The physiology of the mammalian urinary outflow tract.

Urinary outflow from the mammalian bladder occurs through the urethra. This outflow tract is a complicated structure composed of striated and smooth muscle and vascular urothelium. It is controlled by somatic and autonomic nerves and has several functions: it generates sustained tone to prevent urinary leakage during bladder filling; it generates transient reflex increases in pressure to prevent opening of the lumen when abdominal pressure rises; it undergoes relaxation preceding micturition and can generate urethral opening and shortening during micturition. A urethral pressure profile shows a peak pressure of > or = 100 cmH2O. The outermost coat is striated muscle, the striated or external sphincter. The fibres are predominantly circularly oriented. The extent varies in different species and between sexes. In the human female it extends the length of the urethra, and is composed mainly of slow twitch fibres. In the male, the sphincter extends from the membranous urethra over the base of the prostate and has nearly equal numbers of slow and fast twitch fibres. In both sexes, the posterior border may be deficient in striated muscle, and filled by circularly oriented smooth muscle. Activity in the slow twitch fibres through somatic nerves may be continuous during bladder filling. Outer circular and inner longitudinal smooth muscle is present Strips from either layer will generate sustained tone particularly if dissected from the high pressure zone. This tone is myogenic, and may be achieved in the absence of action potentials, but relies on influx of calcium through L-type calcium channels. Both layers receive sympathetic and parasympathetic excitatory innervation and nitrergic inhibitory innervation. Normal urethral pressure requires blood flow to the urothelium (lamina propria). Striated and smooth muscles are both thought to contribute to the resting urethral pressure in the human. The precise role of the smooth muscles during micturition is as yet unresolved.