Effects of selective spinal cord lesions on the spinal motor evoked potential (MEP) in the rat

The effects of selective spinal cord lesions on the motor evoked potential (MEP) in 21 rats were investigated. No significant change in peak amplitude was observed following lesions of the pyramidal tract. There was a significant decrease in peaks 1 and 2 with ventral funiculi lesions. All 4 peaks of the MEP were significantly reduced following lesions of the lateral funiculus. The most marked decrease in peak amplitude followed lateral funiculi lesions that involved the lateral grey of the spinal cord. In one animal where the lesion was confined to the grey matter in the cord there was a marked decrease in all peaks of the MEP. In 3 additional animals interruption of the descending tracts of the spinal cord via bilateral hemisections of the spinal cord failed to completely abolish the MEP. Increases in peak latency were also noted following spinal lesions. In some animals the increase in latency occurred in the absence of significant peak amplitude changes. The findings in this study refute the previously held position that the MEP in the rat arises from pyramidal tract activation. The role of the reticulospinal and propriospinal tracts in the generation and propagation of the MEP are discussed.