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describing the bulbospongiosus reflex
 
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{{Short description|Flat plane of skeletal muscle fibers}}
{{Muscle infobox |
{{Infobox muscle
⚫
Name = Sphincter ani externus muscle |
Latin = musculus sphincter ani externus |
| Name = External anal sphincter
⚫
| Latin = sphincter ani externus
GraySubject = 120 |
GrayPage = 425 |
| Image = Anorectum-en.svg
Image = Anorectum.gif |
| Caption =
Caption = |
| Image2 = Gray1079.png
⚫
| Caption2 = 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]].
Image2 = Gray1079.png |
⚫
| Origin =
⚫
Caption2 = 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. [[Sphinear ani externus]]. |
Origin = |
| Insertion =
⚫
| Action = Keep the [[anal canal]] and [[anal orifice|orifice]] closed
Insertion = |
| Blood =
⚫
Action = keep the [[anal canal]] and [[anal orifice|orifice]] closed|
⚫
| Nerve = Branch from the fourth sacral and contributions from the inferior hemorrhoidal branch of the [[pudendal nerve]]
⚫
Blood = |
⚫
Nerve = branch from the fourth sacral and twigs from the inferior hemorrhoidal branch of the [[pudendal nerve]] |
MeshName = |
MeshNumber = |
DorlandsPre = m_22 |
DorlandsSuf = 12550801 |
}}
}}
The '''external anal sphincter''' (or '''sphincter ani externus''') is an oval tube of [[Skeletal muscle#Skeletal muscle cells|skeletal muscle fibers]].<ref name=":2242">{{Cite book |last=Standring |first=Susan |url=https://www.worldcat.org/oclc/1201341621 |title=Gray's Anatomy: The Anatomical Basis of Clinical Practice |year=1201 |isbn=978-0-7020-7707-4 |edition=42th |location=New York |pages=683 |oclc=1201341621}}</ref> Distally, it is adherent to the [[integument|skin]] surrounding the margin of the [[anus]].'''<ref name=":02">{{Cite book |last=Gray |first=Henry |url=https://archive.org/details/anatomyofhumanbo1918gray/page/424/mode/2up?view=theater |title=Gray's Anatomy |year=1918 |edition=20th |pages=424–425}}</ref>''' It exhibits a resting state of tonical contraction<ref name=":2242" /> and also contracts during the [[bulbospongiosus reflex]].<ref name=Vodusek_2002>{{cite journal | vauthors = Vodušek DB, Deletis V | title = Intraoperative Neurophysiological Monitoring of the Sacral Nervous System | journal = Neurophysiology in Neurosurgery, A Modern Intraoperative Approach | pages = 153–165 | year = 2002 | doi = 10.1016/B978-012209036-3/50011-1 | isbn = 9780122090363 | s2cid = 78605592 }}</ref><ref name=Sarica_1987>{{cite journal | vauthors = Sarica Y, Karacan I | title = Bulbocavernosus reflex to somatic and visceral nerve stimulation in normal subjects and in diabetics with erectile impotence | journal = The Journal of Urology | volume = 138 | issue = 1 | pages = 55–58 | date = July 1987 | pmid = 3599220 | doi = 10.1016/S0022-5347(17)42987-9 }}</ref><ref name=Jiang_2009>{{cite journal | vauthors = Jiang XZ, Zhou CK, Guo LH, Chen J, Wang HQ, Zhang DQ, Shi BK, Xu ZS | display-authors = 6 | title = [Role of bulbocavernosus reflex to stimulation of prostatic urethra in pathologic mechanism of primary premature ejaculation] | language = zh | journal = Zhonghua Yi Xue Za Zhi | volume = 89 | issue = 46 | pages = 3249–3252 | date = December 2009 | pmid = 20193361 }}</ref><ref name=Podnar_2012>{{cite journal | vauthors = Podnar S | title = Clinical elicitation of the penilo-cavernosus reflex in circumcised men | journal = BJU International | volume = 109 | issue = 4 | pages = 582–585 | date = February 2012 | pmid = 21883821 | pmc = | doi = 10.1111/j.1464-410X.2011.10364.x | s2cid = 27143105 }}</ref>
The '''Sphincter ani externus''' ('''external anal sphincter''') is a flat plane of muscular fibers, elliptical in shape and intimately adherent to the [[integument]] surrounding the margin of the [[anus]].


==Anatomy==
==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]].<ref name=":2242" />
It measures about 8 to 10&nbsp;cm in length, from its anterior to its posterior extremity, and is about 2.5&nbsp;cm opposite the anus, when you defecate the sphincter muscle retracts.


=== Structure ===
It consists of two strata, superficial and deep.
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).<ref name=":2242" />


Some of the muscles fibres decussate at the anterior midline and posterior midline, so forming an anterior commissure and posterior commissure.<ref name=":2242" />
* The ''superficial'', constituting the main portion of the muscle, arises from a narrow tendinous band, the [[anococcygeal raphé]], which stretches from the tip of the [[coccyx]] to the posterior margin of the anus; it forms two flattened planes of muscular tissue, which encircle the anus and meet in front to be inserted into the [[central tendinous point of the perineum]], joining with the [[Superficial transverse perineal muscle|Transversus perinæi superficialis]], the [[Levator ani]], and the [[Bulbospongiosus muscle|Bulbocavernosus]].


