Understand the Components of the

Muscular System

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OUR MUSCULAR SYSTEM controls all of your body movement

with coordinated teams of muscles. Muscle tissue moves

the body, powers internal processes, maintains posture, and

produces body heat. There are up to 700 large and small

muscles in your body that bulge and ripple under your skin and

are arranged in crisscrossing layers.

bones and cause bones to move when muscles contract (shorten).

There are three types of muscles: voluntary, involuntary, and cardiac.

Muscles you consciously control are voluntary skeletal muscles; they are attached to

bones. Arm and leg muscles are voluntary and move only when you move them. When you

move the big muscle in your arm (biceps), it contracts, and your lower arm moves up. Skeletal

muscles have long, thin fibers that are striated (striped or streaked). These striations,or

ment of densely packed muscle

fibers. There are more than 600

skeletal bones in your body.

Involuntary smooth muscles

work automatically and are in the

walls of your stomach, intestines,

blood vessels, and other body

organs. They are smooth, without

striations. They contract slowly

and control your chest movement as you breathe. Other flat sheets of smooth muscles stretch

your stomach when you eat a big meal. Some involuntary muscles contract constantly to main-

tain body posture. Strong, stabilizing muscles in your neck, inner shoulders, and upper back

are in constant activity tensing to steady your head and contracting in coordinated teams to

permit neck movements. Upper back muscles that attach to your shoulder blades (scapula)

help stabilize your shoulders.

Cardiac muscle is the involuntary muscle of your heart that is striated like skeletal muscle

and is found only in your heart. It contracts automatically and constantly. It never tires or rests.

It contracts about 70 times a minute and 100,000 times per day. Blood carries oxygen to your

Healthy young skin has resilient fibers made of protein elastin that helps skin return to its

original position, as after smiling. With age, elastin breaks down. As a result, the skin’s dermis

becomes loosely attached to the muscles beneath. Wrinkles appear when the skin can no longer

stretch or shrink easily. “Crow’s feet” radiate from the corners of the eyes, followed by lines

around the brow and mouth, between the eyebrows, on the chin, and on the bridge of the nose.

Facial wrinkles are at right angles to the muscle fibers so they reveal the pattern of facial

muscles. Exposure to excessive sunlight and temperatures hastens wrinkling.

cells, including your muscle fibers.

The striations in skeletal and cardiac muscles are evenly spaced stripes that run perpendicu

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lar (vertical) to the length of the muscle cell. Skeletal muscles have long cells. In contrast, car

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diac muscles have short, branching cells. All muscles are made of the same proteins, but the

Cardiac Involuntary Striated, short Heart

Muscles are dependent on the nervous system to stimulate and integrate your activities.

Your brain controls your body and sends messages to every part along nerves that reach into

every muscle and touch every muscle fiber. When a message reaches a muscle fiber, the tiny

strands inside it move. Strands slide toward each other. The muscle fiber gets shorter, and the

whole muscle contracts and moves the body

part to which it is attached.

thinnest strands overlap each other and look

crisscrossed like your fingers laced together.

Some skeletal muscles are attached to other

muscles; some are attached to skin. A typical

muscle spans a joint and tapers at each end,

attaching to a fibrous tendon that attaches to

a bone. Each tendon connects to at least two

bones.

upper arm, your biceps muscle is paired with

your triceps muscle, which is attached to bones in

your lower arm and shoulder.

Nearly one-fourth of your skeletal muscles are

in your face. They raise your eyebrows and eye

lids. Others pull the corners of your mouth into a

smile. Six muscles control the movement of each

of your eyeballs, allowing you to focus in many

muscles are arranged in groups. For example,

when there is a large muscle, there is a small

muscle. Your leg has a combination of all three

sizes. Big, fan-shaped muscles in your chest help

you move your arms and your shoulders. Twenty

small muscles in your hands allow you to make a

fist, pick things up, and play the piano.

More than 300 skeletal muscles work together

when you take one step. Calf muscles are some of

the strongest muscles. They help you walk, run,

motion is generated by the contraction of skeletal

muscles and tendons. Exercise makes muscles

bigger and stronger. Without exercise, muscles

shrink and lose strength. The harder muscles work, the more oxygen they need. When you

exercise, you breathe faster and more deeply to receive more oxygen. When you experience

shortness of breath, your muscles have lost oxygen and are too tired to contract. Your body is