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 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; 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 via four cranial nerves: the trigeminal (V), facial (VII), glossopharyngeal (IX), and vagus (X). These four cranial nerves transmit visceral sensory information from the internal face and head (V), tongue (taste, VII), hard palate, upper part of the oropharynx, and carotid body (IX), and lower part of the oropharynx, larynx, trachea, esophagus, and thoracic and abdominal organs (X), with the exception of the pelvic viscera, which 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).
Sensory afferents from visceral organs also enter the CNS via the spinal nerves. Those concerned with muscle chemosensation may arise at all spinal levels; sympathetic visceral sensory afferents generally arise at the thoracic levels where sympathetic preganglionic neurons are found. Pelvic sensory afferents from spinal segments S2—S4 enter at that level and are important for the regulation of sacral parasympathetic outflow. In general, visceral afferents that enter the spinal nerves convey information concerned with temperature as well as nociceptive visceral inputs.
Neurotransmitters that mediate transmission from sensory fibers have not been characterized unequivocally. Substance P and calcitonin gene—related peptide are leading candidates for
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