ABSTRACT
Next consider the condition where the membrane is permeable only to sodium. Because sodium is in a greater concentration outside of the cell, the Na+ ions initially diffuse into the cell down their concentration gradient. As a result, an excess of these positively charged ions accumulates in the ICF along the internal surface of the plasma membrane, and an excess of negative charges in the form of the impermeable extracellular anion, chloride (Cl-), remains outside the cell along the external surface of the plasma membrane. This inward movement of positive charges creates a membrane potential that is positive because the inside of the cell is now positive relative to the outside. However, as the positively charged Na+ ions continue to diffuse inward, once again an electrical gradient develops. The initial (+) charges that have accumulated in the ICF begin to repel any additional Na+ ions and oppose the further movement of (+) charges inward. Instead, the positively charged Na+ ions are now attracted to the negatively charged Cl-ions remaining outside the cell. Eventually, the initial force moving Na+ ions inward down their concentration gradient is exactly balanced by the subsequent force moving Na+ ions outward down their electrical gradient, and there is no further net diffusion of sodium. The membrane potential at this point has reached the equilibrium potential for Na+ (ENa+) and is equal to 60 mV. Therefore, when the permeability of the plasma membrane to sodium is high compared to that of potassium, the membrane potential approaches 60 mV.
