THE EFFECTS OF MUSCLE SPINDLE AFFERENTS ON SYNERGIST MOTONEURONS ACTIVITY The effects of ellectrical stimulation of the group I and II afferents on the synergist motoneurons were studied in decerebrate unanesthetized cats. Single motor axons were isolated functionally. The incoming afferent wolley was recorded from the ventral root filaments. Thus the group I and II effects were judged. It was es¬tablished that the stimulation of the extensor group I and/or II afferents activated the synergist motoneurons. These results were discussed under the light of previous investigations.