Parasympathetic preganglionic neurons and visceral primary afferents in monkey sacral spinal cord revealed following application of horseradish peroxidase to pelvic nerve.

Abstract

AbstractHorseradish peroxidase (HRP) applied to the central stump of the transected pelvic nerve of rhesus monkeys labeled afferent and efferent neurons innervating the pelvic viscera. Preganglionic neurons forming the sacral parasympathetic nucleus (SPN) were located in a narrow band 150–200 μm wide on the border of the ipsilateral intermediate gray matter. The SPN contained an average of 820 neurons and characteristically extended over two or three sacral segments (average length: 14.5 mm) with the majority of cells in S2. It was composed of medium sized, elongated somata (15.7 × 30.7 μm) with their major axes in the transverse plane and oriented parallel to the lateral border of the gray matter. Dendrites extended deep into the lateral funiculus, horizontally into the medial dorsal gray commissure, and dorso‐laterally along the lateral edge of the dorsal horn. Preganglionic axons followed the lateral border of the ventral horn to the ventral rootlets.Pelvic nerve afferent axons carried HRP to neurons in the dorsal root ganglia (DRG) and by transganglionic transport to the spinal cord. The average number of labeled DRG cells was 2, 992 (range: 2, 193–4, 363) with the largest fraction (80%) in S2. The numbers of DRG cells correlated with the numbers of SPN cells in corresponding spinal cord segments. Within the spinal cord labeled primary afferent fibers were prominent in the sacral segments and were also observed in decreasing numbers within two to three segments beyond the rostral and caudal limits of the SPN. In transverse sections, primary afferents were found throughout Lissauer's tract (LT) and in the lateral half of the dorsal columns. A prominent collateral fiber bundle from LT was located in lamina I along the lateral edge of the dorsal horn. This bundle extended into the SPN and a few collaterals continued medially into the dorsal gray commissure and the contralateral side of the cord. A much less prominent collateral fiber group from LT passed medially over the apex of the dorsal horn in lamina 1 and descended into the dorsal gray commissure. Horizontal sections revealed that these two collateral pathways were organized into periodically spaced bundles of fibers about 250 μm apart. HRP reaction product in terminal fields related to these afferent collateral projections were found in the lateral marginal zone and bilaterally in the dorsal gray commissure.