DIFFERENCES IN SYNAPTIC OUTPUT BETWEEN EXCITATORY AND INHIBITORY MOTONEURONS IN A CRAYFISH MUSCLE

A Fair of antagonistic motoneurons, one excitatory and one inhibitory, innervates the distal accessory flexor muscle in the walking limb of the crayfish Procambarus clarkii. The number and size of synapses form ed by these two axons on the muscle fibers (neuromuscular synapses) an d on each other (axe-axonal synapses) were estimated using thin-sectio n electron microscopy. Although profiles of nerve terminals of the two axons occur in roughly equal proportions, the frequency of occurrence of neuromuscular synapses differed markedly. 73% were excitatory and 27% were inhibitory. However, inhibitory synapses were 4-5 times large r than excitatory ones, and consequently, the total contact areas devo ted to neuromuscular synapses were similar for both axons. Axe-axonal synapses were predominantly from the inhibitory axon to the excitatory axon (86%), and a few were from the excitatory axon to the inhibitory axon (14%). The role of the inhibitory axe-axonal synapse is presynap tic inhibition, but that of the excitatory axo-axonal synapse is not k nown. The differences in size of neuromuscular synapses between the tw o axons may reflect intrinsic determinants of the neuron, while the si milarity in total synaptic area may reflect retrograde influences from the muscle for regulating synapse number.