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ANATOMY AND GENERAL FUNCTIONS

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The autonomic nervous system (ANS; a.k.a. the visceral, vegetative, or involuntary nervous system) 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.

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DIFFERENCES BETWEEN AUTONOMIC AND SOMATIC NERVES
  • The efferent nerves of the ANS 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. Somatic nerves contain no peripheral ganglia, and the synapses are located entirely within the cerebrospinal axis.

  • Many autonomic nerves form extensive peripheral plexuses; such networks are absent from the somatic system.

  • Postganglionic autonomic nerves generally are nonmyelinated; motor nerves to skeletal muscles are myelinated.

  • When the spinal efferent nerves are interrupted, smooth muscles and glands generally retain some level of spontaneous activity, whereas the denervated skeletal muscles are paralyzed.

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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).

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Information on the status of the visceral organs is transmitted to the CNS through 2 main sensory systems: the cranial nerve (parasympathetic) visceral sensory system and the spinal (sympathetic) visceral afferent system. 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.

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Cranial visceral sensory information enters the CNS by 4 cranial nerves: the trigeminal (V), facial (VII), glossopharyngeal (IX), and vagus (X) nerves. These 4 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 4 cranial nerves terminate topographically in the solitary tract nucleus.

Sensory afferents from visceral organs also enter the CNS from the spinal nerves and convey information concerned with temperature as well as nociceptive visceral inputs related to mechanical, chemical, and thermal stimulation. Those concerned with muscle chemosensation may arise at all spinal levels, whereas sympathetic visceral sensory afferents generally arise at the thoracic levels where sympathetic preganglionic neurons are found. The neurotransmitters that mediate transmission from sensory fibers have not been characterized unequivocally. Substance P and calcitonin gene-related peptide (CGRP), are leading candidates for neurotransmitters that communicate nociceptive stimuli from the periphery. Somatostatin (SST), vasoactive intestinal polypeptide ...

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