The fluid outside the membrane is a saline solution which,from Deutsch and Deutsch [18, Table 1-2, p. 19], has a con-centration of 145 sodiumions, 4 potassiumions, and 120chlorideions in 55 200 molecules of water. The axoplasminside the membrane has 12 sodiumions, 155 potassiumions, and 145 large negative organic ions in the same amountof water. With the addition of small numbers of other ions tomake both solutions electrically neutral, these are the normalion concentrations in the resting state of an unmyelinatedmembrane. They are maintained by a pumping action of themembrane, powered by metabolic energy, that ejects sodiumions from and returns potassium ions to the inside to maintainthe resting state concentration. It leaves the inside axoplasmnegative with respect to the outside so that there is an electricfield in the thin membrane that attracts positive ions to itsouter surface. The magnitude of the radial electric field inthe membrane is 1210V/m. When this is multiplied bythe thickness7510m, the potential differencebetween the inside and outside surfaces of the membrane is90 mV.