The effects of spinal gray activation by strychnine on the motor evoked potential in the rat.

Spinal motor evoked potentials were elicited by electrical stimulation of the motor cortex in 14 rats before and after the application of strychnine to the surface of the spinal cord. Strychnine applied to the high cervical cord resulted in the emergence of additional peaks at the site of application and in electrodes positioned distally on the middle and lower thoracic cord. The strychnine-induced peaks occurred earlier and were larger in amplitude (P less than 0.01) in the distal spinal cord. Strychnine applied to the lower thoracic cord resulted in similar peak generation in the lower thoracic cord and in the spinal cord proximal to the application of strychnine. These findings demonstrate that strychnine-induced motor evoked potential changes arise from spinal gray activity induced by efferent pathways activated during transcortical stimulation. The role of the propriospinal tract in mediating the distal effects of strychnine is discussed. In conclusion, the strychnine-induced peaks of the motor evoked potential may be used as a measure of spinal gray integrity in experimental spinal cord injury models.