The Anal Sphincter Has Two Layers with Separate Innervation
The anal opening is constricted by two sphincters: an internal Sphincterof smooth muscle, which is a direct extension of the circular muscle layer of the rectum; and an external sphincter of striated muscle.
The internal anal sphincter usually remains tonically contracted and is responsible for anal continence. The internal sphincter receives parasympathetic innervation from the sacral spinal segments through the pelvic nerve and sympathetic innervation from the lumbar spinal segments through the hypogastric nerve. In most species, sympathetic stimulation results in constriction of the sphincter, and parasympathetic stimulation results in relaxation.The external sphincter maintains some degree of tonic contraction, but the consistent tone of the anus is primarily regulated by the internal sphincter. The external sphincter is innervated by general somatic efferent fibers that have cell bodies in the cranial sacral spinal segments and course in the pudendal nerve.
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