The action of the neurotransmitter acetylcholine is mediated through two types of receptors, the ionotrophic nicotinic receptors and the metabotrophic muscarinic receptors. The muscarinic receptors belong to the superfamily of 7-TM G-protein-coupled receptors. Five subtypes of muscarinic receptors have been cloned and named m1-m5.
The muscarinic receptors are widely distributed throughout the body, but are predominantly expressed within the parasympathetic nervous system and exerts both excitatory and inhibitory control over central and peripheral tissues.