Pre-Synaptic Inhibition of Afferent Feedback in the Macaque Spinal Cord Does Not Modulate with Cycles of XXXX Around 10 Hz.

@entity1897 @entity717 @entity400 @entity17558
Spinal interneurons are partially phase-locked to physiological @entity400 around 10 Hz. The phase of spinal interneuron activity is approximately opposite to descending drive to motoneurons, leading to partial phase cancellation and @entity400 reduction. Pre-synaptic inhibition of afferent feedback modulates during @entity717 , but it is not known whether it tracks more rapid fluctuations in motor output such as during @entity400 . In this study, dorsal root potentials (DRPs) were recorded from the C8 and T1 roots in two macaque monkeys following intra-spinal micro-stimulation (random inter-stimulus interval 1.5-2.5 s, 30-100  A), whilst the animals performed an index finger flexion task which elicited @entity1897 around 10 Hz. Forty one responses were identified with latency < 5 ms; these were narrow (mean width 0.59 ms), and likely resulted from antidromic activation of afferents following stimulation near terminals. Significant modulation during task performance occurred in 16/41 responses, reflecting terminal excitability changes generated by pre-synaptic inhibition (Wall's excitability test). Stimuli falling during @entity17558 in finger acceleration were extracted, and sub-averages of DRPs constructed for stimuli delivered at different oscillation phases. Although some apparent phase-dependent modulation was seen, this was not above the level expected by chance. We conclude that, although terminal excitability reflecting pre-synaptic inhibition of afferents modulates over the timescale of a @entity717 , it does not follow more rapid changes in motor output. This suggests that pre-synaptic inhibition is not part of the spinal systems for @entity400 reduction described previously, and that it plays a role in overall-but not moment-by-moment-regulation of feedback gain.
