Control and Regulation of the Human Motor System

Current theory considers motor control to be effected over two pathways, one via the alpha motoneurons to the muscle and the other via the gamma motoneurons to the muscle spindles. This mechanism, described as the "alpha-gamma linkage," permits the control of the willed muscle output with the simultaneous maintenance of postural equilibrium by regulation of the muscle's length sensors. The stretch reflex thus remains a functional component of voluntary motor activity. We describe an additional mechanism which can play a role in this control task. This mechanism is control of the gain of the stretch reflex loop. Experimental evidence for the existence of this mechanism is given, and its significance discussed. Not only are the control system dynamics strongly affected, but an additional feature is that the computations required of the central nervous system are simplified.