In the presence of several different ions, the equilibrium of the cell depends on the relative permeability of the ions. For this, we use the Goldman-Hodgkin-Katz equation:
Permeability of an ion is dependent on a number of factors such as the size of the ion, its mobility, etc. During rest in the squid giant axon, the permeabilities have the ratio so that
Since dominates, this is close to During an action potential the ratio is so that
Later on we will approximate the GHK equations by a linearized version:
where the conductances g are proportional to the permeabilities.