External anal sphincter

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External anal sphincter
Anorectum-en.svg
1116 Muscle of the Male Perineum.png
Muscles of Male Perineum.
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

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 [1] and also contracts during the bulbospongiosus reflex. [3] [4] [5] [6]

Contents

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 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). [1]

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

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. [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]

See also

References

  1. 1 2 3 4 5 6 7 8 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.{{cite book}}: ISBN / Date incompatibility (help)
  2. ↑ Gray, Henry (1918). Gray's Anatomy (20th ed.). pp. 424–425.
  3. ↑ Vodušek DB, Deletis V (2002). "Intraoperative Neurophysiological Monitoring of the Sacral Nervous System". Neurophysiology in Neurosurgery, A Modern Intraoperative Approach: 153–165. doi:10.1016/B978-012209036-3/50011-1. ISBN   9780122090363. S2CID   78605592.
  4. ↑ 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.
  5. ↑ Jiang XZ, Zhou CK, Guo LH, Chen J, Wang HQ, Zhang DQ, et al. (December 2009). "[Role of bulbocavernosus reflex to stimulation of prostatic urethra in pathologic mechanism of primary premature ejaculation]". Zhonghua Yi Xue Za Zhi (in Chinese). 89 (46): 3249–3252. PMID   20193361.
  6. ↑ Podnar S (February 2012). "Clinical elicitation of the penilo-cavernosus reflex in circumcised men". BJU International. 109 (4): 582–585. doi:10.1111/j.1464-410X.2011.10364.x. PMID   21883821. S2CID   27143105.

PD-icon.svgThis article incorporates text in the public domain from page 425 of the 20th edition of Gray's Anatomy (1918)

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