External anal sphincter explained

External anal sphincter
Latin:sphincter ani externus
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.[1] Distally, it is adherent to the skin surrounding the margin of the anus.[2] It exhibits a resting state of tonical contraction and also contracts during the bulbospongiosus reflex.[3] [4] [5] [6]

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 subcontinuous). 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.

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.

See also

External links

Notes and References

  1. Book: Standring, Susan . Gray's Anatomy: The Anatomical Basis of Clinical Practice . 1201 . 978-0-7020-7707-4 . 42th . New York . 683 . 1201341621.
  2. Book: Gray, Henry . Gray's Anatomy . 1918 . 20th . 424–425.
  3. Vodušek DB, Deletis V . Intraoperative Neurophysiological Monitoring of the Sacral Nervous System . Neurophysiology in Neurosurgery, A Modern Intraoperative Approach . 153–165 . 2002 . 10.1016/B978-012209036-3/50011-1 . 9780122090363 . 78605592 .
  4. Sarica Y, Karacan I . 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 . July 1987 . 3599220 . 10.1016/S0022-5347(17)42987-9 .
  5. Jiang XZ, Zhou CK, Guo LH, Chen J, Wang HQ, Zhang DQ, Shi BK, Xu ZS . 6 . [Role of bulbocavernosus reflex to stimulation of prostatic urethra in pathologic mechanism of primary premature ejaculation] . zh . Zhonghua Yi Xue Za Zhi . 89 . 46 . 3249–3252 . December 2009 . 20193361 .
  6. Podnar S . Clinical elicitation of the penilo-cavernosus reflex in circumcised men . BJU International . 109 . 4 . 582–585 . February 2012 . 21883821 . 10.1111/j.1464-410X.2011.10364.x . 27143105 .