Conduction pathways and generators of magnetic evoked spinal cord potentials: a study in monkeys.

Evoked spinal cord potentials (ESCPs) following transcranial magnetic stimulation were recorded from the spinal cord in monkeys anesthetized with ketamine. Isopotential maps of the earlier negative deflection of the magnetic ESCP (N1 wave) revealed a distribution of negative field potentials, the maximum of which were located within the medial dorsolateral funiculus, which corresponds to the dorsolateral corticospinal tracts. The N1 wave of the magnetic ESCP had the same latency as the D-wave of the electrical ESCP elicited by either direct cortical or transcranial electrical stimulation. We assumed that the N1 wave was generated by direct excitation of pyramidal axons. Isopotential maps of the waves that followed the N1 peak (waves N2, N3, N4, and N5) of the magnetic ESCP showed a negative field potential distribution, the maximum of which was at the ventromedial funiculus as well as within the medial dorsolateral funiculus. Later waves of magnetic ESCP were suggested to reflect not only the dorsal corticospinal tracts but also the ventromedial spinal cord function.