Changes in motor cortex excitability during ipsilateral hand muscle activation in humans.

OBJECTIVES: To test whether unilateral hand muscle activation involves changes in ipsilateral primary motor cortex (M1) excitability. METHODS: Single- and paired-pulse transcranial magnetic stimulation (TMS) of the right hemisphere was used to evoke motor evoked potentials (MEPs) from the resting left abductor pollicis brevis (APB) in 9 normal volunteers. We monitored changes in motor threshold (MT), MEP recruitment, intracortical inhibition (ICI) and intracortical facilitation (ICF) while the ipsilateral right APB was either at rest or voluntarily activated. Spinal motoneuron excitability was assessed using F-wave recording procedures. RESULTS: Voluntary muscle activation of the ipsilateral APB significantly facilitated the MEPs and F-waves recorded from the contralateral APB. Facilitation was observed with muscle activation >50% of the maximum voluntary force and with stimulus intensities >20% above the individual resting motor threshold. Intracortical inhibition significantly decreased in the ipsilateral M , while there was no significant change in intracortical facilitation during this maneuver. CONCLUSIONS: Unilateral hand muscle activation changes the excitability of homotopic hand muscle representations in both the ipsilateral M1 and the contralateral spinal cord. While the large proportion of MEP facilitation most likely occurred at a spinal level, involvement of the ipsilateral hemisphere may have contributed to the enlargement of magnetic responses.     

Latency of motor evoked potentials to focal transcranial stimulation varies as a function of scalp positions stimulated.

We recorded motor evoked potentials (MEP) to transcranial magnetic stimulation from abductor pollicis brevis (APB), flexor carpi radialis (FCR), biceps brachii and deltoid muscles at rest and during slight voluntary activation. An 8-shaped coil connected to a Cadwell MES-10 magnetic stimulator was positioned over different scalp positions 1 cm apart. At least 24 stimuli were delivered at each location. Latencies of MEPs were compared with those obtained by electrical and magnetic stimulation during muscle activation. Progressively longer MEP latencies were obtained in 5 groups depending on the type and position of stimulation. The shortest latencies were obtained with (1) electrical stimulation during muscle contraction and (2) non-focal magnetic stimulation during muscle contraction; magnetic stimulation at rest produced longer latencies with stimulation of (3) an optimal position, (4) a suboptimal position, and (5) a non-optimal position. Mean latency differences between successive groups were 1.9, 2.0, 1.6, and 2.6 msec for APB. Similar latency differences were found for the other arm muscles. The results are compatible with the hypothesis that the different latencies evoked by stimulation at different scalp locations are determined by the summation at spinal motoneurons of excitatory postsynaptic potentials generated by successive numbers of I waves.     

Modulation of motor activity by cutaneous input: inhibition of the magnetic motor evoked potential by digital electrical stimulation

We examined the inhibitory effect of a brief train of digital (D2) electrical stimuli at 4 times perception threshold on transcranial magnetic motor evoked potentials (MEPs) recorded from abductor pollicis brevis (APB) and flexor carpi radialis (FCR) muscles ipsilateral to the side of D2 stimulation. We compared this to the inhibitory effect of ipsilateral D2 stimulation on averaged rectified EMG recorded at 10% maximum voluntary contraction and on F-responses and H-reflexes recorded from these same muscles. We also compared MEPs recorded following D2 stimulation just above perception threshold to MEPs following higher intensity D2 stimulation. As well, we assessed the effect of preceding D2 stimulation on MEPs recorded from a relaxed versus tonically contracted hand muscle. D2 stimulation elicited a triphasic response of modest MEP facilitation followed by inhibition and further facilitation. The duration and onset of MEP inhibition correlated with those of the initial period of rectified EMG inhibition, however, the magnitude of MEP inhibition was generally less than the magnitude of EMG inhibition, consistent with a greater inhibitory effect of digital afferents on smaller motor neurons. MEN were not facilitated during the rebound of EMG activity (the E2 period) that usually followed the initial period of EMG inhibition (I1 period). The behavior of H-reflexes and F-responses following ipsilateral D2 stimulation suggested that inhibition of both EMG and MEPs is not mediated via presynaptic inhibition of la afferents, and that inhibition is augmented by descending rather than segmental input to spinal motor neurons. Tonic contraction of the target muscle during D2 stimulation decreased the inhibitory effect of the preceding digital stimulus possibly due to recruitment of larger spinal motor neurons less likely to be inhibited by cutaneous input.     

