Inferior rectal nerve
The inferior rectal nerve, also known as the inferior anal nerve or inferior hemorrhoidal nerve, is a branch of the pudendal nerve which is derived from S2, S3 and S4 nerve roots of the sacral plexus. The nerve provides sensory innervation to the anal canal inferior to the pectinate line and motor innervation to the external anal sphincter.
The inferior rectal nerve is typically the most proximal of the three branches of the pudendal nerve (the terminal branches of the nerve being the perineal nerve and the dorsal nerve of the penis or clitoris). The inferior rectal nerve branches from the pudendal nerve as it passes forwards in the lateral wall of the ischioanal fossa. The inferior rectal nerve usually branches from the pudendal nerve within the pudendal (Alcock’s) canal.
The branches of the inferior rectal nerve provide somatic cutaneous sensation to the anal canal inferior to the pectinate line and therefore this part of the anal canal is sensitive to pain, touch and temperature. Somatic efferent fibres from the inferior rectal nerve also stimulate contraction of the voluntary external sphincter.
The inferior rectal nerve may contribute to the innervation of levator ani though the pudendal nerve and the perineal branch of the pudendal nerve typically supply these muscles.
At its origin the inferior rectal nerve branches from the pudendal nerve within the pudendal canal in the lateral wall of the ischioanal fossa, also travelling within this canal are the internal pudendal artery and vein. The lateral wall of the ischioanal fossa also houses the tuberosity of the ischium, the obturator internus muscle and the obturator fascia (that composes the pudendal canal). Outside the pudendal canal and crossing the lateral wall of the ischioanal fossa transversely (within the ischiorectal fat) are the paired inferior rectal arteries, which supply the inferior part of the anal canal as well as the surrounding muscles and perineal skin. Similarly, inferior to the pectinate line the internal rectal plexus drains into the inferior rectal veins around the margin of the external anal sphincter.
Significant variability in the structure of the pudendal and associated inferior rectal nerve has been demonstrated:
- the inferior rectal nerve may have an origin directly from the sacral plexus independent of the pudendal nerve
- cadaveric studies have demonstrated that the inferior rectal nerve does not always enter the pudendal canal and instead may pass behind the sacrospinous ligament to enter the ischioanal fossa; dissections have also demonstrated that the inferior rectal nerve may pierce the sacrospinous ligament
- the inferior rectal nerve has also been shown to innervate the levator ani muscles independent of the pudendal nerve
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