 45-year longitudinal study. Journal of Research in Personality, 33, 208–232.
Thorne, B. M., & Henley, T. B. (2005). Connections in the history and systems of psychology (3rd ed.). Boston, MA: Houghton Mifflin Company.
Tolman, E. C. (1938). The determiners of behavior at a choice point. Psychological Review, 45, 1–41.
U.S. Department of Education, National Center for Education Statistics. (2013). Digest of Education Statistics, 2012 (NCES 2014-015).
Weisstein, N. (1993). Psychology constructs the female: Or, the fantasy life of the male psychologist (with some attention to the fantasies of his friends, the male biologist and the male anthropologist). Feminism and Psychology, 3, 195–210.
Westen, D. (1998). The scientific legacy of Sigmund Freud, toward a psychodynamically informed psychological science. Psychological Bulletin, 124, 333–371.
Psychology is the scientific study of mind and behavior. (credit "background": modification of work by Nattachai Noogure; credit "top left": modification of work by U.S. Navy; credit "top middle-left": modification of work by Peter Shanks; credit "top middle-right": modification of work by "devinf"/Flickr; credit "top right": modification of work by Alejandra Quintero Sinisterra; credit "bottom left": modification of work by Gabriel Rocha; credit "bottom middle-left": modification of work by Caleb Roenigk; credit "bottom middle-right": modification of work by Staffan Scherz; credit "bottom right": modification of work by Czech Provincial Reconstruction Team)
Clive Wearing is an accomplished musician who lost his ability to form new memories when he became sick at the age of 46. While he can remember how to play the piano perfectly, he cannot remember what he ate for breakfast just an hour ago (Sacks, 2007). James Wannerton experiences a taste sensation that is associated with the sound of words. His former girlfriend’s name tastes like rhubarb (Mundasad, 2013). John Nash is a brilliant mathematician and Nobel Prize winner. However, while he was a professor at MIT, he would tell people that the New York Times contained coded messages from extraterrestrial beings that were intended for him. He also began to hear voices and became suspicious of the people around him. Soon thereafter, Nash was diagnosed with schizophrenia and admitted to a state-run mental institution (O’Connor & Robertson, 2002). Nash was the subject of the 2001 movie A Beautiful Mind. Why did these people have these experiences? How does the human brain work? And what is the connection between the brain’s internal processes and people’s external behaviors? This textbook will introduce you to various ways that the field of psychology has explored these questions.
|
oercommons
|
2025-03-18T00:39:13.108458
|
Module
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15293/overview",
"title": "Psychology, Introduction to Psychology, Introduction",
"author": null
}
|
https://oercommons.org/courseware/lesson/79259/overview
|
Readings
Overview
This chapter will explore the interests and career exploration.
Introduction
College can be as scary as it is liberating. One of the biggest things college students love about starting their college careers is the level of freedom they have. Many students have never known freedom to such a degree before moving on campus. They can eat whatever they want, spend time with whomever they choose, and stay up as late as they want. This feeling is at its absolute peak in the brief window between moving for college and before classes start. Amid the changes and excitement, there are also likely to be many questions—especially by family members. Examples might include questions about what classes you are taking and how many hours you will set aside per week for studying, but the most dominant questions of all can be the most stressful to answer. What will be your major? What career will you pursue? I have dealt with these questions daily in my previous role as a career counselor and in my current role as academic advisor. When I was a university career counselor, the majority of students with whom I met were a semester or less away from graduation. This is when most students begin to have these career conversations.
Students can be quite unique in the way they handle stress and anxiety. Some students pick a plan—any plan—just to have something written down on paper. Others put off thinking about such problems because it is not their problem—it is their future selves’ problem. Students pick majors for all sorts of reasons. Some students pick a major because their parents have told them to choose the same occupation as they did, one that will provide financial success. Others may pick their major based on their favorite television show. I have even talked with students who picked their major because of past professors or teachers who taught them to be passionate about the subject. Students pick their major for all sorts of reasons, and most of these reasons are good. Career exploration always starts somewhere, and it can be the smallest of ideas that grows into something magnificent. In Texas, formal exploration starts in seventh or eighth grade and continues through high school, especially with Texas' new K-12 push for students to graduate with career-focused “endorsements.” But students who attempt to pick a career path at 13 may realize when they arrive to college that they are no longer interested in that field. What truly matters is what you do with an idea to help it grow and develop. Do you have an occupational goal in mind and pursue it without assessing the fullness of what it is? Do you put off thinking about career aspirations out of fear of picking the wrong path? Or, do you stumble upon something and dig deeper to reveal how it may align with your own story? This chapter is for you.
Learning Objectives
- The student will familiarize themselves with the general process of career exploration.
- The student will learn about career interest and the impact of interests on choosing a career.
- The student will learn about values and how values affect career choices.
- The student will use online career research tools to help them build an understanding of career options.
- The student will learn about the career decision-making process and how they can use it effectively in their own career decisions.
- The student will learn hands-on ways to research careers and find “good fit” for possible options, including volunteering, internships, and information interviews.
- The student will learn how to integrate these various components to assist in future career decision making to locate possible career options that lean into the core concepts of interest, values, and career options.
Understanding the Self – Part of the Foundation
Interests
Interest can be a powerful force in shaping what we pursue and what we do not, and the same can be said for career interests. This is often a route that yields greater success with students. However, just because people may be aware of some of their interests does not necessarily mean they know how to apply this information and connect it to potential career paths. To assist with this process, the Holland Codes, developed by Dr. Holland in the 1970s, can be a valuable resource. Another prominent area of interest to this chapter is the importance of personality in career decision making and development. For the sake of brevity, this chapter is unable to cover the topic of personality and its relation to career choices. For more information on this, investigate the 16 Personalities link at the end of this chapter under the Instructor/Student Resources section.
Many people can figure out that if they like music, they may very well be interested in it. Some students have clearly defined interests that make it easy to pick possible careers to consider, and others feel stuck in this area of conflicting or competing interests, which makes it very difficult and frustrating to define their interests. Instead of understanding interest through a dichotomous approach of either being interested in something or not, I recommend for those struggling with career exploration to look toward career exploration theories. One prominent theory, trait and factor theory, proposes certain measurable characteristics can be compared with possible career paths. The theory stemming from this parent theory is Holland’s theory on career development, which we cover in greater detail as the chapter progresses. Being grounded in Trait and Factor Theory, Holland’s approach asserts that our interests can be matched through personality characteristics and occupations and that people seek out work environments that align with their skills and abilities, while allowing them to express their values (Nauta, 2010). These categories are broken down into six areas:
Attributions
RIASEC Wheel Image, by Rogue Community College, http://www.roguecc.edu/counseling/hollandcodes/images/RIASEC-hexagon.png. Reused with permission.
Holland Inventory Themes
Let’s dig deeper to explore what the different categories of the Holland Inventory.
Realistic, the Doers, are very practical people and might enjoy activities that are very “hands on.” They typically prefer working with things more than people. Others may see doers as genuine, sensible, and honest. They may have an interest in areas such as building or repairing things, tinkering with computers, enforcing the law, playing sports, or even spending time in nature.
Investigative, the Thinkers, tend to be analytical and curious individuals. A vast majority of your professors might very well have investigative as one of their three dominant Holland codes. They prefer to work with ideas rather than people or things. Thinkers are often seen as intellectual, curious, and independent. They may have an interest in reading, writing, science, research, and math. An important point to note is someone can identify more heavily with an interest category without necessarily agreeing with each point made. Investigative is one of the interest categories with which I identify. I greatly enjoy reading books, both professionally and for pleasure. I enjoy writing, as indicated by this current book chapter. I even enjoy research, because it provides me with more information, which I can use to further help the students I serve. However, I am not a fan of math. I commonly make the joke that if a number was more than 10 (referring to counting on my fingers), or if my phone calculator was not working, I would be out. Even though I have a particular disposition toward math, I still align with the Investigative category, so it remains in my top three.
Artistic, the Creators, tend to be creative and prefer to work with ideas more than things. They are commonly described as independent, emotional, and original. It is important to consider creativity in a more abstract lens than just the classics: painting, drawing, music, theater, etc. Although Picasso would most likely identify with the Artistic category, it is not reserved for Picassos alone. Artistic is one of my categories, despite the fact I do not particularly excel at any of these areas in a creative sense. I am not a great artist—grading my skills as a step above stick figures. I cannot dance nor can I sing. However, I try to use creativity in the ways I approach students with ideas and helping them to map out their possible narrative.
Social, the Helpers, prefer to work with people. They typically enjoy work that incorporates helping, teaching, or caring for others. They are commonly drawn to various work environments, such as education, health care, and nonprofits, but they can also engage in helping roles in other industries that may be less associated with “helping.” At the end of the day, the helper is drawn toward helping. Alternatively, people may be in roles not usually associated with helping. For instance, marketing is not traditionally known as a social occupation, but if a person is in this occupation for an agency or department that is geared toward helping others, they may be tapping into this interest indirectly.
Enterprising, the Persuaders, prefer to work with people. They typically enjoy influencing and leading others toward goals in the workplace, valuing entrepreneurship and competition. Persuaders may have good people skills along with their Social counterparts, but with a swing toward a more business-oriented approach. Someone with this interest may be drawn toward self-employment, politics or government, and industries of business that work at a fast pace. Leadership is a hallmark value for persuaders.
Conventional, the Organizers, prefer working with information. They typically enjoy managing written and numerical data, scheduling, administrative work, and ordering information. Efficiency, stability, and structure are important values for people with this interest type.
| Consider: What Holland code sounds the most like you? Second most? Third? Also contemplate whether these decisions were easy to make or difficult. If difficult, why? |
Together these areas can take on different combinations, which make up both a person’s interests and occupations. Although some careers may be comprised of one interest group, most occupations take on varying combinations. For example, the Occupational Information Network from the U.S. Department of Labor (O*NET), a resource we will explore shortly, categorizes Lawyer as having Enterprising, Investigative, and Artistic qualities. An alignment between an occupation’s three categories and an individual’s three categories does not suggest someone should pursue this role, only that this role could potentially be a good fit from an interest perspective.
Values
It is perfectly normal if you have been struggling with pinpointing your career path. My hope is, by this point, it is becoming clear that people are very complex and that there are competing elements inherent to our interests and values. Someone may have an interest in an area, but it is not as compatible with their personality. Alternatively, someone may be great at something but have no interest in doing it. This section covers Values, and it will showcase another quality you can investigate to dig deeper into career exploration.
Values are linked to a professional purpose, which is more than what a person does but why they do what they do (Cooper & Cottrell, 2010). When people reflect on their “why,” they can build the “what” and “how” around it. For example, I always wanted to help people. Originally, I thought I wanted to be a police officer, but after going on a ride-along and speaking with people in that role, I decided that was not my form of helping. Then I found my way into mental health counseling, then career counseling, and eventually academic advising. Although these are all different careers, they shape around the “why” of my narrative—helping to empower others and build them up for success. People tend to be very values-driven, both in the sense of motivation and personal identity. It only makes sense that values would play a key part in our story. This listing is just a snapshot of possible values:
Common Career Values | |
Practicality | Originality |
Beauty | Service to Others |
Stability | Independence |
Efficiency | Tradition |
Generosity | Influence |
Competition | Risk Taking |
Status | Imagination |
Learning | Accuracy |
Curiosity | Cooperation |
Common Sense | Leadership |
References
Cooper, H., & Cottrell, R. R. (2009). Charting your career path through clear professional values and purpose. Health Promotion Practice, 11(1), 13–15. https://doi.org/10.1177/1524839909352840
Nauta, M. M. (2010). The development, evolution, and status of holland’s theory of vocational personalities: Reflections and future directions for counseling psychology. Journal of Counseling Psychology, 57(1), 11–22. https://doi.org/10.1037/a0018213
Occupational Outlook Handbook
Understanding the self is a foundational to building a career narrative, but it proves insufficient if not paired with further tools for success. As stated previously, when it comes to picking a career and choice of major, students may pick careers earlier in middle school or high school, only to find out it may not be a great fit upon coming to college. This is related to the core truth that people are always evolving and developing, but that we also learn more along the way. We can only know what we know. This shows the immense importance in taking steps to research careers. Sometimes researching careers can pose its own problems because there is so much information on the internet, much of which is conflicting. The purpose of the first major section was to cover understanding of self. Once you have this information in a more concrete place, it is time to connect that information to occupations of interest.
The Occupational Outlook Handbook is a great resource in which the data are taken directly from the Department of Labor and the Bureau of Labor Statistics to use when researching career options and opportunities. To reach this resource, visit https://www.bls.gov/ooh/
When using this resource, you can either search through the “Search Handbook” option in the top right or you can search in “Occupational Groups” located on the left side of the website. Use the search bar only when you know the specific occupation of interest. If the specific occupation of interest is not yet known, begin the search by occupational group, as this will give you a broader perspective.
Follow Along: Go to https://www.bls.gov/ooh/ Then pick an example occupation you might be interested in. This will assist you in understanding the following discussion on this resource. |
"Using the Occupational Outlook Handbook Website" by Bureau of Labor Statistics, PBS Digital Studios, is licensed CC BY 4.0, and is located at https://youtu.be/9s9Hd2hc7pM
Much of this website is quite intuitive, as are these sections. If you are familiar with specific jobs, you can use the tool to explore if the job fits with your interest and values. This website can assist you in gauging how much an occupation aligns with your interest in working with data and customers. If you are a people person and want a job where you interact with others constantly, this tool can help you explore that type of role. If you investigate how to become something, this tool can help you gauge best fit for a major. This tool can tell you what internships and other experience to seek out. If you investigate similar occupations, you can look at jobs related to similar areas. For example, I once had a student who wanted a job in health care but really struggled with finding “their thing.” When the student and I used this site and continually looked through similar occupations, we came across medical services managers. Although this was not an entry-level role, the student and I pinpointed that this person was more interested in the business side of health care, and then they could begin formulating a pathway to get there.
The previous video and description of the Occupational Outlook Handbook is a surface-level explanation of this powerful resource. I encourage you to take the time to further investigate occupations on this website to see what steps you can take to make your goals happen. You are the author of your story, and no one can work harder than you to write it.
O*Net
For individuals who like more technical information, O*NET is the website for you. Visit this website at https://www.onetonline.org/. This website serves as a complement to the Occupational Outlook Handbook. It has a lot of information that can be useful to students in their career-building exercises, and the handbook also works well with the Holland Codes we discussed in a previous section over Interests. You can search this database’s occupations a variety of different ways, including by Holland Codes.
Follow Along: Go to https://www.onetonline.org/ Then pick an example occupation you might be interested in. This will assist you in understanding the following discussion on this resource. |
"O*NET TUTORIAL" by Jason Seward, and is located at https://youtu.be/9mcB_p3k4J8
O*NET is a resource with much of the same information as the Occupational Outlook Handbook, but it contains additional tools and resources. For instance, this site provides viewers with alternate job titles for desired occupations. This website can be helpful when searching for potential jobs. Some jobs have multiple titles, depending on the company or organization. It can give you more keywords to search when looking for opportunities in each field. There is also a section in O*NET that shows the percentages of education associated with an occupation. I have had many students tell me about their plan to go to graduate school because they need it for a job, but that assumption is not always true. There are some occupations that require advanced education (master’s level+), but O*NET can be a great resource to help fact check whether that is hearsay or supported by facts and numbers by showing the percentage of individuals that hold that specific degree level in a given job.
This discussion has scratched the surface and covered the highlights of O*NET. It is important to investigate this resource by using it to research your own interest and career goals.
Professional Associations
It is important to be actively involved in your career journey. No one can better write your own story. Professional associations can be an excellent way to engage in career research. Some career fields may not require that you consult your designated professional association for guidance, but other professions will necessitate exposure. Previously working toward my career path as a mental health counselor and then as a career counselor, I became very intimate with the American Counseling Association and the National Career Development Association. I located accredited graduate programs through these websites to ensure a valuable and more valid education, pertinent research articles, steps for state licensure, and more. Both sites also possess job boards and offer student memberships, scholarships for professional development, and conferences for continuing education. I had the privilege of attending a conference for the National Career Development Association during Summer 2019, as professional associations will often have conferences for likeminded professionals to network and keep each other up to date on current trends and practices in the field. Connecting with professionals through a professional association is a great way to network and learn more about your profession of interest as well as network for potential job opportunities.
When researching more about career options, I commonly found myself showing students these websites because of the rich information in them. In addition to joining relevant organizations and groups on campus, make sure to check out professional associations. If you check out a professional association and find yourself bored and not wanting to keep up with that type of information, reflect on why that might be. If these are not things you care about, why is that? It is important to reflect on occupational identities and career values. There are clues in many of the things we do—the things we like and do not like. The hard part is taking a moment to delve into awareness and ask why.
Career Decision-Making
In the career counseling room, I learned quickly that giving people resources and example job titles is rarely sufficient. Many students still do not know where to go, even when resources are shared. It is important to have conversations and help students unpack their own narratives through aspects of the self. However, there were other times even this was not the case, in which equipping the student with knowledge of relevant occupations was sufficient for them figure out their career goals. Even though a lot of progress can be made in the initial steps of throwing out job titles like darts on a dart board or just trying out a bunch of different ideas with nothing sticking. Sometimes the difficulty comes with struggling to make decisions on a grander scale. There is no one-size-fits-all for decision-making problems. As with many areas in life, understanding and awareness is a foundational piece that can help shed light on the decision-making process.
There are three primary decision styles when it comes to decision difficulties: rational style, avoidant style, and dependent style. Although these styles each present with a degree of varying complexity, they also share common characteristics. Shin and Kelly (2015) stated:
The rational style is an active and planful approach to decision making. The avoidant style is characterized by failure to attain and process career information and postponement of decisions. The dependent style involves ceding responsibility for decisions to external sources, such as significant others. The rational style is viewed favorably because it is a systematic approach that yields information relevant to decisions. (p. 293)
Although engaging in avoidant and dependent styles can decrease immediate career anxiety, by either delaying the perceived threat or seeking to put responsibility on others, these strategies raise the overall career anxiety in the long run. Only by adopting and engaging in a rational style can people appropriately pursue the steps discussed next.
Career Decision-Making Process
The first step in the process is Knowing I Need to Make a Choice. This becomes apparent when one is faced with a career issue or difficulty, which requires the individual to start the process toward making a career decision. Once someone realizes they need to make a career decision, the best place to start is Knowing About Myself. This can involve reflection on personality characteristics, work interests, leisure interests, skills, and values. Values often play an important role in decision making. For instance, I know justice and helping are important qualities to me. Although finances and income are very important, too, they are not as important to me as the distinct feeling of making a difference for the people I serve. It is also important to me to have a job that balances working with people and having time to work on ideas. This goes together with the next step, which is Knowing About My Options. One can use the tools previously discussed to gather more information during this step. Only after someone has evaluated the self and has undertaken career research to learn about opportunities can they begin to know how they relate to those opportunities. Create a list once you have ironed out aspects of the self and occupations that might complement them.
Your career list takes shape in the phase of Expanding and Narrowing Options. This is where the heavy lifting occurs—by matching values and other aspects of the self with possible occupations. This process naturally causes some occupations to be cut from the list and others to be added for further consideration. Refining the list often takes the form of running a cost-benefit analysis or examining the pros and cons of each occupation. This is where you spend time reflecting on the costs and benefits for you, your family, peers, finances, etc. For this step, create a list that leans into the previously mentioned values. This list might be made up of careers, such as career counselor, mental health counselor, teacher, and social worker.
The final phase of the decision-making process can be the most difficult for people because it implies action, referred to as Implementing My Choice. The action taken can vary for individuals, but individuals must commit to making that choice. The successful reaction to implementation leads to the final step in the decision, Knowing I Made a Good Choice. If implementation leads to no positive resolution, then this might suggest the career choice should be reviewed with the new information. Even if you successfully complete this decision-making process once in your life, there is no guarantee this will remain a constant. The ultimate goal is to build up the skill of making informed career decisions, because the workplace will change and new opportunities will come along (Krumboltz et al., 2013). Knowing and understanding how the process works can serve as a helpful guide to facilitating the change moving forward, as the need arises.
References
Krumboltz, J., Foley, P., & Cotter, E. (2013). Applying the happenstance learning theory to involuntary career transitions. The Career Development Quarterly, 61, 15–26.
Sampson, J. P., Jr., Peterson, G. W., Lenz, J. G., & Reardon, R. C. (1992). A cognitive approach to career services: Translating concepts into practice. The Career Development Quarterly, 41, 67–74.
Shin, Y., & Kelly, K. R. (2015). Resilience and decision-making strategies as predictors of career decision difficulties. The Career Development Quarterly, 63(4), 291–305. https://doi.org/10.1002/cdq.12029
Taking Action
Implementation
Implementation can take many forms when being actualized in our lives. As previously stated, no one will put forth as much agency in your career path as you, so it is important to not only think about it and do the research but also to act on that research. It is only through action that steps are taken to make those goals a reality and write your story. Many students become nervous when contemplating a change of major or major declaration because they feel they do not have enough information. Many students also worry about changing a major because they are afraid this will become a repeating pattern. They may also be reluctant because they will have to explain their “why” to their friends and family. It is important to build a strong understanding of your interests, values, and your “why.” Sometimes finding other ways to get involved and take a more action-oriented approach to gathering information can build a case for whether to change majors or not. This section will cover various action-oriented approaches you can take to gather more information on your options.
Informational Interviews
Informational interviews are excellent tools to gather more information about a career path. These are interviews where you can ask a professional in your field of interest various questions you want to know about their position. This can be an invaluable opportunity to seek answers that may not be found in other resources for career research. You can ask questions such as, “What do you love about your job? What is something you do not like about your job or would change about it? Why do you do what you do?” Questions such as these can transcend factual information online because professionals can speak to the meaning behind their careers and why they do it. As a student, if you are unsure how to engage in informational interviews, meet with the university’s career services office for tips and more information on how to seek these interviews out. It is also a great idea to reach out to family and friends who work in your field of interest. The key to seeking informational interviews (and networking) is to not limit yourself to your own bubble. Some people may consult with church groups, other community groups, and professors. Start with familiar people to build confidence in having these conversations.
Groups & Organizations
Being involved in groups and organizations is a significant way students can begin to engage in the social and professional world of their career interests. There is a myriad of groups on campus with many different topics and areas of interest. This can be a great way to exercise your networking muscles and explore areas of interest. For instance, if you think you might be interested in public relations, get involved in such a group. If you find you love the meetings and are passionate about the content area, then that can be a sign of a possible career option. It is important when trying out groups and organizations to reflect on why you like or did not like something. This can provide invaluable information moving forward as you discover your possible career path.
Internships/Volunteering
Internships and volunteering are truly worthwhile opportunities that can help you build marketable skills and experiences to add to your resume. Although internships are commonly reserved for juniors and seniors with a certain minimum GPA, it is never too early to investigate these opportunities because they can be another great way to try an area of interest to see how it fits with your own narrative. If it is not an area of interest, then that might suggest this is not the right career choice for you. It is important to emphasize that there is a difference between not liking the core of an occupation versus disliking a certain part of it. All careers have tasks that are more enjoyable than others. It is also important to note that some occupations will require entry-level candidates to “pay their dues.” You will rarely find your dream job right away and internships may not always reflect the work done after graduation either. This is why common orators and speakers talk about career “building.” Careers are not like picking something from a menu. Careers must be built and are a story that must be written, which takes time and effort. If I want to be a lawyer and am pursuing an internship during my undergraduate education, I should not expect to find a lawyer internship. An internship in such a setting would reflect the previously mentioned clerk internship. So, although it is important to evaluate the pros and cons for internships to gauge if a career is your best fit, it is important to maintain a level of perspective. Volunteering can be similar to what was discussed about internships. However, volunteering can also show community engagement and can be a strong introduction for those interested in various careers, such as work in the community, at nonprofits, and more. Volunteering can be an important part of the journey, given that some internships may even require experience to be competitive for the positions. Even if an internship does not require prior experience, it can still give you a competitive edge over others. Some roles may not allow you to volunteer in the same capacity as you wish to work one day. For example, people who wish to be mental health counselors cannot volunteer as practitioners, but they can seek out other volunteering opportunities that provide similar help or volunteering in a different way, which allows them to work in the same work environment or with the same population of interest as they would in their desired career.
Chapter Summary
This chapter shows the importance of taking your career planning into your own hands by doing research, both related to the job market and to yourself, and being open to new opportunities as they arise. Someone’s major does not always directly relate to their career choice. I started by pursuing mental health counseling, then shifted to career counseling, and now serve as an academic advisor with career elements streamlined into my work. I have enjoyed every role and will enjoy many other roles that proceed from this point forward. You can be the greatest champion of your narrative, and it is worth using these resources to invest in that story. Career exploration is founded on the pillars of being aware of yourself, knowing your options, and employing decision-making strategies. Once this information has been reflected upon and researched further, it is time to put it to action. Experience can be the greatest teacher, but it is important to look for ways to explore occupational compatibility, which may demand less commitment upfront, such as through volunteering, internships, joining groups or organizations, etc. Engaging in these strategies will strengthen your ability to navigate the difficult but meaningful nature of career exploration. Now that you have read this chapter, go back, and try the Career Options worksheet earlier in the chapter.
|
oercommons
|
2025-03-18T00:39:13.168402
|
Heather F. Adair
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/79259/overview",
"title": "Foundations for College Success, Career Exploration, Readings",
"author": "Forrest Lane"
}
|
https://oercommons.org/courseware/lesson/79245/overview
|
Learning Activities
Overview
Learning Activities for Unit 7
Activity 7.1
Practice the Search Process
Try an experiment with a group of classmates. Without looking on the Internet, try to brainstorm a list of 10 topics that you may all be interested in but know very little or nothing at all about. Try to make the topics somewhat obscure rather than ordinary - for example, the possibility of the non-planet Pluto being reclassified again as opposed to something like why we need to drink water.
After you have this random list, think of ways you could find information about these weird topics. Our short answer is always: Google. But think of other ways as well. How else could you read about these topics if you don’t know anything about them? You may well be in a similar circumstance in some of your college classes, so listen carefully to your classmates’ ideas on this one. Think beyond standard answers like “I’d go to the library,” and press for what a researcher would do once they are at the library.
What types of articles or books would you try to find? One reason that you should not ignore the idea of doing research at the library is because once you are there and looking for information, you have a vast number of other sources readily available to you in a highly organized location. You also can tap into the human resources represented by the research librarians who likely can redirect you if you cannot find appropriate sources.
Once you have the resources to answer your questions, what do you do with them? What would be your plan of attack, so to speak?
Attributions
Content on this page is a derivative of “Reading and Notetaking: Summary” and “Reading and Notetaking: Rethinking” by Amy Baldwin, published by OpenStax, and is licensed CC BY 4.0. Access for free at https://openstax.org/books/college-success/pages/1-introduction.
Activity 7.2
Explore Reading & Notetaking Resources
What resources can you find about reading and notetaking that will help you develop these crucial skills? How do you go about deciding what resources are valuable for improving your reading and notetaking skills?
The selection of study guides and books about notetaking vary dramatically. Ask your instructors or a campus librarian for recommendations. Understand the list below is not comprehensive but will give you a starting point.
- College Rules!: How to Study, Survive, and Succeed in College, by Sherri Nist-Olejnik and Jodi Patrick Holschuh. More than just notetaking, this book covers many aspects of transitioning into the rigors of college life and studying.
- Effective Notetaking, by Fiona McPherson. This small volume has suggestions for using your limited time wisely before, during, and after notetaking sessions.
- How to Study in College, by Walter Pauk. This is the book that introduced Pauk’s notetaking suggestions we now call the Cornell Method. It is a bit dated (from the 1940s), but still contains some valuable information.
- Learn to Listen, Listen to Learn 2: Academic Listening and Note-taking, by Roni S. Lebauer. The main point of this book is to help students get the most from college lectures by watching for clues to lecture organization and adapting this information into strong notes.
- Study Skills: Do I Really Need this Stuff?, by Steve Piscitelli. Written in a consistently down-to-earth manner, this book will help you with the foundations of strong study skills, including time management, effective notetaking, and seeing the big picture.
- “What Reading Does for the Mind,” by Anne Cunningham and Keith Stanovich, 1998, https://www.aft.org/sites/default/files/periodicals/cunningham.pdf
- Adler, Mortimer J. and Charles Van Doren. How to Read a Book: The Classic Guide to Intelligent Reading. NY: Simon & Schuster, 1940.
- Berns, Gregory S., Kristina Blaine, Michael J. Prietula, and Brandon E. Pye. Brain Connectivity. Dec 2013.ahead of print http://doi.org/10.1089/brain.2013.0166
Attributions
Content on this page is a derivative of “Reading and Notetaking: Summary” and “Reading and Notetaking: Rethinking” by Amy Baldwin, published by OpenStax, and is licensed CC BY 4.0. Access for free at https://openstax.org/books/college-success/pages/1-introduction.
|
oercommons
|
2025-03-18T00:39:13.194445
|
Heather F. Adair
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/79245/overview",
"title": "Foundations for College Success, Reading Strategies, Learning Activities",
"author": "Forrest Lane"
}
|
https://oercommons.org/courseware/lesson/79241/overview
|
Digging Deeper
Improving Critical Thinking
Watch the video, 5 Steps to Improve Critical Thinking, by Samantha Agoos (2015):
"5 Tips to Improve Your Critical Thinking" by Samantha Agoos is licensed under CC BY-ND 4.0
Complete the questionnaire, titled "Think," on the TedEd website and discuss your responses with the class:
- What is one advantage of critical thinking?
- What is the first step (of five) in the critical thinking process?
- When making a decision, what is the main purpose of gathering facts and information?
- Whas does it mean to consider the implications of a decision?
- How can critical thinking improve your chances of making better choices?
- Consider the example of the diet craze presented in the video. In your own words, explain how you would apply information to determine whether the weight-loss cliams are logical and accurate. Be specific about each step you would take.
- Read the scenario below and respond to the prompt that follows. Two candidates are running for President of the United States. One candidate advocates for reducing corporate taxes to incentivize businesses to open in the US, fostering the economy. The opposing candidate wishes to incrase corporate tax rates to use the revenue to address domestic issues like education. Describe why it is valuable and beneficial to the decision-making process to explore both political perspectives prior to casting your vote?
- Silently reflect on an upcoming choice you may encounter in the future. Compose a pagraph (5 sentences maximum) that discusses how you will employ the critical thinking to enhance your decision-making process. Be specific about how you would apply at least three of the five steps mentioned in the video.
https://ed.ted.com/lessons/5-tips-to-improve-your-critical-thinking-samantha-agoos#review
|
oercommons
|
2025-03-18T00:39:13.210802
|
Heather F. Adair
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/79241/overview",
"title": "Foundations for College Success, Critical Thinking, Digging Deeper",
"author": "Forrest Lane"
}
|
https://oercommons.org/courseware/lesson/87628/overview
|
5.3 Psychological Benefits of Plants
5.4 Physiological Benefits of Plants
5.5 Medicinal Benefits of Plants
5_The-Role-of-Plants-in-Human-Well-Being
Hall & Knuth: A Review of the Emotional and Mental Health Benefits of Plants
Hall & Knuth: Available Resources and Usage of Plant Benefits Information
Hall & Knuth: Physiological Health Benefits
Hall & Knuth: Social Benefits
International Center of Ethnobiology
TeachEthnobotany
The Role of Plants in Human Well-Being
Overview
Title image: "Appalachian Trail, Smoky Mountain National Park, TN" by Abhishek Chinchalkar is marked with CC BY-NC-ND 2.0.
Did you have an idea for improving this content? We’d love your input.
Introduction
Lesson Objectives
Describe the various ways plants impact human well-being.
Distinguish between the terms psychological and physiological.
List research-based psychological and physiological benefits of plants.
Key Terms
medicinal - substances and other treatments that are used to cure illnesses
physical - relates to the body
physiological - the way that living bodies function
psychological - relates to the mind and feelings
restorative - refers to the ability to restore consciousness, vigor, or health
Introduction
For most of human history, our health, wellbeing, and success have been intertwined with our ability to interpret the environmental cues around us, which are often provided by plants. People who were better able to interpret these signals from plants had an easier time finding food, water, shelter, and refuge from predators. There is a growing body of research that demonstrates the physiological, physical, medicinal, and psychological benefits of green nature to humans. Several such studies are referenced in this lesson.
Some studies explore benefits from active gardening activities, such as tending a vegetable or flower garden or taking a nature hike; while others attempt to quantify benefits from passive experiences, including viewing nature from a window or the presence of a houseplant in a room. The amount of plant life varies between areas, and many of these studies use the term “green space” rather than “plants” or “gardens.” What is “green space”? In their comprehensive series of literature reviews on the benefits of plants and horticulture, Dr. Charles Hall and Dr. Melinda Knuth write, “The term ‘green spaces’ has been used extensively to refer to areas of urban vegetation including public and private parks and gardens, residential landscapes, and urban forests and other municipal landscapes.”
Readers should keep in mind that many of the findings referenced in the first three sections of this lesson are from correlational studies rather than true experiments, and correlation does not necessarily mean causation. Correlational studies are an important first step researchers take to determine whether future controlled studies are worthwhile.
Prominent Theories
When researchers explore the relationship between plants and nature on human health, they develop a theory to explain their findings. Three prominent theories in the fields of environmental psychology, environmental sociology, and socio-horticulture include Dr. Stephen and Dr. Rachel Kaplan’s Attention Restoration Theory, Dr. Roger Ulrich’s Stress Recovery Theory (a Psycho-Evolutionary Theory), and Dr. Edward O. Wilson’s Biophilia Hypothesis. While some people may have learned to love plants, gardening, and nature through taught experiences, each of these three theories proposes that unlearned, evolutionary factors are also important considerations when describing our positive response to nature.
Stress Recovery Theory (Psycho-Evolutionary Theory)
The framework for Stress Recovery Theory was proposed by Dr. Roger Ulrich (formerly of Texas A&M University, currently with the Chalmers University of Technology in Sweden); this framework can be found in his landmark work “Aesthetic and affective response to natural environments” (1983).
Ulrich and co-author Russ Parsons later described the theory in “Influences of passive experiences with plants on well-being and health” (1992):
The long evolutionary development of humankind in natural environments has left its mark on our species in the form of unlearned predispositions to pay attention and respond positively to certain contents (e.g., vegetation, water) and configurations that comprise those environments. People respond especially positively to combinations of contents and forms characteristic of natural settings that were most readily exploited by premodern humans, or were most favorable to ongoing well-being or survival… Ulrich postulates that quick-onset affective or emotional reactions – not cognitive responses – constitute the first level of response to nature, and are central to subsequent thoughts, memory, meaning, and behavior with respect to environment.
Dr. Ulrich has measured physiological and psychophysiological responses to stress, including heart rate, blood pressure, muscle tension and brain waves, after exposure to different stimuli. He and other proponents of Stress Recovery Theory have found that exposure to natural environments—even just a view from a window or a poster of a natural scene—can reduce tension and enhance recovery from stress (Ulrich, 1984; Ulrich et al., 1991).
Attention Restoration Theory
Dr. Stephen Kaplan and Dr. Rachel Kaplan of the University of Michigan proposed Attention Restoration Theory in their book The Experience of Nature: A Psychological Perspective as an explanation for why natural environments seem to have a restorative effect on attention.
Any discussion of this theory should include a description of “mental fatigue.” Mental fatigue is caused by spending time in a state of directed attention where focus must be maintained by suppressing distracting stimuli. This is especially common in modern environments where cell phone notifications and advertisements are constantly vying for attention. School environments also require a great deal of directed attention to successfully complete assignments and learn new information. The ability to maintain directed attention decreases over time. The result of prolonged directed attention is mental fatigue (Parsons, 1991). Symptoms of mental fatigue include irritability, increased incidence of mistakes (Kaplan, 2001), stress (Han, 2009), aggression and decreased impulse control (Kuo & Sullivan, 2001). As a person becomes more mentally fatigued, they become less able to evaluate a situation rationally and more likely to have an unnecessary outburst (Kuo & Sullivan, 2001).
Natural environments that are rich in “fascinating” stimuli that intrigue the senses can be suitable treatment for mental fatigue. According Attention Restoration Theory, natural environments that spark human fascination provide an opportunity for the mind to recover from mental fatigue (Parsons, 1991). Other treatments for mental fatigue include taking a vacation and, to some extent, sleep (Kaplan, 1993). Attention restoration is facilitated by a landscape that meets certain criteria (Kaplan, 1984). An example of a restorative landscape would be a “mysterious” environment, where participants in the environment feel drawn in to explore around a bend of a curving path or over a hill just out of view.
Since the type of environment is of primary importance in attention restoration theory, much of the research supporting the theory typically involves some type analysis of the qualities of the landscape (like mystery). However, Rachel Kaplan (1984) once asked, “Is presence in the setting sufficient to reap the benefits? Or is some involvement or commitment [activity in the environment] on the part of the individual essential?” Kaplan went on to note three types of involvement in the landscape that could also contribute to Attention Restoration Theory. The first type of involvement is active involvement in the natural environment, which could include gardening or a walk through the neighborhood. The second type of involvement is observing (passive involvement), such as a looking out on a natural scene from a window or watching plants grow and develop. The third level of involvement is on a conceptual nature. Conceptual involvement has to do with knowledge and imagining one’s participation in a natural environment through an activity like planning a garden or reflecting on a prior outdoor experience.
The Biophilia Hypothesis
Dr. Edward O. Wilson (1929 – 2021) studied at the University of Alabama and Harvard University, where he went on to hold a faculty position from 1956 until 1996. He began his career as a biologist focused on the study of ants. In fact, he discovered the first colony of fire ants in the U.S. near the port of Mobile in Alabama.
Wilson’s work as a biologist led him to study social behavior of insects, animals, and humans. He became one of the foremost naturalists of the 20th century, writing several books on science and conservation. In 1984, Wilson wrote Biophilia, in which he proposed that humans have an innate tendency to seek connections with nature and other forms of life. Wilsons’ Biophilia Hypothesis attempts to explain why humans have a preference for the natural world and makes a strong argument in favor of conservation.
The Biophilia Hypothesis has led to an increase in biophilic design. Biophilic designs incorporate biophilia into the built environment and are becoming more common in architecture, interior design, and other related fields as a complement to green design. While green design strives to decrease the environmental impact of the built environment, biophilic design includes natural elements and features as a way to facilitate human connection with nature.
Psychological Benefits of Plants
Research in environmental psychology, socio-horticulture, and related fields have shown that even in our modern society, humans still experience psychological benefits from spending time with plants. Several studies have shown that access to green nature, a view of green space from a window, the presence of living houseplants, and even images of nature have positive psychological benefits. Humans in modern societies spend most of their time indoors (US Dept. of Labor, 2006). And, with expanding rates of urbanization, more people live in areas that are further removed from natural environments (Van den Berg et al., 2010), and access to green space is an important consideration for human development.
Many of studies have focused on benefits in terms of stress reduction and recovery from mental fatigue. Dr. Rita Berto defined “stress” in her literature review of the role that nature plays in coping with stress (2014):
“Stress” can be defined as the condition that results when person-environment transactions lead the individual to perceive a discrepancy (whether real or not) between the demands of a situation and the biological, psychological or social resources of the individual [1]. The negative effects of stress can be measured in various ways inside and out of the laboratory and these measures fall into three categories: those that rely on (1) neuro-physiological or bodily changes in the individual experiencing stress, (2) performance or behavioral changes and (3) self-report by individuals.
As you will read in the following section, researchers have attempted to measure the benefits of green nature in many different ways, from proximity of trees, gardens, and natural areas to view from a window.
Access to Green Space
Spending time with living, green plants in natural settings, viewing them from a window, or just living near green areas is associated with reducing stress and recovery from mental fatigue (Abraham et al., 2010; Carrus et al., 2015; Watts, 2017; Wolf & Housley, 2014). Access to nature is related to happiness. One study found that after controlling for other variables, access to nature is related to several indicators of happiness (Zelenski & Nisbet, 2014). In fact, more accessibility to parks and natural, forest-like environments is related to increased happiness, better concentration, and less stress, anger, depression, and tension (Van den berg et al., 2003).
Access to green space can also improve memory retention. One experiment tested the working memory of participants who either walked through an arboretum or who walked along a busy urban street. Those who walked through the arboretum had a 20% greater improvement in working memory than those who walked on the urban street (Berman et al., 2012). Another similar study found that those who went on a 50-minute walk in green nature had better working memory and less anxiety than participants who went on a 50-minute walk on a busy street (Berman et al., 2012).
As tree canopy in a community increases, crime tends to decrease. One study found that a 10% increase in tree cover is related to a 12% decrease in crime (Troy et al., 2012).
In the Workplace
A view of nature is also related to satisfaction with work and life. A longitudinal study of employees over a 6-month period found that individuals who had a view of nature in their workspace were more satisfied with their jobs than individuals who did not have a view of nature (Kaplan, 1983). Another survey found that of 615 office workers, individuals with a view of nature were more satisfied with their lives and were more enthusiastic with their jobs than workers who did not have a view of nature (Kaplan, 1983). Workers who have a view of green nature are more productive, have higher workplace satisfaction, and tend to be happier than those who do not (Lottrup et al., 2015).
Interior plants in the workplace (Figure 9.5.4) are associated with increased productivity, decreased stress, improved attention, and higher rates of workplace satisfaction (Gilchrist et al., 2015, Hartig et al., 2014, Raanaas et al., 2011). One study found that the presence of green plants in the workplace increased worker productivity by 15% (Korpela et al., 2017; Nieuwenhuis et al., 2014). Researchers found that as few as three small to medium sized plants can positively impact reaction time and perceived air quality and that as the number of plants in a room increased, so did the mood of the study participants (Lee & Maheswaran, 2011).
In Schools
Living plants in the classroom can improve student performance and influence classroom evaluations. One study found that when plants were placed in a classroom, students advanced through the curriculum 20 to 26% times more quickly (van Duijin et al. 2011). Students who have a view of green space during school have better attentional capacity and lower stress (Kuo, 2015; Becker et al., 2017), or are better able to recover from stress (Li & Sullivan, 2016). A view of green space from the classroom may also be related to academic achievement (Benfield et al., 2015; Browning & Rigolon, 2019). Even the amount of green space on a playground can impact students in the classroom. Students who play in areas with high levels of green nature tend to experience less physiological stress and have improved psychological well-being when compared to children who play in areas with low levels of green space (Kelz et al., 2015)
Students who are diagnosed with attention disorders such as ADD and ADHD may benefit from time spent in nature. One study found that children who have ADHD concentrated better after a walk through the park when compared to children who walked through a downtown neighborhood (Taylor & Kuo, 2009).
At Home
Plants and nature are related to neighborhood satisfaction. One study compared medical records of households in areas with different amounts of green space near their home. When compared to participants living in an area with the greatest amount of green space, those who had only 10% of green space within half a mile of their home had 30% more of a risk of developing anxiety disorders and 25% greater risk of depression (Wolf & Housley, 2014). Additionally, individuals that actively engage in gardening are more satisfied with their neighborhood than those who do not. One survey of apartment dwellers found that permitting gardening activities within or near a neighborhood increased resident satisfaction. In addition to the benefits from gardening, researchers found a strong positive correlation between merely having a view of nature from the home and residential satisfaction (Kaplan, 2001).
Access to nature may also be related to self-control. One study focused on a group of girls who lived in the same housing complex. The girls who had a better view of green space from their windows showed better discipline, concentration, impulsivity, and ability to delay gratification (Taylor et al., 2002). Furthermore, aggression and violence decrease for apartment residents who have access to nearby nature when compared to residents of apartments in a barren environment (Kuo & Sullivan, 2001). Adolescents who live in an area where green space is within 1000 meters of their residence exhibit less aggressive behavior than those without close access to nature (Younan et a., 2016).
Horticultural Therapy
In Green Nature Human Nature, Charles Lewis writes that the primary purpose of horticulture therapy is to “promote the wellbeing of individual patients, and plants become byproducts of the healing process.” Horticulture therapy has both physiological and psychological benefits to patients. The American Horticulture Therapy Association describes horticulture therapy as follows:
Horticultural therapy techniques are employed to assist participants to learn new skills or regain those that are lost. Horticultural therapy helps improve memory, cognitive abilities, task initiation, language skills, and socialization. In physical rehabilitation, horticultural therapy can help strengthen muscles and improve coordination, balance, and endurance. In vocational horticultural therapy settings, people learn to work independently, problem solve, and follow directions. Horticultural therapists are professionals with specific education, training, and credentials in the use of horticulture for therapy and rehabilitation.
Participation in horticulture therapy programs has been found to help people cope with post-traumatic stress disorder (PTSD) (Figure 9.5.5). Veterans with post-traumatic stress who participate in Nature Adventure Rehabilitation report feeling more hope for the future as well as improvements in emotional and social quality of life (Gelkopf et al., 2013). Victims of natural disasters are also at high risk of PTSD. Nature disaster victims who participated in a horticulture therapy program showed fewer symptoms of the disorder than victims who participated in a standard stress-education program (Kotozaki et al., 2015; Sekiguchi et al., 2015).
Researchers have found that horticulture therapy can reduce the effects of dementia by improving cognitive capacity and reducing instances of aggressive behavior (Gigliotti & Jarrott, 2005).
Physiological Benefits of Plants
Spending time with green nature not only benefits our mental wellbeing, but our physical health as well. As with the psychological benefits of plants, several studies have explored the relationship between access to green space and physiological health.
Access to Green Space
People who have access to green space tend to experience less stress and engage in more physical activity (Thompson et al., 2012). Access to green space can improve sleep quality and duration (Astell-Burt et al., 2013; Morita et al., 2011), which is important because insufficient sleep is associated with serious, chronic health issues (Cappuccio et al., 2011; Cappuccio et al., 2008; Chaput et al., 2007; Hislop & Arber, 2003; Hublin et al., 2007). Exposure to plants can also positively impact diabetes by increasing anti-diabetic hormones adiponecitin and didehydroepiandrosterone (DHEA) (Bhasin et al., 2013; Ohtsuka, 1998).
Access to Trees
The number of trees, amount of tree canopy, and access to trees can impact health (Figure 9.5.6). One study found that people who live in neighborhoods with a high density of street trees tend to report significantly fewer cardio-metabolic health conditions (Kardan et al., 2015). The same study also found that having an average of 11 more trees on a city block, on average, has cardio-metabolic health benefits usually associated with an increase of $20,000 in personal income and moving to a neighborhood that has a $20,000 higher median income or being 1.4 years younger (Kardan et al., 2015).
The city of Portland, Oregon explored the health benefits associated with trees. They found that when trees improve air quality by reducing the amount of NO2, the healthcare benefits from fewer respiratory problems are estimated at $7 million (Rao et al., 2014).
Researchers have attempted to gauge whether the loss of trees would have an impact on human mortality. After controlling for many other factors that could impact mortality, communities where trees have been lost to the emerald ash borer (an invasive insect pest) have experienced a corresponding increase in mortality related to lower-respiratory-tract and cardiovascular illnesses. For the 15 states included in the study, tree loss due to the emerald ash borer was linked with 6,113 lower-respiratory-system related deaths and 15,080 deaths related to cardiovascular health problems (Donovan et al., 2013). The implication of this study is that trees are associated with cardiovascular and lower-respiratory-tract health, and the loss of trees is connected with mortality due to cardiovascular and respiratory illnesses.
Access to Gardening
Participation in hands-on gardening activities is linked to physical health. Gardening is exercise. Digging holes for planting, pushing mulch in a wheelbarrow, raking leaves, pulling a hose or carrying a watering can are just a few examples of physical activities common to gardening.
Edible gardening is related to fruit and vegetable consumption. One study found that while non-gardeners only ate fruits and vegetables on average 3.9 times per day, home gardeners consumed produce 4.6 times per day and community gardeners 4.6 times per day (Litt et al., 2011).
Horticulture Therapy
Horticulture therapy can be used to improve coordination and strengthen muscles. One study followed a group of elderly women who participated in a 15-week gardening program and found that participants had an improvement in dexterity and muscle mass and a decrease in waist circumference when compared to an indoor control group (Park et al., 2016).
Horticulture therapy has also been used to help people manage chronic musculoskeletal pain. Participants were less dependent on pain medication, exhibited better coping skills, and had better mental and physical health (Verra et al., 2012).
In Hospitals
Hospital patients who have a view of green nature from their window tend to recover from surgery more quickly and require less pain medication (Mehaffy & Salingaros, 2015; Park et al., 2013). They also are more likely to have more positive interactions with hospital staff (Ulrich, 1983). Hospital patients who have living plants in their rooms or posters of plants may also experience less stress (Beukeboom et al., 2012)
In Schools
The CDC encourages schools to provide farm-to-school activities that provide hands-on education through school garden programs and field trips to local farms, classroom nutrition education, and alternative fundraising using local produce (Harmon, 2011). School garden programs (Figure 9.5.7) have the potential to strengthen the healthy development of students through improved knowledge about fruits and vegetables, increased preference for fruits and vegetables (Morris & Zidenberg-Cherr, 2002; Parmer et al., 2009; Robinson-Obrien et al., 2009), and increased consumption of fruits and vegetables (McAlleese & Rankin, 2007; Parmer et al., 2009; Robinson-Obrien et al., 2009; Ozer, 2007). Children who play in natural environments tend to develop better balance and coordination, which are predictors of physical activity (Fjørtoft, 2001; Fjørtoft, 2004).
Medicinal Benefits of Plants
In addition to providing food, textile fiber, building material, physical exercise, psychological benefits, plants have been used as a source of medicine for most of human history. Even today, researchers and pharmaceutical companies are searching for plants for medicinal properties, and many people grow herbs in their gardens for basic remedies.
How Long Have People Been Using Medicinal Plants?
Excerpt from "Medicinal Botany" by the USDA Forest Service is in the Public Domain
Our earliest human ancestors found plants to heal wounds, cure diseases, and ease troubled minds. People on all continents have long used hundreds, if not thousands, of indigenous plants, for treatment of various ailments dating back to prehistory. Knowledge about the healing properties or poisonous effects of plants, mineral salts, and herbs accumulated from these earliest times to provide health and predates all other medical treatment.
Evidence exists that plants were used for medicinal purposes some 60,000 years ago. A burial site of a Neanderthal man was uncovered in 1960. Eight species of plants had been buried with him, some of which are still used for medicinal purposes today.
By 3500 BC, Ancient Egyptians began to associate less magic with the treatment of disease, and by 2700 BC the Chinese had started to use herbs in a more scientific sense. Egyptians recorded their knowledge of illnesses and cures on temple walls and in the Ebers papyrus (1550 BC), which contains over 700 medicinal formulas.
Hippocrates, 460 – 380 BC, known as the “Father of Medicine,” classified herbs into their essential qualities of hot and cold, moist and dry, and developed a system of diagnosis and prognosis using herbs. The number of effective medicinal plants he discussed was between 300 and 400 species.
Aristotle, the philosopher, also compiled a list of medicinal plants. His best student, Theophrastus discussed herbs as medicines, the kinds and parts of plants used, collection methods, and effects on humans and animals. He started the science of botany with detailed descriptions of medicinal plants growing in the botanical gardens in Athens.
The most significant contribution to the medicinal plant descriptions was made by Dioscorides. While serving as a Roman army physician, he wrote De Materia Medica in about AD 60. This five-volume work is a compilation concerning approximately 500 plants and describes the preparation of about 1000 simple drugs. Written in Greek, it contains good descriptions of plants giving their origins and medical virtues and remained the standard text for 1,500 years.
The earliest Ayurvedic texts on medicine from India date from about 2,500 BC. In Ayurvedic theory, illness is seen in terms of imbalance, with herbs and dietary controls used to restore equilibrium. Abdullah Ben Ahmad Al Bitar (1021 – 1080 AD) an Arabic botanist and pharmaceutical scientist, wrote the Explanation of Dioscorides Book on Herbs. Later, his book, The Glossary of Drugs and Food Vocabulary, contained the names of 1,400 drugs. The drugs were listed by name in alphabetical order in Arabic, Greek, Persian, or Spanish.
Galen, a physician considered the “medical pope” of the Middle Ages, wrote extensively about the body’s four “humors”—the four fluids that were thought to permeate the body and influence its health. Drugs developed by Galen were made from herbs that he collected from all over the world.
The studies of botany and medicine became very closely linked during the Middle Ages. Virtually all reading and writing were carried out in monasteries. Monks laboriously copied and compiled the manuscripts. Following the format of Greek botanical compilations, the monks prepared herbals that described identification and preparation of plants with reported medicinal characteristics. At this time though, healing was as much a matter of prayer as medicine. Early herbalists frequently combined religious incantations with herbal remedies believing that with “God’s help” the patient would be cured.
With time, practitioners began to focus on healing skills and medicines. By the 1530s, Paracelsus (born Philippus Theophrasts Bombastus von Hohenheim, near Zurich in 1493), was changing Europe’s attitudes toward health care. Many physicians and apothecaries were dishonest and took advantage from those they should be helping. Paracelsus was a physician and alchemist who believed that medicine should be simple and straight forward. He was greatly inspired by the Doctrine of Signatures, which maintained that the outward appearance of a plant gave an indication of the problems it would cure. The Doctrine of Signatures is evident in many common names of plants today. For example, lungwort (Pulmonaria spp.) was once used to treat respiratory illnesses because its leaves somewhat resemble human lungs.
In 1775, Dr. William Withering was treating a patient with severe dropsy caused by heart failure. He was unable to bring about any improvement with traditional medicines. The patient’s family administered an herbal brew based on an old family recipe and the patient started to recover. Dr. Withering experimented with the herbs contained in the recipe and identified foxglove (Digitalis purpurea) as the most significant. In 1785, he published his Account of the Foxglove and Some of Its Medical Uses. He detailed 200 cases where foxglove had successfully been used to treat dropsy and heart failure along with his research on the parts of the plant and harvest dates that produced the strongest effect. Withering also realized that therapeutic dose of foxglove is very close to the toxic level where side effects develop. After further analysis, the cardiac glycosides digoxin and digitoxin were eventually extracted. These are still used in treating heart conditions today.
In 1803, morphine became one of the first drugs to be isolated from a plant. It was identified by Frederich Serturner in Germany. He was able to extract white crystal from crude opium poppy. Scientists soon used similar techniques to produce aconitine from monkshood, emetine from ipecacuanha, atropine from deadly nightshade, and quinine from Peruvian bark.
In 1852, scientists were able to synthesize salicin, an active ingredient in willow bark, for the first time. By 1899, the drug company Bayer, modified salicin into a milder form of aectylsalicylic acid and lauched asprin into our modern world.
The synthetic age was born and in the following 100 years, plant extracts have filled pharmacy shelves. Although many medicines have been produced from plant extracts, chemists sometimes find that the synthetic versions do not carry the same therapeutic effects or may have negative side effects not found when using the whole plant source.
A full 40 percent of the drugs behind the pharmacist’s counter in the Western world are derived from plants that people have used for centuries, including the top 20 best-selling prescription drugs in the United States today. For example, quinine extracted from the bark of the South American cinchona tree (Cinchona calisaya) relieves malaria, and licorice root (Glycyrrhiza glabra) has been an ingredient in cough drops for more than 3,500 years. The species native to the United States, Glycyrrhiza lepidota, has a broad range from western Ontario to Washington, south to Texas, Mexico and Missouri. Eastward, there are scattered populations. The leaves and roots have been used for treating sores on the backs of horses, toothaches, and fever in children, sore throats and cough.
Medicinal interest in mints dates from at least the first century A.D., when it was recorded by the Roman naturalist Pliny. In Elizabethan times more than 40 ailments were reported to be remedied by mints. The foremost use of mints today in both home remedies and in pharmaceutical preparations is to relieve the stomach and intestinal gas that is often caused by certain foods.
Modern Ethnobotany
Most United States residents have easy access to pharmacies that are fully stocked with neatly labelled bottles of uniform pills and syrups; therefore, it can be difficult to appreciate the role plants and other natural materials continue to play in modern medicine.
Ethnobotanists like Dr. Cassandra Quave of Emory University are modern-day plant hunters who work hard to identify, test, and introduce new plant-based medicines. In her book The Plant Hunter, Dr. Quave writes “Of the estimated 374,000 species of plants on earth, records exist for the medicinal use of at least 33,443[...] That means that around 9% of all plants on earth have been – and in many cases, continue to be – used as a major form of medicine for people.” Yet of the estimated 9% of plants with medicinal value, fewer than 5% have been studied in a lab.
That’s where ethnobotanists come in. They begin by interviewing people who have traditional knowledge of medicinal plants that grow in their region or reading historic accounts of plants that have healing properties. These scientists work with indigenous community members to identify and collect the correct species of plants. According to Dr. Quave,
As of January 2021, the global population is 7.8 billion people, and roughly 80 percent of them, or 6.2 billion, live in economically underdeveloped countries. Medicinal plants constitute the primary pharmacopoeia, or primary form of medicine, for 70-95 percent of people living in most developing countries. In other words, at least 4 billion people are dependent on plants for medicine, and the key ingredients in their medicine chests are getting more and more difficult to find.
Once the plants have been identified and collected, a portion of the samples are preserved in an herbarium. An herbarium is a library of preserved plant specimens that have been dried, pressed, and labelled with information about where they were collected. Herbarium specimens are an important reference for research, education, and identification.
The remainder of the samples are transported to a lab, where they are processed as ground and dried material or as liquid extracts. These samples are tested to determine their chemical and molecular composition. Samples are tested for their efficacy in combating various bacterial, fungal, and viral diseases. Doctors and pharmaceutical companies learn the results of these studies when they are published and presented at conferences. A new medicine can be introduced after further testing and product development.
Even once a new medicine has been identified and introduced, the search is far from over. Over time and with increased exposure, the organisms responsible for diseases can adapt and become resistant to tried-and-true treatments. Identifying medicinal plants is becoming more difficult, both due to loss of information and habitat destruction. Traditionally, older members of the community would pass these traditions to the younger members; however, this knowledge is at risk of being lost forever as young members of these communities move away in search of better opportunities and as older members grow in age. Even if these traditions are recorded, the native ranges of medicinal plants around the world are threatened by human development. The race is on for ethnobotanists to preserve both the records of plants used and the genetic information of the plants themselves. Investments in research and nature conservation are the keys to ensure our health now and in the future.
Dig Deeper
For more information about the role plants play on human wellbeing, check out Dr. Charles Hall and Dr. Melinda Knuth's comprehensive series of literature reviews on the benefits of plants and horticulture that were published in the Journal of Environmental Horticulture:
- 1: A Review of the Emotional and Mental Health Benefits of Plants
- 2: Physiological Health Benefits
- 3: Social Benefits
- 4: Available Resources and Usage of Plant Benefits Information
To learn more about the importance of nature to children’s development, check out the Children and Nature Network website.
To learn more about modern plant hunters and the search for new plant-based medicines, check out Dr. Cassandra Quave’s TeachEthnobotany YouTube Channel
To learn more about the field of ethnobiology, visit the International Center of Ethnobiology website.
Attribution and References
Attribution
Excerpt from "Medicinal Botany" by the USDA Forest Service is in the Public Domain
Title image: "Appalachian Trail, Smoky Mountain National Park, TN" by Abhishek Chinchalkar is marked with CC BY-NC-ND 2.0.
References
Abraham, A., K. Sommerhalder & T. Abel. (2010). Landscape and well-being: a scoping study on the health-promoting impact of outdoor environments. International Journal of Public Health, 55(1): 59–69.
Astell-Burt, T., Feng, X., & Kolt, G. S. (2013). Does access to neighbourhood green space promote a healthy duration of sleep? Novel findings from a cross-sectional study of 259 319 Australians. BMJ Open, 3(8), e003094–. https://doi.org/10.1136/bmjopen-2013-003094
Berman, M.G., Kross, E., Krpan, K. M., Askren, M. K., Burson, A., Deldin, P. J., Kaplan, S., Sherdell, L., Gotlib, I. H., & Jonides, J. (2012). Interacting with nature improves cognition and affect for individuals with depression. Journal of Affective Disorders, 140(3), 300–305. https://doi.org/10.1016/j.jad.2012.03.012
Becker, C., Lauterbach, G., Spengler, S., Dettweiler, U., & Mess, F. (2017). Effects of Regular Classes in Outdoor Education Settings: A Systematic Review on Students’ Learning, Social and Health Dimensions. International Journal of Environmental Research and Public Health, 14(5), 485–. https://doi.org/10.3390/ijerph14050485
Benfield, J.A., Rainbolt, G. N., Bell, P. A., & Donovan, G. H. (2015). Classrooms With Nature Views: Evidence of Differing Student Perceptions and Behaviors. Environment and Behavior, 47(2), 140–157. https://doi.org/10.1177/0013916513499583
Berto, R. (2014). The role of nature in coping with psycho-physiological stress: a literature review on restorativeness. Behavioral Sciences, 4(4), 394–409. https://doi.org/10.3390/bs4040394
Beukeboom, C.J., Langeveld, D., & Tanja-Dijkstra, K. (2012). Stress- reducing effects of real and artificial nature in a hospital waiting room. The Journal of Alternative and Complementary Medicine, 18(4): 329–333.
Bhasin, M.K., Dusek, J. A., Chang, B.-H., Joseph, M. G., Denninger, J. W., Fricchione, G. L., Benson, H., & Libermann, T. A. (2013). Relaxation response induces temporal transcriptome changes in energy metabolism, insulin secretion and inflammatory pathways. PloS One, 8(5), e62817–e62817. https://doi.org/10.1371/journal.pone.0062817
Browning, M. & Rigolon, A. (2019). School Green Space and Its Impact on Academic Performance: A Systematic Literature Review. International Journal of Environmental Research and Public Health, 16(3), 429–. https://doi.org/10.3390/ijerph16030429
Cappuccio, F.P., Cooper, D., D’Elia, L., Strazzullo, P., & Miller, M. A. (2011). Sleep duration predicts cardiovascular outcomes: a systematic review and meta-analysis of prospective studies. European Heart Journal, 32(12), 1484–1492. https://doi.org/10.1093/eurheartj/ehr007
Cappuccio, F.P., Taggart, F. M., Kandala, N.-B., Currie, A., Peile, E., Stranges, S., & Miller, M. A. (2008). Meta-analysis of short sleep duration and obesity in children and adults. Sleep (New York, N.Y.), 31(5), 619–626. https://doi.org/10.1093/sleep/31.5.619
Carrus, G., M. Scopelliti, R. Lafortezza, G. Colangelo, F. Ferrini, F. Salbitano, M. Agrimi, L. Portoghesi, P. Semenzato & G. Sanesi. (2015). Go greener, feel better? The positive effects of biodiversity on the well-being of individuals visiting urban and peri-urban green areas. Landscape Urban Planing, 134(0): 221–228.
Chaput, J.P., Després, J., Bouchard, C., & Tremblay, A. (2007). Short Sleep Duration is Associated with Reduced Leptin Levels and Increased Adiposity: Results from the Québec Family Study. Obesity (Silver Spring, Md.), 15(1), 253–261. https://doi.org/10.1038/oby.2007.512
Donovan, G.H., Butry, D.T., Michael, Y.L., Prestemon, J.P., Liebhold, A.M., Gatziolis, D. & Mao, M.Y. (2013). The Relationship Between Trees and Human Health: Evidence from the Spread of the Emerald Ash Borer. American Journal of Preventive Medicine, 44(2), 139–145. https://doi.org/10.1016/j.amepre.2012.09.066
Fjørtoft, I. (2001). The natural environment as a playground for children: The impact of outdoor play activities in pre-primary school children. Early Childhood Education Journal. 29(2):111–117.
Fjørtoft, I. (2004). Landscape as playscape: The effects of natural environments on children’s play and motor development. Children, Youth and Environments, 14(2): 1–44.
Gelkopf, M., Hasson-Ohayon, I., Bikman, M., & Kravetz, S. (2013). Nature adventure rehabilitation for combat-related posttraumatic chronic stress disorder: A randomized control trial. Psychiatry Research, 209(3), 485–493. https://doi.org/10.1016/j.psychres.2013.01.026
Gigliotti, C.M. & Jarrott, S. E. (2005). Effects of horticulture therapy on engagement and affect. Canadian Journal on Aging, 24(4), 367–377. https://doi.org/10.1353/cja.2006.0008
Gilchrist, K., Brown, C. & Montarzino, A. (2015). Workplace settings and wellbeing: Greenspace use and views contribute to employee wellbeing at peri-urban business sites. Landscape Urban Planning, 138: 32-40.
Hall, & Knuth, M. (2019). An Update of the Literature Supporting the Well-Being Benefits of Plants: A Review of the Emotional and Mental Health Benefits of Plants. Journal of Environmental Horticulture, 37(1), 30–38. https://doi.org/10.24266/0738-2898-37.1.30
Hall, & Knuth, M. J. (2019). An Update of the Literature Supporting the Well-Being Benefits of Plants: Part 2 Physiological Health Benefits. Journal of Environmental Horticulture, 37(2), 63–73. https://doi.org/10.24266/0738-2898-37.2.63
Hall, & Knuth, M. J. (2019). An Update of the Literature Supporting the Well-Being Benefits of Plants: Part 3 - Social Benefits. Journal of Environmental Horticulture, 37(4), 136–142. https://doi.org/10.24266/0738-2898-37.4.136
Han, K.T. (2009). Influence of limitedly visible leafy indoor plants on the psychology,
behavior, and health of students at a junior-high school. Environment and
Behavior, 41: 658.
Harmon A. (2003). Farm to school: an introduction for food service professionals, food educators, parents and community leaders. Los Angeles, CA: National Farm to School Program, Center for Food and Justice, Urban and Environmental Policy Institute. Retrieved from http://www.foodroutes.org/eflyers/FarmtoSchoolGuide.pdf
Hartig, T., Mitchell, R., De Vries S. & Frumkin, H. (2014). Nature and health. Annual Review of Public Health, 35: 207-228.
Hislop, J. & Arber, S. (2003). Understanding women’s sleep management: beyond medicalization‐healthicization? Sociology of Health & Illness, 25(7), 815–837. https://doi.org/10.1046/j.1467-9566.2003.00371.x
Horticultural therapy: history and practice. (n.d.). American Horticultural Therapy Association. Retrieved 8 March 2022 from https://www.ahta.org/horticultural-therapy
Hublin, C., Partinen, M., Koskenvuo, M. & Kaprio, J. (2007). Sleep and mortality: A population-based 22-year follow-up study. Sleep (New York, N.Y.), 30(10), 1245–1253. https://doi.org/10.1093/sleep/30.10.1245
Kaplan, R. (1984). Impact of urban nature: A theoretical analysis. Urban Ecology, 8:
189-197.
Kaplan, R (1993). The role of nature in the context of the workplace. Landscape and
Planning, 26: 193-201.
Kaplan, R. (2001). The nature of the view from home, psychological benefits.
Environment and Behavior, 33: 507.
Kardan, O., Gozdyra, P., Misic, B., Moola, F., Palmer, L. J., Paus, T., & Berman, M. G. (2015). Neighborhood greenspace and health in a large urban center. Scientific Reports, 5(1), 11610–11610. https://doi.org/10.1038/srep11610
Kellert, S.R. & Wilson, E. O. (1993). The Biophilia hypothesis. Island Press.
Kelz, C., Evans, G. W., & Röderer, K. (2015). The Restorative Effects of Redesigning the Schoolyard: A Multi-Methodological, Quasi-Experimental Study in Rural Austrian Middle Schools. Environment and Behavior, 47(2), 119–139. https://doi.org/10.1177/0013916513510528
Korpela, K., De Bloom, J., Sianoja, M., Pasanen, T., & Kinnunen, U. (2017). Nature at home and at work: Naturally good? Links between window views, indoor plants, outdoor activities and employee well-being over one year. Landscape and Urban Planning, 160, 38–47. https://doi.org/10.1016/j.landurbplan.2016.12.005
Kotozaki, Y., Takeuchi, H., Sekiguchi, A. Araki, T. Takahashi, K. Yamamoto, Y. Nozawa, K. Taki, Y., & Kawashima, R. (2015). Positive effects of the victim by the growing of plants after great East Japan earthquake. International Journal of Recent Scientific Research, 6(2): 2850–2858.
Kuo, M. (2015). How might contact with nature promote human health? Promising mechanisms and a possible central pathway. Frontiers in Psychology, 6.
Kuo, F.E., & Sullivan, W.C. (2001). Aggression and violence in the inner city: Effects
of environment via mental fatigue. Environment and Behavior, 33(4): 543-571.
Lee, A.C. & Maheswaran, R. (2011). The health benefits of urban green spaces: a review of the evidence. Journal of Public Health. 33(2): 212–222.
Lewis, C.A. (1996). Green nature/human nature: the meaning of plants in our lives. University of Illinois Press.
Li, D. & Sullivan, W. C. (2016). Impact of views to school landscapes on recovery from stress and mental fatigue. Landscape and Urban Planning, 148, 149–158. https://doi.org/10.1016/j.landurbplan.2015.12.015
Litt, J.S., Soobader, M.J., Turbin, M.S., Hale, J.W., Buchenau, M. & Marshall, J.A. (2011). The Influence of Social Involvement, Neighborhood Aesthetics, and Community Garden Participation on Fruit and Vegetable Consumption. American Journal of Public Health (1971), 101(8), 1466–1473. https://doi.org/10.2105/AJPH.2010.300111
Lottrup, L., Stigsdotter, U. K., Meilby, H., & Claudi, A. G. (2015). The Workplace Window View: A Determinant of Office Workers’ Work Ability and Job Satisfaction. Landscape Research, 40(1), 57–75. https://doi.org/10.1080/01426397.2013.829806
Morita, E., Imai, M., Okawa, M., Miyaura, T., & Miyazaki, S. (2011). A before and after comparison of the effects of forest walking on the sleep of a community-based sample of people with sleep complaints. BioPsychoSocial Medicine, 5(1), 13–13. https://doi.org/10.1186/1751-0759-5-13
Morris, J.L. & Zidenberg-Cherr, S. (2002). Garden-enhanced nutrition curriculum improves fourth-grade school children’s knowledge of nutrition and preferences for some vegetables. Journal of the American Dietetic Association, 102(1), 91–93. https://doi.org/10.1016/S0002-8223(02)90027-1
Nieuwenhuis, M., Knight, C., Postmes, T. & Haslam, S.A. (2014). The relative benefits of green versus lean office space: three field experiments. Journal of Environmental Psychology – Applied, 20(3): 199–214.
Ohtsuka, Y., Yabunaka, N., & Takayama, S. (1998). Shinrin-yoku (forest-air bathing and walking) effectively decreases blood glucose levels in diabetic patients. International Journal of Biometeorology, 41(3), 125–127. https://doi.org/10.1007/s004840050064
Ozer, E.J. (2007). The Effects of School Gardens on Students and Schools: Conceptualization and Considerations for Maximizing Healthy Development. Health Education & Behavior, 34(6), 846–863. https://doi.org/10.1177/1090198106289002
Park, S.A., Lee, A.-Y., Son, K.-C., Lee, W.-L., & Kim, D.-S. (2016). Gardening Intervention for Physical and Psychological Health Benefits in Elderly Women at Community Centers. HortTechnology (Alexandria, Va.), 26(4), 474–483. https://doi.org/10.21273/HORTTECH.26.4.474
Park, S.A., Oh, S.-R., Lee, K.-S., & Son, K.-C. (2013). Electromyographic Analysis of Upper Limb and Hand Muscles during Horticultural Activity Motions. HortTechnology (Alexandria, Va.), 23(1), 51–56. https://doi.org/10.21273/HORTTECH.23.1.51
Parmer, S.M., Salisbury-Glennon, J., Shannon, D., & Struempler, B. (2009). School Gardens: An Experiential Learning Approach for a Nutrition Education Program to Increase Fruit and Vegetable Knowledge, Preference, and Consumption among Second-grade Students. Journal of Nutrition Education and Behavior, 41(3), 212–217. https://doi.org/10.1016/j.jneb.2008.06.002
Parsons, R. (1991). The potential influences of environmental perception on human
health. Journal of Environmental Psychology, 11: 1-23.
Plante, A.D., (2014). Do plants play a part in student satisfaction? The University of Tennessee, Knoxville. Retrieved from https://trace.tennessee.edu/utk_gradthes/2745
Quave, C.L. (2021). The plant hunter: a scientist’s quest for nature’s next medicines. Viking.
Raanaas, R.K., Evensen, K. H., Rich, D., Sjøstrøm, G., & Patil, G. (2011). Benefits of indoor plants on attention capacity in an office setting. Journal of Environmental Psychology, 31(1), 99–105. https://doi.org/10.1016/j.jenvp.2010.11.005
Rao, M., George, L. A., Rosenstiel, T. N., Shandas, V., & Dinno, A. (2014). Assessing the relationship among urban trees, nitrogen dioxide, and respiratory health. Environmental Pollution (1987), 194, 96–104. https://doi.org/10.1016/j.envpol.2014.07.011
Robinson-O’Brien, R., Story, M., & Heim, S. (2009). Impact of Garden-Based Youth Nutrition Intervention Programs: A Review. Journal of the American Dietetic Association, 109(2), 273–280. https://doi.org/10.1016/j.jada.2008.10.051
School Health Guidelines to Promote Healthy Eating and Physical Activity. (2011). MMWR. Recommendations and Reports, 60(5), 1–76.
Sekiguchi, A. Kotozaki, Y., Sugiura, M., Nouchi, R., Takeuchi, H., Hanawa, S., Nakagawa, S., Miyauchi, C. M., Araki, T., Sakuma, A., Taki, Y., & Kawashima, R. (2015). Resilience after 3/11: structural brain changes 1 year after the Japanese earthquake. Molecular Psychiatry, 20(5), 553–554. https://doi.org/10.1038/mp.2014.28
Taylor, A.F. & Kuo, F. E. (2009). Children With Attention Deficits Concentrate Better After Walk in the Park. Journal of Attention Disorders, 12(5), 402–409. https://doi.org/10.1177/1087054708323000
Taylor, A.F., Kuo, F.E. & Sullivan, W.C. (2002). Views of nature and self-discipline: Evidence from inner city children. Journal of Environmental Psychology, 22(1-2), 49–63. https://doi.org/10.1006/jevp.2001.0241
Thompson, C.W., Roe, J., Aspinall, P., Mitchell, R., Clow, A., & Miller, D. (2012). More green space is linked to less stress in deprived communities: Evidence from salivary cortisol patterns. Landscape and Urban Planning, 105(3), 221–229. https://doi.org/10.1016/j.landurbplan.2011.12.015
Troy, A., Morgan Grove, J., & O’Neil-Dunne, J. (2012). The relationship between tree canopy and crime rates across an urban–rural gradient in the greater Baltimore region. Landscape Urban Planning, 106(3):262– 270.
Ulrich, R.S. (1983). Aesthetic and affective response to natural environment. Behavior and the Natural Environment (pp. 85–125). Springer US. https://doi.org/10.1007/978-1-4613-3539-9_4
Ulrich, R.S. (1984). View through a Window May Influence Recovery from Surgery. Science (American Association for the Advancement of Science), 224(4647), 420–421. https://doi.org/10.1126/science.6143402
Ulrich, R.S., Simons, R. F., Losito, B. D., Fiorito, E., Miles, M. A., & Zelson, M. (1991). Stress recovery during exposure to natural and urban environments. Journal of Environmental Psychology, 11(3), 201–230. https://doi.org/10.1016/S0272-4944(05)80184-7
Ulrich, R.S. & Parsons, R. (1992). Influences of passive experiences with plants on individual well-being and health. The Role of Horticulture in Human Well-Being and Social Development (pp. 93-105). Portland, OR: Timber Press.
United States Department of Labor. (2006). American Time Use Survey (ATUS). Retrieved from http://www.bls.gov/tus/
Van den Berg, A.E., Koole, S.L. & van der Wulp, N.Y. (2003). Environmental preference and restoration: (How) are they related? Journal of Environmental Psychology, 23: 135–146.
Van den Berg, A.E., Maas, J., Verheij, R.A., & Groenewegen, P.P. (2010). Green space as a buffer between stressful life events and health. Social Science & Medicine (1982), 70(8), 1203–1210.
van Duijin, B., Klein Hesselink, J. Kester, M. & Jansen en Hilde Spitters, J. (2011). ‘Planten in de klas’ [plants in the classroom]. Productschap Tuinbouw (Product Board for Horticulture), Rapport Project.
Watts, G. (2017). The effects of ‘‘greening’’ urban areas on the perceptions of tranquillity. Urban Forestry Urban Greening, 26: 11-17.
Wolf, K. & Housley, E. (2014). Reflect & restore: urban green space for mental wellness. The TKF Foundation, Annapolis, MD. 14 p.
Younan, D., Tuvblad, C., Li, L., Wu, J., Lurmann, F., Franklin, M., Berhane, K., McConnell, R., Wu, A. H., Baker, L., & Chen, J.-C. (2016). Environmental Determinants of Aggression in Adolescents: Role of Urban Neighborhood Greenspace. Journal of the American Academy of Child and Adolescent Psychiatry, 55(7), 591–601. https://doi.org/10.1016/j.jaac.2016.05.002
Zelenski, J.M. & Nisbet, E.K. (2014). Happiness and feeling connected: the distinct role of nature relatedness. Environment Behavior 46 (1):3–23.
|
oercommons
|
2025-03-18T00:39:13.321626
|
Anna McCollum
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/87628/overview",
"title": "Statewide Dual Credit Introduction to Plant Science, Impact of Plants and Horticulture on People, The Role of Plants in Human Well-Being",
"author": "Textbook"
}
|
https://oercommons.org/courseware/lesson/85011/overview
|
1.3 Artificial Selection and Early Hybridization
1_Plant-Domestication
Plant Domestication
Overview
Image credit: “CIMMYT- CESBatan - MEX - 06082019 - 0037.jpg” by Alfonso Cortés/CIMMYT is licensed under CC BY-NC-SA 2.0
Did you have an idea for improving this content? We’d love your input.
Introduction
Lesson Objectives
Explain the domestication of plants for agriculture.
Explain the transition to the agrarian lifestyle.
