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/16341/overview
|
Cool Breeze - Oregon Science Project Hybrid 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 3-4 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 grealy 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 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 #5.
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
|
2025-03-18T00:37:57.488664
|
08/17/2017
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/16341/overview",
"title": "Cool Breeze - Oregon Science Project Hybrid Module #2 - Talk & Equity",
"author": "Steve West"
}
|
https://oercommons.org/courseware/lesson/74008/overview
|
Stem Education
Overview
The demand for STEM qualified employees has grown significantly over the years. That is one of the main reasons schools are pushing STEM programs, especially for female students. The number of job vacancies has reach the millions point. This has caused a large gap in qualified employees to head the charge in artificial intelligence.
Why is stem education important?
Many schools are focusing on gaining female students in the STEM field. It is a growing concept that is providing a multitude of programs, grants, and scholarships.
|
oercommons
|
2025-03-18T00:37:57.506409
|
10/28/2020
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/74008/overview",
"title": "Stem Education",
"author": "Kuwanna Moore"
}
|
https://oercommons.org/courseware/lesson/64917/overview
|
Type 2 Diabetes, Our Genes, and the Polyphenols that Improve their Function
Overview
Nutrition, polyphenols, nutrigenomics
|
oercommons
|
2025-03-18T00:37:57.523311
|
Renae Foisy
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/64917/overview",
"title": "Type 2 Diabetes, Our Genes, and the Polyphenols that Improve their Function",
"author": "Diagram/Illustration"
}
|
https://oercommons.org/courseware/lesson/93393/overview
|
Micrograph Staphylococcus aureus Gram stain 1000x p000028
Overview
This micrograph was taken at 1000X total magnifcation on a brightfield microscope. The subject is Staphylococcus aureus cells grown on nutrient agar at 37 degrees Celsius. The cells were heat-fixed to a slide and Gram stained prior to visualization.
Image credit: Emily Fox
Micrograph
Dozens of dark purple, round cells on a light background.
|
oercommons
|
2025-03-18T00:37:57.540320
|
Diagram/Illustration
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/93393/overview",
"title": "Micrograph Staphylococcus aureus Gram stain 1000x p000028",
"author": "Health, Medicine and Nursing"
}
|
https://oercommons.org/courseware/lesson/98821/overview
|
Descriptive Guidelines for OER Content
Overview
Descriptive guidelines to use when uploading content into the Massachusetts Community College OER Hub. Please take careful note of each field, and if you would like greater clarification or have any questions, please contact the Hub Coordinator Rachel Oleaga at Roleaga@necc.mass.edu
Understanding the Guidelines
This resource was created to assist with uploading new materials to the Massachusetts Community College OER Commons. Populating each field with the appropriate metadata is important to ensure access to your resource! If you have any questions or need additional assistance, save your resource and contact OER Hub Coordinator, Rachel Oleaga at Roleaga@necc.mass.edu
A downloadable PDF version of this document is also available below.
Overview - Describe the Resource
A short description of the resource. Include any keywords one might use to locate your resource, for example, relevant subject areas, lesson topics and proper nouns.
Conditions of Use
In the dropdown bar, you have a choice between the following licenses:
Attribution
Attribution No Derivative Works
Attribution Share Alike
Attribution Non Commercial
Attribution Non - Commercial Share Alike
Attribution Non - Commercial No Derivative
Public Domain
Required Fields
There are four required fields needed to describe your resource and complete the upload to the Hub. Each field is populated via a drop down menu. See below for details
Subjects
You must select at least one subject from the dropdown bar. Add all subjects which apply. For example: should your resource fall under Arts and Humanities > Art History > World Cultures, select all three.
Education Levels
Materials should fall within the higher education options provided. More than one level may be selected.
Material Types
To understand where your resource falls within the selections, see the attached document created by OER Commons.
Languages
Additional Descriptions (Optional)
There are several additional description fields available to describe your resource. Although they are not required the more information provided about your resource, the more discoverable.
Media Formats
Select the appropriate format, for example, Graphics/photos or Video. If you are typing your resource directly into Open Author, select “text/HTML.” If you are uploading a document, select “downloadable docs.”
Adding content directly into the Main Content area is preferred over attaching documents. The Open Author tool allows you to upload from OneDrive or Google Drive directly into the content areas, and edit the text as needed from there. If you must attach a document, do not link to a Google Doc, as this can create accessibility issues.
Keywords
Add any keyword that accurately describes your resource. This is important for helping users discover your resource. In addition, all resources should be tagged with the appropriate collection tag phrase. This should be typed in the "Keyword" section along with your other keywords. Find the collection tag phrase with best corresponds to your resource's subject below.
Collection Tag Phrases by Subject
Applied Science: Mass CC Applied Sci
Arts and Humanities: Mass CC Art
Business: Mass CC Business
Career and Technical Education: Mass CC CTE
Criminal Justice and Law: Mass CC Law
Computer and Information Science: Mass CC CIS
Education: Mass CC Edu
Engineering: Mass CC Engineering
English and Writing: Mass CC ELA
History: Mass CC History
Libraries and Information Literacy: Mass CC Libraries
Life Science: Mass CC Life Sci
Mathematics: Mass CC Math
OER, Copyright and Licensing: Mass CC OER
Physical Science: Mass CC Physical Sci
Psychology: Mass CC Psych
Nursing and Allied Health: Mass CC Nursing
Social Sciences: Mass CC Social Sci
|
oercommons
|
2025-03-18T00:37:57.566326
|
11/17/2022
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/98821/overview",
"title": "Descriptive Guidelines for OER Content",
"author": "Rachel Oleaga"
}
|
https://oercommons.org/courseware/lesson/14622/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
rg
rge
or
|
oercommons
|
2025-03-18T00:37:57.591558
|
06/13/2017
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/14622/overview",
"title": "pic",
"author": "Vaibhav Vasant"
}
|
https://oercommons.org/courseware/lesson/73865/overview
|
JULY 11TH IS HARD
Overview
As a 5 year old I lived in a town called Srebrenica in Bosnia and the town was torn by war, 8372 men and boys were killed in the genocide on just July 11th, 1995. The worst Genocide in recent European history with the exception of Holocaust. My childhood memories were not the best, and as an adult and a parent now I want to make sure that it is not forgotten so very often I write about it and hope to share it with my children one day.
POEM
JULY 11TH
BY: Semira Salihovic
July 11th is hard!
It is hard becayse of mother's tears and sister's cries.
It is hard because of every son, husband and brother that still have not been found.
It is hard because of fathers that gave their life for Srebrenica.
it is hard becaue of kids who would love to have their grandfather, father, brother or uncle.
July 11th is hard, hard because of spilled blood.
Hard because of injustice.
Hard because of GENOCIDE!
So tell your children what happened they need to know.
Tell them about the beauty of Bosnia and the fight of our people.
Tell them they should be proud to belong to people that were put through the worst
trials but have not given up!
Tell them about that with pride.
Tell them why July 11th is hard.
|
oercommons
|
2025-03-18T00:37:57.603934
|
10/25/2020
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/73865/overview",
"title": "JULY 11TH IS HARD",
"author": "Semira Salihovic"
}
|
https://oercommons.org/courseware/lesson/93100/overview
|
5 minute search(1)
The Five Minute Search
Overview
Finally, test yourself with this five minute search activity. Using what you have learned regarding Boolean Operators and some of the other strategies listed, you should be able to successfully find a relevant resource in five minutes or less. Set a timer and see if you can do it.
The Five Minute Search
Finally, test yourself with this five minute search activity. Using what you have learned regarding Boolean Operators and some of the other strategies listed, you should be able to successfully find a relevant resource in five minutes or less. Set a timer and see if you can do it.
This work "The Five Minute Search" by Andrea Bearman is licensed under CC-BY 4.0 International License.
|
oercommons
|
2025-03-18T00:37:57.621438
|
05/27/2022
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/93100/overview",
"title": "The Five Minute Search",
"author": "Andrea Bearman"
}
|
https://oercommons.org/courseware/lesson/15247/overview
|
Introduction
When U.S. citizens think of governmental power, they most likely think of the presidency. The framers of the Constitution, however, clearly intended that Congress would be the cornerstone of the new republic. After years of tyranny under a king, they had little interest in creating another system with an overly powerful single individual at the top. Instead, while recognizing the need for centralization in terms of a stronger national government with an elected executive wielding its own authority, those at the Constitutional Convention wanted a strong representative assembly at the national level that would use careful consideration, deliberate action, and constituent representation to carefully draft legislation to meet the needs of the new republic. Thus, Article I of the Constitution grants several key powers to Congress, which include overseeing the budget and all financial matters, introducing legislation, confirming or rejecting judicial and executive nominations, and even declaring war.
Today, however, Congress is the institution most criticized by the public, and the most misunderstood. How exactly does Capitol Hill operate (Figure)? What are the different structures and powers of the House of Representatives and the Senate? How are members of Congress elected? How do they reach their decisions about legislation, budgets, and military action? This chapter addresses these aspects and more as it explores “the first branch” of government.
|
oercommons
|
2025-03-18T00:37:57.635584
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15247/overview",
"title": "American Government, Delivering Collective Action: Formal Institutions",
"author": null
}
|
https://oercommons.org/courseware/lesson/15248/overview
|
The Institutional Design of Congress
Learning Objectives
By the end of this section, you will be able to:
- Describe the role of Congress in the U.S. constitutional system
- Define bicameralism
- Explain gerrymandering and the apportionment of seats in the House of Representatives
- Discuss the three kinds of powers granted to Congress
The origins of the U.S. Constitution and the convention that brought it into existence are rooted in failure—the failure of the Articles of Confederation. After only a handful of years, the states of the union decided that the Articles were simply unworkable. In order to save the young republic, a convention was called, and delegates were sent to assemble and revise the Articles. From the discussions and compromises in this convention emerged Congress in the form we recognize today. In this section, we will explore the debates and compromises that brought about the bicameral (two-chamber) Congress, made up of a House of Representatives and Senate. We will also explore the goals of bicameralism and how it functions. Finally, we will look at the different ways seats are apportioned in the two chambers.
THE GREAT COMPROMISE AND THE BASICS OF BICAMERALISM
Only a few years after the adoption of the Articles of Confederation, the republican experiment seemed on the verge of failure. States deep in debt were printing increasingly worthless paper currency, many were mired in interstate trade battles with each other, and in western Massachusetts, a small group of Revolutionary War veterans angry over the prospect of losing their farms broke into armed open revolt against the state, in what came to be known as Shays’ Rebellion. The conclusion many reached was that the Articles of Confederation were simply not strong enough to keep the young republic together. In the spring of 1787, a convention was called, and delegates from all the states (except Rhode Island, which boycotted the convention) were sent to Philadelphia to hammer out a solution to this central problem.
The meeting these delegates convened became known as the Constitutional Convention of 1787. Although its prescribed purpose was to revise the Articles of Confederation, a number of delegates charted a path toward disposing of the Articles entirely. Under the Articles, the national legislature had been made up of a single chamber composed of an equal number of delegates from each of the states. Large states, like Virginia, felt it would be unfair to continue with this style of legislative institution. As a result, Virginia’s delegates proposed a plan that called for bicameralism, or the division of legislators into two separate assemblies. In this proposed two-chamber Congress, states with larger populations would have more representatives in each chamber. Predictably, smaller states like New Jersey were unhappy with this proposal. In response, they issued their own plan, which called for a single-chamber Congress with equal representation and more state authority (Figure).
The storm of debate over how to allocate power between large and small states was eventually calmed by a third proposal. The Connecticut Compromise, also called the Great Compromise, proposed a bicameral congress with members apportioned differently in each house. The upper house, the Senate, was to have two members from each state. This soothed the fears of the small states. In the lower house, the House of Representatives, membership would be proportional to the population in each state. This measure protected the interests of the large states.
In the final draft of the U.S. Constitution, the bicameral Congress established by the convention of 1787 was given a number of powers and limitations. These are outlined in Article I (Appendix B). This article describes the minimum age of congresspersons (Section 2), requires that Congress meet at least once a year (Section 4), guarantees members’ pay (Section 6), and gives Congress the power to levy taxes, borrow money, and regulate commerce (Section 8). These powers and limitations were the Constitutional Convention’s response to the failings of the Articles of Confederation.
Although the basic design of the House and Senate resulted from a political deal between large and small states, the bicameral legislature established by the convention did not emerge from thin air. The concept had existed in Europe as far back as the medieval era. At that time, the two chambers of a legislature were divided based on class and designed to reflect different types of representation. The names of the two houses in the United Kingdom’s bicameral parliament still reflect this older distinction today: the House of Lords and the House of Commons. Likewise, those at the Constitutional Convention purposely structured the U.S. Senate differently from the House of Representatives in the hopes of encouraging different representative memberships in the two houses. Initially, for example, the power to elect senators was given to the state legislatures instead of to the voting public as it is now. The minimum age requirement is also lower for the House of Representatives: A person must be at least twenty-five years old to serve in the House, whereas one must be at least thirty to be a senator.
The bicameral system established at the Constitutional Convention and still followed today requires the two houses to pass identical bills, or proposed items of legislation. This ensures that after all amending and modifying has occurred, the two houses ultimately reach an agreement about the legislation they send to the president. Passing the same bill in both houses is no easy feat, and this is by design. The framers intended there to be a complex and difficult process for legislation to become law. This challenge serves a number of important and related functions. First, the difficulty of passing legislation through both houses makes it less likely, though hardly impossible, that the Congress will act on fleeting instincts or without the necessary deliberation. Second, the bicameral system ensures that large-scale dramatic reform is exceptionally difficult to pass and that the status quo is more likely to win the day. This maintains a level of conservatism in government, something the landed elite at the convention preferred. Third, the bicameral system makes it difficult for a single faction or interest group to enact laws and restrictions that would unfairly favor it.
The website of the U.S. Congress Visitor Center contains a number of interesting online exhibits and informational tidbits about the U.S. government’s “first branch” (so called because it is described in Article I of the Constitution).
SENATE REPRESENTATION AND HOUSE APPORTIONMENT
The Constitution specifies that every state will have two senators who each serve a six-year term. Therefore, with fifty states in the Union, there are currently one hundred seats in the U.S. Senate. Senators were originally appointed by state legislatures, but in 1913, the Seventeenth Amendment was approved, which allowed for senators to be elected by popular vote in each state. Seats in the House of Representatives are distributed among the states based on each state’s population and each member of the House is elected by voters in a specific congressional district. Each state is guaranteed at least one seat in the House (Table).
| The 114th Congress | ||
|---|---|---|
| House of Representatives | Senate | |
| Total Number of Members | 435 | 100 |
| Number of Members per State | 1 or more, based on population | 2 |
| Length of Term of Office | 2 years | 6 years |
| Minimum Age Requirement | 25 | 30 |
Congressional apportionment today is achieved through the equal proportions method, which uses a mathematical formula to allocate seats based on U.S. Census Bureau population data, gathered every ten years as required by the Constitution. At the close of the first U.S. Congress in 1791, there were sixty-five representatives, each representing approximately thirty thousand citizens. Then, as the territory of the United States expanded, sometimes by leaps and bounds, the population requirement for each new district increased as well. Adjustments were made, but the roster of the House of Representatives continued to grow until it reached 435 members after the 1910 census. Ten years later, following the 1920 census and with urbanization changing populations across the country, Congress failed to reapportion membership because it became deadlocked on the issue. In 1929, an agreement was reached to permanently cap the number of seats in the House at 435.
Redistricting occurs every ten years, after the U.S. Census has established how many persons live in the United States and where. The boundaries of legislative districts are redrawn as needed to maintain similar numbers of voters in each while still maintaining a total number of 435 districts. Because local areas can see their population grow as well as decline over time, these adjustments in district boundaries are typically needed after ten years have passed. Currently, there are seven states with only one representative (Alaska, Delaware, Montana, North Dakota, South Dakota, Vermont, and Wyoming), whereas the most populous state, California, has a total of fifty-three congressional districts (Figure).
Two remaining problems in the House are the size of each representative’s constituency—the body of voters who elect him or her—and the challenge of Washington, DC. First, the average number of citizens in a congressional district now tops 700,000. This is arguably too many for House members to remain very close to the people. George Washington advocated for thirty thousand per elected member to retain effective representation in the House. The second problem is that the approximately 675,000 residents of the federal district of Washington (District of Columbia) do not have voting representation. Like those living in the U.S. territories, they merely have a non-voting delegate.There are six non-voting delegations representing American Samoa, the District of Columbia, Guam, the Northern Mariana Islands, Puerto Rico, and the U.S. Virgin Islands. While these delegates are not able to vote on legislation, they may introduce it and are able to vote in congressional committees and on procedural matters.
The stalemate in the 1920s wasn’t the first time reapportionment in the House resulted in controversy (or the last). The first incident took place before any apportionment had even occurred, while the process was being discussed at the Constitutional Convention. Representatives from large slave-owning states believed their slaves should be counted as part of the total population. States with few or no slaves predictably argued against this. The compromise eventually reached allowed for each slave (who could not vote) to count as three-fifths of a person for purposes of congressional representation. Following the abolition of slavery and the end of Reconstruction, the former slave states in the South took a number of steps to prevent former slaves and their children from voting. Yet because these former slaves were now free persons, they were counted fully toward the states’ congressional representation.
Attempts at African American disenfranchisement continued until the civil rights struggle of the 1960s finally brought about the Voting Rights Act of 1965. The act cleared several final hurdles to voter registration and voting for African Americans. Following its adoption, many Democrats led the charge to create congressional districts that would enhance the power of African American voters. The idea was to create majority-minority districts within states, districts in which African Americans became the majority and thus gained the electoral power to send representatives to Congress.
While the strangely drawn districts succeeded in their stated goals, nearly quintupling the number of African American representatives in Congress in just over two decades, they have frustrated others who claim they are merely a new form of an old practice, gerrymandering. Gerrymandering is the manipulation of legislative district boundaries as a way of favoring a particular candidate. The term combines the word salamander, a reference to the strange shape of these districts, with the name of Massachusetts governor Elbridge Gerry, who in 1812, signed a redistricting plan designed to benefit his party. Despite the questionable ethics behind gerrymandering, the practice is legal, and both major parties have used it to their benefit. It is only when political redistricting appears to dilute the votes of racial minorities that gerrymandering efforts can be challenged under the Voting Rights Act. Other forms of gerrymandering are frequently employed in states where a dominant party seeks to maintain that domination. As we saw in the chapter on political parties, gerrymandering can be a tactic to draw district lines in a way that creates “safe seats” for a particular political party. In states like Maryland, these are safe seats for Democrats. In states like Louisiana, they are safe seats for Republicans (Figure).
Racial Gerrymandering and the Paradox of Minority Representation
In Ohio, one skirts the shoreline of Lake Erie like a snake. In Louisiana, one meanders across the southern part of the state from the eastern shore of Lake Ponchartrain, through much of New Orleans and north along the Mississippi River to Baton Rouge. And in Illinois, another wraps around the city of Chicago and its suburbs in a wandering line that, when seen on a map, looks like the mouth of a large, bearded alligator attempting to drink from Lake Michigan.
These aren’t geographical features or large infrastructure projects. Rather, they are racially gerrymandered congressional districts. Their strange shapes are the product of careful district restructuring organized around the goal of enhancing the votes of minority groups. The alligator-mouth District 4 in Illinois, for example, was drawn to bring a number of geographically autonomous Latino groups in Illinois together in the same congressional district.
While the strategy of creating majority-minority districts has been a success for minorities’ representation in Congress, its long-term effect has revealed a disturbing paradox: Congress as a whole has become less enthusiastic about minority-specific issues. How is this possible? The problem is that by creating districts with high percentages of minority constituents, strategists have made the other districts less diverse. The representatives in those districts are under very little pressure to consider the interests of minority groups. As a result, they typically do not.Steven Hill, “How the Voting Rights Act Hurts Democrats and Minorities,” The Atlantic, 17 June 2013, http://www.theatlantic.com/politics/archive/2013/06/how-the-voting-rights-act-hurts-democrats-and-minorities/276893/.
What changes might help correct this problem? Are majority-minority districts no longer an effective strategy for increasing minority representation in Congress? Are there better ways to achieve a higher level of minority representation?
CONGRESSIONAL POWERS
The authority to introduce and pass legislation is a very strong power. But it is only one of the many that Congress possesses. In general, congressional powers can be divided into three types: enumerated, implied, and inherent. An enumerated power is a power explicitly stated in the Constitution. An implied power is one not specifically detailed in the Constitution but inferred as necessary to achieve the objectives of the national government. And an inherent power, while not enumerated or implied, must be assumed to exist as a direct result of the country’s existence. In this section, we will learn about each type of power and the foundations of legitimacy they claim. We will also learn about the way the different branches of government have historically appropriated powers not previously granted to them and the way congressional power has recently suffered in this process.
Article I, Section 8, of the U.S. Constitution details the enumerated powers of the legislature. These include the power to levy and collect taxes, declare war, raise an army and navy, coin money, borrow money, regulate commerce among the states and with foreign nations, establish federal courts and bankruptcy rules, establish rules for immigration and naturalization, and issue patents and copyrights. Other powers, such as the ability of Congress to override a presidential veto with a two-thirds vote of both houses, are found elsewhere in the Constitution (Article II, Section 7, in the case of the veto override). The first of these enumerated powers, to levy taxes, is quite possibly the most important power Congress possesses. Without it, most of the others, whether enumerated, implied, or inherent, would be largely theoretical. The power to levy and collect taxes, along with the appropriations power, gives Congress what is typically referred to as “the power of the purse” (Figure). This means Congress controls the money.
Some enumerated powers invested in the Congress were included specifically to serve as checks on the other powerful branches of government. These include Congress’s sole power to introduce legislation, the Senate’s final say on many presidential nominations and treaties signed by the president, and the House’s ability to impeach or formally accuse the president or other federal officials of wrongdoing (the first step in removing the person from office; the second step, trial and removal, takes place in the U.S. Senate). Each of these powers also grants Congress oversight of the actions of the president and his or her administration—that is, the right to review and monitor other bodies such as the executive branch. The fact that Congress has the sole power to introduce legislation effectively limits the power of the president to develop the same laws he or she is empowered to enforce. The Senate’s exclusive power to give final approval for many of the president’s nominees, including cabinet members and judicial appointments, compels the president to consider the needs and desires of Congress when selecting top government officials. Finally, removing a president from office who has been elected by the entire country should never be done lightly. Giving this responsibility to a large deliberative body of elected officials ensures it will occur only very rarely.
Despite the fact that the Constitution outlines specific enumerated powers, most of the actions Congress takes on a day-to-day basis are not actually included in this list. The reason is that the Constitution not only gives Congress the power to make laws but also gives it some general direction as to what those laws should accomplish. The “necessary and proper cause” directs Congress “to make all Laws which shall be necessary and proper for carrying into Execution the foregoing Powers, and all other Powers vested by this Constitution in the Government of the United States, or in any Department or Officer thereof.” Laws that regulate banks, establish a minimum wage, and allow for the construction and maintenance of interstate highways are all possible because of the implied powers granted by the necessary and proper clause. Today, the overwhelming portion of Congress’s work is tied to the necessary and proper clause.
Finally, Congress’s inherent powers are unlike either the enumerated or the implied powers. Inherent powers are not only not mentioned in the Constitution, but they do not even have a convenient clause in the Constitution to provide for them. Instead, they are powers Congress has determined it must assume if the government is going to work at all. The general assumption is that these powers were deemed so essential to any functioning government that the framers saw no need to spell them out. Such powers include the power to control borders of the state, the power to expand the territory of the state, and the power to defend itself from internal revolution or coups. These powers are not granted to the Congress, or to any other branch of the government for that matter, but they exist because the country exists.
Understanding the Limits of Congress’s Power to Regulate
One of the most important constitutional anchors for Congress’s implicit power to regulate all manner of activities within the states is the short clause in Article I, Section 8, which says Congress is empowered to “to regulate Commerce with foreign Nations, and among the several States, and with Indian Tribes.” The Supreme Court’s broad interpretation of this so-called commerce clause has greatly expanded the power and reach of Congress over the centuries.
From the earliest days of the republic until the end of the nineteenth century, the Supreme Court consistently handed down decisions that effectively broadened the Congress’s power to regulate interstate and intrastate commerce.Lainie Rutkow and Jon S. Vernick. 2011. “The U.S. Constitution’s Commerce Clause, the Supreme Court, and Public Health,” Public Health Report 126, No. 5 (September–October): 750–753. The growing country, the demands of its expanding economy, and the way changes in technology and transportation contributed to the shrinking of space between the states demanded that Congress be able to function as a regulator. For a short period in the 1930s when federal authority was expanded to combat the Great Depression, the Court began to interpret the commerce clause far more narrowly. But after this interlude, the court’s interpretation swung in an even-broader direction. This change proved particularly important in the 1960s, when Congress rolled back racial segregation throughout much of the South and beyond, and in the 1970s, as federal environmental regulations and programs took root.
But in United States v. Lopez, a decision issued in 1995, the Court changed course again and, for the first time in half a century, struck down a law as an unconstitutional overstepping of the commerce clause.United States v. Lopez, 514 U.S. 549 (1995). Five years later, the Court did it again, convincing many that the country may be witnessing the beginning of a rollback in Congress’s power to regulate in the states. When the Patient Protection and Affordable Care Act (also known as the ACA, or Obamacare) came before the Supreme Court in 2012, many believed the Court would strike it down. Instead, the justices took the novel approach of upholding the law based on the Congress’s enumerated power to tax, rather than the commerce clause. The decision was a shock to many.National Federation of Independent Businesses v. Sebelius, 567 U.S. ___ (2012). And, by not upholding the law on the basis of the commerce clause, the Court left open the possibility that it would continue to pursue a narrower interpretation of the clause.
What are the advantages of the Supreme Court’s broad interpretation of the commerce clause? How do you think this interpretation affects the balance of power between the branches of government? Why are some people concerned that the Court’s view of the clause could change?
In the early days of the republic, Congress’s role was rarely if ever disputed. However, with its decision in Marbury v. Madison (1803), the Supreme Court asserted its authority over judicial review and assumed the power to declare laws unconstitutional.Marbury v. Madison, 5 U.S. 137 (1803). Yet, even after that decision, the Court was reluctant to use this power and didn’t do so for over half a century. Initially, the presidency was also a fairly weak branch of government compared with the legislature. But presidents have sought to increase their power almost from the beginning, typically at the expense of the Congress. By the nature of the enumerated powers provided to the president, it is during wartime that the chief executive is most powerful and Congress least powerful. For example, President Abraham Lincoln, who oversaw the prosecution of the Civil War, stretched the bounds of his legal authority in a number of ways, such as by issuing the Emancipation Proclamation that freed slaves in the confederate states.“Abraham Lincoln: Impact and Legacy,” http://millercenter.org/president/biography/lincoln-impact-and-legacy (May 24, 2016).
In the twentieth century, the modern tussle over power between the Congress and the president really began. There are two primary reasons this struggle emerged. First, as the country grew larger and more complex, the need for the government to assert its regulatory power grew. The executive branch, because of its hierarchical organization with the president at the top, is naturally seen as a more smoothly run governmental machine than the cumbersome Congress. This gives the president advantages in the struggle for power and indeed gives Congress an incentive to delegate authority to the president on processes, such as trade agreements and national monument designations, that would be difficult for the legislature to carry out. The second reason has to do with the president’s powers as commander-in-chief in the realm of foreign policy.
The twin disasters of the Great Depression in the 1930s and World War II, which lasted until the mid-1940s, provided President Franklin D. Roosevelt with a powerful platform from which to expand presidential power. His popularity and his ability to be elected four times allowed him to greatly overshadow Congress. As a result, Congress attempted to restrain the power of the presidency by proposing the Twenty-Second Amendment to the Constitution, which limited a president to only two full terms in office.David M. Jordan. 2011. FDR, Dewey, and the Election of 1944. Bloomington: Indiana University Press, 290; Paul G. Willis and George L. Willis. 1952. “The Politics of the Twenty-Second Amendment,” The Western Political Quarterly 5, No. 3: 469–82; Paul B. Davis. 1979. “The Results and Implications of the Enactment of the Twenty-Second Amendment,” Presidential Studies Quarterly 9, No. 3: 289–303. Although this limitation is a significant one, it has not held back the tendency for the presidency to assume increased power.
In the decades following World War II, the United States entered the Cold War, a seemingly endless conflict with the Soviet Union without actual war, and therefore a period that allowed the presidency to assert more authority, especially in foreign affairs. In an exercise of this increased power, in the 1950s, President Harry Truman effectively went around an enumerated power of Congress by sending troops into battle in Korea without a congressional declaration of war (Figure). By the time of the Kennedy administration in the 1960s, the presidency had assumed nearly all responsibility for creating foreign policy, effectively shutting Congress out.
Following the twin scandals of Vietnam and Watergate in the early 1970s, Congress attempted to assert itself as a coequal branch, even in creating foreign policy, but could not hold back the trend. The War Powers Resolution (covered in the foreign policy chapter) was intended to strengthen congressional war powers but ended up clarifying presidential authority in the first sixty days of a military conflict. The war on terrorism after 9/11 has also strengthened the president’s hand. Today, the seemingly endless bickering between the president and the Congress is a reminder of the ongoing struggle for power between the branches, and indeed between the parties, in Washington, DC.
Summary
The weaknesses of the Articles of Confederation convinced the member states to send delegates to a new convention to revise them. What emerged from the debates and compromises of the convention was instead a new and stronger constitution. The Constitution established a bicameral legislature, with a Senate composed of two members from each state and a House of Representatives composed of members drawn from each state in proportion to its population. Today’s Senate has one hundred members representing fifty states, while membership in the House of Representatives has been capped at 435 since 1929. Apportionment in the House is based on population data collected by the U.S. Census Bureau.
The Constitution empowers Congress with enumerated, implied, and inherent powers. Enumerated powers are specifically addressed in the text of the Constitution. Implied powers are not explicitly called out but are inferred as necessary to achieve the objectives of the national goverment. Inherent powers are assumed to exist by virtue of the fact that the country exists. The power of Congress to regulate interstate and intrastate commerce has generally increased, while its power to control foreign policy has declined over the course of the twentieth century.
The Great Compromise successfully resolved differences between ________.
- large and small states
- slave and non-slave states
- the Articles of Confederation and the Constitution
- the House and the Senate
Hint:
A
While each state has two senators, members of the House are apportioned ________.
- according to the state’s geographic size
- based on the state’s economic size
- according to the state’s population
- based on each state’s need
The process of redistricting can present problems for congressional representation because ________.
- districts must include urban and rural areas
- states can gain but never lose districts
- districts are often drawn to benefit partisan groups
- states have been known to create more districts than they have been apportioned
Hint:
C
Which of the following is an implied power of Congress?
- the power to regulate the sale of tobacco in the states
- the power to increase taxes on the wealthiest one percent
- the power to put the president on trial for high crimes
- the power to override a presidential veto
Briefly explain the benefits and drawbacks of a bicameral system.
Hint:
A primary benefit of a bicameral system is the way it demands careful consideration and deliberate action on the part of the legislators. A primary drawback is that it is tougher overall to pass legislation and makes it extremely difficult to push through large-scale reforms.
What are some examples of the enumerated powers granted to Congress in the Constitution?
Why does a strong presidency necessarily sap power from Congress?
Hint:
The executive and legislative branches complement and check each other. The purpose of dividing their roles is to prevent either from becoming too powerful. As a result, when one branch assumes more power, it necessarily assumes that power from the other branch.
|
oercommons
|
2025-03-18T00:37:57.672276
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15248/overview",
"title": "American Government, Delivering Collective Action: Formal Institutions",
"author": null
}
|
https://oercommons.org/courseware/lesson/15249/overview
|
Congressional Elections
Learning Objectives
By the end of this section, you will be able to:
- Explain how fundamental characteristics of the House and Senate shape their elections
- Discuss campaign funding and the effects of incumbency in the House and Senate
- Analyze the way congressional elections can sometimes become nationalized
The House and Senate operate very differently, partly because their members differ in the length of their terms, as well as in their age and other characteristics. In this section, we will explore why constitutional rules affect the elections for the two types of representatives and the reason the two bodies function differently by design. We also look at campaign finance to better understand how legislators get elected and stay elected.
UNDERSTANDING THE HOUSE AND SENATE
The U.S. Constitution is very clear about who can be elected as a member of the House or Senate. A House member must be a U.S. citizen of at least seven years’ standing and at least twenty-five years old. Senators are required to have nine years’ standing as citizens and be at least thirty years old when sworn in. Representatives serve two-year terms, whereas senators serve six-year terms. Per the Supreme Court decision in U.S. Term Limits v. Thornton (1995), there are currently no term limits for either senators or representatives, despite efforts by many states to impose them in the mid-1990s.U.S. Term Limits, Inc. v. Thornton, 514 U.S. 779 (1995). House members are elected by the voters in their specific congressional districts. There are currently 435 congressional districts in the United States and thus 435 House members, and each state has a number of House districts roughly proportional to its share of the total U.S. population, with states guaranteed at least one House member. Two senators are elected by each state.
The structural and other differences between the House and Senate have practical consequences for the way the two chambers function. The House of Representatives has developed a stronger and more structured leadership than the Senate. Because its members serve short, two-year terms, they must regularly answer to the demands of their constituency when they run for election or reelection. Even House members of the same party in the same state will occasionally disagree on issues because of the different interests of their specific districts. Thus, the House can be highly partisan at times.
In contrast, members of the Senate are furthest from the demands and scrutiny of their constituents. Because of their longer six-year terms, they will see every member of the House face his or her constituents multiple times before they themselves are forced to seek reelection. Originally, when a state’s two U.S. senators were appointed by the state legislature, the Senate chamber’s distance from the electorate was even greater. Also, unlike members of the House who can seek the narrower interests of their district, senators must maintain a broader appeal in order to earn a majority of the votes across their entire state. In addition, the rules of the Senate allow individual members to slow down or stop legislation they dislike. These structural differences between the two chambers create real differences in the actions of their members. The heat of popular, sometimes fleeting, demands from constituents often glows red hot in the House. The Senate has the flexibility to allow these passions to cool. Dozens of major initiatives were passed by the House and had a willing president, for example, only to be defeated in the Senate. In 2012, the Buffett Rule would have implemented a minimum tax rate of 30 percent on wealthy Americans. Sixty senators had to agree to bring it to a vote, but the bill fell short of that number and died.http://dailysignal.com/2015/11/11/12-bills-that-the-filibuster-stopped-from-becoming-law/ (May 15, 2016). Similarly, although the ACA became widely known as “Obamacare,” the president did not send a piece of legislation to Capitol Hill; he asked Congress to write the bills. Both the House and Senate authored their own versions of the legislation. The House’s version was much bolder and larger in terms of establishing a national health care system. However, it did not stand a chance in the Senate, where a more moderate version of the legislation was introduced. In the end, House leaders saw the Senate version as preferable to doing nothing and ultimately supported it.
CONGRESSIONAL CAMPAIGN FUNDING
Modern political campaigns in the United States are expensive, and they have been growing more so. For example, in 1986, the costs of running a successful House and Senate campaign were $776,687 and $6,625,932, respectively, in 2014 dollars. By 2014, those values had shot to $1,466,533 and $9,655,660 (Figure).“The Cost of Winning a House and Senate Seat, 1986–2014,” http://www.cfinst.org/pdf/vital/VitalStats_t1.pdf (May 15, 2016). Raising this amount of money takes quite a bit of time and effort. Indeed, a presentation for incoming Democratic representatives suggested a daily Washington schedule of five hours reaching out to donors, while only three or four were to be used for actual congressional work. As this advice reveals, raising money for reelection constitutes a large proportion of the work a congressperson does. This has caused many to wonder whether the amount of money in politics has truly become a corrupting influence. However, overall, the lion’s share of direct campaign contributions in congressional elections comes from individual donors, who are less influential than the political action committees (PACs) that contribute the remainder.http://www.opensecrets.org/overview/wherefrom.php (May 15, 2016).
Nevertheless, the complex problem of funding campaigns has a long history in the United States. For nearly the first hundred years of the republic, there were no federal campaign finance laws. Then, between the late nineteenth century and the start of World War I, Congress pushed through a flurry of reforms intended to bring order to the world of campaign finance. These laws made it illegal for politicians to solicit contributions from civil service workers, made corporate contributions illegal, and required candidates to report their fundraising. As politicians and donors soon discovered, however, these laws were full of loopholes and were easily skirted by those who knew the ins and outs of the system.
Another handful of reform attempts were therefore pushed through in the wake of World War II, but then Congress neglected campaign finance reform for a few decades. That lull ended in the early 1970s when the Federal Election Campaign Act was passed. Among other things, it created the Federal Election Commission (FEC), required candidates to disclose where their money was coming from and where they were spending it, limited individual contributions, and provided for public financing of presidential campaigns.
Another important reform occurred in 2002, when Senators John McCain (R-AZ) and Russell Feingold (D-WI) drafted, and Congress passed, the Bipartisan Campaign Reform Act (BCRA), also referred to as the McCain-Feingold Act. The purpose of this law was to limit the use of “soft money,” which is raised for purposes like party-building efforts, get-out-the-vote efforts, and issue-advocacy ads. Unlike “hard money” contributed directly to a candidate, which is heavily regulated and limited, soft money had almost no regulations or limits. It had never been a problem before the mid-1990s, when a number of very imaginative political operatives developed a great many ways to spend this money. After that, soft-money donations skyrocketed. But the McCain-Feingold bill greatly limited this type of fundraising.
McCain-Feingold placed limits on total contributions to political parties, prohibited coordination between candidates and PAC campaigns, and required candidates to include personal endorsements on their political ads. Until 2010, it also limited advertisements run by unions and corporations thirty days before a primary and sixty days before a general election.“Bipartisan Campaign Reform Act of 2002,” http://www.fec.gov/pages/bcra/bcra_update.shtml (May 15, 2016); Greg Scott and Gary Mullen, “Thirty Year Report,” September 2005, http://www.fec.gov/info/publications/30year.pdf (May 15, 2016). The FEC’s enforcement of the law spurred numerous court cases challenging it. The most controversial decision was handed down by the Supreme Court in 2010, whose ruling on Citizens United v. Federal Election Commission led to the removal of spending limits on corporations. Justices in the majority argued that the BCRA violated a corporation’s free-speech rights.Citizens United v. Federal Election Commission, 558 U.S. 310 (2010).
The Citizens United case began as a lawsuit against the FEC filed by Citizens United, a nonprofit organization that wanted to advertise a documentary critical of former senator and Democratic hopeful Hillary Clinton on the eve of the 2008 Democratic primaries. Advertising or showing the film during this time window was prohibited by the McCain-Feingold Act. But the Court found that this type of restriction violated the organization’s First Amendment right to free speech. As critics of the decision predicted at the time, the Court thus opened the floodgates to private soft money flowing into campaigns again.
In the wake of the Citizens United decision, a new type of advocacy group emerged, the super PAC. A traditional PAC is an organization designed to raise hard money to elect or defeat candidates. Such PACs tended to be run by businesses and other groups, like the Teamsters Union and the National Rifle Association, to support their special interests. They are highly regulated in regard to the amount of money they can take in and spend, but super PACs aren’t bound by these regulations. While they cannot give money directly to a candidate or a candidate’s party, they can raise and spend unlimited funds, and they can spend independently of a campaign or party. In the 2012 election cycle, for example, super PACs spent just over $600 million dollars and raised about $200 million more.“2012 Outside Spending, by Super PAC,” https://www.opensecrets.org/outsidespending/summ.php?cycle=2012&chrt=V&type=S (May 15, 2016).
At the same time, several limits on campaign contributions have been upheld by the courts and remain in place. Individuals may contribute up to $2700 per candidate per election. Individuals may also give $5000 to PACs and $33,400 to a national party committee. PACs that contribute to more than one candidate are permitted to contribute $5000 per candidate per election, and up to $15,000 to a national party. PACs created to give money to only one candidate are limited to only $2700 per candidate, however (Figure).“Contribution Limits for the 2015-2016 Federal Elections,” http://www.fec.gov/info/contriblimitschart1516.pdf (May 15, 2016). The amounts are adjusted every two years, based on inflation. These limits are intended to create a more equal playing field for the candidates, so that candidates must raise their campaign funds from a broad pool of contributors.
The Center for Responsive Politics reports donation amounts that are required by law to be disclosed to the Federal Elections Commission. One finding is that, counter to conventional wisdom, the vast majority of direct campaign contributions come from individual donors, not from PACs and political parties.
INCUMBENCY EFFECTS
Not surprisingly, the jungle of campaign financing regulations and loopholes is more easily navigated by incumbents in Congress than by newcomers. Incumbents are elected officials who currently hold an office. The amount of money they raise against their challengers demonstrates their advantage. In 2014, for example, the average Senate incumbent raised $12,144,933, whereas the average challenger raised only $1,223,566.“Incumbent Advantage,” http://www.opensecrets.org/overview/incumbs.php?cycle=2014 (May 15, 2016). This is one of the many reasons incumbents win a large majority of congressional races each electoral cycle. Incumbents attract more money because people want to give to a winner. In the House, the percentage of incumbents winning reelection has hovered between 85 and 100 percent for the last half century. In the Senate, there is only slightly more variation, given the statewide nature of the race, but it is still a very high majority of incumbents who win reelection (Figure). As these rates show, even in the worst political environments, incumbents are very difficult to defeat.
The historical difficulty of unseating an incumbent in the House or Senate is often referred to as the incumbent advantage or the incumbency effect. The advantage in financing is a huge part of this effect, but it is not the only important part. Incumbents often have a much higher level of name recognition. All things being equal, voters are far more likely to select the name of the person they recall seeing on television and hearing on the radio for the last few years than the name of a person they hardly know. And donors are more likely to want to give to a proven winner.
But more important is the way the party system itself privileges incumbents. A large percentage of congressional districts across the country are “safe seats” in uncompetitive districts, meaning candidates from a particular party are highly likely to consistently win the seat. This means the functional decision in these elections occurs during the primary, not in the general election. Political parties in general prefer to support incumbents in elections, because the general consensus is that incumbents are better candidates, and their record of success lends support to this conclusion. That said, while the political parties themselves to a degree control and regulate the primaries, popular individual candidates and challengers sometimes rule the day. This has especially been the case in recent years as conservative incumbents have been “primaried” by challengers more conservative than they.
The End of Incumbency Advantage?
At the start of 2014, House majority whip Eric Cantor, a representative from Virginia, was at the top of his game. He was handsome, popular with talk show hosts and powerful insiders, an impressive campaign fundraiser and speaker, and apparently destined to become Speaker of the House when the current speaker stepped down. Four months later, Cantor lost the opportunity to run for his own congressional seat in a shocking primary election upset that shook the Washington political establishment to its core.
What happened? How did such a powerful incumbent lose a game in which the cards had been stacked so heavily in his favor? Analyses of the stunning defeat quickly showed there were more chinks in Cantor’s polished armor than most wanted to admit. But his weakness wasn’t that he was unable to play the political game. Rather, he may have learned to play it too well. He became seen as too much of a Washington insider.
Cantor’s ambition, political skill, deep connections to political insiders, and ability to come out squeaky clean after even the dirtiest political tussling should have given him a clear advantage over any competitor. But in the political environment of 2014, when conservative voices around the country criticized the party for ignoring the people and catering to political insiders, his strengths became weaknesses. Indeed, Cantor was the only highest-level Republican representative sacrificed to conservative populism.
Were the winds of change blowing for incumbents? Between 1946 and 2012, only 5 percent of incumbent senators and 2 percent of House incumbents lost their party primaries.Larry J. Sabato, Kyle Kondik, and Geoffrey Skelley, “Long Odds for Most Senate Primary Challenges,” 30 January 2014, http://www.centerforpolitics.org/crystalball/articles/long-odds-for-most-senate-primary-challenges/ (May 1, 2016). In 2014, Cantor was one of four House incumbents who did so, while no incumbent senators suffered defeat. All evidence suggests the incumbent advantage, especially in the primary system, is alive and well. The story of Eric Cantor may very well be the classic case of an exception proving the rule.
If you are a challenger running against an incumbent, what are some strategies you could use to make the race competitive? Would Congress operate differently if challengers defeated incumbents more often?
Another reason incumbents wield a great advantage over their challengers is the state power they have at their disposal.David R. Mayhew. 1974. Congress: The Electoral Connection. New Haven, CT: Yale University Press. One of the many responsibilities of a sitting congressperson is constituent casework. Constituents routinely reach out to their congressperson for powerful support to solve complex problems, such as applying for and tracking federal benefits or resolving immigration and citizenship challenges.R. Eric Petersen, “Casework in a Congressional Office: Background, Rules, Laws, and Resources,” 24 November 2014, https://www.fas.org/sgp/crs/misc/RL33209.pdf (May 1, 2016). Incumbent members of Congress have paid staff, influence, and access to specialized information that can help their constituents in ways other persons cannot. And congresspersons are hardly reticent about their efforts to support their constituents. Often, they will publicize their casework on their websites or, in some cases, create television advertisements that boast of their helpfulness. Election history has demonstrated that this form of publicity is very effective in garnering the support of voters.
LOCAL AND NATIONAL ELECTIONS
The importance of airing positive constituent casework during campaigns is a testament to the accuracy of saying, “All politics is local.” This phrase, attributed to former Speaker of the House Tip O’Neill (D-MA), essentially means that the most important motivations directing voters are rooted in local concerns. In general, this is true. People naturally feel more driven by the things that affect them on a daily basis. These are concerns like the quality of the roads, the availability of good jobs, and the cost and quality of public education. Good senators and representatives understand this and will seek to use their influence and power in office to affect these issues for the better. This is an age-old strategy for success in office and elections.
Political scientists have taken note of some voting patterns that appear to challenge this common assumption, however. In 1960, political scientist Angus Campbell proposed the surge-and-decline theory to explain these patterns.Angus Campbell. 1960. “Surge and Decline: A Study of Electoral Change.” The Public Opinion Quarterly 24, No. 3: 397–418. Campbell noticed that since the Civil War, with the exception of 1934, the president’s party has consistently lost seats in Congress during the midterm elections. He proposed that the reason was a surge in political stimulation during presidential elections, which contributes to greater turnout and brings in voters who are ordinarily less interested in politics. These voters, Campbell argued, tend to favor the party holding the presidency. In contrast, midterm elections witness the opposite effect. They are less stimulating and have lower turnout because less-interested voters stay home. This shift, in Campbell’s theory, provides an advantage to the party not currently occupying the presidency.
In the decades since Campbell’s influential theory was published, a number of studies have challenged his conclusions. Nevertheless, the pattern of midterm elections benefiting the president’s opposition has persisted.“Midterm congressional elections explained: Why the president’s party typically loses,” 1 October 2014, http://journalistsresource.org/studies/politics/elections/voting-patterns-midterm-congressional-elections-why-presidents-party-typically-loses (May 1, 2016). Only in exceptional years has this pattern been broken: first in 1998 during President Bill Clinton’s second term and the Monica Lewinsky scandal, when exit polls indicated most voters opposed the idea of impeaching the president, and then again in 2002, following the 9/11 terrorist attacks and the ensuing declaration of a “war on terror.”
The evidence does suggest that national concerns, rather than local ones, can function as powerful motivators at the polls. Consider, for example, the role of the Iraq War in bringing about a Democratic rout of the Republicans in the House in 2006 and in the Senate in 2008. Unlike previous wars in Europe and Vietnam, the war in Iraq was fought by a very small percentage of the population.“A Profile of the Modern Military,” 5 October 2011, http://www.pewsocialtrends.org/2011/10/05/chapter-6-a-profile-of-the-modern-military/ (May 1, 2016). The vast majority of citizens were not soldiers, few had relatives fighting in the war, and most did not know anyone who directly suffered from the prolonged conflict. Voters in large numbers were motivated by the political and economic disaster of the war to vote for politicians they believed would end it (Figure).
Congressional elections may be increasingly driven by national issues. Just two decades ago, straight-ticket, party-line voting was still relatively rare across most of the country.Dhrumil Mehta and Harry Enten, “The 2014 Senate Elections Were the Most Nationalized In Decades,” 2 December 2014, http://fivethirtyeight.com/datalab/the-2014-senate-elections-were-the-most-nationalized-in-decades/ (May 1, 2016); Gregory Giroux, “Straight-Ticket Voting Rises As Parties Polarize,” Bloomberg, 29 November 2014, http://www.bloomberg.com/politics/articles/2014-11-29/straightticket-voting-rises-as-parties-polarize (May 1, 2016). In much of the South, which began to vote overwhelmingly Republican in presidential elections during the 1960s and 1970s, Democrats were still commonly elected to the House and Senate. The candidates themselves and the important local issues, apart from party affiliation, were important drivers in congressional elections. This began to change in the 1980s and 1990s, as Democratic representatives across the region began to dwindle. And the South isn’t alone; areas in the Northeast and the Northwest have grown increasingly Democratic. Indeed, the 2014 midterm election was the most nationalized election in many decades. Voters who favor a particular party in a presidential election are now much more likely to also support that same party in House and Senate elections than was the case just a few decades ago.
Summary
Since the House is closest to its constituents because reelection is so frequent a need, it tends to be more easily led by fleeting public desires. In contrast, the Senate’s distance from its constituents allows it to act more deliberately. Each type of representative, however, must raise considerable sums of money in order to stay in office. Attempts by Congress to rein in campaign spending have largely failed. Nevertheless, incumbents tend to have the easiest time funding campaigns and retaining their seats. They also benefit from the way parties organize primary elections, which are designed to promote incumbency.
Senate races tend to inspire ________.
- broad discussion of policy issues
- narrow discussion of specific policy issues
- less money than House races
- less media coverage than House races
The saying “All politics is local” roughly means ________.
- the local candidate will always win
- the local constituents want action on national issues
- the local constituents tend to care about things that affect them
- the act of campaigning always occurs at the local level where constituents are
Hint:
C
What does Campbell’s surge-and-decline theory suggest about the outcome of midterm elections?
Explain the factors that make it difficult to oust incumbents.
Hint:
Incumbents chase off would-be challengers because they are able to raise more money given that people want to back a winner and that voters know incumbents by name because they won the office in a previous election. The challengers who do take on incumbents typically lose soundly for the same reasons.
|
oercommons
|
2025-03-18T00:37:57.704789
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15249/overview",
"title": "American Government, Delivering Collective Action: Formal Institutions",
"author": null
}
|
https://oercommons.org/courseware/lesson/15250/overview
|
Congressional Representation
Learning Objectives
By the end of this section, you will be able to:
- Explain the basics of representation
- Describe the extent to which Congress as a body represents the U.S. population
- Explain the concept of collective representation
- Describe the forces that influence congressional approval ratings
The tension between local and national politics described in the previous section is essentially a struggle between interpretations of representation. Representation is a complex concept. It can mean paying careful attention to the concerns of constituents, understanding that representatives must act as they see fit based on what they feel best for the constituency, or relying on the particular ethnic, racial, or gender diversity of those in office. In this section, we will explore three different models of representation and the concept of descriptive representation. We will look at the way members of Congress navigate the challenging terrain of representation as they serve, and all the many predictable and unpredictable consequences of the decisions they make.
TYPES OF REPRESENTATION: LOOKING OUT FOR CONSTITUENTS
By definition and title, senators and House members are representatives. This means they are intended to be drawn from local populations around the country so they can speak for and make decisions for those local populations, their constituents, while serving in their respective legislative houses. That is, representation refers to an elected leader’s looking out for his or her constituents while carrying out the duties of the office.Steven S. Smith. 1999. The American Congress. Boston, MA: Houghton Mifflin.
Theoretically, the process of constituents voting regularly and reaching out to their representatives helps these congresspersons better represent them. It is considered a given by some in representative democracies that representatives will seldom ignore the wishes of constituents, especially on salient issues that directly affect the district or state. In reality, the job of representing in Congress is often quite complicated, and elected leaders do not always know where their constituents stand. Nor do constituents always agree on everything. Navigating their sometimes contradictory demands and balancing them with the demands of the party, powerful interest groups, ideological concerns, the legislative body, their own personal beliefs, and the country as a whole can be a complicated and frustrating process for representatives.
Traditionally, representatives have seen their role as that of a delegate, a trustee, or someone attempting to balance the two. A representative who sees him- or herself as a delegate believes he or she is empowered merely to enact the wishes of constituents. Delegates must employ some means to identify the views of their constituents and then vote accordingly. They are not permitted the liberty of employing their own reason and judgment while acting as representatives in Congress. This is the delegate model of representation.
In contrast, a representative who understands their role to be that of a trustee believes he or she is entrusted by the constituents with the power to use good judgment to make decisions on the constituents’ behalf. In the words of the eighteenth-century British philosopher Edmund Burke, who championed the trustee model of representation, “Parliament is not a congress of ambassadors from different and hostile interests . . . [it is rather] a deliberative assembly of one nation, with one interest, that of the whole.”Edmund Burke, “Speech to the Electors of Bristol,” 3 November 1774, http://press-pubs.uchicago.edu/founders/documents/v1ch13s7.html (May 1, 2016). In the modern setting, trustee representatives will look to party consensus, party leadership, powerful interests, the member’s own personal views, and national trends to better identify the voting choices they should make.
Understandably, few if any representatives adhere strictly to one model or the other. Instead, most find themselves attempting to balance the important principles embedded in each. Political scientists call this the politico model of representation. In it, members of Congress act as either trustee or delegate based on rational political calculations about who is best served, the constituency or the nation.
For example, every representative, regardless of party or conservative versus liberal leanings, must remain firm in support of some ideologies and resistant to others. On the political right, an issue that demands support might be gun rights; on the left, it might be a woman’s right to an abortion. For votes related to such issues, representatives will likely pursue a delegate approach. For other issues, especially complex questions the public at large has little patience for, such as subtle economic reforms, representatives will tend to follow a trustee approach. This is not to say their decisions on these issues run contrary to public opinion. Rather, it merely means they are not acutely aware of or cannot adequately measure the extent to which their constituents support or reject the proposals at hand. It could also mean that the issue is not salient to their constituents. Congress works on hundreds of different issues each year, and constituents are likely not aware of the particulars of most of them.
DESCRIPTIVE REPRESENTATION IN CONGRESS
In some cases, representation can seem to have very little to do with the substantive issues representatives in Congress tend to debate. Instead, proper representation for some is rooted in the racial, ethnic, socioeconomic, gender, and sexual identity of the representatives themselves. This form of representation is called descriptive representation.
At one time, there was relatively little concern about descriptive representation in Congress. A major reason is that until well into the twentieth century, white men of European background constituted an overwhelming majority of the voting population. African Americans were routinely deprived of the opportunity to participate in democracy, and Hispanics and other minority groups were fairly insignificant in number and excluded by the states. While women in many western states could vote sooner, all women were not able to exercise their right to vote nationwide until passage of the Nineteenth Amendment in 1920, and they began to make up more than 5 percent of either chamber only in the 1990s.
Many advances in women’s rights have been the result of women’s greater engagement in politics and representation in the halls of government, especially since the founding of the National Organization for Women in 1966 and the National Women’s Political Caucus (NWPC) in 1971. The NWPC was formed by Bella Abzug (Figure), Gloria Steinem, Shirley Chisholm, and other leading feminists to encourage women’s participation in political parties, elect women to office, and raise money for their campaigns. For example, Patsy Mink (D-HI) (Figure), the first Asian American woman elected to Congress, was the coauthor of the Education Amendments Act of 1972, Title IX of which prohibits sex discrimination in education. Mink had been interested in fighting discrimination in education since her youth, when she opposed racial segregation in campus housing while a student at the University of Nebraska. She went to law school after being denied admission to medical school because of her gender. Like Mink, many other women sought and won political office, many with the help of the NWPC. Today, EMILY’s List, a PAC founded in 1985 to help elect pro-choice Democratic women to office, plays a major role in fundraising for female candidates. In the 2012 general election, 80 percent of the candidates endorsed by EMILY’s List won a seat.“Claire McCaskill, Emily’s List Celebrate Women’s Wins in 2012,” 14 November 2012, http://abcnews.go.com/blogs/politics/2012/11/claire-mccaskill-emilys-list-celebrate-womens-wins-in-2012/ (May 1, 2016).
In the wake of the Civil Rights Movement, African American representatives also began to enter Congress in increasing numbers. In 1971, to better represent their interests, these representatives founded the Congressional Black Caucus (CBC), an organization that grew out of a Democratic select committee formed in 1969. Founding members of the CBC include John Conyers (D-MI), currently the longest-serving member of the House of Representatives, Charles Rangel (D-NY), and Shirley Chisholm, a founder of the NWPC and the first African American woman to be elected to the House of Representatives (Figure).
In recent decades, Congress has become much more descriptively representative of the United States. The 114th Congress, which began in January 2015, had a historically large percentage of racial and ethnic minorities. African Americans made up the largest percentage, with forty-eight members, while Latinos accounted for thirty-two members, up from nineteen just over a decade before.Jennifer E. Manning, “Membership of the 114th Congress: A Profile,” 1 December 2015, http://www.senate.gov/CRSReports/crs-publish.cfm?pid=%260BL*RLC2%0A (May 15, 2016); “The Congressional Hispanic Caucus and Conference,” http://history.house.gov/Exhibitions-and-Publications/HAIC/Historical-Essays/Strength-Numbers/Caucus-Conference/ (May 15, 2016). Yet, demographically speaking, Congress as a whole is still a long way from where the country is and remains largely white, male, and wealthy. For example, although more than half the U.S. population is female, only 20 percent of Congress is. Congress is also overwhelmingly Christian (Figure).
REPRESENTING CONSTITUENTS
Ethnic, racial, gender, or ideological identity aside, it is a representative’s actions in Congress that ultimately reflect his or her understanding of representation. Congress members’ most important function as lawmakers is writing, supporting, and passing bills. And as representatives of their constituents, they are charged with addressing those constituents’ interests. Historically, this job has included what some have affectionately called “bringing home the bacon” but what many (usually those outside the district in question) call pork-barrel politics. As a term and a practice, pork-barrel politics—federal spending on projects designed to benefit a particular district or set of constituents—has been around since the nineteenth century, when barrels of salt pork were both a sign of wealth and a system of reward. While pork-barrel politics are often deplored during election campaigns, and earmarks—funds appropriated for specific projects—are no longer permitted in Congress (see feature box below), legislative control of local appropriations nevertheless still exists. In more formal language, allocation, or the influencing of the national budget in ways that help the district or state, can mean securing funds for a specific district’s project like an airport, or getting tax breaks for certain types of agriculture or manufacturing.
Language and Metaphor
The language and metaphors of war and violence are common in politics. Candidates routinely “smell blood in the water,” “battle for delegates,” go “head-to-head,” “cripple” their opponent, and “make heads roll.” But references to actual violence aren’t the only metaphorical devices commonly used in politics. Another is mentions of food. Powerful speakers frequently “throw red meat to the crowds;” careful politicians prefer to stick to “meat-and-potato issues;” and representatives are frequently encouraged by their constituents to “bring home the bacon.” And the way members of Congress typically “bring home the bacon” is often described with another agricultural metaphor, the “earmark.”
In ranching, an earmark is a small cut on the ear of a cow or other animal to denote ownership. Similarly, in Congress, an earmark is a mark in a bill that directs some of the bill’s funds to be spent on specific projects or for specific tax exemptions. Since the 1980s, the earmark has become a common vehicle for sending money to various projects around the country. Many a road, hospital, and airport can trace its origins back to a few skillfully drafted earmarks.
Relatively few people outside Congress had ever heard of the term before the 2008 presidential election, when Republican nominee Senator John McCain touted his career-long refusal to use the earmark as a testament to his commitment to reforming spending habits in Washington.“Statement by John McCain on Banning Earmarks,” 13 March 2008, http://www.presidency.ucsb.edu/ws/?pid=90739 (May 15, 2016); “Press Release - John McCain’s Economic Plan,” 15 April 2008, http://www.presidency.ucsb.edu/ws/?pid=94082 (May 15, 2016). McCain’s criticism of the earmark as a form of corruption cast a shadow over a previously common legislative practice. As the country sank into recession and Congress tried to use spending bills to stimulate the economy, the public grew more acutely aware of its earmarking habits. Congresspersons then were eager to distance themselves from the practice. In fact, the use of earmarks to encourage Republicans to help pass health care reform actually made the bill less popular with the public.
In 2011, after Republicans took over the House, they outlawed earmarks. But with deadlocks and stalemates becoming more common, some quiet voices have begun asking for a return to the practice. They argue that Congress works because representatives can satisfy their responsibilities to their constituents by making deals. The earmarks are those deals. By taking them away, Congress has hampered its own ability to “bring home the bacon.”
Are earmarks a vital part of legislating or a corrupt practice that was rightly jettisoned? Pick a cause or industry, and investigate whether any earmarks ever favored it, or research the way earmarks have hurt or helped your state or district, and decide for yourself.
Follow-up activity: Find out where your congressional representative stands on the ban on earmarks and write to support or dissuade him or her.
Such budgetary allocations aren’t always looked upon favorably by constituents. Consider, for example, the passage of the ACA in 2010. The desire for comprehensive universal health care had been a driving position of the Democrats since at least the 1960s. During the 2008 campaign, that desire was so great among both Democrats and Republicans that both parties put forth plans. When the Democrats took control of Congress and the presidency in 2009, they quickly began putting together their plan. Soon, however, the politics grew complex, and the proposed plan became very contentious for the Republican Party.
Nevertheless, the desire to make good on a decades-old political promise compelled Democrats to do everything in their power to pass something. They offered sympathetic members of the Republican Party valuable budgetary concessions; they attempted to include allocations they hoped the opposition might feel compelled to support; and they drafted the bill in a purposely complex manner to avoid future challenges. These efforts, however, had the opposite effect. The Republican Party’s constituency interpreted the allocations as bribery and the bill as inherently flawed, and felt it should be scrapped entirely. The more Democrats dug in, the more frustrated the Republicans became (Figure).
The Republican opposition, which took control of the House during the 2010 midterm elections, promised constituents they would repeal the law. Their attempts were complicated, however, by the fact that Democrats still held the Senate and the presidency. Yet, the desire to represent the interests of their constituents compelled Republicans to use another tool at their disposal, the symbolic vote. During the 112th and 113th Congresses, Republicans voted more than sixty times to either repeal or severely limit the reach of the law. They understood these efforts had little to no chance of ever making it to the president’s desk. And if they did, he would certainly have vetoed them. But it was important for these representatives to demonstrate to their constituents that they understood their wishes and were willing to act on them.
Historically, representatives have been able to balance their role as members of a national legislative body with their role as representatives of a smaller community. The Obamacare fight, however, gave a boost to the growing concern that the power structure in Washington divides representatives from the needs of their constituency.Kathleen Parker, “Health-Care Reform’s Sickeningly Sweet Deals,” The Washington Post, 10 March 2010, http://www.washingtonpost.com/wp-dyn/content/article/2010/03/09/AR2010030903068.html (May 1, 2016); Dana Milbank, “Sweeteners for the South,” The Washington Post, 22 November 2009, http://www.washingtonpost.com/wp-dyn/content/article/2009/11/21/AR2009112102272.html (May 1, 2016); Jeffry H. Anderson, “Nebraska’s Dark-Horse Candidate and the Cornhusker Kickback,” The Weekly Standard, 4 May 2014. This has exerted pressure on representatives to the extent that some now pursue a more straightforward delegate approach to representation. Indeed, following the 2010 election, a handful of Republicans began living in their offices in Washington, convinced that by not establishing a residence in Washington, they would appear closer to their constituents at home.Phil Hirschkorn and Wyatt Andrews, “One-Fifth of House Freshmen Sleep in Offices,” CBS News, 22 January 2011, http://www.cbsnews.com/news/one-fifth-of-house-freshmen-sleep-in-offices/ (May 1, 2016).
COLLECTIVE REPRESENTATION AND CONGRESSIONAL APPROVAL
The concept of collective representation describes the relationship between Congress and the United States as a whole. That is, it considers whether the institution itself represents the American people, not just whether a particular member of Congress represents his or her district. Predictably, it is far more difficult for Congress to maintain a level of collective representation than it is for individual members of Congress to represent their own constituents. Not only is Congress a mixture of different ideologies, interests, and party affiliations, but the collective constituency of the United States has an even-greater level of diversity. Nor is it a solution to attempt to match the diversity of opinions and interests in the United States with those in Congress. Indeed, such an attempt would likely make it more difficult for Congress to maintain collective representation. Its rules and procedures require Congress to use flexibility, bargaining, and concessions. Yet, it is this flexibility and these concessions, which many now interpret as corruption, that tend to engender the high public disapproval ratings experienced by Congress.
After many years of deadlocks and bickering on Capitol Hill, the national perception of Congress is near an all-time low. According to Gallup polls, Congress has a stunningly poor approval rating of about 16 percent. This is unusual even for a body that has rarely enjoyed a high approval rating. For example, for nearly two decades following the Watergate scandal in the early 1970s, the national approval rating of Congress hovered between 30 and 40 percent.“Congress and the Public,” http://www.gallup.com/poll/1600/congress-public.aspx (May 15, 2016).
Yet, incumbent reelections have remained largely unaffected. The reason has to do with the remarkable ability of many in the United States to separate their distaste for Congress from their appreciation for their own representative. Paradoxically, this tendency to hate the group but love one’s own representative actually perpetuates the problem of poor congressional approval ratings. The reason is that it blunts voters’ natural desire to replace those in power who are earning such low approval ratings.
As decades of polling indicate, few events push congressional approval ratings above 50 percent. Indeed, when the ratings are graphed, the two noticeable peaks are at 57 percent in 1998 and 84 percent in 2001 (Figure). In 1998, according to Gallup polling, the rise in approval accompanied a similar rise in other mood measures, including President Bill Clinton’s approval ratings and general satisfaction with the state of the country and the economy. In 2001, approval spiked after the September 11 terrorist attacks and the Bush administration launched the "War on Terror," sending troops first to Afghanistan and later to Iraq. War has the power to bring majorities of voters to view their Congress and president in an overwhelmingly positive way.“Congress and the Public,” http://www.gallup.com/poll/1600/congress-public.aspx (May 15, 2016).
Nevertheless, all things being equal, citizens tend to rate Congress more highly when things get done and more poorly when things do not get done. For example, during the first half of President Obama’s first term, Congress’s approval rating reached a relative high of about 40 percent. Both houses were dominated by members of the president’s own party, and many people were eager for Congress to take action to end the deep recession and begin to repair the economy. Millions were suffering economically, out of work, or losing their jobs, and the idea that Congress was busy passing large stimulus packages, working on finance reform, and grilling unpopular bank CEOs and financial titans appealed to many. Approval began to fade as the Republican Party slowed the wheels of Congress during the tumultuous debates over Obamacare and reached a low of 9 percent following the federal government shutdown in October 2013.
One of the events that began the approval rating’s downward trend was Congress’s divisive debate over national deficits. A deficit is what results when Congress spends more than it has available. It then conducts additional deficit spending by increasing the national debt. Many modern economists contend that during periods of economic decline, the nation should run deficits, because additional government spending has a stimulative effect that can help restart a sluggish economy. Despite this benefit, voters rarely appreciate deficits. They see Congress as spending wastefully during a time when they themselves are cutting costs to get by.
The disconnect between the common public perception of running a deficit and its legitimate policy goals is frequently exploited for political advantage. For example, while running for the presidency in 2008, Barack Obama slammed the deficit spending of the George W. Bush presidency, saying it was “unpatriotic.” This sentiment echoed complaints Democrats had been issuing for years as a weapon against President Bush’s policies. Following the election of President Obama and the Democratic takeover of the Senate, the concern over deficit spending shifted parties, with Republicans championing a spendthrift policy as a way of resisting Democratic policies.
Find your representative at the U.S. House website and then explore his or her website and social media accounts to see whether the issues on which your representative spends time are the ones you think are most appropriate.
Summary
Some representatives follow the delegate model of representation, acting on the expressed wishes of their constituents, whereas others take a trustee model approach, acting on what they believe is in their constituents’ best interests. However, most representatives combine the two approaches and apply each as political circumstances demand. The standard method by which representatives have shown their fidelity to their constituents, namely “bringing home the bacon” of favorable budget allocations, has come to be interpreted as a form of corruption, or pork-barrel politics.
Representation can also be considered in other ways. Descriptive representation is the level at which Congress reflects the nation’s constituents in terms of race, ethnicity, gender, sexuality, and socioeconomic status. Collective representation is the extent to which the institutional body of Congress represents the population as a whole. Despite the incumbency advantage and high opinion many hold of their own legislators, Congress rarely earns an approval rating above 40 percent, and for a number of years the rating has been well below 20 percent.
A congressperson who pursued a strict delegate model of representation would seek to ________.
- legislate in the way he or she believed constituents wanted, regardless of the anticipated outcome
- legislate in a way that carefully considered the circumstances and issue so as to reach a solution that is best for everyone
- legislate in a way that is best for the nation regardless of the costs for the constituents
- legislate in the way that he or she thinks is best for the constituents
The increasing value constituents have placed on descriptive representation in Congress has had the effect of ________.
- increasing the sensitivity representatives have to their constituents demands
- decreasing the rate at which incumbents are elected
- increasing the number of minority members in Congress
- decreasing the number of majority minority districts
Hint:
C
How has the growing interpretation of earmarks and other budget allocations as corruption influenced the way congresspersons work?
What does polling data suggest about the events that trigger exceptionally high congressional approval ratings?
Hint:
The peaks of congressional approval ratings have each occurred when the United States began military involvements overseas. This suggests that the start of a foreign war is one of the few things that triggers a positive reevaluation of Congress.
|
oercommons
|
2025-03-18T00:37:57.738254
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15250/overview",
"title": "American Government, Delivering Collective Action: Formal Institutions",
"author": null
}
|
https://oercommons.org/courseware/lesson/15251/overview
|
House and Senate Organizations
Learning Objectives
By the end of this section, you will be able to:
- Explain the division of labor in the House and in the Senate
- Describe the way congressional committees develop and advance legislation
Not all the business of Congress involves bickering, political infighting, government shutdowns, and Machiavellian maneuvering. Congress does actually get work done. Traditionally, it does this work in a very methodical way. In this section, we will explore how Congress functions at the leadership and committee levels. We will learn how the party leadership controls their conferences and how the many committees within Congress create legislation that can then be moved forward or die on the floor.
PARTY LEADERSHIP
The party leadership in Congress controls the actions of Congress. Leaders are elected by the two-party conferences in each chamber. In the House of Representatives, these are the House Democratic Conference and the House Republican Conference. These conferences meet regularly and separately not only to elect their leaders but also to discuss important issues and strategies for moving policy forward. Based on the number of members in each conference, one conference becomes the majority conference and the other becomes the minority conference. Independents like Senator Bernie Sanders will typically join one or the other major party conference, as a matter of practicality and often based on ideological affinity. Without the membership to elect their own leadership, independents would have a very difficult time getting things done in Congress unless they had a relationship with the leaders.
Despite the power of the conferences, however, the most important leadership position in the House is actually elected by the entire body of representatives. This position is called the Speaker of the House and is the only House officer mentioned in the Constitution. The Constitution does not require the Speaker to be a member of the House, although to date, all fifty-four Speakers have been. The Speaker is the presiding officer, the administrative head of the House, the partisan leader of the majority party in the House, and an elected representative of a single congressional district (Figure). As a testament to the importance of the Speaker, since 1947, the holder of this position has been second in line to succeed the president in an emergency, after the vice president.
The Speaker serves until his or her party loses, or until he or she is voted out of the position or chooses to step down. Republican Speaker John Boehner became the latest Speaker to walk away from the position when it appeared his position was in jeopardy. This event shows how the party conference (or caucus) oversees the leadership as much as, if not more than, the leadership oversees the party membership in the chamber. The Speaker is invested with quite a bit of power, such as the ability to assign bills to committees and decide when a bill will be presented to the floor for a vote. The Speaker also rules on House procedures, often delegating authority for certain duties to other members. He or she appoints members and chairs to committees, creates select committees to fulfill a specific purpose and then disband, and can even select a member to be speaker pro tempore, who acts as Speaker in the Speaker’s absence. Finally, when the Senate joins the House in a joint session, the Speaker presides over these sessions, because they are usually held in the House of Representatives.
Below the Speaker, the majority and minority conferences each elect two leadership positions arranged in hierarchical order. At the top of the hierarchy are the floor leaders of each party. These are generally referred to as the majority and minority leaders. The minority leader has a visible if not always a powerful position. As the official leader of the opposition, he or she technically holds the rank closest to that of the Speaker, makes strategy decisions, and attempts to keep order within the minority. However, the majority rules the day in the House, like a cartel. On the majority side, because it holds the speakership, the majority leader also has considerable power. Historically, moreover, the majority leader tends to be in the best position to assume the speakership when the current Speaker steps down.
Below these leaders are the two party’s respective whips. A whip’s job, as the name suggests, is to whip up votes and otherwise enforce party discipline. Whips make the rounds in Congress, telling members the position of the leadership and the collective voting strategy, and sometimes they wave various carrots and sticks in front of recalcitrant members to bring them in line. The remainder of the leadership positions in the House include a handful of chairs and assistantships.
Like the House, the Senate also has majority and minority leaders and whips, each with duties very similar to those of their counterparts in the House. Unlike the House, however, the Senate doesn’t have a Speaker. The duties and powers held by the Speaker in the House fall to the majority leader in the Senate. Another difference is that, according to the U.S. Constitution, the Senate’s president is actually the elected vice president of the United States, but he or she may vote only in case of a tie. Apart from this and very few other exceptions, the president of the Senate does not actually operate in the Senate. Instead, the Constitution allows for the Senate to choose a president pro tempore—usually the most senior senator of the majority party—who presides over the Senate. Despite the title, the job is largely a formal and powerless role. The real power in the Senate is in the hands of the majority leader (Figure) and the minority leader. Like the Speaker of the House, the majority leader is the chief spokesperson for the majority party, but unlike in the House he or she does not run the floor alone. Because of the traditions of unlimited debate and the filibuster, the majority and minority leaders often occupy the floor together in an attempt to keep things moving along. At times, their interactions are intense and partisan, but for the Senate to get things done, they must cooperate to get the sixty votes needed to run this super-majority legislative institution.
THE COMMITTEE SYSTEM
With 535 members in Congress and a seemingly infinite number of domestic, international, economic, agricultural, regulatory, criminal, and military issues to deal with at any given moment, the two chambers must divide their work based on specialization. Congress does this through the committee system. Specialized committees (or subcommittees) in both the House and the Senate are where bills originate and most of the work that sets the congressional agenda takes place. Committees are roughly approximate to a bureaucratic department in the executive branch. There are well over two hundred committees, subcommittees, select committees, and joint committees in the Congress. The core committees are called standing committees. There are twenty standing committees in the House and sixteen in the Senate (Table).
| Congressional Standing and Permanent Select Committees | |
|---|---|
| House of Representatives | Senate |
| Agriculture | Agriculture, Nutrition, and Forestry |
| Appropriations | Appropriations |
| Armed Services | Armed Services |
| Budget | Banking, Housing, and Urban Affairs |
| Education and the Workforce | Budget |
| Energy and Commerce | Commerce, Science, and Transportation |
| Ethics | Energy and Natural Resources |
| Financial Services | Environment and Public Works |
| Foreign Affairs | Ethics (select) |
| Homeland Security | Finance |
| House Administration | Foreign Relations |
| Intelligence (select) | Health, Education, Labor and Pensions |
| Judiciary | Homeland Security and Governmental Affairs |
| Natural Resources | Indian Affairs (select) |
| Oversight and Government Reform | Intelligence (select) |
| Rules | Judiciary |
| Science, Space, and Technology | Rules and Administration |
| Small Business | Small Business and Entrepreneurship |
| Transportation and Infrastructure | Veterans’ Affairs |
| Veterans’ Affairs | |
| Ways and Means |
Members of both parties compete for positions on various committees. These positions are typically filled by majority and minority members to roughly approximate the ratio of majority to minority members in the respective chambers, although committees are chaired by members of the majority party. Committees and their chairs have a lot of power in the legislative process, including the ability to stop a bill from going to the floor (the full chamber) for a vote. Indeed, most bills die in committee. But when a committee is eager to develop legislation, it takes a number of methodical steps. It will reach out to relevant agencies for comment on resolutions to the problem at hand, such as by holding hearings with experts to collect information. In the Senate, committee hearings are also held to confirm presidential appointments (Figure). After the information has been collected, the committee meets to discuss amendments and legislative language. Finally, the committee will send the bill to the full chamber along with a committee report. The report provides the majority opinion about why the bill should be passed, a minority view to the contrary, and estimates of the proposed law’s cost and impact.
Four types of committees exist in the House and the Senate. The first is the standing, or permanent, committee. This committee is the first call for proposed bills, fewer than 10 percent of which are reported out of committee to the floor. The second type is the joint committee. Joint committee members are appointed from both the House and the Senate, and are charged with exploring a few key issues, such as the economy and taxation. However, joint committees have no bill-referral authority whatsoever—they are informational only. A conference committee is used to reconcile different bills passed in both the House and the Senate. The conference committees are appointed on an ad hoc basis as necessary when a bill passes the House and Senate in different forms. Finally, ad hoc, special, or select committees are temporary committees set up to address specific topics. These types of committees often conduct special investigations, such as on aging or ethics.
Committee hearings can become politically driven public spectacles. Consider the House Select Committee on Benghazi, the committee assembled by Republicans to further investigate the 2011 attacks on the U.S. Consulate in Benghazi, Libya. This prolonged investigation became particularly partisan as Republicans trained their guns on then-secretary of state Hillary Clinton, who was running for the presidency at the time. In two multi-hour hearings in which Secretary Clinton was the only witness, Republicans tended to grandstand in the hopes of gaining political advantage or tripping her up, while Democrats tended to use their time to ridicule Republicans (Figure).Amy Davidson, “The Hillary Hearing,” The New Yorker, 2 November 2015; David A. Graham, “What Conservative Media Say About the Benghazi Hearing,” The Atlantic, 23 October 2015. In the end, the long hearings uncovered little more than the elevated state of partisanship in the House, which had scarcely been a secret before.
Members of Congress bring to their roles a variety of specific experiences, interests, and levels of expertise, and try to match these to committee positions. For example, House members from states with large agricultural interests will typically seek positions on the Agriculture Committee. Senate members with a background in banking or finance may seek positions on the Senate Finance Committee. Members can request these positions from their chambers’ respective leadership, and the leadership also selects the committee chairs.
Committee chairs are very powerful. They control the committee’s budget and choose when the committee will meet, when it will hold hearings, and even whether it will consider a bill (Figure). A chair can convene a meeting when members of the minority are absent or adjourn a meeting when things are not progressing as the majority leadership wishes. Chairs can hear a bill even when the rest of the committee objects. They do not remain in these powerful positions indefinitely, however. In the House, rules prevent committee chairs from serving more than six consecutive years and from serving as the chair of a subcommittee at the same time. A senator may serve only six years as chair of a committee but may, in some instances, also serve as a chair or ranking member of another committee.
Because the Senate is much smaller than the House, senators hold more committee assignments than House members. There are sixteen standing committees in the Senate, and each position must be filled. In contrast, in the House, with 435 members and only twenty standing committees, committee members have time to pursue a more in-depth review of a policy. House members historically defer to the decisions of committees, while senators tend to view committee decisions as recommendations, often seeking additional discussion that could lead to changes.
Take a look at the scores of committees in the House and Senate. The late House Speaker Tip O’Neill once quipped that if you didn’t know a new House member’s name, you could just call him Mr. Chairperson.
Summary
The leader of the House is the Speaker, who also typically the leader of the majority party. In the Senate, the leader is called the majority leader. The minorities in each chamber also have leaders who help create and act on party strategies. The majority leadership in each chamber controls the important committees where legislature is written, amended, and prepared for the floor.
House leaders are more powerful than Senate leaders because of ________.
- the majoritarian nature of the House—a majority can run it like a cartel
- the larger size of the House
- the constitutional position of the House
- the State of the Union address being delivered in the House chamber
A select committee is different from a standing committee because ________.
- a select committee includes member of both chambers, while a standing committee includes only members of the House
- a select committee is used for bill reconciliation, while a standing committee is used for prosecutions
- a select committee must stay in session, while a standing committee goes to recess
- a select committee is convened for a specific and temporary purpose, while a standing committee is permanent
Hint:
D
Explain how the committees demonstrate a division of labor in Congress based on specialization.
|
oercommons
|
2025-03-18T00:37:57.767976
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15251/overview",
"title": "American Government, Delivering Collective Action: Formal Institutions",
"author": null
}
|
https://oercommons.org/courseware/lesson/15252/overview
|
The Legislative Process
Learning Objectives
By the end of this section, you will be able to:
- Explain the steps in the classic bill-becomes-law diagram
- Describe the modern legislative processes that alter the classic process in some way
A dry description of the function of congressional leadership and the many committees and subcommittees in Congress may suggest that the drafting and amending of legislation is a finely tuned process that has become ever more refined over the course of the last few centuries. In reality, however, committees are more likely to kill legislation than to pass it. And the last few decades have seen a dramatic transformation in the way Congress does business. Creative interpretations of rules and statues have turned small loopholes into the large gateways through which much congressional work now gets done. In this section, we will explore both the traditional legislative route by which a bill becomes a law and the modern incarnation of the process. We will also learn how and why the transformation occurred.
THE CLASSIC LEGISLATIVE PROCESS
The traditional process by which a bill becomes a law is called the classic legislative process. First, legislation must be drafted. Theoretically, anyone can do this. Much successful legislation has been initially drafted by someone who is not a member of Congress, such as a think tank or advocacy group, or the president. However, Congress is under no obligation to read or introduce this legislation, and only a bill introduced by a member of Congress can hope to become law. Even the president must rely on legislators to introduce his or her legislative agenda.
Technically, bills that raise revenue, like tax bills, must begin in the House. This exception is encoded within the Constitution in Article I, Section 7, which states, “All Bills for raising Revenue shall originate in the House of Representatives; but the Senate may propose or concur with amendments as on other Bills.” Yet, despite the seemingly clear language of the Constitution, Congress has found ways to get around this rule.
Once legislation has been proposed, however, the majority leadership consults with the parliamentarian about which committee to send it to. Each chamber has a parliamentarian, an advisor, typically a trained lawyer, who has studied the long and complex rules of the chamber. While Congress typically follows the advice of its parliamentarians, it is not obligated to, and the parliamentarian has no power to enforce his or her interpretation of the rules. Once a committee has been selected, the committee chair is empowered to move the bill through the committee process as he or she sees fit. This occasionally means the chair will refer the bill to one of the committee’s subcommittees.
Whether at the full committee level or in one of the subcommittees, the next step is typically to hold a hearing on the bill. If the chair decides to not hold a hearing, this is tantamount to killing the bill in committee. The hearing provides an opportunity for the committee to hear and evaluate expert opinions on the bill or aspects of it. Experts typically include officials from the agency that would be responsible for executing the bill, the bill’s sponsors from Congress, and industry lobbyists, interest groups, and academic experts from a variety of relevant fields. Typically, the committee will also accept written statements from the public concerning the bill in question. For many bills, the hearing process can be very routine and straightforward.
Once hearings have been completed, the bill enters the markup stage. This is essentially an amending and voting process. In the end, with or without amendments, the committee or subcommittee will vote. If the committee decides not to advance the bill at that time, it is tabled. Tabling a bill typically means the bill is dead, but there is still an option to bring it back up for a vote again. If the committee decides to advance the bill, however, it is printed and goes to the chamber, either the House or the Senate. For the sake of example, we will assume that a bill goes first to the House (although the reverse could be true, and, in fact, bills can move simultaneously through both chambers). Before it reaches the House floor, it must first go through the House Committee on Rules. This committee establishes the rules of debate, such as time limits and limits on the number and type of amendments. After these rules have been established, the bill moves through the floor, where it is debated and amendments can be added. Once the limits of debate and amendments have been reached, the House holds a vote. If a simple majority, 50 percent plus 1, votes to advance the bill, it moves out of the House and into the Senate.
Once in the Senate, the bill is placed on the calendar so it can be debated. Or, more typically, the Senate will also consider the bill (or a companion version) in its own committees. Since the Senate is much smaller than the House, it can afford to be much more flexible in its rules for debate. Typically, senators allow each other to talk and debate as long as the speaker wants, though they can agree as a body to create time limits. But without these limits, debate continues until a motion to table has been offered and voted on.
This flexibility about speaking in the Senate gave rise to a unique tactic, the filibuster. The word “filibuster” comes from the Dutch word vrijbuiter, which means pirate. And the name is appropriate, since a senator who launches a filibuster virtually hijacks the floor of the chamber by speaking for long periods of time, thus preventing the Senate from closing debate and acting on a bill. The tactic was perfected in the 1850s as Congress wrestled with the complicated issue of slavery. After the Civil War, the use of the filibuster became even more common. Eventually, in 1917, the Senate passed Rule 22, which allowed the chamber to hold a cloture vote to end debate. To invoke cloture, the Senate had to get a two-thirds majority. This was difficult to do, but it generally did prevent anyone from hijacking the Senate floor, with the salient exception of Senator Strom Thurmond’s record twenty-four-hour filibuster of the Civil Rights Act.
In 1975, after the heightened partisanship of the civil rights era, the Senate further weakened the filibuster by reducing the number needed for cloture from two-thirds to three-fifths, or sixty votes, where it remains today (except for judicial nominations for which only fifty-five votes are needed to invoke cloture). Moreover, filibusters are not permitted on the annual budget reconciliation act (the Reconciliation Act of 2010 was the act under which the implementing legislation for Obamacare was passed).
The Noble History of the Filibuster?
When most people think of the Senate filibuster, they probably picture actor Jimmy Stewart standing exasperated at a podium and demanding the Senate come to its senses and do the right thing. Even for those not familiar with the classic Frank Capra film Mr. Smith Goes to Washington, the image of a heroic single senator sanding up to the power of the entire chamber while armed only with oratorical skill naturally tends to inspire. Unfortunately, the history of the filibuster is less heartwarming.
This is not to say that noble causes haven’t been championed by filibustering senators; they most certainly have. But they have largely been overshadowed by the outright ridiculous and sometimes racist filibusters of the twentieth century. In the first category, the fifteen-and-a-half-hour marathon of Senator Huey Long of Louisiana stands out: Hoping to retain the need for Senate confirmation of some jobs he wanted to keep from his political enemies, Long spent much of his filibuster analyzing the Constitution, talking about his favorite recipes, and telling amusing stories, as was his custom.
In a defining moment for the filibuster, Senator Strom Thurmond of South Carolina spoke for twenty-four hours and eighteen minutes against a weak civil rights bill in 1957. A vocal proponent of segregation and white supremacy, Thurmond had made no secret of his views and had earlier run for the presidency on a segregationist platform. Nor was Thurmond the first to use the filibuster to preserve segregation and prevent the expansion of civil rights for African Americans. Groups of dedicated southern senators used the filibuster to prevent the passage of anti-lynching legislation on multiple occasions during the first half of the twentieth century. Later, when faced with the 1964 Civil Rights Act, southern senators staged a fifty-seven-day filibuster to try and kill it. But the momentum of the nation was against them. The bill passed over their obstructionism and helped to reduce segregation.
Is the filibuster the tool of the noble minority attempting to hold back the tide of a powerful minority? Or does its history as a weapon supporting segregation expose it as merely a tactic of obstruction?
Because both the House and the Senate can and often do amend bills, the bills that pass out of each chamber frequently look different. This presents a problem, since the Constitution requires that both chambers pass identical bills. One simple solution is for the first chamber to simply accept the bill that ultimately makes it out of the second chamber. Another solution is for first chamber to further amend the second chamber’s bill and send it back to the second chamber. Congress typically takes one of these two options, but about one in every eight bills cannot be resolved in this way. These bills must be sent to a conference committee that negotiates a reconciliation both chambers can accept without amendment. Only then can the bill progress to the president’s desk for signature or veto. If the president does veto the bill, both chambers must muster a two-thirds vote to overcome the veto and force the president to sign it. If the two-thirds threshold in each chamber cannot be reached, the bill dies (Figure).
For one look at the classic legislative process, visit YouTube to view “I’m Just a Bill” from the ABC Schoolhouse Rock! series.
MODERN LEGISLATION IS DIFFERENT
For much of the nation’s history, the process described above was the standard method by which a bill became a law. Over the course of the last three and a half decades, however, changes in rules and procedure have created a number of alternate routes. Collectively, these different routes constitute what some political scientists have described as a new but unorthodox legislative process. According to political scientist Barbara Sinclair, the primary trigger for the shift away from the classic legislative route was the budget reforms of the 1970s. The 1974 Budget and Impoundment Control Act gave Congress a mechanism for making large, all-encompassing, budget decisions. In the years that followed, the budget process gradually became the vehicle for creating comprehensive policy changes. One large step in this transformation occurred in 1981 when President Ronald Reagan’s administration suggested using the budget to push through his economic reforms.
The benefit of attaching the reforms to the budget resolution was that Congress could force an up or down (yea or nay) vote on the whole package. Such a packaged bill is called an omnibus bill.Glen S. Krutz. 2001. Hitching a Ride: Omnibus Legislating in the U.S. Congress. Columbus, OH: Ohio State University Press. Creating and voting for an omnibus bill allows Congress to quickly accomplish policy changes that would have taken many votes and the expending of great political capital over a long period of time. This and successive similar uses of the budget process convinced many in Congress of the utility of this strategy. During the contentious and ideologically divided 1990s, the budget process became the common problem-solving mechanism in the legislature, thus laying the groundwork for the way legislation works today.
An important characteristic feature of modern legislating is the greatly expanded power and influence of the party leadership over the control of bills. One reason for this change was the heightened partisanship that stretches back to the 1980s and is still with us today. With such high political stakes, the party leadership is reluctant to simply allow the committees to work things out on their own. In the House, the leadership uses special rules to guide bills through the legislative process and toward a particular outcome. Uncommon just a few decades ago, these now widely used rules restrict debate and options, and are designed to focus the attention of members.
The practice of multiple referrals, with which entire bills or portions of those bills are referred to more than one committee, greatly weakened the different specialization monopolies committees held primarily in the House but also to an extent in the Senate. With less control over the bills, committees naturally reached out to the leadership for assistance. Indeed, as a testament to its increasing control, the leadership may sometimes avoid committees altogether, preferring to work things out on the floor. And even when bills move through the committees, the leadership often seeks to adjust the legislation before it reaches the floor.
Another feature of the modern legislative process, exclusively in the Senate, is the application of the modern filibuster. Unlike the traditional filibuster, in which a senator took the floor and held it for as long as possible, the modern filibuster is actually a perversion of the cloture rules adopted to control the filibuster. When partisanship is high, as it has been frequently, the senators can request cloture before any bill can get a vote. This has the effect of increasing the number of votes needed for a bill to advance from a simple majority of fifty-one to a super majority of sixty. The effect is to give the Senate minority great power to obstruct if it is inclined to do so.
The Library of Congress’s Thomas website has provided scholars, citizens, and media with a bounty of readily available data on members and bills for more than two decades.
Summary
In the classic legislative process, bills are introduced and sent to the appropriate committee. Within the committees, hearings are held and the bill is debated and ultimately sent to the floor of the chamber. On the floor, the bill is debated and amended until passed or voted down. If passed, it moves to the second chamber where the debating and amending begins anew. Eventually, if the bill makes it that far, the two chambers meet in a joint committee to reconcile what are now two different bills. Over the last few decades, however, Congress has adopted a very different process whereby large pieces of legislation covering many different items are passed through the budgeting process. This method has had the effect of further empowering the leadership, to the detriment of the committees. The modern legislative process has also been affected by the increasing number of filibuster threats in the Senate and the use of cloture to forestall them.
Stopping a filibuster requires that ________.
- a majority of senators agree on the bill
- the speaker steps away from the podium
- the chamber votes for cloture
- the Speaker or majority leader intervenes
Hint:
C
Saying a bill is being marked up is just another way to say it is being ________.
- tabled
- neglected
- vetoed
- amended
The key means of advancing modern legislation is now ________.
- committees
- the actions of the leadership
- the budget process
- the filibuster
Hint:
C
Briefly explain the difference between the classic model of legislating and the modern process.
The framers of the Constitution designed the Senate to filter the output of the sometimes hasty House. Do you think this was a wise idea? Why or why not?
Congress has consistently expanded its own power to regulate commerce among and between the states. Should Congress have this power or should the Supreme Court reel it in? Why?
What does the trend toward descriptive representation suggest about what constituents value in their legislature? How might Congress overcome the fact that such representation does not always best serve constituents’ interests?
What factors contributed most to the transformation away from the classic legislative process and toward the new style?
Books:
Binder, Sarah A. 1997. Minority Rights, Majority Rule: Partisanship and the Development of Congress. Cambridge, UK: Cambridge University Press.
Davidson, Roger H. and Walter J. Oleszek. 1981. Congress and Its Members. Washington, DC: Congressional Quarterly Press.
Dodd, Lawrence C. and Bruce Ian Oppenheimer. 1981. Congress Reconsidered. Washington, DC: Congressional Quarterly Press.
Hofstadter, Richard. 1965. The Paranoid Style in American Politics, and Other Essays. New York: Knopf.
Mann, Thomas E. and Norman J. Ornstein. 2012. It’s Even Worse Than It Looks: How the American Constitutional System Collided with the New Politics of Extremism. New York: Basic Books.
Mayhew, David R. 1974. Congress: The Electoral Connection. New Haven, CT: Yale University Press.
Mutch, Robert E. 2014. Buying the Vote: A History of Campaign Finance Reform. Oxford: Oxford University Press.
Oleszek, Walter J. 1978. Congressional Procedures and the Policy Process. Washington: Congressional Quarterly Press.
Sinclair, Barbara. 1997. Unorthodox Lawmaking: New Legislative Processes in the U.S. Congress. Washington, DC: CQ Press.
Films:
1939. Mr. Smith Goes to Washington.
1957. A Face in the Crowd.
1962. Advise and Consent.
1972. The Candidate.
|
oercommons
|
2025-03-18T00:37:57.799815
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15252/overview",
"title": "American Government, Delivering Collective Action: Formal Institutions",
"author": null
}
|
https://oercommons.org/courseware/lesson/15253/overview
|
Introduction
The presidency is the most visible position in the U.S. government (Figure). During the Constitutional Convention of 1787, delegates accepted the need to empower a relatively strong and vigorous chief executive. But they also wanted this chief executive to be bound by checks from the other branches of the federal government as well as by the Constitution itself. Over time, the power of the presidency has grown in response to circumstances and challenges. However, to this day, a president must still work with the other branches to be most effective. Unilateral actions, in which the president acts alone on important and consequential matters, such as President Barack Obama’s strategy on the Iran nuclear deal, are bound to be controversial and suggest potentially serious problems within the federal government. Effective presidents, especially in peacetime, are those who work with the other branches through persuasion and compromise to achieve policy objectives.
What are the powers, opportunities, and limitations of the presidency? How does the chief executive lead in our contemporary political system? What guides his or her actions, including unilateral actions? If it is most effective to work with others to get things done, how does the president do so? What can get in the way of this goal? This chapter answers these and other questions about the nation’s most visible leader.
|
oercommons
|
2025-03-18T00:37:57.815071
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15253/overview",
"title": "American Government, Delivering Collective Action: Formal Institutions",
"author": null
}
|
https://oercommons.org/courseware/lesson/15254/overview
|
The Design and Evolution of the Presidency
Learning Objectives
By the end of this section, you will be able to:
- Explain the reason for the design of the executive branch and its plausible alternatives
- Analyze the way presidents have expanded presidential power and why
- Identify the limitations on a president's power
Since its invention at the Constitutional Convention of 1787, the presidential office has gradually become more powerful, giving its occupants a far-greater chance to exercise leadership at home and abroad. The role of the chief executive has changed over time, as various presidents have confronted challenges in domestic and foreign policy in times of war as well as peace, and as the power of the federal government has grown.
INVENTING THE PRESIDENCY
The Articles of Confederation made no provision for an executive branch, although they did use the term “president” to designate the presiding officer of the Confederation Congress, who also handled other administrative duties.Articles of Confederation, Article XI, 1781. The presidency was proposed early in the Constitutional Convention in Philadelphia by Virginia’s Edmund Randolph, as part of James Madison’s proposal for a federal government, which became known as the Virginia Plan. Madison offered a rather sketchy outline of the executive branch, leaving open whether what he termed the “national executive” would be an individual or a set of people. He proposed that Congress select the executive, whose powers and authority, and even length of term of service, were left largely undefined. He also proposed a “council of revision” consisting of the national executive and members of the national judiciary, which would review laws passed by the legislature and have the power of veto.Jack Rakove and Susan Zlomke. 1987. “James Madison and the Independent Executive,” Presidential Studies Quarterly 17, No. 2: 293–300.
Early deliberations produced agreement that the executive would be a single person, elected for a single term of seven years by the legislature, empowered to veto legislation, and subject to impeachment and removal by the legislature. New Jersey’s William Paterson offered an alternate model as part of his proposal, typically referred to as the small-state or New Jersey Plan. This plan called for merely amending the Articles of Confederation to allow for an executive branch made up of a committee elected by a unicameral Congress for a single term. Under this proposal, the executive committee would be particularly weak because it could be removed from power at any point if a majority of state governors so desired. Far more extreme was Alexander Hamilton’s suggestion that the executive power be entrusted to a single individual. This individual would be chosen by electors, would serve for life, and would exercise broad powers, including the ability to veto legislation, the power to negotiate treaties and grant pardons in all cases except treason, and the duty to serve as commander-in-chief of the armed forces (Figure).
Debate and discussion continued throughout the summer. Delegates eventually settled upon a single executive, but they remained at a loss for how to select that person. Pennsylvania’s James Wilson, who had triumphed on the issue of a single executive, at first proposed the direct election of the president. When delegates rejected that idea, he responded with the suggestion that electors, chosen throughout the nation, should select the executive. Over time, Wilson’s idea gained ground with delegates who were uneasy at the idea of an election by the legislature, which presented the opportunity for intrigue and corruption. The idea of a shorter term of service combined with eligibility for reelection also became more attractive to delegates. The framers of the Constitution struggled to find the proper balance between giving the president the power to perform the job on one hand and opening the way for a president to abuse power and act like a monarch on the other.
By early September, the Electoral College had emerged as the way to select a president for four years who was eligible for reelection. This process is discussed more fully in the chapter on elections. Today, the Electoral College consists of a body of 538 people called electors, each representing one of the fifty states or the District of Columbia, who formally cast votes for the election of the president and vice president (Figure). In forty-eight states and the District of Columbia, the candidate who wins the popular vote in November receives all the state’s electoral votes. In two states, Nebraska and Maine, the electoral votes are divided: The candidate who wins the popular vote in the state gets two electoral votes, but the winner of each congressional district also receives an electoral vote.
In the original design implemented for the first four presidential elections (1788–89, 1792, 1796, and 1800), the electors cast two ballots (but only one could go to a candidate from the elector’s state), and the person who received a majority won the election. The second-place finisher became vice president. Should no candidate receive a majority of the votes cast, the House of Representatives would select the president, with each state casting a single vote, while the Senate chose the vice president.
While George Washington was elected president twice with this approach, the design resulted in controversy in both the 1796 and 1800 elections. In 1796, John Adams won the presidency, while his opponent and political rival Thomas Jefferson was elected vice president. In 1800, Thomas Jefferson and his running mate Aaron Burr finished tied in the Electoral College. Jefferson was elected president in the House of Representatives on the thirty-sixth ballot. These controversies led to the proposal and ratification of the Twelfth Amendment, which couples a particular presidential candidate with that candidate’s running mate in a unified ticket.Tadahisa Kuroda. 1994. The Origins of the Twelfth Amendment: The Electoral College in the Early Republic, 1787-1804. Westport, CT: Greenwood Publishing.
For the last two centuries or so, the Twelfth Amendment has worked fairly well. But this doesn’t mean the arrangement is foolproof. For example, the amendment created a separate ballot for the vice president but left the rules for electors largely intact. One of those rules states that the two votes the electors cast cannot both be for “an inhabitant of the same state with themselves.”U.S. Constitution, Article II, Section 1. This rule means that an elector from, say, Louisiana, could not cast votes for a presidential candidate and a vice presidential candidate who were both from Louisiana; that elector could vote for only one of these people. The intent of the rule was to encourage electors from powerful states to look for a more diverse pool of candidates. But what would happen in a close election where the members of the winning ticket were both from the same state?
The nation almost found out in 2000. In the presidential election of that year, the Republican ticket won the election by a very narrow electoral margin. To win the presidency or vice presidency, a candidate must get 270 electoral votes (a majority). George W. Bush and Dick Cheney won by the skin of their teeth with just 271. Both, however, were living in Texas. This should have meant that Texas’s 32 electoral votes could have gone to only one or the other. Cheney anticipated this problem and had earlier registered to vote in Wyoming, where he was originally from and where he had served as a representative years earlier.Alan Clendenning, “Court: Cheney Is Wyoming Resident,” ABC News, 7 December 2000, http://abcnews.go.com/Politics/story?id=122289&page=1 (May 1, 2016). It’s hard to imagine that the 2000 presidential election could have been even more complicated than it was, but thanks to that seemingly innocuous rule in Article II of the Constitution, that was a real possibility.
Despite provisions for the election of a vice president (to serve in case of the president’s death, resignation, or removal through the impeachment process), and apart from the suggestion that the vice president should be responsible for presiding over the Senate, the framers left the vice president’s role undeveloped. As a result, the influence of the vice presidency has varied dramatically, depending on how much of a role the vice president is given by the president. Some vice presidents, such as Dan Quayle under President George H. W. Bush, serve a mostly ceremonial function, while others, like Dick Cheney under President George W. Bush, become a partner in governance and rival the White House chief of staff in terms of influence.
Read about James Madison’s evolving views of the presidency and the Electoral College.
In addition to describing the process of election for the presidency and vice presidency, the delegates to the Constitutional Convention also outlined who was eligible for election and how Congress might remove the president. Article II of the Constitution lays out the agreed-upon requirements—the chief executive must be at least thirty-five years old and a “natural born” citizen of the United States (or a citizen at the time of the Constitution’s adoption) who has been an inhabitant of the United States for at least fourteen years.U.S. Constitution, Article II, Section 1. While Article II also states that the term of office is four years and does not expressly limit the number of times a person might be elected president, after Franklin D. Roosevelt was elected four times (from 1932 to 1944), the Twenty-Second Amendment was proposed and ratified, limiting the presidency to two four-year terms.
An important means of ensuring that no president could become tyrannical was to build into the Constitution a clear process for removing the chief executive—impeachment. Impeachment is the act of charging a government official with serious wrongdoing; the Constitution calls this wrongdoing high crimes and misdemeanors. The method the framers designed required two steps and both chambers of the Congress. First, the House of Representatives could impeach the president by a simple majority vote. In the second step, the Senate could remove him or her from office by a two-thirds majority, with the chief justice of the Supreme Court presiding over the trial. Upon conviction and removal of the president, if that occurred, the vice president would become president.
Three presidents have faced impeachment proceedings in the House; none has been both impeached by the House and removed by the Senate. In the wake of the Civil War, President Andrew Johnson faced congressional contempt for decisions made during Reconstruction. President Richard Nixon faced an overwhelming likelihood of impeachment in the House for his cover-up of key information relating to the 1972 break-in at the Democratic Party’s campaign headquarters at the Watergate hotel and apartment complex. Nixon likely would have also been removed by the Senate, since there was strong bipartisan consensus for his impeachment and removal. Instead, he resigned before the House and Senate could exercise their constitutional prerogatives.
The most recent impeachment was of President Bill Clinton, brought on by his lying about an extramarital affair with a White House intern named Monica Lewinsky. House Republicans felt the affair and Clinton’s initial public denial of it rose to a level of wrongdoing worthy of impeachment. House Democrats believed it fell short of an impeachable offense and that a simply censure made better sense. Clinton's trial in the Senate went nowhere because too few Senators wanted to move forward with removing the president.
Thus, impeachment remains a rare event indeed and removal has never occurred. Still, the fact that a president could be impeached and removed is an important reminder of the role of the executive in the broader system of shared powers. The same outcome occurred in the case of Andrew Johnson in the nineteenth century though he came closer to the threshold of votes needed for removal than did Clinton.
The Constitution that emerged from the deliberations in Philadelphia treated the powers of the presidency in concise fashion. The president was to be commander-in-chief of the armed forces of the United States, negotiate treaties with the advice and consent of the Senate, and receive representatives of foreign nations (Figure). Charged to “take care that the laws be faithfully executed,” the president was given broad power to pardon those convicted of federal offenses, except for officials removed through the impeachment process.U.S. Constitution, Article II, Section 3. The chief executive would present to Congress information about the state of the union; call Congress into session when needed; veto legislation if necessary, although a two-thirds supermajority in both houses of Congress could override that veto; and make recommendations for legislation and policy as well as call on the heads of various departments to make reports and offer opinions.
Finally, the president’s job included nominating federal judges, including Supreme Court justices, as well as other federal officials, and making appointments to fill military and diplomatic posts. The number of judicial appointments and nominations of other federal officials is great. In recent decades, two-term presidents have nominated well over three hundred federal judges while in office.“Judgeship Appointments By President,” http://www.uscourts.gov/judges-judgeships/authorized-judgeships/judgeship-appointments-president (May 1, 2016). Moreover, new presidents nominate close to five hundred top officials to their Executive Office of the President, key agencies (such as the Department of Justice), and regulatory commissions (such as the Federal Reserve Board), whose appointments require Senate majority approval.G. Calvin Mackenzie, “The Real Invisible Hand: Presidential Appointees in the Administration of George W. Bush,” http://www.whitehousetransitionproject.org/wp-content/uploads/2016/03/PresAppt-GWB.pdf (May 1, 2016).
THE EVOLVING EXECUTIVE BRANCH
No sooner had the presidency been established than the occupants of the office, starting with George Washington, began acting in ways that expanded both its formal and informal powers. For example, Washington established a cabinet or group of advisors to help him administer his duties, consisting of the most senior appointed officers of the executive branch. Today, the heads of the fifteen executive departments serve as the president’s advisers.https://www.justice.gov/about (May 1, 2016). And, in 1793, when it became important for the United States to take a stand in the evolving European conflicts between France and other European powers, especially Great Britain, Washington issued a neutrality proclamation that extended his rights as diplomat-in-chief far more broadly than had at first been conceived.
