Overview of Voiding
For normal urinary function, the autonomic and voluntary nervous systems must be intact, and muscles of the urinary tract must be functional. Normally, bladder filling stimulates stretch receptors in the bladder wall to send impulses via spinal nerves S2 to S4 to the spinal cord, then to the sensory cortex, where the need to void is perceived. A threshold volume, which differs from person to person, triggers awareness of the need to void. However, the external urinary sphincter at the bladder outlet is under voluntary control and usually remains contracted until a person decides to urinate.
The micturition inhibitory center in the frontal lobe also helps control urination. When the decision is made, voluntary signals in the motor cortex initiate urination. These impulses are transmitted to the pontine micturition center, which coordinates simultaneous signals to contract detrusor smooth muscle throughout the bladder (via parasympathetic cholinergic nerve fibers) and to relax the internal sphincter (via alpha sympathetic nerve fibers) and striated muscle of the external sphincter and pelvic floor (see Figure: Normal micturition occurs when bladder contraction is coordinated with urethral sphincter relaxation). In addition to normal urinary function, continence and normal voiding require normal cognitive function (including motivation), mobility, access to a toilet, and manual dexterity.
Damage to or dysfunction of any of the components involved in voiding can cause urinary incontinence or retention.