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//MUSCLES OF THE ANAL CANAL

MUSCLES OF THE ANAL CANAL

The anal canal is encircled by internal and external anal sphincters, separated by the longitudinal layer, and has connections superiorly to puborectalis and the transverse perineii (Fig. 67.47).

  

Fig. 67.47  A–C, Axial views of the anal canal at three levels on endoanal ultrasound in a woman. The endoanal ultrasound probe is the central black structure. A, Upper anal canal. The ‘U’ shape of puborectalis (PR) is visible. Ias, internal anal sphincter. B, Middle anal canal. The external anal sphincter (Eas) is now a complete ring anteriorly (arrowhead). Lm, longitudinal muscle; S, subepithelial tissues. C, Lower anal canal. Below the termination of the internal anal sphincter, the longitudinal layer extends through the subcutaneous external anal sphincter (between arrowheads). D–F, MRI of the anal canal. D, At upper anal canal level, the sling of puborectalis (PR) extends anteriorly to the pubic bones. Vag, vagina; Ur, urethra. E, At mid anal canal level, the transverse perineii (Tp) fuse into the external anal sphincter anteriorly. The superficial (middle) external anal sphincter (SpEas) is attached either side of the anococcygeal ligament (Acl). Ias, internal anal sphincter. F, Low anal canal level, below the internal anal sphincter. ScEas, subcutaneous (lower) part of the external anal sphincter. G, Key for levels of the anal canal.

Internal anal sphincter

The internal anal sphincter is a well-defined ring of obliquely orientated smooth muscle fibres that is continuous with the circular muscle of the rectum, and which terminates at the junction of the superficial and subcutaneous components of the external sphincter. Its thickness varies between 1.5 and 3.5 mm, depending upon the height within the anal canal and whether the canal is distended. It is usually thinner in females and becomes thicker with age. It may also be thickened in disease processes such as rectal prolapse and chronic constipation. The lower portion of the sphincter is crossed by fibres from the conjoint longitudinal coat which pass into the submucosa of the lower canal.

Vascular supply

The internal anal sphincter is supplied from the terminal branches of the superior rectal vessels and branches of the inferior rectal vessels.

Innervation

The internal anal sphincter is supplied by the sympathetic and parasympathetic systems by fibres that extend down from the lower rectum. Sympathetic fibres originate in the lower two lumbar spinal segments, are distributed via the inferior hypogastric plexus, and cause contraction of the sphincter. Parasympathetic fibres originate in the second to fourth sacral spinal segments, are distributed via the inferior hypogastric plexus, and cause relaxation of the sphincter.

External anal sphincter

The external anal sphincter is an oval tube-shaped complex of striated muscle, composed mainly of type 1 (slow twitch) skeletal muscle fibres, which are well suited to prolonged contraction. It has been described as consisting of deep, superficial and subcutaneous parts but the external anal sphincter should be considered as a single functional and anatomical entity although the upper middle and lower thirds show different features and attachments. Endoanal ultrasound and magnetic resonance imaging reveal that the uppermost fibres blend with the lowest fibres of puborectalis. In the upper third some of these upper fibres decussate anteriorly into the superficial transverse perineal muscles and posteriorly some fibres are attached to the anococcygeal raphe. The majority of the fibres of the middle third of the external anal sphincter surround the lower part of the internal sphincter. The middle third is attached anteriorly to the perineal body and posteriorly to the coccyx via the anococcygeal ligament: some fibres from each side of the sphincter decussate in these areas to form a commissure in the anterior and posterior midline. The fibres of the lower third lie below the level of the internal anal sphincter and are separated from the lowest anal epithelium by submucosa.

The length and thickness of the external anal sphincter varies between the sexes. In females, the anterior portion tends to be shorter, the wall may be slightly thinner, and the tube may take the form of an asymmetrical cone (Rociu et al 2000) and the transverse perineii and bulbospongiosus fuse with the external sphincter in the lower part of the perineum. In males, the external sphincter is more anular and is separate from the central point of the perineum into which the transverse perineii and bulbospongiosus fuse, so that there is a surgical plane of cleavage between the external sphincter and perineum (Fig. 67.48).

  

Fig. 67.48  Typical anatomy of male and female external anal sphincters. Puborectalis is shown in isolation with the external anal sphincter viewed from the superolateral aspect. The anterior portion of the external anal sphincter is typically shorter and thinner in females.

Vascular supply

The external anal sphincter is supplied from the terminal branches of the inferior rectal vessels with a small contribution from the median sacral artery.

Innervation

The external anal sphincter is innervated mainly by the inferior rectal branch of the pudendal nerve (anterior divisions of the second, third and fourth sacral spinal nerves). It may also receive some direct supply via fibres which leave the ventral rami of these nerves as they exit the sacral foramina and run beneath the fascia over levator ani to reach the anorectal junction.

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