The detrusor is the primary smooth muscle of the bladder wall, responsible for voiding contraction. It is unique in that its three layers (inner longitudinal, middle circular, outer longitudinal) are interwoven rather than distinct, allowing isometric contraction in any direction to expel urine efficiently.
| Origin | Bladder wall — the smooth muscle forms the entire bladder wall as three interwoven layers |
|---|---|
| Insertion | Internal urethral meatus — the detrusor funnel produces the bladder neck opening |
| Nerve Supply | Parasympathetic (pelvic splanchnic nerves S2-S4) — for contraction during voiding; Sympathetic (hypogastric nerve L1-L2) — for relaxation during filling; Pudendal nerve (S2-S4) — sensory afferents |
| Blood Supply | Vesical arteries from the internal iliac |
| Actions | Contracts to empty the bladder during voiding — coordinated with external urethral sphincter relaxation; Relaxes during filling — accommodates up to 500ml at low pressure; Forms the bladder neck funnel during voiding |
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Detrusor overactivity (formerly unstable bladder) produces urgency urinary incontinence from involuntary contractions. Urodynamic studies demonstrate uninhibited detrusor contractions. Antimuscarinic medications (oxybutynin, tolterodine) reduce detrusor overactivity by blocking parasympathetic M3 receptors. Botulinum toxin injection directly into the detrusor via cystoscopy provides 6-9 months of overactivity suppression. Detrusor underactivity (acontractile bladder) produces urinary retention.
Not externally palpable — assessed by urodynamic cystometry measuring detrusor pressure during filling and voiding.
Uninhibited detrusor contractions producing urgency and urge incontinence, managed by anticholinergic medication, bladder training, and botulinum toxin cystoscopic injection for refractory cases.
