A type of ion channel that opens when a signal molecule (ligand) binds to an extracellular receptor region of the channel protein. This changes the conformation of the channel protein and hence opens the channel. Many neurotransmitters act as ligands for ion channels. For example, at neuromuscular junctions, nicotinic acetylcholine receptors form a cation channel that opens when two molecules of acetylcholine bind to the channel’s extracellular region. The resultant influx of sodium ions causes depolarization of the muscle cell membrane. Other types of ligand-gated ion channels are found at synapses and in the brain. Compare voltage-gated ion channel.