Main Page | See live article | Alphabetical index An acetylcholine receptor (abbreviated AChR) is an integral membrane protein that responds to the binding of the neurotransmitter acetylcholine by opening a pathway in the membrane for the diffusion of ions across the cell membrane. Like other transmembrane receptors, acetylcholine receptors are classified according to their "pharmacology," or according to their relative affinities and sensitivities to different molecules. Although all acetylcholine receptors, by definition, respond to acetylcholine, they respond to other molecules as well. nicotinic acetylcholine receptors (nAChR, also known as "ionotropic" acetylcholine receptors) are particularly responsive to nicotine muscarinic acetylcholine receptors (mAChR, also known as metabotropic acetylcholine receptors) are particularly responsive to muscarine. Generally nicotinic AChRs respond more rapidly to acetylcholine and are therefore found in muscle and brain tissues, whereas muscarinic receptors activate slowly in the presence of acetylcholine and are found mainly in the parasympathetic part of the autonomic nervous system. Molecular biology has shown that the nicotinic and muscarinic receptors belong to distinct protein superfamilies. The nAChr is an ion channel, and like other members of the ligand-gated ion channel superfamily, is composed of five protein subunits arranged like staves around a barrel. The subunit composition is highly variable across different tissues. In contrast, the mAChR is not itself an ion channel, but belongs to the superfamily of G-protein coupled receptors. It activates ionic currents by initiating a second messenger cascade. Muscarinic acetylcholine receptors can be blocked by the drugs atropine and scopolamine. This article is from Wikipedia. All text is available under the terms of the GNU Free Documentation License.