Identified motor terminals in Drosophila larvae show distinct differences in morphology and physiology

In Drosophila, the type I motor terminals innervating the larval ventral lo ngitudinal muscle fibers 6 and 7 have been the most popular preparation for combining synaptic studies with genetics. We have further characterized th e normal morphological and physiological properties of these motor terminal s and the influence of muscle size on terminal morphology, Using dye-inject ion and physiological techniques, we show that the two axons supplying thes e terminals have different innervation patterns: axon 1 innervates only mus cle fibers 6 and 7, whereas axon 2 innervates all of the ventral longitudin al muscle fibers. This difference in innervation pattern allows the two axo ns to be reliably identified. The terminals formed by axons 1 and 2 on musc le fibers 6 and 7 have the same number of branches; however, axon 2 termina ls are approximately 30% Longer than axon 1 terminals, resulting in a corre sponding greater number of boutons for axon 2. The axon 1 boutons are appro ximately 30% wider than the axon 2 boutons, The excitatory postsynaptic pot ential (EPSP) produced by axon 1 is generally smaller than that produced by axon 2, although the size distributions show considerable overlap. Consist ent with vertebrate studies, there is a correlation between muscle fiber si ze and terminal size. For a single axon, terminal area and length, the numb er of terminal branches, and the number of boutons are all correlated with muscle fiber size, but bouton size is not. During prolonged repetitive stim ulation, axon 2 motor terminals show synaptic depression, whereas axon 1 EP SPs facilitate. The response to repetitive stimulation appears to be simila r at all motor terminals of an axon, (C) 2000 John Wiley & Sons. Inc. J Neu robiol 43: 186-197. 2000.