Nitella fluctuation and instability in the membrane potential near threshold.

Excitation of Nitella internodal cell was investigated as an example of the phase transition in an open system far more thermal equilibrium. The power density spectrum of the membrane potential fluctuation had a bulge in a frequency range lower than 1 Hz at the resting state and a peak at approximately 0.03 Hz at a depolarized state near the threshold. A critical oscillation in the membrane potential was observed when threshold was gradually approached from the resting state. Repetitive firing was observed under a step-current of the superthreshold value. The frequency of spectral peaking, critical oscillation, and repetitive firing agree well with each other. The result suggests that the hard-mode instability occurs in the Nitella internodal cell. The membrane impedance had no peak in the same frequency region as the peak of the voltage spectrum. The spectral peak may be ascribed to be electrogenic pump modulated by the metabolic feedback system in photosynthesis.