Functional and structural parallels in crustacean and Drosophila neuromuscular systems

Comparison of morphological and physiological phenotypes of representative crustacean motor neurons, and selected motor neurons of Drosophila larval abdominal muscles, shows several features in common. Crustacean motor nerve terminals, and those of Drosophila, possess numerous small synapses with well-defined active zones. In crustaceans, neurons that are more tonically active have markedly varicose terminals; synapses and mitochondria are selectively localized in the varicosities. Phasic motor axons have filiform terminals, sometimes with small varicosities; mitochondrial content is less than for tonic axons, and synapses are distributed along the terminals. Tonic axons generate small excitatory potentials which facilitate strongly at higher frequencies, and which are resistant to depression. Thephasic neurons generate large excitatory potentials which exhibit relatively little frequency facilitation, and depress rapidly. In Drosophila, counterparts of crustacean phasic and tonic motor neurons have been found, but the differentiation is less pronounced. It isinferred that cellular factors regulating the number of participating synapses and the probability of quantal release are similar in crustaceans and Drosophila, and that advantage can be taken of this in future to develop experiments addressing the regulation of synaptic plasticity.