Postexercise facilitation of motor evoked potentials following transcranial magnetic stimulation: a study in normal subjects

The size of the motor evoked potential (MEP) elicited by transcranial magnetic stimulation increases soon after a nonexhaustive voluntary contraction of the target muscle (postexercise facilitation). Our aim was to determine whether the duration or intensity of voluntary muscle contraction influenced postexercise facilitation in normal subjects. We recorded the MEP from the thenar muscles following contractions of different durations (5, 15, and 30 s) and intensities (10%, 25%, and 50% of maximal voluntary contraction). We found that every combination of the tested intensities and durations of physical effort could induce postexercise MEP facilitation. Although the degree of postexercise MEP facilitation was comparable across the different durations and intensities, the maximal facilitation was observed with the shortest and strongest muscle contraction. Our study thus defines the optimal setting to study postexercise facilitation for clinical purposes.     

Evidence for facilitation of motor evoked potentials (MEPs) induced by motor imagery

This study examined the extent to which motor imagery can facilitate to specific pools of motoneurons. Motor commands induced by motor imagery were subthreshold for muscle activity and were presumably not associated with any change in background afferent activity. To estimate excitability changes of flexor carpi radialis (FCR) muscle motoneuron in spinal and cortical level, electric stimuli for recording H-reflex and transcranial magnetic stimulation (TMS) for recording motor evoked potentials (MEPs) were used. During motor imagery of wrist flexion, remarkable increases in the amplitude of the MEP of FCR were observed with no change in the H-reflex. Furthermore, facilitation of antagonist (extensor carpi radialis; ECR) was also observed. Therefore, it is concluded that internal motor command can activate precisely cortical excitability with no change in spinal level without recourse to afferent feedback.     

Topographic differences in task-dependent facilitation of magnetic motor evoked potentials

OBJECTIVE: To look for differences in task-dependent facilitation of magnetic motor evoked potentials (MEPs) in proximal and distal upper extremity muscles. BACKGROUND: Postexercise facilitation of MEPs has been demonstrated repeatedly in forearm muscles. Proximal muscles are prominently involved in limb stabilization, a largely isometric activity. In contrast, distal hand muscles specialize in precision movements. Based on these functional differences between proximal and distal muscles, we postulated that there might be topographic differences in task-dependent facilitation of MEPs. METHODS: We studied the effects of isometric exercise and a precision grip task on MEPs in proximal and distal upper extremity muscles. RESULTS: Isometric exercise of the target muscle was associated with significant facilitation of MEPs in biceps and extensor carpi radialis muscles, but not in first dorsal interosseous or abductor digiti minimi muscles. In contrast, a precision grip task was associated with significant post-task facilitation of MEPs in first dorsal interosseous, but not in extensor carpi radialis. CONCLUSIONS: These differences in the facilitation of MEPs depending on the motor task and whether the muscle is proximal or distal may reflect the relative importance of proximal muscles in maintaining posture.     

Mirror movements and ipsilateral motor evoked potentials in ALS.

It can be difficult to detect upper motor neuron (UMN) involvement in early amyotrophic lateral sclerosis (ALS). The present study tested the usefulness of mirror movements (MMs), i.e., contralateral coactivation of hand muscles, as a UMN sign in ALS. Thirty-seven patients with possible, probable or definite ALS and 19 patients with suspected ALS without clinical signs of UMN involvement, 21 disease controls and 15 healthy volunteers were included. MMs were studied clinically, electromyographically and by transcranial magnetic stimulation (TMS), looking for ipsilateral motor evoked potentials (IMEPs). MMs were observed clinically in 15 ALS patients (27%) and electromyographically in 28 (50%). IMEPs in the abductor pollicis brevis muscle following TMS were recorded in 34 (61%) of all ALS patients and in nine (47%) out of 19 patients with suspected ALS, but not in healthy controls. Central motor conduction times were prolonged in 15 ALS patients (27%). It is concluded that MMs, especially if studied by EMG and TMS (IMEPs) can essentially improve detection of UMN involvement, especially in early stages of ALS.     

