Human muscle spindles and their role in guiding goal-directed motor behaviour

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Muscle spindles are the primary source of sensory information underlying our perception of body position and movement. Their input also provides sensory feedback for adapting and correcting goal-directed motor behaviour. Unlike other somatosensory receptors, muscle spindle sensitivity is under the direct control of a dedicated motor system (the fusimotor system). The role of muscle spindle feedback in humans, however, is often inferred indirectly from vibratory illusions and stretch reflex responses. Direct evidence for how muscle spindle feedback is modulated during adaptive, goal-directed motor behaviour remains elusive. Furthermore, much of what is known about muscle spindle anatomy and physiology derives from research on animal models (mainly cat and monkey, also rat, toad, and snake). However, differences in muscle spindle anatomy and physiology reported between species jeopardize the translatability of this animal research. For example, human muscle spindles exhibit dynamic and static firing rates that are much lower than those observed in animal models. Human muscle spindles also have higher intrafusal fibre counts, which in theory should endow them with greater fusimotor control than other animals. Combining exoskeleton robotics with human microneurography will enable my lab to quantify single muscle spindle activity with participants passive or performing active movements under precisely controlled stimulus and task parameters. To probe basic aspects of human muscle spindle function, the primary short-term goal of the proposed research will be to construct population average response estimates from a large dataset of single unit recordings under robotically controlled task and stimulation parameters, both with and without skeletomotor activation. Additionally, the ability for the fusimotor system to modulate muscle spindle responses based on task demands and behavioural contexts has intrigued researchers for well over a century now, yet evidence for fusimotor activation occurring independently from skeletomotor activation, particularly in humans, remains unclear and unconvincing. To investigate if strong, consistent fusimotor effects can be observed that reflect the goal of a task or the context in which a task is performed, the secondary short-term goal of the proposed research will be to use various experimental manipulations for altering fusimotor activation, including increasing spatial and temporal task demands, having participants perform a proprioceptive learning task, and exposing participants to virtual height environments using a head-mounted VR display. In summary, the combined 5-year research project will enable me to make excellent progress towards the long-term goal of my research program: elucidating the fundamental role of sensory feedback in goal-directed human motor behaviour.