Micturition (mictur=urinate), also called urination or voiding, is the act of emptying the urinary bladder. In order for micturition to occur, three things must happen simultaneously : (1)the detrusor muscle must contract (2)the internal urethral sphincter must open and (3)the external urethral sphincter must open. The detrusor muscle and its internal urethral sphincter are composed of smooth muscle and are innervated by both parasympathetic and symphatetic nervous system, which have opposing actions. The external urethral sphincter, in contrast, is skeletal muscle, and therefore is innervated by the somatic nervous system.
How are the three events required for micturition coordinated? Micturition is most easily understood in infants where a spinal reflex coordinates the process. As urine accumulates, distension of the bladder activates stretch receptors in its walls. Impulses from the activated receptors travel via visceral afferent fibers to the sacral region of the spinal cord. Visceral afferent impulses, relayed by sets of interneurons, excite parasymphatetic neurons and inhibit symphatetic neurons. As a result, the detrusor muscle contracts and the internal spincter opens. Visceral afferent impulses also inhibit tonically active somatic efferents that keep the external urethral sphincter closed, allowing it to relax and urine to flow.
Between ages two and three, descending circuits from the brain have matured enough to begin to override reflexive urination. The pons has two centers that participate in control of micturition. The pontine storage center inhibits micturition, whereas the pontine micturition centers promotes this refelex. Afferent impulses from bladder stretch receptors are relayed to the pons, as well as to higher brain centers that provide the conscious awareness of bladder fullness.
Lower bladder volumes primarily activate the pontine storage center, which acts to inhibit urination by suppressing parasympatetic and enhancing sympathetic output to the bladder. When a person chooses not to void, reflex bladder contractions subside within a minute or so and urine continues to accumulate. Because the external sphincter is voluntarily controlled, we can choose to keep it closed and postpone bladder emptying temporarily. After additional urine has collected, the micturition reflex occurs again and, if urination is delayed again, is damped once more. The urge to void gradually becomes greater and greater, and micturition usually occurs before urine volume exceed 400 ml. After normal micturition, only about 10 ml of urine remains in bladder.