Although you doubtless appreciate the role your muscular system plays in your daily activities, you may be unaware of the different types of muscle tissue in your body and the special role each plays in keeping you functioning to your best advantage. These different muscle types have some features in common, but each is highly specialized for its function and responds to changes in your body in different ways.
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Voluntary muscle tissue is also called skeletal muscle, because it makes up the muscles that attach to bones and help move parts of your skeleton, such as your arms and legs. Skeletal muscle is voluntary, because it responds to your conscious thoughts and intentions. When viewed microscopically, skeletal muscle cells have regular patterns, called striations, made up of specialized proteins that facilitate strong muscle contraction. These muscle cells are rectangular and tightly attached to one another end to end. Groups of these joined cells, called fibers, bundle together in larger and larger groups attached to one another by connective tissue. The largest groups of fibers eventually end in tendons, which attach your muscles to your bones.
The muscular system contains two types of involuntary muscle that function automatically without conscious thought. One kind, called smooth muscle, is mostly found in the walls of hollow organs, such as the stomach, intestines and bladder. It's also found in the walls of arteries, which are vessels that carry blood away from the heart and receive a surge of pressure each time your heart beats. Under a microscope, smooth muscle lacks the striations of skeletal muscles, although its contractile proteins are similar to those in skeletal muscle. These proteins produce slower, more rhythmic contractions than those in skeletal muscle. This type of contraction helps smooth muscle carry out its functions, such as moving food through your gastrointestinal tract and emptying your bladder. Smooth muscle also contracts or relaxes to adjust the diameter of arteries in response to changes in your circulatory system.
The third type of muscle in your body is cardiac muscle, which is highly specialized and found only in the walls of your heart. Microscopically, cardiac muscle fibers are rectangular and have striations like those in skeletal muscle. But cardiac cells branch and join with neighboring cells and have specialized connections with one another that allow for both tight attachment and rapid cell-to-cell communication. Cardiac muscle fibers also contain exceptionally high numbers of mitochondria, which are energy-producing components. They're also surrounded by an extensive network of capillaries containing oxygen-rich blood. These and other adaptations help cardiac muscle cells contract constantly and steadily, keeping the heart pumping blood without interruption.
Your body regulates contraction of the three types of muscle in different ways. Skeletal muscles contract in response to impulses from nerves, called motor nerves, whose endings contact muscle cells and release neurotransmitters. Smooth muscle cells are stimulated to contract in response to activity of nearby nerve cells or hormones and other molecules in their vicinity. Smooth muscle also contracts as a natural response to stretching. Contraction of cardiac muscle and the rate of your heartbeat are controlled by nerve endings in the heart wall and by hormones, such as epinephrine and norepinephrine, that circulate in the blood.
Both skeletal and smooth muscle tissue can repair themselves after injury and can increase the number of cells they contain when needed, such as when you exercise and build your muscle mass. Smooth muscle cells divide when new cells are needed. Although skeletal muscle cells can't divide, special cells called satellite cells can develop into new muscle cells as needed. This mechanism remains functional throughout life. Cardiac muscle cells can enlarge but can't divide to produce new cells. Many research studies, such as one published in the June 2001 issue of "Journal of Clinical Investigation," suggest that undifferentiated cells called adult stem cells might help replace damaged cardiac muscle. But additional work is needed to determine whether this strategy will eventually be used to treat people with heart disease.
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