Pudendal afferent innervation of the guinea pig external anal sphincter

Sensory nerves to the external anal sphincter (EAS) contribute to mechanisms promoting continence and defecation, yet we know little about their function. We investigated the function of pudendal mechanoreceptors to the guinea pig EAS.

Extracellular recordings from pudendal nerve branches to 14 EAS preparations, in vitro, were used to characterize extrinsic primary afferent nerve endings activated by circumferential distension.

All 42 pudendal nerve afferents were silent under non-distended conditions. Thirty-three of 42 afferents had slowly adapting, low-threshold responses to circumferential stretch that correlated with stretch length (R(2) = 0.40, P<0.001). Twenty of 20 slowly adapting afferents reduced firing when stretch was maintained for 60 s (P<0.0001). They had low thresholds to von Frey hairs (0.1-0.5mN). Firing frequency correlated with degree of compression (R(2) =0.40, P<0.0001). Nine of 42 afferents had rapidly adapting responses at the onset/offset of isometric stretch. During ramp stretch, small vibrations from the stepper motor evoked rapid bursts of firing at frequencies up to 200Hz. Instantaneous frequency was unrelated to either the rate or degree of stretch. Rapidly adapting units had low thresholds (0.1-0.2mN) to von Frey hairs and small punctate mechanotransduction sites. Responses to von Frey hair compression were also rapidly adapting, and instantaneous frequency was unrelated to the degree of compression.

The EAS has two functional classes of mechanoreceptors: slowly adapting low-threshold and rapidly adapting low-threshold mechanoreceptors. These two classes of afferents are likely to be involved in the maintenance of continence, and the process of defecation.