Micturition is fundamentally a spinobulbospinal reflex facilitated and inhibited by higher brain centers such as the pontine micturition center and, like defecation, subject to voluntary facilitation and inhibition.In healthy individuals, the lower urinary tract has two discrete phases of activity: the storage (or guarding) phase, when urine is stored in the bladder; and the voiding phase, when urine is released through the urethra.
During micturition, parasympathetic stimulation causes the detrusor muscle to contract and the internal urethral sphincter to relax.
The external urethral sphincter (sphincter urethrae) is under somatic control and is consciously relaxed during micturition.
Part of the urethra is surrounded by the male or female external urethral sphincter, which is innervated by the somatic pudendal nerve originating in the cord, in an area termed Onuf's nucleus.
Smooth muscle bundles pass on either side of the urethra, and these fibers are sometimes called the internal urethral sphincter, although they do not encircle the urethra.
In infants, some elderly individuals, and those with neurological injury, urination may occur as a reflex.