Role of Presynaptic inhibition for controlling vilitional movement

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we examined the modulation of primary afferent depolarization (PAD) in monkeys performing a wrist flexion-extension task. We stimulated the superficial radial (SR) and Deep radial (DR) nerve and recorded evoked responses of single INs and local field potentials (LFP) in spinal cord [C6-T1]. When a monosynaptic response was observed, we delivered micro stimuli (1-20μA: 3-10Hz) continuously through the recording microelectrode and recorded antidromic volleys (AVs) in the SR through a tripolar cuff electrode. Single stimuli sometimes evoked multiple (2-8) AVs, possibly by activating afferent terminals with different conduction velocity. We calculated the size (area) of individual volleys (n=270) and averaged in each behavioral epoch[rest, cue, delay, movement, hold, etc]. As for SR, the size of most volleys (58%) increased or decreased significantly relative to control period in specific phases of the behavioral task, indicating that PAD was modulated in a task-dependent manner. AVs showing modulation had faster conduction velocities than those without modulation (57.04+-13.04 vs. 53.20+-13.65, p<0.01), suggesting the primary target of PAD modulation could be faster conducting fibers (i.e. A-beta). The size of many volleys increased during active wrist movement against an elastic load, when monosynaptic unit responses and LFPs decreased. These data suggest that low-threshold cutaneous input can be modulated presynaptically by changing the amount of PAD in a behaviorally relevant way (Fig.3). Comparable result was also obtained in the DR nerve.