The pelvic viscera are innervated by nerves from the second through fourth sacral spinal segments. 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. Cranial visceral sensory information enters the CNS by four cranial nerves: the trigeminal (V), facial (VII), glossopharyngeal (IX), and vagus (X) nerves. 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. 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). 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. It reduces blood pressure and heart and respiratory rates and conserves energy through relaxation and rest. It stimulates the production of digestive enzymes and stimulates the processes of digestion, urination, and defecation. The resultant decrease in blood flow to the salivary glands causes. Whereas motor nerves to skeletal muscles are myelinated, postganglionic autonomic nerves generally are nonmyelinated. The parasympathetic nervous system is active during periods of digestion and rest. Sympathetic nerves stimulate the constriction of blood vessels through the digestive tract. Many autonomic nerves form extensive peripheral plexuses, but such networks are absent from the somatic system. Somatic nerves contain no peripheral ganglia, and the synapses are located entirely within the cerebrospinal axis. Somatic responses are solely based on skeletal muscle contraction. The main difference between the somatic and autonomic systems is in what target tissues are effectors. These ganglia are small but complex structures that contain axodendritic synapses between preganglionic and postganglionic neurons. Postganglionic sympathetic nerves also synapse with intrinsic neurons and generally induce relaxation by inhibiting the release of ACh. In addition to regulating the voluntary movements of the body, the somatic nervous system is also responsible for a specific type of involuntary muscle. The autonomic nervous system regulates organ systems through circuits that resemble the reflexes described in the somatic nervous system. The most distal synaptic junctions in the autonomic reflex arc occur in ganglia that are entirely outside the cerebrospinal axis. The efferent nerves of the involuntary system supply all innervated structures of the body except skeletal muscle, which is served by somatic nerves.
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