Although endothelium-dependent relaxation was first described in response to acetylcholine, no evidence exists that muscarinic neurons innervate peripheral arteries or veins such as the saphenous vein. In the periphery, the primary innervation is sympathetic adrenergic neurotransmission ( Figure 2-1). The second regulator, autonomic innervation, responds to activation of peripheral baroreceptors, chemoreceptors, and temperature receptors this causes higher brain centers to trigger neurotransmitter release, causing contraction of medial smooth muscle cells. The first of these regulators is the endothelium, which influences the tone and growth of the underlying smooth muscle through inhibitory and stimulatory factors released in response to blood flow, oxygen tension, hormones, and cytokines and chemokines in the blood. In terms of physiological control, vascular smooth muscle is layered between two regulatory systems. The thickness of the medial layer and the density of innervation differ among blood vessels in various anatomical locations within the body (e.g., arteries have thicker media compared to veins and arterioles and cutaneous veins are more highly innervated than conduit arteries and capacitance veins). The adventia contains undifferentiated dendritic cells, connective tissue (through which course the autonomic innervation to the vascular wall), and the vasa vasorum. The blood vessel wall consists of a single layer of endothelial cells that provides an interface between the blood and the smooth muscle forming the medial layer. Miller PhD, in Comprehensive Vascular and Endovascular Surgery (Second Edition), 2009 BASIC ANATOMY
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