Proceedings of The Physiological Society

University of Oxford (2011) Proc Physiol Soc 23, PC177

Poster Communications

Some observations on the innervation of the female rat anal sphincters

J. F. Jones1, M. Buffini1, C. O'Herlihy1,2, P. R. O'Connell1,3

1. Physiology, University College Dublin, Dublin, Dublin, Ireland. 2. Obstetrics and Gynaecology, Holles Street, National Maternity Hospital, Dublin, Ireland. 3. Surgery, St Vincent's University Hospital, Dublin, Ireland.


The aim of this study was to examine the innervation of the rat sphincters of faecal continence. Female Wistar rats were anesthetised with urethane (1.5 g. kg-1 i.p.) and the femoral vein cannulated for administration of supplemental anaesthetic. The anal canal was arranged as an in vivo ring preparation of both external anal sphincter (EAS) and internal anal sphincter (IAS) both of which were connected to a single force transducer. All data are normally distributed, expressed as mean ± S.D. and analysed statistically with paired Student’s t tests. Lumbar sympathetic stimulation (between L3 and L4 ganglia) (8V, 20-30Hz, 1ms pulse duration) had no effect on either EAS or IAS but was effective in eliciting a contraction of the anococcygeus muscle (16.4 ± 5.4 mN; n=5). Repetitive single twitch contractions of the EAS were elicited by stimulating the left inferior rectal nerve at 1Hz (3-6V, 1ms pulse duration). These EAS striated muscle twitches were superimposed on spontaneous oscillations of the IAS smooth muscle (ultra slow waves; amplitude: 10.7 ± 3.7 mN; frequency: 6 every ten minutes). The amplitude of striated contractions at the peak of the ultra slow wave was 7.0 ± 3.6 mN. As the smooth muscle relaxed, the amplitude of the striated twitch increased to 9.2 ± 4.5 mN which represents a 32.1 ± 8.8% rise (n=5, p=0.01). When the canal was decentralized by transecting both inferior rectal nerves (IRNs), this modulation of EAS force by underlying slow wave activity was still evident in all animals. The percentage increase following IRN transection was not significantly different to the values of the intact anal canal: 34.7 ± 27.9% (n=5, p=0.8). Standard immunocytochemical methods were used to visualise motor endplate complexes (primary antibodies to neurofilament and synaptic vesicle 2 were combined with Alexa fluor-594 labelled α-bungarotoxin). The animals were anaesthetised with 5% isofluorane in oxygen (0.5l/min). Once anaesthetised intraperitoneal gentamycin (6mg/kg) and subcutaneous caprofen (5mg/kg) were administered. When both IRNs were transected 55 ± 30.5% (n=6) of endplates were denervated 5 days later. In conclusion, the rat anal sphincters are not affected by sympathetic stimulation. When the smooth muscle of the IAS relaxes spontaneously neurally evoked EAS contractions increase in amplitude. This modulation of the EAS by slow waves in the IAS is unaffected by severing pudendal nerve motor fibres. As only half the motor endplate are denervated after bilateral IRN section these findings provide tentative evidence for a dual innervation of the rat EAS. The first of these is from the pudendal nerve via its inferior rectal branches and the source of the other may be the enteric nervous system or pelvic nerves.

Where applicable, experiments conform with Society ethical requirements