Visceral Afferent Pathways
There are both general and special visceral afferent pathways. Special visceral afferent pathways are concerned with taste and smell and are discussed later. The receptors of the general visceral afferent pathway are located within viscera and blood vessels; most of these receptors are mechanoreceptors responsive to pressure, stretch, and, less commonly, flow, although a minority are chemoreceptors responsive to the carbon dioxide content of the blood.
The axons that convey impulses from these receptors travel within peripheral nerves and may travel alongside other axons of visceral or somatic origin. The cell bodies of primary sensory neurons carrying visceral afferent information are located within the dorsal root ganglia of all spinal nerves (and the equivalent ganglia of certain cranial nerves) alongside those transmitting somatic afferent information; the axons from these cell bodies project on interneurons and projection neurons within the visceral afferent column of the spinal cord and brainstem (Fig. 8.12/2).Within the central nervous system, short chains of interneurons provide for simple local visceral reflexes that utilize visceral efferent pathways, discussed in the next section, and thus have their last two relays within the visceral efferent column and the peripheral autonomic ganglia. Visceral afferent information also reaches the brain via projection neurons that form ascending pathways that follow somatic systems, both lemniscal and extralemniscal, to end, like the somatic afferent system, within nuclei of the ventrocaudal thalamus. A final projection to the cortex may give rise to conscious perception of visceral sensation, although most visceral activity goes unnoticed. (The sense of fullness arising from digestive organs or the bladder is among the visceral activities of which awareness is most common.) Pronounced contraction and serious overdistention of visceral organs may be perceived as pain. Pain of visceral origin may be "referred" to the surface of the body, presumably as a consequence of the convergence of the cutaneous somatic and visceral afferent pathways on the same neurons at some point along their course.
The special visceral afferent pathway concerned with taste follows a similar route to that taken by the general visceral sensory modalities. The axons course from the taste buds in the oral cavity within the facial, glossopharyngeal, and vagus nerves and terminate in the nucleus of the solitary tract. The more complicated olfactory pathways are described elsewhere (see Fig. 8.40).