Nernst equation + neurons

[Jiller1994]

I need some good ol' fashioned concept clarification

Let's ASSUME [ion] of K+ and Na+ are perfectly equal, and there is a little bit of residual K+ outside neuronal cell membrane, and a bit Na+ inside.
Assuming no net Na ion movement:
1) If the neuron at rest is permeable to K+ leaving, then the interior loses some + charge, thus rendering it more negative.
2) Alternatively, if the neuron at rest reabsorbs some K+, then the interior become more positive.
My confusion is this:
In the Nernst equation [K+ intracellar]/ [K+ extracellular] makes it so that the MORE K+ there is inside, the MORE NEGATIVE the membrane potential becomes.
My examples above show that as K+ conc. increases extracellularly, then the internal membrane voltage decreases.
I need some clarification...

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[MrRed]

So as the concentration of K+ inside the cell increases (Cin) relative to the extracellular concentration of K+ (Cout) then we have an increase in the magnitude of potential for the given ion across the membrane. What does this mean? Well this all has shown us that the Nernst equation gives the equlibrium potential of an ion for only a single ion. It does not consider the complex ionic interactions and relative concentrations that compose the symphony that we think of as a typical resting membrane potential, -70mV. The value given by the nernst equation is not the membrane potential, it is the voltage necessary (or rather voltage difference across the membrane) such that at any given intracellular vs. extracellular ion concentration, diffusion of ion's in and out of the cell will be in equilibrium.

Remember, the Nernst equation is for calculating only when the ion's diffusion is at equilibrium; when the change in Gibb's free energy is 0.

You're right in thinking that if K+ inside increases than the membrane potential overall will become less positive. However if K+ increases inside then the voltage necessary to maintain equilibrium between influx and efflux also increases in magnitude.

Higher K+ intracellular means that more K+ wants to efflux, the membrane potential becoming more negative counters this, repelling the K+ to an extent and keeping more of them in the cell.


For more accurate, and realistic, equilibrium potential calculations see the Goldman–Hodgkin–Katz voltage equation.
Here is a little drawing I made showing the membrane potential with electric field lines and the effect this would have on K+ diffusion.This doesn't directly relate to the Nernst equation so much as the Goldman equation, but I wanted to draw.