The innervation of a ventral abdominal protractor muscle in Locusta

The innervation of the external ventral protractor muscle of the VIIth abdominal segment (M234) of Locusta migratoria is described using a combination of neurophysiological and neuroanatomical techniques. Cobalt backfills of the nerve innervating M234 revealed two neurons each with soma and fields of arborization in the VIIth abdominal ganglion. In addition, extracellular stimulation of the nerve while gradually increasing the stimulus amplitude resulted in a stepwise increase in both the excitatory junction potential amplitude and twitch amplitude so that two different amplitudes of each were observed indicating the sequential recruitment of two motor neurons. 4-Di-2-ASP stains of M234 revealed pre-synaptic boutons at M234 and a neurohaemal plexus covering the nerve the latter being corroborated by electron microscopic examination of nerve sections. Electron microscopic examination of M234 revealed two axon terminal types, one which is similar to the neurohaemal varicosities over the nerve, containing granules of high electron-density, and one which contains larger granules of medium electron-density. Both terminals types also contained small electron lucent vesicles. Finally, twitch contractions of M234 were modulated by glutamate, proctolin, octopamine, and SchistoF-LRFamide.Abbreviations EGAA Enhanced Graphics Acquisition and Analysis system by RC-Electronics, California - EJP excitatory junction potential - OA octopamine