Later presidents built on the foundation of these powers. Some waged undeclared wars, as John Adams did against the French in the Quasi-War (1798–1800). Others agreed to negotiate for significant territorial gains, as Thomas Jefferson did when he oversaw the purchase of Louisiana from France. Concerned that he might be violating the powers of the office, Jefferson rationalized that his not facing impeachment charges constituted Congress’s tacit approval of his actions. James Monroe used his annual message in 1823 to declare that the United States would consider it an intolerable act of aggression for European powers to intervene in the affairs of the nations of the Western Hemisphere. Later dubbed the Monroe Doctrine, this declaration of principles laid the foundation for the growth of American power in the twentieth century. Andrew Jackson employed the veto as a measure of policy to block legislative initiatives with which he did not agree and acted unilaterally when it came to depositing federal funds in several local banks around the country instead of in the Bank of the United States. This move changed the way vetoes would be used in the future. Jackson’s twelve vetoes were more than those of all prior presidents combined, and he issued them due to policy disagreements (their basis today) rather than as a legal tool to protect against encroachments by Congress on the president’s powers.
Of the many ways in which the chief executive’s power grew over the first several decades, the most significant was the expansion of presidential war powers. While Washington, Adams, and Jefferson led the way in waging undeclared wars, it was President James K. Polk who truly set the stage for the broad growth of this authority. In 1846, as the United States and Mexico were bickering over the messy issue of where Texas’s southern border lay, Polk purposely raised anxieties and ruffled feathers through his envoy in Mexico. He then responded to the newly heightened state of affairs by sending U.S. troops to the Rio Grande, the border Texan expansionists claimed for Texas. Mexico sent troops in response, and the Mexican-American War began soon afterward.Fred Greenstein. 2010. “The Policy-Driven Leadership of James K. Polk: Making the Most of a Weak Presidency,” Presidential Studies Quarterly 40, No. 4: 725–33.
Abraham Lincoln, a member of Congress at the time, was critical of Polk’s actions. Later, however, as president himself, Lincoln used presidential war powers and the concepts of military necessity and national security to undermine the Confederate effort to seek independence for the Southern states. In suspending the privilege of the writ of habeas corpus, Lincoln blurred the boundaries between acceptable dissent and unacceptable disloyalty. He also famously used a unilateral proclamation to issue the Emancipation Proclamation, which cited the military necessity of declaring millions of slaves in Confederate-controlled territory to be free. His successor, Andrew Johnson, became so embroiled with Radical Republicans about ways to implement Reconstruction policies and programs after the Civil War that the House of Representatives impeached him, although the legislators in the Senate were unable to successfully remove him from office.Michael Les Benedict. 1973. “A New Look at the Impeachment of Andrew Johnson,” Political Science Quarterly 88, No. 3: 349–67.
Over the course of the twentieth century, presidents expanded and elaborated upon these powers. The rather vague wording in Article II, which says that the “executive power shall be vested” in the president, has been subject to broad and sweeping interpretation in order to justify actions beyond those specifically enumerated in the document.U.S. Constitution, Article II, Section 1. As the federal bureaucracy expanded, so too did the president’s power to grow agencies like the Secret Service and the Federal Bureau of Investigation. Presidents also further developed the concept of executive privilege, the right to withhold information from Congress, the judiciary, or the public. This right, not enumerated in the Constitution, was first asserted by George Washington to curtail inquiry into the actions of the executive branch.Mark J. Rozel. 1999. “’The Law': Executive Privilege: Definition and Standards of Application,” Presidential Studies Quarterly 29, No. 4: 918–30. The more general defense of its use by White House officials and attorneys ensures that the president can secure candid advice from his or her advisors and staff members.
Increasingly over time, presidents have made more use of their unilateral powers, including executive orders, rules that bypass Congress but still have the force of law if the courts do not overturn them. More recently, presidents have offered their own interpretation of legislation as they sign it via signing statements (discussed later in this chapter) directed to the bureaucratic entity charged with implementation. In the realm of foreign policy, Congress permitted the widespread use of executive agreements to formalize international relations, so long as important matters still came through the Senate in the form of treaties.Glen S. Krutz and Jeffrey S. Peake. 2009. Treaty Politics and the Rise of Executive Agreements: International Commitments in a System of Shared Powers. Ann Arbor: University of Michigan Press. Recent presidents have continued to rely upon an ever more expansive definition of war powers to act unilaterally at home and abroad. Finally, presidents, often with Congress's blessing through the formal delegation of authority, have taken the lead in framing budgets, negotiating budget compromises, and at times impounding funds in an effort to prevail in matters of policy.
The Budget and Accounting Act of 1921
Developing a budget in the nineteenth century was a chaotic mess. Unlike the case today, in which the budgeting process is centrally controlled, Congresses in the nineteenth century developed a budget in a piecemeal process. Federal agencies independently submitted budget requests to Congress, and these requests were then considered through the congressional committee process. Because the government was relatively small in the first few decades of the republic, this approach was sufficient. However, as the size and complexity of the U.S. economy grew over the course of the nineteenth century, the traditional congressional budgeting process was unable to keep up.Charles Stewart. 1989. Budget Reform Politics: The Design of the Appropriations Process in the House of Representatives, 1865-1921. New York: Cambridge University Press.
Things finally came to a head following World War I, when federal spending and debt skyrocketed. Reformers proposed the solution of putting the executive branch in charge of developing a budget that could be scrutinized, amended, and approved by Congress. However, President Woodrow Wilson, owing to a provision tacked onto the bill regarding presidential appointments, vetoed the legislation that would have transformed the budgeting process in this way. His successor, Warren Harding, felt differently and signed the Budget and Accounting Act of 1921. The act gave the president first-mover advantage in the budget process via the first “executive budget.” It also created the first-ever budget staff at the disposal of a president, at the time called the Bureau of the Budget but decades later renamed the Office of Management and Budget (Figure). With this act, Congress willingly delegated significant authority to the executive and made the president the chief budget agenda setter.
The Budget Act of 1921 effectively shifted some congressional powers to the president. Why might Congress have felt it important to centralize the budgeting process in the executive branch? What advantages could the executive branch have over the legislative branch in this regard?
The growth of presidential power is also attributable to the growth of the United States and the power of the national government. As the nation has grown and developed, so has the office. Whereas most important decisions were once made at the state and local levels, the increasing complexity and size of the domestic economy have led people in the United States to look to the federal government more often for solutions. At the same time, the rising profile of the United States on the international stage has meant that the president is a far more important figure as leader of the nation, as diplomat-in-chief, and as commander-in-chief. Finally, with the rise of electronic mass media, a president who once depended on newspapers and official documents to distribute information beyond an immediate audience can now bring that message directly to the people via radio, television, and social media. Major events and crises, such as the Great Depression, two world wars, the Cold War, and the war on terrorism, have further contributed to presidential stature.
Summary
The delegates at the Constitutional Convention proposed creating the office of the president and debated many forms the role might take. The president is elected for a maximum of two four-year terms and can be impeached by Congress for wrongdoing and removed from office. The presidency and presidential power, especially war powers, have expanded greatly over the last two centuries, often with the willing assistance of the legislative branch. Executive privilege and executive orders are two of the presidency’s powerful tools. During the last several decades, historical events and new technologies such as radio, television, and the Internet have further enhanced the stature of the presidency.
Many at the Continental Congress were skeptical of allowing presidents to be directly elected by the legislature because ________.
- they were worried about giving the legislature too much power
- they feared the opportunities created for corruption
- they knew the weaknesses of an electoral college
- they worried about subjecting the commander-in-chief to public scrutiny
Hint:
B
Which of the following is a way George Washington expanded the power of the presidency?
- He refused to run again after serving two terms.
- He appointed the heads of various federal departments as his own advisors.
- He worked with the Senate to draft treaties with foreign countries.
- He submitted his neutrality proclamation to the Senate for approval.
How did presidents who served in the decades directly after Washington expand the powers of the presidency?
Hint:
John Adams expanded the war powers by waging undeclared war, Thomas Jefferson negotiated the purchase of Louisiana from France, and James Monroe took direct control of foreign policymaking when he issued the Monroe Doctrine.
What factors contributed to the growth of presidential power in the twentieth century?
|
oercommons
|
2025-03-18T00:37:57.844870
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15254/overview",
"title": "American Government, Delivering Collective Action: Formal Institutions",
"author": null
}
|
https://oercommons.org/courseware/lesson/15255/overview
|
The Presidential Election Process
Learning Objectives
By the end of this section, you will be able to:
- Describe changes over time in the way the president and vice president are selected
- Identify the stages in the modern presidential selection process
- Assess the advantages and disadvantages of the Electoral College
The process of electing a president every four years has evolved over time. This evolution has resulted from attempts to correct the cumbersome procedures first offered by the framers of the Constitution and as a result of political parties’ rising power to act as gatekeepers to the presidency. Over the last several decades, the manner by which parties have chosen candidates has trended away from congressional caucuses and conventions and towards a drawn-out series of state contests, called primaries and caucuses, which begin in the winter prior to the November general election.
SELECTING THE CANDIDATE: THE PARTY PROCESS
The framers of the Constitution made no provision in the document for the establishment of political parties. Indeed, parties were not necessary to select the first president, since George Washington ran unopposed. Following the first election of Washington, the political party system gained steam and power in the electoral process, creating separate nomination and general election stages. Early on, the power to nominate presidents for office bubbled up from the party operatives in the various state legislatures and toward what was known as the king caucus or congressional caucus. The caucus or large-scale gathering was made up of legislators in the Congress who met informally to decide on nominees from their respective parties. In somewhat of a countervailing trend in the general election stage of the process, by the presidential election of 1824, many states were using popular elections to choose their electors. This became important in that election when Andrew Jackson won the popular vote and the largest number of electors, but the presidency was given to John Quincy Adams instead. Out of the frustration of Jackson’s supporters emerged a powerful two-party system that took control of the selection process.Daniel Myron Greene. 1908. “The Evolution of the National Political Convention,” The Sewanee Review 16, No. 2: 228–32.
In the decades that followed, party organizations, party leaders, and workers met in national conventions to choose their nominees, sometimes after long struggles that took place over multiple ballots. In this way, the political parties kept a tight control on the selection of a candidate. In the early twentieth century, however, some states began to hold primaries, elections in which candidates vied for the support of state delegations to the party’s nominating convention. Over the course of the century, the primaries gradually became a far more important part of the process, though the party leadership still controlled the route to nomination through the convention system. This has changed in recent decades, and now a majority of the delegates are chosen through primary elections, and the party conventions themselves are little more than a widely publicized rubber-stamping event.
The rise of the presidential primary and caucus system as the main means by which presidential candidates are selected has had a number of anticipated and unanticipated consequences. For one, the campaign season has grown longer and more costly. In 1960, John F. Kennedy declared his intention to run for the presidency just eleven months before the general election. Compare this to Hillary Clinton, who announced her intention to run nearly two years before the 2008 general election. Today’s long campaign seasons are seasoned with a seemingly ever-increasing number of debates among contenders for the nomination. In 2016, when the number of candidates for the Republican nomination became large and unwieldy, two debates among them were held, in which only those candidates polling greater support were allowed in the more important prime-time debate. The runners-up spoke in the other debate.
Finally, the process of going straight to the people through primaries and caucuses has created some opportunities for party outsiders to rise. Neither Ronald Reagan nor Bill Clinton was especially popular with the party leadership of the Republicans or the Democrats (respectively) at the outset. The outsider phenomenon has been most clearly demonstrated, however, in the 2016 presidential nominating process, as those distrusted by the party establishment, such as Senator Ted Cruz and Donald Trump, who never before held political office, raced ahead of party favorites like Jeb Bush early in the primary process (Figure).
The rise of the primary system during the Progressive Era came at the cost of party regulars’ control of the process of candidate selection. Some party primaries even allow registered independents or members of the opposite party to vote. Even so, the process tends to attract the party faithful at the expense of independent voters, who often hold the key to victory in the fall contest. Thus, candidates who want to succeed in the primary contests seek to align themselves with committed partisans, who are often at the ideological extreme. Those who survive the primaries in this way have to moderate their image as they enter the general election if they hope to succeed among the rest of the party adherents and the uncommitted.
Primaries offer tests of candidates’ popular appeal, while state caucuses testify to their ability to mobilize and organize grassroots support among committed followers. Primaries also reward candidates in different ways, with some giving the winner all the state’s convention delegates, while others distribute delegates proportionately according to the distribution of voter support. Finally, the order in which the primary elections and caucus selections are held shape the overall race.Marty Cohen. 2008. The Party Decides: Presidential Nominations before and after Reform. Chicago: University of Chicago. Currently, the Iowa caucuses and the New Hampshire primary occur first. These early contests tend to shrink the field as candidates who perform poorly leave the race. At other times in the campaign process, some states will maximize their impact on the race by holding their primaries on the same day that other states do. The media has dubbed these critical groupings “Super Tuesdays,” “Super Saturdays,” and so on. They tend to occur later in the nominating process as parties try to force the voters to coalesce around a single nominee.
The rise of the primary has also displaced the convention itself as the place where party regulars choose their standard bearer. Once true contests in which party leaders fought it out to elect a candidate, by the 1970s, party conventions more often than not simply served to rubber-stamp the choice of the primaries. By the 1980s, the convention drama was gone, replaced by a long, televised commercial designed to extol the party’s greatness (Figure). Without the drama and uncertainty, major news outlets have steadily curtailed their coverage of the conventions, convinced that few people are interested. The 2016 elections seem to support the idea that the primary process produces a nominee rather than party insiders. Outsiders Donald Trump on the Republican side and Senator Bernie Sanders on the Democratic side had much success despite significant concerns about them from party elites. Whether this pattern could be reversed in the case of a closely contested selection process remains to be seen.
ELECTING THE PRESIDENT: THE GENERAL ELECTION
Early presidential elections, conducted along the lines of the original process outlined in the Constitution, proved unsatisfactory. So long as George Washington was a candidate, his election was a foregone conclusion. But it took some manipulation of the votes of electors to ensure that the second-place winner (and thus the vice president) did not receive the same number of votes. When Washington declined to run again after two terms, matters worsened. In 1796, political rivals John Adams and Thomas Jefferson were elected president and vice president, respectively. Yet the two men failed to work well together during Adams’s administration, much of which Jefferson spent at his Virginia residence at Monticello. As noted earlier in this chapter, the shortcomings of the system became painfully evident in 1800, when Jefferson and his running mate Aaron Burr finished tied, thus leaving it to the House of Representatives to elect Jefferson.James Roger Sharp. 2010. The Deadlocked Election of 1800: Jefferson, Burr, and the Union in the Balance. Lawrence: University Press of Kansas.
The Twelfth Amendment, ratified in 1804, provided for the separate election of president and vice president as well as setting out ways to choose a winner if no one received a majority of the electoral votes. Only once since the passage of the Twelfth Amendment, during the election of 1824, has the House selected the president under these rules, and only once, in 1836, has the Senate chosen the vice president. In several elections, such as in 1876 and 1888, a candidate who received less than a majority of the popular vote has claimed the presidency, including cases when the losing candidate secured a majority of the popular vote. A recent case was the 2000 election, in which Democratic nominee Al Gore won the popular vote, while Republican nominee George W. Bush won the Electoral College vote and hence the presidency. The 2016 election brought another such irregularity as Donald Trump comfortably won the Electoral College by narrowly winning the popular vote in several states, while Hillary Clinton collected at least 600,000 more votes nationwide.
Not everyone is satisfied with how the Electoral College fundamentally shapes the election, especially in cases such as those noted above, when a candidate with a minority of the popular vote claims victory over a candidate who drew more popular support. Yet movements for electoral reform, including proposals for a straightforward nationwide direct election by popular vote, have gained little traction.
Supporters of the current system defend it as a manifestation of federalism, arguing that it also guards against the chaos inherent in a multiparty environment by encouraging the current two-party system. They point out that under a system of direct election, candidates would focus their efforts on more populous regions and ignore others.John Samples, “In Defense of the Electoral College,” 10 November 2000, http://www.cato.org/publications/commentary/defense-electoral-college (May 1, 2016). Critics, on the other hand, charge that the current system negates the one-person, one-vote basis of U.S. elections, subverts majority rule, works against political participation in states deemed safe for one party, and might lead to chaos should an elector desert a candidate, thus thwarting the popular will. Despite all this, the system remains in place. It appears that many people are more comfortable with the problems of a flawed system than with the uncertainty of change.Clifton B. Parker, “Now We Know Why It’s Time to Dump the Electoral College,” The Fiscal Times, 12 April 2016, http://www.thefiscaltimes.com/2016/04/12/Now-We-Know-Why-It-s-Time-Dump-Electoral-College.
Electoral College Reform
Following the 2000 presidential election, when then-governor George W. Bush won by a single electoral vote and with over half a million fewer individual votes than his challenger, astonished voters called for Electoral College reform. Years later, however, nothing of any significance had been done. The absence of reform in the wake of such a problematic election is a testament to the staying power of the Electoral College.
Those who insist that the Electoral College should be reformed argue that its potential benefits pale in comparison to the way the Electoral College depresses voter turnout and fails to represent the popular will. In addition to favoring small states, since individual votes there count more than in larger states due to the mathematics involved in the distribution of electors, the Electoral College results in a significant number of “safe” states that receive no real electioneering, such that nearly 75 percent of the country is ignored in the general election.
One potential solution to the problems with the Electoral College is to scrap it all together and replace it with the popular vote. The popular vote would be the aggregated totals of the votes in the fifty states and District of Columbia, as certified by the head election official of each state. A second solution often mentioned is to make the Electoral College proportional. That is, as each state assigns it electoral votes, it would do so based on the popular vote percentage in their state, rather with the winner-take-all approach almost all the states use today.
A third alternative for Electoral College reform has been proposed by an organization called National Popular Vote. The National Popular Vote movement is an interstate compact between multiple states that sign onto the compact. Once a combination of states constituting 270 Electoral College votes supports the movement, each state entering the compact pledges all of its Electoral College votes to the national popular vote winner. This reform does not technically change the Electoral College structure, but it results in a mandated process that makes the Electoral College reflect the popular vote. Thus far, eleven states with a total of 165 electoral votes among them have signed onto the compact.
In what ways does the current Electoral College system protect the representative power of small states and less densely populated regions? Why might it be important to preserve these protections?
Follow-up activity: View the National Popular Vote website to learn more about their position. Consider reaching out to them to learn more, offer your support, or even to argue against their proposal.
See how the Electoral College and the idea of swing states fundamentally shapes elections by experimenting with the interactive Electoral College map at 270 to Win.
The general election usually features a series of debates between the presidential contenders as well as a debate among vice presidential candidates. Because the stakes are high, quite a bit of money and resources are expended on all sides. Attempts to rein in the mounting costs of modern general-election campaigns have proven ineffective. Nor has public funding helped to solve the problem. Indeed, starting with Barack Obama’s 2008 decision to forfeit public funding so as to skirt the spending limitations imposed, candidates now regularly opt to raise more money rather than to take public funding.Jason Scott-Sheets, “Public financing is available for presidential candidates. So what’s not to like about free money?” 14 April 2016, http://www.opensecrets.org/news/2016/04/public-financing-is-available-for-presidential-candidates-so-whats-not-to-like-about-free-money/. In addition, political action committees (PACs), supposedly focused on issues rather than specific candidates, seek to influence the outcome of the race by supporting or opposing a candidate according to the PAC’s own interests. But after all the spending and debating is done, those who have not already voted by other means set out on the first Tuesday following the first Monday in November to cast their votes. Several weeks later, the electoral votes are counted and the president is formally elected (Figure).
Summary
The position of president of the United States was created during the Constitutional Convention. Within a generation of Washington’s administration, powerful political parties had overtaken the nominating power of state legislatures and created their own systems for selecting candidates. At first, party leaders kept tight control over the selection of candidates via the convention process. By the start of the twentieth century, however, primary and caucus voting had brought the power to select candidates directly to the people, and the once-important conventions became rubber-stamping events.
How did the election of 1824 change the way presidents were selected?
- Following this election, presidents were directly elected.
- Jackson’s supporters decided to create a device for challenging the Electoral College.
- The election convinced many that the parties must adopt the king caucus as the primary method for selecting presidents.
- The selection of the candidate with fewer electoral votes triggered the rise of party control over nominations.
Hint:
D
Which of the following is an unintended consequence of the rise of the primary and caucus system?
- Sometimes candidates unpopular with the party leadership reach the top.
- Campaigns have become shorter and more expensive.
- The conventions have become more powerful than the voters.
- Often incumbent presidents will fail to be renominated by the party.
What problems exist with the Electoral College?
Hint:
There are many problems with the Electoral College. First, small states are over-represented in the Electoral College. Second, the state by state set-up of the college, in the modern era, leads to states that are safe wins for one party, leaving a handful of states that get all the attention. Finally, its outcomes can differ from the outcome of actual citizen voting (also known as the national popular vote.
|
oercommons
|
2025-03-18T00:37:57.873068
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15255/overview",
"title": "American Government, Delivering Collective Action: Formal Institutions",
"author": null
}
|
https://oercommons.org/courseware/lesson/15256/overview
|
Organizing to Govern
Learning Objectives
By the end of this section, you will be able to:
- Explain how incoming and outgoing presidents peacefully transfer power
- Describe how new presidents fill positions in the executive branch
- Discuss how incoming presidents use their early popularity to advance larger policy solutions
It is one thing to win an election; it is quite another to govern, as many frustrated presidents have discovered. Critical to a president’s success in office is the ability to make a deft transition from the previous administration, including naming a cabinet and filling other offices. The new chief executive must also fashion an agenda, which he or she will often preview in general terms in an inaugural address. Presidents usually embark upon their presidency benefitting from their own and the nation’s renewed hope and optimism, although often unrealistic expectations set the stage for subsequent disappointment.
TRANSITION AND APPOINTMENTS
In the immediate aftermath of the election, the incoming and outgoing administrations work together to help facilitate the transfer of power. While the General Services Administration oversees the logistics of the process, such as office assignments, information technology, and the assignment of keys, prudent candidates typically prepare for a possible victory by appointing members of a transition team during the lead-up to the general election. The success of the team’s actions becomes apparent on inauguration day, when the transition of power takes place in what is often a seamless fashion, with people evacuating their offices (and the White House) for their successors.
Read about presidential transitions as well as explore other topics related to the transfer of power at the White House Transition Project website.
Among the president-elect’s more important tasks is the selection of a cabinet. George Washington’s cabinet was made up of only four people, the attorney general and the secretaries of the Departments of War, State, and the Treasury. Currently, however, there are fifteen members of the cabinet, including the Secretaries of Labor, Agriculture, Education, and others (Figure). The most important members—the heads of the Departments of Defense, Justice, State, and the Treasury (echoing Washington’s original cabinet)—receive the most attention from the president, the Congress, and the media. These four departments have been referred to as the inner cabinet, while the others are called the outer cabinet. When selecting a cabinet, presidents consider ability, expertise, influence, and reputation. More recently, presidents have also tried to balance political and demographic representation (gender, race, religion, and other considerations) to produce a cabinet that is capable as well as descriptively representative, meaning that those in the cabinet look like the U.S. population (see the chapter on bureaucracy and the term “representative bureaucracy”). A recent president who explicitly stated this as his goal was Bill Clinton, who talked about an “E.G.G. strategy” for senior-level appointments, where the E stands for ethnicity, G for gender, and the second G for geography.
Once the new president has been inaugurated and can officially nominate people to fill cabinet positions, the Senate confirms or rejects these nominations. At times, though rarely, cabinet nominations have failed to be confirmed or have even been withdrawn because of questions raised about the past behavior of the nominee.Glen S. Krutz, Richard Fleisher, and Jon R. Bond. 1998. “From Abe Fortas to Zoe Baird.” American Political Science Review 92, No. 4: 871–882. Prominent examples of such withdrawals were Senator John Tower for defense secretary (George H. W. Bush) and Zoe Baird for attorney general (Bill Clinton): Senator Tower’s indiscretions involving alcohol and womanizing led to concerns about his fitness to head the military and his rejection by the Senate,Michael Oreskes. 1989. “Senate Rejects Tower, 53–47; First Cabinet Veto since ‘59; Bush Confers on New Choice,” New York Times, 10 March 1989, http://www.nytimes.com/1989/03/10/us/senate-rejects-tower-53-47-first-cabinet-veto-since-59-bush-confers-new-choice.html. whereas Zoe Baird faced controversy and withdrew her nomination when it was revealed, through what the press dubbed “Nannygate,” that house staff of hers were undocumented workers. However, these cases are rare exceptions to the rule, which is to give approval to the nominees that the president wishes to have in the cabinet. Other possible candidates for cabinet posts may decline to be considered for a number of reasons, from the reduction in pay that can accompany entrance into public life to unwillingness to be subjected to the vetting process that accompanies a nomination.
Also subject to Senate approval are a number of non-cabinet subordinate administrators in the various departments of the executive branch, as well as the administrative heads of several agencies and commissions. These include the heads of the Internal Revenue Service, the Central Intelligence Agency, the Office of Management and Budget, the Federal Reserve, the Social Security Administration, the Environmental Protection Agency, the National Labor Relations Board, and the Equal Employment Opportunity Commission. The Office of Management and Budget (OMB) is the president’s own budget department. In addition to preparing the executive budget proposal and overseeing budgetary implementation during the federal fiscal year, the OMB oversees the actions of the executive bureaucracy.
Not all the non-cabinet positions are open at the beginning of an administration, but presidents move quickly to install their preferred choices in most roles when given the opportunity. Finally, new presidents usually take the opportunity to nominate new ambassadors, whose appointments are subject to Senate confirmation. New presidents make thousands of new appointments in their first two years in office. All the senior cabinet agency positions and nominees for all positions in the Executive Office of the President are made as presidents enter office or when positions become vacant during their presidency. Federal judges serve for life. Therefore, vacancies for the federal courts and the U.S. Supreme Court occur gradually as judges retire.
Throughout much of the history of the republic, the Senate has closely guarded its constitutional duty to consent to the president’s nominees, although in the end it nearly always confirms them. Still, the Senate does occasionally hold up a nominee. Benjamin Fishbourn, President George Washington’s nomination for a minor naval post, was rejected largely because he had insulted a particular senator.Mark J. Rozell, William D. Pederson, Frank J. Williams. 2000. George Washington and the Origins of the American Presidency. Portsmouth, NH: Greenwood Publishing Group, 17. Other rejected nominees included Clement Haynsworth and G. Harrold Carswell, nominated for the U.S. Supreme Court by President Nixon; Theodore Sorensen, nominated by President Carter for director of the Central Intelligence Agency; and John Tower, discussed earlier. At other times, the Senate has used its power to rigorously scrutinize the president’s nominees (Figure). Supreme Court nominee Clarence Thomas, who faced numerous sexual harassment charges from former employees, was forced to sit through repeated questioning of his character and past behavior during Senate hearings, something he referred to as “a high-tech lynching for uppity blacks.”“Hearing of the Senate Judiciary Committee on the Nomination of Clarence Thomas to the Supreme Court,” Electronic Text Center, University of Virginia Library, 11 October 1991.
More recently, the Senate has attempted a new strategy, refusing to hold hearings at all, a strategy of defeat that scholars have referred to as “malign neglect.”Jon R. Bond, Richard Fleisher, and Glen S. Krutz. 2009. “Malign Neglect: Evidence That Delay Has Become the Primary Method of Defeating Presidential Appointments” Congress & the Presidency 36, No. 3: 226–243. Despite the fact that one-third of U.S. presidents have appointed a Supreme Court justice in an election year, when Associate Justice Antonin Scalia died unexpectedly in early 2016, Senate majority leader Mitch McConnell declared that the Senate would not hold hearings on a nominee until after the upcoming presidential election.Barbara Perry, “One-third of all U.S. presidents appointed a Supreme Court justice in an election year,” Washington Post, 29 February 2016, https://www.washingtonpost.com/news/monkey-cage/wp/2016/02/29/one-third-of-all-u-s-presidents-appointed-a-supreme-court-justice-in-an-election-year/. McConnell remained adamant even after President Barack Obama, saying he was acting in fulfillment of his constitutional duty, nominated Merrick Garland, longtime chief judge of the federal Circuit Court of Appeals for the DC Circuit. Garland is highly respected by senators from both parties and won confirmation to his DC circuit position by a 76–23 vote in the Senate. When Republican Donald Trump was elected president in the fall, this strategy appeared to pay off. The Republican Senate and Judiciary Committee will welcome a Trump nominee in early 2017.
Other presidential selections are not subject to Senate approval, including the president’s personal staff (whose most important member is the White House chief of staff) and various advisers (most notably the national security adviser). The Executive Office of the President, created by Franklin D. Roosevelt (FDR), contains a number of advisory bodies, including the Council of Economic Advisers, the National Security Council, the OMB, and the Office of the Vice President. Presidents also choose political advisers, speechwriters, and a press secretary to manage the politics and the message of the administration. In recent years, the president’s staff has become identified by the name of the place where many of its members work: the West Wing of the White House. These people serve at the pleasure of the president, and often the president reshuffles or reforms the staff during his or her term. Just as government bureaucracy has expanded over the centuries, so has the White House staff, which under Abraham Lincoln numbered a handful of private secretaries and a few minor functionaries. A recent report pegged the number of employees working within the White House over 450.Jennifer Liberto, “It pays to work for the White House,” CNN Money, 2 July 2014, http://money.cnn.com/2014/07/02/news/economy/white-house-salaries/ (May 1, 2016). When the staff in nearby executive buildings of the Executive Office of the President are added in, that number increases four-fold.
No Fun at Recess: Dueling Loopholes and the Limits of Presidential Appointments
When Supreme Court justice Antonin Scalia died unexpectedly in early 2016, many in Washington braced for a political sandstorm of obstruction and accusations. Such was the record of Supreme Court nominations during the Obama administration and, indeed, for the last few decades. Nor is this phenomenon restricted to nominations for the highest court in the land. The Senate has been known to occasionally block or slow appointments not because the quality of the nominee was in question but rather as a general protest against the policies of the president and/or as part of the increasing partisan bickering that occurs when the presidency is controlled by one political party and the Senate by the other. This occurred, for example, when the Senate initially refused to nominate anyone to head the Consumer Financial Protection Bureau, established in 2011, because Republicans disliked the existence of the bureau itself.
Such political holdups, however, tend to be the exception rather than the rule. For example, historically, nominees to the presidential cabinet are rarely rejected. And each Congress oversees the approval of around four thousand civilian and sixty-five thousand military appointments from the executive branch.Gary P. Gershman. 2008. The Legislative Branch of Federal Government: People, Process, and Politics. Santa Barbara, CA: ABC-CLIO. The overwhelming majority of these are confirmed in a routine and systematic fashion, and only rarely do holdups occur. But when they do, the Constitution allows for a small presidential loophole called the recess appointment. The relevant part of Article II, Section 2, of the Constitution reads:
“The President shall have Power to fill up all Vacancies that may happen during the Recess of the Senate, by granting Commissions which shall expire at the End of their next Session.”
The purpose of the provision was to give the president the power to temporarily fill vacancies during times when the Senate was not in session and could not act. But presidents have typically used this loophole to get around a Senate that’s inclined to obstruct. Presidents Bill Clinton and George W. Bush made 139 and 171 recess appointments, respectively. President Obama has made far fewer recess appointments; as of May 1, 2015, he had made only thirty-two.Bruce Drake, “Obama lags his predecessors in recess appointments,” 13 January 2014, http://www.pewresearch.org/fact-tank/2014/01/13/obama-lags-his-predecessors-in-recess-appointments/ (May 1, 2016). One reason this number is so low is another loophole the Senate began using at the end of George W. Bush’s presidency, the pro forma session.
A pro forma session is a short meeting held with the understanding that no work will be done. These sessions have the effect of keeping the Senate officially in session while functionally in recess. In 2012, President Obama decided to ignore the pro forma session and make four recess appointments anyway. The Republicans in the Senate were furious and contested the appointments. Eventually, the Supreme Court had the final say in a 2014 decision that declared unequivocally that “the Senate is in session when it says it is.”National Labor Relations Board v. Canning, 573 U.S. ___ (2014). For now at least, the court’s ruling means that the president’s loophole and the Senate’s loophole cancel each other out. It seems they’ve found the middle ground whether they like it or not.
What might have been the legitimate original purpose of the recess appointment loophole? Do you believe the Senate is unfairly obstructing by effectively ending recesses altogether so as to prevent the president from making appointments without its approval?
The most visible, though arguably the least powerful, member of a president’s cabinet is the vice president. Throughout most of the nineteenth and into the twentieth century, the vast majority of vice presidents took very little action in the office unless fate intervened. Few presidents consulted with their running mates. Indeed, until the twentieth century, many presidents had little to do with the naming of their running mate at the nominating convention. The office was seen as a form of political exile, and that motivated Republicans to name Theodore Roosevelt as William McKinley’s running mate in 1900. The strategy was to get the ambitious politician out of the way while still taking advantage of his popularity. This scheme backfired, however, when McKinley was assassinated and Roosevelt became president (Figure).
Vice presidents were often sent on minor missions or used as mouthpieces for the administration, often with a sharp edge. Richard Nixon’s vice president Spiro Agnew is an example. But in the 1970s, starting with Jimmy Carter, presidents made a far more conscious effort to make their vice presidents part of the governing team, placing them in charge of increasingly important issues. Sometimes, as in the case of Bill Clinton and Al Gore, the partnership appeared to be smooth if not always harmonious. In the case of George W. Bush and his very experienced vice president Dick Cheney, observers speculated whether the vice president might have exercised too much influence. Barack Obama’s choice for a running mate and subsequent two-term vice president, former Senator Joseph Biden, was picked for his experience, especially in foreign policy. President Obama relied on Vice President Biden for advice throughout his tenure. In any case, the vice presidency is no longer quite as weak as it once was, and a capable vice president can do much to augment the president’s capacity to govern across issues if the president so desires.Amy C. Gaudion and Douglas Stuart, “More Than Just a Running Mate,” The New York Times, 19 July 2012, http://campaignstops.blogs.nytimes.com/2012/07/19/more-than-just-a-running-mate/.
FORGING AN AGENDA
Having secured election, the incoming president must soon decide how to deliver upon what was promised during the campaign. The chief executive must set priorities, chose what to emphasize, and formulate strategies to get the job done. He or she labors under the shadow of a measure of presidential effectiveness known as the first hundred days in office, a concept popularized during Franklin Roosevelt’s first term in the 1930s. While one hundred days is possibly too short a time for any president to boast of any real accomplishments, most presidents do recognize that they must address their major initiatives during their first two years in office. This is the time when the president is most powerful and is given the benefit of the doubt by the public and the media (aptly called the honeymoon period), especially if he or she enters the White House with a politically aligned Congress, as Barack Obama did. However, recent history suggests that even one-party control of Congress and the presidency does not ensure efficient policymaking. This difficulty is due as much to divisions within the governing party as to obstructionist tactics skillfully practiced by the minority party in Congress. Democratic president Jimmy Carter’s battles with a Congress controlled by Democratic majorities provide a good case in point.
The incoming president must deal to some extent with the outgoing president’s last budget proposal. While some modifications can be made, it is more difficult to pursue new initiatives immediately. Most presidents are well advised to prioritize what they want to achieve during the first year in office and not lose control of their agenda. At times, however, unanticipated events can determine policy, as happened in 2001 when nineteen hijackers perpetrated the worst terrorist attack in U.S. history and transformed U.S. foreign and domestic policy in dramatic ways.
Moreover, a president must be sensitive to what some scholars have termed “political time,” meaning the circumstances under which he or she assumes power. Sometimes, the nation is prepared for drastic proposals to solve deep and pressing problems that cry out for immediate solutions, as was the case following the 1932 election of FDR at the height of the Great Depression. Most times, however, the country is far less inclined to accept revolutionary change. Being an effective president means recognizing the difference.Stephen Skowronek. 2011. Presidential Leadership in Political Time: Reprise and Reappraisal. Lawrence: University Press of Kansas.
The first act undertaken by the new president—the delivery of an inaugural address—can do much to set the tone for what is intended to follow. While such an address may be an exercise in rhetorical inspiration, it also allows the president to set forth priorities within the overarching vision of what he or she intends to do. Abraham Lincoln used his inaugural addresses to calm rising concerns in the South that he would act to overturn slavery. Unfortunately, this attempt at appeasement fell on deaf ears, and the country descended into civil war. Franklin Roosevelt used his first inaugural address to boldly proclaim that the country need not fear the change that would deliver it from the grip of the Great Depression, and he set to work immediately enlarging the federal government to that end. John F. Kennedy, who entered the White House at the height of the Cold War, made an appeal to talented young people around the country to help him make the world a better place. He followed up with new institutions like the Peace Corps, which sends young citizens around the world to work as secular missionaries for American values like democracy and free enterprise.
Listen to clips of the most famous inaugural address in presidential history at the Washington Post website.
Summary
It can be difficult for a new president to come to terms with both the powers of the office and the limitations of those powers. Successful presidents assume their role ready to make a smooth transition and to learn to work within the complex governmental system to fill vacant positions in the cabinet and courts, many of which require Senate confirmation. It also means efficiently laying out a political agenda and reacting appropriately to unexpected events. A new president has limited time to get things done and must take action with the political wind at his or her back.
The people who make up the modern president’s cabinet are the heads of the major federal departments and ________.
- must be confirmed by the Senate
- once in office are subject to dismissal by the Senate
- serve two-year terms
- are selected base on the rules of patronage
A very challenging job for new presidents is to ______.
- move into the White House
- prepare and deliver their first State of the Union address
- nominate and gain confirmation for their cabinet and hundreds of other officials
- prepare their first executive budget
Hint:
C
How do presidents work to fulfill their campaign promises once in office?
|
oercommons
|
2025-03-18T00:37:57.901859
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15256/overview",
"title": "American Government, Delivering Collective Action: Formal Institutions",
"author": null
}
|
https://oercommons.org/courseware/lesson/15257/overview
|
The Public Presidency
Learning Objectives
By the end of this section, you will be able to:
- Explain how technological innovations have empowered presidents
- Identify ways in which presidents appeal to the public for approval
- Explain how the role of first ladies changed over the course of the twentieth century
With the advent of motion picture newsreels and voice recordings in the 1920s, presidents began to broadcast their message to the general public. Franklin Roosevelt, while not the first president to use the radio, adopted this technology to great effect. Over time, as radio gave way to newer and more powerful technologies like television, the Internet, and social media, other presidents have been able magnify their voices to an even-larger degree. Presidents now have far more tools at their disposal to shape public opinion and build support for policies. However, the choice to “go public” does not always lead to political success; it is difficult to convert popularity in public opinion polls into political power. Moreover, the modern era of information and social media empowers opponents at the same time that it provides opportunities for presidents.
THE SHAPING OF THE MODERN PRESIDENCY
From the days of the early republic through the end of the nineteenth century, presidents were limited in the ways they could reach the public to convey their perspective and shape policy. Inaugural addresses and messages to Congress, while circulated in newspapers, proved clumsy devices to attract support, even when a president used plain, blunt language. Some presidents undertook tours of the nation, notably George Washington and Rutherford B. Hayes. Others promoted good relationships with newspaper editors and reporters, sometimes going so far as to sanction a pro-administration newspaper. One president, Ulysses S. Grant, cultivated political cartoonist Thomas Nast to present the president’s perspective in the pages of the magazine Harper’s Weekly.Wendy Wick Reaves. 1987. “Thomas Nast and the President,” American Art Journal 19, No. 1: 61–71. Abraham Lincoln experimented with public meetings recorded by newspaper reporters and public letters that would appear in the press, sometimes after being read at public gatherings (Figure). Most presidents gave speeches, although few proved to have much immediate impact, including Lincoln’s memorable Gettysburg Address.
Rather, most presidents exercised the power of patronage (or appointing people who are loyal and help them out politically) and private deal-making to get what they wanted at a time when Congress usually held the upper hand in such transactions. But even that presidential power began to decline with the emergence of civil service reform in the later nineteenth century, which led to most government officials being hired on their merit instead of through patronage. Only when it came to diplomacy and war were presidents able to exercise authority on their own, and even then, institutional as well as political restraints limited their independence of action.
Theodore Roosevelt came to the presidency in 1901, at a time when movie newsreels were becoming popular. Roosevelt, who had always excelled at cultivating good relationships with the print media, eagerly exploited this new opportunity as he took his case to the people with the concept of the presidency as bully pulpit, a platform from which to push his agenda to the public. His successors followed suit, and they discovered and employed new ways of transmitting their message to the people in an effort to gain public support for policy initiatives. With the popularization of radio in the early twentieth century, it became possible to broadcast the president’s voice into many of the nation’s homes. Most famously, FDR used the radio to broadcast his thirty “fireside chats” to the nation between 1933 and 1944.
In the post–World War II era, television began to replace radio as the medium through which presidents reached the public. This technology enhanced the reach of the handsome young president John F. Kennedy and the trained actor Ronald Reagan. At the turn of the twentieth century, the new technology was the Internet. The extent to which this mass media technology can enhance the power and reach of the president has yet to be fully realized.
Other presidents have used advances in transportation to take their case to the people. Woodrow Wilson traveled the country to advocate formation of the League of Nations. However, he fell short of his goal when he suffered a stroke in 1919 and cut his tour short. Both Franklin Roosevelt in the 1930s and 1940s and Harry S. Truman in the 1940s and 1950s used air travel to conduct diplomatic and military business. Under President Dwight D. Eisenhower, a specific plane, commonly called Air Force One, began carrying the president around the country and the world. This gives the president the ability to take his or her message directly to the far corners of the nation at any time.
GOING PUBLIC: PROMISE AND PITFALLS
The concept of going public involves the president delivering a major television address in the hope that Americans watching the address will be compelled to contact their House and Senate member and that such public pressure will result in the legislators supporting the president on a major piece of legislation. Technological advances have made it more efficient for presidents to take their messages directly to the people than was the case before mass media (Figure). Presidential visits can build support for policy initiatives or serve political purposes, helping the president reward supporters, campaign for candidates, and seek reelection. It remains an open question, however, whether choosing to go public actually enhances a president’s political position in battles with Congress. Political scientist George C. Edwards goes so far as to argue that taking a president’s position public serves to polarize political debate, increase public opposition to the president, and complicate the chances to get something done. It replaces deliberation and compromise with confrontation and campaigning. Edwards believes the best way for presidents to achieve change is to keep issues private and negotiate resolutions that preclude partisan combat. Going public may be more effective in rallying supporters than in gaining additional support or changing minds.George C. Edwards. 2016. Predicting the Presidency: The Potential of Persuasive Leadership. Princeton: Princeton University Press; George C. Edwards and Stephen J. Wayne. 2003. Presidential Leadership: Politics and Policy Making. Belmont, CA: Wadsworth/Thomson Learning.
Today, it is possible for the White House to take its case directly to the people via websites like White House Live, where the public can watch live press briefings and speeches.
THE FIRST LADY: A SECRET WEAPON?
The president is not the only member of the First Family who often attempts to advance an agenda by going public. First ladies increasingly exploited the opportunity to gain public support for an issue of deep interest to them. Before 1933, most first ladies served as private political advisers to their husbands. In the 1910s, Edith Bolling Wilson took a more active but still private role assisting her husband, President Woodrow Wilson, afflicted by a stroke, in the last years of his presidency. However, as the niece of one president and the wife of another, it was Eleanor Roosevelt in the 1930s and 1940s who opened the door for first ladies to do something more.
Eleanor Roosevelt took an active role in championing civil rights, becoming in some ways a bridge between her husband and the civil rights movement. She coordinated meetings between FDR and members of the NAACP, championed antilynching legislation, openly defied segregation laws, and pushed the Army Nurse Corps to allow black women in its ranks. She also wrote a newspaper column and had a weekly radio show. Her immediate successors returned to the less visible role held by her predecessors, although in the early 1960s, Jacqueline Kennedy gained attention for her efforts to refurbish the White House along historical lines, and Lady Bird Johnson in the mid- and late 1960s endorsed an effort to beautify public spaces and highways in the United States. She also established the foundations of what came to be known as the Office of the First Lady, complete with a news reporter, Liz Carpenter, as her press secretary.
Betty Ford took over as first lady in 1974 and became an avid advocate of women’s rights, proclaiming that she was pro-choice when it came to abortion and lobbying for the ratification of the Equal Rights Amendment (ERA). She shared with the public the news of her breast cancer diagnosis and subsequent mastectomy. Her successor, Rosalynn Carter, attended several cabinet meetings and pushed for the ratification of the ERA as well as for legislation addressing mental health issues (Figure).
The increasing public political role of the first lady continued in the 1980s with Nancy Reagan’s “Just Say No” antidrug campaign and in the early 1990s with Barbara Bush’s efforts on behalf of literacy. The public role of the first lady reach a new level with Hillary Clinton in the 1990s when her husband put her in charge of his efforts to achieve health care reform, a controversial decision that did not meet with political success. Her successors, Laura Bush in the first decade of the twenty-first century and Michelle Obama in the second, returned to the roles played by predecessors in advocating less controversial policies: Laura Bush advocated literacy and education, while Michelle Obama has emphasized physical fitness and healthy diet and exercise. Nevertheless, the public and political profiles of first ladies remain high, and in the future, the president’s spouse will have the opportunity to use that unelected position to advance policies that might well be less controversial and more appealing than those pushed by the president.
A New Role for the First Lady?
While running for the presidency for the first time in 1992, Bill Clinton frequently touted the experience and capabilities of his wife. There was a lot to brag about. Hillary Rodham Clinton was a graduate of Yale Law School, had worked as a member of the impeachment inquiry staff during the height of the Watergate scandal in Nixon’s administration, and had been a staff attorney for the Children’s Defense Fund before becoming the first lady of Arkansas. Acknowledging these qualifications, candidate Bill Clinton once suggested that by electing him, voters would get “two for the price of one.” The clear implication in this statement was that his wife would take on a far larger role than previous first ladies, and this proved to be the case.Rupert Cornwell, “Bill and Hillary’s double trouble: Clinton’s ’two for the price of one’ pledge is returning to haunt him,” Independent, 8 March 1994, http://www.independent.co.uk/voices/bill-and-hillarys-double-trouble-clintons-two-for-the-price-of-one-pledge-is-returning-to-haunt-him-1427937.html (May 1, 2016).
Shortly after taking office, Clinton appointed the first lady to chair the Task Force on National Health Care Reform. This organization was to follow through on his campaign promise to fix the problems in the U.S. healthcare system. Hillary Clinton had privately requested the appointment, but she quickly realized that the complex web of business interests and political aspirations combined to make the topic of health care reform a hornet’s nest. This put the Clinton administration’s first lady directly into partisan battles few if any previous first ladies had ever faced.
As a testament to both the large role the first lady had taken on and the extent to which she had become the target of political attacks, the recommendations of the task force were soon dubbed “Hillarycare” by opponents. In a particularly contentious hearing in the House, the first lady and Republican representative Dick Armey exchanged pointed jabs with each other. At one point, Armey suggested that the reports of her charm were “overstated” after the first lady likened him to Dr. Jack Kevorkian, a physician known for helping patients commit suicide (Figure).Tamar Lewin, “First Person; A Feminism That Speaks For Itself,” New York Times, 3 October 1993, http://www.nytimes.com/1993/10/03/weekinreview/first-person-a-feminism-that-speaks-for-itself.html. The following summer, the first lady attempted to use a national bus tour to popularize the health care proposal, although distaste for her and for the program had reached such a fevered pitch that she sometimes was compelled to wear a bulletproof vest. In the end, the efforts came up short and the reform attempts were abandoned as a political failure. Nevertheless, Hillary Clinton remained a political lightning rod for the rest of the Clinton presidency.
What do the challenges of First Lady Hillary Clinton’s foray into national politics suggest about the dangers of a first lady abandoning the traditionally safe nonpartisan goodwill efforts? What do the actions of the first ladies since Clinton suggest about the lessons learned or not learned?
Summary
Despite the obvious fact that the president is the head of state, the U.S. Constitution actually empowers the occupant of the White House with very little authority. Apart from the president’s war powers, the office holder’s real advantage is the ability to speak to the nation with one voice. Technological changes in the twentieth century have greatly expanded the power of the presidential bully pulpit. The twentieth century also saw a string of more public first ladies. Women like Eleanor Roosevelt and Lady Bird Johnson greatly expanded the power of the first lady’s role, although first ladies who have undertaken more nontraditional roles have encountered significant criticism.
President Theodore Roosevelt’s concept of the bully pulpit was the office’s ________.
- authority to use force, especially military force
- constitutional power to veto legislation
- premier position to pressure through public appeal
- ability to use technology to enhance the voice of the president
Hint:
C
In what ways have first ladies expanded the role of their office over the twentieth century?
How were presidents in the eighteenth and nineteenth centuries likely to reach the public? Were these methods effective?
Hint:
Presidents of the eighteenth and nineteenth centuries might make speeches or publish letters in newspapers across the country. These methods may have been effective in their day, but not in comparison to the ability of modern presidents with television, radio, and the Internet at their disposal.
|
oercommons
|
2025-03-18T00:37:57.929234
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15257/overview",
"title": "American Government, Delivering Collective Action: Formal Institutions",
"author": null
}
|
https://oercommons.org/courseware/lesson/15258/overview
|
Presidential Governance: Direct Presidential Action
Learning Objectives
By the end of this section, you will be able to:
- Identify the power presidents have to effect change without congressional cooperation
- Analyze how different circumstances influence the way presidents use unilateral authority
- Explain how presidents persuade others in the political system to support their initiatives
- Describe how historians and political scientists evaluate the effectiveness of a presidency
A president’s powers can be divided into two categories: direct actions the chief executive can take by employing the formal institutional powers of the office and informal powers of persuasion and negotiation essential to working with the legislative branch. When a president governs alone through direct action, it may break a policy deadlock or establish new grounds for action, but it may also spark opposition that might have been handled differently through negotiation and discussion. Moreover, such decisions are subject to court challenge, legislative reversal, or revocation by a successor. What may seem to be a sign of strength is often more properly understood as independent action undertaken in the wake of a failure to achieve a solution through the legislative process, or an admission that such an effort would prove futile. When it comes to national security, international negotiations, or war, the president has many more opportunities to act directly and in some cases must do so when circumstances require quick and decisive action.
DOMESTIC POLICY
The president may not be able to appoint key members of his or her administration without Senate confirmation, but he or she can demand the resignation or removal of cabinet officers, high-ranking appointees (such as ambassadors), and members of the presidential staff. During Reconstruction, Congress tried to curtail the president’s removal power with the Tenure of Office Act (1867), which required Senate concurrence to remove presidential nominees who took office upon Senate confirmation. Andrew Johnson’s violation of that legislation provided the grounds for his impeachment in 1868. Subsequent presidents secured modifications of the legislation before the Supreme Court ruled in 1926 that the Senate had no right to impair the president’s removal power.Myers v. United States, 272 U.S. 52 (1925). In the case of Senate failure to approve presidential nominations, the president is empowered to issue recess appointments (made while the Senate is in recess) that continue in force until the end of the next session of the Senate (unless the Senate confirms the nominee).
The president also exercises the power of pardon without conditions. Once used fairly sparingly—apart from Andrew Johnson’s wholesale pardons of former Confederates during the Reconstruction period—the pardon power has become more visible in recent decades. President Harry S. Truman issued over two thousand pardons and commutations, more than any other post–World War II president.“Bush Issues Pardons, but to a Relative Few,” New York Times, 22 December 2006, http://www.nytimes.com/2006/12/22/washington/22pardon.html. President Gerald Ford has the unenviable reputation of being the only president to pardon another president (his predecessor Richard Nixon, who resigned after the Watergate scandal) (Figure). While not as generous as Truman, President Jimmy Carter also issued a great number of pardons, including several for draft dodging during the Vietnam War. President Reagan was reluctant to use the pardon as much, as was President George H. W. Bush. President Clinton pardoned few people for much of his presidency, but did make several last-minute pardons, which led to some controversy. To date, Barack Obama has seldom used his power to pardon.U.S. Department of Justice. “Clemency Statistics.” https://www.justice.gov/pardon/clemency-statistics (May 1, 2016).
Presidents may choose to issue executive orders or proclamations to achieve policy goals. Usually, executive orders direct government agencies to pursue a certain course in the absence of congressional action. A more subtle version pioneered by recent presidents is the executive memorandum, which tends to attract less attention. Many of the most famous executive orders have come in times of war or invoke the president’s authority as commander-in-chief, including Franklin Roosevelt’s order permitting the internment of Japanese Americans in 1942 and Harry Truman’s directive desegregating the armed forces (1948). The most famous presidential proclamation was Abraham Lincoln’s Emancipation Proclamation (1863), which declared slaves in areas under Confederate control to be free (with a few exceptions).
Executive orders are subject to court rulings or changes in policy enacted by Congress. During the Korean War, the Supreme Court revoked Truman’s order seizing the steel industry.Youngstown Sheet & Tube Co. v. Sawyer, 343 U.S. 579 (1952). These orders are also subject to reversal by presidents who come after, and recent presidents have wasted little time reversing the orders of their predecessors in cases of disagreement. Sustained executive orders, which are those not overturned in courts, typically have some prior authority from Congress that legitimizes them. When there is no prior authority, it is much more likely that an executive order will be overturned by a later president. For this reason, this tool has become less common in recent decades (Figure).
Executive Order 9066
Following the devastating Japanese attacks on the U.S. Pacific fleet at Pearl Harbor in 1941, many in the United States feared that Japanese Americans on the West Coast had the potential and inclination to form a fifth column (a hostile group working from the inside) for the purpose of aiding a Japanese invasion. These fears mingled with existing anti-Japanese sentiment across the country and created a paranoia that washed over the West Coast like a large wave. In an attempt to calm fears and prevent any real fifth-column actions, President Franklin D. Roosevelt signed Executive Order 9066, which authorized the removal of people from military areas as necessary. When the military dubbed the entire West Coast a military area, it effectively allowed for the removal of more than 110,000 Japanese Americans from their homes. These people, many of them U.S. citizens, were moved to relocation centers in the interior of the country. They lived in the camps there for two and a half years (Figure).Julie Des Jardins, “From Citizen to Enemy: The Tragedy of Japanese Internment,” http://www.gilderlehrman.org/history-by-era/world-war-ii/essays/from-citizen-enemy-tragedy-japanese-internment (May 1, 2016).
The overwhelming majority of Japanese Americans felt shamed by the actions of the Japanese empire and willingly went along with the policy in an attempt to demonstrate their loyalty to the United States. But at least one Japanese American refused to go along. His name was Fred Korematsu, and he decided to go into hiding in California rather than be taken to the internment camps with his family. He was soon discovered, turned over to the military, and sent to the internment camp in Utah that held his family. But his challenge to the internment system and the president’s executive order continued.
In 1944, Korematsu’s case was heard by the Supreme Court. In a 6–3 decision, the Court ruled against him, arguing that the administration had the constitutional power to sign the order because of the need to protect U.S. interests against the threat of espionage.Korematsu v. United States, 323 U.S. 214 (1944). Forty-four years after this decision, President Reagan issued an official apology for the internment and provided some compensation to the survivors. In 2011, the Justice Department went a step further by filing a notice officially recognizing that the solicitor general of the United States acted in error by arguing to uphold the executive order. (The solicitor general is the official who argues cases for the U.S. government before the Supreme Court.) However, despite these actions, in 2014, the late Supreme Court justice Antonin Scalia was documented as saying that while he believed the decision was wrong, it could occur again.Ilya Somin, “Justice Scalia on Kelo and Korematsu,” Washington Post, 8 February 2014, https://www.washingtonpost.com/news/volokh-conspiracy/wp/2014/02/08/justice-scalia-on-kelo-and-korematsu/.
What do the Korematsu case and the internment of over 100,000 Japanese Americans suggest about the extent of the president’s war powers? What does this episode in U.S. history suggest about the weaknesses of constitutional checks on executive power during times of war?
To learn more about the relocation and confinement of Japanese Americans during World War II, visit Heart Mountain online.
Finally, presidents have also used the line-item veto and signing statements to alter or influence the application of the laws they sign. A line-item veto is a type of veto that keeps the majority of a spending bill unaltered but nullifies certain lines of spending within it. While a number of states allow their governors the line-item veto (discussed in the chapter on state and local government), the president acquired this power only in 1996 after Congress passed a law permitting it. President Clinton used the tool sparingly. However, those entities that stood to receive the federal funding he lined out brought suit. Two such groups were the City of New York and the Snake River Potato Growers in Idaho.Glen S. Krutz. 2001. Hitching a Ride: Omnibus Legislating in the U.S. Congress. Columbus, OH: Ohio State University Press. The Supreme Court heard their claims together and just sixteen months later declared unconstitutional the act that permitted the line-item veto.Clinton v. City of New York, 524 U.S. 417 (1998). Since then, presidents have asked Congress to draft a line-item veto law that would be constitutional, although none have made it to the president’s desk.
On the other hand, signing statements are statements issued by a president when agreeing to legislation that indicate how the chief executive will interpret and enforce the legislation in question. Signing statements are less powerful than vetoes, though congressional opponents have complained that they derail legislative intent. Signing statements have been used by presidents since at least James Monroe, but they became far more common in this century.
NATIONAL SECURITY, FOREIGN POLICY, AND WAR
Presidents are more likely to justify the use of executive orders in cases of national security or as part of their war powers. In addition to mandating emancipation and the internment of Japanese Americans, presidents have issued orders to protect the homeland from internal threats. Most notably, Lincoln ordered the suspension of the privilege of the writ of habeas corpus in 1861 and 1862 before seeking congressional legislation to undertake such an act. Presidents hire and fire military commanders; they also use their power as commander-in-chief to aggressively deploy U.S. military force. Congress rarely has taken the lead over the course of history, with the War of 1812 being the lone exception. Pearl Harbor was a salient case where Congress did make a clear and formal declaration when asked by FDR. However, since World War II, it has been the president and not Congress who has taken the lead in engaging the United States in military action outside the nation’s boundaries, most notably in Korea, Vietnam, and the Persian Gulf (Figure).
Presidents also issue executive agreements with foreign powers. Executive agreements are formal agreements negotiated between two countries but not ratified by a legislature as a treaty must be. As such, they are not treaties under U.S. law, which require two-thirds of the Senate for ratification. Treaties, presidents have found, are particularly difficult to get ratified. And with the fast pace and complex demands of modern foreign policy, concluding treaties with countries can be a tiresome and burdensome chore. That said, some executive agreements do require some legislative approval, such as those that commit the United States to make payments and thus are restrained by the congressional power of the purse. But for the most part, executive agreements signed by the president require no congressional action and are considered enforceable as long as the provisions of the executive agreement do not conflict with current domestic law.
The American Presidency Project has gathered data outlining presidential activity, including measures for executive orders and signing statements.
THE POWER OF PERSUASION
The framers of the Constitution, concerned about the excesses of British monarchial power, made sure to design the presidency within a network of checks and balances controlled by the other branches of the federal government. Such checks and balances encourage consultation, cooperation, and compromise in policymaking. This is most evident at home, where the Constitution makes it difficult for either Congress or the chief executive to prevail unilaterally, at least when it comes to constructing policy. Although much is made of political stalemate and obstructionism in national political deliberations today, the framers did not want to make it too easy to get things done without a great deal of support for such initiatives.
It is left to the president to employ a strategy of negotiation, persuasion, and compromise in order to secure policy achievements in cooperation with Congress. In 1960, political scientist Richard Neustadt put forward the thesis that presidential power is the power to persuade, a process that takes many forms and is expressed in various ways.Richard E. Neustadt. 1960. Presidential Power and the Modern Presidents New York: Wiley. Yet the successful employment of this technique can lead to significant and durable successes. For example, legislative achievements tend to be of greater duration because they are more difficult to overturn or replace, as the case of health care reform under President Barack Obama suggests. Obamacare has faced court cases and repeated (if largely symbolic) attempts to gut it in Congress. Overturning it will take a new president who opposes it, together with a Congress that can pass the dissolving legislation.
In some cases, cooperation is essential, as when the president nominates and the Senate confirms persons to fill vacancies on the Supreme Court, an increasingly contentious area of friction between branches. While Congress cannot populate the Court on its own, it can frustrate the president’s efforts to do so. Presidents who seek to prevail through persuasion, according to Neustadt, target Congress, members of their own party, the public, the bureaucracy, and, when appropriate, the international community and foreign leaders. Of these audiences, perhaps the most obvious and challenging is Congress.
Read “Power Lessons for Obama” at this website to learn more about applying Richard Neustadt’s framework to the leaders of today.
Much depends on the balance of power within Congress: Should the opposition party hold control of both houses, it will be difficult indeed for the president to realize his or her objectives, especially if the opposition is intent on frustrating all initiatives. However, even control of both houses by the president’s own party is no guarantee of success or even of productive policymaking. For example, neither Bill Clinton nor Barack Obama achieved all they desired despite having favorable conditions for the first two years of their presidencies. In times of divided government (when one party controls the presidency and the other controls one or both chambers of Congress), it is up to the president to cut deals and make compromises that will attract support from at least some members of the opposition party without excessively alienating members of his or her own party. Both Ronald Reagan and Bill Clinton proved effective in dealing with divided government—indeed, Clinton scored more successes with Republicans in control of Congress than he did with Democrats in charge.
It is more difficult to persuade members of the president’s own party or the public to support a president’s policy without risking the dangers inherent in going public. There is precious little opportunity for private persuasion while also going public in such instances, at least directly. The way the president and his or her staff handle media coverage of the administration may afford some opportunities for indirect persuasion of these groups. It is not easy to persuade the federal bureaucracy to do the president’s bidding unless the chief executive has made careful appointments. When it comes to diplomacy, the president must relay some messages privately while offering incentives, both positive and negative, in order to elicit desired responses, although at times, people heed only the threat of force and coercion.
While presidents may choose to go public in an attempt to put pressure on other groups to cooperate, most of the time they “stay private” as they attempt to make deals and reach agreements out of the public eye. The tools of negotiation have changed over time. Once chief executives played patronage politics, rewarding friends while attacking and punishing critics as they built coalitions of support. But the advent of civil service reform in the 1880s systematically deprived presidents of that option and reduced its scope and effectiveness. Although the president may call upon various agencies for assistance in lobbying for proposals, such as the Office of Legislative Liaison with Congress, it is often left to the chief executive to offer incentives and rewards. Some of these are symbolic, like private meetings in the White House or an appearance on the campaign trail. The president must also find common ground and make compromises acceptable to all parties, thus enabling everyone to claim they secured something they wanted.
Complicating Neustadt’s model, however, is that many of the ways he claimed presidents could shape favorable outcomes require going public, which as we have seen can produce mixed results. Political scientist Fred Greenstein, on the other hand, touted the advantages of a “hidden hand presidency,” in which the chief executive did most of the work behind the scenes, wielding both the carrot and the stick.Fred I. Greenstein. 1982. The Hidden-Hand Presidency: Eisenhower as Leader. New York: Basic Books. Greenstein singled out President Dwight Eisenhower as particularly skillful in such endeavors.
OPPORTUNITY AND LEGACY
What often shapes a president’s performance, reputation, and ultimately legacy depends on circumstances that are largely out of his or her control. Did the president prevail in a landslide or was it a closely contested election? Did he or she come to office as the result of death, assassination, or resignation? How much support does the president’s party enjoy, and is that support reflected in the composition of both houses of Congress, just one, or neither? Will the president face a Congress ready to embrace proposals or poised to oppose them? Whatever a president’s ambitions, it will be hard to realize them in the face of a hostile or divided Congress, and the options to exercise independent leadership are greater in times of crisis and war than when looking at domestic concerns alone.
Then there is what political scientist Stephen Skowronek calls “political time.”Stephen Skowronek. 2011. Presidential Leadership in Political Time: Reprise and Reappraisal. Lawrence, KS: University Press of Kansas. Some presidents take office at times of great stability with few concerns. Unless there are radical or unexpected changes, a president’s options are limited, especially if voters hoped for a simple continuation of what had come before. Other presidents take office at a time of crisis or when the electorate is looking for significant changes. Then there is both pressure and opportunity for responding to those challenges. Some presidents, notably Theodore Roosevelt, openly bemoaned the lack of any such crisis, which Roosevelt deemed essential for him to achieve greatness as a president.
People in the United States claim they want a strong president. What does that mean? At times, scholars point to presidential independence, even defiance, as evidence of strong leadership. Thus, vigorous use of the veto power in key situations can cause observers to judge a president as strong and independent, although far from effective in shaping constructive policies. Nor is such defiance and confrontation always evidence of presidential leadership skill or greatness, as the case of Andrew Johnson should remind us. When is effectiveness a sign of strength, and when are we confusing being headstrong with being strong? Sometimes, historians and political scientists see cooperation with Congress as evidence of weakness, as in the case of Ulysses S. Grant, who was far more effective in garnering support for administration initiatives than scholars have given him credit for.
These questions overlap with those concerning political time and circumstance. While domestic policymaking requires far more give-and-take and a fair share of cajoling and collaboration, national emergencies and war offer presidents far more opportunity to act vigorously and at times independently. This phenomenon often produces the rally around the flag effect, in which presidential popularity spikes during international crises. A president must always be aware that politics, according to Otto von Bismarck, is the art of the possible, even as it is his or her duty to increase what might be possible by persuading both members of Congress and the general public of what needs to be done.
Finally, presidents often leave a legacy that lasts far beyond their time in office (Figure). Sometimes, this is due to the long-term implications of policy decisions. Critical to the notion of legacy is the shaping of the Supreme Court as well as other federal judges. Long after John Adams left the White House in 1801, his appointment of John Marshall as chief justice shaped American jurisprudence for over three decades. No wonder confirmation hearings have grown more contentious in the cases of highly visible nominees. Other legacies are more difficult to define, although they suggest that, at times, presidents cast a long shadow over their successors. It was a tough act to follow George Washington, and in death, Abraham Lincoln’s presidential stature grew to extreme heights. Theodore and Franklin D. Roosevelt offered models of vigorous executive leadership, while the image and style of John F. Kennedy and Ronald Reagan influenced and at times haunted or frustrated successors. Nor is this impact limited to chief executives deemed successful: Lyndon Johnson’s Vietnam and Richard Nixon’s Watergate offered cautionary tales of presidential power gone wrong, leaving behind legacies that include terms like Vietnam syndrome and the tendency to add the suffix “-gate” to scandals and controversies.
Summary
While the power of the presidency is typically checked by the other two branches of government, presidents have the unencumbered power to pardon those convicted of federal crimes and to issue executive orders, which don’t require congressional approval but lack the permanence of laws passed by Congress. In matters concerning foreign policy, presidents have at their disposal the executive agreement, which is a much-easier way for two countries to come to terms than a treaty that requires Senate ratification but is also much narrower in scope.
Presidents use various means to attempt to drive public opinion and effect political change. But history has shown that they are limited in their ability to drive public opinion. Favorable conditions can help a president move policies forward. These conditions include party control of Congress and the arrival of crises such as war or economic decline. But as some presidencies have shown, even the most favorable conditions don’t guarantee success.
The passage of the Tenure of Office Act of 1867 was just one instance in a long line of ________.
- struggles for power between the president and the Congress
- unconstitutional presidential power grabbing
- impeachment trials
- arguments over presidential policy
Which of the following is an example of an executive agreement?
- The president negotiates an agreement with China and submits it to the Senate for ratification.
- The president changes a regulation on undocumented immigrant status without congressional approval.
- The president signs legally binding nuclear arms terms with Iran without seeking congressional approval.
- The president issues recommendations to the Department of Justice on what the meaning of a new criminal statute is.
Hint:
C
How have the methods presidents use to negotiate with their party and the opposition changed over time?
What strategies can presidents employ to win people over to their way of thinking?
Hint:
Presidents can use road trips across the country, major speeches, and rewards to people in their camp. Historically, however, these techniques have only rarely been successful. What works best is for a president find a popular position to get out in front of.
What are the opportunities and limitations for presidential leadership in the contemporary political system?
How have presidents used their position to increase the power of the office?
What role has technology played increasing the power and reach of presidents?
Under what conditions will presidents use direct action? When might they prefer passing a formal policy through Congress as a bill?
What do the conditions under which presidents decide to make public pleas suggest about the limits of presidential power?
Edwards, George C. 2016. Predicting the Presidency: The Potential of Persuasive Leadership. Princeton: Princeton University Press.
Edwards, George C. and Stephen J. Wayne. 2003. Presidential Leadership: Politics and Policy Making. Belmont, CA: Wadsworth/Thomson Learning.
Erickson, Robert S. and Christopher Wlezien. 2012. The Timeline of Presidential Elections: How Campaigns Do (and Do Not) Matter. Chicago: Chicago University Press.
Greenstein, Fred I. 1982. The Hidden-Hand Presidency: Eisenhower as Leader. New York: Basic Books.
Kernell, Samuel. 1986. Going Public: New Strategies of Presidential Leadership. Washington, DC: CQ Press.
McGinnis, Joe. 1988. The Selling of the President. New York: Penguin Books.
Nelson, Michael. 1984. The Presidency and the Political System. Washington, DC: CQ Press.
Neustadt, Richard E. 1990. Presidential Power and the Modern Presidents: The Politics of Leadership from Roosevelt to Reagan. New York: Free Press.
Pfiffner, James P. 1994. The Modern Presidency. New York: St. Martin’s Press.
Pika, Joseph August, John Anthony Maltese, Norman C. Thomas, and Norman C. Thomas. 2002. The Politics of the Presidency. Washington, DC: CQ Press.
Porter, Roger B. 1980. Presidential Decision Making: The Economic Policy Board. Cambridge: Cambridge University Press.
Skowronek, Stephen. 2011. Presidential Leadership in Political Time: Reprise and Reappraisal. Lawrence, KS: University Press of Kansas.
|
oercommons
|
2025-03-18T00:37:57.966369
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15258/overview",
"title": "American Government, Delivering Collective Action: Formal Institutions",
"author": null
}
|
https://oercommons.org/courseware/lesson/15259/overview
|
Introduction
If democratic institutions struggle to balance individual freedoms and collective well-being, the judiciary is arguably the branch where the individual has the best chance to be heard. For those seeking protection on the basis of sexual orientation, for example, in recent years, the courts have expanded rights, culminating in 2015 when the Supreme Court ruled that same-sex couples have the right to marry in all fifty states (Figure).Obergefell v. Hodges, 576 U.S. __ (2015).
The U.S. courts pride themselves on two achievements: (1) as part of the framers’ system of checks and balances, they protect the sanctity of the U.S. Constitution from breaches by the other branches of government, and (2) they protect individual rights against societal and governmental oppression. At the federal level, nine Supreme Court judges are nominated by the president and confirmed by the Senate for lifetime appointments. Hence, democratic control over them is indirect at best, but this provides them the independence they need to carry out their duties. However, court power is confined to rulings on those cases the courts decide to hear.In cases of original jurisdiction the courts cannot decide—the U.S. Constitution mandates that the U.S. Supreme Court must hear cases of original jurisdiction.
How do the courts make decisions, and how do they exercise their power to protect individual rights? How are the courts structured, and what distinguishes the Supreme Court from all others? This chapter answers these and other questions in delineating the power of the judiciary in the United States.
|
oercommons
|
2025-03-18T00:37:57.982553
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15259/overview",
"title": "American Government, Delivering Collective Action: Formal Institutions",
"author": null
}
|
https://oercommons.org/courseware/lesson/15260/overview
|
Guardians of the Constitution and Individual Rights
Learning Objectives
By the end of this section, you will be able to:
- Describe the evolving role of the courts since the ratification of the Constitution
- Explain why courts are uniquely situated to protect individual rights
- Recognize how the courts make public policy
Under the Articles of Confederation, there was no national judiciary. The U.S. Constitution changed that, but its Article III, which addresses “the judicial power of the United States,” is the shortest and least detailed of the three articles that created the branches of government. It calls for the creation of “one supreme Court” and establishes the Court’s jurisdiction, or its authority to hear cases and make decisions about them, and the types of cases the Court may hear. It distinguishes which are matters of original jurisdiction and which are for appellate jurisdiction. Under original jurisdiction, a case is heard for the first time, whereas under appellate jurisdiction, a court hears a case on appeal from a lower court and may change the lower court’s decision. The Constitution also limits the Supreme Court’s original jurisdiction to those rare cases of disputes between states, or between the United States and foreign ambassadors or ministers. So, for the most part, the Supreme Court is an appeals court, operating under appellate jurisdiction and hearing appeals from the lower courts. The rest of the development of the judicial system and the creation of the lower courts were left in the hands of Congress.
To add further explanation to Article III, Alexander Hamilton wrote details about the federal judiciary in Federalist No. 78. In explaining the importance of an independent judiciary separated from the other branches of government, he said “interpretation” was a key role of the courts as they seek to protect people from unjust laws. But he also believed “the Judiciary Department” would “always be the least dangerous” because “with no influence over either the sword or the purse,” it had “neither force nor will, but merely judgment.” The courts would only make decisions, not take action. With no control over how those decisions would be implemented and no power to enforce their choices, they could exercise only judgment, and their power would begin and end there. Hamilton would no doubt be surprised by what the judiciary has become: a key component of the nation’s constitutional democracy, finding its place as the chief interpreter of the Constitution and the equal of the other two branches, though still checked and balanced by them.
The first session of the first U.S. Congress laid the framework for today’s federal judicial system, established in the Judiciary Act of 1789. Although legislative changes over the years have altered it, the basic structure of the judicial branch remains as it was set early on: At the lowest level are the district courts, where federal cases are tried, witnesses testify, and evidence and arguments are presented. A losing party who is unhappy with a district court decision may appeal to the circuit courts, or U.S. courts of appeals, where the decision of the lower court is reviewed. Still further, appeal to the U.S. Supreme Court is possible, but of the thousands of petitions for appeal, the Supreme Court will typically hear fewer than one hundred a year.“The U.S. Supreme Court.” The Judicial Learning Center. http://judiciallearningcenter.org/the-us-supreme-court/ (March 1, 2016).
This public site maintained by the Administrative Office of the U.S. Courts provides detailed information from and about the judicial branch.
HUMBLE BEGINNINGS
Starting in New York in 1790, the early Supreme Court focused on establishing its rules and procedures and perhaps trying to carve its place as the new government’s third branch. However, given the difficulty of getting all the justices even to show up, and with no permanent home or building of its own for decades, finding its footing in the early days proved to be a monumental task. Even when the federal government moved to the nation’s capital in 1800, the Court had to share space with Congress in the Capitol building. This ultimately meant that “the high bench crept into an undignified committee room in the Capitol beneath the House Chamber.”Bernard Schwartz. 1993. A History of the Supreme Court. New York: Oxford University Press, 16.
It was not until the Court’s 146th year of operation that Congress, at the urging of Chief Justice—and former president—William Howard Taft, provided the designation and funding for the Supreme Court’s own building, “on a scale in keeping with the importance and dignity of the Court and the Judiciary as a coequal, independent branch of the federal government.”“Washington D.C. A National Register of Historic Places Travel Itinerary.” U.S. Department of the Interior, National Park Service. http://www.nps.gov/nr/travel/wash/dc78.htm (March 1, 2016). It was a symbolic move that recognized the Court’s growing role as a significant part of the national government (Figure).
But it took years for the Court to get to that point, and it faced a number of setbacks on the way to such recognition. In their first case of significance, Chisholm v. Georgia (1793), the justices ruled that the federal courts could hear cases brought by a citizen of one state against a citizen of another state, and that Article III, Section 2, of the Constitution did not protect the states from facing such an interstate lawsuit.Chisholm v. Georgia, 2 U.S. 419 (1793). However, their decision was almost immediately overturned by the Eleventh Amendment, passed by Congress in 1794 and ratified by the states in 1795. In protecting the states, the Eleventh Amendment put a prohibition on the courts by stating, “The Judicial power of the United States shall not be construed to extend to any suit in law or equity, commenced or prosecuted against one of the United States by Citizens of another State, or by Citizens or Subjects of any Foreign State.” It was an early hint that Congress had the power to change the jurisdiction of the courts as it saw fit and stood ready to use it.
In an atmosphere of perceived weakness, the first chief justice, John Jay, an author of The Federalist Papers and appointed by President George Washington, resigned his post to become governor of New York and later declined President John Adams’s offer of a subsequent term.Associated Press. “What You Should Know About Forgotten Founding Father John Jay,” PBS Newshour. July 4, 2015. http://www.pbs.org/newshour/rundown/forgotten-founding-father. In fact, the Court might have remained in a state of what Hamilton called its “natural feebleness” if not for the man who filled the vacancy Jay had refused—the fourth chief justice, John Marshall. Often credited with defining the modern court, clarifying its power, and strengthening its role, Marshall served in the chief’s position for thirty-four years. One landmark case during his tenure changed the course of the judicial branch’s history (Figure).“Life and Legacy.” The John Marshall Foundation. http://www.johnmarshallfoundation.org (March 1, 2016).
In 1803, the Supreme Court declared for itself the power of judicial review, a power to which Hamilton had referred but that is not expressly mentioned in the Constitution. Judicial review is the power of the courts, as part of the system of checks and balances, to look at actions taken by the other branches of government and the states and determine whether they are constitutional. If the courts find an action to be unconstitutional, it becomes null and void. Judicial review was established in the Supreme Court case Marbury v. Madison, when, for the first time, the Court declared an act of Congress to be unconstitutional.Marbury v. Madison, 5 U.S. 137 (1803). Wielding this power is a role Marshall defined as the “very essence of judicial duty,” and it continues today as one of the most significant aspects of judicial power. Judicial review lies at the core of the court’s ability to check the other branches of government—and the states.
Since Marbury, the power of judicial review has continually expanded, and the Court has not only ruled actions of Congress and the president to be unconstitutional, but it has also extended its power to include the review of state and local actions. The power of judicial review is not confined to the Supreme Court but is also exercised by the lower federal courts and even the state courts. Any legislative or executive action at the federal or state level inconsistent with the U.S. Constitution or a state constitution can be subject to judicial review.Stephen Hass. “Judicial Review.” National Juris University. http://juris.nationalparalegal.edu/(X(1)S(wwbvsi5iswopllt1bfpzfkjd))/JudicialReview.aspx (March 1, 2016).
Marbury v. Madison (1803)
The Supreme Court found itself in the middle of a dispute between the outgoing presidential administration of John Adams and that of incoming president (and opposition party member) Thomas Jefferson. It was an interesting circumstance at the time, particularly because Jefferson and the man who would decide the case—John Marshall—were themselves political rivals.
President Adams had appointed William Marbury to a position in Washington, DC, but his commission was not delivered before Adams left office. So Marbury petitioned the Supreme Court to use its power under the Judiciary Act of 1789 and issue a writ of mandamus to force the new president’s secretary of state, James Madison, to deliver the commission documents. It was a task Madison refused to do. A unanimous Court under the leadership of Chief Justice John Marshall ruled that although Marbury was entitled to the job, the Court did not have the power to issue the writ and order Madison to deliver the documents, because the provision in the Judiciary Act that had given the Court that power was unconstitutional.Marbury v. Madison, 5 U.S. 137 (1803).
Perhaps Marshall feared a confrontation with the Jefferson administration and thought Madison would refuse his directive anyway. In any case, his ruling shows an interesting contrast in the early Court. On one hand, it humbly declined a power—issuing a writ of mandamus—given to it by Congress, but on the other, it laid the foundation for legitimizing a much more important one—judicial review. Marbury never got his commission, but the Court’s ruling in the case has become more significant for the precedent it established: As the first time the Court declared an act of Congress unconstitutional, it established the power of judicial review, a key power that enables the judicial branch to remain a powerful check on the other branches of government.
Consider the dual nature of John Marshall’s opinion in Marbury v. Madison: On one hand, it limits the power of the courts, yet on the other it also expanded their power. Explain the different aspects of the decision in terms of these contrasting results.
THE COURTS AND PUBLIC POLICY
Even with judicial review in place, the courts do not always stand ready just to throw out actions of the other branches of government. More broadly, as Marshall put it, “it is emphatically the province and duty of the judicial department to say what the law is.”Marbury v. Madison, 5 U.S. 137 (1803). The United States has a common law system in which law is largely developed through binding judicial decisions. With roots in medieval England, the system was inherited by the American colonies along with many other British traditions.“The Common Law and Civil Law Traditions.” The Robbins Collection. School of Law (Boalt Hall). University of California at Berkeley. https://www.law.berkeley.edu/library/robbins/CommonLawCivilLawTraditions.html (March 1, 2016). It stands in contrast to code law systems, which provide very detailed and comprehensive laws that do not leave room for much interpretation and judicial decision-making. With code law in place, as it is in many nations of the world, it is the job of judges to simply apply the law. But under common law, as in the United States, they interpret it. Often referred to as a system of judge-made law, common law provides the opportunity for the judicial branch to have stronger involvement in the process of law-making itself, largely through its ruling and interpretation on a case-by-case basis.
In their role as policymakers, Congress and the president tend to consider broad questions of public policy and their costs and benefits. But the courts consider specific cases with narrower questions, thus enabling them to focus more closely than other government institutions on the exact context of the individuals, groups, or issues affected by the decision. This means that while the legislature can make policy through statute, and the executive can form policy through regulations and administration, the judicial branch can also influence policy through its rulings and interpretations. As cases are brought to the courts, court decisions can help shape policy.
Consider health care, for example. In 2010, President Barack Obama signed into law the Patient Protection and Affordable Care Act (ACA), a statute that brought significant changes to the nation’s healthcare system. With its goal of providing more widely attainable and affordable health insurance and health care, “Obamacare” was hailed by some but soundly denounced by others as bad policy. People who opposed the law and understood that a congressional repeal would not happen any time soon looked to the courts for help. They challenged the constitutionality of the law in National Federation of Independent Business v. Sebelius, hoping the Supreme Court would overturn it.National Federation of Independent Business v. Sebelius, 567 U.S. __ (2012). The practice of judicial review enabled the law’s critics to exercise this opportunity, even though their hopes were ultimately dashed when, by a narrow 5–4 margin, the Supreme Court upheld the health care law as a constitutional extension of Congress’s power to tax.
Since this 2012 decision, the ACA has continued to face challenges, the most notable of which have also been decided by court rulings. It faced a setback in 2014, for instance, when the Supreme Court ruled in Burwell v. Hobby Lobby that, for religious reasons, some for-profit corporations could be exempt from the requirement that employers provide insurance coverage of contraceptives for their female employees.Burwell v. Hobby Lobby, 573 U.S. __ (2014). But the ACA also attained a victory in King v. Burwell, when the Court upheld the ability of the federal government to provide tax credits for people who bought their health insurance through an exchange created by the law.King v. Burwell, 576 U.S. __ (2015).
With each ACA case it has decided, the Supreme Court has served as the umpire, upholding the law and some of its provisions on one hand, but ruling some aspects of it unconstitutional on the other. Both supporters and opponents of the law have claimed victory and faced defeat. In each case, the Supreme Court has further defined and fine-tuned the law passed by Congress and the president, determining which parts stay and which parts go, thus having its say in the way the act has manifested itself, the way it operates, and the way it serves its public purpose.
In this same vein, the courts have become the key interpreters of the U.S. Constitution, continuously interpreting it and applying it to modern times and circumstances. For example, it was in 2015 that we learned a man’s threat to kill his ex-wife, written in rap lyrics and posted to her Facebook wall, was not a real threat and thus could not be prosecuted as a felony under federal law.Elonis v. United States, 13-983 U.S. __ (2015). Certainly, when the Bill of Rights first declared that government could not abridge freedom of speech, its framers could never have envisioned Facebook—or any other modern technology for that matter.
But freedom of speech, just like many constitutional concepts, has come to mean different things to different generations, and it is the courts that have designed the lens through which we understand the Constitution in modern times. It is often said that the Constitution changes less by amendment and more by the way it is interpreted. Rather than collecting dust on a shelf, the nearly 230-year-old document has come with us into the modern age, and the accepted practice of judicial review has helped carry it along the way.
COURTS AS A LAST RESORT
While the U.S. Supreme Court and state supreme courts exert power over many when reviewing laws or declaring acts of other branches unconstitutional, they become particularly important when an individual or group comes before them believing there has been a wrong. A citizen or group that feels mistreated can approach a variety of institutional venues in the U.S. system for assistance in changing policy or seeking support. Organizing protests, garnering special interest group support, and changing laws through the legislative and executive branches are all possible, but an individual is most likely to find the courts especially well-suited to analyzing the particulars of his or her case.
The adversarial judicial system comes from the common law tradition: In a court case, it is one party versus the other, and it is up to an impartial person or group, such as the judge or jury, to determine which party prevails. The federal court system is most often called upon when a case touches on constitutional rights. For example, when Samantha Elauf, a Muslim woman, was denied a job working for the clothing retailer Abercrombie & Fitch because a headscarf she wears as religious practice violated the company’s dress code, the Supreme Court ruled that her First Amendment rights had been violated, making it possible for her to sue the store for monetary damages.
Elauf had applied for an Abercrombie sales job in Oklahoma in 2008. Her interviewer recommended her based on her qualifications, but she was never given the job because the clothing retailer wanted to avoid having to accommodate her religious practice of wearing a headscarf, or hijab. In so doing, the Court ruled, Abercrombie violated Title VII of the Civil Rights Act of 1964, which prohibits employers from discriminating on the basis of race, color, religion, sex, or national origin, and requires them to accommodate religious practices.Equal Employment Opportunity Commission v. Abercrombie & Fitch Stores, 575 U.S. __ (2015).
Rulings like this have become particularly important for members of religious minority groups, including Muslims, Sikhs, and Jews, who now feel more protected from employment discrimination based on their religious attire, head coverings, or beards.Liptak, Adam. “Muslim Woman Denied Job Over Head Scarf Wins in Supreme Court.” New York Times. 1 June 2015. http://www.nytimes.com/2015/06/02/us/supreme-court-rules-in-samantha-elauf-abercrombie-fitch-case.html?_r=0. Such decisions illustrate how the expansion of individual rights and liberties for particular persons or groups over the years has come about largely as a result of court rulings made for individuals on a case-by-case basis.
Although the United States prides itself on the Declaration of Independence’s statement that “all men are created equal,” and “equal protection of the laws” is a written constitutional principle of the Fourteenth Amendment, the reality is less than perfect. But it is evolving. Changing times and technology have and will continue to alter the way fundamental constitutional rights are defined and applied, and the courts have proven themselves to be crucial in that definition and application.
Societal traditions, public opinion, and politics have often stood in the way of the full expansion of rights and liberties to different groups, and not everyone has agreed that these rights should be expanded as they have been by the courts. Schools were long segregated by race until the Court ordered desegregation in Brown v. Board of Education (1954), and even then, many stood in opposition and tried to block students at the entrances to all-white schools.Brown v. Board of Education of Topeka, 347 U.S. 483 (1954). Factions have formed on opposite sides of the abortion and handgun debates, because many do not agree that women should have abortion rights or that individuals should have the right to a handgun. People disagree about whether members of the LGBT community should be allowed to marry or whether arrested persons should be read their rights, guaranteed an attorney, and/or have their cell phones protected from police search.
But the Supreme Court has ruled in favor of all these issues and others. Even without unanimous agreement among citizens, Supreme Court decisions have made all these possibilities a reality, a particularly important one for the individuals who become the beneficiaries (Table). The judicial branch has often made decisions the other branches were either unwilling or unable to make, and Hamilton was right in Federalist No. 78 when he said that without the courts exercising their duty to defend the Constitution, “all the reservations of particular rights or privileges would amount to nothing.”
| Examples of Supreme Court Cases Involving Individuals | ||
|---|---|---|
| Case Name | Year | Court’s Decision |
| Brown v. Board of Education | 1954 | Public schools must be desegregated. |
| Gideon v. Wainwright | 1963 | Poor criminal defendants must be provided an attorney. |
| Miranda v. Arizona | 1966 | Criminal suspects must be read their rights. |
| Roe v. Wade | 1973 | Women have a constitutional right to abortion. |
| McDonald v. Chicago | 2010 | An individual has the right to a handgun in his or her home. |
| Riley v. California | 2014 | Police may not search a cell phone without a warrant. |
| Obergefell v. Hodges | 2015 | Same-sex couples have the right to marry in all states. |
The courts seldom if ever grant rights to a person instantly and upon request. In a number of cases, they have expressed reluctance to expand rights without limit, and they still balance that expansion with the government’s need to govern, provide for the common good, and serve a broader societal purpose. For example, the Supreme Court has upheld the constitutionality of the death penalty, ruling that the Eighth Amendment does not prevent a person from being put to death for committing a capital crime and that the government may consider “retribution and the possibility of deterrence” when it seeks capital punishment for a crime that so warrants it.Gregg v. Georgia, 428 U.S. 153 (1976). In other words, there is a greater good—more safety and security—that may be more important than sparing the life of an individual who has committed a heinous crime.
Yet the Court has also put limits on the ability to impose the death penalty, ruling, for example, that the government may not execute a person with cognitive disabilities, a person who was under eighteen at the time of the crime, or a child rapist who did not kill his victim.Atkins v. Virginia, 536 U.S. 304 (2002); Roper v. Simmons, 543 U.S. 551 (2005); Kennedy v. Louisiana, 554 U.S. 407 (2008). So the job of the courts on any given issue is never quite done, as justices continuously keep their eye on government laws, actions, and policy changes as cases are brought to them and then decide whether those laws, actions, and policies can stand or must go. Even with an issue such as the death penalty, about which the Court has made several rulings, there is always the possibility that further judicial interpretation of what does (or does not) violate the Constitution will be needed.
This happened, for example, as recently as 2015 in a case involving the use of lethal injection as capital punishment in the state of Oklahoma, where death-row inmates are put to death through the use of three drugs—a sedative to bring about unconsciousness (midazolam), followed by two others that cause paralysis and stop the heart. A group of these inmates challenged the use of midazolam as unconstitutional. They argued that since it could not reliably cause unconsciousness, its use constituted an Eighth Amendment violation against cruel and unusual punishment and should be stopped by the courts. The Supreme Court rejected the inmates’ claims, ruling that Oklahoma could continue to use midazolam as part of its three-drug protocol.Glossip v. Gross, 576 U.S. __ (2015). But with four of the nine justices dissenting from that decision, a sharply divided Court leaves open a greater possibility of more death-penalty cases to come. The 2015–2016 session alone includes four such cases, challenging death-sentencing procedures in such states as Florida, Georgia, and Kansas.“October Term 2015.” SCOTUSblog. http://www.scotusblog.com/case-files/terms/ot2015/?sort=mname (March 1, 2016).
Therefore, we should not underestimate the power and significance of the judicial branch in the United States. Today, the courts have become a relevant player, gaining enough clout and trust over the years to take their place as a separate yet coequal branch.
Summary
From humble beginnings, the judicial branch has evolved over the years to a significance that would have been difficult for the Constitution’s framers to envision. While they understood and prioritized the value of an independent judiciary in a common law system, they could not have predicted the critical role the courts would play in the interpretation of the Constitution, our understanding of the law, the development of public policy, and the preservation and expansion of individual rights and liberties over time.
The Supreme Court’s power of judicial review ________.
- is given to it in the original constitution
- enables it to declare acts of the other branches unconstitutional
- allows it to hear cases
- establishes the three-tiered court system
Hint:
B
The Supreme Court most typically functions as ________.
- a district court
- a trial court
- a court of original jurisdiction
- an appeals court
In Federalist No. 78, Alexander Hamilton characterized the judiciary as the ________ branch of government.
- most unnecessary
- strongest
- least dangerous
- most political
Hint:
C
Explain one positive and one negative aspect of the lifetime term of office for judges and justices in the federal court system. Why do you believe the constitution’s framers chose lifetime terms?
What do you find most significant about having a common law system?
Hint:
The judicial branch is involved in the system of law-making in the United States. Through their interpretation of the law, judges are an important part of the legal system and influence the way law is made and interpreted. They don’t just apply the law; they also make it.
|
oercommons
|
2025-03-18T00:37:58.021284
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15260/overview",
"title": "American Government, Delivering Collective Action: Formal Institutions",
"author": null
}
|
https://oercommons.org/courseware/lesson/15261/overview
|
The Dual Court System
Learning Objectives
By the end of this section, you will be able to:
- Describe the dual court system and its three tiers
- Explain how you are protected and governed by different U.S. court systems
- Compare the positive and negative aspects of a dual court system
Before the writing of the U.S. Constitution and the establishment of the permanent national judiciary under Article III, the states had courts. Each of the thirteen colonies had also had its own courts, based on the British common law model. The judiciary today continues as a dual court system, with courts at both the national and state levels. Both levels have three basic tiers consisting of trial courts, appellate courts, and finally courts of last resort, typically called supreme courts, at the top (Figure).
To add to the complexity, the state and federal court systems sometimes intersect and overlap each other, and no two states are exactly alike when it comes to the organization of their courts. Since a state’s court system is created by the state itself, each one differs in structure, the number of courts, and even name and jurisdiction. Thus, the organization of state courts closely resembles but does not perfectly mirror the more clear-cut system found at the federal level.Bureau of International Information Programs, United States Department of State. Outline of the U.S. Legal System. 2004. Still, we can summarize the overall three-tiered structure of the dual court model and consider the relationship that the national and state sides share with the U.S. Supreme Court, as illustrated in Figure.
Cases heard by the U.S. Supreme Court come from two primary pathways: (1) the circuit courts, or U.S. courts of appeals (after the cases have originated in the federal district courts), and (2) state supreme courts (when there is a substantive federal question in the case). In a later section of the chapter, we discuss the lower courts and the movement of cases through the dual court system to the U.S. Supreme Court. But first, to better understand how the dual court system operates, we consider the types of cases state and local courts handle and the types for which the federal system is better designed.
COURTS AND FEDERALISM
Courts hear two different types of disputes: criminal and civil. Under criminal law, governments establish rules and punishments; laws define conduct that is prohibited because it can harm others and impose punishment for committing such an act. Crimes are usually labeled felonies or misdemeanors based on their nature and seriousness; felonies are the more serious crimes. When someone commits a criminal act, the government (state or national, depending on which law has been broken) charges that person with a crime, and the case brought to court contains the name of the charging government, as in Miranda v. Arizona discussed below.Miranda v. Arizona, 384 U.S. 436 (1966). On the other hand, civil law cases involve two or more private (non-government) parties, at least one of whom alleges harm or injury committed by the other. In both criminal and civil matters, the courts decide the remedy and resolution of the case, and in all cases, the U.S. Supreme Court is the final court of appeal.
This site provides an interesting challenge: Look at the different cases presented and decide whether each would be heard in the state or federal courts. You can check your results at the end.
Although the Supreme Court tends to draw the most public attention, it typically hears fewer than one hundred cases every year. In fact, the entire federal side—both trial and appellate—handles proportionately very few cases, with about 90 percent of all cases in the U.S. court system being heard at the state level.“State Courts vs. Federal Courts.” The Judicial Learning Center. http://judiciallearningcenter.org/state-courts-vs-federal-courts/ (March 1, 2016). The several hundred thousand cases handled every year on the federal side pale in comparison to the several million handled by the states.
State courts really are the core of the U.S. judicial system, and they are responsible for a huge area of law. Most crimes and criminal activity, such as robbery, rape, and murder, are violations of state laws, and cases are thus heard by state courts. State courts also handle civil matters; personal injury, malpractice, divorce, family, juvenile, probate, and contract disputes and real estate cases, to name just a few, are usually state-level cases.
The federal courts, on the other hand, will hear any case that involves a foreign government, patent or copyright infringement, Native American rights, maritime law, bankruptcy, or a controversy between two or more states. Cases arising from activities across state lines (interstate commerce) are also subject to federal court jurisdiction, as are cases in which the United States is a party. A dispute between two parties not from the same state or nation and in which damages of at least $75,000 are claimed is handled at the federal level. Such a case is known as a diversity of citizenship case.“State Courts vs. Federal Courts.” The Judicial Learning Center. http://judiciallearningcenter.org/state-courts-vs-federal-courts/ (March 1, 2016).
However, some cases cut across the dual court system and may end up being heard in both state and federal courts. Any case has the potential to make it to the federal courts if it invokes the U.S. Constitution or federal law. It could be a criminal violation of federal law, such as assault with a gun, the illegal sale of drugs, or bank robbery. Or it could be a civil violation of federal law, such as employment discrimination or securities fraud. Also, any perceived violation of a liberty protected by the Bill of Rights, such as freedom of speech or the protection against cruel and unusual punishment, can be argued before the federal courts. A summary of the basic jurisdictions of the state and federal sides is provided in Table.
| Jurisdiction of the Courts: State vs. Federal | |
|---|---|
| State Courts | Federal Courts |
| Hear most day-to-day cases, covering 90 percent of all cases | Hear cases that involve a “federal question,” involving the Constitution, federal laws or treaties, or a “federal party” in which the U.S. government is a party to the case |
| Hear both civil and criminal matters | Hear both civil and criminal matters, although many criminal cases involving federal law are tried in state courts |
| Help the states retain their own sovereignty in judicial matters over their state laws, distinct from the national government | Hear cases that involve “interstate” matters, “diversity of citizenship” involving parties of two different states, or between a U.S. citizen and a citizen of another nation (and with a damage claim of at least $75,000) |
While we may certainly distinguish between the two sides of a jurisdiction, looking on a case-by-case basis will sometimes complicate the seemingly clear-cut division between the state and federal sides. It is always possible that issues of federal law may start in the state courts before they make their way over to the federal side. And any case that starts out at the state and/or local level on state matters can make it into the federal system on appeal—but only on points that involve a federal law or question, and usually after all avenues of appeal in the state courts have been exhausted.“U.S. Court System.” Syracuse University. http://www2.maxwell.syr.edu/plegal/scales/court.html (March 1, 2016).
Consider the case Miranda v. Arizona.Miranda v. Arizona, 384 U.S. 436 (1966). Ernesto Miranda, arrested for kidnapping and rape, which are violations of state law, was easily convicted and sentenced to prison after a key piece of evidence—his own signed confession—was presented at trial in the Arizona court. On appeal first to the Arizona Supreme Court and then to the U.S. Supreme Court to exclude the confession on the grounds that its admission was a violation of his constitutional rights, Miranda won the case. By a slim 5–4 margin, the justices ruled that the confession had to be excluded from evidence because in obtaining it, the police had violated Miranda’s Fifth Amendment right against self-incrimination and his Sixth Amendment right to an attorney. In the opinion of the Court, because of the coercive nature of police interrogation, no confession can be admissible unless a suspect is made aware of his rights and then in turn waives those rights. For this reason, Miranda’s original conviction was overturned.
Yet the Supreme Court considered only the violation of Miranda’s constitutional rights, but not whether he was guilty of the crimes with which he was charged. So there were still crimes committed for which Miranda had to face charges. He was therefore retried in state court in 1967, the second time without the confession as evidence, found guilty again based on witness testimony and other evidence, and sent to prison.
Miranda’s story is a good example of the tandem operation of the state and federal court systems. His guilt or innocence of the crimes was a matter for the state courts, whereas the constitutional questions raised by his trial were a matter for the federal courts. Although he won his case before the Supreme Court, which established a significant precedent that criminal suspects must be read their so-called Miranda rights before police questioning, the victory did not do much for Miranda himself. After serving prison time, he was stabbed to death in a bar fight in 1976 while out on parole, and due to a lack of evidence, no one was ever convicted in his death.
THE IMPLICATIONS OF A DUAL COURT SYSTEM
From an individual’s perspective, the dual court system has both benefits and drawbacks. On the plus side, each person has more than just one court system ready to protect his or her rights. The dual court system provides alternate venues in which to appeal for assistance, as Ernesto Miranda’s case illustrates. The U.S. Supreme Court found for Miranda an extension of his Fifth Amendment protections—a constitutional right to remain silent when faced with police questioning. It was a right he could not get solely from the state courts in Arizona, but one those courts had to honor nonetheless.
The fact that a minority voice like Miranda’s can be heard in court, and that his or her grievance can be resolved in his or her favor if warranted, says much about the role of the judiciary in a democratic republic. In Miranda’s case, a resolution came from the federal courts, but it can also come from the state side. In fact, the many differences among the state courts themselves may enhance an individual’s potential to be heard.
State courts vary in the degree to which they take on certain types of cases or issues, give access to particular groups, or promote certain interests. If a particular issue or topic is not taken up in one place, it may be handled in another, giving rise to many different opportunities for an interest to be heard somewhere across the nation. In their research, Paul Brace and Melinda Hall found that state courts are important instruments of democracy because they provide different alternatives and varying arenas for political access. They wrote, “Regarding courts, one size does not fit all, and the republic has survived in part because federalism allows these critical variations.”Paul R. Brace and Melinda Gann Hall. 2005. “Is Judicial Federalism Essential to Democracy? State Courts in the Federal System.” In Institutions of American Democracy, The Judicial Branch, eds. Kermit L. Hall and Kevin T. McGuire. New York: Oxford University Press.
But the existence of the dual court system and variations across the states and nation also mean that there are different courts in which a person could face charges for a crime or for a violation of another person’s rights. Except for the fact that the U.S. Constitution binds judges and justices in all the courts, it is state law that governs the authority of state courts, so judicial rulings about what is legal or illegal may differ from state to state. These differences are particularly pronounced when the laws across the states and the nation are not the same, as we see with marijuana laws today.
Marijuana Laws and the Courts
There are so many differences in marijuana laws between states, and between the states and the national government, that uniform application of treatment in courts across the nation is nearly impossible (Figure). What is legal in one state may be illegal in another, and state laws do not cross state geographic boundary lines—but people do. What’s more, a person residing in any of the fifty states is still subject to federal law.
For example, a person over the age of twenty-one may legally buy marijuana for recreational use in four states and for medicinal purpose in nearly half the states, but could face charges—and time in court—for possession in a neighboring state where marijuana use is not legal. Under federal law, too, marijuana is still regulated as a Schedule 1 (most dangerous) drug, and federal authorities often find themselves pitted against states that have legalized it. Such differences can lead, somewhat ironically, to arrests and federal criminal charges for people who have marijuana in states where it is legal, or to federal raids on growers and dispensaries that would otherwise be operating legally under their state’s law.
Differences among the states have also prompted a number of lawsuits against states with legalized marijuana, as people opposed to those state laws seek relief from (none other than) the courts. They want the courts to resolve the issue, which has left in its wake contradictions and conflicts between states that have legalized marijuana and those that have not, as well as conflicts between states and the national government. These lawsuits include at least one filed by the states of Nebraska and Oklahoma against Colorado. Citing concerns over cross-border trafficking, difficulties with law enforcement, and violations of the Constitution’s supremacy clause, Nebraska and Oklahoma have petitioned the U.S. Supreme Court to intervene and rule on the legality of Colorado’s marijuana law, hoping to get it overturned.States of Nebraska and Oklahoma v. State of Colorado. Motion for Leave to File Complaint, Complaint and Brief in Support. December 2014. http://www.scribd.com/doc/250506006/Nebraska-Oklahoma-Lawsuit. The Supreme Court has yet to take up the case.
How do you think differences among the states and differences between federal and state law regarding marijuana use can affect the way a person is treated in court? What, if anything, should be done to rectify the disparities in application of the law across the nation?
Where you are physically located can affect not only what is allowable and what is not, but also how cases are judged. For decades, political scientists have confirmed that political culture affects the operation of government institutions, and when we add to that the differing political interests and cultures at work within each state, we end up with court systems that vary greatly in their judicial and decision-making processes.Joel B. Grossman and Austin Sarat. 1971. “Political Culture and Judicial Research.” Washington University Law Review. 1971 (2) Symposium: Courts, Judges, Politics—Some Political Science Perspectives. http://openscholarship.wustl.edu/cgi/viewcontent.cgi?article=2777&context=law_lawreview. Each state court system operates with its own individual set of biases. People with varying interests, ideologies, behaviors, and attitudes run the disparate legal systems, so the results they produce are not always the same. Moreover, the selection method for judges at the state and local level varies. In some states, judges are elected rather than appointed, which can affect their rulings.
Just as the laws vary across the states, so do judicial rulings and interpretations, and the judges who make them. That means there may not be uniform application of the law—even of the same law—nationwide. We are somewhat bound by geography and do not always have the luxury of picking and choosing the venue for our particular case. So, while having such a decentralized and varied set of judicial operations affects the kinds of cases that make it to the courts and gives citizens alternate locations to get their case heard, it may also lead to disparities in the way they are treated once they get there.
Summary
The U.S. judicial system features a dual court model, with courts at both the federal and state levels, and the U.S. Supreme Court at the top. While cases may sometimes be eligible for both state and federal review, each level has its own distinct jurisdiction. There are trial and appellate courts at both levels, but there are also remarkable differences among the states in their laws, politics, and culture, meaning that no two state court systems are exactly alike. The diversity of courts across the nation can have both positive and negative effects for citizens, depending on their situation. While it provides for various opportunities for an issue or interest to be heard, it may also lead to case-by-case treatment of individuals, groups, or issues that is not always the same or even-handed across the nation.
Of all the court cases in the United States, the majority are handled ________.
- by the U.S. Supreme Court
- at the state level
- by the circuit courts
- by the U.S. district courts
Both state and federal courts hear matters that involve ________.
- civil law only
- criminal law only
- both civil and criminal law
- neither civil nor criminal law
Hint:
C
A state case is more likely to be heard by the federal courts when ________.
- it involves a federal question
- a governor requests a federal court hearing
- it involves a criminal matter
- the state courts are unable to come up with a decision
The existence of the dual court system is an unnecessary duplication to some but beneficial to others. Provide at least one positive and one negative characteristic of having overlapping court systems in the United States.
Hint:
Overlapping court systems provide each individual with more than just one court to protect his or her rights. A person seeking a wrong to be righted may have alternate places to pursue his or her case. On the other hand, having overlapping court systems opens the door to the possibility of unequal or disparate administration of justice.
