Abstract
The role of presynaptic inhibition in normal behavior is largely unknown. We recently reported that the monosynaptic input from cutaneous afferents to spinal interneurons is suppressed during active movement by presynaptic inhibition. To further investigate this phenomenon, we examined the modulation of primary afferent depolarization (PAD) in monkeys performing a wrist flexion-extension task. We delivered microstimuli (1-20μA: 3-10Hz) continuously to the intraspinal sites where orthodromic monosynaptic response were evoked from the superficial radial (SR) nerve (containing purely cutaneous afferents), and recorded antidromic volleys (AVs) in the SR through a tripolar cuff electrode. We calculated the average size (area) of individual volleys in each behavioral epoch. Single stimuli sometimes evoked multiple (2-8) AVs with different latencies, possibly by activating afferent fibers with different conduction velocities. Task-dependent modulation of the volley size was found in 56% of recorded AVs, usually in shorter latency volleys. Facilitation during active movement (flexion and extension) was most common (p< 0.05). Suppression of PAD was also found during active extension. Facilitation of PAD started before EMG onset in both flexion and extension trials, suggesting that PAD is induced in part by descending commands. These data suggest that low-threshold cutaneous input can be modulated presynaptically in a behaviorally relevant way by central control of PAD. [Jpn J Physiol 55 Suppl:S178 (2005)]
