Cervicomedullary Evoked Potentials Research Articles

Corticospinal and spinal excitability during peripheral or central cooling in humans

Cold exposure can impair fine and gross motor control and threaten survival. Most motor task decrement is due to peripheral neuromuscular factors. Less is known about cooling on central neural factors. Corticospinal and spinal excitability were determined during cooling of the skin (Tsk) and core (Tco). Eight subjects (four female) were actively cooled in a liquid perfused suit for 90 min (2 °C inflow temperature), passively cooled for 7 min, and then rewarmed for 30 min (41 °C inflow temperature). Stimulation blocks included 10 transcranial magnetic stimulations [eliciting motor evoked potentials (MEPs) which indicate corticospinal excitability], 8 trans-mastoid electrical stimulations [eliciting cervicomedullary evoked potentials (CMEPs) which indicate spinal excitability] and 2 brachial plexus electrical stimulations [eliciting maximal compound motor action potentials (Mmax)]. These stimulations were delivered every 30 min. Cooling for 90 min reduced Tsk to 18.2 °C while Tco did not change. At the end of rewarming Tsk returned to baseline while Tco decreased by 0.8 °C (afterdrop) (P < 0.001). Metabolic heat production was higher than baseline at the end of passive cooling (P = 0.01), and 7 min into rewarming (P = 0.04). MEP/Mmax remained unchanged throughout. CMEP/Mmax increased by 38% at end cooling (although increased variability at this time rendered the increase insignificant, P = 0.23) and 58% at end warming when Tco was 0.8 °C below baseline (P = 0.02). Cooling increased spinal excitability but not corticospinal excitability. Cooling may decrease cortical and/or supraspinal excitability which is compensated for by increased spinal excitability. This compensation is key to providing a motor task and survival advantage.

Effects of prolonged local vibration superimposed to muscle contraction on motoneuronal and cortical excitability.

Introduction: Acute effects of prolonged local vibration (LV) at the central nervous system level have been well investigated demonstrating an altered motoneuronal excitability with a concomitant increase in cortical excitability. While applying LV during isometric voluntary contraction is thought to optimize the effects of LV, this has never been addressed considering the acute changes in central nervous system excitability. Materials and Methods: In the present study, nineteen healthy participants were engaged in four randomized sessions. LV was applied for 30min to the relaxed flexor carpi radialis muscle (VIBRELAXED) or during wrist flexions (i.e. intermittent contractions at 10% of the maximal voluntary contraction: 15s ON and 15s OFF; VIBCONTRACT). A control condition and a condition where participants only performed repeated low-contractions at 10% maximal force (CONTRACT) were also performed. For each condition, motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation and cervicomedullary evoked potentials (CMEPs) elicited by corticospinal tract electrical stimulation were measured before (PRE) and immediately after prolonged LV (POST) to investigate motoneuronal and corticospinal excitability, respectively. We further calculated the MEP/CMEP ratio as a proxy of cortical excitability. Results: No changes were observed in the control nor CONTRACT condition. At POST, CMEP decreased similarly in VIBRELAXED (-32% ± 42%, p < .001) and VIBCONTRACT (-41% ± 32%, p < .001). MEP/CMEP increased by 110% ± 140% (p = .01) for VIBRELAXED and by 120% ± 208% (p = .02) for VIBCONTRACT without differences between those conditions. Discussion: Our results suggest that LV to the flexor carpi radialis muscle, either relaxed or contracted, acutely decreases motoneuronal excitability and induces some priming of cortical excitability.

Corticospinal excitability to the biceps and triceps brachii during forward and backward arm cycling is direction- and phase-dependent.

