muscle

the contractile tissue of the body, which produces movements of various structures both internally and externally. There are three types of muscle: 1. striated or VOLUNTARY MUSCLE , which has a striped appearance when viewed under a microscope and is attached to the SKELETON . It is called 'voluntary' because it is under the control of the will and produces movements, e.g. in the limbs. 2. smooth or INVOLUNTARY MUSCLE , which has a plain appearance when viewed microscopically and is not under conscious control but is supplied by the AUTONOMIC NERVOUS SYSTEM . Examples are muscles that supply the digestive and respiratory tracts. 3. CARDIAC MUSCLE , the specialized muscle of the walls of the HEART , which is composed of a network of branching, elongated fibres that rejoin and interlock, each having a nucleus. It has a somewhat striated appearance, and where there are junctions between fibres, irregular transverse bands occur, known as intercalated discs. This muscle is involuntary, and contracts and expands rhythmically throughout an individual's life. However, rate of heartbeat is influenced by activity within the VAGUS NERVE .