Bladder, urethra and genitalia

Target & receptor · M3 detrusor β3 detrusor α1 bladder neck NO erection

Storage is sympathetic (hypogastric nerve T11–L2: β3 relaxes detrusor, α1 closes the internal urethral sphincter) and voiding is parasympathetic (pelvic nerve S2–S4: M3 contracts detrusor, NO opens the urethra). Erection is parasympathetic S2–S4 and emission/ejaculation is sympathetic L1–L2 — 'point and shoot' — with the pudendal nerve supplying the external sphincter and rhythmic expulsion.

Traced from the start

Detail

Storage (sympathetic T11–L2): β3 detrusor relaxation and α1 internal sphincter contraction — mirabegron (β3 agonist) treats overactive bladder
Voiding (parasympathetic S2–S4): M3 detrusor contraction with NO-mediated urethral relaxation, triggered by the pontine micturition centre once stretch afferents report ~300–400 mL
Sexual function: Erection: parasympathetic S2–S4 via cavernous nerves (NO); emission and bladder neck closure: sympathetic L1–L2 (α1); expulsion: somatic pudendal S2–S4

When it goes wrong

Suprasacral spinal cord injury once spinal shock has resolved

Reflex (upper motor neurone) bladder: detrusor overactivity with detrusor–sphincter dyssynergia, high-pressure voiding, incomplete emptying and risk of reflux nephropathy and autonomic dysreflexia

α1-blocker (tamsulosin) for benign prostatic hyperplasia, or an antimuscarinic in a man with an enlarged prostate

Tamsulosin improves flow by relaxing the bladder neck but causes retrograde ejaculation and postural hypotension; antimuscarinics precipitate acute urinary retention

Practise this structure

1 question in the bank tagged Autonomic Control.

A 46-year-old man attends the clinic nine months after cardiac transplantation. He feels well, has no chest pain and walks four kilometres most days. He takes n…

Foundations · middle twist