Internal anal sphincter

Internal anal sphincter
Coronal section through the anal canal. B. Cavity of urinary bladder V.D. Vas deferens. S.V. Seminal vesicle. R. Second part of rectum. A.C. Anal canal. L.A. Levator ani. I.S. Internal anal sphincter. E.S. External anal sphincter.
Coronal section of rectum and anal canal
Details
NervePelvic splanchnic nerves (S4), thoracicolumbar outflow of the spinal cord
ActionsKeeps the anal canal and orifice closed, aids in the expulsion of the feces
Identifiers
Latinmusculus sphincter ani internus
TA98A05.7.05.011
TA23018
FMA15710
Anatomical terms of muscle

The internal anal sphincter, IAS, or sphincter ani internus is a ring of smooth muscle that surrounds about 2.5–4.0 cm of the anal canal. It is about 5 mm thick, and is formed by an aggregation of the smooth (involuntary) circular muscle fibers of the rectum. It terminates distally about 6 mm from the anal orifice.[citation needed]

The internal anal sphincter aids the sphincter ani externus to occlude the anal aperture and aids in the expulsion of the feces. Its action is entirely involuntary. It is normally in a state of continuous maximal contraction to prevent leakage of faeces or gases. Sympathetic stimulation stimulates and maintains the sphincter's contraction, and parasympathetic stimulation inhibits it. It becomes relaxed in response to distention of the rectal ampulla, requiring voluntary contraction of the puborectalis and external anal sphincter to maintain continence,[1] and also contracts during the bulbospongiosus reflex.[2][3][4][5]

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  3. ^ Sarica Y, Karacan I (July 1987). "Bulbocavernosus reflex to somatic and visceral nerve stimulation in normal subjects and in diabetics with erectile impotence". The Journal of Urology. 138 (1): 55–58. doi:10.1016/S0022-5347(17)42987-9. PMID 3599220.
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