The organization of giant horizontal-motion-sensitive neurons and their synaptic relationships in the lateral deutocerebrum of Calliphora erythrocephala and Musca domestica

Three giant horizontal-motion-sensitive (HS) neurons arise in the lobula plate. Their axons terminate ipsilaterally in the medial deutocerebrum and suboesophageal ganglion. Both Golgi impregnations and cobalt fills demonstrate that endings of the two HS cells, representing the upper and middle third of the retina, differ in shape and location from that of the HS cell subtending the lower third of the eye. This dichotomy is reflected by the terminals of a pair of centrifugal horizontal cells (CH), one of which invades lobula plate neuropil subtending the upper two-thirds of the retina. The other overlaps the dendrites of the HS cell subtending the lower one-third of the retina. The HS cells are cobalt-coupled to a variety of complexly arborizing descending neurons. In Musca domestica, gap-junction-like apposition areas have been observed between HS axon collaterals and descending neuron dendrites. The three HS cells also share conventional chemical synapses with postsynaptic elements, which include the dendritic spines of descending neurons. Unlike the giant vertical-motion-sensitive neurons of the lobula plate, whose relationships with descending neurons appear to be relatively simple, the horizontal cells end on a large number of descending neurons where they comprise one of several different populations of terminals. These descending neurons terminate within various centres of the thoracic ganglia, including neuropil supplying leg, neck, and flight muscle.