# Micturition Reflex and Bladder Control

> Micturition reflex and bladder control in MBBS Physiology: pelvic and pudendal nerves, S2-S4 segments, storage versus voiding, spinal shock and dyssynergia.

- Canonical URL: https://prepelephant.com/topics/mbbs/physiology/micturition-reflex-and-bladder-control
- Exam / course: MBBS · Subject: Physiology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Micturition Reflex and Bladder Control", PrepElephant, https://prepelephant.com/topics/mbbs/physiology/micturition-reflex-and-bladder-control

## 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.