Which court would you consider to be closest to the people? Why?
|
oercommons
|
2025-03-18T00:37:58.053144
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15261/overview",
"title": "American Government, Delivering Collective Action: Formal Institutions",
"author": null
}
|
https://oercommons.org/courseware/lesson/15262/overview
|
The Federal Court System
Learning Objectives
By the end of this section, you will be able to:
- Describe the differences between the U.S. district courts, circuit courts, and the Supreme Court
- Explain the significance of precedent in the courts’ operations
- Describe how judges are selected for their positions
Congress has made numerous changes to the federal judicial system throughout the years, but the three-tiered structure of the system is quite clear-cut today. Federal cases typically begin at the lowest federal level, the district (or trial) court. Losing parties may appeal their case to the higher courts—first to the circuit courts, or U.S. courts of appeals, and then, if chosen by the justices, to the U.S. Supreme Court. Decisions of the higher courts are binding on the lower courts. The precedent set by each ruling, particularly by the Supreme Court’s decisions, both builds on principles and guidelines set by earlier cases and frames the ongoing operation of the courts, steering the direction of the entire system. Reliance on precedent has enabled the federal courts to operate with logic and consistency that has helped validate their role as the key interpreters of the Constitution and the law—a legitimacy particularly vital in the United States where citizens do not elect federal judges and justices but are still subject to their rulings.
THE THREE TIERS OF FEDERAL COURTS
There are ninety-four U.S. district courts in the fifty states and U.S. territories, of which eighty-nine are in the states (at least one in each state). The others are in Washington, DC; Puerto Rico; Guam; the U.S. Virgin Islands; and the Northern Mariana Islands. These are the trial courts of the national system, in which federal cases are tried, witness testimony is heard, and evidence is presented. No district court crosses state lines, and a single judge oversees each one. Some cases are heard by a jury, and some are not.
There are thirteen U.S. courts of appeals, or circuit courts, eleven across the nation and two in Washington, DC (the DC circuit and the federal circuit courts), as illustrated in Figure. Each court is overseen by a rotating panel of three judges who do not hold trials but instead review the rulings of the trial (district) courts within their geographic circuit. As authorized by Congress, there are currently 179 judges. The circuit courts are often referred to as the intermediate appellate courts of the federal system, since their rulings can be appealed to the U.S. Supreme Court. Moreover, different circuits can hold legal and cultural views, which can lead to differing outcomes on similar legal questions. In such scenarios, clarification from the U.S. Supreme Court might be needed.
Today’s federal court system was not an overnight creation; it has been changing and transitioning for more than two hundred years through various acts of Congress. Since district courts are not called for in Article III of the Constitution, Congress established them and narrowly defined their jurisdiction, at first limiting them to handling only cases that arose within the district. Beginning in 1789 when there were just thirteen, the district courts became the basic organizational units of the federal judicial system. Gradually over the next hundred years, Congress expanded their jurisdiction, in particular over federal questions, which enables them to review constitutional issues and matters of federal law. In the Judicial Code of 1911, Congress made the U.S. district courts the sole general-jurisdiction trial courts of the federal judiciary, a role they had previously shared with the circuit courts.“The U.S. District Courts and the Federal Judiciary.” Federal Judicial Center. http://www.fjc.gov/history/home.nsf/page/courts_district.html (March 1, 2016).
The circuit courts started out as the trial courts for most federal criminal cases and for some civil suits, including those initiated by the United States and those involving citizens of different states. But early on, they did not have their own judges; the local district judge and two Supreme Court justices formed each circuit court panel. (That is how the name “circuit” arose—judges in the early circuit courts traveled from town to town to hear cases, following prescribed paths or circuits to arrive at destinations where they were needed.“Circuit Riding.” Encyclopedia Britannica. http://www.britannica.com/topic/circuit-riding (March 1, 2016).) Circuit courts also exercised appellate jurisdiction (meaning they receive appeals on federal district court cases) over most civil suits that originated in the district courts; however, that role ended in 1891, and their appellate jurisdiction was turned over to the newly created circuit courts, or U.S. courts of appeals. The original circuit courts—the ones that did not have “of appeals” added to their name—were abolished in 1911, fully replaced by these new circuit courts of appeals.“The U.S. Circuit Courts and the Federal Judiciary.” Federal Judicial Center. http://www.fjc.gov/history/home.nsf/page/courts_circuit.html (March 1, 2016).
While we often focus primarily on the district and circuit courts of the federal system, other federal trial courts exist that have more specialized jurisdictions, such as the Court of International Trade, Court of Federal Claims, and U.S. Tax Court. Specialized federal appeals courts include the Court of Appeals for the Armed Forces and the Court of Appeals for Veterans Claims. Cases from any of these courts may also be appealed to the Supreme Court, although that result is very rare.
On the U.S. Supreme Court, there are nine justices—one chief justice and eight associate justices. Circuit courts each contain three justices, whereas federal district courts have just one judge each. As the national court of last resort for all other courts in the system, the Supreme Court plays a vital role in setting the standards of interpretation that the lower courts follow. The Supreme Court’s decisions are binding across the nation and establish the precedent by which future cases are resolved in all the system’s tiers.
The U.S. court system operates on the principle of stare decisis (Latin for stand by things decided), which means that today’s decisions are based largely on rulings from the past, and tomorrow’s rulings rely on what is decided today. Stare decisis is especially important in the U.S. common law system, in which the consistency of precedent ensures greater certainty and stability in law and constitutional interpretation, and it also contributes to the solidity and legitimacy of the court system itself. As former Supreme Court justice Benjamin Cardozo summarized it years ago, “Adherence to precedent must then be the rule rather than the exception if litigants are to have faith in the even-handed administration of justice in the courts.”Benjamin N. Cardozo. 1921. The Nature of the Judicial Process. New Haven: Yale University Press. http://www.constitution.org/cmt/cardozo/jud_proc.htm.
With a focus on federal courts and the public, this website reveals the different ways the federal courts affect the lives of U.S. citizens and how those citizens interact with the courts.
When the legal facts of one case are the same as the legal facts of another, stare decisis dictates that they should be decided the same way, and judges are reluctant to disregard precedent without justification. However, that does not mean there is no flexibility or that new precedents or rulings can never be created. They often are. Certainly, court interpretations can change as times and circumstances change—and as the courts themselves change when new judges are selected and take their place on the bench. For example, the membership of the Supreme Court had changed entirely between Plessey v. Ferguson (1896), which brought the doctrine of “separate but equal” and Brown v. Board of Education (1954), which required integration.Plessy v. Ferguson, 163 U.S. 537 (1896); Brown v. Board of Education of Topeka, 347 U.S. 483 (1954).
THE SELECTION OF JUDGES
Judges fulfill a vital role in the U.S. judicial system and are carefully selected. At the federal level, the president nominates a candidate to a judgeship or justice position, and the nominee must be confirmed by a majority vote in the U.S. Senate, a function of the Senate’s “advice and consent” role. All judges and justices in the national courts serve lifetime terms of office.
The president sometimes chooses nominees from a list of candidates maintained by the American Bar Association, a national professional organization of lawyers.American Bar Association Coalition for Justice. 2008. “Judicial Selection.” In American Bar Association, eds. American Judicature Society and Malia Reddick. http://www.americanbar.org/content/dam/aba/migrated/JusticeCenter/Justice/PublicDocuments/judicial_selection_roadmap.authcheckdam.pdf. The president’s nominee is then discussed (and sometimes hotly debated) in the Senate Judiciary Committee. After a committee vote, the candidate must be confirmed by a majority vote of the full Senate. He or she is then sworn in, taking an oath of office to uphold the Constitution and the laws of the United States.
When a vacancy occurs in a lower federal court, by custom, the president consults with that state’s U.S. senators before making a nomination. Through such senatorial courtesy, senators exert considerable influence on the selection of judges in their state, especially those senators who share a party affiliation with the president. In many cases, a senator can block a proposed nominee just by voicing his or her opposition. Thus, a presidential nominee typically does not get far without the support of the senators from the nominee’s home state.
Most presidential appointments to the federal judiciary go unnoticed by the public, but when a president has the rarer opportunity to make a Supreme Court appointment, it draws more attention. That is particularly true now, when many people get their news primarily from the Internet and social media. It was not surprising to see not only television news coverage but also blogs and tweets about President Obama’s most recent nominees to the high court, Sonia Sotomayor and Elena Kagan (Figure).
Presidential nominees for the courts typically reflect the chief executive’s own ideological position. With a confirmed nominee serving a lifetime appointment, a president’s ideological legacy has the potential to live on long after the end of his or her term.American Bar Association Coalition for Justice. 2008. “Judicial Selection.” In American Bar Association, eds. American Judicature Society and Malia Reddick. http://www.americanbar.org/content/dam/aba/migrated/JusticeCenter/Justice/PublicDocuments/judicial_selection_roadmap.authcheckdam.pdf. President Obama surely considered the ideological leanings of his two Supreme Court appointees, and both Sotomayor and Kagan have consistently ruled in a more liberal ideological direction. The timing of the two nominations also dovetailed nicely with the Democratic Party’s gaining control of the Senate in the 111th Congress of 2009–2011, which helped guarantee their confirmations.
But some nominees turn out to be surprises or end up ruling in ways that the president who nominated them did not anticipate. Democratic-appointed judges sometimes side with conservatives, just as Republican-appointed judges sometimes side with liberals. Republican Dwight D. Eisenhower reportedly called his nomination of Earl Warren as chief justice—in an era that saw substantial broadening of civil and criminal rights—“the biggest damn fool mistake” he had ever made. Sandra Day O’Connor, nominated by Republican president Ronald Reagan, often became a champion for women’s rights. David Souter, nominated by Republican George H. W. Bush, more often than not sided with the Court’s liberal wing. And even on the present-day court, Anthony Kennedy, a Reagan appointee, has become notorious as the Court’s swing vote, sometimes siding with the more conservative justices but sometimes not. Current chief justice John Roberts, though most typically an ardent member of the Court’s more conservative wing, has twice voted to uphold provisions of the Affordable Care Act.
Once a justice has started his or her lifetime tenure on the Court and years begin to pass, many people simply forget which president nominated him or her. For better or worse, sometimes it is only a controversial nominee who leaves a president’s legacy behind. For example, the Reagan presidency is often remembered for two controversial nominees to the Supreme Court—Robert Bork and Douglas Ginsburg, the former accused of taking an overly conservative and “extremist view of the Constitution”John M. Broder. “Edward M. Kennedy, Senate Stalwart, Is Dead at 77.” New York Times. 26 August 2009. and the latter of having used marijuana while a student and then a professor at Harvard University (Figure). President George W. Bush’s nomination of Harriet Miers was withdrawn in the face of criticism from both sides of the political spectrum, questioning her ideological leanings and especially her qualifications, suggesting she was not ready for the job.Michael A. Fletcher and Charles Babington. “Miers, Under Fire From Right, Withdrawn as Court Nominee.” Washington Post. 28 October 2005. http://www.washingtonpost.com/wp-dyn/content/article/2005/10/27/AR2005102700547.html. After Miers’ withdrawal, the Senate went on to confirm Bush’s subsequent nomination of Samuel Alito, who remains on the Court today. The 2016 presidential election between Hillary Clinton and Donald Trump was especially important because the next president is likely to choose three justices.
Presidential legacy and controversial nominations notwithstanding, there is one certainty about the overall look of the federal court system: What was once a predominately white, male, Protestant institution is today much more diverse. As a look at Table reveals, the membership of the Supreme Court has changed with the passing years.
| Supreme Court Justice Firsts | |
|---|---|
| First Catholic | Roger B. Taney (nominated in 1836) |
| First Jew | Louis J. Brandeis (1916) |
| First (and only) former U.S. President | William Howard Taft (1921) |
| First African American | Thurgood Marshall (1967) |
| First Woman | Sandra Day O’Connor (1981) |
| First Hispanic American | Sonia Sotomayor (2009) |
The lower courts are also more diverse today. In the past few decades, the U.S. judiciary has expanded to include more women and minorities at both the federal and state levels.Bureau of International Information Programs. United States Department of State. Outline of the U.S. Legal System. 2004. However, the number of women and people of color on the courts still lags behind the overall number of white men. As of 2009, the federal judiciary consists of 70 percent white men, 15 percent white women, and between 1 and 8 percent African American, Hispanic American, and Asian American men and women.Russell Wheeler. “The Changing Face of the Federal Judiciary.” Governance Studies at Brookings. August 2009. http://www.brookings.edu/~/media/research/files/papers/2009/8/federal-judiciary-wheeler/08_federal_judiciary_wheeler.pdf.
Summary
The structure of today’s three-tiered federal court system, largely established by Congress, is quite clear-cut. The system’s reliance on precedent ensures a consistent and stable institution that is still capable of slowly evolving over the years—such as by increasingly reflecting the diverse population it serves. Presidents hope their judicial nominees will make rulings consistent with the chief executive’s own ideological leanings. But the lifetime tenure of federal court members gives them the flexibility to act in ways that may or may not reflect what their nominating president intended. Perfect alignment between nominating president and justice is not expected; a judge might be liberal on most issues but conservative on others, or vice versa. However, presidents have sometimes been surprised by the decisions made by their nominees, such as President Eisenhower was by Justice Earl Warren and President Reagan by Justice Anthony Kennedy.
Besides the Supreme Court, there are lower courts in the national system called ________.
- state and federal courts
- district and circuit courts
- state and local courts
- civil and common courts
Hint:
B
In standing by precedent, a judge relies on the principle of ________.
- stare decisis
- amicus curiae
- judicial activism
- laissez-faire
The justices of the Supreme Court are ________.
- elected by citizens
- chosen by the Congress
- confirmed by the president
- nominated by the president and confirmed by the Senate
Hint:
D
Do you believe federal judges should be elected rather than appointed? Why or why not?
When it comes to filling judicial positions in the federal courts, do you believe race, gender, religion, and ethnicity should matter? Why or why not?
Hint:
The United States has become much more diverse, and it is only fitting that the judicial branch more accurately reflects the demographic composition of the population. At the same time, judicial positions should be filled by the most competent and qualified candidates.
|
oercommons
|
2025-03-18T00:37:58.083856
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15262/overview",
"title": "American Government, Delivering Collective Action: Formal Institutions",
"author": null
}
|
https://oercommons.org/courseware/lesson/15263/overview
|
The Supreme Court
Learning Objectives
By the end of this section, you will be able to:
- Analyze the structure and important features of the Supreme Court
- Explain how the Supreme Court selects cases to hear
- Discuss the Supreme Court’s processes and procedures
The Supreme Court of the United States, sometimes abbreviated SCOTUS, is a one-of-a-kind institution. While a look at the Supreme Court typically focuses on the nine justices themselves, they represent only the top layer of an entire branch of government that includes many administrators, lawyers, and assistants who contribute to and help run the overall judicial system. The Court has its own set of rules for choosing cases, and it follows a unique set of procedures for hearing them. Its decisions not only affect the outcome of the individual case before the justices, but they also create lasting impacts on legal and constitutional interpretation for the future.
THE STRUCTURE OF THE SUPREME COURT
The original court in 1789 had six justices, but Congress set the number at nine in 1869, and it has remained there ever since. There is one chief justice, who is the lead or highest-ranking judge on the Court, and eight associate justices. All nine serve lifetime terms, after successful nomination by the president and confirmation by the Senate.
The current court is fairly diverse in terms of gender, religion (Christians and Jews), ethnicity, and ideology, as well as length of tenure. Some justices have served for three decades, whereas others were only recently appointed by President Obama. Figure lists the names of the eight justices serving on the Court as of November 2016, along with their year of appointment and the president who nominated them.
With the death of Associate Justice Antonin Scalia in February 2016, there remain three current justices who are considered part of the Court’s more conservative wing—Chief Justice Roberts and Associate Justices Thomas and Alito, while four are considered more liberal-leaning—Justices Ginsburg, Breyer, Sotomayor, and Kagan (Figure). Justice Kennedy has become known as the “swing” vote, particularly on decisions like the Court’s same-sex marriage rulings in 2015, because he sometimes takes a more liberal position and sometimes a more conservative one. Had the Democrats retained the presidency in 2016, the replacement for Scalia’s spot on the court could have swung many key votes in a moderate or liberal direction. However, with Republican Donald Trump winning the election and the Republicans retaining Senate control, it is likely that the replacement in 2017 will be more conservative.
While not formally connected with the public the way elected leaders are, the Supreme Court nonetheless offers visitors a great deal of information at its official website.
For unofficial summaries of recent Supreme Court cases or news about the Court, visit the Oyez website or SCOTUS blog.
In fact, none of the justices works completely in an ideological bubble. While their numerous opinions have revealed certain ideological tendencies, they still consider each case as it comes to them, and they don’t always rule in a consistently predictable or expected way. Furthermore, they don’t work exclusively on their own. Each justice has three or four law clerks, recent law school graduates who temporarily work for him or her, do research, help prepare the justice with background information, and assist with the writing of opinions. The law clerks’ work and recommendations influence whether the justices will choose to hear a case, as well as how they will rule. As the profile below reveals, the role of the clerks is as significant as it is varied.
Profile of a United States Supreme Court Clerk
A Supreme Court clerkship is one of the most sought-after legal positions, giving “thirty-six young lawyers each year a chance to leave their fingerprints all over constitutional law.”Dahlia Lithwick. “Who Feeds the Supreme Court?” Slate.com. September 14, 2015. http://www.slate.com/articles/news_and_politics/jurisprudence/2015/09/supreme_court_feeder_judges_men_and_few_women_send_law_clerks_to_scotus.html. A number of current and former justices were themselves clerks, including Chief Justice John Roberts, Justices Stephen Breyer and Elena Kagan, and former chief justice William Rehnquist.
Supreme Court clerks are often reluctant to share insider information about their experiences, but it is always fascinating and informative to hear about their jobs. Former clerk Philippa Scarlett, who worked for Justice Stephen Breyer, describes four main responsibilities:“Role of Supreme Court Law Clerk: Interview with Philippa Scarlett.” IIP Digital. United States of America Embassy. http://iipdigital.usembassy.gov/st/english/publication/2013/02/20130211142365.html#axzz3grjRwiG (March 1, 2016).
Review the cases: Clerks participate in a “cert. pool” (short for writ of certiorari, a request that the lower court send up its record of the case for review) and make recommendations about which cases the Court should choose to hear.
Prepare the justices for oral argument: Clerks analyze the filed briefs (short arguments explaining each party’s side of the case) and the law at issue in each case waiting to be heard.
Research and draft judicial opinions: Clerks do detailed research to assist justices in writing an opinion, whether it is the majority opinion or a dissenting or concurring opinion.
Help with emergencies: Clerks also assist the justices in deciding on emergency applications to the Court, many of which are applications by prisoners to stay their death sentences and are sometimes submitted within hours of a scheduled execution.
Explain the role of law clerks in the Supreme Court system. What is your opinion about the role they play and the justices’ reliance on them?
HOW THE SUPREME COURT SELECTS CASES
The Supreme Court begins its annual session on the first Monday in October and ends late the following June. Every year, there are literally thousands of people who would like to have their case heard before the Supreme Court, but the justices will select only a handful to be placed on the docket, which is the list of cases scheduled on the Court’s calendar. The Court typically accepts fewer than 2 percent of the as many as ten thousand cases it is asked to review every year.“Supreme Court Procedures.” United States Courts. http://www.uscourts.gov/about-federal-courts/educational-resources/about-educational-outreach/activity-resources/supreme-1 (March 1, 2016).
Case names, written in italics, list the name of a petitioner versus a respondent, as in Roe v. Wade, for example.Roe v. Wade, 410 U.S. 113 (1973). For a case on appeal, you can tell which party lost at the lower level of court by looking at the case name: The party unhappy with the decision of the lower court is the one bringing the appeal and is thus the petitioner, or the first-named party in the case. For example, in Brown v. Board of Education (1954), Oliver Brown was one of the thirteen parents who brought suit against the Topeka public schools for discrimination based on racial segregation.
Most often, the petitioner is asking the Supreme Court to grant a writ of certiorari, a request that the lower court send up its record of the case for review. Once a writ of certiorari (cert. for short) has been granted, the case is scheduled on the Court’s docket. The Supreme Court exercises discretion in the cases it chooses to hear, but four of the nine Justices must vote to accept a case. This is called the Rule of Four.
For decisions about cert., the Court’s Rule 10 (Considerations Governing Review on Writ of Certiorari) takes precedence.”Rule 10. Considerations Governing Review on Certiorari.” Rules of the Supreme Court of the United States. Adopted April 19, 2013, Effective July 1, 2013. http://www.supremecourt.gov/ctrules/2013RulesoftheCourt.pdf. The Court is more likely to grant certiorari when there is a conflict on an issue between or among the lower courts. Examples of conflicts include (1) conflicting decisions among different courts of appeals on the same matter, (2) decisions by an appeals court or a state court conflicting with precedent, and (3) state court decisions that conflict with federal decisions. Occasionally, the Court will fast-track a case that has special urgency, such as Bush v. Gore in the wake of the 2000 election.Bush v. Gore, 531 U.S. 98 (2000).
Past research indicated that the amount of interest-group activity surrounding a case before it is granted cert. has a significant impact on whether the Supreme Court puts the case on its agenda. The more activity, the more likely the case will be placed on the docket.Gregory A. Caldeira and John R. Wright. 1988. “Organized Interests and Agenda-Setting in the U.S. Supreme Court,” American Political Science Review 82: 1109–1128. But more recent research broadens that perspective, suggesting that too much interest-group activity when the Court is considering a case for its docket may actually have diminishing impact and that external actors may have less influence on the work of the Court than they have had in the past.Gregory A. Caldeira, John R. Wright, and Christopher Zorn. 2012. “Organized Interests and Agenda Setting in the U.S. Supreme Court Revisited.” Presentation at the Second Annual Conference on Institutions and Lawmaking, Emory University. http://polisci.emory.edu/home/cslpe/conference-institutions-law-making/2012/papers/caldeira_wright_zorn_cwzpaper.pdf. Still, the Court takes into consideration external influences, not just from interest groups but also from the public, from media attention, and from a very key governmental actor—the solicitor general.
The solicitor general is the lawyer who represents the federal government before the Supreme Court: He or she decides which cases (in which the United States is a party) should be appealed from the lower courts and personally approves each one presented (Figure). Most of the cases the solicitor general brings to the Court will be given a place on the docket. About two-thirds of all Supreme Court cases involve the federal government.“About the Office.” Office of the Solicitor General. The United States Department of Justice. http://www.justice.gov/osg/about-office-1 (March 1, 2016).
The solicitor general determines the position the government will take on a case. The attorneys of his or her office prepare and file the petitions and briefs, and the solicitor general (or an assistant) presents the oral arguments before the Court.
In other cases in which the United States is not the petitioner or the respondent, the solicitor general may choose to intervene or comment as a third party. Before a case is granted cert., the justices will sometimes ask the solicitor general to comment on or file a brief in the case, indicating their potential interest in getting it on the docket. The solicitor general may also recommend that the justices decline to hear a case. Though research has shown that the solicitor general’s special influence on the Court is not unlimited, it remains quite significant. In particular, the Court does not always agree with the solicitor general, and “while justices are not lemmings who will unwittingly fall off legal cliffs for tortured solicitor general recommendations, they nevertheless often go along with them even when we least expect them to.”Ryan C. Black and Ryan J. Owens. “Solicitor General Influence and the United States Supreme Court.” Vanderbilt University. http://www.vanderbilt.edu/csdi/archived/working%20papers/Ryan%20Owens.pdf (March 1, 2016).
Some have credited Donald B. Verrilli, the solicitor general under President Obama, with holding special sway over the five-justice majority ruling on same-sex marriage in June 2015. Indeed, his position that denying homosexuals the right to marry would mean “thousands and thousands of people are going to live out their lives and go to their deaths without their states ever recognizing the equal dignity of their relationships” became a foundational point of the Court’s opinion, written by Justice Kennedy.Mark Joseph Stern., “If SCOTUS Decides in Favor of Marriage Equality, Thank Solicitor General Don Verrilli,” Slate.com. April 29, 2015. http://www.slate.com/blogs/outward/2015/04/29/don_verrilli_solicitor_general_was_the_real_hero_of_scotus_gay_marriage.html. With such power over the Court, the solicitor general is sometimes referred to as “the tenth justice.”
SUPREME COURT PROCEDURES
Once a case has been placed on the docket, briefs, or short arguments explaining each party’s view of the case, must be submitted—first by the petitioner putting forth his or her case, then by the respondent. After initial briefs have been filed, both parties may file subsequent briefs in response to the first. Likewise, people and groups that are not party to the case but are interested in its outcome may file an amicus curiae (“friend of the court”) brief giving their opinion, analysis, and recommendations about how the Court should rule. Interest groups in particular can become heavily involved in trying to influence the judiciary by filing amicus briefs—both before and after a case has been granted cert. And, as noted earlier, if the United States is not party to a case, the solicitor general may file an amicus brief on the government’s behalf.
With briefs filed, the Court hears oral arguments in cases from October through April. The proceedings are quite ceremonial. When the Court is in session, the robed justices make a formal entrance into the courtroom to a standing audience and the sound of a banging gavel. The Court’s marshal presents them with a traditional chant: “The Honorable, the Chief Justice and the Associate Justices of the Supreme Court of the United States. Oyez! Oyez! Oyez! [Hear ye!] All persons having business before the Honorable, the Supreme Court of the United States, are admonished to draw near and give their attention, for the Court is now sitting. God save the United States and this Honorable Court!”“The Court and its Procedures.” Supreme Court of the United States. May 26, 2015. It has not gone unnoticed that the Court, which has defended the First Amendment’s religious protection and the traditional separation of church and state, opens its every public session with a mention of God.
During oral arguments, each side’s lawyers have thirty minutes to make their legal case, though the justices often interrupt the presentations with questions. The justices consider oral arguments not as a forum for a lawyer to restate the merits of his or her case as written in the briefs, but as an opportunity to get answers to any questions they may have.“Supreme Court Procedures.” United States Courts. http://www.uscourts.gov/about-federal-courts/educational-resources/about-educational-outreach/activity-resources/supreme-1 (March 1, 2016). When the United States is party to a case, the solicitor general (or one of his or her assistants) will argue the government’s position; even in other cases, the solicitor general may still be given time to express the government’s position on the dispute.
When oral arguments have been concluded, the justices have to decide the case, and they do so in conference, which is held in private twice a week when the Court is in session and once a week when it is not. The conference is also a time to discuss petitions for certiorari, but for those cases already heard, each justice may state his or her views on the case, ask questions, or raise concerns. The chief justice speaks first about a case, then each justice speaks in turn, in descending order of seniority, ending with the most recently appointed justice.“Supreme Court Procedures.” United States Courts. http://www.uscourts.gov/about-federal-courts/educational-resources/about-educational-outreach/activity-resources/supreme-1 (March 1, 2016). The judges take an initial vote in private before the official announcement of their decisions is made public.
Oral arguments are open to the public, but cameras are not allowed in the courtroom, so the only picture we get is one drawn by an artist’s hand, an illustration or rendering. Cameras seem to be everywhere today, especially to provide security in places such as schools, public buildings, and retail stores, so the lack of live coverage of Supreme Court proceedings may seem unusual or old-fashioned. Over the years, groups have called for the Court to let go of this tradition and open its operations to more “sunshine” and greater transparency. Nevertheless, the justices have resisted the pressure and remain neither filmed nor photographed during oral arguments.Jonathan Sherman. “End the Supreme Court's Ban on Cameras.” New York Times. 24 April 2015. http://www.nytimes.com/2015/04/24/opinion/open-the-supreme-court-to-cameras.html.
Summary
A unique institution, the U.S. Supreme Court today is an interesting mix of the traditional and the modern. On one hand, it still holds to many of the formal traditions, processes, and procedures it has followed for many decades. Its public proceedings remain largely ceremonial and are never filmed or photographed. At the same time, the Court has taken on new cases involving contemporary matters before a nine-justice panel that is more diverse today than ever before. When considering whether to take on a case and then later when ruling on it, the justices rely on a number of internal and external players who assist them with and influence their work, including, but not limited to, their law clerks, the U.S. solicitor general, interest groups, and the mass media.
The Supreme Court consists of ________.
- nine associate justices
- one chief justice and eight associate justices
- thirteen judges
- one chief justice and five associate justices
A case will be placed on the Court’s docket when ________ justices agree to do so.
- four
- five
- six
- all
Hint:
A
One of the main ways interest groups participate in Supreme Court cases is by ________.
- giving monetary contributions to the justices
- lobbying the justices
- filing amicus curiae briefs
- protesting in front of the Supreme Court building
The lawyer who represents the federal government and argues cases before the Supreme Court is the ________.
- solicitor general
- attorney general
- U.S. attorney
- chief justice
Hint:
A
What do the appointments of the Supreme Court’s two newest justices, Sonia Sotomayor and Elena Kagan, reveal about the changing court system?
|
oercommons
|
2025-03-18T00:37:58.115552
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15263/overview",
"title": "American Government, Delivering Collective Action: Formal Institutions",
"author": null
}
|
https://oercommons.org/courseware/lesson/15264/overview
|
Judicial Decision-Making and Implementation by the Supreme Court
Learning Objectives
By the end of this section, you will be able to:
- Describe how the Supreme Court decides cases and issues opinions
- Identify the various influences on the Supreme Court
- Explain how the judiciary is checked by the other branches of government
The courts are the least covered and least publicly known of the three branches of government. The inner workings of the Supreme Court and its day-to-day operations certainly do not get as much public attention as its rulings, and only a very small number of its announced decisions are enthusiastically discussed and debated. The Court’s 2015 decision on same-sex marriage was the exception, not the rule, since most court opinions are filed away quietly in the United States Reports, sought out mostly by judges, lawyers, researchers, and others with a particular interest in reading or studying them.
Thus, we sometimes envision the justices formally robed and cloistered away in their chambers, unaffected by the world around them, but the reality is that they are not that isolated, and a number of outside factors influence their decisions. Though they lack their own mechanism for enforcement of their rulings and their power remains checked and balanced by the other branches, the effect of the justices’ opinions on the workings of government, politics, and society in the United States is much more significant than the attention they attract might indicate.
JUDICIAL OPINIONS
Every Court opinion sets precedent for the future. The Supreme Court’s decisions are not always unanimous, however; the published majority opinion, or explanation of the justices’ decision, is the one with which a majority of the nine justices agree. It can represent a vote as narrow as five in favor to four against. A tied vote is rare but can occur at a time of vacancy, absence, or abstention from a case, perhaps where there is a conflict of interest. In the event of a tied vote, the decision of the lower court stands.
Most typically, though, the Court will put forward a majority opinion. If he or she is in the majority, the chief justice decides who will write the opinion. If not, then the most senior justice ruling with the majority chooses the writer. Likewise, the most senior justice in the dissenting group can assign a member of that group to write the dissenting opinion; however, any justice who disagrees with the majority may write a separate dissenting opinion. If a justice agrees with the outcome of the case but not with the majority’s reasoning in it, that justice may write a concurring opinion.
Court decisions are released at different times throughout the Court’s term, but all opinions are announced publicly before the Court adjourns for the summer. Some of the most controversial and hotly debated rulings are released near or on the last day of the term and thus are avidly anticipated (Figure).
One of the most prominent writers on judicial decision-making in the U.S. system is Dr. Forrest Maltzman of George Washington University. Maltzman’s articles, chapters, and manuscripts, along with articles by other prominent authors in the field, are downloadable at this site.
INFLUENCES ON THE COURT
Many of the same players who influence whether the Court will grant cert. in a case, discussed earlier in this chapter, also play a role in its decision-making, including law clerks, the solicitor general, interest groups, and the mass media. But additional legal, personal, ideological, and political influences weigh on the Supreme Court and its decision-making process. On the legal side, courts, including the Supreme Court, cannot make a ruling unless they have a case before them, and even with a case, courts must rule on its facts. Although the courts’ role is interpretive, judges and justices are still constrained by the facts of the case, the Constitution, the relevant laws, and the courts’ own precedent.
A justice’s decisions are influenced by how he or she defines his role as a jurist, with some justices believing strongly in judicial activism, or the need to defend individual rights and liberties, and they aim to stop actions and laws by other branches of government that they see as infringing on these rights. A judge or justice who views the role with an activist lens is more likely to use his or her judicial power to broaden personal liberty, justice, and equality. Still others believe in judicial restraint, which leads them to defer decisions (and thus policymaking) to the elected branches of government and stay focused on a narrower interpretation of the Bill of Rights. These justices are less likely to strike down actions or laws as unconstitutional and are less likely to focus on the expansion of individual liberties. While it is typically the case that liberal actions are described as unnecessarily activist, conservative decisions can be activist as well.
Critics of the judiciary often deride activist courts for involving themselves too heavily in matters they believe are better left to the elected legislative and executive branches. However, as Justice Anthony Kennedy has said, “An activist court is a court that makes a decision you don’t like.”Matt Sedensky. “Justice questions way court nominees are grilled.” The Associated Press. May 14, 2010. http://www.boston.com/news/nation/articles/2010/05/14/justice_questions_way_court_nominees_are_grilled/.
Justices’ personal beliefs and political attitudes also matter in their decision-making. Although we may prefer to believe a justice can leave political ideology or party identification outside the doors of the courtroom, the reality is that a more liberal-thinking judge may tend to make more liberal decisions and a more conservative-leaning judge may tend toward more conservative ones. Although this is not true 100 percent of the time, and an individual’s decisions are sometimes a cause for surprise, the influence of ideology is real, and at a minimum, it often guides presidents to aim for nominees who mirror their own political or ideological image. It is likely not possible to find a potential justice who is completely apolitical.
And the courts themselves are affected by another “court”—the court of public opinion. Though somewhat isolated from politics and the volatility of the electorate, justices may still be swayed by special-interest pressure, the leverage of elected or other public officials, the mass media, and the general public. As times change and the opinions of the population change, the court’s interpretation is likely to keep up with those changes, lest the courts face the danger of losing their own relevance.
Take, for example, rulings on sodomy laws: In 1986, the Supreme Court upheld the constitutionality of the State of Georgia’s ban on sodomy,Bowers v. Hardwick, 478 U.S. 186 (1986). but it reversed its decision seventeen years later, invalidating sodomy laws in Texas and thirteen other states.Lawrence v. Texas, 539 U.S. 558 (2003). No doubt the Court considered what had been happening nationwide: In the 1960s, sodomy was banned in all the states. By 1986, that number had been reduced by about half. By 2002, thirty-six states had repealed their sodomy laws, and most states were only selectively enforcing them. Changes in state laws, along with an emerging LGBT movement, no doubt swayed the Court and led it to the reversal of its earlier ruling with the 2003 decision, Lawrence v. Texas (Figure).Lawrence v. Texas, 539 U.S. 558 (2003).
Heralded by advocates of gay rights as important progress toward greater equality, the ruling in Lawrence v. Texas illustrates that the Court is willing to reflect upon what is going on in the world. Even with their heavy reliance on precedent and reluctance to throw out past decisions, justices are not completely inflexible and do tend to change and evolve with the times.
The Importance of Jury Duty
Since judges and justices are not elected, we sometimes consider the courts removed from the public; however, this is not always the case, and there are times when average citizens may get involved with the courts firsthand as part of their decision-making process at either the state or federal levels. At some point, if you haven’t already been called, you may receive a summons for jury duty from your local court system. You may be asked to serve on federal jury duty, such as U.S. district court duty or federal grand jury duty, but service at the local level, in the state court system, is much more common.
While your first reaction may be to start planning a way to get out of it, participating in jury service is vital to the operation of the judicial system, because it provides individuals in court the chance to be heard and to be tried fairly by a group of their peers. And jury duty has benefits for those who serve as well. You will no doubt come away better informed about how the judicial system works and ready to share your experiences with others. Who knows? You might even get an unexpected surprise, as some citizens in Dallas, Texas did recently when former President George W. Bush showed up to serve jury duty with them.
Have you ever been called to jury duty? Describe your experience. What did you learn about the judicial process? What advice would you give to someone called to jury duty for the first time? If you’ve never been called to jury duty, what questions do you have for those who have?
THE COURTS AND THE OTHER BRANCHES OF GOVERNMENT
Both the executive and legislative branches check and balance the judiciary in many different ways. The president can leave a lasting imprint on the bench through his or her nominations, even long after leaving office. The president may also influence the Court through the solicitor general’s involvement or through the submission of amicus briefs in cases in which the United States is not a party.
President Franklin D. Roosevelt even attempted to stack the odds in his favor in 1937, with a “court-packing scheme” in which he tried to get a bill passed through Congress that would have reorganized the judiciary and enabled him to appoint up to six additional judges to the high court (Figure). The bill never passed, but other presidents have also been accused of trying similar moves at different courts in the federal system. Most recently, some members of Congress suggested that President Obama was attempting to “pack” the District of Columbia Circuit Court of Appeals with three nominees. Obama was filling vacancies, not adding judges, but the “packing” term was still bandied about.Louis Jacobson. “Is Barack Obama trying to ‘pack’ the D.C. Circuit Court of Appeals?” Tampa Bay Times, PolitiFact.com. June 5, 2013. http://www.politifact.com/truth-o-meter/statements/2013/jun/05/chuck-grassley/barack-obama-trying-pack-dc-circuit-court-appeals/.
Likewise, Congress has checks on the judiciary. It retains the power to modify the federal court structure and its appellate jurisdiction, and the Senate may accept or reject presidential nominees to the federal courts. Faced with a court ruling that overturns one of its laws, Congress may rewrite the law or even begin a constitutional amendment process.
But the most significant check on the Supreme Court is executive and legislative leverage over the implementation and enforcement of its rulings. This process is called judicial implementation. While it is true that courts play a major role in policymaking, they have no mechanism to make their rulings a reality. Remember it was Alexander Hamilton in Federalist No. 78 who remarked that the courts had “neither force nor will, but merely judgment.” And even years later, when the 1832 Supreme Court ruled the State of Georgia’s seizing of Native American lands unconstitutional,Worcester v. Georgia, 31 U.S. (6 Pet.) 515 (1832). President Andrew Jackson is reported to have said, “John Marshall has made his decision, now let him enforce it,” and the Court’s ruling was basically ignored.“Court History.” Supreme Court History: The First Hundred Years. http://www.pbs.org/wnet/supremecourt/antebellum/history2.html (March 1, 2016). Abraham Lincoln, too, famously ignored Chief Justice Roger B. Taney’s order finding unconstitutional Lincoln’s suspension of habeas corpus rights in 1861, early in the Civil War. Thus, court rulings matter only to the extent they are heeded and followed.
The Court relies on the executive to implement or enforce its decisions and on the legislative branch to fund them. As the Jackson and Lincoln stories indicate, presidents may simply ignore decisions of the Court, and Congress may withhold funding needed for implementation and enforcement. Fortunately for the courts, these situations rarely happen, and the other branches tend to provide support rather than opposition. In general, presidents have tended to see it as their duty to both obey and enforce Court rulings, and Congress seldom takes away the funding needed for the president to do so.
For example, in 1957, President Dwight D. Eisenhower called out the military by executive order to enforce the Supreme Court’s order to racially integrate the public schools in Little Rock, Arkansas. Eisenhower told the nation: “Whenever normal agencies prove inadequate to the task and it becomes necessary for the executive branch of the federal government to use its powers and authority to uphold federal courts, the president’s responsibility is inescapable.”Dwight D. Eisenhower. “Radio and Television Address to the American People on the Situation in Little Rock.” Public Papers of the Presidents of the United States: Eisenhower, Dwight D., The American Presidency Project. September 24, 1957. http://www.presidency.ucsb.edu/ws/?pid=10909. Executive Order 10730 nationalized the Arkansas National Guard to enforce desegregation because the governor refused to use the state National Guard troops to protect the black students trying to enter the school (Figure).
So what becomes of court decisions is largely due to their credibility, their viability, and the assistance given by the other branches of government. It is also somewhat a matter of tradition and the way the United States has gone about its judicial business for more than two centuries. Although not everyone agrees with the decisions made by the Court, rulings are generally accepted and followed, and the Court is respected as the key interpreter of the laws and the Constitution. Over time, its rulings have become yet another way policy is legitimately made and justice more adequately served in the United States.
Summary
Like the executive and legislative branches, the judicial system wields power that is not absolute. There remain many checks on its power and limits to its rulings. Judicial decisions are also affected by various internal and external factors, including legal, personal, ideological, and political influences. To stay relevant, Court decisions have to keep up with the changing times, and the justices’ decision-making power is subject to the support afforded by the other branches of government in implementation and enforcement. Nevertheless, the courts have evolved into an indispensable part of our government system—a separate and coequal branch that interprets law, makes policy, guards the Constitution, and protects individual rights.
When using judicial restraint, a judge will usually ________.
- refuse to rule on a case
- overrule any act of Congress he or she doesn’t like
- defer to the decisions of the elected branches of government
- make mostly liberal rulings
Hint:
C
When a Supreme Court ruling is made, justices may write a ________ to show they agree with the majority but for different reasons.
- brief
- dissenting opinion
- majority opinion
- concurring opinion
Which of the following is a check that the legislative branch has over the courts?
- Senate approval is needed for the appointment of justices and federal judges.
- Congress may rewrite a law the courts have declared unconstitutional.
- Congress may withhold funding needed to implement court decisions.
- all of the above
Hint:
D
What are the core factors that determine how judges decide in court cases?
Discuss some of the difficulties involved in the implementation and enforcement of judicial decisions.
Hint:
The judicial branch has no power of its own over implementation of enforcement of its rulings and is thus dependent on the other two branches to make this happen, relying on the executive to enforce its decisions and on the legislature to fund it. Hamilton said the judiciary has “no influence over either the sword or the purse” and “neither force nor will, but merely judgment,” stressing the court system’s reliance on assistance from the other two branches.
In what ways is the court system better suited to protect the individual than are the elected branches of the government?
On what types of policy issues do you expect the judicial branch to be especially powerful, and on which do you expect it to exert less power?
Discuss the relationship of the judicial branch to the other branches of government. In what ways is the judicial more powerful than other branches? In what ways is SCOTUS less powerful than other branches? Explain.
What should be the most important considerations when filling judge and justice positions at the federal level? Why?
The shirking of jury duty is a real problem in the United States. Give some reasons for this and suggest what can be done about it.
Take a closer look at some of the operational norms of the Supreme Court, such as the Rule of Four or the prohibition on cameras in the courtroom. What is your opinion about them as long-standing traditions, and which (if any), do you believe should be changed? Explain your answer.
Books written by current and former justices:
Breyer, Stephen. 2006. Active Liberty: Interpreting the Democratic Constitution. New York: Vintage; 2010; Making Democracy Work: A Judge’s View. New York: Knopf.
O’Connor, Sandra Day. 2004. The Majesty of the Law: Reflections of a Supreme Court Justice. New York: Random House.
Rehnquist, William. 2002. The Supreme Court. New York: Vintage.
Scalia, Antonin. 1998. A Matter of Interpretation: The Federal Courts and the Law. Princeton, NJ: Princeton University Press.
Sotomayor, Sonia. 2014. My Beloved World. New York: Vintage Books.
Stevens, John Paul. 2011. Five Chiefs: A Supreme Court Memoir. New York: Little, Brown.
Thomas, Clarence. 2008. My Grandfather’s Son: A Memoir. New York: Harper.
Books about the U.S. court system:
Coyle, Marcia. 2013. The Roberts Court: The Struggle for the Constitution. New York: Simon and Schuster.
Ferguson, Andrew G. 2013. Why Jury Duty Matters: A Citizen’s Guide to Constitutional Action. New York: New York University Press.
Millhiser, Ian. 2015. Injustices: The Supreme Court’s History of Comforting the Comfortable and Afflicting the Afflicted. New York: Nation Books.
Peppers, Todd C., and Artemus Ward. 2012. In Chambers: Stories of Supreme Court Law Clerks and Their Justices. Charlottesville: University of Virginia Press.
Tobin, Jeffrey. 2012. The Oath: The Obama White House and the Supreme Court. New York: Doubleday.
Vile, John R. 2014. Essential Supreme Court Decisions: Summaries of Leading Cases in U.S. Constitutional Law, 16th ed. Lanham: Rowman & Littlefield.
Films:
1981. The First Monday in October.
1993. The Pelican Brief.
HBO. 2000. Recount.
2015. Confirmation.
2015. On the Basis of Sex.
|
oercommons
|
2025-03-18T00:37:58.151770
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15264/overview",
"title": "American Government, Delivering Collective Action: Formal Institutions",
"author": null
}
|
https://oercommons.org/courseware/lesson/15265/overview
|
Introduction
Controversial national policy decisions by lawmakers and justices tend to grab headlines and dominate social media, while state and local government matters often evoke less enthusiasm. Yet, if we think about which level of government most directly affects us on a daily basis, it is undoubtedly the level closest to us, including our city, county, school districts, and state government. Whether it is by maintaining roads we drive on each day, supplying clean water with which we brush our teeth, or allocating financial support to higher education, state and local government provides resources that shape our everyday lives, including your final tuition bill (Figure).
How do state and local governments gain the authority to make these decisions, and how are their actions guided by cultural and other differences between the states? What tensions exist between national and state governments on policy matters, and what unique powers do mayors and governors enjoy? By answering these and other questions, this chapter explores the role of state and local governments in our lives.
|
oercommons
|
2025-03-18T00:37:58.167197
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15265/overview",
"title": "American Government, Delivering Collective Action: Formal Institutions",
"author": null
}
|
https://oercommons.org/courseware/lesson/15266/overview
|
State Power and Delegation
Learning Objectives
By the end of this section, you will be able to:
- Explain how the balance of power between national and state governments shifted with the drafting and ratification of the Constitution
- Identify parts of the Constitution that grant power to the national government and parts that support states’ rights
- Identify two fiscal policies by which the federal government exerts control over state policy decisions
When the framers met at the Constitutional Convention in 1787, they had many competing tensions to resolve. For instance, they had to consider how citizens would be represented in the national government, given population differences between the states. In addition, they had to iron out differences of opinion about where to concentrate political power. Would the legislative branch have more authority than the executive branch, and would state governments retain as many rights as they had enjoyed under the Articles of Confederation?
Here we look at the manner in which power was divided between the national and state governments, first under the Articles of Confederation and then under the U.S. Constitution. As you read, observe the shifting power dynamic between the national government and subnational governments at the state and local level.
STATE POWER AT THE FOUNDING
Before the ratification of the Constitution, the state governments’ power far exceeded that held by the national government. This distribution of authority was the result of a conscious decision and was reflected in the structure and framework of the Articles of Confederation. The national government was limited, lacking both a president to oversee domestic and foreign policy and a system of federal courts to settle disputes between the states.
Restricting power at the national level gave the states a great deal of authority over and independence from the federal government. Each state legislature appointed its own Congressional representatives, subject to recall by the states, and each state was given the authority to collect taxes from its citizens. But limiting national government power was not the delegates’ only priority. They also wanted to prevent any given state from exceeding the authority and independence of the others. The delegates ultimately worked to create a level playing field between the individual states that formed the confederation. For instance, the Articles of Confederation could not be amended without the approval of each state, and each state received one vote in Congress, regardless of population.“Articles of Confederation,” https://www.gpo.gov/fdsys/pkg/SMAN-107/pdf/SMAN-107-pg935.pdf (March 14, 2016).
It wasn’t long after the Articles of Confederation were established that cracks began to appear in their foundation. Congress struggled to conduct business and to ensure the financial credibility of the new country’s government. One difficulty was its inability to compel the individual states to cover their portion of Revolutionary War debt. Attempts to recoup these funds through the imposition of tariffs were vetoed by states with a vested financial interest in their failure.“Tax History Museum: The Revolutionary War to the War of 1812 (1777–1815),” http://www.taxhistory.org/www/website.nsf/Web/THM1777?OpenDocument (March 14, 2016).
Given the inherent weaknesses in the system set up by the Articles, in 1787 the delegates came together once again to consider amendments to the Articles, but they ended up instead considering a new design for the government (Figure). To produce more long-term stability, they needed to establish a more effective division of power between the federal and state governments. Ultimately, the framers settled on a system in which power would be shared: The national government had its core duties, the state governments had their duties, and other duties were shared equally between them. Today this structure of power sharing is referred to as federalism.
The Constitution allocated more power to the federal government by effectively adding two new branches: a president to head the executive branch and the Supreme Court to head the judicial branch. The specific delegated or expressed powers granted to Congress and to the president were clearly spelled out in the body of the Constitution under Article I, Section 8, and Article II, Sections 2 and 3.
In addition to these expressed powers, the national government was given implied powers that, while not clearly stated, are inferred. These powers stem from the elastic clause in Article I, Section 8, of the Constitution, which provides Congress the authority “to make all Laws which shall be necessary and proper for carrying into Execution the Foregoing powers.” This statement has been used to support the federal government’s playing a role in controversial policy matters, such as the provision of healthcare, the expansion of power to levy and collect taxes, and regulation of interstate commerce. Finally, Article VI declared that the U.S. Constitution and any laws or treaties made in connection with that document were to supersede constitutions and laws made at the state level. This clause, better known as the supremacy clause, makes clear that any conflict in law between the central (or federal) government and the regional (or state) governments is typically resolved in favor of the central government.
Although the U.S. Constitution clearly allocated more power to the federal government than had been the case under the Articles of Confederation, the framers still respected the important role of the states in the new government. The states were given a host of powers independent of those enjoyed by the national government. As one example, they now had the power to establish local governments and to account for the structure, function, and responsibilities of these governments within their state constitutions. This gave states sovereignty, or supreme and independent authority, over county, municipal, school and other special districts.
States were also given the power to ratify amendments to the U.S. Constitution. Throughout U.S. history, all amendments to the Constitution except one have been proposed by Congress and then ratified by either three-fourths of the state legislatures or three-fourths of the state conventions called for ratification purposes. This process ensures that the states have a voice in any changes to the Constitution. The Twenty-First Amendment (repealing the Eighteenth Amendment’s prohibition on alcohol) was the only amendment ratified using the state ratifying convention method. Although this path has never been taken, the U.S. Constitution even allows for state legislatures to take a direct and very active role in the amendment proposal process. If at least two-thirds of the state legislatures apply for a national convention, constitutional amendments can be proposed at the convention.
Debating the Need for a National Convention
As of 2015, twenty-seven states had passed applications to hold a national convention. These states are pushing for the opportunity to propose a constitutional amendment requiring the national government to balance its budget in the same way most states are mandated to do. For a national convention to be held, at least thirty-four states must submit applications. Thus, only seven states currently stand in the way of the first national convention in U.S. history.Reid Wilson, “Conservative Lawmakers Weigh Bid to Call for Constitutional Convention,” Washington Post, 4 April 2015. http://www.washingtonpost.com/politics/conservative-lawmakers-weigh-bid-to-call-for-constitutional-convention/2015/04/04/b25d4f1e-db02-11e4-ba28-f2a685dc7f89_story.html.
Proponents see the convention as an opportunity to propose an amendment they argue is necessary to reduce federal spending and promote fiscal responsibility. Conservatives and Tea Party members believe reducing the deficit is important to maintaining the country’s future economic health and its competitive strength in global markets. They also believe the growing roster of states favoring a convention may encourage Congress to take action on its own.
Opponents feel a balanced budget amendment is not realistic given the need for emergency spending in the event of an economic recession. They also worry about the spending cuts and/or tax increases the federal government would have to impose to consistently balance the budget. Some states fear a balanced-budget requirement would limit the federal government’s ability to provide them with continued fiscal support. Finally, other opponents argue that states balance only their operating budgets, while themselves assuming massive amounts of debt for capital projects.
But perhaps the greatest fear is of the unknown. A national convention is unprecedented, and there is no limit to the number of amendments delegates to such a convention might propose. However, such changes would still need to be ratified by three-fourths of the state legislatures or state conventions before they could take effect.
What are the potential benefits of a national constitutional convention? What are the risks? Are the benefits worth the risks? Why or why not?
Despite the Constitution’s broad grants of state authority, one of the central goals of the Anti-Federalists, a group opposed to several components of the Constitution, was to preserve state government authority, protect the small states, and keep government power concentrated in the hands of the people. For this reason, the Tenth Amendment was included in the Bill of Rights to create a class of powers, known as reserved powers, exclusive to state governments. The amendment specifically reads, “The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.” In essence, if the Constitution does not decree that an activity should be performed by the national government and does not restrict the state government from engaging in it, then the state is seen as having the power to perform the function. In other words, the power is reserved to the states.
Besides reserved powers, the states also retained concurrent powers, or responsibilities shared with the national government. As part of this package of powers, the state and federal governments each have the right to collect income tax from their citizens and corporate tax from businesses. They also share responsibility for building and maintaining the network of interstates and highways and for making and enforcing laws (Figure). For instance, many state governments have laws regulating motorcycle and bicycle helmet use, banning texting and driving, and prohibiting driving under the influence of drugs or alcohol.
THE EVOLUTION OF STATE POWER
Throughout U.S. history, the national and state governments have battled for dominance over the implementation of public policy and the funding of important political programs. Upon taking office in 1933 during the Great Depression (1929–1939), President Franklin D. Roosevelt initiated a series of legislative proposals to boost the economy and put people back to work. The enacted programs allowed the federal government to play a broader role in revitalizing the economy while greatly expanding its power. However, this result was not without its critics. Initially, the Supreme Court overturned several key legislative proposals passed under Roosevelt, reasoning that they represented an overreach of presidential authority and were unconstitutional, such as Schechter Poultry Corp. v. United States.A. L. A. Schechter Poultry Corp. v. United States, 295 U.S. 495 (1935). Eventually, however, the Supreme Court shifted direction to reflect public opinion, which was decisively behind the president and the need for government intervention in a time of economic turmoil.William E. Leuchtenburg, “When Franklin Roosevelt Clashed with the Supreme Court—and Lost,” Smithsonian Magazine, May 2005. http://www.smithsonianmag.com/history/when-franklin-roosevelt-clashed-with-the-supreme-court-and-lost-78497994/.
Just three decades later, during the 1964 presidential election campaign, incumbent President Lyndon B. Johnson declared a “War on Poverty,” instituting a package of Great Society programs designed to improve circumstances for lower-income Americans across the nation. The new programs included Medicare and Medicaid, which are health insurance programs for seniors and low-income citizens respectively, and the food stamp program, which provides food assistance to low-income families. These initiatives greatly expanded the role of the federal government in providing a social safety net.Karen Tumulty, “‘Great Society’ Agenda Led to Great—and Lasting—Philosophical Divide,” Washington Post, 8 January 2014. http://www.washingtonpost.com/politics/great-society-agenda-led-to-great--and-lasting--philosophical-divide/2014/01/08/b082e5d0-786d-11e3-b1c5-739e63e9c9a7_story.html. State and local governments became partners in their implementation and also came to rely on the financial support they received from the federal government in the form of program grants.Michael Schuyler. 19 February 2014. “A Short History of Government Taxing and Spending in the United States,” http://taxfoundation.org/article/short-history-government-taxing-and-spending-united-states.
As the federal government’s role in policy creation expanded, so did its level of spending. Spending by the federal government began to surpass that of state and local governments shortly after 1940 (Figure). It spiked temporarily during the Great Depression and again during World War II, resuming a slow climb with the implementation of Johnson’s Great Society programs noted above.
Growing financial resources gave the federal government increased power over subnational governments. This increased power was because it could use categorical grants to dictate the terms and conditions state governments had to meet to qualify for financial assistance in a specific policy area. Over time, the federal government even began to require state and local governments to comply with legislative and executive authorizations when funding was not attached. These requests from the federal government are referred to as unfunded mandates and are a source of dissatisfaction to political actors at the state and local level. To provide more transparency to state and local governments and reduce the federal government’s use of mandates, the Unfunded Mandates Reform Act was passed in 1995. This act requires the Congressional Budget Office to provide information about the cost of any proposed government mandate that exceeds a specified threshold before the bill can be considered in Congress.Philip Joyce, “Is the Era of Unfunded Federal Mandates Over?” Governing, 16 April 2014. http://www.governing.com/columns/smart-mgmt/col-is-era-unfunded-federal-mandates-over.html.
Explore the latest news on federal mandates at the Congressional Budget Office and the Catalog of Cost Shifts to States at the National Conference of State Legislatures website.
Despite the national government’s power to pass and fund policy that affects lower-level governments, states still have gained considerable headway since the late twentieth century. For instance, with the passage of the Personal Responsibility and Work Opportunity Reconciliation Act in 1996, known as the welfare reform bill, states were given great discretion over the provision of welfare. The federal government reduced its level of monetary support for the program and, in exchange, the states gained more authority over its implementation. States were able to set more restrictive work requirements, to place caps on the number of family members who could receive aid, and to limit the length of time someone could qualify for government assistance.“State Policy Choices under Welfare Reform,” http://www.brookings.edu/research/papers/2002/04/welfare-gais (March 14, 2016).
Since then, states have been granted the flexibility to set policy across a number of controversial policy areas. For instance, a wide array of states require parental consent for abortions performed on minors, set waiting periods before an abortion can be performed, or require patients to undergo an ultrasound before the procedure. As another example, currently, almost half the states allow for the use of medical marijuana and three states have fully legalized it, despite the fact that this practice stands in contradiction to federal law that prohibits the use and distribution of marijuana.
For more on these two controversial policy areas, explore ”An Overview of State Abortion Laws” and ”State Medical Marijuana Laws.”
Today, it is not uncommon to see a patchwork of legal decisions granting states more discretion in some policy areas, such as marijuana use, while providing the federal government more authority in others, such as same-sex marriage. Decisions about which level controls policy can reflect the attitudes of government officials and the public, political ideology and the strategic advantage of setting policy on a state-by-state basis, and the necessity of setting uniform policy in the face of an economic downturn or unanticipated national security threat. What has not changed over time is the central role of the U.S. Supreme Court’s views in determining how power should be distributed in a federalist system.
POWER AT THE SUBSTATE LEVEL
The U.S. Constitution is silent on the dispersion of power between states and localities within each state. The fact that states are mentioned specifically and local jurisdictions are not has traditionally meant that power independent of the federal government resides first with the state. Through their own constitutions and statutes, states decide what to require of local jurisdictions and what to delegate. This structure represents the legal principle of Dillon’s Rule, named for Iowa Supreme Court justice John F. Dillon. Dillon argued that state actions trump those of the local government and have supremacy.“Why Existing Law Won’t Stop Corporations from Harming Your Community,” August 31, 2015. http://celdf.org/2015/08/why-existing-law-wont-stop-corporations-from-harming-your-community/ (March 14, 2016). In this view, cities and towns exist at the pleasure of the state, which means the state can step in and dissolve them or even take them over. Indeed, most states have supremacy clauses over local governments in their constitutions.
However, for practical purposes, state and local governments must work together to ensure that citizens receive adequate services. Given the necessity of cooperation, many states have granted local governments some degree of autonomy and given them discretion to make policy or tax decisions.Jesse J. Richardson, Jr. 5 August 2011. “Dillon’s Rule is from Mars, Home Rule is from Venus: Local Government Autonomy and the Rules of Statutory Construction,” Publius 41, No. 4: 662–685. This added independence is called home rule, and the transfer of power is typically spelled out within a charter. Charters are similar to state constitutions: they provide a framework and a detailed accounting of local government responsibilities and areas of authority. Potential conflicts can come up over home rule. For example, in 2015, the State of Texas overruled a fracking ban imposed by the City of Denton.Max B. Baker, “Denton City Council Repeals Fracking Ban,” Fort Worth Star-Telegram, 16 June 2015. http://www.star-telegram.com/news/business/barnett-shale/article24627469.html.
Like state governments, local governments prioritize spending on building and maintaining the transportation infrastructure, supporting educational institutions, promoting community protection, and funding healthcare.Roberton Williams and Yuri Shadunsky. “State and Local Tax Policy: What are the Sources of Revenue for Local Governments?” http://www.taxpolicycenter.org/briefing-book/state-local/revenues/local_revenue.cfm (March 14, 2016). As shown in Figure, local governments, just like state governments, receive a sizeable chunk of their revenue from grants and transfers from other levels of the government. The next biggest source of revenue for local governments is property tax collections.
Property taxes can be assessed on homes, land, and businesses. The local government’s reliance on property tax revenue can be problematic for a number of reasons. First, unlike sales tax, the collection of which is spaced out in small increments across multiple transactions, property tax is collected in one or two lump sums and is therefore highly visible and unpopular.Charles E. Gilliland. November 2013. “Property Taxes: The Bad, the Good, and the Ugly,” Texas A&M University - Real Estate Center, TR 2037. https://assets.recenter.tamu.edu/documents/articles/2037.pdf. In fact, in response to tax rate increases, many states have placed legal or constitutional limits on regional governments’ ability to raise property taxes. The trend began in California with the 1978 passage of Proposition 13. This citizen-driven initiative capped the real estate tax at 1 percent of the cash value of property and stopped the practice of reassessing properties for tax purposes whenever a home in the neighborhood was sold.“What is Proposition 13?” http://www.californiataxdata.com/pdf/Prop13.pdf (March 14, 2016). After its passage, a number of other states followed suit, making it more difficult for states to reap the rewards of sharp increases in the market value of property.
Another drawback to local governments’ reliance on property tax is that property values vary with the economic health of a given area, the quality of school districts, and the overall desirability of a state, municipality, or county. Significant parcels of land in many cities are also tax-exempt, including property occupied by colleges, churches, and other nonprofit organizations. Boston is a good example as almost 50 percent of the assessed value of property is tax-exempt.Yolanda Perez, John Avault, and Jim Vrabel. December 2002. “Tax Exempt Property in Boston,” Boston Redevelopment Authority Policy Development and Research Report 562, http://www.californiataxdata.com/pdf/Prop13.pdf. College towns face the same challenge.
When the mortgage crisis began in 2007, property values decreased in many areas of the country, and many homeowners defaulted on their mortgages because their homes were now worth less than they had borrowed to buy them. With the decline in property values, local governments faced a loss in tax revenue at the same time states were cutting back on aid; tax collections were also down because of economic conditions and the inability to derive income tax from internet sales. A number of municipalities filed for bankruptcy in the face of fiscal distress during the economic recession. Perhaps the best known municipality was Detroit, Michigan, which filed for Chapter 9 bankruptcy in 2013 (Figure).
Detroit filed for bankruptcy due to massive debt obligations and demands for repayment that it could not meet due to a perfect storm of economic and democratic factors. The city owed money to investors who had loaned it money, and it had liabilities resulting from its failure to fulfill its pension and healthcare obligations to city workers. The bankruptcy allowed the city time to develop an exit strategy and negotiate with creditors and union representatives in an effort to restructure its debt load.Channon Hodge and David Gillen, “What Bankruptcy Means for Detroit,” New York Times, 4 December 2013. http://www.nytimes.com/video/business/100000002583690/what-bankruptcy-means-for-detroit.html. Indeed, Detroit recently emerged from bankruptcy and has started to rebuild economically.
Detroit’s fiscal condition only highlights the unique challenges municipalities face. Local governments have to provide many of the same services as state and national governments, but they are often constrained by the boundaries the state prescribes. They may not have the authority to raise revenue above a certain threshold, and they do not have the ability to pass expenses on to another level of government because they lack sovereignty.
Summary
The power structure of government established in the Articles of Confederation was rebalanced in the Constitution to ensure that both the central and the regional governments had some degree of authority and autonomy. Federal and state governments have managed to work out sharing power throughout history, with the federal government often using fiscal policy to encourage compliance from the states. The taxing power of local governments means they face unique pressures during economic downturns.
________ dictate the terms and conditions state governments would have to meet in order to qualify for financial assistance in a specific policy area.
- Categorical grants
- Block grants
- Unfunded mandates
- Crossover sanctions
Hint:
A
The Tenth Amendment created a class of powers exclusive to state governments. These powers are referred to as ________.
- enumerated powers
- implied powers
- reserved powers
- none of the above
Dillon’s Rule gives local governments the freedom and flexibility to make decisions for themselves.
- True
- False
Hint:
B
Under the Articles of Confederation, the federal government was quite weak relative to the states. What changes were made to strengthen the role of the federal government under the U.S. Constitution?
|
oercommons
|
2025-03-18T00:37:58.197696
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15266/overview",
"title": "American Government, Delivering Collective Action: Formal Institutions",
"author": null
}
|
https://oercommons.org/courseware/lesson/15267/overview
|
State Political Culture
Learning Objectives
By the end of this section, you will be able to:
- Compare Daniel Elazar’s three forms of political culture
- Describe how cultural differences between the states can shape attitudes about the role of government and citizen participation
- Discuss the main criticisms of Daniel Elazar’s theory
Some states, such as Alaska, are endowed with natural resources. They can use their oil or natural gas reserves to their advantage to fund education or reduce taxes. Other states, like Florida, are favored with a climate that attracts tourists and retirees each winter, drawing in revenues to support infrastructure improvements throughout the state. These differences can lead to strategic advantages in the economic fortunes of a state, which can translate into differences in the levels of taxes that must be collected from citizens.
But their economic fortunes are only one component of what makes individual states unique. Theorists have long proposed that states are also unique as a function of their differing political cultures, or their attitudes and beliefs about the functions and expectations of the government. In the book, American Federalism: A View from the States, Daniel Elazar first theorized in 1966 that the United States could be divided into three distinct political cultures: moralistic, individualistic, and traditionalistic (Figure). The diffusion of these cultures throughout the United States is attributed to the migratory patterns of immigrants who settled in and spread out across the country from the east to the west coast. These settlers had distinct political and religious values that influenced their beliefs about the proper role of government, the need for citizen involvement in the democratic process, and the role of political parties.
MORALISTIC POLITICAL CULTURE
In Elazar’s framework, states with a moralistic political culture see the government as a means to better society and promote the general welfare. They expect political officials to be honest in their dealings with others, put the interests of the people they serve above their own, and commit to improving the area they represent. The political process is seen in a positive light and not as a vehicle tainted by corruption. In fact, citizens in moralistic cultures have little patience for corruption and believe that politicians should be motivated by a desire to benefit the community rather than by a need to profit financially from service.
Moralistic states thus tend to support an expanded role for government. They are more likely to believe government should promote the general welfare by allocating funds to programs that will benefit the poor. In addition, they see it as the duty of public officials to advocate for new programs that will benefit marginal citizens or solve public policy problems, even when public pressure to do so is nonexistent.
The moralistic political culture developed among the Puritans in upper New England. After several generations, these settlers moved westward, and their values diffused across the top of the United States to the upper Great Lakes. In the middle of the 1800s, Scandinavians and Northern Europeans joined this group of settlers and reinforced the Puritans’ values. Together, these groups pushed further west through the northern portion of the Midwest and West and then along the West Coast.Daniel Elazar. 1972. American Federalism: A View from the States, 2nd ed. New York: Thomas Y. Crowell Company.
States that identify with this culture value citizen engagement and desire citizen participation in all forms of political affairs. In Elazar’s model, citizens from moralistic states should be more likely to donate their time and/or resources to political campaigns and to vote. This occurs for two main reasons. First, state law is likely to make it easier for residents to register and to vote because mass participation is valued. Second, citizens who hail from moralistic states should be more likely to vote because elections are truly contested. In other words, candidates will be less likely to run unopposed and more likely to face genuine competition from a qualified opponent. According to Elazar, the heightened competition is a function of individuals’ believing that public service is a worthwhile endeavor and an honorable profession.
Oregon’s Efforts to Expand the Voting Franchise
In 1998, Oregon became the first state to switch to mail-in voting when citizens passed a ballot measure for it to take effect. In March 2015, Governor Kate Brown took another step to expand the voting franchise. She signed a bill into law that makes voter registration automatic for all citizens in the state with a driver’s license. These citizens will now be automatically registered to vote in elections and will receive a mail-in ballot before Election Day unless they specifically opt out with the Oregon secretary of state’s office.
In the United States, Oregon is the first to institute automatic voter registration, and it anticipates adding several hundred thousand residents to its voter participation list with the passage of this bill.Maria L. La Ganga, “Under New Oregon law, All Eligible Voters are Registered Unless They Opt Out,” Los Angeles Times, 17 March 2015. http://www.latimes.com/nation/la-na-oregon-automatic-voter-registration-20150317-story.html. However, the new law lacks the support of Republicans in the state legislature. These party members believe automatic registration makes the voting process too easy for citizens and coerces them into voting.Jeff Guo, “It’s Official: New Oregon Law Will Automatically Register People to Vote,” Washington Post, 17 March 2015. http://www.washingtonpost.com/blogs/govbeat/wp/2015/03/17/its-official-new-oregon-law-will-automatically-register-people-to-vote/. Others argue that Oregon’s new law is a positive move. They believe the change is a step in the right direction for democracy and will encourage participation in elections. If Oregon’s law were to be adopted across the United States, it would affect about fifty million citizens, the number who are believed to be eligible to vote but who remain unregistered.Alec MacGillis. 18 March 2015. “The Oregon Trail: The State’s New Governor is Going on the Offensive in the Battle for Voting Rights,” http://www.slate.com/articles/news_and_politics/politics/2015/03/kate_brown_and_automatic_voter_registration_oregon_s_new_governor_has_gone.html.
What are the benefits of Oregon’s automatic voter registration policy? Are there any drawbacks? What advantages and disadvantages might arise if this policy were adopted nationwide?
Finally, in Elazar’s view, citizens in moralistic cultures are more likely to support individuals who earn their positions in government on merit rather than as a reward for party loyalty. In theory, there is less incentive to be corrupt if people acquire positions based on their qualifications. In addition, moralistic cultures are more open to third-party participation. Voters want to see political candidates compete who are motivated by the prospect of supporting the broader community, regardless of their party identification.
INDIVIDUALISTIC POLITICAL CULTURE
States that align with Elazar’s individualistic political culture see the government as a mechanism for addressing issues that matter to individual citizens and for pursuing individual goals. People in this culture interact with the government in the same manner they would interact with a marketplace. They expect the government to provide goods and services they see as essential, and the public officials and bureaucrats who provide them expect to be compensated for their efforts. The focus is on meeting individual needs and private goals rather than on serving the best interests of everyone in the community. New policies will be enacted if politicians can use them to garner support from voters or other interested stakeholders, or if there is great demand for these services on the part of individuals.
According to Elazar, the individualist political culture originated with settlers from non-Puritan England and Germany. The first settlements were in the mid-Atlantic region of New York, Pennsylvania, and New Jersey and diffused into the middle portion of the United States in a fairly straight line from Ohio to Wyoming.
Given their focus on pursuing individual objectives, states with an individualistic mindset will tend to advance tax breaks as a way of trying to boost a state’s economy or as a mechanism for promoting individual initiative and entrepreneurship. For instance, New Jersey governor Chris Christie made headlines in 2015 when discussing the incentives he used to attract businesses to the state. Christie encouraged a number of businesses to move to Camden, where unemployment has risen to almost 14 percent, by providing them with hundreds of millions of dollars in tax breaks.Dean DeChiaro, “$830M in Tax Breaks Later, Christie Says His Camden Plan Won’t Work for America,” U.S. News and World Report, 19 August 2015. http://www.usnews.com/news/articles/2015/08/19/830m-in-tax-breaks-later-christie-says-his-camden-plan-wont-work-for-america. The governor hopes these corporate incentives will spur job creation for citizens who need employment in an economically depressed area of the state.
Since this theoretical lens assumes that the objective of politics and the government is to advance individual interests, Elazar argues that individuals are motivated to become engaged in politics only if they have a personal interest in this area or wish to be in charge of the provision of government benefits. They will tend to remain involved if they get enjoyment from their participation or rewards in the form of patronage appointments or financial compensation. As a result of these personal motivations, citizens in individualistic states will tend to be more tolerant of corruption among their political leaders and less likely to see politics as a noble profession in which all citizens should engage.
Finally, Elazar argues that in individualistic states, electoral competition does not seek to identify the candidate with the best ideas. Instead it pits against each other political parties that are well organized and compete directly for votes. Voters are loyal to the candidates who hold the same party affiliation they do. As a result, unlike the case in moralistic cultures, voters do not pay much attention to the personalities of the candidates when deciding how to vote and are less tolerant of third-party candidates.
TRADITIONALISTIC POLITICAL CULTURE
Given the prominence of slavery in its formation, a traditionalistic political culture, in Elazar’s argument, sees the government as necessary to maintaining the existing social order, the status quo. Only elites belong in the political enterprise, and as a result, new public policies will be advanced only if they reinforce the beliefs and interests of those in power.
Elazar associates traditionalistic political culture with the southern portion of the United States, where it developed in the upper regions of Virginia and Kentucky before spreading to the Deep South and the Southwest. Like the individualistic culture, the traditionalistic culture believes in the importance of the individual. But instead of profiting from corporate ventures, settlers in traditionalistic states tied their economic fortunes to the necessity of slavery on plantations throughout the South.
When elected officials do not prioritize public policies that benefit them, those on the social and economic fringes of society can be plagued by poverty and pervasive health problems. For example, although Figure shows that poverty is a problem across the entire United States, the South has the highest incidence. According to the Centers for Disease Control and Prevention, the South also leads the nation in self-reported obesity, closely followed by the Midwest.“Division of Nutrition, Physical Activity, and Obesity: Data, Trends and Maps,” http://www.cdc.gov/obesity/data/prevalence-maps.html (March 14, 2016). These statistics present challenges for lawmakers not only in the short term but also in the long term, because they must prioritize fiscal constraints in the face of growing demand for services.
While moralistic cultures expect and encourage political participation by all citizens, traditionalistic cultures are more likely to see it as a privilege reserved for only those who meet the qualifications. As a result, voter participation will generally be lower in a traditionalistic culture, and there will be more barriers to participation (e.g., a requirement to produce a photo ID at the voting booth). Conservatives argue that these laws reduce or eliminate fraud on the part of voters, while liberals believe they disproportionally disenfranchise the poor and minorities and constitute a modern-day poll tax.
Visit the National Conference of State Legislatures for an overview of Voter Identification Requirements by state.
Finally, under a traditionalistic political culture, Elazar argues that party competition will tend to occur between factions within a dominant party. Historically, the Democratic Party dominated the political structure in the South before realignment during the civil rights era. Today, depending on the office being sought, the parties are more likely to compete for voters.
CRITIQUES OF ELAZAR’S THEORY
Several critiques have come to light since Elazar first introduced his theory of state political culture fifty years ago. The original theory rested on the assumption that new cultures could arise with the influx of settlers from different parts of the world; however, since immigration patterns have changed over time, it could be argued that the three cultures no longer match the country’s current reality. Today’s immigrants are less likely to come from European countries and are more likely to originate in Latin American and Asian countries.Jie Zong and Jeanne Batalova. 26 February 2015. “Frequently Requested Statistics on Immigrants and Immigration in the United States,” http://www.migrationpolicy.org/article/frequently-requested-statistics-immigrants-and-immigration-united-states. In addition, advances in technology and transportation have made it easier for citizens to travel across state lines and to relocate. Therefore, the pattern of diffusion on which the original theory rests may no longer be accurate, because people are moving around in more, and often unpredictable, directions.
It is also true that people migrate for more reasons than simple economics. They may be motivated by social issues such as widespread unemployment, urban decay, or low-quality health care of schools. Such trends may aggravate existing differences, for example the difference between urban and rural lifestyles (e.g., the city of Atlanta vs. other parts of Georgia), which are not accounted for in Elazar’s classification. Finally, unlike economic or demographic characteristics that lend themselves to more precise measurement, culture is a comprehensive concept that can be difficult to quantify. This can limit its explanatory power in political science research.
Summary
Daniel Elazar’s theory argues, based on the cultural values of early immigrants who settled in different regions of the country, the United States is made up of three component cultures: individualistic, moralistic, and traditionalistic. Each culture views aspects of government and politics differently, particularly the nature and purpose of political competition and the role of citizen participation. Critics of the theory say the arrival of recent immigrants from other parts of the globe, the divide between urban and rural lifestyles in a particular state, and new patterns of diffusion and settlement across states and regions mean the theory is no longer an entirely accurate description of reality.
In a ________ political culture, the government is seen as a mechanism for maintaining the existing social order or status quo.
- moralistic
- individualistic
- traditionalistic
- nativistic
Hint:
C
Under a ________ political culture, citizens will tend to be more tolerant of corruption from their political leaders and less likely to see politics as a noble profession in which all citizens should engage.
- moralistic
- individualistic
- traditionalistic
- nativistic
________ was the first state to institute all mail-in voting and automatic voter registration.
- California
- Oregon
- Washington
- New York
Hint:
B
|
oercommons
|
2025-03-18T00:37:58.226086
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15267/overview",
"title": "American Government, Delivering Collective Action: Formal Institutions",
"author": null
}
|
https://oercommons.org/courseware/lesson/15268/overview
|
Governors and State Legislatures
Learning Objectives
By the end of this section, you will be able to:
- Identify the formal powers and responsibilities of modern-day governors
- List the basic functions performed by state legislatures
- Describe how state legislatures vary in size, diversity, party composition, and professionalism
Public opinion regarding Congress has reached a dismal low, with more than 80 percent of those surveyed in 2014 saying they do not feel most members of Congress deserve to be reelected.Lindsey Cook, “Americans Still Hate Congress,” U.S. News and World Report, 18 August 2014. http://www.usnews.com/news/blogs/data-mine/2014/08/18/americans-still-hate-congress. This attitude stems from partisan rivalry, media coverage that has capitalized on the conflict, fiscal shutdowns, and the general perception that Congress is no longer engaged in lawmaking.
The picture looks quite different at the subnational level, at least where lawmaking is concerned. State representatives and senators have been actively engaged in the lawmaking function, grabbing national attention at times for their controversial and highly partisan policies. Governors have been active in promoting their own policy agendas, either in cooperation with the state legislature or in opposition to it. Among the early 2016 Republican presidential contenders, nine were current or former state governors.Wilson Andrews, Alicia Parlapiano, and Karen Yourish, “Who is Running for President?” New York Times, 4 March 2016. http://www.nytimes.com/interactive/2016/us/elections/2016-presidential-candidates.html?_r=0. Increasingly, governors are using their office and the policies they have signed into law as a platform to gain national attention and to give voters a sense of their priorities should they ascend to the highest office in the country, the presidency.
GOVERNORS IN CHARGE
Anyone elected to the office of governor assumes tremendous responsibility overnight. He or she becomes the spokesperson for the entire state and their political party, accepts blame or praise for handling decision-making in times of crisis, oversees the implementation of public policy, and helps shepherd legislation through the lawmaking process. These tasks require a great deal of skill and demand that governors exhibit different strengths and personality traits. Governors must learn to work well with other lawmakers, bureaucrats, cabinet officials, and with the citizens who elected them to office in the first place. The water crisis in Flint, Michigan, provides a good case in point.
Governors have tremendous power over the legislative branch because they serve year-round and hold office alone. They also command wide press coverage by virtue of being the leading elected official in their state. Finally, while there are variations in degree across the states, most governors have more power relative to their state legislatures than does the U.S. president relative to the U.S. Congress. State executive power flows from factors such as the propensity of state legislatures to meet for only part of the year and their resulting reliance for information on the governor and his/her administration, stronger formal tools (e.g., line-item vetoes), budget-cutting discretion, and the fact that state legislators typically hold down another job besides that of legislator.
Three of the governor’s chief functions are to influence the legislative process through an executive budget proposal, a policy agenda, and vetoes. Just as the president gives a State of the Union address once a year, so too do governors give an annual State of the State address before the state legislature (Figure). In this speech, they discuss economic and political achievements, cite data that supports their accomplishments, and overview the major items on their legislative agenda. This speech signals to members of the state legislature what priorities are high on the governor’s list. Those who share the governor’s party affiliation will work with him or her to see these goals achieved. Given that governors need the cooperation of state legislators to get their bills introduced and steered through the lawmaking process, they make developing good relationships with lawmakers a priority. This can entail helping lawmakers address the concerns of their constituents, inviting legislators to social events and meals, and scheduling weekly meetings with legislative leaders and committee chairs to discuss policy.Alan Rosenthal. 2013. The Best Job in Politics; Exploring How Governors Succeed as Policy Leaders. Thousand Oaks, CA: CQ Press.
In addition to providing a basic list of policy priorities, governors also initiate a budget proposal in most states. Here they indicate funding priorities and spell out the amounts that will be appropriated to various state agencies under their discretion. When the economy is strong, governors may find themselves in the enviable position of having a surplus of tax revenue. That allows them some flexibility to decide whether they want to reduce taxes, direct funds toward a new initiative or program, allocate more funds to current programs, restore funds that were cut during times of fiscal distress, or save surplus revenue in a rainy-day account.Elaine S. Povich, “Many State Governors Have Budget Problems with Their Own Parties,” Governing, 4 February 2013. http://www.governing.com/news/headlines/many-state-governors-have-budget-problems-with-their-own-parties.html. Moreover, when cuts must be made, especially when the legislature is not in session, it is typically the governor or his or her finance director who makes the call on what gets cut.
Having introduced his or her priorities, the governor will work on the sidelines to steer favored bills through the legislative process. This may entail holding meetings with committee chairs or other influential lawmakers concerning their legislative priorities, working with the media to try to get favorable coverage of legislative priorities, targeting advocacy organizations to maintain pressure on resistant lawmakers, or testifying in legislative hearings about the possible impacts of the legislation.“Governors’ Powers and Authority,” http://www.nga.org/cms/home/management-resources/governors-powers-and-authority.html (March 14, 2016).
Once legislation has made its way through the lawmaking process, it comes to the governor’s desk for signature. If a governor signs the bill, it becomes law, and if the governor does not like the terms of the legislation he or she can veto, or reject, the entire bill. The bill can then become law only if a supermajority of legislators overrides the veto by voting in favor of the bill. Since it is difficult for two-thirds or more of state legislators to come together to override a veto (it requires many members of the governor’s own party to vote against him or her), the simple act of threatening to veto can be enough to get legislators to make concessions to the governor before he or she will pass the legislation.
The ability to veto legislation is just one of the formal powers governors have at their disposal. Formal powers are powers the governor may exercise that are specifically outlined in state constitutions or state law.Laura van Assendelft. 1997. Governors, Agenda Setting, and Divided Government. Lanham, MD: University Press of America. Unlike U.S. presidents, many governors also have additional veto powers at their disposal, which enhances their ability to check the actions of the legislative branch. For instance, most states provide governors the power of the line-item veto. The line-item veto gives governors the ability to strike out a line or individual portions of a bill while letting the remainder pass into law. In addition, approximately 30 percent of governors have the power of an amendatory veto, which allows them to send a bill back to the legislature and request a specific amendment to it. Finally, a small number of governors, including the governor of Texas, also have the power of a reduction veto, which allows them to reduce the budget proposed in a piece of legislation.National Conference of State Legislatures. “The Veto Process.” In General Legislative Procedures. Washington, DC: National Conference of State Legislatures, 6-29–6-64. http://www.ncsl.org/documents/legismgt/ilp/98tab6pt3.pdf (March 14, 2016).
The Vanna White and Frankenstein Vetoes in Wisconsin
Although the line-item, reduction, and amendatory vetoes give governors tremendous power to adjust legislation and to check the legislative branch, the most powerful and controversial vetoes, which have allowed governors to make selective deletions from a bill before signing, are dubbed the “Vanna White” veto and the “Frankenstein” veto. (Vanna White hosts the popular game show “Wheel of Fortune,” in which contestants guess what a phrase is based on a limited number of letters. As they guess the letters, White indicates the correct letters within the puzzle.) These powers have a colorful history in the state of Wisconsin, where voters have limited their influence on two occasions.
The first occurred in 1990 when voters passed a provision restricting the governor’s ability to use the “Vanna White” veto to change a bill by crossing out specific letters within a given word in order to create a new word. After this restriction took effect, the “Frankenstein” veto came into practice, which allowed a governor to remove individual words, numbers, or passages from a bill and string the remaining text together (like the fictional Dr. Frankenstein’s monster) in an effort to alter the original intent of the legislation.Monica Davey, “Wisconsin Voters Excise Editing from Governor’s Veto Powers,” New York Times, 3 April 2008. http://www.nytimes.com/2008/04/03/us/03wisconsin.html?_r=0.
As an example of the Frankenstein veto, when an appropriations bill was sent to Wisconsin governor James E. Doyle for signature in 2005, Doyle scrapped over seven hundred words from a passage that would have appropriated millions of dollars to transportation. The words that remained in the bill redirected those funds to education. Lawmakers were outraged, but they were not able to override the veto.Daniel C. Vock. 24 April 2007. “Govs Enjoy Quirky Veto Power,” http://www.pewtrusts.org/en/research-and-analysis/blogs/stateline/2007/04/24/govs-enjoy-quirky-veto-power.
Then, in 2007, Governor Doyle used the veto once again to raise property taxes almost 2 percent.Steven Walters, “Voters Drive Stake into ‘Frankenstein Veto’,” Milwaukee Journal Sentinel, 2 April 2008. http://www.jsonline.com/news/wisconsin/29395824.html. As a result of these controversial moves, the state house and senate passed a referendum to end the ability of governors to create a new sentence by combining words from two or more other sentences. A legislative referendum is a measure passed by the state legislature, such as a constitutional amendment, that goes to the voters for final approval.National Conference of State Legislatures. 20 September 2012. “Initiative, Referendum and Recall,” http://www.ncsl.org/research/elections-and-campaigns/initiative-referendum-and-recall-overview.aspx. This referendum went to the voters for approval or rejection in the 2008 election, and the voters banned the practice. Governors in Wisconsin and all the states still have tremendous power to shape legislation, however, through the other types of vetoes discussed in this chapter.
Should any state governor have the powers referred to as the “Vanna White” and “Frankenstein” vetoes? What advantage, if any, might state residents gain from their governor’s ability to alter the intent of a bill the legislature has approved and then sign it into law?
Besides the formal power to prepare the budget and veto legislation, legislators also have the power to call special sessions of the legislature for a wide array of reasons. For instance, sessions may be called to address budgetary issues during an economic downturn, to put together a redistricting plan, or to focus intensively on a particular issue the governor wants rectified immediately.National Conference of State Legislatures. 6 May 2009. “Special Sessions,” http://www.ncsl.org/research/about-state-legislatures/special-sessions472.aspx. In some states, only the governor has the power to call a special session, while in other states this power is shared between the legislative and the executive branches.
For more details on the calling of legislative Special Sessions, visit the National Conference of State Legislatures website.
Although governors have a great deal of power in the legislative arena, this is not their only area of influence. First, as leaders in their political party, governors often work to raise money for other political figures who are up for reelection. A governor who has high public approval ratings may also make campaign appearances on behalf of candidates in tough reelection fights across the state. Governors can draw in supporters, contributions, and media attention that can be beneficial to other political aspirants, and the party will expect them to do their part to ensure the greatest possible number of victories for their candidates. Second, as the spokesperson for their state, governors make every effort to sell the state’s virtues and unique characteristics, whether to the media, to other citizens across the United States, to potential business owners, or to legislative leaders in Washington, DC. Governors want to project a positive image of their state that will encourage tourism, relocation, and economic investment within its boundaries. Collectively, governors make a mark through the National Governors Association, which is a powerful lobbying force in the nation’s capital.
For example, Texas governor Greg Abbott made headlines in 2015 for writing to the CEO of General Electric (GE), urging the company to relocate its corporate headquarters from Connecticut, which had just raised its corporate tax rate, to Texas.Patrick Svitek, “Abbott Tries Wooing General Electric to Texas,” The Texas Tribune, 10 June 2015. https://www.texastribune.org/2015/06/10/abbott-looks-woo-general-electric-connecticut/. As his state’s spokesperson, Abbott promoted Texas’s friendly corporate tax structure and investment in transportation and education funding in hopes of enticing GE to relocate there and bring economic opportunities with it. The company has since decided to relocate to Boston, after receiving incentives, worth up to $145 million, from Massachusetts officials.Ted Mann and Jon Kamp, “General Electric to Move Headquarters to Boston,” The Wall Street Journal, 13 January 2016. http://www.wsj.com/articles/general-electric-plans-to-move-headquarters-to-boston-1452703676. Another example involved Texas governor Rick Perry touring California in 2014 in order to bring prospective businesses from the Golden State to Texas.
In March 2015, the governor of Virginia, Terry McAuliffe, and the mayor of Chicago, Rahm Emanuel, both sent letters to corporate heads in Indiana after controversy erupted around the passage of that state’s Religious Freedom Restoration Act.“Virginia Governor Tries to Woo Indiana Businesses,” http://www.nbcwashington.com/blogs/first-read-dmv/Virginia-Governor-Tries-to-Woo-Indiana-Businesses-298087131.html (March 14, 2016). This bill is designed to restrict government intrusion into people’s religious beliefs unless there is a compelling state interest. It also provides individuals and businesses with the ability to sue if they feel their religious rights have been violated. However, opponents feared the law would be used as a means to discriminate against members of the LGBT community, based on business owners’ religious objections to providing services for same-sex couples.Stephanie Wang, “What the ‘Religious Freedom’ Law Really Means for Indiana,” Indy Star, 3 April 2015. http://www.indystar.com/story/news/politics/2015/03/29/religious-freedom-law-really-means-indiana/70601584/ In the media firestorm that followed the Indiana law’s passage, several prominent companies announced they would consider taking their business elsewhere or cancelling event contracts in the state if the bill were not amended.James Gherardi. March 25, 2015. “Indiana Businesses Concerned Over Economic Impact of Religious Freedom Bill,” http://cbs4indy.com/2015/03/25/indiana-businesses-concerned-over-economic-impact-of-religious-freedom-bill/. This led opportunistic leaders in the surrounding area to make appeals to these companies in the hope of luring them out of Indiana. Ultimately, the bill was clarified, likely due in part to corporate pressure on the state to do so.Tony Cook, Tom LoBianco, and Brian Eason, “Gov. Mike Pence signs RFRA Fix,” Indy Star, 2 April 2015. http://www.indystar.com/story/news/politics/2015/04/01/indiana-rfra-deal-sets-limited-protections-for-lgbt/70766920/. The clarification made it clear that the law could not be used to refuse employment, housing, or service based on an individual’s sexual orientation or gender identity.German Lopez. April 2, 2015. “How Indiana’s Religious Freedom Law Sparked a Battle Over LGBT Rights,” http://www.vox.com/2015/3/31/8319493/indiana-rfra-lgbt.
Controversial legislation like the Religious Freedom Restoration Act is only one of the many environmental factors that can make or break a governor’s reputation and popularity. Other challenges and crises that may face governors include severe weather, terrorist attacks, immigration challenges, and budget shortfalls.
New Jersey governor Chris Christie gained national attention in 2012 over his handling of the aftermath of Hurricane Sandy, which caused an estimated $65 billion worth of damage and cost the lives of over 150 individuals along the East Coast of the United States.29 October 2014. “These Images Show Just How Much Some Neighborhoods Were Changed by Hurricane Sandy,” http://www.huffingtonpost.com/2014/10/29/hurricane-sandy-second-anniversary-images_n_6054274.html. Christie was famously photographed with President Obama during their joint tour of the damaged areas, and the governor subsequently praised the president for his response (Figure). Some later criticized Christie for his remarks because of the close proximity between the president’s visit and Election Day, along with the fact that the Republican governor and Democratic president were from opposite sides of the political aisle. Critics felt the governor had betrayed his party and that the publicity helped the president win reelection.Michael Barbaro, “After Obama, Christie Wants a G.O.P. Hug,” New York Times, 19 November 2012. http://www.nytimes.com/2012/11/20/us/politics/after-embrace-of-obama-chris-christie-woos-a-wary-gop.html?_r=0. Others praised the governor for cooperating with the president and reaching across the partisan divide to secure federal support for his state in a time of crisis.Teresa Welsh, “Is Chris Christie a GOP Traitor for His Obama Hurricane Praise?” U.S. News and World Report, 1 November 2012. http://www.usnews.com/opinion/articles/2012/11/01/is-chris-christie-a-gop-traitor-for-praising-obamas-response-to-hurricane-sandy
If severe winter weather is forecasted or in the event of civil unrest, governors also have the power to call upon the National Guard to assist residents and first responders or aid in storm recovery (Figure). When governors declare a state of emergency, National Guard troops can be activated to go into local areas and assist with emergency efforts in whatever capacity they are needed.Susan Gardner, “Baltimore Erupts into Chaos: Governor activates National Guard,” 27 April 2015. http://www.dailykos.com/story/2015/04/27/1380756/-Baltimore-erupts-into-chaos-Governor-activates-National-Guard#. In 2015, many governors in the New England region called press conferences, worked with snow-removal crews and local government officials, set up emergency shelters, and activated travel bans or curfews in the face of crippling snowstorms.Shira Schoenberg. 2 February 2015. “Governor Calls on 500 Massachusetts National Guard Troops to Dig State Out from Snowstorms,” http://www.masslive.com/news/boston/index.ssf/2015/02/in_unprecedented_move_500_nati.html. When winter storms fail to bring predicted levels of snow, however, politicians can be left to field criticism that they instigated unnecessary panic.Leslie Larson and Jennifer Fermino, “Cuomo and de Blasio Tell Storm Critics ‘Better Safe Than Sorry’,” New York Daily News, 27 January 2015. http://www.nydailynews.com/news/politics/cuomo-de-blasio-critics-better-safe-article-1.2093306. Governors feel the weight of their decisions as they try to balance the political risks of overreacting and the human costs of letting the state be caught unprepared for these and other major natural disasters. As the chief spokesperson, they take all the blame or all the credit for their actions. With that said, it is important to note that presidents can enlist the National Guard for federal service as well.
Governors also have the power to spare or enhance the lives of individuals convicted of crimes in their state. Although they may choose to exercise this formal power only during the closing days of their term, if at all, most governors have the authority to grant pardons just as U.S. presidents do. A pardon absolves someone of blame for a crime and can secure his or her release from prison. Governors can also commute sentences, reducing the time an individual must serve,“Pardons, Reprieves, Commutations and Respites,” http://www.sos.wv.gov/public-services/execrecords/Pages/Pardons.aspx (March 14, 2016). if there are doubts about the person’s guilt, concerns about his or her mental health, or reason to feel the punishment was inappropriately harsh. In the past ten years, the governors of New Jersey and Illinois have commuted the sentences of all inmates on death row before repealing the death penalty in their states.“Clemency Process by State,” http://www.deathpenaltyinfo.org/clemency?did=126&scid=13#process (March 14, 2016).
Despite the tremendous formal powers that go with the job, being governor is still personally and professionally challenging. The demands of the job are likely to restrict time with family and require forgoing privacy. In addition, governors will often face circumstances beyond their control. For instance, the state legislature may include a majority of members who do not share the governor’s party affiliation. This can make working together more challenging and lead to less cooperation during the legislative session. Another challenge for governors is the plural executive, which refers to the fact that many state officials, such as the lieutenant governor, attorney general, and secretary of state are elected independently from the governor; hence, the governor has no direct control over them the way a president might have sway over U.S. executive officials. Governors can also face spending restrictions due to the economic climate in their state. They may have to make unpopular decisions that weaken their support among voters. The federal government can mandate that states perform some function without giving them any funds to do so. Finally, as we saw above, governors can be swept up in crises or natural disasters they did not anticipate and could not have foreseen. This can drain their energy and hamper their ability to generate good public policy.Rosenthal, The Best Job in Politics; Exploring How Governors Succeed as Policy Leaders.
THE FUNCTIONS OF STATE LEGISLATURES
State legislatures serve three primary functions. They perform a lawmaking function by researching, writing, and passing legislation. Members represent their districts and work to meet requests for help from citizens within it. Finally, legislatures perform an oversight function for the executive branch.
All state representatives and senators serve on committees that examine, research, investigate, and vote on legislation that relates to the committee’s purpose, such as agriculture, transportation, or education. The number of bills introduced in any given session varies. Some state legislatures have more restrictive rules concerning the number of bills any one member can sponsor. Legislators get ideas for bills from lobbyists of various types of interest groups, ranging from corporate groups to labor unions to advocacy organizations. Ideas for bills also come from laws passed in other state legislatures, from policy that diffuses from the federal government, from constituents or citizens in the officeholder’s district who approach them with problems they would like to see addressed with new laws, and from their own personal policy agenda, which they brought to office with them. Finally, as we explored previously, legislators also work with the governor’s agenda in the course of each legislative session, and they must pass a budget for their state either every year or every two years.
Most bills die in committee and never receive a second or third reading on the floor of the legislature. Lawmaking requires frequent consensus, not just among the legislators in a given house but also between the two chambers. In order for a bill to become law, it must pass through both the state house and the state senate in identical form before going to the governor’s desk for final signature.
Besides generating public policy, state legislatures try to represent the interests of their constituents. Edmund Burke was a political philosopher who theorized that representatives are either delegates or trustees.Edmund Burke. 1969. “The English Constitutional System.” In Representation. Hanna Pitkin. New York: Atherton Press. A delegate legislator represents the will of those who elected him or her to office and acts in their expressed interest, even when it goes against personal belief about what is ultimately in the constituency’s best interest. On the other hand, trustees believe they were elected to exercise their own judgment and know best because they have the time and expertise to study and understand an issue. Thus, a trustee will be willing to vote against the desire of the constituency so long as he or she believes it is in the people’s best interest. A trustee will also be more likely to vote his or her conscience on issues that are personal to him or her, such as on same-sex marriage or abortion rights.
Regardless of whether representatives adopt a delegate or a trustee mentality, they will all see it as their duty to address the concerns and needs of the people they represent. Typically, this will entail helping members in the district who need assistance or have problems with the government they want addressed. For instance, a constituent may write an elected official asking for help dealing with the bureaucracy such as in a decision made by tax commission, requesting a letter of recommendation for acceptance into a military academy, or proposing a piece of legislation the member can help turn into a law.
Legislators also try to bring particularized benefits back to their district. These benefits might include money that can be spent on infrastructure improvements or grants for research. Finally, members will accept requests from local government officials or other constituents to attend parades, ribbon-cutting ceremonies, or other celebratory events within their district (Figure). They will also work with teachers and faculty to visit classes or meet with students on field trips to the state capitol.
The last primary function of state legislators is to oversee the bureaucracy’s implementation of public policy, ensuring it occurs in the manner the legislature intended. State legislatures may request that agency heads provide testimony about spending in hearings, or they may investigate particular bureaucratic agencies to ensure that funds are being disbursed as desired.Ohio Legislative Service Commission. 2015–2016. “Legislative Oversight.” In A Guidebook for Ohio Legislators, 14th ed. Columbus, OH: Ohio Legislative Service Commission. Since legislators have many other responsibilities and some meet for only a few months each year, they may wait to investigate until a constituent or lobbyist brings a problem to their attention.
THE COMPOSITION OF STATE LEGISLATURES
In most states, the legislative function is divided between two bodies: a state house and a state senate. The only exception is Nebraska, which has a unicameral state senate of forty-nine members. State legislatures vary a great deal in terms of the number of legislators in the house and senate, the range of diversity across the membership, the partisan composition of the chamber relative to the governor’s affiliation, and the degree of legislative professionalism. This variation can lead to differences in the type of policies passed and the amount of power legislatures wield relative to that of the governor.
According to the National Conference of State Legislatures, at forty members, Alaska’s is the smallest state (or lower) house, while New Hampshire’s is the largest at four hundred. State senates range in size from twenty members in Alaska to sixty-seven members in Minnesota. The size of the institution can have consequences for the number of citizens each member represents; larger bodies have a smaller legislator-to-constituent ratio (assuming even populations). Larger institutions can also complicate legislative business because reaching consensus is more difficult with more participants.National Conference of State Legislatures. 11 March 2013. “Number of Legislators and Length of Terms in Years,” http://www.ncsl.org/research/about-state-legislatures/number-of-legislators-and-length-of-terms.aspx.
The term length in the state house is frequently two years, while in the state senate it is more commonly four years. These differences have consequences, too, because representatives in the state house, with the next election always right around the corner, will need to focus on their reelection campaigns more frequently than senators. On the other hand, state senators may have more time to focus on public policy and become policy generalists because they each must serve on multiple committees due to their smaller numbers.
The number of legislators and term length varies by state.
In 2015, according to the National Conference of State Legislatures, women made up 24.3 percent of the nation’s state legislators. However, the number varies a great deal across states (Figure). For instance, in Colorado and Vermont, women account for just over 40 percent of the state legislative membership. However, they make up less than 15 percent of the legislatures in Alabama, Louisiana, Oklahoma, South Carolina, West Virginia, and Wyoming.National Conference of State Legislatures. 4 September 2015. “Women in State Legislatures for 2015,” http://www.ncsl.org/legislators-staff/legislators/womens-legislative-network/women-in-state-legislatures-for-2015.aspx.
Data on minority representatives is more difficult to obtain, but 2009 estimates from the National Conference of State Legislatures paired with census estimates from 2010 show that African Americans and Latinos are both underrepresented in state government relative to their percentage of the population. In 2009, African Americans made up approximately 9 percent of state legislators, compared to the 13 percent of the population they constitute nationwide. On the other hand, Latino representatives made up approximately 3 percent of state legislators relative to their 14 percent of the total population in the United States.National Conference of State Legislatures. 10 January 2008. “African-American Legislators 2009,” http://www.ncsl.org/research/about-state-legislatures/african-american-legislators-in-2009.aspx; “2009 Latino Legislators.” http://www.ncsl.org/research/about-state-legislatures/latino-legislators-overview.aspx (March 14, 2016). The proportion of Latinos in the legislature is highest in Arizona, California, New Mexico, and Texas, while the proportion of African Americans is highest in Alabama, Georgia, and Mississippi.
Scholars in political science have spent a great deal of time researching the impact of women and minorities on the legislative process and on voter participation and trust. Some research demonstrates that female and minority representatives are more likely to advocate for policies that are of interest to or will benefit minorities, women, and children.Chris T. Owens. 2005. “Black Substantive Representation in State Legislatures from 1971–1999,” Social Science Quarterly 84, No. 5: 779–791; Robert R. Preuhs. 2005. “Descriptive Representation, Legislative Leadership, and Direct Democracy: Latino Influence on English Only Laws in the States, 1984–2002,” State Politics and Policy Quarterly 5, No. 3: 203–224; Sue Thomas. 1991. “The Impact of Women on State Legislative Policies.” The Journal of Politics 53, No. 4: 958–976. Other research suggests that the presence of African American and Latino representatives increases voter turnout by these groups.Rene Rocha, Caroline Tolbert, Daniel Bowen, and Christopher Clark. 2010. “Race and Turnout: Does Descriptive Representation in State Legislatures Increase Minority Voting?” Political Research Quarterly 63, No. 4: 890–907. Thus, increased diversity in state legislatures can have consequences for voter engagement and for the type of legislation pursued and passed within these bodies.
You can compare the numbers and percentages of women in state legislature, state by state.
You can also compare the numbers and percentages of African American representatives.
Similar information about Latino representation in state legislatures is also available.
In 2014, twenty-six states had Republican majorities in the state house and senate, while in twenty states Democratic majorities were the norm. In just four states, party control was split so that the Democratic Party maintained control of one house while the Republican Party maintained control of the other.“2014 Legislative Partisan Composition,” http://www.ncsl.org/portals/1/ImageLibrary/WebImages/Elections/2014_Leg_Party_Control_map.gif (March 14, 2016). Figure illustrates the partisan composition across the United States. Note that states in New England and the West Coast are more likely to be unified behind the Democratic Party, while Republicans control legislatures throughout the South and in large parts of the Midwest. This alignment largely reflects differing political ideologies, with the more liberal, urban areas of the country leaning Democratic while the more conservative, rural areas are Republican.
Like diversity, party composition has consequences for policymaking. Governors who are not from the same party as the one controlling the legislature can find it more difficult to achieve their agenda. This governing circumstance is popularly referred to as divided government. In a time of divided government, a governor may have to work harder to build relationships and to broker consensus. In addition, when state party control is divided between the legislative and executive branches, the governor may find that legislators are more likely to muster the numbers to overturn at least some of their vetoes. In contrast, when the governor’s own party controls the legislature—a situation known as unified government—conventional wisdom suggests that they will have a smoother and more productive relationship with the legislature.
Party composition also matters for the overall legislative agenda. The party in power will elect party members to the top leadership posts in the state house and senate, and it will determine who sits on each of the committees. Committees are chaired by members of the majority party, and the composition of these committees is skewed toward members affiliated with the party in power. This gives the majority party an advantage in meeting its policy objectives and relegates the minority party to the position of obstructionists. In addition, while Republicans and Democrats are both concerned about education, health care, transportation, and other major policy areas, the two parties have different philosophies about what is in the best interest of their citizens and where funds should be allocated to meet those needs. The result is vastly different approaches to handling pressing public policy problems across the states.
As a whole, state legislatures have become progressively more professional. Political scientist Peverill Squire, at several points throughout his career, has measured the degree of state legislative professionalism with a ranking across the fifty states.Peverill Squire. 2007. “Measuring State Legislative Professionalism: The Squire Index Revisited.” State Politics & Policy Quarterly 7, No. 2: 211–227. Legislative professionalism is assessed according to three key factors: state legislators’ salary, the length of time they are in session, and the number of staff at their disposal. Members of professional or full-time legislatures tend to consider legislative service their full-time occupation, and they are paid enough not to require a second occupation. They also have larger staffs to assist with their work, and they tend to be in session for much of the year. On the other end of the spectrum are citizen, or part-time, legislatures. Representatives and senators in these legislatures do not enjoy the same perks as their counterparts in professional legislatures. Generally, salary is much lower and so is staff assistance. Members typically need to seek outside employment to supplement their income from legislative work, and the legislature will meet for only a brief period of time during the year.
Between these two extremes are hybrid legislatures. Their members are compensated at a higher rate than in citizen legislatures, but they are still likely to need outside employment to make an income equal to what they were making prior to taking office. These representatives and senators will have some staff assistance but not as much as in a professional legislature. Finally, members in hybrid legislatures will not consider their service to constitute a full-time occupation, but they will spend more than part of their time conducting legislative business. As Figure shows, California, New York, and Pennsylvania are home to some of the most professional legislatures in the country. On the other hand, New Hampshire, North Dakota, Wyoming, and South Dakota are among the states that rank lowest on legislative professionalism.National Conference of State Legislatures. 1 June 2014. “Table 2. Average Job Time, Compensation and Staff Size by Category of Legislature,” http://www.ncsl.org/research/about-state-legislatures/full-and-part-time-legislatures.aspx#average.
Like the other indicators discussed above, legislative professionalism also affects the business of state legislatures. In professional legislatures, elections tend to be more competitive, and the cost of running for a seat is higher because the benefits of being elected are greater. This makes these seats more attractive, and candidates will tend not to run unless they perceive themselves as well qualified. Since the benefits are more generous, elected officials will tend to stay in office longer and develop more policy expertise as a result. This experience can give professional legislatures an edge when dealing with the governor, because they are likely to be in session for about the same amount of time per year as the governor and have the necessary staff to assist them with researching and writing public policy.Peverill, “Measuring State Legislative Professionalism: The Squire Index Revisited.”
