Biochemistry. A membrane protein that forms a regulable pathway through which specific ions can cross the membrane. Ion channels play a vital role in depolarization and repolarization of nerve and muscle fibers. They can tranport 1 million ions per second when they are open.
Ion channels are classified by the ion selectivity, i.e. they allow the flow of a particular ion at a very high rate. Examples are potassium channels, sodium channels, calcium channels, and chloride channels.
Ion channels are classified by gating, i.e. what opens and closes the channels. The two major types are voltage-gated ion channels (stimulated by membrane potential) and ligand-gated ion channels (stimulated by neurotransmitters such as acetylcholine, GABA, or NMDA).