Pelvic short postganglionic fibres (including cavernous nerves)
Postganglionic · M3 detrusor NO erection
Short cholinergic fibres to the detrusor and rectal wall, and nitrergic fibres in the cavernous nerves that pass beside the prostate to the corpora cavernosa. The pelvic nerves also relax the urethral smooth muscle through nitric oxide during voiding.
Traced from the start
- Parasympathetic (craniosacral) outflow
- Sacral parasympathetic outflow S2–S4 (pelvic splanchnic nerves)
- Pelvic (inferior hypogastric plexus) intramural ganglia
- Pelvic short postganglionic fibres (including cavernous nerves)
Detail
- Erection
- Nitric oxide → guanylate cyclase → cGMP → smooth muscle relaxation of cavernous arterioles and sinusoids; PDE5 degrades cGMP
- Voiding
- M3 detrusor contraction with NO-mediated urethral relaxation, coordinated by the pontine micturition centre
- Defaecation
- Rectal wall contraction with reflex internal anal sphincter relaxation via NO
When it goes wrong
Sildenafil (PDE5 inhibitor) taken with a nitrate
Profound hypotension because both raise cGMP in vascular smooth muscle — an absolute contraindication
Nerve-sparing prostatectomy failing to preserve the cavernous nerves
Erectile dysfunction that may respond poorly to PDE5 inhibitors because no NO is released to start the cascade
Practise this structure
1 question in the bank tagged Autonomic.