The physiological function of lower urinary tract smooth muscle

The lower urinary tract is a muscular system composed of the urinary bladder and the outflow tract. During filling with urine the bladder is relaxed and the outflow tract offers a high resistance; during emptying the outflow resistance falls and the bladder wall generates a high wall tension to raise intravesical pressure. The coordination of these responses is organized in the brainstem and sacral spinal cord to control the activity of autonomic and somatic efferents to the smooth muscle of the bladder (detrusor) and the smooth and skeletal muscle of the bladder base and urethra. Detrusor contraction is predominantly controlled by parasympathetic fibres releasing acetylcholine and ATP; the outflow tract is controlled by parasympathetic and sympathetic fibres to the bladder base (trigone) and urethral smooth muscle (including a nitregic component) and somatic fibres to the urethral rhabdosphincter. The smooth muscles also develop spontaneous contractions that determine the tone of the musculature. The cellular signaling pathways that evoke contraction due to neurotransmitter release and the origin of spontaneous activity are discussed, as well as the electrical properties of the smooth muscle relevant to the propagation of electrical signals. Finally the interaction of muscle cells with other cell types (epithelium and interstitial cells) is considered, relevant to their ability to regulate muscle contractility. Throughout, the basic physiological processes are considered in relation to pathological developments that are prevalent in the human lower urinary tract, in particular the overactive bladder and urinary incontinence, and the identification of drug targets to manage these conditions.