Changes in the dimensions of release sites along terminal branches at amphibian neuromuscular synapses

The probability of transmitter secretion from release sites declines along the length of most long terminal branches (greater than 78 micron) at toad (Bufo marinus) neuromuscular junctions; in contrast, few short terminal branches (less than 78 micron) show such a decline. The present study was carried out to see if any of the dimensions of release sites change along the length of terminal branches in a way that can be correlated with the decrease in secretion probability. The size of presynaptic release site structures was determined by examining serial transverse sections through entire terminal branches with the transmission electron microscope; the size of postsynaptic release site structures was determined by examining terminal gutters with the scanning electron microscope after the removal of terminal branches. Long terminal branches showed a significant decrease in the length of their synaptic contact and cross-sectional area (terminal size) with distance from the origin of the branch. In contrast, there was no significant difference in the length of close apposition (less than 0.2 micron) between the nerve terminal and postsynaptic muscle membrane; furthermore, neither the length of postsynaptic folds nor the frequency of the folds along the length of the terminal gutter changed. Short terminal branches showed no significant differences in the dimensions of either presynaptic or postsynaptic release site structures. The decline in the length of synaptic contacts whilst the length of close apposition remains relatively constant is due to the progressive encroachment of Schwann cell processes between presynaptic and postsynaptic membranes along the length of long terminal branches.