Cortical and spinal motor excitability during the transcranial magnetic stimulation silent period in humans

Contraction intensity modulates spinal excitability during transcranial magnetic stimulation-evoked silent period in rectus femoris muscle

PURPOSE

Reduced spinal excitability during the transcranial magnetic stimulation (TMS) silent period (SP) has recently been shown to last longer than previously thought in the upper limbs, as assessed via spinal electrical stimulation. Further, there is reason to expect that contraction intensity affects the duration of the reduced spinal excitability.

METHODS

This study investigated spinal excitability at different time delays within the TMS-evoked SP in m.rectus femoris. Fifteen participants performed non-fatiguing isometric knee extensions at 25%, 50% and 75% of maximum voluntary contraction (MVC). Lumbar stimulation (LS) induced a lumbar-evoked potential (LEP) of 50% resting M-max. TMS stimulator output induced a SP lasting ~ 200 ms. In each contraction, a LEP (unconditioned) was delivered ~ 2-3 s prior to TMS, which was followed by a second LEP (conditioned) 60, 90, 120 or 150 ms into the silent period. Five contractions were performed at each contraction intensity and for each time delay in random order.

RESULTS

Compared to the unconditioned LEP, the conditioned LEP amplitude was reduced (- 28 ± 34%, p = 0.007) only at 60 ms during 25% of MVC. Conditioned LEP amplitudes during 50% and 75% of MVC were reduced at 60 ms (- 37 ± 47%, p = 0.009 and - 37 ± 42%, p = 0.005, respectively) and 150 ms (- 30% ± 37%, p = 0.0083 and - 37 ± 43%, p = 0.005, respectively). LEP amplitude at 90 ms during 50% of MVC also reduced (- 25 ± 35%, p = 0.013).

CONCLUSION

Reduced spinal excitability is extended during 50% and 75% of MVC. In future, paired TMS-LS could be a potential method to understand changes in spinal excitability during SP (at different contraction intensities) when testing various neurophysiological phenomena.

Brain Neuroplasticity Related to Lateral Ankle Ligamentous Injuries: A Systematic Review

BACKGROUND

Lateral ankle sprains are the most common ankle injuries in sports and have the highest recurrence rates. Almost half of the patients experiencing lateral ankle sprains develop chronic ankle instability. Patients with chronic ankle instability experience persistent ankle dysfunctions and detrimental long-term sequelae. Changes at the brain level are put forward to explain these undesirable consequences and high recurrence rates partially. However, an overview of possible brain adaptations related to lateral ankle sprains and chronic ankle instability is currently lacking.

OBJECTIVE

The primary purpose of this systematic review is to provide a comprehensive overview of the literature on structural and functional brain adaptations related to lateral ankle sprains and in patients with chronic ankle instability.

METHODS

PubMed, Web of Science, Scopus, Embase, EBSCO-SPORTDiscus and Cochrane Central Register of Controlled Trials were systematically searched until 14 December, 2022. Meta-analyses, systematic reviews and narrative reviews were excluded. Included studies investigated functional or structural brain adaptations in patients who experienced a lateral ankle sprain or with chronic ankle instability and who were at least 18 years of age. Lateral ankle sprains and chronic ankle instability were defined following the recommendation of the International Ankle Consortium. Three authors independently extracted the data. They extracted the authors' name, publication year, study design, inclusion criteria, participant characteristics, the sample size of the intervention and control groups, methods of neuroplasticity testing, as well as all means and standard deviations of primary and secondary neuroplasticity outcomes from each study. Data reported on copers were considered as part of the control group. The quality assessment tool for observational and cross-sectional studies was used for the risk of bias assessment. This study is registered on PROSPERO, number CRD42021281956.

RESULTS

Twenty articles were included, of which only one investigated individuals who experienced a lateral ankle sprain. In all studies combined, 356 patients with chronic ankle instability, 10 who experienced a lateral ankle sprain and 46 copers were included. White matter microstructure changes in the cerebellum have been related to lateral ankle sprains. Fifteen studies reported functional brain adaptations in patients with chronic ankle instability, and five articles found structural brain outcomes. Alterations in the sensorimotor network (precentral gyrus and supplementary motor area, postcentral gyrus and middle frontal gyrus) and dorsal anterior cingulate cortex were mainly found in patients with chronic ankle instability.

DISCUSSION

The included studies demonstrated structural and functional brain adaptations related to lateral ankle sprains and chronic ankle instability compared to healthy individuals or copers. These adaptations correlate with clinical outcomes (e.g. patients' self-reported function and different clinical assessments) and might contribute to the persisting dysfunctions, increased re-injury risk and long-term sequelae seen in these patients. Thus, rehabilitation programmes should integrate sensorimotor and motor control strategies to cope with neuroplasticity related to ligamentous ankle injuries.

The effect of acute and chronic nicotine consumption on intra-cortical inhibition and facilitation: A transcranial magnetic stimulation study

OBJECTIVE

The aim of the present study was to explore the impact of acute and chronic nicotine consumption on measures of intracortical inhibition and facilitation.

METHODS

This study involved 50 chronic heavy cigarette smokers and 40 healthy subjects matched for age, sex and educational level, with no history of chronic nicotine intake. Intracortical inhibition and facilitation were assessed using transcranial magnetic stimulation (TMS) measures of motor threshold (MT), short- and long-interval intra-cortical inhibition (SICI, LICI), cortical silent period (CSP) and intra-cortical facilitation (ICF). Basal serum levels of cotinine were measured in the healthy group and at ½ and 2 h after smoking a single cigarette in the chronic smokers.

RESULTS

There was enhanced SICI and reduced ICF in smokers (independent of time after smoking) compared with non-smokers. The former suggests a chronic effect of increased nicotine levels on GABA-A neurotransmission whereas the latter suggests an additional effect on glutamatergic transmission. There were no significant differences between smokers and non-smokers in other TMS parameters. There was a significant negative correlation between cotinine levels at ½ h after smoking and SICI at 3 ms ISI (P < 0.001). There were no significant differences in any of the neurophysiological measures between smokers at ½ h versus 2 h after smoking a single cigarette.

CONCLUSION

Chronic nicotine consumption enhances SICI, and reduces ICF, supporting the hypothesis that nicotine acts as a neuromodulator of GABA-A and glutamate neurotransmission.

