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# 21.3: The Muscular System

Difficulty Level: At Grade Created by: CK-12

## Lesson 21.3: True or False

Name___________________ Class______________ Date________

Write true if the statement is true or false if the statement is false.

_____ 1. Muscles are organs composed mainly of muscle cells.

_____ 2. Each muscle fiber is a very short, thin cell.

_____ 3. There are four types of muscle tissues in the human body.

_____ 4. Both skeletal and cardiac muscles appear striated, or striped.

_____ 5. Contractions of smooth muscle are voluntary.

_____ 6. Skeletal muscle is the most common type of muscle in the human body.

_____ 7. Cardiac muscle contains a great many mitochondria.

_____ 8. There are well over 600 skeletal muscles in the human body.

_____ 9. Each skeletal muscle consists of 125 skeletal muscle fibers.

_____ 10. Muscles can contract, actively extend, and lengthen.

_____ 11. In exercises such as weight lifting, skeletal muscle contracts against a resisting force.

_____ 12. Continued exercise is necessary to maintain bigger, stronger muscles.

_____ 13. Each muscle fiber contains hundreds of organelles called myofibrils.

_____ 14. The region between two Z lines is called a sarcomere.

_____ 15. Voluntary contractions of cardiac and smooth muscles are also controlled by nerves.

Name___________________ Class______________ Date________

Read these passages from the text and answer the questions that follow.

What Are Muscles?

Muscles are organs composed mainly of muscle cells, which are also called muscle fibers. Each muscle fiber is a very long, thin cell that can do something no other cell can do. It can contract, or shorten. Muscle contractions are responsible for virtually all the movements of the body, both inside and out. There are three types of muscle tissues in the human body: cardiac, smooth, and skeletal muscle tissues. They are shown and described below.

Types of Muscle Tissue. Both skeletal and cardiac muscles appear striated, or striped, because their cells are arranged in bundles. Smooth muscles are not striated because their cells are arranged in sheets instead of bundles. (From left to right, images courtesy of the Department of Histology at Jagiellonian University Medical College, Polarlys, and Nathanael Reveal (Nathanael). All images under GNU-FDL 1.2.)

Smooth Muscle

Muscle tissue in the walls of internal organs such as the stomach and intestines is smooth muscle. When smooth muscle contracts, it helps the organs carry out their functions. For example, when smooth muscle in the stomach contracts, it squeezes the food inside the stomach, which helps break the food into smaller pieces. Contractions of smooth muscle are involuntary. This means they are not under conscious control.

Skeletal Muscle

Muscle tissue that is attached to bone is skeletal muscle. Whether you are blinking your eyes or running a marathon, you are using skeletal muscle. Contractions of skeletal muscle are voluntary, or under conscious control. Skeletal muscle is the most common type of muscle in the human body, so it is described in more detail below.

Cardiac Muscle

Cardiac muscle is found only in the walls of the heart. When cardiac muscle contracts, the heart beats and pumps blood. Cardiac muscle contains a great many mitochondria, which produce ATP for energy. This helps the heart resist fatigue. Contractions of cardiac muscle are involuntary, like those of smooth muscle.

Questions

1. Describe what a muscle fiber looks like and what it can do.

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2. What are the three types of muscle tissues in the human body and where are they located?

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3. Explain how cell arrangement causes a muscle to appear either striated or not striated.

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4. For each type of muscle tissue, list whether it is voluntary or involuntary.

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5. Why is it important for cardiac muscle to have many mitochondria?

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## Lesson 21.3: Multiple Choice

Name___________________ Class______________ Date________

Circle the letter of the correct choice.

1. Smooth muscle is found in the
1. heart.
2. stomach.
3. upper leg.
4. middle ear.
2. Skeletal muscle fibers are wrapped in
1. fat.
2. bone.
3. connective tissue.
4. none of the above.
3. Skeletal muscles need a rich blood supply to provide them with
1. oxygen.
2. carbon dioxide.
3. water.
4. none of the above.
4. The biceps and triceps muscles are located in the
1. lower arm.
2. upper arm.
3. knee.
4. stomach.
5. Each myofibril is made up of how many types of protein filaments?
1. one
2. two
3. three
4. four
6. Actin filaments are anchored to structures called
1. W lines.
2. X lines.
3. Y lines.
4. Z lines.
7. Muscles need a stimulus from which of the following to “tell” them to contract?
1. a muscle cell
2. the skeleton
3. a nerve cell
4. none of the above
8. Two main proteins found in muscle are
1. smooth and skeletal.
2. tendons and ligaments.
3. actin and myosin.
4. myofibrils and fibers.

## Lesson 21.3: Vocabulary I

Name___________________ Class______________ Date________

Match the vocabulary word with the proper definition.

Definitions

_____ 1. muscle tissue that is attached to bone

_____ 2. long, thin muscle cell that has the ability to contract, or shorten

_____ 3. theory that explains muscle contraction by the sliding of myosin filaments over actin filaments within muscle fibers

_____ 4. tough connective tissue that attaches skeletal muscle to bones of the skeleton

_____ 5. human body system that includes all the muscles of the body

_____ 6. organs composed mainly of muscle cells

_____ 7. the region between two Z lines

_____ 8. involuntary, nonstriated muscle that is found in the walls of internal organs such as the stomach

_____ 9. organelles contained within each muscle fiber

_____ 10. involuntary, striated muscle found only in the walls of the heart

_____ 11. tiny structures within the myosin filaments, that can attach to actin filaments

Terms

a. cardiac muscle

b. cross bridges

c. muscle fiber

d. muscles

e. muscular system

f. myofibril

g. sarcomere

h. skeletal muscle

i. sliding filament theory

j. smooth muscle

k. tendon

## Lesson 21.3: Vocabulary II

Name___________________ Class______________ Date________

Fill in the blank with the appropriate term.

1. Both skeletal and ___________________ muscles appear striated.

2. Muscle tissue that is attached to ________________ is skeletal muscle.

3. Contractions of skeletal muscle are voluntary, or under ________________ control

4. Cardiac muscle contains a great many mitochondria, which produce ___________________ for energy.

5. Skeletal muscles need a rich blood supply to provide them with __________________ and oxygen and to carry away their wastes.

6. Many skeletal muscles are attached to the ends of bones that meet at a ___________________.

7. Muscles can only contract and cannot actively ____________________.

8. To move bones in opposite directions, pairs of _________________ must work in opposition.

9. In exercises such as weight lifting, skeletal muscle contracts against a _________________ force.

10. If you don’t use a muscle, it will get smaller and weaker—so use it or _______________ it.

11. Muscle contraction occurs when muscle fibers get __________________.

12. Within a sarcomere, myosin filaments __________________ the actin filaments.

## Lesson 21.3: Critical Writing

Name___________________ Class______________ Date________

Thoroughly answer the question below. Use appropriate academic vocabulary and clear and complete sentences.

Describe the Sliding Filament Theory.

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