Key Terms
artificial selection - occurs when humans select an organism for desired characteristics (phenotype), which can lead to changes on a genetic level (genotype)
cultivation - the growing or tending of crops
domestication - the process of modifying wild plants and animals by selective breeding into forms more suited to cultivation by humans
hybridization - the creation of offspring from two unlike parents, often the product of two different species or two different varieties
The Transition to the Agrarian Lifestyle
Archeologists believe that for much of our history, humans lived a nomadic lifestyle: hunting animals and gathering plants for food. These early humans would follow wild game, collecting fruit, nuts, grains, and other plant-based foods along the way. Their movement was largely driven by seasonal changes in food availability.
Some of the earliest plants that were harvested for long-term storage were grains, such as wheat and barley. These cereals grew wild throughout much of an area in the Middle East known as the Fertile Crescent. Using simple tools, such as flint-bladed sickles, early humans could harvest as much as 2 lbs. of wheat an hour. With just a few weeks of work, a small family could store enough grain to sustain itself through an entire year (Standage, 2005). Dried grains were able to be stored for long periods of time, which provided a consistent source of food through lean seasons. However, stored food needed to be monitored and guarded to prevent loss due to pests and environmental damage.
About 10,000 years ago, some groups of humans began to deliberately cultivate their food by planting seeds with the intent to harvest. This development led to the formation of agricultural villages centered around cultivating and harvesting crops (Figure 9.1.1). Agricultural concepts and practices seem to have independently arisen in several areas across the globe over a span of only about six thousand years, rather than originating in just one area and spreading from there (Diamond, 2002; Diamond, 2005). Agriculture spread from those centers as populations and farmable land use increased. Knowledge of agricultural techniques and use of tools and seeds also spread by trade.
Artificial Selection and Early Hybridization
The process of artificial selection happens when humans select a plant or animal, based on qualities of appearance (phenotype), for extensive cultivation or further breeding, which can lead to changes on a genetic level (genotype). To help us better understand the process of artificial selection, let us consider what researchers believe were likely the steps early humans made in domesticating three common crops: wheat, maize, and the cole crops.
A Brief History of Wheat
Primitive einkhorn wheat (Trititicum monococcum) is similar to modern wheat in that it is edible and relatively easy to harvest when compared to other crops. However, there are some differences between the wild ancestors of our modern wheat and the wheat that we grow today.
For example, wild wheat tends to produce much smaller grains held on smaller heads. These heads of grain are delicate and prone to scatter their seeds in response to the slightest touch. This characteristic, known as “shattering seed heads,” makes it easier for wild grains to spread their seeds in their natural environment. However, this adaptation makes it more difficult for humans to collect grains without dropping a good portion of the harvest.
As early humans selected which heads of grain to harvest, they showed preference for wheat that naturally produced larger grains that did not shatter when collected. These improved forms were probably the result of natural spontaneous hybridization in the field. Scientists believe that einkhorn wheat crossed with a related wild grass (T. searsii), resulting in the improved emmer wheat species (T. turgidum). These natural crosses increased the number of chromosomes, or the ploidy level, in the plant. For example, diploid einkhorn wheat has 7 pairs of chromosomes, while the tetraploid emmer wheat has 17 pairs of chromosomes.
The modified forms may have initially sprouted closer to villages from seed that was accidentally spilled, but humans eventually figured out that these harvested grains could also be planted for an improved and more uniform crop. While these new forms of wheat may not have been the product of deliberate hybridization by plant breeders, the choice to primarily grow improved forms (such as emmer wheat rather than einkhorn wheat) was a form of artificial selection (Figure 9.1.2). Improved strains of wheat quickly spread from the Near East to northern Africa, southern Europe, and parts of Asia. As humans cultivated wheat in new environments, they continued to select improved forms that performed better in their region. This tradition continues to this day.
Norman Borlaug (1914 – 2009) is a notable modern wheat breeder who rigorously crossed, trialed, and selected new strains of wheat for characteristics, such as improved yield, stout forms that are less prone to lodging or falling over, as well as the ability to grow in a variety of environments (Figure 9.1.3). His developments contributed to the Green Revolution, which made use of these improved varieties, better crop management practices, and synthetic fertilizers and pesticides to increase crop yields across the globe (Raven et al., 2005).
A Brief History of Maize
While the first evidence of plant domestication is found in the Fertile Crescent, artificial selection also happened independently in the New World. Before the arrival of European settlers, maize was a staple crop for many people living in North, Central, and South America (Figure 9.1.4), but its origins are still somewhat of a mystery. Researchers believe that maize was developed in Central America sometime around 6000 BCE. Unlike progenitors of wheat and most other cereal crops, there doesn’t appear to be a wild form of maize (Zea mays sbsp. mays). There have been several hypotheses as to the origin of maize. We will explore just one of those hypotheses in this text.
Some researchers believe modern corn is the domesticated form of the wild grass teosinte (Zea mays sbsp. parviglumis). Teosinte is a very large grass that looks like a taller, multi-stemmed version of modern corn (Figure 9.1.5). However, there are many important differences between teosinte and maize, including the fact that teosinte’s kernels are basically inedible (Mann, 2005).
Teosinte produces much smaller, extremely woody grains that are held on narrow, two-rowed ears. Each ear is contained in its own husked chamber, with 5 to 12 chambers contained in one larger husk. These chambers split open and the grains shatter, effectively scattering the seed that will sprout the next season. In contrast, maize produces large, succulent grains that are held tightly on a large cob contained by a husk (Raven et al., 2005). These characteristics make maize ideal for harvesting and human consumption, but they would prevent maize from being able to scatter seed independent of human intervention. That is, maize and corn could not survive in the wild without people to harvest and plant its seeds (Mann, 2005).
With an ancestor so markedly different from the cultivated form, it is difficult to understand how exactly maize was selected from teosinte. Scientists are still trying to figure out exactly how this happened, but it is clear that the selection process for more edible grains, larger ears, and non-shattering seed heads was the result of deliberate actions over many generations (Mann, 2005). This process occurred over generations of deliberate breeding, an impressive feat even to accomplished scientists today. In fact, Dr. Nina Federoff, a geneticist at Pennsylvania State University, began her article “Prehistoric GM Corn” with the bold statement: “Corn (maize) is arguably man’s first, and perhaps his greatest, feat of genetic engineering.”
Most of our modern corn is the product of hybridization between inbred selections. In fact, nearly all corn produced in the United States is hybrid corn. These hybrid selections produce a more uniform crop where individuals have identical growth and development characteristics, which makes mechanized harvesting easier. Hybrid corn is usually very vigorous, and strains can be selected based on their ability to grow well in a variety of climates. Most importantly, hybrid corn produces significantly higher yields than traditional corn and maize (Raven et al., 2005).
These characteristics mean that more corn can be produced per acre with less water, fertilizer, pesticides, and labor. While intensive breeding and hybridization has unquestionably led to higher yields, monoculture stands of hybrid corn are especially vulnerable to pests and diseases (Figure 9.1.6). Genetic diversity is the key to overcoming these health issues. Saving and studying landraces (Figure 9.1.7) and preserving wild relatives (like teosinte), in order to use this material for future breeding efforts, will help protect our ability to produce food for ourselves in the future (Raven et al., 2005).
A Brief History of Cole Crops
The Cole Crops (vegetables in the species Brassica oleracea) provide an excellent example of artificial selection. European kale, broccoli, Brussels sprouts, cabbage, cauliflower, collard greens, and kohlrabi, vary widely from one another in appearance Figure 9.1.8). However, they are all members of the same species that have been selected by humans over time for their different physical attributes.
- Both kale and collard greens were selected for their large, tender leaves, and these were probably the first domesticated forms of the B. oleracea. Modern varieties of these plants may have green, purple, or red leaves that may be flat or frilly. Scientists believe kale most closely resembles the wild ancestor of this species.
- Broccoli was bred for its flower buds and tender stems. Plants can form large heads or small spears that can be green, purple, or yellow in color.
- Brussels sprouts was selected for its lateral buds, and it is believed to have been bred from cabbage in Brussels, Belgium in the 16th century.
- Cabbage was developed to produce tightly wrapped leaves around a large terminal bud. Cabbages are also available in a variety of shapes (rounded or savoyed leaves) and colors (white, green, red, and purple).
- Cauliflower is believed to have been developed from broccoli by selecting for easily blanched, tender flower buds and shoots. While white cauliflower is the most familiar form, heads can also be chartreuse, orange, or purple in color.
- Kohlrabi was bred to produce an edible stem that is swollen and fleshy. Plants are available in green or purple.
While each of these crops may look quite different from one another, they are all members of the same species and are purely the product of human selection. While it’s unclear exactly when the first Brassica oleracea was domesticated, there is evidence of early versions of broccoli, cabbage, cauliflower, collards, and kale being grown by the Greeks as early as 300 BCE. The Romans brought cabbage and kale with them on their conquests of and to their settlements in northern Europe between 40 and 450 CE. Kohlrabi and Brussels sprouts were developed in northern Europe around 1500 BCE. New and improved varieties of these crops continue to be introduced in modern times (Colley et al., 2015).
Attribution and References
Attribution
Image credit: “CIMMYT- CESBatan - MEX - 06082019 - 0037.jpg” by Alfonso Cortés/CIMMYT is licensed under CC BY-NC-SA 2.0
References
Colley, M., Zystro, J., Buttala, L. A., & Siegel, S. (2015). The seed garden: The art and practice of seed saving. Seed Savers Exchange.
Diamond, J. (2002). Evolution, consequences and future of plant and animal domestication. Nature (London), 418(6898), 700–707. https://doi.org/10.1038/nature01019
Diamond, J. (2005). Guns, germs, and steel: The fates of human societies. W.W. Norton.
Fedoroff, N.V. (2003). Prehistoric GM Corn. Science, 302(5648), 1158–1159. https://doi.org/10.1126/science.1092042
Mann, C. (2005). 1491: New revelations of the Americas before Columbus. 1st ed. Knopf.
Raven, P.H., R.F. Evert, and S.E. Eichhorn. (2005). Plants and People. Biology of plants. 7th ed (pp. 475-495). W.H. Freeman and Company, Worth Publishers, New York.
Standage, T. (2005). A history of the world in 6 glasses. Walker & Co.
|
oercommons
|
2025-03-18T00:39:13.371170
|
Anna McCollum
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/85011/overview",
"title": "Statewide Dual Credit Introduction to Plant Science, Impact of Plants and Horticulture on People, Plant Domestication",
"author": "Textbook"
}
|
https://oercommons.org/courseware/lesson/85005/overview
|
1.3 Development of Male and Female Gametophyte
1.4 Self Pollination vs. Cross Pollination
1.5 Double Fertilization
1.6 Development of the Seed
1.7 Development of Fruit and Fruit Type
1.8 Fruit and Seed Dispersal
1.9 Seed Dormancy & Germination
1_Sexual-Reproduction-in-Plants
Sexual Reproduction in Plants
Overview
Flowers of different families
Alvesgaspar, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
Students must have knowledge about mitosis and meiosis before studying sexual reproduction in plants. Please refer to chapter 10 & 11 of OpenStax Biology 2e. Links are provided below.
OpenStax Biology 2e (Chapter 10 Cell reproduction)
https://openstax.org/books/biology-2e/pages/10-introduction
OpenStax Biology 2e (Chapter 11 Meiosis & Sexual reproduction)
https://openstax.org/books/biology-2e/pages/11-introduction
Did you have an idea for improving this content? We’d love your input.
Introduction
Learning Objectives
Discuss alternation of generations.
Describe the components of a flower.
Describe the development of male and female gametophytes.
Define pollination.
Contrast self-pollination and cross-pollination.
Describe the process of double fertilization.
Explain the stages of seed development.
Key Terms
alternation of generation - alteration of haploid gametophyte stage with diploid sporophyte stage in the life cycle of an organism
anther - sac-like structure at the tip of a stamen in which pollen grains are produced
carpel - the female part of the flower includes stigma, style, and ovary
cotyledon - the fleshy part of the seed that provides nutrition to the seed
cross-pollination - transfer of pollen from the anther of one flower to the stigma of a different flower
diploid - cell, nucleus, or organisms containing two sets of chromosomes (2n)
double fertilization - two fertilization events in angiosperms; one sperm fuses with the egg, forming the zygote, whereas the other sperm fuses with the polar nuclei, forming the endosperm
egg - female haploid germ cell
embryo - the young plant confined in a seed with endosperm and is viable to germinate
endosperm - triploid structure resulting from the fusion of a sperm with polar nuclei, which serves as a nutritive tissue for the embryo
epicotyl - the part of an embryonic axis that projects above the cotyledons
female gametophyte - multicellular part of the plant that gives rise to the haploid ovule
flower - branches specialized for reproduction found in some seed-bearing plants, containing either specialized male or female organs or both male and female organs
gametophyte - multicellular stage of the plant that gives rise to haploid gametes or spores
generative cell - a cell within the tube cell that divides to produce two sperm nuclei in angiosperms
male gametophyte - multicellular part of a plant that gives rise to haploid pollens
ovary - a chamber that contains and protects the ovule or female megasporangium
ovule - female gametophyte
petal - modified leaf interior to the sepals; colorful petals attract animal pollinators
pollen - structure containing the male gametophyte of the plant
pollen tube - extension from the pollen grain that delivers sperm to the egg cell
pollination - transfer of pollen to the stigma
radicle - the original root that develops from the germinating seed
seed coat - the outer covering of a seed
self-pollination - transfer of pollen from the anther to the stigma of the same flower
sporophyte - multicellular diploid stage in plants that is formed after the fusion of male and female gametes
stamen - the male part of the flower includes filament and anthers
stigma - the uppermost structure of the carpel where pollen is deposited
suspensor - part of the growing embryo that makes the connection with the maternal tissues
synergid - a type of cell found in the ovule sac that secretes chemicals to guide the pollen tube toward the egg
tube cell - the cell in the pollen grain that develops into the pollen tube
zygote - diploid cell produced after fertilization of egg cell by the sperm nuclei delivered by tube cell into the ovule
Introduction
Sexual reproduction takes place with slight variations in different groups of plants. Plants have two distinct stages in their lifecycle: the gametophyte stage and the sporophyte stage. The haploid gametophyte produces the male and female gametes by mitosis in distinct multicellular structures. Fusion of the male and female gametes forms the diploid zygote, which develops into the sporophyte. After reaching maturity, the diploid sporophyte produces spores by meiosis, which in turn divide by mitosis to produce the haploid gametophyte. The new gametophyte produces gametes, and the cycle continues. This is the alternation of generation and is typical of plant reproduction (Figure 3.1.1.).
The life cycle of higher plants is dominated by the sporophyte stage, with the gametophyte borne on the sporophyte. In ferns, the gametophyte is free-living and very distinct in structure from the diploid sporophyte. In bryophytes, such as mosses, the haploid gametophyte is more developed than the sporophyte.
During the vegetative phase of growth, plants increase in size and produce a shoot system and a root system. As they enter the reproductive phase, some of the branches start to bear flowers. Many flowers are borne singly, whereas some are borne in clusters. The flower is borne on a stalk known as a receptacle. Flower shape, color, and size are unique to each species and are often used by taxonomists to classify plants.
Access for free at https://openstax.org/books/biology-2e/pages/32-1-reproductive-development-and-structure
Sexual Reproduction in Angiosperms
The lifecycle of angiosperms follows the alternation of generation explained in the previous section. The haploid gametophyte alternates with the diploid sporophyte during the sexual reproduction process of angiosperms. The male and female reproductive structures of a plant are housed in a flower. Let us revisit the structure of a flower (unit 1: Plant Form, lesson 2: Parts of a plant, section 6.
Flower Structure
A typical flower has four “layers,” illustrated and described below from external to internal structures (Figure 3.1.2.):
- The outermost layer consists of sepals, the green, leafy structures which protect the developing flower bud before it opens.
- The next layer is comprised of petals, the modified leaves which are usually brightly colored, which help attract pollinators.
- The third layer contains the male reproductive structures—the stamen. Stamens are composed of anther and filaments. Anthers contain the microsporangia—the structures that produce the microspores, which go on to develop into male gametophytes. Filaments are structures that support the anthers.
- The innermost layer—the carpel—contains one or more female reproductive structures. Each carpel contains a stigma, style, and ovary. The ovaries contain the megasporangia—the structures that produce the megaspores, which go on to develop into female gametophyte. The stigma is the location where pollen (the male gametophyte) is deposited by wind or by pollinators. The style is a structure that connects the stigma to the ovary.
Access for free at https://openstax.org/books/biology-2e/pages/32-1-reproductive-development-and-structure
Development of Male and Female Gametophyte
Male Gametophyte (The Pollen Grain)
The male gametophyte develops and reaches maturity in an immature anther. In a plant’s male reproductive organs, the development of pollen takes place in a structure known as the microsporangium (Figure 3.1.3.). The microsporangia, which are usually bilobed, are pollen sacs in which the microspores develop into pollen grains. These are found in the anther, which is at the end of the stamen—the long filament that supports the anther.
Within the microsporangium, each of the microspore mother cells divides by meiosis to give rise to four microspores, each of which will ultimately form a pollen grain (Figure 3.1.4.). An inner layer of cells, known as the tapetum, provides nutrition to the developing microspores and contributes key components to the pollen wall. Mature pollen grains contain two cells: a generative cell and a pollen tube cell. The generative cell is contained within the larger pollen tube cell. Upon germination, the tube cell forms the pollen tube through which the generative cell migrates to enter the ovary. During its transit inside the pollen tube, the generative cell divides to form two male gametes (sperm cells). Upon maturity, the microsporangia burst, releasing the pollen grains from the anther.
Each pollen grain has two coverings: the exine (thicker, outer layer) and the intine (Figure 3.1.4.). The exine contains sporopollenin, a complex waterproofing substance supplied by the tapetal cells. Sporopollenin allows the pollen to survive under unfavorable conditions and to be carried by the wind, water, or biological agents without undergoing damage.
Female Gametophyte (The Embryo Sac)
While the details may vary between species, the overall development of the female gametophyte has two distinct phases. First, in the process of mega-sporogenesis, a single cell in the diploid mega-sporangium—an area of tissue in the ovules—undergoes meiosis to produce four megaspores, only one of which survives. During the second phase, mega-gametogenesis, the surviving haploid megaspore undergoes mitosis to produce an eight-nucleate, seven-cell female gametophyte, also known as the megagametophyte or embryo sac. Two of the nuclei—the polar nuclei—move to the equator and fuse, forming a single, diploid central cell. This central cell later fuses with sperm to form the triploid endosperm. Three nuclei position themselves on the end of the embryo sac opposite the micropyle and develop into antipodal cells, which later degenerate. The nucleus closest to the micropyle becomes the female gamete—or egg cell, and the two adjacent nuclei develop into synergid cells (Figure 3.1.5.). The synergids help guide the pollen tube for successful fertilization, after which they disintegrate. Once fertilization is complete, the resulting diploid zygote develops into the embryo and the fertilized ovule forms the other tissues of the seed.
A double-layered integument protects the megasporangium and, later, the embryo sac. The integument will develop into the seed coat after fertilization and protect the entire seed. The ovule wall will become part of the fruit. The integuments, while protecting the megasporangium, do not enclose it completely, but leave an opening called the micropyle. The micropyle allows the pollen tube to enter the female gametophyte for fertilization.
Access for free at https://openstax.org/books/biology-2e/pages/32-1-reproductive-development-and-structure
Self Pollination vs. Cross Pollination
In angiosperms, pollination is defined as the placement or transfer of pollen from the anther to the stigma of the same flower or another flower. In gymnosperms, pollination involves pollen transfer from the male cone to the female cone. Upon transfer, the pollen germinates to form the pollen tube and the sperm for fertilizing the egg. Pollination takes two forms: self-pollination and cross-pollination. Self pollination occurs when the pollen from the anther is deposited on the stigma of the same flower, or another flower on the same plant. Cross pollination is the transfer of pollen from the anther of one flower to the stigma of another flower on a different individual of the same species. Self-pollination occurs in flowers where the stamen and carpel mature at the same time and are positioned so that the pollen can land on the flower’s stigma. This method of pollination does not require an investment from the plant to provide nectar and pollen as food for pollinators.
Self-pollination leads to the production of plants with less genetic diversity, since genetic material from the same plant is used to form gametes, and eventually, the zygote. In contrast, cross-pollination—or out-crossing—leads to greater genetic diversity because the microgametophyte and megagametophyte are derived from different plants.
Because cross-pollination allows for more genetic diversity, plants have developed many ways to promote cross-pollination. In some species, the pollen and the ovary mature at different times. These flowers make self-pollination nearly impossible. By the time pollen matures and has been shed, the stigma of this flower is mature and can only be pollinated by pollen from another flower. Some flowers have developed physical features that prevent self-pollination. Primrose is one such flower. Primroses have evolved two flower types with differences in anther and stigma length: the pin-eyed flower has anthers positioned at the pollen tube’s halfway point, and the thrum-eyed flower’s stigma is likewise located at the halfway point. Insects easily cross-pollinate while seeking the nectar at the bottom of the pollen tube. This phenomenon is also known as heterostyly. Many plants, such as cucumber, have male and female flowers located on different parts of the plant (monoecious, Unit 1 lesson 2), thus making self-pollination difficult. In yet other species, the male and female flowers are borne on different plants (dioecious, Unit 1 lesson 2). All of these are barriers to self-pollination; therefore, the plants depend on pollinators to transfer pollen. Most pollinators are biotic agents such as insects (like bees, flies, and butterflies), bats, birds, and other animals. Other plant species are pollinated by abiotic agents, such as wind and water.
Pollination by Insects
Bees are perhaps the most important pollinator of many garden plants and most commercial fruit trees (Figure 3.1.6.). The most common species of bees are bumblebees and honeybees. Bees collect energy-rich pollen or nectar for their survival and energy needs. They visit flowers that are open during the day, are brightly colored, have a strong aroma or scent, and have a tubular shape, typically with the presence of a nectar guide. A nectar guide includes regions on the flower petals that are visible only to bees, and not to humans; it helps to guide bees to the center of the flower, thus making the pollination process more efficient. The pollen sticks to the bees’ fuzzy hair, and when the bee visits another flower, some of the pollen is transferred to the second flower. We perceive colors based on reflection. When light hits an object, some wavelengths are absorbed, and some wavelengths are reflected. Bees perceive UV light and blue and green wavelengths. Thus, bee-pollinated flowers usually have shades of blue, yellow, or other colors.
Recently, there have been many reports about the declining population of honeybees called colony collapse disorder (CCD). The impact on commercial fruit growers could be devastating. Many flowers will remain unpollinated and not bear seed if honeybees disappear, crops such as almonds, pumpkins, apples, melons, cranberries, squash, and broccoli. Factors such as the use of pesticides, parasitic fungi, mites, viral pathogens, climate change, destruction of natural habitats, and agricultural monocrops are a few of the many factors that affect honeybee populations.
Bees are not the only insects that aid the pollination. Many flies are attracted to flowers that have a decaying smell or an odor of rotting flesh. These flowers, which produce nectar, usually have dull colors, such as brown or purple. They are found on the corpse flower or voodoo lily (Amorphophallus), dragon arum (Dracunculus), and carrion flower (Stapleia, Rafflesia). The nectar provides energy, whereas the pollen provides protein. Wasps are also important insect pollinators and pollinate many species of figs. Butterflies, such as the monarch, pollinate many garden flowers and wildflowers, which usually occur in clusters. These flowers are brightly colored, have a strong fragrance, are open during the day, and have nectar guides to make access to nectar easier. The pollen is picked up and carried on the butterfly’s limbs. Moths, on the other hand, pollinate flowers during the late afternoon and night; the flowers pollinated by moths are pale or white and are flat, enabling the moths to land. One well-studied example of a moth-pollinated plant is the yucca plant, which is pollinated by the yucca moth. The shape of the flower and moth have adapted in such a way as to allow successful pollination. The moth deposits pollen on the sticky stigma for fertilization to occur later. The female moth also deposits eggs into the ovary. As the eggs develop into larvae, they obtain food from the flower and develop seeds. Thus, both the insect and the flower benefit from each other in this symbiotic relationship. The corn earworm moth and Gaura plant have a similar relationship (Figure 3.1.7.).
Pollination by Bats
In the tropics and deserts, bats are often the pollinators of nocturnal flowers, such as agave, guava, and morning glory. The flowers are usually large and white or pale-colored; thus, they can be distinguished from the dark surroundings at night. The flowers have a strong, fruity, or musky fragrance and produce large amounts of nectar. They are naturally large and wide-mouthed to accommodate the head of the bat. As the bats seek the nectar, their faces and heads become covered with pollen, which is then transferred to the next flower.
Pollination by Birds
Brightly colored, odorless flowers that are open during the day are pollinated by birds. As a bird seeks energy-rich nectar, pollen is deposited on the bird’s head and neck and is then transferred to the next flower it visits. Many species of small birds, such as the hummingbird (Figure 3.1.8.) and sunbirds, are pollinators for plants such as orchids and other wildflowers. Flowers visited by birds are usually sturdy and are oriented in such a way as to allow the birds to stay near the flower without getting their wings entangled in the nearby flowers. The flower typically has a curved, tubular shape, which allows access to the bird’s beak. Botanists have been known to determine the range of extinct plants by collecting and identifying pollen from 200-year-old bird specimens from the same site.
Pollination by Wind
Most species of conifers and many angiosperms—such as grasses, maples, and oaks—are pollinated by wind. Pinecones are brown and unscented, while the flowers of wind-pollinated angiosperm species are usually green and small, with tiny or no petals, and produce large amounts of pollen. Unlike the typical insect-pollinated flowers, flowers adapted to pollination by the wind do not produce nectar or scent. In wind-pollinated species, the microsporangia hang out of the flower, and, as the wind blows, the lightweight pollen is carried with it (Figure 3.1.9.). The flowers usually emerge early in the spring, before the leaves, so that the leaves do not block the movement of the wind. The pollen is deposited on the exposed feathery stigma of the flower (Figure 3.1.10.).
Pollination by Water
Some weeds, such as Australian seagrass and pondweeds, are pollinated by water. The pollen floats on water, and when it comes into contact with the flower, it is deposited inside the flower.
EVOLUTION CONNECTION
Pollination by Deception
Orchids are highly valued flowers, with many rare varieties (Figure 3.1.11.) They grow in a range of specific habitats, mainly in the tropics of Asia, South America, and Central America. At least 25,000 species of orchids have been identified.
Flowers often attract pollinators with food rewards, in the form of nectar. However, some species of orchid are an exception to this standard: they have evolved different ways to attract the desired pollinators. They use a method known as food deception, in which bright colors and perfume are offered, but no food. Anacemptis morio, commonly known as the green-winged orchid, bears bright purple flowers and emits a strong scent. The bumblebee, its main pollinator, is attracted to the flower because of the strong scent – which usually indicates food for a bee - and in the process, picks up the pollen to be transported to another flower.
Other orchids use sexual deception. Chiloglottis trapeziformis emits a compound that smells the same as the pheromone emitted by a female wasp to attract male wasps. The male wasp is attracted to the scent, lands on the orchid flower, and in the process, transfer pollen. Some orchids, like the Australian hammer orchid, use scent as well as visual trickery in yet another sexual deception strategy to attract wasps. The flower of this orchid mimics the appearance of a female wasp and emits a pheromone. The male wasp tries to mate with what appears to be a female wasp, and in the process, picks up pollen, which is then transferred to the next counterfeit mate.
Access for free at https://openstax.org/books/biology-2e/pages/32-2-pollination-and-fertilization
Double Fertilization
After pollen is deposited on the stigma, it must germinate and grow through the style to reach the ovule. The microspores, or the pollen, contain two cells: the pollen tube cell and the generative cell. The pollen tube cell grows into a pollen tube through which the generative cell travels. The germination of the pollen tube requires water, oxygen, and certain chemical signals. As it travels through the style to reach the embryo sac, the pollen tube’s growth is supported by the tissues of the style. In the meantime, if the generative cell has not already split into two cells, it now divides to form two sperm cells. The pollen tube is guided by the chemicals secreted by the synergid present in the embryo sac, and it enters the ovule sac through the micropyle. Of the two sperm cells, one sperm fertilizes the egg cell, forming a diploid zygote; the other sperm fuses with the two polar nuclei, forming a triploid cell that develops into the endosperm which serves as a nutritive tissue for the embryo. Together, these two fertilization events in angiosperms are known as double fertilization (Figure 3.1.12.). After fertilization is complete, no other sperm can enter. The fertilized ovule forms the seed, whereas the tissues of the ovary become the fruit, usually enveloping the seed.
After fertilization, the zygote divides to form two cells: the upper cell—or terminal cell—and the lower cell—or basal cell. The division of the basal cell gives rise to the suspensor, which eventually makes a connection with the maternal tissue. The suspensor provides a route for nutrition to be transported from the mother plant to the growing embryo. The terminal cell also divides, giving rise to a globular-shaped proembryo (Figure 3.1.13a.). In dicots (eudicots), the developing embryo has a heart shape, due to the presence of the two rudimentary cotyledon (Figure 3.1.13b.). In non-endospermic dicots, such as Capsella bursa, the endosperm develops initially but is then digested, and the food reserves are moved into the two cotyledons. As the embryo and cotyledons enlarge, they run out of room inside the developing seed and are forced to bend (Figure 3.1.13c). Ultimately, the embryo and cotyledons fill the seed (Figure 3.1.13d), and the seed is ready for dispersal. Embryonic development is suspended after some time, and growth is resumed only when the seed germinates. The developing seedling will rely on the food reserves stored in the cotyledons until the first set of leaves begin photosynthesis.
View an animation of the double fertilization process of angiosperms.
Access for free at https://openstax.org/books/biology-2e/pages/32-2-pollination-and-fertilization
Development of the Seed
The mature ovule develops into the seed. A typical seed contains a seed coat, cotyledons, an endosperm, and a single embryo (Figure 3.1.14). Let us look at the development of each of these components in a seed.
Endosperm and cotyledon: The storage of food reserves in angiosperm seeds differs between monocots and dicots. In monocots, such as corn and wheat, the single cotyledon is called a scutellum; the scutellum is connected directly to the embryo via vascular tissue (xylem and phloem). Food reserves are stored in the large endosperm. Monocot seeds are also identified as endospermic seeds. Upon germination, enzymes are secreted by the aleurone—a single layer of cells just inside the seed coat that surrounds the endosperm and embryo. The enzymes degrade the stored carbohydrates, proteins, and lipids; the products of which are absorbed by the scutellum and transported via a vasculature strand to the developing embryo. Therefore, the scutellum can be seen to be an absorptive organ, not a storage organ.
The two cotyledons in the dicot seed also have vascular connections to the embryo. In endospermic dicots, the food reserves are stored in the endosperm During germination, the two cotyledons, therefore, act as absorptive organs to take up the enzymatically released food reserves. Tobacco (Nicotiana tabaccum), tomato (Solanum lycopersicum), and pepper (Capsicum annuum) are examples of endospermic dicots. In non-endospermic dicots, the triploid endosperm develops normally following double fertilization, but the endosperm food reserves are quickly remobilized and moved into the developing cotyledon for storage. The two halves of a peanut seed (Arachis hypogaea) and the split peas (Pisum sativum) are individual cotyledons loaded with food reserves.
Seed coat: The seed, along with the ovule, is protected by a seed coat that is formed from the integuments of the ovule sac. In dicots, the seed coat is further divided into an outer coat known as the testa and the inner coat known as the tegmen.
Embryo: The embryonic axis consists of three parts: the plumule, the radicle, and the hypocotyl. The portion of the embryo between the cotyledon attachment point and the radicle is known as the hypocotyl (hypocotyl means “below the cotyledons”). The embryonic axis terminates in a radicle (the embryonic root), which is the region from which the root will develop. In dicots, the hypocotyls extend above ground, giving rise to the stem of the plant. In monocots, the hypocotyl does not show above ground because monocots do not exhibit stem elongation. The part of the embryonic axis that projects above the cotyledons are known as the epicotyl. The plumule is composed of the epicotyl, young leaves, and the shoot apical meristem.
Access for free at https://openstax.org/books/biology-2e/pages/32-2-pollination-and-fertilization
Development of Fruit and Fruit Type
Fruits are of many types, depending on their origin and texture. The sweet tissue of the blackberry, the red flesh of the tomato, the shell of the peanut, and the hull of corn (the tough, thin part that gets stuck in your teeth when you eat popcorn) are all fruits. Botanically, the term “fruit” is used for a ripened ovary. In most cases, fruit formation occurs after fertilization. The fruit encloses the seeds and the developing embryo, thereby providing it with protection. As the fruit matures, the seeds also mature. Some fruits develop from the ovary and are known as true fruits, whereas others develop from other parts of the female gametophyte and are known as accessory fruits.
Fruits may be classified as simple, aggregate, multiple, or accessory, depending on their origin (Figure 3.1.15). If the fruit develops from a single carpel or fused carpel of a single ovary, it is known as a simple fruit, as seen in nuts and beans. An aggregate fruit is one that develops from more than one carpel, but all are in the same flower: the mature carpels fuse together to form the entire fruit, as seen in the raspberry. Multiple fruit develops from an inflorescence or a cluster of flowers. An example is a pineapple, where the flowers fuse together to form the fruit. Accessory fruits (sometimes called false fruits) are not derived from the ovary but from another part of the flower, such as the receptacle (strawberry) or the hypanthium (apples and pears).
Fruits generally have three parts: the exocarp (the outermost skin or covering), the mesocarp (middle part of the fruit), and the endocarp (the inner part of the fruit). Together, all three are known as the pericarp. The mesocarp is usually the fleshy, edible part of the fruit; however, in some fruits, such as the almond, the endocarp is the edible part. In many fruits, two or all three of the layers are fused and indistinguishable at maturity. Fruits can be dry or fleshy. Furthermore, fruits can be divided into dehiscent or indehiscent types. Dehiscent fruits, such as peas, readily release their seeds, while indehiscent fruits, like peaches, rely on decay to release their seeds.
Access for free at https://openstax.org/books/biology-2e/pages/32-2-pollination-and-fertilization
Fruit and Seed Dispersal
The fruit has a single purpose: seed dispersal. Seeds contained within fruits need to be dispersed far from the mother plant, so they may find favorable and less competitive conditions in which to germinate and grow.
Some fruit has built-in mechanisms so they can disperse by themselves, whereas others require the help of agents like wind, water, and animals (Figure 3.1.16) Modifications in seed structure, composition, and size help in dispersal. Wind-dispersed fruits are lightweight and may have wing-like appendages that allow them to be carried by the wind. Some have a parachute-like structure to keep them afloat. Some fruits—for example, the dandelion—have hairy, weightless structures that are suited to dispersal by wind.
Seeds dispersed by water are contained in light and buoyant fruit, giving them the ability to float. Coconuts are well known for their ability to float on water to reach the land where they can germinate. Similarly, willow and silver birches produce lightweight fruit that can float on water.
Animals and birds eat fruits, and the seeds that are not digested are excreted in their droppings some distance away. Some animals, like squirrels, bury seed-containing fruits for later use; if the squirrel does not find its stash of fruit, and if conditions are favorable, the seeds germinate. Some fruits, like the cocklebur, have hooks or sticky structures that stick to an animal's coat and are then transported to another place. Humans also play a big role in dispersing seeds when they carry fruits to new places and throw away the inedible part that contains the seeds.
All the above mechanisms allow for seeds to be dispersed through space, much like an animal’s offspring can move to a new location. Seed dormancy, which was described earlier, allows plants to disperse their progeny through time, which is something animals cannot do. Dormant seeds can wait months, years, or even decades for the proper conditions for germination and propagation of the species.
Access for free at https://openstax.org/books/biology-2e/pages/32-2-pollination-and-fertilization
Seed Dormancy & Germination
Many mature seeds enter a period of inactivity, or extremely low metabolic activity: a process is known as dormancy; this may last for months, years, or even centuries. Dormancy helps keep seeds viable during unfavorable conditions. Upon a return to favorable conditions, seed germination takes place. Favorable conditions could be as diverse as moisture, light, cold, fire, or chemical treatments. After heavy rains, many new seedlings emerge. Forest fires also lead to the emergence of new seedlings.
The requirements for germination depend on the species. Common environmental requirements include light, the proper temperature, the presence of oxygen, and the presence of water. Seeds of small-seeded species usually require light as a germination cue. This ensures the seeds only germinate at or near the soil surface (where the light is greatest). If they were to germinate too far underneath the surface, the developing seedling would not have enough food reserves to reach the sunlight.
Not only do some species require a specific temperature to germinate, but they may also require a prolonged cold period (vernalization) prior to germination. In this case, cold conditions gradually break down a chemical inhibitor to germination. This mechanism prevents seeds from germinating during an unseasonably warm spell in the autumn or winter in temperate climates. Similarly, plants growing in hot climates may have seeds that need heat treatment to germinate, which is an adaptation to avoid germination in the hot, dry summers. Horticulturists can improve germination rates of species that have a vernalization requirement by exposing seeds to a stratification treatment, where seeds imbibe water and then are kept in cold storage until vernalization requirements are met.
In many seeds, the presence of a thick seed coat retards the ability to germinate. Scarification, which includes mechanical or chemical processes to soften the seed is often employed before germination. Seeds of many species may need to pass through an animal's digestive tract to remove inhibitors prior to germination. Similarly, some species require mechanical abrasion of the seed coat, which could be achieved by water dispersal. Other species are fire-adapted, requiring fire to break dormancy (Figure 3.1.17).
The Mechanism of Germination
The first step in germination starts with the uptake of water, also known as imbibition. Imbibition activates enzymes that start to break down starch into sugars consumed by the embryo for cell division and growth. This process is irreversible.
Depending on seed size, the time taken for a seedling to emerge may vary. Species with large seeds have enough food reserves to germinate deep below ground, and still, extend their epicotyl all the way to the soil surface while the seedlings of small-seeded species emerge more quickly (and can only germinate close to the surface of the soil).
During epigeous germination, the hypocotyl elongates, and the cotyledons extend above ground. During hypogeous germination, the epicotyl elongates, and the cotyledon(s) remain below ground (Figure 3.1.18). Some species (like beans and onions) have epigeous germination while others (like peas and corn) have hypogeous germination. In many epigeous species, the cotyledons not only transfer their food stores to the developing plant but also turn green and make more food by photosynthesis until they drop off.
Germination in Eudicots
Upon germination in eudicot seeds, the radicle emerges from the seed coat while the seed is still buried in the soil.
For epigeous eudicots (like beans), the hypocotyl is shaped like a hook with the plumule pointing downwards. This shape is called the plumule hook, and it persists as long as germination proceeds in the dark. Therefore, as the hypocotyl pushes through the tough and abrasive soil, the plumule is protected from damage. Additionally, the two cotyledons additionally protect them from mechanical damage. Upon exposure to light, the hypocotyl hook straightens out, the young foliage leaves face the sun and expand, and the epicotyl elongates (Figure 3.1.19; 3.1.20).
In hypogeous eudicots (like peas), the epicotyl rather than the hypocotyl forms a hook, and the cotyledons and hypocotyl thus remains underground. When the epicotyl emerges from the soil, the young foliage leaves expand. The epicotyl continues to elongate (Figure 3.1.21). The radicle continues to grow downwards and ultimately produces the tap root. Lateral roots then branch off to all sides, producing the typical eudicot tap root system.
Germination in Monocots
As the seed germinates, the radicle emerges and forms the first root. In epigeous monocots (such as onion), the single cotyledon will bend, forming a hook and emerge before the coleoptile (Figure 3.1.22). In hypogeous monocots (such as corn), the cotyledon remains below ground, and the coleoptile emerges first. In either case, once the coleoptile has exited the soil and is exposed to light, it stops growing. The first leaf of the plumule then pieces the coleoptile (Figure 3.1.23), and additional leaves expand and unfold. At the other end of the embryonic axis, the first root soon dies while adventitious roots (roots that arise directly from the shoot system) emerge from the base of the stem (Figure 3.1.24). This gives the monocot a fibrous root system.
Glossary
accessory fruit - fruit derived from tissues other than the ovary
aggregate fruit - fruit that develops from multiple carpels in the same flower
aleurone - a single layer of cells just inside the seed coat that secretes enzymes upon germination
androecium - the sum of all the stamens in a flower
antipodals - the three cells away from the micropyle
cotyledon - the fleshy part of the seed that provides nutrition to the seed
cross-pollination - transfer of pollen from the anther of one flower to the stigma of a different flower
double fertilization - two fertilization events in angiosperms; one sperm fuses with the egg, forming the zygote, whereas the other sperm fuses with the polar nuclei, forming the endosperm
endocarp - the innermost part of the fruit
endosperm - triploid structure resulting from the fusion of a sperm with polar nuclei, which serves as a nutritive tissue for the embryo
endospermic dicot - dicot that stores food reserves in the endosperm
exine - outermost covering of pollen
exocarp - outermost covering of a fruit
gametophyte - multicellular stage of the plant that gives rise to haploid gametes or spores
gynoecium - the sum of all the carpels in a flower
intine - the inner lining of the pollen
mega-gametogenesis - the second phase of female gametophyte development, during which the surviving haploid megaspore undergoes mitosis to produce an eight-nucleate, seven-cell female gametophyte, also known as the megagametophyte or embryo sac
megasporangium - tissue found in the ovary that gives rise to the female gamete or egg
megasporogenesis - the first phase of female gametophyte development, during which a single cell in the diploid megasporangium undergoes meiosis to produce four megaspores, only one of which survives
megasporophyll - bract (a type of modified leaf) on the central axis of a female gametophyte
mesocarp - middle part of a fruit
micropropagation - propagation of desirable plants from a plant part; carried out in a laboratory
micropyle - opening on the ovule sac through which the pollen tube can gain entry
microsporangium - tissue that gives rise to the microspores or the pollen grain
microsporophyll - central axis of a male cone on which bracts (a type of modified leaf) are attached
monocarpic - plants that flower once in their lifetime
multiple fruit - fruit that develops from multiple flowers on an inflorescence
nectar guide - pigment pattern on a flower that guides an insect to the nectaries
non-endospermic dicot - dicot that stores food reserves in the developing cotyledon
perianth - also known as petal or sepal; part of the flower consisting of the calyx and/or corolla; forms the outer envelope of the flower
pericarp - a collective term describing the exocarp, mesocarp, and endocarp; the structure that encloses the seed and is a part of the fruit
plumule - shoot that develops from the germinating seed
polar nuclei – diploid nuclei found in the ovule or embryo sac; produce endosperm after fusion with one of the two sperm cells
pollination - transfer of pollen to the stigma
polycarpic - plants that flower several times in their lifetime
radicle - the original root that develops from the germinating seed
scutellum - a type of cotyledon found in monocots, as in grass seeds
self-pollination - transfer of pollen from the anther to the stigma of the same flower
simple fruit - fruit that develops from a single carpel or fused carpels
sporophyte - multicellular diploid stage in plants that is formed after the fusion of male and female gametes
suspensor - part of the growing embryo that makes the connection with the maternal tissues
synergid - a type of cell found in the ovule sac that secretes chemicals to guide the pollen tube toward the egg
tegmen - the inner layer of the seed coat
testa - the outer layer of the seed coat
Attributions
Flowers of different families; Alvesgaspar, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
"Germination" by Melissa Ha, Maria Morrow, & Kammy Algiers, LibreTexts is licensed under CC BY-SA .
Morrow, M. H., Maria, & Algiers, K. (2022, February 19). Germination. https://bio.libretexts.org/@go/page/32044
Biology 2e by OpenStax is licensed under Creative Commons Attribution License v4.0
|
oercommons
|
2025-03-18T00:39:13.519953
|
Anna McCollum
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/85005/overview",
"title": "Statewide Dual Credit Introduction to Plant Science, Plant Reproduction and Propagation, Sexual Reproduction in Plants",
"author": "Full Course"
}
|
https://oercommons.org/courseware/lesson/87612/overview
|
5.11 Animal Manures
5.12 90-120 Day Rule
5.13 Compost
5.14 Compost Tea
5.15 Vermicompost
5.16 Processed Animal Manures
5.17 Ratooning
5.18 Soil Conservation
5.19 Cover Cropping
5.20 Organic Mulches
5.21 Conservation Tillage
5.22 Contour Conservation and Strip Cropping
5.23 Mixed Cropping
5.24 Food Forests
5.25 Nutrient Management_Nitrogen and Trace Minerals
5.26 Genetic Engineering
5.2 Crop Rotation in Annual Crops
5.3 Crop Rotation in Perennial Crops
5.4 Biodiversity
5.5 Disease and Pest Management
5.6 Environmental Benefits and Considerations of Crop Rotations
5.7 Pesticide and Fertilizer Use Under Different Crop Rotations
5.8 Alley Cropping
5.9 Soil Building
5_Crop-Biodiversity
Crop Biodiversity
Overview
Title Image: Strips of oats and hay are interspersed with strips of corn to save soil and improve water quality and wildlife habitat on this field in northeast Iowa Credit: United States Department of Agriculture – Natural Resources Conservation Service; Public Domain
Did you have an idea for improving this content? We’d love your input.
Introduction
Lesson Objectives
Defend the need for genetic diversity in cropping systems.
Identify various cropping systems that promote genetic diversity.
Recognize the advantages and disadvantages related to genetic diversity.
Key Terms
conservation tillage - the minimal use of soil cultivation with crops
crop rotation - cycling through planting different types of crops for several years
mixed cropping - growing different types of plants in the same land
monoculture - a sequence where the same crop is planted for 3 consecutive years
polyculture - the process of growing multiple crops in a designated area to mimic the natural environment
ratooning - the process of cutting plant stems down to stimulate another round of growth
sequential cropping - growing various crops on the same land in different years, one after the other
vulnerability - plants’ susceptibility to pests and environmental conditions
Introduction
Expanding markets, new production technologies, and economic competition in recent decades have resulted in crop specialization, increased purchase of off-farm inputs, and production practices that often have adverse environmental consequences. Monoculture (successively growing the same crop on the same land), continuous row crops, and other intensive cropland uses have increased with the availability of commercial fertilizers to supply nutrient needs and chemicals to control pests. Crop rotations that include hay, grass sod, and other soil-conserving crops were abandoned by many producers as the demand for hay and forages declined. The choice between monoculture and rotating different crops on the same land depends on a broad range of economic and physical factors. And the choice of rotation frequently affects the use of fertilizer and pesticides.
Crop Rotation in Annual Crops
For producers of annual crops, complying with crop rotation standards is straightforward and often beneficial for crop health. Crop rotation refers to the sequencing of crops over time on a field or planting bed. Rotations typically mean that crops are not followed by a member of the same crop family. Sequential cropping is not unique to organic systems, as it is also practiced by many conventional farmers. However, organic systems are unique in that crop rotation is specifically required in the USDA organic regulations.
Crop rotation can
• interrupt insect life cycles.
• suppress soilborne plant diseases.
• prevent soil erosion.
• build organic matter.
• fix nitrogen.
• increase biodiversity of the farm.
Crop rotations are an important way to suppress insects and diseases. For example, farmers who raise potatoes will rotate the field out of solanaceous crops for at least 2 years before replanting potatoes. This helps reduce populations of insects, such as the Colorado potato beetle, and prevent diseases, such as late blight. Rotations with 3 to 5 years between the same crop may be needed to effectively reduce insect and disease levels.
Rotations also can be designed to increase soil fertility. A crop sequence that features soil-improving crops can counterbalance soil-depleting crops. Soil-improving crops include sod crops dominated by perennial grasses and perennial legumes. Sod crops in rotation build soil organic matter and reverse the decline that typically occurs when cultivated annual crops are grown year after year. Legumes, such as alfalfa, clovers, beans, and peas, are especially beneficial because they fix nitrogen from the atmosphere and make it available to subsequent crops. Even short-term, nonleguminous cover crops can provide benefits when used as part of the crop-rotation plan.
The best cover crops are specific varieties adapted to the soil, climate, and season. They are sown at a fairly high rate to cover the soil quickly and prevent erosion. When planning crop rotations, it is important to remember that cultivated row crops, such as vegetables, tend to degrade soil. Since the soil is open and cultivated between rows, microbes break down organic matter at a more rapid pace. Furthermore, row crops have modest root systems and consequently do not contribute enough new organic matter to replace that lost from the open soil between rows. In most cases, above-ground crop residues make only minor contributions to replacing lost organic matter.
In contrast, cereals and cover crops are more closely spaced and have more extensive root systems than row crops, greatly reducing the amount of soil exposed to degradation. In addition, these crops receive little or no cultivation after planting, which reduces organic-matter loss even more. As a result, cereals and green manures can be considered neutral crops, replacing soil organic matter at roughly the same rate at which it breaks down. Crops that make a perennial sod cover, such as grasses, clovers, and alfalfa, not only keep the soil entirely covered but also have massive root systems that produce far more organic matter than is lost.
Incorporating sod crops as a fundamental part of a crop rotation not only builds soil but also supports weed-control strategies. Weed control improves because the types of weeds encouraged by row-cropping systems are usually adapted to growing in a sod/hay crop. To make the most efficient use of sod crops, it is necessary to include livestock in the system or to find a market for the hay. Livestock will assist in transferring (via manure) nutrients from one part of the farm to another. The major drawback to selling hay is that the nutrients it contains are shipped off the farm.
Crop Rotation in Perennial Crops
For producers of organic perennial crops, the requirement for crop rotation can be confusing. Farmers should implement practices that will maintain soil organic matter, control pests, conserve nutrients, and protect the soil against erosion. For growers of annual crops, those practices typically include crop rotation, but other practices can be substituted if rotation is not practical.
Some perennials will be part of a long-term crop rotation, which may last a few years or even decades. Asparagus, for example, is a perennial that can be productive for 15 years or more. When a field is taken out of asparagus production, it is typically planted with another crop to reduce the incidence of soilborne disease. That practice is considered a long crop rotation. Several other perennials, such as strawberries, Echinacea, and lavender, are not required to have a cover crop because they are typically part of a long crop rotation.
Other types of perennials—those that will not be part of a crop rotation—may require additional practices to ensure soil conservation and biodiversity in the cropping system. This is important with large perennials, such as trees, that have large drive rows between the crop rows. For example, organic farmers must have a cover crop (often grass) between the rows of trees in an orchard. Crops that are required to have a cover crop between crop rows include caneberries, grapevines, blueberries, tree fruits, and nut trees.
Some perennial crops, such as alfalfa, develop a canopy that covers the ground and prevents soil erosion. Such crops are not required to be rotated with other crops.
Biodiversity
Many organic farmers actively manage their farms to increase biodiversity, due to its many benefits. Biodiversity plays a particularly crucial role in pest management. Although farmers are encouraged to have diverse systems, there are no specific requirements, standards, or monitoring practices. Diverse agricultural systems support strong populations of predators and parasites that help keep pest populations at manageable levels. This approach is proactive rather than reactive because a diverse system reaches an equilibrium that prevents pest outbreaks from becoming too severe. Birds and bats can keep insect populations low. Raptors can scare away fruit-eating birds. Coyotes, owls, and foxes can keep rodent populations under control. These animals can be encouraged to improve the plants' vulnerability because plants are providing needed shelter, water, and habitat.
Organic producers increase biological diversity in the plant canopy by planting a diversity of crops and plant varieties in any given season. Use of cover crops and hedgerows also adds biodiversity. The diversity of vegetation, combined with reduced use of broad-spectrum pesticides, increases the diversity of insects and spiders in the plant canopy. Introducing beneficial insects and providing habitat for them to become established will increase biodiversity. To promote biodiversity in the soil, it is helpful to minimize tillage, introduce microorganisms in compost, and avoid broad-spectrum pesticides. These practices will increase the variety of bacteria, fungi, and invertebrates in the soil.
Disease and Pest Management
In many field crop and vegetable systems, maintaining a diverse, healthy ecosystem and using well-timed cultural practices are sufficient for pest management. Pests may not be eliminated, but damage levels are low enough to be tolerated. Organic producers maintain that organic soil-building practices will result in crops that are properly nourished and thereby less susceptible to attack by pests and diseases. Natural biological pest control arises in a healthy organic system in the form of an active complex of natural predators and parasites that suppress pest populations. Incorporating habitat and food sources for beneficial insects into the farm, known as farmscaping, can provide long-term benefits.
Environmental Benefits and Considerations of Crop Rotations
Crops face danger of extensive damage or destruction from a variety of sources including weeds, pests, diseases, adverse environmental conditions, and unfavorable weather. Potential crop yields can be seriously restricted by a lack of crop protection.. Planned crop rotations can increase yields, improve soil structure, reduce soil loss, conserve soil moisture, reduce fertilizer and pesticide needs, and provide other environmental and economic benefits. However, crop rotations may reduce profits when the acreage and frequency of highly profitable crops are replaced with crops earning lower returns. Many crop rotations reduce soil loss and are an option for meeting conservation compliance on highly erodible land. The growth of hay, small grain crops, or grass sod in rotation with conventionally tilled row crops reduces the soil’s exposure to wind and water and decreases total soil loss. While beneficial, crop sequencing can be complex and require more knowledge about plants and growing (Figure 5.5.2).
These rotations, however, are a desirable option to farmers only when profitable markets exist or the conservation crops can be utilized by on farm livestock enterprises. Alternating wheat and fallow is a common practice for conserving soil moisture in regions with low rainfall. Applying tillage practices to minimize evaporation or transpiration from idle land in one season increases the amount of stored soil moisture
available for the crop in the following season.
The ability of legume crops to fix atmospheric nitrogen and supply soil nitrogen needs for subsequent crops is well documented. The plowdown of established alfalfa or other legumes can provide carryover nitrogen for a crop that requires high levels of nitrogen, such as corn. Research has shown that soybeans can be managed to fix 90 percent of their nitrogen needs and provide a soil nitrogen credit of 20 pounds or more per acre for a subsequent crop (Heichel, 1987). However, soybeans grown in rotation with corn where soils are already rich in nitrogen have not been shown to fix significant amounts of nitrogen.
Crop rotations affect pest populations and can reduce the need for pesticides. Different crops often break pest cycles and prevent pest and disease organisms from building to damaging levels. Treatment for corn rootworm, the most common insecticide treatment on corn, normally only requires alternating another crop to sufficiently reduce root-worm survival rates to levels that do not require insecticide treatment. Hay and grass sod grown in rotation with corn, however, may increase the need for other corn insecticides to treat other pests.
Besides providing erosion control, small grains, hay, and grass sod are competitive with broadleaf weeds and may help control weed populations in subsequent crops. These crops are usually harvested or can be cut before weeds reach maturity and produce seed for germination the following season. Weeds on prior idle acres or fallow land may be controlled by either cutting or tilling to reduce weed infestations the following year. Sometimes, herbicides are used to kill existing vegetation on idle land (chemical fallow)
in lieu of mechanical methods.
Rotations also can reduce financial risk and provide a more sustainable production system. Since adverse weather or low market prices are less likely to affect all crops simultaneously, the diversity of products resulting from crop rotation can reduce risk.
Pesticide and Fertilizer Use Under Different Crop Rotations
Crop rotation is often key to a sustainable agricultural production system and can reduce the need for fertilizer and pesticides. Fertilizer applications are often adjusted for prior nitrogen-fixing crops. Fewer pesticides may be needed when rotations break pest cycles or reduce infestation levels.
Alley Cropping
Alley cropping is defined as the planting of rows of trees and/or shrubs to create alleys within which agricultural or horticultural crops are produced. Alley cropping systems are sometimes called intercropping, especially in tropical areas. The trees produced through alley cropping may include valuable hardwood veneer or lumber species; fruit, nut or other specialty crop trees/shrubs; or desirable softwood species for wood fiber production. As trees and shrubs grow, they influence the light, water, and nutrient regimes in the field. These interactions are what sets alley cropping apart from more common monocropping systems.
Alley cropping can vary from simple systems, such as an annual grain rotation between timber tree species, to complex multilayered systems that can produce a diverse range of agricultural products. It is especially attractive to producers interested in growing multiple crops on the same acreage to improve whole-farm yield. Growing a variety of crops in close proximity to each other can create significant benefits to producers, such as improved crop production and microclimate benefits and help them manage risk.
Soil Building
For centuries before the advent of chemical fertilizers, farmers supplied all the nutrients for their crops solely by adding organic matter to the soil. As fresh organic matter, such as crop residues, decomposes, it forms a stable substance called humus. Organic matter can be added to soils with compost, animal manures, or green manures. Adding organic matter is a fundamental way to build soils. Organic matter provides food for microorganisms, such as fungi and bacteria, and macroorganisms, such as earthworms. As these diverse soil organisms decompose organic matter, they convert nutrients into forms that are available to plants. Soils high in organic matter also have improved water-holding capacity, helping plants resist drought.
Green Manures
Green manures are crops grown specifically for soil improvement. They are typically incorporated into the soil after they have produced a large amount of biomass or fixed a significant amount of nitrogen in the case of legumes. Managing green manure crops to increase organic matter and provide the maximum amount of nitrogen to the following crop is both an art and a science.
Annual grasses, small grains, legumes, and other useful plants like buckwheat can be inserted into the cropping sequence to serve as green manures. Their roots pull nutrients from deeper soil layers, and the tops are plowed into the soil to add organic matter and a stable source of nutrients. In particular, deep tap-rooted crops such as alfalfa, sweet clover, rape, and mustard are known to extract and use minerals from the deeper layers of soil.
Legumes add nitrogen to the soil. Nitrogen accumulations by leguminous cover crops can range from 40 to 200 pounds of nitrogen per acre. The amount of nitrogen captured by legumes depends on the species of legume grown, the total biomass produced, and the percentage of nitrogen in the plant tissue. Cultural and environmental conditions that limit legume growth—such as a delayed planting date, poor stand establishment, and drought—will reduce the amount of nitrogen produced. Conditions that favor high nitrogen information production include a good stand, optimum soil nutrient levels and soil pH, good nodulation, and adequate soil moisture.
Animal Manures
Conservation of manure and its proper application are key means of recycling nutrients and building soil. Farms without livestock often buy manure or compost because they are considered to be among the best fertilizers available, though sole reliance on fertilizers from other farms can have its drawbacks like cost, availability, and transportation.
Manures from conventional systems are allowed in organic production, including manure from livestock grown in confinement and from those that have been fed genetically engineered feeds. Manure sources containing excessive levels of pesticides, heavy metals, or other contaminants may be prohibited from use. Such contamination is likely present in manure obtained from industrial-scale feedlots and other confinement facilities. Certifiers may require testing for these contaminants if there is reason to suspect a problem.
Herbicide residues have been found in manures and manure-based composts. One type—aminopyralid—is used in pastures for control of broadleaf weeds. Grass and corn are not affected by the herbicide, and cows are not affected when they eat the grass or silage. However, the herbicide can be present in their manure in concentrations high enough to stunt the growth of tomatoes, peppers, and other susceptible broadleaf crops.
If a manure source is suspected of being contaminated with excessive amounts of prohibited substances, appropriate testing should be conducted. If test results indicate that the manure is free of excessive contamination, and it is subsequently used in production, the test results should be kept on file.
Used properly, manures can replace all or most needs for purchased fertilizer, especially when combined with a whole-system fertility plan that includes crop rotation and cover cropping with nitrogen-fixing legumes. Manure is typically applied just ahead of a crop requiring high fertility, such as corn or squash. Manures also can be applied just prior to a cover crop planting. Incorporating the manure as soon as possible after application, rather than allowing it to remain on the soil surface, will conserve the maximum amount of the nitrogen.
Although manure is an excellent fertilizer for crops, and it has been used that way for centuries, manure may harbor microorganisms that are pathogenic to humans. To minimize the possibility of illness due to organic foods, there are strict regulations on the use of manure in organic crops.
90-120 Day Rule
Application of manure to organic crops is restricted by what is known as the 90–120-day rule, as described in § 205.203(c)(1):
“You may not apply raw, uncomposted livestock manure to food crops unless it is:
1. Incorporated into the soil a minimum of 120 days prior to harvest when the edible portion of the crop has soil contact; OR
2. Incorporated into the soil a minimum of 90 days prior to harvest of all other food crops.”
Incorporation is generally assumed to mean mechanical tillage to mix the manure into the soil. This is important for crops that have soil contact which include leafy greens, melons, squash, peas, and many other vegetables. Any harvestable portion of a crop that can be splashed with soil during precipitation or irrigation might be considered to have soil contact. Crops that do not have soil contact include tree fruits and sweet corn.
The 90- and 120-day restrictions apply only to food crops; they do not apply to fiber crops, cover crops, or to crops used as livestock feed.
Compost
Perhaps no other process is more closely associated with organic agriculture than composting. Composting is one of the most reliable and time-honored means of conserving nutrients to build soil fertility. Because matured, well-made compost is a stable fertilizer that will not burn plants and because composting kills most human and plant pathogens, compost can safely be used as a side-dress fertilizer on food crops.
Animal manures used in organic crop production often are composted before use, in part because some types of raw manure will burn plants if applied directly to crops. Composting reduces the number of viable weed seeds, creates a uniform product with predictable nutrient levels, and eliminates worries about human pathogens. If manures are composted according to USDA organic regulations, then they are considered compost, not manure, and may be applied without restrictions. If manure is aged but not composted according to the regulations, then the material is still considered manure and must be applied in accordance with the 90–120-day rule explained above.
The composting procedures are adapted from U.S. Environmental Protection Agency (EPA) and USDA’s Natural Resources Conservation Service (NRCS) guidelines for composting biosolids. This policy was established to ensure the elimination of pathogens that cause illness in humans. The regulations define compost as “the product of a managed process through which microorganisms break down plant and animal materials into more available forms suitable for application to the soil...” Compost used in organic production must be made according to the criteria set out in § 205.203(c)(2). This section of the regulations specifies that:
- “The initial carbon: nitrogen ratio of the blended feedstocks must be between 25:1 and 40:1.
- The temperature must remain between 131 °F and 170 °F for 3 days when an in-vessel or a static-aerated-pile system is used.
- The temperature must remain between 131 and 170°F for 15 days when a windrow composting system is used, during which period the windrow must be turned at least five times.”
Organic farmers often maintain a compost pile on the farm as an efficient and cost-effective way to retain nutrients on the farm and build soil. If compost feedstocks include raw manure, they must be composted in the method detailed above. This composting process must be explained in a system plan and documented with temperature records. If those requirements are not met, then the resulting compost must be applied according to the 90-120-day raw manure rule. If compost feedstocks do not include raw animal manures, then the resulting compost is considered plant material and there are no restrictions on its use.
Compost Tea
Some organic farmers apply compost teas to crops or soil to increase the populations of beneficial microbes. If compost tea will be applied to organic crops, it is critical that the compost used to produce the extract has been made according to USDA organic regulations. The procedures for making both the compost and the compost tea must be explained in your OSP. Applications of teas made from uncomposted manure must follow the 90-120-day rule. The tea extract may need to be tested to ensure that it is free of dangerous pathogens, particularly if the tea has been made with compost tea additives. The additives, such as molasses, provide nutrients for microbes and thereby increase their rate of growth. There is some concern that any human pathogens present will grow more abundantly in a tea made with these additives. Further details on the recommendations for the use of compost tea are available in the NOP publications listed at the end of this chapter.
Vermicompost
Vermicompost is compost that uses worms to digest the feedstocks. Since feedstocks may include animal manures, there has been debate as to whether the 90-120-day rule should apply to vermicompost. The NOP has issued the following guidance: feedstocks for vermicompost materials may include organic matter of plant or animal origin.
Feedstocks should be thoroughly macerated and mixed before processing. Vermicomposting systems depend upon regular additions of thin layers of organic matter at 1- to 3-day intervals. Doing so will maintain an aerobic environment and avoid temperature increases above 35 °C (95 °F), which will kill the earthworms. The composting process must be described in the OSP, reviewed by the certifier, and well documented on the farm. Further details are available in the NOP publications listed at the end of this chapter.
Processed Animal Manures
Heat-treated, processed manure products may be used in organic production. There is no required interval between application of processed manure and crop harvest. From the standpoint of the farmer, of course, these inputs would be applied well before harvest, so that the nutrients would be available to the crop. To be considered processed, the manure must be heated to 150 °F for 1 hour and dried to 12 percent moisture or less.
Ratooning
Ratooning is a production practice that is sometimes used on plants like sugarcane and okra. The process involves cutting stems down in mid-summer. Plants are then fertilized after being ratooned to support plant growth. This process rejuvenates the plant to stimulate another round of harvest on new growth in the later summer to early fall and is common in commercial growing.
Soil Conservation
Careful conservation and management of crop residues is part of organic soil management, since this residue plays a valuable role in improving and protecting the soil. The key to soil conservation is to keep the ground covered for as much of the year as possible. Organic farmers have long recognized the value of basic soil conservation. There are many practices that help conserve soil, including cover crops, mulches, conservation tillage, contour plowing, and strip cropping.
Since water erosion is initiated by raindrop impact on bare soil, any management practice that protects the soil from raindrop impact will decrease erosion and increase water entry into the soil. Mulches, cover crops, and crop residues all serve this purpose well. A major limitation of organic row-crop farming is that cultivation is used for weed control, since herbicides are not allowed. This cultivation creates and maintains bare ground, which increases the likelihood of soil erosion. By contrast, soil that is covered with an organic mulch of crop residue, such as that typically found in no-till fields, is less likely to erode. Organic no-till systems have yet to be perfected for annual row crops, but they work well for perennial fruit crops and pasture, allowing for year-round ground cover and virtually no soil erosion.
Cover Cropping
Cover crops are single species or mixtures of plants grown to provide a vegetative cover between perennial trees, vines, or bushes; between annual crop rows; or on fields between cropping seasons. The vegetative cover on the land prevents soil erosion by wind and water, builds soil fertility, suppresses weeds, and provides habitat for beneficial organisms. Cover crops also can help reduce insect pests and diseases, and legume cover crops fix nitrogen.
Any crop grown to provide soil cover is considered a cover crop, regardless of whether that crop is later incorporated into the soil as a green manure. Both green manures and other types of cover crops can consist of annual, biennial, or perennial herbaceous plants grown in a pure or mixed stand during all or part of the year. When cover crops are planted to reduce nutrient leaching following a cash crop, they are termed “catch crops.” This type of cover crop is typically grown over the winter when the field would otherwise be unoccupied.
Organic Mulches
Organic mulches cover the soil and provide many of the same benefits as cover crops, especially the prevention of soil erosion. Many organic materials—such as straw, leaves, pine needles, and wood chips—can be effective mulches. Straw and other materials that are easily decomposed are applied to strawberries and vegetables during the growing season.
The mulch can be tilled in at the end of the season, where it will quickly decompose. Wood chips, because they decompose very slowly, are more commonly applied to perennial crops such as blueberries, where they will not be tilled in. Applying organic mulch can be labor-intensive. Tree fruit growers sometimes mow the drive rows and blow the green clippings into the tree rows, which automates the mulching process.
Heavy mulches can be a benefit by suppressing weed growth, or a nuisance by providing a haven for slugs. Organic mulches keep the soil cool, which may be a boon for blueberries in hot climates and a drawback for tomatoes in cool spring weather. Organic mulches have a beneficial long-term effect because they add nutrients to the soil as they decompose.
Mulches of high-carbon material may have the opposite effect because they tie up nitrogen during the decomposition process. However, this should not be a problem if mulches are used properly—that is, placed on top of the soil, and not incorporated.
Conservation Tillage
In conservation tillage, crops are grown with minimal soil cultivation. This is also known as no-till, minimum till, incomplete tillage, or reduced tillage. When the amount of tillage is reduced, the residues from the plant canopy are not completely incorporated into the soil after harvest. Crop residues remain on top of the soil and prevent soil erosion, a practice known as crop residue cover. The new crop is planted into this stubble or small strips of tilled soil within the stubble.
Contour Conservation and Strip Cropping
Slope plays a role in soil conservation, in that flat ground erodes less than sloping ground with equal amounts of ground cover. Contour plowing is the practice of plowing across a slope following its elevation contour lines, rather than straight up and down the slope. The cross-slope rows formed by contour plowing slow water runoff during rainstorms to prevent soil erosion.
Strip farming, also known as strip cropping, alternates strips of closely sown crops, such as hay or small grains, with strips of row crops, such as corn, soybeans, or cotton. Strip farming helps prevent soil erosion by creating natural dams for water, helping to preserve the soil.
Mixed Cropping
The growing of several crops simultaneously in the same field but not in rows is called mixed cropping. Mixed cropping, including intercropping, is the oldest form of systemized agricultural production and involves the growing of two or more species or cultivars of the same species simultaneously in the same field. However, mixed cropping has been little by little replaced by sole crop systems, especially in developed countries. Some of the advantages of mixed cropping are, for example, resource use efficiency and yield stability, but there are also several challenges, such as weed management and competition.
Food Forests
Modern agriculture has leaned heavily on monoculture field cropping. Many have found polyculture to be a natural solution for modern issues like soil water conservation, nutrient deficiencies in soil, and disease and pest management. Trees can provide many benefits in gardens and in urban environments. They produce fruit, like apples, peaches and figs, and also provide shade and wildlife habitat.
Food forests support forest ecosystems and connect communities with nature. Trees of different sizes produce nuts and fruit, while their shade can support a variety of fresh, flavorful mushrooms, herbs, and berries. Trees improve air quality and help soil retain water.
Nutrient Management: Nitrogen and Trace Minerals
Although organic matter plays an important role in building productive soils, there are specific crops and soil types that will benefit from additional applications of specific nutrients. Organic farmers are allowed to use a variety of fertilizers to provide micronutrients to their crops. Before applying micronutrients, soil deficiencies must be documented through soil tests, plant tissue tests, observing the condition of plants, or evaluating crop quality at harvest.
Nitrogen is often a limiting nutrient, especially for vegetables and other row crops. Including legumes in the rotation can help to ensure sufficient nitrogen for the following crop. Biological nitrogen fixation in legumes results from a symbiotic relationship between the plant and Rhizobium bacteria. These bacteria “infect” the roots of legumes, forming nodules. The bacteria then fix nitrogen from the air, which results in sufficient nitrogen both for their own needs and for subsequent crops.
The inoculation of legume seed may be necessary to optimize nitrogen fixation. It is important to purchase an inoculant appropriate to the kind of legume being planted to ensure it is not genetically modified. Genetically modified inoculants are prohibited in organic production.
Genetic Engineering
The planting of GM crops is regulated—new varieties may not be widely planted until they’ve been approved by USDA. If conventional seed is planted, the certifier will request proof that it is not genetically engineered. This verification is becoming more important each year, as the number of genetically modified (GM) crops increases. The use of GM seeds is prohibited in organic agriculture, and it is the responsibility of organic growers to make certain that the crops they grow are not genetically engineered. GM crops that are now being planted or will soon be available include alfalfa, beets, corn, soybeans, papaya, plum, rapeseed, tobacco, potato, tomato, squash, cotton, and rice. This list is expected to change, as genetically engineered versions of several other crops have been developed but have not yet been released for commercial production. The most current information about GM crops is maintained by the USDA Animal and Plant Health Inspection Service (APHIS).
Seed companies that develop a new variety of genetically modified seeds must submit a petition to APHIS before that seed can be distributed to the public. Genetic engineering is considered an excluded method and is defined as a variety of methods used to genetically modify organisms or influence their growth and development by means that are not possible under natural conditions or processes and are not considered compatible with organic production. Such methods include cell fusion, microencapsulation and macroencapsulation, and recombinant DNA technology (including gene deletion, gene doubling, introducing a foreign gene, and changing the positions of genes when achieved by recombinant DNA technology). Such methods do not include the use of traditional breeding, conjugation, fermentation, hybridization, in vitro fertilization, or tissue culture.
With certified organic production, if it is necessary to use conventional seeds, it is essential to verify that the variety has not been genetically engineered and to keep documentation of this verification, as your inspector will ask to see it. Seed companies that have taken the Safe Seed Pledge may be convenient sources of non-GMO seeds. The Safe Seed Pledge was developed by the Council for Responsible Genetics and has been signed by numerous seed companies.
Dig Deeper
Attributions
Title Image: Strips of oats and hay are interspersed with strips of corn to save soil and improve water quality and wildlife habitat on this field in northeast Iowa, Credit: United States Department of Agriculture – Natural Resources Conservation Service; Public Domain
4.2 Crop Rotations by the United States Department of Agriculture is in the Public Domain.
Alley Cropping by the United States Department of Agriculture is in the Public Domain.
Guide for Organic Crop Producers by the United States Department of Agriculture is in the Public Domain.
"Sustainable Mixed Cropping Systems for the Boreal-Nemoral Region" by Lizarazo, et. al. is licensed CC BY 4.0.
Trees and Food Forests by the United States Department of Agriculture is in the Public Domain.
|
oercommons
|
2025-03-18T00:39:13.645744
|
Amanda Spangler
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/87612/overview",
"title": "Statewide Dual Credit Introduction to Plant Science, Plant Classification and Use, Crop Biodiversity",
"author": "Anna McCollum"
}
|
https://oercommons.org/courseware/lesson/85008/overview
|
1.3 Layout and Development
1_Nursery-Site-Selection
6a - Nursery-Site Selection, Layout, and Development PUBLIC DOMAIN
https://ufdcimages.uflib.ufl.edu/IR/00/00/32/68/00001/EP03400.pdf
Nursery Site Selection
Overview
Title image "20150515-NRCS-LSC-0923" by USDAgov is licensed under CC PDM 1.0
Did you have an idea for improving this content? We’d love your input.
Introduction
Lesson Objectives
Evaluate the factors that influence site selection of a production nursery.
Identify factors that influence nursery site selection.
Explain the influence of climate, soil, water, topography, previous land use, site potential, and location to nursery site selection.
Key Terms
climate - an average of the long-term, prevailing weather conditions of a region
drainage - the natural or artificial removal of surface water
soil - the layer(s) of generally loose mineral and/or organic material that are affected by physical, chemical, and/or biological processes at or near the planetary surface, and usually hold water, air, and organisms and support plants
supply chain - the system of operations that collaborate to plan, produce, and provide a product or service to a market
water supply - the water available for a region which may be delivered via natural or man-made waterways
Site Selection Criteria
Modified from "Nursery-Site Selection, Layout, and Development" by F.E. Morby, USDA Forest Service is in the Public Domain
Climate
Requirements for climate and growing season will vary depending on the species and ecotypic variants that will be grown. An ecotype is a genetically distinct population that has adapted to its particular environment.
For example, red maple (Acer rubrum) has a wide natural distribution throughout Eastern North America, as far north as Canada and south into Florida. Tennessee growers would have a difficult time growing a Canadian ecotype of red maple that is adapted to the longer daylengths and shorter growing season common at a northerly latitude. A Canadian ecotype would come out of winter dormancy later in the season than a Tennessee variant. Conversely, a Florida ecotype of red maple may break dormancy too early in the growing season, resulting in cold damage (Raulston, 1994).
Climatic concerns for species or ecotypes go beyond the seasonal daylengths or the overall length of growing season. Nursery growers should also consider the temperature, precipitation, wind, and light conditions of a site.
Temperature
Extremely hot periods reduce plant growth and can damage appearance. As temperatures increase, the rate of transpiration rapidly rises, increasing the amount of water needed from irrigation. Short periods of daytime temperatures of 110°F or more can tax irrigation systems, but properly designed irrigation systems can protect plants from burning during those periods. Growth of most species is greatly impeded by ambient temperatures of 90°F and above.
Extremely low temperatures can be detrimental to unprotected young material or container plants. The site may need to be modified by incorporating a pot-in-pot system or constructing a high tunnel. Extreme cold can drive frost deep into the soil for field-grown plants, delaying harvesting and processing into spring (see Unit 6, Lesson 2: Growing Methods for Nursery Production for more information).
Precipitation
High rainfall areas are best avoided. However, the season in which the precipitation occurs is important. Heavy spring rains can delay spring operations, such as adding soil amendments, starting a cover or green manure crop, or sowing tree seed. Summer rains tend to be a problem only when they occur as cloudbursts and result in flooding, erosion, or wash-out. Frequent summer rains may be detrimental because rains may disrupt stock hardening processes already induced by withholding irrigation. Areas with heavy winter rains should be avoided; heavy rain saturates nursery soil to the point of hindering lifting, damaging soil structure, and causing flooding and erosion.
Wind
Wind can damage plants, blow over containerized material, and limit operations, such as pesticide applications. Winds will affect irrigation application and uniformity and may cause erosion. High winds can desiccate plants. In areas with high winds, choose a site with natural windbreaks or install artificial windbreaks (Acquaah, 2009).
Light
Plants grown in outdoor nurseries rely on natural sunlight for healthy growth. Sites with heavy tree cover or tall structures may need to be modified to increase light penetration to the growing environment. Exposed sites may require shade houses to protect shade-loving plants (Acquaah, 2009).
Soil
Containerized nurseries tend to use soilless growing media to fill pots rather than native soil (see Unit 6, Lesson 2: Growing Methods for Nursery Production for more information). Field production requires plants be grown in the site’s native soil. Soil qualities, such as texture, pH, and fertility, can vary across the property. Soil testing is a crucial step in choosing a site that has the most suitable soil for the crop and production method.
Texture
Soils that have good drainage, proper aeration, and a sufficient water-holding capacity are ideal for in-ground nursery production. The ideal soil texture will vary depending on whether field-grown plants will be harvested as ball-and-burlap or bare root. Ball-and-burlap plants should be grown in soil that is cohesive enough to hold around the root ball. Bare root trees should be grown in a loose sand or loam that will be easier to remove from the roots (Acquaah, 2009). Sandy loams or loamy sands with good drainage are excellent for field-grown nurseries.
Soil pH
The optimum soil reaction, or pH, for most tree species is between pH 5.0 and 6.0. Nutrients are less available to plants at excessively low and high pH levels. Soil pH can be altered with soil additives, such as sulfur, or by injecting phosphoric or sulfuric acid into irrigation water.
Fertility
While nutrients can be added in the form of fertilizer or organic matter, the soil should be responsive to fertilization (Acquaah, 2009). Soil testing will help the grower identify the nutrient holding capacity, nutrient availability, and organic matter content of the native soil.
Water
Securing an adequate water supply for domestic or irrigation reasons use can be a major problem. Water rights must be obtained for any water source. Therefore, special consideration must be given to a site where the quantity and quality of water are adequate for current and possible future requirements. All water needs and the timing of those needs must be considered. For example, in most nurseries, irrigation is necessary during the growing season and for frost protection. Restrictions on flow and on periods of delivery must be closely scrutinized.
