Extensive modifications to ascending somatosensory pathways, descending motor pathways, and sensorimotor cortices follow incomplete spinal cord injury (SCI).Reference Levy, Amassian, Traad and Cadwell 1 , Reference Curt, Bruehlmeier, Leenders, Roelcke and Dietz 2 Following experimentally induced deafferentation, inhibitory GABAergic cortical activity in sensorimotor cortices is reduced,Reference Levy, Ziemann, Chen and Cohen 3 - Reference Welker, Soriano and Van der Loos 5 which is thought to mediate rapid cortical plasticity.Reference Levy, Ziemann, Chen and Cohen 3 , Reference Jacobs and Donoghue 6 In SCI, reductions in GABAergic cortical activity are suggested to promote rapid functional gains.Reference Smith, Savic, Frankel, Ellaway, Maskill and Jamous 7 However, there is evidence of sustained alterations in cortical inhibition,Reference Smith, Davey, Savic, Maskill, Ellaway and Jamous 8 , Reference Shimizu, Hino, Komori and Hirai 9 which may lead to alterations in GABA-mediated or GABA-modulated motor cortical circuitry.
Short-interval intracortical inhibition (SICI) is an inhibitory motor cortical circuit evoked by a subthreshold transcranial magnetic stimulation (TMS) pulse (i.e. conditioning stimulus [CS]) followed by a suprathreshold TMS pulse (i.e. test stimulus [TS]). When the TS follows the CS by 1 to 5 ms, the motor-evoked potential (MEP) is suppressed.Reference Kujirai, Caramia, Rothwell, Day, Thompson and Ferbert 10 SICI is thought to be cortically mediatedReference Di Lazzaro, Restuccia, Oliviero, Profice, Ferrara and Insola 11 via GABAA receptor transmissionReference Ziemann, Lonnecker, Steinhoff and Paulus 12 - Reference Ziemann, Lonnecker, Steinhoff and Paulus 14 because late and not early indirect waves (I-waves) are inhibited.Reference Di Lazzaro, Restuccia, Oliviero, Profice, Ferrara and Insola 11 , Reference Hanajima, Ugawa, Terao, Sakai, Furubayashi and Machii 15 These data suggest that SICI modifies intracortical neuron activity, which inhibits the I-3 and later I-waves instead of acting directly on the pyramidal neurons, which results in a net reduction of the corticospinal output to the spinal cord. In controls, SICI is modulated in advance of and during active muscle contraction and is therefore suggested to participate in the control of movement.Reference Ridding, Taylor and Rothwell 16 - Reference Sohn, Wiltz and Hallett 18
In incomplete SCI, the depth of SICI is reduced in the actively contracted tibialis anterior (TA) muscle when compared with controls, although the circuit is elicited at similar intensities in the two groups.Reference Roy, Zewdie and Gorassini 19 The same report also indicates the presence of SICI in the contracted first dorsal interosseous muscle, although it is unknown how this differs from uninjured populations, and specifically in SCI with cervical injury. Collectively, these data contribute to the hypothesis that cortical inhibition is reduced as a result of incomplete SCI and suggest an altered state of GABAA-mediated transmission. However, in SCI, it remains unknown whether the SICI profile is abnormal in a muscle of the upper limb such as the flexor carpi radialis (FCR) muscle. In our experience, chronic incomplete tetraplegia with injury ranging from C3 to C7 retains the ability to perform isometric contraction of the FCR muscle, albeit this ability varies greatly across individuals. This is an important experimental consideration because reliably evoked MEPs of discernable amplitude typically require SCI participants to actively contract the targeted muscle. The present study is focused on FCR, a muscle important in wrist control to achieve grasping, pulling, and writing,Reference Brorsson, Nilsdotter, Thorstensson and Bremander 20 movements that may be lost as a result of cervical SCI.Reference Smith, Savic, Frankel, Ellaway, Maskill and Jamous 7
Long-interval intracortical inhibition (LICI) is elicited by a suprathreshold CS depressing the MEP evoked by a subsequent suprathreshold TS delivered 50 to 200 ms later.Reference Valls-Sole, Pascual-Leone, Wassermann and Hallett 21 , Reference Wassermann, Samii, Mercuri, Ikoma, Oddo and Grill 22 Cervical epidural recordings of descending corticospinal volleys suggest a cortical origin for the LICI circuit; later I-waves of the second suprathreshold stimulus are significantly reduced at interstimulus intervals (ISIs) of 100 to 150 ms, whereas the early I-wave is unaltered.Reference Nakamura, Kitagawa, Kawaguchi and Tsuji 23 - Reference Di Lazzaro, Oliviero, Mazzone, Pilato, Saturno and Insola 25 LICI appears to be mediated via GABAB receptors such that baclofenReference McDonnell, Orekhov and Ziemann 26 and other drugs acting on GABA transmission increase LICI.Reference Werhahn, Kunesch, Noachtar, Benecke and Classen 27 , Reference Pierantozzi, Marciani, Palmieri, Brusa, Galati and Caramia 28 One study demonstrated that LICI can be elicited in SCI at rest and continues to persist during active contraction of first dorsal interosseous.Reference Barry, Bunday, Chen and Perez 29 However, in SCI, the LICI recruitment profile remains unknown.
