Neuro Question

[mrmandrake]

This is taken from Pretest Neuro:

If a membrane is permeable only to sodium ions and the concentra-
tion of sodium ions on one side of the membrane is the same as that on the
other side, then which of the following statements best characterizes the
resting membrane potential for that cell?

a. A pump mechanism will cause the cell to become hyperpolarized
b. The membrane potential would be zero
c. The tendency would be for current to be directed inwardly
d. The membrane potential could not be predicted from the Nernst equation
e. There would be an initial decrease followed by an increase in membrane potential

Highlight the following section of space to see the answer: b

Can someone explain to me why this is the answer?

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[vicinihil]

So membrane potential is due to the difference in charge between the inside and the outside cell compartments. If you have no concentration difference, then you have no flow of ions, and subsequently no charge generated. Simply put...just look at the Nernst Equation

Eq = blah blah blah ln( Concentration of ion inside/concentration of ion outside)

As you can see....ln(1) = 0 so Eq = 0

 

[hartbot]

I think that partly answers the question. Membrane potential is determined only by the ions which can move freely across the membrane. In this case, that is Na, and since the concentrations are equal both inside and outside, the charge across the membrane is 0.

In a normal resting cell, the only permeable ion is K. The concentration of K inside the cell is much greater than the conc of K on the outside, and for that reason the potential is -75 and not 0. The conc of K in this situation is balanced by the negative charge keeping it in (or I suppose the positive extracellular pressure preventing its movement), and the diffusion pressure pushing it out.

 

[mrmandrake]

Thanks for the response guys. I was confused because I thought to myself "What about the other ions that are not permeable? Don't they contribute to the potential since I am assuming there is a concentration difference for K or Cl?"

I guess the answer to that is membrane potential is determined only by the ions which can move freely across the membrane. It's strange to me still, but I guess it's right.