Internal Anal Sphincter
The Sphincter ani internus (or internal anal sphincter, IAS) is a muscular ring that surrounds about 2.5-4.0cm 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. 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. Parasympathetic fibers from the pelvic segments of the spinal cord supply the internal sphincter. It is not innervated by the pudendal nerve, which carries somatic (motor and sensory) fibers that provide the innervation to the external anal sphincter. Role in continence The IAS contributes 55% of the resting pressure of the anal canal. It is very important for bowel continence, especially for liquid and gas. When the rectum fills beyond a certain capacity, the rectal walls are distended, triggering the defecation cycle. This begins with the rectoanal inhibitory reflex, RAIR, where the IAS relaxes. This is thought to allow a small amount of rectal contents to descend into the anal canal where specialized mucosa samples whether it is gas, liquid or solid. Problems with the IAS often present as degrees of fecal incontinence (especially partial incontinence to liquid) or mucous rectal discharge.