Anatomy

Origin

The Sphincter ani internus (Internal sphincter ani) is a muscular ring which surrounds about 2.5 cm. of the anal canal; its inferior border is in contact with, but quite separate from, the Sphincter ani externus. It is about 5 mm. thick, and is formed by an aggregation of the involuntary circular fibers of the intestine. Its lower border is about 6 mm. from the orifice of the anus.

Nerve

Sympathetic fibers from the superior rectal and hypogastric plexuses stimulate and maintain internal anal sphincter contraction. Its contraction is inhibited by parasympathetic fiber stimulation.

Actions

Its action is entirely involuntary. It helps the Sphincter ani externus to occlude the anal aperture and aids in the expulsion of the feces.