SUMMARY

A motor programme is described for defensive kicking in the locust which is also probably the programme for jumping. The method of analysis has been to make intracellular recordings from the somata of identified motorneurones which control the metathoracic tibiae while defensive kicks are made in response to tactile stimuli. Three stages are recognized in the programme. (1) Initial flexion of the tibiae results from the low spike thresholds of tibial flexor motorneurones to tactile stimulation of the body. (2) Co-contraction of flexor and extensor muscles follows in which flexor and extensor excitor motorneurones spike at high frequency for 300-600 me. Flexor inhibitory motorneurones are inhibited. The flexor muscle thus holds the tibia flexed while the extensor muscle develops tension isometrically to the level required for a kick or jump. (3) trigger activity terminates the co-contraction by inhibiting the flexor excitor motorneurones and simultaneously exciting the flexor inhibitors. This causes relaxation of the flexor muscle and allows the tibiae to extend. If the trigger activity does not occur, the jump or kick is aborted, and the tibiae remain flexed