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At rest, the equilibrium potential for potassium given by the Nernst equation is ~ -80mV. Since the cell is mainly permeable to potassium, this is the reason for the cell membrane's rest potential to be close to it, towards ~-70mV.

Now, suppose we increase the concentration of potassium outside of the cell. The equilibrium potential for potassium now increases when calculated through the Nernst equation. Since the cell is mainly permeable to potassium, I'd assume it's resting potential to stay close to potassium's equilibrium potential, and thus increase as well - Therefore depolarizing.

However, this result confuses me a lot. After all, potassium is positively charged. The cell membrane is negatively charged on its inside and positively charged on its outside, so shouldn't adding more positive charges on the outside actually hyperpolarize it?

I'm not sure what part I'm missing / not understanding. The following answer on this site seems to confirm my calculation that the cell would depolarize, but then what's wrong about the reasoning above? How does extracellular potassium ion concentration and calcium ion concentration affect the excitability of a cell?

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  • $\begingroup$ You're pretty much on the right track, just one little misconception is separating what the equations are telling you from your conceptualization. Does this answer to another question help you? biology.stackexchange.com/a/79623/27148 It's talking about sodium, but the important principles are the same; the 3rd and 5th paragraphs are most relevant to you. $\endgroup$ – Bryan Krause May 28 at 22:14

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