![]() The neurotransmitters that mediate transmission from sensory fibers have not been characterized unequivocally. ![]() At the level of the forebrain, the spinal system appears to form a posterolateral continuation of the cranial nerve visceral sensory thalamus and cortex ( Saper, 2000). At the brainstem level, collaterals from the spinal system converge with the cranial nerve sensory system in the STN, the ventrolateral medulla, and the parabrachial nucleus. An important feature of the ascending pathways is that they provide collaterals that converge with the cranial visceral sensory pathway at virtually every level ( Saper, 2000). Most probably converge with musculoskeletal and cutaneous afferents and ascend by the spinothalamic and spinoreticular tracts. The primary pathways taken by ascending spinal visceral afferents are complex and controversial ( Saper, 2002). In general, visceral afferents that enter the spinal nerves convey information concerned with temperature as well as nociceptive visceral inputs related to mechanical, chemical, and thermal stimulation. Pelvic sensory afferents from spinal segments S2–S4 enter at that level and are important for the regulation of sacral parasympathetic outflow. 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. ![]() Sensory afferents from visceral organs also enter the CNS from the spinal nerves. 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. Whereas motor nerves to skeletal muscles are myelinated, postganglionic autonomic nerves generally are nonmyelinated. 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. These ganglia are small but complex structures that contain axodendritic synapses between preganglionic and postganglionic neurons. 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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