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| CHAPTER | 34 |
The Interoceptive, or Visceral, Sensations
Interoceptive sensations are general visceral sensations that arise from the internal organs. The special visceral sensations (smell and taste) are discussed with the cranial nerves. General visceral afferent fibers are found in cranial nerves VII, IX, and X and in the thoracolumbar and sacral autonomic nerves. Visceral afferent fibers run with autonomic efferent fibers to the viscera. Cell bodies are in the dorsal root and associated cranial ganglia; impulses enter the central nervous system through the posterior roots and ascend to higher centers through pathways close to those that carry general somatic afferent impulses.
Visceral afferent fibers are involved with unconscious visceral and autonomic reflexes and also likely convey visceral sensations such as hunger, nausea, sexual excitement, vesical distention, and visceral pain. Afferent impulses from the viscera may reach consciousness by a variety of routes. Some travel in somatic nerves and some with efferent autonomic nerves. Some synapse in the dorsal horn, and axons of the next-order neurons cross to the opposite spinothalamic tract, where the fibers that carry visceral pain lie medial to those that carry superficial pain and temperature sensations. Others may travel in the ipsilateral spinothalamic tract. Many ascend for a great distance in Lissauer tract before synapsing, and some ascend by long intersegmental fibers in the white matter at the border of the dorsal horn, reaching the hypothalamus and thalamus without decussating. As a consequence of the multiple pathways and redundancy, localization of visceral pain is not precise. The gyms rectus, rather than the parietal cortex, may be the end station for visceral afferent sensation.
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