The junctions between nerve cells and either other nerve cells or the organs which they innervate are called synapses and form major sites for the targetting of new drugs with therapeutic effects and for the actions of toxic substances, including those produced by living systems. The synapses act as transducers, transforming the fairly constant energy of the action potential, which conducts nerve impulses from one end of the nerve to the other, into a variety of events upon the next cell in the chain. It is the variety of transmitters and modulators and the effects which they produce which enables drugs to act in relatively selective ways, either directly on the transmission process or the events which follow it. Side-effects are often due to unwanted actions upon some other system. However, it is perhaps less obvious that even if the drugs are highly specific in their actions then they may still have serious side-effects arising from the fact that individual processes are common to many nervous pathways, not all of which are equally affected by disease.

Ideally, the development and design of new drugs for specific applications would arise from a thorough understanding of the basic physiology of the system and the pathology of the process which has led to dysfunction, together with a thorough understanding of the action of drugs and transmitters which affect the system. One would also know how modification to the structures of existing drugs would bring about changes in their pharmacological actions: the modern techniques of structure–activity relationships, as resolved by sophisticated computer simulations and calculations, can be considered to be in their infancy and have yet to lead to therapeutically useful drugs.