Micturition Reflex and Bladder Control
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Direct answer
Micturition is a spinobulbospinal reflex: bladder wall stretch receptors fire from about 150 mL (first sensation) and reach threshold for urgency near 400-500 mL against a functional capacity of 400-600 mL; afferents travel in the pelvic nerve to the sacral cord (S2-S4) and up to the pontine micturition centre, which coordinates detrusor contraction (parasympathetic, pelvic nerve, M3 receptors) with relaxation of the internal sphincter (sympathetic hypogastric, T11-L2, switching off alpha-1 tone) and external sphincter (somatic pudendal nerve from Onuf's nucleus). Storage is the sympathetic-dominant state — beta-3-mediated detrusor relaxation with alpha-1-mediated bladder neck closure — held in check by descending inhibition until socially appropriate.
What you must remember
- The three circuits: parasympathetic S2-S4 pelvic nerve (contracts detrusor via M3 for voiding), sympathetic T11-L2 hypogastric nerve (beta-3 relaxes body, alpha-1 closes bladder neck for storage), somatic S2-S4 pudendal nerve (striated external sphincter, voluntary).
- Volumes: first desire 150-300 mL, strong urgency by 400-500 mL, functional capacity 400-600 mL, sharp pain beyond 500 mL — cystometry quotes these as I and II on the cystometrogram.
- Pontine micturition centre in the brainstem runs the coordination (detrusor contraction with synchronous sphincter relaxation); its frontal lobe connections provide the social brake that develops by 3-5 years of age.
- Spinal shock: immediately after cord injury the bladder is atonic and overflows — retention with overflow incontinence; the reflex arc below the lesion is temporarily silent.
- Automatic (reflex) bladder: returns after spinal shock when the lesion is above S2 — reflex voiding without sensation or control.
- Autonomous bladder: when the conus or sacral roots (S2-S4) are destroyed, no reflex remains — a flaccid bladder needing catheterisation; the same lesion abolishes the anal reflex.
- Detrusor sphincter dyssynergia: sphincter contracts against a contracting detrusor in suprasacral cord lesions, generating high pressures, residuals and hydronephrosis.
- Autonomic dysreflexia: in lesions above T6, bladder distension or catheter blockage triggers mass sympathetic discharge — pounding headache, hypertension, reflex bradycardia; sitting the patient up and draining the bladder are the immediate responses.
- Pharmacology map: antimuscarinics (oxybutynin, solifenacin) for overactive bladder; mirabegron (beta-3 agonist) relaxes detrusor; alpha-1 blockers (tamsulosin) reduce outlet resistance in prostatic enlargement.
A typical case: cord injury at T10
A young man sustained a complete T10 cord injury. In the first weeks his bladder is flaccid and overfills — the spinal shock phase, managed by catheter drainage. As cord shock recedes, sacral reflexes re-emerge and he develops an automatic bladder: filling to a threshold triggers reflex detrusor contraction and (imperfect) sphincter relaxation, so he voids by reflex without warning or sensation. Two dangers follow from the physiology. First, dyssynergia — the external sphincter, also refexively driven, contracts against the contracting detrusor, raising intravesical pressure enough to back-pressure the kidneys; timed voiding and anticholinergics protect the upper tract. Second, any bladder distension in a lesion above T6 can fire mass sympathetic discharge below the level — autonomic dysreflexia with systolic pressures past 200 mmHg — and the fastest treatment is draining the bladder that caused it. Contrast a cauda equina lesion: the reflex centre itself is destroyed, the bladder stays autonomous and areflexic, and saddle anaesthesia with loss of the anal reflex anchors the level.
Where students slip
The storage-versus-voiding transmitter swap is the commonest viva stumble: sympathetic activity stores (beta-3 relaxation, alpha-1 closure), parasympathetic voids (M3 contraction) — students reverse them. The second slip is the trio of late-bladder names: atonic (spinal shock, overflows), automatic (suprasacral lesion, reflex but uncontrolled), autonomous (sacral lesion, no reflex) — examiners ask for all three with the lesion level attached. Third, the pontine centre's role is coordination, not generation: the sacral cord can micturate reflexly without it, but voiding is then unsynchronised — which is exactly what dyssynergia demonstrates.
Frequently asked questions
Which nerves and segments mediate micturition?
Pelvic nerve parasympathetics from S2-S4 contract the detrusor; hypogastrical sympathetics from T11-L2 promote storage; the pudendal nerve (S2-S4, Onuf's nucleus) controls the voluntary external sphincter.
At what bladder volume does the first urge to void occur?
Around 150-300 mL for the first sensation, with urgency by 400-500 mL against a functional capacity of 400-600 mL.
What happens to the bladder during spinal shock?
It becomes atonic and overdistends, producing retention with overflow incontinence until reflex activity returns below the lesion.
Why does bladder filling cause severe hypertension in high cord injuries?
In lesions above T6, distension triggers mass sympathetic discharge (autonomic dysreflexia) that the baroreflex cannot modulate — pounding headache, hypertension and bradycardia, relieved by draining the bladder.
Which drugs act on bladder physiology, and where?
Antimuscarinics block M3-mediated detrusor contraction (overactive bladder), mirabegron agonises beta-3 to relax the detrusor, and alpha-1 blockers reduce bladder neck outlet resistance.