Q5 LONG ANSWER 10M ★ High-yield

Describe the nerve supply of the urinary bladder and the mechanism of micturition. Add a note on the abnormalities of micturition.

Physiology · 1st Professional — Pre-Clinical

Definition

Micturition = the process by which the urinary bladder empties when it becomes filled.

Structure of the Bladder

Detrusor muscle — smooth muscle of the bladder wall
Internal sphincter — smooth muscle, involuntary
External sphincter — skeletal muscle, voluntary
Capacity 300–400 mL; first desire to void at 150 mL; marked discomfort at 400 mL

Nerve Supply

Nerve	Segments	Supplies	Action
Parasympathetic — pelvic nerve	S2, S3, S4	Detrusor	Contracts detrusor, relaxes internal sphincter → micturition
Sympathetic — hypogastric nerve	L1, L2	Internal sphincter, trigone	Relaxes detrusor, contracts internal sphincter → storage
Somatic — pudendal nerve	S2, S3, S4	External sphincter	Voluntary control
Afferents	Pelvic nerve	Stretch receptors in bladder wall	Sense of fullness

CLINICAL PEARL

Note: "S2, 3, 4 keep the urine off the floor." Both the parasympathetic and the somatic supply arise from these segments.

Cystometrogram

Segment	Volume	Pressure	Explanation
Ia	0–50 mL	Rises slightly to 10 cm H2O	Initial filling
Ib	50–300 mL	Almost flat	Plasticity / receptive relaxation of detrusor
II	> 400 mL	Rises steeply	Limit of accommodation; micturition reflex triggered

Storage (filling) Phase

Bladder fills slowly → Wall tension stays low — plasticity of smooth muscle → Weak afferent discharge → Sympathetic (hypogastric) relaxes detrusor, contracts internal sphincter → Guarding reflex — pudendal keeps external sphincter contracted → continence maintained

Plasticity (stress relaxation) allows the bladder to fill from 50 to 300 mL with almost no rise in pressure
The guarding reflex is reinforced during coughing or straining, when abdominal pressure rises suddenly

Micturition Reflex

Bladder fills to about 300–400 mL → Stretch receptors stimulated → Afferents via pelvic nerve → S2–S4 sacral cord → Efferent via parasympathetic (pelvic nerve) → Detrusor contracts + internal sphincter relaxes → Voluntary relaxation of external sphincter (pudendal inhibited) → micturition

It is a self-regenerative (positive feedback) reflex — contraction further stimulates the stretch receptors, so the reflex builds to completion
Assisted by abdominal muscle contraction and the Valsalva manoeuvre

Other Reflexes Involving the Bladder

Reflex	Pathway	Effect
Guarding reflex	Afferent → pudendal efferent	Contracts external sphincter during sudden rises in abdominal pressure
Bulbocavernosus reflex	S2–S4	Squeezing the glans contracts the anal sphincter; tests sacral cord integrity
Cutaneous bladder reflex	Skin of thigh → sacral cord	Stroking the thigh triggers voiding in an automatic bladder

Development and Age Changes

Infant — micturition is a purely spinal reflex; the bladder empties automatically when full
Voluntary control develops by 2–3 years as corticospinal connections mature
Nocturnal enuresis is normal up to about 4–5 years
Elderly — ↓ bladder capacity, ↑ residual urine, detrusor overactivity → frequency and nocturia
Prostatic enlargement in men and pelvic floor weakness in women add to this

Higher Control

Pontine micturition centre (Barrington nucleus) — coordinates detrusor contraction with sphincter relaxation
Cerebral cortex (paracentral lobule) — provides voluntary inhibition, which is the dominant influence in the adult
Voluntary control develops by 2–3 years of age; before this, micturition is a purely spinal reflex

Applied Aspects

Lesion	Bladder type	Features
Spinal shock (immediately after cord transection)	Atonic	Reflex abolished → retention with overflow incontinence; needs catheterisation
After recovery from spinal shock (lesion above S2)	Automatic (reflex) bladder	Reflex returns but no voluntary control; small capacity, empties automatically
Destruction of sacral segments or cauda equina	Autonomous bladder	Reflex arc destroyed → large flaccid bladder, overflow incontinence
Loss of afferents only (tabes dorsalis, diabetes)	Sensory (atonic) bladder	No sensation of fullness → grossly distended bladder
Cortical lesion (stroke, frontal lobe)	Uninhibited bladder	Loss of voluntary inhibition → urgency and precipitate micturition

Benign prostatic hyperplasia → outflow obstruction → hesitancy, poor stream, retention, and eventually hydronephrosis
Cystometry is used to distinguish these neurogenic bladders clinically
Stress incontinence — weak pelvic floor and sphincter; leakage on coughing or sneezing; common after childbirth
Urge incontinence — overactive detrusor; sudden uncontrollable desire to void
Overflow incontinence — chronic retention with a distended bladder, as in prostatic obstruction or an atonic bladder
Residual urine — normally < 50 mL; a raised volume predisposes to infection and stone formation
Anticholinergics (oxybutynin) relax the detrusor in urge incontinence; α-blockers (tamsulosin) relax the internal sphincter in prostatic obstruction

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