FUSIMOTOR - INDUCED POST-CONTRACTION CHANGES IN MUSCLE-SPINDLE AFFERENT OUTFLOW AND RESPONSIVENESS IN MUSCLE FATIGUE IN DECEREBRATE CATS - EFFECTS OF AFFERENT INFLOW FROM OUTSIDE THE CONTRACTING MUSCLE

Changes in discharge rate of muscle spindle afferents from triceps sur ae muscles were recorded and their responsiveness to muscle length cha nges tested during long-lasting fatiguing isometric contractions of ei ther medial gastrocnemius of lateral gastrocnemius and soleus muscles in decerebrate cats. The whole hind limb was either denervated, except for the contracting muscle, or its innervation was preseved. The expe riments were aimed to compare the effects on muscle spindles of the re flex increase in fusimotor activity developing in parallel with muscle fatigue when fusimotor neurones are deprived of afferent inflow origi nating from outside the contracting muscle with those exerted when fus imotor reflex responses are affected by this inflow. In denervated pre parations a long-lasting postcontraction increase in discharge rate oc curred in the majority of muscle spindle primary endings and in all th e secondary afferents studied. However, in innervated preparations it was replaced in the majority of the primary endings by a short-lasting burst, occurring immediately after the end of muscle contraction and in secondary afferents it was either absent or shorter-lasting than in denervated prepartions. In spite of these differences in changes in d ischarge rate, similar changes in responsiveness to muscle length chan ges of muscle spindle endings were found in both innervated and denerv ated preparations. They indicated influences of both static and dynami c fusimotor neurones.