Irrigation Water Sources
Lakes are a good source of irrigation water. Storage capacity, draw-down, other uses, and contaminants must be examined before any commitment is made. Screening may be necessary to remove water-borne debris.
Streams are sometimes used for nursery irrigation and must be checked for water rights, other uses, and quality. In addition, attention must be paid to intakes, diversions for pumping stations, protection during runoff periods, and maintenance of the stream channel to ensure maximum carrying capacity. Stream water may need to be screened to alleviate contamination by vegetation, weed seeds, animals, algae, and other water-borne debris.
Irrigation water delivered through open ditches is usually controlled by irrigation districts and is subject to specific short delivery periods. Such a source is not reliable unless storage is made available on site; therefore, irrigation water is not recommended.
Water drawn from wells is probably one of the best irrigation sources for most locations. Draw-down and pumping capacity must be checked to ensure that water is available in reliable quantities when it is required.
Domestic or irrigation pipelines are reliable. In many instances, clean water will be supplied with adequate pressure and volume to eliminate the need for pumping. The two types of pipelines are similar, and both generally well designed and constructed. However, domestic water lines usually have more connections creating a high demand for water and more concern for failure of the system. Systems must be reviewed to ensure that maintenance is adequate and repairs are timely.
Water Quality
Chemical contaminants may infiltrate an irrigation source through the soil or from precipitation or surface runoff. Contamination by minerals, such as calcium or boron, will usually be found in well water. However, because streams, lakes, and ditches also may have mineral contaminants, any potential site must have its water sources evaluated for mineral content and concentration.
Water from any open source (lake, stream, or ditch) may contain weed seeds. High concentrations of these can lead to unwanted vegetation in seedbeds and cover crops, which is a major problem. Special, well-designed screening devices can alleviate this problem.
Water-borne diseases can infect root systems and foliage. Chemical water treatment may be necessary if pathogens are present, such as Phytophthora—a fungus causing root disease.
Water Runoff
Modern nurseries must also consider how surface water runoff will be handled. Excess water from irrigation or precipitation can carry chemical or biological contaminants or eroded soil. While there are several ways to limit surface runoff, some municipalities may require growers to construct a retention pond to capture runoff in an effort to prevent contamination of waterways or groundwater (Figure 6.1.1) (Fulcher, 2013). This runoff can be re-used for irrigation (Figure 6.1.2).
Topography
The area for nursery beds should be level, or nearly so. A slight slope (2% maximum) is beneficial for better surface drainage, but slopes greater than 2% can cause erosion, necessitating expensive control measures, as well as result in leaching of soluble fertilizer salts (Wilde, 1958). Topography can also impact the ability to use farm equipment (Figure 6.1.3) and irrigation systems. Low areas may be poorly drained and can be more susceptible damage caused by frost pockets (Acquaah, 2009).
The importance of aspect will depend on the latitude and altitude of the nursery site. In most of the temperate zone, eastern and southeastern aspects should be avoided because of greater frost danger, as well as southern and southwestern aspects because of excessive dryness during periods of drought. Where irrigation is available, southern aspects in northern latitudes at high elevations are best because of their greater warmth. For most sites, though, a northwestern aspect is best because vegetative growth starts later in spring and is not subjected to injury by frost. Water loss through evaporation from the soil surface is not so rapid on northwestern aspects.
Previous Land Use
Past use of the land may influence its value as a potential nursery site. For example, past practices that have altered soil acidity or caused toxic chemicals to accumulate will be detrimental to field-grown production. If the site has been altered, the grower should determine what was done when.
Site Production Potential
Many nursery sites have been selected and developed with little or no allowance made for future expansion. Regardless of how remote it may seem; expansion should be considered. To do so, the site-selection team must examine areas adjacent or close to the property.
Proximity to Customers, Labor, and Services
Proximity of the nursery to customers, work force, transportation, utilities, and facilities for people are all important components of the supply chain. These factors should be evaluated by the site-selection team. Locating the site geographically close to customers seems to be most judicious, although, with the advent of transportation systems and refrigerated trucks, this is not as necessary as it once was. Often, other criteria prevail.
Customers
Without customer demand, the nursery business cannot succeed. The grower should ask themselves several questions as they plan the nursery, including: Will the business serve local customers or ship plants to online customers? Will the material be sold wholesale to other nurseries or landscapers, or directly to individuals as retail? Does the local area support a large enough population with sufficient income for targeted sales? (McMahon, 2020).
Labor Force
The nursery should be within easy commuting distance—about 35 miles—of an adequate, dependable labor supply. The number of workers needed varies widely, depending on size of the nursery, extent of mechanization, amount of work contracted out, degree of chemical weed control, and type of stock grown.
Transportation
A good transportation network is essential. In the case of a retail nursery, there must be a way to receive plant material from growers. In the case of a wholesale or an online order nursery, there must be a way to deliver material to customers. Climate-controlled transportation equipment is critical when delivering plants over long distances. County or state roads that are well traveled, maintained, and connected to freeways will aid the transport of both plants and people.
Utilities
Telephone, electric power, and other utilities required for nursery operation must be already available or easily secured. The history of these utilities must be evaluated, along with their current cost, supply, and reliability.
Land Availability and Cost
Are the sites under consideration actually for sale and within the price range given to the selection team? What are the owners' sale stipulations? Look at total developed cost. Unimproved land may initially cost less but require such large capital outlays for development that ultimate total cost may be more. Land that may initially cost more, on the other hand, may be developed to the point that few subsequent improvements are needed, and total cost may be less.
Layout and Development
Modified from "Nursery-Site Selection, Layout, and Development" by F.E. Morby, USDA Forest Service is in the Public Domain
The Team Approach
Like site selection, layout and development benefit from the team approach. The development team should consist of the nursery manager; civil, electrical, and mechanical engineers; landscape and structural architects; and consultants for soils, irrigation, subsurface drainage, or other areas where on-site team expertise is weak or lacking.
It's a good idea to visit similar facilities for comparison. It is expensive to develop a new nursery, and any new technology either already developed or under consideration must be evaluated. New ideas always surface when other nurseries are visited and when both positive and negative sides of a particular site or procedure are discussed.
Access and Traffic Flow
The nursery should be as compact as possible to minimize the length of the boundary fence and reduce the time lost moving from one part of the nursery to another (Aldhous, 1972).
Roads provide access to the site and to growing fields. When the site is developed, all access roads should be paved; they must be capable of taking heavy "semi" truck and tractor traffic in all kinds of weather. Parking areas must be evaluated and particular attention given to pedestrian and vehicle traffic flows. While considering connecting points (entries and exits) to existing road systems, the development team should solicit input from the local community.
Administrative Site
The administrative site includes administrative offices; storage areas for equipment, trees, seed, pesticides, other chemicals, and fuels; shops; a fuel-dispensing station; an employee center; and processing facilities. The type, number, and location of required buildings can be determined with the team approach. Other administrative development could include employee-enrichment areas (in the form of parklike surroundings), holding areas for irrigation water or soil amendments, a culled-plant disposal area, and an area for holding scrap material and used equipment until sale is possible (potential aesthetic conflicts with neighbors may arise in this last case).
Although possible future expansion must always be kept in mind, the administrative complex must optimize the use of space to avoid being spread out. The results of poor or inadequate planning can cause the manager and staff considerable anxiety in future years.
The Master Plan
Once agreement has been reached on placement of all structures and development begins, a master plan—a dynamic tool—must be made to document the team decision. Once the development team has disbanded, this plan will stand as an illustrated document of site layout, indicating growing areas, roads, buildings, outdoor storage areas, reservoirs, streams, fences, neighbors, possible expansion areas for buildings, and other site development. The master plan is not cast in concrete and should be updated as management needs change.
Development Program
To properly develop a site, an action plan must be prepared. One approach is to construct a critical-path chart that shows events and operations on a timeline (Figure 6.1.4). Tree-production scheduling must be coordinated with site development. Structures that are needed first must be built first.
Throughout nursery development, the action plan is continuously reviewed—by an individual, a team, or a contractor—and revised, as needed. Critical factors that may have been overlooked initially are identified and incorporated. It is important for everything to be viewed objectively and in proper perspective.
Budgeting and Accountability
Budgeting is critical and must have highest priority in the development process. Budgets should be planned 2 to 3 years in advance to ensure that funding, people, and facilities will be available when needed. The budget and the action plan must be developed together. If shortages of funds or people are anticipated, construction may have to be delayed or other alternatives sought.
The process of "fixing accountability" identifies objectives and action steps (Morrisey, 1976), as well as the individuals responsible for their accomplishment in the outlined time frames. Responsibilities must be reasonable, however, and should be adjusted if necessary to ensure that the work can realistically be completed.
Dig Deeper
"Nursery-Site Selection, Layout, and Development" by F.E. Morby, USDA Forest Service is in the Public Domain
"Layout and Design Considerations for a Wholesale Container Nursery" by T.H. Yeager & D.L. Ingram, University of Florida IFAS Extension. Copyright © University of Florida IFAS Extension. Used with permission.
Attribution and References
Attribution
Lesson modified from "Nursery-Site Selection, Layout, and Development" by F.E. Morby, USDA Forest Service is in the Public Domain
Title image "20150515-NRCS-LSC-0923" by USDAgov is licensed under CC PDM 1.0
References
Acquaah, G. (2009). Horticulture principles and practices (Fourth edition). Pearson Education, Inc.
Aldhous, J. R. (1972). Nursery practice. Her Majesty's Stationery Office, London. Forestry Commun. Bull. 43. 184 p.
Chavasse, C. G. R. (1980). The means to excellence through plantation establishment: the New Zealand experience. Pages 119- 139 in Proc., Forest plantations, the shape of the future. Weyerhaeuser Science symposium, April 30-May 3, 1979. Weyerhaeuser Co., Tacoma, Washington.
Fulcher, A. & Fernandez, T. (2013). Sustainable nursery irrigation management series: Part III. Strategies to manage nursery runoff. University of Tennessee Extension. Retrieved June 2021 from, https://extension.tennessee.edu/publications/Documents/W280.pdf
Krugman, S. L., and E. C. Stone. (1966). The effect of cold nights on the root-generating potential of ponderosa pine seedlings. Forest Science. 12:451-459.
Kepner-Tregoe. Inc. (1973). Problem analysis and decision making. Princeton Research Press, Princeton, New Jersey. 99 p.
McMahon, M. (2020). Plant science: Growth, development, and utilization of cultivated plants (Sixth edition). Pearson Education, Inc.
Morby, F.E. (1984). Nursery-site selection, layout, and development. Forest nursery manual: Production of bareroot seedlings. Retrieved June 2021 from https://rngr.net/publications/nursery-manuals/fnm/Chapter%202
Morrisey, G. L. (1976). Management by objectives and results in the public sector. Addison-Wesley Publishing Co., Reading, Massachusetts. 278 p.
Raulston, J.C. & Tripp, K.E. (1994). Exploring the complexities of plant hardiness. Arnolida, 54(3): 22-31.
Van den Driessche, R. (1969). Forest nursery handbook. B. C. Forest Service. Victoria. Res. Note 48. 44 p.
Wilde, S. A. (1958). Forest soils - Their properties and relation to silviculture. Ronald Press, New York. 537 p.
|
oercommons
|
2025-03-18T00:39:13.719444
|
Anna McCollum
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/85008/overview",
"title": "Statewide Dual Credit Introduction to Plant Science, Nursery Production, Nursery Site Selection",
"author": "Textbook"
}
|
https://oercommons.org/courseware/lesson/91236/overview
|
Retailers as Channels of Distribution
Overview
Provided by: Lumen Learning. License: CC BY: Attribution
Outcome: Retailers As Channels of Distribution
What you’ll learn to do: describe types of retailers and explain how they are used as a channel of distribution
Retailing is important for marketing students to understand for two main reasons. First, most channel structures end with a retailer. While products may pass through a wholesaler or involve a broker or agent, they also include a retailer. Second, retail offers an immense number of job opportunities. Today in the U.S., there are 3,793,621 retail establishments that support 42 million jobs. Retail also contributes $2.6 trillion to the U.S. gross domestic product.1
You can view the number of jobs and retail presence in your state at the National Retail Federation (NRF).
Who are these retailers? The NRF posts an annual list of the top one hundred retailers by retail sales. The top ten are listed in the table below.2
| Rank | Retailer | U.S. Headquarters | 2018 Retail Sales (billions) |
| 1 | Walmart Stores | Bentonville, Arkansas | $387.66 |
| 2 | Amazon.com | Seattle, Washington | $120.93 |
| 3 | The Kroger Co. | Cincinnati, Ohio | $119.70 |
| 4 | Costco | Issaquah, Washington | $101.43 |
| 5 | Walgreens | Deerfield, Illinois | $98.39 |
| 6 | The Home Depot | Atlanta, Georgia | $97.27 |
| 7 | CVS Health Corporation | Woonsocket, Rhode Island | $83.79 |
| 8 | Target | Minneapolis, Minnesota | $74.48 |
| 9 | Lowe’s Companies | Mooresville, North Carolina | $64.09 |
| 10 | Albertsons Companies | Boise, Idaho | $59.71 |
In this section you’ll learn more about the retail channel and the strategies that drive its growth.
Learning Activities
- Reading: Define Retailing
- Reading: Types of Retailers
- Reading: Retail Strategy
- “Retail's Impact.” NRF. Accessed September 24, 2019. https://nrf.com/retails-impact.
- “STORES Top Retailers 2019.” NRF. NRF. Accessed September 24, 2019. https://stores.org/stores-top-retailers-2019/.
LICENSES AND ATTRIBUTIONS
CC LICENSED CONTENT, ORIGINAL
- Retailers As Channels of Distribution. Provided by: Lumen Learning. License: CC BY: Attribution
Reading: Define Retailing
Introduction
Retailing involves all activities required to market consumer goods and services to ultimate consumers who are purchasing for individual or family needs.
By definition, B2B purchases are not included in the retail channel since they are not made for individual or family needs. In practice this can be confusing because many retail outlets do serve both consumers and business customers—like Home Depot, which has a Pro Xtra program for selling directly to builders and contractors. Generally, retailers that have a significant B2B or wholesale business report these numbers separately in their financial statements, acknowledging that they are separate lines of business within the same company. Those with a pure retail emphasis do not seek to exclude business purchasers. They simply focus their offering to appeal to individual consumers, knowing that some businesses may also choose to purchase from them.
We typically think of a store when we think of a retail sale, even though retail sales occur in other places and settings. For instance, they can be made by a Pampered Chef salesperson in someone’s home. Retail sales also happen online, through catalogs, by automatic vending machines, and in hotels and restaurants. Nonetheless, despite tremendous growth in both nontraditional retail outlets and online sales, most retail sales still take place in brick-and-mortar stores.
The Retail Industry
The retail industry covers an enormous range of consumer needs. In reporting on common trends across the major retail segments, the National Retail Federation covers sixteen different categories. As shown below, these categories are not necessarily store types, but they show the breadth of products offered through the retail chain.1
| Category | Sample Retailers |
|---|---|
| Auto Aftermarket | Advance Auto Parts, AutoZone, Pep Boys |
| Department Stores | Kohl’s, Macy’s, Nordstrom, Saks Fifth Avenue |
| Drug Stores | CVS, Rite Aid, Walgreen’s |
| Entertainment and Consumer Electronics | AT&T, Apple, Barnes & Noble, BestBuy, GameStop, Toys R Us |
| Footwear | DSW, Foot Locker |
| General Apparel | Forever 21, Gap, H&M, Old Navy, TJ Maxx, Urban Outfitters |
| Health and Beauty | Bath and Body Works, Sally Beauty, Sephora, Ulta |
| Hobby and Craft | Michael’s, Guitar Center, Jo-Ann Fabrics |
| Home Improvement and Hardware | Home Depot, Ikea, Pier 1 Imports, True Value, Williams-Sonoma |
| Jewelry and Accessories | Charming Charlie’s, Coach, Piercing Pagoda, Signet, Tiffany & Co. |
| Mass Merchants | Amazon, Costco, Target, Walmart |
| Restaurants | Chipotle, KFC, McDonald’s, Olive Garden, Starbucks |
| Small-Format Value | Big Lots, Dollar General, Dollar Tree, Family Dollar |
| Sporting Goods and Outdoor | Bass Pro Shops, Cabela’s, Dick’s, Sports Authority, REI |
| Supermarkets | Albertson’s, Kroger, QFC, Safeway, Publix, Whole Foods |
| Women’s Apparel | Ann Taylor, Lane Bryant, Talbot’s, Victoria’s Secret |
The retail industry is designed to create contact efficiency—allowing shoppers to buy what they want efficiently with a smaller number of transactions. This design doesn’t come from a master retail plan. It’s driven by market forces. When a retailer sees an opportunity to expand its offering to increase purchases from customers in one location, it will expand its offering to meet the opportunity. When Barnes & Noble adds Starbucks coffee shops to its locations, customers visit more frequently and stay longer, increasing the chance of additional purchases. Costco recognized that busy holiday shoppers would rather buy a Christmas tree as part of a larger convenience purchase than have a focused (and less convenient) buying experience at a Christmas tree lot. Such opportunities cause retailers to expand their offerings, creating greater contact efficiency for consumers.
Given this logic and opportunity, why doesn’t every retailer become a Walmart Super Store filled with every possible product? Like all organizations that market effectively, retailers shape their offerings to a target buyer. Retailers must also consider the particular shopping experience a buyer is seeking in that moment or context. One experience isn’t right for everyone at the same time; nor are all “experiences” compatible. For example, a buyer is expecting a different buying experience when she fills her car’s gas tank and when she stays at a luxury resort.
Retailers define their target buyer segments, identify the service outputs that those segments require, and match their offerings to provide value to each target segment.
LICENSES AND ATTRIBUTIONS
CC LICENSED CONTENT, ORIGINAL
- The Retail Industry. Provided by: Lumen Learning. License: CC BY: Attribution
- Revision and adaptation. Provided by: Lumen Learning. License: CC BY: Attribution
CC LICENSED CONTENT, SHARED PREVIOUSLY
- Chapter 10, Channel Concepts: Distributing the Product, from Introducing Marketing. Authored by: John Burnett. Located at: . License: CC BY: Attribution
- Powell's Books. Authored by: Thomas Hawk. Located at: https://www.flickr.com/photos/thomashawk/2987249389/. License: CC BY-NC: Attribution-NonCommercial
Reading: Types of Retailers
Beyond the distinctions in the products they provide, there are structural differences among retailers that influence their strategies and results. One of the reasons the retail industry is so large and powerful is its diversity. For example, stores vary in size, in the kinds of services that are provided, in the assortment of merchandise they carry, and in their ownership and management structures.
The U.S. Census Bureau indicates that 94.5 percent of retail companies have only one location or store.1 More than one million retail businesses in the U.S. have fewer than one hundred employees. Most retail outlets are small and have weekly sales of just a few hundred dollars. A few are extremely large, having sales of $500,000 or more on a single day. In fact, on special sale days, some stores exceed $1 million in sales.
This diversity in size and earnings is reflected in the range of different ownership and management structures, discussed below.
Department Stores
Department stores are characterized by their very wide product mixes. That is, they carry many different types of merchandise, which may include hardware, clothing, and appliances. Each type of merchandise is typically displayed in a different section or department within the store. The depth of the product mix depends on the store, but department stores’ primary distinction is the ability to provide a wide range of products within a single store. For example, people shopping at Macy’s can buy clothing for a woman, a man, and children, as well as house wares such as dishes and luggage.
Chain Stores
The 1920s saw the evolution of the chain store movement. Because chains were so large, they were able to buy a wide variety of merchandise in large quantity discounts. The discounts substantially lowered their cost compared to costs of single unit retailers. As a result, they could set retail prices that were lower than those of their small competitors and thereby increase their share of the market. Furthermore, chains were able to attract many customers because of their convenient locations, made possible by their financial resources and expertise in selecting locations.
Supermarkets
Supermarkets evolved in the 1920s and 1930s. For example, Piggly Wiggly Food Stores, founded by Clarence Saunders around 1920, introduced self-service and customer checkout counters. Supermarkets are large, self-service stores with central checkout facilities. They carry an extensive line of food items and often nonfood products. There are 37,459 supermarkets operating in the United States, and the average store now carries nearly 44,000 products in roughly 46,500 square feet of space. The average customer visits a store just under twice a week, spending just over $30 per trip. Supermarkets’ entire approach to the distribution of food and household cleaning and maintenance products is to offer large assortments these goods at each store at a minimal price.
Discount Retailers
Discount retailers, like Ross Dress for Less and Grocery Outlet, are characterized by a focus on price as their main sales appeal. Merchandise assortments are generally broad and include both hard and soft goods, but assortments are typically limited to the most popular items, colors, and sizes. Traditional stores are usually large, self-service operations with long hours, free parking, and relatively simple fixtures. Online retailers such as Overstock.com have aggregated products and offered them at deep discounts. Generally, customers sacrifice having a reliable assortment of products to receive deep discounts on the available products.
Warehouse Retailers
Warehouse retailers provide a bare-bones shopping experience at very low prices. Costco is the dominant warehouse retailer, with $138.4 billion in sales in 2018. Warehouse retailers streamline all operational aspects of their business and pass on the efficiency savings to customers. Costco generally uses a cost-plus pricing structure and provides goods in wholesale quantities.
Franchises
The franchise approach brings together national chains and local ownership. An owner purchases a franchise which gives her the right to use the firm’s business model and brand for a set period of time. Often, the franchise agreement includes well-defined guidance for the owner, training, and on-going support. The owner, or franchisee, builds and manages the local business. Entrepreneur magazine posts a list each year of the 500 top franchises according to an evaluation of financial strength and stability, growth rate, and size. The 2019 Top 500 Franchises list by Entrepreneur magazine is led by McDonald’s, Dunkin’ Donuts, Sonic Drive-In, Taco Bell, and the UPS Store.
Malls and Shopping Centers
Malls and shopping centers are successful because they provide customers with a wide assortment of products across many stores. If you want to buy a suit or a dress, a mall provides many alternatives in one location. Malls are larger centers that typically have one or more department stores as major tenants. Strip malls are a common string of stores along major traffic routes, while isolated locations are freestanding sites not necessarily in heavy traffic areas. Stores in isolated locations must use promotion or some other aspect of their marketing mix to attract shoppers.
Online Retailing
Online retailing is unquestionably a dominant force in the retail industry, but today it accounts for only a small percentage of total retail sales. Companies like Amazon and Geico complete all or most of their sales online. Many other online sales result from online sales from traditional retailers, such as purchases made at Nordstrom.com. Online marketing plays a significant role in preparing the buyers who shop in stores. In a similar integrated approach, catalogs that are mailed to customers’ homes drive online orders. In a survey on its Web site, Land’s End found that 75 percent of customers who were making purchases had reviewed the catalog first.2
Catalog Retailing
Catalogs have long been used as a marketing device to drive phone and in-store sales. As online retailing began to grow, it had a significant impact on catalog sales. Many retailers who depended on catalog sales—Sears, Land’s End, and J.C. Penney, to name a few—suffered as online retailers and online sales from traditional retailers pulled convenience shoppers away from catalog sales. Catalog mailings peaked in 2009 and saw a significant decrease through 2012. In 2013, there was a small increase in catalog mailings. Industry experts note that catalogs are changing, as is their role in the retail marketing process. Despite significant declines, U.S. households still receive 11.9 billion catalogs each year.3
Nonstore Retailing
Beyond those mentioned in the categories above, there’s a wide range of traditional and innovative retailing approaches. Although the Avon lady largely disappeared at the end of the last century, there are still in-home sales from Arbonne facial products, cabi women’s clothing, WineShop at Home, and others. Many of these models are based on the idea of a woman using her personal network to sell products to her friends and their friends, often in a party setting.
Vending machines and point-of-sale kiosks have long been a popular retail device. Today they are becoming more targeted, such as companies selling easily forgotten items—such as small electronics devices and makeup items—to travelers in airports.
Each of these retailing approaches can be customized to meet the needs of the target buyer or combined to span a range of needs.
- U.S. Census Bureau, 2007 Economic Census.
- Ruiz, Rebecca R. “Catalogs, After Years of Decline, Are Revamped for Changing Times.” The New York Times. The New York Times, January 25, 2015. http://www.nytimes.com/2015/01/26/business/media/catalogs-after-years-of-decline-are-revamped-for-changing-times.html.
- Geller, Lois. “Why Are Printed Catalogs Still Around?” Forbes. Forbes Magazine, October 16, 2012. http://www.forbes.com/sites/loisgeller/2012/10/16/why-are-printed-catalogs-still-around/.
LICENSES AND ATTRIBUTIONS
CC LICENSED CONTENT, ORIGINAL
- Revision and Adaptation. Provided by: Lumen Learning. License: CC BY: Attribution
CC LICENSED CONTENT, SHARED PREVIOUSLY
- Chapter 10, Channel Concepts: Distributing the Product, from Introducing Marketing. Authored by: John Burnett. Provided by: Global Text. Located at: . License: CC BY: Attribution
- Bike & Skate. Authored by: Karol Franks. Located at: https://www.flickr.com/photos/karolfranks/6586652573/. License: CC BY-NC-ND: Attribution-NonCommercial-NoDerivatives
- Piggly Wiggly VA. Authored by: Acroterion. Provided by: Wikimedia Commons. Located at: https://commons.wikimedia.org/wiki/File:Piggly_Wiggly_VA1.jpg. License: CC BY-SA: Attribution-ShareAlike
- Ankara: Panora Shopping Mall. Authored by: Jorge Franganillo. Provided by: flickr. Located at: https://flic.kr/p/eDYfQW. License: CC BY: Attribution
- iPod vending machine. Authored by: Greg Hewgill. Provided by: flickr. Located at: https://flic.kr/p/68Mbr5. License: CC BY: Attribution
Reading: Retail Strategy
Just when we have finally mastered the marketing mix that includes the four Ps, we arrive at the retail strategy. The retail marketing strategy includes all of the elements of the traditional marketing mix:
- Retailers buy product from producers or wholesalers that will most appeal to their target market.
- Retailers set a price that delivers value for the product and the complete shopping experience.
- Retailers promote their offering, which includes the shopping experience, the products, the pricing, and broadly, the retail brand.
- Retailers create the right place, which is the point of purchase for the buyer.
In delivering the best retail experience through the right place, two additional Ps come into play: presentation and personnel.
Presentation
Think of a physical store where you enjoy shopping. What is it about the store that you like? You might like the way the store looks, feels, sounds, or smells. It might have products that draw you in and make you want to interact with them. You may just like the store because it’s familiar and convenient—you know where to find the things you need. All of these descriptions fall into two categories. They refer either to the atmosphere of the store or the layout of the store.
The atmosphere describes the feeling, tone, or mood of the store. Often, as a shopper it is difficult to identify exactly what creates the atmosphere in a good shopping experience. (It is much easier in a bad shopping experience.) The store’s decor plays a role in the atmosphere. Are the fixtures decorative or merely functional? Is the shopper invited to linger on a couch or inviting chair, or is he encouraged to simply purchase and leave?
One important element of the atmosphere is density. How has the retailer packed elements into the space? Retailers manage the density of employees, fixtures, and merchandise. The shopping experience requires more employees if there is a high need for service or information. High-end clothing sales generally provide a higher level of service, with sales associates available to advise on fit and fashion choices and to bring the shoppers different sizes and clothing options in the dressing room. A car purchase is not one that generally involves the same type or style of service, but there is a high need for information that translates to a higher density of sales employees to explain features, financing, and availability.
The density of merchandise and fixtures also has a significant impact on the atmosphere of the store. If the shoppers value service, or the retail brand requires a high-end experience, then the retailer generally has less density of merchandise and fixtures. If the shopper most values service outputs of assortment and convenience, then the retailer will use a higher density of merchandise. For example, grocery shoppers may have different standards for the quality of fixtures they prefer relative to the price of the grocery items, but generally they prefer a higher-density shopping experience. The shopper is trying to collect many different products from all areas of the store and would rather have shelves stacked than have to wander much farther through a store with more empty space. Convenience is the dominant factor driving the presentation of products.
Finally, the layout, display, and positioning of the merchandise have a significant impact on sales behaviors. Grocers have conducted studies to optimize the layout of the store and the position of items on the shelves. Stores are designed in a logical pattern, so that they are easy to navigate and optimize spending. Higher-margin items are placed at eye level, while those that are inexpensive and commonly purchased are at the bottom of the shelf. The produce section was once the entry point for every grocery store. Today, that spot is more likely to be occupied by high-end novelty items (expensive chocolates, clothing, paper items, floral arrangements). Still, the produce section continues to be the first food section that buyers are steered toward. This is intended to facilitate meal planning before the shopper arrives at the meat and dairy departments.
In a retail environment, the layout is designed to create comfort and convenience and, at the same time, drive sales.
Online Presentation
Moving the presentation to an online shopping experience can be even more difficult. Retail Web sites emphasize site design, navigation, information, and checkout experience. Amazon has set the standard for ease of purchase with its one-click checkout solution. Zappos is well known for providing through, accurate product photos that give a complete view of each product from every angle. Still, the online atmosphere is more difficult to differentiate than the traditional in-store experience.
Personnel
Retail employees are the face of the brand to the shopper. This is true of a sales associate who helps with a purchase decision, a waitperson in a restaurant, a hotel check-in clerk, or a checker in a grocery store who efficiently rings up purchases. Retail employees fill a weighty role in the brand for two reasons. First, they do work that has the potential to add immense value to the purchase process. When an employee is helpful and efficient with the selection and/or purchase of a product, it’s an important and necessary aspect of the buyer’s retail experience. The retail employees working directly with customers have a much more personal and profound impact on the brand experience of each shopper than the senior executives of the company or even store managers, who have less customer contact.
In order to support employees to be successful, effective retailers will:
- Demonstrate care in hiring to ensure that customer-facing employees will represent the retailer’s brand values
- Train employees to be knowledgeable about the products and efficient in their jobs
- Carefully manage operations so that staffing levels match the desired retail experience
- Compensate employees in a way that rewards good service and effective sales
Sales employees are most likely to have some variable compensation or have some portion of their paycheck tied to their ability to drive sales. These incentives can be a direct commission on sales or a less direct financial or benefits bonus for the store meeting its goals.
The following video shares how one retail giant, Costco, understands the importance of treating its employees well in order to ensure good customer service and a positive shopping experience every time.
You can view the transcript for “What’s the secret to Costco’s big box success?” (opens in new window).
LICENSES AND ATTRIBUTIONS
CC LICENSED CONTENT, ORIGINAL
- Retail Strategy. Provided by: Lumen Learning. License: CC BY: Attribution
CC LICENSED CONTENT, SHARED PREVIOUSLY
- Anthropologie Store. Authored by: FASTILY. Provided by: Wikimedia Commons. Located at: https://commons.wikimedia.org/wiki/File:Anthropologie_Walnut_Creek_2_2017-04-29.jpg. License: CC BY-SA: Attribution-ShareAlike
- Grocery Store. Authored by: Diane Webb. Provided by: Pexels. Located at: https://www.pexels.com/photo/grocery-store-piggly-wiggly-produce-small-town-1055443/. License: CC0: No Rights Reserved
- What's the secret to Costco's big box success?. Provided by: CBS News. Located at: https://youtu.be/cysRawnadPc. License: All Rights Reserved. License Terms: Standard YouTube License
|
oercommons
|
2025-03-18T00:39:13.768040
|
03/22/2022
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/91236/overview",
"title": "Statewide Dual Credit Principles of Marketing, Place: Distribution Channels, Retailers as Channels of Distribution",
"author": "Anna McCollum"
}
|
https://oercommons.org/courseware/lesson/93468/overview
|
Defining the Message
Overview
Provided by: Lumen Learning. License: CC BY: Attribution
Outcome: Defining the Message
What you’ll learn to do: discuss how to develop effective messaging for marketing communications
At the center of any successful marketing activity is a message. Without a solid, consistent message, your marketing efforts are like a compass without an arrow: there is nothing to point your target audiences in the direction you want them to go.
Good messaging takes time and attention to develop, but this effort pays a huge dividend down the road—when your marketing activities have their desired effect on the hearts, minds, and wallets of the people you want to reach.
The specific things you’ll learn in this section include:
- Explain the role of consistent messaging in strengthening the impact of marketing communications
- Outline a standard framework for developing messaging for marketing communications
- Explain the importance of including a clear call to action in marketing communications
Learning Activities
- Reading: Defining the Message
LICENSES AND ATTRIBUTIONS
CC LICENSED CONTENT, ORIGINAL
- Outcome: Defining the Message. Provided by: Lumen Learning. License: CC BY: Attribution
Reading: Defining the Message
Why the Message Matters
A clear, consistent message can be the difference between a phenomenally successful marketing campaign and an utter waste of time and money. If you, as a marketer, have not defined your message clearly, how likely is it that your target audiences will get the message you want them to hear? Answer: Not very likely.
With IMC campaigns bringing together multiple communication tools and touch points, the impact of a consistent, effective message is compounded when it reaches the people you’re targeting again and again through different channels. Conversely, in the absence of a clear message, a campaign results in miss after miss after miss in terms of getting your message to your target audience—and it means wasted effort and resources.
The Role of Messaging
In marketing, the term “messaging” refers to how an organization talks about itself and the value it provides. Related to positioning, messaging is an approved set of key points or messages an organization uses to communicate about something with a target audience. Messaging translates a positioning statement into a set of convincing “key message” statements. Marketers use these statements to develop materials for marketing communications such as ad slogans, advertising copy, social media posts, press releases, presentation scripts, and so forth. Messaging documents are a blueprint for what all the other materials–and people–should communicate.
Organizations may create messaging for different purposes. Corporate messaging communicates about the purpose and value a company provides to the market. Brand messaging focuses on how and what to communicate about a company, product, or service brand. Product messaging expresses key selling points about a product. Crisis messaging outlines talking points for how an organization communicates about an unfortunate development, such as a service interruption or a public scandal.
Messaging ensures that everyone in an organization who needs to communicate something with the market can do it with a common set of messages and a common understanding of what the market should hear from them. While messaging is usually created by the marketing team, it may be used by individuals and teams across a company, from executive leaders to product managers, sales representatives and other groups, in addition to the marketing team itself.
Messaging is an essential ingredient for a successful marketing campaign. A campaign may use existing messaging if its goal is tied to a topic for which messaging has already been developed. For example, existing brand messaging might be used to develop a brand-awareness campaign. If no suitable messaging exists, marketers may need to develop key messages expressly for a campaign.
Developing Key Messages
The key messages that make up a messaging document should do the following:
- Express the main idea you want people to understand and remember about your offering
- Resonate with the audience you are targeting, such that they pay attention and feel what you are saying matters
- Articulate clearly and concisely what you need to communicate about: e.g., what you stand for, why you are different, what value you offer, what problems you solve, etc.
The message content, as well as the voice, tone, and style of the message, may vary widely, depending on the organization’s identity and what it wants to accomplish with the communication. All of these elements factor into the key messages and the creation of marketing communication artifacts based on the messaging.
Start with the Basics: What, Who, Why?
Message development starts with doing your homework about what the organization needs to accomplish. Revisit the company goals, objectives, and the marketing strategy to confirm the outcomes that the messaging is designed to help achieve. Consult any related positioning statements the organization has developed, because positioning lays out the foundation for what the organization wants to communicate. As you develop messaging, it’s also a good idea to review any brand-platform content, since that content can help reinforce the organization’s identity, voice, and values.
Next, confirm the audience(s) for the messaging: who are the target segments and stakeholders you need to reach? Some messaging documents outline different sets of key messages for different audiences, depending on what points are most important or convincing for the audience. For example, when company leaders must communicate publicly about poor quarterly earnings, they develop one set of key messages for investors, another set of key messages for employees, and a third set for customers. All these messages are related to one another, but the most important messages for an investor to hear may be different from what employees need to hear.
Identify key words and ideas you want to associate with your organization, product, service, or offering. These words and phrases may figure prominently in the messaging you develop, to help it stand out and differentiate your organization. Also, conduct a competitive messaging analysis to capture what key messages, words, and concepts other organizations are using. Your messaging should avoid sounding like everybody else.
Draft Message Statements
With your audience and objectives in mind, begin drafting key message statements. If you could make only a few key points to your target audience, what would those points be? As you write these message statements, keep the following criteria in mind. Key messages should be:
- Concise: Key message statements should be clear and concise, ideally just one sentence long–but not a long, run-on sentence.
- Simple: Key messages should use language that is easy for target audiences to understand. You should avoid acronyms, jargon, and flowery or bureaucratic-sounding language.
- Strategic: Key messages should differentiate your organization and what you stand for, while articulating the value proposition or key benefits you offer.
- Convincing: Messaging should include believable, meaningful information that creates a sense of urgency and stimulates action. Message wording should be decisive and active, rather than passive.
- Relevant: Key messages should matter to the audience; they should communicate useful, relevant information that the audience finds appealing not only on a logical or rational level but also on an emotional level.
- Memorable: Key messages should stick in the mind, so the impression they make is easy to recall.
- Tailored: Messaging must communicate effectively with intended target audiences. This means the messaging should reflect the target audience’s unique needs, priorities, issues, terminology, relationship to the organization, and other distinguishing factors that might help the messaging better communicate with that audience.
A tip: Don’t worry too much about word-smithing as you develop a first draft of key messages. Get your initial thinking down on paper quickly, and then go back to check against the criteria above as you refine the wording. Remember, you only need a handful of key messages—just one to three well-crafted statements—so don’t slave over trying to fill an entire page.
Organize a Messaging Framework
Once you have drafted an initial set of key messages, it is helpful to prioritize and organize them into a framework that helps you tell a coherent story. Marketers use a variety of different frameworks for this purpose. A simple, standard messaging framework is illustrated in the figure below.
This framework includes key messaging components introduced elsewhere in this course: the brand promise, positioning statement, and target audience. By bringing these elements into the messaging document, it is easy to spot disconnects or confirm alignment between the day-to-day talking points (the primary message and message pillars), the audience, and what the organization stands for (as expressed in the brand promise and positioning statement).
The primary message is sometimes referred to as an elevator pitch. Think of it as the one to three sentences you would say to a member of your target audience if you had just thirty seconds with them in an elevator. In that short time, you need to get across the core ideas. As you review the initial key messages you drafted, identify the most important ideas. Refine them into a concise statement that expresses your primary message.
To support this primary message, identify one to three message pillars that further substantiate the primary message or elevator pitch. When the elevator pitch is expressing a value proposition, the message pillars are usually the key benefits delivered by the value proposition. When the elevator pitch is arguing a position, the message pillars are the key reasons the target audience should believe what is being argued. To identify your message pillars, review the initial messages you drafted. It is likely that your initial work captures some of those pillars or arguments that provide great support for your primary message.
For each message pillar, identify at least three convincing proof points, or reasons the target audience should believe what you tell them. Proof points may come from a variety of sources: actual statistics or data points from research or your customer base; product features and the benefits they deliver; customers’ success stories; and so forth. Their purpose is to provide evidence and add credibility to the messages you want to communicate.
As marketers turn messaging into marketing communication artifacts, the proof points also provide ideas for marketing content: case studies, white papers, advertisement copy, and so forth. They help fill out details around the messaging story you are telling to your target segment(s).
Finally, add a call to action. A call to action is an instruction to the target audience about what you want them to do, once they have heard and digested your messages. Usually it is an imperative verb: Register now. Try this new product. Visit this place. Vote for this person. Although each individual marketing communication piece you create for an IMC campaign might have its own specific call to action, it is helpful to decide on an overarching call to action that identifies the behavioral change you want to incite in your target audience. This call to action serves an important role of making sure that the messages do a good job of convincing the target audience to change their behavior and do what you want them to do. If the messaging doesn’t seem powerful enough to convince people to take action, you need to revisit the messaging and make it more compelling.
The primary purpose of message architecture is to help you make sure that everything you communicate ultimately ties back to the major points you want audiences to know and believe about you. As you finish filling out your message architecture, review it and check for alignment at each level. Each level of the architecture should provide consistency and support for the other levels. If you spot disconnects, work to refine the messaging so there is strong alignment.
Refine Your Work
After completing your message architecture, set it aside for a day or so. Then come back and go through the following checklist. Make revisions and refinements where needed.
- Alignment: Recheck your messaging for alignment. Make sure all levels of the messaging framework are consistent with one another.
- Hearts and Minds: Identify where your messaging is working at a rational level and where it’s working at an emotional level. To be compelling enough to spark a change in behavior, it must appeal to both.
- Strategy: Confirm that the messaging complements your organizational, marketing, and brand strategies. If it isn’t getting you further along those paths, it isn’t doing its job.
- Differentiation: Review your messaging with competitors in mind. Your messaging should set you apart and express messages only you can credibly own. It should be more than just “me, too” catch-up to the competition.
- Tone: When you read your messages out loud, your language should sound natural and conversational. Your messaging should ring true; it should sound like it genuinely comes from your organization and the people who represent you.
- Clarity: If parts of your messaging sound vague or unclear, look for ways to reword them to make them more concise and concrete. People hearing the message should easily understand exactly what you mean.
- Inspiration: Your messaging should motivate and inspire your target audiences to take action. If it isn’t compelling enough to do that, you need to make it stronger.
Once you have completed your messaging framework, test the messages with colleagues and internal stakeholders, as well as with members of your target segment(s). You can do this formally using marketing research techniques, or you can test the messages informally by using them in conversations and gauging whether they produce the desired reaction. Testing helps you figure out quickly what’s working and where there is room for improvement.
Once your messaging framework is complete, you can apply it immediately to marketing campaigns and IMC activity. You should revisit the messaging periodically to make sure it’s still having an impact on your target audiences and helping you achieve your goals.
A Messaging Framework Example
The simple messaging framework shown in Figure 1, above, is easy to use for a variety of different messaging purposes. It can also easily be adapted to include other elements that marketers decide are important to the organization and alignment of messaging.
HIGHFIVE’S MESSAGING
Let’s take a look at the messaging framework for Highfive, a video conferencing company. The goal of this messaging is to convey the central value proposition of the company and its conferencing product, and it demonstrates good alignment across different components of the messaging.1
Brand Promise: Video conferencing you can actually love
Positioning Statement: Highfive is the first video conferencing product designed to connect every employee and every conference room in your entire company
Target Audience: 1, C-level Executive (influencer); 2, Director of IT (buyer); and 3, End-user (user)
Mission: Our mission is to make every conversation face-to-face
Tone of Voice: Empowering, progressive, human, and cheeky
Elevator Pitch: Highfive is video conferencing you can actually love. We believe teams work best face-to-face. That’s why we designed the first video conferencing product designed to connect every person and room in an organization. Highfive provides an all-in-one video conferencing hardware device that plugs into any TV screen, turning any ordinary meeting room into a video room. Highfive also provides cloud apps, which allow employees and guests to simply click a link from any laptop or mobile device and instantly connect face-to-face with anyone, anywhere. The hardware device costs the same as a high-end iPad and the cloud apps are free. We think video shouldn’t be a boardroom luxury. It should be available everywhere.
Brand Pillars
Easy
Headline benefits: Highfive is beautifully simple video conferencing you can start or join with a single click.
Supporting examples: Join calls from your calendar, SMS, or email by clicking a URL, hand off video calls from your personal device to a meeting room TV with a swipe or click—no remote control needed. 5-minute plug and play setup.
Everywhere
Headline Benefits: Twenty conference rooms for the price of one Cisco or Polycom system.
Supporting Examples: Comparable systems cost, about 15 thousand per room. At the price of an iPad, Highfive can be deployed in every room. Free apps let people stay connected at their desks or on the go.
Enterprise
Headline Benefits: Built for businesses, not social networking.
Supporting Examples: Must sign up with work email address, domain-based security model, enterprise reliability and security built by the same people that built Google Apps for Business.
In this example, marketers have left out the call to action, but they have introduced other components around which they want strong alignment: the company mission and brand voice. The rather long elevator pitch is well supported by clear, compelling message pillars, and the messaging offers ample proof points in the form of product features that substantiate the messaging claims. The messaging itself offers both rational reasons to believe the message (e.g., “simple conferencing you can start or join with a single click”) and emotional benefits to inspire action (e.g., “video conferencing you can actually love . . .”). Finally, the overall tone of the messaging demonstrates strong alignment with the company’s brand identity.
- “How to Create Brand Messaging That Really Resonates.” Salesforce Pardot, February 2, 2015. https://www.pardot.com/blog/how-to-create-brand-messaging-that-really-resonates/.
LICENSES AND ATTRIBUTIONS
CC LICENSED CONTENT, ORIGINAL
- Reading: Defining the Message. Authored by: Lumen Learning. License: CC BY: Attribution
CC LICENSED CONTENT, SHARED PREVIOUSLY
- Important Message. Authored by: Patrick Denker. Located at: https://www.flickr.com/photos/pdenker/6001236724/. License: CC BY: Attribution
|
oercommons
|
2025-03-18T00:39:13.807264
|
06/06/2022
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/93468/overview",
"title": "Statewide Dual Credit Principles of Marketing, Promotion: Integrated Marketing Communication (IMC), Defining the Message",
"author": "Anna McCollum"
}
|
https://oercommons.org/courseware/lesson/93465/overview
|
Why it Matters
Overview
Teacher resources for Unit 13 can be found on the next page.
Provided by: Lumen Learning. License: CC BY: Attribution
Why It Matters: Promotion: Integrated Marketing Communication (IMC)
Resources for Unit 13: Promotion: Integrated Marketing Communication (IMC)
Slide Deck - Unit 13: Promotion: Integrated Marketing Communication (IMC)
Simulation Unit 13: “Simulation IMC Hero”
Discussion Assignments and Alignment: Marketing Campaign Concept
Unit 13 Assignment: Complete Marketing Plan
Pacing
The Principles of Marketing textbook contains sixteen units—roughly one unit per week for a 16-week semester. If you need to modify the pace and cover the material more quickly, the following units work well together:
- Unit 1: What Is Marketing? and Unit 2: Marketing Function. Both are lighter, introductory units.
- Unit 15: Global Marketing and Unit 16: Marketing Plan. Unit 16 has more course review and synthesis information than new material per se.
- Unit 5: Ethics can be combined with any unit. You can also move it around without losing anything.
- Unit 8: Positioning and Unit 9: Branding. Companion modules that can be covered in a single week.
- Unit 6: Marketing Information & Research and Unit 7: Consumer Behavior. Companion units that can be covered in a single week.
We recommend NOT doubling up the following units, because they are long and especially challenging. Students will need more time for mastery and completion of assignments.
- Unit 4: Marketing Strategy
- Unit 10: Product Marketing
- Unit 13: Promotion: Integrated Marketing Communication
Did you have an idea for improving this content? We’d love your input.
Learning Outcomes
- Explain integrated marketing communication (IMC) and its connection to the organization’s marketing strategy
- Discuss how to develop effective messaging for marketing communications
- Explain factors to consider when selecting marketing communication methods to execute the strategy
- Describe common methods of marketing communication, their advantages and disadvantages
- Explain how IMC tools support the sales process
- Describe the uses of Customer Relationship Management (CRM) systems for marketing communication purposes
- Explain common tools and approaches used to measure marketing communication effectiveness
- Create a marketing campaign and budget using multiple IMC tools to execute a marketing strategy
Why demonstrate how organizations use integrated marketing communication (IMC) to support their marketing strategies?
The fourth P, promotion, focuses on communicating with target audiences about something: a product, service, organization, idea, or brand. Communication is how you let people know about your offering (product) and why it matters, how much it costs (price), and where to find it (place).
A very wide array of tools is available today to help marketers communicate with their target audiences. Selecting the right tools for the job and combining them into a successful marketing effort is a critically important task for modern marketers. In fact, it has a special name: integrated marketing communication (IMC).
The best way to start learning about IMC is to see it in action.
As you watch the following videos, consider the following questions:
- Who is the target of this IMC effort?
- What core message is being communicated?
- How many and which communication tools are being used?
- How does this IMC activity turn people into active participants instead of remaining passive audience members?
- How is the whole impact of this marketing effort more than just the sum of the individual parts?
IMC Example #1: Small Business Saturday
In 2010, American Express teamed up with millions of small businesses to create a marketing event that quickly became a tradition during the holiday shopping season in the U.S.: Small Business Saturday. To make it successful, American Express and its small business network had to create something out of nothing and then convince consumers to show up.
IMC Example #2: Ariel Fashion Shoot
A jam-squirting robot. A busy mall. Designer clothes. Facebook. No, this isn’t the plot of a sci-fi action movie targeting “tween” girls. It was, at the time in 2011, the largest and most interactive product demonstration ever undertaken, for a laundry detergent called Ariel Actilift. It grabbed attention across Scandinavia and induced thousands of people to participate by playing a silly remote-controlled game. In the process, it also proved the remarkable stain-fighting powers of the laundry detergent at the center of it all.
Understanding Integrated Marketing Communication (IMC)
Not every IMC effort is as elaborate or creative as these examples. The marketers responsible for them imagined and brought into being something that never existed before. But they also help you begin to see what’s possible when you combine creative ideas with the right set of communication tools focused on a common message and particular target segments.
What makes these marketing programs work? When you pull things apart, you see that each of these campaigns starts with clearly articulated goals and audiences. To make their big ideas happen, they use several different marketing tools and techniques that, together, have a larger impact than any of them could manage separately. Each of these marketing activities is also decidedly participatory. It wasn’t enough to simply deliver a message. Each project invited members of the target audience to get involved in the marketing process, and they made the invitations so compelling that people actually did it!
As a marketer, how do you go about creating this type of promotional experience? What elements come together to make it possible?
That’s what this module is about: how marketers design powerful opportunities to engage their target audiences and shape their perceptions and behaviors. The name of this game is IMC.
LICENSES AND ATTRIBUTIONS
CC LICENSED CONTENT, ORIGINAL
- Why It Matters: Integrated Marketing Communication (IMC) Strategy. Provided by: Lumen Learning. License: CC BY: Attribution
ALL RIGHTS RESERVED CONTENT
- AMERICAN EXPRESS OPEN: Small Business Gets An Official Day. Authored by: CannesPredictions. Located at: https://youtu.be/NgmLC6jbxfg?list=PLGhn_uYiaIc8injQ6hgMPXHwmXc3l66Hx. License: All Rights Reserved. License Terms: Standard YouTube license
- Ariel Fashion Shoot case study. Authored by: kristerka. Located at: https://youtu.be/tDQQlT1P_oY. License: All Rights Reserved. License Terms: Standard YouTube License
|
oercommons
|
2025-03-18T00:39:13.842535
|
06/06/2022
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/93465/overview",
"title": "Statewide Dual Credit Principles of Marketing, Promotion: Integrated Marketing Communication (IMC), Why it Matters",
"author": "Anna McCollum"
}
|
https://oercommons.org/courseware/lesson/93470/overview
|
Marketing Communication Methods
Overview
Provided by: Lumen Learning. License: CC BY: Attribution
Outcome: Marketing Communication Methods
What you’ll learn to do: describe common methods of marketing communication, their advantages and disadvantages
A common challenge for people new to marketing is learning about the many marketing communication tools and methods now available and understanding how to use them effectively. Fortunately, most of us have first-hand experience of being on the receiving end of IMC—whether you like it or not, you are a consumer, and you’ve been the target of all kinds of marketing communication. You know the difference between an ad that gets your attention and one you just tune out, for example. You are familiar with the line between “persistent” and “annoying” when it comes to getting marketing-related emails or text messages. You recognize which buy-one-get-one-free offers are a great deal, and which ones seem like a racket.
All this experience will come in handy in this section and later in the course. You’re getting closer to being on the other side of the wall, where you’ll be tasked with using marketing communication methods and tools to devise your own marketing plan (in the last module of this course!). In this section, though, you’ll get a chance to examine each of these marketing communication methods one by one. Fortunately, the underlying principles we’ve discussed up to this point apply to all of them: knowing your audience, defining strategy, setting objectives, crafting the message. Where paths diverge is in the tools themselves: how to design a great ad, how to produce a memorable event, how to get coverage for your organization in the news media, how to use email and social media skillfully for marketing purposes, and so on.
The next several readings provide a general overview of seven important marketing communication methods (shown in Figure 1, below) in common use today. This section will help you become familiar with each method, common tools associated with each method, how to use these methods effectively, and the advantages and disadvantages of each one. Some marketing professionals spend entire careers becoming specialists in one or more of these areas. Other marketers become generalists who are skilled at bringing together different tools–and experts–to execute effective IMC programs.
Figure 1. The Promotion Mix
Whether you think you’re more of a generalist or a specialist, marketing offers great opportunities for creativity and experimentation. There will always be a new idea, strategy, tool, or combination of tactics that marketers can turn into IMC magic for their companies and their customers. As you learn about and gain experience with the basic tools and approaches, you’ll see opportunities to try something new. And you should: in the marketing world, fresh is good!
The specific things you’ll learn in this section include:
- Explain Advertising
- Explain Public Relations
- Explain Sales Promotions
- Explain Personal Selling
- Explain Direct Marketing
- Explain Digital Marketing
- Explain Guerrilla Marketing
Learning Activities
- Reading: Advertising
- Reading: Public Relations
- Reading: Sales Promotions
- Reading: Personal Selling
- Reading: Direct Marketing
- Reading: Digital Marketing
- Reading: Guerrilla Marketing
- Simulation: Integrated Marketing
LICENSES AND ATTRIBUTIONS
CC LICENSED CONTENT, ORIGINAL
- Outcome: Marketing Communication Methods. Provided by: Lumen Learning. License: CC BY: Attribution
Reading: Advertising
Advertising: Pay to Play
Advertising is any paid form of communication from an identified sponsor or source that draws attention to ideas, goods, services or the sponsor itself. Most advertising is directed toward groups rather than individuals, and advertising is usually delivered through media such as television, radio, newspapers and, increasingly, the Internet. Ads are often measured in impressions (the number of times a consumer is exposed to an advertisement).
Advertising is a very old form of promotion with roots that go back even to ancient times. In recent decades, the practices of advertising have changed enormously as new technology and media have allowed consumers to bypass traditional advertising venues. From the invention of the remote control, which allows people to ignore advertising on TV without leaving the couch, to recording devices that let people watch TV programs but skip the ads, conventional advertising is on the wane. Across the board, television viewership has fragmented, and ratings have fallen.
Print media are also in decline, with fewer people subscribing to newspapers and other print media and more people favoring digital sources for news and entertainment. Newspaper advertising revenue has declined steadily since 2000.[1] Advertising revenue in television is also soft, and it is split across a growing number of broadcast and cable networks. Clearly companies need to move beyond traditional advertising channels to reach consumers. Digital media outlets have happily stepped in to fill this gap. Despite this changing landscape, for many companies advertising remains at the forefront of how they deliver the proper message to customers and prospective customers.
The Purpose of Advertising
Advertising has three primary objectives: to inform, to persuade, and to remind.
- Informative Advertising creates awareness of brands, products, services, and ideas. It announces new products and programs and can educate people about the attributes and benefits of new or established products.
- Persuasive Advertising tries to convince customers that a company’s services or products are the best, and it works to alter perceptions and enhance the image of a company or product. Its goal is to influence consumers to take action and switch brands, try a new product, or remain loyal to a current brand.
- Reminder Advertising reminds people about the need for a product or service, or the features and benefits it will provide when they purchase promptly.
When people think of advertising, often product-focused advertisements are top of mind—i.e., ads that promote an organization’s goods or services. Institutional advertising goes beyond products to promote organizations, issues, places, events, and political figures. Public service announcements (PSAs) are a category of institutional advertising focused on social-welfare issues such as drunk driving, drug use, and practicing a healthy lifestyle. Usually PSAs are sponsored by nonprofit organizations and government agencies with a vested interest in the causes they promote.
Advantages and Disadvantages of Advertising
As a method of marketing communication, advertising has both advantages and disadvantages. In terms of advantages, advertising creates a sense of credibility or legitimacy when an organization invests in presenting itself and its products in a public forum. Ads can convey a sense of quality and permanence, the idea that a company isn’t some fly-by-night venture. Advertising allows marketers to repeat a message at intervals selected strategically. Repetition makes it more likely that the target audience will see and recall a message, which improves awareness-building results. Advertising can generate drama and human interest by featuring people and situations that are exciting or engaging. It can introduce emotions, images, and symbols that stimulate desire, and it can show how a product or brand compares favorably to competitors. Finally, advertising is an excellent vehicle for brand building, as it can create rational and emotional connections with a company or offering that translate into goodwill. As advertising becomes more sophisticated with digital media, it is a powerful tool for tracking consumer behaviors, interests, and preferences, allowing advertisers to better tailor content and offers to individual consumers. Through the power of digital media, memorable or entertaining advertising can be shared between friends and go viral—and viewer impressions skyrocket.
The primary disadvantage of advertising is cost. Marketers question whether this communication method is really cost-effective at reaching large groups. Of course, costs vary depending on the medium, with television ads being very expensive to produce and place. In contrast, print and digital ads tend to be much less expensive. Along with cost is the question of how many people an advertisement actually reaches. Ads are easily tuned out in today’s crowded media marketplace. Even ads that initially grab attention can grow stale over time. While digital ads are clickable and interactive, traditional advertising media are not. In the bricks-and-mortar world, it is difficult for marketers to measure the success of advertising and link it directly to changes in consumer perceptions or behavior. Because advertising is a one-way medium, there is usually little direct opportunity for consumer feedback and interaction, particularly from consumers who often feel overwhelmed by competing market messages.
Developing Effective Ads: The Creative Strategy
Effective advertising starts with the same foundational components as any other IMC campaign: identifying the target audience and the objectives for the campaign. When advertising is part of a broader IMC effort, it is important to consider the strategic role advertising will play relative to other marketing communication tools. With clarity around the target audience, campaign strategy, and budget, the next step is to develop the creative strategy for developing compelling advertising. The creative strategy has two primary components: the message and the appeal.
The message comes from the messaging framework discussed earlier in this module: what message elements should the advertising convey to consumers? What should the key message be? What is the call to action? How should the brand promise be manifested in the ad? How will it position and differentiate the offering? With advertising, it’s important to remember that the ad can communicate the message not only with words but also potentially with images, sound, tone, and style.
Marketers also need to consider existing public perceptions and other advertising and messages the company has placed in the market. Has the prior marketing activity resonated well with target audiences? Should the next round of advertising reinforce what went before, or is it time for a fresh new message, look, or tone?
Along with message, the creative strategy also identifies the appeal, or how the advertising will attract attention and influence a person’s perceptions or behavior. Advertising appeals can take many forms, but they tend to fall into one of two categories: informational appeal and emotional appeal.
The informational appeal offers facts and information to help the target audience make a purchasing decision. It tries to generate attention using rational arguments and evidence to convince consumers to select a product, service, or brand. For example:
- More or better product or service features: Ajax “Stronger Than Dirt”
- Cost savings: Wal-Mart “Always Low Prices”
- Quality: John Deere “Nothing runs like a Deere”
- Customer service: Holiday Inn “Pleasing people the world over”
- New, improved: Verizon “Can you hear me now? Good.”
The following Black+Decker commercial relies on an informational appeal to promote its product. (Note: There is no speech in this video; only instrumental music.)
Text alternative for “Black and Decker 20V MAX” (opens in new window).
The emotional appeal targets consumers’ emotional wants and needs rather than rational logic and facts. It plays on conscious or subconscious desires, beliefs, fears, and insecurities to persuade consumers and influence their behavior. The emotional appeal is linked to the features and benefits provided by the product, but it creates a connection with consumers at an emotional level rather than a rational level. Most marketers agree that emotional appeals are more powerful and differentiating than informational appeals. However, they must be executed well to seem authentic and credible to the target audience. A poorly executed emotional appeal can come across as trite or manipulative. Examples of emotional appeals include:
- Self-esteem: L’Oreal “Because I’m worth it”
- Happiness: Coca-Cola “Open happiness”
- Anxiety and fear: World Health Organization “Smoking Kills”
- Achievement: Nike “Just Do It”
- Attitude: Apple “Think Different”
- Freedom: Southwest “You are now free to move about the country”
- Peace of Mind: Allstate “Are you in good hands?”
- Popularity: NBC “Must-see TV”
- Germophobia: Chlorox “For life’s bleachable moments, there’s Chlorox”
The following Heinz Ketchup commercial offers a humorous example of an ad based entirely on an emotional appeal:
Developing the Media Plan
The media plan is a document that outlines the strategy and approach for an advertising campaign, or for the advertising component in an IMC campaign. The media plan is developed simultaneously with the creative strategy. A standard media plan consists of four stages: (a) stating media objectives; (b) evaluating media; (c) selecting and implementing media choices; and (d) determining the media budget.
Media objectives are normally started in terms of three dimensions:
- Reach: number of different persons or households exposed to a particular media vehicle or media schedule at least once during a specified time period.
- Frequency: the number of times within a given time period that a consumer is exposed to a message.
- Continuity: the timing of media assertions (e.g. 10 per cent in September, 20 per cent in October, 20 per cent in November, 40 per cent in December and 10 per cent the rest of the year).
The process of evaluating media involves considering each type of advertising available to a marketer, and the inherent strengths and weaknesses associated with each medium. The table below outlines key strengths and weaknesses of major types of advertising media. Television advertising is a powerful and highly visible medium, but it is expensive to produce and buy air time. Radio is quite flexible and inexpensive, but listenership is lower and it typically delivers fewer impressions and a less-targeted audience. Most newspapers and magazines have passed their advertising heydays and today struggle against declining subscriptions and readership. Yet they can be an excellent and cost-effective investment for reaching some audiences. Display ads offer a lot of flexibility and creative options, from wrapping busses in advertising to creating massive and elaborate 3-D billboards. Yet their reach is limited to their immediate geography. Online advertising such as banner ads, search engine ads, paid listings, pay-per-click links and similar techniques offers a wide selection of opportunities for marketers to attract and engage with target audiences online. Yet the internet is a very crowded place, and it is difficult for any individual company to stand out in the crowd.
| Advertising Media Type | Strengths | Weaknesses |
|---|---|---|
| Television | · Strong emotional impact
· Mass coverage/small cost per impression · Repeat message · Creative flexibility · Entertaining/prestigious | · High costs
· High clutter (too many ads) · Short-lived impression · Programming quality · Schedule inflexibility |
| Radio | · Immediacy
· Low cost per impression · Highly flexible | · Limited national coverage
· High clutter · Less easily perceived during drive time · Fleeting message |
| Newspapers | · Flexibility (size, timing, etc.)
· Community prestige · Market coverage · Offer merchandising services · Reader involvement | · Declining readership
· Short life · Technical quality · Clutter |
| Magazines | · Highly segmented audiences
· High-profile audiences · Reproduction quality | · Inflexible
· Narrow audiences · Waste circulation |
| Display Ads:
Billboards, Posters, Flyers, etc. | · Mass coverage/small cost per impression
· Repeat message · Creative flexibility | · High clutter
· Short-lived impression |
| Online Ads (including mobile):
Banner ads, search ads, paid listings, pay-per-click links, etc. | · Highly segmented audiences
· Highly measurable · Low cost per impression · Immediacy; link to interests, behavior · Click-thru and code allow further interaction · Timing flexibility | · High clutter
· Short-lived impression · Somewhat less flexibility in size, format |
The evaluation process requires research to assess options for reaching their target audience with each medium, and how well a particular message fits the audience in that medium. Many advertisers rely heavily on the research findings provided by the medium, by their own experience, and by subjective appraisal to determine the best media for a given campaign.
To illustrate, if a company is targeting young-to-middle-aged professional women to sell beauty products, the person or team responsible for the media plan should evaluate what options each type of media offers for reaching this audience. How reliably can television, radio, newspapers or magazines deliver this audience? Media organizations maintain carefully-researched information about the size, demographics and other characteristics of their viewership or readership. Cable and broadcast TV networks know which shows are hits with this target demographic and therefore which advertising spots to sell to a company targeting professional women. Likewise newspapers know which sections attract the eyeballs of female audiences, and magazines publishers understand very well the market niches their publications fit. Online advertising becomes a particularly powerful tool for targeted advertising because of the information it captures and tracks about site visitors: who views and clicks on ads, where they visit and what they search for. Not only does digital advertising provide the opportunity to advertise on sites that cater to a target audience of professional women, but it can identify which of these women are searching for beauty products, and it can help a company target these individuals more intensely and provide opportunities for follow-up interaction.
The following video further explains how digital advertising targets and tracks individuals based on their expressed interests and behaviors.
You can view the transcript for “Behavioral Targeting” here (opens in new window).
Selection and Implementation
The media planner must make decisions about the media mix and timing, both of which are restricted by the available budget. The media-mix decision involves choosing the best combination of advertising media to achieve the goals of the campaign. This is a difficult task, and it usually requires evaluating each medium quantitatively and qualitatively to select a mix that optimizes reach and budget.
Unfortunately, there are few valid rules of thumb to guide this process, in part because it is difficult to compare audiences across different types of advertising media. For example, Nielsen ratings measure audiences based on TV viewer reports of the programs watched, while outdoor (billboard) audience-exposure estimates are based on counts of the number of automobiles that pass particular outdoor poster locations. The “timing of media” refers to the actual placement of advertisements during the time periods that are most appropriate, given the selected media objectives. It includes not only the scheduling of advertisements, but also the size and position of the advertisement.
There are three common patterns for advertising scheduling:
- Continuous advertising runs ads steadily at a given level indefinitely. This schedule works well for products and services that are consumed on a steady basis throughout the year, and the purpose of advertising is to nudge consumers, remind them and keep a brand or product top-of-mind.
- Flighting involves heavy spurts of advertising, followed by periods with no advertising. This type of schedule makes sense for products or services that are seasonal in nature, like tax services, as well as one-time or occasional events.
- Pulsing mixes continuous scheduling with flighting, to create a constant drum-beat of ads, with periods of greater intensity. This approach matches products and services for which there is year-round appeal, but there may be some seasonality or periods of greater demand or intensity. Hotels and airlines, for example, might increase their advertising presence during the holiday season.
Budget
When considering advertising as a marketing communication method, companies need to balance the cost of advertising–both of producing the advertising pieces and buying placement—against the total budget for the IMC program. The selection and scheduling of media have a huge impact on budget: advertising that targets a mass audience is generally more expensive than advertising that targets a local or niche audience. It is important for marketers to consider the contribution advertising will make to the whole. Although advertising is generally one of the more expensive parts of the promotion mix, it may be a worthwhile investment if it contributes substantially to the reach and effectiveness of the whole program. Alternatively, some marketers spend very little on advertising because they find other methods are more productive and cost-effective for reaching their target segments.
Anatomy of an Advertisement
Advertisements use several common elements to deliver the message. The visual is the picture, image, or situation portrayed in the advertisement. The visual also considers the emotions, style, or look-and-feel to be conveyed: should the ad appear tender, businesslike, fresh, or supercool? All of these considerations can be conveyed by the visual, without using any words.
The headline is generally what the viewer reads first—i.e., the words in the largest typeface. The headline serves as a hook for the appeal: it should grab attention, pique interest, and cause the viewer to keep reading or paying attention. In a radio or television ad, the headline equivalent might be the voice-over of a narrator delivering the primary message, or it might be a visual headline, similar to a print ad.
In print ads, a subhead is a smaller headline that continues the idea introduced in the headline or provides more information. It usually appears below the headline and in a smaller typeface. The body copy provides supporting information. Generally it appears in a standard, readable font. The call to action may be part of the body copy, or it may appear elsewhere in a larger typeface or color treatment to draw attention to itself.
A variety of brand elements may also appear in an advertisement. These include the name of the advertiser or brand being advertised, the logo, a tagline, hashtag, Web site link, or other standard “branded” elements that convey brand identity. These elements are an important way of establishing continuity with other marketing communications used in the IMC campaign or developed by the company. For example, print ads for an IMC campaign might contain a campaign-specific tagline that also appears in television ads, Website content, and social media posts associated with the campaign.
Ad Testing and Measurement
When organizations are poised to make a large investment in any type of advertising, it is wise to conduct marketing research to test the advertisements with target audiences before spending lots of money on ads and messages that may not hit the mark. Ad testing may preview messages and preliminary ad concepts with members of a target segment to see which ones resonate best and get insight about how to fine-tune messages or other aspects of the ad to make them more effective. Organizations may conduct additional testing with near-final advertising pieces to do more fine-tuning of the messages and visuals before going public.
To gauge the impact of advertising, organizations may conduct pre-tests and post-tests of their target audience to measure whether advertising has its intended effect. A pre-test assesses consumer attitudes, perceptions, and behavior before the advertising campaign. A post-test measures the same things afterward to determine how the ads have influenced the target audience, if at all.
Companies may also measure sales before, during, and after advertising campaigns run in the geographies or targets where the advertising appeared. This provides information about the return on investment for the campaign, which is how much the advertising increased sales relative to how much money it cost to execute. Ideally advertising generates more revenue and, ultimately profits, than it costs to mount the advertising campaign.
LICENSES AND ATTRIBUTIONS
CC LICENSED CONTENT, SHARED PREVIOUSLY
- Revision and adaptation of Advertising. Provided by: Wikipedia. Located at: https://en.wikipedia.org/wiki/Advertising. License: CC BY-SA: Attribution-ShareAlike
- Communicating to Mass Markets, from Introducing Marketing. Authored by: John Burnett. Project: Global Text. License: CC BY: Attribution
- Behavioral Targeting. Provided by: BBC. Located at: https://youtu.be/HtOkaAMOmAc. License: CC BY-NC-ND: Attribution-NonCommercial-NoDerivatives
ALL RIGHTS RESERVED CONTENT
- HEINZ Ketchup Wiener Stampede. Provided by: Heinz. Located at: https://youtu.be/LOlfhBT8i9I. License: All Rights Reserved. License Terms: Standard YouTube license
- Naked Juice Ad. Provided by: Naked Juice. Located at: https://www.nakedjuice.com/. License: All Rights Reserved. License Terms: Fair Use
- Puma Ad. Provided by: PUMA. Located at: https://us.puma.com/en/us/home. License: All Rights Reserved. License Terms: Fair Use
- Screenshot Rogue Voodoo Porter Ad. Provided by: Rogue Ales and Spirits. Located at: https://www.rogue.com/. License: All Rights Reserved. License Terms: Fair Use
- BLACK + DECKER 20V MAX*- Lithium Drill/Driver with AutoSense Technology. Provided by: Lowe's Canada. Located at: https://youtu.be/w6tqDoJQokM. License: All Rights Reserved. License Terms: Standard YouTube license
- Hoover Ad. Provided by: Hoover. Located at: https://www.hoover.com/. License: All Rights Reserved. License Terms: Fair Use
PUBLIC DOMAIN CONTENT
- Got Milk?. Provided by: Wikimedia. Located at: https://commons.wikimedia.org/wiki/File:Gotmilk.png. License: Public Domain: No Known Copyright
- Pears Soap ad. Provided by: Wikimedia. Located at: https://commons.wikimedia.org/wiki/File:Pears_Soap_1900.jpg. License: Public Domain: No Known Copyright
Reading: Public Relations
Public Relations: Getting Attention to Polish Your Image
Public relations (PR) is the process of maintaining a favorable image and building beneficial relationships between an organization and the public communities, groups, and people it serves. Unlike advertising, which tries to create favorable impressions through paid messages, public relations does not pay for attention and publicity. Instead, PR strives to earn a favorable image by drawing attention to newsworthy and attention-worthy activities of the organization and its customers. For this reason, PR is often referred to as “free advertising.”
In fact, PR is not a costless form of promotion. It requires salaries to be paid to people who oversee and execute PR strategy. It also involves expenses associated with events, sponsorships and other PR-related activities.
The Purpose of Public Relations
Like advertising, public relations seeks to promote organizations, products, services, and brands. But PR activities also play an important role in identifying and building relationships with influential individuals and groups responsible for shaping market perceptions in the industry or product category where an organization operates. Public relations efforts strive to do the following:
- Build and maintain a positive image
- Inform target audiences about positive associations with a product, service, brand, or organization
- Maintain good relationships with influencers—the people who strongly influence the opinions of target audiences
- Generate goodwill among consumers, the media, and other target audiences by raising the organization’s profile
- Stimulate demand for a product, service, idea, or organization
- Head off critical or unfavorable media coverage
When to Use Public Relations
Public relations offers an excellent toolset for generating attention whenever there is something newsworthy that marketers would like to share with customers, prospective customers, the local community, or other audiences. PR professionals maintain relationships with reporters and writers who routinely cover news about the company, product category, and industry, so they can alert media organizations when news happens. At times, PR actually creates activities that are newsworthy, such as establishing a scholarship program or hosting a science fair for local schools. PR is involved in publishing general information about an organization, such as an annual report, a newsletter, an article, a white paper providing deeper information about a topic of interest, or an informational press kit for the media. PR is also responsible for identifying and building relationships with influencers who help shape opinions in the marketplace about a company and its products. When an organization finds itself facing a public emergency or crisis of some sort, PR professionals play an important role strategizing and managing communications with various stakeholder groups, to help the organization respond in effective, appropriate ways and to minimize damage to its public image.
To illustrate, PR techniques can help marketers turn the following types of events into opportunities for media attention, community relationship building, and improving the organization’s public image:
- Your organization develops an innovative technology or approach that is different and better than anything else available.
- One of your products wins a “best in category” prize awarded by a trade group.
- You enter into a partnership with another organization to collaborate on providing broader and more complete services to a target market segment.
- You sponsor and help organize a 10K race to benefit a local charity.
- You merge with another company.
- You conduct research to better understand attitudes and behaviors among a target segment, and it yields insights your customers would find interesting and beneficial.
- A customer shares impressive and well-documented results about the cost savings they have realized from using your products or services.
- Your organization is hiring a new CEO or other significant executive appointment.
- A quality-assurance problem leads your company to issue a recall for one of your products.
It is wise to develop a PR strategy around strengthening relationships with any group that is important in shaping or maintaining a positive public image for your organization: reporters and media organizations; industry and professional associations; bloggers; market or industry analysts; governmental regulatory bodies; customers and especially leaders of customer groups, and so forth. It is also wise to maintain regular, periodic communications with these groups to keep them informed about your organization and its activities. This helps build a foundation of familiarity and trust, so these relationships are established and resilient through the ups and downs of day-to-day business.
The following video, about Tyson Foods’ “Meals That Matter” program, shows how one company cooked up an idea that is equal parts public relations and corporate social responsibility (CSR). The video covers the Tyson disaster-relief team delivering food to the residents of Moore, Oklahoma, shortly after tornados struck the area on May 20, 2013. The company received favorable publicity following the inauguration of the program in 2012. (You can read one of the articles here: “Tyson Foods Unveils Disaster Relief Mobile Feeding Unit.”)
You can view the transcript for “Tyson Foods Meals That Matter – Moore, Okla., June 2013” here (opens in new window).
Standard Public Relations Techniques
Public relations encompasses a variety of marketing tactics that all share a common focus: managing public perceptions. The most common PR tools are listed in the following table and discussed below.
| Public Relations Technique | Role and Description | Examples |
|---|---|---|
| Media Relations | Generate positive news coverage about the organization, its products, services, people, and activities | Press release, press kit, and interview leading to a news article about a new product launch; press conference |
| Influencer/Analyst Relations | Maintain strong, beneficial relationships with individuals who are thought leaders for a market or segment | Product review published by a renowned blogger; company profile by an industry analyst; celebrity endorsement |
| Publications and Thought Leadership | Provide information about the organization, showcase its expertise and competitive advantages | Organization’s annual report; newsletters; white papers focused on research and development; video case study about a successful customer |
| Events | Engage with a community to present information and an interactive “live” experience with a product, service, organization or brand | User conference; presentation of a keynote address; day-of-community-service event |
| Sponsorships | Raise the profile of an organization by affiliating it with specific causes or activities | Co-sponsoring an industry conference; sponsoring a sports team; sponsoring a race to benefit a charity |
| Award Programs | Generate recognition for excellence within the organization and/or among customers | Winning an industry “product of the year” award; nominating customer for an outstanding achievement award |
| Crisis Management | Manage perceptions and contain concerns in the face of an emergency situation | Oversee customer communication during a service outage or a product recall; execute action plan associated with an environmental disaster |
Media relations is the first thing that comes to mind when many people think of PR: public announcements about company news, talking to reporters, and articles about new developments at a company. But media relations is the tip of the iceberg. For many industries and product categories, there are influential bloggers and analysts writing about products and the industry. PR plays an important role in identifying and building relationships with these individuals. Offering periodic “company update” briefings, newsletters, or email updates helps keep these individuals informed about your organization, so you are top of mind.
The people responsible for PR are also involved in developing and distributing general information about an organization. This information may be in the form of an annual report, a “state of the company” briefing call, video pieces about the company or its customers, and other publications that convey the company’s identity, vision, and goals. “Thought leadership” publications assert the company’s expertise and position of leading thought, practice, or innovation in the field. These publications should always be mindful of the same messaging employed for other marketing activities to ensure that everything seems consistent and well aligned.
While some consider event marketing a marketing communication method of its own, others categorize it with public relations as we have done here. Events, such as industry conferences or user group meetings, offer opportunities to present the company’s value proposition, products, and services to current and prospective customers. Themed events, such as a community service day or a healthy lifestyle day, raise awareness about causes or issues with the organization wants to be affiliated in the minds of its employees, customers, and other stakeholder groups. A well-designed and well-produced event also offers opportunities for an organization to provide memorable interaction and experiences with target audiences. An executive leader can offer a visionary speech to generate excitement about a company and the value it provides—now or in the future. Events can help cement brand loyalty by not only informing customers but also forging emotional connections and goodwill.
Sponsorships go hand-in-hand with events, as organizations affiliate themselves with events and organizations by signing on to co-sponsor something available to the community. Sponsorships cover the gamut: charitable events, athletes, sports teams, stadiums, trade shows and conferences, contests, scholarships, lectures, concerts, and so forth. Marketers should select sponsorships carefully to make sure that they are affiliating with activities and causes that are well managed and strategically aligned with the public image they are trying to cultivate.
Award programs are another common PR tool. Organizations can participate in established award programs managed by trade groups and media, or they can create award programs that target their customer community. Awards provide opportunities for public recognition of great work by employees and customers. They can also help organizations identify great targets for case studies and public announcements to draw attention to how customers are benefitting from an organization’s products and services.
