The influence of vibration on the excitability of alpha motoneurones

Modulation of the excitatory phase following the cutaneous silent period by vibration

INTRODUCTION

The post-inhibition excitatory phase (E3) of the cutaneous silent period (CSP) is attributed to the resynchronization of motoneuron activity following the inhibitory period but there is also evidence that a somatosensory startle reflex may contribute to this phase. We hypothesized that the startle reflex component contained in E3 will decrease during vibration.

METHODS

Sixteen healthy individuals were included in the study. CSP was recorded from slightly contracted right thenar muscles after painful index finger stimulation, before, during, and immediately after vibration. The values of the percentage change of E3 relative to pre-stimulus baseline (E3%) were compared before, during, and after vibration for each individual.

RESULTS

There was a reduction in E3% during vibration and the values returned to normal immediately after vibration (153.1 ± 43.5%, 115.2 ± 30.2%, 154.9 ± 68.2%, respectively; p = 0.030).

DISCUSSION

E3 is reduced during vibration in healthy individuals, presumably due to suppression of a reflex component, which is superimposed upon the known resynchronization of motoneurons.

Using Cutaneous Receptor Vibration to Uncover the Effect of Transcranial Magnetic Stimulation (TMS) on Motor Cortical Excitability

Background

Little is known about how vibrational stimuli applied to hand digits affect motor cortical excitability. The present transcranial magnetic stimulation (TMS) study investigated motor evoked potentials (MEPs) in the upper extremity muscle following high-frequency vibratory digit stimulation.

Material/Methods

High-frequency vibration was applied to the upper extremity digit II utilizing a miniature electromagnetic solenoid-type stimulator-tactor in 11 healthy study participants. The conditioning stimulation (C) preceded the test magnetic stimulation (T) by inter-stimulus intervals (ISIs) of 5–500 ms in 2 experimental sessions. The TMS was applied over the primary motor cortex for the hand abductor pollicis-brevis (APB) muscle.

Results

Dunnett’s multiple comparisons test indicated significant suppression of MEP amplitudes at ISIs of 200 ms (P=0.001), 300 ms (P=0.023), and 400 ms (P=0.029) compared to control.

Conclusions

MEP amplitude suppression was observed in the APB muscle at ISIs of 200–400 ms, applying afferent signaling that originates in skin receptors following the vibratory stimuli. The study provides novel insight on the time course and MEP modulation following cutaneous receptor vibration of the hand digit. The results of the study may have implications in neurology in the neurorehabilitation of patients with increased amplitude of MEPs.

Differential Effect of Visual and Proprioceptive Stimulation on Corticospinal Output for Reciprocal Muscles

This study investigated the corticospinal excitability of reciprocal muscles during tasks involving sensory difference between proprioceptive and visual inputs. Participants were instructed to relax their muscles and to observe a screen during vibratory stimulation. A video screen was placed on the board covering the right hand and forearm. Participants were randomly tested in four conditions: resting, control, static, and dynamic. The resting condition involved showing a black screen, the control condition, a mosaic patterned static videoclip; the static condition, a static videoclip of wrist flexion 0°; and the dynamic condition, a videoclip that corresponded to each participant's closely-matched illusory wrist flexion angle and speed by vibration. Vibratory stimulation (frequency 80 Hz and duration 4 s) was applied to the distal tendon of the dominant right extensor carpi radialis (ECR) using a tendon vibrator in the control, static, and dynamic conditions. Four seconds after the vibratory stimulation (end of vibration), the primary motor cortex at the midpoint between the centers of gravity of the flexor carpi radialis (FCR) and ECR muscles was stimulated by transcranial magnetic stimulation (TMS). The ECR motor evoked potential (MEP) amplitudes significantly increased in the control condition compared to the resting condition, whereas the FCR MEP amplitudes did not change between the resting and control conditions. In addition, the ECR MEP amplitudes significantly increased in the static condition compared to the dynamic condition. However, the FCR MEP amplitudes significantly increased in the dynamic condition compared to the static condition. These results imply that the difference between visuo-proprioceptive information had an effect on corticospinal excitability for the muscle. In conclusion, we found that proprioceptive and visual information differentially altered the corticospinal excitability of reciprocal muscles.

Use of imperceptible wrist vibration to modulate sensorimotor cortical activity

Peripheral sensory stimulation has been used as a method to stimulate the sensorimotor cortex, with applications in neurorehabilitation. To improve delivery modality and usability, a new stimulation method has been developed in which imperceptible random-frequency vibration is applied to the wrist concurrently during hand activity. The objective of this study was to investigate effects of this new sensory stimulation on the sensorimotor cortex. Healthy adults were studied. In a transcranial magnetic stimulation (TMS) study, resting motor threshold, short-interval intracortical inhibition, and intracortical facilitation for the abductor pollicis brevis muscle were compared between vibration on vs. off, while subjects were at rest. In an electroencephalogram (EEG) study, alpha and beta power during rest and event-related desynchronization (ERD) for hand grip were compared between vibration on vs. off. Results showed that vibration decreased EEG power and decreased TMS short-interval intracortical inhibition (i.e., disinhibition) compared with no vibration at rest. Grip-related ERD was also greater during vibration, compared to no vibration. In conclusion, subthreshold random-frequency wrist vibration affected the release of intracortical inhibition and both resting and grip-related sensorimotor cortical activity. Such effects may have implications in rehabilitation.

