Normal bladder function is the product of the complex interaction between storage and emptying. It requires a compliant bladder of adequate volume, and effective sphincter and detrusor muscles with appropriately coordinated contraction and relaxation.

Storage is facilitated by release of noradrenaline from the branches of the sympathetic nervous system stimulating contraction of the sphincters in the bladder neck and posterior urethra accompanied by relaxation of the detrusor. Bladder emptying is effected by the parasympathetic nervous system. Acetylcholine, released from the preganglionic nerves (S2–S4), stimulates contraction of the detrusor. In infancy, this is accompanied by simultaneous sphincter relaxation. Toilet training superimposes supraspinal polysynaptic reflexes on this system such that inhibition and initiation of detrusor contractions as well as sphincteric relaxation are essentially under voluntary control.

A bladder is defined as unstable if urodynamic investigation shows detrusor contractions during the filling phase while the patient is attempting to inhibit voiding. Contractions occurring as a result of a diagnosed neurological disease are ascribed to detrusor hyperreflexia; those unrelated to proven disease are caused by detrusor instability.1