The key feature in maintaining a voltage difference between a neuron
and the external world is the cell membrane which has the property of
selectively allowing different ions in and out of the cell. There are
many different ions in the intra- and extracellular media. For our
purposes, the most important are calcium [Ca], potassium [K], sodium
[Na], chloride [Cl],
and magnesium [Mg]. Potassium and sodium have a single positive
charge per ion, chloride has a single negative charge, and calcium and
magnesium have two positive charges per ion. Positive ions are called
cations and negative are called anions.