Effects of vibration on cutaneous silent period

Suppression of an ongoing muscle contraction following noxious digital stimulation is called cutaneous silent period (CSP) which is under the influence of several physiological factors. In this study, we aimed to evaluate the influence of group Ia afferents on the cutaneous silent period (CSP) by applying 2-min vibration. CSP was obtained from abductor pollicis brevis muscle after stimulating index finger. The recordings were repeated three times-before, during and after vibration-which was applied over the tendon of flexor carpi radialis muscle. Onset latency, duration and magnitude of total CSP, inhibitory phases I1 and I2, and of the long-loop reflex were measured and compared. Suppression indices of CSP, I1 and I2 increased significantly during and after vibration, indicating significantly less exteroceptive EMG suppression outlasting the time of vibration. Vibration also caused mild shortening of I2 end latency (p = 0.048) and I2 duration (p = 0.019). Our findings indicate that vibration exerts a powerful influence on CSPs and causes reduction in the magnitude of exteroceptive EMG suppression during and after vibration. Although vibration is known to activate Ia afferents, we cannot exclude contribution of other afferents, e.g. mechanoreceptors, as well as pre- or postsynaptic inhibitory effects on ensuing interneurons, or enhanced vibration-related excitatory influence.

Somatosensory impairment and its association with balance limitation in people with multiple sclerosis

BACKGROUND

Somatosensory impairments are common in multiple sclerosis. However, little data are available to characterize the nature and frequency of these problems in people with multiple sclerosis.

OBJECTIVE

To investigate the frequency of somatosensory impairments and identify any association with balance limitations in people with multiple sclerosis.

METHODS

The design was a prospective cross-sectional study, involving 82 people with multiple sclerosis and 30 healthy controls. Tactile and proprioceptive sensory acuity were measured using the Rivermead Assessment of Somatosensory Performance. Vibration duration was assessed using a tuning fork. Duration for the Timed Up and Go Test and reaching distance of the Functional Reach Test were measured to assess balance limitations. The normative range of sensory modalities was defined using cut-off points in the healthy participants. The multivariate linear regression was used to identify the significant predictors of balance in people with multiple sclerosis.

RESULTS

Proprioceptive impairments (66.7%) were more common than tactile (60.8%) and vibration impairments (44.9%). Somatosensory impairments were more frequent in the lower limb (78.2%) than the upper limb (64.1%). All sensory modalities were significantly associated with the Timed Up and Go and Functional Reach tests (p<0.05). The Timed Up and Go test was independently predicted by the severity of the neurological lesion, Body Mass Index, ataxia, and tactile sensation (R2=0.58), whereas the Functional Reach test was predicted by the severity of the neurological lesion, lower limb strength, and vibration sense (R2=0.49).

CONCLUSIONS

Somatosensory impairments are very common in people with multiple sclerosis. These impairments are independent predictors of balance limitation.

Whole-hand water flow stimulation increases motor cortical excitability: a study of transcranial magnetic stimulation and movement-related cortical potentials

Previous studies examining the influence of afferent stimulation on corticospinal excitability have demonstrated that the intensity of afferent stimulation and the nature of the afferents targeted (cutaneous/proprioceptive) determine the effects. In this study, we assessed the effects of whole-hand water immersion (WI) and water flow stimulation (WF) on corticospinal excitability and intracortical circuits by measuring motor evoked potential (MEP) recruitment curves and conditioned MEP amplitudes. We further investigated whether whole-hand WF modulated movement-related cortical activity. Ten healthy subjects participated in three experiments, comprising the immersion of participants' right hands with (whole-hand WF) or without (whole-hand WI) water flow, and no immersion (control). We evaluated MEP recruitment curves produced by a single transcranial magnetic stimulation (TMS) pulse at increasing stimulus intensities, short-interval intracortical inhibition (SICI), and intracortical facilitation (ICF) using the paired TMS technique before and after 15 min of intervention. Movement-related cortical potentials (MRCPs) were evaluated to examine primary motor cortex, supplementary motor area, and somatosensory cortex excitability upon movement before and after whole-hand WF. After whole-hand WF, the slope of the MEP recruitment curve significantly increased, whereas SICI decreased and ICF increased in the contralateral motor cortex. The amplitude of the Bereitschaftspotential, negative slope, and motor potential of MRCPs significantly increased after whole-hand WF. We demonstrated that whole-hand WF increased corticospinal excitability, decreased SICI, and increased ICF, although whole-hand WI did not change corticospinal excitability and intracortical circuits. Whole-hand WF modulated movement-related cortical activity, increasing motor cortex activation for the planning and execution of voluntary movements.

Nonphysiological factors in navigated TMS studies; confounding covariates and valid intracortical estimates

Brain stimulation is used to induce transient alterations of neural excitability to probe or modify brain function. For example, single-pulse transcranial magnetic stimulation (TMS) of the motor cortex can probe corticospinal excitability (CSE). Yet, CSE measurements are confounded by a high level of variability. This variability is due to physical and physiological factors. Navigated TMS (nTMS) systems can record physical parameters of the TMS coil (tilt, location, and orientation) and some also estimate intracortical electric fields (EFs) on a trial-by-trial basis. Thus, these parameters can be partitioned with stepwise regression.

PURPOSE

The primary objective was to dissociate variance due to physical parameters from variance due to physiological factors for CSE estimates. The secondary objective was to establish the predictive validity of EF estimates from spherical head models.

HYPOTHESIS

Variability of physical parameters of TMS predicts CSE variability.

METHODS

Event-related measurements of physical parameters were analyzed in stepwise regression. Partitioned parameter variance and predictive validity were compared for a target-controlled and a nontarget-controlled experiment. A control experiment (preinnervation) confirmed the validity of linear data analysis. A bias-free model quantified the effect of divergence from optimum.

RESULTS

Partitioning physical parameter variance reduces CSE variability. EF estimates from spherical models were valid. Post hoc analyses showed that even small physical fluctuations can confound the statistical comparison of CSE measurements.

CONCLUSIONS

It is necessary to partition physical and physiological variance in TMS studies to make confounded data interpretable. The spatial resolution of nTMS is <5 mm and the EF-estimates are valid.

