Increased intravesical pressure is the stimulus
responsible for initiating a bladder contraction. This stimulus is
sensed by the cerebral cortex, and the center of the micturition reflex
is in the brain stem. Bladder contraction is a coordinated neuromuscular
action characterized by the following events: relaxation of the external
(striated) sphincter, decrease in urethral pressure, increase in detrusor
pressure, opening of the bladder neck, and voiding.