The GHK voltage equation for  monovalent positive ionic species and  negative:
This results in the following if we consider a membrane separating two -solutions:
It is "Nernst-like" but has a term for each permeant ion:
* = the membrane potential (in volts, equivalent to joules per coulomb)
* = the selectivity for that ion (in meters per second)
* = the extracellular concentration of that ion (in moles per cubic meter, to match the other  SI units)
* = the intracellular concentration of that ion (in moles per cubic meter)
* = the ideal gas constant (joules per kelvin per mole)
* = the temperature in kelvins
* = Faraday's constant (coulombs per mole)
 is approximately 26.7 mV at human body temperature (37 °C); when factoring in the change-of-base formula between the natural logarithm, ln, and logarithm with base 10 , it becomes , a value often used in neuroscience.
The ionic charge determines the sign of the membrane potential contribution. During an action potential, although the membrane potential changes about 100mV, the concentrations of ions inside and outside the cell do not change significantly. They are always very close to their respective concentrations when the membrane is at their resting potential.