Variation in the size of synaptic contacts along developing and mature motor terminal branches.

The secretion of a quantum from groups of release sites (me) declines along the length of terminal branches at the amphibian neuromuscular junction. The morphological basis of this decline in me has been studied at neuromuscular junctions in juvenile muscles (fibre length 4 mm) and adult muscles (fibre length 22 mm). Serial sections cut through the length of the junctions have been examined with both light and electron microscopy. Juvenile junctions consist of two short (less than 50 micron) terminal branches; adult junctions often consist of 4 long (100-500 micron) terminal branches. Synaptic contacts are largest near the origins of terminal branches and decline in size towards the end of branches. The number of horseradish peroxidase-labelled synaptic vesicles at release sites, following stimulation in the presence of the enzyme, is largest for sites closest to the origin of the terminal branches. The results suggest that the decline in me along the length of terminal branches is due to decline in the size of release sites.