Localization of ventral giant interneuron connections to the ventral median branch of thoracic interneurons in the cockroach

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Abstract
A detailed morphological study was performed to localize the probable sites of connections between two identified populations of interneurons (ventral giant interneurons and type‐A thoracic interneurons) in the cockroach. Type‐A thoracic interneurons (TIAs) appear to play an important role in orienting the cockroach during windmediated escape. However, their large number, approximately 100 neurons, precludes analyzing each cell's role electrophysiologically. The TIAs are characterized by a prominent branch located on one or both sides of the ventral median (VM) region of the thoracic ganglion in which their soma resides. The presence of this ventral median branch can be used to predict connectivity with left or right ventral giant interneurons (vGIs) (Ritzmann and Pollack, 1988) and is correlated with the TIA's directional response to wind (Westin, Ritzmann, and Goddard, 1988), suggesting that this is the locus of synaptic connection. Two approaches were employed to address this hypothesis. Morphological overlap of differentially labelled cells (ethidium bromide, Lucifer Yellow) was examined at the light microscopic level to locate areas of possible synaptic contact. Experiments were also performed in which one‐half of the vGI input to the TIAs was surgically removed early in postembryonic development. Although no changes in the overall branching pattern were observed, the VM branches on the operated side were significantly shorter than were those on the unoperated side. Thoracic interneurons that do not receive inputs from vGIs were unaffected by this surgery. The data reported here thereby confirm previous observations by localizing the vGI inputs specifically to the VM branch, and provide a morphological cue for predicting connectivity and function.