The pudendal nerve leaves the pelvis and then loops straight back in
One nerve on each side of the pelvis handles sensation from the genitals and the skin around the anus, and also runs the muscles that keep us continent. Its route is oddly roundabout: it exits the pelvis, hooks around a ligament and re-enters, which helps explain why stretching during childbirth can injure it.
Each pudendal nerve is assembled from three roots drawn from the second, third and fourth sacral spinal nerves, with the fourth contributing most. The roots merge into two cords and then a single trunk. Its fibres start in Onuf's nucleus, in the sacral part of the spinal cord. The nerve slips out through the greater sciatic foramen, crosses the sacrospinous ligament, and comes back in through the lesser foramen before running forward in a fascial tunnel alongside the internal pudendal artery and vein. Anatomy can vary: sometimes it branches off the sciatic nerve, so sciatic damage can reach it too.
That tunnel is Alcock's canal, named after the Irish anatomist Benjamin Alcock, who described it in 1836. The nerve's name comes from the Latin pudenda, literally parts to be ashamed of. Inside the canal it gives off branches to the rectum and perineum before ending as the dorsal nerve of the penis or clitoris. It carries both motor and sensory signals, plus sympathetic but not parasympathetic fibres.
Its motor role centres on the external urethral and anal sphincters. Unlike the internal sphincter, the external one is skeletal muscle and under voluntary control; acetylcholine released by the nerve keeps it contracted, and when that release drops the bladder can empty. Doctors can check the nerve through the anal wink reflex, or time how quickly its muscles respond to an electrical pulse from electrodes worn on a gloved finger.
Damage tends to show as altered sensation or pain more than weakness. Nerves can be permanently harmed once stretched to 12 percent or more beyond their normal length, a risk in long, difficult deliveries or chronic straining. Entrapment in the canal is rare, estimated at 1 in 100,000, and sometimes linked with professional cycling. Because the two sides overlap in supplying the anal sphincter, injury on one side causes incontinence in some people but not others. In a study of 70 adults with neurogenic urinary retention, external pudendal stimulation improved bladder emptying more than a standard sacral test over four weeks.
Source: Pudendal nerve
