Multi-joint coordination during walking and foothold searching in the Blaberus cockroach. II. Extensor motor neuron pattern

In a previous study, we combined joint kinematics and electromyograms (EMGs ) to examine the change in the phase relationship of two principal leg join ts during walking and searching. In this study, we recorded intracellularly from motor neurons in semi-intact behaving animals to examine mechanisms c oordinating extension at these leg joints. In particular, we examined the c hange in the phase of the coxa-trochanter (CTr) and femur-tibia (FT) joint extension during walking and searching. In doing so, we discovered marked s imilarities in the activity of CTr and FT joint extensor motor neurons at t he onset of extension during searching and at the end of stance during walk ing. The data suggest that the same interneurons may be involved in coordin ating the CTr and FT extensor motor neurons during walking and searching. P revious studies in stick insects have suggested that extensor motor neuron activity during the stance phase of walking results from an increase in ton ic excitation of the neuron leading to spiking that is periodically interru pted by centrally generated inhibition. However, the CTr and FT extensor mo tor neuron activity during walking consists of characteristic phasic modula tions in motor neuron frequency within each step cycle. The phasic increase s and decreases in extensor EMG frequency during stance are associated with kinematic events (i.e., foot set-down and joint cycle transitions) during walking. Sensory feedback associated with these events might be responsible for phasic modulation of the extensor motor neuron frequency. However, our data rule out the possibility that sensory cues resulting from foot set-do wn are responsible for a decline in CTr extensor activity that is character istic of the Blaberus step cycle. Our data also suggest that both phasic ex citation and inhibition contribute to extensor motor neuron activity during the stance phase of walking.