The legislative pay varies across states.
Compare the size of legislative staffs across states for the years 1979, 1988, 1996, 2003, and 2009.
Summary
Governors are called upon to work with the state legislature in the lawmaking process, to be the head of their political party, and to be the chief spokespersons and crisis managers for their states. State constitution or state statutes give many governors the power to veto legislation, pardon or commute the sentences of convicted criminals, author a state budget, and call a special session of the state legislature. The three key functions performed by state legislatures are lawmaking, constituency service, and oversight. Legislatures differ in size, diversity, party composition, and level of professionalism across the fifty states.
A ________ is an officeholder who represents the will of those who elected him or her and acts in constituents’ expressed interest.
- delegate
- trustee
- politico
- citizen
In a ________ legislature, members tend to have low salaries, shorter sessions, and few staff members to assist them with their legislative functions.
- professional
- citizen
- hybrid
- unicameral
Hint:
B
A(n) ________ veto allows the governor to cross out budget lines in the legislature-approved budget, while signing the remainder of the budget into law.
- amendatory
- line-item
- reduction
- Frankenstein
Which branch would you consider to be closest to the people? Why?
Hint:
The state legislature, particularly the state house, where members represent fewer people per district. Constituency service is part of the job of a state representative or senator, and house members’ need to be frequently reelected means they will have to pay attention to the electorate.
|
oercommons
|
2025-03-18T00:37:58.265450
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15268/overview",
"title": "American Government, Delivering Collective Action: Formal Institutions",
"author": null
}
|
https://oercommons.org/courseware/lesson/15269/overview
|
State Legislative Term Limits
Learning Objectives
By the end of this section, you will be able to:
- Describe the history of state legislative term limits
- Compare the costs and benefits of term limits
Term limits restrict the length of time a member can serve in the state legislature by capping either lifetime service or the number of consecutive terms. The term limits movement gained momentum in the 1990s, spreading across a wide array of state legislative institutions. Today, fifteen states have imposed term limits on their state house and state senate members. On the other hand, six states, one as recently as 2004, have repealed the term limits imposed on them by the electorate, through either judicial action in the state Supreme Courts or through legislative action in the state legislature.National Conference of State Legislatures. 13 March 2015. “The Term-Limited States,” http://www.ncsl.org/research/about-state-legislatures/chart-of-term-limits-states.aspx.
THE BASICS OF TERM LIMITS
Under consecutive term limits, a member can serve for only a specified period of time in either the state house or the state senate, most commonly eight years. To try to regain a seat in the legislature once the limit has been met, the member will have to wait to run for office again. If the member succeeds, the clock will reset and the legislator may once again serve up to the limit set by the state. In states with a lifetime ban, such as Oklahoma, members can serve only one time for the number of years allotted, and they are not permitted to run for office again (Figure).See note 65.
The first term limits were enacted in 1990 in California, Colorado, and Oklahoma. In 1992, eight more states followed suit in one large wave. The last state to enact term limits on legislative members was Nebraska in 2000.See note 65. However, term limits did not stay in effect in all these states; many state supreme courts repealed them and declared them unconstitutional for a variety of reasons (Figure). For instance, in Massachusetts and Washington, term limits were deemed unconstitutional because they affected candidate qualifications to compete for a given office. The courts ruled that changes to those qualifications could be made only by amending the state constitution, not by voters changing the state law.National Conference of State Legislatures. “Term Limits and the Courts,” http://www.ncsl.org/research/about-state-legislatures/summaries-of-term-limits-cases.aspx (March 14, 2016).
ADVANTAGES OF TERM LIMITS
In many cases, the movement to institute term limits was initiated by voters and passed through citizen initiatives, which allow citizens to place a proposed law or constitutional amendment on the ballot for a popular vote.National Conference of State Legislatures. 20 September 2012. “Initiative, Referendum and Recall,” http://www.ncsl.org/research/elections-and-campaigns/initiative-referendum-and-recall-overview.aspx. Proponents of term limits felt new blood was needed in state legislatures to bring fresh ideas and perspectives to lawmaking. In addition, they hoped term limits would compel turnover among members by shortening the time anyone could serve and by reducing the tendency for elected officials to make legislative service their career. In conjunction with this thinking, some supporters hoped term limits would increase the motivation to make good public policy. If members were less focused on reelection and knew they could not serve more than a certain number of years, perhaps they would get right down to the business of making laws and produce innovative policy within a narrow window of time.John Carey, Richard Niemi, and Lynda Powell. 2000. Term Limits in State Legislatures. Ann Arbor: University of Michigan Press.
For other proponents, the hope was that term limits would increase diversity within the chamber by encouraging more women, members of racial and ethnic minority groups, members of the minority party, and people with unconventional occupations to run for office because seats would be open more frequently. In addition, supporters speculated that increased turnover might prompt higher rates of electoral competition and voter interest. Finally, they believed the loss of long-term legislators due to term limits would allow new members and younger legislators to assume leadership positions within the chamber and committees, creating another way to bring fresh approaches to the lawmaking process. See note 70.
Working to Expand Term Limits
One pro–term limits advocacy group, U.S. Term Limits, is dedicated to the expansion of term limits across the United States. Its members work to prevent states from repealing limits that are already in place. They also support efforts by citizens to institute term limits in states where they are not currently in place, and in Congress, where the Supreme Court declared them unconstitutional.“The U.S. Term Limits Pledge,” http://ustermlimitsamendment.org/about-us/ (March 14, 2016).
If you support their cause, you can follow the link below to learn more about these efforts or to participate directly. Write a letter to the editor encouraging the adoption of term limits in a given state, or encourage your member of Congress to sign a pledge agreeing to cosponsor and vote for an amendment to the Constitution to adopt term limits. You can also sign an online petition to support the adoption of term limits at the federal level or make a donation to a term-limit advocacy group.
What is your state’s policy on term limits? If limits are in place, how have they changed your representation in the state capitol? If they are not in place, what effect would adopting them have on your representation? There is no comparable national movement against term limits, why do you think that is the case? Based on your answers, do you favor term limits or not, and why?
For more information about supporting term limits, visit U.S. Terms, an advocacy group for term limits.
DISADVANTAGES OF TERM LIMITS
Although proponents have many reasons for supporting term limits, opponents also have compelling reasons for not supporting their implementation in the state legislature. In addition, research by political scientists has uncovered a number of negative consequences since term limits took effect.
Although proponents argued that term limits would increase legislative diversity, research comparing the rate of female and minority representation in term-limited and non-term-limited states does not bear out this expectation. There is no statistically significant difference in diversity between the two groups of states.Stanley Caress and Todd Kunioka. 2012. Term Limits and Their Consequences: The Aftermath of Legislative Reform. New York: State University of New York Press. Although term limits may have produced more open seats, additional barriers to holding office can still exist and affect the willingness of women and minorities to run for office. In addition, women and minorities are subject to the same term limits as men, and given their low numbers among candidates for office, on balance a legislature can lose more women or minorities than it gains.
Term limits also affect the power structure between the legislative and executive branches and the key sources from whom legislators draw information about bills before the chamber. Research demonstrates that, post-term limits, legislators became more likely to consult with lobbyists to gain information about legislation under consideration than had been the case before term limits.Lyke Thompson, Charles Elder, and Richard Elling. 2004. Political and Institutional Effects of Term Limits. New York: Palgrave Macmillan. This is likely the result of legislators having less policy expertise and political experience as a function of having fewer years in office, being younger when they first enter legislative service, reducing institutional memory and expertise within the chamber as a whole due to member turnover, or all the above. Interest groups may thus enjoy greater ability to set the agenda and push for policy that favors their organization. This same research also found that under term limits state legislators feel they have lost power relative to the governor and to various bureaucratic agency officials.See note above. This presumed loss of power could damage the state legislature’s ability to adequately check the actions of the executive branch and to perform legislative functions, such as oversight.
Finally, term limits could affect voter enthusiasm and turnout if voters are disappointed they cannot retain legislators they like or have developed a positive relationship with. Once term limits take effect, all legislators are at the voters’ mercy, regardless of the skill or talent they may bring to the office.
Summary
Whether they cap lifetime service or consecutive terms, term limits have become popular in many states, though some have overturned them as unconstitutional. Proponents believe term limits increase voter participation, encourage more women and minorities to run for office, and help bring diversity and fresh ideas to the legislature. Opponents point to research showing that diversity has not increased in term-limit states, and that younger and less experienced legislators tend to rely more on lobbyists for information about proposed bills. Finally, voters disappointed at losing their favorites may fail to go to the polls.
Under consecutive term limits, legislators can serve one time for the number of years allotted and are not permitted to ever compete for the office again.
- True
- False
The most common term limit across the states that have imposed them is ________ years.
- four
- six
- eight
- twelve
Hint:
C
When term limits have been overturned, the most common method was ________.
- a bill passed by the state legislature
- a decision by the state Supreme Court
- a voter referendum
- a governor’s decree
Term limits have produced a statistically significant increase in the number of women serving in state legislatures.
- True
- False
Hint:
B
Currently, ________ states have term limits in place.
- five
- ten
- fifteen
- twenty
|
oercommons
|
2025-03-18T00:37:58.292927
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15269/overview",
"title": "American Government, Delivering Collective Action: Formal Institutions",
"author": null
}
|
https://oercommons.org/courseware/lesson/15270/overview
|
County and City Government
Learning Objectives
By the end of this section, you will be able to:
- Identify the differences between county and municipal governments in terms of their responsibilities and funding sources
- Describe the two primary types of municipal government and the three basic types of county government
County and city governments make up an important component of the overall structure of the government. Not only do they affect citizens directly; it is also easier for citizens to interact with local government officials because their offices and the community’s school board or city council meetings are often close by. Despite this fact, voter turnout in local elections tends to be lower than in state and national elections. Municipal and county governments differ in structure and purpose in several ways.
COUNTY GOVERNMENT
County governments serve a larger geographical area than cities and towns, but a smaller area than states. They are created by the state government and typically operate under provisions set out in the state constitution. As such, they are essentially administrative units of the state. Census estimates from 2012 indicate that there are just over three thousand counties in the United States.Brian Lavin. 30 August 2012. “Census Bureau Reports There are 89,004 Local Governments in the United States (CB12-161),” https://www.census.gov/newsroom/releases/archives/governments/cb12-161.html. County systems usually take one of three basic forms: the commission system, the council-administrator system, and the council-elected executive system.
The most common form of county government is the commission system. Under this structure, an elected commission, which generally consists of a small number of commissioners, serves as the governing body within the county, performing all legislative and executive functions. These include adopting a budget, passing county resolutions, and hiring and firing county officials.Frank Coppa. 2000. County Government: A Guide to Efficient and Accountable Government. Westport, CT: Greenwood Publishing.
Under the council-administrator system, the voters elect council members to serve for a specified period of time, and the council in turn appoints an administrator to oversee the operation of the government. The administrator serves at the directive of the council and can be terminated by the council. The goal of this arrangement is to divide administrative and policymaking responsibilities between the elected council and the appointed administrator.Coppa, County Government: A Guide to Efficient and Accountable Government.
Under a council-elected executive system, the voters elect both the members of the council and the executive. The executive performs functions similar to those of the state governor. For instance, he or she can veto the actions of the council, draft a budget, and provide suggestions regarding public policy.Coppa, County Government: A Guide to Efficient and Accountable Government.
Although the tasks they perform can vary from state to state, most counties have a courthouse that houses county officials, such as the sheriff, the county clerk, the assessor, the treasurer, the coroner, and the engineer. These officials carry out a variety of important functions and oversee the responsibilities of running a county government. For instance, the county coroner investigates the cause of death when suspicious circumstances are present. The county clerk oversees the registration of voters and also certifies election results for the county. In addition, this officeholder typically keeps the official birth, death, and marriage records. The county treasurer oversees the collection and distribution of funds within the county, while the county assessor conducts property tax evaluations and informs individual citizens or business owners of their right to contest the appraised value of their property. Finally, a county engineer will oversee the maintenance and construction of county infrastructure.Coppa, County Government: A Guide to Efficient and Accountable Government. In short, counties help to maintain roads and bridges, courthouses and jails, parks and pools, and public libraries, hospitals, and clinics.http://www.naco.org/counties (March 14, 2016). To provide these services, county governments typically rely on property tax revenue, a portion of sales tax receipts, and funds from intergovernmental transfers by way of federal or state grants.
CITY GOVERNMENT
Municipal governments oversee the operation and functions of cities and towns. Census estimates for 2012 show just over 19,500 municipal governments and nearly 16,500 township governments in the United States.Lavin, “Census Bureau Reports There are 89,004 Local Governments in the United States (CB12-161).” The vast majority of municipal governments operate on one of two governing models: a mayor-council system or a council-manager system.
Under the mayor-council system voters elect both a mayor and members of the city council. The city council performs legislative functions and the mayor the executive functions. Under this system, the mayor may be given a great deal of authority or only limited powers.“Forms of Municipal Government,” http://www.nlc.org/build-skills-and-networks/resources/cities-101/city-structures/forms-of-municipal-government (March 14, 2016). Under a strong mayor system, the mayor will be able to veto the actions of the council, appoint and fire the heads of city departments, and produce a budget. Under a weak mayor system, the mayor has little authority compared to the council and acts in a ceremonial capacity as a spokesperson for the city.“Mayoral Powers,” http://www.nlc.org/build-skills-and-networks/resources/cities-101/city-officials/mayoral-powers (March 14, 2016).
In a council-manager system of government, either the members of the city council are elected by voters along with a mayor who presides over the council, or the voters elect members of the city council and the mayor is chosen from among them. In either case, the city council will then appoint a city manager to carry out the administrative functions of the municipal government. This frees the city council to address political functions such as setting policy and formulating the budget.“Forms of Municipal Government.”
Municipal governments are responsible for providing clean water as well as sewage and garbage disposal. They must maintain city facilities, such as parks, streetlights, and stadiums (Figure). In addition, they address zoning and building regulations, promote the city’s economic development, and provide law enforcement, public transportation, and fire protection. Municipal governments typically rely on property tax revenue, user fees from trash collection and the provision of water and sewer services, a portion of sales tax receipts, and taxes on business.
The International City/County Management Association (ICMA) provides networking opportunities, professional development, and statistical data in order to support local government leaders and other individuals throughout the world. Visit the ICMA Priorities page to learn what makes a better leader and how you might improve your local community.
Summary
County governments can adopt the commission system, the council-administrator system, and the council-elected executive system of government to carry out their functions, which usually include the work of the sheriff, the county clerk, the assessor, the treasurer, the coroner, and the engineer. Municipal governments can use the mayor-council system or the council-manager system and manage services such as the provision of clean water, park maintenance, and local law enforcement. Cities and counties both rely on tax revenues, especially property taxes, to fund their provision of services.
Under the mayor-council system, the ________.
- legislative and executive responsibilities are separated
- political and administrative functions are separated
- mayor chairs the city council
- city council selects the mayor
Hint:
A
Which of the following is not one of the three forms of county government?
- the commission system
- the council-elected executive system
- the mayor-council system
- the council-administrator system
What are the primary responsibilities of municipal governments?
Hint:
Municipal governments are responsible for providing clean water as well as sewage and garbage disposal. They maintain city facilities, such as parks, streetlights, and stadiums. In addition, they address zoning and building regulations, promote economic development, and provide law enforcement, public transportation, and fire protection.
What are the advantages and disadvantages of having so many levels of subnational governments in the United States? Explain.
In which level of substate government would you be most likely to get involved? Why?
Is it preferable for representatives in the state legislature to behave as trustees or as delegates? Why?
Do term limits seem to have more advantages or disadvantages? Defend your answer.
Council of State Governments. 2014. The Book of the States. Lexington, KY: The Council of State Governments.
Elazar, Daniel. 1972. American Federalism: A View from the States, 2nd ed. New York: Thomas Y. Crowell Company.
Governing: The State and Localities (http://www.governing.com/).
National Association of Counties (http://www.naco.org/).
National Conference of State Legislatures (http://www.ncsl.org/).
National Governors Association (http://www.nga.org/cms/home.html).
National League of Cities (http://www.nlc.org/).
Rosenthal, Alan. 2013. The Best Job in Politics; Exploring How Governors Succeed as Policy Leaders. Thousand Oaks, CA: CQ Press.
———. 2004. Heavy Lifting: The Job of State Legislatures. Thousand Oaks, CA: CQ Press.
Wright, Ralph. 2005. Inside the Statehouse: Lessons from the Speaker. Washington, DC: CQ Press.
United States Census Bureau, “Quick Facts: United States” (http://quickfacts.census.gov/qfd/index.html).
|
oercommons
|
2025-03-18T00:37:58.319679
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15270/overview",
"title": "American Government, Delivering Collective Action: Formal Institutions",
"author": null
}
|
https://oercommons.org/courseware/lesson/15309/overview
|
Introduction
Our lives involve regular, dramatic changes in the degree to which we are aware of our surroundings and our internal states. While awake, we feel alert and aware of the many important things going on around us. Our experiences change dramatically while we are in deep sleep and once again when we are dreaming.
This chapter will discuss states of consciousness with a particular emphasis on sleep. The different stages of sleep will be identified, and sleep disorders will be described. The chapter will close with discussions of altered states of consciousness produced by psychoactive drugs, hypnosis, and meditation.
References
Aggarwal, S. K., Carter, G. T., Sullivan, M. D., ZumBrunnen, C., Morrill, R., & Mayer, J. D. (2009). Medicinal use of cannabis in the United States: Historical perspectives, current trends, and future directions. Journal of Opioid Management, 5, 153–168.
Alhola, P. & Polo-Kantola, P. (2007). Sleep Deprivation: Impact on cognitive performance. Neuropsychiatric Disease and Treatment, 3, 553–557.
Alladin, A. (2012). Cognitive hypnotherapy for major depressive disorder. The American Journal of Clinical Hypnosis, 54, 275–293.
American Psychiatric Association. (2013). Diagnostic and statistical manual of mental disorders (5th ed.). Arlington, VA: Author.
Aquina, C. T., Marques-Baptista, A., Bridgeman, P., & Merlin, M. A. (2009). Oxycontin abuse and overdose. Postgraduate Medicine, 121, 163–167.
Arnulf, I. (2012). REM sleep behavior disorder: Motor manifestations and pathophysiology. Movement Disorders, 27, 677–689.
Augustinova, M., & Ferrand, L. (2012). Suggestion does not de-automatize word reading: Evidence from the semantically based Stroop task. Psychonomic Bulletin & Review, 19, 521–527.
Banks, S., & Dinges, D. F. (2007). Behavioral and physiological consequences of sleep restriction. Journal of Clinical Sleep Medicine, 3, 519–528.
Bartke, A., Sun, L. Y., & Longo, V. (2013). Somatotropic signaling: Trade-offs between growth, reproductive development, and longevity. Physiological Reviews, 93, 571–598.
Berkowitz, C. D. (2012). Sudden infant death syndrome, sudden unexpected infant death, and apparent life-threatening events. Advances in Pediatrics, 59, 183–208.
Berry, R. B., Kryger, M. H., & Massie, C. A. (2011). A novel nasal excitatory positive airway pressure (EPAP) device for the treatment of obstructive sleep apnea: A randomized controlled trial. Sleep, 34, 479–485.
Bixler, E. O., Kales, A., Soldatos, C. R., Kales, J. D., & Healey, S. (1979). Prevalence of sleep disorders in the Los Angeles metropolitan area. American Journal of Psychiatry, 136, 1257–1262.
Bostwick, J. M. (2012). Blurred boundaries: The therapeutics and politics of medical marijuana. Mayo Clinic Proceedings, 87, 172–186.
Brook, R. D., Appel, L. J., Rubenfire, M., Ogedegbe, G., Bisognano, J. D., Elliott, W. K., . . . Rajagopalan, S. (2013). Beyond medications and diet: Alternative approaches to lowering blood pressure: A scientific statement from the American Heart Association. Hypertension, 61, 1360–1383.
Broughton, R., Billings, R., Cartwright, R., Doucette, D., Edmeads, J., Edwardh, M., . . . Turrell, G. (1994). Homicidal somnambulism: A case report. Sleep, 17, 253–264.
Brown, L. K. (2012). Can sleep deprivation studies explain why human adults sleep? Current Opinion in Pulmonary Medicine, 18, 541–545.
Burgess, C. R., & Scammell, T. E. (2012). Narcolepsy: Neural mechanisms of sleepiness and cataplexy. Journal of Neuroscience, 32, 12305–12311.
Cai, D. J., Mednick, S. A., Harrison, E. M., Kanady, J. C., & Mednick, S. C. (2009). REM, not incubation, improves creativity by priming associative networks. Proceedings of the National Academy of Sciences, USA, 106, 10130–10134.
Caldwell, K., Harrison, M., Adams, M., Quin, R. H., & Greeson, J. (2010). Developing mindfulness in college students through movement based courses: Effects on self-regulatory self-efficacy, mood, stress, and sleep quality. Journal of American College Health, 58, 433–442.
Capellini, I., Barton, R. A., McNamara, P., Preston, B. T., & Nunn, C. L. (2008). Phylogenetic analysis of the ecology and evolution of mammalian sleep. Evolution, 62, 1764–1776.
Cartwright, R. (2004). Sleepwalking violence: A sleep disorder, a legal dilemma, and a psychological challenge. American Journal of Psychiatry, 161, 1149–1158.
Cartwright, R., Agargun, M. Y., Kirkby, J., & Friedman, J. K. (2006). Relation of dreams to waking concerns. Psychiatry Research, 141, 261–270.
Casati, A., Sedefov, R., & Pfeiffer-Gerschel, T. (2012). Misuse of medications in the European Union: A systematic review of the literature. European Addiction Research, 18, 228–245.
Chen, K. W., Berger, C. C., Manheimer, E., Forde, D., Magidson, J., Dachman, L., & Lejuez, C. W. (2013). Meditative therapies for reducing anxiety: A systematic review and meta-analysis of randomized controlled trials. Depression and Anxiety, 29, 545–562.
Chokroverty, S. (2010). Overview of sleep & sleep disorders. Indian Journal of Medical Research, 131, 126–140.
Christensen, A., Bentley, G. E., Cabrera, R., Ortega, H. H., Perfito, N., Wu, T. J., & Micevych, P. (2012). Hormonal regulation of female reproduction. Hormone and Metabolic Research, 44, 587–591.
CNN. (1999, June 25). ‘Sleepwalker’ convicted of murder. Retrieved from http://www.cnn.com/US/9906/25/sleepwalker.01/
Cropley, M., Theadom, A., Pravettoni, G., & Webb, G. (2008). The effectiveness of smoking cessation interventions prior to surgery: A systematic review. Nicotine and Tobacco Research, 10, 407–412.
De la Herrán-Arita, A. K., & Drucker-Colín, R. (2012). Models for narcolepsy with cataplexy drug discovery. Expert Opinion on Drug Discovery, 7, 155–164.
Del Casale, A., Ferracuti, S., Rapinesi, C., Serata, D., Sani, G., Savoja, V., . . . Girardi, P. (2012). Neurocognition under hypnosis: Findings from recent functional neuroimaging studies. International Journal of Clinical and Experimental Hypnosis, 60, 286–317.
Elkins, G., Johnson, A., & Fisher, W. (2012). Cognitive hypnotherapy for pain management. The American Journal of Clinical Hypnosis, 54, 294–310.
Ellenbogen, J. M., Hu, P. T., Payne, J. D., Titone, D., & Walker, M. P. (2007). Human relational memory requires time and sleep. Proceedings of the National Academy of Sciences, USA, 104, 7723–7728.
Fell, J., Axmacher, N., & Haupt, S. (2010). From alpha to gamma: Electrophysiological correlates meditation-related states of consciousness. Medical Hypotheses, 75, 218–224.
Fenn, K. M., Nusbaum, H. C., & Margoliash, D. (2003). Consolidation during sleep of perceptual learning of spoken language. Nature, 425, 614–616.
Ferini-Strambi, L. (2011). Does idiopathic REM sleep behavior disorder (iRBD) really exist? What are the potential markers of neurodegeneration in iRBD [Supplemental material]? Sleep Medicine, 12(2 Suppl.), S43–S49.
Fiorentini, A., Volonteri, L.S., Dragogna, F., Rovera, C., Maffini, M., Mauri, M. C., & Altamura, C. A. (2011). Substance-induced psychoses: A critical review of the literature. Current Drug Abuse Reviews, 4, 228–240.
Fogel, S. M., & Smith, C. T. (2011). The function of the sleep spindle: A physiological index of intelligence and a mechanism for sleep-dependent memory consolidation. Neuroscience and Biobehavioral Reviews, 35, 1154–1165.
Frank, M. G. (2006). The mystery of sleep function: Current perspectives and future directions. Reviews in the Neurosciences, 17, 375–392.
Freeman, M. P., Fava, M., Lake, J., Trivedi, M. H., Wisner, K. L., & Mischoulon, D. (2010). Complementary and alternative medicine in major depressive disorder: The American Psychiatric Association task force report. The Journal of Clinical Psychiatry, 71, 669–681.
Giedke, H., & Schwärzler, F. (2002). Therapeutic use of sleep deprivation in depression. Sleep Medicine Reviews, 6, 361–377.
Gold, D. R., Rogacz, S. R., Bock, N., Tosteson, T. D., Baum, T. M., Speizer, F. M., & Czeisler, C. A. (1992). Rotating shift work, sleep, and accidents related to sleepiness in hospital nurses. American Journal of Public Health, 82, 1011–1014.
Golden, W. L. (2012). Cognitive hypnotherapy for anxiety disorders. The American Journal of Clinical Hypnosis, 54, 263–274.
Gómez, R. L., Bootzin, R. R., & Nadel, L. (2006). Naps promote abstraction in language-learning infants. Psychological Science, 17, 670–674.
Guilleminault, C., Kirisoglu, C., Bao, G., Arias, V., Chan, A., & Li, K. K. (2005). Adult chronic sleepwalking and its treatment based on polysomnography. Brain, 128, 1062–1069.
Gujar, N., Yoo, S., Hu, P., & Walker, M. P. (2011). Sleep deprivation amplifies reactivity of brain reward networks, biasing the appraisal of positive emotional experiences. The Journal of Neuroscience, 31, 4466–4474.
Guldenmund, P., Vanhaudenhuyse, A., Boly, M., Laureys, S., & Soddu, A. (2012). A default mode of brain function in altered states of consciousness. Archives Italiennes de Biologie, 150, 107–121.
Halász, P. (1993). Arousals without awakening—Dynamic aspect of sleep. Physiology and Behavior, 54, 795–802.
Han, F. (2012). Sleepiness that cannot be overcome: Narcolepsy and cataplexy. Respirology, 17, 1157–1165.
Hardeland, R., Pandi-Perumal, S. R., & Cardinali, D. P. (2006). Melatonin. International Journal of Biochemistry & Cell Biology, 38, 313–316.
Haasen, C., & Krausz, M. (2001). Myths versus experience with respect to cocaine and crack: Learning from the US experience. European Addiction Research, 7, 159–160.
Henry, D., & Rosenthal, L. (2013). “Listening for his breath:” The significance of gender and partner reporting on the diagnosis, management, and treatment of obstructive sleep apnea. Social Science & Medicine, 79, 48–56.
Hicks, R. A., Fernandez, C., & Pelligrini, R. J. (2001). The changing sleep habits of university students: An update. Perceptual and Motor Skills, 93, 648.
Hicks, R. A., Johnson, C., & Pelligrini, R. J. (1992). Changes in the self-reported consistency of normal habitual sleep duration of college students (1978 and 1992). Perceptual and Motor Skills, 75, 1168–1170.
Hilgard, E. R., & Hilgard, J. R. (1994). Hypnosis in the Relief of Pain. New York: Brunner/Mazel.
Hishikawa, Y., & Shimizu, T. (1995). Physiology of REM sleep, cataplexy, and sleep paralysis. Advances in Neurology, 67, 245–271.
Herman, A., & Herman, A. P. (2013). Caffeine’s mechanism of action and its cosmetic use. Skin Pharmacology and Physiology, 26, 8–14.
Hobson, J. A. (2009). REM sleep and dreaming: Towards a theory of protoconsciousness. Nature Reviews Neuroscience, 10, 803–814.
Horikawa,T., Tamaki, M., Miyawaki, Y. & Kamitani, Y. (2013). Neural Decoding of Visual Imagery During Sleep. Science, 340(6132), 639–642. doi:10.1126/science.1234330
Hossain, J. L., & Shapiro, C. M. (2002). The prevalence, cost implications, and management of sleep disorders: An overview. Sleep and Breathing, 6, 85–102.
Huang, L. B., Tsai, M. C., Chen, C. Y., & Hsu, S. C. (2013). The effectiveness of light/dark exposure to treat insomnia in female nurses undertaking shift work during the evening/night shift. Journal of Clinical Sleep Medicine, 9, 641–646.
Huber, R., Ghilardi, M. F., Massimini, M., & Tononi, G. (2004). Local sleep and learning. Nature, 430, 78–81.
Jayanthi, L. D., & Ramamoorthy, S. (2005). Regulation of monoamine transporters: Influence of psychostimulants and therapeutic antidepressants. The AAPS Journal, 7, E728–738.
Julien, R. M. (2005). Opioid analgesics. In A primer of drug action: A comprehensive guide to the actions, uses, and side effects of psychoactive drugs (pp. 461–500). Portland, OR: Worth.
Kihlstrom, J. F. (2013). Neuro-hypnotism: Prospects for hypnosis and neuroscience. Cortex, 49, 365–374.
Klein, D. C., Moore, R. Y., & Reppert, S. M. (Eds.). (1991). Suprachiasmatic nucleus: The mind’s clock. New York, NY: Oxford University Press.
Kogan, N. M., & Mechoulam, R. (2007). Cannabinoids in health and disease. Dialogues in Clinical Neuroscience, 9, 413–430.
Kromann, C. B., & Nielson, C. T. (2012). A case of cola dependency in a woman with recurrent depression. BMC Research Notes, 5, 692.
Lang, A. J., Strauss, J. L., Bomeya, J., Bormann, J. E., Hickman, S. D., Good, R. C., & Essex, M. (2012). The theoretical and empirical basis for meditation as an intervention for PTSD. Behavior Modification, 36, 759–786.
LaBerge, S. (1990). Lucid dreaming: Psychophysiological studies of consciousness during REM sleep. In R. R. Bootzen, J. F. Kihlstrom, & D. L. Schacter (Eds.), Sleep and cognition (pp. 109–126). Washington, DC: American Psychological Association.
Lesku, J. A., Roth, T. C., 2nd, Amlaner, C. J., & Lima, S. L. (2006). A phylogenetic analysis of sleep architecture in mammals: The integration of anatomy, physiology, and ecology. The American Naturalist, 168, 441–453.
Levitt, C., Shaw, E., Wong, S., & Kaczorowski, J. (2007). Systematic review of the literature on postpartum care: Effectiveness of interventions for smoking relapse prevention, cessation, and reduction in postpartum women. Birth, 34, 341–347.
Lifshitz, M., Aubert Bonn, N., Fischer, A., Kashem, I. F., & Raz, A. (2013). Using suggestion to modulate automatic processes: From Stroop to McGurk and beyond. Cortex, 49, 463–473.
Luppi, P. H., Clément, O., Sapin, E., Gervasoni, D., Peyron, C., Léger, L., . . . Fort, P. (2011). The neuronal network responsible for paradoxical sleep and its dysfunctions causing narcolepsy and rapid eye movement (REM) behavior disorder. Sleep Medicine Reviews, 15, 153–163.
Mage, D. T., & Donner, M. (2006). Female resistance to hypoxia: Does it explain the sex difference in mortality rates? Journal of Women’s Health, 15, 786–794.
Mahowald, M. W., & Schenck, C. H. (2000). Diagnosis and management of parasomnias. Clinical Cornerstone, 2, 48–54.
Mahowald, M. W., Schenck, C. H., & Cramer Bornemann, M. A. (2005). Sleep-related violence. Current Neurology and Neuroscience Reports, 5, 153–158.
Mayo Clinic. (n.d.). Sleep terrors (night terrors). Retrieved from http://www.mayoclinic.org/diseases-conditions/night-terrors/basics/treatment/con-20032552
Mather, L. E., Rauwendaal, E. R., Moxham-Hall, V. L., & Wodak, A. D. (2013). (Re)introducing medical cannabis. The Medical Journal of Australia, 199, 759–761.
Maxwell, J. C. (2006). Trends in the abuse of prescription drugs. Gulf Coast Addiction Technology Transfer Center. Retrieved from http://asi.nattc.org/userfiles/file/GulfCoast/PrescriptionTrends_Web.pdf
McCarty, D. E. (2010). A case of narcolepsy with strictly unilateral cataplexy. Journal of Clinical Sleep Medicine, 15, 75–76.
McDaid, C., Durée, K. H., Griffin, S. C., Weatherly, H. L., Stradling, J. R., Davies, R. J., . . . Westwood, M. E. (2009). A systematic review of continuous positive airway pressure for obstructive sleep apnoea-hypopnoea syndrome. Sleep Medicine Reviews, 13, 427–436.
McKim, W. A., & Hancock, S. D. (2013). Drugs and behavior: An introduction to behavioral pharmacology, 7th edition. Boston, MA: Pearson.
Mignot, E. J. M. (2012). A practical guide to the therapy of narcolepsy and hypersomnia syndromes. Neurotherapeutics, 9, 739–752.
Miller, N. L., Shattuck, L. G., & Matsangas, P. (2010). Longitudinal study of sleep patterns of United States Military Academy cadets. Sleep, 33, 1623–1631.
Mitchell, E. A. (2009). SIDS: Past, present and future. Acta Paediatrica, 98, 1712–1719.
Montgomery, G. H., Schnur, J. B., & Kravits, K. (2012). Hypnosis for cancer care: Over 200 years young. CA: A Cancer Journal for Clinicians, 63, 31–44.
National Institutes of Health. (n.d.). Information about sleep. Retrieved from http://science.education.nih.gov/supplements/nih3/sleep/guide/info-sleep.htm
National Research Council. (1994). Learning, remembering, believing: Enhancing human performance. Washington, DC: The National Academies Press.
National Sleep Foundation. (n.d.). How much sleep do we really need? Retrieved from http://sleepfoundation.org/how-sleep-works/how-much-sleep-do-we-really-need
Ohayon, M. M. (1997). Prevalence of DSM-IV diagnostic criteria of insomnia: Distinguishing insomnia related to mental disorders from sleep disorders. Journal of Psychiatric Research, 31, 333–346.
Ohayon, M. M. (2002). Epidemiology of insomnia: What we know and what we still need to learn. Sleep Medicine Reviews, 6, 97–111.
Ohayon, M. M., Carskadon, M. A., Guilleminault, C., & Vitiello, M. V. (2004). Meta-analysis of quantitative sleep parameters from childhood to old age in healthy individuals: Developing normative sleep values across the human lifespan. Sleep, 27, 1255–1273.
Ohayon, M. M., & Roth, T. (2002). Prevalence of restless legs syndrome and periodic limb movement disorder in the general population. Journal of Psychosomatic Research, 53, 547–554.
Poe, G. R., Walsh, C. M., & Bjorness, T. E. (2010). Cognitive neuroscience of sleep. Progress in Brain Research, 185, 1–19.
Porkka-Heiskanen, T. (2011). Methylxanthines and sleep. Handbook of Experimental Pharmacology, 200, 331–348.
Presser, H. B. (1995). Job, family, and gender: Determinants of nonstandard work schedules among employed Americans in 1991. Demography, 32, 577–598.
Pressman, M. R. (2007). Disorders of arousal from sleep and violent behavior: The role of physical contact and proximity. Sleep, 30, 1039–1047.
Provini, F., Tinuper, P., Bisulli, F., & Lagaresi, E. (2011). Arousal disorders [Supplemental material]. Sleep Medicine, 12(2 Suppl.), S22–S26.
Rattenborg, N. C., Lesku, J. A., Martinez-Gonzalez, D., & Lima, S. L. (2007). The non-trivial functions of sleep. Sleep Medicine Reviews, 11, 405–409.
Raz, A. (2011). Hypnosis: A twilight zone of the top-down variety: Few have never heard of hypnosis but most know little about the potential of this mind-body regulation technique for advancing science. Trends in Cognitive Sciences, 15, 555–557.
Raz, A., Shapiro, T., Fan, J., & Posner, M. I. (2002). Hypnotic suggestion and the modulation of Stroop interference. Archives of General Psychiatry, 59, 1151–1161.
Reiner, K., Tibi, L., & Lipsitz, J. D. (2013). Do mindfulness-based interventions reduce pain intensity? A critical review of the literature. Pain Medicine, 14, 230–242.
Restless Legs Syndrome Foundation. (n.d.). Restless legs syndrome: Causes, diagnosis, and treatment for the patient living with Restless legs syndrome (RSL). Retrieved from www.rls.org
Rial, R. V., Nicolau, M. C., Gamundí, A., Akaârir, M., Aparicio, S., Garau, C., . . . Esteban, S. (2007). The trivial function of sleep. Sleep Medicine Reviews, 11, 311–325.
Riemann, D., Berger, M., & Volderholzer, U. (2001). Sleep and depression—Results from psychobiological studies: An overview. Biological Psychology, 57, 67–103.
Reinerman, C. (2007, October 14). 5 myths about that demon crack. Washington Post. Retrieved from http://www.washingtonpost.com/wp-dyn/content/article/2007/10/09/AR2007100900751.html
Reissig, C. J., Strain, E. C., & Griffiths, R. R. (2009). Caffeinated energy drinks—A growing problem. Drug and Alcohol Dependence, 99, 1–10.
Robson, P. J. (2014). Therapeutic potential of cannabinoid medicines. Drug Testing and Analysis, 6, 24–30.
Roth, T. (2007). Insomnia: Definition, prevalence, etiology, and consequences [Supplemental material]. Journal of Clinical Sleep Medicine, 3(5 Suppl.), S7–S10.
Rothman, R. B., Blough, B. E., & Baumann, M. H. (2007). Dual dopamine/serotonin releasers as potential medications for stimulant and alcohol addictions. The AAPS Journal, 9, E1–10.
Sánchez-de-la-Torre, M., Campos-Rodriguez, F., & Barbé, F. (2012). Obstructive sleep apnoea and cardiovascular disease. The Lancet Respiratory Medicine, 1, 31–72.
Savard, J., Simard, S., Ivers, H., & Morin, C. M. (2005). Randomized study on the efficacy of cognitive-behavioral therapy for insomnia secondary to breast cancer, part I: Sleep and psychological effects. Journal of Clinical Oncology, 23, 6083–6096.
Schicho, R., & Storr, M. (2014). Cannabis finds its way into treatment of Crohn’s disease. Pharmacology, 93, 1–3.
Shukla, R. K, Crump, J. L., & Chrisco, E. S. (2012). An evolving problem: Methamphetamine production and trafficking in the United States. International Journal of Drug Policy, 23, 426–435.
Siegel, J. M. (2008). Do all animals sleep? Trends in Neuroscience, 31, 208–213.
Siegel, J. M. (2001). The REM sleep-memory consolidation hypothesis. Science, 294, 1058–1063.
Singh, G. K., & Siahpush, M. (2006). Widening socioeconomic inequalities in US life expectancy, 1980–2000. International Journal of Epidemiology, 35, 969–979.
Smedslund, G., Fisher, K. J., Boles, S. M., & Lichtenstein, E. (2004). The effectiveness of workplace smoking cessation programmes: A meta-analysis of recent studies. Tobacco Control, 13, 197–204.
Sofikitis, N., Giotitsas, N., Tsounapi, P., Baltogiannis, D., Giannakis, D., & Pardalidis, N. (2008). Hormonal regulation of spermatogenesis and spermiogenesis. Journal of Steroid Biochemistry and Molecular Biology, 109, 323–330.
Steriade, M., & Amzica, F. (1998). Slow sleep oscillation, rhythmic K-complexes, and their paroxysmal developments [Supplemental material]. Journal of Sleep Research, 7(1 Suppl.), 30–35.
Stickgold, R. (2005). Sleep-dependent memory consolidation. Nature, 437, 1272–1278.
Stone, K. C., Taylor, D. J., McCrae, C. S., Kalsekar, A., & Lichstein, K. L. (2008). Nonrestorative sleep. Sleep Medicine Reviews, 12, 275–288.
Suchecki, D., Tiba, P. A., & Machado, R. B. (2012). REM sleep rebound as an adaptive response to stressful situations. Frontiers in Neuroscience, 3. doi: 10.3389/fneur.2012.00041
Task Force on Sudden Infant Death Syndrome. (2011). SIDS and other sleep-related infant deaths: Expansion of recommendations for a safe infant sleeping environment. Pediatrics, 128, 1030–1039.
Taillard, J., Philip, P., Coste, O., Sagaspe, P., & Bioulac, B. (2003). The circadian and homeostatic modulation of sleep pressure during wakefulness differs between morning and evening chronotypes. Journal of Sleep Research, 12, 275–282.
Thach, B. T. (2005). The role of respiratory control disorders in SIDS. Respiratory Physiology & Neurobiology, 149, 343–353.
U.S. Food and Drug Administration. (2013, October 24). Statement on Proposed Hydrocodone Reclassification from Janet Woodcock, M.D., Director, Center for Drug Evaluation and Research. Retrieved from http://www.fda.gov/drugs/drugsafety/ucm372089.htm
Vogel, G. W. (1975). A review of REM sleep deprivation. Archives of General Psychiatry, 32, 749–761.
Vøllestad, J., Nielsen, M. B., & Nielsen, G. H. (2012). Mindfulness- and acceptance-based interventions for anxiety disorders: A systematic review and meta-analysis. The British Journal of Clinical Psychology, 51, 239–260.
Wagner, U., Gais, S., & Born, J. (2001). Emotional memory formation is enhanced across sleep intervals with high amounts of rapid eye movement sleep. Learning & Memory, 8, 112–119.
Wagner, U., Gais, S., Haider, H., Verleger, R., & Born, J. (2004). Sleep improves insight. Nature, 427, 352–355.
Walker, M. P. (2009). The role of sleep in cognition and emotion. Annals of the New York Academy of Sciences, 1156, 168–197.
Wark, D. M. (2011). Traditional and alert hypnosis for education: A literature review. The American Journal of Clinical Hypnosis, 54(2), 96–106.
Waterhouse. J., Fukuda, Y., & Morita, T. (2012). Daily rhythms of the sleep-wake cycle [Special issue]. Journal of Physiological Anthropology, 31(5). doi:10.1186/1880-6805-31-5
Welsh, D. K. Takahashi, J. S., & Kay, S. A. (2010). Suprachiasmatic nucleus: Cell autonomy and network properties. Annual Review of Physiology, 72, 551–577.
West, S., Boughton, M., & Byrnes, M. (2009). Juggling multiple temporalities: The shift work story of mid-life nurses. Journal of Nursing Management, 17, 110–119.
White, D. P. (2005). Pathogenesis of obstructive and central sleep apnea. American Journal of Respiratory and Critical Care Medicine, 172, 1363–1370.
Williams, J., Roth, A., Vatthauer, K., & McCrae, C. S. (2013). Cognitive behavioral treatment of insomnia. Chest, 143, 554–565.
Williamson, A. M., & Feyer, A. M. (2000). Moderate sleep deprivation produces impairments in cognitive and motor performance equivalent to legally prescribed levels of alcohol intoxication. Occupational and Environmental Medicine, 57, 649–655.
Wolt, B. J., Ganetsky, M., & Babu, K. M. (2012). Toxicity of energy drinks. Current Opinion in Pediatrics, 24, 243–251.
Zangini, S., Calandra-Buonaura, G., Grimaldi, D., & Cortelli, P. (2011). REM behaviour disorder and neurodegenerative diseases [Supplemental material]. Sleep Medicine, 12(2 Suppl.), S54–S58.
Zeidan, F., Grant, J. A., Brown, C. A., McHaffie, J. G., & Coghill, R. C. (2012). Mindfulness meditation-related pain relief: Evidence for unique brain mechanisms in the regulation of pain. Neuroscience Letters, 520, 165–173.
|
oercommons
|
2025-03-18T00:37:58.355139
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15309/overview",
"title": "Psychology, States of Consciousness",
"author": null
}
|
https://oercommons.org/courseware/lesson/15310/overview
|
What Is Consciousness?
Overview
By the end of this section, you will be able to:
- Understand what is meant by consciousness
- Explain how circadian rhythms are involved in regulating the sleep-wake cycle, and how circadian cycles can be disrupted
- Discuss the concept of sleep debt
Consciousness describes our awareness of internal and external stimuli. Awareness of internal stimuli includes feeling pain, hunger, thirst, sleepiness, and being aware of our thoughts and emotions. Awareness of external stimuli includes seeing the light from the sun, feeling the warmth of a room, and hearing the voice of a friend.
We experience different states of consciousness and different levels of awareness on a regular basis. We might even describe consciousness as a continuum that ranges from full awareness to a deep sleep. Sleep is a state marked by relatively low levels of physical activity and reduced sensory awareness that is distinct from periods of rest that occur during wakefulness. Wakefulness is characterized by high levels of sensory awareness, thought, and behavior. In between these extremes are states of consciousness related to daydreaming, intoxication as a result of alcohol or other drug use, meditative states, hypnotic states, and altered states of consciousness following sleep deprivation. We might also experience unconscious states of being via drug-induced anesthesia for medical purposes. Often, we are not completely aware of our surroundings, even when we are fully awake. For instance, have you ever daydreamed while driving home from work or school without really thinking about the drive itself? You were capable of engaging in the all of the complex tasks involved with operating a motor vehicle even though you were not aware of doing so. Many of these processes, like much of psychological behavior, are rooted in our biology.
BIOLOGICAL RHYTHMS
Biological rhythms are internal rhythms of biological activity. A woman’s menstrual cycle is an example of a biological rhythm—a recurring, cyclical pattern of bodily changes. One complete menstrual cycle takes about 28 days—a lunar month—but many biological cycles are much shorter. For example, body temperature fluctuates cyclically over a 24-hour period (Figure). Alertness is associated with higher body temperatures, and sleepiness with lower body temperatures.
This pattern of temperature fluctuation, which repeats every day, is one example of a circadian rhythm. A circadian rhythm is a biological rhythm that takes place over a period of about 24 hours. Our sleep-wake cycle, which is linked to our environment’s natural light-dark cycle, is perhaps the most obvious example of a circadian rhythm, but we also have daily fluctuations in heart rate, blood pressure, blood sugar, and body temperature. Some circadian rhythms play a role in changes in our state of consciousness.
If we have biological rhythms, then is there some sort of biological clock? In the brain, the hypothalamus, which lies above the pituitary gland, is a main center of homeostasis. Homeostasis is the tendency to maintain a balance, or optimal level, within a biological system.
The brain’s clock mechanism is located in an area of the hypothalamus known as the suprachiasmatic nucleus (SCN). The axons of light-sensitive neurons in the retina provide information to the SCN based on the amount of light present, allowing this internal clock to be synchronized with the outside world (Klein, Moore, & Reppert, 1991; Welsh, Takahashi, & Kay, 2010) (Figure).
PROBLEMS WITH CIRCADIAN RHYTHMS
Generally, and for most people, our circadian cycles are aligned with the outside world. For example, most people sleep during the night and are awake during the day. One important regulator of sleep-wake cycles is the hormone melatonin. The pineal gland, an endocrine structure located inside the brain that releases melatonin, is thought to be involved in the regulation of various biological rhythms and of the immune system during sleep (Hardeland, Pandi-Perumal, & Cardinali, 2006). Melatonin release is stimulated by darkness and inhibited by light.
There are individual differences with regards to our sleep-wake cycle. For instance, some people would say they are morning people, while others would consider themselves to be night owls. These individual differences in circadian patterns of activity are known as a person’s chronotype, and research demonstrates that morning larks and night owls differ with regard to sleep regulation (Taillard, Philip, Coste, Sagaspe, & Bioulac, 2003). Sleep regulation refers to the brain’s control of switching between sleep and wakefulness as well as coordinating this cycle with the outside world.
Watch this brief video describing circadian rhythms and how they affect sleep.
Disruptions of Normal Sleep
Whether lark, owl, or somewhere in between, there are situations in which a person’s circadian clock gets out of synchrony with the external environment. One way that this happens involves traveling across multiple time zones. When we do this, we often experience jet lag. Jet lag is a collection of symptoms that results from the mismatch between our internal circadian cycles and our environment. These symptoms include fatigue, sluggishness, irritability, and insomnia (i.e., a consistent difficulty in falling or staying asleep for at least three nights a week over a month’s time) (Roth, 2007).
Individuals who do rotating shift work are also likely to experience disruptions in circadian cycles. Rotating shift work refers to a work schedule that changes from early to late on a daily or weekly basis. For example, a person may work from 7:00 a.m. to 3:00 p.m. on Monday, 3:00 a.m. to 11:00 a.m. on Tuesday, and 11:00 a.m. to 7:00 p.m. on Wednesday. In such instances, the individual’s schedule changes so frequently that it becomes difficult for a normal circadian rhythm to be maintained. This often results in sleeping problems, and it can lead to signs of depression and anxiety. These kinds of schedules are common for individuals working in health care professions and service industries, and they are associated with persistent feelings of exhaustion and agitation that can make someone more prone to making mistakes on the job (Gold et al., 1992; Presser, 1995).
Rotating shift work has pervasive effects on the lives and experiences of individuals engaged in that kind of work, which is clearly illustrated in stories reported in a qualitative study that researched the experiences of middle-aged nurses who worked rotating shifts (West, Boughton & Byrnes, 2009). Several of the nurses interviewed commented that their work schedules affected their relationships with their family. One of the nurses said,
If you’ve had a partner who does work regular job 9 to 5 office hours . . . the ability to spend time, good time with them when you’re not feeling absolutely exhausted . . . that would be one of the problems that I’ve encountered. (West et al., 2009, p. 114)
While disruptions in circadian rhythms can have negative consequences, there are things we can do to help us realign our biological clocks with the external environment. Some of these approaches, such as using a bright light as shown in Figure, have been shown to alleviate some of the problems experienced by individuals suffering from jet lag or from the consequences of rotating shift work. Because the biological clock is driven by light, exposure to bright light during working shifts and dark exposure when not working can help combat insomnia and symptoms of anxiety and depression (Huang, Tsai, Chen, & Hsu, 2013).
Watch this video to hear tips on how to overcome jet lag.
Insufficient Sleep
When people have difficulty getting sleep due to their work or the demands of day-to-day life, they accumulate a sleep debt. A person with a sleep debt does not get sufficient sleep on a chronic basis. The consequences of sleep debt include decreased levels of alertness and mental efficiency. Interestingly, since the advent of electric light, the amount of sleep that people get has declined. While we certainly welcome the convenience of having the darkness lit up, we also suffer the consequences of reduced amounts of sleep because we are more active during the nighttime hours than our ancestors were. As a result, many of us sleep less than 7–8 hours a night and accrue a sleep debt. While there is tremendous variation in any given individual’s sleep needs, the National Sleep Foundation (n.d.) cites research to estimate that newborns require the most sleep (between 12 and 18 hours a night) and that this amount declines to just 7–9 hours by the time we are adults.
If you lie down to take a nap and fall asleep very easily, chances are you may have sleep debt. Given that college students are notorious for suffering from significant sleep debt (Hicks, Fernandez, & Pelligrini, 2001; Hicks, Johnson, & Pelligrini, 1992; Miller, Shattuck, & Matsangas, 2010), chances are you and your classmates deal with sleep debt-related issues on a regular basis. Table shows recommended amounts of sleep at different ages.
| Age | Nightly Sleep Needs |
|---|---|
| 0–3 months | 12–18 hours |
| 3 months–1 year | 14–15 hours |
| 1–3 years | 12–14 hours |
| 3–5 years | 11–13 hours |
| 5–10 years | 10–11 hours |
| 10–18 years | 8–10 hours |
| 18 and older | 7–9 hours |
Sleep debt and sleep deprivation have significant negative psychological and physiological consequences Figure. As mentioned earlier, lack of sleep can result in decreased mental alertness and cognitive function. In addition, sleep deprivation often results in depression-like symptoms. These effects can occur as a function of accumulated sleep debt or in response to more acute periods of sleep deprivation. It may surprise you to know that sleep deprivation is associated with obesity, increased blood pressure, increased levels of stress hormones, and reduced immune functioning (Banks & Dinges, 2007). A sleep deprived individual generally will fall asleep more quickly than if she were not sleep deprived. Some sleep-deprived individuals have difficulty staying awake when they stop moving (example sitting and watching television or driving a car). That is why individuals suffering from sleep deprivation can also put themselves and others at risk when they put themselves behind the wheel of a car or work with dangerous machinery. Some research suggests that sleep deprivation affects cognitive and motor function as much as, if not more than, alcohol intoxication (Williamson & Feyer, 2000).
To assess your own sleeping habits, read this article about sleep needs.
The amount of sleep we get varies across the lifespan. When we are very young, we spend up to 16 hours a day sleeping. As we grow older, we sleep less. In fact, a meta-analysis, which is a study that combines the results of many related studies, conducted within the last decade indicates that by the time we are 65 years old, we average fewer than 7 hours of sleep per day (Ohayon, Carskadon, Guilleminault, & Vitiello, 2004). As the amount of time we sleep varies over our lifespan, presumably the sleep debt would adjust accordingly.
Summary
States of consciousness vary over the course of the day and throughout our lives. Important factors in these changes are the biological rhythms, and, more specifically, the circadian rhythms generated by the suprachiasmatic nucleus (SCN). Typically, our biological clocks are aligned with our external environment, and light tends to be an important cue in setting this clock. When people travel across multiple time zones or work rotating shifts, they can experience disruptions of their circadian cycles that can lead to insomnia, sleepiness, and decreased alertness. Bright light therapy has shown to be promising in dealing with circadian disruptions. If people go extended periods of time without sleep, they will accrue a sleep debt and potentially experience a number of adverse psychological and physiological consequences.
Review Questions
The body’s biological clock is located in the ________.
- hippocampus
- thalamus
- hypothalamus
- pituitary gland
Hint:
C
________ occurs when there is a chronic deficiency in sleep.
- jet lag
- rotating shift work
- circadian rhythm
- sleep debt
Hint:
D
________ cycles occur roughly once every 24 hours.
- biological
- circadian
- rotating
- conscious
Hint:
B
________ is one way in which people can help reset their biological clocks.
- Light-dark exposure
- coffee consumption
- alcohol consumption
- napping
Hint:
A
Critical Thinking Questions
Healthcare professionals often work rotating shifts. Why is this problematic? What can be done to deal with potential problems?
Hint:
Given that rotating shift work can lead to exhaustion and decreased mental efficiency, individuals working under these conditions are more likely to make mistakes on the job. The implications for this in the health care professions are obvious. Those in health care professions could be educated about the benefits of light-dark exposure to help alleviate such problems.
Generally, humans are considered diurnal which means we are awake during the day and asleep during the night. Many rodents, on the other hand, are nocturnal. Why do you think different animals have such different sleep-wake cycles?
Hint:
Different species have different evolutionary histories, and they have adapted to their environments in different ways. There are a number of different possible explanations as to why a given species is diurnal or nocturnal. Perhaps humans would be most vulnerable to threats during the evening hours when light levels are low. Therefore, it might make sense to be in shelter during this time. Rodents, on the other hand, are faced with a number of predatory threats, so perhaps being active at night minimizes the risk from predators such as birds that use their visual senses to locate prey.
Personal Application Questions
We experience shifts in our circadian clocks in the fall and spring of each year with time changes associated with daylight saving time. Is springing ahead or falling back easier for you to adjust to, and why do you think that is?
What do you do to adjust to the differences in your daily schedule throughout the week? Are you running a sleep debt when daylight saving time begins or ends?
|
oercommons
|
2025-03-18T00:37:58.389206
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15310/overview",
"title": "Psychology, States of Consciousness",
"author": null
}
|
https://oercommons.org/courseware/lesson/15311/overview
|
Sleep and Why We Sleep
Overview
By the end of this section, you will be able to:
- Describe areas of the brain involved in sleep
- Understand hormone secretions associated with sleep
- Describe several theories aimed at explaining the function of sleep
We spend approximately one-third of our lives sleeping. Given the average life expectancy for U.S. citizens falls between 73 and 79 years old (Singh & Siahpush, 2006), we can expect to spend approximately 25 years of our lives sleeping. Some animals never sleep (e.g., several fish and amphibian species); other animals can go extended periods of time without sleep and without apparent negative consequences (e.g., dolphins); yet some animals (e.g., rats) die after two weeks of sleep deprivation (Siegel, 2008). Why do we devote so much time to sleeping? Is it absolutely essential that we sleep? This section will consider these questions and explore various explanations for why we sleep.
WHAT IS SLEEP?
You have read that sleep is distinguished by low levels of physical activity and reduced sensory awareness. As discussed by Siegel (2008), a definition of sleep must also include mention of the interplay of the circadian and homeostatic mechanisms that regulate sleep. Homeostatic regulation of sleep is evidenced by sleep rebound following sleep deprivation. Sleep rebound refers to the fact that a sleep-deprived individual will tend to take a shorter time to fall asleep during subsequent opportunities for sleep. Sleep is characterized by certain patterns of activity of the brain that can be visualized using electroencephalography (EEG), and different phases of sleep can be differentiated using EEG as well (Figure).
Sleep-wake cycles seem to be controlled by multiple brain areas acting in conjunction with one another. Some of these areas include the thalamus, the hypothalamus, and the pons. As already mentioned, the hypothalamus contains the SCN—the biological clock of the body—in addition to other nuclei that, in conjunction with the thalamus, regulate slow-wave sleep. The pons is important for regulating rapid eye movement (REM) sleep (National Institutes of Health, n.d.).
Sleep is also associated with the secretion and regulation of a number of hormones from several endocrine glands including: melatonin, follicle stimulating hormone (FSH), luteinizing hormone (LH), and growth hormone (National Institutes of Health, n.d.). You have read that the pineal gland releases melatonin during sleep (Figure). Melatonin is thought to be involved in the regulation of various biological rhythms and the immune system (Hardeland et al., 2006). During sleep, the pituitary gland secretes both FSH and LH which are important in regulating the reproductive system (Christensen et al., 2012; Sofikitis et al., 2008). The pituitary gland also secretes growth hormone, during sleep, which plays a role in physical growth and maturation as well as other metabolic processes (Bartke, Sun, & Longo, 2013).
WHY DO WE SLEEP?
Given the central role that sleep plays in our lives and the number of adverse consequences that have been associated with sleep deprivation, one would think that we would have a clear understanding of why it is that we sleep. Unfortunately, this is not the case; however, several hypotheses have been proposed to explain the function of sleep.
Adaptive Function of Sleep
One popular hypothesis of sleep incorporates the perspective of evolutionary psychology. Evolutionary psychology is a discipline that studies how universal patterns of behavior and cognitive processes have evolved over time as a result of natural selection. Variations and adaptations in cognition and behavior make individuals more or less successful in reproducing and passing their genes to their offspring. One hypothesis from this perspective might argue that sleep is essential to restore resources that are expended during the day. Just as bears hibernate in the winter when resources are scarce, perhaps people sleep at night to reduce their energy expenditures. While this is an intuitive explanation of sleep, there is little research that supports this explanation. In fact, it has been suggested that there is no reason to think that energetic demands could not be addressed with periods of rest and inactivity (Frank, 2006; Rial et al., 2007), and some research has actually found a negative correlation between energetic demands and the amount of time spent sleeping (Capellini, Barton, McNamara, Preston, & Nunn, 2008).
Another evolutionary hypothesis of sleep holds that our sleep patterns evolved as an adaptive response to predatory risks, which increase in darkness. Thus we sleep in safe areas to reduce the chance of harm. Again, this is an intuitive and appealing explanation for why we sleep. Perhaps our ancestors spent extended periods of time asleep to reduce attention to themselves from potential predators. Comparative research indicates, however, that the relationship that exists between predatory risk and sleep is very complex and equivocal. Some research suggests that species that face higher predatory risks sleep fewer hours than other species (Capellini et al., 2008), while other researchers suggest there is no relationship between the amount of time a given species spends in deep sleep and its predation risk (Lesku, Roth, Amlaner, & Lima, 2006).
It is quite possible that sleep serves no single universally adaptive function, and different species have evolved different patterns of sleep in response to their unique evolutionary pressures. While we have discussed the negative outcomes associated with sleep deprivation, it should be pointed out that there are many benefits that are associated with adequate amounts of sleep. A few such benefits listed by the National Sleep Foundation (n.d.) include maintaining healthy weight, lowering stress levels, improving mood, and increasing motor coordination, as well as a number of benefits related to cognition and memory formation.
Cognitive Function of Sleep
Another theory regarding why we sleep involves sleep’s importance for cognitive function and memory formation (Rattenborg, Lesku, Martinez-Gonzalez, & Lima, 2007). Indeed, we know sleep deprivation results in disruptions in cognition and memory deficits (Brown, 2012), leading to impairments in our abilities to maintain attention, make decisions, and recall long-term memories. Moreover, these impairments become more severe as the amount of sleep deprivation increases (Alhola & Polo-Kantola, 2007). Furthermore, slow-wave sleep after learning a new task can improve resultant performance on that task (Huber, Ghilardi, Massimini, & Tononi, 2004) and seems essential for effective memory formation (Stickgold, 2005). Understanding the impact of sleep on cognitive function should help you understand that cramming all night for a test may be not effective and can even prove counterproductive.
Watch this brief video describing sleep deprivation in college students.
Here’s another brief video describing sleep tips for college students.
Sleep has also been associated with other cognitive benefits. Research indicates that included among these possible benefits are increased capacities for creative thinking (Cai, Mednick, Harrison, Kanady, & Mednick, 2009; Wagner, Gais, Haider, Verleger, & Born, 2004), language learning (Fenn, Nusbaum, & Margoliash, 2003; Gómez, Bootzin, & Nadel, 2006), and inferential judgments (Ellenbogen, Hu, Payne, Titone, & Walker, 2007). It is possible that even the processing of emotional information is influenced by certain aspects of sleep (Walker, 2009).
Watch this brief video describing the relationship between sleep and memory.
Summary
We devote a very large portion of time to sleep, and our brains have complex systems that control various aspects of sleep. Several hormones important for physical growth and maturation are secreted during sleep. While the reason we sleep remains something of a mystery, there is some evidence to suggest that sleep is very important to learning and memory.
Review Questions
Growth hormone is secreted by the ________ while we sleep.
- pineal gland
- thyroid
- pituitary gland
- pancreas
Hint:
C
The ________ plays a role in controlling slow-wave sleep.
- hypothalamus
- thalamus
- pons
- both a and b
Hint:
D
________ is a hormone secreted by the pineal gland that plays a role in regulating biological rhythms and immune function.
- growth hormone
- melatonin
- LH
- FSH
Hint:
B
________ appears to be especially important for enhanced performance on recently learned tasks.
- melatonin
- slow-wave sleep
- sleep deprivation
- growth hormone
Hint:
B
Critical Thinking Questions
If theories that assert sleep is necessary for restoration and recovery from daily energetic demands are correct, what do you predict about the relationship that would exist between individuals’ total sleep duration and their level of activity?
Hint:
Those individuals (or species) that expend the greatest amounts of energy would require the longest periods of sleep.
How could researchers determine if given areas of the brain are involved in the regulation of sleep?
Hint:
Researchers could use lesion or brain stimulation techniques to determine how deactivation or activation of a given brain region affects behavior. Furthermore, researchers could use any number of brain imaging techniques like fMRI or CT scans to come to these conclusions.
Differentiate the evolutionary theories of sleep and make a case for the one with the most compelling evidence.
Hint:
One evolutionary theory of sleep holds that sleep is essential for restoration of resources that are expended during the demands of day-to-day life. A second theory proposes that our sleep patterns evolved as an adaptive response to predatory risks, which increase in darkness. The first theory has little or no empirical support, and the second theory is supported by some, though not all, research.
Personal Application Question
Have you (or someone you know) ever experienced significant periods of sleep deprivation because of simple insomnia, high levels of stress, or as a side effect from a medication? What were the consequences of missing out on sleep?
|
oercommons
|
2025-03-18T00:37:58.418305
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15311/overview",
"title": "Psychology, States of Consciousness",
"author": null
}
|
https://oercommons.org/courseware/lesson/15312/overview
|
Stages of Sleep
Overview
By the end of this section, you will be able to:
- Differentiate between REM and non-REM sleep
- Describe the differences between the four stages of non-REM sleep
- Understand the role that REM and non-REM sleep play in learning and memory
Sleep is not a uniform state of being. Instead, sleep is composed of several different stages that can be differentiated from one another by the patterns of brain wave activity that occur during each stage. These changes in brain wave activity can be visualized using EEG and are distinguished from one another by both the frequency and amplitude of brain waves (Figure). Sleep can be divided into two different general phases: REM sleep and non-REM (NREM) sleep. Rapid eye movement (REM) sleep is characterized by darting movements of the eyes under closed eyelids. Brain waves during REM sleep appear very similar to brain waves during wakefulness. In contrast, non-REM (NREM) sleep is subdivided into four stages distinguished from each other and from wakefulness by characteristic patterns of brain waves. The first four stages of sleep are NREM sleep, while the fifth and final stage of sleep is REM sleep. In this section, we will discuss each of these stages of sleep and their associated patterns of brain wave activity.
NREM STAGES OF SLEEP
The first stage of NREM sleep is known as stage 1 sleep. Stage 1 sleep is a transitional phase that occurs between wakefulness and sleep, the period during which we drift off to sleep. During this time, there is a slowdown in both the rates of respiration and heartbeat. In addition, stage 1 sleep involves a marked decrease in both overall muscle tension and core body temperature.
In terms of brain wave activity, stage 1 sleep is associated with both alpha and theta waves. The early portion of stage 1 sleep produces alpha waves, which are relatively low frequency (8–13Hz), high amplitude patterns of electrical activity (waves) that become synchronized (Figure). This pattern of brain wave activity resembles that of someone who is very relaxed, yet awake. As an individual continues through stage 1 sleep, there is an increase in theta wave activity. Theta waves are even lower frequency (4–7 Hz), higher amplitude brain waves than alpha waves. It is relatively easy to wake someone from stage 1 sleep; in fact, people often report that they have not been asleep if they are awoken during stage 1 sleep.
As we move into stage 2 sleep, the body goes into a state of deep relaxation. Theta waves still dominate the activity of the brain, but they are interrupted by brief bursts of activity known as sleep spindles (Figure). A sleep spindle is a rapid burst of higher frequency brain waves that may be important for learning and memory (Fogel & Smith, 2011; Poe, Walsh, & Bjorness, 2010). In addition, the appearance of K-complexes is often associated with stage 2 sleep. A K-complex is a very high amplitude pattern of brain activity that may in some cases occur in response to environmental stimuli. Thus, K-complexes might serve as a bridge to higher levels of arousal in response to what is going on in our environments (Halász, 1993; Steriade & Amzica, 1998).
Stage 3 and stage 4 of sleep are often referred to as deep sleep or slow-wave sleep because these stages are characterized by low frequency (up to 4 Hz), high amplitude delta waves (Figure). During this time, an individual’s heart rate and respiration slow dramatically. It is much more difficult to awaken someone from sleep during stage 3 and stage 4 than during earlier stages. Interestingly, individuals who have increased levels of alpha brain wave activity (more often associated with wakefulness and transition into stage 1 sleep) during stage 3 and stage 4 often report that they do not feel refreshed upon waking, regardless of how long they slept (Stone, Taylor, McCrae, Kalsekar, & Lichstein, 2008).
REM SLEEP
As mentioned earlier, REM sleep is marked by rapid movements of the eyes. The brain waves associated with this stage of sleep are very similar to those observed when a person is awake, as shown in Figure, and this is the period of sleep in which dreaming occurs. It is also associated with paralysis of muscle systems in the body with the exception of those that make circulation and respiration possible. Therefore, no movement of voluntary muscles occurs during REM sleep in a normal individual; REM sleep is often referred to as paradoxical sleep because of this combination of high brain activity and lack of muscle tone. Like NREM sleep, REM has been implicated in various aspects of learning and memory (Wagner, Gais, & Born, 2001), although there is disagreement within the scientific community about how important both NREM and REM sleep are for normal learning and memory (Siegel, 2001).
If people are deprived of REM sleep and then allowed to sleep without disturbance, they will spend more time in REM sleep in what would appear to be an effort to recoup the lost time in REM. This is known as the REM rebound, and it suggests that REM sleep is also homeostatically regulated. Aside from the role that REM sleep may play in processes related to learning and memory, REM sleep may also be involved in emotional processing and regulation. In such instances, REM rebound may actually represent an adaptive response to stress in nondepressed individuals by suppressing the emotional salience of aversive events that occurred in wakefulness (Suchecki, Tiba, & Machado, 2012).
While sleep deprivation in general is associated with a number of negative consequences (Brown, 2012), the consequences of REM deprivation appear to be less profound (as discussed in Siegel, 2001). In fact, some have suggested that REM deprivation can actually be beneficial in some circumstances. For instance, REM sleep deprivation has been demonstrated to improve symptoms of people suffering from major depression, and many effective antidepressant medications suppress REM sleep (Riemann, Berger, & Volderholzer, 2001; Vogel, 1975).
It should be pointed out that some reviews of the literature challenge this finding, suggesting that sleep deprivation that is not limited to REM sleep is just as effective or more effective at alleviating depressive symptoms among some patients suffering from depression. In either case, why sleep deprivation improves the mood of some patients is not entirely understood (Giedke & Schwärzler, 2002). Recently, however, some have suggested that sleep deprivation might change emotional processing so that various stimuli are more likely to be perceived as positive in nature (Gujar, Yoo, Hu, & Walker, 2011). The hypnogram below (Figure) shows a person’s passage through the stages of sleep.
View this video that describes the various stages of sleep.
Dreams
The meaning of dreams varies across different cultures and periods of time. By the late 19th century, German psychiatrist Sigmund Freud had become convinced that dreams represented an opportunity to gain access to the unconscious. By analyzing dreams, Freud thought people could increase self-awareness and gain valuable insight to help them deal with the problems they faced in their lives. Freud made distinctions between the manifest content and the latent content of dreams. Manifest content is the actual content, or storyline, of a dream. Latent content, on the other hand, refers to the hidden meaning of a dream. For instance, if a woman dreams about being chased by a snake, Freud might have argued that this represents the woman’s fear of sexual intimacy, with the snake serving as a symbol of a man’s penis.
Freud was not the only theorist to focus on the content of dreams. The 20th century Swiss psychiatrist Carl Jung believed that dreams allowed us to tap into the collective unconscious. The collective unconscious, as described by Jung, is a theoretical repository of information he believed to be shared by everyone. According to Jung, certain symbols in dreams reflected universal archetypes with meanings that are similar for all people regardless of culture or location.
The sleep and dreaming researcher Rosalind Cartwright, however, believes that dreams simply reflect life events that are important to the dreamer. Unlike Freud and Jung, Cartwright’s ideas about dreaming have found empirical support. For example, she and her colleagues published a study in which women going through divorce were asked several times over a five month period to report the degree to which their former spouses were on their minds. These same women were awakened during REM sleep in order to provide a detailed account of their dream content. There was a significant positive correlation between the degree to which women thought about their former spouses during waking hours and the number of times their former spouses appeared as characters in their dreams (Cartwright, Agargun, Kirkby, & Friedman, 2006). Recent research (Horikawa, Tamaki, Miyawaki, & Kamitani, 2013) has uncovered new techniques by which researchers may effectively detect and classify the visual images that occur during dreaming by using fMRI for neural measurement of brain activity patterns, opening the way for additional research in this area.
Recently, neuroscientists have also become interested in understanding why we dream. For example, Hobson (2009) suggests that dreaming may represent a state of protoconsciousness. In other words, dreaming involves constructing a virtual reality in our heads that we might use to help us during wakefulness. Among a variety of neurobiological evidence, John Hobson cites research on lucid dreams as an opportunity to better understand dreaming in general. Lucid dreams are dreams in which certain aspects of wakefulness are maintained during a dream state. In a lucid dream, a person becomes aware of the fact that they are dreaming, and as such, they can control the dream’s content (LaBerge, 1990).
Summary
The different stages of sleep are characterized by the patterns of brain waves associated with each stage. As a person transitions from being awake to falling asleep, alpha waves are replaced by theta waves. Sleep spindles and K-complexes emerge in stage 2 sleep. Stage 3 and stage 4 are described as slow-wave sleep that is marked by a predominance of delta waves. REM sleep involves rapid movements of the eyes, paralysis of voluntary muscles, and dreaming. Both NREM and REM sleep appear to play important roles in learning and memory. Dreams may represent life events that are important to the dreamer. Alternatively, dreaming may represent a state of protoconsciousness, or a virtual reality, in the mind that helps a person during consciousness.
Review Questions
________ is(are) described as slow-wave sleep.
- stage 1
- stage 2
- stage 3 and stage 4
- REM sleep
Hint:
C
Sleep spindles and K-complexes are most often associated with ________ sleep.
- stage 1
- stage 2
- stage 3 and stage 4
- REM
Hint:
B
Symptoms of ________ may be improved by REM deprivation.
- schizophrenia
- Parkinson’s disease
- depression
- generalized anxiety disorder
Hint:
C
The ________ content of a dream refers to the true meaning of the dream.
- latent
- manifest
- collective unconscious
- important
Hint:
A
Critical Thinking Questions
Freud believed that dreams provide important insight into the unconscious mind. He maintained that a dream’s manifest content could provide clues into an individual’s unconscious. What potential criticisms exist for this particular perspective?
Hint:
The subjective nature of dream analysis is one criticism. Psychoanalysts are charged with helping their clients interpret the true meaning of a dream. There is no way to refute or confirm whether or not these interpretations are accurate. The notion that “sometimes a cigar is just a cigar” (sometimes attributed to Freud but not definitively shown to be his) makes it clear that there is no systematic, objective system in place for dream analysis.
Some people claim that sleepwalking and talking in your sleep involve individuals acting out their dreams. Why is this particular explanation unlikely?
Hint:
Dreaming occurs during REM sleep. One of the hallmarks of this particular stage of sleep is the paralysis of the voluntary musculature which would make acting out dreams improbable.
Personal Application Question
Researchers believe that one important function of sleep is to facilitate learning and memory. How does knowing this help you in your college studies? What changes could you make to your study and sleep habits to maximize your mastery of the material covered in class?
|
oercommons
|
2025-03-18T00:37:58.448154
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15312/overview",
"title": "Psychology, States of Consciousness",
"author": null
}
|
https://oercommons.org/courseware/lesson/15313/overview
|
Sleep Problems and Disorders
Overview
By the end of this section, you will be able to:
- Describe the symptoms and treatments of insomnia
- Recognize the symptoms of several parasomnias
- Describe the symptoms and treatments for sleep apnea
- Recognize risk factors associated with sudden infant death syndrome (SIDS) and steps to prevent it
- Describe the symptoms and treatments for narcolepsy
Many people experience disturbances in their sleep at some point in their lives. Depending on the population and sleep disorder being studied, between 30% and 50% of the population suffers from a sleep disorder at some point in their lives (Bixler, Kales, Soldatos, Kaels, & Healey, 1979; Hossain & Shapiro, 2002; Ohayon, 1997, 2002; Ohayon & Roth, 2002). This section will describe several sleep disorders as well as some of their treatment options.
INSOMNIA
Insomnia, a consistent difficulty in falling or staying asleep, is the most common of the sleep disorders. Individuals with insomnia often experience long delays between the times that they go to bed and actually fall asleep. In addition, these individuals may wake up several times during the night only to find that they have difficulty getting back to sleep. As mentioned earlier, one of the criteria for insomnia involves experiencing these symptoms for at least three nights a week for at least one month’s time (Roth, 2007).
It is not uncommon for people suffering from insomnia to experience increased levels of anxiety about their inability to fall asleep. This becomes a self-perpetuating cycle because increased anxiety leads to increased arousal, and higher levels of arousal make the prospect of falling asleep even more unlikely. Chronic insomnia is almost always associated with feeling overtired and may be associated with symptoms of depression.
There may be many factors that contribute to insomnia, including age, drug use, exercise, mental status, and bedtime routines. Not surprisingly, insomnia treatment may take one of several different approaches. People who suffer from insomnia might limit their use of stimulant drugs (such as caffeine) or increase their amount of physical exercise during the day. Some people might turn to over-the-counter (OTC) or prescribed sleep medications to help them sleep, but this should be done sparingly because many sleep medications result in dependence and alter the nature of the sleep cycle, and they can increase insomnia over time. Those who continue to have insomnia, particularly if it affects their quality of life, should seek professional treatment.
Some forms of psychotherapy, such as cognitive-behavioral therapy, can help sufferers of insomnia. Cognitive-behavioral therapy is a type of psychotherapy that focuses on cognitive processes and problem behaviors. The treatment of insomnia likely would include stress management techniques and changes in problematic behaviors that could contribute to insomnia (e.g., spending more waking time in bed). Cognitive-behavioral therapy has been demonstrated to be quite effective in treating insomnia (Savard, Simard, Ivers, & Morin, 2005; Williams, Roth, Vatthauer, & McCrae, 2013).
PARASOMNIAS
A parasomnia is one of a group of sleep disorders in which unwanted, disruptive motor activity and/or experiences during sleep play a role. Parasomnias can occur in either REM or NREM phases of sleep. Sleepwalking, restless leg syndrome, and night terrors are all examples of parasomnias (Mahowald & Schenck, 2000).
Sleepwalking
In sleepwalking, or somnambulism, the sleeper engages in relatively complex behaviors ranging from wandering about to driving an automobile. During periods of sleepwalking, sleepers often have their eyes open, but they are not responsive to attempts to communicate with them. Sleepwalking most often occurs during slow-wave sleep, but it can occur at any time during a sleep period in some affected individuals (Mahowald & Schenck, 2000).
Historically, somnambulism has been treated with a variety of pharmacotherapies ranging from benzodiazepines to antidepressants. However, the success rate of such treatments is questionable. Guilleminault et al. (2005) found that sleepwalking was not alleviated with the use of benzodiazepines. However, all of their somnambulistic patients who also suffered from sleep-related breathing problems showed a marked decrease in sleepwalking when their breathing problems were effectively treated.
A Sleepwalking Defense?
On January 16, 1997, Scott Falater sat down to dinner with his wife and children and told them about difficulties he was experiencing on a project at work. After dinner, he prepared some materials to use in leading a church youth group the following morning, and then he attempted repair the family’s swimming pool pump before retiring to bed. The following morning, he awoke to barking dogs and unfamiliar voices from downstairs. As he went to investigate what was going on, he was met by a group of police officers who arrested him for the murder of his wife (Cartwright, 2004; CNN, 1999).
Yarmila Falater’s body was found in the family’s pool with 44 stab wounds. A neighbor called the police after witnessing Falater standing over his wife’s body before dragging her into the pool. Upon a search of the premises, police found blood-stained clothes and a bloody knife in the trunk of Falater’s car, and he had blood stains on his neck.
Remarkably, Falater insisted that he had no recollection of hurting his wife in any way. His children and his wife’s parents all agreed that Falater had an excellent relationship with his wife and they couldn’t think of a reason that would provide any sort of motive to murder her (Cartwright, 2004).
Scott Falater had a history of regular episodes of sleepwalking as a child, and he had even behaved violently toward his sister once when she tried to prevent him from leaving their home in his pajamas during a sleepwalking episode. He suffered from no apparent anatomical brain anomalies or psychological disorders. It appeared that Scott Falater had killed his wife in his sleep, or at least, that is the defense he used when he was tried for his wife’s murder (Cartwright, 2004; CNN, 1999). In Falater’s case, a jury found him guilty of first degree murder in June of 1999 (CNN, 1999); however, there are other murder cases where the sleepwalking defense has been used successfully. As scary as it sounds, many sleep researchers believe that homicidal sleepwalking is possible in individuals suffering from the types of sleep disorders described below (Broughton et al., 1994; Cartwright, 2004; Mahowald, Schenck, & Cramer Bornemann, 2005; Pressman, 2007).
REM Sleep Behavior Disorder (RBD)
REM sleep behavior disorder (RBD) occurs when the muscle paralysis associated with the REM sleep phase does not occur. Individuals who suffer from RBD have high levels of physical activity during REM sleep, especially during disturbing dreams. These behaviors vary widely, but they can include kicking, punching, scratching, yelling, and behaving like an animal that has been frightened or attacked. People who suffer from this disorder can injure themselves or their sleeping partners when engaging in these behaviors. Furthermore, these types of behaviors ultimately disrupt sleep, although affected individuals have no memories that these behaviors have occurred (Arnulf, 2012).
This disorder is associated with a number of neurodegenerative diseases such as Parkinson’s disease. In fact, this relationship is so robust that some view the presence of RBD as a potential aid in the diagnosis and treatment of a number of neurodegenerative diseases (Ferini-Strambi, 2011). Clonazepam, an anti-anxiety medication with sedative properties, is most often used to treat RBD. It is administered alone or in conjunction with doses of melatonin (the hormone secreted by the pineal gland). As part of treatment, the sleeping environment is often modified to make it a safer place for those suffering from RBD (Zangini, Calandra-Buonaura, Grimaldi, & Cortelli, 2011).
Other Parasomnias
A person with restless leg syndrome has uncomfortable sensations in the legs during periods of inactivity or when trying to fall asleep. This discomfort is relieved by deliberately moving the legs, which, not surprisingly, contributes to difficulty in falling or staying asleep. Restless leg syndrome is quite common and has been associated with a number of other medical diagnoses, such as chronic kidney disease and diabetes (Mahowald & Schenck, 2000). There are a variety of drugs that treat restless leg syndrome: benzodiazepines, opiates, and anticonvulsants (Restless Legs Syndrome Foundation, n.d.).
Night terrors result in a sense of panic in the sufferer and are often accompanied by screams and attempts to escape from the immediate environment (Mahowald & Schenck, 2000). Although individuals suffering from night terrors appear to be awake, they generally have no memories of the events that occurred, and attempts to console them are ineffective. Typically, individuals suffering from night terrors will fall back asleep again within a short time. Night terrors apparently occur during the NREM phase of sleep (Provini, Tinuper, Bisulli, & Lagaresi, 2011)Generally, treatment for night terrors is unnecessary unless there is some underlying medical or psychological condition that is contributing to the night terrors (Mayo Clinic, n.d.).
SLEEP APNEA
Sleep apnea is defined by episodes during which a sleeper’s breathing stops. These episodes can last 10–20 seconds or longer and often are associated with brief periods of arousal. While individuals suffering from sleep apnea may not be aware of these repeated disruptions in sleep, they do experience increased levels of fatigue. Many individuals diagnosed with sleep apnea first seek treatment because their sleeping partners indicate that they snore loudly and/or stop breathing for extended periods of time while sleeping (Henry & Rosenthal, 2013). Sleep apnea is much more common in overweight people and is often associated with loud snoring. Surprisingly, sleep apnea may exacerbate cardiovascular disease (Sánchez-de-la-Torre, Campos-Rodriguez, & Barbé, 2012). While sleep apnea is less common in thin people, anyone, regardless of their weight, who snores loudly or gasps for air while sleeping, should be checked for sleep apnea.
While people are often unaware of their sleep apnea, they are keenly aware of some of the adverse consequences of insufficient sleep. Consider a patient who believed that as a result of his sleep apnea he “had three car accidents in six weeks. They were ALL my fault. Two of them I didn’t even know I was involved in until afterwards” (Henry & Rosenthal, 2013, p. 52). It is not uncommon for people suffering from undiagnosed or untreated sleep apnea to fear that their careers will be affected by the lack of sleep, illustrated by this statement from another patient, “I’m in a job where there’s a premium on being mentally alert. I was really sleepy… and having trouble concentrating…. It was getting to the point where it was kind of scary” (Henry & Rosenthal, 2013, p. 52).
There are two types of sleep apnea: obstructive sleep apnea and central sleep apnea. Obstructive sleep apnea occurs when an individual’s airway becomes blocked during sleep, and air is prevented from entering the lungs. In central sleep apnea, disruption in signals sent from the brain that regulate breathing cause periods of interrupted breathing (White, 2005).
One of the most common treatments for sleep apnea involves the use of a special device during sleep. A continuous positive airway pressure (CPAP) device includes a mask that fits over the sleeper’s nose and mouth, which is connected to a pump that pumps air into the person’s airways, forcing them to remain open, as shown in Figure. Some newer CPAP masks are smaller and cover only the nose. This treatment option has proven to be effective for people suffering from mild to severe cases of sleep apnea (McDaid et al., 2009). However, alternative treatment options are being explored because consistent compliance by users of CPAP devices is a problem. Recently, a new EPAP (expiratory positive air pressure) device has shown promise in double-blind trials as one such alternative (Berry, Kryger, & Massie, 2011).
SIDS
In sudden infant death syndrome (SIDS) an infant stops breathing during sleep and dies. Infants younger than 12 months appear to be at the highest risk for SIDS, and boys have a greater risk than girls. A number of risk factors have been associated with SIDS including premature birth, smoking within the home, and hyperthermia. There may also be differences in both brain structure and function in infants that die from SIDS (Berkowitz, 2012; Mage & Donner, 2006; Thach, 2005).
The substantial amount of research on SIDS has led to a number of recommendations to parents to protect their children (Figure). For one, research suggests that infants should be placed on their backs when put down to sleep, and their cribs should not contain any items which pose suffocation threats, such as blankets, pillows or padded crib bumpers (cushions that cover the bars of a crib). Infants should not have caps placed on their heads when put down to sleep in order to prevent overheating, and people in the child’s household should abstain from smoking in the home. Recommendations like these have helped to decrease the number of infant deaths from SIDS in recent years (Mitchell, 2009; Task Force on Sudden Infant Death Syndrome, 2011).
NARCOLEPSY
Unlike the other sleep disorders described in this section, a person with narcolepsy cannot resist falling asleep at inopportune times. These sleep episodes are often associated with cataplexy, which is a lack of muscle tone or muscle weakness, and in some cases involves complete paralysis of the voluntary muscles. This is similar to the kind of paralysis experienced by healthy individuals during REM sleep (Burgess & Scammell, 2012; Hishikawa & Shimizu, 1995; Luppi et al., 2011). Narcoleptic episodes take on other features of REM sleep. For example, around one third of individuals diagnosed with narcolepsy experience vivid, dream-like hallucinations during narcoleptic attacks (Chokroverty, 2010).
Surprisingly, narcoleptic episodes are often triggered by states of heightened arousal or stress. The typical episode can last from a minute or two to half an hour. Once awakened from a narcoleptic attack, people report that they feel refreshed (Chokroverty, 2010). Obviously, regular narcoleptic episodes could interfere with the ability to perform one’s job or complete schoolwork, and in some situations, narcolepsy can result in significant harm and injury (e.g., driving a car or operating machinery or other potentially dangerous equipment).
Generally, narcolepsy is treated using psychomotor stimulant drugs, such as amphetamines (Mignot, 2012). These drugs promote increased levels of neural activity. Narcolepsy is associated with reduced levels of the signaling molecule hypocretin in some areas of the brain (De la Herrán-Arita & Drucker-Colín, 2012; Han, 2012), and the traditional stimulant drugs do not have direct effects on this system. Therefore, it is quite likely that new medications that are developed to treat narcolepsy will be designed to target the hypocretin system.
There is a tremendous amount of variability among sufferers, both in terms of how symptoms of narcolepsy manifest and the effectiveness of currently available treatment options. This is illustrated by McCarty’s (2010) case study of a 50-year-old woman who sought help for the excessive sleepiness during normal waking hours that she had experienced for several years. She indicated that she had fallen asleep at inappropriate or dangerous times, including while eating, while socializing with friends, and while driving her car. During periods of emotional arousal, the woman complained that she felt some weakness in the right side of her body. Although she did not experience any dream-like hallucinations, she was diagnosed with narcolepsy as a result of sleep testing. In her case, the fact that her cataplexy was confined to the right side of her body was quite unusual. Early attempts to treat her condition with a stimulant drug alone were unsuccessful. However, when a stimulant drug was used in conjunction with a popular antidepressant, her condition improved dramatically.
Summary
Many individuals suffer from some type of sleep disorder or disturbance at some point in their lives. Insomnia is a common experience in which people have difficulty falling or staying asleep. Parasomnias involve unwanted motor behavior or experiences throughout the sleep cycle and include RBD, sleepwalking, restless leg syndrome, and night terrors. Sleep apnea occurs when individuals stop breathing during their sleep, and in the case of sudden infant death syndrome, infants will stop breathing during sleep and die. Narcolepsy involves an irresistible urge to fall asleep during waking hours and is often associated with cataplexy and hallucination.
Review Questions
________ is loss of muscle tone or control that is often associated with narcolepsy.
- RBD
- CPAP
- cataplexy
- insomnia
Hint:
C
An individual may suffer from ________ if there is a disruption in the brain signals that are sent to the muscles that regulate breathing.
- central sleep apnea
- obstructive sleep apnea
- narcolepsy
- SIDS
Hint:
A
The most common treatment for ________ involves the use of amphetamine-like medications.
- sleep apnea
- RBD
- SIDS
- narcolepsy
Hint:
D
________ is another word for sleepwalking.
- insomnia
- somnambulism
- cataplexy
- narcolepsy
Hint:
B
Critical Thinking Questions
One of the recommendations that therapists will make to people who suffer from insomnia is to spend less waking time in bed. Why do you think spending waking time in bed might interfere with the ability to fall asleep later?
Hint:
Answers will vary. One possible explanation might invoke principles of associative learning. If the bed represents a place for socializing, studying, eating, and so on, then it is possible that it will become a place that elicits higher levels of arousal, which would make falling asleep at the appropriate time more difficult. Answers could also consider self-perpetuating cycle referred to when describing insomnia. If an individual is having trouble falling asleep and that generates anxiety, it might make sense to remove him from the context where sleep would normally take place to try to avoid anxiety being associated with that context.
How is narcolepsy with cataplexy similar to and different from REM sleep?
Hint:
Similarities include muscle atony and the hypnagogic hallucinations associated with narcoleptic episodes. The differences involve the uncontrollable nature of narcoleptic attacks and the fact that these come on in situations that would normally not be associated with sleep of any kind (e.g., instances of heightened arousal or emotionality).
Personal Application Question
What factors might contribute to your own experiences with insomnia?
|
oercommons
|
2025-03-18T00:37:58.482170
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15313/overview",
"title": "Psychology, States of Consciousness",
"author": null
}
|
https://oercommons.org/courseware/lesson/15314/overview
|
Substance Use and Abuse
Overview
By the end of this section, you will be able to:
- Describe the diagnostic criteria for substance use disorders
- Identify the neurotransmitter systems affected by various categories of drugs
- Describe how different categories of drugs effect behavior and experience
While we all experience altered states of consciousness in the form of sleep on a regular basis, some people use drugs and other substances that result in altered states of consciousness as well. This section will present information relating to the use of various psychoactive drugs and problems associated with such use. This will be followed by brief descriptions of the effects of some of the more well-known drugs commonly used today.
SUBSTANCE USE DISORDERS
The fifth edition of the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) is used by clinicians to diagnose individuals suffering from various psychological disorders. Drug use disorders are addictive disorders, and the criteria for specific substance (drug) use disorders are described in DSM-5. A person who has a substance use disorder often uses more of the substance than they originally intended to and continues to use that substance despite experiencing significant adverse consequences. In individuals diagnosed with a substance use disorder, there is a compulsive pattern of drug use that is often associated with both physical and psychological dependence.
Physical dependence involves changes in normal bodily functions—the user will experience withdrawal from the drug upon cessation of use. In contrast, a person who has psychological dependence has an emotional, rather than physical, need for the drug and may use the drug to relieve psychological distress. Tolerance is linked to physiological dependence, and it occurs when a person requires more and more drug to achieve effects previously experienced at lower doses. Tolerance can cause the user to increase the amount of drug used to a dangerous level—even to the point of overdose and death.
Drug withdrawal includes a variety of negative symptoms experienced when drug use is discontinued. These symptoms usually are opposite of the effects of the drug. For example, withdrawal from sedative drugs often produces unpleasant arousal and agitation. In addition to withdrawal, many individuals who are diagnosed with substance use disorders will also develop tolerance to these substances. Psychological dependence, or drug craving, is a recent addition to the diagnostic criteria for substance use disorder in DSM-5. This is an important factor because we can develop tolerance and experience withdrawal from any number of drugs that we do not abuse. In other words, physical dependence in and of itself is of limited utility in determining whether or not someone has a substance use disorder.
DRUG CATEGORIES
The effects of all psychoactive drugs occur through their interactions with our endogenous neurotransmitter systems. Many of these drugs, and their relationships, are shown in Figure. As you have learned, drugs can act as agonists or antagonists of a given neurotransmitter system. An agonist facilitates the activity of a neurotransmitter system, and antagonists impede neurotransmitter activity.
Alcohol and Other Depressants
Ethanol, which we commonly refer to as alcohol, is in a class of psychoactive drugs known as depressants (Figure). A depressant is a drug that tends to suppress central nervous system activity. Other depressants include barbiturates and benzodiazepines. These drugs share in common their ability to serve as agonists of the gamma-Aminobutyric acid (GABA) neurotransmitter system. Because GABA has a quieting effect on the brain, GABA agonists also have a quieting effect; these types of drugs are often prescribed to treat both anxiety and insomnia.
Acute alcohol administration results in a variety of changes to consciousness. At rather low doses, alcohol use is associated with feelings of euphoria. As the dose increases, people report feeling sedated. Generally, alcohol is associated with decreases in reaction time and visual acuity, lowered levels of alertness, and reduction in behavioral control. With excessive alcohol use, a person might experience a complete loss of consciousness and/or difficulty remembering events that occurred during a period of intoxication (McKim & Hancock, 2013). In addition, if a pregnant woman consumes alcohol, her infant may be born with a cluster of birth defects and symptoms collectively called fetal alcohol spectrum disorder (FASD) or fetal alcohol syndrome (FAS).
With repeated use of many central nervous system depressants, such as alcohol, a person becomes physically dependent upon the substance and will exhibit signs of both tolerance and withdrawal. Psychological dependence on these drugs is also possible. Therefore, the abuse potential of central nervous system depressants is relatively high.
Drug withdrawal is usually an aversive experience, and it can be a life-threatening process in individuals who have a long history of very high doses of alcohol and/or barbiturates. This is of such concern that people who are trying to overcome addiction to these substances should only do so under medical supervision.
Stimulants
Stimulants are drugs that tend to increase overall levels of neural activity. Many of these drugs act as agonists of the dopamine neurotransmitter system. Dopamine activity is often associated with reward and craving; therefore, drugs that affect dopamine neurotransmission often have abuse liability. Drugs in this category include cocaine, amphetamines (including methamphetamine), cathinones (i.e., bath salts), MDMA (ecstasy), nicotine, and caffeine.
Cocaine can be taken in multiple ways. While many users snort cocaine, intravenous injection and ingestion are also common. The freebase version of cocaine, known as crack, is a potent, smokable version of the drug. Like many other stimulants, cocaine agonizes the dopamine neurotransmitter system by blocking the reuptake of dopamine in the neuronal synapse.
Crack Cocaine
Crack (Figure) is often considered to be more addictive than cocaine itself because it is smokable and reaches the brain very quickly. Crack is often less expensive than other forms of cocaine; therefore, it tends to be a more accessible drug for individuals from impoverished segments of society. During the 1980s, many drug laws were rewritten to punish crack users more severely than cocaine users. This led to discriminatory sentencing with low-income, inner-city minority populations receiving the harshest punishments. The wisdom of these laws has recently been called into question, especially given research that suggests crack may not be more addictive than other forms of cocaine, as previously thought (Haasen & Krausz, 2001; Reinerman, 2007).
Read this interesting newspaper article describing myths about crack cocaine.
Amphetamines have a mechanism of action quite similar to cocaine in that they block the reuptake of dopamine in addition to stimulating its release (Figure). While amphetamines are often abused, they are also commonly prescribed to children diagnosed with attention deficit hyperactivity disorder (ADHD). It may seem counterintuitive that stimulant medications are prescribed to treat a disorder that involves hyperactivity, but the therapeutic effect comes from increases in neurotransmitter activity within certain areas of the brain associated with impulse control.
In recent years, methamphetamine (meth) use has become increasingly widespread. Methamphetamine is a type of amphetamine that can be made from ingredients that are readily available (e.g., medications containing pseudoephedrine, a compound found in many over-the-counter cold and flu remedies). Despite recent changes in laws designed to make obtaining pseudoephedrine more difficult, methamphetamine continues to be an easily accessible and relatively inexpensive drug option (Shukla, Crump, & Chrisco, 2012).
The cocaine, amphetamine, cathinones, and MDMA users seek a euphoric high, feelings of intense elation and pleasure, especially in those users who take the drug via intravenous injection or smoking. Repeated use of these stimulants can have significant adverse consequences. Users can experience physical symptoms that include nausea, elevated blood pressure, and increased heart rate. In addition, these drugs can cause feelings of anxiety, hallucinations, and paranoia (Fiorentini et al., 2011). Normal brain functioning is altered after repeated use of these drugs. For example, repeated use can lead to overall depletion among the monoamine neurotransmitters (dopamine, norepinephrine, and serotonin). People may engage in compulsive use of these stimulant substances in part to try to reestablish normal levels of these neurotransmitters (Jayanthi & Ramamoorthy, 2005; Rothman, Blough, & Baumann, 2007).
Caffeine is another stimulant drug. While it is probably the most commonly used drug in the world, the potency of this particular drug pales in comparison to the other stimulant drugs described in this section. Generally, people use caffeine to maintain increased levels of alertness and arousal. Caffeine is found in many common medicines (such as weight loss drugs), beverages, foods, and even cosmetics (Herman & Herman, 2013). While caffeine may have some indirect effects on dopamine neurotransmission, its primary mechanism of action involves antagonizing adenosine activity (Porkka-Heiskanen, 2011).
While caffeine is generally considered a relatively safe drug, high blood levels of caffeine can result in insomnia, agitation, muscle twitching, nausea, irregular heartbeat, and even death (Reissig, Strain, & Griffiths, 2009; Wolt, Ganetsky, & Babu, 2012). In 2012, Kromann and Nielson reported on a case study of a 40-year-old woman who suffered significant ill effects from her use of caffeine. The woman used caffeine in the past to boost her mood and to provide energy, but over the course of several years, she increased her caffeine consumption to the point that she was consuming three liters of soda each day. Although she had been taking a prescription antidepressant, her symptoms of depression continued to worsen and she began to suffer physically, displaying significant warning signs of cardiovascular disease and diabetes. Upon admission to an outpatient clinic for treatment of mood disorders, she met all of the diagnostic criteria for substance dependence and was advised to dramatically limit her caffeine intake. Once she was able to limit her use to less than 12 ounces of soda a day, both her mental and physical health gradually improved. Despite the prevalence of caffeine use and the large number of people who confess to suffering from caffeine addiction, this was the first published description of soda dependence appearing in scientific literature.
Nicotine is highly addictive, and the use of tobacco products is associated with increased risks of heart disease, stroke, and a variety of cancers. Nicotine exerts its effects through its interaction with acetylcholine receptors. Acetylcholine functions as a neurotransmitter in motor neurons. In the central nervous system, it plays a role in arousal and reward mechanisms. Nicotine is most commonly used in the form of tobacco products like cigarettes or chewing tobacco; therefore, there is a tremendous interest in developing effective smoking cessation techniques. To date, people have used a variety of nicotine replacement therapies in addition to various psychotherapeutic options in an attempt to discontinue their use of tobacco products. In general, smoking cessation programs may be effective in the short term, but it is unclear whether these effects persist (Cropley, Theadom, Pravettoni, & Webb, 2008; Levitt, Shaw, Wong, & Kaczorowski, 2007; Smedslund, Fisher, Boles, & Lichtenstein, 2004).
Opioids
An opioid is one of a category of drugs that includes heroin, morphine, methadone, and codeine. Opioids have analgesic properties; that is, they decrease pain. Humans have an endogenous opioid neurotransmitter system—the body makes small quantities of opioid compounds that bind to opioid receptors reducing pain and producing euphoria. Thus, opioid drugs, which mimic this endogenous painkilling mechanism, have an extremely high potential for abuse. Natural opioids, called opiates, are derivatives of opium, which is a naturally occurring compound found in the poppy plant. There are now several synthetic versions of opiate drugs (correctly called opioids) that have very potent painkilling effects, and they are often abused. For example, the National Institutes of Drug Abuse has sponsored research that suggests the misuse and abuse of the prescription pain killers hydrocodone and oxycodone are significant public health concerns (Maxwell, 2006). In 2013, the U.S. Food and Drug Administration recommended tighter controls on their medical use.
Historically, heroin has been a major opioid drug of abuse (Figure). Heroin can be snorted, smoked, or injected intravenously. Like the stimulants described earlier, the use of heroin is associated with an initial feeling of euphoria followed by periods of agitation. Because heroin is often administered via intravenous injection, users often bear needle track marks on their arms and, like all abusers of intravenous drugs, have an increased risk for contraction of both tuberculosis and HIV.
Aside from their utility as analgesic drugs, opioid-like compounds are often found in cough suppressants, anti-nausea, and anti-diarrhea medications. Given that withdrawal from a drug often involves an experience opposite to the effect of the drug, it should be no surprise that opioid withdrawal resembles a severe case of the flu. While opioid withdrawal can be extremely unpleasant, it is not life-threatening (Julien, 2005). Still, people experiencing opioid withdrawal may be given methadone to make withdrawal from the drug less difficult. Methadone is a synthetic opioid that is less euphorigenic than heroin and similar drugs. Methadone clinics help people who previously struggled with opioid addiction manage withdrawal symptoms through the use of methadone. Other drugs, including the opioid buprenorphine, have also been used to alleviate symptoms of opiate withdrawal.
Codeine is an opioid with relatively low potency. It is often prescribed for minor pain, and it is available over-the-counter in some other countries. Like all opioids, codeine does have abuse potential. In fact, abuse of prescription opioid medications is becoming a major concern worldwide (Aquina, Marques-Baptista, Bridgeman, & Merlin, 2009; Casati, Sedefov, & Pfeiffer-Gerschel, 2012).
Hallucinogens
A hallucinogen is one of a class of drugs that results in profound alterations in sensory and perceptual experiences (Figure). In some cases, users experience vivid visual hallucinations. It is also common for these types of drugs to cause hallucinations of body sensations (e.g., feeling as if you are a giant) and a skewed perception of the passage of time.
As a group, hallucinogens are incredibly varied in terms of the neurotransmitter systems they affect. Mescaline and LSD are serotonin agonists, and PCP (angel dust) and ketamine (an animal anesthetic) act as antagonists of the NMDA glutamate receptor. In general, these drugs are not thought to possess the same sort of abuse potential as other classes of drugs discussed in this section.
To learn more about some of the most commonly abused prescription and street drugs, check out the Commonly Abused Drugs Chart and the Commonly Abused Prescription Drugs Chart from the National Institute on Drug Abuse.
Medical Marijuana
While the possession and use of marijuana is illegal in most states, it is now legal in Washington and Colorado to possess limited quantities of marijuana for recreational use (Figure). In contrast, medical marijuana use is now legal in nearly half of the United States and in the District of Columbia. Medical marijuana is marijuana that is prescribed by a doctor for the treatment of a health condition. For example, people who undergo chemotherapy will often be prescribed marijuana to stimulate their appetites and prevent excessive weight loss resulting from the side effects of chemotherapy treatment. Marijuana may also have some promise in the treatment of a variety of medical conditions (Mather, Rauwendaal, Moxham-Hall, & Wodak, 2013; Robson, 2014; Schicho & Storr, 2014).
While medical marijuana laws have been passed on a state-by-state basis, federal laws still classify this as an illicit substance, making conducting research on the potentially beneficial medicinal uses of marijuana problematic. There is quite a bit of controversy within the scientific community as to the extent to which marijuana might have medicinal benefits due to a lack of large-scale, controlled research (Bostwick, 2012). As a result, many scientists have urged the federal government to allow for relaxation of current marijuana laws and classifications in order to facilitate a more widespread study of the drug’s effects (Aggarwal et al., 2009; Bostwick, 2012; Kogan & Mechoulam, 2007).
Until recently, the United States Department of Justice routinely arrested people involved and seized marijuana used in medicinal settings. In the latter part of 2013, however, the United States Department of Justice issued statements indicating that they would not continue to challenge state medical marijuana laws. This shift in policy may be in response to the scientific community’s recommendations and/or reflect changing public opinion regarding marijuana.
Summary
Substance use disorder is defined in DSM-5 as a compulsive pattern of drug use despite negative consequences. Both physical and psychological dependence are important parts of this disorder. Alcohol, barbiturates, and benzodiazepines are central nervous system depressants that affect GABA neurotransmission. Cocaine, amphetamine, cathinones, and MDMA are all central nervous stimulants that agonize dopamine neurotransmission, while nicotine and caffeine affect acetylcholine and adenosine, respectively. Opiate drugs serve as powerful analgesics through their effects on the endogenous opioid neurotransmitter system, and hallucinogenic drugs cause pronounced changes in sensory and perceptual experiences. The hallucinogens are variable with regards to the specific neurotransmitter systems they affect.
Review Questions
________ occurs when a drug user requires more and more of a given drug in order to experience the same effects of the drug.
- withdrawal
- psychological dependence
- tolerance
- reuptake
Hint:
C
Cocaine blocks the reuptake of ________.
- GABA
- glutamate
- acetylcholine
- dopamine
Hint:
D
________ refers to drug craving.
- psychological dependence
- antagonism
- agonism
- physical dependence
Hint:
A
LSD affects ________ neurotransmission.
- dopamine
- serotonin
- acetylcholine
- norepinephrine
Hint:
B
Critical Thinking Questions
The negative health consequences of both alcohol and tobacco products are well-documented. A drug like marijuana, on the other hand, is generally considered to be as safe, if not safer than these legal drugs. Why do you think marijuana use continues to be illegal in many parts of the United States?