Longer Cortical Silent Period Length Is Associated to Binge Eating Disorder: An Exploratory Study

Introduction: Although binge eating disorder (BED) is an eating disorder and obesity is a clinical disease, it is known that both conditions present overlapped symptoms related to, at least partially, the disruption of homeostatic and hedonistic eating behavior pathways. Therefore, the understanding of neural substrates, such as the motor cortex excitability assessed by transcranial magnetic stimulation (TMS), might provide new insights into the pathophysiology of BED and obesity. Objectives: (i) To compare, among BED, obesity, ex-obese, and HC (healthy control) subjects, the cortical excitability indexed by TMS measures, such as CSP (cortical silent period; primary outcome), SICI (intracortical inhibition), and ICF (intracortical facilitation; secondary outcome). (ii) To explore the relationship of the CSP, eating behavior (e.g., restraint, disinhibition, and hunger), depressive symptoms, and sleep quality among the four groups (BED, obesity, ex-obese, and HC). Methods: Fifty-nine women [BED (n = 13), obese (n = 20), ex-obese (n = 12), and HC (n = 14)] comprise the total sample for this study. Assessments: cortical excitability measures (CSP, SICI, and ICF), inhibition response task by the Go/No-go paradigm, and instruments to assess the eating psychopathology (Three-Factor Eating Questionnaire, Eating Disorder Examination Questionnaire, and Binge Eating Scale) were used. Results: A MANCOVA analysis revealed that the mean of CSP was longer in the BED group compared with other three groups: 24.10% longer than the obesity group, 25.98% longer than the HC group, and 25.41% longer than the ex-obese group. Pearson's correlations evidenced that CSP was positively associated with both eating concern and binge eating scores. Conclusion: The findings point out that BED patients present longer CSP, which might suggest an upregulation of intracortical inhibition. Additionally, CSP was positively correlated with Binge Eating Scale and eating concern scores. Further studies are needed.

Myths and Methodologies: How loud is the story told by the transcranial magnetic stimulation-evoked silent period?

NEW FINDINGS

What is the topic of this review? The origin, interpretation and methodological constraints of the silent period induced by transcranial magnetic stimulation are reviewed. What advances does it highlight? The silent period is generated by both cortical and spinal mechanisms. Therefore, it seems inappropriate to preface silent period with 'cortical' unless additional measures are taken. Owing to many confounding variables, a standardized approach to the silent period measurement cannot be suggested. Rather, recommendations of best practice are provided based on the available evidence and the context of the research question.

ABSTRACT

Transcranial magnetic stimulation (TMS) of the motor cortex evokes a response in the muscle that can be recorded via electromyography (EMG). One component of this response, when elicited during a voluntary contraction, is a period of EMG silence, termed the silent period (SP), which follows a motor evoked potential (MEP). Modulation of SP duration was long thought to reflect the degree of intracortical inhibition. However, the evidence presented in this review suggests that both cortical and spinal mechanisms contribute to generation of the SP, which makes prefacing SP with 'cortical' misleading. Further investigations with multi-methodological approaches, such as TMS-EEG coupling or interaction of TMS with neuroactive drugs, are needed to make such inferences with greater confidence. A multitude of methodological factors can influence the SP and thus confound the interpretation of this measure; namely, background muscle activity, instructions given to the participant, stimulus intensity and the size of the MEP preceding the SP, and the approach to analysis. A systematic understanding of how the confounding factors influence the interpretation of SP is lacking, which makes standardization of the methodology difficult to conceptualize. Instead, the methodology should be guided through the lens of the research question and the population studied, ensuring greater reproducibility, repeatability and comparability of data sets. Recommendations are provided for the best practice within a given context of the experimental design.

Neural Substrates of Motor Recovery in Severely Impaired Stroke Patients With Hand Paralysis

In well-recovered stroke patients with preserved hand movement, motor dysfunction relates to interhemispheric and intracortical inhibition in affected hand muscles. In less fully recovered patients unable to move their hand, the neural substrates of recovered arm movements, crucial for performance of daily living tasks, are not well understood. Here, we evaluated interhemispheric and intracortical inhibition in paretic arm muscles of patients with no recovery of hand movement (n = 16, upper extremity Fugl-Meyer Assessment = 27.0 ± 8.6). We recorded silent periods (contralateral and ipsilateral) induced by transcranial magnetic stimulation during voluntary isometric contraction of the paretic biceps and triceps brachii muscles (correlates of intracortical and interhemispheric inhibition, respectively) and investigated links between the silent periods and motor recovery, an issue that has not been previously explored. We report that interhemispheric inhibition, stronger in the paretic triceps than biceps brachii muscles, significantly correlated with the magnitude of residual impairment (lower Fugl-Meyer scores). In contrast, intracortical inhibition in the paretic biceps brachii, but not in the triceps, correlated positively with motor recovery (Fugl-Meyer scores) and negatively with spasticity (lower Modified Ashworth scores). Our results suggest that interhemispheric inhibition and intracortical inhibition of paretic upper arm muscles relate to motor recovery in different ways. While interhemispheric inhibition may contribute to poorer recovery, muscle-specific intracortical inhibition may relate to successful motor recovery and lesser spasticity.

Does the region of epileptogenicity influence the pattern of change in cortical excitability?

OBJECTIVE

To investigate whether cortical excitability measures on transcranial magnetic stimulation (TMS) differed between groups of patients with different focal epilepsy syndromes.

METHODS

85 Patients with focal epilepsy syndromes divided into temporal and extra-temporal lobe epilepsy were studied. The cohorts were further divided into drug naïve-new onset, refractory and seizure free groups. Motor threshold (MT) and paired pulse TMS at short (2, 5, 10, 15 ms) and long (100-300 ms) interstimulus intervals (ISIs) were measured. Results were compared to those of 20 controls.

RESULTS

Cortical excitability was higher at 2 & 5 ms and 250, 300 ms ISIs (p<0.01) in focal epilepsy syndromes compared to controls however significant inter-hemispheric differences in MT and the same ISIs were only seen in the drug naïve state early at onset and were much more prominent in temporal lobe epilepsy.

CONCLUSION

Disturbances in cortical excitability are more confined to the affected hemisphere in temporal lobe epilepsy but only early at onset in the drug naïve state.

SIGNIFICANCE

Group TMS studies show that cortical excitability measures are different in temporal lobe epilepsy and can be distinguished from other focal epilepsies early at onset in the drug naïve state. Further studies are needed to determine whether these results can be applied clinically as the utility of TMS in distinguishing between epilepsy syndromes at an individual level remains to be determined.

