Motor evoked potential (MEP) monitoring is a relatively new test for spinal cord motor function. The motor evoked potential has become a valuable component of neurophysiological monitoring. A better understanding of the characteristics of the normal MEP is needed before one can fully appreciate the effects of injury on the MEP. We describe characteristic patterns of spinal cord MEPs, recorded epidurally, in response to transcranial (dura-to palate) brain stimulation in a rat model. In the rat, the MEP consists of the five major identifiable components, which can be grouped into either early latency or late latency potentials.

Transcortical stimulation was achieved using a Compact Four (Nicolet, WI, U.S.A). Constant current square wave pulses were applied to the cortex for a duration of 50 to 500 μsec at an intensity of 2 to 10 mA. We studied the gradual development of the EMP wave form using smaller increments of current strength. We confirmed in rats that long latency peaks appear first at low intensities while short latency peaks appear with higher intensities.

Conduction velocities of MEP peak (N1) were calculated, and they range from 35 to 50 m/sec. Their velocities were consistent with values reported in the literature for extrapyramidal pathways. The stimulus frequency was 0.3 Hz to 8.7 Hz, however the EMPs were not affected by the stimulus frequency. EMPs were amplified, filtered (1 Hz to 50 KHz) and averaged by an evoked potential system. After recording in 13 controls, we also studied the EMPs after motor cortex removal subsequently.

The animals were subjected to a impact to the dorsal surface of the spinal cord. The force was applied via a measured weight (5 gm) that was rounded at the surface, which impact on the cord after being dropped vertically through a calibrated tube at a height of 5 cm, 10 cm and 15 cm individually. MEPs were obtained immediately, 15 min, 30 min, 45 min, and 60 min after injury.

The pattern of MEPs were greatly attenuated just after injury, and started showing partial improvement within 15 minutes. However, MEPs were not recovered when the weight of 75 gm·cm was used.

Finally, the animals were sacrified and the spinal cord were carefully removed. The cords were placed in 10% formalin, sectioned and stained for neurohistopathological verification of the lesion. Histopathologically we could only observed edema, hemorrhage or tissue destruction.

In summary, the MEPs of the rat consisted of the five major identifiable components, which can be grouped into early latency and late latency. The pattern of MEPs were greatly attenuated just after injury, and started showing partial improvement when the weight of 25 gm·cm and 50 gm·cm were used, however MEPs were not recovered when the weight of 75 gm·cm was used.