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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 autonomic functions 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 in various tissues.

Differences Between Autonomic and Somatic Nerves.

The efferent nerves of the involuntary system supply all innervated structures of the body except skeletal muscle, which is served 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 structures that 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 nerves generally 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 generally retain 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 the autonomic 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 is transmitted 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 visceral sensory system carries mainly mechanoreceptor and chemosensory information, whereas the afferents of the spinal visceral system principally convey sensations related to temperature and tissue injury of mechanical, chemical, or thermal origin. Cranial visceral sensory information enters the CNS by four cranial nerves: the trigeminal (V), facial (VII), glossopharyngeal (IX), and vagus (X) nerves. These four cranial nerves transmit visceral sensory information from the internal face and head (V); tongue (taste, VII); hard palate and upper part of the oropharynx (IX); and carotid body, lower part of the oropharynx, larynx, trachea, esophagus, and thoracic and abdominal organs (X), with the exception of the pelvic viscera. The pelvic viscera are innervated by nerves from the second through fourth sacral spinal segments.

The visceral afferents from these four cranial nerves terminate topographically in the solitary tract nucleus (STN) (Altschuler et al., 1989). The most massive site for termination of the fibers from the STN is the parabrachial nucleus, which is thus the major relay station. The parabrachial nucleus consists of at least 13 separate subnuclei, which in turn project extensively to a wide range of sites in the brainstem, hypothalamus, basal forebrain, thalamus, and cerebral ...

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