Feasibility of automated analysis and inter-examiner variability of cortical silent period induced by transcranial magnetic stimulation

Cortical silent period (cSP) is a short interruption in electromyography (EMG) during active muscle contraction induced with transcranial magnetic stimulation (TMS). The cSP is a measure of cortical inhibition and is believed to represent inhibitory interneuron effects on excited motor cortical areas. Several pathological conditions and pharmacological manipulations induce changes to cSP duration indicating alterations in intracortical inhibition. At present, it is common to manually analyse the cSP duration from measured EMG. However, to avoid inter-examiner effects on cSP interpretation and detection, as well as to allow for quick measurement online, automated routine would be preferable. In this study, we evaluate the feasibility of a straight-forward cSP detection routine based on analysing the rectified first derivative of the EMG signal following TMS. Previously measured cSPs of 54 healthy subjects were reanalysed manually by two of the authors and using the automated routine. Furthermore, we recruited one subject for whom the cSPs were induced with several stimulation intensities, and those cSPs were analysed manually by two of the authors as well as using the automated routine. We found that cSPs were detected correctly by the automated cSP detection routine, and agreement with manually analysed subject-specific mean cSPs was excellent (ICC=0.992, p<0.001). The inter-examiner variability was similar to the variability between manual and automated analysis. Hence, we believe the introduced cSP detection routine would be feasible for online cSP detection, in such a way that is presently used to detect the motor evoked potentials.

Prognostic value of cortically induced motor evoked activity by TMS in chronic stroke: caveats from a revealing single clinical case

BACKGROUND

We report the case of a chronic stroke patient (62 months after injury) showing total absence of motor activity evoked by transcranial magnetic stimulation (TMS) of spared regions of the left motor cortex, but near-to-complete recovery of motor abilities in the affected hand.

CASE PRESENTATION

Multimodal investigations included detailed TMS based motor mapping, motor evoked potentials (MEP), and Cortical Silent period (CSP) as well as functional magnetic resonance imaging (fMRI) of motor activity, MRI based lesion analysis and Diffusion Tensor Imaging (DTI) Tractography of corticospinal tract (CST). Anatomical analysis revealed a left hemisphere subinsular lesion interrupting the descending left CST at the level of the internal capsule. The absence of MEPs after intense TMS pulses to the ipsilesional M1, and the reversible suppression of ongoing electromyographic (EMG) activity (indexed by CSP) demonstrate a weak modulation of subcortical systems by the ipsilesional left frontal cortex, but an inability to induce efficient descending volleys from those cortical locations to right hand and forearm muscles. Functional MRI recordings under grasping and finger tapping patterns involving the affected hand showed slight signs of subcortical recruitment, as compared to the unaffected hand and hemisphere, as well as the expected cortical activations.

CONCLUSIONS

The potential sources of motor voluntary activity for the affected hand in absence of MEPs are discussed. We conclude that multimodal analysis may contribute to a more accurate prognosis of stroke patients.

Older adults exhibit more intracortical inhibition and less intracortical facilitation than young adults

BACKGROUND

Aging results in decreased neuromuscular function, which is likely associated with neurologic alterations. At present little is known regarding age-related changes in intracortical properties.

METHODS

In this study we used transcranial magnetic stimulation (TMS) to measure intracortical facilitation (ICF), short- and long-interval intracortical inhibition (SICI and LICI), motor evoked potential amplitude, and silent period duration in young and older adults (21.4+/-0.8years and 70.9+/-1.8years). These variables were assessed from the flexor carpi radialis muscle of the non-dominant arm under resting conditions, and during a submaximal contraction (intensity 15% maximum strength).

RESULTS

Older adults exhibited increased SICI and LICI in comparison to young adults (SICI: 29.0+/-9.2% vs. 46.2+/-4.8% of unconditioned pulse; LICI: 6.5+/-1.7% vs. 15.8+/-3.3% of unconditioned pulse; P=0.04), and less ICF under resting conditions (74.6+/-8.7% vs. 104.9+/-6.9% of unconditioned pulse; P=0.02). These age-related differences disappeared during contraction, although the older adults did exhibit a longer silent period during contraction (112.5+/-6.5 vs. 84.0+/-3.9ms; P<0.01).

CONCLUSIONS

Collectively, these findings suggest increased GABA mediated intracortical inhibition with age.

Transcranial magnetic stimulation in ALS: utility of central motor conduction tests

OBJECTIVE

To investigate transcranial magnetic stimulation (TMS) measures as clinical correlates and longitudinal markers of amyotrophic lateral sclerosis (ALS).

METHODS

We prospectively studied 60 patients with ALS subtypes (sporadic ALS, familial ALS, progressive muscular atrophy, and primary lateral sclerosis) using single pulse TMS, recording from abductor digiti minimi (ADM) and tibialis anterior (TA) muscles. We evaluated three measures: 1) TMS motor response threshold to the ADM, 2) central motor conduction time (CMCT), and 3) motor evoked potential amplitude (correcting for peripheral changes). Patients were evaluated at baseline, compared with controls, and followed every 3 months for up to six visits. Changes were analyzed using generalized estimation equations to test linear trends with time.

RESULTS

TMS threshold, CMCT, and TMS amplitude correlated (p < 0.05) with clinical upper motor neuron (UMN) signs at baseline and were different (p < 0.05) from normal controls in at least one response. Seventy-eight percent of patients with UMN (41/52) and 50% (4/8) of patients without clinical UMN signs had prolonged CMCT. All three measures revealed significant deterioration over time: TMS amplitude showed the greatest change, decreasing 8% per month; threshold increased 1.8% per month; and CMCT increased by 0.9% per month.

CONCLUSIONS

Transcranial magnetic stimulation (TMS) findings, particularly TMS amplitude, can objectively discriminate corticospinal tract involvement in amyotrophic lateral sclerosis (ALS) from controls and assess the progression of ALS. While central motor conduction time and response threshold worsen by less than 2% per month, TMS amplitude decrease averages 8% per month, and may be a useful objective marker of disease progression.

Altered cortical inhibitory function in children with spastic diplegia: a TMS study

Periventricular leukomalacia (PVL) is the most frequent cause of spastic diplegia. The movement disorder is attributed to damage to the corticospinal tract, but there is increasing evidence of additional cortical dysfunction associated with PVL. Aim of the present study was to evaluate the integrity of the corticospinal tract and cortical inhibitory function using transcranial magnetic stimulation. Fifteen children with bilateral PVL and spastic diplegia and twenty-two healthy children underwent single-pulse stimulations to the right tibial anterior muscle. We compared central motor conduction time and amplitudes of motor evoked potentials as markers for corticospinal integrity and the postexcitatory silent period (SP), representing cortical inhibitory interneurons. The patients' parameters of corticospinal tract function did not differ significantly from those in the control children. In contrast, the SP was significantly shortened in children with PVL (mean 25.6 +/- 6.9 ms; controls: mean 47.6 +/- 23.2 ms, P = 0.018). This suggests cortical involvement with reduced cortical inhibitory function in PVL. This could be due to impaired functioning of the cortical interneurons themselves, or to decreased input from activating fibres, e.g. thalamocortical or cortico-cortical connections.

