Recovery functions of spinal cord and subcortical somatosensory evoked potentials to posterior tibial nerve stimulation: intrasurgical recordings

The recovery function of evoked potentials to posterior tibial nerve stimulation was studied. Intrasurgical recordings were made from interspinous ligaments at lumbar levels and from high thoracic-low cervical level. In addition, surface recordings were obtained from neck-scalp derivations. The recovery function of the potentials recorded from lumbar and from high thoracic-low cervical spinal cord were very similar, showing an early period of supernormality (5-20 ms) followed by a period of subnormality which reached its lowest point at 40-60 ms. Assuming that the potentials recorded at the lumbar level reflect activity in the cauda equina, we conclude that the results support the hypothesis that the potentials recorded from the thoraco-cervical level reflect activity in the dorsal columns. The recovery curve of the amplitude between the far field potentials P27 (which most probably reflects activity of the afferent volley at the level of foramen magnum) and N30 (which, by latency criteria, would reflect lemniscal or thalamic activity) showed a similar shape but with a shorter duration of the periods of super- and subnormality. It is likely that this modification was due to the synapse at the gracilis nucleus. The first cortical component (P32) recorded in the neck-scalp derivation was totally abolished within the recovery period studied (50 ms interval).