FUNCTIONAL AND STRUCTURAL PARALLELS IN CRUSTACEAN AND DROSOPHILA NEUROMUSCULAR SYSTEMS

Comparison of morphological and physiological phenotypes of representa tive crustacean motor neurons, and selected motor neurons of Drosophil a larval abdominal muscles, shows several features in common. Crustace an motor nerve terminals, and those of Drosophila, possess numerous sm all synapses with well-defined active zones. In crustaceans, neurons t hat are more tonically active have markedly varicose terminals; synaps es and mitochondria are selectively localized in the varicosities. Pha sic motor axons have filiform terminals, sometimes with small varicosi ties; mitochondrial content is less than for tonic axons, and synapses are distributed along the terminals. Tonic axons generate small excit atory potentials which facilitate strongly at higher frequencies, and which are resistant to depression. The phasic neurons generate large e xcitatory potentials which exhibit relatively little frequency facilit ation, and depress rapidly. In Drosophila, counterparts of crustacean phasic and tonic motor neurons have been found, but the differentiatio n is less pronounced. It is inferred that cellular factors regulating the number of participating synapses and the probability of quantal re lease are similar in crustaceans and Drosophila, and that advantage ca n be taken of this in future to develop experiments addressing the reg ulation of synaptic plasticity.