It was hypothesized that uninjured control participants would exhibit SICI at CS intensities of 70%, 80%, and 90% AMT as described in other studies.Reference Kujirai, Caramia, Rothwell, Day, Thompson and Ferbert 10 , Reference Ridding, Taylor and Rothwell 16 In accordance with Roy et al, (2011) who studied SICI in the TA muscle in SCI, we hypothesized SICI to exist at CS of 80% AMT, thus representing a narrow range of SICI recruitment. It was further hypothesized that LICI would exist in uninjured controls at suprathreshold intensities (i.e. 100%, 110%, 120%, and 130% AMT).Reference McNeil, Martin, Gandevia and Taylor 30 In SCI, we hypothesize LICI to exist at a reduced range of only the highest CS intensity (i.e. 130% AMT), in support of the very large intensity required to elicit LICI in SCI elsewhere.Reference Barry, Bunday, Chen and Perez 29
In this study, we obtained SICI and LICI recruitment curves from the contracted FCR muscle in incomplete cervical SCI and age-matched controls. Thorough characterization of the intracortical inhibitory circuits following incomplete SCI is important for determining changes in motor cortical GABAergic transmission. Identifying aberrations within intracortical GABAergic circuits is fundamental for experimental approaches that promote plasticity in motor cortex to improve arm function in SCI.
Methods
Participants
Eight incomplete SCI (mean age, 29.5 years; standard deviation, 6.7; seven males; eight right-handed participants) and thirteen aged-matched uninjured individuals (mean age, 28.8 years; standard deviation, 5.6; eight males; nine right-handed participants) participated. Because of the asymmetry of the lesion location and the heterogenic nature of the damage to the spinal cord following SCI, each limb from a SCI participant was treated as an independent limb, resulting in the participation of 13 SCI limbs. Demographic, lesion, and medication information for all SCI participants is found within Table 1. SCI limbs that were unable to perform the required isometric contraction were excluded from analysis. All participants provided written informed consent and the study was approved by the Office of Research Ethics at McMaster University and conformed to the Declaration of Helsinki.
Table 1 Demographics of participants
The age of subject, the number of years after the subject sustained a spinal cord injury measured at the time of the experiment, and the cause of injury are shown; sides tested are indicated as left or right flexor carpi radialis and medications taken by subjects are also listed. AMT, active motor threshold; ASIA, American Spinal Injury Association; MSO, mean stimulator output.
Electromyography
Muscle activity was recorded using 9-mm diameter silver–silver chloride surface electrodes with the active electrode being placed over the muscle belly of the FCR and the reference electrode placed over the tendon at the wrist. The ground electrode was placed over the medial styloid process at the wrist. Electromyography (EMG) was amplified at 1000×gain, bandpass filtered (20 to 2.5 kHz, Intronix Technologies Corporation Model 2024F, Bolton, Ontario, Canada) and digitized (5 kHz, Micro 1401, Cambridge Electronics Design, Cambridge, UK). Signal software (v5, Cambridge Electronic Design Limited, Cambridge, UK) was used to acquire and analyze EMG data.