Factors influencing cortical silent period: optimized stimulus location, intensity and muscle contraction

Inhibitory silent period (SP) is a transient suppression of voluntary muscle activity after depolarization of representative motor neuronal populations following transcranial magnetic stimulation (TMS). Our aim was to evaluate and present an optimal protocol for the measurement of SP by (1) determining the impact of muscle activation level and stimulus intensity (SI) on the duration of SP, and, (2) studying the relationship between motor evoked potential (MEP) and SP, using targeted stimulus delivery. Single magnetic pulses were focused on the optimal representation area of the thenar musculature on primary motor cortex. We utilized real-time 3D-positioning of TMS-evoked electric field on anatomical structures derived from individual MR-images. The SI varied from 80% to 120% of individual resting motor threshold (MT). Muscle activation levels varied from 20% to 80% of the maximal voluntary contraction (MVC). Contralateral SP lengthened significantly with increasing SI independent of target muscle activation. The peak amplitude of the MEP was affected by SI and force. Latency and duration of the MEP were practically unaffected by SI or force. Focal stimulation at 110-120% MT and approximately 50% MVC (with only negligible need for control) provides most stable and informative SP. MEP should be included in SP as the error from marking the onset diminishes. This study provides a guideline for the consistent measurement of SP, which is applicable when using navigated or traditional TMS.

Motor and somatosensory evoked potentials in Autosomal Dominant Hereditary Spastic Paraparesis (ADHSP) linked to chromosome 2p, SPG4

The aim of our study was to evaluate Motor Evoked Potentials (MEPs) and cortical excitability, using Transcranial Magnetic Stimulation (TMS) as well as short latency Somatosensory Evoked Potentials (SEPs) in Autosomal Dominant Hereditary Spastic Paraparesis (ADHSP) patients. MEPs were recorded from upper and lower limb muscles in 12 patients (7 m and 5f) affected by ADHSP with spastin mutation (SPG4). We measured: (i) motor threshold (MTh); (ii) total motor conduction time (TMCT); (iii) direct and indirect central motor conduction time (d-CMCT and i-CMCT) calculated by subtracting from the cortical latency those obtained on magnetic spinal stimulation (d-PMCT) and via the F-wave method (i-PMCT); (iv) MEP amplitude (MEP/Mmax ratio%) and (v) duration of the cortical silent period (CSP). Latency, amplitude and persistence of the F-wave obtained with electrical nerve stimulation were also considered; H reflex was also tested from lower extremities. SEPs were recorded from spine and scalp sites following median and posterior tibial nerve stimulation; conventional latency and amplitude measurements were performed. In a comparison with the control group, the MTh recording from lower limbs was significantly higher (67.5 +/- 7.7% versus 52.5 +/- 6.9%), MEPs were absent in one case and showed reduced amplitude in the remainders (22.9 +/- 12.6% versus 66.3 +/- 25.9% of M wave); TMCT resulted to be abnormal (36.5 +/- 3.9 ms versus 27.1 +/- 1.4 ms) and d-CMCT as well as i-CMCT were significantly prolonged (23.1 +/- 3.5 ms versus 13.8 +/- 1.3 ms; and 20.1 +/- 3.4 ms versus 10.6 +/- 1.3 ms, respectively). The CSP, which was normal from the hands, was significantly shortened from the legs and correlated with spasticity scoring (Ashworth scale). Cortical SEPs from lower limbs were abnormal in all cases, whereas SEPs by stimulation of median nerves were normal; F-wave parameters from upper limbs showed no abnormalities, whereas an increased persistence was detected from lower limbs; H reflex amplitudes resulted larger compared with controls. Moreover, shortening of the CSP, being correlated with the Ashworth scale, can be considered an electrophysiological marker of spasticity that seems to arise from impairment of the supraspinal or intracortical inhibitory pathways with an additional contribution of increased segmental motor neuron excitability. These data prove the existence of comparable neurophysiological abnormalities in ADHSP with spastin mutation (SPG4) when long ascending and descending pathways are involved.

Transcranial magnetic stimulation over sensorimotor cortex disrupts anticipatory reflex gain modulation for skilled action

Skilled interactions with new environments require flexible changes to the transformation from somatosensory signals to motor outputs. Transcortical reflex gains are known to be modulated according to task and environmental dynamics, but the mechanism of this modulation remains unclear. We examined reflex organization in the sensorimotor cortex. Subjects performed point-to-point arm movements into predictable force fields. When a small perturbation was applied just before the arm encountered the force field, reflex responses in the shoulder muscles changed according to the upcoming force field direction, indicating anticipatory reflex gain modulation. However, when a transcranial magnetic stimulation (TMS) was applied before the reflex response to such perturbations so that the silent period caused by TMS overlapped the reflex processing period, this modulation was abolished, while the reflex itself remained. Loss of reflex gain modulation could not be explained by reduced reflex amplitudes nor by peripheral effects of TMS on the muscles themselves. Instead, we suggest that TMS disrupted interneuronal networks in the sensorimotor cortex, which contribute to reflex gain modulation rather than reflex generation. We suggest that these networks normally provide the adaptability of rapid sensorimotor reflex responses by regulating reflex gains according to the current dynamical environment.

Disinhibition of upper limb motor area by voluntary contraction of the lower limb muscle

It is well known that monosynaptic spinal reflexes and motor evoked potentials following transcranial magnetic stimulation (TMS) are reinforced during phasic and intensive voluntary contraction in the remote segment (remote effect). However, the remote effect on the cortical silent period (CSP) is less known. The purpose of the present study is to determine to what extent the CSP in the intrinsic hand muscle following TMS is modified by voluntary ankle dorsiflexion and to elucidate the origin of the modulation of CSP by the remote effect. CSP was recorded in the right first dorsal interosseous while subjects performed phasic dorsiflexion in the ipsilateral side under self-paced and reaction-time conditions. Modulation of the peripherally-induced silent period (PSP) induced by electrical stimulation of the ulnar nerve was also investigated under the same conditions. In addition, modulation of the CSP was investigated during ischemic nerve block of the lower limb and during application of vibration to the tibialis anterior tendon. The duration of CSP was significantly shortened by phasic dorsiflexion, and the extent of shortening was proportional to dorsiflexion force. Shortening of the CSP duration was also observed during tonic dorsiflexion. In contrast, the PSP duration following ulnar nerve stimulation was not altered during phasic dorsiflexion. Furthermore, the remote effect on the CSP duration was seen during ischemic nerve block of the lower limb and the pre-movement period in the reaction-time paradigm, but shortening of the CSP was not observed during tendon vibration. These findings suggest that phasic muscle contraction in the remote segment results in a decrease in intracortical inhibitory pathways to the corticospinal tract innervating the muscle involved in reflex testing and that the remote effect on the CSP is predominantly cortical in origin.

