Central representation of the inferior rectal nerve of the rat

Obstetric injury to the pudendal nerve contributes significantly to fecal incontinence. The inferior rectal nerve, a terminal branch of the motor division of the pudendal nerve, innervates the external anal sphincter. Animal models have been developed to establish the scientific basis of sacral neuromodulation. The aims of this study were to determine the spinal location of inferior rectal nerve motoneurons projecting to the external anal sphincter and whether the inferior rectal nerve carries sensory fibers.

Ten female virgin Wistar rats were used; 7 underwent bilateral inferior rectal nerve section and application of the neuronal tracer fluorogold. Five days later dorsal root ganglia L5 to S2 and the lumbosacral spinal cord were harvested and stained for activating transcription factor 3, a molecular marker of nerve injury. Three animals were used to confirm the specificity of activating transcription factor 3 nuclear labeling as a marker of axotomy.

Fluorogold-labeled motoneurons were found exclusively at L6 in the dorsomedial sections of Onuf's nuclei (left and right), which contained 30 +/- 9 motoneurons with a median diameter of 28.3 microm (24.4-31.0) (25th-75th centiles). Double-labeled sensory neurons (fluorogold-labeled cytoplasm and activating transcription factor 3-labeled nuclei) were found in dorsal root ganglia L6 to S2 and were smaller in diameter (20.5 microm (17.8-26.7); median (25th-75th centiles)) than motoneurons (P < .0,001).

The external anal sphincter receives both motor and sensory innervation from the inferior rectal nerve. Activating transcription factor 3 nuclear staining may prove useful for quantifying the degree of nerve injury in animal models of fecal incontinence.