A Study on Monitoring of Pyramidal Motor Evoked Potential Excluding Extrapyramidal Systems in the Rat Spinal Cord

Cho, Joon; Park, Young Gou; Lee, Moo Sam

Abstract

The motor evoked potential (MEP) elicited by transcranial or transcortical stimulation has been advocated as a method of monitoring the integrity of spinal efferent pathways in the various animal models. It was also defined that MEPs were composed of a short latency, direct, synapse free D-wave and a later latency, indirect, synapse mediated I-wave. The authors designed to study pyramidal MEP in rat, because we would understand and use pyramidal MEPs as human efferent spinal pathway monitoring tools during the spinal cord operation in the future. Thirty-two Sprague-Dawley rats were involved in this study. A stainless steel electrode was placed in the hindlimb motor cortex. It was stimulated with 1~4 Hz mono-rectangular pulse waves and 0.1~5 mA in stimulation intensity during short duration. Teflon coated wire electrode was used to record MEP in the spinal cord. MEPs after internal capsule cutting and post-mortem MEPs were recorded finally to exclude the possibility of extrapyramidal MEPs. At the level of medulla oblongata and seventh cervical segment (C7), the recorded MEPs showed positive-negative-positive complex D-wave and a large I-wave activated by presynaptic fibers and monosynaptic depolarization of pyramidal cells. But, at eighth thoracic segment (T8), only large negative I wave, in which prolonged D wave would be included, was recorded. From cortex to seventh cervical spinal cord at 1 mA stimulation intensity, the estimated conduction velocity of D-wave was approximately 11.01+/-0.20 m/sec and that of I-wave was 2.53+/-0.02 m/sec in this study. After internal capsule cutting and postmortem state, both D and I waves were disappeared. Loss of waves indicated that not the extrapyramidal pathway potentials but the pyramidal pathway potentials were recorded selectively. This successful preferential activation of pyramidal MEP in rat demonstrated the possibility of clinical availability during the spinal, especially cervical motor tract monitoring and evaluation. If repeated study would be continued in human, MEP will be more available in clinical field.