The Micturition Reflex

Micturition or urination primarily involves coordination between the bladder detrusor muscle and two sphincters to control bladder emptying.

It is regulated by two spinal reflexes and higher brain centers for voluntary urination.

When the incoming urine stretches the bladder wall, the sacral spinal segments and pontine storage center trigger the urine storage reflex.

As a result, efferent sympathetic impulses inhibit the detrusor muscle and stimulate the internal urethral sphincter.

The relaxation of the detrusor with contraction of the internal urethral sphincter facilitates the bladder filling.

When the bladder fills above 200 mL, the sacral spinal segments and the pontine micturition center initiate the urine voiding reflex.

This center increases parasympathetic impulses, which counteract the sympathetic signals, leading to detrusor contraction and internal sphincter relaxation.

Under voluntary control, the micturition center inhibits the somatic motor neurons innervating the external urethral sphincter, relaxing it for urination.

Children under 2 years of age lack voluntary control over the external sphincter. They develop corticospinal connections for sphincter control and bladder awareness to control urination.