Assessment of cortical excitability using threshold tracking techniques

Conventional paired-pulse transcranial magnetic stimulation (TMS) techniques of assessing cortical excitability are limited by fluctuations in the motor evoked potential (MEP) amplitude. The aim of the present study was to determine the feasibility of threshold tracking TMS for assessing cortical excitability in a clinical setting and to establish normative data. Studies were undertaken in 26 healthy controls, tracking the MEP response from abductor pollicis brevis. Short-interval intracortical inhibition (SICI) occurred up to an interstimulus interval (ISI) of 7-10 ms, with two distinct peaks evident, at ISIs of < or =1 and 3 ms, followed by intracortical facilitation to an ISI of 30 ms. Long-interval intracortical inhibition (LICI) occurred at ISIs of 50-300 ms, peaking at 150 ms. The present study has confirmed the effectiveness of the threshold tracking TMS technique in reliably and reproducibly measuring cortical excitability. Simultaneous assessment of upper and lower motor neuronal function with threshold tracking techniques may help to determine the site of disease onset and patterns of progression in neurodegenerative diseases.

Exteroceptive suppression of temporalis muscle activity in subjects with high and low aggression traits

STUDY AIM

There is evidence that the second exteroceptive suppression period of temporalis muscle activity (ES2) is modulated by the 5-HT neuronal activity in the brainstem, and the aggression trait is also connected with the cerebral 5-HT neuronal innervation. We therefore studied the temporalis ES2 in subjects with high and low aggression traits.

METHODS

Sixty-five subjects with either low or high aggression trends, judged by clinical interview, answered the Zuckerman-Kuhlman Personality Questionnaire (ZKPQ), the Zuckerman Sensation Seeking Scales (SSS) and the Plutchik-van Praag Depression Inventory (PVP). These also underwent a temporalis ES2 test procedure. Twenty-two subjects with a high ZKPQ aggression trait score and 27 with a low score were selected for data analysis.

RESULTS

On average, the high aggression group displayed significantly reduced temporalis ES2 duration, elevated ZKPQ Impulsive Sensation Seeking, Neuroticism-Anxiety and Aggression-Hostility, and PVP scores. The personality traits were not related with either latency or duration of temporalis ES2 in any group. The PVP score, however, was negatively correlated with ES2 duration in all 49 subjects.

CONCLUSIONS

This study demonstrates neurophysiologic signs of brainstem dysfunction in subjects with high aggression traits.

Excitability profile of motor evoked potentials and silent periods

The aim of this study was to confirm the excitability profile of human cortical circuits on the motor evoked potential (MEP) and the silent period (SP) after paired transcranial magnetic stimulation (TMS) with variable interstimulus intervals (ISI), and to compare the time courses of MEP and SP after paired TMS at variable ISIs. MEPs were elicited at the hypothenar muscles at rest, and during tonic muscle contraction by applying paired TMS to the motor cortex. The authors measured the MEP amplitude during rest and the duration of SP during tonic muscle contraction at various ISIs. The response to paired stimuli was inhibited by an ISI of 15 ms and facilitated by an ISI of 1020 ms. The SP at an ISI of 15 ms was shorter than that at the single suprathreshold stimulus, but the SP at an ISI of 1525 ms was longer than this. A significant correlation was observed between the MEP amplitude and the duration of SP at ISIs of 120 ms and for a CS of 80% of threshold. These results may provide useful data for the study of the function of cortical excitability in disease states and suggest that the neural circuits underlying MEP and SP differ partly.

How salient is the silent period? The role of the silent period in the prognosis of upper extremity motor recovery after severe stroke

Transcranial magnetic stimulation (TMS) has been successful in the prediction of motor recovery in acute stroke patients with initially severe paresis or paralysis of the upper extremity. Motor evoked potentials (MEP) appear to have a high specificity but a rather low sensitivity with regard to motor recovery. The silent period (SP) has been proposed as an additional factor to the MEP for predicting motor recovery that might optimize the sensitivity of TMS. The authors reviewed the literature and case series focusing on the additional value of the SP to the MEP for predicting poststroke hand motor recovery. Studies that have analyzed the SP for predicting poststroke motor recovery have rather inconsistent results and suffer from heterogeneity in technical methods, methodology, and patient characteristics. In most studies, prolonged SPs have been found immediately after stroke, whereas in the (sub)acute phase thereafter, different patterns of SP duration have been found. These differences are thought to be related to stroke localization, though contraction-induced reduction phenomena and recovery-related intracortical phenomena may also be responsible. Although the SP might be used to identify clinically silent or minor strokes, in acute stroke patients with initial severe paresis or paralysis, the SP seems to have no additional value to MEP for predicting poststroke motor recovery. Nevertheless, the SP (poststroke-reduced SPs and contraction-induced inhibitory phenomena) has been proposed as a prognostic factor for poststroke spasticity. This review emphasizes the significance of the SP in predicting poststroke motor recovery and spasticity. Although the relation among the SP, recovery-related intracortical phenomena, and spasticity remains unclear, a neurophysiologic model underlying the SP is discussed. However, more research is needed on the value of the SP for predicting poststroke spasticity.

Transcranial magnetic stimulation in lower motor neuron diseases

OBJECTIVE

To study the diagnostic value of transcranial magnetic stimulation (TMS) in a group of patients with lower motor neuron disease (LMND). Among LMND, several chronic immune mediate motor neuropathies may simulate amyotrophic lateral sclerosis (ALS).

METHODS

Forty patients with LMND were included TMS was performed at the first visit. The patients were seen prospectively every 3 months for a period of 1-4 years.

RESULTS

Three different groups were distinguished at the end of follow-up: (1) ALS group with 7 patients, (2) Pure motor neuropathy with 14 patients and (3) Other LMND including 12 patients with hereditary spinal amyotrophy, 3 patients with Kennedy's disease and 4 patients with post-poliomyelitis. On the basis of the results of TMS variables, 6 out of 7 ALS patients had abnormality of silent period (SP) associated or not with abnormality of excitatory threshold or amplitude ratio. Patients with pure motor neuropathy had normal SP and amplitude ratio. Four out of 14 patients had increased central motor conduction time (CMCT), one had increased CMCT and excitatory threshold, and one patient had a slightly increased excitatory threshold. Considering the abnormality of TMS variables in the groups, SP, excitatory threshold, and amplitude ratio were chosen in a post-hoc attempt to select variables yielding high sensitivity and specificity. The overall sensitivity of TMS for diagnosis of ALS among LMND was 85.7%, its specificity was 93.9%. When only the abnormality of SP was taken into account, the sensitivity was unchanged. But the specificity was improved to 100%.

CONCLUSIONS

TMS helped to distinguish suspected ALS from pure motor neuropathy.

