The muscarinic M2 receptor is a 466-amino acid, 7-transmembrane protein. In heart muscle, M2 receptors represent the prevailing subtype of muscarinic receptors. They can also be found in neurons of central and peripheral nervous system. In neuronal cells, mainly on synaptic terminals, stimulation of the M2 autoreceptors is responsible for presynaptic muscarinic autoinhibition of acetylcholine release in both central and peripheral cholinergic neurons. Loss of function of these M2 receptors, as occurs in some patients with asthma and in animal models of inflammation, leads to an increase in vagally mediated hyperreactivity.