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I learnt that the permeability of an ion across the membrane contributes to the membrane potential as much as(or even more than) its concentration and electrical gradients. And so far I've made peace with the fact that Sodium, having a very low permeability, doesn't contribute much (through its own movement) to membrane potential despite having a high electrochemical gradient directed towards the cell's interior. Now, the confusing thing is that, if I imagine dumping some sodium ions (with their positive charge) on the cell's exterior, this would affect the electrical gradient of potassium (and in fact other ions)--because more positivity is recorded externally-- making potassium in particular less "ready" to diffuse out of the cell. The result of which would be a depression (that is a more negative value ) of the resting membrane potential. If potassium's extracellular concentration were increased instead, a similar effect would be observed.

What exactly is wrong with this reasoning?

I'm putting this forward because my Professor mentioned only extracellular potassium ion as having an effect on membrane's resting potential.

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