Some Physiology

The detrusor, whose smooth muscle fibres comprise the body of the bladder, is innervated mainly by parasympathetic nerves which are excitatory and cause the muscle to contract. The internal sphincter, a concentration of smooth muscle at the bladder neck, is well developed only in the male and its principal function is to prevent retrograde flow of semen during ejaculation. It is rich in o^-adrenoceptors, activation of which causes contraction. There is an abundant supply of oestrogen receptors in the distal two-thirds of the female urethral epithelium which degenerates after the menopause causing loss of urinary control.

When the detrusor relaxes and the sphincters close, urine is stored; this is achieved by central inhibition of parasympathetic tone accompanied by a reflex increase in a-adrenergic activity. Voiding requires contraction of the detrusor, accompanied by relaxation of the sphincters. These acts are coordinated by a micturition centre probably in the pons.

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