# A 3D Atlas of Visceral and Somatic Pelvic Motor Neurons in Whole Mounts of Female and Male Rat Spinal Cords

## TL;DR

This paper creates a detailed 3D map of motor neurons in rat spinal cords, revealing differences between males and females and how these neurons control pelvic organs.

## Contribution

The study introduces a novel 3D atlas of pelvic motor neurons in rat spinal cords, highlighting sexual dimorphism and new axon projections.

## Key findings

- Pelvic motor neurons in rats show sexual dimorphism in counts and dendritic arborizations.
- 3D visualization reveals overlapping dendritic bundles between visceral and somatic motor neurons.
- Retrograde tracing identifies preganglionic neurons in specific spinal cord regions controlling pelvic organs.

## Abstract

The sexually dimorphic genitourinary organs and specialized striated muscles in the pelvis are controlled by a spinal cord motor neuron (MN) subsystem that produces reflexogenic and psychogenic (conscious) visceral and somatic motor activity during urinary continence and voiding, scent marking (in some species), defecation, reproduction, and sexual activity. To produce a three‐dimensional (3D) atlas of the pelvic MN system, we performed whole‐mount immunostaining and advanced 3D microscopy on the caudal spinal cord of adult female and male rats. We used choline acetyltransferase immunolabeling to study the macroscopic topology of visceral (autonomic preganglionic; VMN) and somatic MN (SMN) subcolumns and retrograde neural tracing from major pelvic ganglia to locate preganglionic neurons required for pelvic organ regulation. This identified parasympathetic VMNs in the sacral intermediolateral nucleus of segments L6–S1 and pelvic sympathetic preganglionic neurons that were mostly found in the medial dorsal commissural nucleus of segments L1–L2. Retrogradely labeled SMNs were also identified in lumbosacral nuclei containing the urinary rhabdosphincter, cremaster, and levator ani motor pools, suggesting that pelvic MNs project through both the pelvic and pudendal nerves. Evidence of sexual dimorphism was provided by VMN counts and measurements of their dendritic arborizations, and the volume of the dorsolateral and dorsomedial SMN columns. Three‐dimensional visualization also revealed areas of overlap between pelvic VMNs and rhabdosphincter SMNs in dendritic bundles, suggesting a possible functional role in coordinating urinary activity. The datasets are available as an open resource (sparc.science) to support 3D visualization of the pelvic motor system in the intact rat spinal cord.

Visceral and somatic motor neurons (MNs) of the lumbosacral (L6–S1) spinal cord control the genitourinary organs and pelvic striated muscles. We produced a three‐dimensional (3D) MN atlas of the lumbosacral spinal cord using whole‐mount immunostaining, tissue clearing, and advanced 3D microscopy in male and female rats. In so doing, we identified novel axon projections and sexually dimorphic features of MN populations, in addition to providing a definitive online open‐access resource for 3D visualization of visceral and somatic MNs within intact lumbosacral cord.