Vascular Smooth Muscle cell (V.S.M.C) plays a major role in the regulation of vasomotion. Sympathetic system play a key role in vascular smooth muscle function. Epinephrine (humoral pathway) and norepinephrine (nervous pathway) can both stimulate V.S.C.M. via delta and beta adrenergic receptors. The function of parasympathetic system is more controversial. Acetylcholine induces the release of a relaxing factor by the endothelial cell (E.D.R.F.) but it has a direct contracting effect on V.S.C.M. Neural vasodilation is also concerned by noradrenergic mechanisms.