Motor nerve terminals on abdominal muscles in larval flesh flies, Sarcophaga bullata: Comparisons with Drosophila

Motor nerve terminals on abdominal body-wall muscles 6A and 7A in larval fl esh flies were investigated to establish their general structural features with confocal microscopy, transmission electron microscopy, and freeze-frac ture procedures. As in Drosophila. and other dipterans, two motor axons sup ply these muscles, and two morphologically different terminals were discern ed with confocal microscopy: thin terminals with relatively small varicosit ies (Type Is), and thicker terminals with larger varicosities (Type Ib). In serial electron micrographs, Type Tb terminals were distinguished from Typ e Is terminals by their larger cross-sectional area, more extensive subsyna ptic reticulum, more mitochondrial profiles, and more clear synaptic vesicl es. Type Ib terminals possessed larger synapses and more synaptic contact a rea per unit terminal length. Although presynaptic dense bars of active zon es were similar in mean length for the two terminal types, there were almos t twice as many dense bars per synapse for Type Ib terminals. Freeze-fractu res through the presynaptic membrane showed particle-free areas indicative of synapses on the P-face, within which were localized aggregations of larg e intramembranous particles indicative of active zones. These particles wer e similar in number to those found at active zones of several other arthrop od neuromuscular junctions. In general, synaptic structural parameters stro ngly paralleled those of the anatomically homologous muscles in Drosophila melanogaster. In live preparations, simultaneous focal recording from ident ified varicosities and intracellular recording indicated that the two termi nals produced excitatory junction potentials of similar amplitude in a phys iological solution similar to that used for Drosophila. J. Comp. Neurol. 40 2:197-209, 1998. (C) 1998 Wiley-Liss, Inc.