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Presynaptic inhibition of muscle afferent in awake, behaving monkey

The size of antidromic volleys (ADVs) in peripheral nerve is known to depend on the level of primary afferent depolarization (excitability test). In the previous study, we applied this method to the cutaneous afferent (Superficial radial N. SR) in awake monkeys and found their task dependent modulation suggesting the presynaptic inhibition of cutaneous input. We recently established the same technique in the muscle afferent. In a monkey performing wrist movements, a recoding chamber was implanted to the lower cervical vertebrae and applied intraspinal microstimuli (ISMS;1-20µA). To record ADVs evoked by these stimuli, a custom-made nerve cuff electrode was implanted to the muscle nerve innervating wrist extensors (deep radial N. DR). So far, 24 volleys were recorded in DR and we found their similarity with the ADV of SR in term of their recruitment profile to increasing stimulus current and their conduction velocities. However, volleys in DR, but SR, were often evoked concomitantly with twitch response in extensor muscles. This observation suggest that volleys in DR were either ADVs in afferent or orthodromic volleys in efferent. To dissociate two possibilities, we classified the volleys based on existence of EMG responses. At least 7 volleys in DR were recorded without EMG response even using larger stimulus current (20µA) indicating that the recorded volley should be the ADVs of DR afferent. We conclude that excitability test is applicable to muscle nerve that will make it possible to study the presynaptic inhibition of muscle afferent input and their behavioral significance. Ongoing study usint this new technique in our laboratory will elucidate the functional relevance of presynaptic inhibition of muscle afferent for controlling voluntary movement.

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