Abstract
Little is known about nociceptive reflex mechanisms in the upper limb in humans. To investigate nociceptive effects on spinal motoneurone excitability, a conditioning noxious stimulus was applied to the index finger of five healthy subjects. Motor evoked potentials (MEPs) following contralateral transcranial magnetic stimulation (TMS) were recorded from thenar eminence (TE) and biceps brachii (BB) muscles ipsilateral to finger stimulation. TMS was randomly applied alone or combined with preceding finger stimulation at an interstimulus interval of 100 ms. MEP amplitudes were profoundly suppressed in TE and augmented in BB. We conclude that nociception produces a differential effect on different spinal motoneurone pools, which may be part of a complex protective reflex mechanism in the upper limb of humans.
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