Analysis of Spike Electrogenesis of Eel Electroplaques with Phase Plane and Impedance Measurements

Eel electroplaques provide experimental conditions in which registration of phase plane trajectories (dV/dt vs. V) and impedance measurements with an AC Wheatstone bridge, in conjunction with spike electrogenesis describe quantitatively the ionic processes of the electrogenesis. Thus, these data employing as they do measurements of transients, permit an independent test of the validity of the assumptions which underlie the Hodgkin-Huxley equivalent circuit: independent ionic channels with fixed ionic batteries and exhibiting time-variant conductance changes with different kinetics for the different channels. The analysis accords with earlier findings on voltage-clamped electroplaques and this agreement confirms the validity of the equivalent circuit despite the fact that the current-voltage characteristics of the axons and electroplaques differ profoundly. As for squid axons, the equivalent circuit of the electroplaques has four branches: a capacity and three ionic channels. One of the latter is an invariant leak channel (G_L) of high conductance. A K channel (G_K) is fully open at rest, but rapidly undergoes inactivation when the cell is depolarized by more than 40 mv. G_L and G_K have a common inside negative emf (E_K). A Na channel (G_Na) with an inside positive emf (E_Na) is closed at rest, but opens transiently upon depolarization.