Electric Fields in Perfused Nerves

Abstract

IN perfusion experiments with squid giant axons, the internal potassium concentration K_i can be reduced either by substitution of other univalent ions, so that both ionic strength s_i and osmolality are kept constant, or by dilution with non-electrolytes at constant osmolality. The importance of non-electrolyte perfusion in testing various models of resting potential difference (p.d.) and action potential is emphasized by the fact that there is a large range of resting potential difference (between − 25 and 0 mV) obtained by reducing K_i, in which action potentials are abolished if K_i alone has been reduced but are still obtained if s_i has been reduced proportionately^1,2. For brevity, we shall call this the conditional range of potential difference. We can envisage various mechanisms by which lowering a high electric field across the axonal membrane can influence ionic permeabilities; all such possibilities would be lost if action potentials could still be obtained by depolarization of a low field. In order to maintain a high field across the membrane, even when the measured potential difference between inner and outer solutions is reduced by dilution of K_i with non-electrolyte, Hodgkin and Chandler^3,4 introduced the new assumption that the inner surface of the membrane binds about thirty times more negative charges than are needed merely to terminate the membrane field. In this communication we propose, as an alternative, that on dilution of internal K_i with non-electrolyte, the membrane field becomes non-uniform in accord with the Planck electrodiffusion model⁵. The field at the inner surface of the membrane remains high even when the overall potential difference is drastically decreased by perfusion with solutions of low K_i and s_i.