The internal anal sphincter, IAS, (or sphincter ani internus) is a muscular ring that surrounds about 2.5–4.0 cm of the anal canal; its inferior border is in contact with, but quite separate from, the external anal sphincter.

It is about 5 mm thick, and is formed by an aggregation of the involuntary circular fibers of the rectum. Its lower border is about 6 mm from the orifice of the anus.

Actions

Its action is entirely involuntary, and it is in a state of continuous maximal contraction. It helps the Sphincter ani externus to occlude the anal aperture and aids in the expulsion of the feces. Sympathetic fibers from the superior rectal and hypogastric plexuses stimulate and maintain internal anal sphincter contraction. Its contraction is inhibited by parasympathetic fiber stimulation. This sphincter is tonically contracted most of the time to prevent leakage of fluid or gas, but is relaxed upon distention of the rectal ampulla, requiring voluntary contraction of the puborectalis and external anal sphincter. The internal anal sphincter is not innervated by the pudendal nerve, which carries somatic (motor and sensory) fibers that provide the innervation to the external anal sphincter.

Anal sphincter muscles can refer to: