The pudendal nerve provides the major somatic innervation of the perineum. Through which opening does it enter and exit the lesser sciatic foramen, and which ligament does it hook around?

• A Exits via the obturator foramen and supplies the medial perineum
• B Exits the pelvis via the greater sciatic foramen, hooks around the ischial spine/sacrospinous ligament, and re-enters the perineum via the lesser sciatic foramen into Alcock's canal ✓
• C Passes through the piriform foramen alongside the inferior gluteal nerve
• D Passes directly from S2–S4 anterior rami through the pelvic floor musculature

Explanation

The pudendal nerve (S2–S4) leaves the pelvis via the greater sciatic foramen (below piriformis), crosses the sacrospinous ligament at its attachment on the ischial spine, then re-enters the perineum through the lesser sciatic foramen into the pudendal (Alcock's) canal in the lateral wall of the ischioanal fossa. This tortuous course is the basis for the pudendal nerve block via the ischial spine landmark. In this canal it gives branches: inferior rectal nerve (external anal sphincter, lower anal canal), perineal nerve (labia/scrotum, perineal muscles, external urethral sphincter), and dorsal nerve of the clitoris/penis.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.