The formation and function of single transmitter release sites at mature amphibian motor-nerve terminals.

The extent of quantal transmitter release from single sites of synaptic vesicle accumulations along the length of motor-nerve terminal branches at the amphibian neuromuscular junction has been investigated. Such a determination involves development of a model for the generation of quantal potential fields at single styryl-dye stained sites along the length of a branch. Successful testing and application of this model indicates that the extent of quantal release at a dye-stained site is proportional to the total length of active zone at the site. The stability of these sites and of their ensheathing terminal Schwann cell processes was also investigated. Following simultaneous injection of the terminal Schwann cell and nerve terminal with different fluorescent dyes, terminal branches were observed to show dynamic changes in their length, with these occurring in relatively short periods of hours or less. Redistribution of styryl dye stained sites at the ends of branches also occurred in such short periods of time. These were accompanied by changes in the configuration of terminal Schwann cells, which generally occurred prior to changes in the length of nerve terminal branches.