Pattern generation for walking and searching movements of a stick insect leg. I. Coordination of motor activity

During walking, the six legs of a stick insect can be coordinated in differ ent temporal sequences or gaits. Leg coordination in each gait is controlle d and stabilized by coordinating mechanisms that affect the action of the s egmental neuronal networks for walking pattern generation. At present, the motor program for single walking legs in the absence of movement-related co ordinating intersegmental influences from the other legs is not known. This knowledge is a prerequisite for the investigation of the segmental neurona l mechanisms that control the movements of a leg and to study the effects o f intersegmental coordinating input. A stick insect single middle leg walki ng preparation has been established that is able to actively perform walkin g movements on a treadband. The walking pattern showed a clear division int o stance and swing phases and, in the absence of ground contact, the leg pe rformed searching movements. We describe the activity patterns of the leg m uscles and motoneurons supplying the coxa-trochanteral joint, the femur-tib ial joint, and the tarsal leg joints of the middle leg during both walking and searching movements. Furthermore we describe the temporal coordination between them. During walking movements, the coupling between the leg joints was phase-constant; in contrast during searching movements, the coupling b etween the leg joints was dependent on cycle period. The motor pattern of t he single leg generated during walking exhibits similarities with the motor pattern generated during a tripod gait in an intact animal. The generation of walking movements also drives the activity of thoraco-coxal motoneurons of the deafferented and de-efferented thoraco-coxal leg joint in a phase-l ocked manner, with protractor motoneurons being active during swing and ret ractor motoneurons being active during stance. These results show that for the single middle leg, a basic walking motor pattern is generated sharing s imilarities with the tripod gait and that the influence of the motor patter n generated in the distal leg joints is sufficient for driving the activity of coxal motoneurons so an overall motor pattern resembling forward walkin g is generated.