=== Attachments ===
* The ''deeper'' portion forms a complete sphincter to the [[anal canal]]. Its fibers surround the canal, closely applied to the [[Sphincter ani internus muscle]], and in front blend with the other muscles at the central point of the [[perineum]].
{{Expand section|date=July 2023}}
The muscle attaches anteriorly onto the [[perineal body]], and posteriorly onto the anococcygeal ligament.<ref name=":2242" />


=== Innervation ===
In a considerable proportion of cases the fibers [[decussate]] in front of the anus, and are continuous with the [[Superficial transverse perineal muscle|Transversi perinæi superficiales]].
The sphincter receives innervation from the bilaterally paired [[Inferior anal nerves|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]].<ref name=":2242" />


=== Histology ===
Posteriorly, they are not attached to the [[coccyx]], but are continuous with those of the opposite side behind the anal canal.
The sphincter consists mostly of [[slow twitch fiber]]s that allow extended continuous contraction.<ref name=":2242" />


==Gallery==
The upper edge of the muscle is ill-defined, since fibers are given off from it to join the [[Levator ani]].

==Actions==
The action of this muscle is peculiar.

(1) It is, like other muscles, always in a state of tonic contraction, and having no [[Antagonist (muscle)|antagonistic muscle]] it keeps the [[anal canal]] and [[anal orifice|orifice]] closed.

(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]].

(3) Taking its fixed point at the [[coccyx]], it helps to fix the central point of the [[perineum]], so that the [[Bulbocavernosus]] may act from this fixed point.

==Pathology==
[[Anismus]] is a paradoxical contraction of the external anal sphincter, when the intent is relaxation, which can result in [[obstructed defecation]] and [[constipation]]. Abnormal function of this muscle is also seen in [[anorectal malformation]] and after certain surgeries, including [[coccygectomy]].

==Additional images==
<gallery>
<gallery>
Image:illu_intestine.jpg|Intestines
File:illu_intestine.jpg|Intestines
File:Human anus-en.svg|Anatomy of the human anus.
Image:Hemorrhoid.png|Schematic demonstrating the anatomy of [[hemorrhoid]]s.
Image:Gray406.png|Muscles of male perineum.
File:1116 Muscle of the Male Perineum.png|Muscles of male perineum.
Image:Gray408.png|Muscles of the female perineum.
File:1116 Muscle of the Female Perineum.png|Muscles of the female perineum.
Image:Gray1156.png|Vertical section of bladder, penis, and urethra.
File:Gray1156.png|Sagittal (vertical) section of bladder, penis, and urethra.
</gallery>
</gallery>


==See also==
==See also==
*[[Internal anal sphincter]]
*[[Puborectalis muscle]]
*[[Puborectalis muscle]]

==References==
{{Reflist}}
⚫
{{Gray's}}


==External links==
==External links==
* {{MuscleLoyola|spha}}
* {{SUNYAnatomyLabs|42|13|01|00}} - "The Male Perineum and the Penis: The External Anal Sphincter"
* {{SUNYAnatomyLabs|42|13|01|00}} - "The Male Perineum and the Penis: The External Anal Sphincter"
* {{NormanAnatomy|perineum}} ({{NormanAnatomyFig|analtriangle3}})
* {{NormanAnatomy|perineum}} ({{NormanAnatomyFig|analtriangle3}})
* {{NormanAnatomy|pelvis}} ({{NormanAnatomyFig|rectum}})
* {{NormanAnatomy|pelvis}} ({{NormanAnatomyFig|rectum}})


⚫
{{Gray's}}
{{Muscles of perineum}}
{{Muscles of perineum}}
{{Digestive tract}}
{{Digestive tract}}
{{Authority control}}
{{Portal bar|Anatomy}}


[[Category:Perineum]]
[[Category:Perineum]]
[[Category:Anus]]

[[Category:Muscular system]]
{{Muscle-stub}}

[[es:Músculo esfínter externo del ano]]
[[hr:Vanjski čmarni mišić podizač]]
[[it:Muscolo sfintere anale esterno]]
[[nl:Externe anaalsfincter]]
[[pl:Mięsień zwieracz zewnętrzny odbytu]]
[[pt:Músculo esfíncter externo do ânus]]
[[ru:Внешний сфинктер ануса]]

Latest revision as of 16:14, 13 May 2024

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
NerveBranch from the fourth sacral and contributions from the inferior hemorrhoidal branch of the pudendal nerve
ActionsKeep the anal canal and orifice closed
Identifiers
Latinsphincter ani externus
TA98A04.5.04.012
TA22426
FMA21930
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]

Anatomy

[edit]

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

[edit]

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).[1]

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

Attachments

[edit]

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

Innervation

[edit]

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

[edit]

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

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

See also

[edit]

References

[edit]
  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.
  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.

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

[edit]
  • 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)
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