Autonomic Control of the Systemic Vasculature

AUTONOMIC CONTROL OF THE SYSTEMIC VASCULATURE

Although the parasympathetic system can exert important influences on the circulation, auto-nomic control of the vasculature is primarily sym-pathetic. Sympathetic outflow to the circulation passes out of the spinal cord at all thoracic seg-ments and the first two lumbar segments. These fibers reach blood vessels via specific autonomic nerves or by traveling along spinal nerves. Sympa-thetic fibers innervate all parts of the vasculature except for capillaries. Their principal function is to regulate vascular tone. Variations of arterial vas-cular tone serve to regulate blood pressure and the distribution of blood flow to the various organs, whereas variations in venous tone alter vascular capacity, venous pooling, and venous return to the heart.

The vasculature has sympathetic vasocon-strictor and vasodilator fibers, but the former are more important physiologically in most tissue beds. Sympathetic-induced vasoconstriction (via α₁-adrenergic receptors) can be potent in skeletalmuscle, kidneys, gut, and skin; it is least active in the brain and heart. The most important vasodilatory fibers are those feeding skeletal muscle, mediating increased blood flow (via β₂-adrenergic receptors) in response to exercise. Vasodepressor (vasovagal) syn-cope, which can occur following intense emotional strain associated with high sympathetic tone, results from reflex activation of both vagal and sympathetic vasodilator fibers.

Vascular tone and autonomic influences on the heart are controlled by vasomotor centers in the reticular formation of the medulla and lower pons. Distinct vasoconstrictor and vasodilator areas have been identified. Vasoconstriction is mediated by the anterolateral areas of the lower pons and upper medulla. They are also responsible for adrenal secre-tion of catecholamines, as well as the enhancement of cardiac automaticity and contractility. Vasodilatory areas, which are located in the lower medulla, are also adrenergic, but function by projecting inhibitory fibers upward to the vasoconstrictor areas. Vasomo-tor output is modified by inputs from throughout the central nervous system, including the hypothalamus, cerebral cortex, and the other areas in the brainstem. Areas in the posterolateral medulla receive input from both the vagal and the glossopharyngeal nerves and play an important role in mediating a variety of circulatory reflexes. The sympathetic system nor-mally maintains some tonic vasoconstriction on the vascular tree. Loss of this tone following induction of anesthesia or sympathectomy frequently contributes to perioperative hypotension.