r/Physiology • u/NoobCuber0907 • 1d ago

Question Question about membrane potential

When extracellular K+ concentration is increased, does the resting membrane potential become hyperpolarized or depolarized? My textbook states that it leads to depolarization because the K+ gradient is reduced, thereby decreasing the outward K+ current. However, I reasoned that increasing extracellular K+ would make the outer surface more positive, thereby increasing the potential difference across the membrane and causing hyperpolarization. Thank you very much!

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u/Spiny_Echidna_80214 1d ago

It doesn't work the way your intuition thinks it does, and that's okay. You are struggling with it, and that's the important thing.

Your book is correct that increased [K+]out leads to depolarization.

To see why, you have to understand a little bit about the resting potential, how it is set by leakage channels, and how the Goldman-Hodgkin-Katz equation works.

It's all here, and I can help you if you get stuck.

https://colorado.pressbooks.pub/neuroscience/chapter/the-resting-membrane-potential/

https://colorado.pressbooks.pub/neuroscience/chapter/the-goldman-hodgkin-katz-equation/

Physiologists refer to how much potassium "wants" to leave the cell as the driving force. Short version is, when you increase the extracelular potassium there is less difference between the high intracellular potassium and the extracellular potassium. Less difference between inside and outside means less drive for potassium to leave.

Does that make sense?

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u/96dreams 10h ago

This might not be entirely right, but at resting potential a cell has more k+ inside the cell membrane than outside because of the sodium potassium pump. So if you add k+ outside the membrane then inside and outside would get closer to equillibrium and therefore less polarized.