Conducted and segmental components of the somatosensory cervical response

Cervical responses evoked by stimulation of the median nerve have been concurrently recorded from C7--Fz and C7--Sn (suprasternal notch). The existence of two different waveforms (RI and RII) has been confirmed. RI (from C7--Fz) consists of four negative peaks (N9, N11, N13, N14) followed by a large positive deflection (P16). RII (from C7--Sn) is characterised by an early positive--negative spike (P1--N1a) followed by a slow negative--positive wave (N1b--P2). The study of the most relevant parameters (polarity, latency and refractory period) of each component of RI and RII did not indicate whether the generators underlying RI differ from those responsible for RII. However, stimulation of the lower limb, which does not involve segmental events at cervical level, showed a clearcut difference: no response was recorded from C7--Sn, while evoked activity equivalent to RI was obtained from C7--Fz. Therefore it is suggested that RII is entirely generated by segmentally evoked potentials while RI is mainly due to conducted potentials.