MUSCLE SPINDLE RESPONSES TO CONCOMITANT VARIATIONS IN LENGTH AND IN FUSIMOTOR ACTIVATION

The capability of the different fusimotor fibres to mediate the governing signals in the proposed ‘follow‐up length’ servo regulation of muscle movements can be judged from the responses of spindle endings to increasing rates of fusimotor fibre stimulation concomitant with passive muscle shortening. Spindle endings in the ankle and the intercostal muscles have been studied. Dynamic and static fusimotor fibres have been stimulated in isolation or in combinations. Length and fusimotor inputs have been modulated in triangular waves, mostly of equal periodicity. For a servo action the afferent impulse frequency should be able to increase in pace with increasing fusimotor drive in spite of concomitant length decrements. Most of the static fibres, but none of the dynamic fibres, could induce an acceleration of the spindle discharge over the physiological range of length changes. This ability of the static fibres was not affected by the co‐activation of a dynamic fibre either with varying rates or with constant rate of stimulation, but when the activation by another static fibre was added, the increase in impulse rate was sometimes transformed into a decline. Some static fibres, most of which had type II effects on the position sensitivity, showed little ability to perform servo action as defined above. Other functional implications of the results are also discussed. © 1968 Scandinavian Physiological Society