Crisis management is an important PR toolset to have on hand whenever it may be needed. Few companies choose this as a promotional technique if other options are available. But when crises emerge, as inevitably they do, PR provides structure and discipline to help company leaders navigate the crisis with communications and actions that address the needs of all stakeholders. Messaging, communication, listening, and relationship building all come to the fore. When handled effectively, these incidents may help an organization emerge from the crisis stronger and more resilient than it was before. This is the power of good PR.
Advantages and Disadvantages of Public Relations
Because PR activity is earned rather than paid, it tends to carry more credibility and weight. For example, when a news story profiles a customer’s successful experience with a company and its products, people tend to view this type of article as less biased (and therefore more credible) than a paid advertisement. The news story comes from an objective reporter who feels the story is worth telling. Meanwhile an advertisement on a similar topic would be viewed with skepticism because it is a paid placement from a biased source: the ad sponsor.
Advantages of Public Relations[1]
- The opportunity to amplify key messages and milestones. When PR activities are well-aligned with other marketing activities, organizations can use PR to amplify the things they are trying to communicate via other channels. A press release about a new product, for example, can be timed to support a marketing launch of the product and conference where the product is unveiled for the first time.
- Believable. Because publicity is seen to be more objective, people tend to give it more weight and find it more credible. Paid advertisements, on the other hand, are seen with a certain amount of skepticism, since people that companies can make almost any kind of product claim they want.
- Employee pride. Organizing and/or sponsoring charitable activities or community events can help with employee morale and pride (both of which get a boost from any related publicity, too). It can also be an opportunity for teamwork and collaboration.
- Engaging people who visit your Web site. PR activities can generate interesting content that can be featured on your organization’s Web site. Such information can be a means of engaging visitors to the site, and it can generate interest and traffic long after the PR event or moment has passed. Industry influencers may visit the site, too, to get updates on product developments, growth plans, or personnel news, etc.
Disadvantages of Public Relations2
- Cost. Although publicity is usually less expensive to organize than advertising, it isn’t “free.” A public relations firm may need to be hired to develop campaigns, write press releases, and speak to journalists. Even if you have in-house expertise for this work, developing publicity materials can take employees away from their primary responsibilities and drain off needed resources.
- Lack of control. There’s no guarantee that a reporter or industry influencer will give your company or product a favorable review—it’s the price you pay for “unbiased” coverage. You also don’t have any control over the accuracy or thoroughness of the coverage. There’s always a risk that the journalist will get some facts wrong or fail to include important details.
- Missing the mark. Even if you do everything right—you pull off a worthy event and it gets written up by a local newspaper, say—your public relations effort can fall short and fail to reach enough or the right part of your target audience. It doesn’t do any good if the reporter’s write-up is very short or it appears in a section of the paper that no one reads. This is another consequence of not being able to fully control the authorship, content, and placement of PR.
PR and Integrated Marketing Communication
Public relations activities can provide significantly greater benefits to organizations when they happen in conjunction with a broader IMC effort, rather than on their own. Because PR focuses heavily on communication with key stakeholder groups, it stands to reason that other marketing communication tools should be used in conjunction with public relations. For example:
- Press releases can be distributed to media contacts, customers, and other stakeholder groups via email marketing campaigns that might also include additional information or offers—such as an invitation to a webinar to learn more about the subject of the press release.
- Press releases are posted to the Web site to update content and provide a greater body of information for Web site visitors
- Event presentations and other activities should align with an organization’s broader marketing strategy, goals, and messaging. Everything should be part of the same, consistent approach and theme—e.g., the topics of speeches, information available in trade show booths, interactions with event participants via email and social media, etc.
- Sponsorship activities often provide an opportunity to advertise at the event, as well. Naturally it is important for there to be good alignment between these advertising opportunities, company messaging, and the audience for the sponsored activity.
- A thought-leadership piece, such as an article or a white paper authored by a company leader, can be published on the Web site and incorporated into an email marketing campaign that targets selected audiences
Smart marketers consider PR tools in concert with other marketing activity to determine how to make the greatest impact with their efforts. Because PR activities often involve working with many other people inside and outside the organization, they usually need a long lead time in order to come together in the desired time frame. Event planning happens months (and sometimes years) in advance of the actual event itself. Press releases and public announcements can be mapped out over several months to give marketers and other stakeholders plenty of time to prepare and execute effectively. PR is undoubtedly a powerful toolset to amplify other marketing efforts.
- http://edwardlowe.org/digital-library/how-to-establish-a-promotional-mix/
- http://edwardlowe.org/digital-library/how-to-establish-a-promotional-mix/
LICENSES AND ATTRIBUTIONS
CC LICENSED CONTENT, ORIGINAL
- Reading: Public Relations. Authored by: Lumen Learning. License: CC BY: Attribution
CC LICENSED CONTENT, SHARED PREVIOUSLY
- Innovation Awards. Provided by: ICT Authority. Located at: https://www.flickr.com/photos/kenyaictboard/14140767473/. License: CC BY: Attribution
- Announcement. Authored by: rawpixel. Provided by: Pixabay. Located at: https://pixabay.com/photos/announcement-announcing-audio-3976318/. License: CC0: No Rights Reserved
ALL RIGHTS RESERVED CONTENT
- Tyson Foods Meals That Matter. Provided by: Tyson Foods. Located at: https://youtu.be/awsB5_H2PlI. License: All Rights Reserved. License Terms: Standard YouTube license
Reading: Sales Promotions
Sales promotion helps make personal selling and advertising more effective. Sales promotions are marketing events or sales efforts—not including traditional advertising, personal selling, and public relations—that stimulate buying. Sales promotion can be developed as part of the social media or e-commerce effort just as advertising can, but the methods and tactics are much different. Sales promotion is a $300 billion—and growing— industry. Sales promotion is usually targeted toward either of two distinctly different markets. Consumer sales promotion is targeted to the ultimate consumer market. Trade sales promotion is directed to members of the marketing channel, such as wholesalers and retailers.
The goal of many promotion tactics is immediate purchase. Therefore, it makes sense when planning a sales-promotion campaign to target customers according to their general behavior. For instance, is the consumer loyal to the marketer’s product or to the competitor’s? Does the consumer switch brands readily in favor of the best deal? Does the consumer buy only the least expensive product, no matter what? Does the consumer buy any products in your category at all?
PROCTOR & GAMBLE
Procter & Gamble believes shoppers make up their mind about a product in about the time it takes to read this paragraph.
This “first moment of truth,” as P&G calls it, is the three to seven seconds when someone notices an item on a store shelf. Despite spending billions on traditional advertising, the consumer-products giant thinks this instant is one of its most important marketing opportunities. It recently created a position entitled Director of First Moment of Truth, or Director of FMOT (pronounced “EFF-mott”), to produce sharper, flashier in-store displays. There is a 15-person FMOT department at P&G headquarters in Cincinnati as well as 50 FMOT leaders stationed around the world.[1]
One of P&G’s most prominent in-store promotions has been for a new line of Pampers. In the United States, P&G came up with what it calls a “shopper concept”—a single promotional theme that allows it to pitch products in a novel way. The theme for Pampers was “Babies First.” In stores, the company handed out information on childhood immunizations, car-seat safety, and healthy diets while promoting its diapers and wipes in other parts of the store. To market Pampers diapers in the United Kingdom, P&G persuaded retailers earlier this year to put fake doorknobs high up on restroom doors, to remind parents how much babies need to stretch.
Sales Promotion Techniques
Most consumers are familiar with common sales promotion techniques including samples, coupons, point-of-purchase displays, premiums, contents, loyalty programs and rebates.
Do you like free samples? Most people do. A sample is a sales promotion in which a small amount of a product that is for sale is given to consumers to try. Samples encourage trial and an increased awareness of the product. You have probably purchased a product that included a small free sample with it—for example, a small amount of conditioner packaged with your shampoo. Have you ever gone to a store that provided free samples of different food items? The motivation behind giving away samples is to get people to buy a product. Although sampling is an expensive strategy, it is usually very effective for food products. People try the product, the person providing the sample tells consumers about it, and mentions any special pricing or offers for the product.
The objectives of a promotion depend on the general behavior of target consumers, as described in Table 1. For example, marketers who are targeting loyal users of their product don’t want to change behavior. Instead, they want to reinforce existing behavior or increase product usage. Frequent-buyer programs that reward consumers for repeat purchases can be effective in strengthening brand loyalty. Other types of promotions are more effective with customers prone to brand switching or with those who are loyal to a competitor’s product. Cents-off coupons, free samples, or an eye-catching display in a store will often entice shoppers to try a different brand.
The use of sales promotion for services products depends on the type of services. Consumer services, such as hairstyling, rely heavily on sales promotions (such as providing half off the price of a haircut for senior citizens on Mondays). Professional services, however, use very little sales promotion. Doctors, for example, do not often use coupons for performing an appendectomy, for example. In fact, service product companies must be careful not to utilize too many sales-promotion tactics because they can lower the credibility of the firm. Attorneys do not have a sale on providing services for divorce proceedings, for example.
| Type of Behavior | Desired Results | Sales Promotion Examples |
|---|---|---|
| Loyal customers: People who buy your product most or all of the time | Reinforce behavior, increase consumption, change purchase timing | Loyalty marketing programs, such as frequent-buyer cards and frequent-shopper clubs Bonus packs that give loyal consumers an incentive to stock up or premiums offered in return for proof of purchase |
| Competitor’s customers: People who buy a competitor’s product most or all of the time | Break loyalty, persuade to switch to your brand | Sweepstakes, contests, or premiums that create interest in the product |
| Brand switchers: People who buy a variety of products in the category | Persuade to buy your brand more often | Sampling to introduce your product’s superior qualities compared to their brand |
| Price buyers: People who consistently buy the least expensive brand | Appeal with low prices or supply added value that makes price less important | Trade deals that help make the product more readily available than competing products Coupons, cents-off packages, refunds, or trade deals that reduce the price of the brand to match that of the brand that would have been purchased |
Two growing areas of sales promotion are couponing and product placement. American consumers receive over $321 billion worth of coupons each year and redeem about $3 billion.2 Almost 85 percent of all Americans redeem coupons. Sunday newspaper supplements remain the number one source, but there has been explosive growth of online or consumer-printed coupons. General Mills, Kimberly-Clark, and General Electric like online coupons because they have a higher redemption rate. Coupons are used most often for grocery shopping. Do they save you money? One study found that people using coupons at the grocery store spent eight percent more than those who didn’t.3
Product placement is paid inclusion of brands in mass media programming. This includes movies, TV, books, music videos, and video games. So when you see Ford vehicles in the latest James Bond movie or Tom Hanks putting on a pair on Nikes on-screen, that is product placement. Product placement has become a huge business. For example, companies paid more than $6 billion in a recent year to have their products placed prominently in a film or television program; that figure is expected to reach more than $11 billion by 2019.4 It is easy to go overboard with this trend and be portrayed as a parody, however. The 2017 Emoji Movie is an example of failed product placements. The theme of the movie centered on various emojis caught in a smartphone as they are forced to play Candy Crush and say glowing things about such apps as Dropbox and Instagram as they make their way through the phone.5 Also, some have suggested that product placement might doom the products and companies. For example, Atari products appeared in the classic 1982 film Blade Runner, but the original company went out of business shortly after the movie was released, while another product, the Cuisinart food processor, had to settle a price-fixing scandal after making an appearance in the film. This has not stopped companies such as Sony, Peugeot, and Coca-Cola from tempting fate by appearing in the recently released Blade Runner 2049.6 Many large companies are cutting their advertising budgets to spend more on product placements. One area of product placement that continues to raise ethical issues is so-called “experts” being paid to mention brands on the air.
Contests and sweepstakes are also popular consumer sales promotions. Contests are games of skill offered by a company, that offer consumers the chance to win a prize. Cheerios’ Spoonfuls of Stories contest, for example, invited people to submit an original children’s story and the chance to win money and the opportunity to have their story published. Sweepstakes are games of chance people enter for the opportunity to win money or prizes. Sweepstakes are often structured as some variation on a random drawing. The companies and organizations that conduct these activities hope consumers will not only enter their games, but also buy more of their products and ideally share their information for future marketing purposes. As the following video shows, marketers have become increasingly sophisticated in the way they approach this “gaming” aspect of sales promotions.
You can view the transcript for “Gamification” (opens in new window).
WHICH SALES PROMOTIONS WORK BEST, AND WHEN?
Although different types of sales promotions work best for different organizations, rebates are very profitable for companies because, as you have learned, many consumers forget to send in their rebate forms. In a weak economy, consumers tend to use more coupons, but they also buy more store brands. Coupons available online or at the point of purchase are being used more often by consumers. Trade shows can be very successful, although the companies that participate in them need to follow-up on the leads generated at the shows.
Advantages and Disadvantages of Sales Promotions7
In addition to their primary purpose of boosting sales in the near term, companies can use consumer sales promotions to help them understand price sensitivity. Coupons and rebates provide useful information about how pricing influences consumers’ buying behavior. Sales promotions can also be a valuable–and sometimes sneaky–way to acquire contact information for current and prospective customers. Many of these offers require consumers to provide their names and other information in order to participate. Electronically-scanned coupons can be linked to other purchasing data, to inform organizations about buying habits. All this information can be used for future marketing research, campaigns and outreach.
Consumer sales promotions can generate loyalty and enthusiasm for a brand, product, or service. Frequent flyer programs, for example, motivate travelers to fly on a preferred airline even if the ticket prices are somewhat higher. If sales have slowed, a promotion such as a sweepstakes or contest can spur customer excitement and (re)new interest in the company’s offering. Sales promotions are a good way of energizing and inspiring customer action.
Trade promotions offer distribution channel partners financial incentives that encourage them to support and promote a company’s products. Offering incentives like prime shelf space at a retailer’s store in exchange for discounts on products has the potential to build and enhance business relationships with important distributors or businesses. Improving these relationships can lead to higher sales, stocking of other product lines, preferred business terms and other benefits.
Sales promotions can be a two-edged sword: if a company is continually handing out product samples and coupons, it can risk tarnishing the company’s brand. Offering too many freebies can signal to customers that they are not purchasing a prestigious or “limited” product. Another risk with too-frequent promotions is that savvy customers will hold off purchasing until the next promotion, thus depressing sales.
Often businesses rush to grow quickly by offering sales promotions, only to see these promotions fail to reach their sales goals and target customers. The temporary boost in short term sales may be attributed to highly price-sensitive consumers looking for a deal, rather than the long-term loyal customers a company wants to cultivate. Sales promotions need to be thought through, designed and promoted carefully. They also need to align well with the company’s larger business strategy. Failure to do so can be costly in terms of dollars, profitability and reputation.
If businesses become overly reliant on sales growth through promotions, they can get trapped in short-term marketing thinking and forget to focus on long-term goals. If, after each sales dip, a business offers another sales promotion, it can be damaging to the long-term value of its brand.
IMC Support for Sales Promotions
Sales promotions are delivered to targeted groups via marketing campaigns during a pre-set, limited amount of time. In order to broaden awareness, impact and participation, sales promotions are often combined with other marketing communication methods in the promotional mix. Examples of IMC support for sales promotions include:
- Weekly email messages to consumers informing them about the week’s sales, special offers, and coupons
- Promotional information on a Web site informing consumers about the availability of a rebate or other special offer
- Posters and other promotional materials to enhance a point-of-purchase display
- Sweepstakes forms incorporated into a magazine advertisement
- Social media campaigns encouraging people to post about entering a sponsored contest on Twitter, Facebook, and Instagram
These types of activities create synergies between the sales promotions and other marketing activities. IMC activities can amplify the message about the sales promotion and encourage active participation from target customers.
Finally, it is important to recognize that sales promotions cannot compensate for a poor product, a declining sales trend, ineffective advertising, or weak brand loyalty. If these fundamentals are not working, sales promotions can serve only as a temporary solution.
- Jim Tincher, “Your Moment of Truth,” Customer Think, http://customerthink.com, August 30, 2016.
- “Coupon Statistics: The Ultimate Collection,” Access Development, http://blog.accessdevelopment.com, May 17, 2017.
- Drew Hendricks, “5 Ways to Enhance Your SEO Campaign with Online Coupons,” Forbes, https://www.forbes.com, May 13, 2015.
- Laurent Muzellec, “James Bond, Dunder Mifflin, and the Future of Product Placement,” Harvard Business Review, https://hbr.org, June 23, 2016.
- Josh Terry, “Unfunny Emoji Movie Is a Sad Echo of 2015’s “Inside Out,” Deseret News, http://www.deseretnews.com, July 31, 2017.
- Don Steinberg, “Science Affliction: Are Companies Cursed by Cameos in Blade Runner?” The Wall Street Journal, https://www.wsj.com, September 25, 2017.
- http://edwardlowe.org/digital-library/how-to-establish-a-promotional-mix/
LICENSES AND ATTRIBUTIONS
CC LICENSED CONTENT, ORIGINAL
- Revision and adaptation. Provided by: Lumen Learning. License: CC BY-SA: Attribution-ShareAlike
CC LICENSED CONTENT, SHARED PREVIOUSLY
- Sales Promotion, from Boundless Marketing. Provided by: Boundless. Located at: https://courses.lumenlearning.com/boundless-marketing/. License: CC BY-SA: Attribution-ShareAlike
- Communicating to Mass Markets, from Introducing Marketing. Authored by: John Burnett. Project: Global Text. License: CC BY: Attribution
- Sales Promotion. Provided by: OpenStax CNX. Located at: http://cnx.org/contents/4e09771f-a8aa-40ce-9063-aa58cc24e77f@8.6. Project: Introduction to Business. License: CC BY: Attribution. License Terms: Download for free at http://cnx.org/contents/4e09771f-a8aa-40ce-9063-aa58cc24e77f@8.6
- Sampler tray of Starbucks new Mocha Toffee Latte. Authored by: Urban Bohemian. Located at: https://www.flickr.com/photos/urbanbohemian/4948232118/. License: CC BY-NC-ND: Attribution-NonCommercial-NoDerivatives
- Gamification. Provided by: BBC. Located at: https://youtu.be/1nikw1v5Zjo. License: CC BY-NC-ND: Attribution-NonCommercial-NoDerivatives
Reading: Personal Selling
People Power
Personal selling uses in-person interaction to sell products and services. This type of communication is carried out by sales representatives, who are the personal connection between a buyer and a company or a company’s products or services. Salespeople not only inform potential customers about a company’s product or services, they also use their power of persuasion and remind customers of product characteristics, service agreements, prices, deals, and much more. In addition to enhancing customer relationships, this type of marketing communications tool can be a powerful source of customer feedback, as well. Later we’ll cover marketing alignment with the sales process in greater detail. This section focuses on personal selling as one possible tool in the promotional mix.
Effective personal selling addresses the buyer’s needs and preferences without making him or her feel pressured. Good salespeople offer advice, information, and recommendations, and they can help buyers save money and time during the decision process. The seller should give honest responses to any questions or objections the buyer has and show that he cares more about meeting the buyer’s needs than making the sale. Attending to these aspects of personal selling contributes to a strong, trusting relationship between buyer and seller.[1]
Common Personal Selling Techniques
Common personal selling tools and techniques include the following:
- Sales presentations: in-person or virtual presentations to inform prospective customers about a product, service, or organization
- Conversations: relationship-building dialogue with prospective buyers for the purposes of influencing or making sales
- Demonstrations: demonstrating how a product or service works and the benefits it offers, highlighting advantageous features and how the offering solves problems the customer encounters
- Addressing objections: identifying and addressing the concerns of prospective customers, to remove any perceived obstacles to making a purchase
- Field selling: sales calls by a sales representative to connect with target customers in person or via phone
- Retail selling: in-store assistance from a sales clerk to help customers find, select, and purchase products that meet their needs
- Door-to-door selling: offering products for sale by going door-to-door in a neighborhood
- Consultative selling: consultation with a prospective customer, where a sales representative (or consultant) learns about the problems the customer wants to solve and recommends solutions to the customer’s particular problem
- Reference selling: using satisfied customers and their positive experiences to convince target customers to purchase a product or service
Personal selling minimizes wasted effort, promotes sales, and boosts word-of-mouth marketing. Also, personal selling measures marketing return on investment (ROI) better than most tools, and it can give insight into customers’ habits and their responses to a particular marketing campaign or product offer.
When to Use Personal Selling
Not every product or service is a good fit for personal selling. It’s an expensive technique because the proceeds of the person-to-person sales must cover the salary of the sales representative—on top of all the other costs of doing business. Whether or not a company uses personal selling as part of its marketing mix depends on its business model. Most often companies use personal selling when their products or services are highly technical, specialized, or costly—such as complex software systems, business consulting services, homes, and automobiles.
In addition, there are certain conditions that favor personal selling:[2]
- Product situation: Personal selling is relatively more effective and economical when a product is of a high unit value, when it is in the introductory stage of its life cycle, when it requires personal attention to match consumer needs, or when it requires product demonstration or after-sales services.
- Market situation: Personal selling is effective when a firm serves a small number of large-size buyers or a small/local market. Also, it can be used effectively when an indirect channel of distribution is used for selling to agents or middlemen.
- Company situation: Personal selling is best utilized when a firm is not in a good position to use impersonal communication media, or it cannot afford to have a large and regular advertising outlay.
- Consumer behavior situation: Personal selling should be adopted by a company when purchases are valuable but infrequent, or when competition is at such a level that consumers require persuasion and follow-up.
It’s important to keep in mind that personal selling is most effective when a company has established an effective sales-force management system together with a sales force of the right design, size, and structure. Recruitment, selection, training, supervision, and evaluation of the sales force also obviously play an important role in the effectiveness of this marketing communication method.3
Advantages and Disadvantages of Personal Selling
The most significant strength of personal selling is its flexibility. Salespeople can tailor their presentations to fit the needs, motives, and behavior of individual customers. A salesperson can gauge the customer’s reaction to a sales approach and immediately adjust the message to facilitate better understanding.
Personal selling also minimizes wasted effort. Advertisers can spend a lot of time and money on a mass-marketing message that reaches many people outside the target market (but doesn’t result in additional sales). In personal selling, the sales force pinpoints the target market, makes a contact, and focuses effort that has a strong probability of leading to a sale.
As mentioned above, an additional strength of personal selling is that measuring marketing effectiveness and determining ROI are far more straightforward for personal selling than for other marketing communication tools—where recall or attitude change is often the only measurable effect.
Another advantage of personal selling is that a salesperson is in an excellent position to encourage the customer to act. The one-on-one interaction of personal selling means that a salesperson can effectively respond to and overcome objections—e.g., concerns or reservations about the product—so that the customer is more likely to buy. Salespeople can also offer many customized reasons that might spur a customer to buy, whereas an advertisement offers a limited set of reasons that may not persuade everyone in the target audience.
A final strength of personal selling is the multiple tasks that the sales force can perform. For example, in addition to selling, a salesperson can collect payments, service or repair products, return products, and collect product and marketing information. In fact, salespeople are often the best resources when it comes to disseminating positive word-of-mouth product information.
High cost is the primary disadvantage of personal selling. With increased competition, higher travel and lodging costs, and higher salaries, the cost per sales contract continues to rise. Many companies try to control sales costs by compensating sales representatives through commissions alone, thereby guaranteeing that salespeople are paid only if they generate sales. However, commission-only salespeople may become risk averse and only call on clients who have the highest potential return. These salespeople, then, may miss opportunities to develop a broad base of potential customers that could generate higher sales revenues in the long run.
Companies can also reduce sales costs by using complementary techniques, such as telemarketing, direct mail, toll-free numbers for interested customers, and online communication with qualified prospects. Telemarketing and online communication can further reduce costs by serving as an actual selling vehicle. Both technologies can deliver sales messages, respond to questions, take payment, and follow up.
A second disadvantage of personal selling is the problem of finding and retaining high-quality people. Experienced salespeople sometimes realize that the only way their income can outpace their cost-of-living increase is to change jobs. Also, because of the push for profitability, businesses try to hire experienced salespeople away from competitors rather than hiring college graduates, who take three to five years to reach the level of productivity of more experienced salespeople. These two staffing issues have caused high turnover in many sales forces.
Another weakness of personal selling is message inconsistency. Many salespeople view themselves as independent from the organization, so they design their own sales techniques, use their own message strategies, and engage in questionable ploys to generate sales. (You’ll recall our discussion in the ethics module about the unique challenges that B2B salespeople face.) As a result, it can be difficult to find a unified company or product message within a sales force or between the sales force and the rest of the marketing mix.
A final disadvantage of personal selling is that sales-force members have different levels of motivation. Salespeople may vary in their willingness to make the desired number of sales calls each day; to make service calls that do not lead directly to sales; or to take full advantage of the technologies available to them.
How IMC Supports Personal Selling4
As with any other marketing communication method, personal selling must be evaluated on the basis of its contribution to the overall marketing mix. The costs of personal selling can be high and carry risks, but the returns may be just as high. In addition, when personal selling is supported by other elements of a well-conceived IMC strategy, it can be very effective indeed.
Consider the following example of Audi, which set out to build a customer-relationship program:
Audi’s goal was to not have the relationship with the customer end after the sale was made. Operating on the assumption that the company’s best potential customers were also its existing customers, the company initiated an online program to maintain contact, while allowing its sales force to concentrate on selling. Based on its television campaign for the new A4 model, Audi offered a downloadable screensaver that frequently broadcasted updated news and information automatically to the consumers’ computers. After displaying the screensaver option on its Web site, Audi sent an email to owners and prospects offering them the opportunity to download it. More than 10,000 people took advantage of the offer. Audi then began to maintain a continuous dialog with the adopters by sending them newsletters and updates. Click-through rates ranged from 25 to 35 percent on various parts of the site—well exceeding the standard rates—and car sales were 25 percent higher than they were the previous year, even in a down economy.5
As a result of several coordinated communication methods (TV advertising, email, downloadable screensaver, newsletters, and product information) and presumably a well-designed customer relationship management (CRM) system, Audi helped its sales force be more effective (by freeing it up to focus on sales and by connecting it with more prospective customers), which, turn, meant higher profits.
- http://smallbusiness.chron.com/strategic-selling-techniques-15747.html
- http://www.smetimes.in/smetimes/in-depth/2010/Sep/02/personal-selling-when-and-how500001.html
- http://www.smetimes.in/smetimes/in-depth/2010/Sep/02/personal-selling-when-and-how500001.html
- http://www.zabanga.us/marketing-communications/how-companies-integrate-personal-selling-into-the-imc-program.html
- http://www.zabanga.us/marketing-communications/how-companies-integrate-personal-selling-into-the-imc-program.html
LICENSES AND ATTRIBUTIONS
CC LICENSED CONTENT, ORIGINAL
- Revision and adaptation. Provided by: Lumen Learning. License: CC BY-SA: Attribution-ShareAlike
CC LICENSED CONTENT, SHARED PREVIOUSLY
- Personal Selling, From Boundless Marketing. Provided by: Boundless. Located at: https://courses.lumenlearning.com/boundless-marketing/. License: CC BY-SA: Attribution-ShareAlike
- Phone call. Provided by: CWCS Managed Hosting. Located at: https://www.flickr.com/photos/122969584@N07/13780153345/. License: CC BY: Attribution
- Communicating to Mass Markets, from Introducing Marketing. Authored by: John Burnett. Project: Global Text. License: CC BY: Attribution
CC LICENSED CONTENT, SPECIFIC ATTRIBUTION
- Handshake. Authored by: Cytonn Photography. Provided by: Unsplash. Located at: https://unsplash.com/photos/n95VMLxqM2I. License: CC0: No Rights Reserved. License Terms: Unsplash License
Reading: Direct Marketing
Direct Marketing: Going Straight to the Customer
Direct marketing activities bypass any intermediaries and communicate directly with the individual consumer. Direct mail is personalized to the individual consumer, based on whatever a company knows about that person’s needs, interests, behaviors, and preferences. Traditional direct marketing activities include mail, catalogs, and telemarketing. The thousands of “junk mail” offers from credit card companies, bankers, and charitable organizations that flood mailboxes every year are artifacts of direct marketing. Telemarketing contacts prospective customers via the telephone to pitch offers and collect information. Today, direct marketing overlaps heavily with digital marketing, as marketers rely on email and, increasingly, mobile communications to reach and interact with consumers.
The Purpose and Uses of Direct Marketing
The purpose of direct marketing is to reach and appeal directly to individual consumers and to use information about them to offer products, services and offers that are most relevant to them and their needs. Direct marketing can be designed to support any stage of the AIDA model, from building awareness to generating interest, desire, and action. Direct marketing, particularly email, also plays a strong role in post-purchase interaction. Email is commonly used to confirm orders, send receipts or warrantees, solicit feedback through surveys, ask customers to post a social media recommendation, and propose new offers.
Direct marketing is an optimal method for marketing communication in the following situations:
- A company’s primary distribution channel is to sell products or services directly to customers
- A company’s primary distribution method is through the mail or other shipping services to send directly to the customer
- A company relies heavily on sales promotions or discounts, and it is important to spread the word about these offers to consumers
- An advertisement cannot sufficiently convey the many benefits of a company’s product or service, and so a longer marketing piece is required to express the value proposition effectively
- A company finds that standard advertising is not reaching its target segments, and so better-targeted marketing communications are required to reach the right individuals; for example, using direct mail to reach wealthier people according to their affluent zip code
- A company sells expensive products that require more information and interaction to make the sale
- A company has a known “universe” of potential customers and access to contact information and other data about these customers
- A company is heavily dependent on customer retention, reorders and/or repurchasing, making it worthwhile to maintain “permissioned” marketing interaction with known customers
Data: The Key to Effective Direct Marketing
The effectiveness of direct marketing activity depends on marketers using databases to capture the information of target customers and the use of this information to extend ever-more-personalized offers and information to consumers. Databases record an individual’s residence, geography, family status, and credit history. When a person moves or makes a significant purchase like a car or a home, these details become part of the criteria marketers use to identify who will be a good target for their products or services. With electronic media, the information flow about consumers opens the floodgates: marketing databases capture when a consumer opens an email message and clicks on a link. They track which links piqued consumers’ interests, what they view and visit, so that the next email offer is informed by what a person found interesting the last time around. These databases also collect credit card information, so marketers can link a person’s purchasing history to shopping patterns to further tailor communications and offers.
Mobile marketing adds another dimension of personalization in direct-to-consumer communications. It allows marketers to incorporate location-sensitive and even activity-specific information into marketing communications and offers. When marketers know you are playing a video game at a mall, thanks to your helpful smart phone, they can send you timing-, location- and activity-specific offers and messages.
Direct Marketing in Action
How does this work in practice? If you’ve ever paid off an auto loan, you may have noticed a torrent of mail offers from car dealerships right around the five-year mark. They know, from your credit history, that you’re nearly done paying off your car and you’ve had the vehicle for several years, so you might be interested in trading up for a newer model. Based on your geography and any voter registration information, you may be targeted during election season to participate via telephone in political polls and to receive “robocalls” from candidates and parties stumping for your vote.
Moving into the digital world, virtually any time you share an email address with an organization, it becomes part of a database to be used for future marketing. Although most organizations that engage in email marketing give the option of opting out, once you become a customer, it is easy for companies to justify continuing to contact you via email or text as part of the customer relationship you’ve established. As you continue to engage with the company, your behavior and any other information you share becomes part of the database record the company uses to segment and target you with offers it thinks will interest you.
Similarly, marketers use SMS (text) for marketing purposes, and direct marketing activity takes place in mobile apps, games, and Web sites. All of these tools use the data-rich mobile environment to capture information about consumers and turn it into productive marketing opportunities. QR codes, another direct-to-consumer mobile marketing tool, enable consumers to scan an image with a mobile phone that takes them to a Web site where they receive special information or offers.
A great illustration of how companies use consumer information for direct marketing purposes comes from a New York Times article that interviewed Andrew Pole, who conducts marketing analytics for the retailer Target. The article discusses how Target uses behavioral data and purchasing history to anticipate customers’ needs and make them offers based on this information:
Target has a baby registry, and Pole started there, observing how shopping habits changed as a woman approached her due date, which women on the registry had willingly disclosed. He ran test after test, analyzing the data, and before long some useful patterns emerged. Lotions, for example. Lots of people buy lotion, but one of Pole’s colleagues noticed that women on the baby registry were buying larger quantities of unscented lotion around the beginning of their second trimester. Another analyst noted that sometime in the first twenty weeks, pregnant women loaded up on supplements like calcium, magnesium, and zinc. Many shoppers purchase soap and cotton balls, but when someone suddenly starts buying lots of scent-free soap and extra-big bags of cotton balls, in addition to hand sanitizers and washcloths, it signals they could be getting close to their delivery date.
As Pole’s computers crawled through the data, he was able to identify about twenty-five products that, when analyzed together, allowed him to assign each shopper a “pregnancy prediction” score. More important, he could also estimate her due date to within a small window, so Target could send coupons timed to very specific stages of her pregnancy.
One Target employee I spoke to provided a hypothetical example. Take a fictional Target shopper named Jenny Ward, who is twenty-three, lives in Atlanta, and in March bought cocoa-butter lotion, a purse large enough to double as a diaper bag, zinc and magnesium supplements, and a bright blue rug. There’s, say, an 87 percent chance that she’s pregnant and that her delivery date is sometime in late August. What’s more, because of the data attached to her Guest ID number, Target knows how to trigger Jenny’s habits. They know that if she receives a coupon via e-mail, it will most likely cue her to buy online. They know that if she receives an ad in the mail on Friday, she frequently uses it on a weekend trip to the store.1
The article goes on to tell the well-documented story of an outraged father who went into his local Target to complain about the mailer his teenage daughter received from Target featuring coupons for infant clothing and baby furniture. He accused Target of encouraging is daughter to get pregnant. The customer-service employee he spoke with was apologetic but knew nothing about the mailer. When this employee phoned the father a few days later to apologize again, it emerged that the girl was, in fact, pregnant, and Target’s marketing analytics had figured it out before her father did.
Advantages and Disadvantages of Direct Marketing
All this data-driven direct marketing might seem a little creepy or even nefarious, and certainly it can be when marketers are insensitive or unethical in their use of consumer data. However, direct marketing also offers significant value to consumers by tailoring their experience in the market to things that most align with their needs and interests. If you’re going to have a baby (and you don’t mind people knowing about it), wouldn’t you rather have Target send you special offers on baby products than on men’s shoes or home improvement goods?
As suggested in the New York Times excerpt, above, direct marketing can be a powerful tool for anticipating and predicting customer needs and behaviors. Over time, as companies use consumer data to understand their target audiences and market dynamics, they can develop more effective campaigns and offers. Organizations can create offers that are more personalized to consumer needs and preferences, and they can reach these consumers more efficiently through direct contact. Because it is so data intensive, it is relatively easy to measure the effectiveness of direct marketing by linking it to outcomes: did a customer request additional information or use the coupons sent? Did he open the email message containing the discount offer? How many items were purchased and when? And so forth. Although the cost of database and information infrastructure is not insignificant, mobile and email marketing tend to be inexpensive to produce once the underlying infrastructure is in place. As a rule, direct marketing tactics can be designed to fit marketing budgets.
Among the leading disadvantages of direct marketing are, not surprisingly, concerns about privacy and information security. Target’s massive data breach in 2013 took a hefty toll on customer confidence, company revenue, and profitability at the time. Direct marketing also takes place in a crowded, saturated market in which people are only too willing to toss junk mail and unsolicited email into trash bins without a second glance. Electronic spam filters screen out many email messages, so people may never even see email messages from many of the organizations that send them. Heavy reliance on data also leads to the challenge of keeping databases and contact information up to date and complete, a perennial problem for many organizations. Finally, direct marketing implies a direct-to-customer business model that inevitably requires companies to provide an acceptable level of customer service and interaction to win new customers and retain their business.
Direct Marketing in the IMC Process
Direct marketing, and email marketing in particular, plays a critical role in many IMC campaigns because it is a primary means of communicating with any named-and-known target audiences. It is a common vehicle for spreading the word about sales promotions and public relations activities. Direct marketing pieces can reuse and reinforce messages and images developed for advertisements, offering another touch point for reaching target segments. QR codes and other mobile marketing tactics may be used at the point of sale to engage customers and persuade them to purchase. Email marketing messages commonly include links to social media, inviting consumers to share experiences, opinions, marketing messages, and offers with their social networks.
Direct marketing can also be a useful tool in personal selling, as it helps marketers and sales representatives efficiently maintain ongoing relationships with customers and prospects as they are nurtured through the sales process. The rich data behind direct marketing also provides insight for sales representatives to help them segment prospective customers and develop offers and sales approaches personalized to their needs and interests.
LICENSES AND ATTRIBUTIONS
CC LICENSED CONTENT, ORIGINAL
- Direct Marketing. Authored by: Lumen Learning. License: CC BY: Attribution
- Screenshot Mobile Advertisement. Provided by: Lumen Learning. License: CC BY: Attribution
- Screenshot Target Baby Registry. Provided by: Lumen Learning. License: CC BY: Attribution
CC LICENSED CONTENT, SHARED PREVIOUSLY
- Email Marketing. Authored by: RaHuL Rodriguez. Located at: https://www.flickr.com/photos/rahulrodriguez/9162677329/. License: CC BY-SA: Attribution-ShareAlike
Reading: Digital Marketing
Digital Marketing: Inform, Entice, Engage
Digital marketing is an umbrella term for using a digital tools to promote and market products, services, organizations and brands. As consumers and businesses become more reliant on digital communications, the power and importance of digital marketing have increased. The direct marketing section of this module already discussed two digital tools: email and mobile marketing, which fit into both categories. This section will discuss other essential tools in the digital marketing tool kit: websites, content marketing and search-engine optimization (SEO), and social media marketing.
What Makes Digital Marketing Tools Unique
In part, digital marketing is critically important because people use digital technologies frequently, and marketing needs to happen where people are. But digital marketing tools also have other unique capabilities that set them apart from traditional (predigital) marketing communication tools. These capabilities make them uniquely suited to the goals of marketing. Digital marketing tools are:
- Interactive: A primary focus of many digital marketing tools and efforts is to interact with target audiences, so they become actively engaged in the process, ideally at multiple points along the way. This may happen by navigating a website, playing a game, responding to a survey, sharing a link, submitting an email address, publishing a review, or even “liking” a post. Asking consumers to passively view an advertisement is no longer enough: now marketers look for ways to interact.
- Mobile and portable: Today’s digital technologies are more mobile and portable than ever before. This means digital marketing tools are also mobile and portable: consumers can access them–and they can access consumers–virtually anytime and anywhere through digital devices. Digital marketing can reach people in places and ways that simply were not possible in the past. A tired mother stuck in traffic might encourage her child to play a game on her smart phone, exposing both child and mother to marketing messages in the process. A text message sent to a remote location can remind an adventurer to renew a subscription or confirm an order. Many physical limitations fall away in the digital world.
- Highly measurable and data driven. Digital technologies produce mountains of data about who is doing what, when, how, and with whom. Likewise, digital marketing tools enable marketers to determine very precisely whom they want to reach, how to reach them, and what happens when people begin the process of becoming a customer. By tracking and analyzing these data, marketers can also identify which channels are most productive for bringing people into the site and what types of interactions are most efficient at turning them into customers.
- Sharable: Because digital marketing tools are digital, it is easy to share them at low or no cost–a benefit for marketers and for consumers who find content they want to share virally. People routinely share videos, games, websites, articles, images, and brands—any number of overt or covert marketing artifacts. In fact, the degree to which something is shared has become a key metric to confirm how successful it is as a marketing vehicle. Sharing has always been a primary means of spreading ideas. Digital marketing tools now facilitate extremely rapid, efficient, global sharing.
- Synergistic with other marketing activities: Digital marketing tools offer quick, easy, and inexpensive ways to repurpose marketing messages and content from other marketing communication methods. They help amplify and reinforce the messages targeting consumers through other media. For example, uploading a TV ad to YouTube creates a piece of digital marketing content that can be posted to Facebook, tweeted on Twitter, embedded in a website page, and shared via an email from a sales representative engaged in personal selling to a target customer.
ALWAYS #LIKEAGIRL
Let’s take a look at this commercial from Always. What did they do to take advantage of digital marketing tools?
You can view the transcript for “Always #LikeAGirl” here (opens in new window).
Not only did Always produce a video on a relevant topic, but they invited people to join the conversation. At the end of the commercial, they invited viewers to share the message, to tweet using #likeagirl, and to visit their website.
The Imperative to Use Digital Marketing
For virtually every organization that wants to do business in the world today, having some level of digital marketing presence is a requirement. A website is quite literally an organization’s digital address and calling card. People of all ages routinely use Web searches for information that shapes their purchasing decisions; using the Web helps them decide where to look, what to buy, where to find it, and how much to pay. Marketers must develop useful Web content and engage in search engine optimization (SEO) strategies in order to make sure their websites will be found when people come looking.
Social media marketing helps organizations tap into the power of word-of-mouth sharing, so that people hear about a company, product, or brand from trusted sources. Social media allow marketers to foster communities and listen in on timely conversations about their brands and products, providing insight into what’s working or not working with their marketing or the customer experiences they provide. Email and mobile marketing reflect the dominant communication patterns in the developed world as well as in many developing countries. Communicating with prospects and customers effectively requires marketers to use these common, everyday tools.
Digital marketing tools are an integral part of most IMC campaigns, as they provide digital communication support to target and reinforce campaign messages and activities in other media. Examples of digital marketing tools supporting broader IMC activity include the following:
- Media companies host and monitor forums for fans to live-tweet during broadcast and cable TV programs, such as The Walking Dead and Empire, including commentary on the programming, advertising, the entertainment “brand,” and nature of the fan community.
- Companies routinely upload television ads to YouTube and then work to create “buzz” by promoting this content through their websites, blogs, Facebook, Twitter, and other social media platforms.
- Well-designed Web content such as research reports, articles, and e-books are used as informational giveaways to generate interest and cultivate leads during trade shows, conferences, and personal selling activities.
Website Marketing
Websites represent an all-in-one storefront, a display counter, and a megaphone for organizations to communicate in the digital world. For digital and bricks-and-mortar businesses, Websites are a primary channel for communicating with current and prospective customers as well as other audiences. A good website provides evidence that an organization is real, credible, and legitimate.
The variety of online website-building services now available make setting up a basic website relatively simple and inexpensive. Once the website is established, it can continue to be fairly easy and inexpensive to maintain if the organization uses cost-effective and user-friendly tools. On the other hand, sophisticated websites can be massively expensive to build and maintain, and populating them with fresh, compelling content can devour time and money. But organizations can adjust the scope, scale, and resources required for their websites in proportion to their business objectives and the value they want their websites to deliver.
Websites As Marketing Tools
Websites are very flexible, allowing organizations to build the kinds of features and capabilities they need to conduct business effectively. Common marketing objectives and website functions include the following:
- Providing general information about an organization such as the value proposition, products and services, and contact information
- Expressing the brand of an organization through design, look and feel, personality, and voice
- Demonstrating products, services, and expertise, including the customer experience, features, benefits, and value they provide
- Proof points about the value a company offers, using evidence in the form of case studies, product reviews, testimonials, return on investment data, etc.
- Lead generation, capturing information about website visitors to use in ongoing sales and marketing activity
- Communities and forums for target audiences to share information and ask/answer questions
- Publishing value-adding content and tools for informational or entertainment purposes to bring people in and draw them back to the website
- Communication about company news, views, culture, developments, and vision through an electronic newsroom or a company blog, for example
- Shopping, providing tools for customers to research, find, and select products or services in the digital environment
- Recommendations that direct customers to information, products, services, and companies that meet their interests and needs
- Sales, the ability to conduct sales and transact business online
- Capturing customer feedback about the organization, its products, services, content, and the website experience itself
Before starting to build a website, the marketing manager should meet with other company leaders to lay out a common vision for what the Website should accomplish and the business functions it should provide. For example, if a business does not plan to handle sales online, there is no need to build a “shopping cart” function or an e-commerce engine. If cultivating lively dialogue with an active customer community is an important business objective, this capability should be incorporated into the website strategy and design decisions from the outset. The website strategy must be effective at achieving the organization’s goals to inform, engage, entertain, explore, support, etc.
Top Tips for Effective Websites
Many factors go into building an effective website. The following table serves as a checklist for key considerations.
| Website Element | Tips and Recommendations |
|---|---|
| Domain name | The domain name is your digital address. Secure a name that is memorable and functional for your business. |
| Look and feel | A site’s look and feel conveys a lot about a company. Make sure your site makes positive impressions about credibility, product quality, the customer experience, etc. |
| Messaging | Messaging and how it is presented can draw people in or turn them off immediately. Find concise, compelling ways to tell your story. |
| Design | Website design is about usability as well as aesthetics. Make conscious choices about how design expresses your brand personality as well as its role in making the user experience intuitive and effective. |
| Structure | Structure the website and organize information so that it is easy for visitors to navigate the site and find what they want. |
| Content quality | To a large degree, the quality of content is what brings traffic into a website (more on this soon). Produce content and organize it so it can drive traffic, move customers through the sales cycle, and generate business. |
| Content variety | Use a mix of professional-quality text, images, video, and other visual content to make your website interesting and readable. |
| Language | Typos and grammatical errors are an immediate website turnoff. Proofread everything with fresh eyes before you publish. |
| Accessibility | Follow basic principles of website accessibility to ensure that people can use your site effectively regardless of device or disability. |
| Call to action | Provide cues for your website visitors about what to do next. Give each page a clear call to action and a path that invites people to keep exploring and moving closer to a purchasing decision. |
| Analytics | Track website traffic and usage patterns using a tool like Google Analytics. Monitor which website pages get attention and which ones flop. Use what you learn to improve how well your website meets your objectives. |
Advantages and Disadvantages of Website Marketing
Websites have so many advantages that there is almost no excuse for a business not to have one. Effective website marketing declares to the world that an organization exists, what value it offers, and how to do business. Websites can be an engine for generating customer data and new business leads. An electronic storefront is often dramatically less expensive than a physical storefront, and it can serve customers virtually anywhere in the world with internet access. Websites are very flexible and easy to alter. Organizations can try out new strategies, content and tactics at relatively low cost to see what works and where the changes pay off.
At the same time, websites carry costs and risks. They do require some investment of time and money to set up and maintain. For many organizations, especially small organizations without a dedicated website team, keeping website content fresh and up-to-date is a continual challenge. Organizations should make wise, well-researched decisions about information infrastructure and website hosting, to ensure their sites remain operational with good performance and uptime. Companies that capture and maintain customer data through their websites must be vigilant about information security to prevent hackers from stealing sensitive customer data. Some company websites suffer from other types of information security challenges, such as electronic vandalism, trolling (offensive or provocative online posts), and denial-of-service attacks mounted by hackers to take websites out of commission.
Search-Engine Optimization and Content Marketing
Search-engine optimization (SEO) is the process of using Internet search engines, such as Google, Bing, and Yahoo, to gain notice, visibility, and traffic from people conducting searches using these tools. SEO works in lockstep with content marketing, which takes a strategic approach to developing and distributing valuable content targeted to the interests of a defined audience, with the goal of driving sales or another profitable customer action. In other words, content marketers create worthwhile Internet content aimed at their target audiences. Then organizations use SEO tactics to get this content noticed and to generate new traffic and sales leads.
Together, SEO and content marketing can help boost awareness and brand perceptions about the value a company provides. Content marketing can help an organization gain visibility as an expert or leader in its competitive set. Together these marketing communications tools help organizations get noticed and stay top of mind among individuals seeking the types of products or services they offer.
How SEO Works
The basic premise behind search-engine optimization is this: People conduct Internet searches. The search terms they use bring up a given set of results. When someone is searching for the types of things your organization offers, as a marketer you want your results to be at the top. You can boost your search rankings by identifying and applying SEO and content marketing strategies to the search terms people use when they are looking for products or services like yours. It may even be worth paying to get their attention, because people searching for the things you offer are likely to be better-qualified prospective customers.
Because the supply of Internet content on any given topic is continually expanding, and because search-engine companies regularly fine-tune their search algorithms to deliver ever more helpful results, SEO is not a one-time task. It’s an ongoing process that companies should incorporate into their entire approach to digital marketing.
In the world of SEO, there are two types of search results: 1) organic (or unpaid) search results, and 2) inorganic (or paid) search results.
Organic search results are the unpaid listings that appear solely because of their relevance to the search terms entered when you conduct an Internet search. These are unpaid listings, and they earn their place because the search engine determines they are most relevant and valuable based on a variety of factors including the content itself and the popularity of that content with other Internet users.
Inorganic, or paid search results, appear because companies have paid the search engine for a high-ranking placement based on the search terms used. Organizations bid for this placement and typically pay per click when someone clicks through to a website. Most search engines mark the paid results as ads, so that Internet users can distinguish between organic and paid search results. In Figure 1, below, the results preceded by the word Ad in yellow indicate paid search results from a Google search of “cats for sale.”
The following short video explains what makes Google AdWords so powerful.
You can view the transcript for “Google AdWords” here (opens in new window).
Marketers use key-word research to guide their efforts to improve their rankings for both organic and inorganic searches. Key-word research helps marketers identify the search terms people are most likely to use when looking for the types of products, services, or information their website offers. Tools such as freely available Google AdWords Keyword Planner and Google Trends help marketers identify and compare popular search terms. Armed with optimal search key words, they can buy high-ranking placement in inorganic, paid search results for their search terms of choice. They can improve their organic (unpaid) search rankings by applying content marketing strategies.
How Content Marketing Works
There is a popular saying among digital marketers: “Content is king.” Good content attracts eyeballs, while poor content does not. Content marketing is based on the premise that marketers can use web content as a strategic asset to attract attention and drive traffic of target audiences. As a marketer, part of your job is to help the organization publish substantive web content–articles, videos, e-books, podcasts, images, infographics, case studies, games, calculators, etc.–that will be interesting for your target segments. When you do this, you should incorporate your optimal search terms into the content, so that it’s more likely to show up in organic search results. You should also look for ways to link to that content from other webpages, so that search-engine “bots” (or computer programs) responsible for cataloguing websites will think your content is popular and well regarded by the Internet-user community. As your content appears in search results, it will rank higher as more and more people click through to your content and link to it from other locations on the Internet.
Top Tips for SEO and Content Marketing
You can use the following simple recommendations to realize the benefits of SEO and content marketing. When the two work together, they can support your organization’s success raising its profile, improving search rankings, and generating traffic and new business.
| SEO/Content Element | Tips and Recommendations |
|---|---|
| Content quality | Make website content substantive, and showcase your expertise. Create material that makes people want to stay on your site to keep reading, interacting, and exploring. |
| Key-word research | Conduct key-word research to learn what actual search terms people are using that relate to your goods, products, services, and brand. |
| Incorporate key words | Make sure your content matches the search terms you want to be associated with. Be sure to use actual, real-world search terms in order to get the bump to higher rankings. |
| Content freshness | Search-engine algorithms like new content, as well as content where there is a flurry of activity. Create and promote fresh new content regularly to get the “freshness boost” in search results. |
| Evergreen content | Be sure to develop some Web content that won’t age and become outdated quickly, such as news releases. Persistently useful, interesting content generates more visits, more external links from other sites, and higher search rankings. |
| Internal links | Create internal links between content pages on your website. This points users to additional material that may interest them. It also helps search engines crawl through your site to reach and discover all of your content. And more sites that link to a page help boost that page’s search rankings. |
| Headlines | Create great headlines for your Web content that grab attention while also helpfully indicating what the content will provide. Also, make sure your content delivers on the headline. |
| Call to action | Include a clear call to action on each Web page or content element, whether that involves sharing information, registering for a webinar, downloading an e-book, or linking to another Web page. Use content and calls to action to move people through the AIDA model toward purchasing decisions. |
| Promoting content | Once content is published, use other marketing communication tools to promote it. Write posts about it on Twitter, Facebook, LinkedIn, Google+, or other social networks of choice. Send email messages to active sales opportunities. Link to it from the Website home page. Create a flurry to help give it an SEO boost. |
Advantages and Disadvantages of SEO and Content Marketing
Internet search is a fact of life in the modern world. It is a critical tool for customer decision making in B2B and B2C markets. Practicing the basic tenets of SEO helps an organization get into the search-engine fray. When marketers do it skillfully, they can easily track the results, see what works, and adjust course to improve outcomes. When organizations generate high-quality content, it can be relatively inexpensive to achieve great SEO results, particularly as search engines themselves increasingly reward the “real deal”: good information and true substance targeted to a specific audience.
While SEO and content marketing are powerful tools, they are also rather like puppies that need ongoing feeding and care. Both require regular monitoring to check whether they are effective and need refreshing. The Internet is a crowded and competitive place, where organizations from around the globe can compete with one another for attention and customer loyalty. It takes persistence and hard work to get on top of the Internet content world and stay there.
Social Media Marketing
Social media marketing is the use of online applications, networks, blogs, wikis, and other collaborative media for communicating brand messaging, conducting marketing, public relations, and lead generation. Social media are distinctive for their networking capabilities: they allow people to reach and interact with one another through interconnected networks. This “social” phenomenon changes the power dynamic in marketing: no longer is the marketer the central gatekeeper for all communication about a product, service, brand, or organization. Social media allows for organic dialogue and activity to happen directly between individuals, unmediated by a company. Companies can (and should) listen, learn, and find ways to participate authentically.
Social media marketing focuses on three primary objectives:
- Creating buzz: Developing and publishing messages (in a variety of formats–e.g., text, video, and images) that is disseminated via user-to-user contact
- Fostering community: Building ways for fans to engage with one another about a shared interest in a brand, product, or service
- Facilitating two-way communication: Online conversations are not controlled by the organizations. Instead, social media promotes and encourages user participation, feedback, and dialogue
How Social Media Marketing Works
Organizations have opportunities to engage in social media for marketing purposes in several ways: paid, earned, and owned social media activity.
- Paid: Paid social media activity includes advertisements on social media (placed in various locations), sponsored posts or content, and retargeting advertisements that target ads based on a consumer’s previous actions. This type of social media activity is best suited for sales, lead generation, event participation, and incorporation into IMC campaigns.
- Earned: Earned social media activity involves news organizations, thought leaders, or other individuals who create content about an organization. It is particularly suited to supporting public relations efforts.
- Owned: Owned social media activity happens through social media accounts that an organization owns (e.g., Facebook page, Twitter handle, Instagram name, etc.). This activity is ideal for brand awareness, lead generation, and goals around engaging target audiences.
Effective use of social media to reach your target audience requires more effort by an organization than the traditional marketing methods. Not only must an organization create unique content and messaging, but it must be prepared to engage in two-way communication regarding the content that it produces and shares on social media. To be effective at using social media to reach target audiences, an organization must:
- Create unique content, often. Social media, unlike traditional methods, cannot rely on static content. An organization must regularly publish new, unique content to stay relevant on any social media platform.
- Ask questions. To foster engagement, an organization must solicit feedback from users, customers, and prospects. This is critical to creating conversation, insight, and discussion on social media platforms.
- Create short-form media. Most social media platforms have character limits per post. Users on social media expect to be able to scan their feed. Long posts (even within character limits) tend to underperform. The more succinct an organization can be, the better.
- Try different formats. Most social media platforms provide users with the option to add images and video to text. Social media is becoming an increasingly visual medium, where content that performs the best usually includes an image or video. Try to convert messages into images and video when possible for maximum reach.
- Use a clear, immediate call to action. Social media works best for achieving marketing goals with a clear call to action that a user can do immediately from their computer or mobile device. Examples include 1) Web traffic (click-through), 2) downloads of content (e.g., white papers, articles, etc.), 3) online purchases, and 4) engagement (comment, like, share, view, read).
Common Social Media Marketing Tools
What’s hot in social media is a moving target, but the following table provides a listing and description of primary social media platforms.
| Tool | Description |
|---|---|
| Blogs | Long- or short-form medium for communicating with audiences |
| YouTube | Video-hosting social media site |
| Short-form (280 character) “microblogging” medium that is intended for text and image sharing | |
| Long-form (up to 2,000 characters per post) medium for sharing text, images, videos, and other multimedia content | |
| Image-based social network that is intended as a visual medium. Does not have capabilities to drive click-through rate (CTR) because posts offer no link option | |
| Google+ | Long-form medium for sharing text, images, videos, and other multimedia content |
| Medium for sharing photos and visual content categorized by theme | |
| Long- or short-form medium for sharing text, images, videos, and other multimedia content targeted to the business community |
Top Tips for Social Media Marketing
The following tips help break down the process of mounting a successful social media marketing strategy.
| Activity | Tips and Recommendations |
|---|---|
| Start with SWOT | Start by conducting a SWOT analysis of your social media activity. Evaluate how your organization is currently using social media, as well as the competition (platforms, messaging, tactics, and campaigns). |
| Establish a baseline | Establish a baseline. Take measurements for current reach and engagement before starting to use social media for marketing. This will help you gauge the impact and improve as you pursue a social media strategy. |
| Set goals | Set specific goals for your social media campaign. Make them S.M.A.R.T. goals that align with your broader marketing strategy. |
| Target audience | Understand how your target audience is using social media (and what platforms). |
| Platforms | Identify which social media platforms you will use and what you want to accomplish in each. |
| Ownership | Identify who within the organization will “own” and share responsibility for social media participation. Work out plans for how to coordinate activity and messaging if there are multiple owners. |
| Testing | A/B test your content using the targeting features of the social media platform. Figure out which types of posts, messages, content, and topics generate better response. |
| Measurement | Regularly take measurements for how much engagement your efforts are producing. Compare them to the benchmark and assess progress toward goals. |
| Monitor | Monitor social media activity regularly and be sure to respond to customers, prospects, and other users. |
Advantages and Disadvantages of Social Media Marketing
The advantages and benefits of social media marketing focus heavily on the two-way and even multidirectional communication between customers, prospects, and advocates for your company or brand. By listening and engaging in social media, organizations are better equipped to understand and respond to market sentiment. Social media helps organizations identify and cultivate advocates for its products, services, and brand, including the emergence of customers who can become highly credible, trusted voices to help you sell. Unlike many other forms of marketing, social media are very measurable, allowing marketers to track online customer behavior and how target audiences respond to content created by the organization. Social media offers a virtually unlimited audience for communicating and sharing key messages in the market. It also offers marketers the ability to relatively easily target and test the effectiveness of content using the various targeting capabilities of social media for location, interests, income, title, industry, and other sociographic differentiators.
Social media also carry a number of inherent challenges. Social media are dynamic environments that requires significant effort to monitor and stay current on. It is also difficult to continually create “share-worthy” content. The variety of social media tools makes it a challenge to understand which platforms to use for which target audiences and calls to action. Crisis communications can be difficult, too, particularly in the public environment of social media, in which it is difficult to contain or control communication. This means it can be difficult to mitigate the impact of a crisis on the brand.
One of the biggest challenges facing organizations is determining who in the organization should “own” the social media platforms for the organization. Too few hands to help means the burden of content creation is high on a single individual. However, too many people often results in duplication of efforts or conflicting content.
Expert Insight on Using Social Media: JetBlue
Airline carrier JetBlue has received attention and accolades for its effective use of social media to foster two-way communication with customers. In this video, JetBlue’s head of social media strategy, Morgan Johnston, explains the company’s approach to social media and how it complements other corporate and marketing communication activity. He also shares insights about how the company used social media to manage crisis communications and respond to customers during Hurricane Sandy, when extreme weather conditions hit the company’s northeastern U.S. travel routes hard.
LICENSES AND ATTRIBUTIONS
CC LICENSED CONTENT, ORIGINAL
- Social Media Marketing. Authored by: Melissa Barker. Provided by: Spokane Falls Community College. License: CC BY: Attribution
- Digital Marketing. Authored by: Lumen Learning. License: CC BY: Attribution
CC LICENSED CONTENT, SHARED PREVIOUSLY
- Google AdWords. Provided by: BBC. Located at: https://youtu.be/NIL2POjhvI8. License: CC BY-NC-ND: Attribution-NonCommercial-NoDerivatives
ALL RIGHTS RESERVED CONTENT
- Always #LikeAGirl. Provided by: Always. Located at: https://youtu.be/XjJQBjWYDTs. License: All Rights Reserved. License Terms: Standard YouTube license
- Social Fresh interview with Morgan Johnston of jetBlue. Provided by: Social Fresh. Located at: https://youtu.be/mzsN3oEV1YE. License: All Rights Reserved
- Screenshot Cats for Sale Search. Provided by: Google. Located at: https://www.google.com/. License: All Rights Reserved. License Terms: Fair Use
- Farmer's Insurance Web Site. Provided by: Farmer's Insurance. Located at: https://www.farmers.com/. License: All Rights Reserved. License Terms: Fair Use
Reading: Guerrilla Marketing
Guerrilla Marketing: Thinking Outside the Box
Guerrilla marketing is a relatively new marketing strategy that relies on unconventional, often low-cost tactics to create awareness of and goodwill toward a brand, product, service, or even a company. The term “guerrilla marketing” itself comes from Jay Conrad Levinson, who coined the term in his 1984 book Guerrilla Advertising. Though “guerrilla” has military connotations (the word means “little war), guerrilla promotion strategies often combine elements of wit, humor, and spectacle to capture people’s attention and engage them in the marketing act. Guerrilla marketing is memorable. And, like the renegade militias it was presumably named for, unexpected.
Practitioners of guerrilla marketing today have used other words to describe it: disruptive, anti-establishment, newsworthy, and a state of mind. By its nature, guerrilla marketing defies precise description, so it may be worthwhile to view an example before going further.
CLASSIC GUERRILLA: NIKE LIVESTRONG AT THE TOUR DE FRANCE
Although this campaign was a full-blown IMC effort, at its core it was really a memorable guerrilla marketing stunt: the spectacle of painting the streets of France during the world-famous Tour de France bicycle race. It ran in 2008 when Lance Armstrong was still one of the most revered athletes of his generation. Designed to generate awareness for Nike, the nonprofit Livestrong Foundation, and the cause of fighting cancer, marketers succeeded in sharing inspiring messages of hope with their target audiences: athletes, sports enthusiasts and people affected by cancer, particularly young people.
You can view the transcript for “Nike Livestrong Chalkbot Web Film” (opens in new window).
Telltale Signs of Guerrilla Marketing
Guerrilla marketing campaigns can be very diverse in their approach and tactics. So what do they have in common? Guerrilla marketing often has the following characteristics:
- It’s imaginative and surprising, but in a very hip or antiestablishment way
- Doesn’t resemble a traditional marketing initiative, such as a straightforward print or TV advertising campaign
- Uses combinations of different marketing communications tactics, in creative ways
- Is experiential, drawing in the target audience to participate
- Takes risks in what it aspires to accomplish, even if it might ruffle some feathers
- Is not 100 percent approved by the establishment (i.e. the city, the event planners, the powers that be)
When to Use Guerrilla Marketing
This edgy marketing approach focuses on two goals: 1) get media attention, and 2) make a positive and memorable connection with your target audience. Many noteworthy guerrilla campaigns, like Nike Livestrong, focus on creating an experience that embodies the spirit of the brand. Often these projects invite people who encounter the campaign to become co-conspirators in achieving the campaign’s vision and reach.
Guerrilla marketing experts assert that this technique can work for virtually any brand or organization, so long as the organization doesn’t mind taking some risks, and so long as the project is true to who you are and what you represent. The right concept for the guerrilla marketing effort should capture your organization’s authentic voice and express what is unique about your brand identity. At some point you may be asked to stand up for your actions if you’re called onto the carpet, so you need to believe in what you are doing. Guerrilla marketing is particularly suited to small, imaginative organizations that may not have much money but have a burning desire to do something memorable—to make an entrance or a splash. Severe budget constraints can encourage creative teams to be very inventive and original.[1]
Because it is inherently spectacle, guerrilla marketing tactics work very well for building brands and generating awareness and interest in an organization, product, service, or idea. They aim to put a company on the map–the mind-share map. It’s interesting that guerrilla marketing often calls on the audience to engage or take action, but turning participants into a paying customers may not be the goal. However, successful guerrilla marketing can make audiences undergo a kind of “conversion” experience: if the impact is powerful enough, it can move consumers further along the path towards brand loyalty.
VOLKSWAGEN: TAKE THE SLIDE!
Take a look at the following guerrilla marketing spectacle organized by Volkswagen. Notice how the event capitalizes on a unique combination of emotional appeal and surprise. (Note: there is no narration to the video; just background music.)
You can access the text alternative for Speed Up Your Life, Take the Slide (opens in new window).
Guerrilla Marketing Tactics: The Usual Suspects
As you saw in the example of the lamppost transformed into a McDonald’s coffeepot, all kinds of spaces and urban environments present opportunities for the guerrilla marketer. In fact, guerrilla marketing initiatives can be executed offline or online. Some companies feel that an edgy, unexpected online campaign with creative guerrilla elements is a little safer than executing a project in the bricks-and-mortar world.
It goes against the very notion of guerrilla marketing to establish a set of tactics or practices that are “conventional” or “typical.” However, the following list describes some examples of guerrilla marketing tactics from noteworthy campaigns, which will give you an idea of what’s been used in the past.[2]
| Guerrilla Tactic | Description |
|---|---|
| Graffiti | Graffiti marketing, a subset of guerrilla marketing, turns walls, alleys, and streets into larger-than-life canvases for marketing activity. |
| Stencil graffiti | Use of stencils to create repeated works of graffiti, with the stencils enabling the project team to rapidly recreate the same work in multiple locations. Stencils tend to be smaller-scale and simpler than classic graffiti art. |
| Undercover, or stealth marketing | Use of marketers or paid actors to go “undercover” among peers to engage unsuspecting people in a marketing activity of some sort. For example, attractive actors are paid to strike up conversations, rave about a new mobile device, and then ask people to take a photo using the device, so that they get hands-on experience with the product in question. |
| Stickers | Inventive use of stickers as a temporary medium for creating an image, posing an illusion, or conveying a message |
| Flash mobs | A group of people organized to perform an action at a predetermined place and time; usually they blend in with bystanders initially and then join the “mob” activity at a designated moment. |
| Publicity stunts | Extraordinary feats to attract the attention of the general public, as well as media |
| Treasure hunts | Placing a series of online and offline “treasure hunt” clues in an urban environment and inviting target audiences to participate in the hunt to win prizes and glory |
| Sham events | Staging an activity or event that appears real, but in fact is a fake, for the purposes of drawing attention and making a statement |
Despite the irreverent, anti-establishment spirit of guerrilla marketing, marketers should use good judgment about seeking permission from building owners, city managers, event planners, or others in a position of authority, to avoid unpleasant or unnecessary complications. Some coordination, or even a heads-up that something is happening, can go far toward earning goodwill and a cooperative spirit in the face of an unexpected spectacle.
How NOT to Guerrilla Market
When three guerrilla marketing veterans spoke with Entrepreneur about their work, they gave their top advice about what NOT to do with these projects:3
- Adam Salacuse of ALT TERRAIN: “Never aim to upset, scare, or provoke people in a negative way. The goal should be to implement something that people will embrace, enjoy, and share with friends.”
- Brett Zaccardi of Street Attack: “Don’t be contrived or too bland. Don’t try to be something you’re not.”
- Drew Neisser of Renegade Marketing: “Try not to annoy your target. [It] is generally not a good idea to do something that will cause someone on the team to go to jail.”
Advantages and Disadvantages of Guerrilla Marketing
Guerrilla marketing has several notable advantages. It can be inexpensive to execute—it’s often much cheaper than traditional advertising when you consider the number of impressions and amount of attention generated. It encourages creativity and inventiveness, since the goal is to create something novel and original. Guerrilla marketing is about buzz: it is designed for viral sharing, and it taps into powerful word-of-mouth marketing as people share their memorable guerrilla-inspired impressions and experiences with friends and acquaintances. A guerrilla marketing phenomenon can take on a life of its own and live in the memories of the people it affected long after the actual event is over. Finally, when executed effectively, guerrilla tactics are designed with media and publicity in mind. Media attention can snowball and generate a larger-than-expected “bounce” as local or even national outlets choose to cover these events.
As suggested above, guerrilla marketing also carries some disadvantages and risks. When an (apparently) spontaneous activity springs up in a public space, property owners, the police, and other authorities may object and try to interfere or stop the event. Unexpected obstacles can arise, which even the best-laid plans may have missed: weather, traffic, current events, timing, etc. Some audiences or bystanders may misinterpret what is happening, or even take offense at provocative actions or messages. When guerrilla projects are cloaked in secrecy or mystery, people may become uncomfortable or fearful, or the aura of mystery may cause them to interpret the message and goals incorrectly. Similarly, if people feel they have been duped by a guerrilla marketing activity, they may come away with negative impressions. If some people disapprove of a given guerrilla marketing activity or campaign, there’s a risk of backlash, anger, and frustration.
Compared to traditional marketing, guerrilla tactics are definitely riskier. Then again, the rewards can be brilliant, when things go as planned.
The Role of IMC in Guerrilla Marketing
As noted above, one telltale sign of guerrilla marketing is the way it blends multiple tactics to create maximum exposure and impact. Most guerrilla marketing campaigns incorporate multiple marketing communication methods and tools to carry out the the full vision. This makes them more than IMC compatible—they are really IMC dependent. For example, organizers of guerrilla stunts and feats frequently film their activities and post them online to generate (hopefully) viral videos and other content. Real-world guerrilla messages and promotional pieces often include information to access company Web sites, where custom-designed landing pages welcome visitors to the online counterpart of the guerrilla experience.
Social media is a staple of guerrilla marketing. Organizing, publicizing, and sharing a campaign’s outcomes and impact may all take place through social channels. Social media also helps generate the buzz that drives guerrilla content to become viral. As guerrilla activities draw media attention, they intersect with PR and media relations.
- http://www.entrepreneur.com/article/206202
- http://www.wordstream.com/blog/ws/2014/09/22/guerrilla-marketing-examples
- http://www.entrepreneur.com/article/206202
LICENSES AND ATTRIBUTIONS
CC LICENSED CONTENT, ORIGINAL
- Guerrilla Marketing. Provided by: Lumen Learning. License: CC BY: Attribution
CC LICENSED CONTENT, SHARED PREVIOUSLY
- Downtown Guerilla Marketing. Authored by: Daniel X. O'Neil. Provided by: Flickr. Located at: https://flic.kr/p/PSjaN. License: CC BY: Attribution
ALL RIGHTS RESERVED CONTENT
- Nike Livestrong - ChalkBot - Web Film. Located at: https://youtu.be/iCLdyKHxBnQ?list=PLGhn_uYiaIc8injQ6hgMPXHwmXc3l66Hx. License: All Rights Reserved. License Terms: Standard YouTube license
- Speed up your life - Take the slide!. Provided by: Volkswagen. Located at: https://youtu.be/W4o0ZVeixYU. License: All Rights Reserved. License Terms: Standard YouTube license
Simulation: IMC Hero
Try It
Congratulations: you’ve been learning a lot about IMC, and if the length of this module is any indicator, there’s a lot to learn!
Are you sick of just reading about integrated marketing communications and ready to actually try it?
You’re in luck. These simulations give you the opportunity to start up your marketing engine and see what you can do with IMC. Play the simulations below multiple times to see how different choices lead to different outcomes. In this simulation environment, you don’t have to shy away from choices that seem a little off: you can learn as much from the wrong choices as the right ones. All simulations allow unlimited attempts so that you can gain experience applying the concepts.
Have fun!
LICENSES AND ATTRIBUTIONS
CC LICENSED CONTENT, ORIGINAL
- Simulation: Integrated Marketing. Authored by: Clark Aldrich for Lumen Learning. License: CC BY: Attribution
|
oercommons
|
2025-03-18T00:39:13.992221
|
06/06/2022
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/93470/overview",
"title": "Statewide Dual Credit Principles of Marketing, Promotion: Integrated Marketing Communication (IMC), Marketing Communication Methods",
"author": "Anna McCollum"
}
|
https://oercommons.org/courseware/lesson/93469/overview
|
Determining IMC Objectives and Approach
Overview
Provided by: Lumen Learning. License: CC BY: Attribution
Outcome: Determining IMC Objectives and Approach
What you’ll learn to do: explain factors to consider when selecting marketing communication methods to execute the strategy
It’s clear that with the growing proliferation of communication tools and methods, integrated marketing communications are the way of the future—and now. Seemingly every day brings a new social media tool or digital marketing technique to engage people in new ways. Traditional marketing communications methods and media are also stepping up their games, offering new ways to create value for companies trying to connect with their target audiences. For example, old-school conferences and trade shows now feature active mobile and social media elements that have been incorporated into their design. TV shows can sell ad space and sponsorship on air, online, and on social media feeds. Radio programs publish their podcast counterparts, complete with ads and sponsors.
For marketers, all this is great news: plenty of choices and ample opportunity to connect with customers in new ways.
But is it great news? The variety of marketing communication methods and tools can be overwhelming. How do you even get started designing an IMC program? And once you have picked an approach, how do you know you’re on the right track?
These are big questions marketers ask themselves regularly. Because marketing is a constantly evolving field, the right answer on one day might be different six months later. However, there are time-tested models that can help you apply a systematic approach to defining what you want to accomplish with IMC and how to select an approach that is best suited to your objectives.
The specific things you’ll learn in this section include:
- Discuss the AIDA model and the role of marketing communications to help move contacts toward a purchasing decision.
- Describe push vs. pull marketing strategies
- Explain the S.M.A.R.T. model for developing IMC goals and objectives
- Discuss the process of selecting marketing communication methods and tactics to fit the target audience and marketing objectives
Learning Activities
- Reading: Determining IMC Objectives and Approach
- Video: Prioritizing Marketing Communications
LICENSES AND ATTRIBUTIONS
CC LICENSED CONTENT, ORIGINAL
- Provided by: Lumen Learning. License: CC BY: Attribution
Reading: Determining IMC Objectives and Approach
Laying the Foundation for Effective Marketing Campaigns
To use integrated marketing communication (IMC) effectively in marketing campaigns, marketers go through several planning steps to define precisely what they want to accomplish and with whom. Only with this information can they be sure they are identifying the right message and promotional mix to achieve their goals.
Standard marketing campaign planning steps include the following:
- Determine the target market
- Determine purpose and objectives for the IMC campaign
- Set S.M.A.R.T. goals
- Define the message
- Select marketing communications methods and tools
- Determine the promotional mix: which tools to use, when, and how much
- Execute the campaign
- Measure results and refine approach, as needed
Step 1: Determine Target Market
In the segmentation and targeting module, as well as in other sections of this course, we’ve discussed the critical importance of clearly identifying the target market or the set of market segments an organization plans to focus on. A marketing plan may include one or more campaigns focused on one or more target segments. Some campaigns may focus on achieving specific goals for a single segment. Other campaigns may focus on a common set of goals using a variety of IMC activities targeting different segments.
In any case, clearly defining the audience for IMC activities is an essential input. This is because different market segments use different types of media, and they may have other distinctive characteristics that impact the effectiveness of a marketing activity. For example, in 2018, 68 percent of all Internet users were also Facebook users. Its usage is growing among older Americans: 41 percent of Americans aged 65 and over used Facebook (compared with 20 percent in 2012). Meanwhile, 35 percent of Facebook users are under 25 (with an additional 30 percent of users aged 25–34)—for a total of 65 percent of users under 35.[1]
Your decision about whether to use Facebook in an IMC campaign should depend, in part, on what proportion of the target audience you can reach with this tool. Understanding your target segment(s) and their communication and media habits will make a huge difference in your ability to design IMC programs to reach the people you want to reach.
Step 2: Determine Marketing Campaign Objectives
Once the audience is defined, the next essential step for a successful marketing campaign is to define what the campaign will accomplish with its IMC efforts. Although many marketing campaigns may be oriented toward a single objective, it is possible for an IMC program to accomplish more than one objective at a time, so long as this doesn’t create confusion for your target audiences.
The objectives should explain the following two items:
- the impact of campaign activity on target audiences
- the ultimate results or outcomes that align with the organization’s marketing strategy and corporate goals
While the objective of a marketing campaign often involves increasing sales, this does not necessarily have to be an objective. An entire campaign might focus primarily on building awareness and persuading people to engage with a product or brand in some way, as a stepping-stone towards generating demand and increasing sales.
A good place to help with thinking through campaign objectives is to consider the cognitive stages a customer goes through as they become aware of and eventually decide to buy a brand, product, or service. Many marketers use the AIDA model to guide this thinking and help them pinpoint campaign objectives for a given audience.
Communicating with Target Segments: The AIDA Model
AIDA is an acronym marketers use to help them develop effective communication strategies and connect with customers in a way that better responds to their needs and desires. Credited to the American advertising and sales pioneer, Elias St. Elmo Lewis, the model originally applied mainly to advertising. AIDA describes a common list of events that occur when a consumer views an advertisement or other marketing communication. As marketing communication methods have evolved, the model has been used to encompass other marketing tools and channels as well.
The letters in the AIDA acronym stand for the following:
- A represents attention or awareness, and the ability to attract the attention of the consumers.
- I is interest and points to the ability to raise the interest of consumers by focusing on and demonstrating advantages and benefits (instead of focusing on features, as in traditional advertising).
- D represents desire. The advertisement convinces consumers that they want and desire the product or service because it will satisfy their needs.
- A is action. Consumers are led to take action by purchasing the product or service.
The system helps guide marketers to refine their objectives and clarify what they want to accomplish with a target segment. As campaign objectives become clearer, marketers gain insight into ways of refining their marketing messages and deciding which tools they can use to deliver these messages effectively.
The table, below, identifies typical campaign objectives associated with each stage of the AIDA model. Note that the largest group of prospective customers appears in the first stage of the model: Awareness. As the sales cycle progresses, a percentage of prospects is lost at each stage.
Let’s take a look at typical campaign objectives in each stage:
- Awareness: Build awareness to motivate further action
- Develop brand awareness and recognition
- Increase traffic to physical or virtual stores, Web sites, or other channels
- Remind customers about a brand, product, service or category
- Interest: Generate interest by informing about benefits; shaping perceptions
- Differentiate a product, stressing benefits and features not available from competitors
- Provide more information about the product or the service because information may be correlated with greater likelihood of purchase
- Increase demand for a specific product or a product category; generate enough interest to research further
- Desire: Create desire; move from “liking” to “wanting”
- Build brand equity by increasing customer perceptions of quality, desirability, and other brand attributes
- Stimulate trial, an important step in building new brands and rejuvenating stagnant brands
- Change or influence customer beliefs and attitudes about a brand, product, or category, ideally creating an emotional connection
- Action: Take action toward purchasing
- Reduce purchase risk to make prospective customers feel more comfortable buying a new or unfamiliar product or brand
- Encourage repeat purchases in the effort to increase usage and brand loyalty
- Increase sales and/or market share, with the goal of broadening reach within a time period, product category, or segment
MINI AND THE AIDA MODEL
Car marketing is a prime example of using the AIDA model to narrow the target market and get results. Marketers in the automotive industry know their advertisements and other marketing communications must grab the attention of consumers, so they use colors, backgrounds, and themes that would appeal to them. Next, automotive marketers pique interest by showing the advantages of owning the car. In the case of the Mini, for instance, marketers imply that a small car can drive the consumer to open spaces and to fun.
