NORMAL URINATION:
The ability to hold urine and maintain continence is dependent on normal function of the lower urinary tract, the kidneys, and the nervous system, plus the physical and psychological ability to recognize and appropriately respond to the urge to urinate. The bladder's ability to fill and store urine requires a functional sphincter (muscle controlling output) and a stable bladder wall muscle (detrusor).

The process of urination involves two phases: the filling and storage phase and the emptying phase.

During the filling and storage phase, the bladder begins to fill, stretching to accommodate the increasing amounts of urine. The bladder of an average person can hold 350 mL to 550 mL of urine.

The first sensation of the need to urinate occurs when approximately 200 mL of urine is stored. The nervous system responds by alerting you to the need to urinate while also allowing the bladder to continue to fill.

The emptying phase requires the ability of the detrusor muscle to appropriately contract, forcing urine out of the bladder. Your body must also be able to simultaneously relax the sphincter to allow the urine to exit.

The bladder of an infant contracts automatically when a certain volume of urine is reached. As the individual learns to control urination, bladder muscle contraction is prevented by constant inhibition from the cerebral cortex (part of the brain). This allows urination to be delayed until the individual is ready.

Undesired bladder muscle contraction may occur as the result of a break in the neurological pathway from the brain to the bladder. It can also occur if the bladder is irritated and the normal neurological impulses to inhibit urination are insufficient to keep the bladder relaxed as it fills.

URGE INCONTINENCE:
Urge incontinence is basically a storage problem in which the bladder muscle contracts inappropriately. Often these contractions occur regardless of the amount of urine that is in the bladder. Urge incontinence may result from neurological injuries (such as spinal cord injury or stroke), neurological diseases (such as multiple sclerosis), infection, bladder cancer, bladder stones, bladder inflammation, or bladder outlet obstruction.

The majority of cases are classified as idiopathic--a specific cause cannot be identified.

Although urge incontinence may occur in anyone at any age, it is more common in women and the elderly. It is second only to stress incontinence as the most common cause of urinary incontinence (involuntary loss of urine). Approximately 1% to 2% of adult females are affected by urge incontinence.

In men, urge incontinence may be due to secondary bladder injuries caused by benign prostatic hypertrophy (BPH) or bladder outlet obstruction from an enlarged prostate.