The bladder is a hollow muscular organ intended to store urine without leakage over several hours and empty completely upon a volitional signal. Urine is expelled by a contracting of the bladder wall, which normally only starts after the brain has sent a conscious start signal.
If, however, the bladder acts independently of the brain's control, it automatically contracts when a certain capacity level is reached and attempts to expel the urine. The consequence is a sudden, strong urge to urinate, frequently associated with the feeling of not being able to control the expulsion and having to empty the bladder immediately. This malfunction of the bladder can lead to incontinence if a toilet cannot be reached within a couple of minutes of this moment. Since the leakage of urine occurred with an urge to urinate, this form of involuntary leakage is called urge incontinence. An overactive bladder is generally the cause if the urge incontinence coincides with frequent urination during the day and at night.