Estimates of quantal-release and binomial statistical-release parameters at rat neuromuscular junction.

Quantal-release and binomial statistical-release parameters were examined in the isolated rat diaphragm phrenic nerve preparation. The muscle resting potentials were reduced by cutting the muscle fibers to prevent muscle action potentials and contractions. The cutting technique was modified to allow persistent observation of miniature end-plate potentials (MEPPs) and end-plate potentials (EPPs) with intracellular recording techniques. Direct estimates of quantal release were examined and compared with predicted binomial and Poisson distributions. The results indicate that release is binomial when the nerve is stimulated with low- or high-frequency stimuli. The indirect method of estimating quantal release (variance method), which assumes release is described by a Poisson distribution, seriously overestimates quantal release. The binomial analysis indicates that the statistical store is small (less than 90 quanta) and that most of these quanta are released with each nerve impulse.