The purpose of this study was to evaluate corticospinal excitability to the biceps and triceps brachii during forward (FWD) and backward (BWD) arm cycling. Corticospinal and spinal excitability were assessed using transcranial magnetic stimulation and transmastoid electrical stimulation to elicit motor evoked potentials (MEPs) and cervicomedullary evoked potentials (CMEPs), respectively. MEPs and CMEPs were recorded from the biceps and triceps brachii during FWD and BWD arm cycling at 2 positions, 6 and 12 o'clock. The 6 o'clock position corresponded to mid-elbow flexion and extension during FWD and BWD cycling, respectively, while 12 o'clock corresponded to mid-elbow extension and flexion during FWD and BWD cycling, respectively. During the flexion phase, MEP and CMEP amplitudes of the biceps brachii were higher during FWD cycling. However, during the extension phase, MEP and CMEP amplitudes were higher during BWD cycling. For the triceps brachii, MEP amplitudes were higher during FWD cycling regardless of phase. However, CMEP amplitudes were phase-dependent. During the flexion phase, CMEPs of the triceps brachii were higher during FWD cycling compared with BWD, but during the extension phase CMEPs were higher during BWD cycling compared with FWD. The data suggest that corticospinal and spinal excitability to the biceps brachii is phase- and direction-dependent. In the triceps brachii, spinal, but not corticospinal, excitability is phase-dependent when comparing FWD and BWD cycling. Novelty This is the first study to assess corticospinal excitability during FWD and BWD locomotor output. Corticospinal excitability during arm cycling depends on the direction, phase, and muscle being assessed.

Corticospinal and Spinal Excitability to the Biceps and Triceps Brachii is Modulated Differently between Forward and Backward Arm Cycling

The purpose of this study was to evaluate supraspinal and spinal excitability to the biceps and triceps brachii when comparing forward (FWD) and backward (BWD) arm cycling. Supraspinal and spinal excitability were assessed non‐invasively using transcranial magnetic stimulation (TMS) to elicit motor evoked potentials (MEPs) and transmastoid electrical stimulation (TMES) to elicit cervicomedullary evoked potentials (CMEPs), respectively. MEPs and CMEPs were recorded from the biceps and triceps brachii during FWD and BWD arm cycling at two positions, the 6 and 12 o'clock position. The 6 o'clock position corresponded to mid elbow flexion and extension during FWD and BWD cycling, respectively, while the 12 o'clock position corresponded to mid elbow extension and flexion during FWD and BWD cycling, respectively. Participants completed four arm cycling trials, two FWD and two BWD, at 60 rpm and 25 W. During the flexion phase MEP (p = .001) and CMEP (p = .001) amplitudes of the biceps brachii were higher during FWD cycling. However, during the extension phase MEP (p = .006) and CMEP (p = .003) amplitudes were higher during BWD cycling. For the triceps brachii MEP amplitudes were higher during FWD cycling compared to BWD (p = .027) regardless of the functional phase (flexion vs. extension) of the movement cycle. However, spinal excitability to the triceps brachii was dependent on the functional phase of the movement cycle. During the flexion phase CMEPs of the triceps brachii were higher during FWD cycling compared to BWD (p = .032), but during the extension phase CMEPs were higher during BWD cycling compared to FWD (p = .001). This data suggests that the modulation of CSE and spinal excitability to the biceps brachii is dependent on the functional phase of the movement cycle and on the cycling direction. Also, spinal excitability but not CSE to the triceps brachii is dependent on the functional phase of the movement cycle when comparing FWD and BWD cycling.Support or Funding InformationNSERC Discovery Grant awarded to K.E. PowerThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Phase- and Workload-Dependent Changes in Corticospinal Excitability to the Biceps and Triceps Brachii during Arm Cycling.

This is the first study to examine corticospinal excitability (CSE) to antagonistic muscle groups during arm cycling. Transcranial magnetic stimulation (TMS) of the motor cortex and transmastoid electrical stimulation (TMES) of the corticospinal tract were used to assess changes in supraspinal and spinal excitability, respectively. TMS induced motor evoked potentials (MEPs) and TMES induced cervicomedullary evoked potentials (CMEPs) were recorded from the biceps and triceps brachii at two positions, mid-elbow flexion and extension, while cycling at 5% and 15% of peak power output. While phase-dependent modulation of MEP and CMEP amplitudes occurred in the biceps brachii, there was no difference between flexion and extension for MEP amplitudes in the triceps brachii and CMEP amplitudes were higher during flexion than extension. Furthermore, MEP amplitudes in both biceps and triceps brachii increased with increased workload. CMEP amplitudes increased with higher workloads in the triceps brachii, but not biceps brachii, though the pattern of change in CMEPs was similar to MEPs. Differences between changes in CSE between the biceps and triceps brachii suggest that these antagonistic muscles may be under different neural control during arm cycling. Putative mechanisms are discussed.

