Neurophysiological evidence of spared upper motor conduction fibers in clinically complete spinal cord injury: discomplete SCI in rats

Motor evoked potentials (MEP) were recorded and characterized by epidural e lectrodes (scMEP) and extracellular microelectrodes (exMEP) on T-13 level f rom 10 normal rats and 40 rats with chronic spinal cord injury (SCI). The s pinal cord of 40 anesthetized rats were injured with various severity (sham , 35, 70, and 100 g/cm impact injury) at T-8-T-9 cord using Allen's drop mo del. The incline plane and Tarlov techniques were investigated to assess cl inical neurological function. MEPs in the normal rats elicited by applying transcortical suprathreshold stimulation consisted of 3-4 early negative pe aks (N-1, N-2, N-3, N-4) followed by several late waves. The N-1 and N-2 pe aks had their maximal amplitudes in the anterior and ventrolateral funiculu s, respectively, irrespective of the polarity of stimulation, which indicat ed that these impulses were conducted mostly through the extrapyramidal pat hways. The 100 g/cm impact injury or transection of the cord caused abolish ment of the MEP signals distal to the lesion, whereas the 35 g/cm injury re sulted in a latency shift and amplitude decrement of the MEP peaks. Out of 20 rats with 70 g/cm injuries, 18 showed clinically paraplegia. Among them, seven had neurophysiological evidence of residual conduction pathways thro ugh the injured cord segment, such as the presence of N-1 and N-2 peaks in scMEP or exMEP. After 4-aminopyridine (4-AP) administration (1 mg/kg), the amplitude of spared exMEP increased significantly and spread more widely. T hese results suggest that MEPs evoked by transcortical stimulation travel m ostly in the extrapyramid tract. The present study provides further direct and objective electrophysiological evidences of spared functional axons aft er discomplete SCI, since many other studies on this field have achieved si milar results previously. Furthermore, pharmaceutical treatment with 4-AP o r other K+ channel blocking agents proved to be a potential therapeutic str ategy for patient with chronic SCI. (C) 2001 Elsevier Science B.V. All righ ts reserved.