Sensory modulation of voluntary and TMS-induced activation in hand muscles

Nociceptive suppression of tonic voluntary electromyographic (EMG) activity in human hand muscles (cutaneous silent period, CSP) is in its functional organization consistent with a spinal protective reflex. Motoneuronal excitability and its modulation may also be investigated by conditioned motor evoked potentials (MEPs). To date, effects of exteroceptive stimuli on tonic EMG and on MEPs have been compared mainly using innocuous stimuli, while noxious stimuli have not been studied in great detail. In ten subjects, we recorded CSPs induced in volitionally activated flexor pollicis brevis muscle (FPB) by noxious digit II (D2) stimulation, and in first dorsal interosseous muscle (FDI) following noxious D2 and digit V (D5) stimulation. Then, transcranial magnetic stimulation (TMS) was used to evoke MEPs in the same hand muscles at rest--conditioned by equal noxious D2 or D5 stimulation and individually delayed--so that the MEPs occurred at times corresponding to immediately before, during, and immediately after the CSP in each subject. Immediately before the CSP, there was no significant difference between nociceptive MEP modulation and tonic EMG modulation in any muscle-finger-combination. In the middle of the CSP, noxious finger stimulation exerted suppression of TMS-induced MEPs in all the three muscle-finger-combinations, but less so as compared to corresponding tonic EMG levels. After the CSP, MEPs remained suppressed when corresponding tonic EMG levels were significantly enhanced. Notably, MEPs were also suppressed in cases when occurring at times corresponding to the excitatory long-loop reflex. Incomplete MEP suppression during the CSP may allow for an "emergency grip" even during noxious stimulation. MEP suppression outlasting the CSP is compatible with a "passive" re-synchronization of volitionally activated motor units rather than an "active" reflex involving recruitment of corticospinal motoneurons. The differences in tonic EMG and MEP modulation favors an effect of noxious digital nerve stimulation on interneurons responsible for presynaptic inhibition rather than a postsynaptic inhibitory effect on the motoneuron pool. The present findings caution against the use of nociceptive MEP modulation at rest to substitute for tonic EMG modulation as tested in CSP studies.