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Lab 3 (ƒ5) - Somatosensory, Viscerosensory and Spinocerebellar Pathways

Viscerosensory Pathways

The vast majority of visceral afferents and the pathways they form are not sensory in function. They are an integral part of the autonomic system and serve to provide information important for regulating cardiovascular, respiratory, digestive, reproductive, etc. functions. Recall that the peripheral axons of visceral 1° afferent fibers travel in sympathetic and parasympathetic nerves to their target sites. Some of the posterior root ganglion cells of the sacral segments give rise to axons that travel in the sacral parasympathetic nerves to the abdominal and pelvic viscera, and convey perceived sensations such as gut and bladder distention. Many visceral sensory neurons at upper lumbar levels (L2, L1) monitor the condition of the blood vessels, glands and viscera in the lower extremities and pelvic region. The visceral 1° afferents of the thoracic segments travel in the thoracic sympathetic nerves to innervate the blood vessels, glands, and visceral organs of the neck, upper extremities, chest and abdominal cavities. There are few visceral afferents in cervical segments. The visceral 1° afferents of the cranial nerves (vagus and glossopharyngeal) travel in parasympathetic nerves to their terminal sites. The visceral 1° afferents of the vagus (X) nerve innervate the thoracic and abdominal viscera, while those of the glossopharyngeal (IX) nerve innervate the pharynx, tonsil, carotid sinus and carotid body.

The cell bodies of the visceral 1° afferents are located in the posterior root ganglia and in the petrosal ganglion (IXth nerve) and nodose ganglion (Xth nerve). The course of the spinal viscerosensory pathway is similar to that of the paleospinothalamics. The central axons of the 1° cranial visceral afferents enter the medulla, travel in the tractus solitarius and synapse in the caudal two-thirds of the nucleus solitarius (2° cranial visceral afferents). Many of the higher order visceral afferents end in the brain stem and are involved in initiating and regulating autonomic functions. Few of the visceral afferents (3° spinal and 2° cranial viscerosensory afferents) reach the thalamus where they terminate in the intralaminar nuclei. While the intralaminar nuclei project diffusely to the cortex, part of the insular cortex has been identified as a cortical visceral receiving area.