Autonomic Nervous System

Autonomic Nervous System, in vertebrate anatomy, one of the two main divisions of the nervous system, supplying impulses to the body's heart muscles, smooth muscles, and glands. The autonomic system controls the action of the glands; the functions of the respiratory, circulatory, digestive, and urogenital systems; and the involuntary muscles in these systems and in the skin. Controlled by nerve centres in the lower part of the brain, the system also has a reciprocal effect on the internal secretions, being controlled to some degree by the hormones and exercising some control, in turn, on hormone production.

Three divisions make up the autonomic nervous system. Two of them, the sympathetic (or thoracicolumbar) and the parasympathetic (or craniosacral), are antagonistic. The sympathetic division stimulates the heart, dilates the bronchi, contracts the arteries, and inhibits the digestive system, preparing the organism for physical action. The parasympathetic division has the opposite effects, and prepares the organism for feeding, digestion, and rest. The sympathetic division consists of a chain of interconnected ganglia (groups of nerve cells) on each side of the vertebral column, which send nerve fibres to several large ganglia, such as the coeliac ganglion. They, in turn, give rise to nerves passing to the internal organs. The ganglia of the sympathetic chains are connected to the central nervous system by fine branches connecting each ganglion with the spinal cord. Fibres of the parasympathetic system arise in the brain and, with the cranial nerves, especially the vagus and accessory nerves, pass to ganglia and plexuses (networks of nerves) within the various organs. The lower part of the body is innervated by fibres arising from the lowest (sacral) segment of the spinal cord and passing to the pelvic ganglion, which gives rise to nerves for such organs as the rectum, bladder, and genital organs.

The third division of the autonomic nervous system is the relatively independent enteric nervous system. The nerves composing the enteric nervous system are located in the walls of the oesophagus, stomach, small intestine, and large intestine and are concerned with the control of gastric secretions and peristaltic movements of the gastrointestinal tract. Interconnected ganglia pass to plexuses, which have specific functions relating to the position occupied in succeeding sections of the gastrointestinal tract, for example control of muscular activity. The enteric nervous system is connected to the central nervous system by the vagus nerve.