Central Nervous Control of Cockroach Walking

Abstract

During the past few years there has been a concerted effort to determine the neuronal circuit of and the neurophysiological basis for rhythmic leg movements in walking insects. In the cockroach there is strong evidence that rhythmic motor patterns, similar to those occurring during normal walking, spontaneously occur after complete deafferentation of the segmental ganglion. This led to the hypothesis that there are segmental rhythm generators that operate independently from the segmental sensory input. Subsequent morphological and physiological investigations of the metathoracic ganglion of the cockroach demonstrated the presence of interneurons whose membrane potentials oscillate during rhythmic leg movements. One interneuron, Interneuron I, oscillates such that its cyclic depolarizations are synchronized with the flexion phase (return stroke) of the rhythmic leg movements. When experimentally depolarized, Interneuron I drives and specifically recruits the flexion motoneurons in the same sequence as in normal walking. Interneuron I also appears to be an integral part of the neuronal circuit generating the rhythmic leg movements since short depolarizing pulses in Interneuron I can reset the cycle time of the rhythmic leg movements. An unusual phenomenon of Interneuron I and indeed of most interneurons within the metathoracic ganglion of the cockroach is that they are not excitable, that is they do not produce nerve spikes. These interneurons modulate activity in motoneurons by minute changes in membrane potentials. This is the first locomotory system in which identified nonspiking neurons have been demonstrated to play an important role in generating a specific motor pattern.