Changes in intracortical excitability induced by stimulation of wrist afferents in man

The effects of local vibration inducing a tonic vibration reflex or movement illusion on acute modulations of corticospinal excitability

Stimulation of muscle afferents by local vibration (LV) can lead to two distinct perceptual and motor responses: the tonic vibration reflex (TVR) or the movement illusion. This study aimed to evaluate the effect of TVR and movement illusion on corticospinal excitability. In two experiments, EMG activity of the vibrated flexor carpi radialis (FCR) muscle (80 Hz, 6 min) and the extensor carpi radialis (ECR) muscle were recorded. Illusion was assessed using questionnaires. LV conditions were adjusted to favour either TVR (visual attention focused on the vibrating wrist) or ILLUSION (hidden hand, visual attention focused on the EMG of the FCR muscle). Motor-evoked potential (MEP) and cervicomedullary motor-evoked potential (CMEP) were recorded at rest for both muscles before (10 and 0 min) and after (0 and 30 min) each LV condition. Only the TVR condition increased EMG of the FCR muscle (+490% compared to resting, P = 0.005), while movement illusion was greater in the ILLUSION condition (P < 0.001). Concerning the vibrated muscle at P0, TVR reduced the amplitude of CMEP (-13.8 ± 15.8%, P = 0.011) without altering MEP (0.3 ± 27.9%, P = 1), whereas the opposite occurred with movement illusion (i.e. CMEP: -4.5 ± 13.7%, P = 0.891; MEP: -25.1 ± 17.2%, P = 0.002). Cortical excitability (MEP/CMEP ratio) of the vibrated muscle was reduced by 24 ± 13.3% on average compared to values obtained before LV, only in the ILLUSION condition. In conclusion, this study highlights the relevance of measuring and reporting the perceptual and motor responses induced during LV, demonstrating that TVR and movement illusion partly determine the acute effects on the neural network. KEY POINTS: Tonic vibration reflex and movement illusion are rarely controlled and measured in studies investigating the effect of LV on corticospinal excitability. The application of LV with visual attention focused on the vibrated muscle promotes the presence of a tonic vibration reflex (TVR). The absence of visual feedback on the latter promotes the presence of an illusion of movement. The cortical excitability of the vibrated muscle is influenced differently according to the perceptual and motor responses induced during LV, with an opposite effect on the cortical excitability of the antagonist muscle. Improved control of LV application conditions, quantification of perceptual and motor responses, and reporting of results (e.g. EMG activity of the vibrated muscle or illusion of movement during the protocol) are required to enhance our understanding of the physiological mechanisms associated with LV use and, consequently, the effectiveness of LV as a therapeutic modality.

Paired associative transspinal and transcortical stimulation produces plasticity in human cortical and spinal neuronal circuits

Anatomical, physiological, and functional connectivity exists between the neurons of the primary motor cortex (M1) and spinal cord. Paired associative stimulation (PAS) produces enduring changes in M1, based on the Hebbian principle of associative plasticity. The present study aimed to establish neurophysiological changes in human cortical and spinal neuronal circuits by pairing noninvasive transspinal stimulation with transcortical stimulation via transcranial magnetic stimulation (TMS). We delivered paired transspinal and transcortical stimulation for 40 min at precise interstimulus intervals, with TMS being delivered after (transspinal-transcortical PAS) or before (transcortical-transspinal PAS) transspinal stimulation. Transspinal-transcortical PAS markedly decreased intracortical inhibition, increased intracortical facilitation and M1 excitability with concomitant decreases of motor threshold, and reduced the soleus Hoffmann's reflex (H-reflex) low frequency-mediated homosynaptic depression. Transcortical-transspinal PAS did not affect intracortical circuits, decreased M1 excitability, and reduced the soleus H-reflex-paired stimulation pulses' mediated postactivation depression. Both protocols affected the excitation threshold of group Ia afferents and motor axons. These findings clearly indicate that the pairing of transspinal with transcortical stimulation produces cortical and spinal excitability changes based on the timing interval and functional network interactions between the two associated inputs. This new PAS paradigm may constitute a significant neuromodulation method with physiological impact, because it can be used to alter concomitantly excitability of intracortical circuits, corticospinal neurons, and spinal inhibition in humans.

Is the Frequency in Somatosensory Electrical Stimulation the Key Parameter in Modulating the Corticospinal Excitability of Healthy Volunteers and Stroke Patients with Spasticity?

Somatosensory electrical stimulation (SES) has been proposed as an approach to treat patients with sensory-motor impairment such as spasticity. However, there is still no consensus regarding which would be the adequate SES parameters to treat those deficits. Therefore, the aim of this study was to evaluate the effects of applying SES over the forearm muscles at four different frequencies of stimulation (3, 30, 150, and 300 Hz) and in two intervals of time (5′ and 30′) by means of transcranial magnetic stimulation and Hoffmann's reflex (H-reflex) in healthy volunteers (Experiments  I and II). A group of stroke patients (Experiment  III) was also preliminary evaluated to ascertain SES effects at a low frequency (3 Hz) applied for 30′ over the forearm spastic flexors muscles by measuring the wrist joint passive torque. Motor evoked potentials and the H-reflex were collected from different forearm and hand muscles immediately before and after SES and up to 5′ (Experiment  I) and 10′ (Experiments  I and II) later. None of the investigated frequencies of SES was able to operate as a key in switching modulatory effects in the central nervous system of healthy volunteers and stroke patients with spasticity.

