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I am reading a journal paper about the cell adhesion molecule NCAM2 and I have come across the following statement:

To confirm the functionality of the reporters, neurons were time-lapse recorded before and after application of 90mM K+ used to depolarize membranes and induce Ca2+ influx via VDCCs.

I know that depolarisation is when the membrane potential of a neuron becomes more positive. I have read that the opening of channels that let positive ions flow into the cell causes depolarisation, such as the opening of channels that let Na+ into the cell.

However, the above statement in the paper seems to imply that extracellular K+ ions can also cause membrane depolarisation. I am not sure what the relationship between potassium ions and membrane depolarisation is and I have not found anything about it online. Any insights are appreciated.

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Yes, this is a very basic neuroscience question, something that would probably come up in the first week of a neurophysiology class or unit.

The polarization of the membrane at rest is due to cells being primarily permeable to potassium at rest, and due to a concentration difference of potassium inside vs. outside the membrane. Inside, potassium is high, outside potassium is low.

Therefore, there is a concentration gradient such that if a cell were at 0 mV, potassium would be flowing out of the cell. There is a membrane voltage where this net flow would stop. We refer to this as the "reversal potential" or "equilibrium potential" for an ion. For potassium in a typical cell, this tends to be around -90mV. The larger the ratio of concentrations in versus out, the larger the potential. You can calculate this potential with the Nernst equation if you know the concentrations (image from Wikipedia):

enter image description here

From this equation you can see that the reversal potential is negative for a positive ion if [outside] is small compared to [inside]. By applying higher potassium outside than usual, the ratio of [outside]/[inside] gets closer to 1, so the reversal potential for potassium is closer to 0mV, and typical neurons that are mainly permeable to potassium at rest will depolarize.

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