Peripheral organization of visceral afferent fibers

Sensation begins with the activation of receptors on the peripheral terminals of primary afferent nerve fibers. Visceral primary afferent fibers differ significantly from somatic primary afferent fibers in both number and pattern of distribution. Visceroreceptive afferent fibers are diffusely organized into web-like plexuses rather than forming distinct peripheral nerve entities. Afferent fibers with endings in a specific visceral site may have cell bodies in the dorsal root ganglia of 10 or more spinal levels in a bilaterally distributed manner. In contrast, somatic pain afferent fibers arise from a limited number of unilateral dorsal root ganglia. Individual visceroreceptive afferent fibers have been demonstrated to branch within the spinal cord and to spread over multiple spinal segments. Quantitative examination has revealed that spinal dorsal horn neurons with visceral inputs have multiple, convergent inputs from other viscera, from joints, from muscles, and from cutaneous structures. Together, these results suggest that visceral primary inputs are imprecisely organized, which would be consistent with imprecise localization by the central nervous system (CNS).

The clinical features of the referred hyperalge-sia show that the referred zones from the diseased viscera form symptomatic acu-reflex points with more predictable zones of localization on the body surface (see "Referred or Reflex Zones of Major Viscera" section).

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