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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.

Chemo-Mike
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  • You can't just add a bunch of positively charged ions in a bucket; they always come with a negative counterpart. If you somehow could, you would create lightning. Electricity is pretty powerful stuff. – Bryan Krause Sep 10 '20 at 18:53
  • Are you sure that adding extracellular $Na^+$ would inhibit $K^+$ migration? – MattDMo Sep 10 '20 at 18:56
  • I've answered a bunch of very similar questions here and on Psych & Neuro. They all end up being approximately duplicates but the underlying question is really just "how do membrane potentials work" and everyone has a slightly different misunderstanding. Some places to start: https://biology.stackexchange.com/questions/88730/when-is-voltage-generated-across-a-membrane/88777#88777 https://biology.stackexchange.com/questions/64762/cell-biology-resting-membrane-potentials/64773#64773 – Bryan Krause Sep 10 '20 at 18:56
  • https://biology.stackexchange.com/questions/76166/ambiguity-about-the-relation-between-membrane-potential-and-concentration-gradie/76167#76167 https://biology.stackexchange.com/questions/52122/why-does-the-side-of-the-mebrane-of-a-neuron-with-a-high-concentration-of-positi/52595#52595 https://biology.stackexchange.com/questions/77919/why-does-resting-potential-not-become-continually-more-negative/77925#77925 https://biology.stackexchange.com/questions/57064/why-is-it-possible-to-calculate-the-equilibrium-potential-of-an-ion-using-the-ne/57066#57066 – Bryan Krause Sep 10 '20 at 18:56
  • @BryanKrause , thanks for the links. But before I check them out, I'll like to make sure I understand your answer. Since we cannot add positive ions without accompanying negative ions, the change in membrane potential resulting from increase in potassium's extracellular concentration is NOT due to change in its electrical gradient (since just about same amount of negative ions from wherever counteracts that). But what then would be responsible? The change in its concentration gradient? – Chemo-Mike Sep 10 '20 at 19:50
  • @Chemo-Mike Yes. If you add extracellular potassium, there is less of a concentration difference between in and out so the net flow of potassium ions from inside to outside is lower at any given voltage. Therefore, the voltage that takes the net to become zero (the "reversal potential for potassium") is less negative. The Nernst equation shows this for potassium considered by itself, and the Goldman equation will tell you what resting potential will be once accounting for other permeable ions. – Bryan Krause Sep 10 '20 at 19:57
  • Ok. Got that! – Chemo-Mike Sep 11 '20 at 05:38

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