Subthreshold Rectifier Properties of Nitella

Abstract

Electrotonic membrane potential and membrane impedance are recorded simultaneously on single cells of the excitable alga Nitella flexilis. “Subthreshold activity” can be demonstrated for Nitella. Reasons are given, why “subthreshold activity” should be looked upon as a growing instability towards threshold rather than as a sign for all-or-none responses taking place in a limited area of the membrane under investigation. Accommodation is found to have electrical signs: At the make of a constant, subthreshold current the electrotonic potential overshoots before slowly reaching its new equilibrium; the apparent d.c. resistance increases at the make of the subthreshold current and then — in parallel to the potential — decreases to reach a steady state value lower than at rest. Two possible interpretations for the observed electrical phenomena in connection with accommodation are suggested: a) cessation of the processes, which are labelled “graded activity” or b) the presence of a time lag in the establishment of a higher (passive) membrane conductance corresponding to the new current value. In Nitella the repolarization phase of the action potential as well as that of the subthreshold overshoot are slow phenomena in comparison with the membrane's initial time constant.