Sympathetic nervous system
Webster, Douglas B. Department of Otorhinolaryngology and Biocommunication, Louisiana State University Medical Center, Baton Rouge, Louisiana.
Last reviewed:August 2020
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The portion of the autonomic nervous system concerned with nonvolitional preparation of the organism for emergency situations. The sympathetic nervous system is part of the autonomic nervous system, which controls (through involuntary means) visceral (internal organ) functions of the body, including those associated with cardiovascular activity, digestion, metabolism, and thermoregulation. Typically, the autonomic nervous system is partitioned into the sympathetic and parasympathetic nervous systems, based on the region of the brain or spinal cord in which the autonomic nerves have their origin. The sympathetic nervous system is best understood in mammals. This system consists of two neuron chains from the thoracic and lumbar regions of the spinal cord to viscera and blood vessels (see illustration). The first or preganglionic neuron has its cell body in the spinal cord and sends its axon (a process or nerve fiber of a neuron that carries the unidirectional nerve impulse away from the cell body) to synapse with a postganglionic sympathetic neuron, which lies either in a chain of sympathetic ganglia paralleling the spinal cord or in a sympathetic ganglion near the base of the large blood vessels vascularizing the alimentary viscera. The postganglionic axons are longer than the preganglionic axons and extend to glands or smooth muscles of viscera and blood vessels. Sensory visceral nerve fibers innervate blood vessels and viscera, and they carry sensory information to the spinal cord, thus providing a visceral reflex arc. See also: Autonomic nervous system; Blood vessel; Ganglion; Mammalia; Nervous system (vertebrate); Neuron; Parasympathetic nervous system; Spinal cord
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