Dominance of the short-latency component in perturbation induced electromyographic responses of long-trained monkeys.

The effects of prolonged training of adult monkeys subjected to random, brief perturbations of alternating elbow flexions and extensions were studied over a period of four years. The training was intensive at first, for about one year, and then irregular, with long pauses, during the following three years. As a consequence of the prolonged training with the brief perturbations, the M2 component of the electromyographic (EMG) response of the biceps and triceps muscles became gradually smaller, and finally disappeared. The M1 component, on the other hand, progressively increased in amplitude and continued to do so after the loss of the M2, until it finally dominated the EMG response. The training had similar effects on the response of the biceps muscle to longer perturbations, but, only under certain conditions, did it affect the triceps muscle response. All changes occurred at earlier stages of the training in the flexor than in the extensor muscle. These observations demonstrate a long-term functional plasticity of the sensorimotor system of adult animals and suggest a growing role for fast segmental mechanisms in the reaction to external disturbances as motor learning progresses. Changes at various levels of the stretch reflex system could underlie the enlargement of the M1 component, while the lack of the M2 component should, at least partially, reflect a reduced cortical effect on alpha-motoneurones and/or changes in spinal systems processing afferent information.