A ligand-gated ion channel opens when:
Ligand-gated ion channels, also called ionotropic receptors, provide fastest chemical to electrical transduction at synapses through direct conformational coupling between binding and pore opening without second messengers. They are multimeric membrane proteins typically pentameric Cys-loop family including nicotinic acetylcholine, GABA-A, glycine and 5-HT3 receptors, or tetrameric glutamate receptors AMPA, NMDA and kainate, each with extracellular ligand-binding domain formed by loops and transmembrane pore domain of four helices M1-M4 surrounding central ion pathway. Binding of specific agonist, such as acetylcholine to nicotinic receptor at neuromuscular junction, glutamate to dorsal root neurons, GABA to interneurons, or ATP to P2X receptors, stabilizes open state through allosteric rotation increasing pore diameter, allowing selective cation or anion flux within submilliseconds, producing excitatory or inhibitory postsynaptic currents. Unlike voltage-gated channels that sense transmembrane electric field change via S4 charges or mechanosensitive channels responding to membrane tension via Piezo blades, opening is triggered purely by chemical recognition and induced fit with high specificity. After ligand dissociation and possible desensitization, channel closes terminating signal. This architecture provides speed superior to metabotropic GPCR cascades, essential for rapid reflexes and central integration, and targeted by anesthetics, benzodiazepines and neurotoxins like curare.
Ref: Alberts et al., Molecular Biology of the Cell, 6th ed., Chapter 15: Ligand-Gated Ion Channels – Mechanism.