Structure of regenerated synaptic connections of allotransplanted phasic motoneurons on a tonic muscle in crayfish

grantor: University of TorontoCrustacean neuromuscular systems are highly defined into phasic and tonic types exemplified by the deep and superficial flexor muscles in the abdomen of the crayfish 'Procambarus clarkii.' The deep phasic muscle is used occasionally and briefly for escape responses while the superficial tonic muscle is used continuously for routine movements and maintaining posture. To assess the relative importance of the motoneuron and the target muscle in determining the synaptic phenotype of regenerating neuromuscular connections, we attempted a mismatch experiment in which the denervated tonic SFM received an allotransplanted phasic flexor nerve. A quantitative study of various structural parameters of these regenerated synapses using serial section electron microscopy revealed that these synapses, on average, possessed two dense bars that were often closely spaced. Mitochondria comprised a relatively small (6%) volume of the terminals. Collectively, these structural features resemble those of native phasic synapses, thereby corroborating the physiological evidence for the regeneration of phasic type synapses. Therefore, the motoneurons and not their target muscle fibers are most influential in specifying the phenotype of regenerated synapses.M.Sc