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External anal sphincter

Flat plane of skeletal muscle fibers

Summary

Flat plane of skeletal muscle fibers

Field	Value
Name	External anal sphincter
Latin	sphincter ani externus
Image	Anorectum-en.svg
Image2	1116 Muscle of the Male Perineum.png
Caption2	Muscles of Male Perineum.
Action	Keep the anal canal and orifice closed
Nerve	Branch from the fourth sacral and contributions from the inferior hemorrhoidal branch of the pudendal nerve

The external anal sphincter (or sphincter ani externus) is an oval tube of skeletal muscle fibers. Distally, it is adherent to the skin surrounding the margin of the anus.**** It exhibits a resting state of tonic contraction and also contracts during the bulbospongiosus reflex.

Anatomy

The external anal sphincter is far more substantial than the internal anal sphincter. The proximal portion of external anal sphincter overlaps the internal anal sphincter (which terminates distally a little distance proximal to the anal orifice) superficially; where the two overlap, they are separated by the intervening conjoint longitudinal muscle.

Structure

Historically, the sphincter was described as consisting of three parts (deep, superficial, and subcutaneous). This is not supported by current anatomical knowledge. Some sources still describe it in two layers, deep (or proximal) and superficial (or distal or subcutaneous).

Some of the muscles fibres decussate at the anterior midline and posterior midline, so forming an anterior commissure and posterior commissure.

Function

The external anal sphincter keeps feces retained inside the rectum and prevents them from coming out of the rectum involuntarily.

Attachments

The muscle attaches anteriorly onto the perineal body, and posteriorly onto the anococcygeal ligament.

Innervation

The sphincter receives innervation from the bilaterally paired inferior anal nerve (each a branch of the pudendal nerve which is derived from ventral rami of S2-S4). It may also receive additional motor innervation from the nerve to levator ani.

Histology

The sphincter consists mostly of slow twitch fibers that allow extended continuous contraction.

Gallery

File:illu_intestine.jpg|Intestines File:Human anus-en.svg|Anatomy of the human anus. File:Gray1156.png|Sagittal (vertical) section of bladder, penis, and urethra.

References

Standring, Susan. (1201). "Gray's Anatomy: The Anatomical Basis of Clinical Practice".
Gray, Henry. (1918). "Gray's Anatomy".
(2002). "Intraoperative Neurophysiological Monitoring of the Sacral Nervous System". Neurophysiology in Neurosurgery, A Modern Intraoperative Approach.
(July 1987). "Bulbocavernosus reflex to somatic and visceral nerve stimulation in normal subjects and in diabetics with erectile impotence". The Journal of Urology.
(December 2009). "[Role of bulbocavernosus reflex to stimulation of prostatic urethra in pathologic mechanism of primary premature ejaculation]". Zhonghua Yi Xue Za Zhi.
(February 2012). "Clinical elicitation of the penilo-cavernosus reflex in circumcised men". BJU International.

Wikipedia Source

This article was imported from Wikipedia and is available under the Creative Commons Attribution-ShareAlike 4.0 License. Content has been adapted to SurfDoc format. Original contributors can be found on the article history page.

perineum anus muscular-system

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