The detrusor vesicae is the smooth muscle coat of the urinary bladder, responsible for bladder storage (by remaining relaxed during filling) and voiding (by contracting synchronously at micturition). Its innervation integrates parasympathetic (cholinergic, M2/M3 receptors), sympathetic (adrenergic, beta-3 for relaxation during filling, alpha-1 for trigone contraction), and somatic systems in a complex pontine micturition reflex arc.
| Origin | Interlacing smooth muscle bundles throughout the bladder wall, forming three indistinct layers (inner longitudinal, middle circular, outer longitudinal) that converge at the bladder neck |
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| Insertion | Bladder neck and proximal urethra (the detrusor fibres are continuous with the smooth muscle of the proximal urethra forming the internal urethral sphincter) |
| Nerve Supply | Parasympathetic: pelvic splanchnic nerves (S2-S4) via the inferior hypogastric plexus — trigger detrusor contraction; Sympathetic: hypogastric nerve (T10-L2) — trigger detrusor relaxation during filling; Somatic: pudendal nerve — controls the external urethral sphincter |
| Blood Supply | Superior and inferior vesical arteries |
| Actions | Contraction of the detrusor produces bladder emptying (micturition) by increasing intravesical pressure; during filling, sympathetic tone maintains detrusor relaxation allowing bladder distension at low pressure (compliance); coordinated detrusor contraction with simultaneous relaxation of the urethral sphincters is required for complete micturition |
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Overactive bladder (OAB) results from detrusor overactivity — involuntary detrusor contractions during filling producing urgency and urge incontinence. Anticholinergic agents (oxybutynin, solifenacin, tolterodine) or beta-3 agonists (mirabegron) reduce detrusor overactivity. Neurogenic detrusor overactivity occurs in MS, Parkinson's, and spinal cord injury. Detrusor underactivity (acontractile detrusor) produces urinary retention and overflow incontinence. Botulinum toxin A injection into the detrusor (intradetrusor injection, 100-200 units across 20-30 sites) provides 6-9 months of detrusor relaxation for refractory OAB.
Spinal cord injury above the sacral cord disrupts the coordinated pontine micturition reflex, producing detrusor-sphincter dyssynergia where detrusor contracts simultaneously with the external sphincter, generating dangerously high intravesical pressures causing upper tract damage; intermittent catheterisation combined with anticholinergics or intradetrusor botulinum toxin manages the neurogenic detrusor overactivity.
