Afferent Autonomic Pathways

Visceral afferents transmit conscious sensations (e.g., gut distention, cardiac ischemia) and unconscious visceral sensations (e.g., blood pressure, chemical composition of the blood). Their most important function is to initiate autonomic reflexes at the local, ganglion, spinal, and supraspinal levels. y Visceral receptors are innervated by small myelinated and unmyelinated fibers that have cell bodies in the dorsal root ganglia of the spinal or cranial nerves. Spinal visceral afferents are carried by thoracolumbar sympathetic and sacral parasympathetic nerve trunks. In the spinal cord, they branch extensively and synapse on viscerosomatic neurons in the dorsal horn and intermediate gray matter. These neurons receive convergent visceral and somatic inputs and are the substrate for referred pain (i.e., visceral pain referred to overlying or nearby somatic structures). Visceral sensation is carried primarily by the spinothalamic and spinoreticular pathways, which transmit visceral pain and sexual sensations. The dorsal columns may relay sensations related to micturition, defecation, and gastric distention. Viscerosensory inputs, after a relay in the thalamic nuclei, project to the insula and other cortical autonomic areas. y

Brain stem visceral afferents are carried by the vagus and glossopharyngeal nerves, and have their cell bodies in the nodose and petrosal ganglia, respectively. Vagal afferents carry information from aortic, cardiac, pulmonary, and gastrointestinal receptors. Glossopharyngeal afferents carry signals from baroreceptors and chemoreceptors in the carotid sinus. These vagal and glossopharyngeal afferents synapse on neurons of the NTS. Taste and general visceral afferent information from the NTS is relayed, either directly or via the parabrachial nucleus, to the ventroposteromedial nucleus of the thalamus, which in turn projects to the insular cortex. Brain stem visceral afferents are important in complex automatic motor acts such as swallowing, vomiting, and coughing. y , y

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