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Supraspinal influences on spinal mechanisms activated prior to learned movement.

Dogs were trained to perform an instrumental avoidance reflex of lifting and holding a limb above a pre-determined level to prevent or to interrupt painful stimulation of the same limb. A tone preceding the stimulation by 0.5 s served as the CS. During the CS the excitability of the motoneuronal pools involved in the execution of the instrumental motor response was monitored as the magnitude of the response of knee flexors to single stimuli applied to sciatic nerve. For 70-80 ms preceding the onset of the EMG of the conditioned instrumental movement, the amplitude of the muscle response progressively increased. This phenomenon is apparently analogous to that of "motor presetting" described by others for man. Bilateral pyramidotomy at the medullary level was performed following the elaboration of instrumental reflexes. It produced obvious changes in the motor performance: the latencies of conditioned movements became longer and the speed of limb lifting decreased. It did not, however, alter the preparatory increase in motoneuronal excitability. On the other hand pyramidotomy prior to elaboration of the conditioned reflex strongly affected the preparatory changes, Destruction of the red nucleus contralateral to the conditioned limb in pyramidotomized dogs resulted in disappearance for 1.5 months of the conditioned motor response to the CS. After the response had been restored, the latency of movement showed a further increase, and the presetting of the spinal system was essentially absent.