Non-linear dielectric properties of nerve membranes

The unique feature of the measurements of membrane capacitance and conductance of nerve axons is the facts that these measurements must be carried out in the presence of a bias potential (-60 mV) across the membrane and also that measurements are performed in the presence of time dependent and non-linear currents. The bias potential creates an electrical field of 80 kV/cm in the membrane which produces a highly ordered structure at the resting state. Because of this, the AC field which is used for capacitance and conductance measurements should be considered a perturbation of the ordered structure rather than the force causing a transition from a random distribution to a polarized state. If the bias potential is removed by depolarization of the membrane, the ionic permeabilities of the membrane increases and Na and K ions will flow across it. Therefore, measurements of capacitance and conductance of depolarized membranes will be carried out in the presence of these ionic currents. First of all, K+ current is known to create an inductive reactance. Also Na current is believed, although still uncertain, to produce a capacitance component. Therefore, these reactances must be differentiated from the real membrane capacitance due to the dielectric properties of the components of the membrane. In order to do so, it is desirable to conduct dielectric measurements in the absence of ionic currents after application of appropriate toxins.