Hint:
One possibility involves the cultural acceptance and long history of alcohol and tobacco use in our society. No doubt, money comes into play as well. Growing tobacco and producing alcohol on a large scale is a well-regulated and taxed process. Given that marijuana is essentially a weed that requires little care to grow, it would be much more difficult to regulate its production. Recent events suggest that cultural attitudes regarding marijuana are changing, and it is quite likely that its illicit status will be adapted accordingly.
Why are programs designed to educate people about the dangers of using tobacco products just as important as developing tobacco cessation programs?
Hint:
Given that currently available programs designed to help people quit using tobacco products are not necessarily effective in the long term, programs designed to prevent people from using these products in the first place may be the best hope for dealing with the enormous public health concerns associated with tobacco use.
Personal Application Question
Many people experiment with some sort of psychoactive substance at some point in their lives. Why do you think people are motivated to use substances that alter consciousness?
|
oercommons
|
2025-03-18T00:37:58.517668
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15314/overview",
"title": "Psychology, States of Consciousness",
"author": null
}
|
https://oercommons.org/courseware/lesson/15315/overview
|
Other States of Consciousness
Overview
By the end of this section, you will be able to:
- Define hypnosis and meditation
- Understand the similarities and differences of hypnosis and meditation
Our states of consciousness change as we move from wakefulness to sleep. We also alter our consciousness through the use of various psychoactive drugs. This final section will consider hypnotic and meditative states as additional examples of altered states of consciousness experienced by some individuals.
HYPNOSIS
Hypnosis is a state of extreme self-focus and attention in which minimal attention is given to external stimuli. In the therapeutic setting, a clinician may use relaxation and suggestion in an attempt to alter the thoughts and perceptions of a patient. Hypnosis has also been used to draw out information believed to be buried deeply in someone’s memory. For individuals who are especially open to the power of suggestion, hypnosis can prove to be a very effective technique, and brain imaging studies have demonstrated that hypnotic states are associated with global changes in brain functioning (Del Casale et al., 2012; Guldenmund, Vanhaudenhuyse, Boly, Laureys, & Soddu, 2012).
Historically, hypnosis has been viewed with some suspicion because of its portrayal in popular media and entertainment (Figure). Therefore, it is important to make a distinction between hypnosis as an empirically based therapeutic approach versus as a form of entertainment. Contrary to popular belief, individuals undergoing hypnosis usually have clear memories of the hypnotic experience and are in control of their own behaviors. While hypnosis may be useful in enhancing memory or a skill, such enhancements are very modest in nature (Raz, 2011).
How exactly does a hypnotist bring a participant to a state of hypnosis? While there are variations, there are four parts that appear consistent in bringing people into the state of suggestibility associated with hypnosis (National Research Council, 1994). These components include:
- The participant is guided to focus on one thing, such as the hypnotist’s words or a ticking watch.
- The participant is made comfortable and is directed to be relaxed and sleepy.
- The participant is told to be open to the process of hypnosis, trust the hypnotist and let go.
- The participant is encouraged to use his or her imagination.
These steps are conducive to being open to the heightened suggestibility of hypnosis.
People vary in terms of their ability to be hypnotized, but a review of available research suggests that most people are at least moderately hypnotizable (Kihlstrom, 2013). Hypnosis in conjunction with other techniques is used for a variety of therapeutic purposes and has shown to be at least somewhat effective for pain management, treatment of depression and anxiety, smoking cessation, and weight loss (Alladin, 2012; Elkins, Johnson, & Fisher, 2012; Golden, 2012; Montgomery, Schnur, & Kravits, 2012).
Some scientists are working to determine whether the power of suggestion can affect cognitive processes such as learning, with a view to using hypnosis in educational settings (Wark, 2011). Furthermore, there is some evidence that hypnosis can alter processes that were once thought to be automatic and outside the purview of voluntary control, such as reading (Lifshitz, Aubert Bonn, Fischer, Kashem, & Raz, 2013; Raz, Shapiro, Fan, & Posner, 2002). However, it should be noted that others have suggested that the automaticity of these processes remains intact (Augustinova & Ferrand, 2012).
How does hypnosis work? Two theories attempt to answer this question: One theory views hypnosis as dissociation and the other theory views it as the performance of a social role. According to the dissociation view, hypnosis is effectively a dissociated state of consciousness, much like our earlier example where you may drive to work, but you are only minimally aware of the process of driving because your attention is focused elsewhere. This theory is supported by Ernest Hilgard’s research into hypnosis and pain. In Hilgard’s experiments, he induced participants into a state of hypnosis, and placed their arms into ice water. Participants were told they would not feel pain, but they could press a button if they did; while they reported not feeling pain, they did, in fact, press the button, suggesting a dissociation of consciousness while in the hypnotic state (Hilgard & Hilgard, 1994).
Taking a different approach to explain hypnosis, the social-cognitive theory of hypnosis sees people in hypnotic states as performing the social role of a hypnotized person. As you will learn when you study social roles, people’s behavior can be shaped by their expectations of how they should act in a given situation. Some view a hypnotized person’s behavior not as an altered or dissociated state of consciousness, but as their fulfillment of the social expectations for that role.
MEDITATION
Meditation is the act of focusing on a single target (such as the breath or a repeated sound) to increase awareness of the moment. While hypnosis is generally achieved through the interaction of a therapist and the person being treated, an individual can perform meditation alone. Often, however, people wishing to learn to meditate receive some training in techniques to achieve a meditative state. A meditative state, as shown by EEG recordings of newly-practicing meditators, is not an altered state of consciousness per se; however, patterns of brain waves exhibited by expert meditators may represent a unique state of consciousness (Fell, Axmacher, & Haupt, 2010).
Although there are a number of different techniques in use, the central feature of all meditation is clearing the mind in order to achieve a state of relaxed awareness and focus (Chen et al., 2013; Lang et al., 2012). Mindfulness meditation has recently become popular. In the variation of meditation, the meditator’s attention is focused on some internal process or an external object (Zeidan, Grant, Brown, McHaffie, & Coghill, 2012).
Meditative techniques have their roots in religious practices (Figure), but their use has grown in popularity among practitioners of alternative medicine. Research indicates that meditation may help reduce blood pressure, and the American Heart Association suggests that meditation might be used in conjunction with more traditional treatments as a way to manage hypertension, although there is not sufficient data for a recommendation to be made (Brook et al., 2013). Like hypnosis, meditation also shows promise in stress management, sleep quality (Caldwell, Harrison, Adams, Quin, & Greeson, 2010), treatment of mood and anxiety disorders (Chen et al., 2013; Freeman et al., 2010; Vøllestad, Nielsen, & Nielsen, 2012), and pain management (Reiner, Tibi, & Lipsitz, 2013).
Feeling stressed? Think meditation might help? This instructional video teaches how to use Buddhist meditation techniques to alleviate stress.
Watch this video describe the results of a brain imaging study in individuals who underwent specific mindfulness-meditative techniques.
Summary
Hypnosis is a focus on the self that involves suggested changes of behavior and experience. Meditation involves relaxed, yet focused, awareness. Both hypnotic and meditative states may involve altered states of consciousness that have potential application for the treatment of a variety of physical and psychological disorders.
Review Questions
________ is most effective in individuals that are very open to the power of suggestion.
- hypnosis
- meditation
- mindful awareness
- cognitive therapy
Hint:
A
________ has its roots in religious practice.
- hypnosis
- meditation
- cognitive therapy
- behavioral therapy
Hint:
B
Meditation may be helpful in ________.
- pain management
- stress control
- treating the flu
- both a and b
Hint:
D
Research suggests that cognitive processes, such as learning, may be affected by ________.
- hypnosis
- meditation
- mindful awareness
- progressive relaxation
Hint:
A
Critical Thinking Questions
What advantages exist for researching the potential health benefits of hypnosis?
Hint:
Healthcare and pharmaceutical costs continue to skyrocket. If alternative approaches to dealing with these problems could be developed that would be relatively inexpensive, then the potential benefits are many.
What types of studies would be most convincing regarding the effectiveness of meditation in the treatment for some type of physical or mental disorder?
Hint:
Ideally, double-blind experimental trials would be best suited to speak to the effectiveness of meditation. At the very least, some sort of randomized control trial would be very informative.
Personal Application Question
Under what circumstances would you be willing to consider hypnosis and/or meditation as a treatment option? What kind of information would you need before you made a decision to use these techniques?
|
oercommons
|
2025-03-18T00:37:58.545677
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15315/overview",
"title": "Psychology, States of Consciousness",
"author": null
}
|
https://oercommons.org/courseware/lesson/15493/overview
|
Introduction
Overview
- Urbanization and Its Challenges
- The African American “Great Migration” and New European Immigration
- Relief from the Chaos of Urban Life
- Change Reflected in Thought and Writing
“We saw the big woman with spikes on her head.” So begins Sadie Frowne’s first memory of arriving in the United States. Many Americans experienced in their new home what the thirteen-year-old Polish girl had seen in the silhouette of the Statue of Liberty (Figure): a wondrous world of new opportunities fraught with dangers. Sadie and her mother, for instance, had left Poland after her father’s death. Her mother died shortly thereafter, and Sadie had to find her own way in New York, working in factories and slowly assimilating to life in a vast multinational metropolis. Her story is similar to millions of others, as people came to the United States seeking a better future than the one they had at home.
The future they found, however, was often grim. While many believed in the land of opportunity, the reality of urban life in the United States was more chaotic and difficult than people expected. In addition to the challenges of language, class, race, and ethnicity, these new arrivals dealt with low wages, overcrowded buildings, poor sanitation, and widespread disease. The land of opportunity, it seemed, did not always deliver on its promises.
|
oercommons
|
2025-03-18T00:37:58.561202
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15493/overview",
"title": "U.S. History, The Growing Pains of Urbanization, 1870-1900",
"author": null
}
|
https://oercommons.org/courseware/lesson/15494/overview
|
Urbanization and Its Challenges
Overview
By the end of this section, you will be able to:
- Explain the growth of American cities in the late nineteenth century
- Identify the key challenges that Americans faced due to urbanization, as well as some of the possible solutions to those challenges
Urbanization occurred rapidly in the second half of the nineteenth century in the United States for a number of reasons. The new technologies of the time led to a massive leap in industrialization, requiring large numbers of workers. New electric lights and powerful machinery allowed factories to run twenty-four hours a day, seven days a week. Workers were forced into grueling twelve-hour shifts, requiring them to live close to the factories.
While the work was dangerous and difficult, many Americans were willing to leave behind the declining prospects of preindustrial agriculture in the hope of better wages in industrial labor. Furthermore, problems ranging from famine to religious persecution led a new wave of immigrants to arrive from central, eastern, and southern Europe, many of whom settled and found work near the cities where they first arrived. Immigrants sought solace and comfort among others who shared the same language and customs, and the nation’s cities became an invaluable economic and cultural resource.
Although cities such as Philadelphia, Boston, and New York sprang up from the initial days of colonial settlement, the explosion in urban population growth did not occur until the mid-nineteenth century (Figure). At this time, the attractions of city life, and in particular, employment opportunities, grew exponentially due to rapid changes in industrialization. Before the mid-1800s, factories, such as the early textile mills, had to be located near rivers and seaports, both for the transport of goods and the necessary water power. Production became dependent upon seasonal water flow, with cold, icy winters all but stopping river transportation entirely. The development of the steam engine transformed this need, allowing businesses to locate their factories near urban centers. These factories encouraged more and more people to move to urban areas where jobs were plentiful, but hourly wages were often low and the work was routine and grindingly monotonous.
Eventually, cities developed their own unique characters based on the core industry that spurred their growth. In Pittsburgh, it was steel; in Chicago, it was meat packing; in New York, the garment and financial industries dominated; and Detroit, by the mid-twentieth century, was defined by the automobiles it built. But all cities at this time, regardless of their industry, suffered from the universal problems that rapid expansion brought with it, including concerns over housing and living conditions, transportation, and communication. These issues were almost always rooted in deep class inequalities, shaped by racial divisions, religious differences, and ethnic strife, and distorted by corrupt local politics.
This 1884 Bureau of Labor Statistics report from Boston looks in detail at the wages, living conditions, and moral code of the girls who worked in the clothing factories there.
THE KEYS TO SUCCESSFUL URBANIZATION
As the country grew, certain elements led some towns to morph into large urban centers, while others did not. The following four innovations proved critical in shaping urbanization at the turn of the century: electric lighting, communication improvements, intracity transportation, and the rise of skyscrapers. As people migrated for the new jobs, they often struggled with the absence of basic urban infrastructures, such as better transportation, adequate housing, means of communication, and efficient sources of light and energy. Even the basic necessities, such as fresh water and proper sanitation—often taken for granted in the countryside—presented a greater challenge in urban life.
Electric Lighting
Thomas Edison patented the incandescent light bulb in 1879. This development quickly became common in homes as well as factories, transforming how even lower- and middle-class Americans lived. Although slow to arrive in rural areas of the country, electric power became readily available in cities when the first commercial power plants began to open in 1882. When Nikola Tesla subsequently developed the AC (alternating current) system for the Westinghouse Electric & Manufacturing Company, power supplies for lights and other factory equipment could extend for miles from the power source. AC power transformed the use of electricity, allowing urban centers to physically cover greater areas. In the factories, electric lights permitted operations to run twenty-four hours a day, seven days a week. This increase in production required additional workers, and this demand brought more people to cities.
Gradually, cities began to illuminate the streets with electric lamps to allow the city to remain alight throughout the night. No longer did the pace of life and economic activity slow substantially at sunset, the way it had in smaller towns. The cities, following the factories that drew people there, stayed open all the time.
Communications Improvements
The telephone, patented in 1876, greatly transformed communication both regionally and nationally. The telephone rapidly supplanted the telegraph as the preferred form of communication; by 1900, over 1.5 million telephones were in use around the nation, whether as private lines in the homes of some middle- and upper-class Americans, or jointly used “party lines” in many rural areas. By allowing instant communication over larger distances at any given time, growing telephone networks made urban sprawl possible.
In the same way that electric lights spurred greater factory production and economic growth, the telephone increased business through the more rapid pace of demand. Now, orders could come constantly via telephone, rather than via mail-order. More orders generated greater production, which in turn required still more workers. This demand for additional labor played a key role in urban growth, as expanding companies sought workers to handle the increasing consumer demand for their products.
Intracity Transportation
As cities grew and sprawled outward, a major challenge was efficient travel within the city—from home to factories or shops, and then back again. Most transportation infrastructure was used to connect cities to each other, typically by rail or canal. Prior to the 1880s, the most common form of transportation within cities was the omnibus. This was a large, horse-drawn carriage, often placed on iron or steel tracks to provide a smoother ride. While omnibuses worked adequately in smaller, less congested cities, they were not equipped to handle the larger crowds that developed at the close of the century. The horses had to stop and rest, and horse manure became an ongoing problem.
In 1887, Frank Sprague invented the electric trolley, which worked along the same concept as the omnibus, with a large wagon on tracks, but was powered by electricity rather than horses. The electric trolley could run throughout the day and night, like the factories and the workers who fueled them. But it also modernized less important industrial centers, such as the southern city of Richmond, Virginia. As early as 1873, San Francisco engineers adopted pulley technology from the mining industry to introduce cable cars and turn the city’s steep hills into elegant middle-class communities. However, as crowds continued to grow in the largest cities, such as Chicago and New York, trolleys were unable to move efficiently through the crowds of pedestrians (Figure). To avoid this challenge, city planners elevated the trolley lines above the streets, creating elevated trains, or L-trains, as early as 1868 in New York City, and quickly spreading to Boston in 1887 and Chicago in 1892. Finally, as skyscrapers began to dominate the air, transportation evolved one step further to move underground as subways. Boston’s subway system began operating in 1897, and was quickly followed by New York and other cities.
The Rise of Skyscrapers
The last limitation that large cities had to overcome was the ever-increasing need for space. Eastern cities, unlike their midwestern counterparts, could not continue to grow outward, as the land surrounding them was already settled. Geographic limitations such as rivers or the coast also hampered sprawl. And in all cities, citizens needed to be close enough to urban centers to conveniently access work, shops, and other core institutions of urban life. The increasing cost of real estate made upward growth attractive, and so did the prestige that towering buildings carried for the businesses that occupied them. Workers completed the first skyscraper in Chicago, the ten-story Home Insurance Building, in 1885 (Figure). Although engineers had the capability to go higher, thanks to new steel construction techniques, they required another vital invention in order to make taller buildings viable: the elevator. In 1889, the Otis Elevator Company, led by inventor James Otis, installed the first electric elevator. This began the skyscraper craze, allowing developers in eastern cities to build and market prestigious real estate in the hearts of crowded eastern metropoles.
Jacob Riis and the Window into “How the Other Half Lives”
Jacob Riis was a Danish immigrant who moved to New York in the late nineteenth century and, after experiencing poverty and joblessness first-hand, ultimately built a career as a police reporter. In the course of his work, he spent much of his time in the slums and tenements of New York’s working poor. Appalled by what he found there, Riis began documenting these scenes of squalor and sharing them through lectures and ultimately through the publication of his book, How the Other Half Lives, in 1890 (Figure).
By most contemporary accounts, Riis was an effective storyteller, using drama and racial stereotypes to tell his stories of the ethnic slums he encountered. But while his racial thinking was very much a product of his time, he was also a reformer; he felt strongly that upper and middle-class Americans could and should care about the living conditions of the poor. In his book and lectures, he argued against the immoral landlords and useless laws that allowed dangerous living conditions and high rents. He also suggested remodeling existing tenements or building new ones. He was not alone in his concern for the plight of the poor; other reporters and activists had already brought the issue into the public eye, and Riis’s photographs added a new element to the story.
To tell his stories, Riis used a series of deeply compelling photographs. Riis and his group of amateur photographers moved through the various slums of New York, laboriously setting up their tripods and explosive chemicals to create enough light to take the photographs. His photos and writings shocked the public, made Riis a well-known figure both in his day and beyond, and eventually led to new state legislation curbing abuses in tenements.
THE IMMEDIATE CHALLENGES OF URBAN LIFE
Congestion, pollution, crime, and disease were prevalent problems in all urban centers; city planners and inhabitants alike sought new solutions to the problems caused by rapid urban growth. Living conditions for most working-class urban dwellers were atrocious. They lived in crowded tenement houses and cramped apartments with terrible ventilation and substandard plumbing and sanitation. As a result, disease ran rampant, with typhoid and cholera common. Memphis, Tennessee, experienced waves of cholera (1873) followed by yellow fever (1878 and 1879) that resulted in the loss of over ten thousand lives. By the late 1880s, New York City, Baltimore, Chicago, and New Orleans had all introduced sewage pumping systems to provide efficient waste management. Many cities were also serious fire hazards. An average working-class family of six, with two adults and four children, had at best a two-bedroom tenement. By one 1900 estimate, in the New York City borough of Manhattan alone, there were nearly fifty thousand tenement houses. The photographs of these tenement houses are seen in Jacob Riis’s book, How the Other Half Lives, discussed in the feature above. Citing a study by the New York State Assembly at this time, Riis found New York to be the most densely populated city in the world, with as many as eight hundred residents per square acre in the Lower East Side working-class slums, comprising the Eleventh and Thirteenth Wards.
Visit New York City, Tenement Life to get an impression of the everyday life of tenement dwellers on Manhattan’s Lower East Side.
Churches and civic organizations provided some relief to the challenges of working-class city life. Churches were moved to intervene through their belief in the concept of the social gospel. This philosophy stated that all Christians, whether they were church leaders or social reformers, should be as concerned about the conditions of life in the secular world as the afterlife, and the Reverend Washington Gladden was a major advocate. Rather than preaching sermons on heaven and hell, Gladden talked about social changes of the time, urging other preachers to follow his lead. He advocated for improvements in daily life and encouraged Americans of all classes to work together for the betterment of society. His sermons included the message to “love thy neighbor” and held that all Americans had to work together to help the masses. As a result of his influence, churches began to include gymnasiums and libraries as well as offer evening classes on hygiene and health care. Other religious organizations like the Salvation Army and the Young Men’s Christian Association (YMCA) expanded their reach in American cities at this time as well. Beginning in the 1870s, these organizations began providing community services and other benefits to the urban poor.
In the secular sphere, the settlement house movement of the 1890s provided additional relief. Pioneering women such as Jane Addams in Chicago and Lillian Wald in New York led this early progressive reform movement in the United States, building upon ideas originally fashioned by social reformers in England. With no particular religious bent, they worked to create settlement houses in urban centers where they could help the working class, and in particular, working-class women, find aid. Their help included child daycare, evening classes, libraries, gym facilities, and free health care. Addams opened her now-famous Hull House (Figure) in Chicago in 1889, and Wald’s Henry Street Settlement opened in New York six years later. The movement spread quickly to other cities, where they not only provided relief to working-class women but also offered employment opportunities for women graduating college in the growing field of social work. Oftentimes, living in the settlement houses among the women they helped, these college graduates experienced the equivalent of living social classrooms in which to practice their skills, which also frequently caused friction with immigrant women who had their own ideas of reform and self-improvement.
The success of the settlement house movement later became the basis of a political agenda that included pressure for housing laws, child labor laws, and worker’s compensation laws, among others. Florence Kelley, who originally worked with Addams in Chicago, later joined Wald’s efforts in New York; together, they created the National Child Labor Committee and advocated for the subsequent creation of the Children’s Bureau in the U.S. Department of Labor in 1912. Julia Lathrop—herself a former resident of Hull House—became the first woman to head a federal government agency, when President William Howard Taft appointed her to run the bureau. Settlement house workers also became influential leaders in the women’s suffrage movement as well as the antiwar movement during World War I.
Jane Addams Reflects on the Settlement House Movement
Jane Addams was a social activist whose work took many forms. She is perhaps best known as the founder of Hull House in Chicago, which later became a model for settlement houses throughout the country. Here, she reflects on the role that the settlement played.
Life in the Settlement discovers above all what has been called ‘the extraordinary pliability of human nature,’ and it seems impossible to set any bounds to the moral capabilities which might unfold under ideal civic and educational conditions. But in order to obtain these conditions, the Settlement recognizes the need of cooperation, both with the radical and the conservative, and from the very nature of the case the Settlement cannot limit its friends to any one political party or economic school.
The Settlement casts side none of those things which cultivated men have come to consider reasonable and goodly, but it insists that those belong as well to that great body of people who, because of toilsome and underpaid labor, are unable to procure them for themselves. Added to this is a profound conviction that the common stock of intellectual enjoyment should not be difficult of access because of the economic position of him who would approach it, that those ‘best results of civilization’ upon which depend the finer and freer aspects of living must be incorporated into our common life and have free mobility through all elements of society if we would have our democracy endure.
The educational activities of a Settlement, as well its philanthropic, civic, and social undertakings, are but differing manifestations of the attempt to socialize democracy, as is the very existence of the Settlement itself.
In addition to her pioneering work in the settlement house movement, Addams also was active in the women’s suffrage movement as well as an outspoken proponent for international peace efforts. She was instrumental in the relief effort after World War I, a commitment that led to her winning the Nobel Peace Prize in 1931.
Section Summary
Urbanization spread rapidly in the mid-nineteenth century due to a confluence of factors. New technologies, such as electricity and steam engines, transformed factory work, allowing factories to move closer to urban centers and away from the rivers that had previously been vital sources of both water power and transportation. The growth of factories—as well as innovations such as electric lighting, which allowed them to run at all hours of the day and night—created a massive need for workers, who poured in from both rural areas of the United States and from eastern and southern Europe. As cities grew, they were unable to cope with this rapid influx of workers, and the living conditions for the working class were terrible. Tight living quarters, with inadequate plumbing and sanitation, led to widespread illness. Churches, civic organizations, and the secular settlement house movement all sought to provide some relief to the urban working class, but conditions remained brutal for many new city dwellers.
Review Questions
Which of the following four elements was not essential for creating massive urban growth in late nineteenth-century America?
- electric lighting
- communication improvements
- skyscrapers
- settlement houses
Hint:
D
Which of the following did the settlement house movement offer as a means of relief for working-class women?
- childcare
- job opportunities
- political advocacy
- relocation services
Hint:
A
What technological and economic factors combined to lead to the explosive growth of American cities at this time?
Hint:
At the end of the nineteenth century, a confluence of events made urban life more desirable and more possible. Technologies such as electricity and the telephone allowed factories to build and grow in cities, and skyscrapers enabled the relatively small geographic areas to continue expanding. The new demand for workers spurred a massive influx of job-seekers from both rural areas of the United States and from eastern and southern Europe. Urban housing—as well as services such as transportation and sanitation—expanded accordingly, though cities struggled to cope with the surging demand. Together, technological innovations and an exploding population led American cities to grow as never before.
|
oercommons
|
2025-03-18T00:37:58.590008
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15494/overview",
"title": "U.S. History, The Growing Pains of Urbanization, 1870-1900",
"author": null
}
|
https://oercommons.org/courseware/lesson/15495/overview
|
The African American “Great Migration” and New European Immigration
Overview
By the end of this section, you will be able to:
- Identify the factors that prompted African American and European immigration to American cities in the late nineteenth century
- Explain the discrimination and anti-immigration legislation that immigrants faced in the late nineteenth century
New cities were populated with diverse waves of new arrivals, who came to the cities to seek work in the businesses and factories there. While a small percentage of these newcomers were white Americans seeking jobs, most were made up of two groups that had not previously been factors in the urbanization movement: African Americans fleeing the racism of the farms and former plantations in the South, and southern and eastern European immigrants. These new immigrants supplanted the previous waves of northern and western European immigrants, who had tended to move west to purchase land. Unlike their predecessors, the newer immigrants lacked the funds to strike out to the western lands and instead remained in the urban centers where they arrived, seeking any work that would keep them alive.
THE AFRICAN AMERICAN “GREAT MIGRATION”
Between the end of the Civil War and the beginning of the Great Depression, nearly two million African Americans fled the rural South to seek new opportunities elsewhere. While some moved west, the vast majority of this Great Migration, as the large exodus of African Americans leaving the South in the early twentieth century was called, traveled to the Northeast and Upper Midwest. The following cities were the primary destinations for these African Americans: New York, Chicago, Philadelphia, St. Louis, Detroit, Pittsburgh, Cleveland, and Indianapolis. These eight cities accounted for over two-thirds of the total population of the African American migration.
A combination of both “push” and “pull” factors played a role in this movement. Despite the end of the Civil War and the passage of the Thirteenth, Fourteenth, and Fifteenth Amendments to the U.S. Constitution (ensuring freedom, the right to vote regardless of race, and equal protection under the law, respectively), African Americans were still subjected to intense racial hatred. The rise of the Ku Klux Klan in the immediate aftermath of the Civil War led to increased death threats, violence, and a wave of lynchings. Even after the formal dismantling of the Klan in the late 1870s, racially motivated violence continued. According to researchers at the Tuskegee Institute, there were thirty-five hundred racially motivated lynchings and other murders committed in the South between 1865 and 1900. For African Americans fleeing this culture of violence, northern and midwestern cities offered an opportunity to escape the dangers of the South.
In addition to this “push” out of the South, African Americans were also “pulled” to the cities by factors that attracted them, including job opportunities, where they could earn a wage rather than be tied to a landlord, and the chance to vote (for men, at least), supposedly free from the threat of violence. Although many lacked the funds to move themselves north, factory owners and other businesses that sought cheap labor assisted the migration. Often, the men moved first then sent for their families once they were ensconced in their new city life. Racism and a lack of formal education relegated these African American workers to many of the lower-paying unskilled or semi-skilled occupations. More than 80 percent of African American men worked menial jobs in steel mills, mines, construction, and meat packing. In the railroad industry, they were often employed as porters or servants (Figure). In other businesses, they worked as janitors, waiters, or cooks. African American women, who faced discrimination due to both their race and gender, found a few job opportunities in the garment industry or laundries, but were more often employed as maids and domestic servants. Regardless of the status of their jobs, however, African Americans earned higher wages in the North than they did for the same occupations in the South, and typically found housing to be more available.
However, such economic gains were offset by the higher cost of living in the North, especially in terms of rent, food costs, and other essentials. As a result, African Americans often found themselves living in overcrowded, unsanitary conditions, much like the tenement slums in which European immigrants lived in the cities. For newly arrived African Americans, even those who sought out the cities for the opportunities they provided, life in these urban centers was exceedingly difficult. They quickly learned that racial discrimination did not end at the Mason-Dixon Line, but continued to flourish in the North as well as the South. European immigrants, also seeking a better life in the cities of the United States, resented the arrival of the African Americans, whom they feared would compete for the same jobs or offer to work at lower wages. Landlords frequently discriminated against them; their rapid influx into the cities created severe housing shortages and even more overcrowded tenements. Homeowners in traditionally white neighborhoods later entered into covenants in which they agreed not to sell to African American buyers; they also often fled neighborhoods into which African Americans had gained successful entry. In addition, some bankers practiced mortgage discrimination, later known as “redlining,” in order to deny home loans to qualified buyers. Such pervasive discrimination led to a concentration of African Americans in some of the worst slum areas of most major metropolitan cities, a problem that remained ongoing throughout most of the twentieth century.
So why move to the North, given that the economic challenges they faced were similar to those that African Americans encountered in the South? The answer lies in noneconomic gains. Greater educational opportunities and more expansive personal freedoms mattered greatly to the African Americans who made the trek northward during the Great Migration. State legislatures and local school districts allocated more funds for the education of both blacks and whites in the North, and also enforced compulsory school attendance laws more rigorously. Similarly, unlike the South where a simple gesture (or lack of a deferential one) could result in physical harm to the African American who committed it, life in larger, crowded northern urban centers permitted a degree of anonymity—and with it, personal freedom—that enabled African Americans to move, work, and speak without deferring to every white person with whom they crossed paths. Psychologically, these gains more than offset the continued economic challenges that black migrants faced.
THE CHANGING NATURE OF EUROPEAN IMMIGRATION
Immigrants also shifted the demographics of the rapidly growing cities. Although immigration had always been a force of change in the United States, it took on a new character in the late nineteenth century. Beginning in the 1880s, the arrival of immigrants from mostly southern and eastern European countries rapidly increased while the flow from northern and western Europe remained relatively constant (Table).
| Region Country | 1870 | 1880 | 1890 | 1900 | 1910 |
|---|---|---|---|---|---|
| Northern and Western Europe | 4,845,679 | 5,499,889 | 7,288,917 | 7,204,649 | 7,306,325 |
| Germany | 1,690,533 | 1,966,742 | 2,784,894 | 2,663,418 | 2,311,237 |
| Ireland | 1,855,827 | 1,854,571 | 1,871,509 | 1,615,459 | 1,352,251 |
| England | 550,924 | 662,676 | 908,141 | 840,513 | 877,719 |
| Sweden | 97,332 | 194,337 | 478,041 | 582,014 | 665,207 |
| Austria | 30,508 | 38,663 | 123,271 | 275,907 | 626,341 |
| Norway | 114,246 | 181,729 | 322,665 | 336,388 | 403,877 |
| Scotland | 140,835 | 170,136 | 242,231 | 233,524 | 261,076 |
| Southern and Eastern Europe | 93,824 | 248,620 | 728,851 | 1,674,648 | 4,500,932 |
| Italy | 17,157 | 44,230 | 182,580 | 484,027 | 1,343,125 |
| Russia | 4,644 | 35,722 | 182,644 | 423,726 | 1,184,412 |
| Poland | 14,436 | 48,557 | 147,440 | 383,407 | 937,884 |
| Hungary | 3,737 | 11,526 | 62,435 | 145,714 | 495,609 |
| Czechoslovakia | 40,289 | 85,361 | 118,106 | 156,891 | 219,214 |
The previous waves of immigrants from northern and western Europe, particularly Germany, Great Britain, and the Nordic countries, were relatively well off, arriving in the country with some funds and often moving to the newly settled western territories. In contrast, the newer immigrants from southern and eastern European countries, including Italy, Greece, and several Slavic countries including Russia, came over due to “push” and “pull” factors similar to those that influenced the African Americans arriving from the South. Many were “pushed” from their countries by a series of ongoing famines, by the need to escape religious, political, or racial persecution, or by the desire to avoid compulsory military service. They were also “pulled” by the promise of consistent, wage-earning work.
Whatever the reason, these immigrants arrived without the education and finances of the earlier waves of immigrants, and settled more readily in the port towns where they arrived, rather than setting out to seek their fortunes in the West. By 1890, over 80 percent of the population of New York would be either foreign-born or children of foreign-born parentage. Other cities saw huge spikes in foreign populations as well, though not to the same degree, due in large part to Ellis Island in New York City being the primary port of entry for most European immigrants arriving in the United States.
The number of immigrants peaked between 1900 and 1910, when over nine million people arrived in the United States. To assist in the processing and management of this massive wave of immigrants, the Bureau of Immigration in New York City, which had become the official port of entry, opened Ellis Island in 1892. Today, nearly half of all Americans have ancestors who, at some point in time, entered the country through the portal at Ellis Island. Doctors or nurses inspected the immigrants upon arrival, looking for any signs of infectious diseases (Figure). Most immigrants were admitted to the country with only a cursory glance at any other paperwork. Roughly 2 percent of the arriving immigrants were denied entry due to a medical condition or criminal history. The rest would enter the country by way of the streets of New York, many unable to speak English and totally reliant on finding those who spoke their native tongue.
Seeking comfort in a strange land, as well as a common language, many immigrants sought out relatives, friends, former neighbors, townspeople, and countrymen who had already settled in American cities. This led to a rise in ethnic enclaves within the larger city. Little Italy, Chinatown, and many other communities developed in which immigrant groups could find everything to remind them of home, from local language newspapers to ethnic food stores. While these enclaves provided a sense of community to their members, they added to the problems of urban congestion, particularly in the poorest slums where immigrants could afford housing.
This Library of Congress exhibit on the history of Jewish immigration to the United States illustrates the ongoing challenge immigrants felt between the ties to their old land and a love for America.
The demographic shift at the turn of the century was later confirmed by the Dillingham Commission, created by Congress in 1907 to report on the nature of immigration in America; the commission reinforced this ethnic identification of immigrants and their simultaneous discrimination. The report put it simply: These newer immigrants looked and acted differently. They had darker skin tone, spoke languages with which most Americans were unfamiliar, and practiced unfamiliar religions, specifically Judaism and Catholicism. Even the foods they sought out at butchers and grocery stores set immigrants apart. Because of these easily identifiable differences, new immigrants became easy targets for hatred and discrimination. If jobs were hard to find, or if housing was overcrowded, it became easy to blame the immigrants. Like African Americans, immigrants in cities were blamed for the problems of the day.
Growing numbers of Americans resented the waves of new immigrants, resulting in a backlash. The Reverend Josiah Strong fueled the hatred and discrimination in his bestselling book, Our Country: Its Possible Future and Its Present Crisis, published in 1885. In a revised edition that reflected the 1890 census records, he clearly identified undesirable immigrants—those from southern and eastern European countries—as a key threat to the moral fiber of the country, and urged all good Americans to face the challenge. Several thousand Americans answered his call by forming the American Protective Association, the chief political activist group to promote legislation curbing immigration into the United States. The group successfully lobbied Congress to adopt both an English language literacy test for immigrants, which eventually passed in 1917, and the Chinese Exclusion Act (discussed in a previous chapter). The group’s political lobbying also laid the groundwork for the subsequent Emergency Quota Act of 1921 and the Immigration Act of 1924, as well as the National Origins Act.
The global timeline of immigration at the Library of Congress offers a summary of immigration policies and the groups affected by it, as well as a compelling overview of different ethnic groups’ immigration stories. Browse through to see how different ethnic groups made their way in the United States.
Section Summary
For both African Americans migrating from the postwar South and immigrants arriving from southeastern Europe, a combination of “push” and “pull” factors influenced their migration to America’s urban centers. African Americans moved away from the racial violence and limited opportunities that existed in the rural South, seeking wages and steady work, as well as the opportunity to vote safely as free men; however, they quickly learned that racial discrimination and violence were not limited to the South. For European immigrants, famine and persecution led them to seek a new life in the United States, where, the stories said, the streets were paved in gold. Of course, in northeastern and midwestern cities, both groups found a more challenging welcome than they had anticipated. City residents blamed recent arrivals for the ills of the cities, from overcrowding to a rise in crime. Activist groups pushed for anti-immigration legislation, seeking to limit the waves of immigrants that sought a better future in the United States.
Review Questions
Why did African Americans consider moving from the rural South to the urban North following the Civil War?
- to be able to buy land
- to avoid slavery
- to find wage-earning work
- to further their education
Hint:
C
Which of the following is true of late nineteenth-century southern and eastern European immigrants, as opposed to their western and northern European predecessors?
- Southern and eastern European immigrants tended to be wealthier.
- Southern and eastern European immigrants were, on the whole, more skilled and able to find better paying employment.
- Many southern and eastern European immigrants acquired land in the West, while western and northern European immigrants tended to remain in urban centers.
- Ellis Island was the first destination for most southern and eastern Europeans.
Hint:
D
What made recent European immigrants the ready targets of more established city dwellers? What was the result of this discrimination?
Hint:
Newer immigrants often had different appearances, spoke unfamiliar languages, and lived their lives—from the religions they practiced to the food they ate—in ways that were alien to many Americans. In all of city life’s more challenging aspects, from competition for jobs to overcrowding in scarce housing, immigrants became easy scapegoats. The Reverend Josiah Strong’s bestselling book, Our Country: Its Possible Future and Its Present Crisis, fueled this discrimination. The American Protective Association, the chief political activist group promoting anti-immigration legislation, formed largely in response to Strong’s call.
|
oercommons
|
2025-03-18T00:37:58.621852
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15495/overview",
"title": "U.S. History, The Growing Pains of Urbanization, 1870-1900",
"author": null
}
|
https://oercommons.org/courseware/lesson/15496/overview
|
Relief from the Chaos of Urban Life
Overview
By the end of this section, you will be able to:
- Identify how each class of Americans—working class, middle class, and upper class—responded to the challenges associated with urban life
- Explain the process of machine politics and how it brought relief to working-class Americans
Settlement houses and religious and civic organizations attempted to provide some support to working-class city dwellers through free health care, education, and leisure opportunities. Still, for urban citizens, life in the city was chaotic and challenging. But how that chaos manifested and how relief was sought differed greatly, depending on where people were in the social caste—the working class, the upper class, or the newly emerging professional middle class—in addition to the aforementioned issues of race and ethnicity. While many communities found life in the largest American cities disorganized and overwhelming, the ways they answered these challenges were as diverse as the people who lived there. Broad solutions emerged that were typically class specific: The rise of machine politics and popular culture provided relief to the working class, higher education opportunities and suburbanization benefitted the professional middle class, and reminders of their elite status gave comfort to the upper class. And everyone, no matter where they fell in the class system, benefited from the efforts to improve the physical landscapes of the fast-growing urban environment.
THE LIFE AND STRUGGLES OF THE URBAN WORKING CLASS
For the working-class residents of America’s cities, one practical way of coping with the challenges of urban life was to take advantage of the system of machine politics, while another was to seek relief in the variety of popular culture and entertainment found in and around cities. Although neither of these forms of relief was restricted to the working class, they were the ones who relied most heavily on them.
Machine Politics
The primary form of relief for working-class urban Americans, and particularly immigrants, came in the form of machine politics. This phrase referred to the process by which every citizen of the city, no matter their ethnicity or race, was a ward resident with an alderman who spoke on their behalf at city hall. When everyday challenges arose, whether sanitation problems or the need for a sidewalk along a muddy road, citizens would approach their alderman to find a solution. The aldermen knew that, rather than work through the long bureaucratic process associated with city hall, they could work within the “machine” of local politics to find a speedy, mutually beneficial solution. In machine politics, favors were exchanged for votes, votes were given in exchange for fast solutions, and the price of the solutions included a kickback to the boss. In the short term, everyone got what they needed, but the process was neither transparent nor democratic, and it was an inefficient way of conducting the city’s business.
One example of a machine political system was the Democratic political machine Tammany Hall in New York, run by machine boss William Tweed with assistance from George Washington Plunkitt (Figure). There, citizens knew their immediate problems would be addressed in return for their promise of political support in future elections. In this way, machines provided timely solutions for citizens and votes for the politicians. For example, if in Little Italy there was a desperate need for sidewalks in order to improve traffic to the stores on a particular street, the request would likely get bogged down in the bureaucratic red tape at city hall. Instead, store owners would approach the machine. A district captain would approach the “boss” and make him aware of the problem. The boss would contact city politicians and strongly urge them to appropriate the needed funds for the sidewalk in exchange for the promise that the boss would direct votes in their favor in the upcoming election. The boss then used the funds to pay one of his friends for the sidewalk construction, typically at an exorbitant cost, with a financial kickback to the boss, which was known as graft. The sidewalk was built more quickly than anyone hoped, in exchange for the citizens’ promises to vote for machine-supported candidates in the next elections. Despite its corrupt nature, Tammany Hall essentially ran New York politics from the 1850s until the 1930s. Other large cities, including Boston, Philadelphia, Cleveland, St. Louis, and Kansas City, made use of political machines as well.
Popular Culture and Entertainment
Working-class residents also found relief in the diverse and omnipresent offerings of popular culture and entertainment in and around cities. These offerings provided an immediate escape from the squalor and difficulties of everyday life. As improved means of internal transportation developed, working-class residents could escape the city and experience one of the popular new forms of entertainment—the amusement park. For example, Coney Island on the Brooklyn shoreline consisted of several different amusement parks, the first of which opened in 1895 (Figure). At these parks, New Yorkers enjoyed wild rides, animal attractions, and large stage productions designed to help them forget the struggles of their working-day lives. Freak “side” shows fed the public’s curiosity about physical deviance. For a mere ten cents, spectators could watch a high-diving horse, take a ride to the moon to watch moon maidens eat green cheese, or witness the electrocution of an elephant, a spectacle that fascinated the public both with technological marvels and exotic wildlife. The treatment of animals in many acts at Coney Island and other public amusement parks drew the attention of middle-class reformers such as the American Society for the Prevention of Cruelty to Animals. Despite questions regarding the propriety of many of the acts, other cities quickly followed New York’s lead with similar, if smaller, versions of Coney Island’s attractions.
The American Experience Timeline of Coney Island shows a timeline, photo gallery, and other elements of Coney Island. Look to see what elements of American culture, from the hot dog to the roller coaster, debuted there.
Another common form of popular entertainment was vaudeville—large stage variety shows that included everything from singing, dancing, and comedy acts to live animals and magic. The vaudeville circuit gave rise to several prominent performers, including magician Harry Houdini, who began his career in these variety shows before his fame propelled him to solo acts. In addition to live theater shows, it was primarily working-class citizens who enjoyed the advent of the nickelodeon, a forerunner to the movie theater. The first nickelodeon opened in Pittsburgh in 1905, where nearly one hundred visitors packed into a storefront theater to see a traditional vaudeville show interspersed with one-minute film clips. Several theaters initially used the films as “chasers” to indicate the end of the show to the live audience so they would clear the auditorium. However, a vaudeville performers’ strike generated even greater interest in the films, eventually resulting in the rise of modern movie theaters by 1910.
One other major form of entertainment for the working class was professional baseball (Figure). Club teams transformed into professional baseball teams with the Cincinnati Red Stockings, now the Cincinnati Reds, in 1869. Soon, professional teams sprang up in several major American cities. Baseball games provided an inexpensive form of entertainment, where for less than a dollar, a person could enjoy a double-header, two hot dogs, and a beer. But more importantly, the teams became a way for newly relocated Americans and immigrants of diverse backgrounds to develop a unified civic identity, all cheering for one team. By 1876, the National League had formed, and soon after, cathedral-style ballparks began to spring up in many cities. Fenway Park in Boston (1912), Forbes Field in Pittsburgh (1909), and the Polo Grounds in New York (1890) all became touch points where working-class Americans came together to support a common cause.
Other popular sports included prize-fighting, which attracted a predominantly male, working- and middle-class audience who lived vicariously through the triumphs of the boxers during a time where opportunities for individual success were rapidly shrinking, and college football, which paralleled a modern corporation in its team hierarchy, divisions of duties, and emphasis on time management.
THE UPPER CLASS IN THE CITIES
The American financial elite did not need to crowd into cities to find work, like their working-class counterparts. But as urban centers were vital business cores, where multi-million-dollar financial deals were made daily, those who worked in that world wished to remain close to the action. The rich chose to be in the midst of the chaos of the cities, but they were also able to provide significant measures of comfort, convenience, and luxury for themselves.
Wealthy citizens seldom attended what they considered the crass entertainment of the working class. Instead of amusement parks and baseball games, urban elites sought out more refined pastimes that underscored their knowledge of art and culture, preferring classical music concerts, fine art collections, and social gatherings with their peers. In New York, Andrew Carnegie built Carnegie Hall in 1891, which quickly became the center of classical music performances in the country. Nearby, the Metropolitan Museum of Art opened its doors in 1872 and still remains one of the largest collections of fine art in the world. Other cities followed suit, and these cultural pursuits became a way for the upper class to remind themselves of their elevated place amid urban squalor.
As new opportunities for the middle class threatened the austerity of upper-class citizens, including the newer forms of transportation that allowed middle-class Americans to travel with greater ease, wealthier Americans sought unique ways to further set themselves apart in society. These included more expensive excursions, such as vacations in Newport, Rhode Island, winter relocation to sunny Florida, and frequent trips aboard steamships to Europe. For those who were not of the highly respected “old money,” but only recently obtained their riches through business ventures, the relief they sought came in the form of one book—the annual Social Register. First published in 1886 by Louis Keller in New York City, the register became a directory of the wealthy socialites who populated the city. Keller updated it annually, and people would watch with varying degrees of anxiety or complacency to see their names appear in print. Also called the Blue Book, the register was instrumental in the planning of society dinners, balls, and other social events. For those of newer wealth, there was relief found simply in the notion that they and others witnessed their wealth through the publication of their names in the register.
A NEW MIDDLE CLASS
While the working class were confined to tenement houses in the cities by their need to be close to their work and the lack of funds to find anyplace better, and the wealthy class chose to remain in the cities to stay close to the action of big business transactions, the emerging middle class responded to urban challenges with their own solutions. This group included the managers, salesmen, engineers, doctors, accountants, and other salaried professionals who still worked for a living, but were significantly better educated and compensated than the working-class poor. For this new middle class, relief from the trials of the cities came through education and suburbanization.
In large part, the middle class responded to the challenges of the city by physically escaping it. As transportation improved and outlying communities connected to urban centers, the middle class embraced a new type of community—the suburbs. It became possible for those with adequate means to work in the city and escape each evening, by way of a train or trolley, to a house in the suburbs. As the number of people moving to the suburbs grew, there also grew a perception among the middle class that the farther one lived from the city and the more amenities one had, the more affluence one had achieved.
Although a few suburbs existed in the United States prior to the 1880s (such as Llewellyn Park, New Jersey), the introduction of the electric railway generated greater interest and growth during the last decade of the century. The ability to travel from home to work on a relatively quick and cheap mode of transportation encouraged more Americans of modest means to consider living away from the chaos of the city. Eventually, Henry Ford’s popularization of the automobile, specifically in terms of a lower price, permitted more families to own cars and thus consider suburban life. Later in the twentieth century, both the advent of the interstate highway system, along with federal legislation designed to allow families to construct homes with low-interest loans, further sparked the suburban phenomenon.
New Roles for Middle-Class Women
Social norms of the day encouraged middle-class women to take great pride in creating a positive home environment for their working husbands and school-age children, which reinforced the business and educational principles that they practiced on the job or in school. It was at this time that the magazines Ladies Home Journal and Good Housekeeping began distribution, to tremendous popularity (Figure).
While the vast majority of middle-class women took on the expected role of housewife and homemaker, some women were finding paths to college. A small number of men’s colleges began to open their doors to women in the mid-1800s, and co-education became an option. Some of the most elite universities created affiliated women’s colleges, such as Radcliffe College with Harvard, and Pembroke College with Brown University. But more importantly, the first women’s colleges opened at this time. Mount Holyoke, Vassar, Smith, and Wellesley Colleges, still some of the best known women’s schools, opened their doors between 1865 and 1880, and, although enrollment was low (initial class sizes ranged from sixty-one students at Vassar to seventy at Wellesley, seventy-one at Smith, and up to eighty-eight at Mount Holyoke), the opportunity for a higher education, and even a career, began to emerge for young women. These schools offered a unique, all-women environment in which professors and a community of education-seeking young women came together. While most college-educated young women still married, their education offered them new opportunities to work outside the home, most frequently as teachers, professors, or in the aforementioned settlement house environments created by Jane Addams and others.
Education and the Middle Class
Since the children of the professional class did not have to leave school and find work to support their families, they had opportunities for education and advancement that would solidify their position in the middle class. They also benefited from the presence of stay-at-home mothers, unlike working-class children, whose mothers typically worked the same long hours as their fathers. Public school enrollment exploded at this time, with the number of students attending public school tripling from seven million in 1870 to twenty-one million in 1920. Unlike the old-fashioned one-room schoolhouses, larger schools slowly began the practice of employing different teachers for each grade, and some even began hiring discipline-specific instructors. High schools also grew at this time, from one hundred high schools nationally in 1860 to over six thousand by 1900.
The federal government supported the growth of higher education with the Morrill Acts of 1862 and 1890. These laws set aside public land and federal funds to create land-grant colleges that were affordable to middle-class families, offering courses and degrees useful in the professions, but also in trade, commerce, industry, and agriculture (Figure). Land-grant colleges stood in contrast to the expensive, private Ivy League universities such as Harvard and Yale, which still catered to the elite. Iowa became the first state to accept the provisions of the original Morrill Act, creating what later became Iowa State University. Other states soon followed suit, and the availability of an affordable college education encouraged a boost in enrollment, from 50,000 students nationwide in 1870 to over 600,000 students by 1920.
College curricula also changed at this time. Students grew less likely to take traditional liberal arts classes in rhetoric, philosophy, and foreign language, and instead focused on preparing for the modern work world. Professional schools for the study of medicine, law, and business also developed. In short, education for the children of middle-class parents catered to class-specific interests and helped ensure that parents could establish their children comfortably in the middle class as well.
“CITY BEAUTIFUL”
While the working poor lived in the worst of it and the wealthy elite sought to avoid it, all city dwellers at the time had to deal with the harsh realities of urban sprawl. Skyscrapers rose and filled the air, streets were crowded with pedestrians of all sorts, and, as developers worked to meet the always-increasing demand for space, the few remaining green spaces in the city quickly disappeared. As the U.S. population became increasingly centered in urban areas while the century drew to a close, questions about the quality of city life—particularly with regard to issues of aesthetics, crime, and poverty—quickly consumed many reformers’ minds. Those middle-class and wealthier urbanites who enjoyed the costlier amenities presented by city life—including theaters, restaurants, and shopping—were free to escape to the suburbs, leaving behind the poorer working classes living in squalor and unsanitary conditions. Through the City Beautiful movement, leaders such as Frederick Law Olmsted and Daniel Burnham sought to champion middle- and upper-class progressive reforms. They improved the quality of life for city dwellers, but also cultivated middle-class-dominated urban spaces in which Americans of different ethnicities, racial origins, and classes worked and lived.
Olmsted, one of the earliest and most influential designers of urban green space, and the original designer of Central Park in New York, worked with Burnham to introduce the idea of the City Beautiful movement at the Columbian Exposition in 1893. There, they helped to design and construct the “White City”—so named for the plaster of Paris construction of several buildings that were subsequently painted a bright white—an example of landscaping and architecture that shone as an example of perfect city planning. From wide-open green spaces to brightly painted white buildings, connected with modern transportation services and appropriate sanitation, the “White City” set the stage for American urban city planning for the next generation, beginning in 1901 with the modernization of Washington, DC. This model encouraged city planners to consider three principal tenets: First, create larger park areas inside cities; second, build wider boulevards to decrease traffic congestion and allow for lines of trees and other greenery between lanes; and third, add more suburbs in order to mitigate congested living in the city itself (Figure). As each city adapted these principles in various ways, the City Beautiful movement became a cornerstone of urban development well into the twentieth century.
Section Summary
The burgeoning cities brought together both rich and poor, working class and upper class; however, the realities of urban dwellers’ lives varied dramatically based on where they fell in the social chain. Entertainment and leisure-time activities were heavily dependent on one’s status and wealth. For the working poor, amusement parks and baseball games offered inexpensive entertainment and a brief break from the squalor of the tenements. For the emerging middle class of salaried professionals, an escape to the suburbs kept them removed from the city’s chaos outside of working hours. And for the wealthy, immersion in arts and culture, as well as inclusion in the Social Register, allowed them to socialize exclusively with those they felt were of the same social status. The City Beautiful movement benefitted all city dwellers, with its emphasis on public green spaces, and more beautiful and practical city boulevards. In all, these different opportunities for leisure and pleasure made city life manageable for the citizens who lived there.
Review Questions
Which of the following was a popular pastime for working-class urban dwellers?
- football games
- opera
- museums
- amusement parks
Hint:
D
Which of the following was a disadvantage of machine politics?
- Immigrants did not have a voice.
- Taxpayers ultimately paid higher city taxes due to graft.
- Only wealthy parts of the city received timely responses.
- Citizens who voiced complaints were at risk for their safety.
Hint:
B
In what way did education play a crucial role in the emergence of the middle class?
Hint:
Better public education and the explosion of high schools meant that the children of the middle class were better educated than any previous generation. While college had previously been mostly restricted to children of the upper class, the creation of land-grant colleges made college available on a wide scale. The curricula at these new colleges matched the needs of the middle class, offering practical professional training rather than the liberal arts focus that the Ivy League schools embraced. Thus, children of the emerging middle class were able to access the education and training needed to secure their place in the professional class for generations to come.
|
oercommons
|
2025-03-18T00:37:58.652504
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15496/overview",
"title": "U.S. History, The Growing Pains of Urbanization, 1870-1900",
"author": null
}
|
https://oercommons.org/courseware/lesson/15497/overview
|
Change Reflected in Thought and Writing
Overview
By the end of this section, you will be able to:
- Explain how American writers, both fiction and nonfiction, helped Americans to better understand the changes they faced in the late nineteenth and early twentieth centuries
- Identify some of the influential women and African American writers of the era
In the late nineteenth century, Americans were living in a world characterized by rapid change. Western expansion, dramatic new technologies, and the rise of big business drastically influenced society in a matter of a few decades. For those living in the fast-growing urban areas, the pace of change was even faster and harder to ignore. One result of this time of transformation was the emergence of a series of notable authors, who, whether writing fiction or nonfiction, offered a lens through which to better understand the shifts in American society.
UNDERSTANDING SOCIAL PROGRESS
One key idea of the nineteenth century that moved from the realm of science to the murkier ground of social and economic success was Charles Darwin’s theory of evolution. Darwin was a British naturalist who, in his 1859 work On the Origin of Species, made the case that species develop and evolve through natural selection, not through divine intervention. The idea quickly drew fire from the Anglican Church (although a liberal branch of Anglicans embraced the notion of natural selection being part of God’s plan) and later from many others, both in England and abroad, who felt that the theory directly contradicted the role of God in the earth’s creation. Although biologists, botanists, and most of the scientific establishment widely accepted the theory of evolution at the time of Darwin’s publication, which they felt synthesized much of the previous work in the field, the theory remained controversial in the public realm for decades.
Political philosopher Herbert Spencer took Darwin’s theory of evolution further, coining the actual phrase “survival of the fittest,” and later helping to popularize the phrase social Darwinism to posit that society evolved much like a natural organism, wherein some individuals will succeed due to racially and ethnically inherent traits, and their ability to adapt. This model allowed that a collection of traits and skills, which could include intelligence, inherited wealth, and so on, mixed with the ability to adapt, would let all Americans rise or fall of their own accord, so long as the road to success was accessible to all. William Graham Sumner, a sociologist at Yale, became the most vocal proponent of social Darwinism. Not surprisingly, this ideology, which Darwin himself would have rejected as a gross misreading of his scientific discoveries, drew great praise from those who made their wealth at this time. They saw their success as proof of biological fitness, although critics of this theory were quick to point out that those who did not succeed often did not have the same opportunities or equal playing field that the ideology of social Darwinism purported. Eventually, the concept fell into disrepute in the 1930s and 1940s, as eugenicists began to utilize it in conjunction with their racial theories of genetic superiority.
Other thinkers of the day took Charles Darwin’s theories in a more nuanced direction, focusing on different theories of realism that sought to understand the truth underlying the changes in the United States. These thinkers believed that ideas and social constructs must be proven to work before they could be accepted. Philosopher William James was one of the key proponents of the closely related concept of pragmatism, which held that Americans needed to experiment with different ideas and perspectives to find the truth about American society, rather than assuming that there was truth in old, previously accepted models. Only by tying ideas, thoughts, and statements to actual objects and occurrences could one begin to identify a coherent truth, according to James. His work strongly influenced the subsequent avant-garde and modernist movements in literature and art, especially in understanding the role of the observer, artist, or writer in shaping the society they attempted to observe. John Dewey built on the idea of pragmatism to create a theory of instrumentalism, which advocated the use of education in the search for truth. Dewey believed that education, specifically observation and change through the scientific method, was the best tool by which to reform and improve American society as it continued to grow ever more complex. To that end, Dewey strongly encouraged educational reforms designed to create an informed American citizenry that could then form the basis for other, much-needed progressive reforms in society.
In addition to the new medium of photography, popularized by Riis, novelists and other artists also embraced realism in their work. They sought to portray vignettes from real life in their stories, partly in response to the more sentimental works of their predecessors. Visual artists such as George Bellows, Edward Hopper, and Robert Henri, among others, formed the Ashcan School of Art, which was interested primarily in depicting the urban lifestyle that was quickly gripping the United States at the turn of the century. Their works typically focused on working-class city life, including the slums and tenement houses, as well as working-class forms of leisure and entertainment (Figure).
Novelists and journalists also popularized realism in literary works. Authors such as Stephen Crane, who wrote stark stories about life in the slums or during the Civil War, and Rebecca Harding Davis, who in 1861 published Life in the Iron Mills, embodied this popular style. Mark Twain also sought realism in his books, whether it was the reality of the pioneer spirit, seen in The Adventures of Huckleberry Finn, published in 1884, or the issue of corruption in The Gilded Age, co-authored with Charles Dudley Warner in 1873. The narratives and visual arts of these realists could nonetheless be highly stylized, crafted, and even fabricated, since their goal was the effective portrayal of social realities they thought required reform. Some authors, such as Jack London, who wrote Call of the Wild, embraced a school of thought called naturalism, which concluded that the laws of nature and the natural world were the only truly relevant laws governing humanity (Figure).
Kate Chopin, widely regarded as the foremost woman short story writer and novelist of her day, sought to portray a realistic view of women’s lives in late nineteenth-century America, thus paving the way for more explicit feminist literature in generations to come. Although Chopin never described herself as a feminist per se, her reflective works on her experiences as a southern woman introduced a form of creative nonfiction that captured the struggles of women in the United States through their own individual experiences. She also was among the first authors to openly address the race issue of miscegenation. In her work Desiree’s Baby, Chopin specifically explores the Creole community of her native Louisiana in depths that exposed the reality of racism in a manner seldom seen in literature of the time.
African American poet, playwright, and novelist of the realist period, Paul Laurence Dunbar dealt with issues of race at a time when most reform-minded Americans preferred to focus on other issues. Through his combination of writing in both standard English and black dialect, Dunbar delighted readers with his rich portrayals of the successes and struggles associated with African American life. Although he initially struggled to find the patronage and financial support required to develop a full-time literary career, Dunbar’s subsequent professional relationship with literary critic and Atlantic Monthly editor William Dean Howells helped to firmly cement his literary credentials as the foremost African American writer of his generation. As with Chopin and Harding, Dunbar’s writing highlighted parts of the American experience that were not well understood by the dominant demographic of the country. In their work, these authors provided readers with insights into a world that was not necessarily familiar to them and also gave hidden communities—be it iron mill workers, southern women, or African American men—a sense of voice.
Mark Twain’s lampoon of author Horatio Alger demonstrates Twain’s commitment to realism by mocking the myth set out by Alger, whose stories followed a common theme in which a poor but honest boy goes from rags to riches through a combination of “luck and pluck.” See how Twain twists Alger’s hugely popular storyline in this piece of satire.
Kate Chopin: An Awakening in an Unpopular Time
Author Kate Chopin grew up in the American South and later moved to St. Louis, where she began writing stories to make a living after the death of her husband. She published her works throughout the late 1890s, with stories appearing in literary magazines and local papers. It was her second novel, The Awakening, which gained her notoriety and criticism in her lifetime, and ongoing literary fame after her death (Figure).
The Awakening, set in the New Orleans society that Chopin knew well, tells the story of a woman struggling with the constraints of marriage who ultimately seeks her own fulfillment over the needs of her family. The book deals far more openly than most novels of the day with questions of women’s sexual desires. It also flouted nineteenth-century conventions by looking at the protagonist’s struggles with the traditional role expected of women.
While a few contemporary reviewers saw merit in the book, most criticized it as immoral and unseemly. It was censored, called “pure poison,” and critics railed against Chopin herself. While Chopin wrote squarely in the tradition of realism that was popular at this time, her work covered ground that was considered “too real” for comfort. After the negative reception of the novel, Chopin retreated from public life and discontinued writing. She died five years after its publication. After her death, Chopin’s work was largely ignored, until scholars rediscovered it in the late twentieth century, and her books and stories came back into print. The Awakening in particular has been recognized as vital to the earliest edges of the modern feminist movement.
Excerpts from interviews with David Chopin, Kate Chopin’s grandson, and a scholar who studies her work provide interesting perspectives on the author and her views.
CRITICS OF MODERN AMERICA
While many Americans at this time, both everyday working people and theorists, felt the changes of the era would lead to improvements and opportunities, there were critics of the emerging social shifts as well. Although less popular than Twain and London, authors such as Edward Bellamy, Henry George, and Thorstein Veblen were also influential in spreading critiques of the industrial age. While their critiques were quite distinct from each other, all three believed that the industrial age was a step in the wrong direction for the country.
In the 1888 novel Looking Backward, 2000-1887, Edward Bellamy portrays a utopian America in the year 2000, with the country living in peace and harmony after abandoning the capitalist model and moving to a socialist state. In the book, Bellamy predicts the future advent of credit cards, cable entertainment, and “super-store” cooperatives that resemble a modern day Wal-Mart. Looking Backward proved to be a popular bestseller (third only to Uncle Tom’s Cabin and Ben Hur among late nineteenth-century publications) and appealed to those who felt the industrial age of big business was sending the country in the wrong direction. Eugene Debs, who led the national Pullman Railroad Strike in 1894, later commented on how Bellamy’s work influenced him to adopt socialism as the answer to the exploitative industrial capitalist model. In addition, Bellamy’s work spurred the publication of no fewer than thirty-six additional books or articles by other writers, either supporting Bellamy’s outlook or directly criticizing it. In 1897, Bellamy felt compelled to publish a sequel, entitled Equality, in which he further explained ideas he had previously introduced concerning educational reform and women’s equality, as well as a world of vegetarians who speak a universal language.
Another author whose work illustrated the criticisms of the day was nonfiction writer Henry George, an economist best known for his 1879 work Progress and Poverty, which criticized the inequality found in an industrial economy. He suggested that, while people should own that which they create, all land and natural resources should belong to all equally, and should be taxed through a “single land tax” in order to disincentivize private land ownership. His thoughts influenced many economic progressive reformers, as well as led directly to the creation of the now-popular board game, Monopoly.
Another critique of late nineteenth-century American capitalism was Thorstein Veblen, who lamented in The Theory of the Leisure Class (1899) that capitalism created a middle class more preoccupied with its own comfort and consumption than with maximizing production. In coining the phrase “conspicuous consumption,” Veblen identified the means by which one class of nonproducers exploited the working class that produced the goods for their consumption. Such practices, including the creation of business trusts, served only to create a greater divide between the haves and have-nots in American society, and resulted in economic inefficiencies that required correction or reform.
Section Summary
Americans were overwhelmed by the rapid pace and scale of change at the close of the nineteenth century. Authors and thinkers tried to assess the meaning of the country’s seismic shifts in culture and society through their work. Fiction writers often used realism in an attempt to paint an accurate portrait of how people were living at the time. Proponents of economic developments and cultural changes cited social Darwinism as an acceptable model to explain why some people succeeded and others failed, whereas other philosophers looked more closely at Darwin’s work and sought to apply a model of proof and pragmatism to all ideas and institutions. Other sociologists and philosophers criticized the changes of the era, citing the inequities found in the new industrial economy and its negative effects on workers.
Review Questions
Which of the following statements accurately represents Thorstein Veblen’s argument in The Theory of the Leisure Class?
- All citizens of an industrial society would rise or fall based on their own innate merits.
- The tenets of naturalism were the only laws through which society should be governed.
- The middle class was overly focused on its own comfort and consumption.
- Land and natural resources should belong equally to all citizens.
Hint:
C
Which of the following was not an element of realism?
- social Darwinism
- instrumentalism
- naturalism
- pragmatism
Hint:
A
In what ways did writers, photographers, and visual artists begin to embrace more realistic subjects in their work? How were these responses to the advent of the industrial age and the rise of cities?
Hint:
The growth of the industrial economy and the dramatic growth of cities created new, harsh realities that were often hidden from the public eye. Writers and artists, responding both to this fact and to the sentimentalism that characterized the writing and art of their predecessors, began to depict subjects that reflected the new truth. Photographers like Jacob Riis sought to present to the public the realities of working-class life and labor. Novelists began to portray true-to-life vignettes in their stories. Visual artists such as George Bellows, Edward Hopper, and Robert Henri formed the Ashcan School of Art, which depicted the often gritty realities of working-class city life, leisure, and entertainment.
Critical Thinking Questions
What triumphs did the late nineteenth century witness in the realms of industrial growth, urbanization, and technological innovation? What challenges did these developments pose for urban dwellers, workers, and recent immigrants? How did city officials and everyday citizens respond to these challenges?
What were the effects of urbanization on the working, middle, and elite classes of American society? Conversely, how did the different social classes and their activities change the scope, character, and use of urban spaces?
How do you think that different classes of city dwellers would have viewed the City Beautiful movement? What potential benefits and drawbacks of this new direction in urban planning might members of each class have cited?
How was Darwin’s work on the evolution of species exploited by proponents of the industrial age? Why might they have latched on to this idea in particular?
Historians often mine the arts for clues to the social, cultural, political, and intellectual shifts that characterized a given era. How do the many works of visual art, literature, and social philosophy that emerged from this period reflect the massive changes that were taking place? How were Americans—both those who created these works and those who read or viewed them—struggling to understand the new reality through art, literature, and scholarship?
|
oercommons
|
2025-03-18T00:37:58.683193
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15497/overview",
"title": "U.S. History, The Growing Pains of Urbanization, 1870-1900",
"author": null
}
|
https://oercommons.org/courseware/lesson/15472/overview
|
Introduction
Overview
- The Origins and Outbreak of the Civil War
- Early Mobilization and War
- 1863: The Changing Nature of the War
- The Union Triumphant
In May 1864, General Ulysses S. Grant ordered the Union’s Army of the Potomac to cross the Rapidan River in Virginia. Grant knew that Confederate general Robert E. Lee would defend the Confederate capital at Richmond at all costs, committing troops that might otherwise be sent to the Shenandoah or the Deep South to stop Union general William Tecumseh Sherman from capturing Atlanta, a key Confederate city. For two days, the Army of the Potomac fought Lee’s troops in the Wilderness, a wooded area along the Rapidan River. Nearly ten thousand Confederate soldiers were killed or wounded, as were more than seventeen thousand Union troops. A few weeks later, the armies would meet again at the Battle of Cold Harbor, where another fifteen thousand men would be wounded or killed. As in many battles, the bodies of those who died were left on the field where they fell. A year later, African Americans, who were often called upon to perform menial labor for the Union army (Figure), collected the skeletal remains of the dead for a proper burial. The state of the graves of many Civil War soldiers partly inspired the creation of Memorial Day, a day set aside for visiting and decorating the graves of the dead.
|
oercommons
|
2025-03-18T00:37:58.699648
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15472/overview",
"title": "U.S. History, The Civil War, 1860–1865",
"author": null
}
|
https://oercommons.org/courseware/lesson/15473/overview
|
The Origins and Outbreak of the Civil War
Overview
By the end of this section, you will be able to:
- Explain the major events that occurred during the Secession Crisis
- Describe the creation and founding principles of the Confederate States of America
The 1860 election of Abraham Lincoln was a turning point for the United States. Throughout the tumultuous 1850s, the Fire-Eaters of the southern states had been threatening to leave the Union. With Lincoln’s election, they prepared to make good on their threats. Indeed, the Republican president-elect appeared to be their worst nightmare. The Republican Party committed itself to keeping slavery out of the territories as the country expanded westward, a position that shocked southern sensibilities. Meanwhile, southern leaders suspected that Republican abolitionists would employ the violent tactics of John Brown to deprive southerners of their slave property. The threat posed by the Republican victory in the election of 1860 spurred eleven southern states to leave the Union to form the Confederate States of America, a new republic dedicated to maintaining and expanding slavery. The Union, led by President Lincoln, was unwilling to accept the departure of these states and committed itself to restoring the country. Beginning in 1861 and continuing until 1865, the United States engaged in a brutal Civil War that claimed the lives of over 600,000 soldiers. By 1863, the conflict had become not only a war to save the Union, but also a war to end slavery in the United States. Only after four years of fighting did the North prevail. The Union was preserved, and the institution of slavery had been destroyed in the nation.
THE CAUSES OF THE CIVIL WAR
Lincoln’s election sparked the southern secession fever into flame, but it did not cause the Civil War. For decades before Lincoln took office, the sectional divisions in the country had been widening. Both the Northern and southern states engaged in inflammatory rhetoric and agitation, and violent emotions ran strong on both sides. Several factors played into the ultimate split between the North and the South.
One key irritant was the question of slavery’s expansion westward. The debate over whether new states would be slave or free reached back to the controversy over statehood for Missouri beginning in 1819 and Texas in the 1830s and early 1840s. This question arose again after the Mexican-American War (1846–1848), when the government debated whether slavery would be permitted in the territories taken from Mexico. Efforts in Congress to reach a compromise in 1850 fell back on the principle of popular sovereignty—letting the people in the new territories south of the 1820 Missouri Compromise line decide whether to allow slavery. This same principle came to be applied to the Kansas-Nebraska territories in 1854, a move that added fuel to the fire of sectional conflict by destroying the Missouri Compromise boundary and leading to the birth of the Republican Party. In the end, popular sovereignty proved to be no solution at all. This was especially true in “Bleeding Kansas” in the mid-1850s, as pro- and antislavery forces battled each another in an effort to gain the upper hand.
The small but very vocal abolitionist movement further contributed to the escalating tensions between the North and the South. Since the 1830s, abolitionists, led by journalist and reformer William Lloyd Garrison, had cast slavery as a national sin and called for its immediate end. For three decades, the abolitionists—a minority even within the antislavery movement—had had a significant effect on American society by bringing the evils of slavery into the public consciousness. By the 1850s, some of the most radical abolitionists, such as John Brown, had resorted to violence in their efforts to destroy the institution of slavery.
The formation of the Liberty Party (1840), the Free-Soil Party (1848), and the Republican Party (1854), all of which strongly opposed the spread of slavery to the West, brought the question solidly into the political arena. Although not all those who opposed the westward expansion of slavery had a strong abolitionist bent, the attempt to limit slaveholders’ control of their human property stiffened the resolve of southern leaders to defend their society at all costs. Prohibiting slavery’s expansion, they argued, ran counter to fundamental American property rights. Across the country, people of all political stripes worried that the nation’s arguments would cause irreparable rifts in the country.
Despite the ruptures and tensions, by the 1860s, some hope of healing the nation still existed. Before Lincoln took office, John Crittenden, a senator from Kentucky who had helped form the Constitutional Union Party during the 1860 presidential election, attempted to diffuse the explosive situation by offering six constitutional amendments and a series of resolutions, known as the Crittenden Compromise. Crittenden’s goal was to keep the South from seceding, and his strategy was to transform the Constitution to explicitly protect slavery forever. Specifically, Crittenden proposed an amendment that would restore the 36°30′ line from the Missouri Compromise and extend it all the way to the Pacific Ocean, protecting and ensuring slavery south of the line while prohibiting it north of the line (Figure). He further proposed an amendment that would prohibit Congress from abolishing slavery anywhere it already existed or from interfering with the interstate slave trade.
Republicans, including President-elect Lincoln, rejected Crittenden’s proposals because they ran counter to the party’s goal of keeping slavery out of the territories. The southern states also rejected Crittenden’s attempts at compromise, because it would prevent slaveholders from taking their human chattel north of the 36°30′ line. On December 20, 1860, only a few days after Crittenden’s proposal was introduced in Congress, South Carolina began the march towards war when it seceded from the United States. Three more states of the Deep South—Mississippi, Florida, and Alabama—seceded before the U.S. Senate rejected Crittenden’s proposal on January 16, 1861. Georgia, Louisiana, and Texas joined them in rapid succession on January 19, January 26, and February 1, respectively (Figure). In many cases, these secessions occurred after extremely divided conventions and popular votes. A lack of unanimity prevailed in much of the South.
Explore the causes, battles, and aftermath of the Civil War at the interactive website offered by the National Parks Service.
THE CREATION OF THE CONFEDERATE STATES OF AMERICA
The seven Deep South states that seceded quickly formed a new government. In the opinion of many Southern politicians, the federal Constitution that united the states as one nation was a contract by which individual states had agreed to be bound. However, they maintained, the states had not sacrificed their autonomy and could withdraw their consent to be controlled by the federal government. In their eyes, their actions were in keeping with the nature of the Constitution and the social contract theory of government that had influenced the founders of the American Republic.
The new nation formed by these men would not be a federal union, but a confederation. In a confederation, individual member states agree to unite under a central government for some purposes, such as defense, but to retain autonomy in other areas of government. In this way, states could protect themselves, and slavery, from interference by what they perceived to be an overbearing central government. The constitution of the Confederate States of America (CSA), or the Confederacy, drafted at a convention in Montgomery, Alabama, in February 1861, closely followed the 1787 Constitution. The only real difference between the two documents centered on slavery. The Confederate Constitution declared that the new nation existed to defend and perpetuate racial slavery, and the leadership of the slaveholding class. Specifically, the constitution protected the interstate slave trade, guaranteed that slavery would exist in any new territory gained by the Confederacy, and, perhaps most importantly, in Article One, Section Nine, declared that “No . . . law impairing or denying the right of property in negro slaves shall be passed.” Beyond its focus on slavery, the Confederate Constitution resembled the 1787 U.S. Constitution. It allowed for a Congress composed of two chambers, a judicial branch, and an executive branch with a president to serve for six years.
The convention delegates chose Jefferson Davis of Mississippi to lead the new provisional government as president and Alexander Stephens of Georgia to serve as vice president until elections could be held in the spring and fall of 1861. By that time, four new states—Virginia, Arkansas, Tennessee, and North Carolina—had joined the CSA. As 1861 progressed, the Confederacy claimed Missouri and Kentucky, even though no ordinance of secession had been approved in those states. Southern nationalism ran high, and the Confederacy, buoyed by its sense of purpose, hoped that their new nation would achieve eminence in the world.
By the time Lincoln reached Washington, DC, in February 1861, the CSA had already been established. The new president confronted an unprecedented crisis. A conference held that month with delegates from the Southern states failed to secure a promise of peace or to restore the Union. On inauguration day, March 4, 1861, the new president repeated his views on slavery: “I have no purpose, directly or indirectly, to interfere with the institution of slavery in the States where it exists. I believe I have no lawful right to do so, and I have no inclination to do so.” His recognition of slavery in the South did nothing to mollify slaveholders, however, because Lincoln also pledged to keep slavery from expanding into the new western territories. Furthermore, in his inaugural address, Lincoln made clear his commitment to maintaining federal power against the secessionists working to destroy it. Lincoln declared that the Union could not be dissolved by individual state actions, and, therefore, secession was unconstitutional.
Read Lincoln’s entire inaugural address at the Yale Avalon project’s website. How would Lincoln’s audience have responded to this speech?
FORT SUMTER
President Lincoln made it clear to Southern secessionists that he would fight to maintain federal property and to keep the Union intact. Other politicians, however, still hoped to avoid the use of force to resolve the crisis. In February 1861, in an effort to entice the rebellious states to return to the Union without resorting to force, Thomas Corwin, a representative from Ohio, introduced a proposal to amend the Constitution in the House of Representatives. His was but one of several measures proposed in January and February 1861, to head off the impending conflict and save the United States. The proposed amendment would have made it impossible for Congress to pass any law abolishing slavery. The proposal passed the House on February 28, 1861, and the Senate passed the proposal on March 2, 1861. It was then sent to the states to be ratified. Once ratified by three-quarters of state legislatures, it would become law. In his inaugural address, Lincoln stated that he had no objection to the amendment, and his predecessor James Buchanan had supported it. By the time of Lincoln’s inauguration, however, seven states had already left the Union. Of the remaining states, Ohio ratified the amendment in 1861, and Maryland and Illinois did so in 1862. Despite this effort at reconciliation, the Confederate states did not return to the Union.
Indeed, by the time of the Corwin amendment’s passage through Congress, Confederate forces in the Deep South had already begun to take over federal forts. The loss of Fort Sumter, in the harbor of Charleston, South Carolina, proved to be the flashpoint in the contest between the new Confederacy and the federal government. A small Union garrison of fewer than one hundred soldiers and officers held the fort, making it a vulnerable target for the Confederacy. Fire-Eaters pressured Jefferson Davis to take Fort Sumter and thereby demonstrate the Confederate government’s resolve. Some also hoped that the Confederacy would gain foreign recognition, especially from Great Britain, by taking the fort in the South’s most important Atlantic port. The situation grew dire as local merchants refused to sell food to the fort’s Union soldiers, and by mid-April, the garrison’s supplies began to run out. President Lincoln let it be known to Confederate leaders that he planned to resupply the Union forces. His strategy was clear: The decision to start the war would rest squarely on the Confederates, not on the Union. On April 12, 1861, Confederate forces in Charleston began a bombardment of Fort Sumter (Figure). Two days later, the Union soldiers there surrendered.
The attack on Fort Sumter meant war had come, and on April 15, 1861, Lincoln called upon loyal states to supply armed forces to defeat the rebellion and regain Fort Sumter. Faced with the need to choose between the Confederacy and the Union, border states and those of the Upper South, which earlier had been reluctant to dissolve their ties with the United States, were inspired to take action. They quickly voted for secession. A convention in Virginia that had been assembled earlier to consider the question of secession voted to join the Confederacy on April 17, two days after Lincoln called for troops. Arkansas left the Union on May 6 along with Tennessee one day later. North Carolina followed on May 20.
Not all residents of the border states and the Upper South wished to join the Confederacy, however. Pro-Union feelings remained strong in Tennessee, especially in the eastern part of the state where slaves were few and consisted largely of house servants owned by the wealthy. The state of Virginia—home of revolutionary leaders and presidents such as George Washington, Thomas Jefferson, James Madison, and James Monroe—literally was split on the issue of secession. Residents in the north and west of the state, where few slaveholders resided, rejected secession. These counties subsequently united to form “West Virginia,” which entered the Union as a free state in 1863. The rest of Virginia, including the historic lands along the Chesapeake Bay that were home to such early American settlements as Jamestown and Williamsburg, joined the Confederacy. The addition of this area gave the Confederacy even greater hope and brought General Robert E. Lee, arguably the best military commander of the day, to their side. In addition, the secession of Virginia brought Washington, DC, perilously close to the Confederacy, and fears that the border state of Maryland would also join the CSA, thus trapping the U.S. capital within Confederate territories, plagued Lincoln.
The Confederacy also gained the backing of the Five Civilized Tribes, as they were called, in the Indian Territory. The Five Civilized Tribes comprised the Choctaws, Chickasaws, Creeks, Seminoles, and Cherokees. The tribes supported slavery and many members owned slaves. These Indian slaveholders, who had been forced from their lands in Georgia and elsewhere in the Deep South during the presidency of Andrew Jackson, now found unprecedented common cause with white slaveholders. The CSA even allowed them to send delegates to the Confederate Congress.
While most slaveholding states joined the Confederacy, four crucial slave states remained in the Union (Figure). Delaware, which was technically a slave state despite its tiny slave population, never voted to secede. Maryland, despite deep divisions, remained in the Union as well. Missouri became the site of vicious fighting and the home of pro-Confederate guerillas but never joined the Confederacy. Kentucky declared itself neutral, although that did little to stop the fighting that occurred within the state. In all, these four states deprived the Confederacy of key resources and soldiers.
Section Summary
The election of Abraham Lincoln to the presidency in 1860 proved to be a watershed event. While it did not cause the Civil War, it was the culmination of increasing tensions between the proslavery South and the antislavery North. Before Lincoln had even taken office, seven Deep South states had seceded from the Union to form the CSA, dedicated to maintaining racial slavery and white supremacy. Last-minute efforts to reach a compromise, such as the proposal by Senator Crittenden and the Corwin amendment, went nowhere. The time for compromise had come to an end. With the Confederate attack on Fort Sumter, the Civil War began.
Review Questions
Which of the following does not represent a goal of the Confederate States of America?
- to protect slavery from any effort to abolish it
- to protect the domestic slave trade
- to ensure that slavery would be allowed to spread into western territories
- to ensure that the international slave trade would be allowed to continue
Hint:
D
Which was not a provision of the Crittenden Compromise?
- that the Five Civilized Tribes would be admitted into the Confederacy
- that the 36°30′ line from the Missouri Compromise would be restored and extended
- that Congress would be prohibited from abolishing slavery where it already existed
- that the interstate slave trade would be allowed to continue
Hint:
A
Why did the states of the Deep South secede from the Union sooner than the states of the Upper South and the border states?
Hint:
Slavery was more deeply entrenched in the Deep South than it was in the Upper South or the border states. The Deep South was home to larger numbers of both slaveholders and slaves. Pro-Union sentiment remained strong in parts of the Upper South and border states, particularly those areas with smaller populations of slaveholders.
|
oercommons
|
2025-03-18T00:37:58.727605
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15473/overview",
"title": "U.S. History, The Civil War, 1860–1865",
"author": null
}
|
https://oercommons.org/courseware/lesson/15474/overview
|
Early Mobilization and War
Overview
By the end of this section, you will be able to:
- Assess the strengths and weaknesses of the Confederacy and the Union
- Explain the strategic importance of the Battle of Bull Run and the Battle of Shiloh
In 1861, enthusiasm for war ran high on both sides. The North fought to restore the Union, which Lincoln declared could never be broken. The Confederacy, which by the summer of 1861 consisted of eleven states, fought for its independence from the United States. The continuation of slavery was a central issue in the war, of course, although abolitionism and western expansion also played roles, and Northerners and Southerners alike flocked eagerly to the conflict. Both sides thought it would be over quickly. Militarily, however, the North and South were more equally matched than Lincoln had realized, and it soon became clear that the war effort would be neither brief nor painless. In 1861, Americans in both the North and South romanticized war as noble and positive. Soon the carnage and slaughter would awaken them to the horrors of war.
THE FIRST BATTLE OF BULL RUN
After the fall of Fort Sumter on April 15, 1861, Lincoln called for seventy-five thousand volunteers from state militias to join federal forces. His goal was a ninety-day campaign to put down the Southern rebellion. The response from state militias was overwhelming, and the number of Northern troops exceeded the requisition. Also in April, Lincoln put in place a naval blockade of the South, a move that gave tacit recognition of the Confederacy while providing a legal excuse for the British and the French to trade with Southerners. The Confederacy responded to the blockade by declaring that a state of war existed with the United States. This official pronouncement confirmed the beginning of the Civil War. Men rushed to enlist, and the Confederacy turned away tens of thousands who hoped to defend the new nation.
Many believed that a single, heroic battle would decide the contest. Some questioned how committed Southerners really were to their cause. Northerners hoped that most Southerners would not actually fire on the American flag. Meanwhile, Lincoln and military leaders in the North hoped a quick blow to the South, especially if they could capture the Confederacy’s new capital of Richmond, Virginia, would end the rebellion before it went any further. On July 21, 1861, the two armies met near Manassas, Virginia, along Bull Run Creek, only thirty miles from Washington, DC. So great was the belief that this would be a climactic Union victory that many Washington socialites and politicians brought picnic lunches to a nearby area, hoping to witness history unfolding before them. At the First Battle of Bull Run, also known as First Manassas, some sixty thousand troops assembled, most of whom had never seen combat, and each side sent eighteen thousand into the fray. The Union forces attacked first, only to be pushed back. The Confederate forces then carried the day, sending the Union soldiers and Washington, DC, onlookers scrambling back from Virginia and destroying Union hopes of a quick, decisive victory. Instead, the war would drag on for four long, deadly years (Figure).
BALANCE SHEET: THE UNION AND THE CONFEDERACY
As it became clearer that the Union would not be dealing with an easily quashed rebellion, the two sides assessed their strengths and weaknesses. At the onset on the war, in 1861 and 1862, they stood as relatively equal combatants.
The Confederates had the advantage of being able to wage a defensive war, rather than an offensive one. They had to protect and preserve their new boundaries, but they did not have to be the aggressors against the Union. The war would be fought primarily in the South, which gave the Confederates the advantages of the knowledge of the terrain and the support of the civilian population. Further, the vast coastline from Texas to Virginia offered ample opportunities to evade the Union blockade. And with the addition of the Upper South states, especially Virginia, North Carolina, Tennessee, and Arkansas, the Confederacy gained a much larger share of natural resources and industrial might than the Deep South states could muster.
Still, the Confederacy had disadvantages. The South’s economy depended heavily on the export of cotton, but with the naval blockade, the flow of cotton to England, the region’s primary importer, came to an end. The blockade also made it difficult to import manufactured goods. Although the secession of the Upper South added some industrial assets to the Confederacy, overall, the South lacked substantive industry or an extensive railroad infrastructure to move men and supplies. To deal with the lack of commerce and the resulting lack of funds, the Confederate government began printing paper money, leading to runaway inflation (Figure). The advantage that came from fighting on home territory quickly turned to a disadvantage when Confederate armies were defeated and Union forces destroyed Southern farms and towns, and forced Southern civilians to take to the road as refugees. Finally, the population of the South stood at fewer than nine million people, of whom nearly four million were black slaves, compared to over twenty million residents in the North. These limited numbers became a major factor as the war dragged on and the death toll rose.
The Union side held many advantages as well. Its larger population, bolstered by continued immigration from Europe throughout the 1860s, gave it greater manpower reserves to draw upon. The North’s greater industrial capabilities and extensive railroad grid made it far better able to mobilize men and supplies for the war effort. The Industrial Revolution and the transportation revolution, beginning in the 1820s and continuing over the next several decades, had transformed the North. Throughout the war, the North was able to produce more war materials and move goods more quickly than the South. Furthermore, the farms of New England, the Mid-Atlantic, the Old Northwest, and the prairie states supplied Northern civilians and Union troops with abundant food throughout the war. Food shortages and hungry civilians were common in the South, where the best land was devoted to raising cotton, but not in the North.
Unlike the South, however, which could hunker down to defend itself and needed to maintain relatively short supply lines, the North had to go forth and conquer. Union armies had to establish long supply lines, and Union soldiers had to fight on unfamiliar ground and contend with a hostile civilian population off the battlefield. Furthermore, to restore the Union—Lincoln’s overriding goal, in 1861—the United States, after defeating the Southern forces, would then need to pacify a conquered Confederacy, an area of over half a million square miles with nearly nine million residents. In short, although it had better resources and a larger population, the Union faced a daunting task against the well-positioned Confederacy.
MILITARY STALEMATE
The military forces of the Confederacy and the Union battled in 1861 and early 1862 without either side gaining the upper hand. The majority of military leaders on both sides had received the same military education and often knew one another personally, either from their time as students at West Point or as commanding officers in the Mexican-American War. This familiarity allowed them to anticipate each other’s strategies. Both sides believed in the use of concentrated armies charged with taking the capital city of the enemy. For the Union, this meant the capture of the Confederate capital in Richmond, Virginia, whereas Washington, DC, stood as the prize for Confederate forces. After hopes of a quick victory faded at Bull Run, the months dragged on without any major movement on either side (Figure).
General George B. McClellan, the general in chief of the army, responsible for overall control of Union land forces, proved especially reluctant to engage in battle with the Confederates. In direct command of the Army of the Potomac, the Union fighting force operating outside Washington, DC, McClellan believed, incorrectly, that Confederate forces were too strong to defeat and was reluctant to risk his troops in battle. His cautious nature made him popular with his men but not with the president or Congress. By 1862, however, both President Lincoln and the new Secretary of War Edwin Stanton had tired of waiting. The Union put forward a new effort to bolster troop strength, enlisting one million men to serve for three-year stints in the Army of the Potomac. In January 1862, Lincoln and Stanton ordered McClellan to invade the Confederacy with the goal of capturing Richmond.
To that end, General McClellan slowly moved 100,000 soldiers of the Army of the Potomac toward Richmond but stopped a few miles outside the city. As he did so, a Confederate force led by Thomas “Stonewall” Jackson moved north to take Washington, DC. To fend off Jackson’s attack, somewhere between one-quarter and one-third of McClellan’s soldiers, led by Major General Irvin McDowell, returned to defend the nation’s capital, a move that Jackson hoped would leave the remaining troops near Richmond more vulnerable. Having succeeding in drawing off a sizable portion of the Union force, he joined General Lee to launch an attack on McClellan’s remaining soldiers near Richmond. From June 25 to July 1, 1862, the two sides engaged in the brutal Seven Days Battles that killed or wounded almost twenty thousand Confederate and ten thousand Union soldiers. McClellan’s army finally returned north, having failed to take Richmond.
General Lee, flush from his success at keeping McClellan out of Richmond, tried to capitalize on the Union’s failure by taking the fighting northward. He moved his forces into northern Virginia, where, at the Second Battle of Bull Run, the Confederates again defeated the Union forces. Lee then pressed into Maryland, where his troops met the much larger Union forces near Sharpsburg, at Antietam Creek. The ensuing one-day battle on September 17, 1862, led to a tremendous loss of life. Although there are varying opinions about the total number of deaths, eight thousand soldiers were killed or wounded, more than on any other single day of combat. Once again, McClellan, mistakenly believing that the Confederate troops outnumbered his own, held back a significant portion of his forces. Lee withdrew from the field first, but McClellan, fearing he was outnumbered, refused to pursue him.
The Union army’s inability to destroy Lee’s army at Antietam made it clear to Lincoln that McClellan would never win the war, and the president was forced to seek a replacement. Lincoln wanted someone who could deliver a decisive Union victory. He also personally disliked McClellan, who referred to the president as a “baboon” and a “gorilla,” and constantly criticized his decisions. Lincoln chose General Ambrose E. Burnside to replace McClellan as commander of the Army of the Potomac, but Burnside’s efforts to push into Virginia failed in December 1862, as Confederates held their position at Fredericksburg and devastated Burnside’s forces with heavy artillery fire. The Union’s defeat at Fredericksburg harmed morale in the North but bolstered Confederate spirits. By the end of 1862, the Confederates were still holding their ground in Virginia. Burnside’s failure led Lincoln to make another change in leadership, and Joseph “Fighting Joe” Hooker took over command of the Army of the Potomac in January 1863.
General Ulysses S. Grant’s Army of the West, operating in Kentucky, Tennessee, and the Mississippi River Valley, had been more successful. In the western campaign, the goal of both the Union and the Confederacy was to gain control of the major rivers in the west, especially the Mississippi. If the Union could control the Mississippi, the Confederacy would be split in two. The fighting in this campaign initially centered in Tennessee, where Union forces commanded by Grant pushed Confederate troops back and gained control of the state. The major battle in the western theater took place at Pittsburgh Landing, Tennessee, on April 6 and 7, 1862. Grant’s army was camped on the west side of the Tennessee River near a small log church called Shiloh, which gave the battle its name. On Sunday morning, April 6, Confederate forces under General Albert Sidney Johnston attacked Grant’s encampment with the goal of separating them from their supply line on the Tennessee River and driving them into the swamps on the river’s western side, where they could be destroyed. Union general William Tecumseh Sherman tried to rally the Union forces as Grant, who had been convalescing from an injured leg when the attack began and was unable to walk without crutches, called for reinforcements and tried to mount a defense. Many of Union troops fled in terror.
Unfortunately for the Confederates, Johnston was killed on the afternoon of the first day. Leadership of the Southern forces fell to General P. G. T. Beauregard, who ordered an assault at the end of that day. This assault was so desperate that one of the two attacking columns did not even have ammunition. Heavily reinforced Union forces counterattacked the next day, and the Confederate forces were routed. Grant had maintained the Union foothold in the western part of the Confederacy. The North could now concentrate on its efforts to gain control of the Mississippi River, splitting the Confederacy in two and depriving it of its most important water route.
Read a first-hand account from a Confederate soldier at the Battle at Shiloh, followed by the perspective of a Union soldier at the same battle.
In the spring and summer of 1862, the Union was successful in gaining control of part of the Mississippi River. In April 1862, the Union navy under Admiral David Farragut fought its way past the forts that guarded New Orleans and fired naval guns upon the below-sea-level city. When it became obvious that New Orleans could no longer be defended, Confederate major general Marshall Lovell sent his artillery upriver to Vicksburg, Mississippi. Armed civilians in New Orleans fought the Union forces that entered the city. They also destroyed ships and military supplies that might be used by the Union. Upriver, Union naval forces also bombarded Fort Pillow, forty miles from Memphis, Tennessee, a Southern industrial center and one of the largest cities in the Confederacy. On June 4, 1862, the Confederate defenders abandoned the fort. On June 6, Memphis fell to the Union after the ships defending it were destroyed.
Section Summary
Many in both the North and the South believed that a short, decisive confrontation in 1861 would settle the question of the Confederacy. These expectations did not match reality, however, and the war dragged on into a second year. Both sides mobilized, with advantages and disadvantages on each side that led to a rough equilibrium. The losses of battles at Manassas and Fredericksburg, Virginia, kept the North from achieving the speedy victory its generals had hoped for, but the Union did make gains and continued to press forward. While they could not capture the Southern capital of Richmond, they were victorious in the Battle of Shiloh and captured New Orleans and Memphis. Thus, the Confederates lost major ground on the western front.
Review Questions
All the following were strengths of the Union except ________.
- a large population
- substantial industry
- an extensive railroad
- the ability to fight defensively, rather than offensively
Hint:
D
All the following were strengths of the Confederacy except ________.
- the ability to wage a defensive war
- shorter supply lines
- the resources of the Upper South states
- a strong navy
Hint:
D
What military successes and defeats did the Union experience in 1862?
Hint:
In the eastern part of the Confederacy, the Army of the Potomac met with mixed success. The Union army failed to capture Richmond and won at Antietam only because the Confederates withdrew from the field first. In the western part of the Confederacy, the Army of the West won the Battle of Shiloh, and the Union navy captured New Orleans and Memphis.
|
oercommons
|
2025-03-18T00:37:58.756288
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15474/overview",
"title": "U.S. History, The Civil War, 1860–1865",
"author": null
}
|
https://oercommons.org/courseware/lesson/15475/overview
|
1863: The Changing Nature of the War
Overview
By the end of this section, you will be able to:
- Explain what is meant by the term “total war” and provide examples
- Describe mobilization efforts in the North and the South
- Explain why 1863 was a pivotal year in the war
Wars have their own logic; they last far longer than anyone anticipates at the beginning of hostilities. As they drag on, the energy and zeal that marked the entry into warfare often wane, as losses increase and people on both sides suffer the tolls of war. The American Civil War is a case study of this characteristic of modern war.
Although Northerners and Southerners both anticipated that the battle between the Confederacy and the Union would be settled quickly, it soon became clear to all that there was no resolution in sight. The longer the war continued, the more it began to affect life in both the North and the South. Increased need for manpower, the issue of slavery, and the ongoing challenges of keeping the war effort going changed the way life on both sides as the conflict progressed.
MASS MOBILIZATION
By late 1862, the course of the war had changed to take on the characteristics of total war, in which armies attempt to demoralize the enemy by both striking military targets and disrupting their opponent’s ability to wage war through destruction of their resources. In this type of war, armies often make no distinction between civilian and military targets. Both the Union and Confederate forces moved toward total war, although neither side ever entirely abolished the distinction between military and civilian. Total war also requires governments to mobilize all resources, extending their reach into their citizens’ lives as never before. Another reality of war that became apparent in 1862 and beyond was the influence of combat on the size and scope of government. Both the Confederacy and the Union governments had to continue to grow in order to manage the logistics of recruiting men and maintaining, feeding, and equipping an army.
Confederate Mobilization
The Confederate government in Richmond, Virginia, exercised sweeping powers to ensure victory, in stark contradiction to the states’ rights sentiments held by many Southern leaders. The initial emotional outburst of enthusiasm for war in the Confederacy waned, and the Confederate government instituted a military draft in April 1862. Under the terms of the draft, all men between the ages of eighteen and thirty-five would serve three years. The draft had a different effect on men of different socioeconomic classes. One loophole permitted men to hire substitutes instead of serving in the Confederate army. This provision favored the wealthy over the poor, and led to much resentment and resistance. Exercising its power over the states, the Confederate Congress denied state efforts to circumvent the draft.
In order to fund the war, the Confederate government also took over the South’s economy. The government ran Southern industry and built substantial transportation and industrial infrastructure to make the weapons of war. Over the objections of slaveholders, it impressed slaves, seizing these workers from their owners and forcing them to work on fortifications and rail lines. Concerned about the resistance to and unhappiness with the government measures, in 1862, the Confederate Congress gave President Davis the power to suspend the writ of habeas corpus, the right of those arrested to be brought before a judge or court to determine whether there is cause to hold the prisoner. With a stated goal of bolstering national security in the fledgling republic, this change meant that the Confederacy could arrest and detain indefinitely any suspected enemy without giving a reason. This growth of the Confederate central government stood as a glaring contradiction to the earlier states’ rights argument of pro-Confederate advocates.
The war efforts were costing the new nation dearly. Nevertheless, the Confederate Congress heeded the pleas of wealthy plantation owners and refused to place a tax on slaves or cotton, despite the Confederacy’s desperate need for the revenue that such a tax would have raised. Instead, the Confederacy drafted a taxation plan that kept the Southern elite happy but in no way met the needs of the war. The government also resorted to printing immense amounts of paper money, which quickly led to runaway inflation. Food prices soared, and poor, white Southerners faced starvation. In April 1863, thousands of hungry people rioted in Richmond, Virginia (Figure). Many of the rioters were mothers who could not feed their children. The riot ended when President Davis threatened to have Confederate forces open fire on the crowds.
One of the reasons that the Confederacy was so economically devastated was its ill-advised gamble that cotton sales would continue during the war. The government had high hopes that Great Britain and France, which both used cotton as the raw material in their textile mills, would ensure the South’s economic strength—and therefore victory in the war—by continuing to buy. Furthermore, the Confederate government hoped that Great Britain and France would make loans to their new nation in order to ensure the continued flow of raw materials. These hopes were never realized. Great Britain in particular did not wish to risk war with the United States, which would have meant the invasion of Canada. The United States was also a major source of grain for Britain and an important purchaser of British goods. Furthermore, the blockade made Southern trade with Europe difficult. Instead, Great Britain, the major consumer of American cotton, found alternate sources in India and Egypt, leaving the South without the income or alliance it had anticipated.
Dissent within the Confederacy also affected the South’s ability to fight the war. Confederate politicians disagreed over the amount of power that the central government should be allowed to exercise. Many states’ rights advocates, who favored a weak central government and supported the sovereignty of individual states, resented President Davis’s efforts to conscript troops, impose taxation to pay for the war, and requisition necessary resources. Governors in the Confederate states often proved reluctant to provide supplies or troops for the use of the Confederate government. Even Jefferson Davis’s vice president Alexander Stephens opposed conscription, the seizure of slave property to work for the Confederacy, and suspension of habeas corpus. Class divisions also divided Confederates. Poor whites resented the ability of wealthy slaveholders to excuse themselves from military service. Racial tensions plagued the South as well. On those occasions when free blacks volunteered to serve in the Confederate army, they were turned away, and enslaved African Americans were regarded with fear and suspicion, as whites whispered among themselves about the possibility of slave insurrections.
Union Mobilization
Mobilization for war proved to be easier in the North than it was in the South. During the war, the federal government in Washington, DC, like its Southern counterpart, undertook a wide range of efforts to ensure its victory over the Confederacy. To fund the war effort and finance the expansion of Union infrastructure, Republicans in Congress drastically expanded government activism, impacting citizens’ everyday lives through measures such as new types of taxation. The government also contracted with major suppliers of food, weapons, and other needed materials. Virtually every sector of the Northern economy became linked to the war effort.
In keeping with their longstanding objective of keeping slavery out of the newly settled western territories, the Republicans in Congress (the dominant party) passed several measures in 1862. First, the Homestead Act provided generous inducements for Northerners to relocate and farm in the West. Settlers could lay claim to 160 acres of federal land by residing on the property for five years and improving it. The act not only motivated free-labor farmers to move west, but it also aimed to increase agricultural output for the war effort. The federal government also turned its attention to creating a transcontinental railroad to facilitate the movement of people and goods across the country. Congress chartered two companies, the Union Pacific and the Central Pacific, and provided generous funds for these two businesses to connect the country by rail.
The Republican emphasis on free labor, rather than slave labor, also influenced the 1862 Land Grant College Act, commonly known as the Morrill Act after its author, Vermont Republican senator Justin Smith Morrill. The measure provided for the creation of agricultural colleges, funded through federal grants, to teach the latest agricultural techniques. Each state in the Union would be granted thirty thousand acres of federal land for the use of these institutions of higher education.
Congress paid for the war using several strategies. They levied a tax on the income of the wealthy, as well as a tax on all inheritances. They also put high tariffs in place. Finally, they passed two National Bank Acts, one in 1863 and one in 1864, calling on the U.S. Treasury to issue war bonds and on Union banks to buy the bonds. A Union campaign to convince individuals to buy the bonds helped increase sales. The Republicans also passed the Legal Tender Act of 1862, calling for paper money—known as greenbacks—to be printed Figure). Some $150 million worth of greenbacks became legal tender, and the Northern economy boomed, although high inflation also resulted.
Like the Confederacy, the Union turned to conscription to provide the troops needed for the war. In March 1863, Congress passed the Enrollment Act, requiring all unmarried men between the ages of twenty and twenty-five, and all married men between the ages of thirty-five and forty-five—including immigrants who had filed for citizenship—to register with the Union to fight in the Civil War. All who registered were subject to military service, and draftees were selected by a lottery system (Figure). As in the South, a loophole in the law allowed individuals to hire substitutes if they could afford it. Others could avoid enlistment by paying $300 to the federal government. In keeping with the Supreme Court decision in Dred Scott v. Sandford, African Americans were not citizens and were therefore exempt from the draft.
Like the Confederacy, the Union also took the step of suspending habeas corpus rights, so those suspected of pro-Confederate sympathies could be arrested and held without being given the reason. Lincoln had selectively suspended the writ of habeas corpus in the slave state of Maryland, home to many Confederate sympathizers, in 1861 and 1862, in an effort to ensure that the Union capital would be safe. In March 1863, he signed into law the Habeas Corpus Suspension Act, giving him the power to detain suspected Confederate operatives throughout the Union. The Lincoln administration also closed down three hundred newspapers as a national security measure during the war.
In both the North and the South, the Civil War dramatically increased the power of the belligerent governments. Breaking all past precedents in American history, both the Confederacy and the Union employed the power of their central governments to mobilize resources and citizens.
Women’s Mobilization
As men on both sides mobilized for the war, so did women. In both the North and the South, women were forced to take over farms and businesses abandoned by their husbands as they left for war. Women organized themselves into ladies’ aid societies to sew uniforms, knit socks, and raise money to purchase necessities for the troops. In the South, women took wounded soldiers into their homes to nurse. In the North, women volunteered for the United States Sanitary Commission, which formed in June 1861. They inspected military camps with the goal of improving cleanliness and reducing the number of soldiers who died from disease, the most common cause of death in the war. They also raised money to buy medical supplies and helped with the injured. Other women found jobs in the Union army as cooks and laundresses. Thousands volunteered to care for the sick and wounded in response to a call by reformer Dorothea Dix, who was placed in charge of the Union army’s nurses. According to rumor, Dix sought respectable women over the age of thirty who were “plain almost to repulsion in dress” and thus could be trusted not to form romantic liaisons with soldiers. Women on both sides also acted as spies and, disguised as men, engaged in combat.
EMANCIPATION
Early in the war, President Lincoln approached the issue of slavery cautiously. While he disapproved of slavery personally, he did not believe that he had the authority to abolish it. Furthermore, he feared that making the abolition of slavery an objective of the war would cause the border slave states to join the Confederacy. His one objective in 1861 and 1862 was to restore the Union.
Lincoln’s Evolving Thoughts on Slavery
President Lincoln wrote the following letter to newspaper editor Horace Greeley on August 22, 1862. In it, Lincoln states his position on slavery, which is notable for being a middle-of-the-road stance. Lincoln’s later public speeches on the issue take the more strident antislavery tone for which he is remembered.
I would save the Union. I would save it the shortest way under the Constitution. The sooner the national authority can be restored the nearer the Union will be “the Union as it was.” If there be those who would not save the Union unless they could at the same time save Slavery, I do not agree with them. If there be those who would not save the Union unless they could at the same time destroy Slavery, I do not agree with them. My paramount object in this struggle is to save the Union, and is not either to save or destroy Slavery. If I could save the Union without freeing any slave, I would do it, and if I could save it by freeing all the slaves, I would do it, and if I could save it by freeing some and leaving others alone, I would also do that. What I do about Slavery and the colored race, I do because I believe it helps to save this Union, and what I forbear, I forbear because I do not believe it would help to save the Union. I shall do less whenever I shall believe what I am doing hurts the cause, and I shall do more whenever I shall believe doing more will help the cause. I shall try to correct errors when shown to be errors; and I shall adopt new views so fast as they shall appear to be true views. I have here stated my purpose according to my view of official duty, and I intend no modification of my oft-expressed personal wish that all men, everywhere, could be free. Yours, A. LINCOLN.
How would you characterize Lincoln’s public position in August 1862? What was he prepared to do for slaves, and under what conditions?
