Object: The authors conducted a study to provide an objective electrophysiological assessment of descending motor pathways in rats, which may become a means for predicting outcome in spinal cord injury research.
Methods: Transcranial magnetic motor evoked potentials (TMMEPs) were recorded under various conditions in awake, nonanesthetized, restrained rats. Normative data were collected to determine the reproducibility of the model and to evaluate the effect of changing the stimulus intensity on the evoked signals. In addition, an experiment was performed to determine if the TMMEPs produced were the result of auditory startle response (ASR) potentials elicited by the sound generated by the movement of the copper coil inside its casing during magnetic stimulation. Transcranial magnetic motor evoked potentials were elicited after magnetic stimulation. At 100% stimulus intensity, the mean forelimb onset latency was 4.2 +/- 0.39 msec, and the amplitude was 9.16 +/- 3.44 mV. The hindlimb onset latency was 6.5 +/- 0.47 msec, and the amplitude was 11.47 +/- 5.25 mV. As the stimulus intensity was decreased, the TMMEP onset latency increased and the response amplitude decreased. The ASR potentials were shown to have longer latencies, smaller amplitudes, and were more variable than those of the TMMEPs.
Conclusions: These experiments demonstrate that TMMEPs can be recorded in awake, nonanesthetized rats. The evoked signals were easy to elicit and reproduce. This paper introduces noninvasive TMMEPs as a new technique for monitoring the physiological integrity of the rat spinal cord.