<P> In the digestive tract, smooth muscle contracts in a rhythmic peristaltic fashion, rhythmically forcing foodstuffs through the digestive tract as the result of phasic contraction . </P> <P> A non-contractile function is seen in specialized smooth muscle within the afferent arteriole of the juxtaglomerular apparatus, which secretes renin in response to osmotic and pressure changes, and also it is believed to secrete ATP in tubuloglomerular regulation of glomerular filtration rate . Renin in turn activates the renin--angiotensin system to regulate blood pressure . </P> <P> The mechanism in which external factors stimulate growth and rearrangement is not yet fully understood . A number of growth factors and neurohumoral agents influence smooth muscle growth and differentiation . The Notch receptor and cell - signaling pathway have been demonstrated to be essential to vasculogenesis and the formation of arteries and veins . The proliferation is implicated in the pathogenesis of atherosclerosis and is inhibited by nitric oxide . </P> <P> The embryological origin of smooth muscle is usually of mesodermal origin, after the creation of muscle fibers in a process known as myogenesis . However, the smooth muscle within the Aorta and Pulmonary arteries (the Great Arteries of the heart) is derived from ectomesenchyme of neural crest origin, although coronary artery smooth muscle is of mesodermal origin . </P>

Why does unitary smooth muscle contract as a single unit