Sacral parasympathetic outflow S2–S4 (pelvic splanchnic nerves)
Preganglionic · S2–S4 Nervi erigentes
Preganglionic neurones in the lateral grey of S2–S4 (conus medullaris, at vertebral level L1–L2) leave in the anterior rami as the pelvic splanchnic nerves (nervi erigentes), which join the inferior hypogastric plexus and also ascend to the hindgut.
Traced from the start
- Parasympathetic (craniosacral) outflow
- Sacral parasympathetic outflow S2–S4 (pelvic splanchnic nerves)
Detail
- Cord versus vertebral level
- S2–S4 segments lie in the conus at vertebral L1–L2; their roots then descend in the cauda equina, so lumbar disc or cauda lesions damage them
- Distribution
- Bladder detrusor, rectum and descending/sigmoid colon (via hypogastric nerves), uterus, erectile tissue via cavernous nerves
- Companion somatic nerve
- Pudendal nerve S2–S4 (Onuf's nucleus) supplies the external urethral and anal sphincters — voluntary, not autonomic
When it goes wrong
Central lumbar disc prolapse or tumour compressing the cauda equina (S2–S4 roots)
Cauda equina syndrome: painless urinary retention with overflow, reduced anal tone and faecal incontinence, saddle anaesthesia and erectile dysfunction with bilateral leg pain — needs emergency MRI and decompression
Spinal anaesthesia or acute spinal shock blocks S2–S4 outflow
Atonic bladder with retention until autonomic function returns — catheterise
Practise this structure
1 question in the bank tagged Autonomic.