Most vascular smooth muscle is innervated solely by the sympathetic (noradrenergic) nervous system, but there are exceptions. Some blood vessels in the face, tongue, and urogenital tract (especially the penis) are innervated by parasympathetic (cholinergic) as well as sympathetic (noradrenergic) neurons. The parasympathetic innervation of blood vessels has only regional importance, for example, in salivary glands, where increased parasympa-thetic activity causes vasodilation that supports salivation.
The primary neural control of total peripheral resistance is through sympathetic nerves. The diameter of blood vessels is controlled by the tonic activity of nor-adrenergic neurons. There is a continuous outflow of noradrenergic impulses to the vascular smooth muscle, and therefore some degree of constant vascular constriction is maintained. An increase in impulse outflow causes further contraction of the smooth muscle, resulting in greater vasoconstriction. A decrease in impulse outflow permits the smooth muscle to relax, leading to vasodilation.
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