12.2 Introduction to the Muscular System

Created by CK-12 Foundation/Adapted by Christine Miller

12.2 Natalia Zabolotnaya
Figure 12.2.1 Natalia Zabolotnaya, 2012 Olympics.

Marvelous Muscles

Does the word muscle make you think of the well-developed muscles of a weightlifter, like the woman in Figure 12.2.1? Her name is Natalia Zabolotnaya, and she’s a Russian Olympian. The muscles that are used to lift weights are easy to feel and see, but they aren’t the only muscles in the human body. Many muscles are deep within the body, where they form the walls of internal organs and other structures. You can flex your biceps at will, but you can’t control internal muscles like these. It’s a good thing that these internal muscles work without any conscious effort on your part, because movement of these muscles is essential for survival. Muscles are the organs of the muscular system.

What Is the Muscular System?

The  consists of all the muscles of the body. The largest percentage of muscles in the muscular system consists of , which are attached to bones and enable voluntary body movements (shown in Figure 12.2.2). There are almost 650 skeletal muscles in the human body, many of them shown in Figure 12.2.2. Besides skeletal muscles, the muscular system also includes , which makes up the walls of the heart, and , which control movement in other internal organs and structures.

11.2.2 Muscular System
Figure 12.2.2 Many of the skeletal muscles in the human muscular system are shown in this drawing of the human body.

Muscle Structure and Function

Muscles are organs composed mainly of muscle cells, which are also called  (mainly in skeletal and cardiac muscle) or  (mainly in smooth muscle). Muscle cells are long, thin cells that are specialized for the function of contracting. They contain protein filaments that slide over one another using energy in . The sliding filaments increase the tension in — or shorten the length of — muscle cells, causing a contraction. Muscle contractions are responsible for virtually all the movements of the body, both inside and out.

Skeletal muscles are attached to bones of the skeleton. When these muscles contract, they move the body. They allow us to use our limbs in a variety of ways, from walking to turning cartwheels. Skeletal muscles also maintain posture and help us to keep balance.

Smooth muscles in the walls of blood vessels contract to cause , which may help conserve body heat. Relaxation of these muscles causes , which may help the body lose heat. In the organs of the digestive system, smooth muscles squeeze food through the gastrointestinal tract by contracting in sequence to form a wave of muscle contractions called . Think of squirting toothpaste through a tube by applying pressure in sequence from the bottom of the tube to the top, and you have a good idea of how food is moved by muscles through the digestive system. Peristalsis of smooth muscles also moves urine through the urinary tract.

Cardiac muscle tissue is found only in the walls of the heart. When cardiac muscle contracts, it makes the heart beat. The pumping action of the beating heart keeps blood flowing through the cardiovascular system.

Muscle Hypertrophy and Atrophy

Muscles can grow larger, or . This generally occurs through increased use, although hormonal or other influences can also play a role. The increase in that occurs in males during puberty, for example, causes a significant increase in muscle size. Physical exercise that involves weight bearing or resistance training can increase the size of skeletal muscles in virtually everyone. Exercises (such as running) that increase the heart rate may also increase the size and strength of cardiac muscle. The size of muscle, in turn, is the main determinant of muscle strength, which may be measured by the amount of force a muscle can exert.

Muscles can also grow smaller, or , which can occur through lack of physical activity or from starvation. People who are immobilized for any length of time — for example, because of a broken bone or surgery — lose muscle mass relatively quickly. People in concentration or famine camps may be so malnourished that they lose much of their muscle mass, becoming almost literally just “skin and bones.” Astronauts on the International Space Station may also lose significant muscle mass because of weightlessness in space (see Figure 12.2.3).

Figure 12.2.3 It is important for astronauts to exercise on board the International Space Station to help counter the loss of muscle mass that occurs because they are weightless without Earth’s gravity.

Many diseases, including and , are often associated with muscle atrophy. Atrophy of muscles also happens with age. As people grow older, there is a gradual decrease in the ability to maintain skeletal muscle mass, known as . The exact cause of sarcopenia is not known, but one possible cause is a decrease in sensitivity to growth factors that are needed to maintain muscle mass. Because muscle size determines strength, muscle atrophy causes a corresponding decline in muscle strength.

In both hypertrophy and atrophy, the number of muscle fibres does not change. What changes is the size of the muscle fibres. When muscles hypertrophy, the individual fibres become wider. When muscles atrophy, the fibres become narrower.

