The dorsal unpaired median neurons of the locust metathoracic ganglion: neuronal structure and diversity, and synapse distribution

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Abstract
Dorsal unpaired median (DUM) neurons are bilaterally symmetrical. A single primary neurite arises from the soma and runs anteriorly through the neuropil before dividing into two lateral neurites which pass to the nerve roots on each side of the ganglion. The primary neurite runs in one of two tracts, one of which lies further from the surface of the ganglion than the other. The primary neurites in the deeper tract belong to DUM1, DUM5 and DUM3,4,5 neurons, and those in the more superficial tract, to DUM3, DUM3,4 and DUM3,4,5 neurons. Previous studies have shown that in the developing embryonic nervous system the primary neurites of DUM neurons can also be observed to lie in one of two tracts, but these do not appear to correspond to those seen in the adult. The results described here differ further from those of other investigations of adult and embryonic locusts in that no DUM4,5 neurons were seen, but DUM3,4 neurons, not found in previous studies, were frequently stained. The secondary neurites of DUM neurons characteristically give rise to fine 0.2–0.5 μm diameter processes which may run for hundreds of microns through the neuropil with very little branching. The problems this may pose for signal transmission along such processes is discussed. Presynaptic processes of several types make inputs on to spines on the lateral neurites of DUM neurons and on to branches from secondary neurites. Output synapses were rarely observed and were found only on lateral neurite spines. It therefore appears unlikely that the DUM neurons examined play a major central role within the metathoracic ganglion. A novel structure, with the appearance of a presynaptic density but which was not associated with synaptic vesicles, was found in certain regions of the neurons.