Facilitation and reciprocal inhibition by imagining thumb abduction.

It is well known that motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) of the motor cortex are facilitated by voluntary muscle contraction. We evaluated the effects of imagination of movements on MEP latencies of agonist and antagonist muscles in the hand using TMS. Twenty-two healthy volunteers were studied. TMS delivered at rest and while imagining tonic abduction of the right thumb. MEPs were recorded in response to magnetic stimulation over the scalp and cervical spine (C7-T1), and central motor conduction times (CMCT) were calculated. MEPs were recorded from right abductor pollicis brevis muscle (APB) and adductor pollicis muscle (AP) simultaneously. Imagination of abduction resulted in a shortened latency of MEPs in the APB muscle, and a prolonged latency in the AP muscle. But the imagination caused no significant change in the latency of MEPs elicited by stimulation over the cervical spine. The changes of the CMCT may account for these latency changes with imagination of movement. These findings indicate that imagination of thumb abduction facilitates motoneurons of agonist muscle and has an inhibitory effect on those of antagonist muscle (reciprocal inhibition).     

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.     

Reorganization of corticospinal pathways following spinal cord injury

To assess changes in the relationship between cortical motor representation areas and their target muscles following spinal cord lesions, we studied motor evoked potentials (MEPs) to transcranial magnetic stimulation in six patients with complete spinal cord injuries at low thoracic levels and eight healthy subjects. Magnetic stimulation at rest activated a larger fraction of the motoneuron pool and evoked MEPs with shorter latencies from a larger number of scalp positions in muscles immediately rostral to the level of a spinal cord injury than in corresponding muscles in controls. The MEPs associated with maximal voluntary activation were not significantly different in the two groups. These results suggest enhanced excitability of motor pathways targeting muscles rostral to the level of a spinal transection, reflecting reorganization of motor pathways either within cortical motor representation areas or at the level of the spinal cord. The data do not allow the determination of the contribution of spinal or cortical mechanisms. However, they support the notion of a limited flexible relationship between primary motor cortex and its target muscles following alterations of normal input-output patterns.     

Modulations in corticomotor excitability during passive upper-limb movement: is there a cortical influence?

Modulations in the excitability of corticomotor pathways to forearm musculature have previously been demonstrated during passive wrist movement [Brain Res. 900 (2001) 282]. Investigations were conducted to determine the level of the neuroaxis at which these modulations arise, and to establish the influence of proprioceptive task constraints on pathway excitability. Forearm motor evoked potentials (MEPs) in response to transcranial magnetic stimulation (TMS) were examined during passive wrist movement while subjects maintained a low-level muscle activation, thus stabilising the excitability of the motoneuron pool. Modulations in response amplitude during movement were evident in both forearm flexor and extensor muscles. The pattern of modulation generally mirrored that seen in quiescent musculature during movement, with responses potentiated during the phases where the muscle was in a shortened position. Variations in MEP amplitude were not detected while the wrist was constrained statically at various joint angles. This suggests a dynamic influence of movement, most likely mediated by spindle receptors, arising at a supraspinal level. We also investigated the influence of a kinesthetic tracking task on corticomotor excitability during passive movement of the wrist joint. MEPs were recorded from the target driven limb while the contralateral limb was stationary, while the contralateral limb actively tracked the movements of the target limb, and while the contralateral limb moved actively in time with a metronome. The results revealed no differences in MEP characteristics in the driven limb between the three conditions. Placing the movement elicited afferent information in an active movement context does not appear to enhance the modulations in cortical excitability.     

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.     

Cortico-motor excitability of the lower limb motor representation: a comparative study in Parkinson's disease and healthy controls.

