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.nThe 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.The m3 receptor is expressed in brain, heart, urinary bladder, salivary gland and airway smooth muscle
