muscle (m) /mus″əl/ [L, musculus] , a kind of tissue composed of fibers or cells that is able to contract, causing movement of body parts and organs. Muscle fibers are richly vascular, excitable, conductive, and elastic. There are two basic kinds—striated muscle and smooth muscle. Striated muscle, which composes all skeletal muscles except the myocardium, is long and voluntary. It responds very quickly to stimulation and is paralyzed by interruption of its innervation. Smooth muscle, of which all visceral muscles are composed, is short and involuntary. It reacts slowly to all stimuli and does not entirely lose its tone if innervation is interrupted. The myocardium is sometimes classified as a third (cardiac) kind of muscle, but it is basically a striated muscle that does not contract as quickly as the striated muscles of the rest of the body. See also cardiac muscle, striated muscle, smooth muscle.
