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Micturition, Neurogenic Bladder & Incontinence

Name: Micturition, Neurogenic Bladder & Incontinence
Creator: Veli Bakalov, MD

Three nerves, four receptors and a pontine switch explain every way the bladder fails

Renal·Urinary Tract, Cystic & Neoplastic Disease·4 min read·9 high-yield points·Reviewed by Veli Bakalov, MD @HemeOncBuddy

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Storage is sympathetic: the hypogastric nerve (T10–L2) relaxes the detrusor via β3 and closes the internal sphincter via α1.
The pudendal nerve (S2–S4) keeps the striated external sphincter closed through nicotinic receptors.
Voiding is parasympathetic: pelvic splanchnic nerves (S2–S4) contract the detrusor via M3 receptors.
The pontine micturition center coordinates detrusor contraction with sphincter relaxation; the frontal cortex inhibits it, so frontal lesions and NPH cause urge incontinence.
Suprasacral spinal cord lesions cause detrusor-sphincter dyssynergia with high bladder pressures; lesions above T6 risk autonomic dysreflexia from bladder distension.
Sacral or peripheral lesions (cauda equina, diabetic neuropathy) produce an atonic bladder with a large residual and overflow incontinence.
Stress incontinence (urethral hypermobility after childbirth or obesity) is treated with pelvic floor training, then a sling.
Urge incontinence: antimuscarinics (anticholinergic burden in older adults; trospium spares the CNS), the β3 agonist mirabegron, or botulinum toxin.
Overflow incontinence: relieve obstruction (α1-blockers), catheterize, or use bethanechol for a hypotonic detrusor, never against mechanical obstruction.

Peripheral wiring of the bladder

Innervation of the lower urinary tract

Pathway	Nerve and level	Receptor	Effect
Sympathetic (storage)	Hypogastric nerve, T10–L2	β3 on detrusor; α1 on bladder neck and internal sphincter	Detrusor relaxes; internal sphincter closes
Parasympathetic (voiding)	Pelvic splanchnic nerves, S2–S4	M3 (Gq → IP3 → Ca2+) on detrusor	Detrusor contracts; nitric oxide relaxes the outlet
Somatic (continence)	Pudendal nerve, S2–S4 (Onuf nucleus)	Nicotinic on striated muscle	External sphincter held closed voluntarily

Storage. As the bladder fills, sympathetic norepinephrine on β3 receptors relaxes the detrusor, so pressure stays low while volume rises, and α1 receptors keep the bladder neck and internal sphincter shut. Pudendal output to the external sphincter, reinforced by the levator ani, adds voluntary continence during coughing and straining. α1-agonist decongestants such as pseudoephedrine can precipitate acute retention in men with BPH, whereas α1-blockers such as tamsulosin ease outflow.

Voiding. Parasympathetic acetylcholine contracts the detrusor through M3 receptors, nitric oxide relaxes the outlet, and pudendal firing stops so the external sphincter opens.

Central coordination

Bladder stretch travels in pelvic nerve afferents to the sacral cord and up to the periaqueductal gray, which relays fullness to the pontine micturition center. The pontine center is the switch that fires detrusor contraction and sphincter relaxation together, so the bladder empties completely at low pressure. The medial frontal cortex tonically inhibits it until voiding is appropriate. Frontal lesions (normal pressure hydrocephalus, tumors, stroke, dementia) remove that brake and cause urgency and urge incontinence. Normal pressure hydrocephalus pairs urinary urgency and incontinence with a magnetic gait and dementia, because enlarging ventricles stretch periventricular frontal fibers.

Neurogenic bladder by lesion level

Lesion level predicts bladder behavior

Level	Examples	Bladder behavior	Residual
Suprapontine	Stroke, dementia, Parkinson disease, NPH, frontal tumor	Uninhibited detrusor, coordinated sphincter; urge incontinence	Low
Suprasacral cord	Trauma, multiple sclerosis, transverse myelitis	Reflex overactive detrusor with detrusor-sphincter dyssynergia	Elevated, high pressure
Sacral or peripheral	Conus or cauda equina lesion, diabetic autonomic neuropathy, radical pelvic surgery	Atonic, large bladder with lost sensation; overflow	Large

Spinal cord injury. Spinal shock first leaves an areflexic bladder in retention that needs catheterization. Over weeks to months, sacral reflexes return without pontine coordination, and the detrusor contracts against a simultaneously contracting external sphincter (detrusor-sphincter dyssynergia). High pressure and incomplete emptying cause vesicoureteral reflux, hydronephrosis, recurrent UTIs and CKD if unmanaged.

With lesions above T6, a distended bladder or blocked catheter can trigger autonomic dysreflexia: severe hypertension, pounding headache, flushing and sweating above the lesion and reflex bradycardia. Relieve the stimulus first.

Cauda equina and diabetes. Cauda equina compression (disc herniation, tumor) adds saddle anesthesia, loss of the anal wink and bulbocavernosus reflexes and bowel dysfunction. Diabetic autonomic neuropathy first dulls bladder sensation; the bladder overdistends and contractility fails.

Incontinence types and treatment

Three kinds of leakage

Feature	Stress	Urge	Overflow
Mechanism	Urethral hypermobility or intrinsic sphincter deficiency	Detrusor overactivity	Outlet obstruction or underactive detrusor
Pattern	Small leaks with cough, sneeze or lifting; no urge	Sudden urge, then leakage; frequency, nocturia	Constant dribbling from a full bladder
Post-void residual	Normal	Normal	Large
Causes	Vaginal childbirth, obesity, menopause, prostatectomy	Idiopathic; neurologic disease; UTI, caffeine	BPH, urethral stricture, fecal impaction, diabetic neuropathy, anticholinergics, opioids
Treatment	Pelvic floor (Kegel) training, weight loss, pessary, midurethral sling	Bladder training, timed voiding, less caffeine and evening fluid; drugs below	Treat the cause; α1-blockers for BPH; intermittent catheterization; bethanechol

Antimuscarinics. Oxybutynin, tolterodine, solifenacin, darifenacin and trospium block detrusor M3 receptors. Adverse effects are dry mouth, constipation, blurred vision, tachycardia and urinary retention, and they are avoided in narrow-angle glaucoma. In older adults the cumulative anticholinergic burden causes confusion and cognitive decline; oxybutynin is on the Beers list, whereas trospium, a charged quaternary amine, barely crosses the blood-brain barrier.

Other urge therapies. The β3 agonists mirabegron and vibegron relax the detrusor during filling without anticholinergic effects; mirabegron can raise blood pressure. Refractory cases receive intradetrusor onabotulinumtoxinA, which cleaves SNAP-25 and blocks acetylcholine release, at the risk of retention requiring self-catheterization.

Overflow therapy. Bethanechol, a cholinesterase-resistant muscarinic agonist, strengthens a hypotonic detrusor, and drugs that weaken contraction, especially anticholinergics, are stopped.

Bethanechol is contraindicated when retention stems from mechanical obstruction: a stronger contraction against a blocked outlet risks bladder injury.