Critical Slowing-Down near the Transition Region from the Resting to Time-Ordered States in Squid Giant Axons

The transition features from the resting to the time-ordered states of squid giant axons were studied experimentally in detail by observing membrane potential changes induced by weak current pulses. The observed features near the transition point are that (1) a critical slowing-down was observed in the relaxation behavior, (2) the subthreshold oscillation frequency remained invariant before and after the transition point, and (3) the transition was of a discontinuous type. These experimental results could be satisfactorily simulated by numerical calculation on the basis of an electric circuit model proposed by FitzHugh and by Nagumo et al.