Advertisers can target a precise market by using the AIDA model to identify a narrow subset of consumers that may be receptive to the product offering. Car advertisements are especially made to grab attention, pique interest, meet desires, and evoke action in consumers.
Third, automotive marketers speak to what their consumers desire. For Mini drivers, it’s the “fun” of driving, while for Prius consumers it may be the fuel economy or the environmental friendliness. Only after evaluating consumer desires are marketers able to create effective campaigns. Lastly, marketers use advertising and other methods, such as sales promotions, to encourage consumers to take action by purchasing the product or service.
Push versus Pull Promotion Mix Strategies
Push and pull strategies are promotional strategies used to get the product to its target market. A push strategy places the product in front of the customer, to make sure the consumer is aware of the existence of the product. Push strategies also create incentives for retailers to stock products and put them in front of the customer. Examples of push tactics include:
- Point-of-sale displays that make a product highly visible to consumers
- Product demonstrations to show off a product’s features to potential customers at trade shows and in showrooms
- Retailer incentives to stock and sell products, such as discounted bulk pricing
- Negotiations with a retailer to stock a specific item in limited store space, along with proof points the product will sell
- Creating a supply chain for distribution that ensures retailers can obtain the product in sufficient quantities
Push strategies work best when companies already have established relationships with users. For example, cell phone providers proactively send (i.e., push) advertisements via text messages to mobile customers regarding promotions and upgrades. This permission-based marketing can become particularly effective when push tactics and offers are personalized to the user based on individual preferences, usage, and buying behavior.
A pull strategy stimulates demand and motivates customers to actively seek out a specific product. It is aimed primarily at the end users, rather than retailers or other middle players in the value chain. Pull strategies can be particularly successful for strong, visible brands with which consumers already have some familiarity. Examples of pull tactics include:
- Mass-media advertising and promotion of a product
- Marketing communications with existing customers to make them aware of new products that will fill a specific need
- Referrals and word-of-mouth recommendations from existing customers
- Product reviews from opinion leaders
- Sales promotions and discounts
Using these strategies creates a demand for a specific product. With pull tactics stoking demand, retailers are then encouraged to seek out the product and stock it on their shelves. For instance, Apple successfully uses a combination of pull strategies to launch iPhones or iPads. The music industry has shifted strongly toward pull strategies due to digitization and the emergence of social networking Web sites. Music platforms such as iTunes, Grooveshark, and Spotify all reflect a power shift toward music consumers exploring and demanding music they want, rather than music producers controlling what is available to whet music lovers’ appetites. Likewise, music retailers have adapted their strategies toward pulling in consumers to seek out products.
Most businesses use a combination of push and pull strategies in order to successfully market their products, services and brands. As marketers define the objectives they want marketing campaigns and IMC to accomplish, they can determine whether “push,” “pull,” or a combination of both will be most effective. This helps guide their choices around which marketing communication methods and tools to use.
Engagement Strategies
In the age of IMC, it is essential for marketers to think creatively about what they are trying to accomplish with target customers through the campaign. Beyond just “pushing” a product through channel partners or “pulling” a customer in through advertising and awareness-building, marketers should consider how the campaign will draw attention, make an impact, and invite target audiences to take action amidst a crowded marketplace. Exposure alone is no longer sufficient to create brand equity and loyalty; interaction is now the name of the game.
Marketers today have many different avenues for creating engagement opportunities focused on making a desired impact in the mind–and behavior–of the customer. By thinking through campaign objectives at this level, marketers can better pinpoint not only a winning strategy for the campaign, but also the types of IMC tactics and tools to help them deliver the desired results. For example:
| Campaign Strategy | Well-suited IMC Tactics, Tools |
|---|---|
| Interact | Social media, events, guerrilla marketing efforts |
| Engage | Word-of-mouth recommendations, viral sharing, social media |
| Embrace | Brand community, social media, events, sales promotions, viral sharing |
| Influence | Public relations, thought leadership activities, personal selling |
| Convince | Case studies, testimonials, comparisons, free trials, samples |
| Educate | Advertising, thought leadership activities, public relations, website and other content marketing |
| Inspire | Testimonials, guerrilla marketing, events, advertising, case studies |
| Nurture | Email marketing, content marketing, personal selling |
Step 3: Set S.M.A.R.T. Goals
After determining campaign objectives, marketers should set specific goals for their IMC programs using S.M.A.R.T. criteria aligned with the marketing strategy. S.M.A.R.T. is an acronym organizations and managers use to set clear, measurable goals. Used in the business world inside and outside marketing, S.M.A.R.T. comes from the work of George T. Doran.[2] He proposed that each level of the organization should set goals that are:
- Specific: target a specific area for improvement
- Measurable: quantify or at least suggest an indicator of progress
- Assignable: specify who will do it
- Realistic: state what results can realistically be achieved, given available resources
- Time-related: specify when the result(s) can be achieved
S.M.A.R.T. goals help ensure clarity about what will be accomplished with a marketing campaign or other activity. They also contribute to good communication between managers and employees, so that there are clear expectations on all sides about the focus of attention, resources, and results.
MAKING A SMART GOAL
Consider the following example of a S.M.A.R.T. marketing campaign goal:
The California Campaign, implemented by the marketing team in conjunction with the California sales lead, will use customer referrals, conference appearances, content marketing tactics, and personal selling to identify and develop five new medium-to-large businesses to pilot our new technology product by September 1, 2016.
This goal is:
Specific: It focuses on identifying new business opportunities to pilot a new product in California
Measurable: It specifies a goal of developing “five new medium-to-large businesses” to pilot the new product
Assignable: It designates the ownership of this goal between the marketing team and the California sales lead
Realistic: It states the resources and techniques that will be used to achieve the goal, and the size of the goal appears to be well proportioned to the time and resources available
Time-related: The end date for achieving the results is clear: September 1, 2016
Using the S.M.A.R.T. format helps marketers map IMC activities directly to broader marketing goals and strategy. It also sets the stage for being able to monitor progress and adjust the campaign’s approach and tactics midstream if the initial efforts are falling short or getting off track.
Step 4: Define the Message
With the marketing campaign’s objectives determined and goals defined, marketers can revisit and refine campaign messaging to fit the approach they have selected. Refer to the “Defining the Message” section of this module for further guidance and recommendations around developing a messaging framework and getting the messaging right.
Part of the messaging is the call to action. As marketers hone in on the marketing communication methods and tools they will use, each touch point should include a call to action aligned with the campaign strategy and goals. The calls to action should be appropriate to the AIDA model stage, the audience, and the tool being used. For example, as a prospective customer progresses through the sales cycle, the following set of appropriate calls to action might be built into Web content:
- Awareness: Click on a paid search ad to visit a Web site and view a product description and comparative product review
- Interest: Download a white paper outlining how a product offers a novel solution to a common business problem
- Desire: Request a product demonstration
- Action Stage: Request a proposal and price quote
Step 5: Select Marketing Communication Methods
As marketers consider marketing communication methods, several factors shape their choices:
Budget
What is the budget for the marketing campaign, and what resources are available to execute it? A large budget can incorporate more expensive marketing communication techniques—such as mass-market advertising and sales promotions—a larger scale, a broader reach, and/or a longer time frame. A small-budget campaign might also be very ambitious, but it would rely primarily on in-house labor and existing tools, such as a company’s Web site and content marketing, email marketing, and social media capabilities. It’s important to figure out how to get the biggest impact from the available budget.
Timing
Some IMC methods and tactics require a longer lead time than others. For example, email and Web marketing activities can usually be executed rapidly, often with in-house resources. Conference presentations and events require significantly longer lead time to orchestrate. It’s important to choose the tools that will make the biggest impact in the time available.
Audience
Effective IMC methods meet audiences where they are. As suggested above, the media habits and behaviors of the target segments should guide marketers’ choices around marketing communication. For example, if you know your target audience subscribes to a particular magazine, visits a short list of Web sites to get information about your product category, and follows a particular set of bloggers, your IMC strategy should build a presence in these media. Alternatively, if you learn that 60 percent of your new business comes as a result of Yelp and FourSquare reviews, your marketing campaign might focus on social-media reputation building and mobile touch points.
Existing Assets and Organizational Strengths
When considering marketing communications and the promotional mix, marketers should always look for ways to build on and make the best use of existing assets. For example, if a company has a physical store or space, how is it being used to full effect to move prospective customers through the sales cycle? If a company has a well-respected founder or thought leader as an employee, how are marketers using this asset to generate interesting content, educate prospects, differentiate the company, and create a desire for their brand, products, or services? Does the organization have a Website and, if so, how does it support each stage of the AIDA model? Organizations should be aware of these strengths and design IMC programs that use them to best advantage. Often these strengths become competitive advantages that competitors cannot easily match or replicate.
Advantages of Various Marketing Communication Methods
Different marketing communication methods lend themselves to particular stages of the AIDA model, push vs. pull strategies, and ways of interacting with customers.
- Advertising is particularly well-suited to awareness-building
- Public relations activities often focus on generating interest, educating prospective customers and sharing stories that create desire for a product or brand. Similarly, experiential events can create memorable opportunities to interact with product, brands and people.
- Personal selling typically focuses at the later stages of the model, solidifying desire and stimulating action
- Sales promotions, depending on their design, can be focused at any step of the AIDA model. For consumer products, they often focus on point-of-sale touch points to induce buying.
- Direct marketing can also be focused at any step of the AIDA model, depending on the design. It is often used to generate interest, providing information or an offer that motivates prospective customers to dig a little deeper and learn more.
- Digital marketing offers a plethora of tools that can be deployed at any stage of the AIDA model. Paid digital ads, search optimization and social media word-of-mouth all support awareness-building and generating interest. Blogs, newsletters, digital case studies and customer testimonials can be powerful tools for stoking desire. How the website engages customers through the purchasing process is key to persuading prospects to become customers.
- Guerrilla marketing, like digital marketing, can be designed to impact any stage of the AIDA model. It is often used by newcomers for awareness-building, to make an impact in a new market. Marketers also use it frequently for engaging experiential activities that solidify desire and create an emotional bond with the consumer.
Marketers should think creatively about the methods available to them and how they can come together to deliver the overall message, experience, goals and objectives of the campaign. Fortunately, if marketers plan well, they also have the opportunity to evaluate effectiveness and revise the approach to improve outcomes.
Step 6: Determine the Promotional Mix
Once marketers have selected marketing communications methods, the next step is to decide which specific tools to employ, when, and how much. IMC programs are very powerful when they layer communication channels and methods upon one another—it’s an approach that amplifies and reinforces the message. The next section of this module goes into much more detail about marketing communication methods, common tools associated with each method, and when/how to use these tools most effectively.
Step 7: Execute the Campaign
The final sections of this module provide recommendations for how to create effective communication and marketing plans that simplify execution and follow-through.
Step 8: Measure Results
Later in this module we will also discuss the process of identifying the best means of measuring the success of IMC efforts. Tracking and understanding results is how marketing teams and managers monitor progress and know when they need to adjust course.
As marketers design their IMC activities and marketing campaigns with an eye toward results, accountability, and outcomes, they will benefit from an approach that emphasizes alignment between organizational strategy, marketing strategy, and the day-to-day marketing tactics that execute this strategy.
- Cooper, Paige. “41 Facebook Stats That Matter to Marketers in 2019.” Hootsuite Social Media Management, November 13, 2018. https://blog.hootsuite.com/facebook-statistics/.
- Wikipedia: https://en.wikipedia.org/wiki/SMART_criteria
LICENSES AND ATTRIBUTIONS
CC LICENSED CONTENT, ORIGINAL
- Revision and adaptation. Provided by: Lumen Learning. License: CC BY-SA: Attribution-ShareAlike
CC LICENSED CONTENT, SHARED PREVIOUSLY
- AIDA Model, from Boundless Marketing. Provided by: Boundless. Located at: https://courses.lumenlearning.com/boundless-marketing/. License: CC BY-SA: Attribution-ShareAlike
- Push and Pull Strategies, from Boundless Marketing. Provided by: Boundless. Located at: https://courses.lumenlearning.com/boundless-marketing/. License: CC BY-SA: Attribution-ShareAlike
ALL RIGHTS RESERVED CONTENT
- Mini Advertisement. Located at: https://www.miniusa.com/. License: All Rights Reserved. License Terms: Fair Use
Video: Prioritizing Marketing Communications
Given all the different marketing communication tools and opportunities out there, it can be hard to prioritize and choose where to focus your attention and marketing efforts. In this TEDx talk, Google’s Nick Scarpino provides a common-sense framework to help you make the biggest impact with whatever marketing resources are available to you.
LICENSES AND ATTRIBUTIONS
ALL RIGHTS RESERVED CONTENT
- A Guide for Prioritizing Marketing Communications: Nick Scarpino at TEDxUofIChicago. Provided by: TEDx. Located at: https://youtu.be/UhQ2T5V2SQE?list=PLzGAKV7EcDXvAMT-rhjYDfBOTmd_2dSiQ. License: All Rights Reserved. License Terms: Standard YouTube license
Self Check: Determining IMC Objectives and Approach
Check Your Understanding
Answer the question(s) below to see how well you understand the topics covered in the previous section. This short quiz does not count toward your grade in the class, and you can retake it an unlimited number of times.
Use this quiz to check your understanding and decide whether to (1) study the previous section further or (2) move on to the next section.
LICENSES AND ATTRIBUTIONS
CC LICENSED CONTENT, ORIGINAL
- Self Check: Determining IMC Objectives and Approach. Provided by: Lumen Learning. License: CC BY: Attribution
|
oercommons
|
2025-03-18T00:39:14.052441
|
06/06/2022
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/93469/overview",
"title": "Statewide Dual Credit Principles of Marketing, Promotion: Integrated Marketing Communication (IMC), Determining IMC Objectives and Approach",
"author": "Anna McCollum"
}
|
https://oercommons.org/courseware/lesson/91229/overview
|
Common Pricing Strategies
Overview
Provided by: Lumen Learning. License: CC BY: Attribution
Outcome: Common Pricing Strategies
What you’ll learn to do: compare common pricing strategies
Thus far we have discussed many pricing considerations: the impact of pricing on value perceptions, the effects of elasticity, and approaches to common pricing objectives. In this section we are going to introduce some very specific, yet standard pricing strategies that organizations use to bring these concepts together. They do not replace the information that we have discussed to this point, but they are important to understand.
The specific things you’ll learn in this section include:
- Explain why a company would use skim pricing
- Explain why a company would use penetration pricing
- Explain why a company would use cost-oriented pricing
- Explain how price discounting is used and why it can be effective
Learning Activities
- Reading: Skim Pricing
- Reading: Penetration Pricing
- Reading: Cost-Oriented Pricing
- Reading: Discounting Strategies
Licenses and Attributions
CC licensed content, Original
- Outcome: Alternative Pricing Strategies. Provided by: Lumen Learning . License: CC BY: Attribution
Reading: Skim Pricing
With a totally new product, competition either doesn’t exist or is minimal, and there’s no market data about customer demand. How should the price be set in such a case? There are two common pricing strategies that organizations use for new products: skim pricing and penetration pricing.
The Economics of Price and Demand
In order to understand these pricing strategies, let’s review the demand curve. In a typical economic analysis of pricing, the demand curve shows the quantity demanded at every price. In our graph below, the demand increases by 100 units each time the price drops by $1. Based on this demand, if a company priced its product at $4, consumers would buy 200 units. If the company wanted to raise its prices, it could charge $5, but then consumers would buy only 100 units. This is an oversimplified example, but it shows an important relationship between price and demand.
The key thing to understand about this model is that when all else is equal, demand decreases as price increases. Fortunately, the marketer does not have to regard everything else as fixed. She can make adjustments to product, promotion, or distribution to increase the value to the customer in order to increase demand without lowering price. Still, once the other elements of the marketing mix are fixed, it’s generally true that a higher price will result in less demand for a product, and a lower price will result in more demand for a product.
What Is Skim Pricing?
Price skimming involves the top part of the demand curve. A firm charges the highest initial price that customers will pay. As the demand of the first customers is satisfied, the firm lowers the price to attract another, more price-sensitive segment.
Using our example of the demand curve, the price might be set at $5 per unit at first, generating a demand of only 100 units.
The skimming strategy gets its name from skimming successive layers of “cream”—or customer segments—as prices are lowered over time.
Why Might Skim Pricing Make Sense?
There are a number of reasons why an organization might consider a skimming strategy. Sometimes a company simply can’t deliver enough of a new product to meet demand. By setting the price high, the company is able to maximize the total revenue that it can generate from the quantity of product that it can make available.
Price skimming can also be part of a customer segmentation strategy. Take a look at the graph, above. You’ll remember from our discussion of the product life cycle and customer adoption patterns that the Innovators—the adventurous customers on the left who are game to try new products—are less price sensitive and place a premium on being first to own a new product. A skim-pricing strategy targets these customers and sets a higher price for them. As the product starts to move through the Early Adopters stage, the marketer will often reduce the price to begin drawing Early Majority buyers.
A skimming strategy is most appropriate for a premium product. Today we can see many examples of skim pricing in the electronics industry when new product innovations are introduced. Electronics companies know that many buyers will wait to purchase new technologies, so they use skim pricing to get the highest possible price from the Innovators and Early Adopters.
Licenses and Attributions
CC licensed content, Original
- Revision and Adaptation. Authored by: Lumen Learning. License: CC BY-SA: Attribution-ShareAlike
CC licensed content, Shared previously
- Chapter 9, Pricing the Product, Introducing Marketing. Authored by: John Burnett. Provided by: Global Text. Located at: http://solr.bccampus.ca:8001/bcc/file/ddbe3343-9796-4801-a0cb-7af7b02e3191/1/Core%20Concepts%20of%20Marketing.pdf. License: CC BY: Attribution
- Price Skimming. Provided by: Wikipedia. Located at: https://en.wikipedia.org/wiki/Price_skimming. License: CC BY-SA: Attribution-ShareAlike
- Skimmer Skimming. Authored by: Andy Morffew. Located at: https://www.flickr.com/photos/andymorffew/16660136473/. License: CC BY-ND: Attribution-NoDerivatives
Reading: Penetration Pricing
What Is Penetration Pricing?
Penetration pricing is a pricing strategy in which the price of a product is initially set low to rapidly reach a wide fraction of the market and initiate word of mouth.[1] The strategy works on the assumption that customers will switch to the new product because of the lower price. Penetration pricing is most commonly associated with marketing objectives of enlarging market share and exploiting economies of scale or experience.
Returning to our economic model, below, you can see that penetration pricing focuses at the bottom of the demand curve. If the initial price is set low, at $2, for instance, the quantity demanded will be high: 400 units.
Penetration pricing offers a lower price in order to draw in a higher demand from consumers.
Why Might Penetration Pricing Make Sense?
Like skim pricing, penetration pricing shows an awareness of the dynamics in the product life cycle. The advantages of penetration pricing to the firm are the following:
- It can result in fast diffusion and adoption across the product life cycle. The strategy can achieve high market penetration rates quickly, taking competitors by surprise and not giving them time to react.
- It can create goodwill among the Innovators and Early Adopters, which can generate more demand via word of mouth.
- It establishes cost-control and cost-reduction pressures from the start, leading to greater efficiency.
- It discourages the entry of competitors.
- It can generate high stock turnover throughout the distribution channel, which creates important enthusiasm and support in the channel.
The main disadvantage of penetration pricing is that it establishes long-term price expectations for the product and image preconceptions for the brand and company. Both can make it difficult to raise prices later. Another potential disadvantage is that the low profit margins may not be sustainable long enough for the strategy to be effective.
- J Dean (1976). "Pricing Policies for New Products." Harvard Business Review 54 (6): 141–153.
Licenses and Attributions
CC licensed content, Original
- Revision and Adaptation. Provided by: Lumen Learning. License: CC BY-SA: Attribution-ShareAlike
CC licensed content, Shared previously
- Penetration Pricing. Provided by: Wikipedia. Located at: https://en.wikipedia.org/wiki/Penetration_pricing#cite_note-4. License: CC BY-SA: Attribution-ShareAlike
- Mallards Feeding. Authored by: Yankech Gary. Located at: https://www.flickr.com/photos/49663413@N08/6965646827/. License: CC BY-ND: Attribution-NoDerivatives
Reading: Cost-Oriented Pricing
Cost-Plus Pricing
Cost-plus pricing, sometimes called gross margin pricing, is perhaps the most widely used pricing method. The manager selects as a goal a particular gross margin that will produce a desirable profit level. Gross margin is the difference between how much the goods cost and the actual price for which it sells. This gross margin is designated by a percent of net sales. The percent chosen varies among types of merchandise. That means that one product may have a goal of 48 percent gross margin while another has a target of 33.5 percent or 2 percent.
A primary reason that the cost-plus method is attractive to marketers is that they don’t have to forecast general business conditions or customer demand. If sales volume projections are reasonably accurate, profits will be on target. Consumers may also view this method as fair, since the price they pay is related to the cost of producing the item. Likewise, the marketer is sure that costs are covered.
A major disadvantage of cost-plus pricing is its inherent inflexibility. For example, department stores often find it hard to meet (and beat) competition from discount stores, catalog retailers, and furniture warehouses because of their commitment to cost-plus pricing. Another disadvantage is that it doesn’t take into account consumers’ perceptions of a product’s value. Finally, a company’s costs may fluctuate, and constant price changing is not a viable strategy.
Markups
When middlemen use the term markup, they are referring to the difference between the average cost and price of all merchandise in stock, for a particular department, or for an individual item. The difference may be expressed in dollars or as a percentage. For example, a man’s tie costs $14.50 and is sold for $25.23. The dollar markup is $10.73. The markup may be designated as a percent of the selling price or as a percent of the cost of the merchandise. In this example, the markup is 74 percent of cost ($10.73 / $14.50) or 42.5 percent of the retail price ($10.73 / $25.23).
There are several reasons why expressing markup as a percentage of selling price is preferred to expressing it as a percentage of cost. One is that many other ratios are expressed as a percentage of sales. For instance, selling expenses are expressed as a percentage of sales. If selling costs are 8 percent, it means that for each $100,000 in net sales, the cost of selling the merchandise is $8,000. Advertising expenses, operating expenses, and other types of expenses are quoted in the same way. Thus, when making comparisons, there is a consistency in expressing all expenses and costs, including markup, as a percentage of sales (selling price).
As an illustration of the cost-based process of pricing, let’s look at Johnnie Walker Black Label Scotch Whisky. This product sells for about $30 in most liquor stores. How was this price derived? The per-bottle costs are shown below:
$5.00 production, distillation, maturation
+ $2.50 advertising
+ $3.11 distribution
+ $4.39 taxes
+ $7.50 markup (retailer)
+ $7.50 net margin (manufacturer)
$30.00 total
Each of the cost elements, including the retailer’s markup, is added to the initial production costs, and the total is the final price.
Cost-Oriented Pricing of New Products
Certainly costs are an important component of pricing. No firm can make a profit until it covers its costs. However, the process of determining costs and setting a price based on costs does not take into account what the customer is willing to pay at the marketplace. This strategy is a bit of a trap for companies that develop products and continually add features to them, thus adding cost. Their cost-based approach leads them to add a percentage to the cost, which they pass on to customers in the form of a new, higher price. Then they are disappointed when their customers do not see sufficient value in the cost-based price.
Licenses and Attributions
CC licensed content, Original
- Revision and adaptation. Provided by: Lumen Learning. License: CC BY: Attribution
CC licensed content, Shared previously
- Pricing the Product, from Introducing Marketing. Authored by: John Burnett. Project: Global Text. License: CC BY: Attribution
- Mangos. Authored by: Quinn Dombrowski. Located at: https://www.flickr.com/photos/quinnanya/2886818380/. License: CC BY-SA: Attribution-ShareAlike
Reading: Discounting Strategies
In addition to deciding about the base price of products and services, marketing managers must also set policies regarding the use of discounts and allowances. There are many different types of price reductions–each designed to accomplish a specific purpose. The major types are described below.
Quantity Discounts
Quantity discounts are reductions in base price given as the result of a buyer purchasing some predetermined quantity of merchandise. A noncumulative quantity discount applies to each purchase and is intended to encourage buyers to make larger purchases. This means that the buyer holds the excess merchandise until it is used, possibly cutting the inventory cost of the seller and preventing the buyer from switching to a competitor at least until the stock is used. A cumulative quantity discount applies to the total bought over a period of time. The buyer adds to the potential discount with each additional purchase. Such a policy helps to build repeat purchases.
Both Home Depot and Lowe’s offer a contractor discount to customers who buy more than $5,000 worth of goods. Home Depot has a tiered discount for painters, who can save as much as 20 percent off of retail once they spend $7,500.1
Seasonal Discounts
Seasonal discounts are price reductions given for out-of-season merchandise—snowmobiles discounted during the summer, for example. The intention of such discounts is to spread demand over the year, which can allow fuller use of production facilities and improved cash flow during the year.
Seasonal discounts are not always straightforward. It seems logical that gas grills are discounted in September when the summer grilling season is over, and hot tubs are discounted in January when the weather is bad and consumers spend less freely. However, the biggest discounts on large-screen televisions are offered during the weeks before the Super Bowl when demand is greatest. This strategy aims to drive impulse purchases of the large-ticket item, rather than spurring sales during the off-season.
Cash Discounts
Cash discounts are reductions on base price given to customers for paying cash or within some short time period. For example, a 2 percent discount on bills paid within 10 days is a cash discount. The purpose is generally to accelerate the cash flow of the organization and to reduce transaction costs.
Generally cash discounts are offered in a business-to-business transaction where the buyer is negotiating a range of pricing terms, including payment terms. You can imagine that if you offered to pay cash immediately instead of using a credit card at a department store, you wouldn’t receive a discount.
Trade Discounts
Trade discounts are price reductions given to middlemen (e.g., wholesalers, industrial distributors, retailers) to encourage them to stock and give preferred treatment to an organization’s products. For example, a consumer goods company might give a retailer a 20 percent discount to place a larger order for soap. Such a discount might also be used to gain shelf space or a preferred position in the store.
Calico Corners offers a 15 percent discount on fabrics to interior designers who are creating designs or products for their customers. They have paired this with a quantity-discounts program that offers gift certificates for buyers who purchase more than $10,000 in a year.
Personal Allowances
Personal allowances are similar strategies aimed at middlemen. Their purpose is to encourage middlemen to aggressively promote the organization’s products. For example, a furniture manufacturer may offer to pay some specified amount toward a retailer’s advertising expenses if the retailer agrees to include the manufacturer’s brand name in the ads.
Some manufacturers or wholesalers also give retailers prize money called “spiffs,” which can be passed on to the retailer’s sales clerks as a reward for aggressively selling certain items. This is especially common in the electronics and clothing industries, where spiffs are used primarily with new products, slow movers, or high-margin items.
When employees in electronics stores recommend a specific brand or product to a buyer they may receive compensation from the manufacturer on top of their wages and commissions from the store.
Trade-In Allowances
Trade-in allowances also reduce the base price of a product or service. These are often used to help the seller negotiate the best price with a buyer. The trade-in may, of course, be of value if it can be resold. Accepting trade-ins is necessary in marketing many types of products. A construction company with a used grader worth $70,000 probably wouldn’t buy a new model from an equipment company that did not accept trade-ins, particularly when other companies do accept them.
Price Bundling
Price bundling is a very popular pricing strategy. The marketer groups similar or complementary products and charges a total price that is lower than if they were sold separately. Comcast and Direct TV both follow this strategy by combining different products and services for a set price. Similarly, Microsoft bundles Microsoft Word, Excel, Powerpoint, OneNote, and Outlook in the Microsoft Office Suite. The underlying assumption of this pricing strategy is that the increased sales generated will more than compensate for a lower profit margin. It may also be a way of selling a less popular product—like Microsoft OneNote—by combining it with popular ones. Industries such as financial services, telecommunications, and software companies make very effective use of this strategy.
Licenses and Attributions
CC licensed content, Original
- Revision and adaptation. Provided by: Lumen Learning. License: CC BY: Attribution
CC licensed content, Shared previously
- Chapter 9, Pricing the Product, Introducing Marketing. Authored by: John Burnett. Provided by: Global Text. License: CC BY: Attribution
- Fabric Bolts. Authored by: Laura. Provided by: Pixabay. Located at: https://www.flickr.com/photos/luckylaura/2749694639/. License: CC BY: Attribution
|
oercommons
|
2025-03-18T00:39:14.139933
|
03/22/2022
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/91229/overview",
"title": "Statewide Dual Credit Principles of Marketing, Pricing Strategies, Common Pricing Strategies",
"author": "Anna McCollum"
}
|
https://oercommons.org/courseware/lesson/91233/overview
|
Why it Matters
Overview
Teacher resources for Unit 12 can be found on the next page.
Provided by: Lumen Learning. License: CC BY: Attribution
Why It Matters: Place: Distribution Channels
Resources for Unit 12: Distribution Channels
Slide Deck - Unit 12: Place: Distribution Channels
Simulation Unit 12: “Simulation: Distribution”
Discussion Assignments and Alignment: Place: Distribution Channels
Pacing
The Principles of Marketing textbook contains sixteen units—roughly one unit per week for a 16-week semester. If you need to modify the pace and cover the material more quickly, the following units work well together:
- Unit 1: What Is Marketing? and Unit 2: Marketing Function. Both are lighter, introductory units.
- Unit 15: Global Marketing and Unit 16: Marketing Plan. Unit 16 has more course review and synthesis information than new material per se.
- Unit 5: Ethics can be combined with any unit. You can also move it around without losing anything.
- Unit 8: Positioning and Unit 9: Branding. Companion modules that can be covered in a single week.
- Unit 6: Marketing Information & Research and Unit 7: Consumer Behavior. Companion units that can be covered in a single week.
We recommend NOT doubling up the following units, because they are long and especially challenging. Students will need more time for mastery and completion of assignments.
- Unit 4: Marketing Strategy
- Unit 10: Product Marketing
- Unit 13: Promotion: Integrated Marketing Communication
Did you have an idea for improving this content? We’d love your input.
Learning Outcomes
Explain what channels of distribution are and why organizations use them
- Explain how channels affect the marketing of products and services
- Describe types of retailers and explain how they are used as a channel of distribution
- Explain how integrated supply chain management supports an effective distribution strategy
Why evaluate how to use distribution channels to market an organization’s products and services effectively?
More Than Just Another P
Of the elements in the marketing mix, product and price are perhaps the easiest to understand. We see products all around us, and we understand that we need to pay a specific price to buy them. Promotion is sometimes a little more difficult to grasp, but if we begin with the concept of advertising and then develop a more complete view of promotion from that, promotion is also fairly easy to understand.
“Place,” on the other hand, is not so straightforward. In fact, using the word “place” can be misleading. If I were to say, “We are going to talk about place related to groceries,” you would likely think about where you buy your groceries—as in, which store and which location. In this module, though, we want to discuss the process of determining where you want to find particular groceries and how to get those groceries to that place in the way that best aligns with your preferences.
While it inconveniently begins with the letter D rather than P, distribution is a more accurate description of this function. Distribution brings the products that you want to the place where you want to buy them, at a cost that supports the customer and company price requirements.
How do your groceries get to the right place at the right cost? To explore this question, let’s look at two high-end grocery stores that use very different methods to manage this process: Whole Foods and Trader Joe’s.
Whole Foods’ Approach to Distribution
Whole Foods’ motto—Whole Foods, Whole People, Whole Planet—emphasizes a vision that reaches beyond food retailing. The company has chosen a strategy of sourcing locally wherever possible. This, in turn, has driven the strategy of how Whole Foods fills its shelves—the distribution strategy. The video below explains how the company sources products.
You can view the transcript for “Forager Elly Truesdell Whole Foods Market” here (opens in new window).
In order to support local sourcing, store managers are empowered to make purchasing decisions for each store, independently of the regional offices. As a result, it is possible for Whole Foods to buy potatoes from a local farmer who would never dream of selling his produce to a large grocery chain. Essentially, Whole Foods is differentiated because all products are sourced locally. The stores operate under minimal governance and are given maximum freedom to source a product mix that is appropriate for their location. Whole Foods stores operate according to the premise that they need these freedoms to meet the unique buying needs of their local customers. The only governing rule put in place by the corporate office is that stores must not stock products with artificial flavors, preservatives, colors, sweeteners, or hydrogenated oils. A downside to this local purchasing policy is that consistency is compromised across the chain. Every retail location carries a variety of products that distinguishes it from other stores in the same chain.
Not surprisingly, it is difficult to achieve economies of scale with this model. Higher distribution costs lead to higher prices, which makes it important for Whole Foods to target customers with high incomes. To ensure ample access to their target consumer segments, Whole Foods opens stores in communities with a large number of college-educated residents with no fewer than two hundred thousand people within a twenty-minute drive.
Trader Joe’s Approach to Distribution
The mission of Trader Joe’s is to give customers the best food and beverage values they can find anywhere and to provide them with the information required to make informed buying decisions. The company strives to provide these with a dedication to the highest quality of customer satisfaction delivered with a sense of warmth, friendliness, fun, individual pride, and company spirit.
At the core of the Trader Joe’s “way” is a focus on cost control, simplicity, and fun. These company objectives are woven throughout each aspect of the business. Trader Joe’s aims to create a truly unique customer experience, offering high-quality gourmet foods at a low cost in a fun environment that keeps customers coming back for more.
Trader Joe’s manages its distribution networks by minimizing the number of hands that touch the product, thereby reducing costs and making products quickly available to their customers. The company orders directly from the manufacturer. The manufacturer, in turn, is responsible for bringing the product to a Trader Joe’s distribution center. At the distribution center, trucks leave on daily resupply trips to local stores. Because the stores are relatively small, there is little room for excess inventory, and orders from distribution centers need to be incredibly precise.
This quick and efficient distribution process is directly responsible for helping the company identify where to locate new retail stores. Trader Joe’s will only enter markets where the region has a distribution infrastructure that allows it to efficiently resupply products to stores. They did not open stores, for instance, in Florida or Texas—both large, lucrative markets—because the distribution networks were not yet strong enough to support their strategy.[1] [2]
Trader Joe’s strategy of implementing a low-cost and efficient distribution network has contributed to the democratization of gourmet foods by making them more readily available to customers at all income levels.
You can see that the distribution strategy for each company has an effect on where they open stores, how they price their products, which customers will buy, and who will have access to gourmet foods.
In this module, you’ll learn more about distribution strategies and their role in the marketing mix.
- Lewis, Len. The Trader Joe’s Adventure: Turning a Unique Approach to Business to a Retail and Cultural Phenomenon. 2005
- http://www.traderjoes.com/our-story/timeline
Licenses and Attributions
CC licensed content, Original
- Why It Matters: Place: Distribution Channels. Provided by: Lumen Learning. License: CC BY-SA: Attribution-ShareAlike
CC licensed content, Shared previously
- Trader Joe's vs. Whole Foods Market: A Comparison of Operational Management. Provided by: MIT Sloan School. Located at: http://ocw.mit.edu/courses/sloan-school-of-management/15-768-management-of-services-concepts-design-and-delivery-fall-2010/projects/MIT15_768F10_paper05.pdf. License: CC BY-NC-SA: Attribution-NonCommercial-ShareAlike
All rights reserved content
- Video: Forager, Elly Truesell, Whole Foods Market. Provided by: WholeFoodsMarket. Located at: https://youtu.be/vgpugYqyKBM. License: All Rights Reserved. License Terms: Standard YouTube License
|
oercommons
|
2025-03-18T00:39:14.177586
|
03/22/2022
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/91233/overview",
"title": "Statewide Dual Credit Principles of Marketing, Place: Distribution Channels, Why it Matters",
"author": "Anna McCollum"
}
|
https://oercommons.org/courseware/lesson/91214/overview
|
Name Selection
Overview
Provided by: Lumen Learning. License: CC BY: Attribution
Outcome: Name Selection
What you’ll learn to do: explain the importance of name selection in the success of a brand
How important is naming in the success of a brand? Very important.
Consider the function of a brand name: It identifies a product, service, or company and differentiates it from competitors. But it does much more than that. It can generate attention or make something utterly forgettable. It can evoke positive or negative feelings and emotions. It can capture the imagination or drive someone to boredom. It can make a remarkable or unremarkable first impression.
Naming can be difficult in the crowded, increasingly global marketplace in which businesses operate today. As you understand the role of naming and the systematic process for selecting a new brand name, you can help lead your organization in making wise, informed choices about this essential element of branding.
The specific things you’ll learn in this section include:
- Discuss the connection between brand and name
- Outline key steps in the naming process
Learning Activities
The learning activities for this section include the following:
- Reading: Name Selection
Licenses and Attributions
CC licensed content, Original
- Outcome: Name Selection. Provided by: Lumen Learning. License: CC BY: Attribution
Reading: Name Selection
What’s in a (Brand) Name?
A brand identifies a company, product, or service as distinct from the competition. The brand is comprised of all the things that create this identity. A brand’s name is an essential part of the package. A brand name may be a product name (like Windows or Gmail), or it may be the name under which the entire organization operates (like Microsoft or Google). Because the name is so central to identity, naming a brand is an integral part of creating the brand’s reputation, development, and future success.
To some extent, a brand name amounts to whatever an organization makes of it: this is the genius of brand building and marketing strategy. Unlikely names have, on occasion, become powerhouse brands, and well-named brands have fizzled out. Naming is important because an ill-conceived or poorly chosen name can torpedo an organization’s chances. At the same time, a great name alone isn’t enough to guarantee success.
Naming a Brand
Apple iPod line as of 2014. From left to right: iPod Shuffle, iPod Nano, iPod Touch. iPod is one of Apple’s products named with the distinctive “i.”
Selecting a brand name is one of the most important product decisions a seller makes. A brand name reflects the overall product image, positioning, and, ideally, its benefits. A successful brand name can enable a product to be meaningfully advertised and distinguished from competitors; tracked down by consumers; and given legal protection. At its best, a brand can provide a carryover effect when customers are able to associate quality products with an established brand name. Attention to naming also helps customers associate products within the same brand family. For example, Apple names its mobile products with a lowercase i—for example, iPad, iPod, iPhone. Starbucks names its coffee sizes in Italian.
Remember that legally protectable brand names are mandatory if an organization plans to produce mass advertising for their product or service. Once an organization starts using a new brand name, it may encounter other organizations’ claim to own the rights to that name and threaten legal action. To avoid the risks and potential expense associated with legal challenges to a brand name, it is important to use a thorough, systematic process for selecting a brand name.
Selecting a Naming Strategy
Before you start brainstorming new brand names and registering domain names, the company should evaluate which naming/branding policy to pursue for the new offering and choose one the following three viable options. This process helps determine whether you even need a new brand name.
- Strategy 1: Own Brand. A strict branding policy under which a company only produces products and services using its own brand. In this scenario, you need a new brand name.
- Strategy 2: Private-Label Brand. An exclusive distributor’s brand policy in which a producer does not have a brand of his own but agrees to sell his products only to a particular distributor and carry that distributor’s brand name (typically employed by private brands). In this scenario, the new offering will carry the distributor’s brand name, so you don’t need to create your own new brand.
- Strategy 3: Mixed Brand. A mixed-brand policy allows both own-branded and private-label versions of the offering. In this scenario, you need a new brand name for the own-branded product, and the distributor’s version of the product will carry the distributor’s brand name.
Steps to Develop a New Brand Name
Once you have confirmed that you need a new brand name, you should follow a systematic approach to developing and selecting one, as described below:
- Define what you’re naming. Define the personality and distinctive attributes of the company or product to be named.
- Check the landscape. Scan the competitive landscape to identify brand names already active in the category, in order to avoid selecting a name that would easily be confused with competitors.
- Brainstorm ideas. Engage a naming team to brainstorm ideas and generate potential brand names. Due to the challenges of identifying a unique, protectable name in today’s global market, the naming team should include some members with prior naming experience. Often companies hire specialty naming firms to add creative power and expertise to the process. The team should generate lots of ideas, knowing that the vast majority will fall out during the screening process.
- Screen and knock out problematic names. Screen favorite names to make sure they are available to use perceptually (no mind-share conflicts with other known brands), legally (no trademark conflicts) and linguistically (no problems in translation).
- Perceptual screening: Start the screening process with thorough Google searches on the names being considered in order to eliminate any that could easily be confused with established players in your product or service category, or a related category. If an established brand name is similar in terms of phonetics (sound), spelling, root word, or meaning, there is probably a conflict. Check with a trademark attorney if you have questions.
- Legal screening: The next screening process is to evaluate potential conflicts with registered trademarks that exist in the product or service categories in question. Each country has its own trademark registry, so this search must be performed in each country where you expect to do business using this brand name. While anyone can attempt this process, due to the legal complexities of global trademark law, it’s advisable to engage an experienced trademark attorney to review the names, conduct an authoritative search, and provide legal clearance for the short list of final names. To learn more about this process, check out the freely available U.S. Patent & Trademark Office (USPTO) Trademark Electronic Search Service (TESS) trademark search tools.
- Linguistic screening: If you plan to use the brand name in different countries and languages, a linguistic screening is a must. Use a naming firm or a linguistic screening firm to screen your final, short-listed name candidates with native speakers from the countries where you plan to operate. The linguistic screening can help you avoid blunders like GM rushing to rename the Buick LaCrosse sedan in Canada when it learned that the word crosse means either rip-off or masturbation in Quebec French, depending on the context.1
- Check domain name and social media availability. If you want to operate a Web site or social media using your new brand name, you will need an Internet domain name for your Web site, as well as social media accounts. As you are refining your short list of cleared names, check on the availability of domain names and social media handles. If you’re lucky, a clear .com domain will be available to reserve or purchase at a reasonable price, and a clear Twitter name will also be available. Here are some tips for navigating this process:
- Use a reputable registry to check availability. When you’re checking on domain-name availability, don’t just google domain names at random. Instead, use a reputable domain-name registry like Godaddy.com or Register.com. When you use Google or other standard search engines, Internet bots track this activity to detect interest in unregistered domain names. Unscrupulous Internet profiteers buy up these domains and then offer them for resale at a significant markup. When you decide to reserve your domain names, be sure to use reputable registries in all the countries where you plan to operate.
- Look at variations of your chosen name(s). Consider reserving domain-name variations of your chosen brand name(s), either because the original names you want are not available, or because you may want to control close variations to avoid letting them fall into the hands of competitors or Internet profiteers. For example, if your chosen brand name is “Chumber,” you may find that chumber.com has been taken, but chumber.net, chumber.org, and chumbercompany.com are all available. Although you don’t need all of these, you might choose to register them so that no one else can “own” the names and make mischief for you. For social media account names, if your first choice isn’t available, explore variations—perhaps a shortened version of your desired name. Remember, for services such as Twitter, shorter names fit better into the limited length of social media posts.
- Check out your Internet “neighbors.” For any domain names that are not available according to a reputable domain-name registry, do google them to see where they take you. Some may be operated by other businesses, while others may be “parked” and inoperative. Before you settle on a final domain name for your brand, make sure you investigate where common misspellings of your name might take site visitors. For example, an education technology company seriously considered the brand name “OpenMind” and the domain openmind.com until a marketing team member discovered that a variant spelling, openminded.com, would take prospective site visitors to an adult entertainment Web site.
- Reserve domains in geographies where you plan to do business. Consider whether to reserve domain names using different extensions. In other words, not just yourbrand.com, but also other extensions including those in other countries where you plan to operate: yourbrand.mx for Mexico, yourbrand.cn for China, yourbrand.ca for Canada, and so forth. If you plan to do business in multiple countries, it is wise to reserve domain names in each of the countries that are strategically important to your company.
- Customer-test your final short-listed names. It is always wise to conduct market research to test short-listed names among your target customers. This gives you insight into how they will hear, interpret, and think about the names you are considering. Customer testing can reveal nuances or connotations of a name that didn’t occur to the naming team earlier–for better or for worse. Customer testing results can also be a great tie-breaker if the naming team is split between finalists.
- Make your final selection. Ultimately the naming team should select the name with the most potential for creating a strong, differentiated brand, combined with the least risk from a trademark ownership perspective.
- Take steps to get trademark protection for your new brand.
Once a final name is chosen, engage a trademark attorney to file a trademark or service mark registration for the new brand. Ask for legal counsel on where to register your marks based on where you plan to operate globally. While this step may seem expensive and time-consuming, it can protect you and diminish risk for the organization if your brand name is ever challenged legally. Down the road, it is easier to enter into licensing and other types of agreements if a brand name is registered. Licensing can be a lucrative strategy for strong brands.
Licenses and Attributions
CC licensed content, Original
- Revision and adaptation. Provided by: Lumen Learning. License: CC BY-SA: Attribution-ShareAlike
CC licensed content, Shared previously
- Branding, from Introduction to Business. Authored by: Linda Williams and Lumen Learning. Located at: https://courses.candelalearning.com/masterybusiness2xngcxmasterfall2015/chapter/reading-branding-labeling-and-packaging/. License: CC BY-SA: Attribution-ShareAlike
- Hello, My Name Is Opportunity. Authored by: One Way Stock. Located at: https://www.flickr.com/photos/paulbrigham/8423157044/. License: CC BY-ND: Attribution-NoDerivatives
- The iPod Line. Authored by: Kyro. Located at: https://commons.wikimedia.org/wiki/File:IPod_line_as_of_2014.png. License: CC BY: Attribution
- RCMP in Formal Dress. Authored by: Kris Kru00fcg. Located at: https://www.flickr.com/photos/kk/100453947/. License: CC BY-SA: Attribution-ShareAlike
- Registered Trademark Symbol. Provided by: Pixabay. Located at: https://pixabay.com/en/registered-trademark-brand-sign-98574/. License: CC0: No Rights Reserved
|
oercommons
|
2025-03-18T00:39:14.211005
|
03/22/2022
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/91214/overview",
"title": "Statewide Dual Credit Principles of Marketing, Branding, Name Selection",
"author": "Anna McCollum"
}
|
https://oercommons.org/courseware/lesson/93473/overview
|
Measuring Marketing Communication Effectiveness
Overview
Provided by: Lumen Learning. License: CC BY: Attribution
Reading: Measuring Marketing Communication Effectiveness
Why Measure?
Measurement is an important aspect of marketing campaigns and other marketing activity. Measurement makes some people very nervous because it brings accountability into marketing activity. In fact, this step can be one of a marketer’s best friends. If you don’t measure the impact of your marketing efforts, you’ll have no idea whether what you are doing is effective or not. On the other hand, if you do measure the impact, it will help you understand what is working, and where and how to improve your efforts. By nature, marketing is a dynamic field because markets change and people change. What works beautifully this year may be a complete flop next year, and vice versa.
Measurement–and the results or “metrics” this process collects–are like a compass that helps marketers adjust course so they can reach their goals more quickly and effectively.
Deciding What to Measure
Measuring just for the sake of having numbers misses the whole point. It’s actually essential to determine the right things to measure first, if you want to get a relevant picture of what’s happening. To do this, marketers typically go through a process of identifying key performance indicators (often called KPIs). A KPI is something measurable that indicates the progress an organization is making toward its business objectives. The KPI is not the same as the actual company goal or objective; instead it is something measurable that helps managers understand how well they are progressing toward the goal.
To understand the importance of KPIs, let’s say you are a track coach who wants to capture data about the sprinters on your team. You could measure all sorts of things about the athletes: their shoe size, how many cups of sweat they produce during a typical workout, how fast their hearts beat during a race, and so on. Would all those measurements be key performance indicators? Maybe not. You might decide that the key performance indicators for sprinters are their best running times and their average running times (or something else).
In a company, KPIs can be determined for many different levels of the organization. These are described below:
- Company-level KPIs indicate the overall company performance on company-wide goals, in terms of total revenue, profitability, customer-satisfaction rating, market share, or percentage of growth in the customer base.
- Department-level KPIs track performance at the department level. For the marketing department, it might be brand awareness, the number of qualified new leads generated, cost per lead generated, or the conversion rate: the percentage of leads who are converted into customers.
- Team-level KPIs track the impact and effectiveness of a team’s activities. A team focused on digital marketing, for example, might track KPIs such as email-marketing click rates, the number of Web-site visits, or SEO sales conversion rate: the percentage of individuals who come to the Web site via a search engine and result in a sale.
- Campaign-level KPIs track the impact of individual campaigns. By tracking similar metrics across multiple campaigns, it is easy to see which ones are most effective with target audiences and then use this information to refine tactics and replicate successful approaches. Campaign-level KPIs are somewhat dependent on the campaign design; for example, campaigns typically track the “open” rate: i.e., how many people open an email message once it is delivered. If a campaign doesn’t use email, the open rate doesn’t exist. However, there are some “common denominator” campaign metrics marketers can track across IMC activities to determine impact and progress. Cost per impression, impressions per campaign, and conversion rate are metrics that can be tracked for virtually any campaign.
- Marketing tactic-level KPIs track the effectiveness of individual marketing tactics and tools. For example, content-marketing KPIs track the effectiveness of individual content pieces used on a Web site and in IMC campaigns. These metrics, such as page views per article and number of social media shares provide insight for marketers about which types of content are most popular with target customers and which content pieces get little interest.
Different companies select different sets of KPIs, depending on what they are trying to accomplish and the strategies they are pursuing to reach their goals. At any given level, it is important to limit the total number of KPIs to those that are most essential and indicative of progress. If too many things are measured, managers have trouble prioritizing and homing in on what is most important. In addition to KPIs–which represent key, strategic indicators of progress–a company may also track a variety of other metrics to inform its operations.
Alignment with Goals and Objectives
Figuring out what to measure starts with considering the organization’s overall goals and objectives, as well as the marketing team’s goals and objectives. The highest-level KPIs should tell managers about how well marketing is doing at meeting its goals as a team, and how the team is contributing to the organization’s overall performance. KPIs may reflect absolute figures, such as total market share. Or they may track progress toward a target, such as progress toward achieving 1,500 new customers over the course of a year. KPIs should provide information to guide managers in their decision making about what is working and where to adjust course.
It is helpful for an organization to define a standard set of KPIs for measuring the effectiveness of marketing campaigns and for the contributions made by different functions within the marketing organization: public relations, advertising, social media marketing, etc. When marketers define S.M.A.R.T. goals at the outset of a campaign, these goals may incorporate KPIs to confirm what the campaign aspires to achieve and how well it does at achieving these goals. KPIs for awareness-building campaigns, for example, should be focused on campaign reach, such as number of impressions or post-campaign brand awareness.
Managers should be attentive to how many KPIs they are tracking to ensure that measurement remains a useful activity rather than a burden that cuts into the productivity and effectiveness of the broader team. Fortunately, as marketing becomes more data rich and technology driven, many KPI-type metrics are calculated automatically by systems that support the marketing function, making them readily available. Tools are also available that create dashboards for marketing managers and team members to help them easily monitor KPIs on an ongoing basis.
Defining the Metric
Every marketing metric or KPI requires some type of measurement, and it should be based on legitimate data. When marketers define a KPI, they should also define what data will be used to calculate the KPI, as well as the source of that data. At times, different people or teams might have different assumptions about how to calculate the metric, so it is wise to clarify this during the definitional stage.
It isn’t uncommon for people to identify KPIs and then discover that they don’t have ready access to the information needed for measurement. This can be a good motivator for defining a process to obtain that information. Or it can be a cue that perhaps a different KPI based on more readily available information would be a better option.
When to Measure
When to measure depends on what is readily available for marketers and managers to track and maintain. If it takes a lot of manual effort to generate a KPI report, or managers are spending hours per day or week compiling and reporting metrics, it could significantly cut into productive work time—and it might be wise to investigate alternatives. Fortunately, CRM and other systems that build KPI dashboard reports into their regular, day-to-day functions are readily available. In these cases, systems automatically calculate KPIs, which makes them easy to monitor over time and adjust course as needed. Typically managers should monitor KPIs at least once per quarter, in order to gauge progress and learn what’s working and how to improve.
Video: Defining KPIs
The following video provides an overview of different types of key performance indicators and the process of defining them.
Examples of Key Performance Indicators
Different types of KPIs focus on measuring progress and effectiveness in different areas related to marketing. In fact, hundreds of possible KPIs exist, so marketing managers should figure out which ones matter most for achieving their goals and focus attention accordingly. The section and table below lists a variety of KPIs that apply to different aspects of marketing communications and the marketing function generally.
Marketing-Related Business Objectives Sample KPIs
Sales/Revenue Generation Sample KPIs
- Total sales/revenue
- New/incremental sales revenue
- Profitability
- Average revenue per customer
- New customer acquisition
- Number of customers
- Customer retention
- Number of registrations/sign-ups
Market Share Sample KPIs
- Market share in category
- Relative market share (share relative to largest competitor).
Lead Generation Sample KPIs
- Number of qualified leads
- Cost per lead (by source/platform)
- Traffic source breakdown.
Build Brand Sample KPIs
- Brand awareness
- Brand equity
- Price premium
- Brand valuation
- Share of voice: mentions of your brand/mentions of others
- Brand community membership.
Foster Dialogue Sample KPIs
- Audience engagement
- Share of voice: mentions of your brand/mentions of others
- Conversion reach.
Develop Customer Advocates Sample KPIs
- Active advocates
- Advocate influence
- Advocacy impact
- Online review ratings.
Customer Support Sample KPIs
- Resolution rate
- Resolution time
- Satisfaction score
- Net Promoter Score (NPS).
Innovation Sample KPIs
- Topic Trends
- Sentiment Ratio
- Idea Impact.
| Business Objective | KPI Examples |
|---|---|
| Sales/Revenue Generation | Total sales/revenue
New/incremental sales revenue Profitability Average revenue per customer New customer acquisition Number of customers Customer retention Number of registrations/sign-ups |
| Market Share | Market share in category
Relative market share (share relative to largest competitor) |
| Lead Generation | Number of qualified leads
Cost per lead (by source/platform) Traffic source breakdown |
| Build Brand | Brand awareness
Brand equity Price premium Brand valuation Share of voice: mentions of your brand/mentions of others Brand community membership |
| Foster Dialogue | Audience engagement
Share of voice: mentions of your brand/mentions of others Conversion reach |
| Develop Customer Advocates | Active advocates
Advocate influence Advocacy impact Online review ratings |
| Customer Support | Resolution rate
Resolution time Satisfaction score Net Promoter Score (NPS) |
| Innovation | Topic Trends
Sentiment Ratio Idea Impact |
Marketing Communications Activity Sample KPIs
Reach: Campaigns, Owned Media, Earned Media, Social Media, Marketing Content
Sample KPIs
Impressions
Potential Reach: Followers, Fans, Subscribers
Confirmed Reach: Views, Post/Page Views, Video Views
Hits/visits/views
Repeat Visits
Conversion rates (from visitor or buyer)
Buzz indicators (web mentions)
Net Promoter Score (NPS)
Customer acquisition cost
Engagement: Owned Media, Earned Media, Social Media, Marketing Content
Sample KPIs
Likes/Stars/Hearts
Comments
Shares
Retweets/Reposts
Positive/negative sentiment
Impressions
Cost per click (CPC)
Cost per impression (CPM)
Click-thru-rate (CTR)
Customer Retention Cost
Profits per customer
Customer acquisition cost
Paid Media: Advertising Sample KPIs
Impressions
Cost per click (CPC)
Cost per impression (CPM)
Click-thru-rate (CTR)
Customer Retention Cost
Profits per customer
Customer acquisition cost
SEO/Web Site Sample KPIs
SEO keyword ranking
SEO sales conversion rate
Number of unique visitors
Total sessions/visits
Average time on site/page
Email Marketing Sample KPIs
Open rate
Click-thru-rate (CTR)
Bounce rate
Unsubscribe rate
Public Relations Sample KPIs
Advertising value equivalency
Clip/article counting
Brand mentions
| Marketing Activity/Tool | KPI Examples |
|---|---|
| Reach:
Campaigns, Owned Media, Earned Media, Social Media, Marketing Content | Impressions
Potential Reach: Followers, Fans, Subscribers Confirmed Reach: Views, Post/Page Views, Video Views Hits/visits/views Repeat Visits Conversion rates (from visitor or buyer) Buzz indicators (web mentions) Net Promoter Score (NPS) Customer acquisition cost |
| Engagement:
Owned Media, Earned Media, Social Media, Marketing Content | Likes/Stars/Hearts
Comments Shares Retweets/Reposts Positive/negative sentiment |
| Paid Media (advertising) | Impressions
Cost per click (CPC) Cost per impression (CPM) Click-thru-rate (CTR) Customer Retention Cost Profits per customer Customer acquisition cost |
| SEO/Web site | SEO keyword ranking
SEO sales conversion rate Number of unique visitors Total sessions/visits Average time on site/page |
| Email Marketing | Open rate
Click-thru-rate (CTR) Bounce rate Unsubscribe rate |
| Public Relations | Advertising value equivalency
Clip/article counting Brand mentions |
Campaign Metrics Case Study: Citizen Watch
Citizen, one of the world’s largest makers of wristwatches, embarked on a digital marketing strategy to build its brand using social media, with a specific focus on expanding its presence on Facebook. The marketing team’s goal for the first year was to gain 100,000 followers on Facebook. Their campaign strategy focused on offering engagement opportunities that pushed people to Facebook to interact with the brand. It incorporated a combination of tactics that included offline and online elements, such as a series of register-to-win contests like a “Win Your Mum a Watch” giveaway. It also offered related online engagement opportunities, like interactive photo galleries on the company Web site for people to browse, with new products to view and share on social media.
To help gauge their progress and understand how well different dimensions of the campaign were working, they tracked a variety of metrics, with one KPI being the number of Facebook followers. Over the course of the campaign, they had impressive results. In addition to blowing through their goal of getting 100,000 followers, Citizen saw the following results from consumers who participated in campaign activities:
- 76 percent lead-submission rate
- 82 percent app-completion rate
- 26 percent social-share rate
By tracking these metrics across different offers and campaigns, Citizen was able to gauge which activities were the best received and use this information to improve the effectiveness of future campaigns. The company has used these insights to expand promotional activities to other forms of social media and other types of engagement activities.[1]
Licenses and Attributions
CC licensed content, Original
- Examples of Key Performance Indicators. Authored by: Melissa Barker. Provided by: Spokane Falls Community College. License: CC BY: Attribution
- Reading: Measuring Marketing Communication Effectiveness. Authored by: Lumen Learning. License: CC BY: Attribution
CC licensed content, Shared previously
- Citizen Watch. Authored by: Kansai explorer. Located at: https://commons.wikimedia.org/wiki/File:Citizen_Attesa_Eco-Drive_ATV53-3023_02.JPG. License: CC BY-SA: Attribution-ShareAlike
- Compass Illustration. Authored by: Alan Klim. Located at: https://www.flickr.com/photos/igraph/8231264538/. License: CC BY: Attribution
All rights reserved content
- What Are Key Performance Indicators (KPIs)?. Authored by: Bernard Marr. Located at: https://youtu.be/9Co8slUvYj0. License: All Rights Reserved. License Terms: Standard YouTube license
Self Check: Measuring Marketing Communication Effectiveness
Check Your Understanding
Answer the question(s) below to see how well you understand the topics covered in the previous section. This short quiz does not count toward your grade in the class, and you can retake it an unlimited number of times.
Use this quiz to check your understanding and decide whether to (1) study the previous section further or (2) move on to the next section.
Licenses and Attributions
CC licensed content, Original
- Self Check: Measuring Marketing Communication Effectiveness. Provided by: Lumen Learning. License: CC BY: Attribution
|
oercommons
|
2025-03-18T00:39:14.264551
|
06/06/2022
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/93473/overview",
"title": "Statewide Dual Credit Principles of Marketing, Promotion: Integrated Marketing Communication (IMC), Measuring Marketing Communication Effectiveness",
"author": "Anna McCollum"
}
|
https://oercommons.org/courseware/lesson/91252/overview
|
Elements of the Marketing Plan
Overview
Title Image: Photo by rawpixel.com form PxHere
Provided by: Lumen Learning. License: CC BY: Attribution
Outcome: Elements of the Marketing Plan
What you’ll learn to do: identify the key elements of the marketing plan
A lot of work goes into developing a marketing plan. But once it’s completed, it provides a detailed roadmap of not only where you’re heading, but also why and how to get there. By putting in significant effort up front to create a good plan, you’ll find that the “doing” part is much simpler, better focused, better organized. Of course, a good plan also increases your likelihood of marketing success.
Success? Well, that makes you look good.
The specific things you’ll learn in this section include:
- Describe the purpose of a marketing plan
- Explain why each key element of the marketing plan is important to the marketing team’s successful implementation of the overall plan
Learning Activities
- Reading: Elements of the Marketing Plan
- Self Check: Elements of the Marketing Plan
Licenses and Attributions
CC licensed content, Original
- Outcome: Elements of the Marketing Plan. Provided by: Lumen Learning. License: CC BY: Attribution
Reading: Elements of the Marketing Plan
Charting the Course Ahead: The Marketing Plan
Marketing exists in order to support an organization in achieving its strategic goals–for growth, profitability, revenue, influence, and so on. As explained at the beginning of this course, the role of marketing is to identify, satisfy, and retain customers. You have learned about many different tools marketers use to fill this role. The marketing plan is the guiding document used by marketing managers and teams to lay out the objectives that marketing efforts will focus on and the actions they will take to achieve these objectives.
A comprehensive marketing plan paints the big picture of what is happening with an organization internally and externally. After analyzing the marketing environment, the plan then recommends strategies and tactics aimed at helping the organization take full advantage of available opportunities and resources to accomplish its goals. When a marketing plan is completed thoughtfully and skillfully, it helps marketers not only present the case for what they recommend doing, but it also creates a common vision within the organization about what’s happening and how people and resources will come together to achieve that vision.
What’s in a Marketing Plan?
You may already be familiar with marketing plans from your job experience or from your prior work in this course. Different marketers may use a variety of different formats to create a marketing plan, but most marketing plans include common elements that answer basic marketing questions such as the following:
- What are our goals and strategy?
- Who are we trying to reach, and how will we reach them?
- What are we trying to communicate?
- What marketing strategies and tactics will we use to achieve our goals?
The key elements of a marketing plan are described in the table below.
Note that these marketing plan elements correspond to a sample marketing plan template provided for use in this course. Because it is a template, or pattern, you can adapt and use it again—perhaps at a future job. This particular marketing plan template was designed to align well with the structure and content of this course. The table also provides a reference to the course module where each marketing plan element was introduced and explained in greater detail.
Marketing Plan: Key Elements
Executive Summary
What is the plan about?
Summary of key points from the marketing plan and what it will accomplish. It’s an at-a-glance overview for a manager who may not have time to look over the whole thing.
Company Profile
What organization are you marketing?
Basic information about the organization, its offerings, and competitive set.
Market Segmentation and Targeting
Who is your target audience?
Description of the market for the product or service in question, segments in this market, and targeting strategy the marketing plan will address.
Course Module Reference: Segmentation and Targeting
Situation and Company Analysis
What is your strategy, and why is it the right approach?
SWOT analysis of the external marketing environment and the internal company environment, and marketing goals aligned with the company mission and objectives.
Course Module Reference: Marketing Strategy
Ethics and Social Responsibility
How will you demonstrate good corporate citizenship?
Recommendations for how to address any issues around ethics, social responsibility, and sustainability.
Course Module Reference: Ethics and Social Responsibility
Marketing Information and Research
What information do you need to be successful, and how will you get it?
Discussion of key questions that need to be answered, the information needed, and recommendations for how marketing research can provide answers.
Course Module Reference: Marketing Information and Research
Customer Decision-Making Profile
Who is your target customer, and what influences their buying decisions?
Profile of the primary buyer(s) targeted in the marketing plan and factors that impact their choices.
Course Module Reference: Consumer Behavior
Positioning and Differentiation
What do you want to be known for?
List of competitive advantages, positioning recommendations, and how to convince the market you are different and better.
Course Module Reference: Positioning
Branding
What is the brand that you’re building?
Brand platform describing the brand: promise, voice, personality, positioning, and strategic recommendations for building the brand.
Course Module Reference: Branding
Marketing Mix (Four Ps)
How will you impact your target market?
This question is addressed by the strategic recommendations around each of the four Ps below.
Course Module Reference: Marketing Function
Product Strategy
What are you offering to your target market?
Description of the product or service being marketed and recommended improvements to fit the needs of target segments.
Course Module Reference: Product Marketing
Pricing Strategy
How are you pricing the offering?
Recommendations on pricing strategy and why this approach makes sense.
Course Module Reference: Pricing Strategies
Place: Distribution Strategy
How are you distributing the offering?
Recommendations on distribution strategy and channel partners to improve the availability of your offering, and explanations of why this approach makes sense.
Course Module Reference: Place: Distribution Channels
Promotion: IMC Strategy
What marketing campaign(s) are your running?
Overview of marketing strategy, objectives, messaging, and tactical approach for marketing campaign(s) to reach your target audiences.
Course Module Reference: Promotion: IMC Strategy
Measurement and KPIs
How will you measure the impact you’re making?
Identification of key performance indicators (KPIs) and other metrics to monitor effectiveness of marketing campaign activities and provide clues about when to adjust course.
Course Module Reference: Promotion: IMC Strategy
Budget
How much will this cost?
List of resources required to execute the marketing plan, how much they will cost, and how to stay within the allocated budget.
Course Module Reference: Promotion: IMC Strategy
Action Plan
What will it take to make this happen?
A detailed, step-by-step plan about what needs to happen, when, and who’s responsible for each step to execute the marketing campaign.
Course Module Reference: Promotion: IMC Strategy
Risk Factors
What are the risks of this approach?
Discussion of any significant risks or threats associated with this plan and contingency plans for addressing them.
Course Module Reference: Promotion: IMC Strategy
After this course, as you have the opportunity to develop marketing strategy and plans in the future, you may choose to use this template in its entirety or adapt it to specific project needs.
Focusing Purpose, Guiding Activity
Marketing plans can be developed to focus in a variety of areas. A corporate marketing plan can be developed to promote the organization as a whole. Marketing plans may also focus on specific brands, products, services, market segments, and even to cover a set period of time, such as a quarterly marketing plan. To illustrate:
Company A might develop and execute three distinct marketing plans that share some common elements, such as the situation and company analysis and the market segmentation. When it comes to specific target audiences, positioning, campaign objectives, and planning, the three marketing plans diverge to focus on different dimensions of the business:
- A corporate marketing plan to direct marketing communications focused on the company as a whole and building its corporate brand
- A marketing plan focused on the launch and rollout of a new product line
- A marketing plan for expanding the customer base and revenue of an established product line
On the other hand, Company B might develop and execute a single marketing plan that incorporates several different campaigns targeting the market segments served by its product and service portfolio. In this case, some sections of the plan are expanded to provide information, strategy, and planning focused on each target segment. This includes segment-specific customer profiles, positioning, IMC campaigns, and so forth.
Either of these approaches could be exactly right for the organization, depending on its goals and objectives. What’s most important is for the marketing plans to do a good job guiding marketing teams to formulate and execute marketing activities that are well aligned what the organization is trying to achieve. If multiple marketing plans are being developed and used, it is essential for marketing managers to make sure internal communication and sharing are happening between the marketing team members executing the plans. By sharing information about goals, messaging, timing, audience touch points, and other elements, marketers can avoid stepping on one another’s toes or creating confusion in the market. Ideally, teams can learn from one another’s successes and experiences so that the entire marketing effort becomes smarter and more efficient over time.
Licenses and Attributions
CC licensed content, Original
- Reading: Elements of the Marketing Plan. Provided by: Lumen Learning. License: CC BY: Attribution
Self Check: Elements of the Marketing Plan
Check Your Understanding
Answer the question(s) below to see how well you understand the topics covered in the previous section. This short quiz does not count toward your grade in the class, and you can retake it an unlimited number of times.
Use this quiz to check your understanding and decide whether to (1) study the previous section further or (2) move on to the next section.
Licenses and Attributions
CC licensed content, Original
- Self Check: Elements of the Marketing Plan. Provided by: Lumen Learning. License: CC BY: Attribution
|
oercommons
|
2025-03-18T00:39:14.308108
|
03/22/2022
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/91252/overview",
"title": "Statewide Dual Credit Principles of Marketing, Marketing Plan, Elements of the Marketing Plan",
"author": "Anna McCollum"
}
|
https://oercommons.org/courseware/lesson/91245/overview
|
Why it Matters
Overview
Teacher resources for Unit 15 can be found on the next page.
Provided by: Lumen Learning. License: CC BY: Attribution
Why It Matters: Marketing Globally
Resources for Unit 15: Marketing Globally
Slide Deck - Module 15: Marketing Globally
Discussion Assignments and Alignment: Global Marketing
Unit 14 Assignment: Marketing Plan Peer Review
Pacing
The Principles of Marketing textbook contains sixteen units—roughly one unit per week for a 16-week semester. If you need to modify the pace and cover the material more quickly, the following units work well together:
- Unit 1: What Is Marketing? and Unit 2: Marketing Function. Both are lighter, introductory units.
- Unit 15: Global Marketing and Unit 16: Marketing Plan. Unit 16 has more course review and synthesis information than new material per se.
- Unit 5: Ethics can be combined with any unit. You can also move it around without losing anything.
- Unit 8: Positioning and Unit 9: Branding. Companion modules that can be covered in a single week.
- Unit 6: Marketing Information & Research and Unit 7: Consumer Behavior. Companion units that can be covered in a single week.
We recommend NOT doubling up the following units, because they are long and especially challenging. Students will need more time for mastery and completion of assignments.
- Unit 4: Marketing Strategy
- Unit 10: Product Marketing
- Unit 13: Promotion: Integrated Marketing Communication
Did you have an idea for improving this content? We’d love your input.
Learning Outcomes
- Describe globalization and the major benefits and challenges it poses for multinational organizations penetrating global markets
- Describe common approaches used by organizations to compete successfully on a global scale
- Explain the importance of understanding how demographic, cultural and institutional factors shape the global marketing environment
Why identify issues that organizations face and approaches they use when marketing to different countries and cultures?
Suppose you’re in the marketing department for a highly successful snack food company in the U.S. You’re in a brainstorming meeting about expanding into China, and the discussion is starting to get heated. Should you lead with your company’s best-selling nacho-cheese-flavored snacks to take China by storm? Or would it be better to start out with the ranch-dressing-flavored snack instead, because it’s so quintessentially American and it’d be a great way to introduce the Chinese to the tastes Americans love?
Or would something else be a better fit?
It’s time to vote: your manager wants everyone on the team to name the flavor they want to lead with. What are you going to choose?
Set aside your top pick while you watch this short but very interesting video.
You can view the transcript for “Chinese Flavors for American Snacks” (opens in new window) or the text alternative for “Chinese Flavors for American Snacks” (opens in new window).
So . . . how did you do? How close did you come to favorite flavors in the video? Were you in the ballpark? Are you ready for a career developing snack foods for global markets?
If you’re like most Americans, your recommendation probably wasn’t very close to the mark, and you’re probably thinking that many of the flavors that are delicious to Chinese consumers sound a bit odd to you. Well, now you know how a lot of Chinese consumers probably feel when presented with Cheetos Crunchy Flamin’ Hot Limon Cheese Flavored Snacks or Zapp’s Spicy Cajun Crawtator potato chips. A little queasy.
Hopefully this scenario helps highlight some of the challenges of global marketing, as companies start selling products in other countries. How should you enter a new market? Are you offering products that consumers in other countries will want to buy? What should you do to make sure your product–and the rest of your marketing mix–is a good fit for the global customers you want to attract?
Global marketing is a complex and fascinating business. In this module, we can’t cover everything about global marketing–not by a long shot. But we will introduce key challenges, opportunities, and factors to consider when marketing to target audiences outside your home country.
Licenses and Attributions
CC licensed content, Original
- Why It Matters: Marketing Globally. Provided by: Lumen Learning. License: CC BY: Attribution
CC licensed content, Shared previously
- Chinese Flavors for American Snacks. Provided by: BBC. Located at: https://youtu.be/BA8bCNiKZsg. License: CC BY-NC-ND: Attribution-NonCommercial-NoDerivatives
|
oercommons
|
2025-03-18T00:39:14.339831
|
03/22/2022
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/91245/overview",
"title": "Statewide Dual Credit Principles of Marketing, Marketing Globally, Why it Matters",
"author": "Anna McCollum"
}
|
https://oercommons.org/courseware/lesson/100031/overview
|
3.2.2 Extent of Production Agriculture
3.2.3 The Global Economy and U.S. Agriculture
3.2.4 The Commodity Market, Trading, and Futures
3.2.5 Foreign Trade Agreements
3.2.6 Foreign Exchange Rates and Firms
Ag and Food Statistics: Charting the Essentials - February 2023
The Impact of Recent Trade Agreements on Japan's Pork Market
USDA Economic Research Service Publications: Economic Research Reports
Global Economic Systems and Agriculture
Overview
Affecting Market Performance
Learning Objectives
7d Understand markets and factors that affect market performance including competition.
Competition and Market Structures
Competition in the marketplace affects price, demand, and supply of goods and services. Market structures describe the nature or degree of competition among companies in the same industries and in a free enterprise economy.
Economists have developed a theoretical model of an ideal situation where “perfect competition” occurs. Of course, this is only a model to compare to other types of market structures that are not “perfect”.
For there to be “perfect competition” certain conditions must prevail in the market, such as:
- a large number of buyers and sellers;
- sellers must deal in identical products;
- buyers and sellers act independently and compete with each other;
- both buyers and sellers must be well informed of the conditions in the markets;
- and both buyers and sellers can enter into and leave the market whenever they choose.
So, as you may have determined from the five conditions that must exist to have “perfect competition,” there really is no such thing.
If any of the five conditions are not met, then the market structure is called “imperfect”. There are three “imperfect markets”: monopolistic competition, oligopoly, and monopoly.
Firms face different competitive situations. At one extreme—perfect competition—many firms are all trying to sell identical products. At the other extreme—monopoly—only one firm is selling the product, and this firm faces no competition. Monopolistic competition and oligopoly fall between the extremes of perfect competition and monopoly.
Universal Generalizations
Perfect competition is a theory used to evaluate types of market structures: perfect competition, monopolistic competition, oligopoly, and monopoly. The type of market structure is determined by the amount of competition among firms operating in the same industry.
Three Economic Questions: What, How, For Whom?
Firms behave in much the same way as consumers behave. If the firm is successful, the outputs are more valuable than the inputs. Production involves a number of important decisions that define the behavior of firms. In order to meet the needs of its people, every society must answer three basic economic questions: What? How? Whom?
- What should we produce? (What product or products should the firm produce? What price should the firm charge for its products?)
- How should we produce it? (How should the products be produced? How much output should the firm produce? How much labor should the firm employ?)
- For whom should we produce it?
Although every society answers the three basic economic questions differently, in doing so, each confronts the same fundamental problems: resource allocation and scarcity.
As we said in previous sections, scarcity means that resources are limited. No country can produce everything, no matter how rich its mines, how massive its forests, or how advanced its technology. Because of the constraints of scarcity, then, decisions must be made about resource allocation.
Perfect Competition and Why It Matters
A perfectly competitive firm is known as a price taker because the pressure of competing firms forces them to accept the prevailing equilibrium price in the market. If a firm in a perfectly competitive market raises the price of its product by so much as a penny, it will lose all of its sales to competitors. When a wheat grower wants to know what the going price of wheat is, he or she has to go to the computer or listen to the radio to check. The market price is determined solely by supply and demand in the entire market and not the individual farmer.
Also, a perfectly competitive firm must be a very small player in the overall market, so that it can increase or decrease output without noticeably affecting the overall quantity supplied and price in the market.
Agricultural markets are often used as an example of perfect competition. The same crops grown by different farmers are largely interchangeable. According to the United States Department of Agriculture monthly reports, in 2012, U.S. corn farmers received an average price of $6.07 per bushel and wheat farmers received an average price of $7.60 per bushel. A corn farmer who attempted to sell at $7.00 per bushel, or a wheat grower who attempted to sell for $8.00 per bushel would not have found any buyers. A perfectly competitive firm will not sell below the equilibrium price either. Why should they when they can sell all they want at the higher price? Other examples of agricultural markets that operate in close to perfectly competitive markets are small roadside produce markets and small organic farmers.
How Perfectly Competitive Firms Make Output Decisions
A perfectly competitive firm has only one major decision to make—namely, what quantity to produce. To understand why this is so, consider a different way of writing out the basic definition of profit:
Profit=Total revenue−Total cost
Profit =(Price)(Quantity produced)−(Average cost)(Quantity produced)
Since a perfectly competitive firm must accept the price for its output as determined by the product’s market demand and supply, it cannot choose the price it charges. This is already determined in the profit equation, and so the perfectly competitive firm can sell any number of units at exactly the same price. A perfectly competitive firm can increase output without affecting the overall quantity supplied in the market.
Determining the Highest Profit by Comparing Total Revenue and Total Cost
A perfectly competitive firm can sell as large a quantity as it wishes, as long as it accepts the prevailing market price. Total revenue is going to increase as the firm sells more, depending on the price of the product and the number of units sold. If you increase the number of units sold at a given price, then total revenue will increase. If the price of the product increases for every unit sold, then total revenue also increases.
As an example of how a perfectly competitive firm decides what quantity to produce, consider the case of a small farmer who produces raspberries and sells them frozen for $4 per pack. Sales of one pack of raspberries will bring in $4, two packs will be $8, three packs will be $12, and so on. If, for example, the price of frozen raspberries doubles to $8 per pack, then sales of one pack of raspberries will be $8, two packs will be $16, three packs will be $24, and so on.
Total revenue and total costs for the raspberry farm, broken down into fixed and variable costs, are shown in Table 1 and also appear in Figure 3. The horizontal axis shows the quantity of frozen raspberries produced in packs; the vertical axis shows both total revenue and total costs, measured in dollars. The total cost curve intersects with the vertical axis at a value that shows the level of fixed costs, and then slopes upward.
