Surface recording of the spinal ventral root discharge in man. An experimental study.

The response evoked by stimulation of the tibial nerve was recorded by surface electrodes over the lumbosacral spine at multiple levels with both monopolar and bipolar recordings. When the stimulus strength was adjusted to elicit a maximal H reflex, bipolar recordings showed a negative-positive deflection (W1), increasing in latency in the rostral direction and classically attributed to the propagated action potentials in the dorsal roots, followed by a previously unreported positive-negative deflection (W3) showing the opposite sequence, namely, with latency decreasing in the rostral direction. A similar pattern was found in monopolar recordings in which, however, the peak corresponding in time to W3 was partially obscured by a fixed latency component (W2), particularly prominent over the upper lumbar vertebrae. When an intensity corresponding to the maximal M wave was employed, W2 showed a remarkable increase in amplitude, while both W3 and the H reflex disappeared. Several experimental techniques, either suppressing or enhancing the H response, confirmed the strict parallelism existing between this reflex and the W3 component. These findings suggest that W3 represents the reflexly evoked ventral root discharge. This was confirmed by concurrent intrathecal and surface recordings of the lumbosacral spinal evoked activity. W2 is probably generated by the postsynaptic activation of spinal neurons located in segments receiving the tibial nerve afferents, the motoneuronal contribution to this fixed latency component being slight or absent.