4: Actions of Drugs on the Autonomic Nervous System

The autonomic nervous system (ANS) is a network of neurones that innervate the organs of the body through branches of the sympathetic and parasympathetic nervous system. Chemical signals between neurones of both systems interact with associated receptors by which cells in the body communicate with one another and are the target of drugs that affect chemical transmission between cells and body organs (e.g., heart, kidney, GI system and the brain/CNS). Stimulation of the body's sympathetic system in general depresses all physiological function, except those that prepare the body to cope with the challenge of an emergency situation, the so-called “fight or flight” response to physical exertion. In contrast, the parasympathetic system, with a few exceptions opposes the effect of sympathetic stimulation and controls those physiological functions that conserve the body's energy stores, although neither is universally “stimulating” or “depressing”. Thus, the focus of this section is to understand the pharmacology of how drugs exert control of chemical transmission in the peripheral autonomic nervous system, with the same general principles applying to the central nervous system (see Chapter 5).