Motor recovery after spinal cord injury enhanced by strengthening corticospinal synaptic transmission

Abstract

The corticospinal tract is an important target for motor recovery after spinal cord injury (SCI) in animals and humans. Voluntary motor output depends on the efficacy of synapses between corticospinal axons and spinal motoneurons, which can be modulated by the precise timing of neuronal spikes. Using noninvasive techniques, we developed tailored protocols for precise timing of the arrival of descending and peripheral volleys at corticospinal-motoneuronal synapses of an intrinsic finger muscle in humans with chronic incomplete SCI. We found that arrival of presynaptic volleys prior to motoneuron discharge enhanced corticospinal transmission and hand voluntary motor output. The reverse order of volley arrival and sham stimulation did not affect or decreased voluntary motor output and electrophysiological outcomes. These findings are the first demonstration that spike timing-dependent plasticity of residual corticospinal-motoneuronal synapses provides a mechanism to improve motor function after SCI. Modulation of residual corticospinal-motoneuronal synapses may present a novel therapeutic target for enhancing voluntary motor output in motor disorders affecting the corticospinal tract.

Copyright © 2012 Elsevier Ltd. All rights reserved.

Figures

Figure 1

Paired-Pulse Stimulation Protocols (A) Illustration of the spike time-dependent plasticity (STDP) protocol. Here, corticospinal neurons were activated at a cortical level by using transcranial magnetic stimulation (TMS volley) delivered over the hand representation of the motor cortex and spinal motoneurons were activated antidromically by peripheral nerve stimulation (PNS volley) delivered to the ulnar nerve at the wrist. The interstimulus interval between paired pulses was designed to allow descending volleys, elicited by TMS, to arrive at the presynaptic terminal of corticospinal neurons (1st, red arrow) 1–2 ms before antidromic PNS volleys in the motoneurons reached the dendrites (2nd, black arrow). (B) Illustration of the control protocol. Here, antidromic PNS volleys were timed to reach motoneuron dendrites (1st, red arrow) 5 ms before the TMS volleys reached the presynaptic terminal (2nd, black arrow). In both protocols, 100 pairs of TMS and PNS pulses were applied at 0.1 Hz for ~17 min. (C and D) Electromyographic recordings from the first dorsal interosseous (FDI) muscle showing a representative average of the maximal motor response (M-max) and a subsequent F wave during each paired-pulse stimulation protocol (black traces) and during isolated PNS without paired-pulse stimulation (gray traces) in a participant with SCI and in a healthy control. (E and F) The graphs show the group data in SCI participants (n = 18) and in healthy controls (n = 10). The abscissa shows the number of paired pulses measured applied during each protocol (a total of 100 paired pulses). At each point, the average of ten F waves is shown. The ordinate shows the size of the F wave in millivolts. The F wave amplitude was significantly larger during the control (open purple circles, SCI; open pink triangles, healthy controls) compared to the STDP (closed purple circles, SCI; closed pink triangles, healthy controls) protocol at all points in both groups of subjects as indicated by the asterisk. Note the difference in scale in traces and graphs. Error bars indicate the SE. *p

Figure 2

Motor Evoked Potentials Transmission in the corticospinal pathway was assessed by examination of the size of MEPs elicited in the resting FDI muscle by TMS and transcranial electrical stimulation (TES) before (Baseline) and after (0, 10, 20, and 30 min) each paired-pulse stimulation protocol. Raw traces from a representative participant with SCI shows an average of 30 MEPs elicited by TMS (A) and 10 to 20 MEPs elicited by TES (B). The gray bar represents the pair stimulation (paired-pulse stimuli; 100 paired pulses at 0.1 Hz for ~17 min). Note that the size of MEPs evoked by TMS and TES was increased at all times after the STDP (upper traces) but not after the control (lower traces) protocol. Graphs show group data. The abscissa shows the time of measurements, and the ordinate shows the peak-to-peak amplitude of the MEPs elicited by TMS and TES in the FDI muscle as a percentage of the baseline MEP in participants with SCI (C and E; closed purple circles, STDP; open purple circles, control; n = 18) and in healthy controls (D and F; closed pink triangles, STDP; open pink triangles, control; n = 10). Note the increase in the size of FDI MEP elicited by TMS and TES at all times in both groups of subjects. Also note that we did not observe a significant difference between the effects reported at time 0 and later time points in (C–F). Error bars indicate the SE. *p

Figure 3

Voluntary Motor Output Voluntary motor output was assessed by examination of changes in mean force and mean rectified EMG during brief, fast, index finger voluntary contractions in the abduction direction before (Baseline) and after (0, 10, 20, and 30 min) the paired-pulse stimulation protocols. Raw force (A) and EMG (B) traces from a representative participant with SCI. At each time point, 20 raw traces are overimposed. The gray bar represents the paired-pulse stimulation (paired-pulse stimuli; 100 paired pulses at 0.1 Hz for ~17 min). Graphs show group data. The abscissa shows the time of measurements, and the ordinate shows the mean force measured during index finger abduction and mean rectified EMG activity in the FDI muscle as a percentage of the baseline in participants with SCI (C and E; closed purple circles, STDP; open purple circles, control; n = 10) and in healthy controls (D and F; closed pink triangles, STDP; open pink triangles, control; n = 10). Note the parallel increase in mean force and EMG activity after the STDP, but not the control, protocol in both groups of subjects. There were no significant differences between the effects reported at time 0 and later time points in (C–F). Error bars indicate the SE. *p

Figure 4

Manual Dexterity Manual dexterity was assessed by examination of changes in the speed to complete the nine-hole peg test (9HPT) before (Baseline) and after (0, 10, 20, and 30 min) the paired-pulse stimulation protocols in participants with SCI. (A) Individual pictures showing the steps to complete the 9HPT. Note that pictures 1–3 show the part of the test were each pin is lifted by a precision grip between the index and thumb and deposited into the reservoir located on the side, while pictures 4–6 show that each pin is pick up and repositioned back into each hole by a precision grip between the index and thumb. (B) Graph shows group data in participant with SCI (n = 8). The abscissa shows the time of measurements, and the ordinate shows the time to complete the 9HPT as a percentage of the baseline after the STDP (closed purple circles) and control (open purple circles) protocols. Note the improvements to complete the 9HPT after the STDP but not the control protocol. Error bars indicate the SE. *p