S4 anterior ramus
Roots · S4
Lowest root of the sacral plexus. Joins the pudendal nerve, supplies levator ani and coccygeus directly, contributes to the pelvic splanchnics and sends a branch to the coccygeal plexus.
Detail
- Dermatome
- Perianal skin (with S5)
- Motor
- External anal and urethral sphincters via the pudendal nerve; levator ani
When it goes wrong
Central lumbar disc prolapse or tumour compressing the sacral roots of the cauda equina
Saddle anaesthesia, painless urinary retention with overflow, reduced anal tone, bilateral leg weakness and absent ankle jerks — cauda equina syndrome, a surgical emergency
Practise this structure
1 question in the bank tagged Lower Motor Neurone.