Membrane potentials in a nerve membrane

[garytse86]

This is about membrane potentials in a nerve membrane.

I've already studied the Nernst equation and the Goldmann equation, but I have found its significance in explaining the resting potential.

The Nernstian potential occurs at equlibrium, when concentration gradient = opposing electric gradient, but how does knowing Nernstian potentials help you?

I know that the Ek is roughly equal E(Resting Potential), but so what? This doesn't explain the basis of resting potential.

Again the Goldmann equation, it assumes a constant field, but how does this relate to the actual events in the resting potential?

The resting potential is caused by concentration gradient and permeability to potassium mainly, and I know that the potassium channels are voltage gated, how does equilibrium relate to this?

"The Nernst equation can essentially calculate the membrane potential of a cell when only one ion is permeant, as long as the concentrations of that ion both inside and outside the cell are known. The Nernst equation cannot, however, deal with cells having permeability to more than one ion."

and

"The usefulness of the GHK equation to electrophysiologists is that it allows one to calculate the predicted membrane potential for any set of specified permeabilities. For example, if one wanted to calculate the resting potential of a cell, they would use the values of ion permeability that are present at rest (e.g. P_{k} >> P_{Na^{+}}). If one wanted to calculate the peak voltage of an action potential, one would simply substitute the permeabilities that are present at that time (e.g. P_{Na^{+}} >> P_{k})."

So the equations are only useful when performing analysis, but not for explaining the ionic basis of membrane potential? I mean, how would you use the Nernst or Goldman equation to describe the processes that occur at the resting potential?

 

[gerben]

The Goldman equation is an extended form of the Nernst equation; it takes more ions into account. With the Goldman equation you can calculate the membrane potential if you know the concentrations of the ions and the permeability of the membrane for those ions. It does however not say anything about the reason for the different concentrations and permeabilities (i.e. ion-pumps and the state, open or close, of specific ion-channels).

 

[somasimple]

Hi,

These old equations are only valid if ions were homogenous concentrated.

• If the membrane seems "polarized" it is because ions are stuck on it by attraction/repulsion forces (Coulomb, Gauss).
• Action potential uses a very little quantity of ions (70,000) (Huxley, MacKinnon).
• Action potential is a "skin" phenomenon.