Influences of transcutaneous electrical stimulation of cutaneous and mixed nerves on subcortical and cortical somatosensory evoked potentials.

Aim of this study was to assess whether transcutaneous electrical nerve stimulation (TENS) 'gates' somatosensory evoked potentials (EPs) peripherally or centrally, and which afferent fibres and sensory nuclei mediate this effect. The following waves were recorded after stimulation of the median nerve at the wrist or of the digital nerves of the index finger: N9, the cervical N11 and N13, the parietal P9, P11, P14, N18, N20, P22, P27, P40. When both median or digital nerve EPs were conditioned by TENS delivered to the median nerve, reduction in amplitude of N9, P14, N18 and later generated cortical waves was observed. To measure the central contribution to this decrease, unconditioned 'reference' EPs were evoked by stimulating with a current strength yielding an N9 potential of an amplitude equal to that obtained during TENS. In this case, the amplitude of P14, N18 and later cortical waves was significantly greater than during TENS. When both median or digital nerve EPs were conditioned by TENS delivered to the digital nerves, waves were only slightly affected. No effects were seen on the EPs elicited from the median or index finger digital nerves when TENS was administered to the contralateral median or digital nerves or to the ipsilateral middle finger. It is concluded that TENS gates the somatosensory volley, both at a peripheral level through a 'busy line-effect' on large afferent fibres, and centrally at the level of the cuneatus nucleus.