Muscle

Muscle is a type of tissue in the bodies of animals and humans capable of contracting and then relaxing. This contraction generates force, which moves bones, moves body parts, and propels substances through body cavities. Muscles also produce heat and participate in regulating body temperature.

Muscle tissue operates through two proteins called actin and myosin. When a nerve signal arrives, calcium is released from storage within the tissue, causing these proteins to slide over one another and shorten the muscle fiber. The energy for this work comes from ATP, the energy molecule of the body.

Types of muscle

Three types are generally distinguished. Skeletal muscle, attached to bones, is striated and responds readily to voluntary control; it is connected to bones by tendons. Cardiac muscle, found in the heart, is striated but operates involuntarily and contracts continuously. Smooth muscle, found in blood vessel walls, the digestive tract, and other organs, operates automatically.

Structure

Skeletal muscle consists of muscle fibers, each containing smaller contractile units. Each fiber contains small repeated structures called sarcomeres, which contain actin and myosin. The arrangement of these units produces the striated appearance visible under a microscope.

Function and adaptation

Muscles can change in response to use. Sustained loading can increase muscle fiber size (hypertrophy), while disuse causes shrinkage (atrophy). Muscles work in groups: one muscle contracts while its opposing muscle relaxes.

Disease and injury

Muscle problems include fiber tears from excessive strain, cramps, and hereditary diseases such as muscular dystrophy, which progressively weakens muscles. Disorders affecting the nerve-muscle connection can also impair contraction, even when the muscle tissue itself remains healthy.