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15.2: Introduction to the Muscular System

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    22554
  • Marvelous Muscles

    Does the word muscle make you think of the well-developed muscles of a weightlifter, like the woman in this photo? 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 the movement of these muscles is essential for survival. Muscles are the organs of the muscular system.

    Natalia Zabolotnaya
    Figure \(\PageIndex{1}\): Image used with permission (CC BY 2.0; Simon Q; wikimedial.org)

    What Is the Muscular System?

    muscles of body
    Figure \(\PageIndex{2}\): Many of the skeletal muscles in the human muscular system are shown in this drawing of the human body. Image used with permission (public domain; Julien Bougle; via wikimedia.org)

    The muscular system consists of all the muscles of the body. The largest percentage of muscles in the muscular system consists of skeletal muscles, which are attached to bones and enable voluntary body movements. There are almost 650 skeletal muscles in the human body, many of them shown in Figure \(\PageIndex{2}\). Besides skeletal muscles, the muscular system also includes cardiac muscle — which makes up the walls of the heart — and smooth muscles, which control movement in other internal organs and structures.

    Muscle Structure and Function

    Muscles are organs composed mainly of muscle cells, which are also called muscle fibers (mainly in skeletal and cardiac muscle) or myocytes (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 ATP. 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 keep balance.

    Smooth muscles in the walls of blood vessels contract to cause vasoconstriction, which may help conserve body heat. Relaxation of these muscles causes vasodilation, 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 peristalsis. 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 heartbeat. The pumping action of the beating heart keeps blood flowing through the cardiovascular system.

    Muscle Hypertrophy and Atrophy

    Muscles can grow larger, or hypertrophy. This generally occurs through increased use, although hormonal or other influences can also play a role. The increase in testosterone 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 a 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 atrophy, 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 \(\PageIndex{3}\)).

    Many diseases, including cancer and AIDS, 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 sarcopenia. 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 fibers does not change. What changes the size of the muscle fibers? When muscles hypertrophy happens, the individual fibers become wider. When muscles atrophy happens, the fibers become narrower.

    Exercise in space
    Figure \(\PageIndex{3}\): 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. Image used with permission (public domain; NASA; via wikimedia.org)

    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 neuromuscular junction. 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 fibers 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 autonomic nervous system (smooth muscle) or specialized cells in the heart (cardiac muscle). Hormones 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 musculoskeletal system. Skeletal muscles are attached to the skeleton by tough connective tissues called tendons. 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. The skeletal system provides a system of levers that allow body movement. The muscular system provides the force that moves the levers.

    Summary

    • The muscular system consists of all the muscles of the body. There are three types of muscle: skeletal muscle (which is attached to bones and enables voluntary body movements), cardiac muscle (which makes up the walls of the heart and makes it beat), and smooth muscle (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 muscle fibers or myocytes. Muscle cells are specialized for the function of contracting, which occurs when protein filaments inside the cells slide over one another using energy in ATP.
    • Muscles can grow larger, or hypertrophy. This generally occurs through increased use (physical exercise), although hormonal or other influences can also play a role. Muscles can also grow smaller, or atrophy. 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 fibers changes. The size of the 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. Involuntary contractions of cardiac and smooth muscles are controlled by special cells in the heart, nerves of the autonomic nervous system, hormones, or other factors.

    Review

    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 the 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 cells, muscle fibers, and myocytes relate to each other.

    11. Muscle tissue in the stomach is considered ___________________.

    A. cardiac muscle

    B. skeletal muscle

    C. smooth muscle

    D. voluntary muscle

    12. Muscle contraction is the __________ of muscle fibers.

    A. hypertrophy

    B. atrophy

    C. lengthening

    D. shortening

    13. True or False: Smooth muscle does not contract.

    14. Name two systems in the body that work together with the muscular system to carry out movements.

    15. Describe one way in which the muscular system is involved in regulating body temperature.

    Explore More

    Learn more about the growing problem of sarcopenia by watching this TED talk:

    Check out this video to learn about peristalsis of the large intestine: