These two diagrams each show a channel protein embedded in the cell membrane. In the left diagram, there is a large number of sodium ions (NA plus) and calcium ions (CA two plus) in the extracellular fluid. Within the cytosol, there is a large number of potassium ions (K plus) but only a few sodium ions. In this diagram, the channel is closed. Two ACH molecules are floating in the extracellular fluid. Their label indicates that a neurotransmitter, a ligand, is required to open the ion channel. The neurotransmitter receptor site on the extracellular fluid side of the channel protein matches the shape of the ACH molecules. In the right diagram, the two ACH molecules attach to the neurotransmitter receptor sites on the channel protein. This opens the channel and the sodium and calcium ions diffuse through the channel and into the cytosol, down their concentration gradient. The potassium ions also diffuse through the channel in the opposite direction down their concentration gradient (out of the cell and into the extracellular fluid).