OBJECTIVE: To compare indices of cortico-motor excitability derived from transcranial magnetic stimulation (TMS) of the lower limb motor representation in patients with Parkinson's diseases (PD) and healthy controls. METHODS: The cortico-motor excitability of the lower limb motor area was studied both at rest (motor threshold, amplitude of motor evoked potentials (MEPs)) and during active contraction of the quadriceps (Quad) muscle (MEPs facilitation and silent period) in 10 PD patients (11 legs) and 11 healthy controls using single pulse TMS. RESULTS: At rest, the motor threshold was found to be significantly lower and the amplitude of MEPs larger in patients than in controls. During active knee contraction, patients produced lower levels of MEP facilitation with respect to baseline values and the silent period was lengthened in comparison to controls. CONCLUSIONS: The present results provide further evidence from the lower limb motor area that enhanced cortico-spinal excitability is an important feature in the pathophysiology of PD.     

Delayed facilitation of motor cortical excitability following repetitive finger movements.

OBJECTIVES: To define motor cortical excitability changes occurring at various times after non-fatiguing bimanual exercise of the index fingers. METHODS: Twenty healthy right-handed subjects were studied with transcranial magnetic stimulation (TMS) of the right non-dominant hemisphere. They performed regular (3-4/s) repetitive opening-closing bilateral movements of the index finger onto the thumb. Motor evoked potentials (MEPs) of the left first dorsal interosseus (FDI) and rate of the repetitive finger movements were determined (1) before exercise, (2) immediately following 3 exercise periods of 30, 60 and 90 s, and (3) over a subsequent 30 min rest period. RESULTS: Rate of movement did not show significant change during any of the exercise periods but did increase significantly when tested after 15 min of rest. MEPs immediately after 30 and 60 s of exercise were facilitated whereas MEPs after 90 s of exercise did not differ from baseline measures. MEP amplitudes were significantly increased after rest of approximately 15 min compared to the baseline MEPs. In contrast, motor potentials evoked by peripheral nerve stimulation were unchanged throughout the experimental test periods. CONCLUSIONS: Motor cortical excitability relating to an intrinsic finger muscle (FDI) was facilitated beginning 15 min after a brief period of non-forceful, repetitive activity of that muscle. This delayed facilitation of motor cortex after exercise may represent a form of short-term potentiation of motor cortical excitability.     

Phasic modulation of corticomotor excitability during passive movement of the upper limb: effects of movement frequency and muscle specificity

Modulations in the excitability of spinal reflex pathways during passive rhythmic movements of the lower limb have been demonstrated by a number of previous studies [4]. Less emphasis has been placed on the role of supraspinal pathways during passive movement, and on tasks involving the upper limb. In the present study, transcranial magnetic stimulation (TMS) was delivered to subjects while undergoing passive flexion-extension movements of the contralateral wrist. Motor evoked potentials (MEPs) of flexor carpi radialis (FCR) and abductor pollicus brevis (APB) muscles were recorded. Stimuli were delivered in eight phases of the movement cycle during three different frequencies of movement. Evidence of marked modulations in pathway excitability was found in the MEP amplitudes of the FCR muscle, with responses inhibited and facilitated from static values in the extension and flexion phases, respectively. The results indicated that at higher frequencies of movement there was greater modulation in pathway excitability. Paired-pulse TMS (sub-threshold conditioning) at short interstimulus intervals revealed modulations in the extent of inhibition in MEP amplitude at high movement frequencies. In the APB muscle, there was some evidence of phasic modulations of response amplitude, although the effects were less marked than those observed in FCR. It is speculated that these modulatory effects are mediated via Ia afferent pathways and arise as a consequence of the induced forearm muscle shortening and lengthening. Although the level at which this input influences the corticomotoneuronal pathway is difficult to discern, a contribution from cortical regions is suggested.     

