Central Pathways

Upon entering the dorsal horn, visceral afferents terminate in spinal cord laminae I, II, V, and X (11). Visceral afferents constitute less than 10% of afferent inflow into the spinal cord. This is a relatively small percentage when one considers the large surface area of some organs. Both anatomical and electrophysiological studies have demonstrated viscerosomatic convergence in both the dorsal horn and supraspinal centers (11-15). There is also evidence of viscero-visceral convergence onto these second-order neurons. Examples include the convergence of pelvic visceral inputs such as colon/rectum, bladder, uterine cervix, and vagina (3,11). Along with the low density of visceral nociceptors and the functional divergence of visceral input within the CNS, viscerovisceral convergence in the spinal cord may explain poorly localized visceral pain.

Visceral information carried by the pelvic nerve converges onto spinal neurons in the lumbosacral segments of the cord and that carried by the splanchnic nerves onto thoracolum-bar segments (16). Centrally, ascending pathways involved in the transmission of visceral nociceptive information include the spinothalamic tract (STT) , spinohypothalamic tract, spi-nosolitary tract, spinoreticular tract, spinoparabrachial tract, and other tracts located in the anterolateral quadrant (ALQ) . In addition, a number of recent studies have pointed to a role

Figure 1 Schematic illustration of ascending spinal pathways and related centers involved in visceral nociceptive processing. Abbreviations: DRG, dorsal root ganglia; PSDC, postsynaptic dorsal column.

of the dorsal column in viscerosensory processing, opening the door for a new role of the dorsal column in visceral pain (17,18). For a schematic illustration of these pathways, see Figure 1.

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