Effects of carotid endarterectomy on motor evoked potentials elicited by transcranial magnetic stimulation

Purpose To investigate the within 3 days effects of carotid endarterectomy (CEA) on functional status of the central motor system in patients with carotid stenosis by means of transcranial magnetic stimulation (TMS). Patients and method We studied 30 consecutive patients, 20 males and 10 females with a mean age of 69.2 ± 7.1 years, who underwent CEA for symptomatic carotid stenosis. All patients had suffered an ischemic attack 6 months prior to the operation. Two TMS studies, one before and one shortly after CEA were performed on both sides in each of the patients. Resting motor threshold, motor evoked potentials (MEP) amplitude at rest, MEP latency at rest and during contraction and silent period duration (SPD) were recorded and analyzed. Two groups of data were collected. Group 1 consisted of data from the operated side in all 30 patients. Group 2 consisted of data from the contralateral side and served as a control. Results Motor resting thresholds were similar in the two groups. Intragroup pre and post CEA comparisons showed no difference in the operated group and significant increased threshold after CEA on the non-operated side. There was no significant difference of TMS intensity for maximal MEP in either side before or after CEA. Latency at rest and during voluntary contraction and amplitude at rest showed no significant differences between or within groups’ comparisons. In group 1 SPD showed a statistically significant increase after CEA as opposed to baseline. In group 2 SPD showed a non significant increase after CEA. Conclusion In the absence of other MEP changes, our finding of prolonged SPD post–operatively suggests preferential influence of the inhibitory cortical circuits. The potential favorable effect of CEA in patients with hyperexcitability such as disabling spasticity after stroke should be further studied.     

Increased post-exercise facilitation of motor evoked potentials in multiple sclerosis.

OBJECTIVES: To study post-exercise facilitation following a non-fatigue exercise in a homogenous group of multiple sclerosis (MS) patients with complaints of muscle fatigue. METHODS: In 15 MS patients and matched controls motor evoked potentials (MEP) were recorded from the biceps brachii muscle at time delays of 0.5-30s after an isometric contraction with a torque of 50% of maximal voluntary contraction (MVC) maintained for 2-6s. In addition, MEP was recorded after isometric torque of 25, 50, and 100% of MVC maintained for 6s. RESULTS: Isometric non-fatigue contraction induced significant post-exercise increase in MEP amplitude in MS patients compared with controls being most pronounced after a contraction for 6s. The post-exercise increase in MEP lasted for longer than 30s in MS patients. CONCLUSIONS: Post-exercise increase of MEP amplitude following a non-fatigue exercise was significant in MS patients with complaints of muscle fatigue compared with healthy subjects.     

Impaired facilitation of motor evoked potentials in incomplete spinal cord injury

Objectives To improve the diagnosis of damaged spinal motor pathways in incomplete spinal cord injury (iSCI) by assessing the facilitation of lower limbs motor evoked potentials (MEP). Methods Control subjects (n = 12) and iSCI patients (n = 21) performed static and dynamic isometric foot dorsiflexions. MEPs induced by transcranial magnetic stimulation and EMG background of tibialis anterior muscle (TA) were analyzed. Static and dynamic muscle activation was performed at comparable levels of maximal voluntary contraction (MVC). The influence of the motor tasks on the excitability and facilitation of MEPs was compared between controls and iSCI patients. Results In the controls an increased facilitation of TA MEP at lower levels of dynamic compared with static activation (10–20% MVC) could be shown. At matched EMG background level the MEP responses were significantly increased. In the iSCI patients at a comparable level of TA activation the MEP responses were significantly reduced and 3 different patterns of MEP responses could be distinguished: i) preserved increment of TA MEP in the dynamic motor task, ii) unchanged MEP size in the dynamic and static motor task, and iii) elicitable MEPs in the dynamic motor task,which were abolished in the static motor task. Conclusions Static and dynamic motor tasks have different effects on TA MEP facilitation. The task–dependent modulation of TA MEPs is comparable to that described for upper limb muscles. Complementary to the MEP delay this approach allows for an estimation of the severity of spinal tract damage. The task–dependent modulation of TA MEPs is an additional diagnostic tool to improve the assessment and monitoring of motor function in iSCI.     

Early and late lower limb motor evoked potentials elicited by transcranial magnetic motor cortex stimulation.

