Ipsi- and contralateral exteroceptive EMG modulation in uni- and bilaterally activated thenar muscles

The cutaneous silent period (CSP) is a spinal inhibitory reflex mediated by A-delta fibers. The exact underlying neural pathway, however, is unknown. This study was undertaken to investigate whether the neural circuitry mediating CSPs is wired unilaterally or whether there is evidence of influence from or upon the contralateral side.

Fifteen healthy subjects underwent bilateral CSP testing following unilateral nociceptive digit II stimulation. Surface electromyographic (EMG) recordings were obtained bilaterally from thenar muscles following unilateral recurrent nociceptive digit II stimulation while activating the ipsilateral or the contralateral or both thenar muscles against resistance.

Nociceptive digit II stimulation evoked consistent CSPs in ipsilateral thenar muscles during voluntary contraction, while there was no consistent influence on EMG activity in contralateral thenar muscles at rest. Furthermore, nociceptive digit II stimulation did neither consistently affect EMG activity in ipsilateral thenar muscles at rest nor in contralateral thenar muscles during voluntary contraction. Finally, there was no significant difference between any CSP parameters obtained during unilateral versus bilateral muscle contraction. Occasional late excitatory EMG activity in relaxed or contracted thenar muscles resembled startle reflexes, which seem to contribute to the post-inhibition EMG-rebound.

The present findings are consistent with unilateral wiring of the spinal circuitry mediating CSPs.

The essential lack of a crossed inhibitory influence of nociceptive digit II stimulation underscores the utility of CSP testing in the assessment of the A-delta fiber system in healthy subjects and patients with various pathologies.