Changes in Corticospinal and Spinal Excitability to the Biceps Brachii with a Neutral vs. Pronated Handgrip Position Differ between Arm Cycling and Tonic Elbow Flexion.

The purpose of this study was to examine the influence of neutral and pronated handgrip positions on corticospinal excitability to the biceps brachii during arm cycling. Corticospinal and spinal excitability were assessed using motor evoked potentials (MEPs) elicited via transcranial magnetic stimulation (TMS) and cervicomedullary-evoked potentials (CMEPs) elicited via transmastoid electrical stimulation (TMES), respectively. Participants were seated upright in front on arm cycle ergometer. Responses were recorded from the biceps brachii at two different crank positions (6 and 12 o’clock positions relative to a clock face) while arm cycling with neutral and pronated handgrip positions. Responses were also elicited during tonic elbow flexion to compare/contrast the results to a non-rhythmic motor output. MEP and CMEP amplitudes were significantly larger at the 6 o’clock position while arm cycling with a neutral handgrip position compared to pronated (45.6 and 29.9%, respectively). There were no differences in MEP and CMEP amplitudes at the 12 o’clock position for either handgrip position. For the tonic contractions, MEPs were significantly larger with a neutral vs. pronated handgrip position (32.6% greater) while there were no difference in CMEPs. Corticospinal excitability was higher with a neutral handgrip position for both arm cycling and tonic elbow flexion. While spinal excitability was also higher with a neutral handgrip position during arm cycling, no difference was observed during tonic elbow flexion. These findings suggest that not only is corticospinal excitability to the biceps brachii modulated at both the supraspinal and spinal level, but that it is influenced differently between rhythmic arm cycling and tonic elbow flexion.

Fatigue diminishes motoneuronal excitability during cycling exercise.

Exercise-induced fatigue influences the excitability of the motor pathway during single-joint isometric contractions. This study sought to investigate the influence of fatigue on corticospinal excitability during cycling exercise. Eight men performed fatiguing constant-load (80% Wpeak; 241 ± 13 W) cycling to exhaustion during which the percent increase in quadriceps electromyography (ΔEMG; vastus lateralis and rectus femoris) was quantified. During a separate trial, subjects performed two brief (∼45 s) nonfatiguing cycling bouts (244 ± 15 and 331 ± 23W) individually chosen to match the ΔEMG across bouts to that observed during fatiguing cycling. Corticospinal excitability during exercise was quantified by transcranial magnetic, electric transmastoid, and femoral nerve stimulation to elicit motor-evoked potentials (MEP), cervicomedullary evoked potentials (CMEP), and M waves in the quadriceps. Peripheral and central fatigue were expressed as pre- to postexercise reductions in quadriceps twitch force (ΔQtw) and voluntary quadriceps activation (ΔVA). Whereas nonfatiguing cycling caused no measureable fatigue, fatiguing cycling resulted in significant peripheral (ΔQtw: 42 ± 6%) and central (ΔVA: 4 ± 1%) fatigue. During nonfatiguing cycling, the area of MEPs and CMEPs, normalized to M waves, similarly increased in the quadriceps (∼40%; P < 0.05). In contrast, there was no change in normalized MEPs or CMEPs during fatiguing cycling. As a consequence, the ratio of MEP to CMEP was unchanged during both trials (P > 0.5). Therefore, although increases in muscle activation promote corticospinal excitability via motoneuronal facilitation during nonfatiguing cycling, this effect is abolished during fatigue. We conclude that the unaltered excitability of the corticospinal pathway from start of intense cycling exercise to exhaustion is, in part, determined by inhibitory influences on spinal motoneurons obscuring the facilitating effects of muscle activation.

Differences in corticospinal excitability to the biceps brachii between arm cycling and tonic contraction are not evident at the immediate onset of movement.

