Overview

The detrusor is often described as three layers — inner longitudinal, middle circular and outer longitudinal — but the bundles interlace extensively and the arrangement is better considered a meshwork that contracts as a functional syncytium, reducing all dimensions of the bladder together. Its fibres extend toward the bladder neck and trigone.

During storage, sympathetic noradrenaline acting on β3-adrenoceptors relaxes the detrusor, and the muscle shows viscoelastic stress relaxation, so pressure rises little as volume increases. During voiding, acetylcholine released by parasympathetic pelvic splanchnic nerves (S2–S4) acts on M3 muscarinic receptors (Gq → IP₃ → Ca²⁺) to produce a sustained contraction; ATP acting on P2X receptors contributes, especially in disease.

Location: Wall of the urinary bladder.

Anatomy

Innervation: Parasympathetic pelvic splanchnic nerves S2–S4 (contraction, M3); sympathetic hypogastric nerve T11–L2 (relaxation, β3).
Histology: Bundles of smooth muscle separated by connective tissue; hypertrophy and collagen deposition with chronic obstruction.

Function

Accommodate urine at low pressure during filling
Contract to expel urine during micturition

Clinical significance

Overactive bladder

Involuntary detrusor contractions during filling cause urgency, frequency, nocturia and urge incontinence. Treated with antimuscarinics (oxybutynin, solifenacin — side effects dry mouth, constipation, cognitive impairment in the elderly), the β3-agonist mirabegron, or intradetrusor botulinum toxin.

Trabeculation and diverticula

Outflow obstruction (BPH, urethral stricture) causes detrusor hypertrophy with trabeculated walls and out-pouchings of mucosa between bundles (diverticula), which predispose to stasis, infection and stones.

Test yourself

Detrusor contraction during voiding is mediated by:

Sympathetic fibres from T11–L2 acting on β3 receptors
Parasympathetic pelvic splanchnic nerves (S2–S4) acting on M3 receptors
The pudendal nerve acting on nicotinic receptors
Sympathetic fibres acting on α1 receptors

Answer: Parasympathetic pelvic splanchnic nerves (S2–S4) acting on M3 receptors

Pelvic splanchnic nerves (S2–S4) release acetylcholine onto muscarinic M3 receptors of the detrusor. Sympathetic β3 stimulation relaxes it (storage); the pudendal nerve supplies the skeletal external sphincter.

A man with a complete spinal cord transection at T6 develops a reflex bladder that contracts against a closed external sphincter. Which structure normally coordinates sphincter relaxation with detrusor contraction?

Pontine micturition centre
Sacral spinal cord (Onuf nucleus) alone
Hypothalamus
Cerebellum

Answer: Pontine micturition centre

The pontine micturition centre (Barrington nucleus) coordinates detrusor contraction with sphincter relaxation. Suprasacral cord lesions below the pons disconnect it, causing detrusor–sphincter dyssynergia with high bladder pressures.