Maximum Voluntary Contraction
To determine the maximum voluntary contraction (MVC) for FCR, participants were asked to maximally contract the forearm during wrist flexion. Three trials (each 3 seconds) separated by 18 seconds of relaxation were performed. This duration of intertrial rest sufficiently allowed the maximum EMG to be obtained in each subsequent trial (see the following analysis). MVC was calculated as the rectified area of a 38-ms window that fell within their maximum contraction period during each of the three trials. In cases where participants expressed discomfort from the high stimulation intensity (i.e. 80%, 90%, or 100% mean stimulator output [MSO]), one trial rather than three would be acquired for the recruitment curves. The duration of 38 ms was chosen to allow direct comparison with background EMG measured during SICI and LICI testing as described in the following section. SICI and LICI recruitment curves were obtained during active isometric contraction of 15% to 20% MVC in FCR in all participants. As reported elsewhere, resting MEPs tend to be small or absent in many individuals with SCI; thus, all responses were evoked during a tonic contraction.Reference Roy, Zewdie and Gorassini 19 , Reference Roy, Yang and Gorassini 31
TMS
TMS was delivered using a custom built 50-mm diameter figure-of-eight branding coil connected to a Magstim 200 stimulator (Magstim, Whitland, UK). The position and orientation of the coil was monitored throughout the experiment using Brainsight Neuronavigation (Rogue Research, Montreal, Canada) with optical sensors placed on the coil and the participant. The same TMS coil was used for all measures in both experiments. The TMS coil delivered a monophasic pulse over the optimal location to elicit MEPs in the relaxed FCR at 45° in relation to the parasagittal plane to induce a posterolateral to anteromedial current in the cortex. The motor hotspot was defined at this location and was where all subsequent measures were taken.
The active motor threshold (AMT) of the SCI group was defined as the stimulator intensity which resulted in MEPs 50 µV larger than the background EMG in 5 of 10 consecutive trials during a constant contraction of 20% MVC in FCR. The age-matched control participants were then asked to maintain a constant contraction equal to 20% MVC of the SCI match participant (using the voltage of the EMG); similarly, AMT was defined as 50 µV larger than the background EMG in 5 of 10 consecutive trials. Although this approach equates the magnitude of AMT based on matched EMG contraction, it remains unclear whether perceived exertion to achieve the 20% MVC was similar between groups.
MSO Required to Elicit a Rectified Area Equal to Half of the Maximum
The maximum MEP in mV*ms was defined as the largest rectified area in a 70-ms window following the TMS artifact and was identified by increasing the MSO in 10% increments from 10% to 100% area was measured to avoid ambiguities in identifying peaks within polyphasic MEPs that are characteristic of the FCR muscle.Reference Byblow, Stinear, Smith, Bjerre, Flaskager and McCambridge 32 Three trials were typically obtained at each increment and averaged. MEPhalfmax was defined as the MSO required to elicit a rectified area equal to half of the maximum. MEPhalfmax was recorded and confirmed in 10 subsequent trials.
SICI and LICI
During SICI and LICI testing, the EMG representing 15% to 20% MVC from FCR was displayed as a bright line on an oscilloscope; participants were required to match the position of the FCR EMG controlled line to the 20% MVC target line. Background EMG was calculated as the rectified area of the first 38 ms of each trial window, the length of time from the start of the trial to the onset of TMS stimulation, and expressed as a percent of MVC that was also acquired over a 38-ms window.
SICI in FCR was tested at a CS-TS ISI of 3 ms. Thirteen SCI limbs and 13 control limbs were tested. The CS was delivered at intensities of 60%, 70%, 80%, 90%, 100%, and 110% AMT. For all participants, the TS intensity was set to elicit MEPhalfmax as performed elsewhere.Reference Roy, Zewdie and Gorassini 19 , Reference Roy, Yang and Gorassini 31 This allows the TS to be set at an intensity that enables either suppression or facilitation of the MEP to be observed, a choice pertinent to studies in SCI where MEPs are generally reduced. Fifteen conditioned MEPs (i.e. MEPCS-TS) were collected for each CS intensity and 15 unconditioned MEPs (MEPTS) were collected. The CS intensities were tested in separate blocks and the order of blocks was randomized for each participant. Consecutive stimuli were delivered every 5 seconds.
LICI in FCR was tested at a CS-TS ISI of 150 ms. Twelve SCI limbs and 13 control limbs were tested. The CS was delivered at an intensity of 90%, 100%, 110%, 120%, and 130% AMT. TS intensity was set at MEPhalfmax. ISI of 150 ms has been shown to evoke strong LICI for CS intensities between 100% and 120% MT.Reference Valls-Sole, Pascual-Leone, Wassermann and Hallett 21 Similar to SICI, 15 conditioned MEPs were acquired for each CS intensity and 15 unconditioned MEPs were acquired. CS intensities were tested in separate blocks and the block order was randomized. Consecutive stimuli were delivered every 5 seconds.
