Non-Poisson distribution in time of small- and large-mode miniature end-plate potentials.

Minature end-plate potentials (MEPP's) were recorded from frog (Rana pipiens) sartorius muscle cells of 60--80 mum diam; MEPP frequency remained constant up to 3 h and was 100 times greater in 80-mum cells than in 6-mum cells. Rapid cooling or heating of preparations initiated or accentuated bursts of MEPP's. Records from cells under control conditions and from cells where the MEPP population had been reduced to the small mode showed many instances of small clusters of MEPP's occurring within 100-ms intervals, indicating quantal release interdependencies. Some small cells initially generated many large MEPP's 2 or more times the average amplitude and fewer small clusters. Analysis of the occurrence of both large- and small-mode MEPP's for both 1.0-s and 100-ms nonoverlapping bins demonstrated a highly significant deviation from a Poisson distribution with significance increasing with decreasing cell diameter and/or MEPP frequency. It is suggested that doublet MEPP's are generated from one release site and that large bursts results from the activity of several release sites, since the size of bursts decreases with cell diameter (junctional area). It is concluded that release characteristics are the same for large-mode (classical) and small-mode MEPP's and that the occurrence of MEPP's is not Poisson.