Action potential
Electrical signals within biological organisms are, in general, driven by ions. The most important cations for the action potential are sodium (Na+) and potassium (K+). Both of these are "monovalent" cations that carry a single positive charge. Action potentials can also involve calcium (Ca2+), which is a "divalent" cation that carries a double positive charge. The chloride anion (Cl−) plays a major role in the action potentials of some algae, but plays a negligible role in the action potentials of most animals.