Lena Ting

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  Force encoding in muscle spindles - implications for sensorimotor control of balance
  Lena Ting

  Proprioceptive sensory information is essential to movement, particularly in sensorimotor responses to external perturbations to the body. Our data show that rapid increase in resistive force of a passive muscle when stretched, i.e. short-range stiffness may cause enhanced sensory signals that facilitate the detection and predictive response to sudden mechanical perturbations to the body. Importantly, this history-dependent property of muscle spindle firing rates does not have a unique relationship to muscle length or velocity, but rather can be predicted in fine detail based on a unique pseudo-linear transformation between muscle force and the first time derivative of force, dF/dt and muscle spindle afferent firing rate. Several history-dependent features of muscle spindle firing rates can be predicted based on muscle force and dF/dt and are likely due to cross-bridge cycling kinetics in muscle fibers. Such history-dependence is lacking in current models of muscle spindles, but could be necessary to explain a number of phenomena from postural response to perturbation, spasticity, and perception of limb position. Moreover, the encoding of force as a proxy for length in muscle spindles has many implications for normal and impaired control of movement.