Total revenue for a perfectly competitive firm is a straight line sloping up. The slope is equal to the price of the good. Total cost also slopes up, but with some curvature. At higher levels of output, total cost begins to slope upward more steeply because of diminishing marginal returns. The maximum profit will occur at the quantity where the gap of total revenue over total cost is largest.
Based on its total revenue and total cost curves, a perfectly competitive firm like the raspberry farm can calculate the quantity of output that will provide the highest level of profit. At any given quantity, total revenue minus total cost will equal profit. One way to determine the most profitable quantity to produce is to see at what quantity total revenue exceeds total cost by the largest amount.
Raspberry Farm Total Revenue and Total Cost Table | |||||
Quantity | Total Cost | Fixed Cost | Variable Cost | Total Revenue | Profit |
(Q) | (TC) | (FC) | (VC) | (TR) | |
0 | $62 | $62 | - | $0 | −$62 |
10 | $90 | $62 | $28 | $40 | −$50 |
20 | $110 | $62 | $48 | $80 | −$30 |
30 | $126 | $62 | $64 | $120 | −$6 |
40 | $144 | $62 | $82 | $160 | $16 |
50 | $166 | $62 | $104 | $200 | $34 |
60 | $192 | $62 | $130 | $240 | $48 |
70 | $224 | $62 | $162 | $280 | $56 |
80 | $264 | $62 | $202 | $320 | $56 |
90 | $324 | $62 | $262 | $360 | $36 |
100 | $404 | $62 | $342 | $400 | −$4 |
In Table 3.2.1a, the vertical gap between total revenue and total cost represents either profit (if total revenues are greater than total costs at a certain quantity) or losses (if total costs are greater than total revenues at a certain quantity). In this example, total costs will exceed total revenues at output levels from 0 to 40, and so over this range of output, the firm will be making losses. At output levels from 50 to 80, total revenues exceed total costs, so the firm is earning profits. But then at an output of 90 or 100, total costs again exceed total revenues, which means the firm is making losses. The highest total profits in the figure occur at an output of 70–80, when profits will be $56.
Entry and Exit Decisions in the Long Run
The line between the short run and the long run cannot be defined precisely with a stopwatch or even with a calendar. It varies according to the specific business. The distinction between the short run and the long run is, therefore, more technical: in the short run, firms cannot change the usage of fixed inputs, while in the long run, the firm can adjust all factors of production.
If a business is making a profit in the short run, it has an incentive to expand existing factories or to build new ones. New firms may start production, as well. When new firms enter the industry in response to increased industry profits it is called entry.
Losses are the black thundercloud that causes businesses to flee. If a business is making losses in the short run, it will either keep limping along or just shut down, depending on whether its revenues are covering its variable costs. But in the long run, firms that are facing losses will shut down at least some of their output, and some firms will cease production altogether. The long run process of reducing production in response to a sustained pattern of losses is called exit.
Promoting Innovation
Innovation takes time and resources to achieve. Suppose a company invests in research and development and finds the cure for the common cold. In this world of abundant information, other companies could take the formula, produce the drug, and because they did not incur the costs of research and development (R&D), undercut the price of the company that discovered the drug. Given this possibility, many firms would choose not to invest in research and development, and as a result, the world would have less innovation.
The U.S. has a combination of patents, trademarks, copyrights, and trade secret laws. These things are collectively called intellectual property because it implies ownership over an idea, concept, or image, not a physical piece of property like a house or a car. Countries around the world have enacted laws to protect intellectual property although the time periods and exact provisions of such laws vary across countries. There are ongoing negotiations, both through the World Intellectual Property Organization (WIPO) and through international treaties, to bring greater harmony to the intellectual property laws of different countries to determine the extent to which patents and copyrights in one country will be respected in other countries.
The combination of improvements in production technologies in the U.S. and a general sense that the markets could provide services adequately led to a wave of deregulation. This deregulation started in the late 1970s and continuing into the 1990s. This movement eliminated or reduced government restrictions on the firms that could enter, the prices that could be charged, and the quantities that could be produced in many industries, including telecommunications, airlines, trucking, banking, and electricity.
Around the world, from Europe to Latin America to Africa and Asia, many governments continue to control and limit competition in what those governments perceive to be key industries, including airlines, banks, steel companies, oil companies, and telephone companies.
Attributions
Title Image: "Interior of REMA 1000 supermarket/grocery store in Tønsberg, Norway. Fruit and vegetables displayed for sale." by Wolfmann, Wikimedia Commons is licensed under CC BY-SA 4.0
"2.9: Competition and Market Structures" by CK-12 Foundation is licensed under CC BY-NC 4.0
Extent of Production Agriculture
Learning Objectives
7a Identify the extent of production agriculture.
What is Agriculture’s Share of the Overall U.S. Economy?
The U.S. agriculture sector not only includes farms and ranches but also includes a range of farm-related industries. According to the 2017 Agricultural Census, the market value of U.S. agricultural products sold was $300,000,000. In 2021, agriculture, food, and related industries contributed 5.4% to U.S. gross domestic product and provided 10.5% of U.S. employment. On average, 12% of American household budgets is assigned to purchasing food. And nutrition assistance far outpaces other federal assistance programs.
Agriculture and GDP
Agriculture, food, and related industries contributed roughly $1.264 trillion to U.S. gross domestic product (GDP) in 2021, which was a 5.4% share. The output of America’s farms contributed $164.7 billion of this sum—about 0.7% of U.S. GDP. The overall contribution of agriculture to GDP is larger than 0.7% because sectors related to agriculture rely on agricultural inputs in order to contribute added value to the economy. Sectors related to agriculture include food and beverage manufacturing; food and beverage stores; food services and eating/drinking places; textiles, apparel, and leather products; and forestry and fishing.
Agriculture and Employment
In 2021, 21.1 million full- and part-time jobs were related to the agricultural and food sectors—10.5% of total U.S. employment. Direct on-farm employment accounted for about 2.6 million of these jobs, or 1.3% of U.S. employment. Employment in agriculture- and food-related industries supported another 18.5 million jobs. Of this, food services accounted for the largest share—11.8 million jobs—and food/beverage stores supported 3.3 million jobs. The remaining agriculture-related industries together added another 3.4 million jobs.
Food and Households
Expenditures on food accounted for 12.4% of U.S. households’ spending in 2021, an increase from 2020 when it was 11.9%. The share of household expenditures on food ranked third behind housing (33.8%) and transportation (16.4%).
Food Employees by Sector
In 2021, the U.S. food and beverage manufacturing sector employed 1.7 million people, or just over 1.1% of all U.S. nonfarm employment. In thousands of food and beverage manufacturing plants throughout the country, these employees were engaged in transforming raw agricultural materials into products for intermediate or final consumption. Meat and poultry plants employed the largest percentage of food and beverage manufacturing workers, followed by bakeries and beverage plants.
USDA Budget Outlays
USDA outlays, money spent on things such as programs, increased by 48% from 2006 to 2015. The largest increase coming from food and nutrition assistance programs, which grew especially fast since 2008, reflecting higher recession-related participation and a temporary increase in per-person benefits from the Supplemental Nutrition Assistance Program (SNAP). An improving economy and expiration of the larger SNAP benefits caused growth of food and nutrition assistance program outlays to slow by 2012 and decrease in 2014.
Outlays on Federal crop insurance also decreased in 2014 as extreme weather events subsided and crop prices declined. Commodity program outlays declined in 2015 with the passing of the new Farm Act in 2014. Food and nutrition assistance accounted for more than 73% of USDA outlays in 2015.
The agricultural industry is a far-reaching industry that uses input from other industries and provides inputs to other industries. Much of our economy is based on the ability to produce nutritious, safe, affordable food for the population. Another portion of our economy relies on jobs being available. No matter what your job is you are involved in agriculture, your food, your attire, your house, all made with things that originate somewhere in the agricultural industry.
Attributions
"Ag and Food Sectors and the Economy" by Kathleen Kassel and Anikka Martin, USDA Economic Research Service is in the Public Domain
"U.S. Agricultural Trade at a Glance" by James Kaufman, USDA Economic Research Service is in the Public Domain
The Global Economy and U.S. Agriculture
Instructor Ideas:
- Students could research trade between the U.S. and various countries. Students could create a presentation over the information or write a short research paper. This could be done in groups or individually.
- Instructor could assign reading material for students, such as the USDA Reports linked below, then lead a class discussion over the far reaching effects of trade.
Learning Objectives
7c Discuss the impact of U.S. agriculture on the global economy.
America’s Farmers and Ranchers
America’s farmers and ranchers make an important contribution to the U.S. economy by ensuring a safe and reliable food supply and improving energy security, as well as supporting job growth and economic development. Agriculture is particularly important to the economies of small towns and rural areas, where farming supports a number of sectors, from farm machinery manufacturers to food processing companies.
Access to world markets will be important for continued success due to increasing agricultural productivity. Population growth in the decades ahead will be concentrated in developing countries, and 95% of the world’s potential consumers live outside of the United States. As these countries grow and their citizens’ incomes rise, their demand for meat, dairy, and other agricultural products will increase.
Agricultural exports currently support nearly one million jobs across the country. But U.S. farmers, ranchers and food producers are well positioned to capture an increasing share of the growing world market for agricultural products. The United States is the world’s leading exporter of agricultural products. At $141.3 billion, agricultural exports made up 10% of U.S. exports in 2012. Since 1960, the United States has posted a trade surplus in agriculture. This surplus totaled $38.5 billion in 2012. Capturing a growing share of the world market for agricultural products will benefit the entire economy.
Despite recent success, challenges remain for U.S. agriculture, including uncertainty about future farm policy. U.S. agricultural exporters often confront barriers imposed by countries that keep U.S. products from reaching their target markets. Small and beginning farmers, ranchers and processors may face added burdens in navigating the complexities involved in exporting their products. America’s deteriorating transportation infrastructure and uncertainty regarding the agricultural workforce because of unsettled immigration policy add to the challenges facing agricultural exporters. Addressing these challenges will benefit U.S. agriculture and the economy overall.
The Economic Impact of U.S. Agriculture
The United States has a robust farm economy. In 2012, total farm cash receipts exceeded $390 billion, including $219.6 billion in cash receipts for crops and $171.7 billion in cash receipts for livestock and related products. Some products such as wheat, coarse grains, cotton, and soybeans are sold in bulk either in the United States or abroad, while most others undergo various levels of processing. Wheat flour, soybean oil, meats, cereals, and dairy products are examples of products that receive additional processing prior to their final sale. After accounting for production expenses, net farm income totaled $112.8 billion in 2012, about 125% higher than a decade prior in 2002.
A successful agricultural sector supports economic growth overall. By producing a wide variety of foods inexpensively—including fruits, vegetables, grains, meat and dairy products, America’s farmers and ranchers ensure a safe and reliable domestic food supply. This sector also improves U.S. energy security and reduces dependence on foreign oil through the production of biofuels and the development of other alternative sources of energy. These new sources of energy can help reduce costs for businesses and consumers. For example, some studies have found that an increased supply of biofuels reduces gas prices, especially as biofuel production technology improves.
A healthy farm economy is especially important to small towns and rural areas. Farmers and ranchers invest in their operations, supporting jobs in farm machinery manufacturing and other industries, and they purchase goods and services from local businesses. High levels of farm production, in turn, improve the prospects for downstream businesses, such as food processing companies and biofuel refineries. Businesses up and down the agricultural product supply chain have benefited in recent years because of the strong agricultural economy. An increase in sales of organic, specialty and bio-based products, as well as a recent expansion in agritourism, has contributed to this success.
Exporting is particularly important for agriculture, since growth in demand for agricultural products in the coming decades is expected to come largely from developing countries. U.S. agriculture has been successful in exporting its products, even as other industries have struggled in the global market. According to a U.S. Department of Agriculture (USDA) model, each $1 billion of agricultural exports supported 6,800 American jobs in 2011. These jobs include positions on farms, in the food processing industry, in the trade and transportation sector and in other supporting industries. In general, high-value (processed) exports supported more jobs and economic activity per dollar of exports than bulk exports of raw products. Assuming the number of jobs supported by each $1 billion of agricultural exports stayed within a range of the values estimated for 2010 and 2011, U.S. agricultural exports supported nearly one million jobs in 2012.
Recent Trends in Agricultural Exports
Since 1990, high-value agricultural exports, which include consumer-ready products and processed goods used as inputs by other industries, have made up the largest share of agricultural exports. The real value of U.S. agricultural exports has increased substantially over the past decades, due largely to rising demand for food and other agricultural products in developing countries. In 2012, U.S. agricultural exports totaled $141.3 billion. Rounding out the top five export destinations in 2012 were Canada, Mexico, Japan and the European Union. Grains and feeds accounted for nearly one-quarter of agricultural exports in 2012, representing $32 billion in export sales. And soybean exports totaled approximately $25 billion and made up 17.5% of export sales. Soybean exports increased by over 35% from the previous year. And global demand for soybeans, as well as soybean oil and soybean meal, is expected to continue to grow substantially. Red meats accounted for nearly 10% of agricultural exports, as did animal feeds and oil meal. The following products each made up roughly 4 - 5% of U.S. exports: tree nuts and preparations, fruits, cotton and linters, vegetables, poultry, sugar and tropical products and dairy products.
Challenges remain that could keep the United States from taking advantage of these growth opportunities. These challenges include uncertainty about long-term farm policy, trade barriers imposed by foreign countries, issues facing small and beginning farmers, ranchers and processors, the deterioration of U.S. transportation infrastructure and uncertainty in the agricultural workforce resulting from an unsettled immigration policy.
Export Issues
Agricultural exporters often encounter trade barriers. Despite some progress, average agricultural tariffs remain substantially higher than those imposed on other products. Consequently, there has been a continual push for lower average tariffs on agricultural products for small and beginning farmers, ranchers and processors, as well as for better export opportunities.
Overseas markets offer tremendous growth opportunities for small and beginning farmers, ranchers, and agricultural processors. These individuals and businesses face particular challenges in exporting their products. They may not be able to finance losses of a shipment at the border if a country imposes trade barriers, and they are more likely to lack the resources to identify and address such barriers. In addition, small farmers and food producers face many of the same challenges that small businesses in other industries face in exporting. For example, compared with larger businesses, they may have limited knowledge of foreign markets or technical expertise regarding export procedures.
The Export Promotion Act (2010)—which was part of the Small Business Jobs Act—connects small businesses with export promotion and outreach resources through the Department of Commerce; these connections are made to help them expand into new markets. This law also expands the outreach program through the Department’s Rural Export Initiative to ensure that small businesses located in rural areas know about available export-promotion services. Improving export opportunities for small farmers and agricultural producers could contribute to increasing exports overall.
Infrastructure Issues
America’s deteriorating transportation infrastructure may inhibit agricultural export growth. The agricultural sector relies on various forms of transportation infrastructure to move products from farms and factories to consumers at home and abroad, including roads, rails, and ports. Inland transportation infrastructure is particularly important for agricultural exporters. However, infrastructure surveys show that the United States is falling behind in investing in and maintaining its transportation infrastructure compared to global competitors. In 2012, inadequate investment in harbor maintenance and other water infrastructure negatively affected exporters who rely on the Mississippi River and the Great Lakes to transport their products.
Workforce Issues
Uncertainty regarding the agricultural workforce stemming from an unsettled immigration policy adds to challenges facing agricultural exporters. Foreign-born workers are critical to U.S. agriculture, making up 72% of the workforce. Seasonal and temporary workers are especially vital. Many of the positions these immigrants and temporary residents fill would not otherwise be filled by native-born workers.
Conclusion
The agricultural sector makes an important contribution to the U.S. economy, from promoting food and energy security to supporting jobs in communities across the country. Exports are critical to the success of U.S. agriculture, and population and income growth in developing countries ensures that this will continue to be the case in the decades to come. U.S. agricultural exporters are well positioned to capture a significant share of the growing world market for agricultural products, but some challenges remain. Taking actions that can facilitate exports would help to strengthen the agricultural sector and promote overall economic growth.
Attributions
"The Economic Contribution of America's Farmers and the Importance of Agricultural Exports." by Joint Economic Committee is in the Public Domain
The Commodity Market, Trading, and Futures
Learning Objectives
7e Discuss commodity market, trading, and futures.
Commodity Markets
Producers—farmers and ranchers, merchandisers, processors, retailers, and consumers rely on each other. Producers need to be able to sell the raw products they produce to processors. Processors then use these raw materials to create goods that are sold to retailers. Producers and processors both require a place to negotiate prices and either buy or sell their agricultural products. And that place is commodity markets—the place where prices are determined.
What is a Commodity?
A commodity is a raw product. Examples of commodities include grains—like corn, wheat and soybeans; livestock—like cattle and hogs; metals—like gold and silver, and energy sources—like crude oil and natural gas. This raw product is typically sold, and then processed and/or packaged in some way. So, corn may be sold to a processor who makes ethanol or to a processor who packages food, while crude oil may be sold to a processor who makes plastic or one makes fuel. These processed goods are then shipped to retailers, who sell finished products to consumers.
To make it easier to buy and sell these raw goods, the quality of the commodity must be uniform from all producers. So, all the bushels of corn, all the bales of cotton, and all the barrels of crude oil are essentially the same, regardless of who produced them.
Marketing Commodities and Managing Risk
Farming is full of risk. In any year, growers can face weather perils that include droughts and floods. Even when producers escape those extremes, conditions must be favorable at key periods during planting, growing, and harvesting. And even after crops are grown and harvested, producers still encounter risk. Changes in consumer demand, unforeseen international events, costs for fuel, and other circumstances can all influence profit. But the greatest risk of all may not be associated with producing commodities; rather, the greatest risk is associated with marketing—or selling for a profit. Two methods that are commonly used to market commodities are cash marketing and forward contracting.
Cash Marketing
The cash sale of the physical commodity is the most common sales method used by farmers, and it is ultimately involved in all grain sales. At times, it is used as a stand-alone transaction; at other times, it represents the completion of a hedge or other strategy.
Cash marketing takes place when a farmer sells his commodity for cash. For a grain farmer, this is usually done at a local cooperative or elevator. The farmer has not entered into any kind of contract to deliver the commodity at a certain time or at a certain price. In fact, cash marketing can take place any time after harvest, and it can be delayed by months if the producer stores his/her/their crop. The primary risk revolves around prices lowering while the commodity is held in storage; then, the farmer will have missed the opportunity to sell at the higher price.
A trade on the cash market always involves transfer of the actual commodity. The farmer delivers their grain to the elevator after harvest or from storage and, then, receives the current price. Every grain marketing transaction, involving price protection, results in the sale of the physical commodity in the cash market. In other words, all spot, forward cash, futures hedges, options, basis, hedge-to-arrive contracts, etc., are not considered complete until the cash sale is made. This is a key point to remember when we discuss the mechanics of alternatives that employ more than one transaction in the cash, futures, or options markets.
The majority of all cash sales do not require any further action in terms of using additional marketing alternatives. Once the cash sale is complete, any further action taken regarding previously sold grain results in the "speculative" use of grain marketing alternatives, futures or options. We will be covering the ideas of “speculation”, hedging, and options in later sections.
It is important to remember that the cash sale often represents the best sale that can be made at a given point in time. Deciding when to use the cash sale as the primary pricing method for a given unit of grain, instead of other marketing alternatives, depends on many factors. Most of the factors are quite similar to those used in making all grain sales decisions.
Spot Marketing
A spot market price is for current purchase, payment, and delivery. In commodity spot contracts, payment is required immediately, as is delivery.
How Does the Spot Sale Work?
- The price for the spot sale is based on the nearby futures contract plus or minus the basis and is stated as a cash price ($/bu. or, in some cases, $/lb. or $/cwt.).
- The farmer agrees to sell a specific quantity of grain at the spot price on the day that the grain is delivered. Note that premiums may be available for special qualities or large volumes. These premiums are negotiated between the seller and the grain merchant.
- Payment for the grain sold may be taken immediately or deferred to a later date.
Advantages of the Spot Sale
- The exact price is known.
- Further downside price risk is eliminated for the quantity sold.
- Carrying charges are eliminated on the quantity sold.
- The sale may be for any quantity of grain.
Disadvantages of the Spot Sale
- Since the price is fixed on the quantity sold, flexibility in pricing is eliminated or greatly reduced.
- Because title and control change hands, USDA's Commodity Credit Corporation (CCC) loan and loan deficiency payment (LDP) are no longer available on the grain.
Best Time to Use the Spot Sale
- When the price represents an acceptable profit.
- When the basis is stronger than normal (in most regions, a positive basis is highly indicative that the spot price represents a good sales opportunity).
Forward Contracting
A forward contract is a way to minimize the risk that the price of a commodity might go down before a farmer sells. A forward contract is an agreement to deliver a specific amount of a specific commodity at a specific time in the future. Because no one really knows whether prices will go up or down, a forward contract "locks-in" a price that is higher than the current cash price.
A farmer who forward contracts with the local elevator is guaranteed a known price for a specific amount of his crop; however, the arrangement doesn't offer much flexibility. If prices move higher before the delivery date, the farmer is still obligated to deliver the contracted grain at the lower, previously agreed to price. Also, the farmer is obligated to deliver the contracted amount of the commodity, even if his yields are lower than expected.
The forward contract is the second most common way to sell grain. This is a cash contract that allows the farmer to sell a specific quantity of grain for a specified cash price for delivery at a later date. It allows the farmer to set a price for a crop that is to be grown, growing in the field, harvested, or being held for later delivery. For example, in July, a farmer contracts to deliver 5,000 bushels of corn to a grain elevator operator in November, and the contract price is $4.00 a bushel. The cash price of corn could go higher or lower between July and November. In November, even if the market price for corn is only $3.60 a bushel, the elevator operator is obligated to pay the farmer $4.00 a bushel. Likewise, if corn sells for $4.75 a bushel, the farmer still receives only $4.00 a bushel.
How does the Forward Contract Work?
- Forward contracts can be made with a local grain dealer (or end user) any time—before planting, during the growing season, at harvest, or after harvest.
- The contract can be written to allow the seller to take payment at the time the grain is delivered or to defer payment until a later date (see section on "Cash Sale with Deferred Payment").
- Forward contracts are made for a specific price, quantity, and delivery date.
Advantages of Forward Contracting
- The exact price is known.
- The exact quantity is known.
- The date of delivery is known.
- Downside price risk is eliminated for the quantity contracted.
- Any quantity can be contracted.
- Premiums can be negotiated for large-volume contracts or special qualities.
- Generally, farmers who irrigate can safely contract up to 100% of intended production.
Disadvantages of Forward Contracting
- The seller is obligated to fill the contract, even in the event of a production shortfall, depending on price and local conditions.
- Upside price potential is eliminated on the quantity contracted.
- You give up flexibility in choosing your delivery point.
- The seller must fill the contract even in the case of a production shortfall. As a result, farmers who produce crops on dry land generally limit the amount they contract to 50% of intended production; crop insurance or the use of options may boost this amount.
Best Time to Use the Forward Contract
- When the contract price represents an acceptable profit.
- When basis is stronger than normal.
- When you expect prices to fall.
The Cash Sale with Deferred Payment
The cash sale for deferred payment—whether a spot sale or forward contract—is generally used for tax management, to defer income into the next tax year.
Advantages of the Cash Sale with Deferred Payment
- The exact price is known.
- Payment is taken in the tax year the seller chooses.
Disadvantages of the Cash Sale with Deferred Payment
- Deferred income can present a tax problem in the event production and commodity prices are higher—or income is up for other reasons—in the following year.
- There is a credit risk. Should the buyer go out of business, the seller may have trouble collecting his or her payment. Some, but not all, states have indemnity funds to protect farmers in the case of elevator bankruptcy, but coverage often is not 100% and the protection does not apply to direct sales to end users such as livestock producers. The credit risk with this contract is less, however, than one with "deferred pricing"—in which the price is not determined at time of delivery.
What are Commodity Markets?
A commodity market is a place where you can buy, sell, or trade these raw products. But imagine having to transport all of the world's grain, gold, crude oil and other commodities to a single place in order to sell them. It would be unwieldy and costly to have a huge central location, to which all the sellers would deliver their commodities and from which all the buyers would haul them away. So, instead of trading the physical commodity, buyers and sellers in a commodity market trade contracts representing specific amounts of each commodity. For example, a producer could sell a contract to deliver 5,000 bushels of grain at a set price at a certain time. In exchange for payment, the contract would require the producer to deliver the grain to a specific location by a certain date. A processor could then use the market to purchase the contract for 5,000 bushels of grain at a set price and time.
It is in the commodities market that the prices of raw commodities, such as grain and livestock, are set. In the example of a grain farmer, it is these markets that set the price a farmer will receive when she sells her grain at the local elevator. By understanding how the markets work, processors attempt to buy their raw goods at the lowest price, and producers attempt to sell their commodity for the highest price.
There are many commodities markets around the world. Regardless of their names or locations, these trading centers all provide the same thing: a central location for buyers and sellers to negotiate prices and execute trades. The world's largest commodities market is the CME Group, which is the combination of the two largest commodity exchanges in the world—the CBOT (Chicago Board of Trade) and the CME (Chicago Mercantile Exchange).
There are a variety of participants in the commodities market. Traders are anyone who buys or sells a contract—also known as “taking a position" in the commodities market. Speculators are those traders who buy or sell in an attempt to profit from price movements. Hedgers are traders who "hedge their bets" for favorable prices in one market by buying or selling a commodity in another.
Market Prices & Decision Making
Commodity markets are big business, and for farmers the rise and fall of commodity prices can have a significant impact on the bottom line. Keeping up to date on prices and factors influencing the market helps producers make informed business decisions. Things that can impact the price of many commodities include the weather, government policies, international events, consumer preferences, shifting input costs, and general supply and demand for the commodity.
Because of all of the different factors that influence prices, buying or selling contracts in a commodity market requires detailed data-gathering, critical thinking, and an ability to tolerate and manage risk. There are many sources a producer or trader can use for this data, including industry publications, weather forecasts, news headlines, and government reports. Many traders rely on personal experience and an understanding of market history and trends to help make decisions.
Attributions
"Understanding Commodity Markets" by Tyler Schau, OER Commons is licensed under CC BY 4.0
Foreign Trade Agreements
Learning Objectives
7b Explain the importance of foreign trade in agribusiness.
7g Discuss NAFTA.
Trade Agreements and Organizations
Free trade is encouraged by a number of agreements and organizations set up to monitor trade policies. The two most important are the General Agreement on Tariffs and Trade (GATT) and the World Trade Organization (WTO).
Business Language
English is the international language of business. The natives of such European countries as France and Spain certainly take pride in their own languages and cultures; nevertheless, English is the business language of the European community. Whereas only a few educated Europeans have studied Italian or Norwegian, most have studied English. Similarly, on the South Asian subcontinent, where hundreds of local languages and dialects are spoken, English is the official language of business. Different cultures have different communication styles, but in most corners of the world, English-only speakers—such as most Americans—have no problem finding competent translators and interpreters because their language is considered the Lingua Franca—common language.
General Agreement on Tariffs and Trade
After the Great Depression and World War II, most countries focused on protecting home industries, so international trade was hindered by rigid trade restrictions. To rectify this situation, twenty-three nations joined together in 1947 and signed the General Agreement on Tariffs and Trade (GATT), which encouraged free trade by regulating and reducing tariffs, as well as by providing a forum for resolving trade disputes. The highly successful initiative achieved substantial reductions in tariffs and quotas, and in 1995 its members founded the World Trade Organization (WTO) to continue the work of GATT in overseeing global trade.
World Trade Organization
Based in Geneva, Switzerland, with nearly 150 members, the World Trade Organization (WTO) encourages global commerce and lower trade barriers, enforces international rules of trade, and provides a forum for resolving disputes. It is empowered, for instance, to determine whether a member nation’s trade policies have violated the organization’s rules, and it can direct “guilty” countries to remove disputed barriers (though it has no legal power to force any country to do anything it doesn’t want to do). If the guilty party refuses to comply, the WTO may authorize the plaintiff nation to erect trade barriers of its own, generally in the form of tariffs.
Affected members aren’t always happy with WTO actions. In 2002, for example, the Bush administration imposed a three-year tariff on imported steel. In ruling against this tariff, the WTO allowed the aggrieved nations to impose counter-tariffs on some politically sensitive American products, such as Florida oranges, Texas grapefruits, Wisconsin cheese, and computers. Reluctantly, the administration lifted its tariff on steel.
Trading Blocs
The complete absence of barriers is an ideal state of affairs that we haven’t yet attained. In the meantime, economists and policymakers tend to focus on a more practical question: Can we achieve the goal of free trade on the regional level? To an extent, the answer is yes. In certain parts of the world, groups of countries have joined together to allow goods and services to flow without restrictions across their mutual borders. Such groups are called trading blocs. Let’s examine two of the most powerful trading blocks—NAFTA, now known as USMCA, and the European Union.
North American Free Trade Association
The North American Free Trade Association (NAFTA) is an agreement among the governments of the United States, Canada, and Mexico to open their borders to unrestricted trade. The effect of this agreement is that three very different economies are combined into one economic zone with almost no trade barriers. From the northern tip of Canada to the southern tip of Mexico, each country benefits from the comparative advantages of its partners: each nation is free to produce what it does best and to trade its goods and services without restrictions.
When the agreement was ratified in 1994, it had no shortage of skeptics. Many people feared, for example, that without tariffs on Mexican goods, more U.S. manufacturing jobs would be lost to Mexico, where labor is cheaper. More than two decades later, most such fears have not been realized, and, by and large, NAFTA was a success. Since it went into effect, the value of trade between the United States and Mexico has grown substantially, and Canada and Mexico are now the United States’ top trading partners.
As of July 1, 2020, NAFTA was renamed the U.S.-Mexico-Canada Agreement (USMCA). Making this change nearly quadrupled exports (by value) to Canada and Mexico. USMCA expands more on topics that were already included in NAFTA and also adds new chapters. Some of the new or changed information for USMCA include policies and regulations surrounding digital trade, anticorruption, intellectual property, rules of origin, and more. The agreement is meant to create more balanced trade supporting high paying jobs for Americans while still growing and supporting the economies of North America. It benefits farmers, ranchers, and other producers by strengthening agricultural trade within the continent. This agreement is beneficial to the economies and workers within North America.
The European Union
The forty-plus countries of Europe have long shown an interest in integrating their economies. The first organized effort to integrate a segment of Europe’s economic entities began in the late 1950s, when six countries joined together to form the European Economic Community (EEC). Over the next four decades, membership grew, and in the late 1990s, the EEC became the European Union. Today, the European Union (EU) is a group of twenty-seven countries that have eliminated trade barriers among themselves (see Figure 3.2.5a).
At first glance, the EU looks similar to USMCA. Both, for instance, allow unrestricted trade among member nations. But the provisions of the EU go beyond those of USMCA in several important ways. Most importantly, the EU is more than a trading organization: it also enhances political and social cooperation and binds its members into a single entity with authority to require them to follow common rules and regulations. It is much like a federation of states with a weak central government, with the effect not only of eliminating internal barriers but also of enforcing common tariffs on trade from outside the EU. In addition, while USMCA allows goods and services, as well as capital, to pass between borders, the EU also allows people to come and go freely: if you possess an EU passport, you can work in any EU nation.
Attributions
"An Introduction to Business v.2.0" by Anonymous , 2012 Book Archive is licensed under CC BY-NC-SA 3.0
Foreign Exchange Rates and Firms
Learning Objectives
7f Discuss exchange rates, trades, and tariffs and their effects on commodities.
Dollarize
Most countries have different currencies, but small economies commonly use an economically larger neighbor's currency. For example, Ecuador, El Salvador, and Panama have decided to dollarize—that is, to use the U.S. dollar as their currency. Nations can also share a common currency. A large-scale example of a common currency is the decision by many European nations—including some very large economies such as France, Germany, and Italy—to replace their former currencies with the Euro at the start of 1999.
Apart from exceptions already mentioned, most of the international economy takes place in a situation of multiple national currencies. People and firms often need to convert from one currency to another when selling, buying, hiring, borrowing, traveling, or investing across national borders. We call the market in which people or firms use one currency to purchase another currency the foreign exchange market. The exchange rate is a price—the price of one currency expressed in terms of units of another currency. The key framework for analyzing prices is the operation of supply and demand in markets.
The Extraordinary Size of the Foreign Exchange Markets
If you travel to a foreign country that uses a different currency, you will undoubtedly need to make a trip to a bank or foreign currency office to exchange your currency for that country’s currency. Even though this is a simple transaction, it is part of a very large market. The quantities traded in foreign exchange markets are breathtaking. A 2019 Bank of International Settlements survey found that $5.3 trillion per day was traded on foreign exchange markets, which makes the foreign exchange market the largest market in the world economy. In contrast, 2019 U.S. real GDP was $21.4 trillion per year. There is an average of $6.6 trillion worth of exchanges per day.
Your transaction is simple enough. Suppose you carry a $100 bill. You bring it into the foreign currency office and look up, and you see a bunch of different numbers on a digital board.
If you are traveling to Turkey, whose national currency is the Turkish Lira, one line of the board might read: “U.S. DOLLARS: BUY 5.50; SELL 5.80.” This means that the office will give you 5.50 Turkish Lira in exchange for 1 U.S. dollar. If you have $100, the office will give you 550 Turkish Lira.
If you want to sell Turkish Lira for U.S. dollars, the office will surely buy them from you. They will not buy it at the same exchange rate since the office will make a profit on the exchange. If you bring 550 Turkish Lira and ask for U.S. dollars, it will not give you 100 dollars, but instead about $95.
Demanders and Suppliers of Currency in Foreign Exchange Markets
In foreign exchange markets, demand and supply become closely interrelated, because a person or firm who demands one currency must at the same time supply another currency—and vice versa. To get a sense of this, it is useful to consider four groups of people or firms who participate in the market:
- firms that are involved in international trade of goods and services;
- tourists visiting other countries;
- international investors buying ownership (or part-ownership) of a foreign firm;
- and international investors making financial investments that do not involve ownership.
Example: Financial Investor
Business people often link portfolio investment to expectations about how exchange rates will shift. Look at a U.S. financial investor who is considering purchasing U.K. issued bonds. For simplicity, ignore any bond interest and focus on exchange rates. Say that a British pound is currently worth $1.50 in U.S. currency. However, the investor believes that in a month, the British pound will be worth $1.60 in U.S. currency. Thus, as Figure 3.2.6a part (a) shows, this investor would change $24,000 for 16,000 British pounds. In a month, if the pound is worth $1.60, then the portfolio investor can trade back to U.S. dollars at the new exchange rate, and you will have $25,600—a nice profit.
Now consider Figure 3.2.6a part (b). An investor expects that the pound, now worth $1.50 in U.S. currency, will decline to $1.40. That investor could start off with £20,000 in British currency (borrowing the money if necessary); convert it to $30,000 in U.S. currency; wait a month; and then convert back to approximately £21,429 in British currency—again making a nice profit. Of course, this kind of investment comes without guarantees, as the market can move in ways not predicted.
Example: Foreign Contract Hedging
Many portfolio investment decisions are not as simple as betting that the currency's value will change in one direction or the other. Instead, they involve firms trying to protect themselves from movements in exchange rates. Imagine you are running a U.S. firm that is exporting to France. You have signed a contract to deliver certain products and will receive 1 million euros a year from now. However, you do not know how much this contract will be worth in U.S. dollars, because the dollar/euro exchange rate can fluctuate in the next year. Let’s say you want to know for sure what the contract will be worth, and not take a risk that the euro will be worth less in U.S. dollars than it currently is. You can hedge, which means using a financial transaction to protect yourself against a risk from one of your investments (in this case, currency risk from the contract).
With hedging, you can sign a financial contract and pay a fee that guarantees you a certain exchange rate one year from now—regardless of what the market exchange rate is at that time. Now, it is possible that the euro will be worth more in dollars a year from now, so your hedging contract will be unnecessary, and you will have paid a fee for nothing. However, if the value of the euro in dollars declines, then you are protected by the hedge.
Attributions
"Principles of Economics 3e" by Steven A. Greenlaw, David Shapiro, Daniel MacDonald, OpenStax is licensed under CC BY 4.0
Access for free at: https://openstax.org/books/principles-economics-3e/pages/29-1-how-the-foreign-exchange-market-works
|
oercommons
|
2025-03-18T00:39:14.514738
|
Anna McCollum
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/100031/overview",
"title": "Statewide Dual Credit Introduction to Agriculture Business Collection, Agriculture, Trade, and the Global Economy, Global Economic Systems and Agriculture",
"author": "Textbook"
}
|
https://oercommons.org/courseware/lesson/100027/overview
|
2.2.11 The Five Cs of Credit
2.2.12 Credit and Risk
2.2.1 Financial and Managerial Accounting
2.2.2 The Importance of Accounting Information
2.2.3 Fixed and Variable Costs
2.2.4 Budgets and Goals
2.2.5 Analyzing Financial Statements
2.2.6 Budgets and the Use in Agribusiness
2.2.7 Cash Flow Budgeting in Agribusiness
2.2.8 Net Worth Statements in Agribusiness
2.2.9 Important Financial Statements for Agribusiness
Agribusiness Budgeting and Accounting
Overview
Financial and Managerial Accounting
Learning Objectives
4a Differentiate financial and managerial accounting.
What is Accounting?
Accounting is the process of organizing, analyzing, and communicating financial information that is used for decision-making. Financial information is typically prepared by accountants—those trained in the specific techniques and practices of the profession. This section explores many of the topics and techniques related to the accounting profession because a solid understanding of accounting can serve as a useful resource for any career. In fact, it is hard to think of a profession where a foundation in the principles of accounting would not be beneficial. While students may directly apply the knowledge gained in this course to continue their education in accounting, others may pursue different career paths in business. Therefore, one of the goals of this section is to provide a solid understanding of how financial information is prepared and used in the workplace, specifically in agribusiness.
A traditional adage states that “accounting is the language of business.” While that is true, you can also say that “accounting is the language of life.” At some point, most people will make a decision that relies on accounting information. For example, you may have to decide whether it is better to lease or buy a vehicle. Likewise, a college graduate may have to decide whether it is better to take a higher-paying job in a bigger city (where the cost of living is also higher) or a job in a smaller community where both the pay and cost of living may be lower. In a professional setting, a theater manager may want to know if the most recent play was profitable. Similarly, the owner of the local plumbing business may want to know whether it is worthwhile to pay an employee to be “on call” for emergencies during off-hours and weekends. Whether personal or professional, accounting information plays a vital role in all of these decisions.
You may have noticed that the decisions in these scenarios would be based on factors that include both financial and nonfinancial information. For instance, when deciding whether to lease or buy a vehicle, you would consider not only the monthly payments but also such factors as vehicle maintenance and reliability. The college graduate considering two job offers might weigh factors such as working hours, ease of commuting, and closeness to existing friends and family. The theater manager would analyze the proceeds from ticket sales and sponsorships, as well as the expenses for production of the play and operating the concessions. In addition, the theater manager should consider how the financial performance of the play might have been influenced by the marketing of the play, the weather during the performances, and other factors such as competing events during the time of the play. the owner of the local plumbing business would consider nonfinancial factors in the decision, in addition to the added cost of having an employee “on call” during evenings and weekends; for instance, if there are no other plumbing businesses that offer services during evenings and weekends, offering emergency service might give the business a strategic advantage that could increase overall sales by attracting new customers. All of these factors, both financial and nonfinancial, are relevant to the possible financial performance in each scenario.
Financial Accounting
Financial accounting measures the financial performance of an organization using standard conventions to prepare and distribute financial reports. Financial accounting is used to generate information for stakeholders outside of an organization, such as owners, stockholders, lenders, and governmental entities such as the Securities and Exchange Commission (SEC) and the Internal Revenue Service (IRS).
Managerial Accounting
Managerial accounting employs both financial and nonfinancial information as a basis for making decisions within an organization for the purpose of equipping decision makers to set and evaluate business goals. Managerial accounting allows decision makers to determine what information is needed, and how to analyze and communicate that information, so they can make a particular business decision. This information tends to be used internally, for such purposes as budgeting, pricing, and determining production costs. Since the information is generally used internally, you do not see the same need for financial oversight in an organization’s managerial data.
Accounting Oversight
There are governmental and organizational entities that oversee the accounting processes and systems that are used in financial accounting. These entities include organizations such as the Securities and Exchange Commission (SEC), the Financial Accounting Standards Board (FASB), the American Institute of Certified Public Accountants (AICPA), and the Public Company Accounting Oversight Board (PCAOB). The PCAOB was created after several major cases of corporate fraud, leading to the Sarbanes-Oxley Act of 2002, known as SOX. If you choose to pursue more advanced accounting courses, especially auditing courses, you will address the SOX in much greater detail. For now, it is not necessary to go into greater detail about the mechanics of these organizations or other accounting and financial legislation. You just need to have a basic understanding that they function to provide a degree of protection for those outside of the organization who rely on the financial information produced by a business.
Both Forms are Useful
Whether or not you aspire to become an accountant, understanding financial and managerial accounting is valuable and necessary for practically any career you will pursue. Management of a car manufacturer, for example, would use both financial and managerial accounting information to help improve the business. Financial accounting information is valuable as it measures whether or not the company is financially successful. Knowing this provides management with an opportunity to repeat activities that have proven effective and to make adjustments in areas in which the company has underperformed. Managerial accounting information is likewise valuable. Managers of the car manufacturer may want to know, for example, how much scrap is generated from a particular area in the manufacturing process. While identifying and improving the manufacturing process (i.e., reducing scrap) helps the company financially, it may also help improve other areas of the production process that are indirectly related, such as poor quality and shipping delays.
Attributions
Title Image: "Analyzing Financial Data" by Dave Dugdale, Wikimedia Commons is licensed under CC BY-SA 2.0
Description: Closeup of calculator, hand, pen and financial statement.
"Principles of Accounting, Volume 1: Financial Accounting" by Mitchell Franklin, Patty Graybeal, Dixon Cooper, OpenStax is licensed under CC BY-NC-SA 4.0
Access for free at https://openstax.org/books/principles-financial-accounting/pages/1-1-explain-the-importance-of-accounting-and-distinguish-between-financial-and-managerial-accounting
The Importance of Accounting Information
Learning Objectives
4b Explain basic accounting considerations.
Accounting and Decision Making
The ultimate goal of accounting is to provide information that is useful for decision-making. Users of accounting information are generally divided into two categories: internal and external.
Internal users are those within an organization who use financial information to make day-to-day decisions. Internal users include managers and other employees who use financial information to confirm past results and help make adjustments for future activities.
External users are those outside of the organization who use the financial information to make decisions or to evaluate an entity’s performance. For example, investors, financial analysts, loan officers, governmental auditors, such as IRS agents, and an assortment of other stakeholders are classified as external users, while still having an interest in an organization’s financial information.
Characteristics, Users, and Sources of Financial Accounting Information
Financial accounting is one of the broad categories in the study of accounting. While some industries and types of organizations have variations in how the financial information is prepared and communicated, accountants generally use the same methodologies—called accounting standards—to prepare the financial information.
Financial information is primarily communicated through financial statements, which include the Income Statement, Statement of Owner’s Equity, Balance Sheet, and Statement of Cash Flows and Disclosures. These financial statements ensure the information is consistent from period to period and generally comparable between organizations. The conventions also ensure that the information provided is both reliable and relevant to the user.
Organizations measure financial performance in monetary terms. In the United States, the dollar is used as the standard measurement basis. Measuring financial performance in monetary terms allows managers to compare the organization’s performance to previous periods, to expectations, and to other organizations or industry standards.
Virtually every activity and event that occurs in a business has an associated cost or value and is known as a transaction. Part of an accountant’s responsibility is to quantify these activities and events. In this course you will learn about the many types of transactions that occur within a business. You will also examine the effects of these transactions, including their impact on the financial position of the entity.
Accountants use formal accounting standards in financial accounting. These accounting standards are referred to as generally accepted accounting principles (GAAP) and are the common set of rules, standards, and procedures that publicly traded companies must follow when composing their financial statements. The Financial Accounting Standards Board (FASB)—an independent, nonprofit organization that sets financial accounting and reporting standards for both public and private sector businesses in the United States—uses the GAAP guidelines as its foundation for its system of accepted accounting methods and practices, reports, and other documents.
Accountants often use computerized accounting systems to record and summarize the financial reports, which offer many benefits. The primary benefit of a computerized accounting system is the efficiency by which transactions can be recorded and summarized, and financial reports prepared. In addition, computerized accounting systems store data, which allows organizations to easily extract historical financial information.
Common computerized accounting systems include QuickBooks, which is designed for small organizations, and SAP, which is designed for large and/or multinational organizations. QuickBooks is popular with smaller, less complex entities. It is less expensive than more sophisticated software packages, such as Oracle or SAP, and the QuickBooks skills that accountants developed at previous employers tend to be applicable to the needs of new employers, which can reduce both training time and costs spent on acclimating new employees to an employer’s software system. Also, being familiar with a common software package such as QuickBooks helps provide employment mobility when workers wish to reenter the job market.
While QuickBooks has many advantages, once a company’s operations reach a certain level of complexity, it will need a more advanced software package or platform, such as Oracle or SAP, which is then customized to meet the unique informational needs of the entity.
Financial accounting information is mostly historical in nature, although companies and other entities also incorporate estimates into their accounting processes. For example, you will learn how to use estimates to determine bad debt expenses or depreciation expenses for assets that will be used over a multiyear period. That is, accountants prepare financial reports that summarize what has already occurred in an organization. This information provides what is called feedback value. The benefit of reporting what has already occurred is the reliability of the information. Accountants can, with a fair amount of confidence, accurately report the financial performance of the organization related to past activities. The feedback value offered by the accounting information is particularly useful to internal users. That is, reviewing how the organization performed in the past can help managers and other employees make better decisions about and adjustments to future activities.
Financial information has limitations as a predictive tool. Business involves a large amount of uncertainty, and accountants cannot predict how the organization will perform in the future. However, by observing historical financial information, users of the information can detect patterns or trends that may be useful for estimating the company’s future financial performance.
Collecting and analyzing a series of historical financial data is useful to both internal and external users. For example, internal users can use financial information as a predictive tool to assess whether the long-term financial performance of the organization aligns with its long-term strategic goals. External users also use the historical pattern of an organization’s financial performance as a predictive tool. For example, when deciding whether to loan money to an organization, a bank may require a certain number of years of financial statements and other financial information from the organization. The bank will assess the historical performance in order to make an informed decision about the organization’s ability to repay the loan and interest (the cost of borrowing money). Similarly, a potential investor may look at a business’s past financial performance in order to assess whether or not to invest money in the company. In this scenario, the investor wants to know if the organization will provide a sufficient and consistent return on the investment. In these scenarios, the financial information provides value to the process of allocating scarce resources (money). If potential lenders and investors determine the organization is a worthwhile investment, money will be provided; if all goes well, those funds will be used by the organization to generate additional value at a rate greater than the alternate uses of the money.
Characteristics, Users, and Sources of Managerial Accounting Information
As you’ve learned, managerial accounting information is different from financial accounting information in several respects. The business environment is constantly changing, and managers and decision makers within organizations need a variety of information in order to view or assess issues from multiple perspectives. Since managerial accounting often includes strategic or competitive decisions, it is often closely protected. Most managerial accounting activities are conducted for internal uses and applications; therefore, managerial accounting is not prepared using a comprehensive, prescribed set of conventions similar to those required by financial accounting. This is because managerial accountants provide managerial accounting information that is intended to serve the needs of internal, rather than external, users. In fact, managerial accounting information is rarely shared with those outside of the organization.
Accountants must be adaptable and flexible in their ability to generate the necessary information for management decision-making. For example, information derived from a computerized accounting system is often the starting point for obtaining managerial accounting information. But accountants must also be able to extract information from other sources (internal and external) and analyze the data using mathematical, formula-driven software (such as Microsoft Excel). Examples of other decisions that require management accounting information include whether an organization should repair or replace equipment, make products internally or purchase the items from outside vendors, and hire additional workers or use automation.
Management accounting information as a term encompasses many activities within an organization. Preparing a budget, for example, allows an organization to estimate the financial performance for the upcoming year or years and plan for adjustments to scale operations according to the projections. Accountants often lead the budgeting process by gathering information from internal (estimates from the sales and engineering departments, for example) and external (trade groups and economic forecasts, for example) sources. All of this data is then compiled and presented to decision makers within the organization.
As you have learned, management accounting information uses both financial and nonfinancial information. This is important because there are situations in which a purely financial analysis might lead to one decision, while considering nonfinancial information might lead to a different decision. For example, suppose a financial analysis indicates that a particular product is unprofitable and should no longer be offered by a company. If the company fails to consider that customers also purchase a complementary good, the company may be making the wrong decision. For example, assume that you have a company that produces and sells both computer printers and the replacement ink cartridges. If the company decided to eliminate the printers, then it would also lose the cartridge sales. The elimination of one component, such as printers, has led customers, in the past, to switch producers for their computers and other peripheral hardware. In the end, an organization needs to consider both the financial and nonfinancial aspects of a decision, and sometimes the effects are not intuitively obvious at the time of the decision.
Overview
Figure 2.2.2a offers an overview of some of the differences between financial and managerial accounting.
Attributions
"Principles of Accounting, Volume 1: Financial Accounting" by Mitchell Franklin, Patty Graybeal, Dixon Cooper, OpenStax is licensed under CC BY-NC-SA 4.0
Access for free at https://openstax.org/books/principles-financial-accounting/pages/1-2-identify-users-of-accounting-information-and-how-they-apply-information
Fixed and Variable Costs
Learning Objectives
4i Describe the fixed and variable expenses.
Fixed Costs
A fixed cost is an unavoidable operating expense that does not change in total over the short term, even if a business experiences variation in its level of activity. Table 2.2.3a illustrates the types of fixed costs for merchandising, service, and manufacturing organizations.
| Type of Business | Fixed Cost |
|---|---|
| Merchandising | Rent, insurance, managers’ salaries |
| Manufacturing | Property taxes, insurance, equipment leases |
| Service | Rent, straight-line depreciation, administrative salaries, and insurance |
We have established that fixed costs do not change in total as the level of activity changes, but what about fixed costs on a per-unit basis? Let’s examine Tony’s screen-printing company to illustrate how costs can remain fixed in total but change on a per-unit basis.
Tony operates a screen-printing company, specializing in custom T-shirts. One of his fixed costs is his monthly rent of $1,000. Regardless of whether he produces and sells any T-shirts, he is obligated under his lease to pay $1,000 per month. However, he can consider this fixed cost on a per-unit basis, as shown in Figure 2.2.3a.
Tony’s information illustrates that, despite the unchanging fixed cost of rent, as the level of activity increases, the per-unit fixed cost falls. In other words, fixed costs remain fixed in total but can increase or decrease on a per-unit basis.
Two specialized types of fixed costs are committed fixed costs and discretionary fixed costs. These classifications are generally used for long-range planning purposes and are covered in upper-level managerial accounting courses, so they are only briefly described here.
Committed fixed costs are fixed costs that typically cannot be eliminated if the company is going to continue to function. An example would be the lease of factory equipment for a production company.
Discretionary fixed costs generally are fixed costs that can be incurred during some periods and postponed during other periods but which cannot normally be eliminated permanently. Examples could include advertising campaigns and employee training. Both of these costs could potentially be postponed temporarily, but the company would probably incur negative effects if the costs were permanently eliminated. These classifications are generally used for long-range planning purposes.
In addition to understanding fixed costs, it is critical to understand variable costs, the second fundamental cost classification. A variable cost is one that varies in direct proportion to the level of activity within the business. Typical costs that are classified as variable costs are the cost of raw materials used to produce a product, labor applied directly to the production of the product, and overhead expenses that change based upon activity. For each variable cost, there is some activity that drives the variable cost up or down. A cost driver is defined as any activity that causes the organization to incur a variable cost. Examples of cost drivers are direct labor hours, machine hours, units produced, and units sold. Table 2.2.3b provides examples of variable costs and their associated cost drivers.
| Variable Cost | Cost Driver | |
|---|---|---|
| Merchandising | Total monthly hourly wages for sales staff | Hours business is open during month |
| Manufacturing | Direct materials used to produce one unit of product | Number of units produced |
| Service | Cost of laundering linens and towels | Number of hotel rooms occupied |
Variable Costs
Unlike fixed costs that remain fixed in total but change on a per-unit basis, variable costs remain the same per unit, but change in total relative to the level of activity in the business. Revisiting Tony’s T-Shirts, Figure 2.2.3b shows how the variable cost of ink behaves as the level of activity changes.
As Figure 2.2.3b shows, the variable cost per unit (per T-shirt) does not change as the number of T-shirts produced increases or decreases. However, the variable costs change in total as the number of units produced increases or decreases. In short, total variable costs rise and fall as the level of activity (the cost driver) rises and falls.
Distinguishing between fixed and variable costs is critical because the total cost is the sum of all fixed costs (the total fixed costs) and all variable costs (the total variable costs). In every business situation, managers should determine their total costs both per unit of activity and in total by combining their fixed and variable costs together. The graphic in Figure 2.2.3c illustrates the concept of total costs.
Remember that the reason that organizations take the time and effort to classify costs as either fixed or variable is to be able to control costs. When they classify costs properly, managers can use cost data to make decisions and plan for the future of the business.
Boeing Example
If you’ve ever flown on an airplane, there’s a good chance you know Boeing. The Boeing Company generates around $90 billion each year from selling thousands of airplanes to commercial and military customers around the world. It employs around 200,000 people, and it’s indirectly responsible for more than a million jobs through its suppliers, contractors, regulators, and others. Its main assembly line in Everett, WA is housed in the largest building in the world, a colossal facility that covers nearly a half-trillion cubic feet. Boeing is, simply put, a massive enterprise.
And yet, Boeing’s managers know the exact cost of everything the company uses to produce its airplanes: every propeller, flap, seat belt, welder, computer programmer, and so forth. Moreover, they know how those costs would change if they produced more airplanes or fewer. They also know the price at which they sold each plane and the profit made from each sale. Boeing’s executives expect their managers to know this information, in real time, so the company can make useful decisions. See Table 2.2.3c for a picture of how cost information affects business decisions.
| Decision | Cost Information |
|---|---|
| Discontinue a product line | Variable costs, overhead directly tied to product, potential reduction in fixed costs |
| Add second production shift | Labor costs, cost of fringe benefits, potential overhead increases (utilities, security personnel) |
| Open additional retail outlets | Fixed costs, variable operating costs, potential increases in administrative expenses at corporate headquarters |
Attributions
"Principles of Accounting, Volume 2: Managerial Accounting" by Mitchell Franklin, Patty Graybeal, Dixon Cooper, OpenStax is licensed under CC BY-NC-SA 4.0
Access for free at https://openstax.org/books/principles-managerial-accounting/pages/2-2-identify-and-apply-basic-cost-behavior-patterns
Budgets and Goals
Learning Objectives
4g Discuss the importance of budgets and written goals.
The Budget—For Planning and Control
Time and money are scarce resources to all individuals and organizations; the efficient and effective use of these resources requires planning. Planning alone, however, is insufficient. Control is also necessary to ensure that plans are actually carried out.
A budget is a plan showing the company's objectives and how management intends to acquire and use resources to attain those objectives. And it is a tool that managers use to control the use of scarce resources.
The period covered by a budget varies according to the nature of the specific activity involved. Cash budgets may cover a week or a month; sales and production budgets may cover a month, a quarter, or a year; and the general operating budget may cover a quarter or a year.
Companies, nonprofit organizations, and governmental units use many different types of budgets. Responsibility budgets are designed to judge the performance of an individual segment or manager. Capital budgets evaluate long-term capital projects, such as the addition of equipment or the relocation of a plant.
This section focuses on the master budget, which consists of a planned operating budget and a financial budget. The planned operating budget helps to plan future earnings and results in a projected income statement. The financial budget helps management plan the financing of assets and results in a projected balance sheet.
The budgeting process involves planning for future profitability because earning a reasonable return on resources used is a primary company objective. A company must devise some method to deal with the uncertainty of the future. A company that does no planning whatsoever chooses to deal with the future by default and can react to events only as they occur. Most businesses, however, devise a blueprint for the actions they will take given the foreseeable events that may occur, and that blueprint is generally the budget.
A budget
- shows management's operating plans for the coming periods;
- formalizes management's plans in quantitative terms;
- forces all levels of management to think ahead, anticipate results, and take action to remedy possible poor results;
- and may motivate individuals to strive to achieve stated goals.
Companies can use budget-to-actual comparisons to evaluate individual performance. For instance, the standard variable cost of producing a personal computer at IBM is a budget figure. This figure can be compared with the actual cost of producing personal computers to help evaluate the performance of the personal computer production managers and employees.
Many other desired benefits result from the preparation and use of budgets, including:
- Businesses can better coordinate their activities.
- Managers can become aware of other managers' plans.
- Employees can become more cost conscious and try to conserve resources.
- The company can review its organization plan and changes it when necessary.
- Managers can foster a vision that otherwise might not be developed. The planning process that results in a formal budget provides an opportunity for various levels of management to think through and commit future plans to writing. In addition, a properly prepared budget allows management to follow the management-by-exception principle by devoting attention to results that deviate significantly from planned levels.
Failing to budget because of the uncertainty of the future is a poor excuse for not budgeting. In fact, the less stable the conditions, the more necessary and desirable budgeting is, although the process becomes more difficult. Obviously, stable operating conditions permit greater reliance on past experience as a basis for budgeting. Remember, however, that budgets involve more than a company's past results. Budgets also consider a company's future plans and express expected activities. As a result, budgeted performance is more useful than past performance as a basis for judging actual results.
Management and the Budget
A budget should describe management's assumptions relating to
- the state of the economy over the planning horizon;
- plans for adding, deleting, or changing product lines;
- the nature of the industry's competition;
- and the effects of existing or possible government regulations.
If these assumptions change during the budget period, management should analyze the effects of the changes and include this in an evaluation of performance based on actual results.
Management should frequently compare accounting data with budgeted projections during the budget period and investigate any differences. Budgeting, however, is not a substitute for good management. Instead, the budget is an important tool of managerial control. Managers make decisions in budget preparation that serve as a plan of action.
Accountant Involvement
The accounting system and the budget are closely related. Budgets are quantitative plans for the future. However, they are based mainly on past experience adjusted for future expectations. Thus, accounting data related to the past plays an important part in budget preparation. The details of the budget must agree with the company's ledger accounts. In turn, the accounts must be designed to provide the appropriate information for preparing the budget, financial statements, and interim financial reports to facilitate operational control.
Within a participatory budgeting process, accountants should be compilers or coordinators of the budget, not preparers. They should be on hand during the preparation process to present and explain significant financial data. Accountants must identify the relevant cost data that enables management's objectives to be quantified in dollars. Accountants are responsible for designing meaningful budget reports. Also, accountants must continually strive to make the accounting system more responsive to managerial needs. That responsiveness, in turn, increases confidence in the accounting system.
Overcoming Budget Stigma
The term budget has negative connotations for many employees. Often in the past, management has imposed a budget from the top without considering the opinions and feelings of the personnel affected. Such a dictatorial process may result in resistance to the budget. A number of reasons may underlie such resistance, including lack of understanding of the process, concern for status, and an expectation of increased pressure to perform. Employees may believe that the performance evaluation method is unfair or that the goals are unrealistic and unattainable. They may lack confidence in the way accounting figures are generated or may prefer a less formal communication and evaluation system. Often these fears are completely unfounded, but if employees believe these problems exist, it is difficult to accomplish the objectives of budgeting.
Problems encountered with such imposed budgets have led accountants and management to adopt participatory budgeting. Participatory budgeting means that all levels of management responsible for actual performance actively participate in setting operating goals for the coming period. Managers and other employees are more likely to understand, accept, and pursue goals when they are involved in formulating them.
Conditions that Affect the Budget
Budgeting involves the coordination of financial and nonfinancial planning to satisfy organizational goals and objectives. No foolproof method exists for preparing an effective budget. However, budget makers should carefully consider several conditions during the proposal stage.
Top Management Support
All management levels must be aware of the budget's importance to the company and must know that the budget has top management's support. Top management, then, must clearly state long-range goals and broad objectives. These goals and objectives must be communicated throughout the organization. Long-range goals include the expected quality of products or services, growth rates in sales and earnings, and percentage-of-market targets. Overemphasis on the mechanics of the budgeting process should be avoided.
Participation in Goal Setting
Management uses budgets to show how it intends to acquire and use resources to achieve the company's long-range goals. Employees are more likely to strive toward organizational goals if they participate in setting them and in preparing budgets. Often, employees have significant information that could help in preparing a meaningful budget. Also, employees may be motivated to perform their own functions within budget constraints if they are committed to achieving organizational goals.
Although many companies have used participatory budgeting successfully, it does not always work. Studies have shown that in many organizations, participation in the budget formulation failed to make employees more motivated to achieve budgeted goals. Whether or not participation works depends on management's leadership style, the attitudes of employees, and the organization's size and structure. Participation is not the answer to all the problems of budget preparation. However, it is one way to achieve better results in organizations that are receptive to the philosophy of participation.
Communicating Results
Managers should effectively and promptly communicate results so employees can make any necessary adjustments to their performance. Effective communication implies
- timeliness,
- reasonable accuracy,
- and improved understanding.
Flexibility
If significant basic assumptions underlying the budget change during the year, the planned operating budget should be restated. For control purposes, after the actual level of operations is known, the actual revenues and expenses can be compared to expected performance at that level of operations.
Follow-up
Budget follow-up and data feedback are part of the control aspect of budgetary control. Since budgets are dealing with projections and estimates for future operating results and financial positions, managers must continuously check their budgets and correct them if necessary. Often management uses performance reports as a follow-up tool to compare actual results with budgeted results.
Attributions
"Accounting Principles: A Business Perspective" by Roger H. Hermanson, PhD, CPA; James Don Edwards PhD, D.H.C, CPA; Michael W. Maher PhD, CPA, Global Text Project, Open Education Network is licensed under CC BY 3.0
Analyzing Financial Statements
Learning Objectives
4f Analyze financial statements.
4h Discuss the importance of ROIC.
Financial Statement Analysis
Financial statement analysis reviews financial information found on financial statements to make informed decisions about the business. The income statement, statement of retained earnings, balance sheet, and statement of cash flows, among other financial information, can be analyzed. The information obtained from this analysis can benefit decision-making for internal and external stakeholders and can give a company valuable information on overall performance and specific areas for improvement. The analysis can help them with budgeting, deciding where to cut costs, how to increase revenues, and future capital investments opportunities.
When considering the outcomes from analysis, it is important for a company to understand that data produced needs to be compared to others within industry and close competitors. The company should also consider their past experience and how it corresponds to current and future performance expectations. Three common analysis tools are used for decision-making; horizontal analysis, vertical analysis, and financial ratios.
For our discussion of financial statement analysis, we will use Banyan Goods. Banyan Goods is a merchandising company that sells a variety of products. The image below shows the comparative income statements and balance sheets for the past two years.
Keep in mind that the comparative income statements and balance sheets for Banyan Goods are simplified for our calculations and do not fully represent all the accounts a company could maintain. Let’s begin our analysis discussion by looking at horizontal analysis.
Horizontal Analysis
Horizontal analysis (also known as trend analysis) looks at trends over time on various financial statement line items. A company will look at one period (usually a year) and compare it to another period. For example, a company may compare sales from their current year to sales from the prior year. The trending of items on these financial statements can give a company valuable information on overall performance and specific areas for improvement. It is most valuable to do horizontal analysis for information over multiple periods to see how change is occurring for each line item. If multiple periods are not used, it can be difficult to identify a trend. The year being used for comparison purposes is called the base year (usually the prior period). The year of comparison for horizontal analysis is analyzed for dollar and percent changes against the base year.
The dollar change is found by taking the dollar amount in the base year and subtracting that from the year of analysis.
Using Banyan Goods as our example, if Banyan wanted to compare net sales in the current year (year of analysis) of $120,000 to the prior year (base year) of $100,000, the dollar change would be as follows:
The percentage change is found by taking the dollar change, dividing by the base year amount, and then multiplying by 100.
Let’s compute the percentage change for Banyan Goods’ net sales.
This means Banyan Goods saw an increase of $20,000 in net sales in the current year as compared to the prior year, which was a 20% increase. The same dollar change and percentage change calculations would be used for the income statement line items as well as the balance sheet line items. The image below shows the complete horizontal analysis of the income statement and balance sheet for Banyan Goods.
Depending on their expectations, Banyan Goods could make decisions to alter operations to produce expected outcomes. For example, Banyan saw a 50% accounts receivable increase from the prior year to the current year. If they were only expecting a 20% increase, they may need to explore this line item further to determine what caused this difference and how to correct it going forward. It could possibly be that they are extending credit more readily than anticipated or not collecting as rapidly on outstanding accounts receivable. The company will need to further examine this difference before deciding on a course of action. Another method of analysis Banyan might consider before making a decision is vertical analysis.
Vertical Analysis
Vertical analysis shows a comparison of a line item within a statement to another line item within that same statement. For example, a company may compare cash to total assets in the current year. This allows a company to see what percentage of cash (the comparison line item) makes up total assets (the other line item) during the period. This is different from horizontal analysis, which compares across years. Vertical analysis compares line items within a statement in the current year. This can help a business to know how much of one item is contributing to overall operations. For example, a company may want to know how much inventory contributes to total assets. They can then use this information to make business decisions such as preparing the budget, cutting costs, increasing revenues, or capital investments.
The company will need to determine which line item they are comparing all items to within that statement and then calculate the percentage makeup. These percentages are considered common-size because they make businesses within industry comparable by taking out fluctuations for size. It is typical for an income statement to use net sales (or sales) as the comparison line item. This means net sales will be set at 100% and all other line items within the income statement will represent a percentage of net sales.
On the balance sheet, a company will typically look at two areas: (1) total assets, and (2) total liabilities and stockholders’ equity. Total assets will be set at 100% and all assets will represent a percentage of total assets. Total liabilities and stockholders’ equity will also be set at 100% and all line items within liabilities and equity will be represented as a percentage of total liabilities and stockholders’ equity. The line item set at 100% is considered the base amount and the comparison line item is considered the comparison amount. The formula to determine the common-size percentage is:
For example, if Banyan Goods set total assets as the base amount and wanted to see what percentage of total assets were made up of cash in the current year, the following calculation would occur.
Cash in the current year is $110,000 and total assets equal $250,000, giving a common-size percentage of 44%. If the company had an expected cash balance of 40% of total assets, they would be exceeding expectations. This may not be enough of a difference to make a change, but if they notice this deviates from industry standards, they may need to make adjustments, such as reducing the amount of cash on hand to reinvest in the business. The image below shows the common-size calculations on the comparative income statements and comparative balance sheets for Banyan Goods.
Even though vertical analysis is a statement comparison within the same year, Banyan can use information from the prior year’s vertical analysis to make sure the business is operating as expected. For example, unearned revenues increased from the prior year to the current year and made up a larger portion of total liabilities and stockholders’ equity. This could be due to many factors, and Banyan Goods will need to examine this further to see why this change has occurred. Let’s turn to financial statement analysis using financial ratios.
Overview of Financial Ratios
Financial ratios help both internal and external users of information make informed decisions about a company. A stakeholder could be looking to invest, become a supplier, make a loan, or alter internal operations, among other things, based in part on the outcomes of ratio analysis. The information resulting from ratio analysis can be used to examine trends in performance, establish benchmarks for success, set budget expectations, and compare industry competitors. There are four main categories of ratios: liquidity, solvency, efficiency, and profitability. Note that while there are more ideal outcomes for some ratios, the industry in which the business operates can change the influence each of these outcomes has over stakeholder decisions. (You will learn more about ratios, industry standards, and ratio interpretation in advanced accounting courses.)
Liquidity Ratios
Liquidity ratios show the ability of the company to pay short-term obligations if they came due immediately with assets that can be quickly converted to cash. This is done by comparing current assets to current liabilities. Lenders, for example, may consider the outcomes of liquidity ratios when deciding whether to extend a loan to a company. A company would like to be liquid enough to manage any currently due obligations but not too liquid where they may not be effectively investing in growth opportunities. Three common liquidity measurements are working capital, current ratio, and quick ratio.
Working Capital
Working capital measures the financial health of an organization in the short-term by finding the difference between current assets and current liabilities. A company will need enough current assets to cover current liabilities; otherwise, they may not be able to continue operations in the future. Before a lender extends credit, they will review the working capital of the company to see if the company can meet their obligations. A larger difference signals that a company can cover their short-term debts and a lender may be more willing to extend the loan. On the other hand, too large of a difference may indicate that the company may not be correctly using their assets to grow the business. The formula for working capital is:
Using Banyan Goods, working capital is computed as follows for the current year:
In this case, current assets were $200,000, and current liabilities were $100,000. Current assets were far greater than current liabilities for Banyan Goods and they would easily be able to cover short-term debt.
The dollar value of the difference for working capital is limited given company size and scope. It is most useful to convert this information to a ratio to determine the company’s current financial health. This ratio is the current ratio.
Current Ratio
Working capital expressed as a ratio is the current ratio. The current ratio considers the amount of current assets available to cover current liabilities. The higher the current ratio, the more likely the company can cover its short-term debt. The formula for current ratio is:
The current ratio in the current year for Banyan Goods is:
A 2:1 ratio means the company has twice as many current assets as current liabilities; typically, this would be plenty to cover obligations. This may be an acceptable ratio for Banyan Goods, but if it is too high, they may want to consider using those assets in a different way to grow the company.
Quick Ratio
The quick ratio, also known as the acid-test ratio, is similar to the current ratio except current assets are more narrowly defined as the most liquid assets, which exclude inventory and prepaid expenses. The conversion of inventory and prepaid expenses to cash can sometimes take more time than the liquidation of other current assets. A company will want to know what they have on hand and can use quickly if an immediate obligation is due. The formula for the quick ratio is:
The quick ratio for Banyan Goods in the current year is:
A 1.6:1 ratio means the company has enough quick assets to cover current liabilities.
Another category of financial measurement uses solvency ratios.
Solvency Ratios
Solvency implies that a company can meet its long-term obligations and will likely stay in business in the future. To stay in business the company must generate more revenue than debt in the long-term. Meeting long-term obligations includes the ability to pay any interest incurred on long-term debt. Two main solvency ratios are the debt-to-equity ratio and the times interest earned ratio.
Debt to Equity Ratio
The debt-to-equity ratio shows the relationship between debt and equity as it relates to business financing. A company can take out loans, issue stock, and retain earnings to be used in future periods to keep operations running. It is less risky and less costly to use equity sources for financing as compared to debt resources. This is mainly due to interest expense repayment that a loan carries as opposed to equity, which does not have this requirement. Therefore, a company wants to know how much debt and equity contribute to its financing. Ideally, a company would prefer more equity than debt financing. The formula for the debt to equity ratio is:
The information needed to compute the debt-to-equity ratio for Banyan Goods in the current year can be found on the balance sheet.
This means that for every $1 of equity contributed toward financing, $1.50 is contributed from lenders. This would be a concern for Banyan Goods. This could be a red flag for potential investors that the company could be trending toward insolvency. Banyan Goods might want to get the ratio below 1:1 to improve their long-term business viability.
Times Interest Earned Ratio
Time interest earned measures the company’s ability to pay interest expense on long-term debt incurred. This ability to pay is determined by the available earnings before interest and taxes (EBIT) are deducted. These earnings are considered the operating income. Lenders will pay attention to this ratio before extending credit. The more times over a company can cover interest, the more likely a lender will extend long-term credit. The formula for times interest earned is:
The information needed to compute times interest earned for Banyan Goods in the current year can be found on the income statement.
The $43,000 is the operating income, representing earnings before interest and taxes. The 21.5 times outcome suggests that Banyan Goods can easily repay interest on an outstanding loan and creditors would have little risk that Banyan Goods would be unable to pay.
Another category of financial measurement uses efficiency ratios.
Efficiency Ratios
Efficiency shows how well a company uses and manages their assets. Areas of importance with efficiency are management of sales, accounts receivable, and inventory. A company that is efficient typically will be able to generate revenues quickly using the assets it acquires. Let’s examine four efficiency ratios: accounts receivable turnover, total asset turnover, inventory turnover, and days’ sales in inventory.
Accounts Receivable Turnover
Accounts receivable turnover measures how many times in a period (usually a year) a company will collect cash from accounts receivable. A higher number of times could mean cash is collected more quickly and that credit customers are of high quality. A higher number is usually preferable because the cash collected can be reinvested in the business at a quicker rate. A lower number of times could mean cash is collected slowly on these accounts and customers may not be properly qualified to accept the debt. The formula for accounts receivable turnover is:
Many companies do not split credit and cash sales, in which case net sales would be used to compute accounts receivable turnover. Average accounts receivable is found by dividing the sum of beginning and ending accounts receivable balances found on the balance sheet. The beginning accounts receivable balance in the current year is taken from the ending accounts receivable balance in the prior year.
When computing the accounts receivable turnover for Banyan Goods, let’s assume net credit sales make up $100,000 of the $120,000 of the net sales found on the income statement in the current year.
An accounts receivable turnover of four times per year may be low for Banyan Goods. Given this outcome, they may want to consider stricter credit lending practices to make sure credit customers are of a higher quality. They may also need to be more aggressive with collecting any outstanding accounts.
Total Asset Turnover
Total asset turnover measures the ability of a company to use their assets to generate revenues. A company would like to use as few assets as possible to generate the most net sales. Therefore, a higher total asset turnover means the company is using their assets very efficiently to produce net sales. The formula for total asset turnover is:
Average total assets are found by dividing the sum of beginning and ending total assets balances found on the balance sheet. The beginning total assets balance in the current year is taken from the ending total assets balance in the prior year.
Banyan Goods’ total asset turnover is:
The outcome of 0.53 means that for every $1 of assets, $0.53 of net sales are generated. Over time, Banyan Goods would like to see this turnover ratio increase.
Inventory Turnover
Inventory turnover measures how many times during the year a company has sold and replaced inventory. This can tell a company how well inventory is managed. A higher ratio is preferable; however, an extremely high turnover may mean that the company does not have enough inventory available to meet demand. A low turnover may mean the company has too much supply of inventory on hand. The formula for inventory turnover is:
Cost of goods sold for the current year is found on the income statement. Average inventory is found by dividing the sum of beginning and ending inventory balances found on the balance sheet. The beginning inventory balance in the current year is taken from the ending inventory balance in the prior year.
Banyan Goods’ inventory turnover is:
1.6 times is a very low turnover rate for Banyan Goods. This may mean the company is maintaining too high an inventory supply to meet a low demand from customers. They may want to decrease their on-hand inventory to free up more liquid assets to use in other ways.
Days’ Sales in Inventory
Days’ sales in inventory expresses the number of days it takes a company to turn inventory into sales. This assumes that no new purchase of inventory occurred within that time period. The fewer the number of days, the more quickly the company can sell its inventory. The higher the number of days, the longer it takes to sell its inventory. The formula for days’ sales in inventory is:
Banyan Goods’ days’ sales in inventory is:
243 days is a long time to sell inventory. While industry dictates what is an acceptable number of days to sell inventory, 243 days is unsustainable long-term. Banyan Goods will need to better manage their inventory and sales strategies to move inventory more quickly.
The last category of financial measurement examines profitability ratios.
Profitability Ratios
Profitability considers how well a company produces returns given their operational performance. The company needs to leverage its operations to increase profit. To assist with profit goal attainment, company revenues need to outweigh expenses. Let’s consider three profitability measurements and ratios: profit margin, return on total assets, and return on equity.
Profit Margin
Profit margin represents how much of sales revenue has translated into income. This ratio shows how much of each $1 of sales is returned as profit. The larger the ratio figure (the closer it gets to 1), the more of each sales dollar is returned as profit. The portion of the sales dollar not returned as profit goes toward expenses. The formula for profit margin is:
For Banyan Goods, the profit margin in the current year is:
This means that for every dollar of sales, $0.29 returns as profit. If Banyan Goods thinks this is too low, the company would try and find ways to reduce expenses and increase sales.
Return on Total Assets
The return on total assets measures the company’s ability to use its assets successfully to generate a profit. The higher the return (ratio outcome), the more profit is created from asset use. Average total assets are found by dividing the sum of beginning and ending total assets balances found on the balance sheet. The beginning total assets balance in the current year is taken from the ending total assets balance in the prior year. The formula for return on total assets is:
For Banyan Goods, the return on total assets for the current year is:
The higher the figure, the better the company is using its assets to create a profit.
Industry standards can dictate what is an acceptable return.
Return on Equity
Return on equity measures the company’s ability to use its invested capital to generate income. The invested capital comes from stockholders investments in the company’s stock and its retained earnings and is leveraged to create profit. The higher the return, the better the company is doing at using its investments to yield a profit. The formula for return on equity is:
Average stockholders’ equity is found by dividing the sum of beginning and ending stockholders’ equity balances found on the balance sheet. The beginning stockholders’ equity balance in the current year is taken from the ending stockholders’ equity balance in the prior year. Keep in mind that the net income is calculated after preferred dividends have been paid.
For Banyan Goods, we will use the net income figure and assume no preferred dividends have been paid. The return on equity for the current year is:
The higher the figure, the better the company is using its investments to create a profit.
Industry standards can dictate what is an acceptable return.
Return on Capital or Return on Invested Capital
ROC or ROIC is a ratio used as a measure of the profitability and value-creating potential of companies relative to the amount of capital invested by shareholders and other debtholders. It indicates how effective a company is at turning capital into profits.
The ratio is calculated by dividing the after tax operating income (NOPAT) by the average book-value of the invested capital (IC).
While ratios such as return on equity and return on assets use net income as the numerator, ROIC uses net operating income after tax (NOPAT), which means that after-tax expenses (income) from financing activities are added back to (deducted from) net income.
While many financial computations use market value instead of book value (for instance, calculating debt-to-equity ratios or calculating the weights for the weighted average cost of capital (WACC)), ROIC uses book values of the invested capital as the denominator. This procedure is done because, unlike market values which reflect future expectations in efficient markets, book values more closely reflect the amount of initial capital invested to generate a return.
