Examining the timing of miniature endplate potential releases at the frog and mouse neuromuscular junctions for deviations from Poisson expectations.

The intervals between miniature endplate potentials (MEPPs) were measured at frog and mouse neuromuscular junctions in several different solutions. Data sets with monotonic trends in MEPP frequency were discarded. The remaining data sets had between 283 and 5024 MEPPs. The fit to the exponential distribution, which describes a Poisson process, was tested with Sherman's statistic. If the intervals are not distributed exponentially, this statistic indicates whether they deviate because they are more concentrated or more diffuse. In 6 of 20 data sets from the frog and in 3 of 7 data sets from the mouse the intervals between MEPPs were not distributed exponentially. Some of these were clustered, while others were diffuse. In one frog data set release was periodic. In all data sets releases appeared to be independent, judging from the integrated power spectra. One data set may show fractal behavior. Calculations of the approximate entropy gave no indication of an underlying regularity, so there is no evidence for a chaotic process. The lack of fit to the exponential distribution due to either concentrated or diffuse interval distributions is mimicked by a model in which release is Poisson, but with a mean rate that randomly shifts to a different level.