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21.3: The Muscular System

Difficulty Level: At Grade Created by: CK-12
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Lesson Objectives

  • Identify and describe the three types of human muscle tissue.
  • Describe the structure of skeletal muscles, and explain how they move bones.
  • Explain how muscles contract according to the sliding filament theory.


  • cardiac muscle
  • muscle fiber
  • muscular system
  • skeletal muscle
  • sliding filament theory
  • smooth muscle
  • tendon


The muscular system consists of all the muscles of the body. Does the word muscle make you think of the bulging biceps of a weightlifter, like the man in Figure below? Muscles such as biceps that move the body are easy to feel and see, but they aren’t the only muscles in the human body. Many muscles are deep within the body. They form the walls of internal organs such as the heart and stomach. You can flex your biceps like a body builder but you cannot control the muscles inside you. It’s a good thing that they work on their own without any conscious effort your part, because movement of these muscles is essential for survival.

This weightlifter works hard to build big muscles in his upper arms.

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 in Figure below and described below. You can also watch an overview of the three types at this link: http://www.youtube.com/watch?v=TermIXEkavY.

Types of Muscle Tissue. This artist's rendition shows that 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.

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.

Skeletal Muscles

There are well over 600 skeletal muscles in the human body, some of which are identified in Figure below. Skeletal muscles vary considerably in size, from tiny muscles inside the middle ear to very large muscles in the upper leg.

Skeletal Muscles. Skeletal muscles enable the body to move.

Structure of Skeletal Muscles

Each skeletal muscle consists of hundreds or even thousands of skeletal muscle fibers. The fibers are bundled together and wrapped in connective tissue as shown Figure below. The connective tissue supports and protects the delicate muscle cells and allows them to withstand the forces of contraction. It also provides pathways for nerves and blood vessels to reach the muscles. Skeletal muscles work hard to move body parts. They need a rich blood supply to provide them with nutrients and oxygen and to carry away their wastes. You can watch a video about skeletal muscle structure and how skeletal muscles work at this link: http://www.youtube.com/watch?v=XoP1diaXVCI.

Skeletal Muscle Structure. A skeletal muscle contains bundles of muscle fibers inside a “coat” of connective tissue.

The Anatomy of a Muscle Cell is available at http://www.youtube.com/watch?v=uY2ZOsCnXIA (16:32).

Skeletal Muscles and Bones

Skeletal muscles are attached to the skeleton by tough connective tissues called tendons (see Figure above). Many skeletal muscles are attached to the ends of bones that meet at a joint. The muscles span the joint and connect the bones. When the muscles contract, they pull on the bones, causing them to move. You can watch a video showing how muscles and bones move together at this link: http://www.youtube.com/watch?v=7Rzi7zYlWno.

Muscles can only contract. They cannot actively extend, or lengthen. Therefore, to move bones in opposite directions, pairs of muscles must work in opposition. For example, the biceps and triceps muscles of the upper arm work in opposition to bend and extend the arm at the elbow (see Figure below). You can watch an animation of these two muscles working in opposition at the link below. What other body movements do you think require opposing muscle pairs? http://www.youtube.com/watch?v=T-ozRNVhGVg

Triceps and biceps muscles in the upper arm are opposing muscles.

Use It or Lose It

In exercises such as weight lifting, skeletal muscle contracts against a resisting force (see Figure below). Using skeletal muscle in this way increases its size and strength. In exercises such as running, the cardiac muscle contracts faster and the heart pumps more blood. Using cardiac muscle in this way increases its strength and efficiency. Continued exercise is necessary to maintain bigger, stronger muscles. If you don’t use a muscle, it will get smaller and weaker—so use it or lose it.

This exercise pits human muscles against a force. What force is it?

Muscle Contraction

Muscle contraction occurs when muscle fibers get shorter. Literally, the muscle fibers get smaller in size. To understand how this happens, you need to know more about the structure of muscle fibers.

Structure of Muscle Fibers

Each muscle fiber contains hundreds of organelles called myofibrils. Each myofibril is made up of two types of protein filaments: actin filaments, which are thinner, and myosin filaments, which are thicker. Actin filaments are anchored to structures called Z lines (see Figure below). The region between two Z lines is called a sarcomere. Within a sarcomere, myosin filaments overlap the actin filaments. The myosin filaments have tiny structures called cross bridges that can attach to actin filaments.

Sarcomere. A sarcomere contains actin and myosin filaments between two Z lines.

Sliding Filament Theory

The most widely accepted theory explaining how muscle fibers contract is called the sliding filament theory. According to this theory, myosin filaments use energy from ATP to “walk” along the actin filaments with their cross bridges. This pulls the actin filaments closer together. The movement of the actin filaments also pulls the Z lines closer together, thus shortening the sarcomere. You can watch this occurring in a video animation at http://www.youtube.com/watch?v=7V-zFVnFkWg.

When all of the sarcomeres in a muscle fiber shorten, the fiber contracts. A muscle fiber either contracts fully or it doesn’t contract at all. The number of fibers that contract determines the strength of the muscular force. When more fibers contract at the same time, the force is greater.

Actin, myosin and muscle contraction are discussed at http://www.youtube.com/watch?v=zopoN2i7ALQ (9:38).

Additional information about muscle contraction is available at http://www.youtube.com/watch?v=LiOfeSsjrB8 (9:22) and http://www.youtube.com/watch?v=SauhB2fYQkM (14:42).

Muscles and Nerves

Muscles cannot contract on their own. They need a stimulus from a nerve cell to “tell” them to contract. Let’s say you decide to raise your hand in class. Your brain sends electrical messages to nerve cells, called motor neurons, in your arm and shoulder. The motor neurons, in turn, stimulate muscle fibers in your arm and shoulder to contract, causing your arm to rise. Involuntary contractions of cardiac and smooth muscles are also controlled by nerves.

Lesson Summary

  • There are three types of human muscle tissue: smooth muscle (in internal organs), skeletal muscle, and cardiac muscle (only in the heart).
  • Skeletal muscles are attached to the skeleton and cause bones to move when they contract.
  • According to the sliding filament theory, a muscle fiber contracts when myosin filaments pull actin filaments closer together and thus shorten sarcomeres within a fiber. When all the sarcomeres in a muscle fiber shorten, the fiber contracts.

Lesson Review Questions


1. What can muscle cells do that other cells cannot?

2. Why are skeletal and cardiac muscles striated?

3. Where is smooth muscle tissue found?

4. What is the function of skeletal muscle?

5. How are skeletal muscles attached to bones?

Apply Concepts

6. A serious neck injury may leave a person paralyzed from the neck down. Explain why.

Think Critically

7. Compare and contrast the three types of muscle tissue.

8. Explain why many skeletal muscles must work in opposing pairs.

9. Explain how muscles contract according to the sliding filament theory.

Points to Consider

Bones and muscles are organs. They are contained within the skin, which is also an organ.

  • Do you know which organ system the skin belongs to?
  • What other organs might belong to the same organ system as the skin?

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