Connectivity pattern of the cercal-to-giant interneuron system of the American cockroach

The pattern of connectivity between identified cercal afferents and the three largest giant interneurons (GIs 1, 2, and 3) of the American cockroach was investigated by intracellular methods. These three GIs all have different directional response sensitivities and appear to be especially important in initiating the short latency escape behavior of the American cockroach. One of the interneurons, GI 1 responds to wind from all four quadrants of space about the animal. However, it clearly shows a greater ipsilateral versus contralateral (relative to the GI's axons within the nerve cord) wind sensitivity. In contrast, the directional sensitivity of GI2 is more nearly bilaterally symmetrical. Both of these interneurons receive excitatory synaptic input from the sensory cells of the nine most prominent columns (a, d, g, f, h, i, k, l, and m) of filiform hairs of the ipsilateral cercus. The nine ipsilateral inputs all made roughly equivalent strength excitatory connections with GI 1. The connectivity pattern to GI 2 was the same as that to GI 1 except that the connection strength for two of the nine columns, h and i, was substantially stronger to GI 2 than to GI 1. The remaining seven sensory columns all make equivalent strength connection with GI 2. Only select columns of contralateral sensory cells made synaptic connection with GIs 1 and 2. All detectable connections produced subthreshold depolarizations. The response curve of GI 3 is more sharply restricted in space than that of either GI 1 or 2 and this interneuron only responds to wind stimuli originating from in front of the animal. GI 3 received excitatory synaptic input only from ipsilateral columns d, f, g, i, and k, all of which have their best excitatory directions well within the boundaries of the response curve of GI 3. Columns a and l with best excitatory directions near the edges of the response curve of GI 3 made no detectable connection. The remaining two columns (h and m) with best excitatory directions well outside the boundaries of the response curve of GI 3 provided inhibitory input. GI 3 received synaptic input from contralateral columns d, f, g, h, i, k, and m. Columns d, f, g, i, and k produced subthreshold depolarizations when six hairs of a column were stimulated. Contralateral columns a and l do not make detectable connection. Contralateral columns h and m made inhibitory connection with GI 3. Plots of the directional sensitivities of GIs 1, 2, and 3 were computer constructed based on a simple connectivity model. Each sensory column was represented in the model by a half cosine function which was multiplied by the functional strength of connection. These weighted curves were then summed and plotted. A close fit exists between the real and calculated curves suggesting 1) the subpopulation of sensory cells studied was representative of the entire population, and 2) the GIs act as passive receivers since no complex postsynaptic integration was needed in the model.