Origins and conducting tracts of evoked spinal cord potentials in cats

The origins and the conducting tracts of the evoked spinal cord potential elicited by sciatic nerve stimulation (spinal somatosensory evoked potential: spinal SEP) were studied in cats. The potentials were recorded at various levels of the intact spinal cord and partially resected spinal cord and analyzed with special reference to four negative potentials (N1, N2, N3, and N4 waves). The conduction velocities of N1, N2, N3, and N4 waves were 101.8, 52.7, 88.7, and 42.7 m/s, respectively. N1 and N3 waves are considered to originate from Group I fibers and to be conducted by second-order neurons ascending the ipsilateral-dorsolateral funiculus (dorsal spinocerebellar tract) after synaptic delays. N3 wave may have a longer synaptic delay than N1 wave. N2 and N4 waves are considered to originate from nerve fibers other than Group I and to be conducted primarily in the posterior funiculus. N1 and N3 waves have synaptic delays at the L3 level, but N2 and N4 appear to be conducted without synaptic delay. Therefore, N2 and N4 waves precede N1 and N3 at the upper lumbar and thoracolumbar junction levels, respectively. After that, N1 and N3 waves are conducted with greater velocity than N2 and N4 waves. Because of different conduction velocities, N1 overtook N2 and N3 overtook N4 at the midthoracic region. N2 and N1 waves had the shortest latency in the upper lumbar to thoracolumbar junction levels and the upper thoracic region, respectively.