ligand-gated ion channel

A ligand-gated ion channel is a receptor and a gate rolled into one. Picture a pore through the cell membrane held shut by a spring-loaded latch; when the right signalling molecule binds, the latch releases and the pore snaps open, letting ions pour through. Because the binding site and the pore are part of the same protein, the response is almost instantaneous — milliseconds.

This speed makes ligand-gated channels the receptors of fast synaptic transmission. When a neurotransmitter such as acetylcholine, glutamate, GABA or glycine binds, the channel opens and ions like sodium, potassium, calcium or chloride flow down their gradients, instantly changing the cell's electrical state and either exciting or inhibiting it.

Many important drugs act here, often not at the neurotransmitter site itself but at separate regulatory sites that tune the channel's behaviour. The honest nuance is that such drugs usually do not open or close the channel on their own; they make the channel more or less responsive to its natural ligand, which is why their effects depend on ongoing synaptic activity.