Plasticity in aimed limb movements

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The overall goal of this proposal is to develop our understanding of how nervous systems generate aimed limb movements. I have developed a particularly powerful invertebrate model system that permits us to couple quantitative movement analyses with a range of extracellular and intracellular recoding techniques in animals that carry out their normal behaviour. We will focus on two main issues. First, we will analyse plasticity in aimed scratching movements and underlying neuronal networks, across three time scales. Second, we will test the hypothesis that regulation of leg joint stiffness is critical for successful aiming towards a target. To analyse plasticity, we will first define the ability of locusts to modify their movement kinematics in response to instantaneous changes in body orientation, leg loading or leg damage. Locusts are marked with retroreflective markers that define the body and leg, and the movements are videotaped. We use customised motion capture software to process the images and automatically detect the limb movements so that we can generate very large datasets for analysis. From the movement kinematics we use inverse dynamics calculations to compute the joint torques, which will be related to the patterns of motor activity recorded extracellularly from the leg muscle using standard techniques. We have already implemented the basic features or the inverse dynamics procedure in a Matlab program. Once we have defined the mechanisms underlying the rapid compensation, we will examine how the behaviour and neuronal control networks are recalibrated over hours and weeks following various perturbations, including limb damage and precise surgical manipulation of proprioceptive feedback from an identified joint receptor. We have already developed the surgical technique, and will use standard electrophysiological methods to record the responses of the sensory neurons. To analyse developmental plasticity at the time of the last moult into adulthood, we will compare movements kinematics and patterns of motor activity in the same individuals before and after moulting. We will use paired intracellular and extracellular recordings, and dye backfilling to define the patterns of branching and synaptic connectively of juvenile and adult wing hair sensory neurones. To analyse joint stiffness we will make direct measurements of the forces required to perturb the limb during an active movement. To do so will require the development of a miniature closed feedback torque-controller. This will be technically challenging, so we also plan to calculate joint stiffness from measurements made using standard force transducers in a much simpler setup. We will also record intracellularly from the leg motor neurones restrained and perhaps reduced preparation so that we can control their firing rates and manipulate joint stiffness directly. These motor neurones are among the largest in the ventral nerve cord, and are routinely recorded in my laboratory. A leg will be loaded by the addition of small lead weights, and the effects on movement kinematics, dynamics, and motor patterns measured as described above. We will use a laser photoablation method to destroy the 3 inhibitory leg motor neurones to test directly their role in modulating joint stiffness. We have proven experience in the key techniques that we wish to use, and have developed new methods to increase the power of our approach. Our results will demonstrate clearly how sensory-motor transformations are adapted over time, and how limb mechanics interact with neuronal control systems to guide aimed limb movements. These outcomes will advance our understanding of nervous system functioning, and will guide the design of autonomous legged robots.