This is the first study to examine changes in corticospinal excitability to the biceps brachii during the onset of arm cycling from a resting position to a point when steady-state arm cycling was obtained. Supraspinal and spinal excitability were assessed using motor-evoked potentials (MEPs) elicited via transcranial magnetic stimulation and cervicomedullary evoked potentials (CMEPs) elicited via transmastoid electrical stimulation, respectively. Evoked responses were recorded from the biceps brachii during elbow flexion (6 o'clock relative to a clock face) for both arm cycling and an intensity-matched tonic contraction at three separate periods: (1) immediately at the onset of motor output and after completion of the (2) 4th revolution and (3) 9th revolution. There was no difference during initiation between tasks for MEP (P=0.79) or CMEP amplitudes (P=0.57). However, MEP amplitudes were significantly larger during arm cycling than an intensity-matched tonic contraction after the completion of the 4th (Cycling 76.48±17.35% of M max, Tonic 63.45±18.45% of M max, P<0.05) and 9th revolutions (Cycling 72.37±15.96% of M max, Tonic 58.1±24.23% of M max, P<0.05). There were no differences between conditions in CMEP amplitudes at the 4th (Cycling 49.6±25.4% of M max, Tonic 41.6±11.2% of M max, P=0.31) or the 9th revolution (Cycling 47.2±17.0% of M max, Tonic 40.8±13.6% of M max, P=0.29). These results demonstrate that corticospinal excitability is not different between arm cycling and a tonic contraction at motor output onset, but supraspinal excitability is enhanced during steady-state arm cycling. This suggests a similarity in the way the corticospinal tract initiates motor outputs in humans, regardless of the differences that present themselves in the later, steady-state stages.

New evidence of corticospinal network modulation induced by motor imagery.

Motor imagery (MI) is the mental simulation of movement, without the corresponding muscle contraction. Whereas the activation of cortical motor areas during MI is established, the involvement of spinal structures is still under debate. We used original and complementary techniques to probe the influence of MI on spinal structures. Amplitude of motor-evoked potentials (MEPs), cervico-medullary-evoked potentials (CMEPs), and Hoffmann (H)-reflexes of the flexor carpi radialis (FCR) muscle and of the triceps surae muscles was measured in young, healthy subjects at rest and during MI. Participants were asked to imagine maximal voluntary contraction of the wrist and ankle, while the targeted limb was fixed (static condition). We confirmed previous studies with an increase of FCR MEPs during MI compared with rest. Interestingly, CMEPs, but not H-reflexes, also increased during MI, revealing a possible activation of subcortical structures. Then, to investigate the effect of MI on the spinal network, we used two techniques: 1) passive lengthening of the targeted muscle via an isokinetic dynamometer and 2) conditioning of H-reflexes with stimulation of the antagonistic nerve. Both techniques activate spinal inhibitory presynaptic circuitry, reducing the H-reflex amplitude at rest. In contrast, no reduction of H-reflex amplitude was observed during MI. These findings suggest that MI has modulatory effects on the spinal neuronal network. Specifically, the activation of low-threshold spinal structures during specific conditions (lengthening and H-reflex conditioning) highlights the possible generation of subliminal cortical output during MI.

Changes in supraspinal and spinal excitability of the biceps brachii following brief, non-fatiguing submaximal contractions of the elbow flexors in resistance-trained males

The purpose of the current study was to assess the effects of 5 brief (2s), intermittent, submaximal elbow flexors voluntary contractions at 50% of maximal voluntary contraction (MVC) on measures of central (i.e. supraspinal and spinal) excitability. Supraspinal and spinal excitability of the biceps brachii were assessed via transcranial magnetic stimulation (TMS) of the motor cortex and transmastoid electrical stimulation (TMES) of the corticospinal tract, respectively. TMS-induced motor-evoked potentials (MEPs), TMES-induced cervicomedullary-evoked potentials (CMEPs), Erb’s point peripheral nerve stimulation and MVC were assessed prior to and following submaximal voluntary contractions at 50% of MVC. The MEP to CMEP ratio increased (584±77.2%; p=0.011) and CMEP amplitudes decreased (62±3.0%; p=0.02) immediately post-exercise. MVC force output did not change immediately post-exercise. The results suggest that brief, non-fatiguing intermittent submaximal voluntary contractions transiently enhance supraspinal excitability while decreasing spinal excitability. The impact of these changes on one's ability to generate or maintain force production remains unknown.

