Muscle

Smooth muscle is found in organs made up also of other tissues, such as the heart and intestines, which contain layers of connective tissue. Skeletal muscle is usually found in bundles, composing muscular structures resembling organs in function. These often ripple the skin visibly during muscular action. The shape of the muscular organ is dependent on its location and function. Such a muscle structure is named scientifically according to its shape, function, or attachments: the trapezius muscle of the back, for example, is so called because it looks like a geometrical figure known as a trapezoid; and the masseter (Greek masētēr, “a chewer”) muscle of the face is so called because it is used in chewing food. Muscle fibers have been classified by function into slow twitch (type I) and fast twitch (type II). Most skeletal muscles are composed of both types of fibers, although one type may predominate. The fast-twitch, darker-hued muscle fibers contract more rapidly and produce bursts of power; the slow-twitch, lighter-hued muscle fibers have greater endurance.

Contraction of a muscle cell is activated by the release of calcium from inside the cell, probably in response to electrical changes at the cell's surface. See Anatomy.

Muscles that are given proper exercise react to stimuli quickly and powerfully, and are said to possess tone. As a result of excessive use, muscles may hypertrophy, that is, increase in size because of an increase in size of the individual muscle cells. As a result of prolonged disuse, muscles may atrophy, or diminish in size, and become weaker. In certain diseases, such as various forms of paralysis, the muscles may atrophy to such a degree that they are reduced to a fraction of their normal size.

See Muscular Dystrophy.