Neuroanatomy of the striated muscular anal continence mechanism

Abstract

The striated pelvic floor musculature and the striated muscle of the external anal sphincter contribute to anal continence by effecting, respectively, the rectoanal angulation of the bowel and an anal high pressure zone. The muscular anatomy of the pelvic floor is generally understood, but the neuroanatomy remains controversial. The authors dissected three male cadavers and traced the sacral nerves from their entrance into the pelvis through the sacral foramina throughout their branching to their final destinations. Deriving from a common source, the sacral nerves S₂ to S₄, the neural supply of the levator ani was distinct from that of the external anal sphincter: the levator is supplied by direct branches splitting from the sacral nerves proximal to the sacral plexus and running on the inner surface; the external anal sphincter is supplied by nerve fibers travelling with the pudendal nerve on the levator's undersurface. To document the functional relevance of these anatomic findings, stimulation of the pudendal and sacral nerves was performed at different levels in five patients with lower urinary tract dysfunction. Stimulation of the pudendal nerve increased the anal pressure, whereas stimulation of S₃ increased it only slightly but caused an impressive decrease of the rectoanal angle; when S₃ was stimulated after bilateral pudendal block, anal pressure did not change but the decrease in the rectoanal angulation persisted. The changes in anal pressure could be obtained without fatigue at stimulation frequencies of 10 to 20 Hz.