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   

Facilitation of motor evoked potentials by postcontraction response (Kohnstamm phenomenon)

We have applied repeated transcranial magnetic stimuli during the involuntary postcontraction muscle activity (Kohnstamm phenomenon) or during a tonic vibration reflex, both presumably arising from subcortical levels. The motor evoked potentials (MEPs) were compared with the MEPs evoked during a comparable voluntary contraction (cortical origin). The MEP amplitudes from the deltoid muscle appeared linearly related to the mean amplitude of the smoothed rectified background EMG preceding the stimulus. No differences in the facilitatory effect between voluntary and involuntary preinnervation manoeuvres were founf. If we accept the hypothesis of a subcortical origin of the involuntary muscle activity in the Kohnstamm phenomenon, the similar facilitatory effect of involuntary and voluntary background EMG supports a predominantly spinal localisation of the facilitatory mechanism in this proximal muscle both during involuntary and during voluntary activity, at least under the present conditions of rather low stimulus strengths. In about 20–30% of all the trials an extra facilitatory effect on the MEP amplitude was observed during the shortening contraction compared to an MEP elicited during the lengthening contraction, in spite of a similar background EMG. This extra facilitatory effect of the shortening contraction was observed during involuntary and voluntary preactivation, suggesting an elevated excitatory state at the spinal level.     

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 by somatosensory afferent stimulation.

The effect of an electrically induced peripheral afferent volley upon electrical and magnetic motor evoked potentials (MEPs) from muscles of the upper and lower extremities was studied in 16 healthy volunteers. A standard conditioning-test (C-T) paradigm was employed whereby the test stimulus (transcranial electric or magnetic) was applied at random time intervals, from 10 msec prior to 90 msec after the conditioning stimulus (peripheral nerve stimulus). MEP amplitude facilitation was observed for the majority of the upper extremity muscles tested at two distinct periods, one occurring at short, and the other at long C-T intervals. This bimodal trend of MEP facilitation was found to be equally as prominent in the lower extremity muscles tested. The period of short C-T interval facilitation is consistent with modifications in the spinal excitability of the segmental motoneuron pool. On the other hand, the period of long C-T interval facilitation is suggested to be due to alterations in excitability of the motor cortex as a result of the arrival of the orthodromic sensory volley. Although most pronounced in muscles innervated by the nerve to which the conditioning stimulus was applied, this bimodal facilitatory effect was also observed in adjacent muscles not innervated by the stimulated nerve. Qualitatively, the conditioned MEPs from the upper and lower extremities responded similarly to both electrical and magnetic trans-cranial stimulation. In addition, our study demonstrates that the C-T paradigm has potential for use in the assessment of spinal and cortical sensorimotor integration by providing quantitative information which cannot be obtained through isolated assessment of sensory and/or motor pathways.(ABSTRACT TRUNCATED AT 250 WORDS)     

Motor cortex excitability in chronic fatigue syndrome.

OBJECTIVE: To use transcranial magnetic stimulation (TMS) to define motor cortical excitability in chronic fatigue syndrome (CFS) subjects during a repetitive, bilateral finger movement task. METHODS: A total of 14 CFS patients were tested and compared with 14 age-matched healthy control subjects. TMS of the motor cortex (5% above threshold) was used to elicit motor evoked potentials (MEPs). Subjects performed regular (3-4/s) repetitive bilateral opening-closing movements of the index finger onto the thumb. MEPs of the first dorsal interosseus (FDI) were measured before, immediately following exercise periods of 30, 60 and 90 s, and after 15 min of rest. RESULTS: Performance, defined by rate of movement, was significantly slower in CFS subjects (3.5/s) than in controls (4. 0/s) independent of the hand measured. The rate, however, was not significantly affected by the exercise duration for either group. The threshold of TMS to evoke MEPs from the FDI muscle was significantly higher in CFS than in control subjects, independent of the hemisphere tested. A transient post-exercise facilitation of MEP amplitudes immediately after the exercise periods was present in controls independent of the hemisphere tested, but was absent in CFS subjects. A delayed facilitation of MEPs after 15-30 min of rest was restricted to the non-dominant hemisphere in controls; delayed facilitation was absent in CFS subjects. CONCLUSIONS: Individuals with CFS do not show the normal fluctuations of motor cortical excitability that accompany and follow non-fatiguing repetitive bimanual finger movements.     

Repetitive magnetic stimulation of cortical motor areas in Parkinson's disease: implications for the pathophysiology of cortical function.