Analysis
MEPs were measured as the rectified area of the EMG response in a 70-ms window following the TMS artifact. Normalized MEP areas (i.e. MEPCS-TS/MEPTS) were subjected to a two-way analysis of variance (ANOVA) with between-subject factor groups (two levels: control and SCI) and within-subject factor CS intensity (SICI with six levels: 60%, 70%, 80%, 90%, 100%, and 110% AMT; LICI with five levels: 90%, 100%, 110%, 120%, and 130% AMT). To test the hypothesis that SICI and LICI exist at CS intensities that typically evoke these circuits (SICI: CS 70%, 80%, and 90%; LICI: 120% and 130%), Bonferroni corrected a priori t-tests compared the unnormalized unconditioned MEP area (i.e. MEPTS) versus the unnormalized conditioned MEP area (i.e. MEPCS-TS). Further, between-group differences were compared using Bonferroni corrected a priori t-tests at 90% AMT (SICI) and at 120% and 130% AMT (LICI). Post-hoc Tukey’s test was used to test for significant main and/or interaction effects following ANOVAs. MVC was analyzed in a two-way repeated measure ANOVA with within factor blocks (three levels: blocks 1, 2, and 3) and between-factor groups (two levels: control and SCI). Background EMG was tested using the previous two-way ANOVA statistical model. MEP recruitment curves were analyzed in a two-factor ANOVA with between-subject factor groups (two levels: control and SCI) and within-subject factor intensities (10 levels: 10 through 100 MSO). To test whether SCI participants experienced fatigue over the course of the testing session, background EMG was further analyzed as a function of the factor time (five levels: blocks 1 through 5). Effect sizes were calculated using Cohen’s D. If the assumptions of sphericity were not met, Greenhouse-Geisser corrections were used. Statistical significance was set at p<0.05.
Results
EMG during MVC was not different between groups, as revealed by two-way ANOVA (F(1,16)=0.415, p=0.529) (SCI, 30.08±31.9 mV*ms; control, 38.22±18.8 mV*ms). Likewise, there was no difference in MVC among the three blocks (F(2,32)=0.144, p=0.866). One-way repeated measures ANOVA indicated that participants with SCI did not fatigue over the duration of the experiment (time, F(2.216,36.146)=2.072, p=0.138). Background EMG was lower in SCI compared with controls (SCI, 2.75±0.56 mV*ms; control, 4.34±0.37 mV*ms; unpaired two-tailed t-test; p=0.033). However, relative EMG, normalized to individual MVCs, was not different between groups (SCI, 24±7.1%; control, 22±2.9%; unpaired two-tailed t-test; p=0.76), indicating that both groups of participants were exerting the same percent of their maximum output of their muscles. The group-averaged MSO (with standard deviation) for delivery of the TS (i.e. unconditioned MEP) was 44±6% in controls and 47±7% MSO in SCI. The MSO values were not statistically different (two-tailed, unpaired t-test, p=0.33). A clear peak-to-peak unconditioned MEP was observable in six SCI participants (nine limbs). From these individuals, the peak-to-peak MEP amplitude ranged from ~1 to 6 mV; however, all analyses were performed on the MEP area.
AMT was significantly greater in SCI compared with controls (SCI, 36±6.8 % MSO; control, 28±1.1% MSO; unpaired two-tailed t-test; p=0.002). Because the effects of SICI and LICI can be explained by the higher CS intensity used in participants with SCI, we analyzed SICI and LICI data only from participants in whom the range of AMT overlapped. The AMT matching criterion was designated as any AMT values that fell within 2 standard deviations of the group-averaged mean AMT of the uninjured controls. Any AMT values from either group that fell outside of that range were excluded from further analysis. Therefore, nine SCI limb data (n=6 SCI participants) and nine aged-matched controls with similar AMT values (SCI, 33±4.8% MSO; control, 30±3.0% MSO; unpaired two-tailed t-test; p=0.20) were subsequently analyzed and the AMT values for all participants pre- and post-AMT matching are shown in Figure 1A. This procedure therefore controlled for group differences in CS intensities that could affect the depth of SICI and LICI. Figure 1B displays the group-averaged MEP recruitment curve for SCI and controls (AMT matched). Two-way ANOVA revealed an effect of INTENSITY (F(1.396,22.337)=41.784, p<0.01) and an intensity×group interaction (F(1.396,22.337)=18.654, p<0.01). Post-hoc Tukey’s revealed greater MEP area in controls versus SCI at all intensities ≥50% MSO (p<0.05).
