mAChRs (muscarinic acetylcholine receptors) are acetylcholine receptors that form G protein-receptor complexes in the cell membranes of certainneurons and other cells. They play several roles, including acting as the main end-receptor stimulated by acetylcholine released from postganglionic fibersin the parasympathetic nervous system. mAChRs are named as such because they are more sensitive to muscarine than to nicotine. Their counterparts are nicotinic acetylcholine receptors (nAChRs), receptor ion channels that are also important in the autonomic nervous system. Many drugs and other substances (for example pilocarpineand scopolamine) manipulate these two distinct receptors by acting as selective agonists or antagonists. Acetylcholine (ACh) is a neurotransmitter found extensively in the brain and the autonomic ganglia.