Central and peripheral control of the trigger mechanism for kicking and jumping in the locust

A crucial stage of the locust kick motor program is the trigger activity th at inhibits the flexor motorneurons at the end of flexor-extensor coactivat ion and releases the tibia. One source of this inhibition is the M interneu ron, which produces a spike burst at the time of the trigger activity. Prev ious work has suggested that sensory input resulting from extensor muscle t ension may contribute to the M spike burst. We find that extensor muscle te nsion produced during thrusting behavior or by direct electrical stimulatio n with the tibia held fixed results in the depolarization of M, but this is not of sufficient amplitude to account for the M spike burst during the tr igger activity. Furthermore, M still produces a spike burst after ablating the sensory systems that produce the response to the muscle stimulation. It is concluded that the major component of the M trigger activity is central in origin, although sensory feedback from extensor muscle tension makes so me contribution. The combination of both central and peripheral paths for M activation may enhance the robustness of the behavior. (C) 1999 Wiley-Liss , Inc.