Cadence-dependent changes in corticospinal excitability of the biceps brachii during arm cycling.

This is the first study to report the influence of different cadences on the modulation of supraspinal and spinal excitability during arm cycling. Supraspinal and spinal excitability were assessed using transcranial magnetic stimulation of the motor cortex and transmastoid electrical stimulation of the corticospinal tract, respectively. Transcranial magnetic stimulation-induced motor evoked potentials and transmastoid electrical stimulation-induced cervicomedullary evoked potentials (CMEPs) were recorded from the biceps brachii at two separate positions corresponding to elbow flexion and extension (6 and 12 o'clock relative to a clock face, respectively) while arm cycling at 30, 60 and 90 rpm. Motor evoked potential amplitudes increased significantly as cadence increased during both elbow flexion (P < 0.001) and extension (P = 0.027). CMEP amplitudes also increased with cadence during elbow flexion (P < 0.01); however, the opposite occurred during elbow extension (i.e., decreased CMEP amplitude; P = 0.01). The data indicate an overall increase in the excitability of corticospinal neurons which ultimately project to biceps brachii throughout arm cycling as cadence increased. Conversely, changes in spinal excitability as cadence increased were phase dependent (i.e., increased during elbow flexion and decreased during elbow extension). Phase- and cadence-dependent changes in spinal excitability are suggested to be mediated via changes in the balance of excitatory and inhibitory synaptic input to the motor pool, as opposed to changes in the intrinsic properties of spinal motoneurons.

Chronic resistance training enhances the spinal excitability of the biceps brachii in the non-dominant arm at moderate contraction intensities

The purpose of the study was to assess corticospinal excitability of the biceps brachii in the non-dominant arm of chronic resistance-trained (RT) and non-RT individuals. Seven chronic-RT and six non-RT male participants performed 4 sets of 5s pseudo-randomized contractions of the non-dominant elbow flexors at 25, 50, 75, 90, and 100% of maximum voluntary contraction (MVC). During each contraction, transcranial magnetic stimulation, transmastoid electrical stimulation, and Erb’s point electrical stimulation were administered to assess the amplitudes of motor evoked potentials (MEPs), cervicomedullary evoked potentials (CMEPs), and maximal muscle compound potentials (Mmax), respectively, in the biceps brachii. MEP and CMEP amplitudes were normalized to Mmax. Training did not affect (p>0.14) MEP amplitudes across any contraction intensity. CMEP amplitudes were significantly (p<0.05) higher in the chronic-RT group at 50% and 75% of MVC by 38% and 27%, respectively, and there was a trend for higher amplitudes at 25%, 90%, and 100% MVC by 25% (p=0.055), 36% (p=0.077), and 35% (p=0.078), respectively, compared to the non-RT group. Corticospinal excitability of the non-dominant biceps brachii was increased in chronic-RT individuals mainly due to changes in spinal excitability.

Corticospinal excitability of the biceps brachii is higher during arm cycling than an intensity-matched tonic contraction.

Human studies have not assessed corticospinal excitability of an upper-limb prime mover during arm cycling. The purpose of the present study was to determine whether supraspinal and/or spinal motoneuron excitability of the biceps brachii was different between arm cycling and an intensity-matched tonic contraction. We hypothesized that spinal motoneuron excitability would be higher during arm cycling than an intensity-matched tonic contraction. Supraspinal and spinal motoneuron excitability were assessed using transcranial magnetic stimulation (TMS) of the motor cortex and transmastoid electrical stimulation (TMES) of the corticospinal tract, respectively. TMS-induced motor-evoked potentials (MEPs) and TMES-induced cervicomedullary-evoked potentials (CMEPs) were assessed at three separate positions (3, 6, and 12 o'clock relative to a clock face) during arm cycling and an intensity-matched tonic contraction. MEP amplitudes were 7.2 and 8.8% maximum amplitude of the compound muscle action potential (Mmax) larger during arm cycling compared with a tonic contraction at the 3 (P < 0.001) and 6 o'clock (P < 0.001) positions, respectively. There was no difference between tasks during elbow extension (12 o'clock). CMEP amplitudes were 5.2% Mmax larger during arm cycling compared with a tonic contraction at the 3 o'clock position (P < 0.001) with no differences seen at midflexion (6 o'clock) or extension (12 o'clock). The data indicate an increase in the excitability of corticospinal neurons, which ultimately project to biceps brachii during the elbow flexion portion of arm cycling, and increased spinal motoneuron excitability at the onset of elbow flexion during arm cycling. We conclude that supraspinal and spinal motoneuron excitability are phase- and task-dependent.

