FUSIMOTOR-INDUCED CHANGES IN MUSCLE-SPINDLE OUTFLOW AND RESPONSIVENESS IN MUSCLE FATIGUE IN DECEREBRATE CATS

Changes in discharge rate and responsiveness of muscle spindle afferen ts from triceps surae muscles were studied during long-lasting fatigue isometric contractions of either medial gastrocnemius or lateral gast rocnemius and soleus muscles in decerebrate cats. The rest of the hind limb was either denervated or its innervation was preserved. In dener vated preparations a long-lasting post-contraction increase in dischar ge 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 sp indle of origin of the afferent recorded. In innervated preparations t he long-lasting increase was replaced in the majority of primary endin gs (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 i n denervated preparations. Changes in responsiveness to sinusoidal mus cle length changes, indicating influences of both static and dynamic f usimotor neurons, were, however, similar in innervated and denervated preparations. The results obtained provide evidence that changes in mu scle spindle outflow and responsiveness are elicited by the reflex inc rease in fusimotor activity developing in response to the fatigue-indu ced afferent discharges from the contracting muscle. Concomitant affer ent inflow of another origin to fusimotor neurons affects the changes in spindle outflow, but not in responsiveness. In this way an appropri ate increase in support to skeletomotor activity as well as in informa tion on the fatigued muscle of higher motor centres, initiated by the fatigue itself, could be achieved through the gamma loop.