Motor Evoked Potentials from Transcranial Stimulation of the Motor Cortex in Cats

Electrical testing of central nervous system pathways is assuming increasing importance in clinical medicine. However, there is no direct monitor of the motor system. We previously reported using a motor evoked potential created by direct excitation of the spinal cord, placing a stimulating electrode over the corticospinal tract area. To produce a less invasive test, we now use direct transcranial stimulation of the motor cortex through the scalp or direct stimulation of the motor cortex itself during operation. A descending signal can be recorded over the spinal cord and in the peripheral nerves where no retrograde sensory signals should be able to descend. This motor cortex stimulation produces contralateral limb movements and selective activation of the peripheral nerves of a limb. The characteristics of this signal are similar to those described in the neurophysiological literature for a descending motor signal. With a depth electrode, it was found that the signal was strongest in the spinal cord near the corticospinal tracts and in the anterior horn cell area. A set of lesioning studies showed that most of the signal travels in the area of the corticospinal tract, with some traveling in the ventral portion of the spinal cord, perhaps in the anterior corticospinal tract. Section of the pyramid essentially abolishes the signal, but lesioning of the red nucleus does not. This test offers an electrical assessment of the motor system that can be useful in experimental work on spinal cord and brain function. It has potential clinical applicability in humans.