Since the beginning of the war, thousands of slaves had fled to the safety of Union lines. In May 1861, Union general Benjamin Butler and others labeled these refugees from slavery contrabands. Butler reasoned that since Southern states had left the United States, he was not obliged to follow federal fugitive slave laws. Slaves who made it through the Union lines were shielded by the U.S. military and not returned to slavery. The intent was not only to assist slaves but also to deprive the South of a valuable source of manpower.
Congress began to define the status of these ex-slaves in 1861 and 1862. In August 1861, legislators approved the Confiscation Act of 1861, empowering the Union to seize property, including slaves, used by the Confederacy. The Republican-dominated Congress took additional steps, abolishing slavery in Washington, DC, in April 1862. Congress passed a second Confiscation Act in July 1862, which extended freedom to runaway slaves and those captured by Union armies. In that month, Congress also addressed the issue of slavery in the West, banning the practice in the territories. This federal law made the 1846 Wilmot Proviso and the dreams of the Free-Soil Party a reality. However, even as the Union government took steps to aid individual slaves and to limit the practice of slavery, it passed no measure to address the institution of slavery as a whole.
Lincoln moved slowly and cautiously on the issue of abolition. His primary concern was the cohesion of the Union and the bringing of the Southern states back into the fold. However, as the war dragged on and many thousands of contrabands made their way north, Republicans in Congress continued to call for the end of slavery. Throughout his political career, Lincoln’s plans for former slaves had been to send them to Liberia. As late as August 1862, he had hoped to interest African Americans in building a colony for former slaves in Central America, an idea that found favor neither with black leaders nor with abolitionists, and thus was abandoned by Lincoln. Responding to Congressional demands for an end to slavery, Lincoln presented an ultimatum to the Confederates on September 22, 1862, shortly after the Confederate retreat at Antietam. He gave the Confederate states until January 1, 1863, to rejoin the Union. If they did, slavery would continue in the slave states. If they refused to rejoin, however, the war would continue and all slaves would be freed at its conclusion. The Confederacy took no action. It had committed itself to maintaining its independence and had no interest in the president’s ultimatum.
On January 1, 1863, Lincoln made good on his promise and signed the Emancipation Proclamation. It stated “That on the first day of January, in the year of our Lord one thousand eight hundred and sixty-three, all persons held as slaves within any State or designated part of a State, the people whereof shall then be in rebellion against the United States, shall be then, thenceforward, and forever free.” The proclamation did not immediately free the slaves in the Confederate states. Although they were in rebellion against the United States, the lack of the Union army’s presence in such areas meant that the president’s directive could not be enforced. The proclamation also did not free slaves in the border states, because these states were not, by definition, in rebellion. Lincoln relied on his powers as commander-in-chief in issuing the Emancipation Proclamation. He knew the proclamation could be easily challenged in court, but by excluding the territories still outside his control, slaveholders and slave governments could not sue him. Moreover, slave states in the Union, such as Kentucky, could not sue because the proclamation did not apply to them. Slaveholders in Kentucky knew full well that if the institution were abolished throughout the South, it would not survive in a handful of border territories. Despite the limits of the proclamation, Lincoln dramatically shifted the objective of the war increasingly toward ending slavery. The Emancipation Proclamation became a monumental step forward on the road to changing the character of the United States.
Read through the full text of the Emancipation Proclamation at the National Archives website.
The proclamation generated quick and dramatic reactions. The news created euphoria among slaves, as it signaled the eventual end of their bondage. Predictably, Confederate leaders raged against the proclamation, reinforcing their commitment to fight to maintain slavery, the foundation of the Confederacy. In the North, opinions split widely on the issue. Abolitionists praised Lincoln’s actions, which they saw as the fulfillment of their long campaign to strike down an immoral institution. But other Northerners, especially Irish, working-class, urban dwellers loyal to the Democratic Party and others with racist beliefs, hated the new goal of emancipation and found the idea of freed slaves repugnant. At its core, much of this racism had an economic foundation: Many Northerners feared competing with emancipated slaves for scarce jobs.
In New York City, the Emancipation Proclamation, combined with unhappiness over the Union draft, which began in March 1863, fanned the flames of white racism. Many New Yorkers supported the Confederacy for business reasons, and, in 1861, the city’s mayor actually suggested that New York City leave the Union. On July 13, 1863, two days after the first draft lottery took place, this racial hatred erupted into violence. A volunteer fire company whose commander had been drafted initiated a riot, and the violence spread quickly across the city. The rioters chose targets associated either with the Union army or with African Americans. An armory was destroyed, as was a Brooks Brothers’ store, which supplied uniforms to the army. White mobs attacked and killed black New Yorkers and destroyed an African American orphanage (Figure). On the fourth day of the riots, federal troops dispatched by Lincoln arrived in the city and ended the violence. Millions of dollars in property had been destroyed. More than one hundred people died, approximately one thousand were left injured, and about one-fifth of the city’s African American population fled New York in fear.
UNION ADVANCES
The war in the west continued in favor of the North in 1863. At the start of the year, Union forces controlled much of the Mississippi River. In the spring and summer of 1862, they had captured New Orleans—the most important port in the Confederacy, through which cotton harvested from all the Southern states was exported—and Memphis. Grant had then attempted to capture Vicksburg, Mississippi, a commercial center on the bluffs above the Mississippi River. Once Vicksburg fell, the Union would have won complete control over the river. A military bombardment that summer failed to force a Confederate surrender. An assault by land forces also failed in December 1862.
In April 1863, the Union began a final attempt to capture Vicksburg. On July 3, after more than a month of a Union siege, during which Vicksburg’s residents hid in caves to protect themselves from the bombardment and ate their pets to stay alive, Grant finally achieved his objective. The trapped Confederate forces surrendered. The Union had succeeded in capturing Vicksburg and splitting the Confederacy (Figure). This victory inflicted a serious blow to the Southern war effort.
As Grant and his forces pounded Vicksburg, Confederate strategists, at the urging of General Lee, who had defeated a larger Union army at Chancellorsville, Virginia, in May 1863, decided on a bold plan to invade the North. Leaders hoped this invasion would force the Union to send troops engaged in the Vicksburg campaign east, thus weakening their power over the Mississippi. Further, they hoped the aggressive action of pushing north would weaken the Union’s resolve to fight. Lee also hoped that a significant Confederate victory in the North would convince Great Britain and France to extend support to Jefferson Davis’s government and encourage the North to negotiate peace.
Beginning in June 1863, General Lee began to move the Army of Northern Virginia north through Maryland. The Union army—the Army of the Potomac—traveled east to end up alongside the Confederate forces. The two armies met at Gettysburg, Pennsylvania, where Confederate forces had gone to secure supplies. The resulting battle lasted three days, July 1–3 (Figure) and remains the biggest and costliest battle ever fought in North America. The climax of the Battle of Gettysburg occurred on the third day. In the morning, after a fight lasting several hours, Union forces fought back a Confederate attack on Culp’s Hill, one of the Union’s defensive positions. To regain a perceived advantage and secure victory, Lee ordered a frontal assault, known as Pickett’s Charge (for Confederate general George Pickett), against the center of the Union lines on Cemetery Ridge. Approximately fifteen thousand Confederate soldiers took part, and more than half lost their lives, as they advanced nearly a mile across an open field to attack the entrenched Union forces. In all, more than a third of the Army of Northern Virginia had been lost, and on the evening of July 4, Lee and his men slipped away in the rain. General George Meade did not pursue them. Both sides suffered staggering losses. Total casualties numbered around twenty-three thousand for the Union and some twenty-eight thousand among the Confederates. With its defeats at Gettysburg and Vicksburg, both on the same day, the Confederacy lost its momentum. The tide had turned in favor of the Union in both the east and the west.
Following the Battle of Gettysburg, the bodies of those who had fallen were hastily buried. Attorney David Wills, a resident of Gettysburg, campaigned for the creation of a national cemetery on the site of the battlefield, and the governor of Pennsylvania tasked him with creating it. President Lincoln was invited to attend the cemetery’s dedication. After the featured orator had delivered a two-hour speech, Lincoln addressed the crowd for several minutes. In his speech, known as the Gettysburg Address, which he had finished writing while a guest in David Wills’ home the day before the dedication, Lincoln invoked the Founding Fathers and the spirit of the American Revolution. The Union soldiers who had died at Gettysburg, he proclaimed, had died not only to preserve the Union, but also to guarantee freedom and equality for all.
Lincoln’s Gettysburg Address
Several months after the battle at Gettysburg, Lincoln traveled to Pennsylvania and, speaking to an audience at the dedication of the new Soldiers’ National Ceremony near the site of the battle, he delivered his now-famous Gettysburg Address to commemorate the turning point of the war and the soldiers whose sacrifices had made it possible. The two-minute speech was politely received at the time, although press reactions split along party lines. Upon receiving a letter of congratulations from Massachusetts politician and orator William Everett, whose speech at the ceremony had lasted for two hours, Lincoln said he was glad to know that his brief address, now virtually immortal, was not “a total failure.”
Four score and seven years ago our fathers brought forth on this continent, a new nation, conceived in Liberty, and dedicated to the proposition that all men are created equal.
Now we are engaged in a great civil war, testing whether that nation, or any nation so conceived and so dedicated, can long endure. We are met on a great battle-field of that war. We have come to dedicate a portion of that field, as a final resting place for those who here gave their lives that that nation might live. It is altogether fitting and proper that we should do this.
It is for us the living . . . to be here dedicated to the great task remaining before us—that from these honored dead we take increased devotion to that cause for which they gave the last full measure of devotion—that we here highly resolve that these dead shall not have died in vain—that this nation, under God, shall have a new birth of freedom—and that government of the people, by the people, for the people, shall not perish from the earth.
—Abraham Lincoln, Gettysburg Address, November 19, 1863
What did Lincoln mean by “a new birth of freedom”? What did he mean when he said “a government of the people, by the people, for the people, shall not perish from the earth”?
Acclaimed filmmaker Ken Burns has created a documentary about a small boys’ school in Vermont where students memorize the Gettysburg Address. It explores the value the address has in these boys’ lives, and why the words still matter.
Section Summary
The year 1863 proved decisive in the Civil War for two major reasons. First, the Union transformed the purpose of the struggle from restoring the Union to ending slavery. While Lincoln’s Emancipation Proclamation actually succeeded in freeing few slaves, it made freedom for African Americans a cause of the Union. Second, the tide increasingly turned against the Confederacy. The success of the Vicksburg Campaign had given the Union control of the Mississippi River, and Lee’s defeat at Gettysburg had ended the attempted Confederate invasion of the North.
Review Questions
Which of the following did the North not do to mobilize for war?
- institute a military draft
- form a military alliance with Great Britain
- print paper money
- pass the Homestead Act
Hint:
B
Why is 1863 considered a turning point in the Civil War?
Hint:
At the beginning of 1863, Abraham Lincoln issued the Emancipation Proclamation, which freed all slaves in areas under rebellion. This changed the war from one in which the North fought to preserve the Union to one in which it fought to free enslaved African Americans. On the battlefield, Union forces led by Grant captured Vicksburg, Mississippi, splitting the Confederacy in two and depriving it of a major avenue of transportation. In the east, General Meade stopped a Confederate invasion of the North at Gettysburg, Pennsylvania.
|
oercommons
|
2025-03-18T00:37:58.845789
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15475/overview",
"title": "U.S. History, The Civil War, 1860–1865",
"author": null
}
|
https://oercommons.org/courseware/lesson/15476/overview
|
The Union Triumphant
Overview
By the end of this section, you will be able to:
- Describe the reasons why many Americans doubted that Abraham Lincoln would be reelected
- Explain how the Union forces overpowered the Confederacy
By the outset of 1864, after three years of war, the Union had mobilized its resources for the ongoing struggle on a massive scale. The government had overseen the construction of new railroad lines and for the first time used standardized rail tracks that allowed the North to move men and materials with greater ease. The North’s economy had shifted to a wartime model. The Confederacy also mobilized, perhaps to a greater degree than the Union, its efforts to secure independence and maintain slavery. Yet the Confederacy experienced ever-greater hardships after years of war. Without the population of the North, it faced a shortage of manpower. The lack of industry, compared to the North, undercut the ability to sustain and wage war. Rampant inflation as well as food shortages in the South lowered morale.
THE RELATIONSHIP WITH EUROPE
From the beginning of the war, the Confederacy placed great hope in being recognized and supported by Great Britain and France. European intervention in the conflict remained a strong possibility, but when it did occur, it was not in a way anticipated by either the Confederacy or the Union.
Napoleon III of France believed the Civil War presented an opportunity for him to restore a French empire in the Americas. With the United States preoccupied, the time seemed ripe for action. Napoleon’s target was Mexico, and in 1861, a large French fleet took Veracruz. The French then moved to capture Mexico City, but the advance came to an end when Mexican forces defeated the French in 1862. Despite this setback, France eventually did conquer Mexico, establishing a regime that lasted until 1867. Rather than coming to the aid of the Confederacy, France used the Civil War to provide a pretext for efforts to reestablish its former eighteenth-century colonial holdings.
Still, the Confederacy had great confidence that it would find an ally in Great Britain despite the antislavery sentiment there. Southerners hoped Britain’s dependence on cotton for its textile mills would keep the country on their side. The fact that the British proved willing to build and sell ironclad ships intended to smash through the Union naval blockade further raised Southern hopes. The Confederacy purchased two of these armored blockade runners, the CSS Florida and the CSS Alabama. Both were destroyed during the war.
The Confederacy’s staunch commitment to slavery eventually worked against British recognition and support, since Great Britain had abolished slavery in 1833. The 1863 Emancipation Proclamation ended any doubts the British had about the goals of the Union cause. In the aftermath of the proclamation, many in Great Britain cheered for a Union victory. Ultimately, Great Britain, like France, disappointed the Confederacy’s hope of an alliance, leaving the outnumbered and out-resourced states that had left the Union to fend for themselves.
AFRICAN AMERICAN SOLDIERS
At the beginning of the war, in 1861 and 1862, Union forces had used contrabands, or escaped slaves, for manual labor. The Emancipation Proclamation, however, led to the enrollment of African American men as Union soldiers. Huge numbers, former slaves as well as free blacks from the North, enlisted, and by the end of the war in 1865, their numbers had swelled to over 190,000. Racism among whites in the Union army ran deep, however, fueling the belief that black soldiers could never be effective or trustworthy. The Union also feared for the fate of captured black soldiers. Although many black soldiers saw combat duty, these factors affected the types of tasks assigned to them. Many black regiments were limited to hauling supplies, serving as cooks, digging trenches, and doing other types of labor, rather than serving on the battlefield (Figure).
African American soldiers also received lower wages than their white counterparts: ten dollars per month, with three dollars deducted for clothing. White soldiers, in contrast, received thirteen dollars monthly, with no deductions. Abolitionists and their Republican supporters in Congress worked to correct this discriminatory practice, and in 1864, black soldiers began to receive the same pay as white soldiers plus retroactive pay to 1863 (Figure).
For their part, African American soldiers welcomed the opportunity to prove themselves. Some 85 percent were former slaves who were fighting for the liberation of all slaves and the end of slavery. When given the opportunity to serve, many black regiments did so heroically. One such regiment, the Fifty-Fourth Regiment of Massachusetts Volunteers, distinguished itself at Fort Wagner in South Carolina by fighting valiantly against an entrenched Confederate position. They willingly gave their lives for the cause.
The Confederacy, not surprisingly, showed no mercy to African American troops. In April 1864, Southern forces attempted to take Fort Pillow in Tennessee from the Union forces that had captured it in 1862. Confederate troops under Major General Nathan Bedford Forrest, the future founder of the Ku Klux Klan, quickly overran the fort, and the Union defenders surrendered. Instead of taking the African American soldiers prisoner, as they did the white soldiers, the Confederates executed them. The massacre outraged the North, and the Union refused to engage in any future exchanges of prisoners with the Confederacy.
THE CAMPAIGNS OF 1864 AND 1865
In the final years of the war, the Union continued its efforts on both the eastern and western fronts while bringing the war into the Deep South. Union forces increasingly engaged in total war, not distinguishing between military and civilian targets. They destroyed everything that lay in their path, committed to breaking the will of the Confederacy and forcing an end to the war. General Grant, mastermind of the Vicksburg campaign, took charge of the war effort. He understood the advantage of having large numbers of soldiers at his disposal and recognized that Union soldiers could be replaced, whereas the Confederates, whose smaller population was feeling the strain of the years of war, could not. Grant thus pushed forward relentlessly, despite huge losses of men. In 1864, Grant committed his forces to destroying Lee’s army in Virginia.
In the Virginia campaign, Grant hoped to use his larger army to his advantage. But at the Battle of the Wilderness, fought from May 5 to May 7, Confederate forces stopped Grant’s advance. Rather than retreating, he pushed forward. At the Battle of Spotsylvania on May 8 through 12, Grant again faced determined Confederate resistance, and again his advance was halted. As before, he renewed the Union campaign. At the Battle of Cold Harbor in early June, Grant had between 100,000 and 110,000 soldiers, whereas the Confederates had slightly more than half that number. Again, the Union advance was halted, if only momentarily, as Grant awaited reinforcements. An attack on the Confederate position on June 3 resulted in heavy casualties for the Union, and nine days later, Grant led his army away from Cold Harbor to Petersburg, Virginia, a rail center that supplied Richmond. The immense losses that Grant’s forces suffered severely hurt Union morale. The war seemed unending, and with the tremendous loss of life, many in the North began to question the war and desire peace. Undaunted by the changing opinion in the North and hoping to destroy the Confederate rail network in the Upper South, however, Grant laid siege to Petersburg for nine months. As the months wore on, both sides dug in, creating miles of trenches and gun emplacements.
The other major Union campaigns of 1864 were more successful and gave President Lincoln the advantage that he needed to win reelection in November. In August 1864, the Union navy captured Mobile Bay. General Sherman invaded the Deep South, advancing slowly from Tennessee into Georgia, confronted at every turn by the Confederates, who were commanded by Johnston. When President Davis replaced Johnston with General John B. Hood, the Confederates made a daring but ultimately costly direct attack on the Union army that failed to drive out the invaders. Atlanta fell to Union forces on September 2, 1864. The fall of Atlanta held tremendous significance for the war-weary Union and helped to reverse the North’s sinking morale. In keeping with the logic of total war, Sherman’s forces cut a swath of destruction to Savannah. On Sherman’s March to the Sea, the Union army, seeking to demoralize the South, destroyed everything in its path, despite strict instructions regarding the preservation of civilian property. Although towns were left standing, houses and barns were burned. Homes were looted, food was stolen, crops were destroyed, orchards were burned, and livestock was killed or confiscated. Savannah fell on December 21, 1864—a Christmas gift for Lincoln, Sherman proclaimed. In 1865, Sherman’s forces invaded South Carolina, capturing Charleston and Columbia. In Columbia, the state capital, the Union army burned slaveholders’ homes and destroyed much of the city. From South Carolina, Sherman’s force moved north in an effort to join Grant and destroy Lee’s army.
Dolly Sumner Lunt on Sherman’s March to the Sea
The following account is by Dolly Sumner Lunt, a widow who ran her Georgia cotton plantation after the death of her husband. She describes General Sherman’s march to Savannah, where he enacted the policy of total war by burning and plundering the landscape to inhibit the Confederates’ ability to keep fighting.
Alas! little did I think while trying to save my house from plunder and fire that they were forcing my boys [slaves] from home at the point of the bayonet. One, Newton, jumped into bed in his cabin, and declared himself sick. Another crawled under the floor,—a lame boy he was,—but they pulled him out, placed him on a horse, and drove him off. Mid, poor Mid! The last I saw of him, a man had him going around the garden, looking, as I thought, for my sheep, as he was my shepherd. Jack came crying to me, the big tears coursing down his cheeks, saying they were making him go. I said: ‘Stay in my room.’
But a man followed in, cursing him and threatening to shoot him if he did not go; so poor Jack had to yield. . . .
Sherman himself and a greater portion of his army passed my house that day. All day, as the sad moments rolled on, were they passing not only in front of my house, but from behind; they tore down my garden palings, made a road through my back-yard and lot field, driving their stock and riding through, tearing down my fences and desolating my home—wantonly doing it when there was no necessity for it. . . .
About ten o’clock they had all passed save one, who came in and wanted coffee made, which was done, and he, too, went on. A few minutes elapsed, and two couriers riding rapidly passed back. Then, presently, more soldiers came by, and this ended the passing of Sherman’s army by my place, leaving me poorer by thirty thousand dollars than I was yesterday morning. And a much stronger Rebel!
According to this account, what was the reaction of slaves to the arrival of the Union forces? What did the Union forces do with the slaves? For Lunt, did the strategy of total war work as planned?
THE ELECTION OF 1864
Despite the military successes for the Union army in 1863, in 1864, Lincoln’s status among many Northern voters plummeted. Citing the suspension of the writ of habeas corpus, many saw him as a dictator, bent on grabbing power while senselessly and uncaringly drafting more young men into combat. Arguably, his greatest liability, however, was the Emancipation Proclamation and the enlistment of African American soldiers. Many whites in the North found this deeply offensive, since they still believed in racial inequality. The 1863 New York City Draft Riots illustrated the depth of white anger.
Northern Democrats railed against Lincoln and the war. Republicans labeled these vocal opponents of the President Copperheads, a term that many antiwar Democrats accepted. As the anti-Lincoln poster below illustrates, his enemies tried to paint him as an untrustworthy and suspect leader (Figure). It seemed to most in the North that the Democratic candidate, General George B. McClellan, who did not support abolition and was replaced with another commander by Lincoln, would win the election.
The Republican Party also split over the issue of reelecting Lincoln. Those who found him timid and indecisive, and favored extending full rights to African Americans, as well as completely refashioning the South after its defeat, earned the name Radicals. A moderate faction of Republicans opposed the Radicals. For his part, Lincoln did not align himself with either group.
The tide of the election campaign turned in favor of Lincoln, however, in the fall of 1864. Above all else, Union victories, including the fall of Atlanta in September and General Philip Sheridan’s successes in the Shenandoah Valley of Virginia, bolstered Lincoln’s popularity and his reelection bid. In November 1864, despite earlier forecasts to the contrary, Lincoln was reelected. Lincoln won all but three states—New Jersey and the border states of Delaware and Kentucky. To the chagrin of his opponent, McClellan, even Union army troops voted overwhelmingly for the incumbent President.
THE WAR ENDS
By the spring of 1865, it had become clear to both sides that the Confederacy could not last much longer. Most of its major cities, ports, and industrial centers—Atlanta, Savannah, Charleston, Columbia, Mobile, New Orleans, and Memphis—had been captured. In April 1865, Lee had abandoned both Petersburg and Richmond. His goal in doing so was to unite his depleted army with Confederate forces commanded by General Johnston. Grant effectively cut him off. On April 9, 1865, Lee surrendered to Grant at Appomattox Court House in Virginia (Figure). By that time, he had fewer than 35,000 soldiers, while Grant had some 100,000. Meanwhile, Sherman’s army proceeded to North Carolina, where General Johnston surrendered on April 19, 1865. The Civil War had come to an end. The war had cost the lives of more than 600,000 soldiers. Many more had been wounded. Thousands of women were left widowed. Children were left without fathers, and many parents were deprived of a source of support in their old age. In some areas, where local volunteer units had marched off to battle, never to return, an entire generation of young women was left without marriage partners. Millions of dollars’ worth of property had been destroyed, and towns and cities were laid to waste. With the conflict finally over, the very difficult work of reconciling North and South and reestablishing the United States lay ahead.
Section Summary
Having failed to win the support it expected from either Great Britain or France, the Confederacy faced a long war with limited resources and no allies. Lincoln won reelection in 1864, and continued to pursue the Union campaign, not only in the east and west, but also with a drive into the South under the leadership of General Sherman, whose March to the Sea through Georgia destroyed everything in its path. Cut off and outnumbered, Confederate general Lee surrendered to Union general Grant on April 9 at Appomattox Court House in Virginia. Within days of Lee’s surrender, Confederate troops had lay down their arms, and the devastating war came to a close.
Review Questions
Which of the following is not a reason why many people opposed Lincoln’s reelection in 1864?
- He appeared to have overstepped his authority by suspending the writ of habeas corpus.
- He issued the Emancipation Proclamation.
- He had replaced General George B. McClellan.
- He was seen as a power-hungry dictator.
Hint:
C
What was General Sherman’s objective on his March to the Sea?
- to destroy military and civilian resources wherever possible
- to free black prisoners of war
- to join his army to that of General Grant
- to capture General Robert E. Lee
Hint:
A
Critical Thinking Questions
Could the differences between the North and South have been worked out in late 1860 and 1861? Could war have been avoided? Provide evidence to support your answer.
Why did the North prevail in the Civil War? What might have turned the tide of the war against the North?
If you were in charge of the Confederate war effort, what strategy or strategies would you have pursued? Conversely, if you had to devise the Union strategy, what would you propose? How does your answer depend on your knowledge of how the war actually played out?
What do you believe to be the enduring qualities of the Gettysburg Address? Why has this two-minute speech so endured?
What role did women and African Americans play in the war?
|
oercommons
|
2025-03-18T00:37:58.878905
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15476/overview",
"title": "U.S. History, The Civil War, 1860–1865",
"author": null
}
|
https://oercommons.org/courseware/lesson/15488/overview
|
Introduction
Overview
- Inventors of the Age
- From Invention to Industrial Growth
- Building Industrial America on the Backs of Labor
- A New American Consumer Culture
“The electric age was ushered into being in this last decade of the nineteenth century today when President Cleveland, by pressing a button, started the mighty machinery, rushing waters and revolving wheels in the World’s Columbian exhibition.” With this announcement about the official start of the Chicago World’s Fair in 1893 (Figure), the Salt Lake City Herald captured the excitement and optimism of the machine age. “In the previous expositions,” the editorial continued, “the possibilities of electricity had been limited to the mere starting of the engines in the machinery hall, but in this it made thousands of servants do its bidding . . . the magic of electricity did the duty of the hour.”
The fair, which commemorated the four hundredth anniversary of Columbus’s journey to America, was a potent symbol of the myriad inventions that changed American life and contributed to the significant economic growth of the era, as well as the new wave of industrialization that swept the country. While businessmen capitalized upon such technological innovations, the new industrial working class faced enormous challenges. Ironically, as the World’s Fair welcomed its first visitors, the nation was spiraling downward into the worst depression of the century. Subsequent frustrations among working-class Americans laid the groundwork for the country’s first significant labor movement.
|
oercommons
|
2025-03-18T00:37:58.896414
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15488/overview",
"title": "U.S. History, Industrialization and the Rise of Big Business, 1870-1900",
"author": null
}
|
https://oercommons.org/courseware/lesson/15489/overview
|
Inventors of the Age
Overview
By the end of this section, you will be able to:
- Explain how the ideas and products of late nineteenth-century inventors contributed to the rise of big business
- Explain how the inventions of the late nineteenth century changed everyday American life
The late nineteenth century was an energetic era of inventions and entrepreneurial spirit. Building upon the mid-century Industrial Revolution in Great Britain, as well as answering the increasing call from Americans for efficiency and comfort, the country found itself in the grip of invention fever, with more people working on their big ideas than ever before. In retrospect, harnessing the power of steam and then electricity in the nineteenth century vastly increased the power of man and machine, thus making other advances possible as the century progressed.
Facing an increasingly complex everyday life, Americans sought the means by which to cope with it. Inventions often provided the answers, even as the inventors themselves remained largely unaware of the life-changing nature of their ideas. To understand the scope of this zeal for creation, consider the U.S. Patent Office, which, in 1790—its first decade of existence—recorded only 276 inventions. By 1860, the office had issued a total of 60,000 patents. But between 1860 and 1890, that number exploded to nearly 450,000, with another 235,000 in the last decade of the century. While many of these patents came to naught, some inventions became lynchpins in the rise of big business and the country’s move towards an industrial-based economy, in which the desire for efficiency, comfort, and abundance could be more fully realized by most Americans.
AN EXPLOSION OF INVENTIVE ENERGY
From corrugated rollers that could crack hard, homestead-grown wheat into flour to refrigerated train cars and garment-sewing machines (Figure), new inventions fueled industrial growth around the country. As late as 1880, fully one-half of all Americans still lived and worked on farms, whereas fewer than one in seven—mostly men, except for long-established textile factories in which female employees tended to dominate—were employed in factories. However, the development of commercial electricity by the close of the century, to complement the steam engines that already existed in many larger factories, permitted more industries to concentrate in cities, away from the previously essential water power. In turn, newly arrived immigrants sought employment in new urban factories. Immigration, urbanization, and industrialization coincided to transform the face of American society from primarily rural to significantly urban. From 1880 to 1920, the number of industrial workers in the nation quadrupled from 2.5 million to over 10 million, while over the same period urban populations doubled, to reach one-half of the country’s total population.
In offices, worker productivity benefited from the typewriter, invented in 1867, the cash register, invented in 1879, and the adding machine, invented in 1885. These tools made it easier than ever to keep up with the rapid pace of business growth. Inventions also slowly transformed home life. The vacuum cleaner arrived during this era, as well as the flush toilet. These indoor “water closets” improved public health through the reduction in contamination associated with outhouses and their proximity to water supplies and homes. Tin cans and, later, Clarence Birdseye’s experiments with frozen food, eventually changed how women shopped for, and prepared, food for their families, despite initial health concerns over preserved foods. With the advent of more easily prepared food, women gained valuable time in their daily schedules, a step that partially laid the groundwork for the modern women’s movement. Women who had the means to purchase such items could use their time to seek other employment outside of the home, as well as broaden their knowledge through education and reading. Such a transformation did not occur overnight, as these inventions also increased expectations for women to remain tied to the home and their domestic chores; slowly, the culture of domesticity changed.
Perhaps the most important industrial advancement of the era came in the production of steel. Manufacturers and builders preferred steel to iron, due to its increased strength and durability. After the Civil War, two new processes allowed for the creation of furnaces large enough and hot enough to melt the wrought iron needed to produce large quantities of steel at increasingly cheaper prices. The Bessemer process, named for English inventor Henry Bessemer, and the open-hearth process, changed the way the United States produced steel and, in doing so, led the country into a new industrialized age. As the new material became more available, builders eagerly sought it out, a demand that steel mill owners were happy to supply.
In 1860, the country produced thirteen thousand tons of steel. By 1879, American furnaces were producing over one million tons per year; by 1900, this figure had risen to ten million. Just ten years later, the United States was the top steel producer in the world, at over twenty-four million tons annually. As production increased to match the overwhelming demand, the price of steel dropped by over 80 percent. When quality steel became cheaper and more readily available, other industries relied upon it more heavily as a key to their growth and development, including construction and, later, the automotive industry. As a result, the steel industry rapidly became the cornerstone of the American economy, remaining the primary indicator of industrial growth and stability through the end of World War II.
ALEXANDER GRAHAM BELL AND THE TELEPHONE
Advancements in communications matched the pace of growth seen in industry and home life. Communication technologies were changing quickly, and they brought with them new ways for information to travel. In 1858, British and American crews laid the first transatlantic cable lines, enabling messages to pass between the United States and Europe in a matter of hours, rather than waiting the few weeks it could take for a letter to arrive by steamship. Although these initial cables worked for barely a month, they generated great interest in developing a more efficient telecommunications industry. Within twenty years, over 100,000 miles of cable crisscrossed the ocean floors, connecting all the continents. Domestically, Western Union, which controlled 80 percent of the country’s telegraph lines, operated nearly 200,000 miles of telegraph routes from coast to coast. In short, people were connected like never before, able to relay messages in minutes and hours rather than days and weeks.
One of the greatest advancements was the telephone, which Alexander Graham Bell patented in 1876 (Figure). While he was not the first to invent the concept, Bell was the first one to capitalize on it; after securing the patent, he worked with financiers and businessmen to create the National Bell Telephone Company. Western Union, which had originally turned down Bell’s machine, went on to commission Thomas Edison to invent an improved version of the telephone. It is actually Edison’s version that is most like the modern telephone used today. However, Western Union, fearing a costly legal battle they were likely to lose due to Bell’s patent, ultimately sold Edison’s idea to the Bell Company. With the communications industry now largely in their control, along with an agreement from the federal government to permit such control, the Bell Company was transformed into the American Telephone and Telegraph Company, which still exists today as AT&T. By 1880, fifty thousand telephones were in use in the United States, including one at the White House. By 1900, that number had increased to 1.35 million, and hundreds of American cities had obtained local service for their citizens. Quickly and inexorably, technology was bringing the country into closer contact, changing forever the rural isolation that had defined America since its beginnings.
Visit the Library of Congress to examine the controversy over the invention of the telephone. While Alexander Graham Bell is credited with the invention, several other inventors played a role in its development; however, Bell was the first to patent the device.
THOMAS EDISON AND ELECTRIC LIGHTING
Although Thomas Alva Edison (Figure) is best known for his contributions to the electrical industry, his experimentation went far beyond the light bulb. Edison was quite possibly the greatest inventor of the turn of the century, saying famously that he “hoped to have a minor invention every ten days and a big thing every month or so.” He registered 1,093 patents over his lifetime and ran a world-famous laboratory, Menlo Park, which housed a rotating group of up to twenty-five scientists from around the globe.
Edison became interested in the telegraph industry as a boy, when he worked aboard trains selling candy and newspapers. He soon began tinkering with telegraph technology and, by 1876, had devoted himself full time to lab work as an inventor. He then proceeded to invent a string of items that are still used today: the phonograph, the mimeograph machine, the motion picture projector, the dictaphone, and the storage battery, all using a factory-oriented assembly line process that made the rapid production of inventions possible.
In 1879, Edison invented the item that has led to his greatest fame: the incandescent light bulb. He allegedly explored over six thousand different materials for the filament, before stumbling upon tungsten as the ideal substance. By 1882, with financial backing largely from financier J. P. Morgan, he had created the Edison Electric Illuminating Company, which began supplying electrical current to a small number of customers in New York City. Morgan guided subsequent mergers of Edison’s other enterprises, including a machine works firm and a lamp company, resulting in the creation of the Edison General Electric Company in 1889.
The next stage of invention in electric power came about with the contribution of George Westinghouse. Westinghouse was responsible for making electric lighting possible on a national scale. While Edison used “direct current” or DC power, which could only extend two miles from the power source, in 1886, Westinghouse invented “alternating current” or AC power, which allowed for delivery over greater distances due to its wavelike patterns. The Westinghouse Electric Company delivered AC power, which meant that factories, homes, and farms—in short, anything that needed power—could be served, regardless of their proximity to the power source. A public relations battle ensued between the Westinghouse and Edison camps, coinciding with the invention of the electric chair as a form of prisoner execution. Edison publicly proclaimed AC power to be best adapted for use in the chair, in the hope that such a smear campaign would result in homeowners becoming reluctant to use AC power in their houses. Although Edison originally fought the use of AC power in other devices, he reluctantly adapted to it as its popularity increased.
Not all of Edison’s ventures were successful. Read about Edison’s Folly to learn the story behind his greatest failure. Was there some benefit to his efforts? Or was it wasted time and money?
Section Summary
Inventors in the late nineteenth century flooded the market with new technological advances. Encouraged by Great Britain’s Industrial Revolution, and eager for economic development in the wake of the Civil War, business investors sought the latest ideas upon which they could capitalize, both to transform the nation as well as to make a personal profit. These inventions were a key piece of the massive shift towards industrialization that followed. For both families and businesses, these inventions eventually represented a fundamental change in their way of life. Although the technology spread slowly, it did spread across the country. Whether it was a company that could now produce ten times more products with new factories, or a household that could communicate with distant relations, the old way of doing things was disappearing.
Communication technologies, electric power production, and steel production were perhaps the three most significant developments of the time. While the first two affected both personal lives and business development, the latter influenced business growth first and foremost, as the ability to produce large steel elements efficiently and cost-effectively led to permanently changes in the direction of industrial growth.
Review Questions
Which of these was not a successful invention of the era?
- high-powered sewing machines
- movies with sound
- frozen foods
- typewriters
Hint:
B
What was the major advantage of Westinghouse’s “alternating current” power invention?
- It was less prone to fire.
- It cost less to produce.
- It allowed machines to be farther from the power source.
- It was not under Edison’s control.
Hint:
C
How did the burst of new inventions during this era fuel the process of urbanization?
Hint:
New inventions fueled industrial growth, and the development of commercial electricity—along with the use of steam engines—allowed industries that had previously situated themselves close to sources of water power to shift away from those areas and move their production into cities. Immigrants sought employment in these urban factories and settled nearby, transforming the country’s population from mostly rural to largely urban.
|
oercommons
|
2025-03-18T00:37:58.922435
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15489/overview",
"title": "U.S. History, Industrialization and the Rise of Big Business, 1870-1900",
"author": null
}
|
https://oercommons.org/courseware/lesson/15490/overview
|
From Invention to Industrial Growth
Overview
By the end of this section, you will be able to:
- Explain how the inventions of the late nineteenth century contributed directly to industrial growth in America
- Identify the contributions of Andrew Carnegie, John Rockefeller, and J. P. Morgan to the new industrial order emerging in the late nineteenth century
- Describe the visions, philosophies, and business methods of the leaders of the new industrial order
As discussed previously, new processes in steel refining, along with inventions in the fields of communications and electricity, transformed the business landscape of the nineteenth century. The exploitation of these new technologies provided opportunities for tremendous growth, and business entrepreneurs with financial backing and the right mix of business acumen and ambition could make their fortunes. Some of these new millionaires were known in their day as robber barons, a negative term that connoted the belief that they exploited workers and bent laws to succeed. Regardless of how they were perceived, these businessmen and the companies they created revolutionized American industry.
RAILROADS AND ROBBER BARONS
Earlier in the nineteenth century, the first transcontinental railroad and subsequent spur lines paved the way for rapid and explosive railway growth, as well as stimulated growth in the iron, wood, coal, and other related industries. The railroad industry quickly became the nation’s first “big business.” A powerful, inexpensive, and consistent form of transportation, railroads accelerated the development of virtually every other industry in the country. By 1890, railroad lines covered nearly every corner of the United States, bringing raw materials to industrial factories and finished goods to consumer markets. The amount of track grew from 35,000 miles at the end of the Civil War to over 200,000 miles by the close of the century. Inventions such as car couplers, air brakes, and Pullman passenger cars allowed the volume of both freight and people to increase steadily. From 1877 to 1890, both the amount of goods and the number of passengers traveling the rails tripled.
Financing for all of this growth came through a combination of private capital and government loans and grants. Federal and state loans of cash and land grants totaled $150 million and 185 million acres of public land, respectively. Railroads also listed their stocks and bonds on the New York Stock Exchange to attract investors from both within the United States and Europe. Individual investors consolidated their power as railroads merged and companies grew in size and power. These individuals became some of the wealthiest Americans the country had ever known. Midwest farmers, angry at large railroad owners for their exploitative business practices, came to refer to them as “robber barons,” as their business dealings were frequently shady and exploitative. Among their highly questionable tactics was the practice of differential shipping rates, in which larger business enterprises received discounted rates to transport their goods, as opposed to local producers and farmers whose higher rates essentially subsidized the discounts.
Jay Gould was perhaps the first prominent railroad magnate to be tarred with the “robber baron” brush. He bought older, smaller, rundown railroads, offered minimal improvements, and then capitalized on factory owners’ desires to ship their goods on this increasingly popular and more cost-efficient form of transportation. His work with the Erie Railroad was notorious among other investors, as he drove the company to near ruin in a failed attempt to attract foreign investors during a takeover attempt. His model worked better in the American West, where the railroads were still widely scattered across the country, forcing farmers and businesses to pay whatever prices Gould demanded in order to use his trains. In addition to owning the Union Pacific Railroad that helped to construct the original transcontinental railroad line, Gould came to control over ten thousand miles of track across the United States, accounting for 15 percent of all railroad transportation. When he died in 1892, Gould had a personal worth of over $100 million, although he was a deeply unpopular figure.
In contrast to Gould’s exploitative business model, which focused on financial profit more than on tangible industrial contributions, Commodore Cornelius Vanderbilt was a “robber baron” who truly cared about the success of his railroad enterprise and its positive impact on the American economy. Vanderbilt consolidated several smaller railroad lines, called trunk lines, to create the powerful New York Central Railroad Company, one of the largest corporations in the United States at the time (Figure). He later purchased stock in the major rail lines that would connect his company to Chicago, thus expanding his reach and power while simultaneously creating a railroad network to connect Chicago to New York City. This consolidation provided more efficient connections from Midwestern suppliers to eastern markets. It was through such consolidation that, by 1900, seven major railroad tycoons controlled over 70 percent of all operating lines. Vanderbilt’s personal wealth at his death (over $100 million in 1877), placed him among the top three wealthiest individuals in American history.
GIANTS OF WEALTH: CARNEGIE, ROCKEFELLER, AND MORGAN
The post-Civil War inventors generated ideas that transformed the economy, but they were not big businessmen. The evolution from technical innovation to massive industry took place at the hands of the entrepreneurs whose business gambles paid off, making them some of the richest Americans of their day. Steel magnate Andrew Carnegie, oil tycoon John D. Rockefeller, and business financier J. P. Morgan were all businessmen who grew their respective businesses to a scale and scope that were unprecedented. Their companies changed how Americans lived and worked, and they themselves greatly influenced the growth of the country.
Andrew Carnegie and The Gospel of Wealth
Andrew Carnegie, steel magnate, has the prototypical rags-to-riches story. Although such stories resembled more myth than reality, they served to encourage many Americans to seek similar paths to fame and fortune. In Carnegie, the story was one of few derived from fact. Born in Scotland, Carnegie immigrated with his family to Pennsylvania in 1848. Following a brief stint as a “bobbin boy,” changing spools of thread at a Pittsburgh clothing manufacturer at age thirteen, he subsequently became a telegram messenger boy. As a messenger, he spent much of his time around the Pennsylvania Railroad office and developed parallel interests in railroads, bridge building, and, eventually, the steel industry.
Ingratiating himself to his supervisor and future president of the Pennsylvania Railroad, Tom Scott, Carnegie worked his way into a position of management for the company and subsequently began to invest some of his earnings, with Scott’s guidance. One particular investment, in the booming oil fields of northwest Pennsylvania in 1864, resulted in Carnegie earning over $1 million in cash dividends, thus providing him with the capital necessary to pursue his ambition to modernize the iron and steel industries, transforming the United States in the process. Having seen firsthand during the Civil War, when he served as Superintendent of Military Railways and telegraph coordinator for the Union forces, the importance of industry, particularly steel, to the future growth of the country, Carnegie was convinced of his strategy. His first company was the J. Edgar Thompson Steel Works, and, a decade later, he bought out the newly built Homestead Steel Works from the Pittsburgh Bessemer Steel Company. By the end of the century, his enterprise was running an annual profit in excess of $40 million (Figure).
Although not a scientific expert in steel, Carnegie was an excellent promoter and salesman, able to locate financial backing for his enterprise. He was also shrewd in his calculations on consolidation and expansion, and was able to capitalize on smart business decisions. Always thrifty with the profits he earned, a trait owed to his upbringing, Carnegie saved his profits during prosperous times and used them to buy out other steel companies at low prices during the economic recessions of the 1870s and 1890s. He insisted on up-to-date machinery and equipment, and urged the men who worked at and managed his steel mills to constantly think of innovative ways to increase production and reduce cost.
Carnegie, more than any other businessman of the era, championed the idea that America’s leading tycoons owed a debt to society. He believed that, given the circumstances of their successes, they should serve as benefactors to the less fortunate public. For Carnegie, poverty was not an abstract concept, as his family had been a part of the struggling masses. He desired to set an example of philanthropy for all other prominent industrialists of the era to follow. Carnegie’s famous essay, The Gospel of Wealth, featured below, expounded on his beliefs. In it, he borrowed from Herbert Spencer’s theory of social Darwinism, which held that society developed much like plant or animal life through a process of evolution in which the most fit and capable enjoyed the greatest material and social success.
Andrew Carnegie on Wealth
Carnegie applauded American capitalism for creating a society where, through hard work, ingenuity, and a bit of luck, someone like himself could amass a fortune. In return for that opportunity, Carnegie wrote that the wealthy should find proper uses for their wealth by funding hospitals, libraries, colleges, the arts, and more. The Gospel of Wealth spelled out that responsibility.
Poor and restricted are our opportunities in this life; narrow our horizon; our best work most imperfect; but rich men should be thankful for one inestimable boon. They have it in their power during their lives to busy themselves in organizing benefactions from which the masses of their fellows will derive lasting advantage, and thus dignify their own lives. . . .
This, then, is held to be the duty of the man of Wealth: First, to set an example of modest, unostentatious living, shunning display or extravagance; to provide moderately for the legitimate wants of those dependent upon him; and after doing so to consider all surplus revenues which come to him simply as trust funds, which he is called upon to administer, and strictly bound as a matter of duty to administer in the manner which, in his judgment, is best calculated to produce the most beneficial results for the community—the man of wealth thus becoming the mere agent and trustee for his poorer brethren, bringing to their service his superior wisdom, experience and ability to administer, doing for them better than they would or could do for themselves. . . .
In bestowing charity, the main consideration should be to help those who will help themselves; to provide part of the means by which those who desire to improve may do so; to give those who desire to use the aids by which they may rise; to assist, but rarely or never to do all. Neither the individual nor the race is improved by alms-giving. Those worthy of assistance, except in rare cases, seldom require assistance. The really valuable men of the race never do, except in cases of accident or sudden change. Every one has, of course, cases of individuals brought to his own knowledge where temporary assistance can do genuine good, and these he will not overlook. But the amount which can be wisely given by the individual for individuals is necessarily limited by his lack of knowledge of the circumstances connected with each. He is the only true reformer who is as careful and as anxious not to aid the unworthy as he is to aid the worthy, and, perhaps, even more so, for in alms-giving more injury is probably done by rewarding vice than by relieving virtue.
—Andrew Carnegie, The Gospel of Wealth
Social Darwinism added a layer of pseudoscience to the idea of the self-made man, a desirable thought for all who sought to follow Carnegie’s example. The myth of the rags-to-riches businessman was a potent one. Author Horatio Alger made his own fortune writing stories about young enterprising boys who beat poverty and succeeded in business through a combination of “luck and pluck.” His stories were immensely popular, even leading to a board game (Figure) where players could hope to win in the same way that his heroes did.
John D. Rockefeller and Business Integration Models
Like Carnegie, John D. Rockefeller was born in 1839 of modest means, with a frequently absent traveling salesman of a father who sold medicinal elixirs and other wares. Young Rockefeller helped his mother with various chores and earned extra money for the family through the sale of family farm products. When the family moved to a suburb of Cleveland in 1853, he had an opportunity to take accounting and bookkeeping courses while in high school and developed a career interest in business. While living in Cleveland in 1859, he learned of Colonel Edwin Drake who had struck “black gold,” or oil, near Titusville, Pennsylvania, setting off a boom even greater than the California Gold Rush of the previous decade. Many sought to find a fortune through risky and chaotic “wildcatting,” or drilling exploratory oil wells, hoping to strike it rich. But Rockefeller chose a more certain investment: refining crude oil into kerosene, which could be used for both heating and lamps. As a more efficient source of energy, as well as less dangerous to produce, kerosene quickly replaced whale oil in many businesses and homes. Rockefeller worked initially with family and friends in the refining business located in the Cleveland area, but by 1870, Rockefeller ventured out on his own, consolidating his resources and creating the Standard Oil Company of Ohio, initially valued at $1 million.
Rockefeller was ruthless in his pursuit of total control of the oil refining business. As other entrepreneurs flooded the area seeking a quick fortune, Rockefeller developed a plan to crush his competitors and create a true monopoly in the refining industry. Beginning in 1872, he forged agreements with several large railroad companies to obtain discounted freight rates for shipping his product. He also used the railroad companies to gather information on his competitors. As he could now deliver his kerosene at lower prices, he drove his competition out of business, often offering to buy them out for pennies on the dollar. He hounded those who refused to sell out to him, until they were driven out of business. Through his method of growth via mergers and acquisitions of similar companies—known as horizontal integration —Standard Oil grew to include almost all refineries in the area. By 1879, the Standard Oil Company controlled nearly 95 percent of all oil refining businesses in the country, as well as 90 percent of all the refining businesses in the world. Editors of the New York World lamented of Standard Oil in 1880 that, “When the nineteenth century shall have passed into history, the impartial eyes of the reviewers will be amazed to find that the U.S. . . . tolerated the presence of the most gigantic, the most cruel, impudent, pitiless and grasping monopoly that ever fastened itself upon a country.”
Seeking still more control, Rockefeller recognized the advantages of controlling the transportation of his product. He next began to grow his company through vertical integration, wherein a company handles all aspects of a product’s lifecycle, from the creation of raw materials through the production process to the delivery of the final product. In Rockefeller’s case, this model required investment and acquisition of companies involved in everything from barrel-making to pipelines, tanker cars to railroads. He came to own almost every type of business and used his vast power to drive competitors from the market through intense price wars. Although vilified by competitors who suffered from his takeovers and considered him to be no better than a robber baron, several observers lauded Rockefeller for his ingenuity in integrating the oil refining industry and, as a result, lowering kerosene prices by as much as 80 percent by the end of the century. Other industrialists quickly followed suit, including Gustavus Swift, who used vertical integration to dominate the U.S. meatpacking industry in the late nineteenth century.
In order to control the variety of interests he now maintained in industry, Rockefeller created a new legal entity, known as a trust. In this arrangement, a small group of trustees possess legal ownership of a business that they operate for the benefit of other investors. In 1882, all thirty-seven stockholders in the various Standard Oil enterprises gave their stock to nine trustees who were to control and direct all of the company’s business ventures. State and federal challenges arose, due to the obvious appearance of a monopoly, which implied sole ownership of all enterprises composing an entire industry. When the Ohio Supreme Court ruled that the Standard Oil Company must dissolve, as its monopoly control over all refining operations in the U.S. was in violation of state and federal statutes, Rockefeller shifted to yet another legal entity, called a holding company model. The holding company model created a central corporate entity that controlled the operations of multiple companies by holding the majority of stock for each enterprise. While not technically a “trust” and therefore not vulnerable to anti-monopoly laws, this consolidation of power and wealth into one entity was on par with a monopoly; thus, progressive reformers of the late nineteenth century considered holding companies to epitomize the dangers inherent in capitalistic big business, as can be seen in the political cartoon below (Figure). Impervious to reformers’ misgivings, other businessmen followed Rockefeller’s example. By 1905, over three hundred business mergers had occurred in the United States, affecting more than 80 percent of all industries. By that time, despite passage of federal legislation such as the Sherman Anti-Trust Act in 1890, 1 percent of the country’s businesses controlled over 40 percent of the nation’s economy.
The PBS video on Robber Barons or Industrial Giants presents a lively discussion of whether the industrialists of the nineteenth century were really “robber barons” or if they were “industrial giants.”
J. Pierpont Morgan
Unlike Carnegie and Rockefeller, J. P. Morgan was no rags-to-riches hero. He was born to wealth and became much wealthier as an investment banker, making wise financial decisions in support of the hard-working entrepreneurs building their fortunes. Morgan’s father was a London banker, and Morgan the son moved to New York in 1857 to look after the family’s business interests there. Once in America, he separated from the London bank and created the J. Pierpont Morgan and Company financial firm. The firm bought and sold stock in growing companies, investing the family’s wealth in those that showed great promise, turning an enormous profit as a result. Investments from firms such as his were the key to the success stories of up-and-coming businessmen like Carnegie and Rockefeller. In return for his investment, Morgan and other investment bankers demanded seats on the companies’ boards, which gave them even greater control over policies and decisions than just investment alone. There were many critics of Morgan and these other bankers, particularly among members of a U.S. congressional subcommittee who investigated the control that financiers maintained over key industries in the country. The subcommittee referred to Morgan’s enterprise as a form of “money trust” that was even more powerful than the trusts operated by Rockefeller and others. Morgan argued that his firm, and others like it, brought stability and organization to a hypercompetitive capitalist economy, and likened his role to a kind of public service.
Ultimately, Morgan’s most notable investment, and greatest consolidation, was in the steel industry, when he bought out Andrew Carnegie in 1901. Initially, Carnegie was reluctant to sell, but after repeated badgering by Morgan, Carnegie named his price: an outrageously inflated sum of $500 million. Morgan agreed without hesitation, and then consolidated Carnegie’s holdings with several smaller steel firms to create the U.S. Steel Corporation. U.S. Steel was subsequently capitalized at $1.4 billion. It was the country’s first billion-dollar firm. Lauded by admirers for the efficiency and modernization he brought to investment banking practices, as well as for his philanthropy and support of the arts, Morgan was also criticized by reformers who subsequently blamed his (and other bankers’) efforts for contributing to the artificial bubble of prosperity that eventually burst in the Great Depression of the 1930s. What none could doubt was that Morgan’s financial aptitude and savvy business dealings kept him in good stead. A subsequent U.S. congressional committee, in 1912, reported that his firm held 341 directorships in 112 corporations that controlled over $22 billion in assets. In comparison, that amount of wealth was greater than the assessed value of all the land in the United States west of the Mississippi River.
Section Summary
As the three tycoons profiled in this section illustrate, the end of the nineteenth century was a period in history that offered tremendous financial rewards to those who had the right combination of skill, ambition, and luck. Whether self-made millionaires like Carnegie or Rockefeller, or born to wealth like Morgan, these men were the lynchpins that turned inventors’ ideas into industrial growth. Steel production, in particular, but also oil refining techniques and countless other inventions, changed how industries in the country could operate, allowing them to grow in scale and scope like never before.
It is also critical to note how these different men managed their businesses and ambition. Where Carnegie felt strongly that it was the job of the wealthy to give back in their lifetime to the greater community, his fellow tycoons did not necessarily agree. Although he contributed to many philanthropic efforts, Rockefeller’s financial success was built on the backs of ruined and bankrupt companies, and he came to be condemned by progressive reformers who questioned the impact on the working class as well as the dangers of consolidating too much power and wealth into one individual’s hands. Morgan sought wealth strictly through the investment in, and subsequent purchase of, others’ hard work. Along the way, the models of management they adopted—horizontal and vertical integration, trusts, holding companies, and investment brokerages—became commonplace in American businesses. Very quickly, large business enterprises fell under the control of fewer and fewer individuals and trusts. In sum, their ruthlessness, their ambition, their generosity, and their management made up the workings of America’s industrial age.
Review Questions
Which of the following “robber barons” was notable for the exploitative way he made his fortune in railroads?
- Jay Gould
- Cornelius Vanderbilt
- Andrew Carnegie
- J. Pierpont Morgan
Hint:
A
Which of the following does not represent one of the management strategies that John D. Rockefeller used in building his empire?
- horizontal integration
- vertical integration
- social Darwinism
- the holding company model
Hint:
C
Why was Rockefeller’s use of horizontal integration such an effective business tool at this time? Were his choices legal? Why or why not?
Hint:
Horizontal integration enabled Rockefeller to gain tremendous control over the oil industry and use that power to influence vendors and competitors. For example, he could pressure railroads into giving him lower rates because of the volume of his products. He undercut competitors, forcing them to set their prices so low that they could barely stay in business—at which point he could buy them out. Through horizontal integration, he was able to create a virtual monopoly and set the terms for business. While his business model of a holding company was technically legal, it held as much power as a monopoly and did not allow for other businesses to grow and compete.
What differentiated a “robber baron” from other “captains of industry” in late nineteenth-century America?
Hint:
“Captains of industry” (such as Carnegie or Rockefeller) are noted for their new business models, entrepreneurial approaches, and, to varying degrees, philanthropic efforts, all of which transformed late nineteenth-century America. “Robber barons” (such as Gould) are noted for their self-centered drive for profit at the expense of workers and the general public, who seldom benefitted to any great degree. The terms, however, remain a gray area, as one could characterize the ruthless business practices of Rockefeller, or some of Carnegie’s tactics with regard to workers’ efforts to organize, as similar to the methods of robber barons. Nevertheless, “captains of industry” are noted for contributions that fundamentally changed and typically improved the nation, whereas “robber barons” can seldom point to such concrete contributions.
|
oercommons
|
2025-03-18T00:37:58.955260
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15490/overview",
"title": "U.S. History, Industrialization and the Rise of Big Business, 1870-1900",
"author": null
}
|
https://oercommons.org/courseware/lesson/15491/overview
|
Building Industrial America on the Backs of Labor
Overview
By the end of this section, you will be able to:
- Explain the qualities of industrial working-class life in the late nineteenth century
- Analyze both workers’ desire for labor unions and the reasons for unions’ inability to achieve their goals
The growth of the American economy in the last half of the nineteenth century presented a paradox. The standard of living for many American workers increased. As Carnegie said in The Gospel of Wealth, “the poor enjoy what the rich could not before afford. What were the luxuries have become the necessaries of life. The laborer has now more comforts than the landlord had a few generations ago.” In many ways, Carnegie was correct. The decline in prices and the cost of living meant that the industrial era offered many Americans relatively better lives in 1900 than they had only decades before. For some Americans, there were also increased opportunities for upward mobility. For the multitudes in the working class, however, conditions in the factories and at home remained deplorable. The difficulties they faced led many workers to question an industrial order in which a handful of wealthy Americans built their fortunes on the backs of workers.
WORKING-CLASS LIFE
Between the end of the Civil War and the turn of the century, the American workforce underwent a transformative shift. In 1865, nearly 60 percent of Americans still lived and worked on farms; by the early 1900s, that number had reversed itself, and only 40 percent still lived in rural areas, with the remainder living and working in urban and early suburban areas. A significant number of these urban and suburban dwellers earned their wages in factories. Advances in farm machinery allowed for greater production with less manual labor, thus leading many Americans to seek job opportunities in the burgeoning factories in the cities. Not surprisingly, there was a concurrent trend of a decrease in American workers being self-employed and an increase of those working for others and being dependent on a factory wage system for their living.
Yet factory wages were, for the most part, very low. In 1900, the average factory wage was approximately twenty cents per hour, for an annual salary of barely six hundred dollars. According to some historical estimates, that wage left approximately 20 percent of the population in industrialized cities at, or below, the poverty level. An average factory work week was sixty hours, ten hours per day, six days per week, although in steel mills, the workers put in twelve hours per day, seven days a week. Factory owners had little concern for workers’ safety. According to one of the few available accurate measures, as late as 1913, nearly 25,000 Americans lost their lives on the job, while another 700,000 workers suffered from injuries that resulted in at least one missed month of work. Another element of hardship for workers was the increasingly dehumanizing nature of their work. Factory workers executed repetitive tasks throughout the long hours of their shifts, seldom interacting with coworkers or supervisors. This solitary and repetitive work style was a difficult adjustment for those used to more collaborative and skill-based work, whether on farms or in crafts shops. Managers embraced Fredrick Taylor’s principles of scientific management, also called “stop-watch management,” where he used stop-watch studies to divide manufacturing tasks into short, repetitive segments. A mechanical engineer by training, Taylor encouraged factory owners to seek efficiency and profitability over any benefits of personal interaction. Owners adopted this model, effectively making workers cogs in a well-oiled machine.
One result of the new breakdown of work processes was that factory owners were able to hire women and children to perform many of the tasks. From 1870 through 1900, the number of women working outside the home tripled. By the end of this period, five million American women were wage earners, with one-quarter of them working factory jobs. Most were young, under twenty-five, and either immigrants themselves or the daughters of immigrants. Their foray into the working world was not seen as a step towards empowerment or equality, but rather a hardship born of financial necessity. Women’s factory work tended to be in clothing or textile factories, where their appearance was less offensive to men who felt that heavy industry was their purview. Other women in the workforce worked in clerical positions as bookkeepers and secretaries, and as salesclerks. Not surprisingly, women were paid less than men, under the pretense that they should be under the care of a man and did not require a living wage.
Factory owners used the same rationale for the exceedingly low wages they paid to children. Children were small enough to fit easily among the machines and could be hired for simple work for a fraction of an adult man’s pay. The image below (Figure) shows children working the night shift in a glass factory. From 1870 through 1900, child labor in factories tripled. Growing concerns among progressive reformers over the safety of women and children in the workplace would eventually result in the development of political lobby groups. Several states passed legislative efforts to ensure a safe workplace, and the lobby groups pressured Congress to pass protective legislation. However, such legislation would not be forthcoming until well into the twentieth century. In the meantime, many working-class immigrants still desired the additional wages that child and women labor produced, regardless of the harsh working conditions.
WORKER PROTESTS AND VIOLENCE
Workers were well aware of the vast discrepancy between their lives and the wealth of the factory owners. Lacking the assets and legal protection needed to organize, and deeply frustrated, some working communities erupted in spontaneous violence. The coal mines of eastern Pennsylvania and the railroad yards of western Pennsylvania, central to both respective industries and home to large, immigrant, working enclaves, saw the brunt of these outbursts. The combination of violence, along with several other factors, blunted any significant efforts to organize workers until well into the twentieth century.
Business owners viewed organization efforts with great mistrust, capitalizing upon widespread anti-union sentiment among the general public to crush unions through open shops, the use of strikebreakers, yellow-dog contracts (in which the employee agrees to not join a union as a pre-condition of employment), and other means. Workers also faced obstacles to organization associated with race and ethnicity, as questions arose on how to address the increasing number of low-paid African American workers, in addition to the language and cultural barriers introduced by the large wave of southeastern European immigration to the United States. But in large part, the greatest obstacle to effective unionization was the general public’s continued belief in a strong work ethic and that an individual work ethic—not organizing into radical collectives—would reap its own rewards. As violence erupted, such events seemed only to confirm widespread popular sentiment that radical, un-American elements were behind all union efforts.
In the 1870s, Irish coal miners in eastern Pennsylvania formed a secret organization known as the Molly Maguires, named for the famous Irish patriot. Through a series of scare tactics that included kidnappings, beatings, and even murder, the Molly Maguires sought to bring attention to the miners’ plight, as well as to cause enough damage and concern to the mine owners that the owners would pay attention to their concerns. Owners paid attention, but not in the way that the protesters had hoped. They hired detectives to pose as miners and mingle among the workers to obtain the names of the Molly Maguires. By 1875, they had acquired the names of twenty-four suspected Maguires, who were subsequently convicted of murder and violence against property. All were convicted and ten were hanged in 1876, at a public “Day of the Rope.” This harsh reprisal quickly crushed the remaining Molly Maguires movement. The only substantial gain the workers had from this episode was the knowledge that, lacking labor organization, sporadic violent protest would be met by escalated violence.
Public opinion was not sympathetic towards labor’s violent methods as displayed by the Molly Maguires. But the public was further shocked by some of the harsh practices employed by government agents to crush the labor movement, as seen the following year in the Great Railroad Strike of 1877. After incurring a significant pay cut earlier that year, railroad workers in West Virginia spontaneously went on strike and blocked the tracks (Figure). As word spread of the event, railroad workers across the country joined in sympathy, leaving their jobs and committing acts of vandalism to show their frustration with the ownership. Local citizens, who in many instances were relatives and friends, were largely sympathetic to the railroad workers’ demands.
The most significant violent outbreak of the railroad strike occurred in Pittsburgh, beginning on July 19. The governor ordered militiamen from Philadelphia to the Pittsburgh roundhouse to protect railroad property. The militia opened fire to disperse the angry crowd and killed twenty individuals while wounding another twenty-nine. A riot erupted, resulting in twenty-four hours of looting, violence, fire, and mayhem, and did not die down until the rioters wore out in the hot summer weather. In a subsequent skirmish with strikers while trying to escape the roundhouse, militiamen killed another twenty individuals. Violence erupted in Maryland and Illinois as well, and President Hayes eventually sent federal troops into major cities to restore order. This move, along with the impending return of cooler weather that brought with it the need for food and fuel, resulted in striking workers nationwide returning to the railroad. The strike had lasted for forty-five days, and they had gained nothing but a reputation for violence and aggression that left the public less sympathetic than ever. Dissatisfied laborers began to realize that there would be no substantial improvement in their quality of life until they found a way to better organize themselves.
WORKER ORGANIZATION AND THE STRUGGLES OF UNIONS
Prior to the Civil War, there were limited efforts to create an organized labor movement on any large scale. With the majority of workers in the country working independently in rural settings, the idea of organized labor was not largely understood. But, as economic conditions changed, people became more aware of the inequities facing factory wage workers. By the early 1880s, even farmers began to fully recognize the strength of unity behind a common cause.
Models of Organizing: The Knights of Labor and American Federation of Labor
In 1866, seventy-seven delegates representing a variety of different occupations met in Baltimore to form the National Labor Union (NLU). The NLU had ambitious ideas about equal rights for African Americans and women, currency reform, and a legally mandated eight-hour workday. The organization was successful in convincing Congress to adopt the eight-hour workday for federal employees, but their reach did not progress much further. The Panic of 1873 and the economic recession that followed as a result of overspeculation on railroads and the subsequent closing of several banks—during which workers actively sought any employment regardless of the conditions or wages—as well as the death of the NLU’s founder, led to a decline in their efforts.
A combination of factors contributed to the debilitating Panic of 1873, which triggered what the public referred to at the time as the “Great Depression” of the 1870s. Most notably, the railroad boom that had occurred from 1840 to 1870 was rapidly coming to a close. Overinvestment in the industry had extended many investors’ capital resources in the form of railroad bonds. However, when several economic developments in Europe affected the value of silver in America, which in turn led to a de facto gold standard that shrunk the U.S. monetary supply, the amount of cash capital available for railroad investments rapidly declined. Several large business enterprises were left holding their wealth in all but worthless railroad bonds. When Jay Cooke & Company, a leader in the American banking industry, declared bankruptcy on the eve of their plans to finance the construction of a new transcontinental railroad, the panic truly began. A chain reaction of bank failures culminated with the New York Stock Exchange suspending all trading for ten days at the end of September 1873. Within a year, over one hundred railroad enterprises had failed; within two years, nearly twenty thousand businesses had failed. The loss of jobs and wages sent workers throughout the United States seeking solutions and clamoring for scapegoats.
Although the NLU proved to be the wrong effort at the wrong time, in the wake of the Panic of 1873 and the subsequent frustration exhibited in the failed Molly Maguires uprising and the national railroad strike, another, more significant, labor organization emerged. The Knights of Labor (KOL) was more able to attract a sympathetic following than the Molly Maguires and others by widening its base and appealing to more members. Philadelphia tailor Uriah Stephens grew the KOL from a small presence during the Panic of 1873 to an organization of national importance by 1878. That was the year the KOL held their first general assembly, where they adopted a broad reform platform, including a renewed call for an eight-hour workday, equal pay regardless of gender, the elimination of convict labor, and the creation of greater cooperative enterprises with worker ownership of businesses. Much of the KOL’s strength came from its concept of “One Big Union”—the idea that it welcomed all wage workers, regardless of occupation, with the exception of doctors, lawyers, and bankers. It welcomed women, African Americans, Native Americans, and immigrants, of all trades and skill levels. This was a notable break from the earlier tradition of craft unions, which were highly specialized and limited to a particular group. In 1879, a new leader, Terence V. Powderly, joined the organization, and he gained even more followers due to his marketing and promotional efforts. Although largely opposed to strikes as effective tactics, through their sheer size, the Knights claimed victories in several railroad strikes in 1884–1885, including one against notorious “robber baron” Jay Gould, and their popularity consequently rose among workers. By 1886, the KOL had a membership in excess of 700,000.
In one night, however, the KOL’s popularity—and indeed the momentum of the labor movement as a whole—plummeted due to an event known as the Haymarket affair, which occurred on May 4, 1886, in Chicago’s Haymarket Square (Figure). There, an anarchist group had gathered in response to a death at an earlier nationwide demonstration for the eight-hour workday. At the earlier demonstration, clashes between police and strikers at the International Harvester Company of Chicago led to the death of a striking worker. The anarchist group decided to hold a protest the following night in Haymarket Square, and, although the protest was quiet, the police arrived armed for conflict. Someone in the crowd threw a bomb at the police, killing one officer and injuring another. The seven anarchists speaking at the protest were arrested and charged with murder. They were sentenced to death, though two were later pardoned and one committed suicide in prison before his execution.
The press immediately blamed the KOL as well as Powderly for the Haymarket affair, despite the fact that neither the organization nor Powderly had anything to do with the demonstration. Combined with the American public’s lukewarm reception to organized labor as a whole, the damage was done. The KOL saw its membership decline to barely 100,000 by the end of 1886. Nonetheless, during its brief success, the Knights illustrated the potential for success with their model of “industrial unionism,” which welcomed workers from all trades.
The Haymarket Rally
On May 1, 1886, recognized internationally as a day for labor celebration, labor organizations around the country engaged in a national rally for the eight-hour workday. While the number of striking workers varied around the country, estimates are that between 300,000 and 500,000 workers protested in New York, Detroit, Chicago, and beyond. In Chicago, clashes between police and protesters led the police to fire into the crowd, resulting in fatalities. Afterward, angry at the deaths of the striking workers, organizers quickly organized a “mass meeting,” per the poster below (Figure).
While the meeting was intended to be peaceful, a large police presence made itself known, prompting one of the event organizers to state in his speech, “There seems to prevail the opinion in some quarters that this meeting has been called for the purpose of inaugurating a riot, hence these warlike preparations on the part of so-called ‘law and order.’ However, let me tell you at the beginning that this meeting has not been called for any such purpose. The object of this meeting is to explain the general situation of the eight-hour movement and to throw light upon various incidents in connection with it.” The mayor of Chicago later corroborated accounts of the meeting, noted that it was a peaceful rally, but as it was winding down, the police marched into the crowd, demanding they disperse. Someone in the crowd threw a bomb, killing one policeman immediately and wounding many others, some of whom died later. Despite the aggressive actions of the police, public opinion was strongly against the striking laborers. The New York Times, after the events played out, reported on it with the headline “Rioting and Bloodshed in the Streets of Chicago: Police Mowed Down with Dynamite.” Other papers echoed the tone and often exaggerated the chaos, undermining organized labor’s efforts and leading to the ultimate conviction and hanging of the rally organizers. Labor activists considered those hanged after the Haymarket affair to be martyrs for the cause and created an informal memorial at their gravesides in Park Forest, Illinois.
This article about the “Rioting and Bloodshed in the Streets of Chicago” reveals how the New York Times reported on the Haymarket affair. Assess whether the article gives evidence of the information it lays out. Consider how it portrays the events, and how different, more sympathetic coverage might have changed the response of the general public towards immigrant workers and labor unions.
During the effort to establish industrial unionism in the form of the KOL, craft unions had continued to operate. In 1886, twenty different craft unions met to organize a national federation of autonomous craft unions. This group became the American Federation of Labor (AFL), led by Samuel Gompers from its inception until his death in 1924. More so than any of its predecessors, the AFL focused almost all of its efforts on economic gains for its members, seldom straying into political issues other than those that had a direct impact upon working conditions. The AFL also kept a strict policy of not interfering in each union’s individual business. Rather, Gompers often settled disputes between unions, using the AFL to represent all unions of matters of federal legislation that could affect all workers, such as the eight-hour workday.
By 1900, the AFL had 500,000 members; by 1914, its numbers had risen to one million, and by 1920 they claimed four million working members. Still, as a federation of craft unions, it excluded many factory workers and thus, even at its height, represented only 15 percent of the nonfarm workers in the country. As a result, even as the country moved towards an increasingly industrial age, the majority of American workers still lacked support, protection from ownership, and access to upward mobility.
The Decline of Labor: The Homestead and Pullman Strikes
While workers struggled to find the right organizational structure to support a union movement in a society that was highly critical of such worker organization, there came two final violent events at the close of the nineteenth century. These events, the Homestead Steel Strike of 1892 and the Pullman Strike of 1894, all but crushed the labor movement for the next forty years, leaving public opinion of labor strikes lower than ever and workers unprotected.
At the Homestead factory of the Carnegie Steel Company, workers represented by the Amalgamated Association of Iron and Steel Workers enjoyed relatively good relations with management until Henry C. Frick became the factory manager in 1889. When the union contract was up for renewal in 1892, Carnegie—long a champion of living wages for his employees—had left for Scotland and trusted Frick—noted for his strong anti-union stance—to manage the negotiations. When no settlement was reached by June 29, Frick ordered a lockout of the workers and hired three hundred Pinkerton detectives to protect company property. On July 6, as the Pinkertons arrived on barges on the river, union workers along the shore engaged them in a gunfight that resulted in the deaths of three Pinkertons and six workers. One week later, the Pennsylvania militia arrived to escort strike-breakers into the factory to resume production. Although the lockout continued until November, it ended with the union defeated and individual workers asking for their jobs back. A subsequent failed assassination attempt by anarchist Alexander Berkman on Frick further strengthened public animosity towards the union.
Two years later, in 1894, the Pullman Strike was another disaster for unionized labor. The crisis began in the company town of Pullman, Illinois, where Pullman “sleeper” cars were manufactured for America’s railroads. When the depression of 1893 unfolded in the wake of the failure of several northeastern railroad companies, mostly due to overconstruction and poor financing, company owner George Pullman fired three thousand of the factory’s six thousand employees, cut the remaining workers’ wages by an average of 25 percent, and then continued to charge the same high rents and prices in the company homes and store where workers were required to live and shop. Workers began the strike on May 11, when Eugene V. Debs, the president of the American Railway Union, ordered rail workers throughout the country to stop handling any trains that had Pullman cars on them. In practicality, almost all of the trains fell into this category, and, therefore, the strike created a nationwide train stoppage, right on the heels of the depression of 1893. Seeking justification for sending in federal troops, President Grover Cleveland turned to his attorney general, who came up with a solution: Attach a mail car to every train and then send in troops to ensure the delivery of the mail. The government also ordered the strike to end; when Debs refused, he was arrested and imprisoned for his interference with the delivery of U.S. mail. The image below (Figure) shows the standoff between federal troops and the workers. The troops protected the hiring of new workers, thus rendering the strike tactic largely ineffective. The strike ended abruptly on July 13, with no labor gains and much lost in the way of public opinion.
George Estes on the Order of Railroad Telegraphers
The following excerpt is a reflection from George Estes, an organizer and member of the Order of Railroad Telegraphers, a labor organization at the end of the nineteenth century. His perspective on the ways that labor and management related to each other illustrates the difficulties at the heart of their negotiations. He notes that, in this era, the two groups saw each other as enemies and that any gain by one was automatically a loss by the other.
I have always noticed that things usually have to get pretty bad before they get any better. When inequities pile up so high that the burden is more than the underdog can bear, he gets his dander up and things begin to happen. It was that way with the telegraphers’ problem. These exploited individuals were determined to get for themselves better working conditions—higher pay, shorter hours, less work which might not properly be classed as telegraphy, and the high and mighty Mr. Fillmore [railroad company president] was not going to stop them. It was a bitter fight. At the outset, Mr. Fillmore let it be known, by his actions and comments, that he held the telegraphers in the utmost contempt.
With the papers crammed each day with news of labor strife—and with two great labor factions at each other’s throats, I am reminded of a parallel in my own early and more active career. Shortly before the turn of the century, in 1898 and 1899 to be more specific, I occupied a position with regard to a certain class of skilled labor, comparable to that held by the Lewises and Greens of today. I refer, of course, to the telegraphers and station agents. These hard-working gentlemen—servants of the public—had no regular hours, performed a multiplicity of duties, and, considering the service they rendered, were sorely and inadequately paid. A telegrapher’s day included a considerable number of chores that present-day telegraphers probably never did or will do in the course of a day’s work. He used to clean and fill lanterns, block lights, etc. Used to do the janitor work around the small town depot, stoke the pot-bellied stove of the waiting-room, sweep the floors, picking up papers and waiting-room litter. . . .
Today, capital and labor seem to understand each other better than they did a generation or so ago. Capital is out to make money. So is labor—and each is willing to grant the other a certain amount of tolerant leeway, just so he doesn’t go too far. In the old days there was a breach as wide as the Pacific separating capital and labor. It wasn’t money altogether in those days, it was a matter of principle. Capital and labor couldn’t see eye to eye on a single point. Every gain that either made was at the expense of the other, and was fought tooth and nail. No difference seemed ever possible of amicable settlement. Strikes were riots. Murder and mayhem was common. Railroad labor troubles were frequent. The railroads, in the nineties, were the country’s largest employers. They were so big, so powerful, so perfectly organized themselves—I mean so in accord among themselves as to what treatment they felt like offering the man who worked for them—that it was extremely difficult for labor to gain a single advantage in the struggle for better conditions.
—George Estes, interview with Andrew Sherbert, 1938
Section Summary
After the Civil War, as more and more people crowded into urban areas and joined the ranks of wage earners, the landscape of American labor changed. For the first time, the majority of workers were employed by others in factories and offices in the cities. Factory workers, in particular, suffered from the inequity of their positions. Owners had no legal restrictions on exploiting employees with long hours in dehumanizing and poorly paid work. Women and children were hired for the lowest possible wages, but even men’s wages were barely enough upon which to live.
Poor working conditions, combined with few substantial options for relief, led workers to frustration and sporadic acts of protest and violence, acts that rarely, if ever, gained them any lasting, positive effects. Workers realized that change would require organization, and thus began early labor unions that sought to win rights for all workers through political advocacy and owner engagement. Groups like the National Labor Union and Knights of Labor both opened their membership to any and all wage earners, male or female, black or white, regardless of skill. Their approach was a departure from the craft unions of the very early nineteenth century, which were unique to their individual industries. While these organizations gained members for a time, they both ultimately failed when public reaction to violent labor strikes turned opinion against them. The American Federation of Labor, a loose affiliation of different unions, grew in the wake of these universal organizations, although negative publicity impeded their work as well. In all, the century ended with the vast majority of American laborers unrepresented by any collective or union, leaving them vulnerable to the power wielded by factory ownership.
Review Questions
What was one of the key goals for which striking workers fought in the late nineteenth century?
- health insurance
- disability pay
- an eight-hour workday
- women’s right to hold factory jobs
Hint:
C
Which of the following was not a key goal of the Knights of Labor?
- an end to convict labor
- a graduated income tax on personal wealth
- equal pay regardless of gender
- the creation of cooperative business enterprises
Hint:
B
What were the core differences in the methods and agendas of the Knights of Labor and the American Federation of Labor?
Hint:
The Knights of Labor (KOL) had a broad and open base, inviting all types of workers, including women and African Americans, into their ranks. The KOL also sought political gains for workers throughout the country, regardless of their membership. In contrast, the American Federation of Labor (AFL) was a loose affiliation of separate unions, with each group remaining intact and distinct. The AFL did not advocate for national labor issues, but restricted its efforts to helping improve economic conditions for its members.
|
oercommons
|
2025-03-18T00:37:58.989844
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15491/overview",
"title": "U.S. History, Industrialization and the Rise of Big Business, 1870-1900",
"author": null
}
|
https://oercommons.org/courseware/lesson/15492/overview
|
A New American Consumer Culture
Overview
By the end of this section, you will be able to:
- Describe the characteristics of the new consumer culture that emerged at the end of the nineteenth century
Despite the challenges workers faced in their new roles as wage earners, the rise of industry in the United States allowed people to access and consume goods as never before. The rise of big business had turned America into a culture of consumers desperate for time-saving and leisure commodities, where people could expect to find everything they wanted in shops or by mail order. Gone were the days where the small general store was the only option for shoppers; at the end of the nineteenth century, people could take a train to the city and shop in large department stores like Macy’s in New York, Gimbel’s in Philadelphia, and Marshall Fields in Chicago. Chain stores, like A&P and Woolworth’s, both of which opened in the 1870s, offered options to those who lived farther from major urban areas and clearly catered to classes other than the wealthy elite. Industrial advancements contributed to this proliferation, as new construction techniques permitted the building of stores with higher ceilings for larger displays, and the production of larger sheets of plate glass lent themselves to the development of larger store windows, glass countertops, and display cases where shoppers could observe a variety of goods at a glance. L. Frank Baum, of Wizard of Oz fame, later founded the National Association of Window Trimmers in 1898, and began publishing The Store Window journal to advise businesses on space usage and promotion.
Even families in rural America had new opportunities to purchase a greater variety of products than ever before, at ever decreasing prices. Those far from chain stores could benefit from the newly developed business of mail-order catalogs, placing orders by telephone. Aaron Montgomery Ward established the first significant mail-order business in 1872, with Sears, Roebuck & Company following in 1886. Sears distributed over 300,000 catalogs annually by 1897, and later broke the one million annual mark in 1907. Sears in particular understood that farmers and rural Americans sought alternatives to the higher prices and credit purchases they were forced to endure at small-town country stores. By clearly stating the prices in his catalog, Richard Sears steadily increased his company’s image of their catalog serving as “the consumer’s bible.” In the process, Sears, Roebuck & Company supplied much of America’s hinterland with products ranging from farm supplies to bicycles, toilet paper to automobiles, as seen below in a page from the catalog (Figure).
The tremendous variety of goods available for sale required businesses to compete for customers in ways they had never before imagined. Suddenly, instead of a single option for clothing or shoes, customers were faced with dozens, whether ordered by mail, found at the local chain store, or lined up in massive rows at department stores. This new level of competition made advertising a vital component of all businesses. By 1900, American businesses were spending almost $100 million annually on advertising. Competitors offered “new and improved” models as frequently as possible in order to generate interest. From toothpaste and mouthwash to books on entertaining guests, new goods were constantly offered. Newspapers accommodated the demand for advertising by shifting their production to include full-page advertisements, as opposed to the traditional column width, agate-type advertisements that dominated mid-nineteenth century newspapers (similar to classified advertisements in today’s publications). Likewise, professional advertising agencies began to emerge in the 1880s, with experts in consumer demand bidding for accounts with major firms.
It may seem strange that, at a time when wages were so low, people began buying readily; however, the slow emergence of a middle class by the end of the century, combined with the growing practice of buying on credit, presented more opportunities to take part in the new consumer culture. Stores allowed people to open accounts and purchase on credit, thus securing business and allowing consumers to buy without ready cash. Then, as today, the risks of buying on credit led many into debt. As advertising expert Roland Marchand described in his Parable on the Democracy of Goods, in an era when access to products became more important than access to the means of production, Americans quickly accepted the notion that they could live a better lifestyle by purchasing the right clothes, the best hair cream, and the shiniest shoes, regardless of their class. For better or worse, American consumerism had begun.
Advertising in the Industrial Age: Credit, Luxury, and the Advent of “New and Improved”
Before the industrial revolution, most household goods were either made at home or purchased locally, with limited choices. By the end of the nineteenth century, factors such as the population’s move towards urban centers and the expansion of the railroad changed how Americans shopped for, and perceived, consumer goods. As mentioned above, advertising took off, as businesses competed for customers.
Many of the elements used widely in nineteenth-century advertisements are familiar. Companies sought to sell luxury, safety, and, as the ad for the typewriter below shows (Figure), the allure of the new-and-improved model. One advertising tactic that truly took off in this era was the option to purchase on credit. For the first time, mail order and mass production meant that the aspiring middle class could purchase items that could only be owned previously by the wealthy. While there was a societal stigma for buying everyday goods on credit, certain items, such as fine furniture or pianos, were considered an investment in the move toward entry into the middle class.
Additionally, farmers and housewives purchased farm equipment and sewing machines on credit, considering these items investments rather than luxuries. For women, the purchase of a sewing machine meant that a shirt could be made in one hour, instead of fourteen. The Singer Sewing Machine Company was one of the most aggressive at pushing purchase on credit. They advertised widely, and their “Dollar Down, Dollar a Week” campaign made them one of the fastest-growing companies in the country.
For workers earning lower wages, these easy credit terms meant that the middle-class lifestyle was within their reach. Of course, it also meant they were in debt, and changes in wages, illness, or other unexpected expenses could wreak havoc on a household’s tenuous finances. Still, the opportunity to own new and luxurious products was one that many Americans, aspiring to improve their place in society, could not resist.
Section Summary
While tensions between owners and workers continued to grow, and wage earners struggled with the challenges of industrial work, the culture of American consumerism was changing. Greater choice, easier access, and improved goods at lower prices meant that even lower-income Americans, whether rural and shopping via mail order, or urban and shopping in large department stores, had more options. These increased options led to a rise in advertising, as businesses competed for customers. Furthermore, the opportunity to buy on credit meant that Americans could have their goods, even without ready cash. The result was a population that had a better standard of living than ever before, even as they went into debt or worked long factory hours to pay for it.
Review Questions
Which of the following did not contribute to the growth of a consumer culture in the United States at the close of the nineteenth century?
- personal credit
- advertising
- greater disposable income
- mail-order catalogs
Hint:
C
Briefly explain Roland Marchand’s argument in the Parable of the Democracy of Goods.
Hint:
Marchand argues that in the new era of consumerism, workers’ desire for access to consumer goods replaces their desire for access to the means of production of those goods. So long as Americans could buy products that advertisers convinced them would make them look and feel wealthy, they did not need to fight for access to the means of wealth.
Critical Thinking Questions
Consider the fact that the light bulb and the telephone were invented only three years apart. Although it took many more years for such devices to find their way into common household use, they eventually wrought major changes in a relatively brief period of time. What effects did these inventions have on the lives of those who used them? Are there contemporary analogies in your lifetime of significant changes due to inventions or technological innovations?
Industrialization, immigration, and urbanization all took place on an unprecedented scale during this era. What were the relationships of these processes to one another? How did each process serve to catalyze and fuel the others?
Describe the various attempts at labor organization in this era, from the Molly Maguires to the Knights of Labor and American Federation of Labor. How were the goals, philosophies, and tactics of these groups similar and different? How did their agendas represent the concerns and grievances of their members and of workers more generally?
Describe the various violent clashes between labor and management that occurred during this era. What do these events reveal about how each group had come to view the other?
How did the new industrial order represent both new opportunities and new limitations for rural and working-class urban Americans?
How did the emergent consumer culture change what it meant to be “American” at the turn of the century?
|
oercommons
|
2025-03-18T00:37:59.016816
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15492/overview",
"title": "U.S. History, Industrialization and the Rise of Big Business, 1870-1900",
"author": null
}
|
https://oercommons.org/courseware/lesson/15456/overview
|
Introduction
Overview
- Political Corruption in Postbellum America
- Key Political Issues: Patronage, Tariffs, and Gold
- Farmers Revolt in the Populist Era
- Social and Labor Unrest in the 1890s
Nine new slave states entered the Union between 1789 and 1860, rapidly expanding and transforming the South into a region of economic growth built on slave labor. In the image above (Figure), innumerable slaves load cargo onto a steamship in the Port of New Orleans, the commercial center of the antebellum South, while two well-dressed white men stand by talking. Commercial activity extends as far as the eye can see.
By the mid-nineteenth century, southern commercial centers like New Orleans had become home to the greatest concentration of wealth in the United States. While most white southerners did not own slaves, they aspired to join the ranks of elite slaveholders, who played a key role in the politics of both the South and the nation. Meanwhile, slavery shaped the culture and society of the South, which rested on a racial ideology of white supremacy and a vision of the United States as a white man’s republic. Slaves endured the traumas of slavery by creating their own culture and using the Christian message of redemption to find hope for a world of freedom without violence.
|
oercommons
|
2025-03-18T00:37:59.032603
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15456/overview",
"title": "U.S. History, Cotton is King: The Antebellum South, 1800–1860",
"author": null
}
|
https://oercommons.org/courseware/lesson/15457/overview
|
The Economics of Cotton
Overview
By the end of this section, you will be able to:
- Explain the labor-intensive processes of cotton production
- Describe the importance of cotton to the Atlantic and American antebellum economy
In the antebellum era—that is, in the years before the Civil War—American planters in the South continued to grow Chesapeake tobacco and Carolina rice as they had in the colonial era. Cotton, however, emerged as the antebellum South’s major commercial crop, eclipsing tobacco, rice, and sugar in economic importance. By 1860, the region was producing two-thirds of the world’s cotton. In 1793, Eli Whitney revolutionized the production of cotton when he invented the cotton gin, a device that separated the seeds from raw cotton. Suddenly, a process that was extraordinarily labor-intensive when done by hand could be completed quickly and easily. American plantation owners, who were searching for a successful staple crop to compete on the world market, found it in cotton.
As a commodity, cotton had the advantage of being easily stored and transported. A demand for it already existed in the industrial textile mills in Great Britain, and in time, a steady stream of slave-grown American cotton would also supply northern textile mills. Southern cotton, picked and processed by American slaves, helped fuel the nineteenth-century Industrial Revolution in both the United States and Great Britain.
KING COTTON
Almost no cotton was grown in the United States in 1787, the year the federal constitution was written. However, following the War of 1812, a huge increase in production resulted in the so-called cotton boom, and by midcentury, cotton became the key cash crop (a crop grown to sell rather than for the farmer’s sole use) of the southern economy and the most important American commodity. By 1850, of the 3.2 million slaves in the country’s fifteen slave states, 1.8 million were producing cotton; by 1860, slave labor was producing over two billion pounds of cotton per year. Indeed, American cotton soon made up two-thirds of the global supply, and production continued to soar. By the time of the Civil War, South Carolina politician James Hammond confidently proclaimed that the North could never threaten the South because “cotton is king.”
The crop grown in the South was a hybrid: Gossypium barbadense, known as Petit Gulf cotton, a mix of Mexican, Georgia, and Siamese strains. Petit Gulf cotton grew extremely well in different soils and climates. It dominated cotton production in the Mississippi River Valley—home of the new slave states of Louisiana, Mississippi, Arkansas, Tennessee, Kentucky, and Missouri—as well as in other states like Texas. Whenever new slave states entered the Union, white slaveholders sent armies of slaves to clear the land in order to grow and pick the lucrative crop. The phrase “to be sold down the river,” used by Harriet Beecher Stowe in her 1852 novel Uncle Tom’s Cabin, refers to this forced migration from the upper southern states to the Deep South, lower on the Mississippi, to grow cotton.
The slaves who built this cotton kingdom with their labor started by clearing the land. Although the Jeffersonian vision of the settlement of new U.S. territories entailed white yeoman farmers single-handedly carving out small independent farms, the reality proved quite different. Entire old-growth forests and cypress swamps fell to the axe as slaves labored to strip the vegetation to make way for cotton. With the land cleared, slaves readied the earth by plowing and planting. To ambitious white planters, the extent of new land available for cotton production seemed almost limitless, and many planters simply leapfrogged from one area to the next, abandoning their fields every ten to fifteen years after the soil became exhausted. Theirs was a world of mobility and restlessness, a constant search for the next area to grow the valuable crop. Slaves composed the vanguard of this American expansion to the West.
Cotton planting took place in March and April, when slaves planted seeds in rows around three to five feet apart. Over the next several months, from April to August, they carefully tended the plants. Weeding the cotton rows took significant energy and time. In August, after the cotton plants had flowered and the flowers had begun to give way to cotton bolls (the seed-bearing capsule that contains the cotton fiber), all the plantation’s slaves—men, women, and children—worked together to pick the crop (Figure). On each day of cotton picking, slaves went to the fields with sacks, which they would fill as many times as they could. The effort was laborious, and a white “driver” employed the lash to make slaves work as quickly as possible.
Cotton planters projected the amount of cotton they could harvest based on the number of slaves under their control. In general, planters expected a good “hand,” or slave, to work ten acres of land and pick two hundred pounds of cotton a day. An overseer or master measured each individual slave’s daily yield. Great pressure existed to meet the expected daily amount, and some masters whipped slaves who picked less than expected.
Cotton picking occurred as many as seven times a season as the plant grew and continued to produce bolls through the fall and early winter. During the picking season, slaves worked from sunrise to sunset with a ten-minute break at lunch; many slaveholders tended to give them little to eat, since spending on food would cut into their profits. Other slaveholders knew that feeding slaves could increase productivity and therefore provided what they thought would help ensure a profitable crop. The slaves’ day didn’t end after they picked the cotton; once they had brought it to the gin house to be weighed, they then had to care for the animals and perform other chores. Indeed, slaves often maintained their own gardens and livestock, which they tended after working the cotton fields, in order to supplement their supply of food.
Sometimes the cotton was dried before it was ginned (put through the process of separating the seeds from the cotton fiber). The cotton gin allowed a slave to remove the seeds from fifty pounds of cotton a day, compared to one pound if done by hand. After the seeds had been removed, the cotton was pressed into bales. These bales, weighing about four hundred to five hundred pounds, were wrapped in burlap cloth and sent down the Mississippi River.
Visit the Internet Archive to watch a 1937 WPA film showing cotton bales being loaded onto a steamboat.
As the cotton industry boomed in the South, the Mississippi River quickly became the essential water highway in the United States. Steamboats, a crucial part of the transportation revolution thanks to their enormous freight-carrying capacity and ability to navigate shallow waterways, became a defining component of the cotton kingdom. Steamboats also illustrated the class and social distinctions of the antebellum age. While the decks carried precious cargo, ornate rooms graced the interior. In these spaces, whites socialized in the ship’s saloons and dining halls while black slaves served them (Figure).
Investors poured huge sums into steamships. In 1817, only seventeen plied the waters of western rivers, but by 1837, there were over seven hundred steamships in operation. Major new ports developed at St. Louis, Missouri; Memphis, Tennessee; and other locations. By 1860, some thirty-five hundred vessels were steaming in and out of New Orleans, carrying an annual cargo made up primarily of cotton that amounted to $220 million worth of goods (approximately $6.5 billion in 2014 dollars).
New Orleans had been part of the French empire before the United States purchased it, along with the rest of the Louisiana Territory, in 1803. In the first half of the nineteenth century, it rose in prominence and importance largely because of the cotton boom, steam-powered river traffic, and its strategic position near the mouth of the Mississippi River. Steamboats moved down the river transporting cotton grown on plantations along the river and throughout the South to the port at New Orleans. From there, the bulk of American cotton went to Liverpool, England, where it was sold to British manufacturers who ran the cotton mills in Manchester and elsewhere. This lucrative international trade brought new wealth and new residents to the city. By 1840, New Orleans alone had 12 percent of the nation’s total banking capital, and visitors often commented on the great cultural diversity of the city. In 1835, Joseph Holt Ingraham wrote: “Truly does New-Orleans represent every other city and nation upon earth. I know of none where is congregated so great a variety of the human species.” Slaves, cotton, and the steamship transformed the city from a relatively isolated corner of North America in the eighteenth century to a thriving metropolis that rivaled New York in importance (Figure).
THE DOMESTIC SLAVE TRADE
The South’s dependence on cotton was matched by its dependence on slaves to harvest the cotton. Despite the rhetoric of the Revolution that “all men are created equal,” slavery not only endured in the American republic but formed the very foundation of the country’s economic success. Cotton and slavery occupied a central—and intertwined—place in the nineteenth-century economy.
In 1807, the U.S. Congress abolished the foreign slave trade, a ban that went into effect on January 1, 1808. After this date, importing slaves from Africa became illegal in the United States. While smuggling continued to occur, the end of the international slave trade meant that domestic slaves were in very high demand. Fortunately for Americans whose wealth depended upon the exploitation of slave labor, a fall in the price of tobacco had caused landowners in the Upper South to reduce their production of this crop and use more of their land to grow wheat, which was far more profitable. While tobacco was a labor-intensive crop that required many people to cultivate it, wheat was not. Former tobacco farmers in the older states of Virginia and Maryland found themselves with “surplus” slaves whom they were obligated to feed, clothe, and shelter. Some slaveholders responded to this situation by freeing slaves; far more decided to sell their excess bondsmen. Virginia and Maryland therefore took the lead in the domestic slave trade, the trading of slaves within the borders of the United States.
The domestic slave trade offered many economic opportunities for white men. Those who sold their slaves could realize great profits, as could the slave brokers who served as middlemen between sellers and buyers. Other white men could benefit from the trade as owners of warehouses and pens in which slaves were held, or as suppliers of clothing and food for slaves on the move. Between 1790 and 1859, slaveholders in Virginia sold more than half a million slaves. In the early part of this period, many of these slaves were sold to people living in Kentucky, Tennessee, and North and South Carolina. By the 1820s, however, people in Kentucky and the Carolinas had begun to sell many of their slaves as well. Maryland slave dealers sold at least 185,000 slaves. Kentucky slaveholders sold some seventy-one thousand individuals. Most of the slave traders carried these slaves further south to Alabama, Louisiana, and Mississippi. New Orleans, the hub of commerce, boasted the largest slave market in the United States and grew to become the nation’s fourth-largest city as a result. Natchez, Mississippi, had the second-largest market. In Virginia, Maryland, the Carolinas, and elsewhere in the South, slave auctions happened every day.
All told, the movement of slaves in the South made up one of the largest forced internal migrations in the United States. In each of the decades between 1820 and 1860, about 200,000 people were sold and relocated. The 1800 census recorded over one million African Americans, of which nearly 900,000 were slaves. By 1860, the total number of African Americans increased to 4.4 million, and of that number, 3.95 million were held in bondage. For many slaves, the domestic slave trade incited the terror of being sold away from family and friends.
Solomon Northup Remembers the New Orleans Slave Market
Solomon Northup was a free black man living in Saratoga, New York, when he was kidnapped and sold into slavery in 1841. He later escaped and wrote a book about his experiences: Twelve Years a Slave. Narrative of Solomon Northup, a Citizen of New-York, Kidnapped in Washington City in 1841 and Rescued in 1853 (the basis of a 2013 Academy Award–winning film). This excerpt derives from Northup’s description of being sold in New Orleans, along with fellow slave Eliza and her children Randall and Emily.
One old gentleman, who said he wanted a coachman, appeared to take a fancy to me. . . .
The same man also purchased Randall. The little fellow was made to jump, and run across the floor, and perform many other feats, exhibiting his activity and condition. All the time the trade was going on, Eliza was crying aloud, and wringing her hands. She besought the man not to buy him, unless he also bought her self and Emily. . . . Freeman turned round to her, savagely, with his whip in his uplifted hand, ordering her to stop her noise, or he would flog her. He would not have such work—such snivelling; and unless she ceased that minute, he would take her to the yard and give her a hundred lashes. . . . Eliza shrunk before him, and tried to wipe away her tears, but it was all in vain. She wanted to be with her children, she said, the little time she had to live. All the frowns and threats of Freeman, could not wholly silence the afflicted mother.
What does Northup’s narrative tell you about the experience of being a slave? How does he characterize Freeman, the slave trader? How does he characterize Eliza?
THE SOUTH IN THE AMERICAN AND WORLD MARKETS
The first half of the nineteenth century saw a market revolution in the United States, one in which industrialization brought changes to both the production and the consumption of goods. Some southerners of the time believed that their region’s reliance on a single cash crop and its use of slaves to produce it gave the South economic independence and made it immune from the effects of these changes, but this was far from the truth. Indeed, the production of cotton brought the South more firmly into the larger American and Atlantic markets. Northern mills depended on the South for supplies of raw cotton that was then converted into textiles. But this domestic cotton market paled in comparison to the Atlantic market. About 75 percent of the cotton produced in the United States was eventually exported abroad. Exporting at such high volumes made the United States the undisputed world leader in cotton production. Between the years 1820 and 1860, approximately 80 percent of the global cotton supply was produced in the United States. Nearly all the exported cotton was shipped to Great Britain, fueling its burgeoning textile industry and making the powerful British Empire increasingly dependent on American cotton and southern slavery.
The power of cotton on the world market may have brought wealth to the South, but it also increased its economic dependence on other countries and other parts of the United States. Much of the corn and pork that slaves consumed came from farms in the West. Some of the inexpensive clothing, called “slops,” and shoes worn by slaves were manufactured in the North. The North also supplied the furnishings found in the homes of both wealthy planters and members of the middle class. Many of the trappings of domestic life, such as carpets, lamps, dinnerware, upholstered furniture, books, and musical instruments—all the accoutrements of comfortable living for southern whites—were made in either the North or Europe. Southern planters also borrowed money from banks in northern cities, and in the southern summers, took advantage of the developments in transportation to travel to resorts at Saratoga, New York; Litchfield, Connecticut; and Newport, Rhode Island.
Section Summary
In the years before the Civil War, the South produced the bulk of the world’s supply of cotton. The Mississippi River Valley slave states became the epicenter of cotton production, an area of frantic economic activity where the landscape changed dramatically as land was transformed from pinewoods and swamps into cotton fields. Cotton’s profitability relied on the institution of slavery, which generated the product that fueled cotton mill profits in the North. When the international slave trade was outlawed in 1808, the domestic slave trade exploded, providing economic opportunities for whites involved in many aspects of the trade and increasing the possibility of slaves’ dislocation and separation from kin and friends. Although the larger American and Atlantic markets relied on southern cotton in this era, the South depended on these other markets for food, manufactured goods, and loans. Thus, the market revolution transformed the South just as it had other regions.
Review Questions
Which of the following was not one of the effects of the cotton boom?
- U.S. trade increased with France and Spain.
- Northern manufacturing expanded.
- The need for slave labor grew.
- Port cities like New Orleans expanded.
Hint:
A
The abolition of the foreign slave trade in 1807 led to _______.
- a dramatic decrease in the price and demand for slaves
- the rise of a thriving domestic slave trade
- a reform movement calling for the complete end to slavery in the United States
- the decline of cotton production
Hint:
B
Why did some southerners believe their region was immune to the effects of the market revolution? Why was this thinking misguided?
Hint:
Some southerners believed that their region’s monopoly over the lucrative cotton crop—on which both the larger American and Atlantic markets depended—and their possession of a slave labor force allowed the South to remain independent from the market revolution. However, the very cotton that provided the South with such economic potency also increased its reliance on the larger U.S. and world markets, which supplied—among other things—the food and clothes slaves needed, the furniture and other manufactured goods that defined the southern standard of comfortable living, and the banks from which southerners borrowed needed funds.
|
oercommons
|
2025-03-18T00:37:59.060836
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15457/overview",
"title": "U.S. History, Cotton is King: The Antebellum South, 1800–1860",
"author": null
}
|
https://oercommons.org/courseware/lesson/15458/overview
|
African Americans in the Antebellum United States
Overview
By the end of this section, you will be able to:
- Discuss the similarities and differences in the lives of slaves and free blacks
- Describe the independent culture and customs that slaves developed
In addition to cotton, the great commodity of the antebellum South was human chattel. Slavery was the cornerstone of the southern economy. By 1850, about 3.2 million slaves labored in the United States, 1.8 million of whom worked in the cotton fields. Slaves faced arbitrary power abuses from whites; they coped by creating family and community networks. Storytelling, song, and Christianity also provided solace and allowed slaves to develop their own interpretations of their condition.
LIFE AS A SLAVE
Southern whites frequently relied upon the idea of paternalism—the premise that white slaveholders acted in the best interests of slaves, taking responsibility for their care, feeding, discipline, and even their Christian morality—to justify the existence of slavery. This grossly misrepresented the reality of slavery, which was, by any measure, a dehumanizing, traumatizing, and horrifying human disaster and crime against humanity. Nevertheless, slaves were hardly passive victims of their conditions; they sought and found myriad ways to resist their shackles and develop their own communities and cultures.
Slaves often used the notion of paternalism to their advantage, finding opportunities within this system to engage in acts of resistance and win a degree of freedom and autonomy. For example, some slaves played into their masters’ racism by hiding their intelligence and feigning childishness and ignorance. The slaves could then slow down the workday and sabotage the system in small ways by “accidentally” breaking tools, for example; the master, seeing his slaves as unsophisticated and childlike, would believe these incidents were accidents rather than rebellions. Some slaves engaged in more dramatic forms of resistance, such as poisoning their masters slowly. Other slaves reported rebellious slaves to their masters, hoping to gain preferential treatment. Slaves who informed their masters about planned slave rebellions could often expect the slaveholder’s gratitude and, perhaps, more lenient treatment. Such expectations were always tempered by the individual personality and caprice of the master.
Slaveholders used both psychological coercion and physical violence to prevent slaves from disobeying their wishes. Often, the most efficient way to discipline slaves was to threaten to sell them. The lash, while the most common form of punishment, was effective but not efficient; whippings sometimes left slaves incapacitated or even dead. Slave masters also used punishment gear like neck braces, balls and chains, leg irons, and paddles with holes to produce blood blisters. Slaves lived in constant terror of both physical violence and separation from family and friends (Figure).
Under southern law, slaves could not marry. Nonetheless, some slaveholders allowed marriages to promote the birth of children and to foster harmony on plantations. Some masters even forced certain slaves to form unions, anticipating the birth of more children (and consequently greater profits) from them. Masters sometimes allowed slaves to choose their own partners, but they could also veto a match. Slave couples always faced the prospect of being sold away from each other, and, once they had children, the horrifying reality that their children could be sold and sent away at any time.
Browse a collection of first-hand narratives of slaves and former slaves at the National Humanities Center to learn more about the experience of slavery.
Slave parents had to show their children the best way to survive under slavery. This meant teaching them to be discreet, submissive, and guarded around whites. Parents also taught their children through the stories they told. Popular stories among slaves included tales of tricksters, sly slaves, or animals like Brer Rabbit, who outwitted their antagonists (Figure). Such stories provided comfort in humor and conveyed the slaves’ sense of the wrongs of slavery. Slaves’ work songs commented on the harshness of their life and often had double meanings—a literal meaning that whites would not find offensive and a deeper meaning for slaves.
African beliefs, including ideas about the spiritual world and the importance of African healers, survived in the South as well. Whites who became aware of non-Christian rituals among slaves labeled such practices as witchcraft. Among Africans, however, the rituals and use of various plants by respected slave healers created connections between the African past and the American South while also providing a sense of community and identity for slaves. Other African customs, including traditional naming patterns, the making of baskets, and the cultivation of certain native African plants that had been brought to the New World, also endured.
African Americans and Christian Spirituals
Many slaves embraced Christianity. Their masters emphasized a scriptural message of obedience to whites and a better day awaiting slaves in heaven, but slaves focused on the uplifting message of being freed from bondage.
The styles of worship in the Methodist and Baptist churches, which emphasized emotional responses to scripture, attracted slaves to those traditions and inspired some to become preachers. Spiritual songs that referenced the Exodus (the biblical account of the Hebrews’ escape from slavery in Egypt), such as “Roll, Jordan, Roll,” allowed slaves to freely express messages of hope, struggle, and overcoming adversity (Figure).
What imagery might the Jordan River suggest to slaves working in the Deep South? What lyrics in this song suggest redemption and a better world ahead?
Listen to a rendition of “Roll, Jordan, Roll” from the movie based on Solomon Northup’s memoir and life.
