Divergence of Visceral Afferents in the Spinal Cord

In addition to the dual innervation of viscera by sensory afferent fibers, the central projection and terminal arborization of these visceral afferent fibers are highly divergent compared to somatic afferents. Somatic afferents project to well-defined regions within the dorsal horn (Fig. 2A) (22-26). Small diameter myelinated and unmyelinated fibers, most of which are associated with nociceptors, terminate in the superficial dorsal horn and to a lesser extent in

Nociceptor Afferent Fiber

Figure 1 The sensory innervation of the gastrointestinal tract. The splanchnic afferent Innervation Is shown on the left, the vagal/pelvic afferent innervation is shown on the right. The splanchnic afferent nerves have cell bodies in the thoracolumbar dorsal root ganglia (DRG) and project centrally through the dorsal roots into the spinal cord. The vagal/pelvic afferents that innervate the esophagus to the middle of the transverse colon project in the vagus nerve with cell bodies in the nodose ganglia. These afferents project centrally to the nucleus tractus solitarius. Pelvic afferents innervating the lower bowel have cell bodies in the lumbosacral DRG and project centrally into the lumbosacral spinal cord. Source: From Ref. 16.

Figure 1 The sensory innervation of the gastrointestinal tract. The splanchnic afferent Innervation Is shown on the left, the vagal/pelvic afferent innervation is shown on the right. The splanchnic afferent nerves have cell bodies in the thoracolumbar dorsal root ganglia (DRG) and project centrally through the dorsal roots into the spinal cord. The vagal/pelvic afferents that innervate the esophagus to the middle of the transverse colon project in the vagus nerve with cell bodies in the nodose ganglia. These afferents project centrally to the nucleus tractus solitarius. Pelvic afferents innervating the lower bowel have cell bodies in the lumbosacral DRG and project centrally into the lumbosacral spinal cord. Source: From Ref. 16.

Visceral Divergence Pain

Figure 2 (A) The central projection of a single somatic C polymodal nociceptor shown In the transverse plane. The Image is collapsed into two dimensions from a serial reconstruction of an intracellularly labeled primary afferent fiber. Note the limited mediolateral and dorsoventral extent of the terminal arborization. The rostrocaudal extent of the arborization was approximately 500 mm. (B) The central projection of a visceral afferent fiber shown in the transverse plane. The image is collapsed into two dimensions as in A. This afferent projected at least 15 mm in the rostrocaudal direction sending at least 14 collateral branches into the grey matter. These collaterals formed the terminal arborization that extends across the superficial dorsal horn, down into the deep dorsal horn and across the midline to the contralateral dorsal horn. Source: From Ref. 22.

Figure 2 (A) The central projection of a single somatic C polymodal nociceptor shown In the transverse plane. The Image is collapsed into two dimensions from a serial reconstruction of an intracellularly labeled primary afferent fiber. Note the limited mediolateral and dorsoventral extent of the terminal arborization. The rostrocaudal extent of the arborization was approximately 500 mm. (B) The central projection of a visceral afferent fiber shown in the transverse plane. The image is collapsed into two dimensions as in A. This afferent projected at least 15 mm in the rostrocaudal direction sending at least 14 collateral branches into the grey matter. These collaterals formed the terminal arborization that extends across the superficial dorsal horn, down into the deep dorsal horn and across the midline to the contralateral dorsal horn. Source: From Ref. 22.

lamina V and VI, while larger diameter low threshold fibers terminate in lamina III and IV (27,28). Intracellular recording and labeling individual afferent fibers further reveal rostro-caudal projections extending two to three segments with terminal arborizations restricted in the mediolateral and dorsoventral planes. This mediolateral specificity helps define the somatotopic map. Thus, precise localization of external stimuli to the body surface can be determined. Visceral afferent fibers, in contrast, have an extensive terminal arborization in the spinal cord that extends rostrocaudally for 5 to 10 segments and covers the mediolateral and dorsoventral extent of the dorsal horn. Horseradish peroxidase applied to the pelvic nerve or splanchnic nerve labels bundles of fibers terminating in the superficial dorsal horn, lamina

V and VI, and the dorsal gray commissure (29-32). Labeled fibers project in Lissauer's tract four to five segments rostrally and many segments caudally. Intra-axonal labeling of visceral afferent fibers provides greater detail of their central projection (22). A single afferent fiber projects into the superficial dorsal horn, the deep dorsal horn, and across the dorsal gray commissure to the contralateral side (Fig. 2B). The medial and lateral paths taken by branches of a single afferent fiber correspond to the medial and lateral collateral pathways reported in whole nerve studies. This extremely divergent terminal arborization of visceral afferent fibers likely contributes to the poor localization of visceral pain.

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  • Holman
    What is divergence of pain fibers from the somatic system and viscera from a spinal segment?
    4 months ago

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