Extracellular ion concentrations and neural membrane potential

[wrongusername]

I'm learning about a very basic model neuron, in which only potassium and chloride is permeable.

Why is it that when the extracellular concentration of potassium is increased, the neuron become depolarized, whereas when extracellular concentration of chloride is reduced, the neural membrane potential remains the same? Intuitively it would seem that whatever applies to potassium also applies to chloride.

Even when sodium permeability is added to the simple neuron model, it seems chloride can be ignored as it more or less takes care of itself no matter where the membrane potential goes.

 

[aroc91]

I believe this is your issue-

basic model

Chloride does contribute to resting membrane potential and any changes in intra- or extracellular conc. of it would definitely alter it.

Since I can't figure out complex fractions inside parentheses in LaTeX, I've just attached a picture. Sorry for the blurriness.

 

[wrongusername]

I believe this is your issue-



Chloride does contribute to resting membrane potential and any changes in intra- or extracellular conc. of it would definitely alter it.

Since I can't figure out complex fractions inside parentheses in LaTeX, I've just attached a picture. Sorry for the blurriness.

Ahh excellent, thanks!

Though I do wonder what my textbook was talking about when it says:

... The model cell membrane is permeable to potassium and chloride, but not to sodium or to the internal anion.

...

In neurons and in many other cells, the resting membrane potential is sensitive to changes in extracellular potassium concentration but is relatively unaffected by changes in extracellular chloride. To understand how this comes about it is useful to consider the consequences of such changes in the model cell.

...

The increase in extracellular potassium reduces the concentration gradient for outward potassium movement, while initially leaving the electrical gradient unchanged. As a result there will be a net inward movement of potassium ions. As positive charges accumulate on its inner surface, the membrane is depolarized. This, in turn, means that chloride ions are no longer in equilibrium, and they move into the cell as well. Potassium and chloride entry continues until a new equilibrium is established, with both ions at a new concentration ratio consistent with the new membrane potential, in this example -68 mV.

...

Similar considerations apply to changes in extracellular chloride concentration, but with a marked difference: When the new steady state is finally reached, the membrane potential is essentially unchanged. ... Chloride leaves the cell, depolarizing the membrane toward the new chloride equilibrium potential (-68 mV). Potassium, being no longer in equilibrium, leaves as well. ... Because the intracellular concentration of potassium is high, the fractional change in concentration produced by the efflux is relatively small. However, the efflux of chloride causes a sizable fractional change in the intracellular chloride concentration, and hence in the chloride equilibrium potential. As chloride continues to leave the cell, the equilibrium potential returns toward its original value. The process continues until the chloride and potassium equilibrium potentials are again equal and the membrane potential is restored.

But if efflux of chloride changes chloride's equilibrium potential a lot because there's not that much of it, then the same must be true for an influx of chloride when potassium is introduced outside the cell. I don't understand the book's explanation for this phenomenon.

Later, when the book explains the constant field equation using potassium and sodium, it says

How do these considerations apply to chloride? As for all other ions, there must be no net chloride current across the resting membrane. As already discussed, chloride is able to reach equilibrium simply by an appropriate adjustment in internal concentration, without affecting the steady-state membrane potential.

Would you happen to know what the book is trying to say here? Thanks.

 

[atyy]

I googled the passage you quoted and got http://homepage.univie.ac.at/andreas.franz.reichelt/intro2cogsci2/protected/nichols_2001_neuron_ch5.pdf .

Looking at Fig. 5.2 and the accompanying text, in both cases, potassium and chloride leave or enter the cell together leave the cell together, so the change of potassium and chloride concentration in the cell must be the same, if the cell volume is unchanged. However, in one case the cell volume increases, while in the other case the cell volume decreases, since water moves to maintain osmotic balance. After taking into account the different final cell volumes, you should be able to get the final concentrations given in the book.