Modulation of upper extremity motor evoked potentials by cutaneous afferents in humans

The excitability of motoneurons controlling upper limb muscles in humans may vary with cutaneous nerve stimulation. We investigated the effect of noxious and non-noxious conditioning stimuli applied to right and left digit II and right digit V on motor evoked potentials (MEPs) recorded from right thenar eminence, abductor digiti minimi, biceps and triceps brachii muscles in twelve healthy subjects. Transcranial magnetic stimulation (TMS) was applied at interstimulus intervals (ISI) ranging from 40 to 160 ms following conditioning distal digital stimulation. TMS and transcranial electrical stimulation (TES) were compared at ISI 80 ms. Painful digital stimulation caused differential MEP amplitude modulation with an early maximum inhibition in hand muscles and triceps brachii followed by a maximum facilitation in arm muscles. Stimulation of different digits elicited a similar pattern of MEP modulation, which largely paralleled the behavior of cutaneous silent periods in the same muscles. Contralateral digital stimulation was less effective. MEPs following TMS and TES did not differ in their response to noxious digital stimulation. MEP latencies were shortened by cutaneous stimuli. The observed effects were stimulus intensity dependent. We conclude that activation of A-α and A-δ fibers gives rise to complex modulatory effects on upper limb motoneuron pools. A-δ fibers initiate a spinal reflex resulting in MEP amplitude reduction in muscles involved in reaching and grasping, and MEP amplitude facilitation in muscles involved in withdrawal. These findings suggest a protective reflex mediated by A-δ fibers that protects the hand from harm. A-α fibers induce MEP latency shortening possibly via a transcortical excitatory loop.