Figure 5. Muscle spindle firing dynamics emerge from cross-bridge mechanisms during simulated muscle stretch.

Simulations assume a low level of muscle cross-bridge cycling consistent with relaxed muscle. Length displacements were imposed on the muscle fiber. (A) Predicted driving potentials (upper traces) during ramp stretches of varying velocity and acceleration (lower traces). (B) Classical data showing Ia afferent firing modulation with different stretch velocity (Matthews, 1963). (C) Dynamic index emergent from cross-bridge mechanisms. Dynamic index is defined classically as the ratio of firing rate at the end of the ramp phase and the firing rate 0.5 s into the hold phase (upper diagram). The muscle spindle model exhibits a sublinear relationship between dynamic index and stretch velocity (middle plot – colors correspond to A), similarly to classical findings (bottom plot). (E) Linear acceleration scaling of initial burst emergent from cross-bridge mechanisms. Initial burst amplitude is defined as the difference between peak firing rate during initial burst and baseline. Muscle spindle model exhibits linear scaling with stretch acceleration at stretch onset (top plot), which is consistent with classical findings (bottom plot – Schäfer and Schäfer, 1969). Figure 5B,C (bottom panel) is reproduced from Matthews and Stein, 1969 with permission from John Wiley and Sons. These images are not covered by the CC-BY 4.0 license and further reproduction of these panels would need permission from the copyright holder. Figure 5D (bottom panel) is redrawn from Schäfer and Schäfer, 1969 with permission from Springer Nature.