The Human Body

Autonomic Nervous System

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Autonomic Nervous System
The autonomic nervous system directs all activities of the body that occur without a person’s conscious control, such as breathing and food digestion. It has two parts: the sympathetic division, which is most active in times of stress, and the parasympathetic division, which controls maintenance activities and helps conserve the body’s energy.

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 centers 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.

Two antagonistic divisions make up the autonomic nervous system: the sympathetic, or thoracicolumbar, division, which stimulates the heart, dilates the bronchi, contracts the arteries, and inhibits the digestive system, preparing the organism for physical action; and the parasympathetic, or craniosacral, division, which 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 fibers 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. Fibers 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 fibers 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.