micturition

Micturition, also known as urination, is the process by which urine is expelled from the urinary bladder through the urethra. It occurs in two coordinated phases: storage, during which the bladder fills at low pressure, and voiding, when urine is actively expelled.

During storage, the detrusor muscle of the bladder wall remains relaxed to accommodate increasing urine volume. Sympathetic input via the hypogastric nerve (T11–L2) promotes detrusor relaxation and internal urethral sphincter contraction, helping maintain continence. The external urethral sphincter, composed of skeletal muscle, is under somatic control and remains contracted under the pudendal nerve (S2–S4) to prevent leakage.

Voiding is initiated when a person decides to urinate or when reflex pathways trigger a voiding response. Parasympathetic signals via the pelvic nerves (S2–S4) stimulate detrusor contraction and inhibit the internal sphincter, while the external sphincter relaxes under voluntary control. The pontine micturition center in the brainstem coordinates the switch from storage to voiding, integrating input from higher brain centers to allow conscious control.

Anatomy-wise, the bladder stores urine and the detrusor muscle Governs contraction. Urine passes from the bladder into the urethra as the internal sphincter (involuntary) and external sphincter (voluntary) regulate flow. Higher centers in the cerebral cortex and hypothalamus modulate this reflex, ensuring appropriate timing and social continence.

Clinical relevance includes urinary retention and incontinence. Neurogenic bladder, resulting from impaired neural control due to spinal cord injury, stroke, or multiple sclerosis, exemplifies how disruption of neural pathways affects micturition. Assessment often involves history, examination, and urodynamic testing.