The oscillatory behavior of the cephalopod giant axons in response to an applied current has been established by previous investigators. In the study reported here the relationship between the familiar "RC" electrotonic response and the oscillatory behavior is examined experimentally and shown to be dependent on the membrane potential. Computations based on the threecurrent system which was inferred from electrical measurements by Hodgkin and Huxley yield subthreshold responses in good agreement with experimental data. The point which is developed explicitly is that since the three currents, in general, have nonzero resting values and two currents, the "Na" system and the "K" system, are controlled by voltage-dependent time-variant conductances, the subthreshold behavior of the squid axon in the small-signal range can be looked upon as arising from phenomenological inductance or capacitance. The total phenomenological impedance as a function of membrane potential is derived by linearizing the empirically fitted equations which describe the time-variant conductances. At the resting potential the impedance consists of three structures in parallel, namely, two series RL elements and one series RC element. The true membrane capacitance acts in parallel with the phenomenological elements, to give a total impedance which is, in effect, a parallel R, L, C system with a "natural frequency" of oscillation. At relatively hyperpolarized levels the impedance "degenerates" to an RC system. The subthreshold behavior of the axonal membrane, exemplified by the classical "electrotonus" of the myelinated nerve (Hermann, 1905; Hodgkin and Rushton, 1946;Davis and Lorente de N6, 1947) and the "local response" and oscillatory behavior of the giant axon in the cephalopods, Loligo and Sepia (Hodgkin, 1938;Arvanitaki, 1939 Arvanitaki, , 1941Brink et al., 1946), have been studied by many investigators. In recent years, perhaps due to the advent of the microelectrode technique with its emphasis on large amplitude recordings, electrophysiologists have shown only moderate interest in the subthreshold be-497
