PROBABILISTIC SECRETION OF QUANTA - SPONTANEOUS RELEASE AT ACTIVE ZONES OF VARICOSITIES, BOUTONS, AND END-PLATES

The amplitude-frequency histogram of spontaneous miniature endplate po tentials follows a Gaussian distribution at mature endplates. This dis tribution gives the mean and variance of the quantum of transmitter. A ccording to the vesicle hypothesis, this quantum is due to exocytosis of the contents of a single synaptic vesicle. Multimodal amplitude-fre quency histograms are observed in varying degrees at developing endpla tes and at peripheral and central synapses, each of which has a specif ic active zone structure. These multimodal histograms may be due to th e near synchronous exocytosis of more than one vesicle. In the present work, a theoretical treatment is given of the rise of intraterminal c alcium after the stochastic opening of a calcium channel within a part icular active zone geometry. The stochastic interaction of this calciu m with the vesicle-associated proteins involved in exocytosis is then used to calculate the probability of quantal secretions from one or se veral vesicles at each active zone type. It is shown that this procedu re can account for multiquantal spontaneous release that may occur at varicosities and boutons, compared with that at the active zones of mo tor nerve terminals.