A Cerebellar-dependent Efference Copy Mechanism for Generating Appropriate Muscle Responses to Limb Perturbations
The generation of appropriate muscle responses to a limb or body perturbation requires correct prediction of the nature and consequences of the perturbation. For example, Nashner (1976) demonstrated that the same stretch of the ankle extensors elicited different muscle responses depending on the postural situation in which this stretch occurred. Given a few trials the responses could be altered such That they were appropriate for maintaining postural stability. Thus training can be used to set up stretch reflexes such that they are context-specific. Motor responses can also be altered by prior instruction or intent. Hammond (1956) showed that the muscle response to stretch could be modulated in magnitude depending on whether the subject intended to resist or give way to the stretch. How does motor set, whether mediated by training or prior instruction, modify the muscular response to stretch? One possibility is that set can modulate or gate the synaptic efficacy of stretch reflexes. In the case of Nashner’s experiment reflex loops could be modulated or even opened or closed depending on the motor set. Another possibility is that set allows the preprogramming of the appropriate response which then is triggered or released by the initial stretch. This mode is suggested in the experiments of Evarts and Tanji (1976) which dissociated reflex from intended muscle activity. Here set-dependent activity in both muscle and motor cortex was triggered by a perturbation independent of the direction of the initial stretch.
KeywordsMotor Cortex Antagonist Muscle Muscle Response Stretch Reflex Return Movement
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