The denominator represents the average value of the invested capital rather than the value of the end of the year. This is because the NOPAT represents a sum of money flows, while the value of the invested capital changes every day (e.g., the invested capital on December 31 could be 30% lower than the invested capital on December 30).
Advantages and Disadvantages of Financial Statement Analysis
There are several advantages and disadvantages to financial statement analysis.
Financial statement analysis can show trends over time, which can be helpful in making future business decisions. Converting information to percentages or ratios eliminates some of the disparity between competitor sizes and operating abilities, making it easier for stakeholders to make informed decisions. It can assist with understanding the makeup of current operations within the business, and which shifts need to occur internally to increase productivity.
A stakeholder needs to keep in mind that past performance does not always dictate future performance. Attention must be given to possible economic influences that could skew the numbers being analyzed, such as inflation or a recession. Additionally, the way a company reports information within accounts may change over time. For example, where and when certain transactions are recorded may shift, which may not be readily evident in the financial statements.
A company that wants to budget properly, control costs, increase revenues, and make long-term expenditure decisions may want to use financial statement analysis to guide future operations. As long as the company understands the limitations of the information provided, financial statement analysis is a good tool to predict growth and company financial strength.
Attributions
"Principles of Accounting, Volume 1: Financial Accounting" by Mitchell Franklin, Patty Graybeal, Dixon Cooper, OpenStax is licensed under CC BY-NC-SA 4.0
Access for free at https://openstax.org/books/principles-financial-accounting/pages/a-financial-statement-analysis
"Return on capital" by Wikipedia is licensed under CC BY-SA 3.0
Insert
Budgets and their Use in Agribusiness
Excerpt used with permission from "Budgets: Their Use in Farm Management" by Roger Sahs, Courtney Bir, Oklahoma State University Extension. Copyright © OSU Extension.
Learning Objectives
4c Understand Operating, Cash Flow, and Capital Expenses Budgets, that components of each budget type and how each budget is utilized in agribusiness.
4f Analyze financial statements.
How to Best Organize and Manage the Farm
Questions of how to best organize and manage the farm business in a manner consistent with the goals and objectives of the farm family must be continually addressed. The decision as to whether the considered alternatives are consistent with established goals and objectives rests upon the farmer and the farm family acting as the manager if no outside management is hired. Budgeting is a management tool that can provide information to answer a multitude of questions if used properly. Combining inputs into products, allocating resources to alternative products and choosing combinations of different products can be analyzed efficiently through the use of budgets.
The purpose of this OSU Extension Fact Sheet is to describe the different types of budgets available to farm managers. Several basic economic principles will be introduced that relate to the budgeting process.
Introduction
The agricultural producer or farm manager is faced with organizing and managing farm resources to maxi-mize economic returns to owned or controlled resources. Resources include land (owned and rented) and associated improvements, capital assets such as machinery and breeding livestock (borrowed and owned) and labor (hired, farm operator and additional family). The manager is responsible for combining available resources and knowledge to best achieve the desired goals and objectives of the farm business.
With budgets, management can begin to answer such questions as:
- How may the available resources best be used?
- What enterprises (crops and/or livestock) can be produced and which will contribute most to returns to owned resources?
- How much of the controlled land should be devoted to each enterprise?
- What equipment and machinery will be needed to produce the potential enterprises?
- What production practices should be used to produce each of the enterprises?
- How much labor (both family and hired) will be needed on the farm?
- What are the capital requirements?
- Farm management skills and knowledge are an integral part of financial success.
Resource Allocation
- The problems of resource use and allocation involve the application of five economic principles. These principles, in a simplified form, consist of:
- Adding units of an input as long as the value of the resulting output or added returns is greater than the added cost.
- Substituting one input for another as long as the cost of the added input is less than the cost of the input that is replaced and the output is maintained.
- Substituting one product for another as long as the value of the added output is greater than the value of the output that is replaced and the cost is constant.
- Using each unit of resource where it gives the greatest returns when resources are limited.
- Basing comparisons upon discounted values when considering different time periods and/or elements of risk.
- The first three principles relate to situations where unlimited resources are available for use by the manager with perfect knowledge. The last two relate to situations where there are limited resources and when there is not perfect knowledge.
Most resource allocation management problems faced by farm managers can be addressed by applying the basic budgeting economic principles. Numerical calculations to assist in making management decisions. No one type of budget is tied to any particular principle. The type of budget relates to the intent of the analysis, while the principles relate to the farm resources and the resource relationships that exist.
Types of Budgets
There are three basic types of budgets that can be used in the farm business management process. Each type of budget provides different information to the manager for use in the decision-making process. The common thread in each type is that, if properly defined and used, the budget format permits the manager to use economic logic to answer questions of what, how much and when resources should be used to achieve the goals and objectives as established by the farm family.
The three types of budgets are:
- Whole-farm budget
- Enterprise budget
- Partial budget
The whole-farm budget is a classified and detailed summary of the major physical and financial features of the entire farm business. Whole-farm budgets identify the component parts of the total farm business and determine the relationships among the different parts, both individually and as a whole.
An enterprise budget is a statement of what generally is expected from a set of particular production practices when producing a specified amount of product. It consists of a statement of revenues from and the expenses incurred in the production of a particular product. An enterprise budget documents variable and fixed costs. It is useful in calculating profitability and break-even values.
The partial budget is useful in analyzing the effects of a change from an existing plan. This budget only considers revenue and expense items that will change with a defined change in the plan.
Whole-Farm Budget
To develop a whole-farm budget:
- List the goals and objectives of the farm firm.
- Inventory the resources available for use in production.
- Determine physical production data that will be used in the input/output process.
- Identify reliable input and output prices.
- Calculate the expected variable and fixed costs and all returns.
Since it is a plan for the future use of farm resources and establishes the future direction of the farm organization, the whole-farm budget must conform to the farm family goals and objectives to be successful. Farm management that is goal-directed integrates the goals and objectives of the farm with those of the family and reduces pressure on competitive uses of family controlled resources. OSU Extension fact sheet AGEC-244, “Goal Setting for Farm and Ranch Families,” can help develop a process for identifying farm and family goals, prioritizing them, and identifying management strategies that will achieve the identified goals (a worksheet is included). The whole-farm budget is the best tool to analyze the farm business and the impacts of the goals and objectives.
The whole-farm budget should start with the inputs the operator has available for use in the farm business. Often the amount of land and operating capital available are limiting factors. Other factors such as buildings, the farmer’s manage-rial skills and available markets can also be relatively fixed. It is important to start with those fixed elements in planning a whole-farm budget. The results of the whole-farm budget should combine the resources, constraints, technical information and price data into a realistic whole-farm budget for the farm being considered. The outcome should be a plan that can provide direction for the farmer and family to follow in maximizing the returns to owned resources.
An Enterprise Budget
Although managers lack information needed to make perfect decisions, they are forced to make decisions using information available, then must accept the risk associated with that decision. An enterprise budget provides a format for the manager to use in classifying information so the economics of alternative enterprises and alternative production systems can be consistently analyzed.
One problem in enterprise budgeting is the lack of information concerning the amount of products that will result from particular combinations of inputs for example, how much forage would be produced with a certain amount of seed and fertilizer. Seldom do managers have certainty regarding technical production information as producers never have complete information with regard to production conditions, such as weather and insects. Typically, more information is available regarding the prices of inputs than on products since inputs are purchased during one time period and products are sold in a later time period. The greater lag between planning and actual use of information on product prices relative to input prices adds uncertainty and product price risk that must be considered when planning.
An enterprise budget should contain several components. A detailed description should include a production goal, the production techniques to be employed, the land resource required and even something about the capital and labor requirements. An enterprise budget should include all costs and all returns associated with the defined enterprise. All variable and fixed costs, both cash and non-cash items, should be included. The returns from products produced for sale (wheat grain crop) plus those produced for use in another enterprise (grazing) should be included in an enterprise budget.
Variable costs are the costs of such input items as seed, feed, fertilizer, normal repairs, custom operations and machinery and equipment operating expenses. These costs also include labor, whether associated with machinery or equipment or as hand labor operations. They are items that will be used during one year’s operation or during one production period and would not be purchased if the enterprise was not produced. Variable costs are always included in an enterprise budget.
Fixed costs are the costs associated with buildings, machinery, and equipment which are prorated over a period of years. Included in this category are depreciation, interest, insurance, and taxes on individual buildings and pieces of machinery and equipment that can be allocated to an individual enterprise. Fixed costs are always included in an enterprise budget.
Some costs of production are difficult to allocate to a specific enterprise. The costs are generally classified as overhead costs and include costs usually associated with buildings, utilities and other miscellaneous items (such as recordkeeping and budgeting) that are used in more than one enterprise and are not easily allocated to an individual enterprise. Overhead costs can include both variable and fixed costs. It is necessary to allocate all costs of producing an enterprise even if an arbitrary method of allocation must be used. The key to allocating costs is to develop a process that is consistent over time.
The OSU Agricultural Economics Department has developed software tools to assist producers in analyzing many Oklahoma crop and livestock enterprises. Information and sample reports are available at Enterprise Budgets Home.
The Partial Budget Concept
Partial budgeting is a procedure where receipts and expenses which increase/decrease with a change in organization or procedures are listed in a systematic order. It is a process to allow a total farm budget to be fine-tuned. It focuses the analysis of a defined change to see if it improves the total farm budget.
The steps in constructing a partial budget are to:
- State the proposed alternative or change that will be analyzed.
- Collect data on all aspects of the business that will be affected by the change.
- Classify or group the types of impacts that will occur by including expenses increased or reduced and receipts increased or reduced.
- The partial budget (example in Table 1) is based on the concept that a change in the organization of the business will have one or more of the following effects:
Partial Budget, Wheat Grazeout versus Harvest for Grain
Situation: Should I leave stockers on wheat pasture for 60 days rather than remove stockers and combine wheat?
Additional Costs |
|
|
| Interest on Investment | $10.00 |
| Additional vet/feed/etc | $3.00 |
Reduced Returns |
|
|
| Steers: 640 lbs x $1.52/lb | $972.80 |
| Wheat Sales: 35 bushel x 4.50/bushel | $157.50 |
Total Annual Additional Costs and Reduced Returns |
| $1,143.30 (A) |
Additional Returns |
|
|
| Steers: 790lbs x $1.35/lb | $1066.50 |
Reduced Costs |
|
|
| Harvesting $24/acre + ($0.24/bushel x14) | $27.36 |
| Hauling $0.22 x 35 bushel/acre | $7.70 |
Total Annual Additional Costs and Reduced Returns |
| $1,101.56 (B) |
Net Change in Income (B – A) $1,101.56 - $1,143.30 = -41.74
1Estimates are based on a stocking rate of one head per acre.
Positive Economic Effects
- The change will eliminate or reduce some costs.
- The change will increase returns.
Negative Economic Effects
- The change will cause some additional costs.
- The change will eliminate or reduce some returns.
- The net change between positive and negative economic effects is an estimate of the net effect of making the proposed change in the total farm budget.
- A positive net change indicates a potential increase in income and a negative net change indicates a potential reduction in income due to the proposed change.
In the example, the total of the Additional Returns/Reduced Costs column is $1,101.56 and the total of the Additional Costs/Reduced Returns is $1,143.30. Subtracting the total of column A from B yields a net value of -$41.74 per acre. This represents the amount of economic loss with grazing out the wheat rather than selling the steers on March 1 and combining the wheat. Note that with different prices or stocking rates, the conclusion could be different. In some years, grazing out is the more profitable option so having accurate price forecasts is critical.
Sources of Budget Information
All budgets should be based upon the best information available. The reliability of the budgets is only as good as the quality of the data used in the process. Data needed for use in budgets includes quantity, price, method and timing of the inputs used.
Some sources of information available for use in preparing budgets are:
- Actual farm records
- Area summary analysis
- County production data
- Typical budgets
- Farm literature
- Information from meetings
- Neighbors
Any or all of these sources should be used in collecting and verifying data or information used in preparing budgets. Good managers verify the reliability of data collected from any source to see that it applies to their situation. Experience from one year is only an indicator and does not assure that same response will result in following years.
Budget Limitations
Careful evaluation of the resource situation must precede the drawing of inferences from budgets. Farms with different owned resource situations can have different management plans given the same basic budget information. Budget data for a 160-acre farm can be used in preparing a budget for a farm of 320 acres; however, differences in resources and organization must be considered and adequately accounted for if the end result is to be reliable and useful.
Budgets are generally constructed to reflect future actions and it is difficult to accurately predict future prices and yields. Historical data provide some basis for establishing initial levels of budget yield, price and timing data. Several options are available in establishing future prices such as forward contracting and hedging techniques.
Production and marketing risks will limit budget reliability. Best estimates should be used to develop budgets for use in farm business analysis. However, high degrees of variability create risk to the operator and put pressure on the reliability of the estimates used in the enterprise budgets. One alternative is to evaluate best- and worst-case scenarios in addition to the expected outcome. Probability distributions on weather events and prices can add valuable insights. Even with careful use, errors can compound themselves to the point where budgets can have little or no value. This element of risk should be considered and evaluated by the manager when determining the solutions that best meet the goals and objectives of the farm family.
Budget preparation is time consuming. It requires pencil and calculator activity as well as searching data sources for information to be used in preparing the budget. Software also is available to assist in budget calculations. As with all problems, this becomes an economic question such that the farmer faces the problem of allocating their time in a manner whereby the returns from budgeting are greater than the cost of gathering the information.
Why Budget?
Using budgets can provide the farm manager a method to:
Experiment through simulation with possible outcomes of a given organizational change before resources are actually committed to the change.
- Uncover cost items that might otherwise be overlooked.
- Refine the present organization.
- Seek credit from lending agencies.
- Learn to better organize and reorganize.
Aids to the Process
The Department of Agricultural Economics has decision aids and materials available to assist farmers in building an information system, using information to develop all types of budgets and using budgets in management decisions. Meetings are held upon request throughout Oklahoma to provide the most current information available. Computer software has been developed to assist with the analysis and assimilation of data into the management framework.
Enterprise budgets developed by the Department of Agricultural Economics are available at Enterprise Budgets Home. Those interested in obtaining enterprise budgets may also contact their Extension Educators — Agriculture, Area Agricultural Economics Specialist, or State Agricultural Economics Specialist, Room 515, Agricultural Hall, OSU, Stillwater, OK 74078, (405) 744-9836 for more information.
Record keeping systems (both manual and electronic) as explained in OSU Extension Fact Sheet AGEC-302, “Information Systems for Oklahoma Farmers,” are available to help organize historical data for use in business management. One such affordable software program that is appropriate for farms and ranches requiring only cash records is Quicken. More information on using Quicken for farm financial record keeping is available from the OSU Department of Agricultural Economics at About Quicken. For smaller or less complex businesses, hand-kept ledgers may still be a satisfactory alternative. The Oklahoma Farm and Ranch Account Book is designed to be a comprehensive, easy to use, manual record-keeping system. A customized book can be built and printed for individual needs at OSU Farmbook. Other types of ledgers are often available from agricultural lenders, farm supply dealers, and farm management firms.
Summary
Budgets (whole-farm, enterprise, and partial) are management tools to help evaluate the farm business. Each type of budget has a different but related purpose and should be used by managers accordingly. The whole-farm budget becomes a starting point that can be used to analyze the farm business over time. Enterprise budgets can be used to analyze components of the farm business and also be a building block for the whole-farm. Once a whole-farm budget has been developed, a partial budget can be valuable in evaluating changes to the total-farm budget. Each type of budget offers useful information to support management decisions.
Attributions
"Budgets: Their Use in Farm Management" by Roger Sahs, Courtney Bir, OSU Extension . Copyright © OSU Extension. Used with permission.
Cash Flow Budgeting in Agribusiness
Excerpt used with permission from "Twelve Steps to Cash Flow Budgeting" by William Edwards, Iowa State University Extension and Outreach. Copyright © ISU Extension and Outreach.
Learning Objectives
4c Understand Operating, Cash Flow, and Capital Expenses Budgets, that components of each budget type and how each budget is utilized in agribusiness.
4f Analyze financial statements.
Twelve Steps to Cash Flow Budgeting
How much financing will your farm business require this year? When will money be needed and from where will it come? A little advance planning can help avoid short-term shortages of cash. One useful tool for planning the use of capital in the farm business is a cash flow budget.
A cash flow budget is an estimate of all cash receipts and all cash expenditures that are expected to occur during a certain time period. Estimates can be made monthly, bimonthly, or quarterly, and can include nonfarm income and expenditures as well as farm items. Cash flow budgeting looks only at money movement, though, not at net income or profitability.
A cash flow budget is a useful management tool because it:
- forces you to think through your farming plans for the year.
- tests your farming plans, such as if you will produce enough income to meet all your cash needs.
- projects how much operating credit you will need and when projects when loans can be repaid.
- provides a guide against which you can compare your actual cash flows.
- helps you communicate your farming plans and credit needs to your lender.
Getting Started on the Budget
Developing a cash flow budget for the first time will not be easy. Following a step-by-step approach can make the task less difficult, though. The pages at the end of this publication contain a format for completing your plan, although other forms can be used. There also are many personal computer programs available for developing cash flow budgets. Or, you may want to develop your own. In any case, the following steps can be applied.
1. Outline your tentative plans for livestock and crop production for the year, as shown in Example 1.
2. Take an inventory of livestock on hand and crops in storage now. If a recent financial statement is available, information found under the current assets section can be used.
3. Estimate feed requirements for the proposed livestock program, as shown in Example 2. Some typical feed requirements are contained in ISU Extension publication FM 1815/AgDM B1-21, Livestock Enterprise Budgets for Iowa. Your own past feed records are also a good guide.
Adjust feed requirements if livestock will complete only part of the feeding program during the budget year. It also is helpful to divide requirements for homegrown feedstuffs between the periods prior to harvest and following harvest.
4. Estimate feed available, as shown in Example 2. List beginning inventories prior to harvest, and expected new crop production after harvest. Remember to exclude grain transferred to the landlord under a cropshare lease. Finally, estimate the quantity of feed purchases needed, if any, and the quantity available to sell. Once your feed supply and feed requirements are estimated, you may want to adjust the livestock program to fit them.
5. Now you are ready to start with the actual cash flow budget.
First estimate livestock sales, based on production and marketing plans, as shown in the top line of Example 3.
Start with livestock on hand, then add livestock to be produced during the year. Exclude animals to be carried over to next year or held back for breeding stock.
- Include sales of breeding stock that will be culled.
- Include livestock product sales, such as for milk or wool.
- Use your best estimate of selling prices based on outlook forecasts or marketing contracts.
- Reflect expected seasonal price patterns when appropriate, rather than using the same price all year.
Stay on the conservative side. If your plan will work at conservative prices, it also will work at better prices.
Some producers prepare budgets at two or three price levels for the major products they sell. This helps them identify the amount of price risk they face.
6. Plan sales of nonfeed crops and excess feed.
- Consider crops in inventory at the beginning of the year as well as crops to be harvested during the year. Plan to carry over grain for feed for next year plus other crops normally sold in the following year.
- Plan timing of sales according to your normal marketing strategy. In Example 3, the farmer plans to sell old-crop soybeans in March and hold new-crop soybeans until after January 1 of next year.
- Follow the same guidelines as in step 5 for estimating crop prices. Look at outlook forecasts, consider seasonal price patterns, and use conservative price estimates.
- Multiply quantities to sell by anticipated prices, and carry the totals to the budget form.
After the initial cash flow budget is completed, you may want to revise your marketing plans to meet capital needs throughout the year.
7. Estimate income from other sources, including:
- USDA farm payments
- custom machine work income
- income from off-farm work, rental property, or other business activities
- interest, dividends, patronage refunds, etc.
Last year’s additional cash income listed on your income tax return is a useful guide.
8. Project crop expenses and other farm operating expenditures.
Last year’s expenditures are a good guide. Adjust for changes in price levels.
If cropping plans will be different this year, detailed field-by-field production plans or field maps can be used to estimate expenses.
Expenses that are determined by contract, agreement, or law can be estimated directly from contract terms, unless rates are expected to change. These include property taxes, property and liability insurance premiums, and fixed cash rents.
Expenses should be spaced through the year based on your best judgment. Some will fall mainly during certain seasons, such as machine hire, part-time labor, and crop expenses.
Remember to place these expenses during the period of payment, not the period of use. Some expenses will be spread through the year but will have definite seasonal peaks. Fuel, machinery and equipment repair, and utilities are examples. Other expenses may be spaced evenly through the year, such as vehicle operating expenses, livestock health and supplies, and purchased feed.
9. Consider capital purchases
such as machinery, equipment, land, or additional breeding livestock. Major machinery expenses such as a tractor overhaul also can be included here, as well as construction or improvement of buildings. Example 3 shows that the farmer is considering trading for a new combine for a cost of $50,000. This amount is entered under the “Purchases of Capital Assets” section. Show only the cash difference to be paid when a trade is involved.
You may want to complete the rest of the cash flow budget first to see if major capital expenditures will be feasible this year. If a portion of the item will be financed by borrowing, then include the anticipated loan amount in the “Financing” section.
10. Summarize debt repayment.
Much of this information can be taken from your most recent net worth statement. Include only those debts that you have already acquired at the beginning of the budgeting period. Calculate the interest that will be due at the time the payment will be made. Remember, the net worth statement may show only interest accrued up to the date of the statement.
11. Estimate nonfarm expenditures.
Adjust last year’s living expenses for changes in family circumstances and inflation. Remember to allow for possible purchases of vehicles, furniture, appliances, or major repairs, and contributions to retirement accounts.
A tax estimate made at the end of the year for tax management is helpful for projecting income tax and Social Security payments to be made for last year’s income. Your estimate can be revised when your actual tax returns have been completed.
12. Sum total cash inflows and total cash outflows.
Add total projected cash inflows for the year and for each period, as shown in the sample budget in Example 3. Add the total inflows for each period to check that they equal the total projected inflows for the year.
Add total projected cash outflows for the year and for each period. Add the total outflows for each period to check that they equal the total projected outflows for the year.
Subtract total cash outflows from total cash inflows to determine the net cash flow for each period. Add the net cash flows for each period to check that they equal the total net cash flow for the year.
If the estimated net cash flows for the entire year and for each period are all positive, you have a feasible cash flow plan. If the net cash flows for some periods are negative, some adjustments may need to be made.
Analyzing Your Budget
In the example, the farm business will have a net cash flow of $16,989 for the year as a whole. The projected cash outflows include:
$36,000 for family living expenses and $23,000 for nonfarm investments.
$61,078 for repayment of borrowed funds plus interest.
$50,000 for trading combines.
$458,720 for operating expenses.
A cash flow budget only indicates whether or not the farm business will produce enough cash income to meet all demands for cash. It does not estimate net income or profit. Remember that net farm income also includes non-cash items such as depreciation and changes in crop and livestock inventories, and that net farm income can be positive even when net cash flow is negative, and vice versa.
Annual Adjustments
The first step in analyzing cash flow is to add cash on hand to net cash flow. If the total projected net cash flow for the year is still negative, some type of annual adjustments must be made. Alternatives include:
Sell more current assets (crops and livestock). Be careful here, though - reducing inventories may solve the cash flow squeeze this year, but could result in even more severe problems next year.
Carry over operating debt to the following year.
Finance capital expenditures with credit, or postpone them until another year. Anticipated borrowing for capital assets can be included in the financing section under cash inflows.
Reduce the size of intermediate and long-term debt payments by lengthening the repayment period or adding a balloon payment at the end.
Convert short-term debt to intermediate or long-term debt by refinancing it as an amortized loan.
Reduce nonfarm expenditures or increase nonfarm income.
Sell intermediate or long-term assets to raise cash.
In the example, financing 50 percent of the $50,000 combine trade with a lender ($25,000) would leave a positive net cash flow for the year of $41,989. The $25,000 would be entered as new borrowing in the period when the purchase was projected (July through August).
Seasonal Adjustments
Even when the yearly net cash flow is positive, sizable deficits can occur in some months. This is due to the seasonal nature of expenses in farming and the tendency to sell large quantities of a product at one time. Some types of enterprises, such as dairy, produce a more constant cash flow than other types.
Shorter term adjustments can be made when projected net cash flow is positive for the whole year but negative for certain months. These include:
Shift the timing of some sales.
Shift the timing of some expenditures.
Increase short-term borrowing in periods with negative cash flow, and project repayment in periods with positive cash flow. Remember to add interest charges to payments.
Delay the due date of fixed debt payments to match periods with positive net cash flows.
Operating Loan Transactions
In Example 3, cash on hand at the beginning of the year is $6,146. Enter this in the January-February column, as well. Then work through the remaining periods to determine the amount of additional new borrowing needed in each period.
The farmer in the example wishes to plan for a cash balance of at least $1,000 at the end of each period. The cash flow can be balanced by planning to borrow $20,000 in operating capital in January, $5,000 in April, $11,000 in June and $22,000 in October. The operating loan balance ($60,000) can be repaid in December, however, plus interest.
Some farmers operate with a line of credit from their lender, with a maximum borrowing limit, instead of borrowing funds in fixed amounts. The cash flow budget also can be used to test if the need for operating capital will exceed this limit, as shown in the lower part of Example 3.
Add the outstanding balance on the line of credit at the beginning of each period to the amount of new borrowing in that period. If operating debt will be repaid instead, subtract the amount to be repaid to arrive at the ending credit balance for that period. Do not include new borrowing to be repaid over several years (such as for the combine) if the borrowing limit applies only to short-term capital.
In this case we are concerned only with the amount of principal borrowed and repaid, not interest.
In the example, the farmer started the year with an annual operating loan balance of $203,200. The loan balance was projected to drop to $201,200 by the end of the year.
If the projected ending credit balance for any period exceeds the credit limit, adjustments to cash flow can be made as discussed above.
Monitoring Cash Flows
Review your cash flow budget from time to time during the year. Prices and costs may differ from your estimates, or production plans may change. Monthly bank statements and canceled checks are a good source of cash flow information against which your budget can be compared. This will help you anticipate changes in your needs for cash and credit later in the year. You may even need to prepare a revised budget for the remainder of the year.
Developing a cash flow budget for the first time will not be easy. Close communication with your lender is important. By planning where you are going financially, you can increase your chances of arriving there safely. Cash flow budgeting is an essential part of sound financial management.
Budgeting Major Investments
A cash flow budget also can be very helpful in evaluating major capital investments or changes in the farm business. Examples are purchasing land, building new hog facilities, or expanding a beef cow herd. Often it will be necessary to develop two budgets: one for a business year after the investment or change in the business is complete, and one for the intermediate or transition year (or years).
As an example, a beef cow-calf producer decides to expand the herd by buying heifer calves. The producer should develop a total cash flow budget for the operation as it will be after the expansion is complete. However, the greatest cash flow problem may be in the transition year. The expenses will increase because there will be more cattle in the herd but income will not increase until calves from the new heifers are sold.
Expansion of a livestock enterprise through construction of new facilities can often create cash flow problems in the construction year, even if the facilities are financed with an intermediate or long-term loan. This is especially true if it will take some time to expand the enterprise up to the capacity of the facility. In the meantime, the producer will have to meet the loan payments on the facility, as well as pay for additional labor and feed. A set of cash flow budgets could help select the best alternative in terms of financial feasibility.
Attributions
"Twelve Steps to Cash Flow Budgeting" by William Edwards, Iowa State University Extension and Outreach. Copyright © ISU Extension and Outreach. Used with permission.
Net Worth Statements in Agribusiness
Excerpt used with permission from "Your Net Worth Statement" by William Edwards, Iowa State University Extension and Outreach Copyright © ISU Extension and Outreach.
Learning Objectives
4d Understand the accounting equation and all components.
4e Understand the components and uses of a balance sheet and profit and Loss Statement.
Your Net Worth Statement
Would you like to know more about the current financial situation of your farming operation? A simple listing of the property you own and the debts you owe can provide valuable insights. Such a listing is called a net worth statement, or sometimes a financial statement, or balance sheet.
The net worth statement is based on the relationship:
assets = liabilities + net worth, or
assets - liabilities = net worth
Most farm businesses are made up of a combination of land, livestock, crops, and machinery acquired with debt (liabilities) or contributed by the operator (net worth or owner’s equity). The net worth statement is like a photograph of these assets and liabilities on a given date.
Comparing net worth statements made at the end of each year over several years can help you measure the progress of your farm business. The net worth statement also helps you judge the ability of the farm operation to pay off current debts and take on additional ones.
Developing the Statement
A net worth statement may include only the farm business, or it may include household and personal assets and debts as well. For business analysis purposes, only information pertaining to the farming operation is needed. Information about nonfarm assets and liabilities can be added in a separate section and used for analyzing debt repayment capacity. For a farm partnership, include only items owned or owed by the partnership, not by the partners individually.
Most families create a net worth statement as of December 31 or January 1 because this is the end of their accounting year. However, it is possible to develop a statement at any date and as often as needed. A blank form for completing a net worth statement is available at the end of this publication. If you want to create your own net worth statement, as well as an income statement, cash flow statement and statement of owner equity, use Decision Tool Comprehensive Farm Financial Statements or the blank worksheets available in ISU Extension and Outreach publication FM 1824/AgDM C3-56 Farm Financial Statements.
Valuing Assets
Assets are generally listed on the left-hand side and liabilities on the right-hand side of the statement. Both assets and liabilities are divided into current and fixed items.
Current assets include cash, bank accounts, crops, livestock, and supplies that will normally be sold or used within a year.
List the current balances for all your savings and checking accounts used for farm receipts and expenses. If you obtain your current checking account balance from your bank, remember to subtract the value of any checks that are still outstanding.
The key to correctly listing current assets is to accurately estimate both the number and value of items on hand. ISU Extension and Outreach publication FM 1490/AgDM C1-40, Suggested Closing Inventory Prices is helpful for valuing current assets.
For market livestock, begin with an up-to-date inventory of the number of head and estimated weight for each class of livestock. Value them at current market prices, minus potential marketing and transportation costs. Check with local markets or use local prices available from newspapers, websites, or other sources of marketing information.
- Value young livestock at feeder animal prices.
- Value heavier livestock at their estimated weight times the current slaughter market price.
- Use an average of feeder and market livestock prices for animals at intermediate weights.
For grain and feed, including hay, silage, straw, and supplements:
- Begin with accurate estimates of bushels, tons, bales, etc., on hand.
- Include grain under warehouse receipt at an elevator. Also include grain delivered under a deferred pricing (price later) contract if the price has not yet been established or payment received.
- Value crops at current market price, or their contracted price, minus marketing costs. Check with local markets or use local prices available through newspapers, websites, or other sources.
- Include grain placed under a USDA marketing loan. Value it at the current market price or the loan rate, whichever is higher, because you have the option of repaying the loan at a lower rate if the price is below the loan rate. Include the marketing loan as a current liability.
- Value crops that have been hedged with a futures contract at their current market price, not the futures price. Any gains or losses incurred in the futures market will be reflected in the balance of the hedging account.
- Value commercial feed at its purchase cost.
Other current assets include:
- Supplies on hand, such as seed corn, chemicals, medications, and fuel.
- Prepaid expenses, such as payment made for feed to be delivered in the coming year. Show this as an asset only if you have already paid for it or if you show the obligation to pay for it as a liability.
- Money invested in a future crop such as for fall-applied fertilizer. Growing crops generally should be given a value equal to the costs of production already incurred.
- Hedging accounts used for forward pricing grain, livestock, or production inputs. Obtain a current estimate of net market value of all futures and options accounts on the date of the net worth statements, including realized gains from closed contracts, plus margin money deposited.
- Accounts receivable, such as the payment a customer might owe you for custom combining, government payments to be received for past production, or crop insurance payments earned but not yet received.
Fixed assets are those used in farm production, but not intended to be sold or converted directly into marketable products during the year (except for breeding livestock to be culled).
For breeding and dairy livestock:
- Begin with an accurate count of each species and type of livestock.
- Cows or ewes should be valued according to a conservative dairy or breeding value. For sows that are replaced more rapidly, an estimated slaughter value is suggested.
- Avoid making large year-to-year changes in values placed on breeding stock, which can cause large paper increases or decreases in net worth. Establishing a base value for each class of breeding stock and using it each year is recommended.
For machinery, equipment, and vehicles:
- Your tax depreciation schedule should provide a complete inventory.
- Use the depreciated or remaining value (cost minus total depreciation allowed, including depreciation for the past year), under the cost value column. However, if very rapid tax depreciation methods have been used, such as “expensing,” you may want to start with a value that is closer to fair market value.
- Once a total remaining value has been determined, it can be adjusted in following years by this formula:
Value of machinery (or equipment or vehicles) at the beginning of the year
+ net cost of machinery added (purchases or cash difference paid on a trade)
- the value of machinery sold or junked
- depreciation expense for the year (economic, not income tax values) (10% of undepreciated value is suggested)
= machinery value at the end of the year
(see Example 2). - Use a conservative market value under the market value column, or adjust the previous year’s value for purchases, sales, and depreciation. Use the same depreciation expense value that you show on your net income statement.
Do not include machinery, equipment, or breeding livestock that you are leasing, unless they are shown on your tax depreciation schedule.
For perennial or long-term crops such as alfalfa, orchard crops, or some vegetables, sum up all the costs incurred for establishing the crop and depreciate that amount over its remaining productive life.
Other fixed assets include land, buildings, and other improvements. They often have the largest dollar value of any assets on the net worth statement. On some statements, fixed assets are divided into intermediate and long-term assets.
List the cost basis of farm real estate under the cost value column:
- Your original basis is the price you paid for the farm.
- If you received the property through gift, you retain the giver’s basis.
- If you inherited the property, the basis is the value that was used for valuing the estate.
- Adjust the original basis by adding the cost of improvements made and subtracting the depreciation taken on improvements.
List owned farm real estate at a conservative current value in the market value column.
- List the value of improvements separately from real estate. Use the remaining value for depreciable improvements.
- Reduce market value land prices to allow for broker’s commission and other selling costs that might have to be paid if the farm were sold.
Shares in other farming entities such as a sow cooperative should also be shown under fixed assets, as well as shares in other farm corporations or LLCs.
Personal assets such as family bank accounts, retirement accounts, stocks and bonds, household goods, vehicles, housing or other real estate can be listed separately at the bottom of the assets side of the statement.
Listing Liabilities
Liabilities are generally listed on the right-hand side of the net worth statement and include all debts and loan obligations to pay that the farm business or family has on the date of the statement. Liabilities are usually listed according to the length of time before they become due. You may want to list the creditor’s name and the purpose of each liability, as well as the amount, on a separate page.
Current liabilities are those due within the next 12 months.
- Include debts such as operating notes, feeder livestock notes, or the outstanding balance on a credit line with a bank or other lender.
- Accounts payable, such as an unpaid open account with a feed mill or attorney, should also be shown, as well as unpaid wages, custom charges, and farm income and property taxes due.
- Contractual obligations, such as a cash rent leasing agreement or a machinery operating lease, are generally not shown unless they are past due. However, if they are included in liabilities, the value of the rights that you have as a result of the contract should also be shown as an asset. These are generally given the same value as the liability.
- List principal payments due on fixed liabilities within the next 12 months (see Example 3).
- Calculate the amount of unpaid interest accrued on all liabilities as of the date of the statement. Multiply the outstanding principal of each debt by its respective interest rate, then multiply by the fraction of a year that has passed since the last payment, or since the loan was received if no payments have been made yet (see Example 4).
- Some accountants show the potential income tax that would be due if all current assets were sold as a current liability, under deferred tax liabilities.
Fixed liabilities include debts payable more than one year in the future.
- Loans for breeding stock, machinery, land, or farm improvements usually fall into the fixed category.
- A mortgage or contract on real estate is usually a fixed liability, too.
- Show the unpaid balance minus the principal due in the coming year (it has already been shown as a current liability).
- Some accountants show the potential income and capital gain taxes that would be due if all fixed assets were sold as a fixed liability, under deferred tax liabilities.
Personal liabilities can be shown at the bottom of the liabilities column. These include consumer debts, credit card balances, home mortgages, and medical bills to pay.
Net Worth
The difference between total farm assets and total farm liabilities is the net worth, or equity, at the time the statement is made. It is the current value of your own investment in the farming operation. Adding net worth to total liabilities (which is the share of assets contributed by creditors) gives you a value equal to total assets and serves as a check on your calculations.
The cost value net worth shows the value of your own investment excluding changes in the market values of machinery or real estate, while market value net worth does include these changes.
Farm and personal net worth can be added together to find the total family net worth.
Analyzing the Statement
Once you have completed your net worth statement, take time to look it over and understand what it can tell you. To begin, look at each major liability listed and see if a corresponding item can be found under the asset side. The corresponding item will usually be listed under the same section (current or fixed). If a corresponding asset cannot be found, you may have forgotten to list something. Or the asset originally acquired with borrowed money may have already been sold or used up before paying the corresponding liability. This is a danger sign. It means that you must generate funds to pay this debt elsewhere in the farm business.
Another danger sign is a liability that appears closer to the top of the statement than its corresponding asset. An example is a machinery item bought on a one-year note. It is usually difficult to pay for an asset over a period of time considerably shorter than its useful life.
Sometimes the value of a particular liability is greater than the value of its corresponding asset. This may mean that the debt is not adequately secured, or it may occur simply because rapid depreciation methods have been used.
Financial Ratios
Debt-to-asset ratio (or percent debt) is equal to total liabilities divided by the market value of total assets. It indicates the portion of total capital supplied by creditors. A successful farm business will have a decreasing ratio over time, except in years when major assets such as land are purchased with borrowed capital. A low debt-to-asset ratio usually leads to less year-to-year variability in net farm income, but may also cause net worth to grow more slowly.
A personal debt-to-asset ratio also can be calculated, using total farm and personal asset and liability values.
A current ratio can be calculated by dividing total current assets by total current liabilities. This is a measure of liquidity, or the ability to pay bills and debts as they come due over the next 12 months.
A farm business with good overall risk-bearing ability can still have liquidity problems. This may be caused by a low income year resulting in carryover operating debt, or too rapid investment of cash into intermediate and long-term assets, such as machinery or land.
Many lenders consider a current ratio of 2.0 or greater to show good short-term risk-bearing ability, while a ratio close to 1.0 or lower indicates potential cash flow problems. However, this is affected by the type of farm, volume of production, and financial structure. For example, farms with regular livestock sales, such as dairy, often can withstand lower current ratios than crop farms that have production only late in the year.
Some lenders prefer to look at the difference between current assets and current liabilities rather than their ratio. This difference is called working capital. It indicates the potential cash available for meeting daily operating costs, consumption expenditures, and other items not listed under current liabilities.
In many cases current liabilities will be paid from income generated from sales of farm products that have not yet been produced and do not appear as current assets. A more accurate analysis of repayment capacity can be made by developing a cash flow budget, as explained in ISU Extension and Outreach publication FM 1792/AgDM C3-15 Twelve Steps to Cash Flow Budgeting.
Year-to-year Comparisons
The financial progress of the farm business can be measured by comparing a current net worth statement with earlier ones.
The change in cost value net worth from one year to the next shows the growth (or loss) due to net income earned from the farm business, and consumption. The following formula summarizes the relation among cost value net worth, income, and consumption expenditures:
net farm income (accrual)
+ non-farm income, gifts, or inheritances invested in the farm business
- farm income used for living expenses, income tax payments, and other consumption
= change in cost value farm net worth
The change in market value net worth is found by subtracting the market net worth shown on last year’s financial statement from that shown on this year’s. It measures the change in the market value of your equity share of the farm business. It also depends on net income and consumption, but includes changes in the market value of land or machinery, as well. It can also be expressed as a percent of net worth at the beginning of the year.
A decrease in net worth from one year to the next may result from low net farm income or high consumption expenditures. It may also result from large changes in inventory prices of current and fixed assets. For this reason, it is useful to compare similar items on the balance sheet from one year to the next. Changes in their values may be due to changes in volume, changes in unit prices, or both.
Many different forms and formats exist for developing a net worth statement. However, all of them contain the same basic information. Completing an annual net worth statement is one of the simplest means available for analyzing the risk-bearing ability and financial progress of your farm business.
Year-to-year Comparisons
The financial progress of the farm business can be measured by comparing a current net worth statement with earlier ones.
The change in cost value net worth from one year to the next shows the growth (or loss) due to net income earned from the farm business, and consumption. The following formula summarizes the relation among cost value net worth, income, and consumption expenditures:
net farm income (accrual)
+ non-farm income, gifts, or inheritances invested in the farm business
- farm income used for living expenses, income tax payments, and other consumption
= change in cost value farm net worth
The change in market value net worth is found by subtracting the market net worth shown on last year’s financial statement from that shown on this year’s. It measures the change in the market value of your equity share of the farm business. It also depends on net income and consumption, but includes changes in the market value of land or machinery, as well.
A decrease in net worth from one year to the next may result from low net farm income or high consumption expenditures. It may also result from large changes in inventory prices of current and fixed assets. For this reason, it is useful to compare similar items on the balance sheet from one year to the next. Changes in their values may be due to changes in volume, changes in unit prices, or both.
Many different forms and formats exist for developing a net worth statement. However, all of them contain the same basic information. Completing an annual net worth statement is one of the simplest means available for analyzing the risk-bearing ability and financial progress of your farm business.
Attributions
"Your Net Worth Statement" by William Edwards, Iowa State University Extension and Outreach Copyright © ISU Extension and Outreach. Used with permission.
Important Financial Statements for Agribusiness
Excerpt used with permission from "Your Net Worth Statement" and "Your Farm Income Statement" by William Edwards, Iowa State University Extension and Outreach. Copyright © ISU Extension and Outreach.
Learning Objectives
4d Understand the accounting equation and all components.
4e Understand the components and uses of a balance sheet and Profit and Loss Statement.
Important Farm Financial Statements
The financial position and performance of a farm business can be summarized by four important financial statements. The relationship of these statements is illustrated below. Information from these statements can be used:
- to make important financing and investment decisions,
- to substantiate credit applications,
- to derive performance measures for analyzing the farm business,
- to develop budgets for planning purposes.
The major statements and their purposes are as follows:
Net Worth Statement - Summarizes the property and financial assets owned, the debts owed, and the net worth of the business at a point in time
Net Farm Income Statement - Summarizes the income generated, the expenses incurred, and the net income earned by the business during a period of time
Statement of Cash Flows - Summarizes all the sources and uses of cash by the business during a period of time
Statement of Owner Equity - Shows how net worth changed from the beginning to the end of the year
Your Farm Income Statement
How much did your farm business earn last year? Was it profitable? There are many ways to answer these questions.
A farm income statement (sometimes called a profit and loss statement) is a summary of income and expenses that occurred during a specified accounting period, usually the calendar year for farmers. It is a measure of input and output in dollar values. It offers a capsule view of the value of what your farm produced for the time period covered and what it cost to produce it. Most farm families do a good job of keeping records of income and expenses for the purpose of filing income tax returns. Values from the tax return, however, may not accurately measure the economic performance of the farm. Consequently, you need to have a clear understanding of the purpose of an income statement, the information needed to prepare the statement, and the way in which it is summarized.
A net farm income, as calculated by the accrual or inventory method, represents the economic return to your contributions to the farm business: labor, management, and net worth in land and other farm assets. Cash net farm income also can be calculated. It shows how much cash was available for purchasing capital assets, debt reduction, family living, and income taxes.
Preparing the Statement
The income statement is divided into two parts: income and expenses. Each of these is further divided into a section for cash entries and a section for noncash (accrual) adjustments.
An example income statement is shown at the end of this publication, along with a blank form. Blank forms for developing your own income statement are also available in ISU Extension and Outreach publication FM 1824/AgDM C3-56, Farm Financial Statements.
Most of the information needed to prepare an income statement can be found in common farm business records. These include a farm account book or program, Internal Revenue Service (IRS) forms 1040F Profit or Loss From Farming and 4797 Sales of Business Property, and your beginning and ending net worth statements for the year. If you use the IRS forms, you will need to organize the information a bit differently to make allowances for capital gains treatment of breeding stock sales, and the income from feeder livestock or other items purchased for resale.
Cash Income
Cash income is derived from sales of livestock, livestock products, crops, government payments, tax credits and refunds, crop insurance proceeds, and other miscellaneous income sources.
- Include total receipts from sales of both raised livestock and market livestock purchased for resale. Remember not to subtract the original cost of feeder livestock purchased in the previous year, even though you do this for income tax purposes. Also include total cash receipts from sales of breeding livestock before adjustments for capital gains treatment of income are made. These are termed “gross sales price” on IRS Form 4797.
- Do not include proceeds from outstanding USDA marketing loans in cash income even if you report these as income for tax purposes.
- Do not include noncash income such as profits or losses on futures contracts and options. However, do include cash withdrawn from hedging accounts.
- Do not include sales of land, machinery, or other depreciable assets; loans received; or income from nonfarm sources in income.
Adjustments to Income
Not all farm income is accounted for by cash sales. Changes in inventory values can either increase or decrease the net farm income for the year. Changes in the values of inventories of feed and grain, market livestock, and breeding livestock can result from increases or decreases in the quantity of these items on hand or changes in their unit values (see Example 1). Adjusting for inventory changes ensures that the value of farm products is counted in the year they are produced rather than the year they are sold. Subtract beginning of the year values from end of the year values to find the net adjustment.
Changes in the market values of land, buildings, machinery, and equipment (except for depreciation) are not included in the income statement unless they are actually sold. Accounts receivable and unpaid patronage dividends are included, however, because they reflect income that has been earned but not yet received.
Cash Expenses
All cash expenses involved in the operation of the farm business during the business year should be entered into the expense section of the income statement. These can come from Part II of IRS Schedule F. Under livestock purchases include the value of breeding livestock as well as market animals.
- Do not include death loss of livestock as an expense. This will be reflected automatically by a lower ending livestock inventory value.
- Income tax and Social Security tax payments are considered personal expenses and should not be included in the farm income statement, unless the statement is for a farm corporation.
- Interest paid on all farm loans or contracts is a cash expense, but principal payments are not.
- Do not include the purchase of capital assets such as machinery and equipment. Their cost is accounted for through depreciation. Land purchases also are excluded.
- You may wish to exclude wages paid to family members, because these also are income to the family.
- Include cash deposits made to hedging accounts.
Adjustments to Expenses
Some cash expenses paid in one year may be for items not actually used until the following year. These include feed and supply inventories, prepaid expenses, and investments in growing crops. Subtract the ending value of these from the beginning value to find the net adjustment (see Example 2).
Other expenses may be incurred in one year but not paid until the following year or later, such as farm taxes due, and other accounts payable. Record accounts payable so that products or services that have been purchased but not paid for are counted. However, do not include any items that already appear under cash expenses. Subtract the beginning total of these items from the ending totals to find the net adjustment. Note that interest expense due is not included until later, after net farm income from operations is calculated.
Depreciation is the amount by which machinery, equipment, buildings, and other capital assets decline in value due to use and obsolescence. The depreciation deduction allowed on your income tax return can be used, but you may want to calculate your own estimate based on more realistic depreciation rates. One simple procedure is to multiply the value of these assets at the end of the year by a fixed rate, such as 10%. This way you can group similar items, such as machinery, rather than maintain separate records for each item.
If you include breeding livestock under beginning and ending inventories, do not include any depreciation expense for them.
The beginning and ending net worth statements for the farm are a good source of information about inventory values and accounts payable and receivable. ISU Extension and Outreach publication FM 1791/AgDM C3-20, Your Net Worth Statement, provides more detail on how to complete a net worth statement. ISU Extension and Outreach publication FM 1824/AgDM C3-56, Farm Financial Statements contains schedules for listing adjustment items for both income and expenses. Use the same values that are shown on your beginning and ending net worth statements for completing adjustments to your net income statement for the year.
Summarizing the Statement
You have now accounted for cash farm income and cash expenses (excluding interest). You also have accounted for depreciation and changes in inventory values of farm products, accounts payable, and prepaid expenses. You are now ready to summarize two measures of farm income.
Net Farm Income from Operations
Subtract total farm expenses from gross farm revenue. The difference is the net income generated from the ordinary production and marketing activities of the farm, or net farm income from operations.
Interest Expense
Interest is considered to be the cost of financing the farm business rather than operating it. Net interest expense is equal to cash interest expense adjusted for beginning and ending accrued interest.
Capital Gains and Losses
Some years income is received from the sale of capital assets such as land, machinery, and equipment. The sale price may be either more or less than the cost value (or basis) of the asset.
For depreciable items the cost value is the original value minus the depreciation taken. For land it is the original value plus the cost of any nondepreciable improvements made. The difference between the sale value and the cost value is a capital gain or loss. For purposes of the farm income statement, capital gain would also include the value of “recaptured depreciation” from the farm tax return. Information for calculating capital gains and losses can come from the depreciation schedule and/or IRS Form 4797.
Sales of breeding livestock can be handled two ways: (1) record sales and purchases as cash income and expenses, and adjust for changes in inventory, or (2) record capital gains or losses when animals are sold and include depreciation as an expense. Either method can be used, but do not mix them.
Net Farm Income
Subtract interest expense, then add capital gains or subtract capital losses from net farm income from operations to calculate net farm income. This represents the income earned by the farm operator’s own capital, labor, and management ability. It also represents the value of everything the farm produced during the year, minus the cost of producing it.
Further Analysis
Net farm income is an important measure of the profitability of your farm business. Even more can be learned by comparing your results with those for other similar farms. ISU Extension and Outreach publication FM 1845/AgDM C3-55, Financial Performance Measures for Iowa Farms, contains information about typical income levels generated by Iowa farms. It also illustrates other important measures and ratios that can help you evaluate the profitability, liquidity, and solvency of your own business over time.
Other Financial Statements
Two other financial statements are often used to summarize the results of a farm business. While they are not as common as the net income statement and the net worth statement, they do provide useful financial information.
Statement of Cash Flows
A statement of cash flows summarizes all the cash receipts and cash expenditures that were received or paid out during the accounting year. It is sometimes called a flow of funds statement. Unlike the net income statement, it does not measure the profitability of the business. It merely shows the sources and uses of cash. The statement of cash flows is divided into five sections:
- cash income and cash expenses
- purchases and sales of capital assets
- new loans received and principal repaid
- nonfarm income and expenses (sole proprietor)
- beginning and ending cash on hand
If all cash flows are accurately recorded, the total sources of cash will be equal to the total uses of cash. If a significant difference exists, the records should be carefully reviewed for errors and omissions.
An example of a statement of cash flows is found at the end of this publication, along with a blank form.
Statement of Owner Equity
The statement of owner equity ties together net farm income and the change in net worth. Net worth will increase or decrease during the accounting year based on three factors:
- net farm income (accrual)
- net nonfarm withdrawals (nonfarm income minus nonfarm expenditures)
- adjustments to the market value of capital assets (affects market value net worth, only)
If these factors are recorded accurately and added to the beginning net worth of the farm, the result will equal the ending net worth.
Attributions
"Your Net Worth Statement" by William Edwards, Iowa State University Extension and Outreach. Copyright © ISU Extension and Outreach. Used with permission.
"Your Farm Income Statement" by William Edwards, Iowa State University Extension and Outreach. Copyright © ISU Extension and Outreach. Used with permission.
How is Interest Computed?
Excerpt used with permission from "Understanding the Time Value of Money" by Don Hofstrand,, Iowa State University Extension and Outreach Copyright © ISU Extension and Outreach.
Learning Objectives
4f Analyze financial statements.
4l Compute Interest.
Understanding the Time Value of Money
If I offered to give you $100, you would probably say yes. Then, if I asked you if you wanted the $100 today or one year from today, you would probably say today. This is a rational decision because you could spend the money now and get the satisfaction from your purchase now rather than waiting a year. Even if you decided to save the money, you would rather receive it today because you could deposit the money in a bank and earn interest on it over the coming year. So there is a time value to money.
Next, let’s discuss the size of the time value of money. If I offered you $100 today or $105 dollars a year from now, which would you take? What if I offered you $110, $115, or $120 a year from today? Which would you take? The time value of money is the value at which you are indifferent to receiving the money today or one year from today. If the amount is $115, then the time value of money over the coming year is $15. If the amount is $110, then the time value is $10. In other words, if you will receive an additional $10 a year from today, you are indifferent to receiving the money today or a year from today.
When discussing the time value of money, it is important to understand the concept of a time line. Time lines are used to identify when cash inflows and outflows will occur so that an accurate financial assessment can be made. A time line is shown below with five time periods. The time periods may represent years, months, days, or any length of time so long as each time period is the same length of time. Let’s assume they represent years. The zero tick mark represents today. The one tick mark represents a year from today. Any time during the next 365 days is represented on the time line from the zero tick mark to the one tick mark. At the one tick mark, a full year has been completed. When the second tick mark is reached, two full years have been completed, and it represents two years from today. Move on to the five tick mark, which represents five years from today.
Because money has a time value, it gives rise to the concept of interest. Interest can be thought of as rent for the use of money. If you want to use my money for a year, I will require that you pay me a fee for the use of the money. The size of the rental rate or user fee is the interest rate. If the interest rate is 10 percent, then the rental rate for using $100 for the year is $10.
Compounding
Compounding is the impact of the time value of money (e.g., interest rate) over multiple periods into the future, where the interest is added to the original amount. For example, if you have $1,000 and invest it at 10 percent per year for 20 years, its value after 20 years is $6,727. This assumes that you leave the interest amount earned each year with the investment rather than withdrawing it. If you remove the interest amount every year, at the end of 20 years the $1,000 will still be worth only $1,000. But if you leave it with the investment, the size of the investment will grow exponentially. This is because you are earning interest on your interest. This process is called compounding. And, as the amount grows, the size of the interest amount will also grow. As shown in Table 2.2.10a, during the first year of a $1,000 investment, you will earn $100 of interest if the interest rate is 10 percent. When the $100 interest is added to the $1,000 investment it becomes $1,100 and 10 percent of $1,100 in year two is $110. This process continues until year 20, when the amount of interest is 10 percent of $6,116 or $611.60. The amount of the investment is $6,727 at the end of 20 years.
| Year | Amount | Computation |
|---|---|---|
| 0 | $1,000 | |
| 1 | $1,100 | $1,000 x 10% = $100 + $1,000 = $1,100 |
| 2 | $1,210 | $1,100 x 10% = $110 + $1,100 = $1,210 |
| 3 | $1,331 | $1,210 x 10% = $121 + $1,210 = $1,331 |
| 4 | $1,464 | $1,331 x 10% = $133 + $1,331 = $1,464 |
| 5 | $1,611 | $1,464 x 10% = $146 + $1,464 = $1,611 |
| 6 | $1,772 | $1,611 x 10% = $161 + $1,611 = $1,772 |
| 7 | $1,949 | $1,772 x 10% = $177 + $1,772 = $1,949 |
| 8 | $2,144 | $1,949 x 10% = $195 + $1949 = $2,144 |
| 9 | $2,358 | $2,144 x 10% = $214 + $2,144 = $2,358 |
| 10 | $2,594 | $2,358 x 10% = $236 + $2,358 = $2,594 |
| 11 | $2,853 | $2,594 x 10% = $259 + $2,594 = $2,853 |
| 12 | $3,138 | $2,853 x 10% = $285 + $2,853 = $3,138 |
| 13 | $3,452 | $3,138 x 10% = $314 + $3,138 = $3,452 |
| 14 | $3,797 | $3,452 x 10% = $345 + $3,452 = $3,797 |
| 15 | $4,177 | $3,797 x 10% = $380 + $3,797 = $4,177 |
| 16 | $4,595 | $4,177 x 10% = $418 + $4,177 = $4,595 |
| 17 | $5,054 | $4,595 x 10% = $456 = $4,595 = $5,054 |
| 18 | $5,560 | $5,054 x 10% = $505 + $5,054 = $5,560 |
| 19 | $6,116 | $5,560 x 10% = $556 + $5,560 = $6,116 |
| 20 | $6,727 | $6,116 x 10% = $612 + $6,116 = $6,727 |
The impact of compounding outlined in Table 2.2.10a is shown graphically in Figure 2.2.10b. The increase in the size of the cash amount over the 20-year period does not increase in a straight line but rather exponentially. Because the slope of the line increases over time it means that each year the size of the increase is greater than the previous year. If the time period is extended to 30 or 40 years, the slope of the line would continue to increase. Over the long-term, compounding is a very powerful financial concept.
The effect of compounding is also greatly impacted by the size of the interest rate. Essentially, the larger the interest rate the greater the impact of compounding. In addition to the compounding impact of a 10 percent interest rate, Figure 2.2.10c shows the impact of a 15 percent interest rate (5 percent higher rate) and 5 percent (5 percent lower rate). Over the 20-year period, the 15 percent rate yields almost $10,000 more than the 10 percent rate, while the 5 percent rate results in about $4,000 less. By examining the last 10 years of the 20-year period you can see that increasing the number of time periods and the size of the interest rate greatly increases the power of compounding.
Table 2.2.10b shows the same compounding impact as Table 2.2.10a but with a different computational process. An easier way to compute the amount of compound interest is to multiply the investment by one plus the interest rate. Multiplying by one maintains the cash amount at its current level and .10 adds the interest amount to the original cash amount. For example, multiplying $1,000 by 1.10 yields $1,100 at the end of the year, which is the $1,000 original amount plus the interest amount of $100 ($1,000 x .10 = $100).
| Year | Amount | Computation |
|---|---|---|
| 0 | $1,000 | |
| 1 | $1,100 | $1,000 x 1.10 = $1,100 |
| 2 | $1,210 | $1,100 x 1.10 = $1,210 |
| 3 | $1,331 | $1,210 x 1.10 = $1,331 |
| 4 | $1,464 | $1,331 x 1.10 = $1,464 |
| 5 | $1,611 | $1,464 x 1.10 = $1,611 |
| 6 | $1,772 | $1,611 x 1.10 = $1,772 |
| 7 | $1,949 | $1,772 x 1.10 = $1,949 |
| 8 | $2,144 | $1,949 x 1.10 = $2,144 |
| 9 | $2,358 | $2,144 x 1.10 = $2,358 |
| 10 | $2,594 | $2,358 x 1.10 = $2,594 |
| 11 | $2,853 | $2,594 x 1.10 = $2,853 |
| 12 | $3,138 | $2,853 x 1.10 = $3,138 |
| 13 | $3,452 | $3,138 x 1.10 = $3,452 |
| 14 | $3,797 | $3,452 x 1.10 = $3,797 |
| 15 | $4,177 | $3,797 x 1.10 = $4,177 |
| 16 | $4,595 | $4,177 x 1.10 = $4,595 |
| 17 | $5,054 | $4,595 x 1.10 = $5,054 |
| 18 | $5,560 | $5,054 x 1.10 = $5,560 |
| 19 | $6,116 | $5,560 x 1.10 = $6,116 |
| 20 | $6,727 | $6,116 x 1.10 = $6,727 |
Another dimension of the impact of compounding is the number of compounding periods within a year. Table 2.2.10c shows the impact of 10 percent annual compounding of $1,000 over 10 years. It also shows the same $1,000 compounded semiannually over the 10-year period. Semiannual compounding means a 10 percent annual interest rate is converted to a 5 percent interest rate and charged for half of the year. The interest amount is then added to the original amount, and the interest during the last half of the year is 5 percent of this larger amount. As shown in Table 2.2.10c, semiannual compounding will result in 20 compounding periods over a 10-year period, while annual compounding results in only 10 compounding period. A shorter compounding period means a larger number of compounding periods over a given time period and a greater compounding impact. If the compounding period is shortened to monthly or daily periods, the compounding impact will be even greater.
| Compounding Annually | Compounding Semiannually | |||
|---|---|---|---|---|
| Year | Amount | Computation | Amount | Computation |
| 0 | $1,000 | $1,000 | ||
| 0.5 | $1,050 | $1,000 x 1.05 = $1,050 | ||
| 1 | $1,100 | $1,000 x 1.10 = $1,100 | $1,103 | $1,050 x 1.05 = $1,103 |
| 1.5 | $1,158 | $1,103 x 1.05 = $1,158 | ||
| 2 | $1,210 | $1,100 x 1.10 = $1,210 | $1,216 | $1,158 x 1.05 = $1,216 |
| 2.5 | $1,276 | $1,216 x 1.05 = $1,276 | ||
| 3 | $1,331 | $1,210 x 1.10 = $1,331 | $1,340 | $1,276 x 1.05 = $1,340 |
| 3.5 | $1,407 | $1,340 x 1.05 = $1,407 | ||
| 4 | $1,464 | $1,331 x 1.10 = $1,464 | $1,477 | $1,407 x 1.05 = $1,477 |
| 4.5 | $1,551 | $1,477 x 1.05 = $1,551 | ||
| 5 | $1,611 | $1,464 x 1.10 = $1,611 | $1,629 | $1,551 x 1.05 = $1,629 |
| 5.5 | $1,710 | $1629 x 1.05 = $1,710 | ||
| 6 | $1,772 | $1,611 x 1.10 = $1,772 | $1,796 | $1,710 x 1.05 = $1,796 |
| 6.5 | $1,886 | $1,796 x 1.05 = $1,886 | ||
| 7 | $1,949 | $1,772 x 1.10 = $1,949 | $1,980 | $1,866 x 1.05 = $1,980 |
| 7.5 | $2,079 | $1,980 x 1.05 = $2,079 | ||
| 8 | $2,144 | $1,949 x 1.10 = $2,144 | $2,183 | $2,079 x 1.05 = $2,183 |
| 8.5 | $2,292 | $2,183 x 1.05 = $2,292 | ||
| 9 | $2,358 | $2,144 x 1.10 = $2,358 | $2,407 | $2,202 x 1.05 = $2,407 |
| 9.5 | $2,527 | $2,407 x 1.05 = $2,527 | ||
| 10 | $2,594 | $2,358 x 1.10 = $2,594 | $2,653 | $2,527 x 1.05 = $2,653 |
Discounting
Although the concept of compounding is straight forward and relatively easy to understand, the concept of discounting is more difficult. However, the important fact to remember is that discounting is the opposite of compounding. As shown below, if we start with a future value of $6,727 at the end of 20 years in the future and discount it back to today at an interest rate of 10 percent, the present value is $1,000.
| Year | Amount | Computation |
|---|---|---|
| 20 | $6,727 | |
| 19 | $6,116 | $6,727 x .91 = $6,116 |
| 18 | $5,560 | $6,116 x .91 = $5,560 |
| 17 | $5,054 | $5,560 x .91 = $5,054 |
| 16 | $4,595 | $5,054 x .91 = $4,595 |
| 15 | $4,177 | $4,595 x .91 = $4,177 |
| 14 | $3,797 | $4,177 x .91 = $3,797 |
| 13 | $3,452 | $3,797 x .91 = $3,452 |
| 12 | $3,138 | $3,452 x .91 = $3,138 |
| 11 | $2,853 | $3,138 x .91 = $2,853 |
| 10 | $2,594 | $2,853 x .91 = $2,594 |
| 9 | $2,358 | $2,594 x .91 = $2,358 |
| 8 | $2,144 | $2,358 x .91 = $2,144 |
| 7 | $1,949 | $2,144 x .91 = $1,949 |
| 6 | $1,772 | $1,949 x .91 = $1,772 |
| 5 | $1,611 | $1,772 x .91 = $1,611 |
| 4 | $1,464 | $1,611 x .91 = $1,464 |
| 3 | $1,331 | $1,464 x .91 = $1,331 |
| 2 | $1,210 | $1,331 x .91 = $1,210 |
| 1 | $1,100 | $1,210 x .91 = $1,100 |
| 0 | $1,000 | $1,100 x .91 = $1,000 |
As shown in Table 2.2.10b, the compounding factor of annually compounding at an interest rate of 10 percent is 1.10 or 1.10/1.00. If discounting is the opposite of compounding, then the discounting factor is 1.00/1.10 = .90909 or .91. As shown in Table 2.2.10d, the discounted amount becomes smaller as the time period moves closer to the current time period. When we compounded $1,000 over 20 years at a 10 percent interest rate, the value at the end of the period is $6,727 (Table 2.2.10b). When we discount $6,727 over 20 years at a 10 percent interest rate, the present value or value today is $1,000.
The discounting impact is shown in Figure 2.2.10d. Note that the curve is the opposite of the compounding curve in Figure 2.2.10b.
The impact of discounting using interest rates of 5 percent, 10 percent, and 15 percent is shown in Figure 2.2.10e. The 15 percent interest rate results in a larger discounting impact than the 10 percent rate, just as the 15 percent interest rate results in a larger compounding impact as shown in Figure 2.2.10b.
Discounting Example
An example of discounting is to determine the present value of a bond. A bond provides a future stream of income. It provides a cash return at a future time period, often called the value at maturity. It may also provide a stream of annual cash flows until the maturity of the bond. Table 2.2.10e shows an example of a $10,000 bond with a 10-year maturity. In other words, the bond will yield $10,000 at maturity, which is received at the end of 10 years. The bond also has an annual annuity (an annuity is a stream of equal cash payments at regular time intervals for a fi xed period of time) equity to 10 percent of the value at maturity. So, the bond yields 10 $1,000 (10% x $10,000) annual payments over the 10-year period. Adding together the 10 $1,000 payments plus the $10,000 value at maturity, the future cash return from the bond is $20,000.
| Year | Annuity | Value at Maturity | Total |
|---|---|---|---|
| 0 | |||
| 1 | $1,000 | $1,000 | |
| 2 | $1,000 | $1,000 | |
| 3 | $1,000 | $1,000 | |
| 4 | $1,000 | $1,000 | |
| 5 | $1,000 | $1,000 | |
| 6 | $1,000 | $1,000 | |
| 7 | $1,000 | $1,000 | |
| 8 | $1,000 | $1,000 | |
| 9 | $1,000 | $1,000 | |
| 10 | $1,000 | $10,000 | $10,000 |
| Total | $10,000 | $10,000 | $20,000 |
To compute the current value of the bond, we must discount the future cash flows back to the time when the bond is purchased. To do this we must select an interest rate (called the discount rate when we are discounting).
In Table 2.2.10f, we have calculated the present value of the bond using discount rates of 5 percent, 10 percent, and 15 percent. First, let’s examine the computation using a 5 percent rate. Each of the $1,000 annuity payments is discounted to the present value. Note that the one year $1,000 annuity payment has a present value of $952 and the 10-year payment has a present value of $614. This is because the first-year payment is only discounted one time and the tenth-year payment is discounted 10 times over 10 years. The present value of all 10 annuity payments is $7,722. The present value of the $10,000 at maturity (after 10 years) is $6,139. Note that the present value of the $10,000 of annual annuity payments is greater than the $10,000 payment received at maturity because most of the annuity payments are discounted over time periods less than 10 years. The total present value of the annuity and the value at maturity is $13,861. So, the $20,000 of future cash payments has a value at the time of purchase of $13,861. Looking at it from a different perspective, if you paid $13,861 for this bond you would receive a 5 percent annual rate of return (called the internal rate of return) over the 10-year period.
| Year | Annuity | Value at Maturity | Total |
| Discount Rate = 5% | |||
| 0 | |||
| 1 | $952 | $952 | |
| 2 | $907 | $907 | |
| 3 | $864 | $864 | |
| 4 | $823 | $823 | |
| 5 | $784 | $784 | |
| 6 | $746 | $746 | |
| 7 | $711 | $711 | |
| 8 | $677 | $677 | |
| 9 | $645 | $645 | |
| 10 | $614 | $6,139 | $6,753 |
| Total | $7,722 | $6,139 | $13,861 |
If we increase the discount rate from 5 percent to 10 percent, the discounting power becomes greater. The present value of the bond drops from $13,861 to $10,000. In other words, if you want a 10 percent rate of return you can only pay $10,000 for the bond that will generate $20,000 in future cash payments. Note that the value at maturity dropped over $2,000 from $6,139 to $3,855. Conversely, the value of the annuity dropped from $7,722 to $6,145, a reduction of about $1,600.
| Year | Annuity | Value at Maturity | Total |
| Discount Rate = 10% | |||
| 0 | |||
| 1 | $909 | $909 | |
| 2 | $826 | $826 | |
| 3 | $751 | $751 | |
| 4 | $683 | $683 | |
| 5 | $621 | $621 | |
| 6 | $564 | $564 | |
| 7 | $513 | $513 | |
| 8 | $467 | $467 | |
| 9 | $424 | $424 | |
| 10 | $386 | $3,855 | $4,241 |
| Total | $6,145 | $3,855 | $10,000 |
If we increase the discount rate to 15 percent, the discounting power becomes even greater. The present value of the bond drops to $7,491. In other words, if you want a 15 percent rate of return you can only pay $7,491 for the bond that will generate $20,000 in future cash payments. Note that the value at maturity dropped from $6,139 (5 percent) to $3,855 (10 percent) to $2,472 (15 percent). The value of the annuity dropped in smaller increments from $7,722 (5 percent) to $6,145 (10 percent) to $5,019 (15 percent).
| Year | Annuity | Value at Maturity | Total |
| Discount Rate = 10% | |||
| 0 | |||
| 1 | $870 | $870 | |
| 2 | $756 | $756 | |
| 3 | $658 | $658 | |
| 4 | $572 | $572 | |
| 5 | $497 | $497 | |
| 6 | $432 | $432 | |
| 7 | $376 | $376 | |
| 8 | $327 | $327 | |
| 9 | $284 | $284 | |
| 10 | $247 | $2,472 | $2,719 |
| Total | $5,019 | $2,472 | $7,491 |
A bond is a simple example of computing the present value of an asset with an annual cash income stream and a terminal value at the end of the time period. This methodology can be used to analyze any investment that has an annual cash payment and a terminal or salvage value at the end of the time period.
Perpetuity
A perpetuity is similar to an annuity except that an annuity has a limited life and a perpetuity is an even payment that has an unlimited life. The computation of a perpetuity is straight forward. The present value of a perpetuity is the payment divided by the discount rate.
Time Value of Money Formulas
There are mathematical formulas for compounding and discounting that simplify the methodology. At right are the formulas, in which:
- “PV” represents the present value at the beginning of the time period.
- “FV” represents the future value at the end of the time period.
- “N” or “Nper” represents the number of compounding or discounting periods. It can represent a specific number of years, months, days, or other predetermined time periods.
- “Rate” or “i” represents the interest rate for the time period specified. For example, if “N” represents a specified number of years, then the interest rate represents an annual interest rate. If “N” represents a specific number of days, then the interest rate represents a daily interest rate.
If we are computing the compounded value of a current amount of money into the future, we will use the following formula. The future value “FV” that we are solving for is the current amount of money “PV” multiplied by one plus the interest rate to the power of the number of compounding periods. We are solving for the future compounded value (FV), in which the present value (PV) is $1,000, the annual interest rate (Rate) is 10 percent, and the number of time periods (Nper) is 20 years. This results in $1,000 multiplied by 6.727 and a future value of $6,727. Note that this is the same result as shown in Tables 2.2.10a and 2.2.10b.
To compute the discounted value of an amount of money to be received in the future, we use the same formula but solve for the present value rather than the future value. To adjust our formula, we divided both sides by (1 + Rate) Nper and the following formula emerges.
The present value (PV) of a future value (FV) of $6,727 discounted over 20 years (Npers) at an annual discount interest rate (Rate) of 10 percent is $1,000, the same as shown in Table 2.2.10d.
Time Value of Money Computation
A financial calculator or an electronic spreadsheet on a personal computer is a useful tool for making time value of money computations. For compounding computations, you enter the present value, interest rate, and the number of time periods, and the calculator or personal computer will compute the future value. The future value for the example below is $6,727, the same as the future value shown in Tables 2.2.10a and 2.2.10b.
Likewise, for discounting computations you enter the future value, interest rate, and the number of time periods, and the calculator or personal computer will compute the present value. The present value for the example below is $1,000, the same as the present value shown in Table 2.2.10d.
If an annuity is involved, you can use the payment function (PMT). In the example below, the present value is $10,000, the same as the present value of the bond example in Table 2.2.10f.
If an annuity is involved, you can use the payment function (PMT). In the example below, the present value is $10,000, the same as the present value of the bond example in Table 2.2.10f.
By using a financial calculator or spreadsheet, any of the values in the examples above can be computed as long as the other values are known. For example, the interest rate can be computed if the future value, present value, and number of time periods are known. The numbef or time periods can be computed if the present value, future value, and interest rate are known. The same is true if an annuity is involved.
Attributions
"Understanding the Time Value of Money" by Don Hofstrand, Iowa State University Extension and Outreach. Copyright © ISU Extension and Outreach. Used with permission.
The Five Cs of Credit
Learning Objectives
4j Discuss the importance of credit.
Five Cs of Credit Analysis
The Five Cs of Credit Analysis is an informal mnemonic of a set of risk factors that are commonly thought to be influential in determining the credit quality of a commercial borrower.
In alphabetical order, the Five Cs are commonly considered to be the following dimensions (with some variations in naming):
- Capacity
- Capital
- Character
- Collateral
- Conditions
Capacity
Capacity (sometimes replaced by Cashflow) refers to a borrower's ability to repay their debt, on the basis of their projected income profile and their other expenditures (including other debt). The key metrics used in evaluating credit capacity are ratios such as Debt to Income Ratio (DTI)—the balance between debt and income—or the Debt Service Coverage Ratio—a financial metric used to assess an entity's ability to generate enough cash to cover its debt service obligations. For individual borrowers, current income and employment history are good indicators of ability to repay outstanding debt. Income amount, stability over time, and type of income are important attributes. For corporate borrowers, important attributes include sources of revenue and profitability over time (such as that captured in net operating income).
In all cases there are two important approaches to evaluating capacity:
- historical ability to service the debt: such as that based on recent years cashflow metrics and compared to projected debt service,
- and projected ability to service debt: based on projected cashflows. Projected cashflows may be more faithful to evolving reality if they incorporate such things as a new project or new employment status. But they may also be subject to more uncertainty.
For longer maturity debt simple ratios may not be accurate indicators of credit capacity if the Contractual Cash Flows of the debt instrument are heavily skewed towards later periods. Contractual Cash Flows consist of the money exchanged between the parties who have signed a legal contract.
It is typically assumed that, all else being equal, a higher repayment capacity implies less credit risk.
Capital
Capital refers in general to the asset base (net worth) of the borrower and the degree to which it is committed to support a given amount of debt. When the borrowing concerns a specific project, capital refers the equity (own means) that the borrower invests in the project. For example, the down-payment on a mortgage for homeowners or the equity funds committed by a commercial borrower.
Capital influences credit risk in two ways:
- It provides a buffer in case income (cashflow) deteriorates.
- And it aligns the interests of the borrower with that of the lender.
A relative metric that captures the Capital dimension (typically used for corporate borrowers) is the Debt to Equity Ratio.
It is typically assumed that all else being equal a higher capital base committed implies less risk.
Character
Character refers to a borrower's overall behavioral profile towards repayment of debt. This assessment is made in relation to the dominant Credit Culture in a given jurisdiction and economic region. Credit Culture is the attitudes, beliefs, and behaviors that surround the use of credit in a society.
The assessment of credit character entails (in principle) both subjective and objective elements. Subjective elements require that the assessor (credit officer) has intimate knowledge of the borrower and may draw on qualitative arguments. Qualitative inputs may involve soliciting feedback from such entities as peers, community, clients, vendors that have had economic relationships with the borrower in the past.
In contrast objective elements do not require special insights and are more quantitative in nature. Objective inputs to the assessment of character include a borrower's Credit History, which provides evidence of past economic activities and also reflects the quality of the borrower's management ability. Credit History is a record of a person's or company's borrowing and repaying of money over time.
It is typically assumed that all else being equal a better character profile implies less credit risk.
Collateral
Collateral refers to any assets the borrower pledges as security for their borrowed funds. Assets may be financial in nature (such as securities) or real assets (such as real estate). It is the only one of the 5 Cs that is actually optional (as depending on the lending product, there might be no collateral pledged).
Collateral can be used in general Secured Lending, which involves business or personal loans that require some type of collateral as a condition of borrowing. But collateral is especially common in the financing of specific assets such as houses or automobiles for individuals, commercial real estate, and transport equipment for commercial borrowers. A key metric capturing the impact of collateral is the Loan to Value Ratio—the percentage of the value of a property that is borrowed as a loan.
It is typically assumed that, all else being equal, more collateral leads to lower realized losses in the case of a Default Event, which leads to a lower Loss Given Default—the share of an asset that is lost if a borrower defaults.
Conditions
Conditions is maybe the least well defined of the 5 Cs as it refers potentially on several distinct and unrelated aspects:
- the intended purpose of the loan (such as consumption or investment)
- the size of the loan and the interest rate (expressing the lender's Risk Appetite—the amount and type of risk that an organization is willing to take or retain in order to meet their strategic objectives)
- the relevant business and economic conditions (such as borrower specific, sectoral outlook, local economy, broader economy)
In general, using the funds for investment in a positive external environment would imply lower risk.
Usage
The Five Cs are typically used, explicitly or implicitly, in the construction of Credit Scorecards, with the significance of each "C" being assigned either subjectively or quantitatively.
Attributions
"Five Cs Of Credit Analysis" by Open Risk Manual contributors, Open Risk Manual is licensed under CC BY-NC-SA 4.0
Credit and Risk
Excerpt used with permission from "Financing Your Farm Business or Enterprise" by Robert E. Mikesell, Lynn Kime, PennState Extension. Copyright © PennState Extension.
Learning Objectives
4k Discuss the importance of returns, repayment ability, and risk.
Creditors and Lenders
In order to provide goods and services to their customers, businesses make purchases from other businesses. These purchases come in the form of materials used to make finished goods or resell, office equipment such as copiers and telephones, utility services such as heating and cooling, and many other products and services that are vital to run the business efficiently and effectively.
It is rare that payment is required at the time of the purchase or when the service is provided. Instead, businesses usually extend “credit” to other businesses. Selling and purchasing on credit means the payment is expected after a certain period of time, following receipt of the goods or provision of the service. The term creditor refers to a business that grants extended payment terms to other businesses. The time frame for extended credit to other businesses for purchases of goods and services is usually very short, typically thirty-day to forty-five-day periods are common.
When businesses need to borrow larger amounts of money and/or for longer periods of time, they will often borrow money from a lender, a bank or other institution that has the primary purpose of lending money with a specified repayment period and stated interest rate. If you or your family own a home, you may already be familiar with lending institutions.
