concentration changes caused by axial current

[biswa]

Hi Ted/Michael,

I seek your advice in modelling a two compartment model (a sphere and a cylinder) with their respective Na/K currents. Both soma and the dendrite have a Na/K pump of the form ipumpsoma[t] = gpump*(3*(vsoma[t]- revpot[nain[t], naout[t]]) - 2*(vsoma[t] - revpot[kin[t], kout[t]])- EATPsoma). The respective soma and dendritic compartments contribute to the internal Na/K concentration for eg. nain'[t] = gamma (-ina[t] - 3*ipumpsoma[t]) and kin'[t] = gamma(-ik[t] - ika[t] - iadapt[t] + 2*ipumpsoma[t]). The coupling between the compartments is of the form icoup[t] = gcoup*(vsoma[t] - vdend[t]).

Now, I am unsure how should I take the contribution of the axial current in determining the respective internal Na/K concentration pool? Certainly, the axial current flowing out of the dendrite to the soma will reduce the internal Na conc. in the dendrite and increase the internal Na conc. in the soma (electro-diffusion)? Am I right? What would you suggest ?

Best,
B.

[ted]

There may be exceptional cases in which longitudinal current flow makes a significant contribution to the spatial distribution of charged solutes in a cell--are you dealing with one, or is your interest primarily theoretical? In either case, it would be useful to become familiar with the existing body of experimental and theoretical literature on electrodiffusion in neurons. For the most part such articles focus on transmembrane current flow through channels, but a few deal with the longitudinal spread of charge carriers in cytoplasm. A good place to start would be the paper by Qian and Sejnowski in Biol. Cyben. 62 (1989).

Potassium is the principal intracellular charge carrier, not sodium.

[biswa]

Thanks Ted for your speedy reply and the link to the reference! Could you let me know of a reference that studies which of the ions is the major intracellular charge carrier?

[ted]

Could you let me know of a reference that studies which of the ions is the major intracellular charge carrier?

Try a basic text on cellular biophysics. Weiss wrote a three volume set that probably gets into this. Maybe Koch's book. Aidley wrote some good books, maybe look in those. It's easy to work out on your own, though: which ions are most plentiful, and which have the highest mobility. Potassium wins hands down.