An acetylcholine receptor (abbreviated AChR) is an integral membrane protein that responds to the binding of acetylcholine, a neurotransmitter. Classification: 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. Nicotinic and muscarinic are two main kinds of "cholinergic" receptors.