Pudendal nerve: S2 to S4, course and perineal function
The pudendal nerve, formed from S2 to S4, supplies the perineum (the area between the genitals and the anus), including the external anal and urethral sphincters.
Remember
After childbirth, loss of voluntary control (muscle contraction on purpose) of both external sphincters, together with reduced sensation in the perineal skin, points to the pudendal nerve (S2 to S4), the somatic nerve of the perineum (somatic nerves supply skeletal muscle and skin).
Memory aid
S2, S3, S4 keeps poo off the floor
It ties the three spinal nerves S2, S3 and S4 to the nerve's job of giving voluntary control over faeces.
The pudendal nerve arises from the anterior rami (the front branches of the spinal nerves) of S2, S3 and S4, in the sacral plexus (the network of nerves beside the sacrum). It is a somatic nerve, meaning it supplies skeletal (voluntary) muscle and skin.
It passes out of the pelvis through the greater sciatic foramen (a large opening at the back of the pelvis), loops round the ischial spine (a bony point on the pelvis) and comes back in through the lesser sciatic foramen, a smaller opening below it. It then runs forward in a passage called the pudendal canal to reach the perineum.
It supplies the skeletal muscles of the perineum, including the external anal sphincter and the external urethral sphincter, the two rings of voluntary muscle that we use to hold urine and faeces. It also carries sensation from the penis or clitoris and from most of the skin of the perineum.
Damage to the nerve can therefore affect control of the sphincters and sensation in the perineum. Childbirth is one recognised cause of injury to this nerve, and injury has also been reported after a pelvic fracture.
Key facts
- Formed from
- the front branches of the spinal nerves S2, S3 and S4.
- Course
- passes out of the pelvis through the greater sciatic foramen, loops round the ischial spine, then comes back in through the lesser sciatic foramen and reaches the perineum.
- Muscles
- the skeletal muscles of the perineum, including the external anal sphincter and the external urethral sphincter.
- Anal branch
- the inferior rectal branch gives the voluntary supply to the external anal sphincter.
- Sensation
- the penis or clitoris and most of the skin of the perineum.
- Nerve type
- somatic, meaning it supplies skeletal (voluntary) muscle and skin.
- Look-alike
- the posterior femoral cutaneous nerve (S1 to S3) supplies the skin of the back of the thigh and also some perineal skin.
Exam traps
- The posterior femoral cutaneous nerve does supply some perineal skin, so it is a near miss, but it has no motor fibres (the fibres that let you contract skeletal muscle on purpose), so it would not cause loss of voluntary sphincter control.
- The sciatic nerve also passes out through the greater sciatic foramen, but it then runs down the back of the thigh, does not re-enter the pelvis and does not supply the perineum.
- The set S1, S2 and S3 belongs to the posterior femoral cutaneous nerve. For the pudendal nerve, S2 and S3 are correct, but S1 does not contribute and S4 is missing.
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