A central pattern-generating network contributes to "reflex-reversal"-like leg motoneuron activity in the locust

We introduce a new rhythmic preparation of the locust mesothoracic segment that exhibits long-lasting rhythmicity without pharmacological treatment. In most experiments, isolation of the locust mesothoracic ganglion from the anterior and posterior ganglia causes episodes of patterned activity to be generated in leg motoneurons that supply the femur-tibia (FT) joint. Flexor and extensor tibiae motoneuron pools exhibit alternating bursts of activity mostly composed of doublets and triplets of bursts. Motor activity during these episodes appears to be centrally generated because it persisted after complete deafferentation in 37% of the preparations; however, proprioceptive signals from the middle leg strongly influenced the patterning of motoneuron activity. Mimicking FT joint flexion by elongating the femoral chordotonal organ during an extensor burst terminates extensor activity and initiates flexor activity. The reverse is true for a mimicked extension during a flexor burst. This motor activity represents a reflex reversal that is typically observed in the locomotor state of the stick insect walking system. Our results provide evidence that this "reversed" reflex is caused by the action of central pattern-generating networks.