Transcranial magnetic motor cortex stimulation can elicit a series of responses recorded with different latencies from relaxed muscles of the lower limbs. In 7 healthy subjects, ranging in age from 16 to 62 years, stimulation was delivered by a 9 cm coil centered over Cz with the subject in the supine position. Surface polyelectromyography was used to record motor evoked potentials (MEPs) from the quadriceps (QD), hamstrings (HS), tibialis anterior (TA) and triceps surae (TS) muscles bilaterally. Three characteristic responses were identified in each muscle group on the basis of amplitude and latency criteria, identified by latencies: the direct oligosynaptic response MEP30 appeared with a latency of 24.3 msec in the QD, 26.3 msec in the HS, 30.5 msec in the TA and 31.3 msec in the TS; MEP70 with latencies of 64 msec in the QD, 59 msec in the HS, 79 msec in the TA and 72 msec in the TS; MEP120 with latencies of 115 msec in the QD, 126 msec in the HS, 117 msec in the TA and 124 msec in the TS. These 3 responses have distinct latencies, amplitudes and durations. MEP70 appears to be the result of activation of long descending tracts which end on spinal interneuronal circuits. As MEP120 has different features, it may have a different mechanism.     

Normal postexercise facilitation and depression of motor evoked potentials in postpolio patients

We studied the effects of exercise on motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation in healthy subjects and postpolio patients. Subjects performed repeated sets of isometric exercise until the muscle fatigued. In both groups, the mean MEP amplitude immediately after each exercise set was approximately twice that of the baseline amplitude, indicating similar postexercise facilitation, and after fatigue was approximately half that of the baseline amplitude, indicating similar postexercise depression. We conclude that the intracortical component of central fatigue is normal in postpolio patients. © 1998 John Wiley & Sons, Inc. Muscle Nerve 21:948–950, 1998.     

Task-dependent facilitation of motor evoked potentials during dynamic and steady muscle contractions

Task-dependent differences in the facilitation of motor evoked potentials (MEPs) following cortex stimulation were studied in a proximal (deltoid) and a distal muscle (abductor digiti minimi; ADM) in 23 healthy subjects during both dynamic and steady contractions of the target muscle under isometric and under nonisometric conditions. In the deltoid, MEP amplitudes were significantly greater if stimulation was performed during dynamic contractions than during steady contractions, despite equal background electromyographic levels just prior to the stimulus. The same task-specific extra facilitation of deltoid MEP amplitudes was also found with magnetic stimulation of the brain stem instead of the cortex in 3 subjects. In the ADM, no such task-dependent extra facilitation of MEPs during dynamic contractions was found. It is concluded that in the deltoid, during dynamic contractions, a greater proportion of the spinal motoneurons is close to depolarization threshold (greater “subliminal fringe”) whereas the number of firing motoneurons is similar to that during steady contraction. The lack of task-dependent extra facilitation of MEPs in the ADM is explained by the predominant recruitment principle for force gradation in small hand muscles, which is in contrast to the predominant frequency principle used in proximal muscles. © 1998 John Wiley & Sons, Inc. Muscle Nerve 21:1309–1316, 1998.     

Facilitation of motor evoked potentials in hand extensor muscles of stroke patients: correlation to the level of voluntary contraction

The influence of ongoing voluntary isometric contractions (ranging from 2.5% to 100% of maximum force production) on motor evoked potentials in the extensor carpi radialis muscle was investigated in 20 healthy subjects and 25 hemiparetic stroke patients using transcranial magnetic stimulation at threshold and at 90% of maximum stimulus intensity. In healthy subjects and in stroke patients, an initial sharp decay in response latencies was observed at low contraction levels. In hemiparetic patients, however, no significant further reduction of response latencies with increasing contraction levels was observed irrespective of whether threshold or 90% stimulus intensities were applied. The continuous decrease in latency in the healthy subjects is supposed to result from an enhanced involvement of rapidly conducting corticospinal neurones that are preferentially damaged in the patient group. In healthy subjects and in hemiparetic patients, however, the increase in response amplitudes runs in parallel with increasing force production, at least with threshold stimulus intensity. Contrary to response latencies, amplitude facilitation appears to be less dependent on the involved corticospinal fibre spectrum but to be predominantly based on temporal and spatial summation effects. The relevance of the latency and amplitude data obtained in healthy subjects and in stroke patients for physiology and localization of facilitatory processes, i.e. whether cortical or spinal, is discussed. For the rehabilitation of stroke patients it is concluded that the effect of slight voluntary contractions is indeed superior to most other facilitatory approaches. The functional relevance is discussed.     

