Neural Mechanisms of Task Specific Changes in Sensorimotor Processing
Final Report Abstract
With respect to the question, how the influence and processing of feedback from leg proprioceptors changes during curve walking, we showed that the transition between part I and part II of the AR always occurs after a specific angular excursion relative to the starting angle in the FTi-joint, i.e. at smaller absolute FTi angles with decreasing starting angles and independent of the turning direction. Flexion signals from the fCO thus promote the generation of AR to assist stance activity, however, without requiring a fixed set point for the transition between the two parts (Schmitz et al. submitted). With respect to body-side specificity of the processing of load feedback, we demonstrated that processing of stimuli mimicking leg loading during touch down is the same during outside and forward stepping. In contrast, the same stimulus elicits either RetCx MN activation, ProCx MN activation, or even no response of either MN pool during inside steps of the same animal. This matches the kinematically observed variability in stepping directions for inside legs, from forward, through sideward to backward steps. We further showed that in inside or outside stepping of the curve stepping animal, the probability for AR increases with increasing starting angle, decreasing stimulus velocity, and it is independent of the total angular excursion. However, for all three tested parameters, at all starting angles, ramp velocities and ramp amplitudes, the probability for AR was significantly higher in inside over outside MLs. This suggests a task-dependent shift in the gain for the generation of ARs. To investigate the underlying neural mechanisms, we focused on potential modifications in the short latency excitatory connections between fCO sensory neurons and tibial extensor MNs. The present results cannot yet identify the origin of the increased likelihood for the generation of the AR on the inside compared to the outside in curve walking, and are still under ongoing research in the lab. Investigation of the influence of load feedback from the different groups of ML CS onto local MN pools revealed the following: 1) different CS groups along the stick insect leg act to activate muscle synergies and show task-dependent changes in their effects. Such task-specific reinforcement of muscle synergies can ensure rapid substrate adhesion to provide a stable point for force generation. 2) femoral CS of the stick insect ML discharge both, to forces directed posteriorly, and in the CTr joint plane, supporting the idea that receptors encode forces in the plane of action of leg muscles used synergistically in body support and propulsion. 3) This principle was found in all legs, albeit with differences in strength of the influence. Applying forces using torque waveforms supports the idea that CS act to synchronize muscle synergies to the range of force magnitudes and dynamics that occur in each leg. 4) We identified all CS groups capable of affecting ProCx and RetCx MNs. Together with results from dye fills of trochanteral, femoral and tibial CS groups, which revealed a common projection pattern with little group specificity, the results support the idea that the influences of CS feedback are determined by the activities of pre-motor interneurons to facilitate fast and task dependent behavioral adaptation.
Publications
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(2015) Force feedback reinforces muscle synergies in insect legs. Arthropod Structure and Development 44: 541-53
Zill SN, Chaudry S, Büschges A, Schmitz J
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(2016) Body side-specific control of motor activity during turning in a walking animal. eLife
Gruhn M, Rosenbaum P, Bockemühl T, Büschges A
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(2017) Effects of force detecting sense organs on muscle synergies are correlated with their response properties. Arthropod, Structure and Development 46: 564-578
Zill SN, Neff D, Chaudry S, Exter A, Schmitz J, Büschges A
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(2018) Force dynamics and synergist muscle activation in stick insects: the effects of using joint torques as mechanical stimuli. J. Neurophysiol. 120, 1807-1823
Zill SN, Dallmann CJ, Büschges A, Chaudhry S, Schmitz J
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(2019) Identification of the origin of force feedback signals influencing motor neurons of the thoraco-coxal joint in an Insect. J. Comp. Physiology A 205(2), 253-270
Haberkorn A, Zill SN, Gruhn M, Büschges A
