Innervation of the external urethral and external anal sphincters in higher primates.

Stimulating electrodes were placed on the terminal branches of the pudendal nerve to the external urethral and external anal sphincters. The proximity of the electrodes to the sphincters assured organ specificity. Evoked responses produced by stimulation of these terminal nerve branches were recorded in the fascicles and rootlets of the lower thoracic, lumbar, and sacral nerve roots. By this method, the segmental spinal cord origin of the innervation of the external urethral and anal sphincters was determined for the Rhesus monkey and chimpanzee. The data indicated that the pudendal nerves to the urethral and anal sphincters in the Rhesus monkey arose from the sixth and seventh lumbar spinal segments and the first and second sacral spinal segments which are homologous to the S-1 and S-4 segments found to give innervation to these structures in the chimpanzee. The primate experiments thus indicate that the spinal origin of the pudendal nerve was more rostrally located by one segment or more than was the origin of the pelvic nerves to the urinary bladder.