The skeleton-fusimotor and fusimotor systems modulate the afferent discharge from the primary and secondary endings of the muscle spindles . They act in the manner of the controls on a transistor radio. Beta- and large gamma- motor neurons are dynamic in control. They act as a dynamic fusimotor system, ‘tuning’ the spindle receptors by increasing the viscosity and stiffness of the intrafusal muscle fibres around which and upon which the sensory endings are placed. The spindle sensitivity to the rate at which itself and of course, the muscle fascicle in which it is situated, is being lengthened is in this way selected.
In the analogy of the radio, the spindle receives a different programme. The small gamma motor neurons increase the sensitivity of the spindle receptors to a steadily maintained stretch. They increase the position sensitivity of the receptors. In our radio analogy, the…
