"The composition of every cell is different from that of the material in which it exists. ...The striking feature of a neuron at rest ...its contents are one-tenth as rich in sodium s as the external fluid and... ten times richer in ions. ...[S]odium ions will leak into the cell and potassium will leak from it. ...[T]he membrane possesses pumps to offset the flow ...sodium-potassium adenosine triphosphotase pumps ...can steadily exchange three sodium ions for two potassium ...Every neuron contains about a million pumps... and every pump can swap about 200 sodium ions for 130 potassium ions every second.. ...The pumps ...maintain that sodium potassium imbalance, and... the inside of each neuron at seventy millivolts negative to the outside. ...[T]hat helps to explain the near-permanent demand for oxygen ..."