Internal urethral sphincter
Musculus sphincter urethrae internus
Also known as: bladder neck sphincter, sphincter vesicae, preprostatic sphincter
An involuntary smooth muscle sphincter at the bladder neck, contracted by sympathetic α1 stimulation, which maintains continence at rest and (in males) prevents retrograde ejaculation.
Overview
In males, a distinct collar of circular smooth muscle surrounds the bladder neck and preprostatic urethra. It is innervated by sympathetic fibres (T11–L2, hypogastric nerve) acting on α1-adrenoceptors: it remains tonically contracted during bladder filling and closes firmly during ejaculation so semen passes distally rather than into the bladder.
In females a discrete internal sphincter is poorly developed; continence at the bladder neck depends more on the smooth muscle and elastic tissue of the proximal urethra, urethral mucosal coaptation, the pelvic floor and the external sphincter. During voiding the bladder neck opens passively as the detrusor contracts and sympathetic tone is inhibited.
- Location
Bladder neck and preprostatic (intramural) urethra.
Anatomy
- Innervation
Sympathetic T11–L2 via the hypogastric nerve and inferior hypogastric plexus (α1-adrenoceptors).
Function
- Involuntary continence at the bladder neck
- Prevention of retrograde ejaculation (males)
Clinical significance
α1-blockers and retrograde ejaculation
Tamsulosin relaxes the bladder neck and prostatic smooth muscle, improving flow in BPH but often causing retrograde (dry) ejaculation; the same occurs after transurethral resection of the prostate or with diabetic autonomic neuropathy.
Compiled from standard anatomy and physiology references — see the Urinary System sources. For education only; not medical advice.