Neural and mechanical contributions to the stretch reflex: a model synthesis

A model for the soleus stretch reflex in the decerebrate cat was synthesized from models of the neural and muscular components, including the two proprioceptors (the muscle spindle and Golgi tendon organ) and their associated afferents (Ia, II, and Ib), the alpha motoneuron pool with its reflex pathways, the branches of the alpha motoneurons to the intrafusal muscles (beta innervation), and the extrafusal muscle. Parameters for the muscle and receptor models were chosen independently to match their responses in isolation. Reflex gains and gamma inputs were estimated to fit the response to stretch measured by Nichols and Houk. The chosen reflex gains and gamma inputs are not unique; many different combinations reproduced the characteristic stretch response. With a single set of fixed parameters, the model predicted many mechanical properties of the stretch reflex, including linearization effects (when the stretch magnitude and direction are varied), as well as the dependence on operating force and initial muscle length. The model did not accurately predict the responses at higher stretch velocities, due to failure of the extrafusal muscle model.