Electrical neuromodulation of the cervical spinal cord facilitates forelimb skilled function recovery in spinal cord injured rats

Abstract

Enabling motor control by epidural electrical stimulation of the spinal cord is a promising therapeutic technique for the recovery of motor function after a spinal cord injury (SCI). Although epidural electrical stimulation has resulted in improvement in hindlimb motor function, it is unknown whether it has any therapeutic benefit for improving forelimb fine motor function after a cervical SCI. We tested whether trains of pulses delivered at spinal cord segments C6 and C8 would facilitate the recovery of forelimb fine motor control after a cervical SCI in rats. Rats were trained to reach and grasp sugar pellets. Immediately after a dorsal funiculus crush at C4, the rats showed significant deficits in forelimb fine motor control. The rats were tested to reach and grasp with and without cervical epidural stimulation for 10weeks post-injury. To determine the best stimulation parameters to activate the cervical spinal networks involved in forelimb motor function, monopolar and bipolar currents were delivered at varying frequencies (20, 40, and 60Hz) concomitant with the reaching and grasping task. We found that cervical epidural stimulation increased reaching and grasping success rates compared to the no stimulation condition. Bipolar stimulation (C6- C8+ and C6+ C8-) produced the largest spinal motor-evoked potentials (sMEPs) and resulted in higher reaching and grasping success rates compared with monopolar stimulation (C6- Ref+ and C8- Ref+). Forelimb performance was similar when tested at stimulation frequencies of 20, 40, and 60Hz. We also found that the EMG activity in most forelimb muscles as well as the co-activation between flexor and extensor muscles increased post-injury. With epidural stimulation, however, this trend was reversed indicating that cervical epidural spinal cord stimulation has therapeutic potential for rehabilitation after a cervical SCI.

Keywords: Cervical spinal cord injury; Corticospinal tract; Epidural electrical stimulation; Motor-evoked potentials; Reaching and grasping.

Figure 1:. Experimental design.

Fifteen rats were trained to reach and grasp sugar pellets with their preferred paw. The 12 rats having the best performance were implanted with stimulating epidural electrodes at spinal cord levels C6 and C8 and recording intramuscular electrodes in several forelimb muscles (Surgery 1). Baseline reaching and grasping performance was determined and spinal motor-evoked potentials (sMEPs) were recorded for 4 weeks post-implantation. Subsequently, all 12 rats received a dorsal funiculi crush at C4 (Surgery 2). In the following 10 weeks post-injury, reaching and grasping performance and sMEPs were tested biweekly. In each testing session, reaching and grasping performance was tested pre-, during, and post-stimulation using four different electrode configurations. Three stimulation frequencies (20, 40, and 60 Hz) were tested on alternate days of each week of testing.

Figure 2:. Cervical electrical stimulation facilitates forelimb reaching and grasping function after a cervical SCI.

(A) Mean (±SEM) threshold current required for each stimulation electrode configuration to elicit spinal motor evoked potentials (sMEPs) in any of the five forelimb muscles tested pre-injury and at different time points post-injury. C6+ Rf− and C8+ Rf− vs. other electrode configurations (*p < 0.05). C6+ C8− configuration at weeks 5 and 10 vs. week 1 (^†p < 0.05). (B) Mean (±SEM) success rates for reaching and grasping when the rats were not receiving epidural stimulation (blue line) and the average of all the success rates obtained with the combination of all the stimulation parameters tested during (red line) and immediately after (black line) receiving epidural stimulation. Pre-injury vs. post-injury without stimulation (*p < 0.05). Pre-stimulation vs. with stimulation (^†p < 0.05) and post-stimulation (^††p < 0.05). Difference between pre-stimulation and post-stimulation at week 1 vs. other post-injury time points (^†††p < 0.05). (C) Effects of the stimulation electrode configuration on reaching and grasping performance. Mean (±SEM) success rates for reaching and grasping with all four electrode configurations were obtained by combining all of the frequencies tested. C6− C8+ vs. C6− Rf+ (^†p < 0.05). C6+ C8− vs. C6− Rf+ (^††p < 0.05) and C8− Rf+ (^†††p < 0.05). (D) Effects of stimulation frequency on reaching and grasping performance. Mean (±SEM) success rates for reaching and grasping for all three stimulation frequencies showed no differences when combining all stimulation electrode configurations tested.

Figure 3:. Effects of cervical electrical stimulation on forelimb muscle synergies post-injury.

(A) Raw EMG signals from the forelimb muscles during reaching and grasping pre-injury (blue trace) and 1 week post-injury (red trace), and the mean (±SD, color shading) rectified (Rec.) EMG signals (n = 20 trials each) from the same rat. Black arrow indicates the initiation of lifting the forelimb paw. (B) Raw EMG signals from the forelimb muscles during reaching and grasping at 10 weeks post-injury pre-stimulation (pink traces) and post-stimulation (black traces), and the mean (±SD, color shading) rectified EMG signals (n = 20 trials each) from the same rat. Black arrow indicates the initiation of lifting the forelimb paw. (C) Comparison of the mean (±SEM) integrated EMG values (n = 40 trials, 5 rats) obtained pre-injury and 1 week post-injury during reaching and grasping without stimulation. Two muscles, i.e., the deltoid and extensor, had significantly higher EMG activity levels 1-week post-injury compared to pre-injury when performing the task. (D) Comparison of the mean (±SEM) integrated EMG values (n = 40 trials, 5 rats) obtained prior (pre-stim) and immediate after (post-stim) receiving epidural stimulation at 10 weeks post-injury. *: significantly different at p < 0.05

Figure 4:. Effects of cervical electrical stimulation on antagonistic distal forelimb muscle activation during reaching and grasping.

Joint probability density distributions for the flexor digitorum (FD) and extensor digitorum (ED) in a rat pre-injury without stimulation (A), and 10 weeks post-injury without (B) and with (C) C6+ C8− stimulation are shown. (D) Mean (±SEM) percentage of co-activation during reaching and grasping (normalized to maximum observed for any of the three experimental conditions). *: lower than all conditions post-injury (p < 0.05). †: lower than the co-activation during failed forelimb reaching attempts post-injury without stimulation (p < 0.05). (E) Qualitative scores of accuracy (mean ±SEM; n = 60 trials) for the seven components of reaching and grasping at 10 weeks post-injury with and without epidural stimulation. *: higher than without stimulation (p < 0.05).

Figure 5:. Changes in spinal excitability after SCI.

(A) Examples of sMEPs evoked in each forelimb muscle during C6− C8+ stimulation (400 μA current intensity) of the same rats preinjury and at 1 and 10 weeks post-injury. Each panel shows 30 superimposed sMEPs (different colors) with the mean shown in black. sMEPs were rectified (Rec.) and the means for each time point for each muscle are shown in the bottom row. The vertical dashed lines in the bottom row separate the early (<10 ms) and late (10-30 ms) sMEP responses. (B) Changes in the mean (±SEM) sMEP magnitudes normalized to the maximum value (area under the rectified curve) of the early and late responses for each muscle pre-injury (black trace) and at 1 (red trace) and 10 (green trace) weeks post-injury are shown as a function of increasing stimulation current intensities.