Probability of quantal secretion and the mobilization of vesicles at the active zones of endplates.

A vesicle mobilization model is developed for active zones of motor-nerve terminals. In the model, vesicles from a storage pool are recruited to an available pool where they are docked at the active zone, the latter pool being replenished by endocytosis. The predictions of this model are then compared with experimental results on these active zones. If a styryl dye (FM1-43) is used to stain all of the vesicles at motor-nerve terminals then the stores of vesicles associated with different active zones are shown to vary considerably in size. Under the assumption that the available pool size is proportional to the storage pool size, the mobilization model shows that the extent of destaining of these stores during a particular period of stimulation is proportional to the size of the storage pool, this size being indicated by the initial staining. This is not so under the alternative assumption that the available pool size does not vary proportionally with store size. If the two pools are proportional then the number of quanta released from the available pool is proportional to the storage pool size. Experimental results indicate that there is a linear relationship between destaining and staining, as well as between quantal release and staining at different sites along motor-nerve terminals, thus favouring the hypothesis that available pool size (or the size of the active zone) and store size are proportional. Given that there are very different store sizes along the length of motor-nerve terminals, this analysis provides further evidence for the non-uniform probability of quantal secretion at different active zones within a single motor-nerve terminal.