Ion channels in animal and plant cells transduce chemical, mechanical or other stimuli into ionic and electric information. The discovery of ion channels in microbes indicates an early origin for these transducers and the search is now on for their physiological roles. Ion channels and their regulation Animal behavior is largely governed by biological electricity. Ions, the carriers of electric currents, cannot pass through the hydrophobic lipid bilayer of the membrane. Instead they go through a special class of integral membrane proteins called ion channels. Whe~l open, channels allow ions to pass passively along the electrochemical gradients and therefore dissipate the gradients which are built with energy expenditure by ion pumps, a different class of membrane proteins. Because open channels drain energy, the channels are 'gated', and the gates are usually closed. The change of the channel protein to the open conformation depends on specific signals. Thus ion channels are switches that transduce signals into electric or ionic information, using the energy stored in the form of ion gradients. More than 60 ion channels are now recognized ~ and these can be classified by the signals that gate them. Some are gated by external ligands (e.g. the nicotinic acetylcholine receptor2), by internal ligands (e.g. the cGMP-gated Na + channel in phototransduction 3) or by other membrane proteins (e.g. the Gprotein-gated Ca 2+ channel4). Other channels are gated by transmembrane voltage (e.g. the depolarization-gated Na + channel of nerves 5) or mechanical forces (e.g. the ion channels 6 of the hair cells in the inner ear). Ion channels underlie smelling, tasting, hearing and vision, as well as responses to hormones and neurotransmitters.