Static and dynamic fusimotor controls in various hindlimb muscles during locomotor activity in the decorticate cat

During stereotyped locomotor movements of the acute decorticate (thalamic) cate, the static and dynamic fusimotor actions upon spindles of various hindlimb muscles were studied in relation to extrafusal activities. For this purpose, the rhythmic alterations in the responses of spindle endings to appropriate repetitive stretches were analyzed. Co-activation of alpha motoneurones with static and dynamic fusimotor neurones throughout the locomotor cycle was demonstrated. Quantitative differences were found between the fusimotor influences on spindles of various muscle groups. The static action was stronger than the dynamic one in pure flexor muscles; a predominantly dynamic action occurred in pure extensor ones while both actions were balanced in proximal bifunctional muscles. These results are discussed in relation to the known characteristics of fusimotor innervation and to the control of locomotor movement.