Ionic Basis of the Membrane Potential (V_m)

        First of all, remember that when talking about V_m, it's the numerical value of V_m that measures the magnitude of V_m. The sign (+ or -) doesn't tell us anything about the magnitude of V_m; it only tells us which direction, into the cell or out of the cell, charges will tend to move through the membrane in response to V_m. Thus, contrary to what you've learned in your math courses, the potential difference across the plasma membrane represented by V_m = - 90 mV is greater ^__ not less ^__ than a V_m of - 80 mV. Likewise, a V_m = + 40 mV represents a smaller potential difference than V_m = - 80 mV)

        With the previous paragraph in mind, there are several key points to remember when interpreting the results of experiments involving the effect of ion concentrations and membrane conductances on V_m:

1. The membrane potential is entirely the result of separated charges, + charges on one side of the membrane, - charges on the other. If there are no charges separated across the membrane, then there will be no measurable membrane potential (i.e., V_m = 0).
   
2. The magnitude of V_m is determined by the amount of separated charge.
   
3. Anything that alters the amount of separated charge will alter V_m. Specifically, the more + charge you have separated from – charge across the plasma membrane, the greater will be the magnitude of V_m.
   
4. The amount of separated charge (and therefore the magnitude of V_m) is primarily determined by the rate at which ions (particularly cations) are moving through the plasma membrane and thereby separating themselves from the negatively-charged non-diffusible anions that remain behind as the cations move through the membrane.
   
5. The rate at which ions are moving through the plasma membrane is determined primarily by:
   
   1. a. the concentration gradient(s) across the membrane ( Dc = the difference between the internal and external ion concentrations), and
      
   2. b. the conductance of the membrane for the ions moving through it ( g_m = the ease with which ions can move through the membrane).
      
6. A cation (say Na⁺) moving in one direction through the membrane will tend to offset or neutralize the V_m effects of a cation (say K⁺) moving in the opposite direction through the membrane.

        Putting points 1 through 5 together, we may conclude that anything that changes Dc or g_m for any particular ion will tend to change V_m. This is because changes in Dc or g_m will change the rate at which ions are moving through the membrane which, in turn, will change the rate at which + and - charges are being separated across the plasma membrane. And, since V_m is determined by the amount of separated + and – charges, changing the rate at which charges are being separated will change V_m.