Distributing coordinated motor outputs to several body segments: escape movements in the cockroach

In the escape behavior of the cockroach, all six legs begin to make directed movements nearly simultaneously. The sensory stimulus that evokes these leg movements is a wind puff. Posterior wind receptors excite giant interneurons that carry a multi-cellular code for stimulus direction — and thus for turn direction-to the three thoracic ganglia, which innervate the three pairs of legs. We have attemptd to discriminate among various possible ways that the directional information in the giant interneurons could be distributed to each leg's motor circuit. Do the giant interneurons, for instance, inform separately each thoracic ganglion of wind direction? Or is there one readout system that conveys this information to all three ganglia, and if so, might the identified thoracic interneurons, which are postsynaptic to the giant interneurons, subserve this function? We made mid-sagittal lesions in one or two thoracic ganglia, thus severing the initial segments of all the known thoracic interneurons in these ganglia, and thus causing their projection axons to the other thoracic ganglia to degenerate. This lesion did not sever the giant interneurons, however (Fig. 5). Following such lesions, the legs innervated by the intact thoracic ganglia made normally directed leg movements (Figs. 4, 6, 7). Thus, the projection axons of the thoracic interneurons are not necessary for normal leg movements. Rather, the giant interneurons appear to specify to each thoracic ganglion in which direction to move the pair of legs it innervates.