Figure 1 (A) Active motor threshold (AMT) of controls and spinal cord injury (SCI; left). Prematching showed significantly greater AMT in SCI than controls (right). Matching the AMT values between the two groups removed any effects caused by differences in conditioning stimulus (CS) intensity because of AMT on short- (SICI) and long-interval intracortical inhibition (LICI). *Significant differences between the two groups. The dashed line represents the average AMT for either controls or SCI. (B) Motor-evoked potential (MEP) recruitment curves of AMT matched controls and SCI. Group-averaged MEP area data for all ten stimulator intensities. *Differences between the control and SCI groups observed at all intensities greater than 50% MSO.
SICI
Nine SCI limbs and nine controls were included in the analysis of the SICI experiment. Figure 2A displays representative SICI data from an uninjured control and an SCI participant displaying a U-shaped recruitment curve for both groups. MEPhalfmax was significantly greater in controls versus SCI (control, 45.12±20.89 mV*ms; SCI, 16.97±10.04 mV*ms; unpaired two-tailed t-test; p=0.0036). The group-averaged recruitment profile of SICI, graphing the normalized MEP data in control and SCI participants during an isometric contraction is shown in Figure 2B. Two-way ANOVA revealed no effect of CS (F(1.79,28.71)=3.267, p=0.057) or group (F(1,16)=0.039, p=0.846) or CS*GROUP interactions (F(1.79,28.71)=1.08, p=0.347). For uninjured controls, a priori Bonferroni corrected t-tests revealed the presence of SICI at all three hypothesized CS intensities of 70% (one-tailed paired t-test, p=0.01, Cohen’s D=0.5), 80% (one-tailed paired t-test, p=0.014, Cohen’s D=0.47), and 90% (one-tailed paired t-test, p=0.013, Cohen’s D=0.5) AMT. For SCI, SICI was not present at 70% (one-tailed paired t-test, p=0.105) or 80% AMT (one-tailed paired t-test, p=0.082), but did demonstrate SICI at 90% AMT (one-tailed paired t-test, p=0.0043, Cohen’s D=0.46). Similar to the analysis performed elsewhere,Reference Roy, Zewdie and Gorassini 19 we subsequently tested differences between SCI and controls at 90% AMT and revealed no significant differences between the two groups (two-tailed paired t-test, p=0.086). Group-averaged background EMG is shown in Figure 2C and two-way ANOVA revealed no effect of CS (F(5,120)=1.017, p=0.41) or group (F(1,24)=0.375, p=0.55) or CS×group interaction (F(5,120)=2.049, p=0.08). In addition to AMT matching analysis was performed on the complete dataset, without AMT matching and is shown in Figure 2D (n=13, both groups). Similar to the above, the two-way ANOVA revealed no effect of CS (F(2.06,49.42)=2.856, p=0.066), group (F(1,24)=1.197, p=0.29), or CS×group interaction (F(2.06,49.42)=0.886, p=0.42). For uninjured controls, SICI was not present at any CS intensity. Similar to the “matched” data, SICI was only present at 90% AMT for SCI (one-tailed paired t-test, p=0.0009) and there no statistical differences between SCI and control at the 90% AMT (two-tailed paired t-test, p=0.10).
Figure 2 Short-interval intracortical inhibition (SICI). (A) Raw traces of SICI recruitment in controls and spinal cord injury (SCI). (B) SICI recruitment curves of controls and SCI with active motor threshold (AMT) matching between groups. Group-averaged motor-evoked potential (MEP) data (with standard error) normalized to the unconditioned MEP for each of the conditioning stimulus (CS) intensities. Symbols represent differences between the unconditioned and conditioned MEP within the control group (*) and within the SCI group (§), indicating presence of SICI. (C) SICI background electromyelography (EMG) of controls and SCI. Group-averaged EMG ratio data (with standard error) normalized to the EMG of the unconditioned state for all CS intensities. (D) SICI recruitment curves of controls and SCI without AMT matching (n=13, both groups). Group-averaged MEP data (with standard error) normalized to the unconditioned MEP for each of the CS intensities. Group-averaged MEP data (with standard error) normalized to the unconditioned MEP for each of the CS intensities. Symbols represent differences between the unconditioned and conditioned MEP within the SCI group (§) indicating presence of SICI.
