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Extracellular ion concentrations and neural membrane potential

  1. Aug 20, 2014 #1
    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.
  2. jcsd
  3. Aug 24, 2014 #2
    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.

    Attached Files:

  4. Aug 24, 2014 #3
    Ahh excellent, thanks!

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

    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

    Would you happen to know what the book is trying to say here? Thanks.
  5. Aug 24, 2014 #4


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    I googled the passage you quoted and got http://homepage.univie.ac.at/andrea...cogsci2/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.
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