Muscles

There are three types of muscles: skeletal, smooth, and cardiac (heart). Two of these kinds--skeletal and smooth--are part of the musculoskeletal system.

Skeletal muscle is what most people think of as muscle, the type that can be contracted to move the various parts of the body. Skeletal muscles are bundles of contractile fibers that are organized in a regular pattern, so that under a microscope they appear as stripes (hence, they are also called striped or striated muscles). Skeletal muscles, which are responsible for posture and movement, are attached to bones and arranged in opposing groups around joints. For example, muscles that bend the elbow (biceps) are countered by muscles that straighten it (triceps). Skeletal muscles are controlled by the brain and are called voluntary muscles because they operate with a person's awareness. The size and strength of skeletal muscles are maintained or increased by regular exercise. In addition, growth hormone and testosterone help muscles grow in childhood and maintain their size in adulthood.

See the figure Musculoskeletal System.

Smooth muscle controls certain bodily functions that are not readily noticeable. Smooth muscle surrounds many arteries and contracts to adjust blood flow. It surrounds the intestines and contracts to move food and feces along the digestive tract. Smooth muscle is controlled by the brain also, but not voluntarily. The triggers for contracting and relaxing smooth muscle are controlled by the body's needs, so smooth muscle operates without a person's awareness. Smooth muscles are also called involuntary muscles.

Cardiac (heart) muscle is the type of muscle that forms the heart; it is not part of the musculoskeletal system. Like skeletal muscle, cardiac muscle has a regular pattern of fibers that also appear as stripes under the microscope. However, cardiac muscle contracts and relaxes rhythmically without a person's awareness.