In real cells, the resting membrane potential is the point at which sodium influx is exactly balanced by potassium efflux. This point depends on the relative membrane permeabilities to sodium and potassium; in most cells pK is much higher than pNa and the balance is struck close to EK. The Goldman equation gives the quantitative expression of the relation between membrane potential on the one hand and ion concentrations and permeabilities on the other. Because the steady-state membrane potential lies between the equilibrium potentials for sodium and potassium, there is a constant exchange ofintracellu-lar potassium for sodium. This would lead to progressive decline of the ion gradients across the membrane if it were not for the action of the sodium-potassium pump. Thus, metabolic energy, in the form of ATP used by the pump, is required for the long-term maintenance of the sodium and potassium gradients. In the absence of chloride pumping, the chloride equilibrium potential will change to come into line with the value of membrane potential established by sodium and potassium. In some cells, however, a chloride pump maintains the internal chloride concentration in a nonequilibrium state, just as the sodium-potassium pump maintains internal sodium and potassium concentrations at nonequilibrium values.

The steady fluxes of potassium and sodium ions constitute electrical currents across the cell membrane, and at the steady-state Em these currents cancel each other so that the net membrane current is zero. The membrane current carried by a particular ion is given by an ionic form of Ohm's law that is, by the product of the driving force for that ion and the membrane conductance to that ion. The driving force is the difference between the actual value of membrane potential and the equilibrium potential for that ion. Conductance is a measure of the ability of the ion to carry electrical current across the membrane, and it is closely related to the membrane permeability to the ion.

Individual ion channels behave as though access to the pore through which ions can cross the membrane is controlled by a gate that may be open or closed. When the gate is open, the channel conducts and electrical current flows across the membrane; when the gate is closed, there is no current flow. The current through a single open channel is again given by the ionic form of Ohm's law that is, the driving force multiplied by the single-channel conductance.

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