Three-dimensional model of the nicotinic acetyl-choline-gated ion channel. The structure is based on a three-dimensional reconstruction of electron microscope images by Un-win and his colleagues. The receptor-channel complex consists of five subunits, all of which contribute to forming the pore. When two molecules of acetylcholine bind to portions of the a-subunits exposed to the membrane surface, the receptor-channel changes conformation. This opens a pore in the portion of the channel embedded in the lipid bilayer, and both K+ and Na+ flow through the open channel down their electrochemical gradients.