The autonomic nervous system (ANS) comprises the sympathetic and parasympathetic nervous systems. Ganglionic synaptic transmission in the ANS is mediated by the release of acetylcholine from the preganglionic neurone. Both muscarinic and nicotinic receptors are involved in mediation of the postganglionic response, as are inhibitory dopaminergic interneurones. In general, sympathetic postganglionic neurones are noradrenergic, and parasympathetic postganglionic neurones are muscarinic (cholinergic). The two systems tend to have opposite actions. Deliberate pharmacological manipulation of the ANS is therefore aimed at sites where physiological or anatomical differences exist between the two systems.