Corticospinal disinhibition during dual action

When attempting to perform two tasks simultaneously, the human motor as well as the cognitive system shows interference. Such interference often causes altered activation of the cortical area representing each task compared to the single task condition. We investigated changes in corticospinal inhibition during dual action by transcranial magnetic stimulation (TMS). Single-pulse TMS was applied to the left motor cortex, triggered by right leg movement (tibialis anterior muscle) while the right abductor digiti minimi (ADM) muscle was moderately activated (10-20% of the maximal voluntary contraction). The background electromyography (EMG) activity of ADM was measured before and during the leg movement. The silent period (SP) and amplitude of motor evoked potential (MEP) following magnetic stimulation in active ADM were compared for the conditions with and without leg movement. The mean area of the rectified EMG activity of ADM did not alter, while the SP was significantly shortened during leg movement compared to that without leg movement. MEP amplitude was comparable between the two conditions. These results suggest that corticospinal inhibition tested by the SP duration is reduced during the movement of another body part, presumably in order to help maintain muscle force by compensating interference-related alteration in motor cortical activation.

Corticospinal control of respiratory muscles in chronic obstructive pulmonary disease

Patients with chronic obstructive pulmonary disease (COPD) face an increased respiratory load and in consequence have an elevated respiratory drive. We used transcranial magnetic stimulation (TMS) to investigate associated changes in corticospinal excitability both at rest and during voluntary facilitation at different levels of inspiratory effort. Diaphragm and abdominal motor thresholds were significantly lower in COPD than healthy controls, but the quadriceps response was the same. In patients there was a significant increase in diaphragm response from rest during 20% inspiratory efforts but no further increase with greater efforts. In controls there was a further stepwise increase at 40% and 60% of inspiratory effort. The cortical silent period was significantly shorter in COPD. Using paired stimulation to study intracortical inhibitory and excitatory circuits we found significantly less excitability of intracortical facilitatory circuits in patients at long (>7 ms) interstimulus intervals. These results suggest that there is a ceiling effect in motor control output to the respiratory muscles of patients with COPD.

Motor evoked potentials

Noninvasive electrical stimulation of the human brain first was attempted in the 1950s. In the early 1980s, the first clinical application method of transcranial electrical stimulation was developed. Investigators in the mid-1980s showed that it was possible to stimulate the nerve and the brain using external magnetic stimulation (transcranial magnetic stimulation [TMS]), with little or no pain. TMS now is used commonly in clinical neurology to study central motor conduction time. Depending on the stimulation techniques and parameters, TMS can excite or inhibit brain activity, allowing functional mapping of cortical regions and creation of transient functional lesions. It now is used widely as a research tool to study aspects of human brain physiology, including motor function and the pathophysiology of various brain disorders.

Responses of single motor units in human masseter to transcranial magnetic stimulation of either hemisphere

The corticobulbar inputs to single masseter motoneurons from the contra- and ipsilateral motor cortex were examined using focal transcranial magnetic stimulation (TMS) with a figure-of-eight stimulating coil. Fine-wire electrodes were inserted into the masseter muscle of six subjects, and the responses of 30 motor units were examined. All were tested with contralateral TMS, and 87 % showed a short-latency excitation in the peristimulus time histogram at 7.0 +/- 0.3 ms. The response was a single peak of 1.5 +/- 0.2 ms duration, consistent with monosynaptic excitation via a single D- or I1-wave volley elicited by the stimulus. Increased TMS intensity produced a higher response probability (n = 13, paired t test, P < 0.05) but did not affect response latency. Of the remaining motor units tested with contralateral TMS, 7 % did not respond at intensities tested, and 7 % had reduced firing probability without any preceding excitation. Sixteen of these motor units were also tested with ipsilateral TMS and four (25 %) showed short-latency excitation at 6.7 +/- 0.6 ms, with a duration of 1.5 +/- 0.3 ms. Latency and duration of excitatory peaks for these four motor units did not differ significantly with ipsilateral vs. contralateral TMS (paired t tests, P > 0.05). Of the motor units tested with ipsilateral TMS, 56 % responded with a reduced firing probability without a preceding excitation, and 19 % did not respond. These data suggest that masseter motoneurons receive monosynaptic input from the motor cortex that is asymmetrical from each hemisphere, with most low threshold motoneurons receiving short-latency excitatory input from the contralateral hemisphere only.

Transcranial magnetic stimulation and epilepsy

Epileptic conditions are characterized by an altered balance between excitatory and inhibitory influences at the cortical level. Transcranial magnetic stimulation (TMS) provides a noninvasive evaluation of separate excitatory and inhibitory functions of the cerebral cortex. In addition, repetitive TMS (rTMS) can modulate the excitability of cortical networks. We review the different ways that TMS has been used to investigate pathophysiological mechanisms and effects of antiepileptic drugs in patients with epilepsy and epileptic myoclonus. The safety of different TMS techniques is discussed too. Finally, we discuss the therapeutic prospects of rTMS in this field.

An automated method to determine the transcranial magnetic stimulation-induced contralateral silent period

BACKGROUND

The transcranial magnetic stimulation (TMS)-induced contralateral silent period (CSP) refers to a period of interruption of voluntary muscle activity measured in tonically active muscles. The length of the CSP is generally interpreted to reflect cortical inhibition. The determination of the return of voluntary motor activity is typically accomplished via visual inspection of the electromyography (EMG) waveform and may be subject to inaccuracy on the part of the rater.

OBJECTIVE

To present and evaluate an automated method (AM) to determine the CSP.

METHODS

The CSP of 11 healthy controls was recorded using stimulus intensities 20 and 50% above the resting motor threshold (RMT). The mean CSP duration obtained by the two raters using visual inspection and our automated approach were compared.

RESULTS

The interclass correlation coefficient (ICC) between the two raters and the AM was 0.99 at 150% of RMT and was 0.97 at 120% of RMT. The level of pre-stimulus EMG amplitude and sampling rate did not affect agreement between the AM and more conventional visually guided methods.

CONCLUSIONS

Our study demonstrates that this AM is a simple, objective and reliable approach for CSP determination.

SIGNIFICANCE

The CSP is an important neurophysiological measure of cortical inhibition and its determination by our AM provides a more objective and automated approach compared to visually guided methods.

Clinical and research methods for evaluating cortical excitability

The evaluation of motor cortical output after transcranial magnetic stimulation (TMS) is a means of investigating how the motor cortex reacts to external stimuli (i.e., a method to assess the excitability of the motor cortex). The recording of the descending volleys at the surface of the spinal cord provides a direct measure of the motor cortical output. However, this approach is highly invasive and can be used only during particular conditions. On the other hand, electromyographic recordings of the motor phenomena induced by TMS provide a completely painless, noninvasive, indirect measure of the cortical output, with these phenomena obviously reflecting the excitability of the spinal motoneurons as well as that of the muscle itself. The authors review how the electromyographic activity induced by TMS can provide valuable information about motor cortical excitability for use in clinical practice and research.

