INTRODUCTION

A full appreciation of the autonomic nervous system (ANS) is central to the understanding of the actions and uses of drugs in a wide range of conditions, in particular conditions affecting the cardiovascular, respiratory, genitourinary and gastrointestinal systems. The actions of key drugs used in the treatment of eye diseases also depend on their ability to modify (mimic or block) the action of chemical mediators produced by the ANS. Drugs acting on the ANS are capable of bringing great benefits to patients but, like many other drugs, the dangers of inappropriate use (and side effects) should not be underestimated, especially in vulnerable patients at extremes of age.

Careful study of this chapter, and further reading (before a study of section 3), will be amply rewarded both in the theoretical context and in a very practical sense.

PERIPHERAL NERVOUS SYSTEM

The peripheral nervous system comprises the ANS and other systems that innervate skeletal (striated) muscle. The anatomy and physiology of the ANS will be described in some detail. The ANS has three anatomical divisions: the sympathetic and parasympathetic systems, which convey the outputs from the central nervous system to the main organ systems of the body, and the enteric nervous system. The enteric nervous system is made up of nerve plexuses of the gastrointestinal tract.

The ANS is responsible for the involuntary activity of the body, for example heartbeat and the contraction and relaxation of smooth muscle in the cardiovascular, respiratory and gastrointestinal systems. Other important actions include the regulation of secretions (endocrine and exocrine) and the production of energy in the liver and skeletal muscle.

The basic anatomical plan of the ANS is illustrated in Figure 9.1. In a more detailed and highly simplified form, Figure 9.2 illustrates the similarities and differences between the sympathetic and parasympathetic systems. It will be seen that in the sympathetic system the ganglia are situated close to the central nervous system, whereas in the parasympathetic system the ganglia are mainly close to or within the organ supplied.

Fig. 9.1 Basic anatomical plan of the autonomic nervous system. C, cervical; GI, gastrointestinal; L, lumbar; M, medullary; S, sacral; T, thoracic. Please see Fig. 9.1, p. 124 in Rang and Dale.

(From Rang HP, Dale MM, Ritter JM et al. 2003 Pharmacology. Churchill Livingstone, Edinburgh. With permission of Elsevier Ltd.)

Fig. 9.2 Simplified structures within the autonomic nervous system.

The neurones of the enteric nervous system are located in the plexuses of the gastrointestinal tract and are supplied by the sympathetic and parasympathetic systems, but they can also act independently to control functions of the gastrointestinal system. The enteric nervous system is more complex than the other elements of the ANS, because it involves other chemical transmitters. The sympathetic and parasympathetic systems have opposing actions on some organs of the body, for example the heart (see Tables 9.1 and 9.2). In very general terms, sympathetic activity is associated with stress and activity; parasympathetic activity is associated with inactivity. Under normal conditions, the ANS exerts control on organs of the body on a continuous basis.

Table 9.2 Main actions of acetylcholine on muscarinic receptors