Recording of spared motor evoked potentials and its augmentation by 4-aminopyridine in chronic spinal cord-injured rats

Objective To research the direct electrophysiological evidence of discomple te spinal card injury (SCI) and the effect of 4-aminopyridine on it. Methods Motor evoked potentials (MEPs), both spinal cord recorded MEPs (scM EPs) and extracellularly recorded MEPs (exMEPs) were recorded and character ized on a T-13 epidural electrode (scMEPs) and an extracellular microelectr ode (exMEPs) for 10 normal rats and 40 rats with lesions of various severit y (sham, 35 g . cm force (gcf), 70 gcf, 100 gcf impact injury) at the T-8-T -9 cord using the Alien's drop model. The incline plane and Tarlov techniqu es were used to assess clinical neurological function. Results MEPs in the normal rats were elicited by applying transcortical sup rathreshold stimulation consisting of 3 - 4 early negative peaks (N-1, N-2, N-3 and N-4) followed by several late waves. The N-1 and N-2 peaks were la rgest in the anterior and ventrolateral funiculus, respectively, which was indicative of extrapyramidal pathways. The 100 gcf impact injuries and the cord transection abolished the MEP distal to the lesion, whereas the 35 gcf injuries resulted in a latency shift and amplitude decrement of the MEP pe aks. Eighteen of the 20 rats with 70 gcf injuries showed clinical paraplegi a. Among them, 7 rats had neurophysiological evidence of residual conductio n pathways through the lesioned cord segment, such as the presence of N-1 a nd N-2 peaks in the scMEPs or exMEPs. After LF-aminopyridine (4-AP) adminis trations (1 mg/kg), the amplitude of the spared exMEP increased significant ly and spread more widely. Conclusions MEPs evoked by transcortical stimulation travel mostly in the e xtrapyramidal tract. MEP monitoring could provide an excellent method of de tecting the functional integrity of the motor tracts after SCI, and could e ven detect spared motor fibers after discomplete SCI. Furthermore, the use of 4-AP or other K+ channel blocking agents may be a potential treatment fo r patients with chronic moderate to severe SCI.