Transcranial magnetic stimulation and epilepsy

Transcranial magnetic stimulation has been used to study generalized and focal epilepsies for more than a decade. The technique appears safe and has yielded important information about the mechanisms underlying epilepsy. Transcranial magnetic stimulation findings differ depending on the epilepsy syndrome, lending support to the concept that there are distinct pathophysiologies underlying each condition. In most studies of generalized epilepsies, transcranial magnetic stimulation has indicated a state of relative hyperexcitability of excitatory cortical interneurons and possibly inhibitory interneurons as well, which can be reversed through the actions of anticonvulsant medications. Transcranial magnetic stimulation studies in patients with a seizure focus in the motor cortex indicate increased cortical excitability and reduced inhibition, but in patients with seizure foci located elsewhere the findings are similar to those in generalized epilepsies. Transcranial magnetic stimulation has also been used to study the mode of action of anticonvulsants and may prove to be a useful means of testing the potential for new drugs to act as anticonvulsants. Repetitive transcranial magnetic stimulation may prove to have a therapeutic role by producing long-lasting cortical inhibition after a train of impulses.

Pharmacologic influences on TMS effects

Transcranial magnetic stimulation (TMS) has been used increasingly to probe the physiology of the human cortex. Besides measuring directly the cortical excitability in motor and visual systems, this noninvasive method can be used to study short- and long-term cortical plasticity. One possible method to examine basic mechanisms underlying cortical excitability and plasticity in humans is the combination of TMS and pharmacologic interventions. In this review the author describes TMS paradigms used to study mechanisms of plasticity in the intact human motor system and its excitability using pharmacologic methods.

Different patterns of excitation and inhibition of the small hand and forearm muscles from magnetic brain stimulation in humans

OBJECTIVES

The objective of the present study is to quantify the effects of voluntary muscle contraction of the small hand (abductor pollicis brevis, first dorsal interosseus (FDI)) and forearm muscles (extensor carpi radialis (ECR), extensor carpi ulnaris (ECU), flexor carpi radialis (FCR) and flexor carpi ulnaris (FCU)) on motor evoked potentials (MEPs).

METHODS

MEPs were recorded in 12 healthy subjects by a circular coil placed over the vertex at 1.2 times the resting motor threshold at different levels of the muscle contraction (0-100% of maximum voluntary contraction (MVC)). The effects of transcranial magnetic stimulation (TMS) on the onset latency, MEP area and silent period (SP) as a function of the %MVC were evaluated using a piecewise linear regression analysis.

RESULTS

The MEP areas for the small hand muscles were almost completely saturated at 20% of MVC. In contrast, the MEP areas for radial muscles (ECR, FCR) had a dual increase at 40% of MVC while the ulnar muscles (ECU, FCU) had a dual increase at 20% of MVC. A uniform latency shift (1.5-3 ms reduction) was observed in all muscles with a changing point at 10% of MVC. The SPs were the longest for FDI and were not significantly influenced by MVC for any muscles.

CONCLUSIONS

The excitatory and inhibitory effects of TMS on the MEPs differed for the small hand and forearm muscles and also between the ulnar and radial muscles. These results probably resulted from the different degrees of direct corticomotoneuronal inputs to each muscle and the inherent properties of the spinal motoneurons.

Reduced excitability of the motor cortex in untreated patients with de novo idiopathic "grand mal" seizures

OBJECTIVES

Transcranial magnetic stimulation (TMS) was used to investigate motor cortex excitability, intracortical excitatory, and inhibitory pathways in 18 patients having experienced a first "grand mal" seizure within 48 hours of the electrophysiological test. All had normal brain MRI, and were free of any treatment, drug, or alcohol misuse. Results were compared with those of 35 age matched normal volunteers.

METHODS

The following parameters of responses to TMS were measured: motor thresholds at rest and with voluntary contraction, amplitudes of responses, cortical silent periods, and responses to paired pulse stimulation with interstimulus intervals of 1 to 20 ms.

RESULTS

In patients, there were significantly increased motor thresholds with normal amplitudes of motor evoked potentials (MEPs), suggesting decreased cortical excitability. Cortical silent periods were not significantly different from those of normal subjects. Paired TMS with short interstimulus intervals (1-5 ms) induced normal inhibition of test MEPs, suggesting preserved function of GABAergic intracortical inhibitory interneurons. On the contrary, the subsequent period of MEP facilitation found in normal subjects (ISIs of 6-20 ms) was markedly reduced in patients. This suggests the existence of abnormally prolonged intracortical inhibition or deficient intracortical excitation. In nine patients retested 2 to 4 weeks after the initial seizure, these abnormalities persisted, although to a lesser extent.

CONCLUSION

The present findings together with abnormally high motor thresholds could represent protective mechanisms against the spread or recurrence of seizures.

Spinal and supraspinal factors in human muscle fatigue

Muscle fatigue is an exercise-induced reduction in maximal voluntary muscle force. It may arise not only because of peripheral changes at the level of the muscle, but also because the central nervous system fails to drive the motoneurons adequately. Evidence for "central" fatigue and the neural mechanisms underlying it are reviewed, together with its terminology and the methods used to reveal it. Much data suggest that voluntary activation of human motoneurons and muscle fibers is suboptimal and thus maximal voluntary force is commonly less than true maximal force. Hence, maximal voluntary strength can often be below true maximal muscle force. The technique of twitch interpolation has helped to reveal the changes in drive to motoneurons during fatigue. Voluntary activation usually diminishes during maximal voluntary isometric tasks, that is central fatigue develops, and motor unit firing rates decline. Transcranial magnetic stimulation over the motor cortex during fatiguing exercise has revealed focal changes in cortical excitability and inhibitability based on electromyographic (EMG) recordings, and a decline in supraspinal "drive" based on force recordings. Some of the changes in motor cortical behavior can be dissociated from the development of this "supraspinal" fatigue. Central changes also occur at a spinal level due to the altered input from muscle spindle, tendon organ, and group III and IV muscle afferents innervating the fatiguing muscle. Some intrinsic adaptive properties of the motoneurons help to minimize fatigue. A number of other central changes occur during fatigue and affect, for example, proprioception, tremor, and postural control. Human muscle fatigue does not simply reside in the muscle.

Transcranial magnetic stimulation and human muscle fatigue

During exercise, changes occur at many sites in the motor pathway, including the muscle fiber, motoneuron, motor cortex, and "upstream" of the motor cortex. Some of the changes result in fatigue, which can be defined as a decrease in ability to produce maximal muscle force voluntarily. Transcranial magnetic stimulation (TMS) over the human motor cortex reveals changes in both motor evoked potentials (MEPs) and the silent period during and after fatiguing voluntary contractions in normal subjects. The relationship of these changes to loss of force or fatigue is unclear. However, during a sustained maximal contraction TMS evokes extra force from the muscle and thus demonstrates the development of suboptimal output from the motor cortex, that is, fatigue at a supraspinal level. In some patients with symptoms of fatigue, the response to TMS after exercise is altered, but the changed MEP behavior is not yet linked to particular symptoms or pathology.

