Muscarinic acetylcholine receptors are G protein-coupled receptors. M1, M3, M5 receptors couple to G proteins of the Gq/11 family, which activate phospholipase C. M2 and M4 receptors couple to Gi/o-type G proteins that inhibit adenylyl cyclase activity.Muscarinic receptors control many effects of acetylcholine in the central and peripheral nervous system. M3 receptor is found in smooth muscles, the endocrine glands, the exocrine glands, as well as the lungs. It is also found in the CNS, where it induces emesis. In general, it causes smooth muscle contraction and increased glandular secretions. M3 receptor is thought to be implicated in Alzheimer′s disease.