Frequency of Miniature End Plate Potentials Determined by an Extension of Campbell’s Theorem

Abstract

A method based upon an extention of Campbell’s theorem is used to measure the time course, amplitude and frequency of occurence of miniature end plate potentials (mepp) at rapidly secreting neuromuscuIar junctions in frog cutaneous pectoris muscles. Measurements of the SKEW (third central moment), variance and power spectrum of the fluctuations in membrane potential are used to determine the mepp parameters. The resulting estimates of mepp amplitude and frequency are insensitive to slow drifts in membrane potential that preclude the conventional application of Campbell’s theorem, which uses the MEAN and variance. The new method fails at very high mepp frequencies because the distribution of the values of the membrane potential approaches a Gaussian, and the measurements of the SKEW lose statistical significance. The frequency at which this failure occurs is raised to about 104s-1 if the data are high-pass filtered. The method has been tested on computer-simulated data, applied to junctions exposed to La3+, and the effects of Ca2+ on the La3+-induced secretion have been explored. Some muscles were fixed after treatment with La3+, and the changes in nerve terminal intrastructure were assessed by morphometric analysis of electron micrographs. Horseradish peroxidase (HRP) was used to obtain direct information about vesicle recycling. Most experiments were done at room temperature (22–25°C) using solutions that contained 0.1mN La3+, a concentration that caused enormous increases in mepp frequency. When the rate of secretion was at its peak, these frequencies sometimes exceeded 5x10+3s-1 and mepp amplitudes were reduced by over 60%. Several million quanta were secreted from single nerve terminals during an hour, ie several times the number of synaptic vesicles in control terminals. The number of vesicles in the experimental terminals fell by 75% or more, the axolemma expanded to form extensive folds that penetrated deep into the axoplasm, and some of the remaining vesicles were labelled with HRP if this extracellular tracer had been present in the bathing solution. Ca2+-free solutions increased the rates at which these changes developed, but had relatively little effect on the final state of the terminals.