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.

  • $\begingroup$ 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. $\endgroup$
    – Bryan Krause
    Commented Sep 10, 2020 at 18:53
  • $\begingroup$ Are you sure that adding extracellular $Na^+$ would inhibit $K^+$ migration? $\endgroup$
    – MattDMo
    Commented Sep 10, 2020 at 18:56
  • $\begingroup$ 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: biology.stackexchange.com/questions/88730/… biology.stackexchange.com/questions/64762/… $\endgroup$
    – Bryan Krause
    Commented Sep 10, 2020 at 18:56
  • $\begingroup$ @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? $\endgroup$
    – Chemo-Mike
    Commented Sep 10, 2020 at 19:50


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