29.22 : The Micturition Reflex

Urination, or micturition involves the coordination of the bladder's detrusor muscle and two sphincters to ensure controlled bladder emptying.

The process begins with bladder filling, where the bladder wall stretches as urine accumulates. This stretching activates the urine storage reflex, mediated by the sacral spinal segments and the pontine storage center. Efferent sympathetic impulses stimulate the detrusor muscle to relax and the internal urethral sphincter to contract, facilitating urine storage.

When the bladder volume exceeds 200 mL, the pontine micturition center and sacral spinal segments initiate the urine-voiding reflex. This reflex increases parasympathetic impulses while inhibiting sympathetic signals, causing the detrusor muscle to contract and the internal sphincter to relax. These actions enable the expulsion of urine through the urethra.

Voluntary control of urination is governed by the micturition center, which inhibits somatic motor neurons innervating the external urethral sphincter. Relaxation of this sphincter under conscious control allows for urination. This voluntary mechanism is responsible for maintaining continence and preventing involuntary urination (incontinence).

In children below the age of two, voluntary control over the external urethral sphincter is absent due to immature corticospinal pathways. As they grow, these pathways develop, enabling voluntary control over urination.