Every source gives the resting membrane potential as about -70 mV, and then also asks me to calculate equilibrium potentials with Nernst, which give different numbers for each ion.
If the answer is -70, what are all the other numbers for?
Accepted Answer
They answer different questions, and the difference between them is what drives every current in the cell.
Nernst gives the equilibrium potential for one ion: the voltage at which that ion has no net tendency to move. Potassium is around -90 mV, sodium around +60 mV.
The resting potential is what the membrane actually sits at, given that several ions are moving at once with different permeabilities. That is the Goldman equation rather than Nernst.
At rest the membrane is far more permeable to potassium than sodium, so -70 sits close to the potassium equilibrium and far from sodium's.
The gap between the two is the point. Sodium is 130 mV away from equilibrium, which is why opening sodium channels produces such a violent inward current.
A correction on something that sounds right and is not: the sodium potassium ATPase is not what holds the resting potential at -70.
It is electrogenic, three out for two in, so it contributes a few millivolts directly. Its real job is maintaining the concentration gradients that Nernst depends on.
Block it and the potential does not collapse immediately. It decays over minutes as the gradients run down. That timing is the evidence for which role it plays.
A quick way to use it in questions: current flows in proportion to the driving force, which is membrane potential minus that ion's equilibrium potential. Whenever they ask which way an ion moves, compute that difference.
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