On the quantal hypothesis of neurotransmitter release: an explanation for the calcium dependence of the binomial parameters

Results for quantal neurotransmitter release can be explained by assuming a binomial distribution with a population of N elements each with a probability p to release a quantum in a given trial. The binomial parameter N was unexpectedly observed to depend on external calcium concentration and (to a lesser extent) on the frequency of stimulation. This observation is explained here by the hypothesis that the release population is not homogeneous. It is shown that the same hypothesis can also account for other experimental findings. A possible cause for this inhomogeneity is suggested.