Effect of vibration on forward split flexibility and pain perception in young male gymnasts

Serious stretching in many sports involves discomfort and is often an early ceiling on improvements.

PURPOSE

To continue investigation of the use of vibration to enhance acute range of motion while assessing the influence of vibration and stretching on pressure-to-pain threshold perception.

METHODS

Ten young male gymnasts were assessed for split range of motion. One side split was randomly assigned as the experimental condition, and the other side split was assigned as the control. Both side splits were performed on a vibration device; the experimental condition had the device turned on and the control condition was performed with the device turned off. In addition, the athletes were assessed for pressure-to-pain transition using an algometer on the biceps femoris (stretched muscle) and vastus lateralis (nonstretched muscle) bilaterally.

RESULTS

Pre-post difference scores between the vibrated split (most improved) and the nonvibrated split were statistically different (P=.001, 95% confidence interval of the difference 2.3 to 5.8 cm). Following the stretching protocol, the force values for the pressure-to-pain threshold comparing the vibrated and nonvibrated biceps femoris muscle were not statistically different. The nonstretched vastus lateralis muscle also showed no statistical difference in pressure-to-pain threshold between the vibration and nonvibration conditions.

CONCLUSION

This study showed that vibration improved split range of motion over stretching alone, but did not show a difference in pressure-to-pain perception in either the stretched or nonstretched muscles.

Contribution of transcranial magnetic stimulation to the understanding of cortical mechanisms involved in motor control

Transcranial magnetic stimulation (TMS) was initially used to evaluate the integrity of the corticospinal tract in humans non-invasively. Since these early studies, the development of paired-pulse and repetitive TMS protocols allowed investigators to explore inhibitory and excitatory interactions of various motor and non-motor cortical regions within and across cerebral hemispheres. These applications have provided insight into the intracortical physiological processes underlying the functional role of different brain regions in various cognitive processes, motor control in health and disease and neuroplastic changes during recovery of function after brain lesions. Used in combination with neuroimaging tools, TMS provides valuable information on functional connectivity between different brain regions, and on the relationship between physiological processes and the anatomical configuration of specific brain areas and connected pathways. More recently, there has been increasing interest in the extent to which these physiological processes are modulated depending on the behavioural setting. The purpose of this paper is (a) to present an up-to-date review of the available electrophysiological data and the impact on our understanding of human motor behaviour and (b) to discuss some of the gaps in our present knowledge as well as future directions of research in a format accessible to new students and/or investigators. Finally, areas of uncertainty and limitations in the interpretation of TMS studies are discussed in some detail.

Transcallosal sensorimotor integration: Effects of sensory input on cortical projections to the contralateral hand

OBJECTIVE

Low amplitude vibration of forearm or hand muscles predominantly activates proprioceptive inputs that influence corticospinal projections in a focal manner, increasing output to the stimulated muscle while reducing output to neighbouring muscles. Modulation of contralateral forearm muscles by vibration has also been reported on one occasion. The aim of the current investigation was to investigate the effects of proprioceptive input from a hand muscle on corticospinal excitability, intracortical inhibition (SICI) and interhemispheric inhibition (IHI) targeting the homologous contralateral muscle.

METHODS

Transcranial Magnetic Stimulation (TMS) was delivered to the left cortical hand area of 10 healthy subjects and surface electromyography (EMG) recordings taken from the right First Dorsal Interosseus (FDI) and Abductor Digiti Minimi (ADM). The effect of low amplitude vibration of the left FDI on MEP amplitudes, SICI and IHI targeting the right hand was assessed.

RESULTS

Vibration of the left FDI caused a significant reduction in MEP amplitudes in the homologous right FDI but not in the right ADM. SICI and IHI targeting both muscles were also significantly increased.

CONCLUSIONS

We conclude that proprioceptive input from a hand muscle reduces the corticospinal excitability of the contralateral homologous muscle. The increases in SICI and IHI suggest that at least some of this effect occurs in the cortex ipsilateral to the stimulus and this may be mediated via transcallosal fibres.

SIGNIFICANCE

These results suggest that sensory input can modulate excitability in both motor cortices simultaneously, as well as the relationship between them. Interventions which modulate this transcallosal relationship may become useful in disorders where abnormal IHI is a potential therapeutic target.

Flexibility Enhancement with Vibration

INTRODUCTION

The most popular method of stretching is static stretching. Vibration may provide a means of enhancing range of motion beyond that of static stretching alone.

PURPOSE

This study sought to observe the effects of vibration on static stretching to determine whether vibration-aided static stretching could enhance range of motion acquisition more than static stretching alone in the forward split position.

METHODS

Ten highly trained male volunteer gymnasts were randomly assigned to experimental (N = 5) and control (N = 5) groups. The test was a forward split with the rear knee flexed to prevent pelvic misalignment. Height of the anterior iliac spine of the pelvis was measured at the lowest split position. Athletes stretched forward and rearward legs to the point of discomfort for 10 s followed by 5 s of rest, repeated four times on each leg and split position (4 min total). The experimental group stretched with the device turned on; the control group stretched with the device turned off. A pretest was followed by an acute phase posttest, then a second posttest measurement was performed following 4 wk of treatment. Difference scores were analyzed.

RESULTS

The acute phase showed dramatic increases in forward split flexibility for both legs (P < 0.05), whereas the long-term test showed a statistically significant increase in range of motion on the right rear leg split only (P < 0.05). Effect sizes indicated large effects in all cases.

CONCLUSION

This study showed that vibration can be a promising means of increasing range of motion beyond that obtained with static stretching in highly trained male gymnasts.

