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

The excitability of motoneurons controlling upper limb muscles in humans ma y vary with cutaneous nerve stimulation. We investigated the effect of noxi ous and non-noxious conditioning stimuli applied to right and left digit II and right digit V on motor evoked potentials (MEPs) recorded from right th enar eminence, abductor digiti minimi, biceps and triceps brachii muscles i n twelve healthy subjects. Transcranial magnetic stimulation (TMS) was appl ied at interstimulus intervals (ISI) ranging from 40 to 160 ms following co nditioning distal digital stimulation. TMS and transcranial electrical stim ulation (TES) were compared at ISI 80 ms. Painful digital stimulation cause d 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 peri ods in the same muscles. Contralateral digital stimulation was less effecti ve. 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 activ ation of A-a and A-S fibers gives rise to complex modulatory effects on upp er limb motoneuron pools. A-delta fibers initiate a spinal reflex resulting in MEP amplitude reduction in muscles involved in reaching and grasping, a nd MEP amplitude facilitation in muscles involved in withdrawal. These find ings suggest a proteotive reflex mediated by A-delta fibers that protects t he hand from harm. A-alpha fibers induce MEP latency shortening possibly vi a a transcortical excitatory loop. (C) 2001 Elsevier Science Ireland Ltd. A ll rights reserved.