Transcranial magnetic stimulation: specific and non-specific facilitation of magnetic motor evoked potentials

Summary Different physiological mechanisms of facilitation of latencies and amplitudes of magnetic motor evoked potentials (MEPs) were evaluated in a cohort of 140 healthy volunteers. The potentials were induced at the vertex and recorded at the abductor pollicis brevis. The aim of the present investigation was to compare physiological mechanisms which presumably facilitate motor pathways at the cortical level with those known to occur during contraction of small hand muscles. When compared with MEPs at rest, the maximum average decrease of latencies (1.5, SD 1.1 ms) as well as the highest increase of peak to peak amplitudes (2.6, SD 2.1 mV) was observed during exertion of a voluntary background force, at the muscle recorded from. Pre-innervation of a neighbouring muscle (abductor digiti minimi) led to a lesser average decrease of latencies by 1.0, SD 1.1 ms and an average increase of amplitudes by only 0.5, SD 1.5 mV. Non-specific manoeuvres, like sticking out the tongue or counting aloud, reduced mean latencies slightly by 0.4 ms, SD 0.8 ms and 0.3 SD 0.85 ms respectively, but increased amplitudes markedly by an average of 1.0, SD 1.6 mV and 0.8, SD 1.4mV respectively. It is concluded that facilitation of MEPs by non-specific manoeuvres occurs and must be taken into account when evaluating MEPs.     

Effect of exercise on motor evoked potentials elicited by transcranial magnetic stimulation in psychiatric patients.

SUMMARY: Under normal conditions, motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation increase in amplitude if the subject exercises the examined muscle immediately before recording. The authors examined the effect of nonfatiguing exercise on the amplitude of MEPs on 42 psychiatric, medicated inpatients (14 with depression, 14 with schizophrenia, and 14 with mania) compared with 14 healthy control subjects. For each subject, a total of 50 baseline and 50 postexercise MEPs were recorded. The mean (+/- standard deviation) postexercise MEP facilitation, expressed as a percentage of mean baseline values, was significantly lower (p 相似文献   

Effect of fatiguing maximal voluntary contraction on excitatory and inhibitory responses elicited by transcranial magnetic motor cortex stimulation

Vertex transcranial magnetic stimulation (TMS) elicited tibialis anterior motor evoked potentials (MEPs) and silent periods (SPs) that were recorded during and following isometric maximal volitional contraction (MVC). During MVC in 6 healthy subjects, MEP amplitudes in the exercised muscle showed an increasing trend from an initial value of 4539 ± 809 μV (mean ± SE) to 550 ± 908 μV (P < 0.13) while force and EMG decreased (P < 0.01). Also, SP duration increased from 165 ± 37 ms to 231 ± 32 ms (P < 0.01). Thus, during a fatiguing MVC both excitatory and inhibitory TMS-induced responses increased. TMS delivered during repeated brief 10% MVC contractions before and after a fatiguing MVC in 5 subjects, showed no change in MEP amplitude but SP duration was prolonged after MVC. This SP prolongation was focal to the exercised muscle. Silent periods recorded after pyramidal tract stimulation were unchanged following the MVC. These results suggest that MEP and SP might have common sources of facilitation during an MVC and that inhibitory mechanisms remain focally augmented following a fatiguing MVC. © 1996 John Wiley & Sons, Inc.     

Absence of facilitation or depression of motor evoked potentials after contralateral homologous muscle activation

We have previously described post-exercise facilitation and post-exercise depression of motor evoked potentials (MEPs) to transcranial magnetic stimulation (TMS). To determine the presence of post-exercise facilitation after exercise of a contralateral muscle, MEPs were recorded from the resting right extensor carpi radialis (ECR) muscle while the left ECR muscle was activated, then immediately after brief left ECR activation, and, finally, immediately after brief right ECR activation. We repeated the experiment using the first dorsal interosseous (FDI) muscle. To determine the presence of post-exercise depression after exercise of a contralateral muscle, MEPs were recorded from the right ECR after prolonged exercise of the left ECR, followed by right ECR recording after its fatigue. The mean MEP amplitudes from the right ECR and the right FDI after brief activation were 187% and 266% of their pre-exercise values, respectively. There were no significant changes in MEPs recorded from the right ECR or FDI muscles during or immediately after brief activation of their left counterparts. The mean amplitude of MEPs recorded from the right ECR after it fatigued was approximately half the pre-exercise value, but there was no significant change in MEPs recorded from the right ECR after prolonged exercise of the left ECR. Therefore, neither post-exercise facilitation nor post-exercise depression occurred after contralateral homologous muscle exercise.     

