NEURONS OF THE DROSOPHILA GIANT FIBER SYSTEM .1. DORSAL LONGITUDINAL MOTOR-NEURONS

The giant fiber system (GFS) mediates the startle response of Drosophi la. This response includes an activation of the dorsal longitudinal wi ng-depressor muscles (DLMs). However, the morphology of the motor neur ons innervating these muscles has not been well studied. Even the loca tion of the somata of these motor neurons has been a source of controv ersy. This paper identifies the somata and provides a morphological de scription of these motoneurons. The DLM is comprised of six muscle fib ers, 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 horsera dish peroxidase (HRP) into individual muscle fibers. The cell body of DLMn a/b is extraordinarily large (32 mu m) and lies dorsal and contra lateral. In this hemiganglion, it does not have a fixed position; it c an be found anywhere from the midline to the extreme lateral edge of t he ganglion. The position is not genetically controlled: We find no st rain 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 feath er. The cell bodies of the four motor neurons c-flie in an ipsilateral and ventral cluster. Each soma occupies a fixed corner of this quadri laterally shaped cluster. The neurites ramify in the same dorsal regio n as DLMn a/b. (C) 1997 Wiley-Liss, Inc.