Interactions with Other Body Systems

Muscles cannot contract on their own. Skeletal muscles need stimulation from motor neurons in order to contract. The point where a motor neuron attaches to a muscle is called a . Let’s say you decide to raise your hand in class. Your brain sends electrical messages through motor neurons to your arm and shoulder. The motor neurons, in turn, stimulate muscle fibres in your arm and shoulder to contract, causing your arm to rise.

Involuntary contractions of smooth and cardiac muscles are also controlled by electrical impulses, but in the case of these muscles, the impulses come from the (smooth muscle) or specialized cells in the heart (cardiac muscle). and some other factors also influence involuntary contractions of cardiac and smooth muscles. For example, the fight-or-flight hormone adrenaline increases the rate at which cardiac muscle contracts, thereby speeding up the heartbeat.

Muscles cannot move the body on their own. They need the skeletal system to act upon. The two systems together are often referred to as the . Skeletal muscles are attached to the skeleton by tough connective tissues called . Many skeletal muscles are attached to the ends of bones that meet at a . The muscles span the joint and connect the bones. When the muscles contract, they pull on the bones, causing them to move. The skeletal system provides a system of levers that allow body movement. The muscular system provides the force that moves the levers.

12.2 Summary

  • The consists of all the muscles of the body. There are three types of muscle: (which is attached to bones and enables body movements), (which makes up the walls of the heart and makes it beat), and (which is found in the walls of internal organs and other internal structures and controls their movements).
  • Muscles are organs composed mainly of muscle cells, which may also be called or . Muscle cells are specialized for the function of contracting, which occurs when protein filaments inside the cells slide over one another using energy in .
  • Muscles can grow larger, or . This generally occurs through increased use (physical exercise), although hormonal or other influences can also play a role. Muscles can also grow smaller, or . This may occur through lack of use, starvation, certain diseases, or aging. In both hypertrophy and atrophy, the size — but not the number — of muscle fibres changes. The size of muscles is the main determinant of muscle strength.
  • Skeletal muscles need the stimulus of motor neurons to contract, and to move the body, they need the skeletal system to act upon. contractions of cardiac and smooth muscles are controlled by special cells in the heart, nerves of the , hormones, or other factors.

12.2 Review Questions

  1. What is the muscular system?
  2. Describe muscle cells and their function.
  3. Identify three types of muscle tissue and where each type is found.
  4. Define muscle hypertrophy and muscle atrophy.
  5. What are some possible causes of muscle hypertrophy?
  6. Give three reasons that muscle atrophy may occur.
  7. How do muscles change when they increase or decrease in size?
  8. How do changes in muscle size affect strength?
  9. Explain why astronauts can easily lose muscle mass in space.
  10. Describe how the terms muscle cellsmuscle fibres, and myocytes relate to each other.
  11. Name two systems in the body that work together with the muscular system to carry out movements.
  12. Describe one way in which the muscular system is involved in regulating body temperature.

12.2 Explore More

How your muscular system works – Emma Bryce, TED-Ed, 2017.

3D Medical Animation – Peristalsis in Large Intestine/Bowel || ABP ©, AnimatedBiomedical, 2013.

Muscle matters: Dr Brendan Egan at TEDxUCD, TEDx Talks, 2014.


Figure 12.2.1

Natalia_Zabolotnaya_2012b by Simon Q on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/deed.en) license.

Figure 12.2.2

Bougle_whole2_retouched by Bouglé, Julien from the National LIbrary of Medicine (NLM) on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 12.2.3

Daniel_Tani_iss016e027910 by NASA/ International Space Station Imagery on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).


AnimatedBiomedical. (2013, January 30). 3D Medical animation – Peristalsis in large intestine/bowel || ABP ©. YouTube. https://www.youtube.com/watch?v=Ujr0UAbyPS4&feature=youtu.be

Bouglé, J. (1899). Le corps humain en grandeur naturelle : planches coloriées et superposées, avec texte explicatif. J. B. Baillière et fils. In Historical Anatomies on the Web. http://www.nlm.nih.gov/exhibition/historicalanatomies/bougle_home.html

TED-Ed. (2017, October 26). How your muscular system works – Emma Bryce. YouTube. https://www.youtube.com/watch?v=VVL-8zr2hk4&feature=youtu.be

TEDx Talks. (2014, June 27). Muscle matters: Dr Brendan Egan at TEDxUCD. YouTube. https://www.youtube.com/watch?v=LkXwfTsqQgQ&feature=youtu.be

Wikipedia contributors. (2020, June 15). Natalya Zabolotnaya. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Natalya_Zabolotnaya&oldid=962630409


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Human Biology Copyright © 2020 by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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