Differential effect of muscle vibration on intracortical inhibitory circuits in humans

Low amplitude muscle vibration (0.5 ms; 80 Hz; duration 1.5 s) was applied in turn to each of three different intrinsic hand muscles (first dorsal interosseus, FDI; abductor pollicis brevis, APB; and abductor digiti minimi, ADM) in order to test its effect on the EMG responses evoked by transcranial magnetic stimulation (TMS). Recordings were also taken from flexor and extensor carpi radialis (FCR and ECR, respectively). We evaluated the amplitude of motor evoked potentials (MEPs) produced by a single TMS pulse, short interval intracortical inhibition and facilitation (SICI and ICF) and long interval intracortical inhibition (LICI). TMS pulses were applied 1 s after the start of vibration with subjects relaxed throughout. Vibration increased the amplitude of MEPs evoked in the vibrated muscle (162 +/- 6 % of MEP with no vibration; mean +/- S.E.M.), but suppressed MEPs in the two non-vibrated hand muscles (72 +/- 9 %). Compared with no vibration (test response reduced to 51 +/- 5 % of control), there was less SICI in the vibrated muscle (test response reduced to 92 +/- 28 % of control) and more in the non-vibrated hand muscles (test response reduced to 27 +/- 5 % of control). The opposite occurred for LICI: compared with the no vibration condition (test response reduced to 33 +/- 6 % control), there was more LICI in the vibrated muscle (test response reduced to 17 +/- 3 % control) than in the non-vibrated hand muscles (test response reduced to 80 +/- 11 % control) even when the intensity of the test stimulus was adjusted to compensate for the changes in baseline MEP. There was no effect on ICF. Cutaneous stimulation of the index finger (80 Hz, 1.5 s duration, twice sensory threshold) had no consistent differential effect on any of the parameters. We conclude that vibratory input from muscle can differentially modulate excitability in motor cortical circuits.

Transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) is a noninvasive and painless method of stimulating the cerebral cortex that allows the study of cortical excitability and inhibition. This article describes the technique, the parameters most commonly studied, the clinical applications of TMS, and potential research applications.

Focal reduction of intracortical inhibition in the motor cortex by selective proprioceptive stimulation

The influence of proprioception on motorcortical excitability was assessed by muscle vibration (MV; 80 Hz, 0.5 mm amplitude) of the flexor carpi radialis muscle (FCR) and compared to voluntary contraction and relaxation conditions. Motor thresholds, motor-evoked potentials (MEPs) in response to single pulses of transcranial magnetic stimulation (TMS) and the intracortical inhibition (ICI) and facilitation (ICF) after paired magnetic stimuli were studied. A control experiment using TMS inducing posteriorly directed current was performed. MEPs were recorded simultaneously from the FCR, the extensor carpi radialis, the abductor pollicis brevis and the first dorsal interosseus. In the FCR, MV led to an increase of excitability shown by a decrease of motor threshold, a facilitation of MEPs in response to single-pulse TMS, a reduction of ICI and an increase of ICF. Since especially the ICI and ICF remain unchanged in other recorded muscles, this increase of excitability is specific for the vibrated muscle. With posteriorly directed current the ICI in the FCR was reduced as well, showing an involvement of later I-waves. We suggest that MV induces a focused motorcortical activation which relies on a reduced activity of intracortical inhibitory interneuronal circuits targeting selectively the motorcortical representation of the vibrated muscle.

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.

Impaired sensorimotor integration in cervical dystonia: a study using transcranial magnetic stimulation and muscle vibration

The authors studied the effects of sensorimotor integration (corticocortical inhibition and facilitation during muscle vibration [MV]) in dystonic patients. Eleven patients with cervical dystonia and 11 age-matched healthy control subjects were enrolled in the study. They were examined using transcranial magnetic stimulation (TMS) and tonic proprioceptive input (MV). Paired-pulse transcranial magnetic stimulation was done at interstimulus intervals of 3 msec (intracortical inhibition) and 13 msec, the intensity of the conditioning stimulus was 70% of the motor threshold, and the test stimulus was 120%. Motor evoked potentials were recorded from the vibrated extensor carpi radialis muscle and its antagonist, the flexor carpi radialis. Duration of MV trains (80 Hz; amplitude, 0.5 mm) was 4 seconds. The authors found differences between patients and healthy control subjects during MV only. Intracortical inhibition was pronounced significantly only in control subjects, whereas intracortical facilitation was significant in patients only (P < 0.05). Furthermore, the significant reduction of motor evoked potentials at 13-msec interstimulus intervals, which can be found in healthy subjects frequently, was observed in one dystonia patient only. The results of the current study suggest that sensorimotor integration is impaired in cervical dystonia, probably by an altered control of proprioceptive (vibratory) input.

Perceptual changes in illusory wrist flexion angles resulting from motor imagery of the same wrist movements

Recent neuroimaging studies have suggested that similar cortical motor areas are recruited both by kinesthetic sensations elicited by tendon vibration and by voluntarily imaging one's own movements of the same joints. Little is known, however, as to whether kinesthetic motor imagery interacts with kinesthetic illusion. We examined such interaction by behavioral analysis in which 19 subjects imagined wrist flexion or extension, with or without illusory flexion induced by tendon vibration. Electromyograms were also recorded to monitor the peripheral modulations caused by the interaction. The kinesthetic motor imagery had a psychophysical effect on kinesthetic illusion in the absence of overt movement. It was confirmed that the subjects could imagine wrist movements without facilitating muscle activities in the absence of vibration stimuli. The electromyogram activity of the vibrated extensor muscles was significantly higher than that of non-vibrated flexor muscles. Motor imagery of wrist extension, when illusory flexion was experienced, reduced the angle of illusory flexion while enhancing extensor muscle activities in comparison with the control. On the other hand, flexion motor imagery increased the angle of illusory flexion with or without enhancement of flexor muscle activities. Our results indicate that motor imagery interacts with kinesthetic illusion with or without enhancement of activities of the related muscles. This suggests (1) that common neural substrates shared by imagery and by illusion exist and (2) that different physiological mechanisms contribute to the enhancement of muscle activities of vibrated muscles and their antagonists.

