1. An unexpected perturbation of a joint gives rise to a sequence of EMG events in a stretched muscle. The first one (M 1 ) comes at a short latency (under 40 ms). Then two peaks (M 2 and M 3 ) come at a latency of between 50 and 100 ms. M 2 and M 3 are addressed as pre- programmed reactions. Later, a voluntary correction comes. EMG Time Perturbation M1M1 M2M2 M3M3 Voluntary reaction Lecture 12: Pre-Programmed Reactions

2. Pre-programmed reactions demonstrate a strong dependence on the instruction. If the subject is instructed to resist perturbations, the pre- programmed reactions are large (solid lines). If the subject is asked to let the limb move, the pre-programmed reactions are much smaller (dashed lines). Note that the M 1 reaction is the same. Effect of Instruction on Muscle Reactions to a Perturbation EMG Time Perturbation M1 M2 M3 Voluntary reaction React Let go

3. Pre-Programmed Reactions  Also called triggered reactions, M2–M3, functional stretch reflex, transcortical reflex, long-loop reflex  Come at an intermediate latency (40–100 ms)  Are involuntary in nature but can be modified by instruction  Can be triggered by stimuli of virtually any modality  Generate a quick, crude compensation for the perturbation  Are followed by voluntary corrections

4. A subject is holding a position in a joint against a load with a central command to a muscle. If the subject knows that a perturbation can occur, he/she can prepare an addition to the central command that would compensate for the predicted perturbation. The pre- programmed command (∆C) is triggered by peripheral signals generated by the perturbation and attenuates the mechanical effects of the perturbation. A Possible Scheme Muscle Central command Tonic stretch reflex mechanism ∆C Afferents

5. The actual source of the triggering signal for a pre-programmed reaction is not important as long as the signal carries sufficient information. It can be provided by proprioceptors, a flash of light, a loud tone, etc. Pre-Programmed Responses Can Be Triggered by Any Sensory Signal Muscle Central command Tonic stretch reflex mechanism ∆C Proprioceptors Vision Hearing

6. If a perturbation occurs during a fast voluntary movement, EMG changes are seen at a characteristic for the pre-programmed reactions latency. Generally, they involve an increase in the activity of a muscle that acts against the perturbation, and a decrease in the activity of a muscle that is assisted by the perturbation (bold lines). Effects of a Perturbation Applied During a Fast Movement

7. Muscle vibration has different effects on different components of the responses to an external perturbation. The early response (M 1 ) is suppressed just like the H- reflex, whereas the pre- programmed response (M 2 -M 3 ) is unchanged. Effects of Vibration on M 1-2-3 Responses Vibration EMG Time Perturbation M1 M2-M3

8. Examples of Pre-Programmed Reactions  The “waiter’s response”  Grasp adjustments  Two-hand object holding  Reactions to postural perturbations (ankle and hip strategies)  Corrective stumbling reaction

9. Pre-programmed postural corrections to a perturbation created by platform movement are context-dependent. If the cup is loaded with play dough, the corrections will be different than if the cup is full of hot tea. Effects of Intention on Quick Muscle Reactions

10. A mechanical or electrical stimulation of the paw during locomotion induces different reactions in the swing and the stance phases. In the swing phase (A), there is a flexor reaction, so that the leg steps over a fictitious obstacle. In the stance phase (B), there is an extensor reaction, leading to the shortening of the stance phase for this limb. Corrective Stumbling Reaction Surface Stim AB