The time frame for borrowing from lenders is typically measured in years rather than days, as was the case with creditors. While lending arrangements vary, typically the borrower is required to make periodic, scheduled payments with the full amount being repaid by a certain date. In addition, since the borrowing is for a long period of time, lending institutions require the borrower to pay a fee (called interest) for the use of borrowing.
Both creditors and lenders use financial information to make decisions. The ultimate decision that both creditors and lenders have to make is whether or not the funds will be repaid by the borrower. The reason this is important is because lending money involves risk. The type of risk creditors and lenders assess is repayment risk—the risk the funds will not be repaid. As a rule, the longer the money is borrowed, the higher the risk involved.
Recall that accounting information is historical in nature. While historical performance is no guarantee of future performance (repayment of borrowed funds, in this case), an established pattern of financial performance using historical accounting information does help creditors and lenders to assess the likelihood the funds will be repaid, which, in turn, helps them to determine how much money to lend, how long to lend the money for, and how much interest (in the case of lenders) to charge the borrower.
Creditor | Lender |
|
|
Credit Score
Before applying for any loan be sure to check your credit score. This includes checking your credit history with all three of the major credit-reporting agencies—Experian, Equifax and TransUnion. Credit score information can be accessed from each agency once annually free of charge, but this service will not include an actual credit score, so check one every four months to catch inaccuracies as soon as possible. Contact the credit reporting agencies immediately about anything on the report that you do not agree with. Keep in mind that scores "purchased" online do not always accurately reflect what the bank sees when it pulls your credit history.
Financing
The type of financing you apply for will depend on the type of farm you are going to buy or enterprise you are considering, as well as your specific needs and circumstances. If you are considering a new business, you will require business financing and at least a rudimentary business plan, which should include:
- a brief narrative that describes your farm business plan and your experience to carry it off
- projected income from each enterprise (For example, stall rent—X number of horses at Y dollars a month each, land or barn rental, or crop income—X acres of corn at Y bushels per acre at Z price.)
- a thorough expense sheet for each enterprise
- any existing business or "projected income" that will remain (It helps to have obtained the seller's tax return to validate actual revenue history).
A thorough business plan that shows income to offset the expenses will strengthen the loan application. Providing as much detail as possible will only strengthen your application so be sure to include both income and expenses, as well as potential profit or lack of profit for the first year. Any off-farm household income that can support loan payments also needs to be included and proven by recent tax returns. You will be required to submit a balance sheet showing all assets and liabilities as a portion of your application.
Most agricultural lenders know that a farm business or enterprise may not be profitable the first year, so having three years of projections will tell the lender when they may expect to see income that will support the loan payments.
Different types of farm loans are set up for different types of farming operations. Be sure to match the loan type and repayment structure to the use of the funds. Main choices are operating loans, term loans, and residential loans.
Operating loans usually provide a line of credit to be paid back (typically at prime rate) once the farmer receives revenue, such as for a crop. These loans are designed to be used on crop or livestock production expenses and repaid each year. You may need to obtain annual operating loans, but this will strengthen your credit and relationship with the lender. An operating loan would typically be used to begin a new enterprise.
Term loans are usually for infrastructure and building improvements. Most banks match the length of the loan to the projected depreciation value or life expectancy of the purchase. For example, a loan for a tractor may have a seven-year term, while real estate may have a twenty to thirty-year repayment schedule.
Residential loans usually have a lower interest rate and are easier to obtain than business loans and may be the way to go if the farm is being purchased primarily for residential purposes with farming as a sideline. If the farm is residential but also commercial then the loan is considered commercial. The required down payment for these types of loans is generally 20 percent, but other lending sources may provide funding for the initial down payment.
Cost of a Loan
A good rule of thumb for figuring how much a loan will cost a borrower in the long run is to consider that a 20-year loan will roughly double the initial cost of that loan. For instance, an initial $100,000 loan will ultimately cost the borrower about $200,000, while a 30-year payback will approximately triple the initial loan amount. While the actual repayment time and cost will fluctuate with interest rates, the above gives a pretty good example of the differences in actual costs associated with typical mortgage loan life spans. You may consider starting with a longer-term loan to keep payments low during the start-up period then refinance when a more predictable and steady income is achieved.
When it comes to loans for farming operations, many choices exist with an even greater number of possibilities. The larger your initial down payment at the time of establishing the loan, the lower your repayment and interest rate will be. The more "skin you have in the game" the less risk the lender is assuming.
Conventional Financing
Beginning farmers who have some equity may qualify for financing from traditional lenders. Undoubtedly, conventional financing through local commercial banks or through the Farm Credit system is the simplest, most straight-forward route. These institutions carry a variety of financial products, but often will not finance more than 80% of a farming venture's start-up cost. Many times loans can and will be combined with funding from various small business administrations, economic development organizations, and funding agencies. Most loan officers should be able to steer you in the right direction for these opportunities. But there are other options for the remainder of the down payment, such as USDA assistance.
US Department of Agriculture (USDA) Financing
The Federal government has noted the ageing demographics of our agricultural producers and offers a variety of affordable financing options to help new producers get started. Farm Service Agency (FSA)—the branch of USDA that distributes and services farm loans—provides a step-by-step approach and resources to plan and finance a farming career. They offer many services to farmers above the loan programs and should be a stop on your farm financing journey.
FSA can provide both farm ownership and operating loans for both new and established producers. Additionally, FSA can co-fund with, or guarantee loans from, conventional lenders. As with any government program, the application process for farm ownership and large operating loans can be a bit cumbersome. FSA also offers a Microloan program for farm ownership or operating expenses, with a $50,000 limit. Microloans have a streamlined application process and can be a nice fit for many producers. You can now apply for two microloans for a total of $100,000: one for land and one for equipment or operating expenses. Another benefit of the Microloan program is that the item being purchased with the loan may be used as collateral.
Other Financing Options
Sometimes new farmers can align themselves with a retiring farmer who agrees to finance the operation until the new farm gets on solid financial footing. PA Farm Link is one such organization that helps align established farmers with new ones.
Final Thoughts
Navigating the financing challenges is just one hurdle for new and beginning farmers, but it is among the more important. There are lots of resources out there but accessing them and determining which ones are applicable in each situation requires some study. Just be careful to work with someone who understands what you are planning. Agriculture is a unique industry and requires some understanding of production cycles and potential hazards. When drafting your business plan be sure to include a risk management plan to show the lender you have considered that something may not go as planned. This will demonstrate that you have conducted your research and understand the industry.
Cash is king, but not always practical. You will need to be sure you have funding for several years as most businesses fail due to the lack of capital funding. Borrowing funds is never easy but is almost always necessary. It will not happen overnight so begin the process several months before you anticipate needing the funds.
Attributions
"Principles of Accounting, Volume 1: Financial Accounting" by Mitchell Franklin, Patty Graybeal, Dixon Cooper, OpenStax is licensed under CC BY-NC-SA 4.0
Access for Free at https://openstax.org/books/principles-financial-accounting/pages/1-4-explain-why-accounting-is-important-to-business-stakeholders
"Financing Your Farm Business or Enterprise" by Robert E. Mikesell, Lynn Kime, PennState Extension.Copyright © PennState Extension. Used with permission.
|
oercommons
|
2025-03-18T00:39:15.022726
|
Carrie Walker
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/100027/overview",
"title": "Statewide Dual Credit Introduction to Agriculture Business Collection, Agribusiness Ownership and Management, Agribusiness Budgeting and Accounting",
"author": "Anna McCollum"
}
|
https://oercommons.org/courseware/lesson/104399/overview
|
Introduction to Biology Syllabus
Synthesis Project
BIOL 1: Introduction to Biology - Open For Antiracism (OFAR)
Overview
The Open for Antiracism (OFAR) Program – co-led by CCCOER and College of the Canyons – emerged as a response to the growing awareness of structural racism in our educational systems and the realization that adoption of open educational resources (OER) and open pedagogy could be transformative at institutions seeking to improve. The program is designed to give participants a workshop experience where they can better understand anti-racist teaching and how the use of OER and open pedagogy can empower them to involve students in the co-creation of an anti-racist classroom. The capstone project involves developing an action plan for incorporating OER and open pedagogy into a course.
I share 3 resources here from my Introduction to Biology course: our syllabus with specifications grading (complete/incomplete grading with unlimited revisions) and no late penalties, our Biologist Biographies project, and our content curation project. Please adapt these resources to make your own course more antiracist!
Action Plan
OER and open pedagogy have helped me make my class more antiracist because it allows for the inclusion of more voices and allows students to choose applications of our course material that are relevant to themselves and their communities.
Open pedagogy projects have allowed me to collaborate with my students to build curriculum. With open pedagogy, students are not just consumers of knowledge, but also producers and co-owners of the knowledge they acquire.
I share 3 resources here from my Introduction to Biology course: our syllabus with specifications grading (complete/incomplete grading with unlimited revisions) and no late penalties, our Biologist Biographies project, and our content curation project. Please adapt these resources to make your own course more antiracist!
Syllabus
In Introduction to Biology you will build a foundation of biology that you can use to understand personal experiences and issues at the intersection of biology and society. You will be able to use your understanding of biological principles to make decisions that affect your health, and the health of your family, community, and planet. I am looking forward to exploring and learning about the natural world with you this semester!
Course Description
4 Units. This course is an introduction to the basic principles of biology, focusing on the flow of genetic information through cells and generations and the flow of energy through cells and ecosystems. Topics include processes of science, cell structure and function, genetics, molecular biology, evolution, ecology, and a survey of the diversity of life. An emphasis is placed on the critical analysis of current biological issues, including threats to biodiversity and applications of biotechnology in agriculture and medicine.
Prerequisites: None. Transfer Status: CSU/UC
Student Learning Objectives (SLOs)
Upon successful completion of this course, the student will be able to:
- Investigate the biographies and contributions of biologists, design a controlled experiment, and communicate experimental results to demonstrate an understanding of the process of science.
- Describe the flow of information in cells and between generations.
- Explain the evolutionary and genetic mechanisms that account for both the unity and the diversity of life.
- Interpret biogeochemical cycles and explain the flow of energy through cells, communities, and ecosystems.
- Compare and contrast the major groups of organisms on our planet and relate their structures to their functions.
- Apply biological principles, critically analyze evidence, and summarize scientific conclusions to explain personal experiences and issues at the intersection of biology and society.
Specifications Grading
Our course grading system is designed around the principles of how we learn - how our brain works! Your course grade will not be determined by how quickly, or how many attempts it takes to complete an activity. We are here to learn, and the best way to learn is to explore, get confused, review feedback, and then revise!
- Everyone can learn biology
- Believe in yourself
- Struggle and mistakes are really important
- Speed is not important
Grading only your first attempt at a new skill does not capture your full capacity to learn and discourages creativity. If you are able to successfully explain a new concept on the first try, then maybe the concept was too easy or you already knew it. Deeper learning occurs when you are given opportunities to take risks, extend beyond what you already know, get frustrated, receive feedback, and then put the feedback into action. Growth and learning happen when we are challenged and have opportunities to practice a skill or concept, with support in the learning process.
This course uses a specifications grading system. Submissions will be recorded in the Canvas Gradebook as complete (✔️) or incomplete (✖️). The specifications used to determine whether a submission is complete can be found highlighted in yellow at the bottom of each activity's instructions. Review these specifications before you submit, but you can still submit incomplete work if you are stuck and would like some feedback. You can think of incomplete as "in progress". Review the feedback provided in submission comments and then revise and resubmit to complete the activity. You can revise and resubmit each activity as many times as you need to complete the activity. I will share feedback on every submission to guide you during the learning process. When the submission is complete, the ✖️ will be updated to a ✔️ to record the activity as complete.
The only letter grade in this course will be your final course grade submitted to the college, and it will be determined by the number of activities you complete over the course of the semester. This final letter grade is not an evaluation of you, but just a description of the number of activities completed during the semester. There are more activities available than you need to complete to give you choice in which activities you complete to secure your grade goal.
Biologist Biographies
Biology students investigate diverse biologists and contribute Biologist Biographies to an openly licensed website that will be shared with future students and other faculty looking for examples of diverse biologists to incorporate into their curriculum. The goal is to continue to add biologists to the website each semester that are contributing to current issues at the intersection of biology and society that represent the diverse, intersectional identities of our students.
This project is designed to allow students to complete the first part of SLO 1: Investigate the biographies and contributions of biologists, design a controlled experiment, and communicate experimental results to demonstrate an understanding of the process of science.
Content Curation Project
In biology, synthesis means to bring smaller components together to build something bigger and more complex. In photosynthesis, plants use light energy to build sugar from carbon dioxide and water. The goal of our project is to curate content on a topic at the intersection of biology and society. You will use your developing foundation of biology to evaluate, sort, interpret, present, and share trustworthy information on your topic in a format of your choice that is understandable and engaging for friends, family, community, and future BIOL 1 students. You will use the understanding of biology you build during our course to make these complex topics accessible and relevant.
This project is designed to allow students to demonstrate SLO 6: Apply biological principles, critically analyze evidence, and summarize scientific conclusions to explain personal experiences and issues at the intersection of biology and society.
|
oercommons
|
2025-03-18T00:39:15.072449
|
Homework/Assignment
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/104399/overview",
"title": "BIOL 1: Introduction to Biology - Open For Antiracism (OFAR)",
"author": "Activity/Lab"
}
|
https://oercommons.org/courseware/lesson/122913/overview
|
Learning Object "Parts of the house"
Overview
Learning Object "Parts of the house"
TESL, TEFL
Learning Object "Parts of the house"
Learning Object "Parts of the house"
|
oercommons
|
2025-03-18T00:39:15.089216
|
12/11/2024
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/122913/overview",
"title": "Learning Object \"Parts of the house\"",
"author": "Lizeth Rojas"
}
|
https://oercommons.org/courseware/lesson/88748/overview
|
Crash Rate for Intersections exercise
Overview
An example of how to calculate the crash rate for an intersection
Calculate the crash rate for the intersection
In this assignment you will practice how to calculate the crash rate of a given intersection. Analyzing the crash rate provides traffic engineers the necessary information needed in order to address the risks of the intersection. Please solve the following problem:
The number of crashes reported for the following intersection in 3 years is 12. Compute the crash rate.
Solution
For Intersections
Where,
R = crash rate (crashes/million entering vehicles)
C = number of crashes
AADT = average annual daily traffic volume (vpd = vehicles per day)
ny = number of years during which the crashes occured (years)
TEV = total number of vehicles entering the intersection (vpd = vehicles per day)
|
oercommons
|
2025-03-18T00:39:15.103829
|
Homework/Assignment
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/88748/overview",
"title": "Crash Rate for Intersections exercise",
"author": "Activity/Lab"
}
|
https://oercommons.org/courseware/lesson/68886/overview
|
Sign in to see your Hubs
Sign in to see your Groups
Create a standalone learning module, lesson, assignment, assessment or activity
Submit OER from the web for review by our librarians
Please log in to save materials. Log in
This visualizes how to describe variables.
or
|
oercommons
|
2025-03-18T00:39:15.124671
|
06/23/2020
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/68886/overview",
"title": "Statistics 101",
"author": "Michelle Lin"
}
|
https://oercommons.org/courseware/lesson/13691/overview
|
Goodness of Fit
Hypothesis Test - Single Mean
Hypothesis Test - Single Proportion
Linear Regression
Normal Distribution
Univariate Data
Spreadsheet-based Statistics Labs
Overview
This collection of spreadsheet-based labs was funded as part of the Digital Learning Research Network (dLRN) made possible by a grant from the Bill and Melinda Gates Foundation. The labs were adapted from the Statistics book, “Introduction to Statistics,” published by OpenStax College. The original labs used graphing calculators and were found within the book after each chapter. These interactive spreadsheet-based labs are effective for online and face-face courses. They may also be used with the book (see Resource: Lab Mapping to Book Chapters) or stand-alone.
Authors: Barbara Illowsky PhD, Foothill-De Anza Community College District; Larry Green PhD, Lake Tahoe Community College; James Sullivan, Sierra College; Lena Feinman,College of San Mateo; Cindy Moss, Skyline College; Sharon Bober, Pasadena Community College; Lenore Desilets, De Anza Community College.
Hyporhesis Test - Single Proportion
Univariarate Data
Learning Objective:
The students will design and carry out a survey.
- The students will analyze and graphically display the results of the survey.
For Student:
Normal Distribution
Learning Objective:
Compare the distribution of empirical data to the Normal Distribution.
For Student
Central Limit Theorem
Learning Objective
Students will examine properties of the Central Limit Theorem by randomly selecting 10 groups of 5 colleges and analyzing the distributions of the means of the student enrollment. Four students will collaborate on this lab. The group should be divided into two teams
For Students
Hypothesis Test - Single Mean
Learning Objective:
Conduct a hypothesis test for a single mean and interpret the results.
For Students
Hypothesis Test - Single Proportion
Learning Objective:
Conduct a hypothesis test for a single proportion and interpret the results.
For Students
Goodness of Fit
Learning Objective
Students will collect and evaluate birth Day data to determine if they fit a Uniform Distribution. Four students will collaborate on this lab.
For Students
Linear Regression
Learning Objective:
Evaluate the relationship between two variables to determine the significance of the linear correlation.
If linear correlation exists, determine and construct the linear regression equation between two variables.
For Students:
|
oercommons
|
2025-03-18T00:39:15.155720
|
Barbara Illowsky
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/13691/overview",
"title": "Spreadsheet-based Statistics Labs",
"author": "lenore desilets"
}
|
https://oercommons.org/courseware/lesson/95523/overview
|
simple cuboidal epi_in kidney tubules_100x, p000131
Overview
simple cuboidal epi_in kidney tubules_100x
simple cuboidal epi_in kidney tubules_100x
simple cuboidal epi_in kidney tubules_100x
simple cuboidal epi_in kidney tubules_100x
simple cuboidal epi_in kidney tubules_100x
|
oercommons
|
2025-03-18T00:39:15.171049
|
Diagram/Illustration
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/95523/overview",
"title": "simple cuboidal epi_in kidney tubules_100x, p000131",
"author": "Health, Medicine and Nursing"
}
|
https://oercommons.org/courseware/lesson/96895/overview
|
All Lecture Videos Subtitles
Overview
All the lecture videos' subtitles
All Lecture Videos Subtitles
In this section you can find all the subtitles for the lecture videos.
|
oercommons
|
2025-03-18T00:39:15.186889
|
08/31/2022
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/96895/overview",
"title": "All Lecture Videos Subtitles",
"author": "Saeid Samadidana"
}
|
https://oercommons.org/courseware/lesson/71125/overview
|
Chapter 4: Manage Your Time
Overview
LEARNING OBJECTIVES
By the end of this chapter, you will be able to:
- Understand the relationship between goals and time management.
- Consider how your priorities impact your time.
- Identify your time management personality.
- Assess your current use of time.
- Understand the basic principles of time management and planning.
- Use a calendar planner and daily to-do list to plan ahead for study tasks and manage your time effectively.
- Explore time management tips and strategies.
- Identify procrastination behaviors and strategies to avoid them.
Manage Your Time
Manage Your Time
Goals And Time
Now that you have spent some time exploring your values and your goals, you can easily see how you need to manage your time well in order to accomplish your goals. When you have a long-term goal and break it down into mid-term and short-term goals, it leads to the question, “What do I need to do today in order to achieve my goal?” This question is at the heart of time management. Your goals guide how you spend your time and resources. Having clearly defined goals with specific deadlines leads you to be deliberate in planning your time and makes it easier to establish an effective time management system.
As most students discover, college is not the same as high school. For many students, college is the first time they are “on their own” in an environment filled with opportunity. And while this can be exciting, you may find that social opportunities and job responsibilities conflict with academic expectations. For example, a free day before an exam, if not wisely spent, can spell trouble for doing well on the exam. It is easy to fall behind when there are so many choices and freedoms.
One of the main goals of a college education is learning how to learn. In this section, we zoom in on learning how to skillfully manage your time. To be successful in college, it’s imperative to be able to effectively manage your time and to manage all aspects of your life including school, work, and social opportunities. Time management isn’t actually difficult, but you do need to learn how to do it well.
In the following Alleyoop Advice video, Alleyoop (Angel Aquino) discusses what many students discover about college: there is a lot of free time—and just as many challenges to balance free time with study time.
Identifying Your Priorities
Thinking about your goals gets you started, but it’s also important to think about priorities. We often use the word “priorities” to refer to how important something is to us. We might think, This is a really important goal, and that is less important. Try this experiment: go back to the goals you wrote in the last chapter and see if you can rank each goal as a 1 (top priority), 2 (middle priority), or 3 (lowest priority).
It sounds easy, but do you actually feel comfortable doing that? Maybe you gave a priority 1 to passing your courses and a priority 3 to playing your guitar. So what does that mean—that you never play guitar again, or at least not while in college? Whenever you have an hour free between class and work, you have to study because that’s the higher priority? What about all your other goals—do you have to ignore everything that’s not a priority 1? And what happens when you have to choose between different goals that are both number 1 priorities?
In reality, priorities don’t work quite that way. It doesn’t make a lot of sense to try to rank goals as always more or less important. The question of priority is really a question of what is more important at a specific time. It is important to do well in your classes, but it’s also important to earn money to support yourself and have a social life. You shouldn’t have to choose between these, except at any given time. Priorities always involve time: what is most important to do right now. Time management is mostly a way to juggle priorities so you can meet all your goals.
When you manage your time well, you don’t have to ignore some goals completely in order to meet other goals. In other words, you don’t have to give up your life when you register for college, but you may need to work on managing your life and time more effectively.
But, time management works only when you’re committed to your goals. Attitude and motivation are very important. If you haven’t yet developed an attitude for success, all the time management skills in the world won’t keep you focused and motivated to succeed.
Identify Your Time Management Style
People’s attitudes toward time vary widely. One person seems to be always rushing around but actually gets less done than another person who seems unconcerned about time and calmly goes about the day. Since there are so many different “time personalities,” it’s important to realize how you approach time. Try the following activity to help you identify your personal time management style.
Activity 1: Identify your Time Management Style
The following self-assessment survey can help you determine your time-management personality type. Read each question in the Questions column. Then read the possible responses. Select one response for each question. Each response should reflect what you probably would do in a given situation, not what you think is the “right” answer. Put a checkmark in the My Time Management Type column next to your likely response.
| QUESTIONS | RESPONSES: Which response most closely matches what you would do? In the right column, check one response (a, b, c or d) for each question. | MY TIME MANAGEMENT TYPE | |
|---|---|---|---|
| 1 | Your instructor just gave your class the prompts for your first essay, which is due in two weeks. How do you proceed from here? | a. Choose a prompt and begin working on a thesis immediately. Better to get it out of the way! | Ο Early bird |
| b. Read over the prompts and let them sink in for a week or so. You’ll still have one more week to finish the assignment, right? | Ο Balancing act | ||
| c. Read the prompts and maybe start playing around with ideas, but wait to really start writing until the day before. You swear it’s all in your head somewhere! | Ο Pressure cooker | ||
| d. Look at the prompts the morning that assignment is due and quickly type up your essay. This makes you late for class, but at least you got it turned in on time. | Ο Improviser | ||
| 2 | You are working on a group assignment that requires you to split up responsibilities with three other classmates. When would you typically finish your part? | a. First. Then you’re done and don’t have to worry about it. Plus it could give you time in case you want to tweak anything later. | Ο Early bird |
| b. After one or two of the others have submitted their materials to the group, but definitely not last. You wanted to see how they approached it first. | Ο Balancing act | ||
| c. Maybe last, but definitely before the assignment due date and hopefully before any of the other group members ask about it. | Ο Pressure cooker | ||
| d. Definitely last. You’ll wait until everyone else has done their work, so you can make sure you are not duplicating efforts. Whatever, this is why you hate group work. | Ο Improviser | ||
| 3 | Your instructor just shared the instructions for your next assignment and you read them but don’t quite understand what he’s asking for in a certain part. What would you probably do? | a. Send the instructor an email that afternoon. When he doesn’t respond that night, email him again. This is your worst nightmare—you just want to know what he wants!! | Ο Early bird |
| b. Send him an email asking for clarification, giving yourself enough time to wait for his response and then complete the assignment. Better to be safe than sorry. | Ο Balancing act | ||
| c. Try to figure it out for yourself. You’re pretty sure what he’s trying to say, and you’ll give it your best shot. | Ο Pressure cooker | ||
| d. Don’t say anything until after the assignment is due. Other people in the class felt the same way too, probably! | Ο Improviser | ||
| 4 | The course you are taking requires you to post in a weekly discussion forum by Sunday night each week so the class can talk about everyone’s posts on Monday. When do you submit your posts? | a. Tuesday night, after the first day of class that week. Then it’s out of the way. | Ο Early bird |
| b. Thursday or Friday night. You want to let the week’s discussion sink in a little so you can collect your thoughts. | Ο Balancing act | ||
| c. Sunday night. You always forget over the weekend! | Ο Pressure cooker | ||
| d. Monday at 3 AM. That still counts as Sunday night, right? | Ο Improviser | ||
| 5 | You have an important assignment due Monday morning, and you have a social/work/family obligation that will keep you busy for most of the weekend. It is now the Wednesday before the assignment is due. How would you approach this dilemma? | a. You already finished it yesterday, the day it was assigned. Done! | Ο Early bird |
| b. You tell yourself that you’ll finish it by Friday night, and you manage this by chipping away at it over those 3 days. …Little. By. Little. | Ο Balancing act | ||
| c. You tell yourself that you’ll finish it by Friday night, so you can have your weekend free, but you still have a little left to do on Sunday—no big deal. | Ο Pressure cooker | ||
| d. You tell yourself that you’ll take the weekend off, then stay up late on Sunday or wake up early on Monday to finish it. It’s not a final or anything, and you have a life. | Ο Improviser | ||
| 6 | You have to read 150 pages before your next class meeting. You have 4 days to do so. What would you most likely do? | a. 150 pages divided by 4 days means… a little less than 40 pages a day. You like to chunk it this way because then you’ll also have time to go over your notes and highlights and come up with questions for the instructor. | Ο Early bird |
| b. 150 pages divided by…well … 2 days (because it’s been a long week), means 75 pages a day. Totally doable. | Ο Balancing act | ||
| c. 150 pages, the day before it is due. You did this to yourself, it’s fine. | Ο Pressure cooker | ||
| d. How much time does it take to skim the text for keywords and/or find a summary online? | Ο Improviser |
Assessing Your Responses
Which of the four basic time-management personality types did you select the most? Which did you select the least? Do you feel like these selections match the student you have been in the past? Has your previous way of doing things worked for you, or do you think it’s time for a change? Remember, we can all always improve!
Learn more below about your tendencies. Review traits, strengths, challenges, and tips for success for each of the four time-management personality types.
The Early Bird
- Traits: You like to make checklists and feel great satisfaction when you can cross something off of your to-do list. When it comes to assignments, you want to get started as soon as possible (and maybe start brainstorming before that), because it lets you stay in control.
- Strengths: You know what you want and are driven to figure out how to achieve it. Motivation is never really a problem for you.
- Challenges: Sometimes you can get more caught up in getting things done as quickly as possible and don’t give yourself enough time to really mull over issues in all of their complexity.
- Tips for Success: You’re extremely organized and on top of your schoolwork, so make sure you take the time to really enjoy learning in your classes. Remember, school isn’t all deadlines and checkboxes—you also have the opportunity to think about big-picture intellectual problems that don’t necessarily have clear answers.
The Balancing Act
- Traits: You really know what you’re capable of and are ready to do what it takes to get the most out of your classes. Maybe you’re naturally gifted in this way or maybe it’s a skill that you have developed over time; in any case, you should have the basic organizational skills to succeed in any class, as long as you keep your balance.
- Strengths: Your strength really lies in your ability to be well rounded. You may not always complete assignments perfectly every time, but you are remarkably consistent and usually manage to do very well in classes.
- Challenges: Because you’re so consistent, sometimes you can get in a bit of a rut and begin to coast in class, rather than really challenging yourself.
- Tips for Success: Instead of simply doing what works, use each class as an opportunity for growth by engaging thoughtfully with the material and constantly pushing the boundaries of your own expectations for yourself.
The Pressure Cooker
- Traits: You always get things done and almost always at the last minute. Hey, it takes time to really come up with good ideas!
- Strengths: You work well under pressure, and when you do finally sit down to accomplish a task, you can sit and work for hours. In these times, you can be extremely focused and shut out the rest of the world in order to complete what’s needed.
- Challenges: You sometimes use your ability to work under pressure as an excuse to procrastinate. Sure, you can really focus when the deadline is tomorrow but is it really the best work you could produce if you had a couple of days of cushion?
- Tips for Success: Give yourself small, achievable deadlines, and stick to them. Make sure they’re goals that you really could (and would) achieve in a day. Then don’t allow yourself to make excuses. You’ll find that it’s actually a lot more enjoyable to not be stressed out when completing schoolwork. Who would have known?
The Improviser
- Traits: You frequently wait until the last minute to do assignments, but it’s because you’ve been able to get away with this habit in many classes. Sometimes you miss an assignment or two, or have to pretend to have done reading that you haven’t, but everyone does that sometimes, right?
- Strengths: You think quickly on your feet, and while this is a true strength, it also can be a crutch that prevents you from being really successful in a class.
- Challenges: As the saying goes, old habits die hard. If you find that you lack a foundation of discipline and personal accountability, it can be difficult to change, especially when the course material becomes challenging or you find yourself struggling to keep up with the pace of the class.
- Tips for Success: The good news is you can turn this around! Make a plan to organize your time and materials in a reasonable way, and really stick with it. Also, don’t be afraid to ask your instructor for help, but be sure to do it before, rather than after, you fall behind.
People also differ in how they respond to schedule changes. Some go with the flow and accept changes easily, while others function well only when following a planned schedule and may become upset if that schedule changes. If you do not react well to an unexpected disruption in your schedule, plan extra time for catching up if something throws you off. This is all part of understanding your time personality.
Another aspect of your time personality involves the time of day. If you need to concentrate, such as when writing a class paper, are you more alert and focused in the morning, afternoon, or evening? Do you concentrate best when you look forward to a relaxing activity later on, or do you study better when you’ve finished all other activities? Do you function well if you get up early, or stay up late, to accomplish a task? How does that affect the rest of your day or the next day? Understanding this will help you better plan your study periods.
While you may not be able to change your “time personality,” you can learn to manage your time more successfully. The key is to be realistic. The best way to improve your time management is to take an honest look at how you are currently spending your time.
Assess Your Use Of Time
The best way to know how you spend your time is to record what you do all day in a time log, every day for a week, and then add that up. First, you want to take your best guess at how you are currently spending your time so you can compare that with how you are actually spending your time. This helps you identify the areas you need to work on.
Activity 2: Where Does the Time Go?
See if you can account for a week’s worth of time. For each of the activity categories listed, make your best estimate of how many hours you spend in a week. (For categories that are about the same every day, just estimate for one day and multiply by seven for that line.)
| Category of activity | Estimated Hours per week | Actual Hours per week |
|---|---|---|
| Sleeping | ||
| Eating (including preparing food) | ||
| Personal hygiene (i.e., bathing, etc.) | ||
| Working (employment) | ||
| Volunteer service or internship | ||
| Chores, cleaning, errands, shopping, etc. | ||
| Attending class | ||
| Studying, homework, reading, and researching (outside of class) | ||
| Transportation to work or school | ||
| Getting to classes (walking, biking, etc.) | ||
| Organized group activities (clubs, church services, etc.) | ||
| Time with friends (include television, video games, etc.) | ||
| Attending events (movies, parties, etc.) | ||
| Time alone (include television, video games, surfing the Web, etc.) | ||
| Exercise or sports activities | ||
| Reading for fun or other interests done alone | ||
| Time on the phone, texting, Facebook, Twitter, etc. | ||
| Other—specify: ________________________ | ||
| Other—specify: ________________________ | ||
| TOTAL (168 hours in a week) |
Now use your calculator to total your estimated hours. Is your number larger or smaller than 168, the total number of hours in a week? If your estimate is higher, go back through your list and adjust numbers to be more realistic. But if your estimated hours total fewer than 168, don’t just go back and add more time in certain categories. Instead, ponder this question: Where does the time go? We’ll come back to this question.
Next, print the Time Log and carry it with you throughout the week. Every few hours, fill in what you have been doing. Do this for a week before adding up the times; then enter the total hours in the categories in Activity 2. You might be surprised that you spend a lot more time than you thought just hanging out with friends, or surfing the Web or playing around with Facebook or any of the many other things people do. You might find that you study well early in the morning even though you thought you are a night person, or vice versa. You might learn how long you can continue at a specific task before needing a break.
Time Log
| SUNDAY | MONDAY | TUESDAY | WEDNESDAY | THURSDAY | FRIDAY | SATURDAY | |
| 6-7 am | |||||||
| 7-8 | |||||||
| 8-9 | |||||||
| 9-10 | |||||||
| 10-11 | |||||||
| 11-12 | |||||||
| 12-1 pm | |||||||
| 1-2 | |||||||
| 2-3 | |||||||
| 3-4 | |||||||
| 4-5 | |||||||
| 5-6 | |||||||
| 6-7 | |||||||
| 7-8 | |||||||
| 8-9 | |||||||
| 9-10 | |||||||
| 10-11 | |||||||
| 11-12 | |||||||
| 12-1 am | |||||||
| 1-2 | |||||||
| 2-3 | |||||||
| 3-4 | |||||||
| 4-5 | |||||||
| 5-6 |
Establishing A Time Management System
Now that you’ve evaluated how you have done things in the past, you’ll want to think about how you might create a schedule for managing your time well going forward. The best schedules have some flexibility built into them, as unexpected situations and circumstances will likely arise during your time as a student.
For every hour in the classroom, college students should spend, on average, about two to three hours on that class reading, studying, writing papers, and so on. Look at the following scenarios to get an idea of how many hours you should be spending on your classes outside of class time.
12 credit hours over a 15-week session = 12 hours a week in class + 24-36 hours outside of class
6 credit hours over a 15-week session = 6 hours a week in class + 12-18 hours outside of class
3 credit hours over a 6-week session = 8 hours a week in class + 16-24 hours outside of class
If you’re a full-time student with twelve hours a week in class plus your study time, that 36-42 hours is about the same as a typical full-time job, which is why you are considered to be a full-time student. If you work part-time or have a family, time management skills are even more essential. To succeed in college, everyone has to develop effective strategies for dealing with time.
Look back at the number of hours you wrote in Activity 2 for a week of studying. Do you have two to three hours of study time for every hour in class? Many students begin college not knowing this much time is needed, so don’t be surprised if you underestimated this number of hours. Remember this is just an average amount of study time—you may need more or less for your own courses. To be safe, and to help ensure your success, add another five to ten hours a week for studying.
To reserve this study time, you may need to adjust how much time you spend on other activities. Activity 3 will help you figure out what your typical week should look like.
Activity 3: Where Should Your Time Go?
Plan for the ideal use of a week’s worth of time. Fill in your hours in this order:
- Hours attending class
- Study hours (2 times the number of class hours plus 5 or more hours extra)
- Work, internships, and fixed volunteer time
- Fixed life activities (sleeping, eating, hygiene, chores, transportation, etc.)
Now subtotal your hours so far and subtract that number from 168. How many hours are left? ____________ Then portion out the remaining hours for “discretionary activities” (things you don’t have to do for school, work, or a healthy life).
- Discretionary activities
| CATEGORY OF ACTIVITY | HOURS PER WEEK |
| Attending class | |
| Studying, reading, and researching (outside of class) | |
| Working (employment) | |
| Transportation to work or school | |
| Sleeping | |
| Eating (including preparing food) | |
| Personal hygiene (i.e., bathing, etc.) | |
| Chores, cleaning, errands, shopping, etc. | |
| Volunteer service or internship | |
| Getting to classes (walking, biking, etc.) | |
| Subtotal: | |
| Discretionary activities: | |
| Organized group activities (clubs, church services, etc.) | |
| Time with friends (include television, video games, etc.) | |
| Attending events (movies, parties, etc.) | |
| Time alone (include television, video games, surfing the Web, etc.) | |
| Exercise or sports activities | |
| Hobbies or other interests done alone | |
| Time on the phone, texting, Facebook, Twitter, etc. | |
| Other—specify: ________________________ | |
| Other—specify: ________________________ |
Activity 3 shows most college students that they do actually have plenty of time for their studies without losing sleep or giving up their social life. But you may have less time for discretionary activities, like video games or watching movies, than in the past. Something, somewhere has to give. That’s part of time management and why it’s important to keep your goals and priorities in mind.
Below is an example of a student’s weekly schedule, with designated times for class, work and study time.
Kai’s Schedule
Since Kai’s Spanish class starts his schedule at 9:00 every day, Kai decides to use that as the base for his schedule. He doesn’t usually have trouble waking up in the mornings (except on the weekends), so he decides that he can do a bit of studying before class. His Spanish practice is often something he can do while eating or traveling, so this gives him a bit of leniency with his schedule.
| Sunday | Monday | Tuesday | Wednesday | Thursday | Friday | Saturday | |
| 7:00 AM | |||||||
| 8:00 AM | Spanish 101 | Spanish 101 | Spanish 101 | Spanish 101 | Spanish 101 | ||
| 9:00 AM | Spanish 101 | Spanish 101 | Spanish 101 | Spanish 101 | Spanish 101 | ||
| 10:00 AM | US History I | Spanish 101 | US History I | Spanish 101 | US History I | Work | |
| 11:00 AM | College Algebra | Intro to Psychology (ends at 12:30) | College Algebra | Intro to Psychology (ends at 12:30) | College Algebra | ||
| 12:00 PM | Spanish 101 | Spanish 101 | Spanish 101 | ||||
| 1:00 PM | Spanish 101 | Work (start 12:30 end 4:30) | Work (start 12:30 end 4:30) | Work (start 12:30 end 4:30) | Spanish 101 | ||
| 2:00 PM | US History I | Work | Work | Intro to Psych | |||
| 3:00 PM | |||||||
| 4:00 PM | |||||||
| 5:00 PM | College Algebra | College Algebra | College Algebra | ||||
| 6:00 PM | |||||||
| 7:00 PM | |||||||
| 8:00 PM | Intro to Psych | Intro to Psych | |||||
| 9:00 PM | US History I | US History I | |||||
| 10:00 PM |
Creating a Planner
Now that you know what you need to be spending your time on, let’s work on getting it put into a schedule or calendar. The first thing you want to do is select what type of planner or calendar you want to use. There are several to choose from. The following chart outlines some pros and cons to different systems. online calendars, weekly calendars, monthly calendars and wall calendars.
| Type | Example | Cost | Pros | Cons |
| Weekly Planner | $5-$10 |
|
| |
| Monthly Planner | $5-$15 |
|
| |
| Daily Planner | $5-$10 |
|
| |
| Electronic Calendar | Free |
|
| |
| Dry Erase Calendar | $15 – $20 |
|
|
What Goes in Your Planner?
Now that you have selected your planner, it’s time to fill it in. But what goes in it? Well, everything! Start by putting in your top priorities and then move on to your discretionary time.
Priorities
- Class time
- Work Time
- Designated study time (2-3 hours per hour in class)
- Assignment due dates (check your syllabus)
- Exam dates and quizzes (check your syllabus)
- Appointments
- Birthdays of family and friends
Discretionary Time
- Social events
- Parties
- Exercise
- Club activities
- Church activities
Reminders
- Birthdays
- Anniversaries
- Holidays
Your schedule will vary depending on the course you’re taking. So pull out your syllabus for each class and try to determine the rhythm of the class by looking at the following factors:
- Will you have tests or exams in this course? When are those scheduled?
- Are there assignments and papers? When are those due?
- Is there any group or collaborative assignments? You’ll want to pay particular attention to the timing of any assignment that requires you to work with others.
Remember your goals. Does your schedule reflect your goals? Set your short and long-term goals accordingly. Ask yourself the following:
- What needs to get done today?
- What needs to get done this week?
- What needs to get done by the end the first month of the semester?
- What needs to get done by the end the second month of the semester?
- What needs to get done by the end of the semester?
Don’t try to micromanage your schedule. Don’t try to estimate exactly how many minutes you’ll need two weeks from today to read a given chapter in a given textbook. Instead, just choose the blocks of time you will use for your studies. Don’t yet write in the exact study activity, just reserve the block. Next, look at the major deadlines for projects and exams that you wrote in earlier. Estimate how much time you may need for each and work backward on the schedule from the due date.
Plan Backwards
As a college student, you will likely have big assignments, papers, or projects that you are expected to work on throughout the semester. These are often tricky for students to schedule since it isn’t a regularly occurring event, like a weekly quiz or a homework assignment. These big projects often feel overwhelming so students have a tendency to shy away from them and procrastinate on them. This often results in a lot of last-minute stress and panic when the deadline is looming. A way to plan for these big projects is to plan backward. Start at the final project and then figure out all the steps that come before it and assign due dates for yourself. For example, you have a research paper due May 1. Start there!
| Assignment | Due Date |
| Research Paper Due | May 1 |
| Final Draft | April 28 |
| Rough Draft | April 21 |
| Final Outline | April 7 |
| Find sources | March 24 |
| Thesis statement | March 17 |
| Select topic | March 10 |
You have now created a series of assignments for yourself that will keep you on track for your project. Put these dates in your planner the same way you would any other assignment.
Establish A To-Do List
People use to-do lists in different ways, and you should find what works best for you. As with your planner, consistent use of your to-do list will make it an effective habit.
Some people prefer not to carry their planner everywhere but instead, copy the key information for the day onto a to-do list. Using this approach, your daily to-do list starts out with your key scheduled activities and then adds other things you hope to do today. This is a good fit for those that prefer to keep a wall calendar at home rather than carry their planner with them.
Some people use their to-do list only for things not included in their planners, such as short errands, phone calls or e-mail, and the like. This still includes important things, but they’re not scheduled out for specific times like your planner is.
Although we call it a daily list, the to-do list can also include things you may not get to today but don’t want to forget about. Keeping these things on the list, even if they’re a low priority, helps ensure that eventually, you’ll get to it.
Just as there are several options for planners, there are different types of to-do lists. Check your planner to see if it has one incorporated. If not, get a small notebook or pad of paper that you will designate as your to-do list. Of course, there’s always an app for that! Your smartphone likely came with a Reminder App or another type of To-Do List app. There are also many free apps to choose from and there are apps to help you manage your homework and assignments. Take a few minutes to look through your options to pick the best one for you.
Your To-Do list should be a reflection of your goals and priorities and should support your planner Your To-Do List should answer the question, “What do I have to do today, this week, this month?”
Here are some examples of different to-do lists.
Use whatever format works best for you to prioritize or highlight the most important activities.
Here are some more tips for effectively using your daily to-do list:
- Be specific: “Read history chapter 2 (30 pages)”—not “History homework.”
- Put important things high on your list where you’ll see them every time you check the list.
- Make your list at the same time every day so that it becomes a habit.
- Don’t make your list overwhelming. If you added everything you eventually need to do, you could end up with so many things on the list that you’d never read through them all. If you worry you might forget something, write it in the margin of your planner’s page a week or two away.
- Use your list. Lists often include little things that may take only a few minutes to do, so check your list anytime during the day you have a moment free.
- Cross out or check off things after you’ve done them—doing this becomes rewarding.
- Don’t use your to-do list to procrastinate. Don’t pull it out to find something else you just “have” to do instead of studying!
Time Management Strategies
Following are some strategies you can begin using immediately to make the most of your time:
- Prepare to be successful. When planning ahead for studying, think yourself into the right mood. Focus on the positive. “When I get these chapters read tonight, I’ll be ahead in studying for the next test, and I’ll also have plenty of time tomorrow to do X.” Visualize yourself studying well!
- Use your best—and most appropriate—time of day. Different tasks require different mental skills. Some kinds of studying you may be able to start first thing in the morning as you wake, while others need your most alert moments at another time.
- Break up large projects into small pieces. Whether it’s writing a paper for class, studying for a final exam, or reading a long assignment or full book, students often feel daunted at the beginning of a large project. It’s easier to get going if you break it up into stages that you schedule at separate times—and then begin with the first section that requires only an hour or two.
- Do the most important studying first. When two or more things require your attention, do the more crucial one first. If something happens and you can’t complete everything, you’ll suffer less if the most crucial work is done.
- If you have trouble getting started, do an easier task first. Like large tasks, complex or difficult ones can be daunting. If you can’t get going, switch to an easier task you can accomplish quickly. That will give you momentum, and often you feel more confident in tackling the difficult task after being successful in the first one.
- If you’re feeling overwhelmed and stressed because you have too much to do, revisit your time planner. Sometimes it’s hard to get started if you keep thinking about other things you need to get done. Review your schedule for the next few days and make sure everything important is scheduled, then relax and concentrate on the task at hand.
- If you’re really floundering, talk to someone. Maybe you just don’t understand what you should be doing. Talk to your instructor or another student in the class to get back on track.
- Take a break. We all need breaks to help us concentrate without becoming fatigued and burned out. As a general rule, a short break every hour or so is effective in helping recharge your study energy. Get up and move around to get your blood flowing, clear your thoughts, and work off stress.
- Use unscheduled times to work ahead. You’ve scheduled that hundred pages of reading for later today, but you have the textbook with you as you’re waiting for the bus. Start reading now, or flip through the chapter to get a sense of what you’ll be reading later. Either way, you’ll save time later. You may be amazed at how much studying you can get done during downtimes throughout the day.
- Keep your momentum. Prevent distractions, such as multitasking, that will only slow you down. Check for messages, for example, only at scheduled break times.
- Reward yourself. It’s not easy to sit still for hours of studying. When you successfully complete the task, you should feel good and deserve a small reward. A healthy snack, a quick video game session, or social activity can help you feel even better about your successful use of time.
- Just say no. Always tell others nearby when you’re studying, to reduce the chances of being interrupted. Still, interruptions happen, and if you are in a situation where you are frequently interrupted by a family member, spouse, roommate, or friend, it helps to have your “no” prepared in advance: “No, I really have to be ready for this test” or “That’s a great idea, but let’s do it tomorrow—I just can’t today.” You shouldn’t feel bad about saying no—especially if you told that person in advance that you needed to study.
- Have a life. Never schedule your day or week so full of work and study that you have no time at all for yourself, your family and friends, and your larger life.
- Use a calendar planner and a daily to-do list.
Watch this supplemental video, College Survival Tips: Time Management for Beginners by MyCollegePalTeam6, for a brief re-cap of effective time management strategies.
Time Management Tips for Students Who Work
If you’re both working and taking classes, you seldom have large blocks of free time. Avoid temptations to stay up very late studying, for losing sleep can lead to a downward spiral in performance at both work and school. Instead, try to follow these guidelines:
- If possible, adjust your work or sleep hours so that you don’t spend your most productive times at work. If your job offers flex time, arrange your schedule to be free to study at times when you perform best.
- Try to arrange your class and work schedules to minimize commuting time. If you are a part-time student taking two classes, taking classes back-to-back two or three days a week uses less time than spreading them out over four or five days. Working four ten-hour days rather than five eight-hour days reduces time lost to travel, getting ready for work, and so on.
- If you can’t arrange an effective schedule for classes and work, consider online courses that allow you to do most of the work on your own time.
- Use your daily and weekly planner conscientiously. Anytime you have thirty minutes or more free, schedule a study activity.
- Consider your “body clock” when you schedule activities. Plan easier tasks for those times when you’re often fatigued and reserve alert times for more demanding tasks.
- Look for any “hidden” time potentials. Maybe you prefer the thirty-minute drive to work over a forty-five-minute train ride. But if you can read on the train, that’s a gain of ninety minutes every day at the cost of thirty minutes longer travel time. An hour a day can make a huge difference in your studies.
- Can you do quick study tasks during slow times at work? Take your class notes with you and use even five minutes of free time wisely.
- Remember your long-term goals. You need to work, but you also want to finish your college program. If you have the opportunity to volunteer for some overtime, consider whether it’s really worth it. Sure, the extra money would help, but could the extra time put you at risk for not doing well in your classes?
- Be as organized on the job as you are academically. Use your planner and to-do list for work matters, too. The better organized you are at work, the less stress you’ll feel—and the more successful you’ll be as a student also.
- If you have a family as well as a job, your time is even more limited. In addition to the previous tips, try some of the strategies that follow.
Time Management Tips for Students with Family
Living with family members often introduces additional time stresses. You may have family obligations that require careful time management. Use all the strategies described earlier, including family time in your daily plans the same as you would hours spent at work. Don’t assume that you’ll be “free” every hour you’re home, because family events or a family member’s need for your assistance may occur at unexpected times. Schedule your important academic work well ahead and in blocks of time you control. See also the earlier suggestions for controlling your space: you may need to use the library or another space to ensure you are not interrupted or distracted during important study times.
Students with their own families are likely to feel time pressures. After all, you can’t just tell your partner or kids that you’ll see them in a couple years when you’re not so busy with job and college! In addition to all the planning and study strategies discussed so far, you also need to manage your family relationships and time spent with family. While there’s no magical solution for making more hours in the day, even with this added time pressure there are ways to balance your life well:
- Talk everything over with your family. If you’re going back to school, your family members may not have realized changes will occur. Don’t let them be shocked by sudden household changes. Keep communication lines open so that your partner and children feel they’re together with you in this new adventure. Eventually, you will need their support.
- Work to enjoy your time together, whatever you’re doing. You may not have as much time together as previously, but cherish the time you do have—even if it’s washing dishes together or cleaning house. If you’ve been studying for two hours and need a break, spend the next ten minutes with family instead of checking e-mail or watching television. Ultimately, the important thing is being together, not going out to movies or dinners or the special things you used to do when you had more time. Look forward to being with family and appreciate every moment you are together, and they will share your attitude.
Overcoming Procrastination
Procrastination Checklist
Do any of the following descriptions apply to you?
- My paper is due in two days and I haven’t really started writing it yet.
- I’ve had to pull an all-nighter to get an assignment done on time.
- I’ve turned in an assignment late or asked for an extension when I really didn’t have a good excuse not to get it done on time.
- I’ve worked right up to the minute an assignment was due.
- I’ve underestimated how long a reading assignment would take and didn’t finish it in time for class.
- I’ve relied on the Internet for information (like a summary of a concept or a book) because I didn’t finish the reading on time.
If these sound like issues you’ve struggled with in the past, you might want to consider whether you have the tendency to procrastinate and how you want to deal with it in your future classes. You’re already spending a lot of time, energy, and money on the classes you’re taking—don’t let all of that go to waste!
Procrastination is a way of thinking that lets one put off doing something that should be done now. This can happen to anyone at any time. It’s like a voice inside your head keeps coming up with these brilliant ideas for things to do right now other than studying: “I really ought to get this room cleaned up before I study” or “I can study anytime, but tonight’s the only chance I have to do X.” That voice is also very good at rationalizing: “I really don’t need to read that chapter now; I’ll have plenty of time tomorrow at lunch.…”
Procrastination is very powerful. Some people battle it daily, others only occasionally. Most college students procrastinate often, and about half say they need help to avoid procrastination. Procrastination can threaten one’s ability to do well on an assignment or test.
People procrastinate for different reasons. Some people are too relaxed in their priorities, seldom worry, and easily put off responsibilities. Others worry constantly, and that stress keeps them from focusing on the task at hand. Some procrastinate because they fear failure; others procrastinate because they fear success or are so perfectionistic that they don’t want to let themselves down. Some are dreamers. Many different factors are involved, and there are different styles of procrastinating.
Strategies to Combat Procrastination
Just as there are different causes, there are different possible solutions to procrastination. Different strategies work for different people. The time management strategies described earlier can help you avoid procrastination. Because this is a psychological issue, some additional psychological strategies can also help:
- Since procrastination is usually a habit, accept that and work on breaking it as you would any other bad habit: one day at a time. Know that every time you overcome feelings of procrastination, the habit becomes weaker and eventually, you’ll have a new habit of being able to start studying right away.
- Schedule times for studying using a daily or weekly planner. Commit to your study schedule in the same way you commit to other obligations like class time or school. Carry it with you and look at it often. Just being aware of the time and what you need to do today can help you get organized and stay on track.
- If you keep thinking of something else you might forget to do later (making you feel like you “must” do it now), write yourself a note about it for later and get it out of your mind.
- Counter a negative with a positive. If you’re procrastinating because you’re not looking forward to a certain task, try to think of the positive future results of doing the work, like getting a good grade or raising your GPA.
- Counter a negative with a worse negative. If thinking about the positive results of completing the task doesn’t motivate you to get started, think about what could happen if you keep procrastinating. You’ll have to study tomorrow instead of doing something fun you had planned. Or you could fail the test. Some people can jolt themselves right out of procrastination.
- On the other hand, fear causes procrastination in some people—so don’t dwell on the thought of failing. If you’re studying for a test, and you’re so afraid of failing it that you can’t focus on studying and you start procrastinating, try to put things in perspective. Even if it’s your most difficult class and you don’t understand everything about the topic, that doesn’t mean you’ll fail, even if you may not receive an A or a B.
- Study with a motivated friend. Form a study group with other students who are motivated and won’t procrastinate along with you. You’ll learn good habits from them while getting the work done now.
- Keep your studying “bite-sized”: When confronted with 150 pages of reading or 50 problems to solve, it’s natural to feel overwhelmed. Try breaking it down: What if you decide that you will read for 45 minutes or that you will solve 10 problems? That sounds much more manageable.
- Turn off your phone, close your chat windows, and block distracting Web sites. Treat your studying as if you’re in a movie theater—just turn it off.
- Set up a reward system: If you read for 40 minutes, you can check your phone for 5 minutes. But keep in mind that reward-based systems only work if you stick to an honor system.
- Study in a place reserved for studying ONLY. Your bedroom may have too many distractions (or temptations, such as taking a nap), so it may be best to avoid it when you’re working on school assignments.
- Use checklists: Make your incremental accomplishments visible. Some people take great satisfaction and motivation from checking items off a to-do list. Be very specific when creating this list, and clearly describe each task one step at a time.
- Get help. If you really can’t stay on track with your study schedule, or if you’re always putting things off until the last minute, see a college counselor. They have lots of experience with this common student problem and can help you find ways to overcome this habit.
In the following video, Joseph Clough shares key strategies for conquering procrastination once and for all.
Pomodoro Technique
A well-known technique for managing time that can help with procrastination is called the Pomodoro Technique, developed by Francesco Cirillo in the 1980s and named after the popular tomato-shaped kitchen timer (pomodoro means “tomato” in Italian.) This simple technique is a method of managing procrastination by breaking down your work periods into small, manageable units. The system operates on the belief that by dividing your work and breaks into regular, short increments you can avoid feeling overwhelmed by a looming task while also avoiding burn out.
Here are the basics:
- Consider in advance how many pomodoros you might need to achieve your task.
- Set a timer for 25 minutes, and start your task.
- It doesn’t have to be a tomato timer. You can use your phone timer (but put it on Do Not Disturb.)
- There are several online versions or apps for Pomodoro Timers – do a quick search to find one that works best for you.
- It doesn’t have to be 25 minutes, you can tweak this as you get more comfortable with the method.
- If a distraction pops into your head, write it down and immediately return to your task.
- When the buzzer rings, you’ve completed one increment, also known as one pomodoro.
- Take a five-minute break. You can check the distractions that popped into your head, stretch, grab a cup of tea, etc.
- After four pomodoros, take a fifteen- to thirty-minute break.
- Repeat!
Give it a try if you’re interested in breaking your studying into manageable tomato-sized bites while developing a greater understanding of time management and how long it will take to complete a task.
Watch this supplemental video that explains the Pomodoro Technique.
KEY TAKEAWAYS
- Your values help shape your goals and your goals help shape your time management.
- Identifying your priorities is an important first step to creating an effective time management system.
- Models like The Eisenhower Method help you prioritize and avoid unnecessarily stressful situations.
- There are unique Time Management Styles and knowing yours will help you create your own system.
- Having an accurate snapshot of how you currently spend your time is the first step in creating an effective time management system.
- Once you know how you spend your time, you can make a specific plan for how you want to spend your time.
- There are different types of planners, including hard-copy and electronic. Find a planner that works best for you and your preferences and habits.
- Your planner should reflect your values, goals, and priorities. It should include class time, work time, appointments, due dates, exams, and reminders of special dates.
- For big projects, plan backward to ensure you have enough time planned for each step.
- There are several options for To-Do Lists including paper and electronic choices. Find a system that works with your planner and that you will actively use.
- Implement Time Management Strategies to support your success and ultimately support your goals.
- Understand procrastination and the reasons you personally procrastinate. Use this information to incorporate proactive strategies to help you avoid procrastinating.
JOURNAL IDEA: AVOIDING TIME TRAPS
Now that you have a better understanding of how you are spending your time, write a journal entry that identifies your time traps and what strategies you can implement to overcome those time traps.
What is a time trap? A time trap is something you end up spending a lot of time that doesn’t support your goals or priorities. They take up your time, energy and focus if you let them. The way to avoid time traps is to be aware of what your personal time traps are and have a plan for how you will avoid or reduce them. We all have our own personal time traps. What are yours? To help you get started, here’s a list of some common time traps:
- Web surfing
- Streaming online videos
- Video games
- Social media
- Checking your phone for texts or alerts
- Television
- Sleeping unnecessarily
- Hanging out with friends when there’s nothing really happening
- Watching movies from streaming services
Directions
Look at your Time Log from Activity #2. What were your two most common time traps? How did these time traps distract you from your priorities?
For each of your two time traps, come up with two specific plans to effectively avoid or reduce those time traps.
What time management strategies can you implement that can support your plans to avoid your time traps?
LICENSES AND ATTRIBUTIONS
LICENSES AND ATTRIBUTIONS
CC LICENSED CONTENT, ORIGINAL
- Manage Your Time. Authored by: Heather Syrett. Provided by: Austin Community College. License: CC BY: Attribution
CC LICENSED CONTENT, SPECIFIC ATTRIBUTION
- Image. Authored by: Flickr. Located at :https://www.flickr.com/photos/bionicteaching/45191993455. License: CC BY-SA: Attribution-ShareAlike
- The Pomodoro Technique: Study More Efficiently, Take More Breaks Authored by: Gena Ellett. Provided by: The University of British Colombia Located at: https://learningcommons.ubc.ca/the-pomodoro-technique-study-more-efficiently-take-more-breaks/ License: CC BY: Attribution
- The Pomodoro Technique. Authored by: Ryan MacGillivray. Provided by: SKETCHPLANTATIONS Located at: https://www.sketchplanations.com/post/179972023741/the-pomodoro-technique-a-super-simple-methodLicense: CC BY: Attribution
PUBLIC DOMAIN CONTENT
- Provided by: Wikipedia. Located at: https://en.wikipedia.org/wiki/Time_management. License: Public Domain: No Known Copyright
- College Success Provided by: University of Minnesota. Located at: http://www.oercommons.org/courses/college-success/view. License: CC BY: Attribution
ALL RIGHTS RESERVED CONTENT
- The Pomodoro Technique Authored by: Cirillo Company. Located at: https://www.youtube.com/watch?time_continue=6&v=VFW3Ld7JO0w License: All Rights Reserved. License Terms: Standard YouTube License
LUMEN LEARNING AUTHORED CONTENT
- Provided by: Lumen Learning. Located at: https://courses.lumenlearning.com/sanjacinto-learningframework/. License: CC BY: Attribution
- Provided by: Lumen Learning. Located at: https://courses.lumenlearning.com/sanjacinto-learningframework/. License: CC BY: Attribution
|
oercommons
|
2025-03-18T00:39:15.271070
|
Mike Mckee
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/71125/overview",
"title": "Chapter 4: Manage Your Time",
"author": "Reading"
}
|
https://oercommons.org/courseware/lesson/88468/overview
|
sentence patterns
Worksheet for Newspaper Reading Assignment
News and Current Events: Intermediate Academic English
Overview
This module will cover Current Events, Paragraph writing,
Reading Assignment and Written Response
The instructor shoud choose three articles on a current events for students to read. The students can then choose one of the articles for the reading assignment. The purpose is to practice reading for the main idea and then writing about the information. Students should utilize the information learned from Reporter's questions.
Interesting Article concerning newspapers and ESL students Information on choosing newspapers for ESL students
In this module, you will discuss current events and information in the news- locally, nationally, and internationally.
Objectives
- To increase ability to write on a variety of topics
- To develop an understanding of sentence structure
- To improve reading comprehension through practice in identifying main ideas and details
- To increase reading skills by practicing predicting and skimming skills
- To instill confidence in speaking by preparing and presenting a power point and giving a speech
Reading and Writing Activity
Reading Assignment with Journal Response (Reading Assignment will be posted the week of the assignment. It will be a state, national, or international article on a current event.)
Choose one of the newspaper articles listed below. Read carefully and then answer questions concerning main idea, use of detail and prediction. Before reading, review the Worksheet for Newspaper Article. You will need to answer questions before, during, and after reading. You will then complete a journal assignment. The worksheet will help you with the assignment.
(Article belongs here.)
Journal Assisgnment: After reading the newspaper article and completing the attached worksheet, write two paragraphs about the assigned reading.
Paragraph One: Write a paragraph telling what the article is about. Use your notes to write the paragraph.
Paragraph Two: Critical Thinking Questions - In the paragraph tell why you think or do not think this story is newsworthy. Predict what you think will happen next.
Listening Activity
This reading assignment could also, of course, be used in the In the Community - Health Module. For using it in the Current Events Module, students can discuss how qucikly news information can change and the importance of keeping up with ourrent events. For more advanced intermediate students, it is a good introduction to comparing sources.
Listening Activity
For this assignment listen to Corona Virus TED TALK Copyright Information: [ESL Video - TEDTALK Education].
- Listen to the Ted Talk without looking at the transcript.
- Then listen to the TED Talk while reading the transcript. Underline any words you do not know.
- Take the quiz on understanding the content of the talk. Review your results.
- Take the Vocabulary Quiz and review your results.
- Discuss the following questions with your classmates:
- This Ted Talk was made in 2020 at the beginning of the Corona Virus outbreak. What information in this video is not longer current? Discuss this information with your classmates,
- With your classmates, disuss changes that have occured in the world because of the Corona Virus.
Oral Presentation Assignment ` `
Directions for Oral Presentation
For this assignment, you are to make a presentation on a current event you are interested in.
Task: Choose a current event. It could be in the United States, your home country, or anywhere in the world. To prepare for the presentation, you must read one to two news stories about the event. You may also watch a news story about the event, but you still must find a news article. In your presentation you must
- explain the current event.
- tell why it is important.
- give your opinion about the current event.
- tell the name of the newspaper, the name of the article, and the author if there is one. You must also include any information about any videos you watched.
Visuals: You must present your information in a Slide Presentation. You should have two to five slides. You should have no more than fifteen words per slide. You must use your own words. Use PowerPoint, Prezi, Google Slides, or presentation tool of your choice.
Other: Your presentation should be less than ten minutes.
Related Discussion
For this topic, discuss the current event you have chosen for your slide Presentation. Tell the current event and tell if it is a local issue, a national issue, or an international issue. Is it an issue from your native country? If someone else has already posted the same topic, choose another topic or address the topic in a different way.
Sentence Structure Basic Sentence Patterns and Simple and Compound Sentences
In this lesson, you will review five basic sentence patterns, simple sentences, and compound sentences.
Sentence Patterns
All sentences in the English language have a subject and a verb. All the other parts of the sentence revolve around the subject and verb since they are the most important part of the sentence. The way that the sentence is arranged is called a sentence pattern. This part of the lesson reviews the five basic sentence patterns. Practicing writing different patterns will improve your writing as well as your conversation skills. You will see an explanation with examples in the presentation Sentence Patterns attached to this lesson. You may also follow this link to review basic patterns or open the file resources: Users\ladpa\Downloads\sentence patterns.pdf.
Writing Activity with Five Basic Sentence Patterns
Directions: Show your understanding of the five sentence patterns by writing an example sentence for each pattern. All of your sentences should be on the same subject.
- Subject plus Verb
- Subject plus Action Verb plus Direct Object
- Subject plus Verb plus Adverb
- Subject plus Linking Verb plus Predicate Noun
- Subject plus Linking Verb plus Predicate Adjective
More Review with Additional Pattern Examples
- HOME (towson.edu). (Source: https:webapps.towson.edu/SentPatt.html)
Simple and Compound Sentences
Sentences can also be studied according to their structure or the types of clauses within the sentence. This lesson focuses on simple and compound sentences.
Terms to Know
- Clause - a group of words with a subject and a verb (I studied for the test.)
- Dependent Clause or Subordinate – a clause that cannot stand alone as a sentence because it starts with a subordinate conjunction (because, although, even though) or a relative pronoun (Because I studied for the test.) A dependent clause is not a sentence.
- Independent Clause – A clause that can stand alone as a sentence (I studied for the test. I made an A.).
- Simple Sentence - One independent clause (I studied for the test. I made an A.).
- Compound Sentence (Two independent clauses joined correctly.
(I studied for the test, and I made an A.).
Methods of Writing Compound Sentences
- Join with a coordinate conjunction – BOYFANS – BUT, OR, YET, FOR, AND, NOR, SO
I studied for the test, and I made an A.
I studied for the test, so I made an A.
- Use a semicolon to join the independent clauses.
I studied for the test; I made an A.
- Use a semicolon + transitional word (however/therefore)+ comma
I studied for the test; therefore, I made an A.
I forgot to study for test; however, I made an A.
For more review on compound sentences visit the websites below.
lEnglish Hints: How to Make Compound Sentences Catherine (Cathy) Simonton
(ahref="https://www.englishhints.com/compound-sentences.html">Compound Sentences: Examples and Practice</a>(Copyright 2011-221)
Basic Grammar and Punctuation: Compound Sentences
(Compound Sentences - Basic Grammar and Punctuation - LibGuides at St. Petersburg College)
information on Paragraph Writing
Information on Paragraph Writing
What is a paragraph? A paragraph is a goup of sentences on a single subject.
What are the parts of a paragraph? Most paragraphs have a topic sentence, supporting sentences, and a concluding sentence
What is a topic sentence? A topic sentence is a sentence that states the main idea of the paragraph, It usually is the first sentence of the paragraph.
What are supporting sentences? Supporting sentences are the sentences in a paragrph that support the topic sentence. The supporting sentences may be examples or a long illustration to explain the topic sentence. They may answer who, what, where, when, why, and how about the topic.
What is a concluding sentence? A concluding sentence is the sentence that summarized the main idea of the paragraph. It is the last sentence of the paragraph.
Follow the links below for more information about writing good paragraphs:
|
oercommons
|
2025-03-18T00:39:15.322343
|
Janet Rosenthal
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/88468/overview",
"title": "News and Current Events: Intermediate Academic English",
"author": "Linda Patterson"
}
|
https://oercommons.org/courseware/lesson/96890/overview
|
Revise Template to Support Teachers in Customizing Pathways Project Activities
Overview
This is a template that teacher educators may use, revise, or remix to support OER teacher education.
You may also make a copy of this template below by clicking here.
Let’s Revise a Pathways Project Activity!
Find an Activity for Your Classroom - The Pathways Project
Name:
Language: Level:
Link to Selected Pathways Project Activity from OER Commons:
To begin, read the following Guiding Principles for Revising an Activity:
|
List adapted from “What is Localization?” from the “How Tos” of OER Commons, licensed under a Creative Commons Attribution License 2.0 license. |
Review the following sections of the activity and complete the steps:
How are Pathways Project Activities Organized?
You’ll focus on the items highlighted in orange.
STEP 1
Can-Do Statements:
- Think…
|
- Will you maintain or revise?
Maintain | Revise Write a new can-do statement or make some tweaks |
|
|
STEP 2
Main Activity:
- Customizing for your learners
Think: How might you adjust this to fit the needs of your students?
| |
Maintain | Revise How will you make changes? |
|
|
- Making Structural Changes
Think: Do you need to make structural changes to better align to your unit?
| |
Maintain | Revise How will you make changes? |
|
|
STEP 3
Warm-Up: Now that you’ve seen the main activity, use the backward-design strategy to think about the warm-up
- Think…
|
- Will you maintain or revise?
Maintain | Revise How will you make changes?
|
|
|
STEP 4
Wrap-Up:
- Think…
|
- Will you maintain or revise?
Maintain | Revise How will you make changes?
|
|
|
STEP 5
Instructional Materials:
- Think…
|
- Will you maintain or revise?
Maintain | Revise How will you make changes?
|
|
|
STEP 6
New: Start putting your revisions into action by filling out the two boxes below.
Include a brief description (1-2 sentences) about why/how you revised this activity. This description will go under the title on OER Commons and help teachers understand how you’ve made it more applicable for a K-12 context. By writing this below now, you be more efficient in Module 3 when you share your revised/remixed activity to OER Commons. |
|
If you didn’t include (links to) new elements that you created (ex: new images, different vocabulary, new worksheet, etc.), you can list them below or go back up to each individual step and add them. In Step 6 you move beyond ideas/brainstorm to putting those ideas into action. |
|
|
oercommons
|
2025-03-18T00:39:15.380238
|
Teaching/Learning Strategy
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/96890/overview",
"title": "Revise Template to Support Teachers in Customizing Pathways Project Activities",
"author": "World Cultures"
}
|
https://oercommons.org/courseware/lesson/84256/overview
|
ENGLISH 1A Syllabus
English 1A
Overview
In this class, you will explore ideas about virtues in our society such as love, success, compassion, happiness, and justice through readings and writings. This course will explore how the phenomenon
of these different ideas manifests in our culture and in our language. How do we define love? What is success? Who desires justice? And how do this definition change in regards to ideas about race, sex, gender, age, and other cultural constructs? What does our subjective understand about our values ultimately say about who we are, individually? We will discuss different arguments about from essayists, poets, and artists. We will also analyze how modern media portrays our value systems. Finally, we will write essays that utilize different modes of composition and argument strategies to write research papers for your own ideas.
Syllabus and Sample Assignment
In this class, you will explore ideas about virtues in our society such as love, success, compassion, happiness, and justice through readings and writings. This course will explore how the phenomenon
of these different ideas manifests in our culture and in our language. How do we define love? What is success? Who desires justice? And how do this definition change in regards to ideas about race, sex, gender, age, and other cultural constructs? What does our subjective understand about our values ultimately say about who we are, individually? We will discuss different arguments about from essayists, poets, and artists. We will also analyze how modern media portrays our value systems. Finally, we will write essays that utilize different modes of composition and argument strategies to write research papers for your own ideas.
|
oercommons
|
2025-03-18T00:39:15.398021
|
Syllabus
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/84256/overview",
"title": "English 1A",
"author": "Homework/Assignment"
}
|
https://oercommons.org/courseware/lesson/72444/overview
|
LIQUID CLEANERS
Overview
Synthesis of Household Products. Household products like hand wash, dishwasher, and disinfectants are prepared using simple ingredients with available items at home.
Science
Description of the Course: This Course enables us to prepare household products like liquid cleaners, disinfectants and fresheners using available ingredients at a low-cost.
|
oercommons
|
2025-03-18T00:39:15.415614
|
09/12/2020
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/72444/overview",
"title": "LIQUID CLEANERS",
"author": "JEELANI BEGUM G"
}
|
https://oercommons.org/courseware/lesson/96335/overview
|
COMM 2025 Informative Demonstration Speech TILTed
Overview
This resource is a COMM 2025 Fundamentals of Communication Informative Demonstration Speech assignment that follows TILT (Transparency in Learning and Teaching.)
Informative Demonstration Speech Assignment
Purpose: By developing delivery, writing, and organization skills for a public speaking presentation, this assignment will help prepare you for professional and social speaking situations.
Skills: The Demonstration Speech will help you practice the following essential skills:
- Select an appropriate topic.
- Use supporting information from appropriate sources.
- Compose a well-organized preparation and delivery outline, using an appropriate organizational pattern.
- Create an introduction and conclusion that shows awareness of audience and purpose.
- Use correct formatting to cite sources orally and create a reference page (Works Cited page)
- Create effective visual/presentation aids.
- Deliver a well-organized, clear and concise speech to the audience.
Knowledge: The Demonstration Speech will give you the opportunity to apply the following important content knowledge:
- Recognizing the importance of audience when communicating.
- Utilizing an effective introduction and conclusion to engage your audience.
- Organizing content to engage your audience to help them understand and retain the information.
- Using delivery skills to create maximum impact.
- Using visual/presentation aids to enhance audience understanding.
- Understanding that effective communication requires providing support for ideas.
- Realizing the importance of acknowledging sources for the purpose of establishing credibility.
Task: To complete this assignment, you should complete each of the steps below. You can check off each step as you complete the step.
- Select a topic for the speech where you can show the audience how to do something, how to make something, or how something works.
- Find a minimum of 1 source for your speech topic. Cite your source in MLA format on your Works Cited page, and cite your source during your speech using an acceptable in-text oral citation.
- Develop a 3 to 5 minute speech demonstrating your speech topic.
- Organize your presentation using the preparation outline format. An example preparation outline is on pages 254-256 in your textbook.
- Create visual/presentation aids that will help the audience better understand your topic. Do not use the whiteboard, chalkboard or handouts as your visual aid.
- Rehearse one time using your preparation outline to time your speech; if your speech does not meet the time requirements, adjust accordingly and rehearse again.
- Use the delivery outline format to create the outline to use while presenting your speech. Be sure the delivery outline is no longer than one 8.5x11” sheet of paper, one side only, or three 4x6” notecards, one side only. An example delivery outline is on page 258-259 in your textbook.
- Rehearse your speech multiple times using your delivery outline and your visual/presentation aids. Focus on your verbal and nonverbal delivery as you rehearse. Continue to time your speech each rehearsal to ensure that you are within the 3 to 5 minute time requirements.
- Submit a digital copy of your preparation outline and delivery outline to the assignment dropbox prior to the speech session. Through the dropbox, TurnItIn will evaluate your submissions for plagiarism. You must submit your Preparation Outline and Delivery Outline to the Dropbox in order to receive their speech grade. If you use three 4x6" note cards for your delivery outline, submit a photo of the note cards front and back to the assignment dropbox. It is required for you to bring a copy of your preparation and delivery outline to the speech session to use while delivering your speech.
- Deliver your speech on your assigned speech day and time (see weekly course schedule for speech session dates). * It is essential that you are time. If you arrive late, wait outside the classroom until you hear applause before entering.
Criteria for Success: Please see the Speech Evaluation Rubric. This rubric is how your speech will be graded. Preparation outline with your Works Cited page and your delivery outline are due when you give your speech.
|
oercommons
|
2025-03-18T00:39:15.449278
|
08/11/2022
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/96335/overview",
"title": "COMM 2025 Informative Demonstration Speech TILTed",
"author": "Amy Bryant"
}
|
https://oercommons.org/courseware/lesson/96375/overview
|
Alkenes Overview Alkenes - Method of preparation and Chemical properties Alkenes - Method of preparation and Chemical properties
|
oercommons
|
2025-03-18T00:39:15.470485
|
08/14/2022
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/96375/overview",
"title": "Alkenes",
"author": "Gulnaz Khan"
}
|
https://oercommons.org/courseware/lesson/97613/overview
|
the-basic-elements-of-music-8.8 html files
The Basic Elements of Music
Overview
This open book by Catherine Schmidt-Jones has units on time elements (rhythm and meter), pitch elements (timbre, melody, and harmony), and the combination of these elements. The textbook is being provided in both PDF and html formats for download.
Downloadable resources
This is an OER Commons-hosted copy of The Basic Elements of Music (cc-by) by Catherine Schmidt-Jones. This could originally be found on OpenStax CNX, the content of which has been migrated to the Internet Archive.
The contents are broken into 3 units (the below table is provided for reference only; at present the content can only be accessed by downloading the resource from the Section Resources.
1 Time Elements
1.1 Rhythm
1.2 Simple Rhythm Activities
1.3 Meter in Music
1.4 Musical Meter Activities
1.5 Tempo
1.6 A Tempo Activity
1.7 Dynamics and Accents in Music
1.8 A Musical Dynamics Activity
1.9 A Musical Accent Activity
2 Pitch Elements
2.1 Timbre
2.2 Melody
2.3 Harmony
3 Combining Time and Pitch
3.1 The Textures of Music
3.2 A Musical Textures Activity
3.3 An Introduction to Counterpoint
3.4 Counterpoint Activities: Listening and Discussion
3.5 Counterpoint Activities: Singing Rounds
3.6 Music Form Activities
3.7 Form in Music
|
oercommons
|
2025-03-18T00:39:15.490321
|
09/30/2022
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/97613/overview",
"title": "The Basic Elements of Music",
"author": "Peter Musser"
}
|
https://oercommons.org/courseware/lesson/56678/overview
|
12.4.2 Non-Mendelian Genetics (video) Genetic recombination, X-linked traits
12.4 Non-Mendelian Genetics
Recombination of genes leads to genetic diversity.
Non-Mendelian Genetics: X-linked inheritance.
In cellular reproduction (meiosis), prophase I, homologous chromosomes align and swap segments of like DNA. This recombination of genes to genetic diversity.
This video covers topics such as - In cellular reproduction (meiosis), prophase I, homologous chromosomes align and swap segments of like DNA. This recombination of genes to genetic diversity.
X-linked inheritance.
|
oercommons
|
2025-03-18T00:39:15.503274
|
08/05/2019
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/56678/overview",
"title": "12.4.2 Non-Mendelian Genetics (video) Genetic recombination, X-linked traits",
"author": "Urbi Ghosh"
}
|
https://oercommons.org/courseware/lesson/111817/overview
|
Education Standards
PronounObjectSubject-answer key
Pronoun Poster Quiz Answer Key
Pronoun Quiz
Pronoun Quiz Options
OER Pronoun Lesson
Overview
This lesson will help students understand pronouns.
Intro and Lesson
Introduce the lesson about how to chose of the right pronoun. Work through the PronounObjectSubject poster.
Give the students an anticipatory quiz / activity to see if they can write or paste the options into the correct box.
Go over the answers. Explain how to talk through each possibility so students come to the correct answer by answering the questions.
Have students look for pronouns in something they are reading. Not where the pronoun fits in the chart.
What do you call yourself when you talk about yourself? What about when you talk to someone else? When you talk about someone, what do you call them when you don't use their name?
The words you use are called prounouns, and the pronoun that you use depends on where the word comes in the sentence and how many people you're talking about.
|
oercommons
|
2025-03-18T00:39:15.530130
|
Student Guide
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/111817/overview",
"title": "OER Pronoun Lesson",
"author": "Lesson"
}
|
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.