mictionsreflex

The micturition reflex, or micturition reflex arc, is the neural mechanism that coordinates bladder filling and urination by linking sensory input from the bladder with motor output to the detrusor muscle and the urethral sphincters. It operates through spinal circuits and brain centers to balance storage and voiding.

During the storage phase, the bladder fills and stretch receptors send afferent signals to the sacral spinal cord (S2–S4). Efferent pathways from the sympathetic system (hypogastric nerve) relax the detrusor and contract the internal urethral sphincter, while the somatic pudendal nerve maintains external sphincter tone to preserve continence. Afferent information is transmitted to higher brain centers that regulate the decision to void.

Voiding is initiated when the central nervous system facilitates a coordinated response: parasympathetic activity via the pelvic nerve causes detrusor contraction, and the internal urethral sphincter relaxes. The external sphincter relaxes under voluntary control, mediated by descending pathways from the brainstem and cortex. The pontine micturition center coordinates these events, integrating autonomic and somatic signals, with higher cortical areas providing voluntary control and social timing.

Development and clinical relevance: In infants, the reflex dominates, with maturation leading to voluntary control. Alterations in the micturition reflex underlie disorders such as neurogenic bladder, detrusor overactivity, detrusor underactivity, and sphincter dyssynergia. Diagnosis often involves urodynamic studies; treatment ranges from behavioral and pharmacologic approaches (antimuscarinics, beta-3 agonists) to neuromodulation or surgical intervention, depending on the underlying dysfunction.