External anal sphincter

Origin and insertion

The external anal sphincter has quite complex origin an insertion, as it is a circular structure and doesn’t follow the typical attachment pattern. It is generally described as originating from the skin and fascia surrounding the anus and the anal canal and having two separate insertions: anteriorly into the perineal body and posteriorly into the anococcygeal ligament, which stretches posteriorly to insert into the coccyx.

The external anal sphincter is comprised of three indistinct parts, or zones: the upper or deep part, the middle or superficial part and the lower or subcutaneous part.

Some fibres from the deep part of the external anal sphincter blend with the puboanalis muscle. In the anterior part of the same region, some fibres cross over the midline to blend with the superficial transverse perineal muscle of the opposite side. Posteriorly, this part inserts into the anococcygeal ligament.

The superficial part inserts into perineal body anteriorly and into the coccyx posteriorly via the anococcygeal ligament. At these midline points, some fibres cross over to the other side. Due to these attachments, the cross section of the muscle is oval rather than round, with its longer axis lying in the midline.

The subcutaneous part of external anal sphincter lies underneath the skin, surrounding the lowest, and most superficial part of the anal canal (anal verge).

Relations

The external anal sphincter is located at the intersection of many intricate muscle fibers and many anatomical spaces are related to it. The deep part forms a tube around the higher portion of the internal anal sphincter, while the superficial part surrounds the lower portion of the internal anal sphincter. The subcutaneous part surrounds the anal verge, with its most inferior fibres extending past the internal anal sphincter. Its position results in the typical creases seen externally around the anus.

The external anal sphincter is contained in the anal triangle, together with the anococcygeal ligament and the inferior anal nerve. It forms part of the posteromedial wall of the ischioanal fossa.

As mentioned before, the external anal sphincter relates to some muscles of the perineum. Its fibers partially blend with the puboanalis muscle, which is part of the pelvic diaphragm, and the superficial transverse perineal muscle, which is a perineal muscle. The layer of the pelvic diaphragm covering the inferior aspect of the levator ani also extends over the external anal sphincter.

The external anal sphincter is also related to the conjoint longitudinal muscle, the latter lying between the former and the internal anal sphincter. The conjoint longitudinal muscle sends fibres through the external anal sphincter. The inferior fibres go through the lower subcutaneous part of the lower subcutaneous part of the external anal sphincter, while the outward facing fibres go through the deep part.

There is a small potential space between the external anal sphincter and the conjoint longitudinal muscle known as the intersphincteric space. This space contains anal glands, and may need to be surgically accessed if the ducts of these glands become blocked.

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Innervation

The external anal sphincter receives somatic innervation from the inferior anal nerve, a branch of the pudendal nerve (S2-S4). As a result, this muscle is under voluntary control.

Blood supply

Inferior rectal arteries follow the inferior rectal nerves and carry blood to the external anal sphincter. They are terminal branches of the internal pudendal arteries, which stem from the internal iliac artery.

Function

The external anal sphincter provides voluntary control for defecation. As the rectum gradually fills with feces at rest and after meals, the continuous tonic contraction of the external anal sphincter helps to prevent defecation. When the urge to defecate passes a certain threshold and the situation is appropriate, the external anal sphincter is voluntarily relaxed, permitting the feces to exit through the anal canal. The function of the external anal sphincter is aided by the sling-like puboanalis muscle, which is one of the levator ani muscles. Together with the perineal muscles, the external anal sphincter helps to support the pelvic floor.