Influence of transcutaneous electrical nerve stimulation on cutaneous silent periods in humans.

The cutaneous silent period (CSP) is a spinal inhibitory reflex mediated by A-delta fibers. Ten healthy subjects underwent assessment of the effect of high-frequency transcutaneous electrical nerve stimulation (TENS) on the CSP. Surface electromyographic (EMG) recordings were obtained from thenar muscles following recurrent nociceptive digit II stimulation. Group average CSP duration was shortened relative to baseline recordings following 15 min of TENS. The amount of exteroceptive EMG inhibition was slightly increased due to a concomitant suppression of transcortical long-loop reflexes, which may be present within the CSP. Thus, TENS exerts an influence on both inhibitory and excitatory circuits involved in protective reflexes. These effects are likely mediated at the spinal segmental level through TENS-associated presynaptic inhibition of nociceptive A-delta fibers. The findings are in agreement with a known opiate-insensitive mechanism of TENS at the spinal level, and a previously reported insensitivity to fentanyl of CSPs.