Changes in the Motor Evoked Potentials Produced by Stimulation of Reticulospinal Tract Neurons in the Chronic Spinal Cord Injury of Rat

Kim, Hak Sun; Kim, Sang Soo; Shim, Dae Moo; Kim, Tae Yo; Lim, Kyung Su; Kim, Jong Hwan

Abstract

STUDY DESIGN: There is a prospective study of 80 Sprague-Dawley rats which were made at the spinal cord lesion T5/6 level, sparing only one ventral quadrant. We monitered medullary reticulospinal neurons(RtN) evoked potentials at the L2/3 level which laminectomy was performed.
OBJECTIVE
to investigate changes in the physiological responses of motor neurons to stimulation of the medullary reticular formation following partial spinal cord lesions sparing only the ventral quadrant. SUMMERY AND BACK GROUND DATA: There were many report that the animals with spinal cord lesion recovered well-coordi-nated fourlimb locomotion within 2-3 weeks. The time course of the functional recovery of this hindlimb locomotion was cor-related with the recovery of motor evoked potentials(MEP), which originate from reticular nuclei. Therefore, it was hypothe-sized that the return of locomotor function after incomplete spinal cord injury may partially rely on the reorganization of descending inputs to ventral horn neurons previously occupied by damaged afferents.
MATERIALS AND METHODS
Total 80 Sprague-Dawley rats were used in this study. Under sterile conditions, spinal cord lesions were made at the T5/6 level using a No. 11 blade, sparing only one ventral quadrant. The animals allowed to survive from one day to 61 days. To monitor RtN evoked potentials, laminectomies were performed at L2/3 level. Field potentials were recorded using a glass microelectrode filled with 2 M NaCl(1.5-2.0 M Ohm). Cord dorsum potentials were also epidurally monitored at L2/3 using a pair of teflon-coated wires. The gigantocellular reticular nucleus ipsilateral to the spared ventral cord was stimulated using a monopolar tungsten microelectrode.
RESULTS
The field potentials generated in the ventral horn of the lumbar cord were recorded bilaterally. In some animals field potentials were monitored just before and right after the spinal cord lesion. 1) Following spinal cord lesion at T5/6, the amplitude of RtN evoked potentials declined significantly in the L2/3 ventral gray matter of the completely lesioned side. Field potentials monitored below the ipsilaterally spared ventral quadrant remained unchanged. Depressed RtN evoked potentials in the ventral cord gradually increased during the next four weeks, and finally reached greater than 4 times of the amplitude monitored on the contralateral side. 2) The sites in which field potentials could be monitored in the lumbar spinal cord were mapped. In normal rats, the largest field was monitored near the ventral margin of the gray matter. On the other hand, in spinal cord injured ani-mals, the largest field potentials were located in more dorsal aspects of the ventral horn, suggesting a structural reorganization of the descending inputs has taken place.
CONCLUSION
The RtN evoked potentials in the ventral horn increased gradually for several weeks after the injury. The returned RtN evoked potentials below the completely lesioned side of spinal cord were larger than those seen in normal spinal cord. The time course of returning evoked potentials below the lesioned side of the spinal cord seems to coincide with the resti-tution of same-side hindlimb locomotion.