Internal anal sphincter

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Sphincter ani internus muscle
Coronal section through the anal canal. B. Cavity of urinary bladder V.D. Ductus deferens. S.V. Seminal vesicle. R. Second part of rectum. A.C. Anal canal. L.A. Levator ani. I.S. Sphincter ani internus. E.S. Sphincter ani externus.
Coronal section of rectum and anal canal.
Latin Musculus sphincter ani internus
Pelvic splanchnic nerves (S4), thoracicolumbar outflow of the spinal cord
Actions Keep the anal canal and orifice closed, aids in the expulsion of the feces
Gray's p.426
TA A05.7.05.011
FMA 15710
Anatomical terms of muscle

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.


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.[1] 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.[2]

Role in continence[edit]

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.[3]

Additional images[edit]


  1. ^ Moore, K., Dalley, A., Ague, A. "Clinically Oriented Anatomy. 6th Edition.
  2. ^
  3. ^ al.], senior editors, Bruce G. Wolff ... [et (2007). The ASCRS textbook of colon and rectal surgery. New York: Springer. ISBN 0-387-24846-3. 

This article incorporates text from a public domain edition of Gray's Anatomy.

External links[edit]