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My textbook says “Among K+, Na+ and Cl-, K+ contributes to the resting membrane potential the most, because it has the greatest permeability across the membrane.”

I agree with this, but I’m just curious: why is the equilibrium potential of Cl- (-75mV) closer to the resting membrane potential (-70mV) than that of K+ (-90mV)?

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This is only true because the chloride concentration happens to be slightly less negative than potassium, therefore the weighted (by permeability) average of all the other ions is near the reversal potential for chloride.

Even if you assume the permeability of every ion except sodium and potassium is zero (i.e., if chloride doesn't contribute at all), you will end up with a resting potential near chloride's reversal potential.

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