A biorobotic structural model of the mammalian muscle spindle primary afferent response

A biorobotic model of the mammalian muscle spindle Ia response was implemented in precision hardware. We derived engineering specifications from displacement, receptor potential, and Ia data in the muscle spindle literature, allowing reproduction of muscle spindle behavior directly in the robot's hardware; a linear actuator replicated intrafusal contractile behavior, a cantilever-based transducer reproduced sensory membrane depolarization, and a voltage-controlled oscillator encoded strain into a frequency signal. Aspects of muscle spindle behavior not intrinsic to the physical design were added in control software using an adaptation of Schaafsma's mathematical model. We tuned the response to biological ramp and hold metrics including peak, mean, dynamic index, time domain response, and sensory region displacement. The model was validated against biological Ia response to ramp and hold, sinusoidal and fusimotor inputs. The response with dynamic or static gamma motorneuron input was excellent across all studies. The passive spindle response matched well in five of the nine measures. Potential applications include basic science muscle spindle research and applied research in prosthetics and robotics.