THE FREE BLACK POPULATION
Complicating the picture of the antebellum South was the existence of a large free black population. In fact, more free blacks lived in the South than in the North; roughly 261,000 lived in slave states, while 226,000 lived in northern states without slavery. Most free blacks did not live in the Lower, or Deep South: the states of Alabama, Arkansas, Florida, Georgia, Louisiana, Mississippi, South Carolina, and Texas. Instead, the largest number lived in the upper southern states of Delaware, Maryland, Virginia, North Carolina, and later Kentucky, Missouri, Tennessee, and the District of Columbia.
Part of the reason for the large number of free blacks living in slave states were the many instances of manumission—the formal granting of freedom to slaves—that occurred as a result of the Revolution, when many slaveholders put into action the ideal that “all men are created equal” and freed their slaves. The transition in the Upper South to the staple crop of wheat, which did not require large numbers of slaves to produce, also spurred manumissions. Another large group of free blacks in the South had been free residents of Louisiana before the 1803 Louisiana Purchase, while still other free blacks came from Cuba and Haiti.
Most free blacks in the South lived in cities, and a majority of free blacks were lighter-skinned women, a reflection of the interracial unions that formed between white men and black women. Everywhere in the United States blackness had come to be associated with slavery, the station at the bottom of the social ladder. Both whites and those with African ancestry tended to delineate varying degrees of lightness in skin color in a social hierarchy. In the slaveholding South, different names described one’s distance from blackness or whiteness: mulattos (those with one black and one white parent), quadroons (those with one black grandparent), and octoroons (those with one black great-grandparent) (Figure). Lighter-skinned blacks often looked down on their darker counterparts, an indication of the ways in which both whites and blacks internalized the racism of the age.
Some free blacks in the South owned slaves of their own. Andrew Durnford, for example, was born in New Orleans in 1800, three years before the Louisiana Purchase. His father was white, and his mother was a free black. Durnford became an American citizen after the Louisiana Purchase, rising to prominence as a Louisiana sugar planter and slaveholder. William Ellison, another free black who amassed great wealth and power in the South, was born a slave in 1790 in South Carolina. After buying his freedom and that of his wife and daughter, he proceeded to purchase his own slaves, whom he then put to work manufacturing cotton gins. By the eve of the Civil War, Ellison had become one of the richest and largest slaveholders in the entire state.
The phenomenon of free blacks amassing large fortunes within a slave society predicated on racial difference, however, was exceedingly rare. Most free blacks in the South lived under the specter of slavery and faced many obstacles. Beginning in the early nineteenth century, southern states increasingly made manumission of slaves illegal. They also devised laws that divested free blacks of their rights, such as the right to testify against whites in court or the right to seek employment where they pleased. Interestingly, it was in the upper southern states that such laws were the harshest. In Virginia, for example, legislators made efforts to require free blacks to leave the state. In parts of the Deep South, free blacks were able to maintain their rights more easily. The difference in treatment between free blacks in the Deep South and those in the Upper South, historians have surmised, came down to economics. In the Deep South, slavery as an institution was strong and profitable. In the Upper South, the opposite was true. The anxiety of this economic uncertainty manifested in the form of harsh laws that targeted free blacks.
SLAVE REVOLTS
Slaves resisted their enslavement in small ways every day, but this resistance did not usually translate into mass uprisings. Slaves understood that the chances of ending slavery through rebellion were slim and would likely result in massive retaliation; many also feared the risk that participating in such actions would pose to themselves and their families. White slaveholders, however, constantly feared uprisings and took drastic steps, including torture and mutilation, whenever they believed that rebellions might be simmering. Gripped by the fear of insurrection, whites often imagined revolts to be in the works even when no uprising actually happened.
At least two major slave uprisings did occur in the antebellum South. In 1811, a major rebellion broke out in the sugar parishes of the booming territory of Louisiana. Inspired by the successful overthrow of the white planter class in Haiti, Louisiana slaves took up arms against planters. Perhaps as many five hundred slaves joined the rebellion, led by Charles Deslondes, a mixed-race slave driver on a sugar plantation owned by Manuel Andry.
The revolt began in January 1811 on Andry’s plantation. Deslondes and other slaves attacked the Andry household, where they killed the slave master’s son (although Andry himself escaped). The rebels then began traveling toward New Orleans, armed with weapons gathered at Andry’s plantation. Whites mobilized to stop the rebellion, but not before Deslondes and the other rebelling slaves set fire to three plantations and killed numerous whites. A small white force led by Andry ultimately captured Deslondes, whose body was mutilated and burned following his execution. Other slave rebels were beheaded, and their heads placed on pikes along the Mississippi River.
The second rebellion, led by the slave Nat Turner, occurred in 1831 in Southampton County, Virginia. Turner had suffered not only from personal enslavement, but also from the additional trauma of having his wife sold away from him. Bolstered by Christianity, Turner became convinced that like Christ, he should lay down his life to end slavery. Mustering his relatives and friends, he began the rebellion August 22, killing scores of whites in the county. Whites mobilized quickly and within forty-eight hours had brought the rebellion to an end. Shocked by Nat Turner’s Rebellion, Virginia’s state legislature considered ending slavery in the state in order to provide greater security. In the end, legislators decided slavery would remain and that their state would continue to play a key role in the domestic slave trade.
SLAVE MARKETS
As discussed above, after centuries of slave trade with West Africa, Congress banned the further importation of slaves beginning in 1808. The domestic slave trade then expanded rapidly. As the cotton trade grew in size and importance, so did the domestic slave trade; the cultivation of cotton gave new life and importance to slavery, increasing the value of slaves. To meet the South’s fierce demand for labor, American smugglers illegally transferred slaves through Florida and later through Texas. Many more slaves arrived illegally from Cuba; indeed, Cubans relied on the smuggling of slaves to prop up their finances. The largest number of slaves after 1808, however, came from the massive, legal internal slave market in which slave states in the Upper South sold enslaved men, women, and children to states in the Lower South. For slaves, the domestic trade presented the full horrors of slavery as children were ripped from their mothers and fathers and families destroyed, creating heartbreak and alienation.
Some slaveholders sought to increase the number of slave children by placing male slaves with fertile female slaves, and slave masters routinely raped their female slaves. The resulting births played an important role in slavery’s expansion in the first half of the nineteenth century, as many slave children were born as a result of rape. One account written by a slave named William J. Anderson captures the horror of sexual exploitation in the antebellum South. Anderson wrote about how a Mississippi slaveholder
divested a poor female slave of all wearing apparel, tied her down to stakes, and whipped her with a handsaw until he broke it over her naked body. In process of time he ravished [raped] her person, and became the father of a child by her. Besides, he always kept a colored Miss in the house with him. This is another curse of Slavery—concubinage and illegitimate connections—which is carried on to an alarming extent in the far South. A poor slave man who lives close by his wife, is permitted to visit her but very seldom, and other men, both white and colored, cohabit with her. It is undoubtedly the worst place of incest and bigamy in the world. A white man thinks nothing of putting a colored man out to carry the fore row [front row in field work], and carry on the same sport with the colored man’s wife at the same time.
Anderson, a devout Christian, recognized and explains in his narrative that one of the evils of slavery is the way it undermines the family. Anderson was not the only critic of slavery to emphasize this point. Frederick Douglass, a Maryland slave who escaped to the North in 1838, elaborated on this dimension of slavery in his 1845 narrative. He recounted how slave masters had to sell their own children whom they had with slave women to appease the white wives who despised their offspring.
The selling of slaves was a major business enterprise in the antebellum South, representing a key part of the economy. White men invested substantial sums in slaves, carefully calculating the annual returns they could expect from a slave as well as the possibility of greater profits through natural increase. The domestic slave trade was highly visible, and like the infamous Middle Passage that brought captive Africans to the Americas, it constituted an equally disruptive and horrifying journey now called the second middle passage. Between 1820 and 1860, white American traders sold a million or more slaves in the domestic slave market. Groups of slaves were transported by ship from places like Virginia, a state that specialized in raising slaves for sale, to New Orleans, where they were sold to planters in the Mississippi Valley. Other slaves made the overland trek from older states like North Carolina to new and booming Deep South states like Alabama.
New Orleans had the largest slave market in the United States (Figure). Slaveholders brought their slaves there from the East (Virginia, Maryland, and the Carolinas) and the West (Tennessee and Kentucky) to be sold for work in the Mississippi Valley. The slave trade benefited whites in the Chesapeake and Carolinas, providing them with extra income: A healthy young male slave in the 1850s could be sold for $1,000 (approximately $30,000 in 2014 dollars), and a planter who could sell ten such slaves collected a windfall.
In fact, by the 1850s, the demand for slaves reached an all-time high, and prices therefore doubled. A slave who would have sold for $400 in the 1820s could command a price of $800 in the 1850s. The high price of slaves in the 1850s and the inability of natural increase to satisfy demands led some southerners to demand the reopening of the international slave trade, a movement that caused a rift between the Upper South and the Lower South. Whites in the Upper South who sold slaves to their counterparts in the Lower South worried that reopening the trade would lower prices and therefore hurt their profits.
John Brown on Slave Life in Georgia
A slave named John Brown lived in Virginia, North Carolina, and Georgia before he escaped and moved to England. While there, he dictated his autobiography to someone at the British and Foreign Anti-Slavery Society, who published it in 1855.
I really thought my mother would have died of grief at being obliged to leave her two children, her mother, and her relations behind. But it was of no use lamenting, the few things we had were put together that night, and we completed our preparations for being parted for life by kissing one another over and over again, and saying good bye till some of us little ones fell asleep. . . . And here I may as well tell what kind of man our new master was. He was of small stature, and thin, but very strong. He had sandy hair, a very red face, and chewed tobacco. His countenance had a very cruel expression, and his disposition was a match for it. He was, indeed, a very bad man, and used to flog us dreadfully. He would make his slaves work on one meal a day, until quite night, and after supper, set them to burn brush or spin cotton. We worked from four in the morning till twelve before we broke our fast, and from that time till eleven or twelve at night . . . we labored eighteen hours a day.
—John Brown, Slave Life in Georgia: A Narrative of the Life, Sufferings, and Escape of John Brown, A Fugitive Slave, Now in England, 1855
What features of the domestic slave trade does Brown’s narrative illuminate? Why do you think he brought his story to an antislavery society? How do you think people responded to this narrative?
Read through several narratives at “Born in Slavery,” part of the American Memory collection at the Library of Congress. Do these narratives have anything in common? What differences can you find between them?
Section Summary
Slave labor in the antebellum South generated great wealth for plantation owners. Slaves, in contrast, endured daily traumas as the human property of masters. Slaves resisted their condition in a variety of ways, and many found some solace in Christianity and the communities they created in the slave quarters. While some free blacks achieved economic prosperity and even became slaveholders themselves, the vast majority found themselves restricted by the same white-supremacist assumptions upon which the institution of slavery was based.
Review Questions
Under the law in the antebellum South, slaves were ________.
- servants
- animals
- property
- indentures
Hint:
C
How did both slaveholders and slaves use the concept of paternalism to their advantage?
Hint:
Southern whites often used paternalism to justify the institution of slavery, arguing that slaves, like children, needed the care, feeding, discipline, and moral and religious education that they could provide. Slaves often used this misguided notion to their advantage: By feigning ignorance and playing into slaveholders’ paternalistic perceptions of them, slaves found opportunities to resist their condition and gain a degree of freedom and autonomy.
|
oercommons
|
2025-03-18T00:37:59.091718
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15458/overview",
"title": "U.S. History, Cotton is King: The Antebellum South, 1800–1860",
"author": null
}
|
https://oercommons.org/courseware/lesson/15459/overview
|
Wealth and Culture in the South
Overview
By the end of this section, you will be able to:
- Assess the distribution of wealth in the antebellum South
- Describe the southern culture of honor
- Identify the main proslavery arguments in the years prior to the Civil War
During the antebellum years, wealthy southern planters formed an elite master class that wielded most of the economic and political power of the region. They created their own standards of gentility and honor, defining ideals of southern white manhood and womanhood and shaping the culture of the South. To defend the system of forced labor on which their economic survival and genteel lifestyles depended, elite southerners developed several proslavery arguments that they levied at those who would see the institution dismantled.
SLAVERY AND THE WHITE CLASS STRUCTURE
The South prospered, but its wealth was very unequally distributed. Upward social mobility did not exist for the millions of slaves who produced a good portion of the nation’s wealth, while poor southern whites envisioned a day when they might rise enough in the world to own slaves of their own. Because of the cotton boom, there were more millionaires per capita in the Mississippi River Valley by 1860 than anywhere else in the United States. However, in that same year, only 3 percent of whites owned more than fifty slaves, and two-thirds of white households in the South did not own any slaves at all (Figure). Distribution of wealth in the South became less democratic over time; fewer whites owned slaves in 1860 than in 1840.
At the top of southern white society stood the planter elite, which comprised two groups. In the Upper South, an aristocratic gentry, generation upon generation of whom had grown up with slavery, held a privileged place. In the Deep South, an elite group of slaveholders gained new wealth from cotton. Some members of this group hailed from established families in the eastern states (Virginia and the Carolinas), while others came from humbler backgrounds. South Carolinian Nathaniel Heyward, a wealthy rice planter and member of the aristocratic gentry, came from an established family and sat atop the pyramid of southern slaveholders. He amassed an enormous estate; in 1850, he owned more than eighteen hundred slaves. When he died in 1851, he left an estate worth more than $2 million (approximately $63 million in 2014 dollars).
As cotton production increased, new wealth flowed to the cotton planters. These planters became the staunchest defenders of slavery, and as their wealth grew, they gained considerable political power.
One member of the planter elite was Edward Lloyd V, who came from an established and wealthy family of Talbot County, Maryland. Lloyd had inherited his position rather than rising to it through his own labors. His hundreds of slaves formed a crucial part of his wealth. Like many of the planter elite, Lloyd’s plantation was a masterpiece of elegant architecture and gardens (Figure).
One of the slaves on Lloyd’s plantation was Frederick Douglass, who escaped in 1838 and became an abolitionist leader, writer, statesman, and orator in the North. In his autobiography, Douglass described the plantation’s elaborate gardens and racehorses, but also its underfed and brutalized slave population. Lloyd provided employment opportunities to other whites in Talbot County, many of whom served as slave traders and the “slave breakers” entrusted with beating and overworking unruly slaves into submission. Like other members of the planter elite, Lloyd himself served in a variety of local and national political offices. He was governor of Maryland from 1809 to 1811, a member of the House of Representatives from 1807 to 1809, and a senator from 1819 to 1826. As a representative and a senator, Lloyd defended slavery as the foundation of the American economy.
Wealthy plantation owners like Lloyd came close to forming an American ruling class in the years before the Civil War. They helped shape foreign and domestic policy with one goal in view: to expand the power and reach of the cotton kingdom of the South. Socially, they cultivated a refined manner and believed whites, especially members of their class, should not perform manual labor. Rather, they created an identity for themselves based on a world of leisure in which horse racing and entertainment mattered greatly, and where the enslavement of others was the bedrock of civilization.
Below the wealthy planters were the yeoman farmers, or small landowners (Figure). Below yeomen were poor, landless whites, who made up the majority of whites in the South. These landless white men dreamed of owning land and slaves and served as slave overseers, drivers, and traders in the southern economy. In fact, owning land and slaves provided one of the only opportunities for upward social and economic mobility. In the South, living the American dream meant possessing slaves, producing cotton, and owning land.
Despite this unequal distribution of wealth, non-slaveholding whites shared with white planters a common set of values, most notably a belief in white supremacy. Whites, whether rich or poor, were bound together by racism. Slavery defused class tensions among them, because no matter how poor they were, white southerners had race in common with the mighty plantation owners. Non-slaveholders accepted the rule of the planters as defenders of their shared interest in maintaining a racial hierarchy. Significantly, all whites were also bound together by the constant, prevailing fear of slave uprisings.
D. R. Hundley on the Southern Yeoman
D. R. Hundley was a well-educated planter, lawyer, and banker from Alabama. Something of an amateur sociologist, he argued against the common northern assumption that the South was made up exclusively of two tiers of white residents: the very wealthy planter class and the very poor landless whites. In his 1860 book, Social Relations in Our Southern States, Hundley describes what he calls the “Southern Yeomen,” a social group he insists is roughly equivalent to the middle-class farmers of the North.
But you have no Yeomen in the South, my dear Sir? Beg your pardon, our dear Sir, but we have—hosts of them. I thought you had only poor White Trash? Yes, we dare say as much—and that the moon is made of green cheese! . . . Know, then, that the Poor Whites of the South constitute a separate class to themselves; the Southern Yeomen are as distinct from them as the Southern Gentleman is from the Cotton Snob. Certainly the Southern Yeomen are nearly always poor, at least so far as this world’s goods are to be taken into account. As a general thing they own no slaves; and even in case they do, the wealthiest of them rarely possess more than from ten to fifteen. . . . The Southern Yeoman much resembles in his speech, religious opinions, household arrangements, indoor sports, and family traditions, the middle class farmers of the Northern States. He is fully as intelligent as the latter, and is on the whole much better versed in the lore of politics and the provisions of our Federal and State Constitutions. . . . [A]lthough not as a class pecuniarily interested in slave property, the Southern Yeomanry are almost unanimously pro-slavery in sentiment. Nor do we see how any honest, thoughtful person can reasonably find fault with them on this account.
—D. R. Hundley, Social Relations in Our Southern States, 1860
What elements of social relations in the South is Hundley attempting to emphasize for his readers? In what respects might his position as an educated and wealthy planter influence his understanding of social relations in the South?
Because race bound all whites together as members of the master race, non-slaveholding whites took part in civil duties. They served on juries and voted. They also engaged in the daily rounds of maintaining slavery by serving on neighborhood patrols to ensure that slaves did not escape and that rebellions did not occur. The practical consequence of such activities was that the institution of slavery, and its perpetuation, became a source of commonality among different economic and social tiers that otherwise were separated by a gulf of difference.
Southern planters exerted a powerful influence on the federal government. Seven of the first eleven presidents owned slaves, and more than half of the Supreme Court justices who served on the court from its inception to the Civil War came from slaveholding states. However, southern white yeoman farmers generally did not support an active federal government. They were suspicious of the state bank and supported President Jackson’s dismantling of the Second Bank of the United States. They also did not support taxes to create internal improvements such as canals and railroads; to them, government involvement in the economic life of the nation disrupted what they perceived as the natural workings of the economy. They also feared a strong national government might tamper with slavery.
Planters operated within a larger capitalist society, but the labor system they used to produce goods—that is, slavery—was similar to systems that existed before capitalism, such as feudalism and serfdom. Under capitalism, free workers are paid for their labor (by owners of capital) to produce commodities; the money from the sale of the goods is used to pay for the work performed. As slaves did not reap any earnings from their forced labor, some economic historians consider the antebellum plantation system a “pre-capitalist” system.
HONOR IN THE SOUTH
A complicated code of honor among privileged white southerners, dictating the beliefs and behavior of “gentlemen” and “ladies,” developed in the antebellum years. Maintaining appearances and reputation was supremely important. It can be argued that, as in many societies, the concept of honor in the antebellum South had much to do with control over dependents, whether slaves, wives, or relatives. Defending their honor and ensuring that they received proper respect became preoccupations of whites in the slaveholding South. To question another man’s assertions was to call his honor and reputation into question. Insults in the form of words or behavior, such as calling someone a coward, could trigger a rupture that might well end on the dueling ground (Figure). Dueling had largely disappeared in the antebellum North by the early nineteenth century, but it remained an important part of the southern code of honor through the Civil War years. Southern white men, especially those of high social status, settled their differences with duels, before which antagonists usually attempted reconciliation, often through the exchange of letters addressing the alleged insult. If the challenger was not satisfied by the exchange, a duel would often result.
The dispute between South Carolina’s James Hammond and his erstwhile friend (and brother-in-law) Wade Hampton II illustrates the southern culture of honor and the place of the duel in that culture. A strong friendship bound Hammond and Hampton together. Both stood at the top of South Carolina’s society as successful, married plantation owners involved in state politics. Prior to his election as governor of the state in 1842, Hammond became sexually involved with each of Hampton’s four teenage daughters, who were his nieces by marriage. “[A]ll of them rushing on every occasion into my arms,” Hammond confided in his private diary, “covering me with kisses, lolling on my lap, pressing their bodies almost into mine . . . and permitting my hands to stray unchecked.” Hampton found out about these dalliances, and in keeping with the code of honor, could have demanded a duel with Hammond. However, Hampton instead tried to use the liaisons to destroy his former friend politically. This effort proved disastrous for Hampton, because it represented a violation of the southern code of honor. “As matters now stand,” Hammond wrote, “he [Hampton] is a convicted dastard who, not having nerve to redress his own wrongs, put forward bullies to do it for him. . . . To challenge me [to a duel] would be to throw himself upon my mercy for he knows I am not bound to meet him [for a duel].” Because Hampton’s behavior marked him as a man who lacked honor, Hammond was no longer bound to meet Hampton in a duel even if Hampton were to demand one. Hammond’s reputation, though tarnished, remained high in the esteem of South Carolinians, and the governor went on to serve as a U.S. senator from 1857 to 1860. As for the four Hampton daughters, they never married; their names were disgraced, not only by the whispered-about scandal but by their father’s actions in response to it; and no man of honor in South Carolina would stoop so low as to marry them.
GENDER AND THE SOUTHERN HOUSEHOLD
The antebellum South was an especially male-dominated society. Far more than in the North, southern men, particularly wealthy planters, were patriarchs and sovereigns of their own household. Among the white members of the household, labor and daily ritual conformed to rigid gender delineations. Men represented their household in the larger world of politics, business, and war. Within the family, the patriarchal male was the ultimate authority. White women were relegated to the household and lived under the thumb and protection of the male patriarch. The ideal southern lady conformed to her prescribed gender role, a role that was largely domestic and subservient. While responsibilities and experiences varied across different social tiers, women’s subordinate state in relation to the male patriarch remained the same.
Writers in the antebellum period were fond of celebrating the image of the ideal southern woman (Figure). One such writer, Thomas Roderick Dew, president of Virginia’s College of William and Mary in the mid-nineteenth century, wrote approvingly of the virtue of southern women, a virtue he concluded derived from their natural weakness, piety, grace, and modesty. In his Dissertation on the Characteristic Differences Between the Sexes, he writes that southern women derive their power not by
leading armies to combat, or of enabling her to bring into more formidable action the physical power which nature has conferred on her. No! It is but the better to perfect all those feminine graces, all those fascinating attributes, which render her the center of attraction, and which delight and charm all those who breathe the atmosphere in which she moves; and, in the language of Mr. Burke, would make ten thousand swords leap from their scabbards to avenge the insult that might be offered to her. By her very meekness and beauty does she subdue all around her.
Such popular idealizations of elite southern white women, however, are difficult to reconcile with their lived experience: in their own words, these women frequently described the trauma of childbirth, the loss of children, and the loneliness of the plantation.
Louisa Cheves McCord’s “Woman’s Progress”
Louisa Cheves McCord was born in Charleston, South Carolina, in 1810. A child of some privilege in the South, she received an excellent education and became a prolific writer. As the excerpt from her poem “Woman’s Progress” indicates, some southern women also contributed to the idealization of southern white womanhood.
Sweet Sister! stoop not thou to be a man!
Man has his place as woman hers; and she
As made to comfort, minister and help;
Moulded for gentler duties, ill fulfils
His jarring destinies. Her mission is
To labour and to pray; to help, to heal,
To soothe, to bear; patient, with smiles, to suffer;
And with self-abnegation noble lose
Her private interest in the dearer weal
Of those she loves and lives for. Call not this—
(The all-fulfilling of her destiny;
She the world’s soothing mother)—call it not,
With scorn and mocking sneer, a drudgery.
The ribald tongue profanes Heaven’s holiest things,
But holy still they are. The lowliest tasks
Are sanctified in nobly acting them.
Christ washed the apostles’ feet, not thus cast shame
Upon the God-like in him. Woman lives
Man’s constant prophet. If her life be true
And based upon the instincts of her being,
She is a living sermon of that truth
Which ever through her gentle actions speaks,
That life is given to labour and to love.
—Louisa Susanna Cheves McCord, “Woman’s Progress,” 1853
What womanly virtues does Louisa Cheves McCord emphasize? How might her social status, as an educated southern woman of great privilege, influence her understanding of gender relations in the South?
For slaveholding whites, the male-dominated household operated to protect gendered divisions and prevalent gender norms; for slave women, however, the same system exposed them to brutality and frequent sexual domination. The demands on the labor of slave women made it impossible for them to perform the role of domestic caretaker that was so idealized by southern men. That slaveholders put them out into the fields, where they frequently performed work traditionally thought of as male, reflected little the ideal image of gentleness and delicacy reserved for white women. Nor did the slave woman’s role as daughter, wife, or mother garner any patriarchal protection. Each of these roles and the relationships they defined was subject to the prerogative of a master, who could freely violate enslaved women’s persons, sell off their children, or separate them from their families.
DEFENDING SLAVERY
With the rise of democracy during the Jacksonian era in the 1830s, slaveholders worried about the power of the majority. If political power went to a majority that was hostile to slavery, the South—and the honor of white southerners—would be imperiled. White southerners keen on preserving the institution of slavery bristled at what they perceived to be northern attempts to deprive them of their livelihood. Powerful southerners like South Carolinian John C. Calhoun (Figure) highlighted laws like the Tariff of 1828 as evidence of the North’s desire to destroy the southern economy and, by extension, its culture. Such a tariff, he and others concluded, would disproportionately harm the South, which relied heavily on imports, and benefit the North, which would receive protections for its manufacturing centers. The tariff appeared to open the door for other federal initiatives, including the abolition of slavery. Because of this perceived threat to southern society, Calhoun argued that states could nullify federal laws. This belief illustrated the importance of the states’ rights argument to the southern states. It also showed slaveholders’ willingness to unite against the federal government when they believed it acted unjustly against their interests.
As the nation expanded in the 1830s and 1840s, the writings of abolitionists—a small but vocal group of northerners committed to ending slavery—reached a larger national audience. White southerners responded by putting forth arguments in defense of slavery, their way of life, and their honor. Calhoun became a leading political theorist defending slavery and the rights of the South, which he saw as containing an increasingly embattled minority. He advanced the idea of a concurrent majority, a majority of a separate region (that would otherwise be in the minority of the nation) with the power to veto or disallow legislation put forward by a hostile majority.
Calhoun’s idea of the concurrent majority found full expression in his 1850 essay “Disquisition on Government.” In this treatise, he wrote about government as a necessary means to ensure the preservation of society, since society existed to “preserve and protect our race.” If government grew hostile to society, then a concurrent majority had to take action, including forming a new government. “Disquisition on Government” advanced a profoundly anti-democratic argument. It illustrates southern leaders’ intense suspicion of democratic majorities and their ability to effect legislation that would challenge southern interests.
Go to the Internet Archive to read John C. Calhoun’s “Disquisition on Government.” Why do you think he proposed the creation of a concurrent majority?
White southerners reacted strongly to abolitionists’ attacks on slavery. In making their defense of slavery, they critiqued wage labor in the North. They argued that the Industrial Revolution had brought about a new type of slavery—wage slavery—and that this form of “slavery” was far worse than the slave labor used on southern plantations. Defenders of the institution also lashed out directly at abolitionists such as William Lloyd Garrison for daring to call into question their way of life. Indeed, Virginians cited Garrison as the instigator of Nat Turner’s 1831 rebellion.
The Virginian George Fitzhugh contributed to the defense of slavery with his book Sociology for the South, or the Failure of Free Society (1854). Fitzhugh argued that laissez-faire capitalism, as celebrated by Adam Smith, benefited only the quick-witted and intelligent, leaving the ignorant at a huge disadvantage. Slaveholders, he argued, took care of the ignorant—in Fitzhugh’s argument, the slaves of the South. Southerners provided slaves with care from birth to death, he asserted; this offered a stark contrast to the wage slavery of the North, where workers were at the mercy of economic forces beyond their control. Fitzhugh’s ideas exemplified southern notions of paternalism.
George Fitzhugh’s Defense of Slavery
George Fitzhugh, a southern writer of social treatises, was a staunch supporter of slavery, not as a necessary evil but as what he argued was a necessary good, a way to take care of slaves and keep them from being a burden on society. He published Sociology for the South, or the Failure of Free Society in 1854, in which he laid out what he believed to be the benefits of slavery to both the slaves and society as a whole. According to Fitzhugh:
[I]t is clear the Athenian democracy would not suit a negro nation, nor will the government of mere law suffice for the individual negro. He is but a grown up child and must be governed as a child . . . The master occupies towards him the place of parent or guardian. . . . The negro is improvident; will not lay up in summer for the wants of winter; will not accumulate in youth for the exigencies of age. He would become an insufferable burden to society. Society has the right to prevent this, and can only do so by subjecting him to domestic slavery.
In the last place, the negro race is inferior to the white race, and living in their midst, they would be far outstripped or outwitted in the chase of free competition. . . . Our negroes are not only better off as to physical comfort than free laborers, but their moral condition is better.
What arguments does Fitzhugh use to promote slavery? What basic premise underlies his ideas? Can you think of a modern parallel to Fitzhugh’s argument?
The North also produced defenders of slavery, including Louis Agassiz, a Harvard professor of zoology and geology. Agassiz helped to popularize polygenism, the idea that different human races came from separate origins. According to this formulation, no single human family origin existed, and blacks made up a race wholly separate from the white race. Agassiz’s notion gained widespread popularity in the 1850s with the 1854 publication of George Gliddon and Josiah Nott’s Types of Mankind and other books. The theory of polygenism codified racism, giving the notion of black inferiority the lofty mantle of science. One popular advocate of the idea posited that blacks occupied a place in evolution between the Greeks and chimpanzees (Figure).
Section Summary
Although a small white elite owned the vast majority of slaves in the South, and most other whites could only aspire to slaveholders’ wealth and status, slavery shaped the social life of all white southerners in profound ways. Southern culture valued a behavioral code in which men’s honor, based on the domination of others and the protection of southern white womanhood, stood as the highest good. Slavery also decreased class tensions, binding whites together on the basis of race despite their inequalities of wealth. Several defenses of slavery were prevalent in the antebellum era, including Calhoun’s argument that the South’s “concurrent majority” could overrule federal legislation deemed hostile to southern interests; the notion that slaveholders’ care of their chattel made slaves better off than wage workers in the North; and the profoundly racist ideas underlying polygenism.
Review Questions
The largest group of whites in the South _______.
- owned no slaves
- owned between one and nine slaves each
- owned between ten and ninety-nine slaves each
- owned over one hundred slaves each
Hint:
A
John C. Calhoun argued for greater rights for southerners with which idea?
- polygenism
- nullification
- concurrent majority
- paternalism
Hint:
C
How did defenders of slavery use the concept of paternalism to structure their ideas?
Hint:
Defenders of slavery, such as George Fitzhugh, argued that only the clever and the bright could truly benefit within a laissez-faire economy. Premising their argument on the notion that slaves were, by nature, intellectually inferior and less able to compete, such defenders maintained that slaves were better off in the care of paternalistic masters. While northern workers found themselves trapped in wage slavery, they argued, southern slaves’ needs—for food, clothing, and shelter, among other things—were met by their masters’ paternal benevolence.
|
oercommons
|
2025-03-18T00:37:59.128767
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15459/overview",
"title": "U.S. History, Cotton is King: The Antebellum South, 1800–1860",
"author": null
}
|
https://oercommons.org/courseware/lesson/15460/overview
|
The Filibuster and the Quest for New Slave States
Overview
By the end of this section, you will be able to:
- Explain the expansionist goals of advocates of slavery
- Describe the filibuster expeditions undertaken during the antebellum era
Southern expansionists had spearheaded the drive to add more territory to the United States. They applauded the Louisiana Purchase and fervently supported Indian removal, the annexation of Texas, and the Mexican-American War. Drawing inspiration from the annexation of Texas, proslavery expansionists hoped to replicate that feat by bringing Cuba and other territories into the United States and thereby enlarging the American empire of slavery.
In the 1850s, the expansionist drive among white southerners intensified. Among southern imperialists, one way to push for the creation of an American empire of slavery was through the actions of filibusters—men who led unofficial military operations intended to seize land from foreign countries or foment revolution there. These unsanctioned military adventures were not part of the official foreign policy of the United States; American citizens simply formed themselves into private armies to forcefully annex new land without the government’s approval.
An 1818 federal law made it a crime to undertake such adventures, which was an indication of both the reality of efforts at expansion through these illegal expeditions and the government’s effort to create a U.S. foreign policy. Nonetheless, Americans continued to filibuster throughout the nineteenth century. In 1819, an expedition of two hundred Americans invaded Spanish Texas, intent on creating a republic modeled on the United States, only to be driven out by Spanish forces. Using force, taking action, and asserting white supremacy in these militaristic drives were seen by many as an ideal of American male vigor. President Jackson epitomized this military prowess as an officer in the Tennessee militia, where earlier in the century he had played a leading role in ending the Creek War and driving Indian peoples out of Alabama and Georgia. His reputation helped him to win the presidency in 1828 and again in 1832.
Filibustering plots picked up pace in the 1850s as the drive for expansion continued. Slaveholders looked south to the Caribbean, Mexico, and Central America, hoping to add new slave states. Spanish Cuba became the objective of many American slaveholders in the 1850s, as debate over the island dominated the national conversation. Many who urged its annexation believed Cuba had to be made part of the United States to prevent it from going the route of Haiti, with black slaves overthrowing their masters and creating another black republic, a prospect horrifying to many in the United States. Americans also feared that the British, who had an interest in the sugar island, would make the first move and snatch Cuba from the United States. Since Britain had outlawed slavery in its colonies in 1833, blacks on the island of Cuba would then be free.
Narisco López, a Cuban who wanted to end Spanish control of the island, gained American support. He tried five times to take the island, with his last effort occurring in the summer of 1851 when he led an armed group from New Orleans. Thousands came out to cheer his small force as they set off to wrest Cuba from the Spanish. Unfortunately for López and his supporters, however, the effort to take Cuba did not produce the hoped-for spontaneous uprising of the Cuban people. Spanish authorities in Cuba captured and executed López and the American filibusters.
Efforts to take Cuba continued under President Franklin Pierce, who had announced at his inauguration in 1853 his intention to pursue expansion. In 1854, American diplomats met in Ostend, Belgium, to find a way to gain Cuba. They wrote a secret memo, known as the Ostend Manifesto (thought to be penned by James Buchanan, who was elected president two years later), stating that if Spain refused to sell Cuba to the United States, the United States was justified in taking the island as a national security measure.
The contents of this memo were supposed to remain secret, but details were leaked to the public, leading the House of Representatives to demand a copy. Many in the North were outraged over what appeared to be a southern scheme, orchestrated by what they perceived as the Slave Power—a term they used to describe the disproportionate influence that elite slaveholders wielded—to expand slavery. European powers also reacted with anger. Southern annexationists, however, applauded the effort to take Cuba. The Louisiana legislature in 1854 asked the federal government to take decisive action, and John Quitman, a former Mississippi governor, raised money from slaveholders to fund efforts to take the island.
Read an 1860 editorial titled Annexation of Cuba Made Easy from the online archives of The New York Times. Does the author support annexation? Why or why not?
Controversy around the Ostend Manifesto caused President Pierce to step back from the plan to take Cuba. After his election, President Buchanan, despite his earlier expansionist efforts, denounced filibustering as the action of pirates. Filibustering caused an even wider gulf between the North and the South (Figure).
Cuba was not the only territory in slaveholders’ expansionist sights: some focused on Mexico and Central America. In 1855, Tennessee-born William Walker, along with an army of no more than sixty mercenaries, gained control of the Central American nation of Nicaragua. Previously, Walker had launched a successful invasion of Mexico, dubbing his conquered land the Republic of Sonora. In a relatively short period of time, Walker was dislodged from Sonora by Mexican authorities and forced to retreat back to the United States. His conquest of Nicaragua garnered far more attention, catapulting him into national popularity as the heroic embodiment of white supremacy (Figure).
Why Nicaragua? Nicaragua presented a tempting target because it provided a quick route from the Caribbean to the Pacific: Only twelve miles of land stood between the Pacific Ocean, the inland Lake Nicaragua, and the river that drained into the Atlantic. Shipping from the East Coast to the West Coast of the United States had to travel either by land across the continent, south around the entire continent of South America, or through Nicaragua. Previously, American tycoon Cornelius Vanderbilt (Figure) had recognized the strategic importance of Nicaragua and worked with the Nicaraguan government to control shipping there. The filibustering of William Walker may have excited expansionist-minded southerners, but it greatly upset Vanderbilt’s business interests in the region.
Walker clung to the racist, expansionist philosophies of the proslavery South. In 1856, Walker made slavery legal in Nicaragua—it had been illegal there for thirty years—in a move to gain the support of the South. He also reopened the slave trade. In 1856, he was elected president of Nicaragua, but in 1857, he was chased from the country. When he returned to Central America in 1860, he was captured by the British and released to Honduran authorities, who executed him by firing squad.
Section Summary
The decade of the 1850s witnessed various schemes to expand the American empire of slavery. The Ostend Manifesto articulated the right of the United States to forcefully seize Cuba if Spain would not sell it, while filibuster expeditions attempted to annex new slave states without the benefit of governmental approval. Those who pursued the goal of expanding American slavery believed they embodied the true spirit of white racial superiority.
Review Questions
Why did southern expansionists conduct filibuster expeditions?
- to gain political advantage
- to annex new slave states
- to prove they could raise an army
- to map unknown territories
Hint:
B
The controversy at the heart of the Ostend Manifesto centered on the fate of:
- Ostend, Belgium
- Nicaragua
- Cuba
- Louisiana
Hint:
C
Why did expansionists set their sights on the annexation of Spanish Cuba?
Hint:
Many slaveholding expansionists believed that the events of the Haitian Revolution could repeat themselves in Cuba, leading to the overthrow of slavery on the island and the creation of an independent black republic. Americans also feared that the British would seize Cuba—which, since Britain had outlawed slavery in its colonies in 1833, would render all slaves on the island free.
Critical Thinking Questions
Compare and contrast the steamboats of the antebellum years with technologies today. In your estimation, what modern technology compares to steamboats in its transformative power?
Does the history of the cotton kingdom support or undermine the Jeffersonian vision of white farmers on self-sufficient farms? Explain your answer.
Based on your reading of William J. Anderson’s and John Brown’s accounts, what types of traumas did slaves experience? How were the experiences of black women and men similar and different?
What strategies did slaves employ to resist, revolt, and sustain their own independent communities and cultures? How did slaves use white southerners’ own philosophies—paternalism and Christianity, for example—to their advantage in these efforts?
What are the major arguments put forward by proslavery advocates? How would you argue against their statements?
Consider filibustering from the point of view of the Cuban or Nicaraguan people. If you lived in Cuba or Nicaragua, would you support filibustering? Why or why not?
|
oercommons
|
2025-03-18T00:37:59.156738
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15460/overview",
"title": "U.S. History, Cotton is King: The Antebellum South, 1800–1860",
"author": null
}
|
https://oercommons.org/courseware/lesson/15287/overview
|
Declaration of Independence
When in the Course of human events, it becomes necessary for one people to dissolve the political bands which have connected them with another, and to assume among the powers of the earth, the separate and equal station to which the Laws of Nature and of Nature's God entitle them, a decent respect to the opinions of mankind requires that they should declare the causes which impel them to the separation.
We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness. —That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed, —That whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their Safety and Happiness. Prudence, indeed, will dictate that Governments long established should not be changed for light and transient causes; and accordingly all experience hath shewn, that mankind are more disposed to suffer, while evils are sufferable, than to right themselves by abolishing the forms to which they are accustomed. But when a long train of abuses and usurpations, pursuing invariably the same Object evinces a design to reduce them under absolute Despotism, it is their right, it is their duty, to throw off such Government, and to provide new Guards for their future security. —Such has been the patient sufferance of these Colonies; and such is now the necessity which constrains them to alter their former Systems of Government. The history of the present King of Great Britain is a history of repeated injuries and usurpations, all having in direct object the establishment of an absolute Tyranny over these States. To prove this, let Facts be submitted to a candid world.
He has refused his Assent to Laws, the most wholesome and necessary for the public good.
He has forbidden his Governors to pass Laws of immediate and pressing importance, unless suspended in their operation till his Assent should be obtained; and when so suspended, he has utterly neglected to attend to them.
He has refused to pass other Laws for the accommodation of large districts of people, unless those people would relinquish the right of Representation in the Legislature, a right inestimable to them and formidable to tyrants only.
He has called together legislative bodies at places unusual, uncomfortable, and distant from the depository of their public Records, for the sole purpose of fatiguing them into compliance with his measures.
He has dissolved Representative Houses repeatedly, for opposing with manly firmness his invasions on the rights of the people.
He has refused for a long time, after such dissolutions, to cause others to be elected; whereby the Legislative powers, incapable of Annihilation, have returned to the People at large for their exercise; the State remaining in the mean time exposed to all the dangers of invasion from without, and convulsions within.
He has endeavoured to prevent the population of these States; for that purpose obstructing the Laws for Naturalization of Foreigners; refusing to pass others to encourage their migrations hither, and raising the conditions of new Appropriations of Lands.
He has obstructed the Administration of Justice, by refusing his Assent to Laws for establishing Judiciary powers.
He has made Judges dependent on his Will alone, for the tenure of their offices, and the amount and payment of their salaries.
He has erected a multitude of New Offices, and sent hither swarms of Officers to harrass our people, and eat out their substance.
He has kept among us, in times of peace, Standing Armies without the Consent of our legislatures.
He has affected to render the Military independent of and superior to the Civil power.
He has combined with others to subject us to a jurisdiction foreign to our constitution, and unacknowledged by our laws; giving his Assent to their Acts of pretended Legislation:
For Quartering large bodies of armed troops among us:
For protecting them, by a mock Trial, from punishment for any Murders which they should commit on the Inhabitants of these States:
For cutting off our Trade with all parts of the world:
For imposing Taxes on us without our Consent:
For depriving us in many cases, of the benefits of Trial by Jury:
For transporting us beyond Seas to be tried for pretended offences
For abolishing the free System of English Laws in a neighbouring Province, establishing therein an Arbitrary government, and enlarging its Boundaries so as to render it at once an example and fit instrument for introducing the same absolute rule into these Colonies:
For taking away our Charters, abolishing our most valuable Laws, and altering fundamentally the Forms of our Governments:
For suspending our own Legislatures, and declaring themselves invested with power to legislate for us in all cases whatsoever.
He has abdicated Government here, by declaring us out of his Protection and waging War against us.
He has plundered our seas, ravaged our Coasts, burnt our towns, and destroyed the lives of our people.
He is at this time transporting large Armies of foreign Mercenaries to compleat the works of death, desolation and tyranny, already begun with circumstances of Cruelty & perfidy scarcely paralleled in the most barbarous ages, and totally unworthy the Head of a civilized nation.
He has constrained our fellow Citizens taken Captive on the high Seas to bear Arms against their Country, to become the executioners of their friends and Brethren, or to fall themselves by their Hands.
He has excited domestic insurrections amongst us, and has endeavoured to bring on the inhabitants of our frontiers, the merciless Indian Savages, whose known rule of warfare, is an undistinguished destruction of all ages, sexes and conditions.
In every stage of these Oppressions We have Petitioned for Redress in the most humble terms: Our repeated Petitions have been answered only by repeated injury. A Prince whose character is thus marked by every act which may define a Tyrant, is unfit to be the ruler of a free people.
Nor have We been wanting in attentions to our Brittish brethren. We have warned them from time to time of attempts by their legislature to extend an unwarrantable jurisdiction over us. We have reminded them of the circumstances of our emigration and settlement here. We have appealed to their native justice and magnanimity, and we have conjured them by the ties of our common kindred to disavow these usurpations, which, would inevitably interrupt our connections and correspondence. They too have been deaf to the voice of justice and of consanguinity. We must, therefore, acquiesce in the necessity, which denounces our Separation, and hold them, as we hold the rest of mankind, Enemies in War, in Peace Friends.
We, therefore, the Representatives of the united States of America, in General Congress, Assembled, appealing to the Supreme Judge of the world for the rectitude of our intentions, do, in the Name, and by Authority of the good People of these Colonies, solemnly publish and declare, That these United Colonies are, and of Right ought to be Free and Independent States; that they are Absolved from all Allegiance to the British Crown, and that all political connection between them and the State of Great Britain, is and ought to be totally dissolved; and that as Free and Independent States, they have full Power to levy War, conclude Peace, contract Alliances, establish Commerce, and to do all other Acts and Things which Independent States may of right do. And for the support of this Declaration, with a firm reliance on the protection of divine Providence, we mutually pledge to each other our Lives, our Fortunes and our sacred Honor.
The 56 signatures on the Declaration appear in the positions indicated:
Column 1
Georgia:
Button Gwinnett
Lyman Hall
George Walton
Column 2
North Carolina:
William Hooper
Joseph Hewes
John Penn
South Carolina:
Edward Rutledge
Thomas Heyward, Jr.
Thomas Lynch, Jr.
Arthur Middleton
Column 3
Massachusetts:
John Hancock
Maryland:
Samuel Chase
William Paca
Thomas Stone
Charles Carroll of Carrollton
Virginia:
George Wythe
Richard Henry Lee
Thomas Jefferson
Benjamin Harrison
Thomas Nelson, Jr.
Francis Lightfoot Lee
Carter Braxton
Column 4
Pennsylvania:
Robert Morris
Benjamin Rush
Benjamin Franklin
John Morton
George Clymer
James Smith
George Taylor
James Wilson
George Ross
Delaware:
Caesar Rodney
George Read
Thomas McKean
Column 5
New York:
William Floyd
Philip Livingston
Francis Lewis
Lewis Morris
New Jersey:
Richard Stockton
John Witherspoon
Francis Hopkinson
John Hart
Abraham Clark
Column 6
New Hampshire:
Josiah Bartlett
William Whipple
Massachusetts:
Samuel Adams
John Adams
Robert Treat Paine
Elbridge Gerry
Rhode Island:
Stephen Hopkins
William Ellery
Connecticut:
Roger Sherman
Samuel Huntington
William Williams
Oliver Wolcott
New Hampshire:
Matthew Thornton
|
oercommons
|
2025-03-18T00:37:59.181432
| null |
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15287/overview",
"title": "American Government, Declaration of Independence",
"author": null
}
|
https://oercommons.org/courseware/lesson/66276/overview
|
Glossary
Overview
Glossary
Glossary: The Texas Legislature
biennial sessions: In Texas, legislative sessions meet once every odd-numbered years, for 140 days
bill: a proposed law that has been sponsored by a member of the legislature and submitted to the clerk of the House or Senate
cracking: occurs when a constituency is divided between several districts in order to prevent it from achieving a majority in any one district.
gerrymandering: the process in which voting districts are redrawn in a way to favor one party during elections
legislative budget: the state budget that is prepared and submitted by the Legislative Budget Board (LBB) and that is fully considered by the House and Senate
packing: occurs when a constituency or voting group is placed within a single district, thereby minimizing its influence in other districts.
redistricting: the process of redrawing election districts and redistributing legislative representatives in the Texas House, Texas Senate, and U.S. House. Redistricting typically occurs every 10 years to reflect shifts in population or in response to legal challenges in existing districts
single-member district: a district in which one official is elected rather than multiple officials.
special session: a legislative session called by the governor that addresses an agenda set by him or her; lasts no longer than 30 days
Voting Rights Act of 1965: mandates that electoral district lines cannot be drawn in such a manner as to “improperly dilute minorities’ voting power”
License and Attribution
CC LICENSED CONTENT, ORIGINAL
The Texas Legislature: Glossary. Authored by: John Osterman. License: CC BY: Attribution
|
oercommons
|
2025-03-18T00:37:59.198584
|
05/05/2020
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/66276/overview",
"title": "Texas Government 2.0, The Texas Legislature, Glossary",
"author": "Kris Seago"
}
|
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
|
2025-03-18T00:37:59.221008
|
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/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
|
2025-03-18T00:37:59.251226
|
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
|
2025-03-18T00:37:59.370172
| 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
|
2025-03-18T00:37:59.524925
| 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
|
2025-03-18T00:37:59.739000
| 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
|
2025-03-18T00:37:59.899886
| 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
|
2025-03-18T00:38:00.024530
| 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
|
2025-03-18T00:38:00.164382
| 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
|
2025-03-18T00:38:00.302909
| 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
|
2025-03-18T00:38:00.401141
| 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
|
2025-03-18T00:38:00.507355
| 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
|
2025-03-18T00:38:00.656774
| 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/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
|
2025-03-18T00:38:00.795886
|
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
|
2025-03-18T00:38:00.908422
|
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/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
|
2025-03-18T00:38:01.029037
|
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/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
|
2025-03-18T00:38:01.161203
|
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
|
2025-03-18T00:38:01.315272
|
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
|
2025-03-18T00:38:01.455276
|
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/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
|
2025-03-18T00:38:01.550676
|
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
|
2025-03-18T00:38:01.683917
|
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/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
|
2025-03-18T00:38:01.736783
|
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/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
|
2025-03-18T00:38:01.751609
|
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
|
2025-03-18T00:38:01.798624
|
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
|
2025-03-18T00:38:01.826914
|
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
|
2025-03-18T00:38:01.849844
|
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/64119/overview
|
Education Standards
English Language Arts Core Instructional Materials Options
Overview
These are full-course openly licensed resources for districts interested in exploring OER options when considering core instructional materials for district adoption. Course materials are available for online viewing or download.
Full Course Openly LIcensed ELA for District Consideration
Grade Band | Developer org. or district | License | Obtain Materials fees involved for different delivery platforms | EdReports Review if available | Comments |
PK-12 | Various developers for EngageNY | See individual developers below | Delivery options: PDF download Professional learning: EngageNY Professional Development Library | ||
K-12 | CC BY NC SA | Delivery options: Online viewing, PDF download Professional learning: Match Fishtank Teacher Tools | |||
K-3 | Core Knowledge Foundation for EngageNY | Delivery options: PDF download, print purchase Professional learning: CKLA Professional Development | |||
K-5 | CC-BY | Delivery options: PDF download, print purchase, digital platforms Professional learning: EL Professional Development Services | |||
K-5 | CC-BY | Delivery options: Online viewing (in beta), PDF download, print purchase | |||
3-12 | CC-BY | soon available again via LA DOE web | Delivery options: online viewing, PDF download, digital platform, LMS integration, print purchase Professional learning: Learn Zillion Guidebooks Professional Development |
Image by Steve Buissinne from Pixabay
Except where otherwise noted, this work by the Washington Office of Superintendent of Public Instruction is licensed under a Creative Commons Attribution License. All logos and trademarks are property of their respective owners.
|
oercommons
|
2025-03-18T00:38:01.901294
|
Module
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/64119/overview",
"title": "English Language Arts Core Instructional Materials Options",
"author": "Lesson"
}
|
https://oercommons.org/courseware/lesson/110078/overview
|
Ch 2 Neolithic
Ch 3 Golden Age
Ch 4 Dark Ages
Ch 5 Renaissance
Ch 6 Enlightenment
Ch 7 Industrial Age
Open History: Full Textbook
Overview
Open History is a free and open World History textbook designed for Adult Basic Education students to practice skills for High School Equivalency (HSE). It can be used as a reading for information text for RLA Reading and to practice important skills for Social Studies.
Full Textbook
Attached you will find all seven chapters of Open History. Open History is a free and open World History textbook designed for Adult Basic Education students to practice skills for High School Equivalency (HSE). It can be used as a reading for information text for RLA Reading and to practice important skills for Social Studies.
|
oercommons
|
2025-03-18T00:38:01.923871
|
Alexander Greengaard
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/110078/overview",
"title": "Open History: Full Textbook",
"author": "Textbook"
}
|
https://oercommons.org/courseware/lesson/64513/overview
|
Grade 6-8 Social Science Learning Resources
Overview
Resource suggestions to explore and consider as a way to support your family learning during school closures.
Civics & Government
Center for Civics Education: The Center’s lesson plans include We the People: The Citizen and the Constitution; Project Citizen; the School Violence Prevention Demonstration Program; Representative Democracy in America; Citizens, Not Spectators; and Foundations of Democracy.
Library of Congress: The Library of Congress offers classroom materials and professional development to help teachers effectively use primary sources from the Library's vast digital collections in their teaching.
Social Studies Cirriculum Maps and Resources: Social Studies School Service is a publisher and distributor of educational materials, from full curricula to supplementary resources.
Teaching Civics: Lessons for civics, government, and law-related education in elementary, middle, and high school classrooms. Includes simulations, discussions, mock trials, case studies and other research based materials.
Photo by Stephen Walker on Unsplash
Economics & Financial Literacy
EconEd: Econ Ed Link provides classroom-tested, Internet-based economic and personal finance lesson materials for K-12 teachers and their students.
Social Studies Cirriculum Maps and Resources: Social Studies School Service is a publisher and distributor of educational materials, from full curricula to supplementary resources.
Geography
National Geographic: This site provides curated collections of activities that have been developed for educators, parents, and caregivers to implement with K–12 learners anywhere, anytime.
Social Studies Cirriculum Maps and Resources: Social Studies School Service is a publisher and distributor of educational materials, from full curricula to supplementary resources.
History
PBS Learning Media U.S. History and World History: These sites provides U.S. and world histories is broken down by time periods using an interactive maps and interactive lessons.
Social Studies Cirriculum Maps and Resources: Social Studies School Service is a publisher and distributor of educational materials, from full curricula to supplementary resources.
Stanford History Education Group: The Reading Like a Historian curriculum engages students in historical inquiry. Each lesson revolves around a central historical question and features a set of primary documents designed for groups of students with a range of reading skills.
Teaching Tolerance: These classroom lessons offer breadth and depth, spanning essential social justice topics and reinforcing critical social emotional learning skills.
|
oercommons
|
2025-03-18T00:38:01.941236
|
Social Science
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/64513/overview",
"title": "Grade 6-8 Social Science Learning Resources",
"author": "Physical Geography"
}
|
https://oercommons.org/courseware/lesson/111941/overview
|
Gamification in Education Toolkit
Overview
Toolkit is an Open Educational Resource on Gamification. Gamification in education is a teaching method that uses game elements and design to motivate students. The goal is to make learning fun and engaging by inspiring students to continue learning. The text, images, video, and quizzes are included to promote gamification in education.
Introduction
Learning environments around the world are changing. Educators no longer rely on the familiar didactic pedagogy of decades before, whether in the classroom or the corporate boardroom. Students and learners yearn for more interactive, adaptable and on-time solutions. Many students have learning situations that are different from what they learn remotely. All these issues demand innovations in education that can engage learners, boost retention and promote abilities.
Enter gamification. Gamification is applying typical elements of game playing, such as leaderboards, point systems, rankings, and badges, to other areas of activity, such as learning. With the advent of new technology (i.e. the internet, tablets, and mobile phones) and new media, gamification is poised to fill the gap that current or past learning environments have left. From a simple game of duck, duck, and goose to a complete learning video game, gamification impacts the education environment. In this paper, we will research the impact of gamification on students learning and achievements.
This toolkit was made to introduce educators to the gamification model. Information, graphics, videos and sources are provided for educators to use in their own classroom environments as they explore gamification and all it's benefits.
Benefits of Gamification
Gamification aids in cognitive development in adolescents.
Gamification aids in physical development.
Gamification increases the level of engagement in classrooms.
Gamification aids in accessibility in the classroom.
Gamification isn't limited to the classroom.
Makes learning fun and interactive
Creates an addiction to learning
Gives learners the opportunity to see real-world applications
Offers real-time feedback
Gamification enhances the learning experience
More engagement with content
Facilitates Mistake-Driven Learning
Enhances The Overall Learning Experience For All Age Group
Design
Define your Audience and model:
Audience: Educators, Teachers, Instructors
Instructional Design Model: Gamification or Game Based Model
Incorporate ISTE and TPACK standards into the design from the beginning and during development and evaluation.
Decide Sequencing
List of a few gamification resources:
https://iste.org/blog/5-ways-to-gamify-your-classroom
https://www.youtube.com/watch?v=W72DnmSZbr4
https://ssec.si.edu/stemvisions-blog/5-benefits-gamification
https://www.youtube.com/watch?v=uGTQRfelUhk&t=527s
https://www.youtube.com/watch?v=3nY33ZbOaVw&t=19s
https://axonpark.com/how-effective-is-gamification-in-education-10-case-studies-and-examples/
https://slejournal.springeropen.com/articles/10.1186/s40561-019-0098-x
https://www.cultofpedagogy.com/pod/
https://www.youtube.com/watch?v=5TOXvl3Vig4
Prepare Activities for Students
- Set goals: Establish goals for your gamification plan.
- Identify behaviors: Identify desired learner behaviors.
- Profile users: Profile users.
- Select game elements: Select effective game elements and mechanics.
- Design experiences: Design meaningful and engaging experiences.
- Evaluate: Implement and evaluate your gamification strategy.
Examples of gamification activities:
Storyboarding: Create a story arc and incorporate learners directly into the plot.
Challenges: Provide learners with challenges.
Rewards: Reward them with medals, badges, or further levels.
Competition: Create competitions for sales, product knowledge, or leads.
Leaderboards: Establish leaderboards within teams, departments, branches, or across the whole company.
Streaks: Use streaks for daily lessons.
Points: Use points for saving money.
Feedback: Offer feedback throughout courses.
Avatar login: Add avatar login features.
Peer setting: Create a peer setting.
Scoring system: Use a scoring system.
Simulation training: Use simulation training.
Timed quizzes: Use timed quizzes.
Develop
Develop using ISTE appropriate standards.
The Empowered Learner 1a: Learning Goals (ISTE Standards for Students)
Using ISTE standards to transform teaching, learning, and administration
Develop Using appropriate TPACK standards.
TPACK is a technology integration framework that identifies three types of knowledge instructors need to combine for successful edtech integration—technological, pedagogical, and content knowledge (a.k.a. TPACK). While TPACK is often compared with the SAMR Model, they are very different in scope
5 key points when developing for gamification in education:
Make it relevant: Gamification should be relevant to the topic being taught.
Make it motivating: Gamification can increase student motivation by offering rewards for completing tasks. Rewards include virtual currency, points, badges, levels, and tangible prizes.
Avoid boredom: Gamification can help students focus by providing feedback and progress-tracking tools.
Keep the feedback loop open: Gamification can help students stay focused by providing feedback and progress-tracking tools.
Create personalized learning experiences: Gamification allows students to learn at their own pace and level.
Another 5 key points when developing for gamification in education:
Step 1: Assess your students.
Step 2: Define learning goals.
Step 3: Structure the gamified learning experience.
Step 4: Identify resources.
Step 5: Apply gamification elements.
Evaluate
Evaluate Using appropriate TPACK standards.
What is gamified assessment and feedback?
Gamified assessment and feedback are ways of measuring and providing information on student learning that incorporate game features, such as points, badges, levels, leaderboards, rewards, challenges, and narratives. Gamified assessment and feedback aim to make learning more engaging, fun, and meaningful for students, as well as to provide them with clear goals, progress indicators, and feedback loops. Gamified assessment and feedback can also motivate students to improve their performance, self-regulate their learning, and collaborate with others.
How to design gamified assessment and feedback?
Designing gamified assessment and feedback involves four main steps: defining your learning outcomes, choosing game elements, aligning assessment and feedback methods, and implementing and evaluating your gamification strategy. To define your learning outcomes, you should identify what you want your students to know, understand, and do as a result of your teaching, and how you will measure their achievement. SMART criteria should be used to ensure the learning outcomes are specific, measurable, achievable, relevant, and timely. When selecting game elements, consider the learning outcomes, student preferences, and teaching context. Points can quantify student performance; badges recognize achievements; levels show progress; leaderboards foster competition or cooperation; rewards incentivize student actions; challenges increase difficulty; and narratives create immersion. Align assessment and feedback methods with the learning outcomes and game elements. For example, use formative assessment for ongoing feedback; summative assessment to evaluate student learning; self-assessment for reflection and self-regulation; peer assessment for collaboration; and rubrics to communicate criteria. Finally, implement the gamification strategy in teaching, monitoring its effectiveness on student learning and motivation. Data collection tools such as surveys, interviews, observations, quizzes, analytics, and dashboards can be used to collect and analyze data. This data can then be used to improve the gamification strategy or address any challenges.
What are the benefits of gamified assessment and feedback?
Gamified assessment and feedback can offer various benefits for both students and teachers, such as increased student engagement, motivation, and learning, as well as empowered student autonomy. Students can interact with the content, teacher, and peers in playful and creative ways, while receiving immediate and constructive feedback, recognition, and rewards for their efforts. Gamified assessment and feedback also provide students with clear objectives, progress indicators, and feedback loops, as well as opportunities to practice, apply, and revise their knowledge. Not only does this reduce teacher workload and stress by automating grading, tracking, and reporting processes; but it also provides teachers with useful data and insights on student learning and motivation.
What are the challenges of gamified assessment and feedback?
Gamified assessment and feedback can also present some challenges and limitations for both students and teachers. For instance, if the game elements are not well-designed, integrated, and aligned with learning outcomes, students may be more focused on earning points than understanding the concepts. Additionally, if the game elements are not adapted to students' needs, preferences, and abilities, some may feel frustrated or anxious. Moreover, ethical concerns may arise if the game elements are not transparent and respectful to student rights. Lastly, technical issues may arise if the game elements are not well-supported by technology and infrastructure.
How to overcome the challenges of gamified assessment and feedback?
To overcome the challenges of gamified assessment and feedback, you need to consider some best practices and recommendations. Involving your students in the design, implementation, and evaluation of your gamified assessment and feedback can help create a more relevant, engaging, and inclusive learning experience. It is also important to balance your game elements with your learning content and objectives. You should also monitor and adjust your gamification strategy based on data and observations, measuring the impact of game elements on student learning and motivation, as well as identifying and addressing any issues that arise. This will help you optimize your assessment and feedback outcomes.
Source: LinkedIn https://www.linkedin.com/advice/0/how-do-you-gamify-assessment-feedback-your
Interactive Quiz
Mutiple choice quiz on TPACK - click on the link
TPACK - Quizziz
Mutiple choice quiz on Gamification - click on the link
Gamification - Quizziz
|
oercommons
|
2025-03-18T00:38:01.971922
|
01/26/2024
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/111941/overview",
"title": "Gamification in Education Toolkit",
"author": "John Verber"
}
|
https://oercommons.org/courseware/lesson/15205/overview
|
The Evolution of American Federalism
Learning Objectives
By the end of this section, you will be able to:
- Describe how federalism has evolved in the United States
- Compare different conceptions of federalism
The Constitution sketches a federal framework that aims to balance the forces of decentralized and centralized governance in general terms; it does not flesh out standard operating procedures that say precisely how the states and federal governments are to handle all policy contingencies imaginable. Therefore, officials at the state and national levels have had some room to maneuver as they operate within the Constitution’s federal design. This has led to changes in the configuration of federalism over time, changes corresponding to different historical phases that capture distinct balances between state and federal authority.
THE STRUGGLE BETWEEN NATIONAL POWER AND STATE POWER
As George Washington’s secretary of the treasury from 1789 to 1795, Alexander Hamilton championed legislative efforts to create a publicly chartered bank. For Hamilton, the establishment of the Bank of the United States was fully within Congress’s authority, and he hoped the bank would foster economic development, print and circulate paper money, and provide loans to the government. Although Thomas Jefferson, Washington’s secretary of state, staunchly opposed Hamilton’s plan on the constitutional grounds that the national government had no authority to create such an instrument, Hamilton managed to convince the reluctant president to sign the legislation.The Lehrman Institute. “The Founding Trio: Washington, Hamilton and Jefferson.” http://lehrmaninstitute.org/history/FoundingTrio.asp
When the bank’s charter expired in 1811, Jeffersonian Democratic-Republicans prevailed in blocking its renewal. However, the fiscal hardships that plagued the government during the War of 1812, coupled with the fragility of the country’s financial system, convinced Congress and then-president James Madison to create the Second Bank of the United States in 1816. Many states rejected the Second Bank, arguing that the national government was infringing upon the states’ constitutional jurisdiction.
A political showdown between Maryland and the national government emerged when James McCulloch, an agent for the Baltimore branch of the Second Bank, refused to pay a tax that Maryland had imposed on all out-of-state chartered banks. The standoff raised two constitutional questions: Did Congress have the authority to charter a national bank? Were states allowed to tax federal property? In McCulloch v. Maryland, Chief Justice John Marshall (Figure) argued that Congress could create a national bank even though the Constitution did not expressly authorize it.McCulloch v. Maryland, 17 U.S. 316 (1819). Under the necessary and proper clause of Article I, Section 8, the Supreme Court asserted that Congress could establish “all means which are appropriate” to fulfill “the legitimate ends” of the Constitution. In other words, the bank was an appropriate instrument that enabled the national government to carry out several of its enumerated powers, such as regulating interstate commerce, collecting taxes, and borrowing money.
This ruling established the doctrine of implied powers, granting Congress a vast source of discretionary power to achieve its constitutional responsibilities. The Supreme Court also sided with the federal government on the issue of whether states could tax federal property. Under the supremacy clause of Article VI, legitimate national laws trump conflicting state laws. As the court observed, “the government of the Union, though limited in its powers, is supreme within its sphere of action and its laws, when made in pursuance of the constitution, form the supreme law of the land.” Maryland’s action violated national supremacy because “the power to tax is the power to destroy.” This second ruling established the principle of national supremacy, which prohibits states from meddling in the lawful activities of the national government.
Defining the scope of national power was the subject of another landmark Supreme Court decision in 1824. In Gibbons v. Ogden, the court had to interpret the commerce clause of Article I, Section 8; specifically, it had to determine whether the federal government had the sole authority to regulate the licensing of steamboats operating between New York and New Jersey.Gibbons v. Ogden, 22 U.S. 1 (1824). Aaron Ogden, who had obtained an exclusive license from New York State to operate steamboat ferries between New York City and New Jersey, sued Thomas Gibbons, who was operating ferries along the same route under a coasting license issued by the federal government. Gibbons lost in New York state courts and appealed. Chief Justice Marshall delivered a two-part ruling in favor of Gibbons that strengthened the power of the national government. First, interstate commerce was interpreted broadly to mean “commercial intercourse” among states, thus allowing Congress to regulate navigation. Second, because the federal Licensing Act of 1793, which regulated coastal commerce, was a constitutional exercise of Congress’s authority under the commerce clause, federal law trumped the New York State license-monopoly law that had granted Ogden an exclusive steamboat operating license. As Marshall pointed out, “the acts of New York must yield to the law of Congress.”Gibbons v. Ogden, 22 U.S. 1 (1824).
Various states railed against the nationalization of power that had been going on since the late 1700s. When President John Adams signed the Sedition Act in 1798, which made it a crime to speak openly against the government, the Kentucky and Virginia legislatures passed resolutions declaring the act null on the grounds that they retained the discretion to follow national laws. In effect, these resolutions articulated the legal reasoning underpinning the doctrine of nullification—that states had the right to reject national laws they deemed unconstitutional.W. Kirk Wood. 2008. Nullification, A Constitutional History, 1776–1833. Lanham, MD: University Press of America.
A nullification crisis emerged in the 1830s over President Andrew Jackson’s tariff acts of 1828 and 1832. Led by John Calhoun, President Jackson’s vice president, nullifiers argued that high tariffs on imported goods benefited northern manufacturing interests while disadvantaging economies in the South. South Carolina passed an Ordinance of Nullification declaring both tariff acts null and void and threatened to leave the Union. The federal government responded by enacting the Force Bill in 1833, authorizing President Jackson to use military force against states that challenged federal tariff laws. The prospect of military action coupled with the passage of the Compromise Tariff Act of 1833 (which lowered tariffs over time) led South Carolina to back off, ending the nullification crisis.
The ultimate showdown between national and state authority came during the Civil War. Prior to the conflict, in Dred Scott v. Sandford, the Supreme Court ruled that the national government lacked the authority to ban slavery in the territories.Dred Scott v. Sandford, 60 U.S. 393 (1857). But the election of President Abraham Lincoln in 1860 led eleven southern states to secede from the United States because they believed the new president would challenge the institution of slavery. What was initially a conflict to preserve the Union became a conflict to end slavery when Lincoln issued the Emancipation Proclamation in 1863, freeing all slaves in the rebellious states. The defeat of the South had a huge impact on the balance of power between the states and the national government in two important ways. First, the Union victory put an end to the right of states to secede and to challenge legitimate national laws. Second, Congress imposed several conditions for readmitting former Confederate states into the Union; among them was ratification of the Fourteenth and Fifteenth Amendments. In sum, after the Civil War the power balance shifted toward the national government, a movement that had begun several decades before with McCulloch v. Maryland (1819) and Gibbons v. Odgen (1824).
The period between 1819 and the 1860s demonstrated that the national government sought to establish its role within the newly created federal design, which in turn often provoked the states to resist as they sought to protect their interests. With the exception of the Civil War, the Supreme Court settled the power struggles between the states and national government. From a historical perspective, the national supremacy principle introduced during this period did not so much narrow the states’ scope of constitutional authority as restrict their encroachment on national powers.Joseph R. Marbach, Troy E. Smith, and Ellis Katz. 2005. Federalism in America: An Encyclopedia. Westport, CT: Greenwood Publishing.
DUAL FEDERALISM
The late 1870s ushered in a new phase in the evolution of U.S. federalism. Under dual federalism, the states and national government exercise exclusive authority in distinctly delineated spheres of jurisdiction. Like the layers of a cake, the levels of government do not blend with one another but rather are clearly defined. Two factors contributed to the emergence of this conception of federalism. First, several Supreme Court rulings blocked attempts by both state and federal governments to step outside their jurisdictional boundaries. Second, the prevailing economic philosophy at the time loathed government interference in the process of industrial development.
Industrialization changed the socioeconomic landscape of the United States. One of its adverse effects was the concentration of market power. Because there was no national regulatory supervision to ensure fairness in market practices, collusive behavior among powerful firms emerged in several industries.Marc Allen Eisner. 2014. The American Political Economy: Institutional Evolution of Market and State. New York: Routledge. To curtail widespread anticompetitive practices in the railroad industry, Congress passed the Interstate Commerce Act in 1887, which created the Interstate Commerce Commission. Three years later, national regulatory capacity was broadened by the Sherman Antitrust Act of 1890, which made it illegal to monopolize or attempt to monopolize and conspire in restraining commerce (Figure 03_02_Commerce). In the early stages of industrial capitalism, federal regulations were focused for the most part on promoting market competition rather than on addressing the social dislocations resulting from market operations, something the government began to tackle in the 1930s.Eisner, The American Political Economy; Stephen Skowronek. 1982. Building a New American State: The Expansion of National Administrative Capacities, 1877–1920. Cambridge, MA: Cambridge University Press.
The new federal regulatory regime was dealt a legal blow early in its existence. In 1895, in United States v. E. C. Knight, the Supreme Court ruled that the national government lacked the authority to regulate manufacturing.United States v. E. C. Knight, 156 U.S. 1 (1895). The case came about when the government, using its regulatory power under the Sherman Act, attempted to override American Sugar’s purchase of four sugar refineries, which would give the company a commanding share of the industry. Distinguishing between commerce among states and the production of goods, the court argued that the national government’s regulatory authority applied only to commercial activities. If manufacturing activities fell within the purview of the commerce clause of the Constitution, then “comparatively little of business operations would be left for state control,” the court argued.
In the late 1800s, some states attempted to regulate working conditions. For example, New York State passed the Bakeshop Act in 1897, which prohibited bakery employees from working more than sixty hours in a week. In Lochner v. New York, the Supreme Court ruled this state regulation that capped work hours unconstitutional, on the grounds that it violated the due process clause of the Fourteenth Amendment.Lochner v. New York, 198 U.S. 45 (1905). In other words, the right to sell and buy labor is a “liberty of the individual” safeguarded by the Constitution, the court asserted. The federal government also took up the issue of working conditions, but that case resulted in the same outcome as in the Lochner case.Hammer v. Dagenhart, 247 U.S. 251 (1918).
COOPERATIVE FEDERALISM
The Great Depression of the 1930s brought economic hardships the nation had never witnessed before (Figure). Between 1929 and 1933, the national unemployment rate reached 25 percent, industrial output dropped by half, stock market assets lost more than half their value, thousands of banks went out of business, and the gross domestic product shrunk by one-quarter.Nicholas Crafts and Peter Fearon. 2010. “Lessons from the 1930s Great Depression,” Oxford Review of Economic Policy 26: 286–287; Gene Smiley. “The Concise Encyclopedia of Economics: Great Depression.” http://www.econlib.org/library/Enc/GreatDepression.html Given the magnitude of the economic depression, there was pressure on the national government to coordinate a robust national response along with the states.
Cooperative federalism was born of necessity and lasted well into the twentieth century as the national and state governments each found it beneficial. Under this model, both levels of government coordinated their actions to solve national problems, such as the Great Depression and the civil rights struggle of the following decades. In contrast to dual federalism, it erodes the jurisdictional boundaries between the states and national government, leading to a blending of layers as in a marble cake. The era of cooperative federalism contributed to the gradual incursion of national authority into the jurisdictional domain of the states, as well as the expansion of the national government’s power in concurrent policy areas.Marbach et al, Federalism in America: An Encyclopedia.
The New Deal programs President Franklin D. Roosevelt proposed as a means to tackle the Great Depression ran afoul of the dual-federalism mindset of the justices on the Supreme Court in the 1930s. The court struck down key pillars of the New Deal—the National Industrial Recovery Act and the Agricultural Adjustment Act, for example—on the grounds that the federal government was operating in matters that were within the purview of the states. The court’s obstructionist position infuriated Roosevelt, leading him in 1937 to propose a court-packing plan that would add one new justice for each one over the age of seventy, thus allowing the president to make a maximum of six new appointments. Before Congress took action on the proposal, the Supreme Court began leaning in support of the New Deal as Chief Justice Charles Evans Hughes and Justice Owen Roberts changed their view on federalism.Jeff Shesol. 2010. Supreme Power: Franklin Roosevelt vs. The Supreme Court. New York: W. W. Norton.
In National Labor Relations Board (NLRB) v. Jones and Laughlin Steel,National Labor Relations Board (NLRB) v. Jones & Laughlin Steel, 301 U.S. 1 (1937). for instance, the Supreme Court ruled the National Labor Relations Act of 1935 constitutional, asserting that Congress can use its authority under the commerce clause to regulate both manufacturing activities and labor-management relations. The New Deal changed the relationship Americans had with the national government. Before the Great Depression, the government offered little in terms of financial aid, social benefits, and economic rights. After the New Deal, it provided old-age pensions (Social Security), unemployment insurance, agricultural subsidies, protections for organizing in the workplace, and a variety of other public services created during Roosevelt’s administration.
In the 1960s, President Lyndon Johnson’s administration expanded the national government’s role in society even more. Medicaid (which provides medical assistance to the indigent), Medicare (which provides health insurance to the elderly and disabled), and school nutrition programs were created. The Elementary and Secondary Education Act (1965), the Higher Education Act (1965), and the Head Start preschool program (1965) were established to expand educational opportunities and equality (Figure). The Clean Air Act (1965), the Highway Safety Act (1966), and the Fair Packaging and Labeling Act (1966) promoted environmental and consumer protection. Finally, laws were passed to promote urban renewal, public housing development, and affordable housing. In addition to these Great Society programs, the Civil Rights Act (1964) and the Voting Rights Act (1965) gave the federal government effective tools to promote civil rights equality across the country.
While the era of cooperative federalism witnessed a broadening of federal powers in concurrent and state policy domains, it is also the era of a deepening coordination between the states and the federal government in Washington. Nowhere is this clearer than with respect to the social welfare and social insurance programs created during the New Deal and Great Society eras, most of which are administered by both state and federal authorities and are jointly funded. The Social Security Act of 1935, which created federal subsidies for state-administered programs for the elderly; people with handicaps; dependent mothers; and children, gave state and local officials wide discretion over eligibility and benefit levels. The unemployment insurance program, also created by the Social Security Act, requires states to provide jobless benefits, but it allows them significant latitude to decide the level of tax to impose on businesses in order to fund the program as well as the duration and replacement rate of unemployment benefits. A similar multilevel division of labor governs Medicaid and Children’s Health Insurance.Lawrence R. Jacobs and Theda Skocpol. 2014. “Progressive Federalism and the Contested Implemented of Obama’s Health Reform,” In The Politics of Major Policy Reform in Postwar America, eds. Jeffrey A. Jenkins and Sidney M. Milkis. New York: Cambridge University Press.
Thus, the era of cooperative federalism left two lasting attributes on federalism in the United States. First, a nationalization of politics emerged as a result of federal legislative activism aimed at addressing national problems such as marketplace inefficiencies, social and political inequality, and poverty. The nationalization process expanded the size of the federal administrative apparatus and increased the flow of federal grants to state and local authorities, which have helped offset the financial costs of maintaining a host of New Deal- and Great Society–era programs. The second lasting attribute is the flexibility that states and local authorities were given in the implementation of federal social welfare programs. One consequence of administrative flexibility, however, is that it has led to cross-state differences in the levels of benefits and coverage.R. Kent Weaver. 2000. Ending Welfare as We Know It. Washington, DC: The Brookings Institution.
NEW FEDERALISM
During the administrations of Presidents Richard Nixon (1969–1974) and Ronald Reagan (1981–1989), attempts were made to reverse the process of nationalization—that is, to restore states’ prominence in policy areas into which the federal government had moved in the past. New federalism is premised on the idea that the decentralization of policies enhances administrative efficiency, reduces overall public spending, and improves policy outcomes. During Nixon’s administration, general revenue sharing programs were created that distributed funds to the state and local governments with minimal restrictions on how the money was spent. The election of Ronald Reagan heralded the advent of a “devolution revolution” in U.S. federalism, in which the president pledged to return authority to the states according to the Constitution. In the Omnibus Budget Reconciliation Act of 1981, congressional leaders together with President Reagan consolidated numerous federal grant programs related to social welfare and reformulated them in order to give state and local administrators greater discretion in using federal funds.Allen Schick. 2007. The Federal Budget, 3rd ed. Washington, DC: The Brookings Institution.
However, Reagan’s track record in promoting new federalism was inconsistent. This was partly due to the fact that the president’s devolution agenda met some opposition from Democrats in Congress, moderate Republicans, and interest groups, preventing him from making further advances on that front. For example, his efforts to completely devolve Aid to Families With Dependent Children (a New Deal-era program) and food stamps (a Great Society-era program) to the states were rejected by members of Congress, who feared states would underfund both programs, and by members of the National Governors’ Association, who believed the proposal would be too costly for states. Reagan terminated general revenue sharing in 1986.Dilger, “Federal Grants to State and Local Governments,” 30–31.
Several Supreme Court rulings also promoted new federalism by hemming in the scope of the national government’s power, especially under the commerce clause. For example, in United States v. Lopez, the court struck down the Gun-Free School Zones Act of 1990, which banned gun possession in school zones.United States v. Lopez, 514 U.S. 549 (1995). It argued that the regulation in question did not “substantively affect interstate commerce.” The ruling ended a nearly sixty-year period in which the court had used a broad interpretation of the commerce clause that by the 1960s allowed it to regulate numerous local commercial activities.See Printz v. United States, 521 U.S. 898 (1997).
However, many would say that the years since the 9/11 attacks have swung the pendulum back in the direction of central federal power. The creation of the Department of Homeland Security federalized disaster response power in Washington, and the Transportation Security Administration was created to federalize airport security. Broad new federal policies and mandates have also been carried out in the form of the Faith-Based Initiative and No Child Left Behind (during the George W. Bush administration) and the Affordable Care Act (during Barack Obama’s administration).
Cooperative Federalism versus New Federalism
Morton Grodzins coined the cake analogy of federalism in the 1950s while conducting research on the evolution of American federalism. Until then most scholars had thought of federalism as a layer cake, but according to Grodzins the 1930s ushered in “marble-cake federalism” (Figure): “The American form of government is often, but erroneously, symbolized by a three-layer cake. A far more accurate image is the rainbow or marble cake, characterized by an inseparable mingling of differently colored ingredients, the colors appearing in vertical and diagonal strands and unexpected whirls. As colors are mixed in the marble cake, so functions are mixed in the American federal system.”Morton Grodzins. 2004. “The Federal System.” In American Government Readings and Cases, ed. P. Woll. New York: Pearson Longman, 74–78.
Cooperative federalism has several merits:
- Because state and local governments have varying fiscal capacities, the national government’s involvement in state activities such as education, health, and social welfare is necessary to ensure some degree of uniformity in the provision of public services to citizens in richer and poorer states.
- The problem of collective action, which dissuades state and local authorities from raising regulatory standards for fear they will be disadvantaged as others lower theirs, is resolved by requiring state and local authorities to meet minimum federal standards (e.g., minimum wage and air quality).
- Federal assistance is necessary to ensure state and local programs (e.g., water and air pollution controls) that generate positive externalities are maintained. For example, one state’s environmental regulations impose higher fuel prices on its residents, but the externality of the cleaner air they produce benefits neighboring states. Without the federal government’s support, this state and others like it would underfund such programs.
New federalism has advantages as well:
- Because there are economic, demographic, social, and geographical differences among states, one-size-fits-all features of federal laws are suboptimal. Decentralization accommodates the diversity that exists across states.
- By virtue of being closer to citizens, state and local authorities are better than federal agencies at discerning the public’s needs.
- Decentralized federalism fosters a marketplace of innovative policy ideas as states compete against each other to minimize administrative costs and maximize policy output.
Which model of federalism do you think works best for the United States? Why?
The leading international journal devoted to the practical and theoretical study of federalism is called Publius: The Journal of Federalism. Find out where its name comes from.
Federalism in the United States has gone through several phases of evolution during which the relationship between the federal and state governments has varied. In the era of dual federalism, both levels of government stayed within their own jurisdictional spheres. During the era of cooperative federalism, the federal government became active in policy areas previously handled by the states. The 1970s ushered in an era of new federalism and attempts to decentralize policy management.
In McCulloch v. Maryland, the Supreme Court invoked which provisions of the constitution?
- Tenth Amendment and spending clause
- commerce clause and supremacy clause
- necessary and proper clause and supremacy clause
- taxing power and necessary and proper clause
Hint:
C
Which statement about new federalism is not true?
- New federalism was launched by President Nixon and continued by President Reagan.
- New federalism is based on the idea that decentralization of responsibility enhances administrative efficiency.
- United States v. Lopez is a Supreme Court ruling that advanced the logic of new federalism.
- President Reagan was able to promote new federalism consistently throughout his administration.
Which is not a merit of cooperative federalism?
- Federal cooperation helps mitigate the problem of collective action among states.
- Federal assistance encourages state and local governments to generate positive externalities.
- Cooperative federalism respects the traditional jurisdictional boundaries between states and federal government.
- Federal assistance ensures some degree of uniformity of public services across states.
Hint:
C
What are the main differences between cooperative federalism and dual federalism?
What were the implications of McCulloch v. Maryland for federalism?
Hint:
The McCulloch decision established the doctrine of implied powers, meaning the federal government can create policy instruments deemed necessary and appropriate to fulfill its constitutional responsibilities. The case also affirmed the principle of national supremacy embodied in Article VI of the Constitution, namely, that the Constitution and legitimate federal laws trump state laws.
|
oercommons
|
2025-03-18T00:38:02.006701
|
07/10/2017
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15205/overview",
"title": "American Government, Students and the System, American Federalism, The Evolution of American Federalism",
"author": null
}
|
https://oercommons.org/courseware/lesson/15208/overview
|
Advantages and Disadvantages of Federalism
Learning Objectives
By the end of this section, you will be able to:
- Discuss the advantages of federalism
- Explain the disadvantages of federalism
The federal design of our Constitution has had a profound effect on U.S. politics. Several positive and negative attributes of federalism have manifested themselves in the U.S. political system.
THE BENEFITS OF FEDERALISM
Among the merits of federalism are that it promotes policy innovation and political participation and accommodates diversity of opinion. On the subject of policy innovation, Supreme Court Justice Louis Brandeis observed in 1932 that “a single courageous state may, if its citizens choose, serve as a laboratory; and try novel social and economic experiments without risk to the rest of the country.”New State Ice Co. v. Liebmann, 285 U.S. 262 (1932). What Brandeis meant was that states could harness their constitutional authority to engage in policy innovations that might eventually be diffused to other states and at the national level. For example, a number of New Deal breakthroughs, such as child labor laws, were inspired by state policies. Prior to the passage of the Nineteenth Amendment, several states had already granted women the right to vote. California has led the way in establishing standards for fuel emissions and other environmental policies (Figure). Recently, the health insurance exchanges run by Connecticut, Kentucky, Rhode Island, and Washington have served as models for other states seeking to improve the performance of their exchanges.Christine Vestal and Michael Ollove, “Why some state-run health exchanges worked,” USA Today, 10 December 2013.
Another advantage of federalism is that because our federal system creates two levels of government with the capacity to take action, failure to attain a desired policy goal at one level can be offset by successfully securing the support of elected representatives at another level. Thus, individuals, groups, and social movements are encouraged to actively participate and help shape public policy.
Federalism and Political Office
Thinking of running for elected office? Well, you have several options. As Table shows, there are a total of 510,682 elected offices at the federal, state, and local levels. Elected representatives in municipal and township governments account for a little more than half the total number of elected officials in the United States. Political careers rarely start at the national level. In fact, a very small share of politicians at the subnational level transition to the national stage as representatives, senators, vice presidents, or presidents.
| Elected Officials at the Federal, State, and Local Levels | ||
|---|---|---|
| Number of Elective Bodies | Number of Elected Officials | |
| Federal Government | 1 | |
| Executive branch | 2 | |
| U.S. Senate | 100 | |
| U.S. House of Representatives | 435 | |
| State Government | 50 | |
| State legislatures | 7,382 | |
| Statewide offices | 1,036 | |
| State boards | 1,331 | |
| Local Government | ||
| County governments | 3,034 | 58,818 |
| Municipal governments | 19,429 | 135,531 |
| Town governments | 16,504 | 126,958 |
| School districts | 13,506 | 95,000 |
| Special districts | 35,052 | 84,089 |
| Total | 87,576 | 510,682 |
If you are interested in serving the public as an elected official, there are more opportunities to do so at the local and state levels than at the national level. As an added incentive for setting your sights at the subnational stage, consider the following. Whereas only 28 percent of U.S. adults trusted Congress in 2014, about 62 percent trusted their state governments and 72 percent had confidence in their local governments.Justin McCarthy. 2014. “Americans Still Trust Local Government More Than State,” September 22. http://www.gallup.com/poll/176846/americans-trust-local-government-state.aspx (June 24, 2015).
If you ran for public office, what problems would you most want to solve? What level of government would best enable you to solve them, and why?
The system of checks and balances in our political system often prevents the federal government from imposing uniform policies across the country. As a result, states and local communities have the latitude to address policy issues based on the specific needs and interests of their citizens. The diversity of public viewpoints across states is manifested by differences in the way states handle access to abortion, distribution of alcohol, gun control, and social welfare benefits, for example.
THE DRAWBACKS OF FEDERALISM
Federalism also comes with drawbacks. Chief among them are economic disparities across states, race-to-the-bottom dynamics (i.e., states compete to attract business by lowering taxes and regulations), and the difficulty of taking action on issues of national importance.
Stark economic differences across states have a profound effect on the well-being of citizens. For example, in 2014, Maryland had the highest median household income ($73,971), while Mississippi had the lowest ($39,680).See http://www.deptofnumbers.com/income/ for more data on household income. There are also huge disparities in school funding across states. In 2013, New York spent $19,818 per student for elementary and secondary education, while Utah spent $6,555.Governing. “Education Spending Per Student by State.” http://www.governing.com/gov-data/education-data/state-education-spending-per-pupil-data.html (June 24, 2015). Furthermore, health-care access, costs, and quality vary greatly across states.The Commonwealth Fund. “Aiming Higher: Results from a Scorecard on State Health System Performance, 2014.” http://www.commonwealthfund.org/publications/fund-reports/2014/apr/2014-state-scorecard (June 24, 2015). Proponents of social justice contend that federalism has tended to obstruct national efforts to effectively even out these disparities.
The National Education Association discusses the problem of inequality in the educational system of the United States. Read its proposed solution and decide whether you agree.
The economic strategy of using race-to-the-bottom tactics in order to compete with other states in attracting new business growth also carries a social cost. For example, workers’ safety and pay can suffer as workplace regulations are lifted, and the reduction in payroll taxes for employers has led a number of states to end up with underfunded unemployment insurance programs.Alexander Hertel-Fernandez. 2012. “Why U.S. Unemployment Insurance is in Financial Trouble,” February. http://www.scholarsstrategynetwork.org/sites/default/files/ssn_basic_facts_hertel-fernandez_on_unemployment_insurance_financing.pdf Nineteen states have also opted not to cover more of their residents under Medicaid, as encouraged by the Patient Protection and Affordable Care Act in 2010, for fear it will raise state public spending and increase employers’ cost of employee benefits, despite provisions that the federal government will pick up nearly all cost of the expansion.Matt Broaddus and January Angeles. 2012. “Federal Government Will Pick Up Nearly All Costs of Health Reform’s Medicaid Expansion,” March 28. http://www.cbpp.org/research/federal-government-will-pick-up-nearly-all-costs-of-health-reforms-medicaid-expansion More than half of these states are in the South.
The federal design of our Constitution and the system of checks and balances has jeopardized or outright blocked federal responses to important national issues. President Roosevelt’s efforts to combat the scourge of the Great Depression were initially struck down by the Supreme Court. More recently, President Obama’s effort to make health insurance accessible to more Americans under the Affordable Care Act immediately ran into legal challengesNational Federation of Independent Business v. Sebelius, 567 U.S. __ (2012). from some states, but it has been supported by the Supreme Court so far. However, the federal government’s ability to defend the voting rights of citizens suffered a major setback when the Supreme Court in 2013 struck down a key provision of the Voting Rights Act of 1965.Shelby County v. Holder, 570 U.S. __ (2013). No longer are the nine states with histories of racial discrimination in their voting processes required to submit plans for changes to the federal government for approval.
The benefits of federalism are that it can encourage political participation, give states an incentive to engage in policy innovation, and accommodate diverse viewpoints across the country. The disadvantages are that it can set off a race to the bottom among states, cause cross-state economic and social disparities, and obstruct federal efforts to address national problems.
Which of the following is not a benefit of federalism?
- Federalism promotes political participation.
- Federalism encourages economic equality across the country.
- Federalism provides for multiple levels of government action.
- Federalism accommodates a diversity of opinion.
Hint:
B
Describe the advantages of federalism.
Describe the disadvantages of federalism.
Hint:
Federalism can trigger a race to the bottom, leading states to reduce workplace regulations and social benefits for employees; it can obstruct federal efforts to address national problems; and it can deepen economic and social disparities among states.
Describe the primary differences in the role of citizens in government among the federal, confederation, and unitary systems.
How have the political and economic relationships between the states and federal government evolved since the early 1800s?
Discuss how the federal government shapes the actions of state and local governments.
What are the merits and drawbacks of American federalism?
What do you see as the upcoming challenges to federalism in the next decade? Choose an issue and outline how the states and the federal government could respond.
Beer, Samuel H. 1998. To Make a Nation: The Rediscovery of American Federalism. Cambridge, MA: Harvard University Press.
Berry, Christopher R. 2009. Imperfect Union: Representation and Taxation in Multilevel Governments. New York: Cambridge University Press.
Derthick, Martha, ed. 1999. Dilemmas of Scale in America’s Federal Democracy. New York: Cambridge University Press.
Diamond, Martin. 1981. The Founding of the American Democratic Republic. Belmont, CA: Wadsworth Cengage Learning.
Elazar, Daniel J. 1992. Federal Systems of the World: A Handbook of Federal, Confederal and Autonomy Arrangements. Harlow, Essex: Longman Current Affairs.
Grodzins, Morton. 2004. “The Federal System.” In American Government Readings and Cases, ed. P. Woll. New York: Pearson Longman, 74–78.
LaCroix, Alison. 2011. The Ideological Origins of American Federalism. Cambridge, MA: Harvard University Press.
Orren, Karen, and Stephen Skowronek. 2004. The Search for American Political Development. New York: Cambridge University Press.
O’Toole, Laurence J., Jr., and Robert K. Christensen, eds. 2012. American Intergovernmental Relations: Foundations, Perspectives, and Issues. Thousand Oaks, CA: CQ Press.
Peterson, Paul E. 1995. The Price of Federalism. Washington, DC: Brookings Institution Press.
Watts, Ronald L. 1999. Comparing Federal Systems. 2nd ed. Kingston, Ontario: McGill-Queen’s University Press.
|
oercommons
|
2025-03-18T00:38:02.037835
|
07/10/2017
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15208/overview",
"title": "American Government, Students and the System, American Federalism, Advantages and Disadvantages of Federalism",
"author": null
}
|
https://oercommons.org/courseware/lesson/108137/overview
|
Primary Source Documents on the Pueblo Revolt
Overview
In summary, Primary Source Documents on the Pueblo Revolt, 1680 includes indigenous testimonies about the historic 1680 Pueblo Revolt in New Mexico, which was then under Spanish rule.
Overview
This document titled "Primary Source Documents on the Pueblo Revolt, 1680" provides valuable insights into the historic Pueblo Revolt that took place in 1680 in New Mexico, a region then under Spanish colonial rule. Led by the Pueblo holy man Popay, this uprising marked a significant turning point in Spanish imperial authority in North America. The document consists of testimonies from various indigenous individuals who were either willing participants or prisoners during the revolt. These testimonies shed light on the reasons for the rebellion, the role of Popay, the cultural and religious aspects of Pueblo life in the 17th century, and the Puebloans' vision for a world free of Spanish influence. These primary sources provide a unique window into the indigenous perspective of the events surrounding the Pueblo Revolt and offer valuable historical context for understanding this pivotal moment in North American history.
Primary Source Documents on the Pueblo Revolt, 1680
Primary Source Documents on the Pueblo Revolt, 1680
Introduction: In 1680 Spanish imperial authority in North America was shaken to its core by an uprising among Pueblo Indians in New Mexico, one of the northernmost provinces of the Viceroyalty of New Spain. The leader of the revolt was a Pueblo holy man named Popay (spelled “Popé” in the following excerpts from the historical interviews) who managed to convince nearly all of the pueblos in New Mexico to participate in a coordinated uprising to expel the Spanish from their land.
The revolt came at the tail end of a tumultuous decade for Spanish New Mexico. In the 1670’s the province found itself afflicted by severe drought, accompanied by famine, and escalating raids by bands of hostile Apache warriors who carried off crops, livestock, and captives, which the Spanish authorities were seemingly powerless to stop. As living conditions in New Mexico deteriorated, popular unrest with Spanish administration seemed to escalate. Many Pueblos, though nominal Catholics, began seeking solace in traditional religious practices. These included participating in Kachina dances, which were rituals designed to summon the assistance of “Kachinas” (spirit beings to whom worshipers can turn for assistance with worldly problems like sickness, drought, etc.), even though these had long been banned by New Mexico’s Catholic (mainly Franciscan) authorities.
In 1675, then governor Juan Francisco Treviño had forty-seven Pueblo medicine men arrested on charges of “sorcery” and stirring up trouble for the Spanish government. Four were sentenced to death, while the rest were whipped and released with a warning. One of the holy men who was whipped was Popay, who then relocated to Taos Pueblo and began laying plans for a revolt to drive out the hated Spaniards.
The uprising commenced in August of 1680, and within a few days hundreds of Spaniards, including roughly two thirds of the province’s Franciscan missionaries, were dead. New Mexico’s governor, Antonio de Otermín, found himself besieged at his residence in the provincial capital of Santa Fe, and had to order a hasty retreat out of the province with about two thousand followers, most of whom ended up gathering at El Paso del Norte (modern Juarez) in Mexico.
The following year, in November, 1681, Otermín would make a belated (and ultimately unsuccessful) attempt to invade and reconquer New Mexico. Most of the Puebloans adopted scorched earth tactics, abandoning their pueblos and fields and withdrawing northward rather than give battle or surrender. In January, 1682, aware that a large Puebloan force was gathering to attack him, Otermín once again retreated out of New Mexico, which remained free of Spanish control for more than a decade thereafter.
Instructions: During his 1691-82 expedition, Otermín took sworn testimony from various Pueblo people who came into his custody -- some willingly, and some captured -- which were compiled as part of regular dispatches to his superior, the Count of Paredes, Viceroy of New Spain. These testimonies represent some of the best written sources we have on the indigenous perspective of the Pueblo Revolt, including how it was carried out and their reasons for rebelling. As you read the following excerpts from their testimony, remember to ask yourself certain questions: What does the testimony of these witnesses reveal about the culture and values of the Pueblo peoples of the 17th century? What concerns did they have that led them to revolt against Spanish authority? Did effective leadership (from Popay and others) make a difference? What did their actions show about the type of world they wanted to create?
Historical Note: In the following deposition, the deponent, Juan, is described as a friendly native who had previously worked as a servant to one of the Spanish officers who accompanied Governor Antonio de Otermín on his expedition to attempt the reconquest of New Mexico in 1681. According to this deposition, Juan came to the Spanish camp willingly to provide them with intelligence.
Declaration [of the Indian, Juan. Place on the Rio del Norte, December 18, 1681][1]
“Having been questioned according to the tenor of the case, and asked for what reasons and causes all the Indians of the kingdom in general rebelled, returning to idolatry, forsaking the law of God and obedience to his Majesty … [Juan] said that what he knows concerning this question is that not all of them joined the said rebellion willingly; that the chief mover of it is an Indian who is a native of the pueblo of San Juan, named El Popé, and that from fear of this Indian all of them joined in the plot that he made. …”
“Asked why they held the said Popé in such fear and obeyed him … he said that the common report that circulated and still is current among all the natives is that the said Indian Popé talks with the devil, and for this reason all held him in terror, obeying his commands although they were contrary to the orders of the [Spanish authorities], he giving them to understand that the word he spoke was better than that of all the rest; and he states that it was a matter of common knowledge that the Indian Popé, talking with the devil, killed in his own house a son-in-law of his named Nicolás Bua, the governor of the pueblo of San Juan. On being asked why he killed him, he said that it was so that he might not warn the Spaniards of the rebellion, as he intended to do. …”
“Asked how the said Indian, Popé, convoked all the people of the kingdom so that they obeyed him in the treason, he said that he took a cord made of maguey fiber[2] and tied some knots in it which indicated the number of days until the perpetration of the treason. He sent it through all the pueblos as far as that of La Isleta, there remaining in the whole kingdom only the nation of the Piros who did not receive it; and the order which the said Popé gave when he sent the said cord was under strict charge of secrecy, commanding that the war captains take it from pueblo to pueblo. …”
“Asked to state and declare what things occurred after they found themselves without religious or Spaniards, he said ... following the departure of the señor governor and captain general, the religious[3], and the Spaniards ... the said Indian, Popé, came down in person ... proclaiming through the pueblos that the devil was very strong and much better than God, and that they should burn all the images and temples, rosaries and crosses, and that all the people should discard the names given them in holy baptism and call themselves whatever they liked. They should leave the wives whom they had taken in holy matrimony and take any one whom they might wish, and that they were not to mention in any manner the name of God, of the most holy Virgin, or of the Saints, on pain of severe punishment ... They were ordered likewise not to teach the Castilian language in any pueblo and to burn the seeds which the Spaniards sowed and to plant only maize and beans, which were the crops of their ancestors. ...”
“Asked whether they thought that perhaps the Spaniards would never return to this kingdom at any time, or that they would have to return as their ancestors did, and in this case what plans or dispositions they would make ... he said that they were of different minds regarding it, because some said that if the Spaniards should come they would have to fight to the death, and others said that in the end they must come and gain the kingdom because they were sons of the land and had grown up with the natives. ...”
Historical Note: In the following deposition, the deponent, Josephe, is described as an Indian prisoner, roughly 20 years of age, who had recently been captured by Otermín and his men. He was a servant of one of the Spanish officers who accompanied Governor Antonio de Otermín on his expedition to attempt the reconquest of New Mexico in 1681. He could also speak Spanish, and thus did not need the aid of an interpreter.
Declaration of Josephe, Spanish-Speaking Indian. [Place of the Rio del Norte, December 19, 1681.]
“Being asked why he fled from his master, the said Sargento Mayor Sebastían de Herrera, and went to live with the treacherous Indian apostates of New Mexico ... he said that the reason why he left was that he was suffering hunger in the plaza de armas of La Toma [del Rio del Norte], and a companion of his named Domingo urged this declarant to go to New Mexico for a while, so as to find out how matters stood with the Indians and to give warning to the Spaniard of any treason. ...”
“Asked what causes or motives the said Indian rebels had for renouncing the law of God and obedience to his Majestoy, and for committing so many kinds of crimes ... he said that the prime movers of the rebellion were two Indians of San Juan, one named El Popé and the other El Taqu, and another from Taos named Saca, and another from San Ildefonso named Francisco. He knows that these were the principals, and the causes they gave were alleged ill treatment and injuries received from the present secretary, Francisco Xavier, and the maestre de campo, Alonso García, and from the sargentos mayores, Luis de Quintana and Diego López, because they beat them, took away what they had, and made them work without pay. ...”
“Asked if he has learned ... why the apostates burned the images, churches, and things pertaining to divine worship, making a mockery and a trophy of them, killing the priests and doing the other things they did, he said ... while they were besieging the villa the rebellious traitors burned the church and shouted in loud voices, ‘Now the God of the Spaniards, who was their father, is dead, and Santa María, who was their mother, and the saints, were rotten pieces of wood,’ saying that only their own god lived. Thus they ordered all the temples and images, crosses and rosaries burned, and this function being over, they all went to bathe in the rivers, saying that they thereby washed away the water of baptism. For their churches, they placed on the four sides and in the center of the plaza some small circular enclosures of stone where they went to offer flour, feathers, and the seed of maguey, maize, and tobacco, and performed other superstitious rites, giving the children to understand that they must all do this in the future. The captains and chiefs ordered that the names of Jesus and Mary should nowhere be uttered, and that they should discard their baptismal names, and abandon the wives whom God had given them in matrimony ... they ordered that all the estufas erected, which are their houses of idolatry, and danced throughout the kingdam the dance of the cazina[4], making man masks for it in the image of the devil.”
“Asked what plans or information the said apostates communicated with regard to the possible return of the Spaniards ... he said that it is true that there were various opinions among them, most of them believing that they would have to fight to the death with the said Spaniards, keeping them out. Ohers, who were not so guilty, said, ‘We are not to blame, and we must await them [the Spaniards] in our pueblos.’ And he said that when the hostile Apaches came they denounced the leaders of the rebellion, saying that when the Spaniards were among they they lived in security and quiet, and afterwards with much uneasiness.”
[Josephe goes on to describe a plot by one of the ringleaders of the revolt to use their young women to catch the Spanish with their guard down, should they return to New Mexico.]
“[A]nother Indian named Alonso Catití, a leader of the uprising ... sent to notify the people that he had already planned to deceive the Spaniards with feigned peace. He had arranged to send to the pueblo of Cochití all the prettiest, most pleasing, and neatest Indian women so that, under pretense of coming down to prepare food for the Spaniards, they could provoke them to lewdness, and that night while they were with the, the said coyote Catití would come down with all the men of the Queres and Jemez nations, only the said Catití attempting to speak with the said Spaniards, and at a shout from him they would all rush down to kill the said Spaniards; and he gave orders that all the rest who were in the other junta ... whould at the same time attack the horse drove, so as to finish that too. ...”
Historical Note: In the following deposition, the deponent, Lucas, is described as an Indian prisoner of the Piro nation, around twenty years of age, and a native of the pueblo of Socorro.
Declaration [of Lucas, Piro Indian. Place of the Rio del Norte, December 19, 1681].
“Asked whether he knows ... the reason why the Indians of this kingdom in general rebelled, forsaking the law of God and renouncing obedience to his Majesty ... he said that of everything contained in the question he knows only that the temples and images, crosses, and rosaries were burned generally by all the Indians of the districts, and he also heard it said that each one was to live according to such law as he wished, forsaking that of the Spaniards, which was not good, and that these commands came from the jurisdictions above here; he does not know who gave them ...”
“Asked if he knows ... that the said apostates have erected houses of idolatry which they call estufas in the pueblos, and have practiced dances and superstitions, he said there is a general report throughout the kingdom that they have done so and he has seen many houses of idolatry which they have built, dancing the dance of the cachina, which this declarant has also danced. ...”
“Asked for what reason the Indian natives have abandoned the pueblos, gathering in the sierras, and what it is that the chiefs and the rest of the people alike are discussing, he said ... he knows only that they say all must fight with the Spaniards to the death ...”
Historical Note: In the following deposition, the deponent, Pedro Naranjo, is described as an Indian prisoner of the Queres nation, around eighty years of age, and a native of the pueblo of San Felipe, who had recently been captured by the Spanish. He could speak Spanish, and thus did not require the help of an interpreter.
Declaration of Pedro Naranjo of the Queres Nation. [Place of the Rio del Norte, December 19, 1681.]
