Physiology · Renal, fluids and acid–base
Micturition and the bladder reflex !
- Regulation
- Exam priority: medium
The bladder fills while relaxed and empties through a reflex coordinated by the sacral cord and brainstem, which the cortex allows or holds back.
Updated 2026-11-01
Also called: Micturition reflex, Neurogenic bladder, Pontine micturition centre
See it in 3D: Urinary bladder · Trigone of bladder · External urethral sphincter · Pelvic plexus and cavernous nerves · Urethra (male) · Spinal cord · Pons
What it is
The bladder switches between two modes: filling (detrusor relaxed, sphincters closed) and voiding (detrusor contracting, sphincters open). The sympathetic system favours filling, the pelvic parasympathetic nerves drive contraction and the pudendal nerve controls the voluntary external sphincter. The pontine micturition centre makes sure the bladder contracts while the sphincters relax; the frontal cortex decides when. A lesion at each level gives a different kind of neurogenic bladder.
How it works, step by step
- As the bladder fills, wall stretch receptors signal through the pelvic nerves to the sacral cord.
- During filling the sympathetic system relaxes the detrusor and closes the neck; the pudendal nerve keeps the external sphincter shut (guarding reflex).
- Past a certain volume, signals reach the pontine micturition centre, which receives permission or inhibition from the frontal cortex.
- If allowed, the pontine centre switches on the sacral parasympathetic outflow: the detrusor contracts.
- At the same time it inhibits sympathetic and pudendal output: the neck and external sphincter open.
- Urine in the urethra reinforces the contraction (positive feedback) until the bladder is empty.
Bladder volumes (adult, approx.)
| Parameter | Value | Note |
|---|---|---|
| First desire to void | ≈150–250 mL | |
| Strong desire | ≈300–400 mL | |
| Functional capacity | ≈400–600 mL | |
| Normal post-void residual | < 50 mL | higher in older adults |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Regulation
- +Pelvic parasympathetic (acetylcholine, M3): Detrusor contraction.
- −Sympathetic (noradrenaline, β3): Relaxes the detrusor during filling.
- +Sympathetic (α1) at bladder neck and prostate: Closes the outlet during filling and ejaculation.
- +Pudendal (acetylcholine, nicotinic): Keeps the external sphincter closed.
- −Frontal cortex: Inhibits the reflex until the right moment.
Where the lesion is and what the bladder does
| Level | Example | Bladder | Key point |
|---|---|---|---|
| Frontal cortex | Stroke, dementia, hydrocephalus | Uninhibited: urgency, complete emptying | Coordination preserved |
| Cord above S2 (after spinal shock) | Traumatic spinal cord injury, multiple sclerosis | Spastic, reflex | Detrusor–sphincter dyssynergia; risk of high pressure reaching the kidneys |
| Sacral cord or cauda equina | Large disc herniation, tumour | Flaccid, retention | Overflow incontinence; saddle anaesthesia |
| Outlet obstruction | Benign prostatic hyperplasia | Hypertrophied detrusor, high residual | Weak stream, overflow |
| Pelvic floor support | Childbirth, menopause | Weak sphincter mechanism | Leakage with coughing or straining |
Clinical notes
- After an acute spinal cord injury there is spinal shock: the bladder stays atonic for weeks and needs drainage; the spastic pattern appears later.
- Urinary retention with perineal numbness and leg weakness is a red flag for cauda equina syndrome.
- Anticholinergic drugs (older antihistamines, tricyclic antidepressants) can precipitate retention, especially with a large prostate.
Test yourself
Think of the answer, then tap the question to see it.
Why does a chronic cervical cord lesion give a spastic bladder with dyssynergia, while a cauda equina lesion gives a flaccid one?
Above S2 the sacral reflex arc is intact but cut off from the pontine centre: the detrusor contracts without coordinating with the sphincter. In the cauda equina the reflex arc itself is cut: the detrusor does not contract.
Which receptor relaxes the detrusor during filling, and which closes the bladder neck?
β3 (sympathetic) relaxes the detrusor; α1 (sympathetic) contracts the bladder neck and prostatic urethra.
Why can an anticholinergic drug cause urinary retention in a man with prostatic hyperplasia?
It blocks detrusor M3 receptors and weakens contraction; with an already narrowed outlet, the bladder cannot overcome the resistance.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
- Urinary bladder
- Trigone of bladder
- External urethral sphincter
- Pelvic plexus and cavernous nerves
- Urethra (male)
- Spinal cord
- Pons
Related topics
- The autonomic nervous system: organization
- Adrenergic and cholinergic receptors
- Sympathetic and parasympathetic effects on each organ
- Spinal cord reflexes
Related conditions
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 25.2 Gross Anatomy of Urine Transport (OpenStax (Rice University), 2022); Anatomy, Abdomen and Pelvis: Bladder (StatPearls (NCBI Bookshelf), 2024); Stress Urinary Incontinence (StatPearls (NCBI Bookshelf), 2024); Cauda Equina and Conus Medullaris Syndromes (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 26). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.