LICI
Nine SCI limbs and nine controls were included in the analysis of the LICI experiment. Figure 3A displays representative LICI data from an uninjured control and an SCI participant displaying an increase in inhibition with increase in CS intensity for SCI and no LICI in control participants. MEPhalfmax was significantly greater in controls than SCI (control, 47.13±20.66 mV*ms; SCI, 16.09±7.47 mV*ms; unpaired two-tailed t-test; p=0.0017). The group-averaged recruitment profile of LICI graphing the normalized MEP data in control and SCI participants during an isometric contraction is shown in Figure 3B. Two-way ANOVA revealed no effect of CS (F(2.46,39.29)=2.627, p=0.074), but demonstrated significant effect of group (F(1,16)=4.353, p=0.05) and CS×group interaction (F(2.46,39.29)=3.404, p=0.035). A priori Bonferroni corrected t-tests test revealed LICI in SCI at CS intensities of 120% (one-tailed paired t-test, p=0.003, Cohen’s D=0.48) and near significance at 130% (one-tailed paired t-test, p=0.03) AMT. No LICI was observed in uninjured controls at either 120% (one-tailed paired t-test, p=0.48) or 130% (one-tailed paired t-test, p=0.44) AMT. Differences between controls and SCI were observed at 120% (one-tailed paired t-test, p=0.012) and 130% AMT (one-tailed paired t-test, p=0.012). Figure 3C displays the group-averaged background EMG for each CS intensity normalized to the background EMG of the unconditioned MEP. The two-way ANOVA performed for the background EMG revealed no effect of CS (F(4,92)=1.319, p=0.27), group (F(1,23)=0.957, p=0.34), and no CS×group interaction (F(4,92)=0.032, p=0.99). The data without AMT matching is shown in Figure 3D (n=13, both groups). Similar to the “matched AMT” results, two-way ANOVA revealed no significance of CS intensity (F(4,92)=1.347, p=0.26), but demonstrated significant effects of group (F(1,23)=5.248, p=0.031) and CS×group (F(4,92)=6.835, p<0.001). Post-hoc Tukey’s test revealed differences between the SCI and controls at CS intensities of 120 % (p=0.001) and 130% (p<0.001) AMT. Follow-up paired two-tailed t-tests were performed at the different CS intensities to identify the presence of the LICI circuit within either the control or SCI groups. Controls did not demonstrate LICI at 120% (p=0.66) or 130% (p=0.60) AMT. In contrast and similar to the “matched AMT” analysis, SCI demonstrate LICI circuit at both 120% (p=0.007) and 130% (p=0.028) AMT.
Figure 3 Long-interval intracortical inhibition (LICI). (A) Raw traces of LICI recruitment in controls and spinal cord injury (SCI). (B) LICI recruitment curves of controls and SCI with active motor threshold (AMT) matching between groups. Group-averaged motor-evoked potential (MEP) data (with standard error) normalized to the unconditioned MEP for each of the conditioning stimulus (CS) intensities. Control MEP ratios are significantly different from SCI MEP ratios at 120% and 130%. *Significant differences between the two groups. (C) Active short-interval intracortical inhibition (SICI) background electromyelography (EMG) of controls and SCI. Group-averaged EMG ratio data (with standard error) normalized to the EMG of the unconditioned state for all CS intensities. (D) LICI recruitment curves of controls and SCI without AMT matching. Group-averaged MEP data (with standard error) normalized to the unconditioned MEP for each of the CS intensities. Control MEP ratios are significantly different from SCI MEP ratios at 120% and 130%. *Significant differences between the two groups.
Discussion
In this study, we observed higher AMT and lower MEPhalfmax in the SCI participants compared with the uninjured controls. However, the relative background contraction was not different between the two groups. Because of the differences in AMT between the two groups, a subset of SCI data was used that allowed AMT matching such that group differences in SICI and LICI cannot be attributed to differences resulting from greater CS intensities in one group compared with the other. In SCI, SICI was evoked at a CS intensity of 90% AMT only. Uninjured controls demonstrated SICI at all three hypothesized CS intensities. Our findings confirm previous observations in the TA muscle of controls and in chronic tetraplegia, where the range of SICI recruitment appears to be different.Reference Roy, Zewdie and Gorassini 19 With an isometric contraction of the FCR muscle, LICI remains intact in SCI but is absent within controls. Previous reports have observed LICI to occur in both uninjured and postspinal cord injured participants at a very high CS intensity during an isometric contraction.Reference Barry, Bunday, Chen and Perez 29 Contrary to these observations, our data demonstrate that none of the hypothesized suprathreshold CS intensities elicited LICI in uninjured controls while LICI was observed in SCI at the highest CS intensity tested. The persistence of LICI in SCI alludes to an altered function of this circuit in the presence of muscle activity.
