Neurons of the Drosophila giant fiber system: I. Dorsal longitudinal motor neurons.

The giant fiber system (GFS) mediates the startle response of Drosophila. This response includes an activation of the dorsal longitudinal wing-depressor muscles (DLMs). However, the morphology of the motor neurons innervating these muscles has not been well studied. Even the location of the somata of these motor neurons has been a source of controversy. This paper identifies the somata and provides a morphological description of these motoneurons. The DLM is comprised of six muscle fibers, named a through f (dorsal to ventral). Each muscle fiber is singly innervated. Each of the four ventral muscle fibers is innervated by a separate motor neuron (DLMn c-f), but the two dorsal fibers share an axon (DLMn a/b). Motor neurons were back filled by introducing horseradish peroxidase (HRP) into individual muscle fibers. The cell body of DLMn a/b is extraordinarily large (32 microm) and lies dorsal and contralateral. In this hemiganglion, it does not have a fixed position; it can be found anywhere from the midline to the extreme lateral edge of the ganglion. The position is not genetically controlled: We find no strain differences, and, within a single individual, the right and left cells may take different positions. The neuritic arborization fills a shallow dorsal cap of the ganglion, with branches arrayed like a feather. The cell bodies of the four motor neurons c-f lie in an ipsilateral and ventral cluster. Each soma occupies a fixed corner of this quadrilaterally shaped cluster. The neurites ramify in the same dorsal region as DLMn a/b.