Sensory contributions to human movement and balance

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The task of maintaining upright stance in the human involves a complex motor control system that has to use and integrate sensory information from primarily three sensory sources: the vestibular system, the somatosensory system, and the visual system. The LONG TERM GOAL of my basic research program is to better understand the fundamental role played by these sensory inputs in the control of standing balance, locomotion and movement in humans. To continue with this direction of work, two main concentration areas or lines of research will be carried out over the next 5 years. The SHORT TERM goal of the first main concentration area of research will focus on the influence of two descending motor pathways that are directly influenced by these sensory systems, on the regulation of single motorneurons of postural muscles in standing human subjects. To look at single motorneuron behaviour, we have developed long term recordings of single motor units, using custom built and extremely stable indwelling wire recording electrodes. The influence of vestibulospinal and reticulospinal inputs onto single motor units recorded during quiet stance will be investigated. Vestibular inputs will be modulated using Galvanic Vestibular Stimulation (GVS), while reticulospinal inputs will be modulated using loud acoustic stimuli to activate the Audiospinal Reflex (ASR). The SHORT TERM goal of the second main concentration area will investigate the regulation of the human muscle spindles (muscle stretch receptors) utilizing a research technique termed microneurography. We plan to investigate the response profile of muscle spindles of the lower limb during realistic physiological active and passive muscle lengthening and shortening. These recordings will be first recorded in sitting subjects, and pilot experiments have demonstrated that, although very difficult, it is possible to record from the single axons from muscle spindles in ankle dorsiflexors in quietly standing human subjects, even during standing perturbations (ankle plantar/dorsiflexion). The experiments in this grant proposal will provide us with valuable new insight into how certain descending inputs regulate muscle spindle and motor unit behaviour during normal stance.