Muscular System

The muscular system is the body's network of tissues for both conscious and unconscious movement. Movement is generated through the contraction and relaxation of specific muscles. Some muscles, like those in the arms and legs, are involved in voluntary movements such as raising a hand or flexing the foot. Other muscles are involuntary and function without conscious effort. Voluntary muscles include skeletal muscles and total about 650 in the whole human body. Skeletal muscles are controlled by the somatic nervous system; whereas the autonomic nervous system controls involuntary muscles. Involuntary muscles include muscles that line internal organs. These smooth muscles are called visceral muscles, and they perform tasks not generally associated with voluntary activity throughout the body even when it is asleep. Smooth muscles control several automatic physiological responses such as pupil constriction when iris muscles contract in bright light and blood vessel dilation when smooth muscles around them relax, or lengthen. In addition to skeletal and smooth muscle, which are considered voluntary and involuntary, respectively, there is cardiac muscle, which is considered neither. Cardiac muscle is not under conscious control, and it can also function without external nervous system regulation.

Smooth muscles derive their name from their appearance when viewed in polarized light microscopy; in contrast to cardiac and skeletal muscles, which have striations (appearance of parallel bands or lines), smooth muscle is unstriated. Striations result from the pattern of the myofilaments, actin and myosin, which line the myofibrils within each muscle cell. When many myofilaments align along the length of a muscle cell, light and dark regions create the striated appearance. This microscopic view of muscle reveals some hint of how muscles alter their shape to induce movement. Because muscle cells tend to be elongated, they are often called muscle fibers. Muscle cells are distinct from other cells in the body in shape, protein composition, and in the fact that they are multi-nucleated (have more than one nucleus per cell).