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The autonomic nervous system, also called the visceral, vegetative, or involuntary nervous system, is distributed widely throughout the body and regulates autonomicfunctions that occur without conscious control. In the periphery, it consists of nerves, ganglia, and plexuses that innervate the heart, blood vessels, glands, other visceral organs, and smooth muscle invarious tissues.

The efferent nerves of the involuntary system supply all innervated structures of the body except skeletal muscle, which isserved by somatic nerves. The most distal synaptic junctions in the autonomic reflex arc occur in ganglia that are entirely outside the cerebrospinal axis. These ganglia are small but complex structuresthat contain axodendritic synapses between preganglionic and postganglionic neurons. Somatic nerves contain no peripheral ganglia, and the synapses are located entirely within the cerebrospinal axis.Many autonomic nerves form extensive peripheral plexuses, but such networks are absent from the somatic system. Whereas motor nerves to skeletal muscles are myelinated, postganglionic autonomic nervesgenerally are nonmyelinated. When the spinal efferent nerves are interrupted, the denervated skeletal muscles lack myogenic tone, are paralyzed, and atrophy, whereas smooth muscles and glands generallyretain some level of spontaneous activity independent of intact innervation.
Visceral Afferent Fibers
The afferent fibers from visceral structures are the first link in the reflex arcs of theautonomic system. With certain exceptions, such as local axon reflexes, most visceral reflexes are mediated through the central nervous system (CNS).

Information on the status of the visceral organs istransmitted to the CNS through two main sensory systems: the cranial nerve (parasympathetic) visceral sensory system and the spinal (sympathetic) visceral afferent system (Saper, 2002). The cranial...
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