Cortically evoked motor action potential in spinal cord injury research

Somatosensory evoked potentials (SEPs) have been used extensively in the neurophysiological assessment of spinal cord integrity. However, SEPs mainly reflect the function of the dorsal column. In an effort to find a reliable method of assessing the integrity of the motor tracts for experimental purposes, we studied cortically evoked motor action potentials (CEMAPs). Twenty white rats used were anesthetized with ketamine, and extradural cortical stimulation was carried out through a parasagittal craniectomy posterior to the coronal suture. A single stimulus of 325 mV for a duration of 0.1 to 0.5 ms was used to obtain a motor action potential from the contralateral lower extremity. Complete transsection of the spinal cord abolished the CEMAP. A progressive increase in pressure applied to the spinal cord with a progressive increase in duration produced loss of the CEMAP. The selective motor tract study made possible by cortical stimulation makes this a good technique for studying the motor tracts in experimental work.