We investigated the neurophysiological and clinical effects of repetitive magnetic stimulation (rTMS) delivered to the cortical motor areas in healthy subjects and patients with Parkinson's disease. rTMS was delivered with a high speed magnetic stimulator (Cadwell, Kennewick, WA) through a figure-eight coil centred on the primary motor area at a stimulus intensity of 120% motor threshold. Trains of 10 stimuli were delivered at frequencies of 5 Hz while subjects were at rest and during a voluntary contraction of the contralateral first dorsal interosseous muscle. In normal subjects at rest, the muscle evoked responses (MEPs) to each stimulus in a train of magnetic stimuli progressively increased in size during the train. rTMS left the MEPs unchanged in patients off therapy and had a small facilitatory effect in those on therapy. In normal subjects and patients, 5-Hz rTMS trains delivered during a voluntary contraction of the target muscle left the MEP unchanged in size. MEPs were followed by a silent period that increased in duration during the course of the train. The silent period duration increased to a similar extent in patients and controls. The reduced rTMS-induced facilitation of MEPs in patients with Parkinson's disease reflects a decreased facilitation of the excitatory cells in the cortical motor areas.     

Pre-movement motor excitability is reduced ipsilateral to low force pinch grips

Motor excitability ipsilateral to pinch grips was investigated during the pre-movement period. Subjects performed right-handed phasic pinch grips with 2% or 20% maximum voluntary contraction (MVC) in response to a visual go-signal. Transcranial magnetic stimulation (TMS) was applied over the right motor cortex at various intervals before the go-signal and 100 msec after movement onset. Motor evoked potentials were recorded from the relaxed left first dorsal interosseous muscle. Immediately prior to and during 2% MVC pinch grips, MEP amplitudes were reduced. In contrast, MEPs obtained by transcranial electrical stimulation tended to be increased, indicating that MEP decreases are mediated at a cortical level. Before and during 20% MVC pinch grips MEP amplitudes were enhanced. TMS delayed reaction time if applied close to the go-signal. We conclude that the motor cortex ipsilateral to low force movements is inhibited prior to and during movement.     

Post-exercise facilitation and depression of motor evoked potentials to transcranial magnetic stimulation: a study in multiple sclerosis.

OBJECTIVE: To evaluate motor cortex excitability changes by transcranial magnetic stimulation (TMS) following repetitive muscle contractions in patients with multiple sclerosis (MS); to state whether a typical pattern of post-exercise motor evoked potentials (MEPs) is related to clinical fatigue in MS. METHODS: In 41 patients with definite MS (32 with fatigue and 9 without fatigue according to Fatigue Severity Scale) and 13 controls, MEPs were recorded at rest: at baseline condition, following repetitive contractions until fatigue, and after fatigue, to evaluate post-exercise MEP facilitation (PEF) and depression (PED). RESULTS: After exercise, MEP amplitude significantly increased both in patients and controls (PEF). When fatigue set in, MEP amplitude was significantly reduced in normal subjects (PED), but not in patients. Post-exercise MEP findings were similar both in patients with and without fatigue. CONCLUSIONS: Our findings suggest an intracortical motor dysfunction following a voluntary contraction in MS patients, possibly due to failure of depression of facilitatory cortical circuits, or alternatively of inhibitory mechanisms.     

Facilitatory effect of thinking about movement on magnetic motor-evoked potentials

To investigate the facilitatory effect of thinking about movement on motor evoked potential (MEP) amplitude, we recorded MEPs in two test muscles during rest, with the subject thinking about contracting the test muscle but without subsequent contraction, and during 10% maximum voluntary contraction. Stimuli were delivered at 10% above resting motor threshold and at 90–100% stimulator output. H-reflexes, recorded in flexor carpi radialis, were obtained during rest and think conditions. MEP threshold was lower during the think condition (P=0.004). At both stimulus intensities, median MEP amplitudes and areas were significantly (P<0.001) larger during the think paradigm compared with rest. This effect was greater at the lower stimulus intensity. There was no significant difference in latency (P=0.15). In 4/8 subjects, H-reflex amplitudes were mildly facilitated (P<0.05) during the think condition. We conclude that thinking about movement without detectable EMG activity has a facilitatory effect on magnetic MEPs. The absence of a MEP latency shift between rest and think conditions and absence of a consistent increase in H-reflex amplitude suggests this effect occurs largely at the cortical level. In some subjects, however, an increase in spinal motoneuron excitability may also contribute.