CENTRAL EXCITATION OF COCKROACH GIANT INTERNEURONS DURING WALKING

Extracellular recordings from the ventral nerve cord of suspended or free-walking cockroaches (Periplaneta americana) reveal that a number of the giant interneurons in the cord become active during locomotion. We have studied this walking-activated excitation of these interneurons. 1. The initiation of firing of the giants as walking begins and the cessation of their activity as it stops is extremely rapid (Fig. 1), occuring within approximately 10 ms in many instances. 2. The average frequency of firing of the excited giants is higher during faster steps than during slower ones-i.e., is positively correlatd with speed of walking. This relationship is weakened by severing the ventral nerve cord in the abdomen (Figs. 2, 3). 3. Pinned-out, deafferented preparations may be induced to produce rhythmic activity in leg motoneurons similar to that observed during walking. One or more giant interneurons is excited during such rhythmic activity (Fig. 4), suggesting that sensory input from the moving legs during locomotion is not the main source of the excitatory signals. All this evidence supports the suggestion (Delcomyn, 1977) that the excited giants are driven by corollary discharge from the locomotor centers in the thorax. 4. Comparisons of the amplitudes of action potentials of those neurons active during spontaneous walking (Fig. 5) and those responding to puffs of air during rest (Fig. 6) suggested that not all the giants were active during walking. Intracellular recordings from individual giants confirmed this, revealing that only the three smaller, dorsal giants were excited (Fig. 7), while the three largest, ventral ones were not (Fig. 8). 5. The possible function of corollary discharge to the dorsal giants and the subsequent excitation of these interneurons during walking is discussed. Two possible functions are considered: 1) to keep the insect responsive to air disturbances during walking, and 2) to provide a general excitatory input to the central nervous system necessary to keep central excitatory state high during walking. © 1979 Springer-Verlag.