External urethral sphincter
Musculus sphincter urethrae externus
Also known as: sphincter urethrae, rhabdosphincter, urogenital sphincter, compressor urethrae, urethrovaginal sphincter
A voluntary, skeletal muscle sphincter surrounding the membranous urethra (males) or middle third of the urethra (females), innervated by the pudendal nerve (S2–S4), which provides conscious control of continence.
Overview
The external sphincter lies in the deep perineal pouch, within and above the perineal membrane. In males it forms a trough-shaped muscle around the membranous urethra, extending upward over the anterior surface of the prostate. In females it is part of the urogenital sphincter complex: the sphincter urethrae proper around the middle urethra, plus the compressor urethrae and urethrovaginal sphincter, which compress the urethra against the anterior vaginal wall.
It contains slow-twitch fibres for sustained resting tone and fast-twitch fibres for rapid closure (e.g. during a cough — the guarding reflex). Its motor neurons lie in Onuf nucleus of the anterior horn at S2–S4, and it is supplied by the perineal branch of the pudendal nerve. During voiding, pontine centres inhibit Onuf nucleus so the sphincter relaxes just before the detrusor contracts.
- Location
Deep perineal pouch, around the membranous urethra (male) or middle urethra (female).
Anatomy
- Innervation
Pudendal nerve (perineal branch), S2–S4 — somatic, nicotinic cholinergic.
- Nerve roots
S2–S4 (Onuf nucleus).
Function
- Voluntary control of continence
- Reflex closure with increased intra-abdominal pressure
- Interruption of the urinary stream
Clinical significance
Post-prostatectomy incontinence
Damage to the external sphincter or its nerve supply during radical prostatectomy causes stress incontinence, because after prostatectomy the bladder-neck sphincter is also lost.
Childbirth injury
Stretch injury to the pudendal nerve and pelvic floor during vaginal delivery weakens the external sphincter and urethral support, a major cause of female stress incontinence.
Compiled from standard anatomy and physiology references — see the Urinary System sources. For education only; not medical advice.