A protein that spans a cell membrane to form a water-filled pore through which ions can pass in or out of the cell or cell compartment. Ion channels are found in the plasma membrane and in certain internal cell membranes. They vary in how they open and close and in their selectivity to different ions: some may be specific for one particular ion, whereas others may admit two or more similar ions (e.g. K⁺ and Na⁺). The electrical and chemical environment inside cells, including the resting potential, is determined largely by the numbers, types, and activity of the cell’s ion channels; they play a crucial role in the excitability of nerve and muscle cells. Ions pass through their respective channels at a rate and in a direction dictated mainly by the electrochemical gradients across the membrane. Ungated ion channels are permanently open, whereas gated ion channels can open and close. Of the latter there are two main types: ligand-gated ion channels, which typically open when a signal molecule binds to a receptor region of the channel protein, and voltage-gated ion channels, which respond to changes in membrane potential. See calcium ion channel; potassium ion channel; sodium ion channel.

http://www2.montana.edu/cftr/ionchannelprimers/beginners.htm Overview of the mechanisms and functions of ion channels, including diagrams