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equilibrium potential of an ion can be calculated by using nernst equation. The resting potential is closer to equilibrium potential of potassium because during resting, Na-K ATPase moves 3 sodium ions out of cell and 2 potassium ions in. Also, there's some leak potassium channel and potassium ion moves out of cell down its concentration gradient. This make inside of cell is more negative than outside. why resting potential is 'closer' to equilibrium potential of potassium but not same? This is because some sodium channels are opened during resting state. Thus, some sodium ion moves into the cell and cancelling the effect of equivalent number of potassium moving out. the peak of action potential is closer to the equilibrium potential of sodium. i hope this helps.
When you sum up all the equilibrium ionic potentials of each ion that is relevant, you would get resting membrane potential?
If you want to consider other ions, you can use Goldman-Hodgkin-Katz equation because it considers ions that contribute to the real membrane potential.
So with the Nernst Equation you get the ionic equilibrium potential for ONE ion, but with the Goldmann eqn, you get an approximation of the true RESTING MEMBRANE POTENTIAL?
@yukitou Suppose a neuronal membrane was only selectively permeable to Cl-. We know that [Cl-] is more concentrated outside than inside of the cell. At (eq), what will be the sign of the membrane potential? I say its (-), b/c the Cl- will go down its gradient into the cell
Also, Goldman equation gives you the RESTING MEMBRANE POTENTIAL AT EQUILIBRIUM approximation, by taking into account the relative permeabilities of ions that neuronal membrane is permeable to?
it's true that chloride moves down its concentration gradient. Eventually, there will be more chloride inside than outside. usually membrane potential is based on voltage relative to outside of the cell. Inside more negative means negative membrane potential. I would use nernst equation to confirm the sign. |dw:1339678430425:dw| Log (chloride outside/chloride inside) will give you positive number because chloride outside/chloride inside ratio is always greater than one. Log this ratio will give you positive number. Z means charge on ion, which means in this case z=-1. Overall gives you negative value. yes, goldmann equation considers the permeabilities of membrane to other ions but nernst equation don't. That's why goldmann equation is slightly 'longer and complicated' than nernst equation.
So we say that the neuronal membrane potential at rest is at EQUILIBRIUM IF: - the electrical forces are equal and opposite to the diffusional forces?
yes, you can say that. But i will prefer to say electrical gradient is equal and opposite to chemical gradient. This is because Na-K pump is an ATPase so the 'diffusion' is not applicable to this pump.