Graded limb targeting in an insect is caused by the shift of a single movement pattern
Journal of Neurophysiology 90: 1754 - 1765 (2003)
V. Dürr and T. Matheson
The task of making an aimed movement of a multi-jointed limb towards a target requires that the movement is regulated against external perturbations such as changing load. In particular, loading one part of a limb leads to altered static forces on all proximal segments, and to additional dynamic joint interaction forces when the limb moves. We have addressed the question of load compensation in an insect preparation in which a locust makes aimed scratching movements with a hind leg in response to tactile stimulation of a wing.
We show that loading the femur or tibia with a load equivalent to 8.5 times the mass of the tibia (corresponding to an increase of up to 11.6 times the rotational moment of inertia at the femur-tibia joint) does not impair the animal’s ability to make well coordinated, aimed movements of that leg towards different targets. The kinematics of the movements are the same, and animals aim the same part of their distal tibia at the target regardless of loading. The movements are carried out with equal accuracy and at the same initial velocity under all load conditions. Because loading of the leg does not change the behavioural performance, there is no indication of a change in aiming strategy. This implies high leg joint stiffness and/or the existence of high gain proprioceptive control loops. We have previously shown that in the unloaded condition, movements elicited by stimuli to different places on the wing are driven by a single underlying movement pattern that shifts depending on stimulus location along the wing surface. Our present data show that leg proprioceptive inputs are also integrated into the leg motor networks, rendering hind limb targeting robust against large changes in moment of inertia.