Potassium is the major cation (K, a positive ion) inside animal cells, while sodium (Na) is the major cation outside animal cells. The difference between the concentrations of these charged particles causes a difference in electric potential between the inside and outside of cells, known as the membrane potential.  The balance between potassium and sodium is maintained by ion transporters in the cell membrane. All potassium ion channels are tetramers with several conserved secondary structural elements. A number of potassium channel structures have been solved including voltage gated, ligand gated, tandem-pore, and inwardly rectifying channels, from prokaryotes and eukaryotes. The cell membrane potential created by potassium and sodium  ions allows the cell to generate an action potential&mdash;a "spike" of electrical discharge. The ability of cells to produce electrical discharge is critical for body functions such as neurotransmission, muscle contraction, and heart function.