Interspike interval fluctuations in aplysia pacemaker neurons

In recent years, several mathematical models have been put forth to explain the time sequence of spike discharges in single neurons, in terms of synaptic inputs or intrinsic mechanisms. All of these models have been hypothetical, in that intracellular events were assumed, and not measured directly. The purpose of the present work was to study the statistics of the discharge from a preparation where intracellular recording was possible, and relate the observed discharge to measurable cell parameters. Regularly firing "pacemaker neurons" in the visceral ganglion of Aplysia californica were studied, using intracellular stimulating and recording techniques. Measurements were obtained of average curves of membrane potential, threshold for spike initiation, membrane resistance, and fluctuations of potential in the intervals between spontaneously occurring spikes. The timing of discharges from these neurons was described quantitatively by interspike-interval histograms, mean and standard deviation of intervals, skewness, and serial correlation coefficients. A mathematical model (contained in a simulation program for the IBM 7094 computer) was constructed, based on discrete fluctuations of membrane potential following each spike and other directly observed intracellular events. It was found that the model could quantitatively account for observed spike trains, including variations in the discharge from one cell to another.