The projection of anorectal afferents to spinal cord and effect of sacral neuromodulation on dorsal horn neurons which receive such input in the rat

Background The rat has served usefully as a model for fecal incontinence and exploration of the mechanism of action of sacral neuromodulation (SNM). There remains a deficit in information regarding the location and type of spinal neurons which receive anorectal input and the effect of SNM on those neurons. Methods Single neuronal extracellular recordings of neurons receiving anorectal input were made at the S1 level of the spinal cord using sharp glass electrodes. SNM at S1 was delivered at 2 Hz for 3 minutes and its effect on discharge was quantified. Key Results In total, 31 units (n = 14 animals) receiving anorectal synaptic input were recorded at the first sacral (S1) segmental level in either lamina III or IV of the dorsal horn. The inputs were classified according to afferent fiber conduction speed (16 A delta, 11 A beta, and 4 C-fiber). The baseline firing frequency (ie, the mean firing frequency before the application of SNM) was 0.48 Hz +/- 0.49 (mean +/- SD) and 58% of units responded to acute SNM with either an increase or decrease in mean firing frequency. Conclusions & Inferences In this study, the majority of spinal neurons receiving anorectal input changed their activity in response to SNM. These findings provide the basis for future studies which aim to explore the precise cellular mechanism of action of SNM on this fecal continence pathway.