Modulation of the excitability of cockroach giant interneurons during walking - I. Simultaneous excitation and inhibition

1. The giant interneurons (GI's) of the cockroach Periplaneta americana are morphologically divisible into dorsal and ventral groups within each connective of the ventral nerve cord (VNC). During walking there is simultaneous excitation of dorsal GI's and inhibition of ventral GI's (Figs. 1, 2, 5 and 6). 2. The mean firing frequency of each of the six dorsal GI's increases as the animal's speed of walking increases. In an analogous manner, the amount of inhibition of the six large ventral GI's increases as the speed of walking increases (Figs. 6 and 7). 3. Computer-aided analysis of intracellular records of GI firing during walking in suspended preparations indicated that at the slow speeds of walking tested, spike activity of all three pairs of dorsal GI's is not closely coupled to any particular phase in the stepping cycle. 4. The inhibition of ventral GI activity doesn't build up over time. It begins when the animal begins stepping and stops when the animal stops stepping (Fig. 5) as has been shown previously for excitatory modulation of dorsal GI's (Delcomyn and Daley, 1979). 5. These results suggest that the morphological division of GI's into dorsal and ventral groups has a physiological correlate. The dual modulation of GI activity during walking is discussed in relation to the animal's escape behavior.