Acamprosate reduces motor cortex excitability determined by transcranial magnetic stimulation

Acamprosate is effective in reducing alcohol intake in weaned alcoholics. We were interested if acamprosate had an effect on the excitability of cortical motoneurons determined by transcranial magnetic stimulation (TMS). We studied 12 male healthy volunteers (mean age 29.5 years, SD = 4.8) who were either treated with 6 tablets of acamprosate (each containing 333 mg verum) per day or placebo (randomized cross-over design) for 1 week. TMS was performed after each treatment session including a paired stimulation paradigm. Motor evoked potentials (MEPs) of the placebo and verum group did not differ with respect to paired stimulation. However, motor threshold increased in the acamprosate group (verum: 61.5% (SD = 7.9) vs. placebo: 58.9% (SD = 8.8), p = 0.036). We conclude that acamprosate leads to a hypoexcitability of the motor cortex. This might be due to subcortical mechanisms, e.g. thalamocortical pathways since intracortical inhibition and facilitation was not affected.

Alteration of sensorimotor integration in musician's cramp: impaired focusing of proprioception

OBJECTIVE

The influence of muscle vibration (MV) as a strong proprioceptive input on motorcortical excitability was studied in 5 patients with musician's cramp, 5 musician controls and 5 non-musician controls.

METHODS

The relaxed flexor carpi radialis (FCR), involved in the dystonic movement in all patients, was vibrated using low frequency (80 Hz) and low amplitude (0.5 mm). Transcranial magnetic stimulation (TMS; intensity, 120% of motor threshold) was applied without MV, 3 and 9 s after the onset of MV. Motor-evoked potentials (MEPs) in the FCR and in the antagonistic extensor carpi radialis (ECR) were recorded.

RESULTS

With MV, musician and non-musician controls showed a facilitation of MEPs in the FCR and a decrease of MEPs in the ECR. In musician's cramp, both phenomena were significantly less pronounced.

CONCLUSIONS

The reduced facilitation of MEPs in musician's cramp indicates a reduced MV-induced activation of motorcortical areas representing the FCR. The less pronounced inhibition by MV reflects a reduced inhibitory control of the antagonistic ECR. As there were no differences between musician and non-musician controls, the observed changes in musician's cramp refer to this special form of focal dystonia. An impairment of focused motorcortical activation by proprioceptive input from a muscle involved in the dystonic movement is suggested.

Effects of somatosensory input on central fatigue: a pilot study

OBJECTIVE

Depression of motor evoked potentials (MEPs) following transcranial magnetic stimulation (TMS) may be a sign of central motor fatigue. As a pilot study, we have examined whether post-exercise MEP depression can be compensated by application of sensory stimuli prior to TMS.

METHODS

We studied 15 healthy volunteers (aged 21-28 years) who were required to perform an exercise protocol of ankle dorsiflexion until force fell below 66% of maximum force. MEPs were recorded from the right tibialis anterior muscle. Prior to TMS, electrical stimuli were applied to the ipsilateral sural nerve with an individual interstimulus interval between 50 and 80 ms.

RESULTS

MEP areas decreased after exercise. When a sensory stimulus was administered MEPs did not change.

CONCLUSION

We conclude that the effects of central fatigue may be influenced by application of sensory stimuli.

Modulation of motor evoked potentials by muscle vibration: the role of vibration frequency

Augmentation of motor evoked potentials (MEPs) by muscle vibration (MV) was studied in 10 healthy subjects with regard to the vibration frequency (VF). The extensor carpi radialis muscle (ECR) was vibrated using VFs of 80, 120, and 160 Hz. Motor evoked potentials following transcranial magnetic stimulation were recorded simultaneously from the vibrated ECR and the antagonist flexor carpi radialis muscle (FCR) without MV, 0.5 s and 3 s after onset of MV and 1 s after offset. Only the VFs of 80 Hz and 120 Hz caused MEP augmentation and latency shortening in ECR, whereas depression of MEPs in FCR was induced by all VFs used. It appears that MEP augmentation and latency shortening in ECR are mediated by the primary muscle spindle endings which respond with optimal discharge rates to VFs of up to 100 Hz. Motor evoked potential depression in FCR, being well expressed also with VF 160 Hz, seems to involve other dynamic mechanoreceptors.

A clinical study of motor evoked potentials using a triple stimulation technique

Amplitudes of motor evoked potentials (MEPs) are usually much smaller than those of motor responses to maximal peripheral nerve stimulation, and show marked variation between normal subjects and from one stimulus to another. Consequently, amplitude measurements have low sensitivity to detect central motor conduction failures due to the broad range of normal values. Since these characteristics are mostly due to varying desynchronization of the descending action potentials, causing different degrees of phase cancellation, we applied the recently developed triple stimulation technique (TST) to study corticospinal conduction to 489 abductor digiti minimi muscles of 271 unselected patients referred for possible corticospinal dysfunction. The TST allows resynchronization of the MEP, and thereby a quantification of the proportion of motor units activated by the transcranial stimulus. TST results were compared with those of conventional MEPs. In 212 of 489 sides, abnormal TST responses suggested conduction failure of various degrees. By contrast, conventional MEPs detected conduction failures in only 77 of 489 sides. The TST was therefore 2.75 times more sensitive than conventional MEPs in disclosing corticospinal conduction failures. When the results of the TST and conventional MEPs were combined, 225 sides were abnormal: 145 sides showed central conduction failure, 13 sides central conduction slowing and 67 sides both conduction failure and slowing. It is concluded that the TST is a valuable addition to the study of MEPs, since it improves detection and gives quantitative information on central conduction failure, an abnormality which appears to be much more frequent than conduction slowing. This new technique will be useful in following the natural course and the benefit of treatments in disorders affecting central motor conduction.

