Fusimotor-induced changes in muscle spindle outflow and responsiveness in muscle fatigue in decerebrate cats.

Changes in discharge rate and responsiveness of muscle spindle afferents from triceps surae muscles were studied during long-lasting fatigue isometric contractions of either medial gastrocnemius or lateral gastrocnemius and soleus muscles in decerebrate cats. The rest of the hind limb was either denervated or its innervation was preserved. In denervated preparations a long-lasting post-contraction increase in discharge rate developed in the majority of primary (15 of 18) and in all (20) secondary afferents. This increase was abolished, while the decrease during muscle contraction was enhanced after application of procaine to the corresponding muscle nerve, to block either the small-diameter afferents from the contracting muscle or the fusimotor axons to the spindle of origin of the afferent recorded. In innervated preparations the long-lasting increase was replaced in the majority of primary endings (14 of 22) by a sharp burst at the end of muscle contraction, while in secondary afferents it was either absent or shorter-lasting than in denervated preparations. Changes in responsiveness to sinusoidal muscle length changes, indicating influences of both static and dynamic fusimotor neurons, were, however, similar in innervated and denervated preparations. The results obtained provide evidence that changes in muscle spindle outflow and responsiveness are elicited by the reflex increase in fusimotor activity developing in response to the fatigue-induced afferent discharges from the contracting muscle. Concomitant afferent inflow of another origin to fusimotor neurons affects the changes in spindle outflow, but not in responsiveness. In this way an appropriate increase in support to skeletomotor activity as well as in information on the fatigued muscle of higher motor centres, initiated by the fatigue itself, could be achieved through the gamma loop.