Thalamic potentials evoked by motor point stimulation

We studied thalamic potentials elicited by stimulation of the extensor digitorum communis muscle motor point during stereotactic surgery for movement disorders. In 6 patients with Parkinson's disease and 1 with cerebral palsy, muscle afferent-evoked thalamic potentials (METPs) were recorded in the ventral intermediate (Vim) nucleus. METPs consisted of three peaks with an average latency of 11.02 +/- 0.80 ms for initial positive peak (PI), 13.04 +/- 0.58 ms for negative (N), and 14.30 +/- 0.87 ms for later positive peak (PII). These METPs were different from those evoked by median nerve stimulation. No peaks corresponding to the N20 component of the somatosensory evoked potential were recorded from C3 or C4 scalp electrodes. The METP amplitudes in the ventrolateral (VL) nucleus were markedly lower than those in the Vim nucleus. These findings suggest that METPs in the Vim nucleus are responses that occur via the fast conducting group Ia afferent fibers rather than somatosensory afferent fibers.     

Significance of coil orientation for motor evoked potentials from nasalis muscle elicited by transcranial magnetic stimulation.

OBJECTIVE: In transcranial magnetic stimulation (TMS) of the motor cortex, the optimal orientation of the coil on the scalp is dependent on the muscle under investigation, but not yet known for facial muscles. METHODS: Using a figure-of-eight coil, we compared TMS induced motor evoked potentials (MEPs) from eight different coil orientations when recording from ipsi- and contralateral nasalis muscle. RESULTS: The MEPs from nasalis muscle revealed three components: The major ipsi- and contra-lateral middle latency responses of approximately 10 ms onset latency proved entirely dependent on voluntary pre-innervation. They were most easily obtained from a coil orientation with posterior inducing current direction, and in this respect resembled the intrinsic hand rather than the masseter muscles. Early short duration responses of around 6 ms onset latency were best elicited with an antero-lateral current direction and not pre-innervation dependent, and therefore most probably due to stimulation of the nerve roots. Late responses (>18 ms) could inconsistently be elicited with posterior coil orientations in pre-innervated condition. CONCLUSIONS: By using the appropriate coil orientation and both conditions relaxed and pre-innervated, cortically evoked MEP responses from nasalis muscle can reliably be separated from peripheral and reflex components and also from cross talk of masseter muscle activation.     

Facilitation of motor evoked potentials after repetitive voluntary hand movements depends on the type of motor activity

Recent neurophysiological studies suggest that repetitive execution of identical movements is crucial for motor learning. During and after repetitive motor action, changes in motor cortical excitability have been demonstrated by means of transcranial magnetic stimulation. Nevertheless, the frequency and intensity of movement repetition that are necessary to achieve an optimal improvement in motor function are unknown. Fourteen healthy volunteers participated in the present study, which deals with the post-exercise facilitatory and/or inhibitory effects of 5 different motor conditions, including repetitive isotonic contractions at the wrist at two different velocities and two different forearm positions, a sustained isometric hand extension and repetitive hand extensions at the wrist induced by means of transcutaneous electrical muscle stimulation. The modification of muscular response potentials in the extensor carpi radialis muscle was measured following the various motor tasks and the electrical muscle stimulation. The only statistically significant facilitatory effect was observed following an extension-relaxation task at low frequency. Furthermore, the duration of transcranially induced silent periods showed a significant reduction after this motor task.     

Visual evoked potentials elicited by a moving unidimensional noise pattern

(1) Motion onset and offset visual evoked potentials (VEPs) were recorded in normal human subjects using a unidimensional noise pattern moving at 1, 8 and 64 degrees/s. The maximum N1-P1 amplitude of the motion onset response was obtained when using a fine noise pattern (maximum energy at 5.2 cpd) moving at 8 degrees/s. (2) At a velocity of 8 degrees/s, the motion onset response (fine pattern, 0.70 contrast) showed a morphology similar to the pattern disappearance response. Both at a lower (1 degrees/s) and a higher velocity (64 degrees/s) the N1-P1 amplitude of the motion onset complex was significantly reduced. The latency of the motion onset response (8 degrees/s) and the pattern disappearance complex were significantly different. (3) The effect of lowering the spatial content of the noise pattern on the amplitude of the motion onset response was different for the 3 velocities tested: the largest effect was at the lower velocity of 1 degrees/s; there was no similar effect on the pattern disappearance response. (4) With decreasing contrast, the N1-P1 amplitude of the motion onset response at 8 degrees/s decreased, but this reduction in amplitude was much less than that of the disappearance response. The contrast dependency of the motion onset complex was identical for binocular and monocular recordings. (5) Increasing the motion duration or the duration of the interstimulus interval did not alter the general morphology of the motion response.