Anatomy => Muscle
Muscle, tissue or organ of the animal body characterized by the ability to contract, usually in response to a stimulus from the nervous system. The basic unit of all muscle is the myofibril, a minute, threadlike structure composed of complex proteins. Each muscle cell, or fiber, contains several myofibrils, which are composed of regularly arranged myofilaments of two types, thick and thin. Each thick myofilament contains several hundred molecules of the protein myosin. Thin filaments contain two strands of the protein actin. The myofibrils are made up of alternating rows of thick and thin myofilaments with their ends interleaved. During muscular contractions, these interdigitated rows of filaments slide along each other by means of cross bridges that act as ratchets. The energy for this motion is generated by densely packed mitochondria that surround the myofibrils.
Three types of muscular tissue are recognized: smooth, skeletal, and cardiac.
Visceral, or involuntary, muscle is composed of spindle-shaped cells, each having a central nucleus. The cells have no cross striations, although they do exhibit faint longitudinal striations. Stimuli for the contractions of smooth muscles are mediated by the autonomic nervous system. Smooth muscle is found in the skin, internal organs, reproductive system, major blood vessels, and excretory system.
SKELETAL, OR STRIATED, MUSCLE TISSUE
This type of muscle is composed of long fibers surrounded by a membranous sheath, the sarcolemma. The fibers are elongated, sausage-shaped cells containing many nuclei and clearly display longitudinal and cross striations. Skeletal muscle is supplied with nerves from the central nervous system, and because it is partly under conscious control, it is also called voluntary muscle. Most skeletal muscle is attached to portions of the skeleton by connective-tissue attachments called tendons. Contractions of skeletal muscle serve to move the various bones and cartilages of the skeleton. Skeletal muscle forms most of the underlying flesh of vertebrates.
This muscle tissue composes most of the vertebrate heart. The cells, which show both longitudinal and imperfect cross striations, differ from skeletal muscle primarily in having centrally placed nuclei and in the branching and interconnecting of fibers. Cardiac muscle is not under voluntary control. It is supplied with nerves from the autonomic nervous system, but autonomic impulses merely speed or slow its action and are not responsible for the continuous rhythmic contraction characteristic of living cardiac muscle. The mechanism of cardiac contraction is not yet understood.
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 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.