Morphology of the giant interneurons and cercal nerve projections of the American cockroach.

We have investigated the morphology of the giant interneurons (GIs) and the main sensory projections to these interneurons in the American cockroach. These neurons are thought to mediate the animal's escape behavior. We describe here the dendritic branching pattern of each of the 14 GIs (7 bilateral pairs) in the terminal ganglion, the pattern of projection of the cercal sensory nerve, and the overlap of the cercal projections with the dendrites of the GIs. Visualization of the GIs and cercal nerve projection was accomplished by single cell injection and axonal backfilling with cobalt. Comparisons of the same identified GI in different animals show the position of the soma and the locations and orientations of the major processes are characteristic for each GI. The axons of the cercal nerve project to a well-defined ipsilateral region of the terminal ganglion. After entering the terminal ganglion, the cercal afferents split into lateral and medial tracts. The projections of the lateral cercal tract overlap extensively with the dendritic fields of the GIs. In contrast, the medial tract does not overlap the dendritic fields of the GIs in the posterior portion of the ganglion and shows only a small degree of overlap in the anterior portion. Correlations between physiological properties of the GIs and cercal afferents are discussed in relation to our anatomical findings.