Within-patient variability of myogenic motor-evoked potentials to multipulse transcranial electrical stimulation during two levels of partial neuromuscular blockade in aortic surgery

Intraoperative recording of myogenic motor responses evoked by transcranial electrical stimulation (tcMEPs) is a method of assessing the integrity of the motor pathways during aortic surgery. To identify conditions for optimal spinal cord monitoring, we investigated the effects of manipulating the level of neuromuscular blockade (T1 response of the train-of-four (TOF) stimulation 5%-15% versus T1 response 45%-55% of baseline), as well as the number of transcranial pulses (two versus six stimuli) on the within-patient variability and amplitude of tcMEPs. Ten patients (30-76 yr) scheduled to undergo surgery on the thoracic and thoracoabdominal aorta were studied. After achieving a stable anesthetic state and before surgery, 10 tcMEPs were recorded from the right extensor digitorum communis muscle and the right tibialis anterior muscle in response to two-pulse and six-pulse transcranial electrical stimulation with an interstimulus interval of 2 ms during two levels of neuromuscular blockade. The right thenar eminence was used for recording the level of relaxation. The tcMEP amplitude using the six-pulse paradigm was larger (P < 0.01; leg and arm) compared with the amplitude evoked by two-pulse stimulation during both levels of relaxation. The within-patient variability, expressed as median coefficient of variation, was less when six-pulse stimulation was used. At a T1 response of 45%-55% of baseline, larger, less variable tcMEPs were recorded than at a T1 response of 5%-15%. Our results suggest that the best quality of tcMEP signals (tibialis anterior muscle) is obtained when the six-pulse paradigm is used with a stable level of muscle relaxation (the first twitch of the TOF-thenar eminence-at 45%-55% of baseline).

IMPLICATIONS

This study shows that six-pulse (rather than two-pulse) transcranial electrical stimulation during a stable anesthetic state and a stable neuromuscular blockade aimed at 45%-55% (rather than 5%-15%) of baseline provides reliable and recordable muscle responses sufficiently robust for spinal cord monitoring in aortic surgery.

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.

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 found. 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.

Age-dependent effects of muscle vibration and the Jendrassik maneuver on the patellar tendon reflex response

OBJECTIVE

To explore possible effects of aging on the excitability of spinal reflexes.

DESIGN

Using a cross-sectional design, the influences of muscle vibration and the Jendrassik maneuver on patellar tendon reflex function were compared between 30 young adults and 15 older adults.

SETTING

Motor control research laboratory.

SUBJECTS

The young adults were volunteers of college age. The older adults (74.5 +/- 4.14 yr) were volunteers from the local community. All subjects were free of medications and neurological conditions that would affect normal neuromuscular responses.

MAIN OUTCOME MEASURES

A force-time curve analysis of the patellar tendon reflex response was used to assess the inhibition and facilitation of spinal reflexes. In the experimental protocol to assess spinal reflex inhibition, 100 Hz vibration was applied to the right quadriceps muscle. In another experimental protocol, spinal reflex facilitation was assessed using the Jendrassik maneuver. To perform the Jendrassik maneuver, subjects were instructed to grasp their hands together and to pull as hard as possible while breathing normally. After a 2-second count, the tendon tap was delivered to the right leg and the subject was instructed to relax. In both experimental protocols, control patellar tendon reflexes were collected.

RESULTS

Analysis of variance for reflex peak force revealed a significant 30% reduction in the amount of vibration-induced reflex inhibition with increasing age, and a similar 33% reduction in the amount of Jendrassik maneuver facilitation observed for the older adults as compared with the younger adults.

CONCLUSION

These results support the hypothesis that inhibitory and excitatory influences acting on the alpha motoneuron pool are different in young and older adults.

Distribution of Ia effects onto human hand muscle motoneurones as revealed using an H reflex technique

1. The possibility of eliciting H reflexes in relaxed hand muscles using a collision between the orthodromic impulses generated by magnetic cortical stimulation and the antidromic motor volley due to a supramaximal (SM) peripheral nerve stimulus was investigated in seven subjects. 2. Magnetic stimuli, applied through a circular coil (outer diameter, 13 cm) centred at the vertex, evoking EMG responses of 3-5 mV amplitude in the relaxed abductor digit minimi (ADM) muscle, and SM test stimuli to the ulnar nerve at the wrist producing a direct maximal motor response (Mmax) in the ADM muscle, were given either alone or combined. 3. In all subjects, combined cortical and SM ulnar stimulation produced a response after the Mmax with the latency of an H reflex evoked by the ulnar stimulus. This response occurred only within interstimulus intervals (1-20 ms) compatible with collision in the motor axons. The response behaved like an H reflex being time-locked to the SM ulnar stimulus, facilitated by voluntary activation of ADM muscle, depressed by vibration (4 s, 100 Hz) of ADM tendon and by a submotor-threshold ulnar nerve stimulus applied 50 and 80 ms before the combined stimulation, respectively. 4. In some subjects, it was also possible to distinguish an earlier response preceding the H reflex by 3 ms. Evidence is given that this response is probably of cortical origin. 5. Varying the intensity of magnetic stimulation resulted in a non-linear relationship between the H reflex size and the size of the cortical response. When the latter was between 5-25% of Mmax, H reflexes were small (2.5-7.5% of Mmax); with cortical responses between 25-50% of Mmax, there was a steep increase in H reflex amplitude (10-30% of Mmax). We suggest that this behaviour is due to an uneven distribution of Ia effects within the motoneurone pool.

