A multi-scale model predicts diverse muscle spindle firing properties

We introduce a multiscale muscle spindle model that provides a framework to simulate the effects of altered muscle properties, neural dynamics, and descending drive on muscle spindle sensory feedback. Our recent work shows that muscle spindle stretch responses in relaxed muscle have a unique relationship with history-dependent muscle fiber force and its first time derivative, i.e. dF/dt, or yank, but not with imposed stretch kinematics (1). Our muscle spindle model simulates cross-bridge population cycling kinetics where pseudolinear combinations of intrafusal fiber force and yank simulate neuron driving potentials. Ours is the first muscle spindle model to exhibit history-dependent firing caused by prior movement. Further, classical and paradoxical muscle spindle firing properties are emergent from the model, including changes in muscle spindle sensitivity due to gamma motor neuron activation.