FUSIMOTOR RESPONSES TO FATIGUING MUSCLE CONTRACTIONS IN NON-DENERVATED HINDLIMB OF DECEREBRATE CATS

Changes in discharge rate of 21 fusimotor neurons to medial gastrocnem ius muscle during long-lasting fatiguing contractions of lateral gastr onemius and soleus muscles were recorded in decerebrate cats with inne rvation of the same hindlimb preserved. Both the spontaneous activity and reflex responses of fusimotor neurons differed from those found pr eviously in preparations with denervated hindlimb. Higher proportion o f units fired at rest at rates above 20 impulses/s, the initial increa se in discharge rate at the onset of muscle contraction was markedly p rolonged, lasting in the majority of units throughout the muscle contr action, while the late increase in discharge rate developing with musc le fatigue was either absent or short-lasting. It is suggested that th e increase in spontaneous firing rate occurs in dynamic fusimotor neur ons being supported by afferent inflow from secondary muscle spindle e ndings from non-contracting muscles, the enhancement of the early resp onses to be primarily due to recurrent disinhibition and the differenc es in changes of early and late responses to reflect their partly diff erent origin. The possibility is raised that the late reflex responses are lacking more often in static than in dynamic fusimotor neurons. F rom the functional point of view the differences in fusimotor reflex r esponses in innervated versus denervated hindlimb may indicate their s usceptibility to modifications by changes in afferent inflow according , supposedly, to tile current demands of motor control of the active a nd/or fatigued muscle.