External anal sphincter

External anal sphincter
External anal sphincter
	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.
Details
Nerve	Branch from the fourth sacral and contributions from the inferior hemorrhoidal branch of the pudendal nerve
Actions	Keep the anal canal and orifice closed
Identifiers
Latin	Sphincter ani externus
TA98	A04.5.04.012
TA2	2426
FMA	21930
	Anatomical terms of muscle [edit on Wikidata]

The external anal sphincter (or sphincter ani externus) is an oval tube skeletal muscle fibers.^[1] Distally, it is adherent to the skin surrounding the margin of the anus.^[2] The sphincter exhibits a resting state of tonical contraction.^[1]

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.^[1]

Structure

Historically, the sphincter was described as consisting of three parts (deep, superficial, and subcontinuous), however, this is not supported by current anatomical knowledge. Some sources still describe it as consisting of two layers: deep (or proximal) superficial (or distal or subcutaneous).^[1]

Some of the muscles fibres decussate at the anterior midline and posterior midline, so forming an anterior commissure and posterior commissure.^[1]

Attachments

The muscle attaches anteriorly onto the perineal body, and posteriorly onto the anococcygeal ligament.^[1]

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.^[1]

Histology

The sphincter consists mostly of slow twitch fibers that allow extended continuous contraction.^[1]

Function

(1) Like other muscles, it is always in a state of tonic contraction, and having no antagonistic muscle it keeps the anal canal and orifice shut.^{[citation needed]}

(2) It can be put into a condition of greater contraction under the influence of the will, so as more firmly to occlude the anal aperture, in expiratory efforts unconnected with defecation.^{[citation needed]}^{[clarification needed]}

(3) Taking its fixed point at the coccyx, it helps to fix the central point of the perineum, so that the bulbospongiosus muscle may act from this fixed point.^{[citation needed]}

Additional images

Intestines
Anatomy of the human anus.
Muscles of male perineum.
Muscles of the female perineum.
Sagittal (vertical) section of bladder, penis, and urethra.

References

^ ^a ^b ^c ^d ^e ^f ^g ^h Standring, Susan (1201). Gray's Anatomy: The Anatomical Basis of Clinical Practice (42th ed.). New York. p. 683. ISBN 978-0-7020-7707-4. OCLC 1201341621.
^ Gray, Henry (1918). Gray's Anatomy (20th ed.). pp. 424–425.

This article incorporates text in the public domain from page 425 of the 20th edition of Gray's Anatomy (1918)

External links

Anatomy photo:42:13-0100 at the SUNY Downstate Medical Center - "The Male Perineum and the Penis: The External Anal Sphincter"
perineum at The Anatomy Lesson by Wesley Norman (Georgetown University) (analtriangle3)
pelvis at The Anatomy Lesson by Wesley Norman (Georgetown University) (rectum)

[:2242-1] ^ ^a ^b ^c ^d ^e ^f ^g ^h Standring, Susan (1201). Gray's Anatomy: The Anatomical Basis of Clinical Practice (42th ed.). New York. p. 683. ISBN 978-0-7020-7707-4. OCLC 1201341621.

[:02-2] Gray, Henry (1918). Gray's Anatomy (20th ed.). pp. 424–425.

[1]

[2]