Temporalis exteroceptive suppression in generalized anxiety disorder and major depression

The duration abnormality of the exteroceptive suppressions, or silent periods, of peripheral or jaw-closing muscle activities induced by transcranial magnetic or trigeminal electrical stimuli in patients suffering from anxiety or depression is ill-defined. We therefore studied the exteroceptive suppression periods of the temporalis muscle electromyography elicited by trigeminal territory electrical stimuli, Zuckerman-Kuhlman's Personality Questionnaire, and Plutchik-van Praag's Depression Inventory (PVP) in 12 patients suffering from generalized anxiety disorder (GAD) and 16 from major depression (MD) as well as in 17 healthy volunteers. Durations of the second suppression period (ES2) sociability scores were decreased in GAD patients, while PVP and neuroticism-anxiety scores were elevated in both GAD and MD patients. There was a positive correlation between ES2 duration and sociability score in the GAD group. This study indicates that anxiety can modify the temporalis ES2 duration through cortical descending inhibitory controls.

Modification of the silent period by double transcranial magnetic stimulation

OBJECTIVES

To study the time course of the changes of the inhibitory network of the human motor system, we investigated the silent period (SP) in 7 healthy subjects by double suprathreshold transcranial magnetic stimulation (TMS).

METHODS

SPs and motor evoked potentials (MEPs) were recorded from the voluntarily activated right abductor digiti minimi muscle. Conditioning and test stimuli were delivered with equal intensity, which was set to yield a baseline SP duration of 130 ms by a single pulse, and with various interstimulus intervals (ISIs). In addition, a control experiment with adjustment of the intensity of single stimuli was performed.

RESULTS

At ISIs of 20 and 30 ms the test pulse SP duration was prolonged, without increasing the MEP amplitude. The SP duration shortened at longer ISIs and showed a significant depression between ISIs of 60-110 ms. The shortened SP was accompanied by a diminished MEP. The control experiment revealed that the SPs evoked by the adjusted pulses were significantly shorter than the test pulse SPs.

CONCLUSIONS

A conditioning stimulus can prolong and shorten the test pulse SP duration at different ISIs. The prolongation is probably cortically generated, whereas the shortening is likely to occur at a cortical and spinal level.

Remote effects of cortical dysgenesis on the primary motor cortex: evidence from the silent period following transcranial magnetic stimulation

OBJECTIVE

In cortical dysgenesis (CD), animal studies suggested abnormal cortico-cortical connections. Cerebral areas projecting to the primary motor cortex (M1) modulate the cortical silent period (CSP) following transcranial magnetic stimulation (TMS). Therefore, we used the CSP to investigate remote effects of CD on the M1.

METHODS

A detailed investigation, including single-pulse TMS and electrical nerve stimulation, was performed in 3 consecutive adults with focal CD located outside the M1 and in 18 controls. Two patients with unilateral CD were epileptic and treated with anti-epileptic drugs. One patient with focal CD on both sides had no history of seizures. Neurological examination was normal in all patients. Recordings were made from both first dorsal interosseous muscles.

RESULTS

In CD patients, the CSP was significantly lengthened contralaterally to the affected hemispheres. In treated patients with unilateral CD, the interside difference of the CSP duration was also significantly increased. In contrast, excitability threshold, peripheral and corticospinal motor conduction studies, and peripheral as well as ipsilateral silent periods were not significantly modified.

CONCLUSIONS

Our findings indicate that focal CD outside the M1 may produce CSP modifications, which are likely due to changes of afferent control.

Supraspinal fatigue during intermittent maximal voluntary contractions of the human elbow flexors

Responses to transcranial magnetic stimulation in human subjects (n = 9) were studied during series of intermittent isometric maximal voluntary contractions (MVCs) of the elbow. Stimuli were given during MVCs in four fatigue protocols with different duty cycles. As maximal voluntary torque fell during each protocol, the torque increment evoked by cortical stimulation increased from approximately 1.5 to 7% of ongoing torque. Thus "supraspinal" fatigue developed in each protocol. The motor evoked potential (MEP) and silent period in the elbow flexor muscles also changed. The silent period lengthened by 20-75 ms (lowest to highest duty cycle protocol) and recovered significantly with a 5-s rest. The MEP increased in area by >50% in all protocols and recovered significantly with 10 s, but not 5 s, of rest. These changes are similar to those during sustained MVC. The central fatigue demonstrated by the torque increments evoked by the stimuli did not parallel the changes in the electromyogram responses. This suggests that part of the fatigue developed during intermittent exercise is "upstream" of the motor cortex.

Loss of the muscle silent period evoked by transcranial magnetic stimulation of the motor cortex in patients with cervical cord lesions

The silent period following motor evoked potentials in small hand muscles after transcranial magnetic stimulation (TMS) of the human motor cortex is considered to be cortical origin. The authors report three patients with cervical spinal cord lesions who showed loss of the cortical silent period (CSP) after TMS. One patient had traumatic cervical cord injury, and the other two patients had cervical spondylosis. All the patients had cervical cord compression on magnetic resonance imaging. TMS study showed loss of the CSP in both the hand and foot muscles in two patients and only in the foot muscle in one patient. Paired TMS study in one patient with pseudoathetotic hands showed reduced inhibition within the motor cortex. The hand weakness or interrupted sensory afferents might have caused motor cortical reorganization or hyperexcitability, leading to the loss of the CSP.

Motor cortex disinhibition in acute stroke

OBJECTIVES

To test whether a disinhibition occurs in the human motor cortex after stroke.

METHODS

Patients with a mild to moderate hemiparesis after an acute unilateral ischemic stroke were compared with age-matched healthy controls. We used paired transcranial magnetic stimuli (TMS) to investigate intracortical inhibition and facilitation. Single TMS were applied to obtain a cortical silent period.

RESULTS

Intracortical inhibition was significantly reduced in the affected hemisphere at interstimulus intervals of 2, 3 and 4 ms. The cortical silent period was significantly prolonged when compared to the unaffected hemisphere of the patients and to the control group. Motor cortex disinhibition observed in stroke patients was associated either with minimal impairment at the onset of symptoms or with rapidly improving motor functions.

CONCLUSIONS

Motor cortex disinhibition occurs in humans after stroke. We suggest that this disinhibition is indicative of compensatory mechanisms, which are involved in recovery-related reorganization.