Neuronal and biomechanical mechanisms underlying the control and plasticity of aimed limb movements

Most animal movements are driven by muscle contractions controlled by the nervous system, but there is now considerable evidence that passive forces, originating in muscles, tendons or other tissues, interact with active forces to generate limb movements in both vertebrates and invertebrates (Page et al. 2008, J Neurosci). We developed a neuromechanical model (simulation) of the locust hind leg that predicted an important role for passive forces in generating aimed limb movements (Zakotnik et al. 2006, J Neurosci), and have now shown experimentally that meaningful movements can indeed be generated by passive forces. Very surprisingly, some of these forces arise within joints themselves and not in the muscles or tendons (Ache & Matheson, 2012 J Neurophysiol, 2013, Curr Biol). These passive joint forces move the leg in the absence of motor control (and even in the absence of muscles). Across species, where antagonist muscles have different strengths, passive joint forces support the weaker muscle. We therefore suggest that passive joint forces are shaped by evolutionary adaptation and form an important component of effective motor control.

In related work, we have shown that locusts ’recalibrate’ their aimed limb movements following experimental disruption of joint proprioceptors. This plasticity allows animals to regain accurate movements in the face of sensory loss. Young adult locusts can similarly adjust their movements following the loss of part of a limb (Gunderson et al. 2011, Proc Göttingen Neurobiol Conf). Older adults do not readjust their movements, indicating that the ability to express plastic changes in motor control varies with age. This is extremely interesting in light of our demonstration that during swarm formation (when locusts undergo remarkable plastic changes in morphology and behaviour collectively known as ‘phase change’) there are marked alterations of walking gait and posture (Blackburn et al. 2010, J Ins Physiol).

This PhD project will merge these strands of research to seek the mechanisms governing plasticity of motor control using our powerful locust phase change model system. The student will measure active and passive limb forces in a range of species to test further our hypothesis that passive joint forces are matched to active forces acting at the same joint. Limb morphology and function will be investigated using micro CT-scanning, coupled with finite element analysis. Our predictive model will be iteratively modified to incorporate passive joint forces, and used to examine experimentally intractable questions. The model will be extended to simulate limbs that have different geometries, behavioural functions and patterns of motor control. Is our current understanding of the interplay of forces sufficiently robust to enable the model to predict the movements generated by different limbs subject to specific patterns of natural motor output? The project will investigate changes in passive forces in animals that learn new movement strategies to deal with limb damage. Is neuronal plasticity matched by plasticity in biomechanical properties? Do passive biomechanical properties change during development? Do our new results generalise across different limb joints? How does inhibitory motor control (Calas-List et al. 2013, J Neurosci) bias passive muscle properties? Does the induction of phase change induce motor plasticity in older adult locusts? Are changes in neuromodulator levels during phase change (Rogers et al. 2004, J Exp Biol; Anstey et al. 2009, Science) important in regulating this motor plasticity?

In this Systems Biology project the student will be trained in both in vivo techniques (behavioural, biomechanical and electrophysiological experiments) and mathematical modelling. The modelling will be supported through a longstanding collaboration with Prof Volker Dürr in the Department of Biological Cybernetics at the University of Bielefeld, and a new collaboration with Jingzhe Pan in the College of Science and Engineering. The project will thus use analysis techniques drawn from distinct behavioural, engineering and neurobiological disciplines to address fundamental questions in limb motor control, viewed from a comparative (evolutionary) functional perspective.