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potential, or voltage, with the right side of the membrane having excess negative charge with respect to the left.

As more and more Na+ ions move through channels across the membrane, the magnitude of this charge difference (i.e., voltage) increases. However, continued right-to-left movement of the Na+ ions eventually is inhibited by the mutual repulsion between the excess positive (Na+) charges accumulated on the left side of the membrane and by the attraction of Na+ ions to the excess negative charges built up on the right side. The system soon reaches an equilibrium point at which the two opposing factors that determine the movement of Na+ ions—the membrane electric potential and the ion concentration gradient—balance each other out. At equilibrium, no net movement of Na+ ions occurs across the membrane. Thus this semipermeable membrane, like all biological membranes, acts like a capacitor—a device consisting of a thin sheet of nonconducting material (the hy-drophobic interior) surrounded on both sides by electrically conducting material (the polar phospholipid head groups and the ions in the surrounding aqueous solution)—that can store positive charges on one side and negative charges on the other.

If a membrane is permeable only to Na+ ions, then at equilibrium the measured electric potential across the membrane equals the sodium equilibrium potential in volts, ENa. The magnitude of ENa is given by the Nernst equation, which is derived from basic principles of physical chemistry:

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