Nerves supplying the internal anal sphincter: an immunohistochemical study using donated elderly cadavers

Abstract

Nerves serving the internal anal sphincter (NIAS) have been described as the lower rectal branches of the pelvic autonomic nerve plexus. However, their topographical anatomy and fiber components have remained unclear. Using histological sections from ten elderly donated cadavers, we investigated the topographical anatomy and composite fibers of the NIAS using immunohistochemistry for S100 protein, neuronal nitric oxide synthase (nNOS), vasoactive intestinal polypeptide (VIP) and tyrosine hydroxylase (TH). At the 2-3 o'clock position in the lower rectum, the NIAS originated from nerves at the posterolateral corner of the prostate in males or in the lower paracolpium in females. The nerves ran inferiorly along the internal aspect of the levator ani muscle, and joined branches of the myenteric plexus at a level slightly above the epithelial junction. The NIAS contained both nNOS-positive parasympathetic nerve fibers and TH-positive sympathetic fibers, but VIP-positive fibers were few in number. The origin of the NIAS at the posterolateral corner of the prostate as well as in the lower paracolpium might be sacrificed or damaged during radical prostatectomy or tension-free vaginal tape insertion. Low anterior resection of rectal cancer will most likely render damage to the NIAS because of its intersphincteric course. Although the nerve composition of the NIAS is characterized by a higher proportion of sympathetic nerve fibers than the myenteric plexus in the large intestine, their role is unclear. However, evaluation of sphincteric function after surgery would appear to be difficult because of the complex control mechanism independent of nerve supply.