How does OAB affect the bladder?

Normally, the muscle of the bladder wall (known as the detrusor muscle) sends a message to your brain to tell you that your bladder is getting full. To empty the bladder, the detrusor muscle contracts and the bladder outlet, the sphincter, relaxes. If you have OAB, the detrusor muscle contracts involuntarily even though your bladder may not be full.

Why does the detrusor muscle contract involuntarily? There are different reasons for different people. It may be caused by changes in the nerves that make the detrusor muscle more sensitive, by changes in the way the brain interprets messages from the muscle, or by changes in the muscle itself. As well, there may be other unknown reasons for involuntary detrusor muscle contraction.