Electrophysiological assessment of central and peripheral motor routes to the lingual muscles

Compound muscle action potentials (CMAPs) of the lingual muscles were recorded by especially devised bipolar surface electrodes placed on the tongue. Distinct responses were evoked in the tongue muscles by peripheral electrical stimulation of the hypoglossal nerve medial to the angle of the jaw and by transcranial magnetic stimulation of the motor cortex. With cortical stimulation during voluntary activation of the tongue muscles it proved easy to obtain responses with the characteristics of centrally evoked responses allowing reliable measurements of latencies and amplitudes. By contrast, responses from magnetic stimulation of the intracranial segment of the hypoglossal nerve were more difficult to obtain and the reproducibility was often not satisfactory. In a group of 20 healthy subjects the average distal motor latency of both sides from peripheral stimulation was 2.4 ms and the corresponding amplitude was 9.3 mV on the left and 8.6 mV on the right side (range 5.1-16.0 mV). Cortical stimulation gave responses with an average onset latency of 8.6 ms and 8.8 ms and an average amplitude of 1.8 mV and 2.6 mV on the left and right sides of the tongue respectively (range 0.7-5.6 mV). From this mean conduction times of 6.2 ms on the left and 6.4 ms on the right side (SD 1.0 ms) between cortex and mandibular angle and relative amplitudes from cortical stimulation as compared with the peripheral CMAP of 29% on the left and 21% on the right side (range 7%-66%) were calculated. In 16 patients it was possible to differentiate between a central (supranuclear) and a peripheral (intranuclear) site for the lesions of the motors routes to the lingual muscles and to show subclinical lesions in some cases. With a recording arrangement allowing selective unilateral recording of muscle activity from both sides of the tongue the assumed bihemispheric motor representation of the lingual muscles was confirmed.

Latency jump of "relaxed" versus "contracted" motor evoked potentials as a marker of cortico-spinal maturation

Magnetic brain stimulation was carried out in 17 children, aged from 2 to 12 years, in order to investigate the latency difference between relaxed and contracted motor evoked potentials (MEPs) as a function of age. While the latency of contracted MEPs increased in a linear fashion with age and body size, the relaxed MEP latency had a much slower "maturation," which gained the adult value at about 10-12 years of age in parallel with the acquisition of manual skills. The age-related variation of this "latency jump" appears to be a specific indicator of maturative phenomena relating to motor systems.

'Excitability changes of muscular responses to magnetic brain stimulation in patients with central motor disorders

The 'excitability' and 'conductivity' of motor pathways during transcranial stimulation (TCS) have been investigated in 49 patients affected by multiple sclerosis (34), amyotrophic lateral sclerosis (7), spino-cerebellar ataxia (3), primary lateral sclerosis (4) and brain metastasis (1). Hyper-reflexia, spasticity and weakness were correlated with the central motor conduction time (CCT) and with the threshold intensity of TCS required to produce a motor evoked potential (MEP). MEPs to magnetic TCS were recorded from hand and foot muscles during relaxation, contraction and after tendon vibration. Thresholds and CCTs of the patients were compared with those of 30 healthy controls. Increased threshold was found in 37 out of 49 patients (75.5%). Prolongation of the CCT was found in 38 out of 63 clinically affected upper limbs (60.3%) and in 56 out of 77 clinically affected lower limbs (72.7%). Absent motor responses to maximal TCS were found in 20 out of 98 lower limbs (20.4%). Excluding ALS patients (in whom there was a lower threshold for MEP elicitation), a significant linear correlation was found between prolonged CCT and increased threshold. While MEPs with prolonged CCTs have elevated TCS threshold, it is important to note that an elevated threshold was found in 14 out of 49 patients (28.5%) despite unchanged CCT. Spasticity and/or hyper-reflexia were more frequently associated with increased threshold than with prolonged CCT, while weakness was correlated equally well with both these parameters. In this respect magnetic TCS proves to represent a new tool for the detection of abnormal 'excitability' of the central motor tracts.

Standardization of facilitation of compound muscle action potentials evoked by magnetic stimulation of the cortex. Results in healthy volunteers and in patients with multiple sclerosis

To establish the importance of standardization of the facilitation of central motor conduction measured by magnetic stimulation we studied the effect of increasing voluntary muscle contraction on the central motor conduction time (CMCT) and motor evoked potential (MEP) amplitudes for 3 upper and 2 lower limb muscles. MEPs were elicited by magnetic stimulation of the cortex and the spinal roots. Muscle force was indirectly assessed from the integrated electrical muscle activity and expressed as the root mean square (RMS) and was varied from 0 to 40% of maximal activity. The central motor conduction time (CMCT) decreased during increasing muscle contraction, reaching constant values at approximately 10-20% RMSmax. Similarly, the increases of MEP amplitude tapered off at about the same RMS level. For each muscle an optimal RMS level was defined. The shortening of the CMCTs at the optimal RMS levels were: the brachial biceps, 3.4 msec; the radial carpal flexor of the wrist, 2.7 msec; the first dorsal interosseus muscle of the hand, 2.9 msec; the anterior tibial, 4.2 msec; and the abductor hallucis, 2.4 msec. The standardizing procedure was applied to 10 patients with multiple sclerosis. The stimulus thresholds were higher in these patients compared with those of the normals. Only the CMCT reduction of the BB was significantly larger (8.1 msec) than in the controls. Using standardized facilitation the diagnostic value of the amplitudes seems to be only a little less than that of the CMCTs.

