Adrenergic receptors: anatomy, physiology, and pharmacological interactions.

The success of anesthesia and the surgical experience is dependent on many factors including an intact autonomic nervous system and the ability of the patient to respond reflexly to stress. Cardiac rate and rhythm, as well as airway resistance, is largely controlled by the antagonistic effects of the sympathetic and parasympathetic nervous systems. Alterations in cardiovascular or pulmonary functions are beacons which warn the anesthetist of imbalances in the involuntary nervous system. Inhibiting or exciting either segment may result in cardiac dysrhythmias, dysfunction or bronchospastic disorders. Dysrhythmias of vagal predominance have been treated for years with drugs such as the anticholinergics, resulting in competitive antagonism of acetylcholine at the cholinergic muscarinic receptors. The majority of problems that develop during the administration of anesthesia, however, is of sympathetic origin. Appropriate management of these potentially hazardous problems is dependent upon an understanding of the adrenergic system and its effector site, the alpha and beta receptor. The adrenergic system The autonomic nervous system plays a major role in maintaining homeostasis. Two major divisions of this system, the sympathetic and parasympathetic, consist of neurons and ganglia which innervate glands, the heart, blood vessels, and visceral smooth muscle. In general, the two divisions have antagonistic effects. The parasympathetic segment functions to conserve or restore the organism while the sympathetic segment prepares the organism for "fight or flight." Pharmacologically, these systems are classified as cholinergic and adrenergic systems. Cholinergic is a term describing those fibers which release acetylcholine as their neurotransmitter. Acetylcholine is effective on both nicotinic and muscarinic receptors. Adrenergic is a term describing those fibers which release norepinephrine as their neurotransmitter. Norepinephrine is effective on alpha and beta receptors. The systems, their receptors, and the effects of receptor stimulation are summarized in Table 1.1.2 Sympathetic division of the autonomic nervous system originates from cells located in the lateral horns of the spinal cord from T 1 to L2-3. 3 These preganglionic fibers leave the cord and synapse with postganglionic nerve cells, located in the ganglia on each side of the vertebral column or in the abdominal cavity. Once a synapse has occurred, the impulse is carried by postganglionic fibers to the adrenergic neuroeffector junction. Anatomically, the postganglionic sympathetic nerve consists of a cell body, long axon, and ex-