Micturition is the process of expelling urine from the urinary bladder. The functional anatomy of the lower urinary tract consists of the detrusor muscle, the trigone, the internal urethral sphincter, and the external urethral sphincter.
The bladder receives nerve supply from three main sets of peripheral nerves originating from the autonomic and somatic nervous systems.
During the storage phase, the bladder accommodates increasing volumes of urine without a significant rise in intravesical pressure, a property known as compliance. Sympathetic activation predominates. Norepinephrine acts on beta-3 adrenergic receptors on the detrusor muscle to cause relaxation and on alpha-1 adrenergic receptors at the bladder neck and internal sphincter to maintain closure. Simultaneously, somatic nerve activity via the pudendal nerve keeps the external urethral sphincter tonically contracted. Afferent sensory signals travel to the spinal cord, but higher centers suppress the micturition reflex until an appropriate time to void presents itself.
When bladder volume reaches approximately 200 to 400 milliliters, stretch receptors in the detrusor wall send impulses via visceral afferent fibers to the sacral segments of the spinal cord (S2-S4). This triggers a spinal reflex loop. Parasympathetic efferent fibers fire, causing the detrusor muscle to contract and the internal sphincter to relax. At the same time, sympathetic pathways and pudendal nerve activity are inhibited, allowing the external sphincter to relax. This coordinated spinal reflex initiates micturition in infants, while adults learn to modulate it through higher brain centers.
Although the micturition reflex is coordinated at the spinal level, conscious control depends on higher brain centers. The pontine micturition center (PMC), located in the brainstem, acts as the primary relay switch that coordinates the synergy between detrusor contraction and urethral relaxation. The cerebral cortex and frontal lobes evaluate social and environmental context, sending inhibitory or facilitatory signals down to the PMC. If voiding is appropriate, the cortex removes inhibition, allowing the PMC to trigger the reflex while the perineal muscles relax.
Disruptions anywhere along the neural or muscular pathway of micturition result in lower urinary tract symptoms. A neurogenic bladder arises from damage to the central or peripheral nervous systems, leading to either failure to store or failure to empty urine. Spinal cord injuries above the sacral reflex arc often result in detrusor-sphincter dyssynergia, where the bladder contracts against a closed, uncoordinated external sphincter, causing high pressures and potential renal damage. Benign prostatic hyperplasia in men causes mechanical outflow obstruction, leading to hesitancy, weak stream, and incomplete emptying.