Muscle

A structure composed of bundles of specialized cells capable of contraction and relaxation to create movement. There are 3 types of muscle: skeletal, smooth, and cardiac. The skeletal muscles are the most prominent in the body (see muscular system). They are called voluntary muscles because they are under conscious control. Skeletal muscles are composed of groups of muscle fibres arranged in bundles called fascicles. A fibre is made up of longitudinal units called myofibrils, the working units of which are filaments of actin and myosin (two proteins that control contraction). A state of partial contraction is constantly maintained – this is muscle tone. Smooth muscle is concerned with the movements of internal organs. It is not under conscious control; for this reason, it is also called involuntary muscle. Smooth muscle is made up of long, spindle-shaped cells, and contracts with the same sliding action of actin and myosin as skeletal muscle. This type of muscle is stimulated by the autonomic nervous system; it also responds to hormones and to levels of chemicals in fluid around the muscle. Cardiac muscle (also called myocardium) is found only in the heart. It is able to contract rhythmically about 100,000 times a day, and has a similar structure to that of skeletal muscle. Contraction is stimulated by the autonomic nervous system, by hormones, and by the stretching of muscle fibres. The most common muscle disorders are injury and lack of blood supply to a muscle. Rare disorders include muscular dystrophy and myasthenia gravis.