AUTONOMIC NERVOUS SYSTEM 

In an earlier module we considered the neurochemistry of the peripheral components of the autonomic nervous system. What was clear from that analysis was that the neurotransmitters used by the postganglionic neurones of the sympathetic and parasympathetic divisions of the autonomic nervous system were different. This has been useful because it has allowed the development of drugs to modify the action of one or the other division to produce clinically beneficial outcomes.

Drugs which mimic the actions of the sympathetic nervous system are called sympathomimetics whilst those that mimic the action of the parasympathetic division are called parasympathomimetics.

Not surprisingly, most sympathomimetics are adrenergic receptor agonists (i.e. they have the same effect as noradrenaline when they combine with adrenergic receptors) and most parasympathomimetics are muscarinic receptor agonists.

In addition to drugs which mimic the actions of the sympathetic and parasympathetic divisions of the autonomic nervous system there are also those that block them. These are known as parasympathetic or sympathetic blockers (or more correctly antagonists). These drugs block the actions of acetylcholine at muscarinic receptors and noradrenaline at adrenergic receptors respectively.

Given the large number of physiological processes that the autonomic nervous system regulates, it is not surprising that sympathomimetic and parasympathomimetic drugs (as well as the antagonists) have found widespread applications in modern medicine. Because of the dual innervation of many organs, antagonists can be used to reduce the effects of one division of the autonomic nervous system or agonists can be used to enhance the effects of the other division.

Finally, the fact that there are 5 different types of adrenergic receptor and 3 different types of muscarinic receptor and that these are expressed by different peripheral tissues has been very useful.

This complex neurochemistry has allowed pharmaceutical companies to design drugs that are reasonable specific for a particular receptor and can therefore be used to target particular tissues with a reasonable amount of specificity. This allows more effective treatment outcomes with the drugs and of course less side-effects.

The table below summarises some of the drugs which modify autonomic nervous system and their clinical applications.