Micturition Reflex

Mnemonic

“S2, 3, 4 keeps the urine off the floor”, for the parasympathetic supply that empties the bladder.

Three nerves, with opposite jobs:

• Parasympathetic (pelvic nerve, S2 to S4) - contracts the detrusor and relaxes the internal sphincter. Emptying
• Sympathetic (hypogastric nerve, T11 to L2) - relaxes the detrusor and contracts the internal sphincter. Filling
• Somatic (pudendal nerve, S2 to S4) - voluntary control of the external sphincter

“Parasympathetic pees, sympathetic stores.”

The reflex is coordinated in the pontine micturition centre, which is why lesions above and below it behave differently:

• Above the pons (stroke, dementia) - urge incontinence, with coordinated but uninhibited voiding
• Spinal cord lesion above the sacral segments - detrusor-sphincter dyssynergia, a reflex neurogenic bladder that contracts against a closed sphincter
• Sacral or cauda equina lesion - flaccid, atonic bladder with overflow incontinence and retention

Expansion

Storage

• Sympathetic (T11 to L2, hypogastric nerve): relaxes the detrusor via beta-3 receptors and contracts the internal sphincter via alpha-1 receptors
• Somatic (pudendal nerve, S2 to S4): contracts the external urethral sphincter voluntarily

Emptying

• Parasympathetic (S2 to S4, pelvic splanchnic nerves): contracts the detrusor via M3 receptors
• Simultaneous relaxation of both sphincters

Coordination is by the pontine micturition centre, which ensures the detrusor contracts only when the sphincter relaxes. Higher cortical centres provide voluntary inhibition until it is socially appropriate.

Lesion patterns follow the anatomy:

• Above the pons: loss of voluntary inhibition, giving urge incontinence with coordinated voiding
• Spinal cord between pons and sacrum: detrusor-sphincter dyssynergia, with high pressures and a risk of reflux and renal damage
• Sacral cord or cauda equina: an areflexic, atonic bladder with overflow incontinence