Motor potentials evoked in tibialis anterior by single and paired cervical stimuli in man

Latencies of compound muscle action potentials evoked in lower limb muscles after transcranial stimulation shorten and amplitudes enlarge when the target muscle is contracted. After transcutaneous stimulation of the cervical cord with single electrical stimuli, the latency is not affected by muscle contraction. This is thought to be due to a lack of temporal summation at the spinal motoneurones. Results are different when paired stimuli are used. With contraction most motoneurones respond to the first cervical stimulus while in a relaxed condition they respond to the second. Temporal summation brings motoneurones to their firing threshold. Therefore, cervical double stimulation is more effective than single stimuli. It is however less powerful than cortical stimulation. Paired stimuli are most powerful at an interval of 2 ms similar to the D I-wave interval after cortical stimulation.

Influence of posture and voluntary background contraction upon compound muscle action potentials from anterior tibial and soleus muscle following transcranial magnetic stimulation

Compound muscle action potentials (CMAPs) were recorded from anterior tibial (TA) and soleus (SOL) muscles following transcranial magnetic stimulation of motor cortex in 10 healthy subjects: (1) while standing upright without support, (2) while sitting, and (3) while lying supine. The results of this study demonstrate a significant influence of posture upon the amplitudes of CMAPs. Postural facilitation presented itself, firstly, in terms of a higher incidence of bilateral activation of distal leg muscles during stance and, secondly, as significant enhancement of the amplitude of CMAPs while standing as compared to lying supine. The onset latency, however, did not disclose a significant shortening during stance. To assess the effects of preinnervation subjects voluntarily adjusted the level of TA activity to 5%, 10% and 20% of maximum isometric force respectively before cortex stimulation. Voluntary background contraction resulted in a significant increase of amplitude of CMAPs but, in contrast to postural facilitation, concomitant with a significant decrease in onset latency. These results point to a somewhat different mechanism of facilitation during stance as compared to voluntary preinnervation. But it cannot be decided whether cortical mechanisms, different descending systems, the spinal circuitry or a combination of these factors is responsible for the observed effects.

Motor evoked potentials and central motor conduction: studies of transcranial magnetic stimulation with recording from the leg

To determine central conduction times in the corticospinal pathways of humans using magnetic stimulation, we have developed a method for consistently recording conduction times between the motor cortex and the L4-5 level of the spinal cord. In 30 subjects, motor evoked potentials (MEPs) were recorded from the tibialis anterior muscle following contralateral motor cortex and peroneal nerve stimulation. In 18 of these subjects, the L4-5 intervertebral space was stimulated. The stimuli consisted of single, painless, short-duration magnetic pulses. In 12 subjects, measurements were made during voluntary ankle dorsiflexion, and during vibration of the TA tendon at rest. All subjects had measureable MEP latencies of 30.3 +/- 2.2 msec (mean +/- S.D.). The central motor conduction time (CMCT) was calculated using both a direct as well as an indirect method. The direct method in 18 subjects had a mean value of 16.2 +/- 1.7 msec, while the indirect method in all 30 subjects was 13.8 +/- 1.8 msec. No significant correlation of the CMCT was found with either age or height in these subjects. Ankle dorsiflexion significantly reduced the MEP latency and increased the amplitude, whereas vibration of the TA tendon significantly increased the amplitude alone. We conclude that MEPs may be consistently and painlessly measured in the lower extremity using magnetic stimulation in adults. Facilitation of the MEPs was produced more consistently by voluntary contraction than by vibratory stimulation of the tibialis anterior muscle tendon. Finally, CMCT was independent of both age and height in our study population.

Effects of electric and magnetic transcranial stimulation on long latency reflexes

The interaction of transcranial electric and magnetic brain stimulation with electrically elicited short- and long latency reflexes (LLR) of hand and forearm flexor muscles has been investigated in normal subjects. In the first paradigm, the motor potential evoked in thenar muscles by transcranial stimulation was conditioned by median nerve stimulation at various conditioning-test intervals. At short intervals (electric: 5-12.5 ms, magnetic: 0-7.5 ms) facilitation occurred that corresponded to the H-reflex and at longer intervals (electric: 25-40 ms, magnetic: 22.5-35 ms) there was a facilitation corresponding to the LLR. Electric and magnetic stimulation resulted in a similar degree of facilitation. A second paradigm investigated the facilitation of the forearm flexor H-reflex by a cutaneo-muscular LLR elicited by radial superficial nerve stimulation and transcranial stimulation used separately or together. When electric and magnetic brain stimulation were compared, magnetic brain stimulation was followed by significant extrafacilitation but electric stimulation was not. This result favours an interaction between the afferent volley eliciting the LLR and transcranial magnetic stimulation most likely at supraspinal level.

Responses of monkey precentral neurones to passive movements and phasic muscle stretch: relevance to man

Single cell recordings were made from movement-related neurones from the precentral cortex of two monkeys, trained to perform a simple lever-pulling task. They were also trained to remain relaxed while the arm was explored with passive movements at different joints, cutaneous stimuli and during the application of two types of phasic muscle stretch: percutaneous vibration and percussion of muscle tendons. Recordings were made of the responses of cortical neurones both to the 'natural' stimuli and to vibration of specific muscle tendons or percussion of the triceps tendon. Both tendon percussion and vibration excited neurones within area 4 with an average latency for tendon percussion of 21.0 msec. There was a high degree of consistency in the effects on single neurones of tendon percussion and vibration at the same site. Although long-term facilitation was not seen. vibration-induced discharge in the motor cortex should be considered as a potential mechanism of its effects in intact man. In contrast to the similarity of the effect of the two forms of phasic stretch, the relationship between a single neurone's response to either tendon percussion or vibration and to passive movement was complex. The dissociation seen between the effects of phasic muscle stretch and that of passive movement may underlie the failure, in man, to find uniformly increased long-latency stretch reflexes in clinical states of extrapyramidal rigidity.