“Asked whether he knows the reason or motives which the Indians of this kingdom had for rebelling, forsaking the law of God and obedience to his Majesty, and committing such grave and atrocious crimes ... he said that since the government of Señor General Hernando Ugarte y la Concha they have planned to rebel on various occasions through the conspiracies of Indian sorcerers, and that although in some pueblos the messages were accepted, in other parts they would not agree to it … but they always kept in their hearts the desire to carry it out, so as to live as they are living today. Finally, in the past years, at the summons of an Indian named Popé who is said to have communication with the devil, it happened that in an estufa of the pueblo of Los Taos there appeared to the said Popé three figures of Indians who never came out of the estufa. … He saw these figures emit fire from all the extremities of their bodies, and that one of them was called Caudi, another Tilini, and the other Tleume; and these three beings … told him to make a cord of maguey fiber and tie some knots in it which would signify the number of days they must wait before the rebellion. He said that the cord was passed through all the pueblos of the kingdom so that the ones which agreed to it [the rebellion] might untie one knot in sign of obedience, and by the other knots they would know the days which were lacking …”
“Everything being thus arranged, two days before the time set of its execution, because his lordship had learned of it and had imprisoned two Indian accomplices from the pueblo of Tesuque, it was carried out prematurely that night … and they killed religious, Spaniards, women, and children. …”
“Finally, the Señor governor and those who were with him escaped from the siege, and later this declarant saw that as soon as the Spaniards had left the kingdom an order came from the said Indian, Popé, in which he commanded all the Indians to break the lands and enlarge their cultivated fields, saying that now they were as they had been in ancient times, free from the labor they had performed for the religious and the Spaniards, who could not now be alive. He said that this is the legitimate cause and the reason they had for rebelling, because they had always desired to live as they had when they came out of the lake of Copola.[5] …”
“Asked for what reason they so blindly burned the images, temples, crosses, and other things of divine worship, he stated that the said Indian, Popé, came down in person, and with him El Saca and El Chato from the pueblo of Los Taos… and he ordered in all the pueblos through which he passed that they instantly break up and burn the images of the holy Christ, the Virgin Mary and the other saints, the crosses, and everything pertaining to Christianity, and that they burn the temples, break up the bells, and separate from the wives whom God had given them in marriage and take those whom they desired. In order to take away their baptismal names, the water, and the holy oils, they were to plunge into the rivers and wash themselves with amole, which is a root native to the country … with the understanding that there would thus be taken from them the character of the holy sacraments. …”
“These things were observed and obeyed by all except some who, moved by the zeal of Christians, opposed it, and such persons the said Popé caused to be killed immediately. He saw to it that they at once erected and rebuilt their houses of idolatry which they call estufas, and made very ugly masks in immigration of the devil in order to dance the dance of the cacina ...”
“Asked what arrangements and plans they had made for the contingency of the Spaniards return, he said that what he knows concerning the question is that they were always saying they would have to fight to the death, for they do not wish to live in any other way than they are living at present; and the demons in the estufa of Taos had given them to understand that as soon as the Spaniards began to move toward this kingdom they would warn them so that they might unite and none of them would be caught.”
Historical Note: In the following deposition, the deponent, who gives his name as Alonso Attuzayo, is described as an “old Indian,” a widower of around sixty years of age, and a native of the pueblo Almeda. During Governor Otermín’s unsuccessful expedition to reconquer New Mexico in 1681, Alonso apparently came to the Spanish camp willingly, in the company of his two grandsons whom he was taking care of, only to flee a few days later. When captured, he admitted his intention of going with his grandsons to live among the “apostate Indians of the kingdom.”
Declaration of an Indian. [House of Captain Francisco de Ortega, December 27, 1681.]
“His lordship asked him why, being restored to the church, absolved, and free, he had committed ... such a serious crime as returning to apostasy, taking wth him his said grandchildren and delivering them over to the apostates so that their souls and his own would be lost. He replied that he knows now that he did wrong, but that the devil deceived him and turned his heard and therefore he committed that folly. Being asked to say truly, before God, what cause of motive he had for forsaking the law of God and returning to apostasy, he said that it is true that he though the lfie the apostates led, living as they liked, was better than life among the Spaniards ...”
Historical Note: The following deposition, the deponent, who gives his name as Juan, is described as a very elderly widower from the pueblo of Alameda. The deposition implausibly suggests he was over a hundred years old because he said he remembered when the Spanish first arrived in New Mexico.
Declaration of an Indian. [House of Captain Francisco de Ortega, December 27, 1681.]
“Asked to state and declare truthfully what reasons or motives the natives of this kingdom had for rebelling, he said that he does not know, nor has he heard an reason given. Asked why they killed religious and Spaniards and burned the church and all the houses ... he said that to him, he being so old, they never communicated anything; that the most he knew ... was that when they committed this destruction it was by order of an Indian from San Juan [Popé] whom he does not know, who [ordered] them to burn the churches, convents, holy crosses, and every object pertaining to Christianity; and that they separate from the wives the religious had given them in marriage and take those whom they wished. ...”
“He said that this Indian of San Juan told and gave all the people to understand in the pueblos where he went that they should do as he said because they would thereby be assured of harvesing much maize, cotton, and an abundance of all crops, and better ones than every, and that they would live with great ease. The people have remained very well content and pleased with all this until now, when they have experieced the contrary, and have seen that the deceived them, for as a matter of fact they have had very small harvests, there has been no rain, and everyone is perishing. ...”
Historical Note: The following deposition, the deponent, Jerónimo, is described as a Tigua Indian of about sixty years of age from the pueblo of Puaray, and a gardener by profession. According to the deposition, he professed to be a Christian and came to the Spanish camp willingly because he said he was “weary of the bad life that he had led among the said apostates.”[6] He also claims that he made his way to the Spanish camp to warn Otermín’s forces of rebel intentions to attack their camp and drive away their horses.
Declaration [of Jerónimo, a Tigua Indian. Place opposite La Isleta, January 1, 1682].
“Asked to state and declare what else he know about the apostate traitors, and why they rebelled ... he said that what has come to his notice is that the said rebellion was motivated by the Taos Indians and another from San Juan, named Popé, whom they regarded as a great sorcerer, and that all the pueblos were summoned to take part ...”
“[Popé] was the one who made them kill priests and Spaniards, together with their women and children, and burn images and churches, and cease living with the wives to whom they were married, leaving them and taking others, and he caused them to wash their heads in order to take away the water of baptism, so that they might be as they had been in ancient times; and he told them that they would gather large crops of grain, maize with large and thick ears, many bundles of cotton, many calabashes and watermelons, and everything else in proportion. Today they are happy without religious or Spaniards ...”
[1] This excerpt, like all of the following excerpts, come from original Spanish archival documents compiled by historian Charles Wilson Hackett in Revolt of the Pueblo Indians of New Mexico and Otermín’s Attempted Reconquest, 1680-82 (Albuquerque, NM: New Mexico University of New Mexico Press, 1942). The translations were done by Hackett’s collaborator Charmion Clair Shelby.
[2] Fiber from the leaves of agave plants.
[3] The term “religious” is here used as a shorthand for Catholic clerics, almost exclusively Franciscan missionaries, who ran the religious life of New Mexico under Spanish authority.
[4] Probably a reference to “Kachinas,” which are spirit beings venerated in the religious traditions of Pueblo peoples of the Southwestern United States. Various spellings of this word appear in these readings.
[5] Probably a reference to a religious belief, current among many Pueblo peoples, that their ancestors emerged from a series of caves near a great body of water, which some scholars identify as Utah Lake near modern day Provo. See, this article from the Utah Historical Quarterly: https://issuu.com/utah10/docs/volume_20_1952/s/91249
[6] An “apostate” is one who rejects or renounces her/her religious beliefs or principles in favor of another or (in some cases) none at all.
|
oercommons
|
2025-03-18T00:38:02.103104
|
Linda Neff
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/108137/overview",
"title": "Primary Source Documents on the Pueblo Revolt",
"author": "Textbook"
}
|
https://oercommons.org/courseware/lesson/90472/overview
|
Numbers addition Video.
Simple Addition
Overview
Simple lesson regarding early childhood education, first grade simple addition math preperation.
Early Childhood Education, 1st Grade: Simple Addition
For this lesson, we will watch the video and students will have a chance to play the game that is offered as an additional resource to learning simple addition.
We will be learning addition up to the number 15 to begin this lesson.
By comparing numbers to items, such as what was provided in the video, students may feel more comfortable or familiar with the idea of adding. Such as, apples, bananas or even toys.
After going over examples and the video in class together, students will complete both worksheets independently or in small groups if additional help is needed to show their understanding of the lesson.
Following the worksheet, students will have a chance to " Color by Sum " with the derby horse picture shown below.
Once students have completed these worksheets, an assessment of their understanding will be completed to analyze how much additional teaching is needed as we continue building the foundation of simple addition.
|
oercommons
|
2025-03-18T00:38:02.123604
|
02/27/2022
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/90472/overview",
"title": "Simple Addition",
"author": "anna campbell"
}
|
https://oercommons.org/courseware/lesson/94766/overview
|
Angles_accessible
Area_accessible
Basic Probability (M1420)_accessible
Basic Statistics Terminology_accessible
Circles_accessible
Complements and Single Trial Conditional Probability (M1420)_accessible
Congruence, Similarity, Symmetry and Constructions_accessible
Congruent and Similar Triangles_accessible
Creating_a_MyOpenMath_Instructor_Account_oNds1ZG
Creating School Mascot_accessible
Data Types_accessible
Dimensional Analysis_accessible
Empirical Rule_accessible
Euclidean Geometry_accessible
Frequency Distributions, and Graphs_accessible
Logo Contest_accessible
MATH 1420 Algebraic Expressions and Equations_accessible
MATH 1420 Algebraic Expressions and Equations_accessible
MATH 1420 Expressions Activity_accessible
MATH 1420 Inequalities_accessible
MATH 1420 Inequalities_accessible
MATH 1420 Multi-step Equations_accessible
MATH 1420 Multi-step Equations_accessible
MATH 1420 One Step Equations_accessible
MATH 1420 One Step Equations_accessible
MATH 1420 Order of Operations_accessible
MATH 1420 Order of Operations_accessible
Math 1420 Reflections, Glide Reflections, and Dilations_accessible
Math 1420 Translations and Rotations_accessible
Measures of Center_accessible
Measures of Variance_accessible
Multiplication Rule (M1420)_accessible
Percentiles, Quartiles, and Boxplots_accessible
Perimeter and Circumference_accessible
Points, Lines, Planes, and Angles_accessible
Probability and Statistics Activity_accessible
Quadrilaterals_accessible
Scatterplots, Correlation, and Regression_accessible
Simple Curves_accessible
Surface Area_accessible
Translate Word Phrases to Equations_accessible
Translate Word Phrases to Equations_accessible
Triangles_accessible
Volume_accessible
MATH 1420: Geometry Concepts for Teachers
Overview
Course Objective: This course is an introduction to basic algebra; elements of probability and statistics; and basic concepts of Euclidean geometry, including congruence, similarity, measurements, areas, and volumes.
Getting Started
MyOpenMath Help Video Playlist
A YouTube playlist has been created to help instructors navigate MyOpenMath. A multitude of videos have been created to help walk new users through everything they need to use MyOpenMath in their courses. Access the playlist at:
https://youtube.com/playlist?list=PL4DaWQ8GB98Q0VLyE9QCmrb697U8UUc4T
Can’t find what you need? Email chambersjh@roanestate.edu to request additional video help.
Hello and welcome to our geometry concepts course developed for teachers. In this course, students will have focus on eight different units of study. The first three units will provide a focus in algebra review with an introduction to basic algebra, function exploration, and graphing figures. The following three units will focus on geometric topics including geometry basics, geometric figures, and dimensional geometry. The last two units provide students an opportunity to learn elements of probability and statistics.
Copying the MyOpenMath
Math 1420: Geometry Concepts for Teachers Template
To copy the MyOpenMath Math 1420: Geometry Concepts for Teachers course that is to be used in conjunction with the materials in the OER Commons, begin by visiting https://www.myopenmath.com/ and then following these three easy steps:
Step 1: Log in to MyOpenMath by typing in the username and password you selected when creating your MyOpenMath account.
Step 2: Once you are logged in, click the “Add a New Course” button under the section titled “Courses you’re teaching”.
Step 3: To copy the promoted course, Math 1420: Geometry Concepts for Teachers select “Copy a template or promoted course”. A box will open with all the available templates and promoted courses. Scroll down until you see Math 1420 Concepts for Teachers. To minimize the number of courses you have to scroll through, you can filter by level by selecting “Level” and checking Prealgebra, Elementary Algebra, Statistics, and Geometry.
Unit 1: Algebra Basics
Keys
MATH_1420_Order_of_Operations_Key_accessible.docx
MATH_1420_Algebraic_Expressions_and_Equations_Key_accessible.docx
MATH_1420_One_Step_Equations_Key_accessible.docx
MATH_1420_Multi-step_Equations_Key_accessible.docx
Topics List
- Order of Operations
- Algebraic Expressions and Equations
- One Step Equations
- Multi-step Equations
- Solving Linear Inequalities
- Translate Word Phrases in Equations
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
All equations have been rendered in EquatIO: https://www.texthelp.com/products/equatio/ .
Homework is available in the accompanying MyOpenMath course.
Unit 2: Exploring Functions
Keys
MATH_1420_Functions_Key_accessible.docx
Composition_of_Functions_Key_accessible.docx
Math_1420_Square_Root_Property_Key_accessible.docx
MATH_1420_Points_Slope_and_Intercepts_Key_accessible.docx
MATH_1420_Graphing_Linear_Equations_Key_accessible.docx
MATH_1420_Modeling_with_Linear_Functions_Key_accessible.docx
Topic List
- Functions
- Composition of Functions
- Square Root Property
- Points, Slope, and Intercepts
- Graphing Linear Equations
- Modeling Linear Functions
- Systems of Equations
- Sequences
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
All equations have been rendered in EquatIO: https://www.texthelp.com/products/equatio/ .
Homework is available in the accompanying MyOpenMath course.
Unit 3: Graphing Figures
Keys
Math_1420_Translations_and_Rotations_Key_accessible.docx
Math_1420_Reflections_Glide_Reflections_and_Dilations_Key_accessible.docx
Topic List
- Translations and Rotations
- Reflections, Glide Reflections, and Dilations
Unit 4: Basic Geometry
Topic List
- Points, Lines, Planes, and Angles
- Euclidean Geometry
- Angles
- Simple Curses
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
All equations have been rendered in EquatIO: https://www.texthelp.com/products/equatio/ .
Homework is available in the accompanying MyOpenMath course.
Unit 5: Geometric Figures
Keys
Congruent_and_Similar_Triangles_Key_accessible.docx
Quadrilaterals_key_accessible.docx
Congruence_Similarity_Symmetry_and_Constructions_Key_accessible.docx
Topic List
- Triangles
- Congruent and Similar Triangles
- Quadrilaterals
- Circles
- Congruence, Similarity, Symmetry, and Constructions
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
All equations have been rendered in EquatIO: https://www.texthelp.com/products/equatio/ .
Homework is available in the accompanying MyOpenMath course.
Unit 6: Dimensional Geometry
Keys
Dimensional_Analysis_Key_accessible.docx
Perimeter_and_Circumference_Key_accessible.docx
Topic List
- Dimensional Analysis
- Perimeter and Circumference
- Area
- Surface Area
- Volume
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
All equations have been rendered in EquatIO: https://www.texthelp.com/products/equatio/ .
Homework is available in the accompanying MyOpenMath course.
Unit 7: Probability
Keys
Basic_Probability_M1420_Key_accessible.docx
Complements_and_Single_Trial_Conditional_Probability_M1420_Key_accessible.docx
Topic List
- Basic Probability
- Complements and Single Trail Conditional Probability
- Addition Rule
- Multiplication Rule
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
All equations have been rendered in EquatIO: https://www.texthelp.com/products/equatio/ .
Homework is available in the accompanying MyOpenMath course.
Unit 8: Statistics
Keys
Basic_Statistics_Terminology_Key__accessible.docx
Data_Types_Key_accessible.docx
Frequency_Distributions_and_Graphs_Key_accessible.docx
Measures_of_Center_Key_accessible.docx
Measures_of_Variance_Key_accessible.docx
Empirical_Rule_Key_accessible.docx
Topic List
- Basic Statistic Terminology
- Data Types
- Frequency Distribution and Graphs
- Measures of Center
- Measure of Variance
- Empirical Rule
- Percentile, Quartiles, and Boxplots
- Scatterplots, Correlation, and Regression
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
All equations have been rendered in EquatIO: https://www.texthelp.com/products/equatio/ .
Homework is available in the accompanying MyOpenMath course.
Projects and Group Activities
Activity List
- Expression Activity
- Logo Contest Activity
- Creating a School Mascot Activity
- Statistics Activity
|
oercommons
|
2025-03-18T00:38:02.206966
|
Ashley Morgan
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/94766/overview",
"title": "MATH 1420: Geometry Concepts for Teachers",
"author": "Lecture Notes"
}
|
https://oercommons.org/courseware/lesson/73086/overview
|
Natural Asset Management
Overview
This course consists of 4 modules and is designed to help government, professionals in multiple disciplines and community organizations understand the fast-emerging field of natural asset management.
Natural Asset Management Fundamentals - Course Overview
Course Overview
"Natural Asset Management Fundamentals - Overview" by Roy Brooke, Adaptation Learning Network is licensed under CC BY 4.0 except where indicated. For external links to resources, review the rights and permission details. |
Welcome to this 4-week course: Natural Asset Management Fundamentals. I’m Roy Brooke and I'll be facilitating the course using videos, discussion forums, and other learning activities in each week/module. Throughout the course, you will also be meeting and interacting with many others who are involved in natural asset management.
We’ve designed this course to help local government staff, professionals in multiple disciplines who work with them, and people involved with community organizations, understand the fast-emerging field of natural asset management. The course is in divided into four modules:
- Introduction to the course, natural asset management & each other to help us understand: what natural asset management is, how it came about, and what is starting to happen at a community level.
The ‘enabling environment’ for natural asset management – what some of the provincial, regional and national opportunities and barriers are that can help or hinder local governments.
- A detailed look at the communities that pioneered natural asset management, why and how they’re doing it, and the results so far.
- How to bring natural asset management into your own work.
At the end of the course, participants will understand:
- What natural asset management is, why it matters, and what conditions enable or hinder it
- How natural asset management is relevant in your own disciplines or community contexts
- What you may be able to do differently as a result of knowing more about natural asset management
- Where you can get additional information on natural asset management
Each module includes one or more video-lectures and discussion forums that are designed to inform, prompt reflection, and help participants consider the relevance of the work to their own contexts. In the final module, participants will apply what they’ve learned in real-world, learner-relevant examples. Each module also contains resources (e.g., relevant reports, video links) in the Readings and Resources folder.
You are not required to engage in the learning activities at any specific time. However, we recommend that you complete the activities for each module within the corresponding week. This will help you maximize learning and provide opportunities for meaningful discussion with the instructor and your fellow learners.
Readings & Resources
This page is a compilation of all of the course readings and resources used in all Modules.
Module 1
Required Reading
MNAI Natural Asset Primer copyrighted
What are Municipal Natural Assets: Defining and Scoping Municipal Natural Assets (2019) copyrighted
Asset Management for Sustainable Service Delivery: A BC Framework
Optional Reading
FCM asset management resources
Summary report for the Americas:
Canada Infrastructure Report Card 2019
Integrating Natural Assets Into Asset Management: A Sustainable Service Delivery Primer
Advancing Natural Infrastructure in Canada (IISD)
Module 2
Required Reading
MNAI Cohort 1 Results – Summary copyrighted
MNAI Cohort 2 Results – Summary copyrighted
Module 3
Required
Public Sector Accounting Board Final Input copyrighted
Diagram on Natural Asset Management, Title and Jurisdiction copyrighted
Advancing natural asset management through collaborative strategies for private lands copyrighted
Optional
The Town of Gibsons experience on financial planning and reporting copyrighted
Course Schedule
Module Date Course Activities | ||
Course Introduction and Module 1 - What are natural assets and why do they matter? | Week 1 | Module 1 Activities Join instructor for a synchronous online meeting if you are available. Introduction to Padlet Post a video on Padlet introducing yourself and why you are taking the course Module 1 Course Content Module 1: Drop-in online session |
Module 2 - Who is doing what: Canadian case examples | Week 2 | Module 2 Activities Module 2 Course Content Module 2: Discussion Forum: a professional or community natural asset management issues you would like to address. |
Module 3 - Building a strong enabling environment | Week 3 | Module 3 Activities Module 3 Course Content Module 3: Discussion Forum: normative and/or professional barriers Module 3: Drop-in online session |
Module 4: Getting to scale | Week 4 | Module 4 Activities Module 4 Course Content Final synchronous discussion |
Natural Asset Management Fundamentals - Module 1 - Introduction to the course, municipal natural asset management & each other
Natural Asset Management Fundamentals - Module 1 - Introduction to the course, municipal natural asset management & each other
"Natural Asset Management Fundamentals - Module 1 - Introduction to the course, municipal natural asset management & each other” by Roy Brooke, Adaptation Learning Network is licensed under CC BY 4.0 except where indicated. For external links to resources, review the rights and permission details. |
Welcome to Module 1. In this Module, we will become familiar with the core concepts of natural asset management - what it is, how it is defined, what it is based on, and how it came to be. We will also take time to understand each others’ interests in natural asset management so that collectively, we can help advance our various journeys in this fast-evolving field.
Module 1 Learning Goals This Module will help you:
- Understand what natural assets are, and why they matter in Canadian communities
- Learn about the 'drivers' for natural asset management
- Learn how natural asset management emerged in Gibsons, BC
- Understand where and how asset management is starting to become an effective tool to help local governments deliver infrastructure services and become more resilient
Readings and Resources
Module 1
Required Reading
MNAI Natural Asset Primer copyrighted
What are Municipal Natural Assets: Defining and Scoping Municipal Natural Assets (2019) copyrighted
Asset Management for Sustainable Service Delivery: A BC Framework
Optional Reading
FCM asset management resources
Summary report for the Americas:
Canada Infrastructure Report Card 2019
Integrating Natural Assets Into Asset Management: A Sustainable Service Delivery Primer
Advancing Natural Infrastructure in Canada (IISD)
Module 1 Course Content
"Natural Asset Management Fundamentals - Module 1 - Introduction to the course, municipal natural asset management & each other” by Roy Brooke, Adaptation Learning Network is licensed under CC BY 4.0 except where indicated. For external links to resources, review the rights and permission details. |
Module 1 Overview
Module 1 consists of several, shorter sub-modules that cover the following topics:
- A synchronous meeting to launch the course
- Postings on Padlet to meet each other and understand our respective professional/community contexts
- Definitions of natural assets and their relevance to local government
- Where it all started: the Town of Gibsons
- An insurance sector perspective on climate change context as a driver for natural asset management
- First Nations and natural assets management
- Asset management 101
- Discussion forum on readiness for natural asset management.
Introductory story
This short (true) story contains many of the themes that will covered over the next 4 modules.
"Natural Asset Management Fundamentals - Module 1.1 Video by Roy Brooke, Adaptation Learning Network is licensed under CC BY 4.0 |
Module 1.1: What are natural assets and why do they matter?
Module 1.1 begins with a video lecture that explains what natural assets are; different ways to think about the services they provide; and, why they matter to Canadian communities.
"Natural Asset Management Fundamentals - Module 1.2 Video by Roy Brooke, Adaptation Learning Network is licensed under CC BY 4.0 |
Module 1.2: Where it all started: the Town of Gibsons
The Town of Gibsons, BC was the first community in Canada - and likely anywhere - to link the practice of modern asset management with the concept of ecosystem services. In this video you will learn more about the Town, what they have done, and why.
Introduction from Emanuel Machado, Chief Administrative Officer, Town of Gibsons, and Chair, Municipal Natural Assets Initiative.
Module 1.2 Video
"Natural Asset Management Fundamentals - Module 1.3 Video by Roy Brooke, Adaptation Learning Network is licensed under CC BY 4.0 |
OPTIONAL:
Module 1.3: An insurance sector perspective on climate change context as a driver for natural asset management
In this video we will learn more about the insurance sector's perspective from Natalia Moudrak, Director of Resilience at the Intact Center on Climate Adaptation.
"Natural Asset Management Fundamentals - Module 1.3 Video by Roy Brooke, Adaptation Learning Network is licensed under CC BY 4.0 |
Module 1.4: A First Nations perspective on natural assets
"Natural Asset Management Fundamentals - Module 1.4 Video by Roy Brooke, Adaptation Learning Network is licensed under CC BY 4.0 |
Module 1.5: An introduction to asset management
Asset management is an integrated process to help local governments make informed decisions about their assets and provide sustainable service delivery. In these videos, you will learn more about asset management and how it is helping communities manage natural as well as engineered assets.
Brief introduction. In this video we will discuss the challenge of putting the concept ecosystem services into practice, and how asset management as a modern, structured process is helping to address that challenge.
"Natural Asset Management Fundamentals - Module 1.5 Intro Video by Roy Brooke, Adaptation Learning Network is licensed under CC BY 4.0 |
Asset management 101: Part 1. What is asset management video
"Natural Asset Management Fundamentals - Module 1.5 Part 1 What is asset management Video by Roy Brooke, Adaptation Learning Network is licensed under CC BY 4.0 |
Asset management 101: Part 2. Asset management as an ongoing decision-making process Video
"Natural Asset Management Fundamentals - Module 1.5 Part 2. Asset management as an ongoing decision making process Video by Roy Brooke, Adaptation Learning Network is licensed under CC BY 4.0 |
Module 1 Discussion
Please share your insights on the natural assets that are relevant in your community, and who benefits from them. Please consider, and also review the answers of your fellow learners to help them consider, the following:
Prompts:
What are the natural assets in your community?
What services do they provide? Who benefits from the services? Are they managed for these services?
Post your thoughts to the discussion forum titled "Natural assets in your community".
Natural Asset Management Fundamentals - Module 2 - Natural asset management on the ground
Module 2 - Natural asset management on the ground
"Natural Asset Management Fundamentals – Module 2 – Natural asset management on the ground by Roy Brooke, Adaptation Learning Network is licensed under CC BY 4.0 except where indicated. For external links to resources, review the rights and permission details. |
Welcome to Module 2. The focus of this module is on the communities that have been leading the way on natural asset management. We will learn about how the original work in Gibsons, BC has spread to communities in 4 provinces; what the results have been so far, and why.
Module 2 Learning Goals
This Module will help you:
Understand the natural asset management methodology in more detail, including some of the current and upcoming tools that can be applied in communities
Consider the results of projects completed in sone of the main communities to date
Readings and Resources
Module 2
Required Reading
MNAI Cohort 1 Results – Summary
MNAI Cohort 2 Results – Summary
Optional Reading
Check out the blog and report from recent work in Northwest New Brunswick
At this link you can watch a webinar with the Fraser Basin Council on the results to date of the Comox Lake Watershed Project.
Module 2 Overview
Module 2 explores how the original work in Gibsons, BC, is beginning to getting refined, replicated, and scaled-up in communities across Canada. Topics include:
- An introduction video
- A review of MNAI Cohort 1 and Cohort 2 projects
- Two communities in New Brunswick that have completed MNAI projects
- A watershed-level initiative underway on Vancouver Island, BC
- The development and piloting of a tool to integrate First Nations knowledge about natural assets and cultural assets into asset management
- The process for developing natural asset inventories
- Efforts to expand the MNAI asset management method to coastal zone assets
- An approach to help local governments optimize their asset management to also consider species at risk and critical habitat.
Module 2.0 Overview of the stormwater projects and methods
This first video describes early efforts to refine and replicate Gibsons' work.
"Natural Asset Management Fundamentals - Module 2.0 Video by Roy Brooke, Adaptation Learning Network is licensed under CC BY 4.0 |
Next, we will hear from Michelle Molnar, MNAI's Technical Director, about what happened in the early project cohorts, and what exactly is being measured when we speak of 'nature's value'.
"Natural Asset Management Fundamentals - Module 2.0a Video by Roy Brooke, Adaptation Learning Network is licensed under CC BY 4.0 |
Your reading resources for the week contain summary documents as well as full technical reports of the 11 of these completed projects.
Module 2.1: connecting with the communities
Now, we will hear directly from some of the local governments who are leading natural asset management projects.
First is James Bornemann from the Southeast Regional Service Commission in New Brunswick who will describe recently completed projects in Riverview and Riverside-Albert, N.B.
"Natural Asset Management Fundamentals - Module 2.1 Video by Roy Brooke, Adaptation Learning Network is licensed under CC BY 4.0 |
Second is Kris LaRose, from the Cowichan Valley Regional District who will explain efforts to manage natural assets in a multi- use, multi-owner watershed on Vancouver Island.
"Natural Asset Management Fundamentals - Module 2.1a Video by Roy Brooke, Adaptation Learning Network is licensed under CC BY 4.0 |
Module 2.2: Integrating First Nations knowledge into asset management
We have discussed already the importance of First Nations knowledge to community resilience. In this segment, we will introduce a new effort to develop and pilot a replicable tool integrate First Nations knowledge of natural and cultural assets into natural asset management. The tool is still in development stages but is nevertheless promising and interesting to know about.
"Natural Asset Management Fundamentals - Module 2.2 Video by Roy Brooke, Adaptation Learning Network is licensed under CC BY 4.0 |
Module 2.3: Taking a plunge with natural asset inventories
Building a natural asset inventory is a first critical step in the assessment phase of natural asset management.
In this module we will discuss different types of inventories, and then "look under the hood" at the asset registry and the dashboard tools that can help make that data insightful and useful as possible for local governments.
Module 2.4: Expanding the methodology
Until recently, MNAI's methodology focussed almost exclusively on stormwater management and surface water quality issues. Now, the methodology is being expanded to include coastal zone issues in two pilot communities. In this segment we will hear from Cedar Morton, Senior Systems Ecologist with ESSA Technologist, who is working with the MNAI technical team on this project.
"Natural Asset Management Fundamentals - Module 2.4 Video by Roy Brooke, Adaptation Learning Network is licensed under CC BY 4.0 |
Module 2.5: Expanding the toolkit: endangered species
MNAI's methodology contains a range of different tools that can be configured for local community contexts.
In this segment we will hear from Tim Ennis, a conservation biologist who lives in the Comox Valley, BC and is working with MNAI to develop develop and pilot a tool that local governments will be able to use to consider species at risk and their critical habitat when they are undertaking natural asset management.
"Natural Asset Management Fundamentals - Module 2.5 Video by Roy Brooke, Adaptation Learning Network is licensed under CC BY 4.0 |
Module 2 Discussion
Consider what you have learned about the projects underway now across Canada. Which examples resonate, and why, and which do not?
Prompts:
Do any of the case examples seem relevant in your community/work?
Do any of the case examples have gaps that would reduce their impact?
If you were to refine and and replicate one in your community, which would it be?
Natural Asset Management Fundamentals - Module 3 - The enabling environment for natural asset management
Module 3 - The enabling environment for natural asset management
"Natural Asset Management Fundamentals – Module 3 – The enabling environment for natural asset management by Roy Brooke, Adaptation Learning Network is licensed under CC BY 4.0 except where indicated. For external links to resources, review the rights and permission details. |
Local governments do not operate in a vacuum and there are many factors that affect their ability to undertake natural asset management. The regulatory environment, financial frameworks, and multi-jurisdictional nature of land ownership mean that multiple actors will influence local government's ability to manage natural assets for municipal service delivery. This ‘enabling environment’ becomes a critical consideration if the ultimate goal is to have Canada’s nearly 4000 local governments all undertaking natural asset management, not just a handful of leaders. In this module we will discuss this context, with particular focus on opportunities and barriers to accelerating the uptake of natural asset management.
Module 3 Learning Goals
This Module will help you:
Understand the challenges and opportunities related to scaling up natural asset management in Canada Explore activities to create, clarify or strengthen norms and standards for natural asset management.
Readings and Resources
Module 3
Required
Public Sector Accounting Board Final Input
Diagram on Natural Asset Management, Title and Jurisdiction
Advancing natural asset management through collaborative strategies for private lands
Optional
The Town of Gibsons experience on financial planning and reporting
Module 3: The enabling environment
In this module we will discuss the issues, opportunities and barriers to making natural asset management a mainstream practice in Canada. Topics include:
- Ways to think about the challenge of achieving scale
- A new project to create standards for engineers, and other professions A perspective from the insurance sector
- Non-municipal land
- Financial planning and reporting for natural asset management
Module 3.1: The challenge of scale
On the positive side, it is really encouraging that Gibsons example is starting to get refined and replicated now in 5 provinces and over 20 communities. On a more sobering note, there are around 3700 local governments in Canada, so there is a long way to go before natural asset management gets anywhere close to a mainstream practice.
In an overview video presentation we will discuss some of the issues involved in scaling up natural asset management.
"Natural Asset Management Fundamentals - Module 3.1 Video by Roy Brooke, Adaptation Learning Network is licensed under CC BY 4.0 |
Module 3.2: Towards engineering norms
A strong enabling environment for municipal natural asset management requires, amongst other things, professional norms that encourage and guide the practice.
In this segment we will hear from Stuart Nash of the Engineers and Geoscientists of British Columbia about a project that will result in the first professional standards anywhere to explicitly recognize and guide natural asset management.
"Natural Asset Management Fundamentals - Module 3.2 Video by Roy Brooke, Adaptation Learning Network is licensed under CC BY 4.0 |
Module 3.3 Thinking about non-municipal land
A common question is whether natural asset management only concerns municipally owned land. The answer is no - and this video explains why.
"Natural Asset Management Fundamentals - Module 3.3 Video by Roy Brooke, Adaptation Learning Network is licensed under CC BY 4.0 |
Module 3.4: Financial planning and reporting
Much can be said about financial planning and reporting for natural assets. One key lesson is that accounting rules do not pose quite the barrier that one might imagine. This video explains more.
"Natural Asset Management Fundamentals - Module 3.4 Video by Roy Brooke, Adaptation Learning Network is licensed under CC BY 4.0 |
Module 3.5: planning and development case example [optional]
If you would like to learn a little more about how the Town of Gibsons is handling planning and development with natural assets in mind, then do listen to this interview with the Town's Chief Administrative Office.
"Natural Asset Management Fundamentals - Module 3.5 Video by Roy Brooke, Adaptation Learning Network is licensed under CC BY 4.0 |
Natural Asset Management Fundamentals - Module 4 - So what?
Module 4 - So what?
"Natural Asset Management Fundamentals – Module 4 – So What? by Roy Brooke, Adaptation Learning Network is licensed under CC BY 4.0 except where indicated. For external links to resources, review the rights and permission details. |
Welcome to Module 4. Some of you work in local government. Others work in professions or organizations that support local governments and could help them advance natural asset management. All of you live in communities that in some way are already relying on natural assets for core services.
In this module we will dig into your individual and group contexts to answer the question - "what could I being doing differently tomorrow than I am doing today, on natural asset management?"
Module 4 Learning Goals
Module 4 is an opportunity to reflect on how Modules 1-3 relate to your own community and professional contexts, synthesize this into project template that is similar to the project documents that underpin all MNAI projects, discuss it in a team format, and then share your ideas in plenary. The completed project template may just turn out to be the basis for Canada's next natural asset management project...
Module 4 Activity
Module 4 integrates everything we have learned to date into what may well be foundation for the next terrific Canadian natural asset project. It will help you to develop a clear picture of a set of natural asset services, risks, condition and potential strategies in your own community.
Here are the steps and suggested schedule.
Step 1 – Monday & Tuesday (or before): Individual work. Download the worksheet at this link. Thinking about your own community context, consider everything you have learned, complete the worksheet.
Step 2 –Wednesday & Thursday: group work. Post your completed worksheet to the appropriate team discussion forum titled "Activity 4 Step 2 – Wednesday & Thursday: group work".The list of teams is below. Then, within your teams, perform a peer review function on each others’ documents so that each person benefits from the insights of every other person on the team. As suggestions, tell each team member (a) what you like best about their worksheet (b) one thing you learned from their worksheet(c) one thing you would suggest they consider adding to the project (d) one barrier they might face and a strategy for overcoming it.
Feel free to self-organize a Zoom call with your teammates.
Step 3: Thursday and Friday. Share group work. The final step is to share the results of your group work to fellow learners in the Week Four Discussion Forum titled "Sharing Group Work". Your team can do this in a number of ways. For example, you can: select one worksheet that you feel is especially promising, and share this with the whole class and explain why the project is so promising; summarize your projects; or pick one element of each worksheet that is compelling and share this.
Step 4: Thursday and Friday. Review each others’ group work. The final step is to review and comment on each other groups work.
Finally, celebrate your work and kindly complete the course evaluation.
Teams
Team Members Team Discussion Forum for Step 2 | |||
Red Red Team Activity 4 Step 2 – Wednesday & Thursday: group work | |||
Blue Blue Team Activity 4 Step 2 – Wednesday & Thursday: group work | |||
Green | Green Team Activity 4 Step 2 – | ||
Wednesday & Thursday: group work | |||
Yellow Yellow Team Activity 4 Step 2 – Wednesday & Thursday: group work | |||
Red Team Activity 4 Step 2 – Wednesday & Thursday: group work
Step 2 –Wednesday & Thursday: group work. Post your completed worksheet to the appropriate team discussion forum titled "Activity 4 Step 2 – Wednesday & Thursday: group work".Then, within your teams, perform a peer review function on each others’ documents so that each person benefits from the insights of every other person on the team. As suggestions, tell each team member (a) what you like best about their worksheet (b) one thing you learned from their worksheet(c) one thing you would suggest they consider adding to the project (d) one barrier they might face and a strategy for overcoming it.
Blue Team Activity 4 Step 2 – Wednesday & Thursday: group work
Step 2 –Wednesday & Thursday: group work. Post your completed worksheet to the appropriate team discussion forum titled "Activity 4 Step 2 – Wednesday & Thursday: group work". Then, within your teams, perform a peer review function on each others’ documents so that each person benefits from the insights of every other person on the team. As suggestions, tell each team member (a) what you like best about their worksheet (b) one thing you learned from their worksheet(c) one thing you would suggest they consider adding to the project (d) one barrier they might face and a strategy for overcoming it.
Green Team Activity 4 Step 2 – Wednesday & Thursday: group work
Step 2 –Wednesday & Thursday: group work. Post your completed worksheet to the appropriate team discussion forum titled "Activity 4 Step 2 – Wednesday & Thursday: group work". Then, within your teams, perform a peer review function on each others’ documents so that each person benefits from the insights of every other person on the team. As suggestions, tell each team member (a) what you like best about their worksheet (b) one thing you learned from their worksheet(c) one thing you would suggest they consider adding to the project (d) one barrier they might face and a strategy for overcoming it.
Yellow Team Activity 4 Step 2 – Wednesday & Thursday: group work
Step 2 –Wednesday & Thursday: group work. Post your completed worksheet to the appropriate team discussion forum titled "Activity 4 Step 2 – Wednesday & Thursday: group work". Then, within your teams, perform a peer review function on each others’ documents so that each person benefits from the insights of every other person on the team. As suggestions, tell each team member (a) what you like best about their worksheet (b) one thing you learned from their worksheet(c) one thing you would suggest they consider adding to the project (d) one barrier they might face and a strategy for overcoming it.
Module 4 Discussion Forum: Sharing Group Work
Liked Your Course? Join the Adaptation Learning Network
Stay in touch
To find out about upcoming courses, events, and adaptation news. You can keep in touch with us on:
We want to hear from you
The Adaptation Learning Network is committed to building a community of learners that exchange ideas across fields to better prepare for the effects of the climate crisis. If you have a story about climate adaptation that you want to share, please get in touch. We produce a podcast and original web content to amplify inspiring stories of climate action.
|
oercommons
|
2025-03-18T00:38:02.503678
|
Full Course
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/73086/overview",
"title": "Natural Asset Management",
"author": "Case Study"
}
|
https://oercommons.org/courseware/lesson/15204/overview
|
The Division of Powers
Learning Objectives
By the end of this section, you will be able to:
- Explain the concept of federalism
- Discuss the constitutional logic of federalism
- Identify the powers and responsibilities of federal, state, and local governments
Modern democracies divide governmental power in two general ways; some, like the United States, use a combination of both structures. The first and more common mechanism shares power among three branches of government—the legislature, the executive, and the judiciary. The second, federalism, apportions power between two levels of government: national and subnational. In the United States, the term federal government refers to the government at the national level, while the term states means governments at the subnational level.
FEDERALISM DEFINED AND CONTRASTED
Federalism is an institutional arrangement that creates two relatively autonomous levels of government, each possessing the capacity to act directly on behalf of the people with the authority granted to it by the national constitution.See John Kincaid. 1975. “Federalism.” In Civitas: A Framework for Civil Education, eds. Charles Quigley and Charles Bahmueller. Calabasas, CA: Center for Civic Education, 391–392; William S. Riker. 1975. “Federalism.” In Handbook of Political Science, eds. Fred Greenstein and Nelson Polsby. Reading, MA: Addison-Wesley, 93–172. Although today’s federal systems vary in design, five structural characteristics are common to the United States and other federal systems around the world, including Germany and Mexico.
First, all federal systems establish two levels of government, with both levels being elected by the people and each level assigned different functions. The national government is responsible for handling matters that affect the country as a whole, for example, defending the nation against foreign threats and promoting national economic prosperity. Subnational, or state governments, are responsible for matters that lie within their regions, which include ensuring the well-being of their people by administering education, health care, public safety, and other public services. By definition, a system like this requires that different levels of government cooperate, because the institutions at each level form an interacting network. In the U.S. federal system, all national matters are handled by the federal government, which is led by the president and members of Congress, all of whom are elected by voters across the country. All matters at the subnational level are the responsibility of the fifty states, each headed by an elected governor and legislature. Thus, there is a separation of functions between the federal and state governments, and voters choose the leader at each level.Garry Willis, ed. 1982. The Federalist Papers by Alexander Hamilton, James Madison and John Jay. New York: Bantam Books, 237.
The second characteristic common to all federal systems is a written national constitution that cannot be changed without the substantial consent of subnational governments. In the American federal system, the twenty-seven amendments added to the Constitution since its adoption were the result of an arduous process that required approval by two-thirds of both houses of Congress and three-fourths of the states. The main advantage of this supermajority requirement is that no changes to the Constitution can occur unless there is broad support within Congress and among states. The potential drawback is that numerous national amendment initiatives—such as the Equal Rights Amendment (ERA), which aims to guarantee equal rights regardless of sex—have failed because they cannot garner sufficient consent among members of Congress or, in the case of the ERA, the states.
Third, the constitutions of countries with federal systems formally allocate legislative, judicial, and executive authority to the two levels of government in such a way as to ensure each level some degree of autonomy from the other. Under the U.S. Constitution, the president assumes executive power, Congress exercises legislative powers, and the federal courts (e.g., U.S. district courts, appellate courts, and the Supreme Court) assume judicial powers. In each of the fifty states, a governor assumes executive authority, a state legislature makes laws, and state-level courts (e.g., trial courts, intermediate appellate courts, and supreme courts) possess judicial authority.
While each level of government is somewhat independent of the others, a great deal of interaction occurs among them. In fact, the ability of the federal and state governments to achieve their objectives often depends on the cooperation of the other level of government. For example, the federal government’s efforts to ensure homeland security are bolstered by the involvement of law enforcement agents working at local and state levels. On the other hand, the ability of states to provide their residents with public education and health care is enhanced by the federal government’s financial assistance.
Another common characteristic of federalism around the world is that national courts commonly resolve disputes between levels and departments of government. In the United States, conflicts between states and the federal government are adjudicated by federal courts, with the U.S. Supreme Court being the final arbiter. The resolution of such disputes can preserve the autonomy of one level of government, as illustrated recently when the Supreme Court ruled that states cannot interfere with the federal government’s actions relating to immigration.Arizona v. United States, 567 U.S. __ (2012). In other instances, a Supreme Court ruling can erode that autonomy, as demonstrated in the 1940s when, in United States v. Wrightwood Dairy Co., the Court enabled the federal government to regulate commercial activities that occurred within states, a function previously handled exclusively by the states.United States v. Wrightwood Dairy Co., 315 U.S. 110 (1942).
Finally, subnational governments are always represented in the upper house of the national legislature, enabling regional interests to influence national lawmaking.Ronald L. Watts. 1999. Comparing Federal Systems, 2nd ed. Kingston, Ontario: McGill-Queen’s University, 6–7; Daniel J. Elazar. 1992. Federal Systems of the World: A Handbook of Federal, Confederal and Autonomy Arrangements. Harlow, Essex: Longman Current Affairs. In the American federal system, the U.S. Senate functions as a territorial body by representing the fifty states: Each state elects two senators to ensure equal representation regardless of state population differences. Thus, federal laws are shaped in part by state interests, which senators convey to the federal policymaking process.
The governmental design of the United States is unusual; most countries do not have a federal structure. Aside from the United States, how many other countries have a federal system?
Division of power can also occur via a unitary structure or confederation (Figure). In contrast to federalism, a unitary system makes subnational governments dependent on the national government, where significant authority is concentrated. Before the late 1990s, the United Kingdom’s unitary system was centralized to the extent that the national government held the most important levers of power. Since then, power has been gradually decentralized through a process of devolution, leading to the creation of regional governments in Scotland, Wales, and Northern Ireland as well as the delegation of specific responsibilities to them. Other democratic countries with unitary systems, such as France, Japan, and Sweden, have followed a similar path of decentralization.
In a confederation, authority is decentralized, and the central government’s ability to act depends on the consent of the subnational governments. Under the Articles of Confederation (the first constitution of the United States), states were sovereign and powerful while the national government was subordinate and weak. Because states were reluctant to give up any of their power, the national government lacked authority in the face of challenges such as servicing the war debt, ending commercial disputes among states, negotiating trade agreements with other countries, and addressing popular uprisings that were sweeping the country. As the brief American experience with confederation clearly shows, the main drawback with this system of government is that it maximizes regional self-rule at the expense of effective national governance.
FEDERALISM AND THE CONSTITUTION
The Constitution contains several provisions that direct the functioning of U.S. federalism. Some delineate the scope of national and state power, while others restrict it. The remaining provisions shape relationships among the states and between the states and the federal government.
The enumerated powers of the national legislature are found in Article I, Section 8. These powers define the jurisdictional boundaries within which the federal government has authority. In seeking not to replay the problems that plagued the young country under the Articles of Confederation, the Constitution’s framers granted Congress specific powers that ensured its authority over national and foreign affairs. To provide for the general welfare of the populace, it can tax, borrow money, regulate interstate and foreign commerce, and protect property rights, for example. To provide for the common defense of the people, the federal government can raise and support armies and declare war. Furthermore, national integration and unity are fostered with the government’s powers over the coining of money, naturalization, postal services, and other responsibilities.
The last clause of Article I, Section 8, commonly referred to as the elastic clause or the necessary and proper cause, enables Congress “to make all Laws which shall be necessary and proper for carrying” out its constitutional responsibilities. While the enumerated powers define the policy areas in which the national government has authority, the elastic clause allows it to create the legal means to fulfill those responsibilities. However, the open-ended construction of this clause has enabled the national government to expand its authority beyond what is specified in the Constitution, a development also motivated by the expansive interpretation of the commerce clause, which empowers the federal government to regulate interstate economic transactions.
The powers of the state governments were never listed in the original Constitution. The consensus among the framers was that states would retain any powers not prohibited by the Constitution or delegated to the national government.Jack Rakove. 2007. James Madison and the Creation of the American Republic. New York: Pearson; Samuel H. Beer. 1998. To Make a Nation: The Rediscovery of American Federalism. Cambridge, MA: Harvard University Press. However, when it came time to ratify the Constitution, a number of states requested that an amendment be added explicitly identifying the reserved powers of the states. What these Anti-Federalists sought was further assurance that the national government’s capacity to act directly on behalf of the people would be restricted, which the first ten amendments (Bill of Rights) provided. The Tenth Amendment affirms the states’ reserved powers: “The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.” Indeed, state constitutions had bills of rights, which the first Congress used as the source for the first ten amendments to the Constitution.
Some of the states’ reserved powers are no longer exclusively within state domain, however. For example, since the 1940s, the federal government has also engaged in administering health, safety, income security, education, and welfare to state residents. The boundary between intrastate and interstate commerce has become indefinable as a result of broad interpretation of the commerce clause. Shared and overlapping powers have become an integral part of contemporary U.S. federalism. These concurrent powers range from taxing, borrowing, and making and enforcing laws to establishing court systems (Figure).Elton E. Richter. 1929. “Exclusive and Concurrent Powers in the Federal Constitution,” Notre Dame Law Review 4, No. 8: 513–542. http://scholarship.law.nd.edu/cgi/viewcontent.cgi?article=4416&context=ndlr
Article I, Sections 9 and 10, along with several constitutional amendments, lay out the restrictions on federal and state authority. The most important restriction Section 9 places on the national government prevents measures that cause the deprivation of personal liberty. Specifically, the government cannot suspend the writ of habeas corpus, which enables someone in custody to petition a judge to determine whether that person’s detention is legal; pass a bill of attainder, a legislative action declaring someone guilty without a trial; or enact an ex post facto law, which criminalizes an act retroactively. The Bill of Rights affirms and expands these constitutional restrictions, ensuring that the government cannot encroach on personal freedoms.
The states are also constrained by the Constitution. Article I, Section 10, prohibits the states from entering into treaties with other countries, coining money, and levying taxes on imports and exports. Like the federal government, the states cannot violate personal freedoms by suspending the writ of habeas corpus, passing bills of attainder, or enacting ex post facto laws. Furthermore, the Fourteenth Amendment, ratified in 1868, prohibits the states from denying citizens the rights to which they are entitled by the Constitution, due process of law, or the equal protection of the laws. Lastly, three civil rights amendments—the Fifteenth, Nineteenth, and Twenty-Sixth—prevent both the states and the federal government from abridging citizens’ right to vote based on race, sex, and age. This topic remains controversial because states have not always ensured equal protection.
The supremacy clause in Article VI of the Constitution regulates relationships between the federal and state governments by declaring that the Constitution and federal law are the supreme law of the land. This means that if a state law clashes with a federal law found to be within the national government’s constitutional authority, the federal law prevails. The intent of the supremacy clause is not to subordinate the states to the federal government; rather, it affirms that one body of laws binds the country. In fact, all national and state government officials are bound by oath to uphold the Constitution regardless of the offices they hold. Yet enforcement is not always that simple. In the case of marijuana use, which the federal government defines to be illegal, twenty-three states and the District of Columbia have nevertheless established medical marijuana laws, others have decriminalized its recreational use, and four states have completely legalized it. The federal government could act in this area if it wanted to. For example, in addition to the legalization issue, there is the question of how to treat the money from marijuana sales, which the national government designates as drug money and regulates under laws regarding its deposit in banks.
Various constitutional provisions govern state-to-state relations. Article IV, Section 1, referred to as the full faith and credit clause or the comity clause, requires the states to accept court decisions, public acts, and contracts of other states. Thus, an adoption certificate or driver’s license issued in one state is valid in any other state. The movement for marriage equality has put the full faith and credit clause to the test in recent decades. In light of Baehr v. Lewin, a 1993 ruling in which the Hawaii Supreme Court asserted that the state’s ban on same-sex marriage was unconstitutional, a number of states became worried that they would be required to recognize those marriage certificates.Baehr v. Lewin. 1993. 74 Haw. 530. To address this concern, Congress passed and President Clinton signed the Defense of Marriage Act (DOMA) in 1996. The law declared that “No state (or other political subdivision within the United States) need recognize a marriage between persons of the same sex, even if the marriage was concluded or recognized in another state.” The law also barred federal benefits for same-sex partners.
DOMA clearly made the topic a state matter. It denoted a choice for states, which led many states to take up the policy issue of marriage equality. Scores of states considered legislation and ballot initiatives on the question. The federal courts took up the issue with zeal after the U.S. Supreme Court in United States v. Windsor struck down the part of DOMA that outlawed federal benefits.United States v. Windsor, 570 U.S. __ (2013). That move was followed by upwards of forty federal court decisions that upheld marriage equality in particular states. In 2014, the Supreme Court decided not to hear several key case appeals from a variety of states, all of which were brought by opponents of marriage equality who had lost in the federal courts. The outcome of not hearing these cases was that federal court decisions in four states were affirmed, which, when added to other states in the same federal circuit districts, brought the total number of states permitting same-sex marriage to thirty.Adam Liptak, “Supreme Court Delivers Tacit Win to Gay Marriage,” New York Times, 6 October, 2014. Then, in 2015, the Obergefell v. Hodges case had a sweeping effect when the Supreme Court clearly identified a constitutional right to marriage based on the Fourteenth Amendment.Obergefell v. Hodges, 576 U.S. ___ (2015).
The privileges and immunities clause of Article IV asserts that states are prohibited from discriminating against out-of-staters by denying them such guarantees as access to courts, legal protection, property rights, and travel rights. The clause has not been interpreted to mean there cannot be any difference in the way a state treats residents and non-residents. For example, individuals cannot vote in a state in which they do not reside, tuition at state universities is higher for out-of-state residents, and in some cases individuals who have recently become residents of a state must wait a certain amount of time to be eligible for social welfare benefits. Another constitutional provision prohibits states from establishing trade restrictions on goods produced in other states. However, a state can tax out-of-state goods sold within its borders as long as state-made goods are taxed at the same level.
THE DISTRIBUTION OF FINANCES
Federal, state, and local governments depend on different sources of revenue to finance their annual expenditures. In 2014, total revenue (or receipts) reached $3.2 trillion for the federal government, $1.7 trillion for the states, and $1.2 trillion for local governments.Data reported by http://www.usgovernmentrevenue.com/federal_revenue. State and local government figures are estimated. Two important developments have fundamentally changed the allocation of revenue since the early 1900s. First, the ratification of the Sixteenth Amendment in 1913 authorized Congress to impose income taxes without apportioning it among the states on the basis of population, a burdensome provision that Article I, Section 9, had imposed on the national government.Pollock v. Farmers’ Loan & Trust Co., 158 U.S. 601 (1895). With this change, the federal government’s ability to raise revenue significantly increased and so did its ability to spend.
The second development regulates federal grants, that is, transfers of federal money to state and local governments. These transfers, which do not have to be repaid, are designed to support the activities of the recipient governments, but also to encourage them to pursue federal policy objectives they might not otherwise adopt. The expansion of the federal government’s spending power has enabled it to transfer more grant money to lower government levels, which has accounted for an increasing share of their total revenue.See Robert Jay Dilger, “Federal Grants to State and Local Governments: A Historical Perspective on Contemporary Issues,” Congressional Research Service, Report 7-5700, 5 March 2015.
The sources of revenue for federal, state, and local governments are detailed in Figure. Although the data reflect 2013 results, the patterns we see in the figure give us a good idea of how governments have funded their activities in recent years. For the federal government, 47 percent of 2013 revenue came from individual income taxes and 34 percent from payroll taxes, which combine Social Security tax and Medicare tax.
For state governments, 50 percent of revenue came from taxes, while 30 percent consisted of federal grants. Sales tax—which includes taxes on purchased food, clothing, alcohol, amusements, insurance, motor fuels, tobacco products, and public utilities, for example—accounted for about 47 percent of total tax revenue, and individual income taxes represented roughly 35 percent. Revenue from service charges (e.g., tuition revenue from public universities and fees for hospital-related services) accounted for 11 percent.
The tax structure of states varies. Alaska, Florida, Nevada, South Dakota, Texas, Washington, and Wyoming do not have individual income taxes. Figure illustrates yet another difference: Fuel tax as a percentage of total tax revenue is much higher in South Dakota and West Virginia than in Alaska and Hawaii. However, most states have done little to prevent the erosion of the fuel tax’s share of their total tax revenue between 2007 and 2014 (notice that for many states the dark blue dots for 2014 are to the left of the light blue numbers for 2007). Fuel tax revenue is typically used to finance state highway transportation projects, although some states do use it to fund non-transportation projects.
The most important sources of revenue for local governments in 2013 were taxes, federal and state grants, and service charges. For local governments the property tax, a levy on residential and commercial real estate, was the most important source of tax revenue, accounting for about 74 percent of the total. Federal and state grants accounted for 37 percent of local government revenue. State grants made up 87 percent of total local grants. Charges for hospital-related services, sewage and solid-waste management, public city university tuition, and airport services are important sources of general revenue for local governments.
Intergovernmental grants are important sources of revenue for both state and local governments. When economic times are good, such grants help states, cities, municipalities, and townships carry out their regular functions. However, during hard economic times, such as the Great Recession of 2007–2009, intergovernmental transfers provide much-needed fiscal relief as the revenue streams of state and local governments dry up. During the Great Recession, tax receipts dropped as business activities slowed, consumer spending dropped, and family incomes decreased due to layoffs or work-hour reductions. To offset the adverse effects of the recession on the states and local governments, federal grants increased by roughly 33 percent during this period.Jeffrey L. Barnett et al. 2014. 2012 Census of Governments: Finance-State and Local Government Summary Report, Appendix Table A-1. December 17. Washington, DC: United States Census Bureau, 2.
In 2009, President Obama signed the American Recovery and Reinvestment Act (ARRA), which provided immediate economic-crisis management assistance such as helping local and state economies ride out the Great Recession and shoring up the country’s banking sector. A total of $274.7 billion in grants, contracts, and loans was allocated to state and local governments under the ARRA.Dilger, “Federal Grants to State and Local Governments,” 4. The bulk of the stimulus funds apportioned to state and local governments was used to create and protect existing jobs through public works projects and to fund various public welfare programs such as unemployment insurance.James Feyrer and Bruce Sacerdote. 2011. “Did the Stimulus Stimulate? Real Time Estimates of the Effects of the American Recovery and Reinvestment Act” (Working Paper No. 16759), Cambridge, MA: National Bureau of Economic Research. http://www.nber.org/papers/w16759.pdf
How are the revenues generated by our tax dollars, fees we pay to use public services and obtain licenses, and monies from other sources put to use by the different levels of government? A good starting point to gain insight on this question as it relates to the federal government is Article I, Section 8, of the Constitution. Recall, for instance, that the Constitution assigns the federal government various powers that allow it to affect the nation as a whole. A look at the federal budget in 2014 (Figure) shows that the three largest spending categories were Social Security (24 percent of the total budget); Medicare, Medicaid, the Children’s Health Insurance Program, and marketplace subsidies under the Affordable Care Act (24 percent); and defense and international security assistance (18 percent). The rest was divided among categories such as safety net programs (11 percent), including the Earned Income Tax Credit and Child Tax Credit, unemployment insurance, food stamps, and other low-income assistance programs; interest on federal debt (7 percent); benefits for federal retirees and veterans (8 percent); and transportation infrastructure (3 percent).Data reported by the Center on Budget and Policy Priorities. 2015. “Policy Basics: Where Do Our Federal Tax Dollars Go?” March 11. http://www.cbpp.org/research/policy-basics-where-do-our-federal-tax-dollars-go It is clear from the 2014 federal budget that providing for the general welfare and national defense consumes much of the government’s resources—not just its revenue, but also its administrative capacity and labor power.
Figure compares recent spending activities of local and state governments. Educational expenditures constitute a major category for both. However, whereas the states spend comparatively more than local governments on university education, local governments spend even more on elementary and secondary education. That said, nationwide, state funding for public higher education has declined as a percentage of university revenues; this is primarily because states have taken in lower amounts of sales taxes as internet commerce has increased. Local governments allocate more funds to police protection, fire protection, housing and community development, and public utilities such as water, sewage, and electricity. And while state governments allocate comparatively more funds to public welfare programs, such as health care, income support, and highways, both local and state governments spend roughly similar amounts on judicial and legal services and correctional services.
Federalism is a system of government that creates two relatively autonomous levels of government, each possessing authority granted to them by the national constitution. Federal systems like the one in the United States are different from unitary systems, which concentrate authority in the national government, and from confederations, which concentrate authority in subnational governments.
The U.S. Constitution allocates powers to the states and federal government, structures the relationship between these two levels of government, and guides state-to-state relationships. Federal, state, and local governments rely on different sources of revenue to enable them to fulfill their public responsibilities.
Which statement about federal and unitary systems is most accurate?
- In a federal system, power is concentrated in the states; in a unitary system, it is concentrated in the national government.
- In a federal system, the constitution allocates powers between states and federal government; in a unitary system, powers are lodged in the national government.
- Today there are more countries with federal systems than with unitary systems.
- The United States and Japan have federal systems, while Great Britain and Canada have unitary systems.
Hint:
B
Which statement is most accurate about the sources of revenue for local and state governments?
- Taxes generate well over one-half the total revenue of local and state governments.
- Property taxes generate the most tax revenue for both local and state governments.
- Between 30 and 40 percent of the revenue for local and state governments comes from grant money.
- Local and state governments generate an equal amount of revenue from issuing licenses and certificates.
What key constitutional provisions define the scope of authority of the federal and state governments?
Hint:
The following parts of the Constitution sketch the powers of the states and the federal government: Article I, Section 8; the supremacy clause of Article VI; and the Tenth Amendment. The following parts of the Constitution detail the limits on their authority: Article I, Sections 9 and 10; Bill of Rights; Fourteenth Amendment; and the civil rights amendments.
What are the main functions of federal and state governments?
|
oercommons
|
2025-03-18T00:38:02.538035
|
07/10/2017
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/15204/overview",
"title": "American Government, Students and the System, American Federalism, The Division of Powers",
"author": null
}
|
https://oercommons.org/courseware/lesson/124669/overview
|
"The Converging Polarities of Frida Kahlo as Tinku”
Overview
A paper that explores ancient Andean cultural roots (which influenced Mesoamerican indigenous culture) as manifest in Frida Kahlo’s life perspective which models the ancient Andean notion of tinku (when two entities merge, such as two streams meeting to form a river). The author characterizes the Mexican identity as a tinku (from the 15th century Spanish influence merging with the indigenous culture to transmute into a new alchemical entity, the Mexican, rather than merely a hybridization). The paper seeks out patterns in Frida’s life and work that reveal ancient heritage, as well as ancestral trauma, with a focus on identifying tinku. Diego Rivera is also discussed to some degree, as their two lives were so intertwined. The paper identifies tinku in Diego’s work, as well, and asserts that the notion of Gesamtkunstwerk is also an echo pattern of the ancient Andean notion of tinku.
Explores Latine identity construct as a "tinku" (ancient Andean term), using Frida Kahlo as a model.
“Searingly Alive/Ever-present Death: The Converging Polarities of Frida Kahlo as Tinku”
by Mahara T. Sinclaire
There are many ideas about the influences and nature of Frida Kahlo’s work. Andre Breton linked her art practice to Surrealism; however, her work was grounded in her real-life experience, so the Surrealist label does not quite suit. As Andrea Kettenmann points out, “It is more important for the artist [Frida] to reproduce her emotional state in a distillation of the reality she had experienced...” 1 Given her emphasis on expressing her raw emotional reality, one can identify parallels to van Gogh’s self-portraits and his legacy of candid communication of his internal experience. Diego Rivera on Frida’s art states, “Frida is the only example in the history of art of an artist who tore open her chest and heart to reveal the biological truth of her feelings.” 2 The vulnerability and sincerity that van Gogh and Kahlo share with the world may well explain their powerful resonance within the global community, capturing the hearts of humanity. Curator Gregory O’Brien accurately notes that Kahlo was “inspired by the primitivism of Rousseau and Gauguin, and most importantly by Mexican folk art and pre-Hispanic culture.” 3 While these assessments contribute significantly to our understanding, I will explore the idea that Frida’s work encompasses many impulses reminiscent of the ancient indigenous cultures of Latin America, most significantly the ancient Andean perspective. In identifying the ancestral heritage to which she was so deeply connected, I will also address the more immediate ancestral traumas that may have colored her life experience.
Kahlo’s work embraces essence (symbol) over appearance (realism) which is a core trait of ancient Andean art. While she employs symbols on one hand to communicate akin to the popular votive practice of her day, (for example, in Time Flies she literally uses the symbols of a clock and an airplane as metaphors), in other later works these symbols are often oblique, conveying sensation rather than orchestrating comprehension. Her work flirts with illegibility and incomprehensibility in that the whole congers more than what the individual elements connote. The viewer, when trying to decode the symbols, often finds some of the elements to be mysterious and inexplicable. “Kahlo’s singular portrait style cuts straight to the heart of deeply felt passions and sorrows. Juxtaposing the familiar with the strange, marrying naturalistic depiction with bizarre symbolism Kahlo is able to convince us of the truthfulness to her inner life shown in her paintings” 4, states editor Anthony White in his astute assessment of Kahlo’s oeuvre.
Adolfo Best Maugard ‘s 1923 book Drawing Method: Tradition, Resurgence, and Evolution of Mexican Art describes Mexican art as stylized using non-perspective as its indigenous component in conjunction with details such as tied-back curtain cords indicating a colonial adaptation typical of 19th century portrait painting. Frida’s work is grounded in the Mexican retablo painting tradition while simultaneously embodying Western art’s visual expression of Saint Sebastian and the Christ figure’s suffering through pain and death as filtered through her own tortured life story to spiritually transcend the trials of the physical plane. “Kahlo was largely concerned to record accurately her physical and psychological situation in a way that would avoid pity… but instead ask for identification and empathy.” 5 This hybridization is characteristic in the original Mexican voice, and Frida used both these visual strategies in her work. Mexicanism (‘mexicanidad’) is not merely a Spanish/ Indigenous hybrid, but rather the evolution of that hybridization into a newly-birthed, original, neo-indigenous, cultural identity. Kahlo psychologically creates a distinction between contemporary Spain (European culture) and the 15th century Spanish infusion of culture (Hispanic influence) that intermingled with the indigenous culture at that point in time. Therefore, “a Mestiza, a ‘true’ Mexican woman” 6 is the embodiment of that marriage. In ancient Latin American culture, this marriage can best be understood as a ‘tinku’, in which the two components (the 15th century Spanish infusion of culture meeting the indigenous culture of that time) converge to make a third culture. This alchemy constitutes the ‘mestizo’. To reiterate, the mestizo is not merely a hybrid, but rather an alchemical transformation into a new entity, unique unto itself.
Kahlo “favored an independent Mexican culture with its Pre-Columbian and Hispanic roots.” 7 She felt that the “pre-Hispanic Indians were her prime-movers, her progenitors” 8 In her self-portraits, she “shares the limelight with the flora and fauna of Mexico, with cacti, plants of the primeval forest, volcanic rock, parrots, deer, monkeys and Itzcuintli dogs” 9 In her powerful1939 painting The Two Fridas, she indicates her shared identity as two halves: European dress (female, societal, the portion rejected by Diego) and the Tehuana Mexican dress that Diego favored (seated in a more masculine posture), representing her indigenous side as a celebration of the natural world. These two halves conjoin to alchemize the Mestizo. The two halves in the emblem of Mexico include the eagle (representing Quetzalcoatl, the domain of the sky, the essence of transcendental freedom) and the serpent (representing Tezcatlipoca, “the sexual energy of the grounded ‘self’’10 “Quetzalcoatl and Tezcatlipoca are mirror images of the unconscious mind. One represents the luminous intellect, the other, the black shadow, the formless chaos of the emotions.” 11 Victor Zamudio-Taylor observes that, “In the double self-portrait, the first of its kind ever, the artist depicts two fundamental sides of her persona that are intrinsic to the mestizo condition, namely plural or polyphonic subjectivities in a permanent state of flux, contradiction, and paradox.” 12 He further states that “Contradictions are allowed to remain as such, resting unresolved in a discursive space…[of] mestizaje –understood as impurity, hybridism, and mixture—which creates a ‘third space’” 13 His statement aligns with the notion of tinku. Frida articulates a fluid, ambiguous, everchanging present that cannot be labeled or pinned down (not an either/or but an either/and), so we have a duality and a unity co-expressing. As Diego describes it, “... The Two Fridas are at once the same person and two different people.” 14 Duality and unity coexist in an uncontradictory, mutually-validating, complexly-divergent yet unified field. Frida Kahlo articulated dynamic contrasts of steeled endurance with inner turmoil, evincing a control/chaos dichotomy. Diego further remarked, “Kahlo’s art is “a combination of the collective and the individual.” 15 She is Mexico. She is herself. These two components unite, transcending her identity into an icon, her great legacy.
Similarly, Diego shared this tinku-exemplifying idea of two merging into one to create something new. Diego’s political dream was to establish a ‘common citizenship’ for everybody in the Americas…a union of the ancient traditions of the South and the industrial activity of the north.” 16 Additionally, Diego’s art practice conjoins plastic art aspects (composition, formal issues, structural organization) with his representational narrative content resulting in truly affecting art of great magnitude (tinku). After Diego’s return from Europe where he was indoctrinated into Cubism, Italian Renaissance frescos, and modernism, he went with Vasconcelos to the Yucatan to immerse himself in his Mexican cultural heritage. Seeing the Mayan pyramids, “Rivera stood in awe inside the precious inner chamber of the Temple of the Tigers still ablaze with 12th century frescos. Those best preserved combine complex geometric planning with lively anecodic storytelling.” 17 In the Mayan culture, we see the melding of the plastic arts (compositional and structural strategies) working in conjunction with subject matter (meaningful content conveying a narrative), blending together into a total art experience, a tinku. The Detroit Industry murals created for the Detroit Institute of Arts commission, “led Rivera to develop a very sophisticated mural style that combined the best of cubist abstraction with his poetic images of men and machines.” 18 Interestingly, one can recognize a parallel between Wagner’s concept of Gesamtkunstwerk characterized by a “seamless melding of a variety of art forms” 19 with the ancient peoples of Latin America’s notion of the tinku. Kandinsky further elucidates the definition of Gesamtkunstwerk as the bringing together of “all the arts to synesthesia” 20, as articulated in his 1910 Concerning the Spiritual in Art. Rivera saw his Detroit Industry murals as “a wonderful symphony.” 21 One can recognize the understanding of the magical transmutation that occurs in the ancient ‘tinku’ echoed in the later European concept of Gesamtkunstwerk.
The polar relationship between life and death was another major theme that threaded through Kahlo’s oeuvre. Frida’s 1931 portrait of Luther Burbank (a ground-breaking grafter and horticulturalist) conceptualized him as a hybridization in which his corpse feeds the tree which then feeds him as a newly regenerating tree suggesting reciprocity. By giving back to the earth and connecting to nature, he feeds himself. To feed the earth is to feed oneself. The painting also conveys her idea of coexistent life/death polarity as she saw herself as dying inside while emphatically and defiantly alive in the world. She asserts that death feeds life. It is not a contradictory adversary but a cooperative partner toward birthing the new. This “life death duality and the fertilization of life by death...” 22 implies the rich life force inherent in death, and this mutually constructive and reciprocal relationship ignites a tinku. Frida’s Portrait of Luther Burbank conveys her respect for Burbank’s life- affirming hybrid creations, “which seemed to defy death, a subject of particular interest to her.” 23 Victor Zamudio-Taylor points out that in Portrait of Luther Burbank “…his legs, transformed into a tree trunk, sprout roots that appear to grow from his own corpse. The duality of life and death as complementary and contradictory facts of human existence is highlighted, as in the dynamic relationship between nature and culture as inseparable entities.” 24 Zamudio-Taylor is noticing the polarities of life/death and nature/culture, which Frida illustrates as forming a tinku, a new entity.
This theme of the life/death polarity is evident in Kahlo’s 1932 My Birth, painted shortly after her miscarriage in New York and the death of her mother. The birth-giving figure represents her mother as well as herself, and the birthed figure represents both herself and the recent miscarriage, linking life and death as a paradoxical and perpetual cycle of sorrow. Both figures are depicted as ambiguously dead and alive (still-born/still-alive), conveying her deep grief.
A Few Small Nips from 1935 harnesses a news item in which a jealous drunken man in a fit of rage stabbed his girlfriend twenty times saying “it was just a few nips”. In Frida’s personal life at the time, she learned that Diego had been having a months-long affair with her younger sister Cristina. Devastated, she felt “murdered by life.” 25 Kahlo experienced significant physical pain throughout her life (it was killing her) and significant emotional pain with Diego’s continual affairs (it was killing her psychologically); therefore, life and death merged into a somewhat synonymous experience.
This simultaneous duality (life and death as separate) and conflation (life/death as a merged unity) parallels the nature of Frida and Diego’s connection. They were distinct individuals who had great respect for each other’s work and values, while at the same time they functioned as one entity like the fusion of two cells merging into one. As a last will request, Diego had left instructions that his ashes be mixed with Frida’s ashes in the urn at the Blue House. This request was not honored, and his body was instead interred in the Rotunda of illustrious Men in the Dolores Cemetery, per instructions of the Mexican President.
Even in Kahlo’s early self-portrait Time Flies, the attention to death is apparent. The center bead on the pre-Hispanic necklace has two crossed bands which “are associated with Mictlantecutli, the Aztec god of death.” 26
Preoccupied with an acute awareness of death, Frida created still lifes toward the end of her life that evoke the presence of death. Historically, the still life genre was initiated after the Black Plague’s ravaging of human life when people valued the tangible aspects of real life over an uncertain spiritual afterlife. Still life painting “depicted death at the heart of life.”27 A still life ‘stops time’, breaking the cycle of death and temporarily ends life’s processes, functioning as a gap, a suspended pause. “’I paint flowers so they will not die’ she confided to her last lover, Josep Bartoli.” 28 Bartoli was a Spanish refugee that Frida met through her sister Cristina toward the end of her life. “Reflecting on Kahlo’s morbid preoccupation with death, Bartoli admitted that ‘all her life was a suicide; she was born suicidal.’” 29 Significantly, Frida’s maternal grandmother was traumatized as a young girl by witnessing her first boyfriend commit suicide right in front of her. This may have reverberated in to Frida’s life perspective through ancestral trauma transmission. Frida herself attempted suicide on a few occasions and some feel that her death was in fact a suicide.
Looking further at ancestral trauma embedded in Frida’s familial heritage, her father Wilhelm (Guillermo) Kahlo, at age 18, while a student at the University of Nuremberg in his homeland Germany “sustained brain injuries in a fall, and began to suffer epileptic seizures” 30 that would plague him for the rest of his life. In a parallel pattern of ancestral trauma transmittal, Frida at age 17 sustained serious bodily injuries in the trolley/bus collision that would plague her with pain for the rest of her life.
At two months pregnant in Detroit, contemplating whether to carry or abort, Frida writes to Dr. Loesser “… with this heredity in my blood I do not think the child could come out very healthy.” 31 Author Herrera speculates that Frida refers to her father’s epilepsy here. However, upon her third incident of pregnancy, Dr. Zollinger “ordered an abortion after 3 months because of the ‘infantilism of Frida’s ovaries’. Both Frida’s older sisters also had ‘insufficient ovaries’; neither bore children (Adrianna had 3 miscarriages) and both eventually had their ovaries removed because of cysts”. 32 There were lineage factors that likely contributed to Frida’s inability to bear children.
As a young child of age 6, Frida had “an accident and hit her right foot against a tree stump. This caused a slight deformation of her foot, which turned outward. Several doctors diagnosed the problem as polio. Others said Frida had a ‘white tumor’. The treatment consisted of sun baths and calcium baths.” 33 Robin Richmond postulates that Frida’s polio state of being the ‘invalid’ imprinted in her a psychological self-image of being ‘in-valid’ resulting in an aggravated need to be seen and validated. The name-calling she endured from the other children calling her “Frida, peg-leg!” etched in her a defiant defense and seemed to cement the defect of her right foot as a central aspect of her self-image. Over the course of her life, she had 32 operations to rectify her spinal injuries from the trolley/bus accident at age18 (other potential causes include scoliosis or “‘spina bifida’, a congenital malformation that occurs when the lower spine does not close during fetal development that sometimes leads to progressive trophic ulcers on the legs and feet.” )34 Despite these numerous spinal surgeries, Frida was still able to sustain her lust for life. It was the amputation of her right leg that extinguished her will to live.
In examining why Kahlo characterized herself as a wounded deer in her 1946 The Little Deer, it is pertinent to note that, “In Aztec belief, the deer was the sign of the right foot.” 35As her health troubles ostensibly initiated with her polio-inflicted right foot, choosing to cast herself as a deer does indeed seem to reference the Aztec Codex Vaticanus A which indicates that the head of a deer is the symbol for a man’s right foot. As she specifically has her face stand-in for the head of the deer, this would indicate the degree to which her injured right foot dominated her self-identity. Another possible impetus for characterizing herself as a deer is that “Tlaloc was the god of the mazatl (deer), the animal that ruled her birthdate.” 36
In addition to Aztec references in Kahlo’s work, there are numerous Catholic elements as well, due to her religious upbringing’s Spanish roots. Frida’s 1937 painting Memory casts her as a martyr akin to Saint Teresa of Avila whose heart was pierced by the arrow of Divine Love. One of Frida’s Catholic names was Carmen. Since Saint Teresa founded the Order of the Discalced Carmelites, Frida would have known Teresa’s transverberation story. Teresa’s Carmelites walked barefoot causing significant pain in their feet, with which Frida could identify, enduring the pain of her own right foot. Other similarities include that both Frida and Saint Teresa, when young girls, were “both ill and nearly died, after which their fathers were their primary caregivers.”37
“In the mid-1940s, Frida’s health declined. She lost weight and had fainting spells.” 38 “There has been quite a lot of speculation about whether Kahlo’s physical suffering and the many operations she endured were not basically a means of tying her husband [Diego] closer to her. Her favorite physician, Dr. Eloesser even went so far as to express the opinion that many of the operations were unnecessary.” 39 Mimicking her mother’s modeled behavior of fainting spells (diagnosed as ‘hysterical hypochondria’) to redirect focus from her husband Guillermo’s epileptic seizures, “… many of her [Frida’s] operations were elective and occurred when she felt a new love in Diego’s life.” 40
Frida’s mother, ill after Frida’s birth and possibly suffering post-partum depression, hired an indigenous woman to be Frida’s wet nurse. The lack of bonding with the mother is considered to have resulted in Frida’s need for attention. An interesting parallel in Diego’s early life: his parents, for fear he might die like his twin brother who had died at 18 months (the mother having had three previous miscarriages already), placed Diego in the care of an Indian nurse who lived in the country. He remained with his nurse for two years (ages 2-4). It seems that both Frida and Diego did not bond with their mothers as infants, which may have contributed to an exacerbated need for attention and validation.
Having addressed some of the more immediate ancestral trauma patterns that colored Frida’s life experience, I will return to the examination of the overarching polarities that Frida conflated, inventing new modes of expression, with attention to male/female binaries, as well as mother/son and father/daughter dynamics that played out in her life and love.
Regarding Frida’s relationship with her father, “She was always at the ready to come to his aid if he suffered an epileptic seizure. She would spring into action, making sure he was alright while guarding his [camera] equipment to prevent its theft.” 41 In this sense, she is mothering her father.
“Guillermo Kahlo adored his fifth child [Frieda] and considered her the most intelligent of his daughters and hence, the one most like him! He treated her like the son he never had…”42 This attitude by the father cultivates Frida’s embrace and expression of her masculine side from an early age. “She was certainly her father’s favourite daughter… Frida in turn lavished affection on him, returning his love with what can only be termed veneration” 43 She repeated this relational pattern with her husband, venerating Diego, as well. After Frida’s childhood polio affliction, her father prescribed sports to strengthen her frail right leg. Frida’s post-polio sports included: soccer, boxing, wrestling, and swimming. Frida stated “’My toys were those of a boy: skates, bicycles’. She liked to climb trees, row on the lakes of Chapultepec Park, and play ball.” 44
Her attire decisions (e.g., wearing her father’s three-piece suit for a family photo) evinced her experimentation with transvestitism, “asserting her right to be different and unconventional.” 45 In this light, her Tehuana costume can be understood as a form of drag as she exercised the power of masquerade to craft a unique presence. Her defiant embrace of being different, steeled after the relentless taunting to which she was subjected as a young child served as a shield to mitigate vulnerability.
In her self-portraits, Frida emphasizes her mustache and unibrow while donning the traditional Tehuana female dress. She highlights her masculinity while celebrating her femininity in such a way that the two poles do not contradict one another but rather coalesce into a third male/female transmutation, birthing a new form of identity (tinku). Frida models a persona that is both male and a female, conflating the polarity and opening new possibilities for expression and behavior. Frida’s face in her self-portraits is androgenous. “To judge from her more obvious feminine prettiness in photographs, it is clear that [in her paintings] she exaggerated her mustache and gave her features a steely cast.”46
Diego was 21 years older than Frida. When they married, she was 22 and he was 43, (but she told him she was only 19, so he would have thought he was 24 years older than her). Their relationship had a father/daughter dynamic, especially at the start. Frida would sign her love letters “your little girl [nina] Frida.” 47 But there was also a mother/son dynamic in their relationship. “He didn’t want any more children [He had children from other relationships]: He wanted to be the only child in the relationship…[He] needed all the attention Frida could spare him. In some ways, she seemed to be marrying a father figure…He called her his ‘little Fisita, my beautiful little girl’, but in other ways, he was an utter baby.” 48 “Just as she loved his [Diego’s] vulnerability and his womanly breasts, he loved her grit and her ‘Zapata’ mustache.” 49 “There was always a definite androgenous aspect to both Frida and Diego.” 50 It is evident that they had a very fluid and complex role-playing dynamic.
In Kahlo’s 1944 Diego and Frida 1929-1944, she presents a composite portrait that is half her face/ half Diego’s face, coalescing them into one entity. Here, Frida combines the energy of Diego with her own, creating an ‘ayni’ (union). These two interdependent though unequal halves can be understood in the late Incan term ‘karihuarmi’, indicating a male/female complementarity. This state of reciprocity provides an apt description of the underlying dynamic of their deep connection.
There are similarities in Frida’s communications with Alejandro Gomez Arias (her first boyfriend) and Diego. When Alejandro ended up falling in love with Frida’s friend Esparanza Ordonez, Frida plead to Alejandro, “Deep down, you understand me, you know I adore you! That you are not only a thing that is mine, but you are me myself!” 51 She composited herself with Diego in this same way, seeing herself unified with him into one entity. In a statement regarding her relationship to Diego, Frida reflected that, “within my difficult and obscure role of ally of an extraordinary being [Diego], …I have the reward of equilibrium.” 52 She needed his presence for her own balance.
In her1949 The Love Embrace of the Universe, the Earth (Mexico), Myself, Diego, and Senor Xolotl, Frida paints an image with Diego as a baby in her arms. There are three levels of female characters in the piece: an overarching Universal Goddess with polarities of black/white, night/day, sun/moon; Mother Earth representing the natural world; and Frida, all holding and nurturing the baby Diego, infusing him with the energy of creation (‘camay’). His activated third eye signifies his male genius/ wisdom and the fire ball in his hand signifies his creative power. “With this piece, Frida again explores themes of duality and the connectedness of all living things” 53 Additionally, the piece can be understood to articulate quick time (human time, as depicted in the Frida and Diego figures) and cosmic time (as depicted in the Universal Goddess) occurring simultaneously, not contradicting each other, but co-existent. The persistent extreme pain that marked Frida’s life experience and Diego’s extreme strain from long hours painting murals (sometimes 16-hour days), pushed their human consciousness into a vivid awareness of the present moment, grounding them into the spirit-world experience akin to the indigenous Latin American life experience of arduous treks through the Andes which catalyzed their cultural values and aspirations. This embrace of intense endurance to evoke a spiritual state can be traced back to the black and white polarity signifying duality in the stone portal design of the New Temple at Chavin de Huantar in the ancient Andean culture, a culture which influenced and permeated the indigenous Mesoamerican cultures.
In The Love Embrace of the Universe, the Earth (Mexico) Myself, Diego, and Senor Xolotl, Frida offers a core message about love as the fundamental driving force of the Universe and celebrates the polarities that comprise this oneness:
- Light/ dark
- Day/ night
- Aztec sun god/ Aztec moon god 54
- Male/ female
- Frida/ Diego
- earth plane (represented by earth goddess Cihuacoatl)/ underworld (represented by Senor Xolotl)
- life/ death
- positive/ negative
- figure of universe as a biracial polarity
- physical (bodily experience)/ spiritual (intuitive, feeling experience)
These polarities parallel those of Mexicanism (an inheritance of Hispanic culture and Indigenous roots). Kahlo maps duality as a route to unity. Duality as a condition of oneness, not a contradiction to oneness. Battling forces as a prerequisite for equilibrium which exemplifies the paradoxical nature of life.
Frida agreed with Octavio Paz who observed that “the traditional Mexican attitude toward time is a passionate feeling of connection with the past. Mexico is a land of super-imposed pasts. [Mexico City atop Tenochtitlan which was built like the Toltec’s Tula, which was built like Teotihuacan] It’s not something known but something lived.” 55 The idea of the past not dying, but echoing in the present, in a state of perpetual rebirth, accumulating wisdom as a genetic strand of the cosmos. We are libraries of our indigenous memories which are rekindled when we come into presence with our essence, our core soul nature.
Rodriguez Prampolini argues that Mexican artists “cling to the real…The Mexican has a magic sense of life and an animistic perception of concrete reality…There is no opposition between subject and object, between conscious and subconscious, between the symbol and the thing symbolized.” 56 Kahlo’s work certainly exemplifies this idea.
A diary entry by Frida in 1947 musing on the nature of her union with Diego states, “I am the embryo, the germ, the first cell that—in potency—engendered him—I am him from the most primitive and most ancient cells, which with ‘time’ became him.”57 Frida had a pantheistic view of life ‘in which ‘everything moves according to only one law—life. No one is apart from anyone. No one fights for himself. All is all and one. Anguish and pain, pleasure and death are nothing but a process in order to exist.’” 58 Frida and Diego both envisioned the world as a continuum and saw themselves as connected to a “microcosm/macrocosm dialectic.” 59
Kahlo’s diary entry in 1950 reflects this cosmic perspective: “No one [person is] more than a functioning or a part of the total function…We direct ourselves toward our own selves through millions of beings—stones—bird creatures—star beings—microbe beings—fountain beings to ourselves… We have always been hate-love-mother-child-plant-earth-light-lightning-etc.-world giver of worlds—universes and universal cells.” 60 Reflecting on Frida’s legacy, artist Lesley Dill commented on “The irrationality of her imagery—it’s as big as the world because it encompasses [everything] from the molecule to death.” 61
Viva la Vida is considered to be Frida’s last painting. At the brink of her own death, she celebrated the experience of life. Kahlo endeavored to integrate the duality of her lifelong partnership with death’s shadow and her fiercely vibrant alegria for the earthly pleasures of life. Following Frida’s blueprint, as our human collective integrates duality consciousness with unity consciousness, we manifest tinku. Viva la Vida!
Dedicated to Profe Manuel Aguilar-Moreno, Ph.D.
Works Cited
- p. 35 Kettenmann, Andrea, Frida Kahlo 1907-1954: Pain and Passion, Barnes & Noble Books, New York, 2004.
- p. 84 Richmond, Robin, Painters & Places: Frida Kahlo in Mexico, Pomegranite Artbooks, San Francisco, 1994.
- p. 10 Gelman, Jacques and Natasha, Frida Kahlo Diego Rivera and Mexican Modernism, National Gallery of Australia, 2001.
- p. 25 Gelman, Jacques and Natasha, Frida Kahlo Diego Rivera and Mexican Modernism, National Gallery of Australia, 2001.
- p. 16 Gelman, Jacques and Natasha, Frida Kahlo Diego Rivera and Mexican Modernism, National Gallery of Australia, 2001.
- p. 26 Kettenmann, Andrea, Frida Kahlo 1907-1954: Pain and Passion, Barnes & Noble Books, New York, 2004.
- p. 82 Kettenmann, Andrea, Frida Kahlo 1907-1954: Pain and Passion, Barnes & Noble Books, New York, 2004.
- p. 26 Richmond, Robin, Painters & Places: Frida Kahlo in Mexico, Pomegranite Artbooks, San Francisco, 1994.
- p. 27 Kettenmann, Andrea, Frida Kahlo 1907-1954: Pain and Passion, Barnes & Noble Books, New York, 2004.
- p. 16 Richmond, Robin, Painters & Places: Frida Kahlo in Mexico, Pomegranite Artbooks, San Francisco, 1994.
- p. 35 Richmond, Robin, Painters & Places: Frida Kahlo in Mexico, Pomegranite Artbooks, San Francisco, 1994.
- p. 33 Carpenter, Elizabeth, Frida Kahlo, Walker Art Center, Minneapolis, 2007.
- p. 17 Carpenter, Elizabeth, Frida Kahlo, Walker Art Center, Minneapolis, 2007.
- p. 33 Carpenter, Elizabeth, Frida Kahlo, Walker Art Center, Minneapolis, 2007.
- p. 101 Rosenthal, Mark, Diego Rivera & Frida Kahlo in Detroit, Detroit Institute of Arts, 2015.
- p. 296 Herrera, Hayden, Frida Kahlo: A Biography of Frida Kahlo, Harper and Row, 1983.
- p. 189 Rosenthal, Mark, Diego Rivera & Frida Kahlo in Detroit, Detroit Institute of Arts, 2015.
- p. 193 Rosenthal, Mark, Diego Rivera & Frida Kahlo in Detroit, Detroit Institute of Arts, 2015.
- p. 243 Rosenthal, Mark, Diego Rivera & Frida Kahlo in Detroit, Detroit Institute of Arts, 2015.
- p. 238 Rosenthal, Mark, Diego Rivera & Frida Kahlo in Detroit, Detroit Institute of Arts, 2015.
- p. 238 Rosenthal, Mark, Diego Rivera & Frida Kahlo in Detroit, Detroit Institute of Arts, 2015.
- p. 123 Herrera, Hayden, Frida Kahlo: A Biography of Frida Kahlo, Harper and Row, 1983.
- p. 29 Grimberg, Salomon, Frida Kahlo: The Still Lifes, Merrill, London, 2008.
- p. 32 Carpenter, Elizabeth, Frida Kahlo, Walker Art Center, Minneapolis, 2007.
- p. 99 Barbezat, Suzanne, Frida Kahlo at Home, Francis Lincoln Limited, London, 2016.
- p. 16 Grimberg, Salomon, Frida Kahlo: The Still Lifes, Merrill, London, 2008.
- p. 12 Grimberg, Salomon, Frida Kahlo: The Still Lifes, Merrill, London, 2008.
- p. 11 Grimberg, Salomon, Frida Kahlo: The Still Lifes, Merrill, London, 2008.
- p. 11 Grimberg, Salomon, Frida Kahlo: The Still Lifes, Merrill, London, 2008.
- p. 5 Herrera, Hayden, Frida Kahlo: A Biography of Frida Kahlo, Harper and Row, 1983.
- p. 138 Herrera, Hayden, Frida Kahlo: A Biography of Frida Kahlo, Harper and Row, 1983 .
- p. 463 Herrera, Hayden, Frida Kahlo: A Biography of Frida Kahlo, Harper and Row, 1983.
- p. 450 Herrera, Hayden, Frida Kahlo: A Biography of Frida Kahlo, Harper and Row, 1983.
- p. 37 Herrera, Hayden, Frida Kahlo: The Paintings, HarperCollins Publishers, New York, 1991.
- p. 190 Herrera, Hayden, Frida Kahlo: The Paintings, HarperCollins Publishers, New York, 1991.
- p. 113 Grimberg, Salomon, Frida Kahlo: The Still Lifes, Merrill, London, 2008.
- p. 154 Grimberg, Salomon, Frida Kahlo: The Still Lifes, Merrill, London, 2008.
- p. 180 Herrera, Hayden, Frida Kahlo: The Paintings, HarperCollins Publishers, New York, 1991.
- p. 78 Alcantara, Isabel and Sandra Egnolff, Frida Kahlo and Diego Rivera, Prestel, Munich, 1999.
- p. 86 Richmond, Robin, Painters & Places: Frida Kahlo in Mexico, Pomegranite Artbooks, San Francisco, 1994.
- p. 23 Barbezat, Suzanne, Frida Kahlo at Home, Francis Lincoln Limited, London, 2016.
- p. 11 Hooks, Margaret, Frida Kahlo: Portraits of an Icon, Artes Graficas Palermo, Madrid, Spain, 2002.
- p. 9 Alcantara, Isabel and Sandra Egnolff, Frida Kahlo and Diego Rivera, Prestel, Munich, 1999.
- p. 15 Herrera, Hayden, Frida Kahlo: A Biography of Frida Kahlo, Harper and Row, 1983.
- p. 41 Carpenter, Elizabeth, Frida Kahlo, Walker Art Center, Minneapolis, 2007.
- p. 138 Herrera, Hayden, Frida Kahlo: The Paintings, HarperCollins Publishers, New York, 1991.
- p. 55 Kettenmann, Andrea, Frida Kahlo 1907-1954: Pain and Passion, Barnes & Noble Books, New York, 2004.
- p. 87 Richmond, Robin, Painters & Places: Frida Kahlo in Mexico, Pomegranite Artbooks, San Francisco, 1994.
- p. 154 Herrera, Hayden, Frida Kahlo: The Paintings, HarperCollins Publishers, New York, 1991.
- p. 370 Herrera, Hayden, Frida Kahlo: A Biography of Frida Kahlo, Harper and Row, 1983.
- p. 80 Herrera, Hayden, Frida Kahlo: A Biography of Frida Kahlo, Harper and Row, 1983.
- p. 366 Herrera, Hayden, Frida Kahlo: A Biography of Frida Kahlo, Harper and Row, 1983.
- p. 150 Barbezat, Suzanne, Frida Kahlo at Home, Francis Lincoln Limited, London, 2016.
- p. 72 Kettenmann, Andrea, Frida Kahlo 1907-1954: Pain and Passion, Barnes & Noble Books, New York, 2004.
- p. 469 Herrera, Hayden, Frida Kahlo: A Biography of Frida Kahlo, Harper and Row, 1983.
- p. 475 Herrera, Hayden, Frida Kahlo: A Biography of Frida Kahlo, Harper and Row, 1983.
- p. 375 Herrera, Hayden, Frida Kahlo: A Biography of Frida Kahlo, Harper and Row, 1983.
- p. 328 Herrera, Hayden, Frida Kahlo: A Biography of Frida Kahlo, Harper and Row, 1983.
- p. 484 Herrera, Hayden, Frida Kahlo: A Biography of Frida Kahlo, Harper and Row, 1983.
- p. 77 Carpenter, Elizabeth, Frida Kahlo, Walker Art Center, Minneapolis, 2007.
- p. 76 Carpenter, Elizabeth, Frida Kahlo, Walker Art Center, Minneapolis, 2007.
Bibliography Page
Alcantara, Isabel and Sandra Egnolff, Frida Kahlo and Diego Rivera, Prestel, Munich, 1999.
Barbezat, Suzanne, Frida Kahlo at Home, Francis Lincoln Limited, London, 2016.
Carpenter, Elizabeth, Frida Kahlo, Walker Art Center, Minneapolis, 2007.
Gelman, Jacques and Natasha, Frida Kahlo Diego Rivera and Mexican Modernism, National Gallery of Australia, 2001.
Grimberg, Salomon, Frida Kahlo: The Still Lifes, Merrill, London, 2008.
Herrera, Hayden, Frida Kahlo: A Biography of Frida Kahlo, Harper and Row, 1983.
Herrera, Hayden, Frida Kahlo: The Paintings, HarperCollins Publishers, New York, 1991.
Hooks, Margaret, Frida Kahlo: Portraits of an Icon, Artes Graficas Palermo, Spain, 2002.
Kettenmann, Andrea, Frida Kahlo 1907-1954: Pain and Passion, Barnes & Noble Books, New York, 2004.
Levine, Barbara with Stephen Jaycox, Finding Frida Kahlo: in Mexico, 55 years after the Death of Frida Kahlo, in San Miguel de Allende, Princeton Architectural Press, New York, 2009.
Richmond, Robin, Painters & Places: Frida Kahlo in Mexico, Pomegranite Artbooks, San Francisco, 1994.
Rosenthal, Mark, Diego Rivera & Frida Kahlo in Detroit, Detroit Institute of Arts, 2015.
|
oercommons
|
2025-03-18T00:38:02.599511
|
02/13/2025
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/124669/overview",
"title": "\"The Converging Polarities of Frida Kahlo as Tinku”",
"author": "Mahara Sinclaire"
}
|
https://oercommons.org/courseware/lesson/125679/overview
|
Flashcards: Climate Change and Human Health
Slides: Student Exploration of the Impacts of Climate Change on Human Health in the United States
Student Exploration of the Impacts of Climate Change on Human Health in the United States
Key Message Four: Climate Change Compromises Human Health and Reshapes Demographics
Overview
Each Key Message features three guiding questions to help educators navigate these topics with students. Each guiding question includes example lessons and supporting videos. The lessons were taken from the Climate Literacy and Energy Awareness Network (CLEAN) educational resources database. The videos were selected from reputable sources to support the lessons.
Increases in extreme heat, drought, flooding, and wildfire activity are negatively impacting the physical health of Southwest residents. Climate change is also shaping the demographics of the region by spurring the migration of people from Central America to the Southwest. Individuals particularly vulnerable to increasing climate change impacts include older adults, outdoor workers, and people with low incomes. Local, state, and federal adaptation initiatives are working to respond to these impacts.
Guiding Question One
Notes From Our Reviewers The CLEAN collection is hand-picked and rigorously reviewed for scientific accuracy and classroom effectiveness.
Teaching Tips
- Teachers may want to have multiple versions of the readings available for different reading levels within a classroom.
- Linked reading for students ranges from full report to abbreviated brochures. Teachers are offered several choices on the instructional design based on time and student reading level. Several topics or one topic can be addressed based on student interest, instructional time and desired depth and breadth of content.
About the Content
- This activity allows students to investigate the US Global Change Research Program's 2016 report, "The Impacts of Climate Change on Human Health in the United States: A Scientific Assessment". Students consider how environmental conditions such as flooding or drought create environmental hazards which impact health effects. They also investigate how climate change will affect environmental conditions and therefore health outcomes. Groups explore the different chapters of the report, making conceptual models of these connections. These are put together to showcase how complex the system is when considering how climate change impacts public health.
- Comments from expert scientist: Provides a lot of good scientific information, has a well laid out lesson plan where students have to engage with the material and use systems thinking. I wonder if all high school science courses could use this lesson plan as the material is so interdisciplinary that it would not fit into many standard high school science classes.
About the Pedagogy
- This activity allows students to read (at different levels) a scientific report and try to synthesize information from that report. They do so through conceptual models and discussion-based activities. The various options for reading materials and evaluation strategies allow teachers to implement this in a variety of classrooms with diverse time frames. In the final evaluation activity, students can write summaries, evaluate mitigation and adaptation strategies, or develop their own resilience building project, adding to the place-based project nature of this activity.
- The extension activities suggest a place-based project for students to develop a resilience building project for their community, allowing many different learners to consider the relevance to their own lives. This strategy of place-based projects has been shown to be engaging for culturally diverse audiences.
- The authors also provide the reading materials at various levels of reading ability to allow for more students to participate.
- Many variations on instructional pathways are provided, including depth of content exploration, accommodations for learning modalities and styles, and links to companion materials to support the learning progression.
- Prior knowledge and misconceptions are addressed. Small group work, enhances the class learning. Students create visuals, models and graphic organizers of the content to structure learning and share new knowledge. Assessments are varied and allow for diverse learning styles.
- This resource engages students in using scientific data.
See other data-rich activities
Technical Details/Ease of Use
- This activity clearly lays out all the details teachers will need to implement this activity. The authors provide various options throughout the module, which helps teachers implement the activity in diverse classrooms.
- Computers for students are useful but materials are downloadable and printable.
Related URLs These related sites were noted by our reviewers but have not been reviewed by CLEAN
- URL for Global Temperature and Precipitation Map should be: https://www.ncdc.noaa.gov/temp-and-precip/global-maps/
- Powerpoint slides for use throughout the lesson.
What populations are most vulnerable to health risks associated with climate change in the Southwest?
Example Lesson
Student Exploration of the Impacts of Climate Change on Human Health in the United States
National Institute of Environmental Health Sciences
Description: This module follows the 5E instructional model to promote student discovery and learning about the complex interactions between climate change, the environment and human health. Students describe the impacts of changing climatic conditions on human health with emphasis on vulnerable populations and apply systems thinking to create a visual model of various health implications arising from climate change. (Please note: NIH Files attached as section-level resources)
Instructional Time: This learning activity takes two to three 45-minute class periods.
Grade Level: Ninth through twelfth
Supporting Videos
How Climate Affects Community Health
Centers for Disease Control and Prevention
Description: This animated video discusses how climate change is altering the environment and increasing disease risk from air pollution, spread of disease vectors, increased high temperatures, violent storms and flooding. Ideas for community preparedness are offered.
Video Length: 4:37 minutes
Guiding Question Two
What adaptation initiatives at the local, state, and federal levels are aimed at responding to the negative health impacts of climate change in the Southwest?
Example Lesson
CDC's Building Resilience Against Climate Effects (BRACE) Framework
Centers for Disease Control and Prevention
Description: Introduce the CDC's BRACE Framework that allows health officials to develop strategies and programs to help communities prepare for the health effects of climate change. Students can brainstorm examples of health impacts in their community and explore strategies for building resilience using the BRACE framework. (Please note: Web page also available as PDF in Section Resources)
Instructional Time: One class period
Grade Level: Sixth through twelfth
Supporting Videos
A new effort to help communities adapt to climate change
PBS NewsHour
Description: Biden's infrastructure bill includes $50 billion for climate resiliency funding to help mitigate and adapt to global warming. Tom Casciato reports on a unique partnership in California that uses behavioral science and cultural awareness in climate studies to help communities cope with extreme weather, as part of our series, 'Peril and Promise: the Challenge of Climate Change.'
Video Length: 8:01 minutes
Guiding Question Three
What determines which communities are considered frontline communities in the Southwest?
Example Lessons
Tackling Climate Change through Environmental Justice High School
EcoRise
Description: This multi-lesson resource set for high school is focused on environmental justice and social science. It asks students to consider inequality and justice in the context of their own lives and the environment through a series of both hands-on and research-focused activities. This unit supports student understanding of the multiple, complex issues and perspectives of environmental justice in the United States. In part one, students complete a group activity under the pressures of environmental discrimination and then evaluate their success. The second and third part uses short videos to explain a real-life example of overcoming environmental discrimination to encourage students to reflect on the complexity of these issues. In the final part, students debate a solution to an issue using assigned roles in a town hall platform.
Instructional Time: This learning activity takes six 60-minute class periods.
Grade Level: Ninth through twelfth
Supporting Videos
Victor Galván discusses environmental impact
PBS
Description: While the state of Colorado is known for its beautiful natural spaces, it also has one of the most polluted zip codes in the country. The majority Latino-community of Commerce City, in northeast Denver, has dealt with the fallout from factories and pollution in their backyards and Victor Galván, an activist from the area, has made it his life's work to advocate for change for the Latino community
Video Length: 3:48 minutes
|
oercommons
|
2025-03-18T00:38:02.633776
|
Melinda Newfarmer
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/125679/overview",
"title": "National Climate Assessment: The Southwest, Key Messages for the Southwest, Key Message Four: Climate Change Compromises Human Health and Reshapes Demographics",
"author": "Lesson Plan"
}
|
https://oercommons.org/courseware/lesson/80422/overview
|
Economics Grade 10 - Demand
Overview
ECONOMICS GRADE 10
TOPIC: Introduction to demand
By the end of this lesson, learners should know how to:
- define Economics
- define demand
- state the law of demand
- list and discuss the factors which determine demand
- draw the demand curve using the demand schedule
- differentiate between changes in quantity demanded and changes in demand
Introduction to Economics
Welcome to Economics Grade 10!
I bet you are all as excited as I am to be a part of this fun and interactive lesson in Economics. What is Economics? How would you define it to a friend or relative?
After discussing or defining Economics by yourself, you can click on the attached links below to view the definition of Economics both in video and written format.
Economics
Table of Content
| Introduction to Economics | What is Economics? |
| Body - Demand | Demand and Law of Demand |
| Pre-activity | Factors that determine demand |
| Classwork activity | Position of the demand curve |
| Changes in quantity demanded and demand | Demand summary |
Definition of demand - Law of demand
Demand has to do with the willingness and ability to buy a good or a service.
It refers to the quantities of goods and services that a consumer is willing and able to buy at a specific price, in any given point in time.
Demand is determined by the buyer (consumer).
See attached images for the definiton of demand and law of demand.
Is the picture below illustrating demand? Why do you say so?
Factors that determine demand
Give the learners the corrections for the pre-activity:
- The quantities demanded for bread will decrease due to an increase in the price of bread. This means that bread has become expensive.
- Dineo will now buy less quantities of rice because she cannot afford more of it anymore since her salary decsreased from R3000 to R2000.
- They will demand more goods and services.
Below is a pictrue that shows illustrations of the factors that have an influence on demand.
You are also welcome to click on the link attached on this section that explains these factors that determine demand.
Pre-activity
Individual activity. Refer to what you have just read and heard about the factors that determine demand and answer the following questions:
- The price of bread increases from R10 to R15. What will happen to the quantities demanded for bread? (2)
- COVID-19 has had a negative effect on Dineo's salary which decreased from R3000 to R2000. What will now happen to Dineo's demand for rice if her salary decreases? (2)
- The population growth rate of South Africa has increased. Will the new members of this country demand to buy more or less goods? (2) TOTAL= 6
Position of the demand curve
CORRECTIONS FOR CLASSWORK ACTIVITY:
The position of the demand curve is downward slopping, from left to right. This is due to the law of demand.
When drawing a graph showing a demand curve, prices should be wriitten on the y-axis (from smallest to biggest price) and quantities on the x-axis (from smallest to biggest price). See the picture below to get a visual representation of the curve:
CLASSWORK ACTIVITY (DURING THE LESSON)
Use the demand schedule below to draw a graph showing a demand curve for chocolate bars. Label all axis carefully. (5)
| PRICE OF CHOCOLATE BARS (P) | QUANTITY OF CHOCOLATE BARS (Q) |
| R10 | 5 |
| R20 | 4 |
| R30 | 3 |
| R40 | 2 |
| R50 | 1 |
Changes in quantity demanded versus changes in demand
CORRECTIONS FOR THE HOMEWORK ACTIVITY:
- Substitute goods are goods that can be used in the place of other goods in order to satisfy a need or a want. Examples: tea and coffe. Complementary goods are goods that can be used together with other goods in order to satisfy a need or a want. Examples: toothbrush and toothpaste.
- The demand curve is downward slopping because of the law of demand.
4.
Changes in quantity demanded:
- Also known as 'movement along the demand curve'.
- Shown by the price of a good or service.
Changes in demand:
- Also known as 'shift of the demand curve'.
- Shown by the other factors of demand (such as level of income) besides the price of a good or a service).
The video below explains in detail the changes in quantity demanded and changes in demand. It also summarizes the content of this lesson. Click on the video below to watch it:
HOMEWORK:
Individual work.
- Differentiate between substitute goods and complementary goods. (4)
- Why is the demand curve downward slopping? (1)
- Price of meat decreases from R50 to R20. What will happen to the portions (quantities) of meat you will be buying? Illustrate this using your own graph. You can use your own quantities. (2)
- Sam's income increases from R2000 to R2500. Will Sam buy more or less litres of milk? Answer this question using a graph. (2) TOTAL = 9
|
oercommons
|
2025-03-18T00:38:02.658225
|
Homework/Assignment
|
{
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"url": "https://oercommons.org/courseware/lesson/80422/overview",
"title": "Economics Grade 10 - Demand",
"author": "Diagram/Illustration"
}
|
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.