"The plasma membrane is a heterogeneous structure whose thickness ia around 75 Å and which bounds the cell. An important constituent is lipid, which often represents as much as 70% of the membrane volume (depending on cell type). The membrane lipid readily excludes the passage of ions; it remains for imbedded proteins to form the channels which permit exchange of ions between intracellular and extracellular space. For nerve and muscle, electrical activation is associated with the movement of sodium and potassium (and other) ions across membranes by means of these channels; the proteins not only facilitate the flow of each ion but they control the flow of each giving rise to the ' of the membrane."