Clonidine Premedication in Anesthetic Practice

Abstract

The autonomic nervous system refers to the nervous and humoral mechanisms which modify the function of the autonomous or autonomic organs. The autonomic nervous system can be divided physiologically into the parasympathetic nervous system and the sympathetic nervous system. The sympathetic nervous system is primarily an adrenergic rsystem which excites those organ functions that prepare the body for "fight or flight". The predominant neurochemical effector for these functions is norepinephrine. Catecholamines act via two principal types of receptors which are termed alpha and beta. Alpha receptors have been classified into a 1 and a2 receptors based upon the antagonist yohimbine and prazocin. a2 receptors have also been classified into subtypes according to autoradiographic and molecular biological techniques and transmiembrane signaling mechanisms. a2 receptors are members of the G-protein-coupled family of receptors. There are at least five separate effector mechanisms that are directly modulated by the activated a2-adrenoceptors. These include adenylate cyclase system, acceleration of Na+/H+ exchange, activation of K+ channels, inhibition of voltage-sensitive Ca2+ channels and phosphatidyl inositol turnover.