Ion flow that establishes the equilibrium potential across the membrane. The difference in concentration across the membrane determines the equilibrium potential as described by the Nernst equation. Click the play button below to follow the Goldman equation in the steady state.
The relative opening of the sodium (red) and potassium (green) channels determine the membrane potential. When only the sodium channels are open, the membrane is at the sodium equilibrium potential of +55 mV. When only the potassium channels are open, the membrane is at the equilibrium potential for potassium at -80 mV. When both channels are open, the membrane is at an intermediate potential determined by the ratio of the openings. This is an expression of the Goldman equation
The resting membrane potential is set by a specific ratio of sodium and potassium channel openings. The equilibrium potential for sodium is +55 mV, and that for potassium is -80 mV. At rest, the potassium channels are more open than the sodium channels. One can think of this fixed ratio of channel openings as representing together a separate single channel, call it the "resting channel, with equilibrium potential at the resting level.