Differences in supraspinal and spinal excitability during various force outputs of the biceps brachii in chronic- and non-resistance trained individuals.

Motor evoked potentials (MEP) and cervicomedullary evoked potentials (CMEP) may help determine the corticospinal adaptations underlying chronic resistance training-induced increases in voluntary force production. The purpose of the study was to determine the effect of chronic resistance training on corticospinal excitability (CE) of the biceps brachii during elbow flexion contractions at various intensities and the CNS site (i.e. supraspinal or spinal) predominantly responsible for any training-induced differences in CE. Fifteen male subjects were divided into two groups: 1) chronic resistance-trained (RT), (n = 8) and 2) non-RT, (n = 7). Each group performed four sets of ∼5 s elbow flexion contractions of the dominant arm at 10 target forces (from 10%–100% MVC). During each contraction, subjects received 1) transcranial magnetic stimulation, 2) transmastoid electrical stimulation and 3) brachial plexus electrical stimulation, to determine MEP, CMEP and compound muscle action potential (Mmax) amplitudes, respectively, of the biceps brachii. All MEP and CMEP amplitudes were normalized to Mmax. MEP amplitudes were similar in both groups up to 50% MVC, however, beyond 50% MVC, MEP amplitudes were lower in the chronic RT group (p<0.05). CMEP amplitudes recorded from 10–100% MVC were similar for both groups. The ratio of MEP amplitude/absolute force and CMEP amplitude/absolute force were reduced (p<0.012) at all contraction intensities from 10–100% MVC in the chronic-RT compared to the non-RT group. In conclusion, chronic resistance training alters supraspinal and spinal excitability. However, adaptations in the spinal cord (i.e. motoneurone) seem to have a greater influence on the altered CE.

Excitability changes in human forearm corticospinal projections and spinal reflex pathways during rhythmic voluntary movement of the opposite limb.

Rhythmic movements brought about by the contraction of muscles on one side of the body give rise to phase-locked changes in the excitability of the homologous motor pathways of the opposite limb. Such crossed facilitation should favour patterns of bimanual coordination in which homologous muscles are engaged simultaneously, and disrupt those in which the muscles are activated in an alternating fashion. In order to examine these issues, we obtained responses to transcranial magnetic stimulation (TMS), to stimulation of the cervicomedullary junction (cervicomedullary-evoked potentials, CMEPs), to peripheral nerve stimulation (H-reflexes and f-waves), and elicited stretch reflexes in the relaxed right flexor carpi radialis (FCR) muscle during rhythmic (2 Hz) flexion and extension movements of the opposite (left) wrist. The potentials evoked by TMS in right FCR were potentiated during the phases of movement in which the left FCR was most strongly engaged. In contrast, CMEPs were unaffected by the movements of the opposite limb. These results suggest that there was systematic variation of the excitability of the motor cortex ipsilateral to the moving limb. H-reflexes and stretch reflexes recorded in right FCR were modulated in phase with the activation of left FCR. As the f-waves did not vary in corresponding fashion, it appears that the phasic modulation of the H-reflex was mediated by presynaptic inhibition of Ia afferents. The observation that both H-reflexes and f-waves were depressed markedly during movements of the opposite indicates that there may also have been postsynaptic inhibition or disfacilitation of the largest motor units. Our findings indicate that the patterned modulation of excitability in motor pathways that occurs during rhythmic movements of the opposite limb is mediated primarily by interhemispheric interactions between cortical motor areas.