Projection patterns of posterior dorsal unpaired median neurons of the locust subesophageal ganglion.

Abstract

Six neurons in a group of dorsal unpaired median (DUM) neurons with cell bodies in the posterior part-maxillary and labial neuromeres-of the subesophageal ganglion of locusts have two axons each that descend into both the left and the right halves of the ganglia of the ventral nerve cord. None of the neurons has peripheral axons, so they are interneurons. Electrophysiology shows that the axons of at least four neurons project to the terminal abdominal ganglion to which they conduct spikes at a velocity of 0.5-0.6 m. second(-1). In the somata, the spikes have a smaller amplitude and briefer duration at half height than the spikes of thoracic, efferent DUM neurons. Each neuron has bilaterally symmetrical branches within the subesophageal ganglion and in the thoracic ganglia. On the basis of the specific patterns of branches, and the neuropiles, tracts, and commissures in which they occur, three types of neurons (DUM SD 1-3) can be recognized. DUM SD 1 and 3 project to ventral regions of neuropile in the thoracic ganglia in which the efferent DUM neurons of these ganglia have no branches. DUM SD 2 projects to dorsal neuropiles. The projection patterns of these putatively octopaminergic neurons suggest that they could be the source of the octopaminergic modulation of networks underlying sensory processing and motor pattern generation within these ganglia. Within this group of posterior DUM neurons, two additional cells were stained that have axons ascending to the brain.