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I'm trying to gain an intuition for the dynamics of across neuronal membranes. The overarching idea here is they are controlled by ion concentration gradients across the membrane (which we can describe mathematically).

I'm looking for someone to delineate every present concentration gradient across neuronal membranes that are relevant here(involving $Na^+$, $Ca^{2+}$, $K^-$, and $Cl^-$).

It's unclear to me what creates a concentration gradient:

  • Are the concentrations of each type of ion independent from eachother, i.e. in a world without charge, diffusion across the membrane is individual to the concentration of each ion, or, is mediated by the net concentrations of mass (which makes less sense)?
  • Is the flow of ions across the membrane driven by net charge on each side, or the charge concentrations of each individual ion?
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    $\begingroup$ Hmm.. chemical gradients are individual while for the electric, we take the over all charge, or rather the potential difference. This is what we are actually doing in the Nernst equation right.. We equate the chemical potential to the electric potential difference to see, at which voltage it would balance out. So finally, chemical gradients of individual ions and net charge (implying potential). But still, lets wait for a well structured answer! $\endgroup$
    – Polisetty
    Commented Apr 5, 2016 at 19:03
  • $\begingroup$ Ok thanks. that's what I was hoping it would be; otherwise Id be really confused. On that last point, I don't understand why chemical gradients work on individual ions – I'd think it's about the net chemical concentrations. What am I missing in my mental model? $\endgroup$
    – theideasmith
    Commented Apr 5, 2016 at 19:16

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I try to answer your two questions briefly:

  • Ion gradients are dependent on charge, but there exist independent transport mechanisms, extending diffusion. Trans-membrane transport proteins can specifically move only one sort of ions. Also symport or antiport exists, that can depend on gradients of other (e.g. non-charged molecules). In addition, the mentioned inorganic ions can form chemical bonding to organic ions for example, which neutralizes the electrical charge.

  • Beside an electrical potential, also chemical (concentration) potential can act as force on ios. In addition active transport, inactivation as mentioned above and other factors have an influence on the molecule flow.

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  • $\begingroup$ You don't mention the Nernst equation at all here. Isn't that somewhat important? $\endgroup$
    – James
    Commented Apr 11, 2016 at 1:47

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