Figure 1.

Model overview and data flow. A, Submodels implemented within the model. The input to the model is the descending input arriving from supraspinal centers at each motoneuron population, while the output is the mechanical action of the limb (represented as the length of the musculotendon segments). The descending drive triggers α motoneuron activity that determines muscle force. Force determines the musculotendon mechanics, based on which activity of the muscle spindles is estimated and fed back to the motoneurons via Ia fibers, thereby closing an afferent loop. Interneurons receiving Ia input provide inhibitory input to the heteronymous motoneuron pool. Gamma motoneuron activity controls the muscle spindle responsiveness to muscle mechanics, while force is also determined by the muscle dynamics (force–length and force–velocity relations). B, Representative simulation example showing, from top to bottom, the membrane potentials of several motoneurons, superimposed to the low-pass-filtered (180-point Blackman–Harris window) CST of the whole pool; the resultant force exerted by that muscle; the muscle-contraction speed; and the discharges of several Ia neurons as a result of muscle contraction, superimposed to the low-pass-filtered (180-point Blackman–Harris window) CST of all the Ia neurons from that muscle. These variables are colored as the “probes” in A. The vertical marks (dotted gray lines) illustrate some of the delays in Table 1. MN, Motoneuron; IN, interneuron; MS, muscle spindle.