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William H. Calvin
University of Washington
Seattle WA 98195-1800 USA
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Cat lumbrosacral motoneurons were studied with intracellular microelectrodes in an attempt to identify the sources of interspikeinterval variability observed in these cells.
All neurons studied exhibited the haphazard membrane potential fluctuations termed synaptic noise. Further, all neurons exhibited marked fluctuations in the interspike interval. The interspike-interval histograms were approximately gaussian with a standard deviation approximately 5% of the mean interval, and successive intervals were uncorrelated.
In one class of cells the spike-generator mechanism appeared to be adequately described by a model consisting of a linearly increasing depolarization that generates a spike on reaching a constant firing level and is then reset. In a second class of cell the firing level increased approximately linearly with time since the last spike, and this feature had to be included in the model of spike generation.
If measured properties of synaptic noise are combined with the measured parameters of the descriptive model for spike generation it is possible to make predictions of the neuron's interspike-interval variability. Close agreement of observed and predicted histograms was obtained for both classes of cells. It may thus be concluded that the presence of synaptic noise is adequate to account for the observed variability, and that other sources of variability make only negligible contributions.
In two instances synaptic noise alone was not adequate to account for the observed interspike-interval variability, but further analysis revealed an additional source that would account for the variability. In one case a multimodal histogram appeared to arise from "local responses" that tended to occur at preferred times. In the other instance the rate of depolarization appeared to change from interval to interval; by including this additional noise source it was possible to account for the observed variability.