AMT and MEPs
The increase in AMTReference Roy, Zewdie and Gorassini 19 , Reference Davey, Smith, Savic, Maskill, Ellaway and Frankel 33 , Reference Freund, Rothwell, Craggs, Thompson and Bestmann 34 and decrease in MEPsReference Edwards, Cortes, Thickbroom, Rykman, Pacsual-Leone and Volpe 35 are consistent with previous literature on the changes to these measurements following an injury to the spinal cord. These changes are likely attributed to the decreases in corticospinal output because of injury.
SICI
The magnitude of SICI in the contracted FCR muscle was not different between SCI and controls. However, we did observe that the range of CS intensities capable of evoking significant SICI within each group were different. In controls, CS intensities of 70%, 80%, and 90% AMT evoked significant SICI. In SCI, however, the only CS intensity to evoke significant SICI was 90% AMT. These data suggest that SICI in FCR may indeed be observed in SCI but requires a relatively high CS intensity. These data support a previous finding whereby SICI was only observed in the TA muscle in SCI using a CS intensity at 80% AMT, whereas a larger range of CS intensities (60% to 90% AMT) evoked SICI in controls.Reference Roy, Zewdie and Gorassini 19 However, these authors also report greater SICI in controls versus SCI, an effect we did not observe. This difference can be attributed to the specific muscle tested (TA vs FCR). One mechanism to explain the high CS intensity required to evoke SICI in SCI relates to reductions in GABA concentration and/or GABAA-mediated transmission. As reported elsewhere, damage to ascending pathways leads to reductions in GABA concentration in sensorimotor corticesReference Levy, Ziemann, Chen and Cohen 3 - Reference Welker, Soriano and Van der Loos 5 and this may contribute to neural plasticity following injury.Reference Levy, Ziemann, Chen and Cohen 3 , Reference Jacobs and Donoghue 6 Such reductions in intracortical GABA concentration are likely to accompany SCI as suggested elsewhere.Reference Smith, Savic, Frankel, Ellaway, Maskill and Jamous 7 , Reference Roy, Zewdie and Gorassini 19 Further, SICI is reduced in the presence of the GABAB receptor agonist baclofenReference McDonnell, Orekhov and Ziemann 26 through presynaptic GABAB receptors inhibiting GABAA interneurons,Reference Sanger, Garg and Chen 36 which is a common medication in our SCI population. Because SICI is reduced because of presynaptic GABAB autoinhibition,Reference McDonnell, Orekhov and Ziemann 26 , Reference Werhahn, Kunesch, Noachtar, Benecke and Classen 27 , Reference Florian, Muller-Dahlhaus, Liu and Ziemann 37 baclofen may be responsible for our observation that SICI was only observed at 90% AMT CS intensity in participants with SCI. Therefore, it is unclear whether the high CS requirement in SCI relates to decreases in GABA concentration and/or the inhibition of GABAA receptors via baclofen, and both of these sources may contribute to reductions in SICI in our SCI population.
LICI
Controls exhibited typical LICI behavior in the presence of an isometric contraction; little to no inhibition caused by LICIReference Sohn, Wiltz and Hallett 18 likely via the reduction of the cortical excitability controlling the LICI circuit. Compared with controls, SCI participants showed greater LICI during contraction at the highest CS intensities used. One study investigated the contraction-related modulation of LICI in controls, in SCI taking baclofen and also in SCI not taking baclofen.Reference Barry, Bunday, Chen and Perez 29 Their data demonstrate modulation of LICI such that the SCI group on baclofen showed modulations that parallel those in controls, namely a contraction-related reduction in LICI. In contrast, the SCI group not taking baclofen did not show contraction-related modulation of LICI. In our study, we exposed group differences that appear similar to the control versus non-baclofen group in Barry et al (2013). However, it is difficult to make direct comparisons with the latter report since within-group effects of contraction-related modulation are exposed rather than differences between groups.
