Overview

First sensation≈150 mL

Strong urge≈200–300 mL and above

Capacity≈400–600 mL

Spinal centresSacral S2–S4; thoracolumbar T11–L2

Storage phase. As the bladder fills, low-level afferent activity in the pelvic nerves activates spinal storage reflexes: sympathetic outflow from T11/T12–L2 (hypogastric nerve) relaxes the detrusor (β3) and contracts the bladder neck (α1), and somatic outflow from Onuf nucleus (S2–S4) via the pudendal nerve contracts the external urethral sphincter (guarding reflex). Parasympathetic outflow is inhibited.

Voiding phase. When bladder volume reaches the threshold (urge ≈200–300 mL), intense afferent activity reaches the periaqueductal grey, which projects to the pontine micturition centre (PMC, Barrington nucleus). If the medial prefrontal cortex and other forebrain areas judge the time and place appropriate, they release the PMC, which (1) activates sacral parasympathetic neurons (S2–S4, pelvic splanchnic nerves → ACh on M3 receptors) so the detrusor contracts, and (2) inhibits sympathetic and Onuf nucleus outflow so the bladder neck and external sphincter relax. The external sphincter relaxes slightly before the detrusor contracts, and the bladder empties completely in a coordinated, self-sustaining contraction.

In infants the reflex operates without cortical control; voluntary control develops at ≈2–4 years.

Location: Bladder and urethra, sacral spinal cord (S2–S4), thoracolumbar cord (T11–L2), pons, periaqueductal grey and prefrontal cortex.

Anatomy

Innervation: Parasympathetic: pelvic splanchnic nerves S2–S4 (detrusor). Sympathetic: hypogastric nerve T11–L2 (detrusor relaxation, bladder neck). Somatic: pudendal nerve S2–S4 (external sphincter). Afferents: pelvic nerves (stretch), hypogastric (pain from dome), pudendal (urethral sensation).

Function

Coordinate continence during storage
Produce complete, coordinated, voluntary emptying of the bladder

Clinical significance

Neurogenic bladder

Cortical/suprapontine lesions (stroke, dementia, normal-pressure hydrocephalus, frontal tumours): loss of inhibition → urge incontinence with coordinated voiding. Suprasacral spinal cord lesions (trauma, multiple sclerosis): after spinal shock, a spastic reflex bladder with detrusor–sphincter dyssynergia → high pressures, incomplete emptying, vesicoureteric reflux and renal damage; treated with intermittent self-catheterisation and antimuscarinics. Sacral cord, cauda equina or peripheral lesions (cauda equina syndrome, diabetic neuropathy, pelvic surgery): atonic, flaccid bladder → painless retention with overflow incontinence — cauda equina syndrome is a surgical emergency.

Autonomic dysreflexia

In spinal cord injury above T6, bladder distension can trigger massive unopposed sympathetic discharge → severe hypertension with reflex bradycardia, headache and flushing above the lesion; treated by draining the bladder.

Mnemonics

Pee vs storage

Parasympathetic = Pee; Sympathetic = Storage; "S2, 3, 4 keeps the pee off the floor."

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.