SN2 SHORT NOTE 5M ★ High-yield

Describe the course, branches and applied anatomy of the pudendal nerve.

Anatomy · 1st Professional — Pre-Clinical

The pudendal nerve is the chief nerve of the perineum, arising from the ventral rami of S2, S3 and S4 in the sacral plexus.

It is the only nerve that leaves the pelvis and then re-enters the perineum — a distinctive course worth remembering
At the ischial spine it lies medial to the internal pudendal artery

Branch	Supplies
Inferior rectal nerve	External anal sphincter; skin around the anus; anal canal below the pectinate line
Perineal nerve	Muscles of the perineum (superficial and deep transverse perinei, bulbospongiosus, ischiocavernosus, sphincter urethrae); posterior scrotal or labial skin
Dorsal nerve of the penis or clitoris	Skin of the penis or clitoris; the chief sensory nerve of the organ

It is both motor and sensory, and carries the somatic supply to both voluntary sphincters — the external anal and the external urethral
Mnemonic — "S2, 3, 4 keeps the faeces off the floor"

It does not relieve the pain of uterine contractions, which is carried by T10–L1; only the perineal (second stage) pain is abolished
The ischial spine is the key landmark, palpable per vaginam and also used to assess the station of the fetal head

Pudendal nerve injury in childbirth — from prolonged second stage or instrumental delivery; contributes to later urinary and faecal incontinence
Pudendal neuralgia — entrapment in Alcock's canal causes perineal pain worse on sitting and relieved on standing or sitting on a lavatory seat
Cauda equina syndrome — involves S2–S4 and gives saddle anaesthesia, urinary retention and loss of anal tone; a surgical emergency requiring decompression within hours
Cycling on a narrow saddle can compress the nerve against the pubic arch, causing perineal numbness and erectile dysfunction
Anal reflex ("anal wink") — stroking the perianal skin contracts the external sphincter; it tests the integrity of S2–S4 and the pudendal nerve
The bulbocavernosus reflex similarly tests S2–S4 and returns after spinal shock resolves, which is why it is checked in spinal injury
The nerve is spared in a lesion above the sacral segments, so the external sphincter retains reflex tone though voluntary control is lost
Perineal sensation must be tested in any suspected cauda equina lesion — saddle anaesthesia is the earliest and most reliable sign

All questions in Pelvis & Perineum