One possible explanation for the presence of LICI in SCI compared with the absence of LICI in controls might be attributed to the effects of baclofen as almost all of the SCI participants were taking baclofen. Baclofen is a GABAB agonist and serves to increase GABAB transmission within the motor cortex.Reference McDonnell, Orekhov and Ziemann 26 Increasing GABAB transmission likely results in an increase in LICI.Reference McDonnell, Orekhov and Ziemann 26 The mechanism of LICI involves GABAB receptor–mediated interneurons,Reference McDonnell, Orekhov and Ziemann 26 which act to inhibit I3- and later I-waves elicited by the TS,Reference Nakamura, Kitagawa, Kawaguchi and Tsuji 23 - Reference Di Lazzaro, Oliviero, Mazzone, Pilato, Saturno and Insola 25 resulting in inhibition of the MEP.Reference Paulus, Classen, Cohen, Large, Di Lazzaro and Nische 38 Facilitating these interneurons by increasing GABAB transmission results in a greater inhibition of the I3- and later I-waves, which results in decreased summation of the corticospinal volleys at the level of the spinal cord and a decreased MEP. Alternatively, it is possible that there are abnormalities in the movement-related modulation of LICI that may have arisen following an injury to the spinal cord. Because the movement-related modulation of SICI has been hypothesized to be a result of changes to cortical excitability,Reference Ridding, Taylor and Rothwell 16 , Reference Reynolds and Ashby 17 a similar mechanism can also be hypothesized for the movement-related modulation of LICI. Because SCI has been observed to result in reductions in GABA concentration within the sensorimotor cortices,Reference Levy, Ziemann, Chen and Cohen 3 - Reference Welker, Soriano and Van der Loos 5 it is a possibility that SCI can also affect the excitability of the primary motor cortex.
Technical Considerations
One important issue to consider in TMS studies in SCI is the reduction in the corticospinal output elicited by TMS. We attempted to match the groups’ unconditioned MEP by selecting an intensity that fell on the sensitive portion of the recruitment curve (i.e. MEPhalfmax). However, this resulted in higher absolute MEP values for the control group. Therefore, the effect of the CS on the TS may be different between groups and may lead to a greater opportunity for conditioning effects to be observed in the group with the lower unconditioned MEP area. Therefore, one limitation is that our findings in SCI may relate to the absolute unconditioned MEP area that was, compared with controls, smaller and therefore may have been more easily conditioned by the CS. Increasing the TS amplitude from 1 to 2 mV reduces LICI (100 ms), but does not alter SICI.Reference Udupa, Ni, Gunraj and Chen 39 It is unclear whether such changes occur in LICI using 150 ms ISI. However, the possibility remains that group differences in LICI may relate to larger and smaller TS MEP amplitudes obtained in controls and SCI, respectively (Figure 1B). Additionally, other reports indicate that increasing the TS amplitude from 0.2 to 1 mV result in an increase in SICI strength.Reference Sanger, Garg and Chen 36 , Reference Daskalakis, Christensen, Chen, Fitzgerald, Zipursky and Kapur 40 , Reference Wagle-Shukla, Ni, Gunraj, Bahl and Chen 41 However, increasing the TS intensity from 1 to 2 mV decreases SICI strength.Reference Ilic, Meintzschel, Cleff, Ruge, Kessler and Ziemann 13 , Reference Florian, Muller-Dahlhaus, Liu and Ziemann 37 Last, one inherent difficulty in studying SCI involves the range of prescribed medications that cannot be withdrawn without caution. Therefore, we have studied individuals in their normally medicated state similar to other studies.Reference Roy, Zewdie and Gorassini 19 , Reference Roy, Yang and Gorassini 31 Further, we did not have access to many SCI participants who were not taking baclofen and thus were unable to explicitly study the contribution of baclofen to the SICI and LICI circuits. Future studies may consider assessing the relationship between the magnitude of SICI/LICI and baclofen dosage.
Conclusions
The present study investigated LICI in the contracted FCR muscle in controls and individuals with SCI. Our main finding is that LICI is increased in individuals with SCI compared with controls and we suggest that baclofen and/or impairment in contraction-related modulation may provide plausible explanations for this effect. Further, the present research benefited from the use of a recruitment curve to elucidate the profile of intracortical inhibitory circuits in SCI, and we observed similarities and differences among our groups that may have been missed with the use of a single CS intensity. These findings have advanced our knowledge of the motor cortical changes that accompany chronic SCI and research focused on rehabilitative strategies in SCI populations may consider interventions focused on restoring contraction-related modulation of LICI for the ultimate purpose of improving arm function.
Disclosures
AJN received funding from the Natural Sciences and Engineering Research Council of Canada. YPM, AB, and AN have nothing to disclose.
