Goldmann-Hodgekin-Katz / Millmann Equation

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CPH-DK
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Goldmann-Hodgekin-Katz / Millmann Equation

Post by CPH-DK » Thu Jan 05, 2006 10:44 pm

Hello!

i have a question regarding the cells resting membrane potentiale. In a normal neuron the resting potentiale is -70mV, 20 mV away from Potassiums nernstpotential at -90 mV. This is due to the Cl- and Na+ permabilities.....

Now suppose you take away all the K+, both intracellular & extracelluar. So you kan remove the k+ from the GHK-equation. Now also suppose that cl- is impermable, so we only have Na+ left.....

Why wont the resting potential achive go towards Na+´s nernst potentiale?
the answer is that it will go to about 0 mV, maybe slightly higher.... My question is why?
because of na+ is as good as impermable? because of the Na+/k+´ elecrogenic activity or???

hope you can help

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Post by sdekivit » Fri Jan 06, 2006 1:03 pm

permeability of Na-ions is almost nothing compared to K-permeability. Thus the resting membranepotential will be more close to the K-Nernst-potential than the Na-Nernst-potential.

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Post by MrMistery » Fri Jan 06, 2006 9:36 pm

true, but he was asking a hypothetical situation when potassium and cloride are no longer present.
Now, i do not know what the nernst potential is, but i do know the Na ions will move according to their electrochemical gradient. and since there are a lot of negatively chargen organic molecules inside the neuron, the sodium ions will go to the inside of the cell according to it's electrical gradient. The difussion will reach equilibrium when the force pulling the ions inside according to their electrical gradient is equal to the one pulling them out occurding to their chemical gradient.
the final result is that the potential is a little bit higher than 0mV and most of the sodium ions are INSIDE the cell...
I think...
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Post by 2810712 » Sat Jan 07, 2006 10:03 am

I don't know what is nernt-potential... hope that is somewhat like the nernst eqn in electrochem...
As the permeability of Na+ is very less than that of K+ and if Cl- is impermeable then the diff bet. the normal nernst poetntial and the hypothetical nernst potential should be less as it is created due to Na+ passage across the membr. and Na the only {H+/H3O+ and others???} permeable ion has less permeability so less Na+ pass across the membr. so lesser should be the potential diff. This seems to be the qn.
But when i think about the eqilibrium in such an only Na+ case would be attained when the conc.s of Na+ on both the sides become equal or slightly hiegher due to presence of -ve ions in the cell interion like aminoAAs etc...So the final charge diff. betn both the sides aproaches 0 or hiegher , thus the Potential aproaches 0 or heigher...



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Re: Goldmann-Hodgekin-Katz / Millmann Equation

Post by sdekivit » Sat Jan 07, 2006 10:58 am

CPH-DK wrote:Hello!

i have a question regarding the cells resting membrane potentiale. In a normal neuron the resting potentiale is -70mV, 20 mV away from Potassiums nernstpotential at -90 mV. This is due to the Cl- and Na+ permabilities.....

Now suppose you take away all the K+, both intracellular & extracelluar. So you kan remove the k+ from the GHK-equation. Now also suppose that cl- is impermable, so we only have Na+ left.....

Why wont the resting potential achive go towards Na+´s nernst potentiale?
the answer is that it will go to about 0 mV, maybe slightly higher.... My question is why?
because of na+ is as good as impermable? because of the Na+/k+´ elecrogenic activity or???

hope you can help


if only Na(+) is permeable through the membrane, then we only have to deal with the Nernst-potential for Na(+), which is normally +60 mV.

--> Now we only have a low permeability, so the voltage will stay above 0 V, but will slightly decrease from the 60 mV

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