REFLEX ACTIVITY OF EXTERNAL ANAL SPHINCTER OF CAT

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Abstract
The activity of the skeletal muscle which encircles the anus, the external anal sphincter, was studied by recording the action potentials from the exposed sphincter of decerebrate cats. By dorsal root rhizotomy it was demonstrated that the tonic, resting discharge of the sphincter is a true spinal reflex. The S2 dorsal roots carry the majority of the afferent impulses maintaining the tonic activity. The sphincter itself proved to be the site of origin of these afferent impulses and the pudendal nerves serve as the centripetal route. Absence of one pudendal nerve does not sidrupt sphincter reflexes. Selective Metycaine (piperocaine) and pressure block of the remaining, intact pudendal nerve, which carries both afferent and efferent impulses between sphincter and cord, demonstrated that the phasic activities of the sphincter are mediated by fibers of larger diameter than those mediating the tonic reflex. Colon distention relaxes (i.e., inhibits) the tonic discharge of the sphincter. Posterior rhizotomy identified S2 as the chief site of entry for these inhibitory impulses also. The persistence of the phasic responses following division of the S2 dorsal roots implies that the phasic reflexes have afferent limbs entering the cord at levels distinct from the tonic and inhibitory reflexes. Though colonic distention silences the sphincter, several stimuli applied to the colon lumen via colostomy augmented sphincter activity. Among these stimuli were the cutting of a colostomy incision, pin-prick of colonic mucosa or introduction of solutions with a pH below 1.2 or above 8.0 into the colon lumen. Even in the presence of these nocuous stimuli colon distention elicited sphincter relaxation indicating that the stimuli were not destructive. The role of these reflex activities in the complex act of defecation has been discussed.