Structural Model of the Muscle Spindle

A model of the muscle spindle was developed based on its anatomical structure. The model contains three intrafusal fibers (bag1, bag2, and chain), two efferents (dynamic efferent to the bag1 fiber and static efferent to bag2 and chain fibers), and two afferents primary (Ia) and secondary (II)]. As in the real muscle spindle, the spindle model, under the modulation of efferents, responds to the extrafusal muscle fiber length. Both outputs (Ia and II afferents) of the model were compared extensively with published data, under both sinusoidal stretch (with different stretch amplitudes and frequencies) and ramp and hold stretch (with different stretch amplitudes and velocities) in three different fusimotor activation conditions (dynamic stimulation, static stimulation, and without stimulation). Model Ia afferent responses fit the published data well with active gamma input, but less well in the passive state. Model II afferent responses also fit the published data, although less quantitative data were available for comparison. The model correctly predicted the fractional power dependence of the primary and secondary ending responses on stretch velocity. The current model provides a powerful tool for simulation studies of neuromusculoskeletal systems, and demonstrates the feasibility of using a structural approach to model complex neurophysiological systems. © 2002 Biomedical Engineering Society. PAC02: 8719Ff, 8719La, 8719St