Transspinal constant-current long-lasting stimulation: a new method to induce cortical and corticospinal plasticity

Functional neuroplasticity in response to stimulation and motor training is a well-established phenomenon. Transcutaneous stimulation of the spine is used mostly to alleviate pain, but it may also induce functional neuroplasticity, because the spinal cord serves as an integration center for descending and ascending neuronal signals. In this work, we examined whether long-lasting noninvasive cathodal (c-tsCCS) and anodal (a-tsCCS) transspinal constant-current stimulation over the thoracolumbar enlargement can induce cortical, corticospinal, and spinal neuroplasticity. Twelve healthy human subjects, blind to the stimulation protocol, were randomly assigned to 40 min of c-tsCCS or a-tsCCS. Before and after transspinal stimulation, we established the afferent-mediated motor evoked potential (MEP) facilitation and the subthreshold transcranial magnetic stimulation (TMS)-mediated flexor reflex facilitation. Recruitment input-output curves of MEPs and transspinal evoked potentials (TEPs) and postactivation depression of the soleus H reflex and TEPs was also established. We demonstrate that both c-tsCCS and a-tsCCS decrease the afferent-mediated MEP facilitation and alter the subthreshold TMS-mediated flexor reflex facilitation in a polarity-dependent manner. Both c-tsCCS and a-tsCCS increased the tibialis anterior MEPs recorded at 1.2 MEP resting threshold, intermediate, and maximal intensities and altered the recruitment input-output curve of TEPs in a muscle- and polarity-dependent manner. Soleus H-reflex postactivation depression was reduced after a-tsCCS and remained unaltered after c-tsCCS. No changes were found in the postactivation depression of TEPs after c-tsCCS or a-tsCCS. Our findings reveal that c-tsCCS and a-tsCCS have distinct effects on cortical and corticospinal excitability. This method can be utilized to induce targeted neuroplasticity in humans.

Convergence of flexor reflex and corticospinal inputs on tibialis anterior network in humans

OBJECTIVE

Integration between descending and ascending inputs at supraspinal and spinal levels is a key characteristic of neural control of movement. In this study, we characterized convergence of the flexor reflex and corticospinal inputs on the tibialis anterior (TA) network in healthy human subjects. Specifically, we characterized the modulation profiles of the spinal TA flexor reflex following subthreshold and suprathreshold transcranial magnetic stimulation (TMS). We also characterized the modulation profiles of the TA motor evoked potentials (MEPs) following medial arch foot stimulation at sensory and above reflex threshold.

METHODS

TA flexor reflexes were evoked following stimulation of the medial arch of the foot with a 30 ms pulse train at innocuous intensities. TA MEPs were evoked following TMS of the leg motor cortex area.

RESULTS

TMS at 0.7 and at 1.2 MEP resting threshold increased the TA flexor reflex when TMS was delivered 40-100 ms after foot stimulation, and decreased the TA flexor reflex when TMS was delivered 25-110 ms before foot stimulation. Foot stimulation at sensory and above flexor reflex threshold induced a similar time-dependent modulation in resting TA MEPs, that were facilitated when foot stimulation was delivered 40-100 ms before TMS. The flexor reflex and MEPs recorded from the medial hamstring muscle were modulated in a similar manner to that observed for the TA flexor reflex and MEP.

CONCLUSION

Cutaneomuscular afferents from the distal foot can increase the output of the leg motor cortex area. Descending motor volleys that directly or indirectly depolarize flexor motoneurons increase the output of the spinal FRA interneuronal network. The parallel facilitation of flexor MEPs and flexor reflexes is likely cortical in origin.

SIGNIFICANCE

Afferent mediated facilitation of corticospinal excitability can be utilized to strengthen motor cortex output in neurological disorders.

30 Hz theta-burst stimulation over primary somatosensory cortex modulates corticospinal output to the hand

BACKGROUND

The primary somatosensory cortex (SI) is important for hand function and has direct connectivity with the primary motor cortex (M1). Much of our present knowledge of this connectivity and its relevance to hand function is based on animal research. In humans, less is known about the neural mechanisms by which SI influences motor circuitry that outputs to the muscles controlling the hand.

OBJECTIVE

The present study investigated the influence of SI on corticospinal excitability, and inhibitory and excitatory intracortical neural circuitry within M1 before and after continuous theta-burst stimulation (cTBS). Motor-evoked potentials (MEPs), short-latency intracortical inhibition (SICI) and intracortical facilitation (ICF) were recorded from the first dorsal interosseous (RFDI) muscle of the right hand following 30 Hz cTBS over left-hemisphere SI and M1 delivered in separate sessions.

RESULTS

cTBS over SI facilitated MEPs and did not alter ICF or SICI. cTBS delivered over M1 suppressed MEPs and ICF and did not alter SICI.

CONCLUSIONS

These findings indicate that SI influences corticospinal output to the hand, possibly via corticocortical projections, and may be one mechanism by which somatosensory information influences hand control.

Modulation of transcallosal inhibition by bilateral activation of agonist and antagonist proximal arm muscles

Transcallosal inhibitory interactions between proximal representations in the primary motor cortex remain poorly understood. In this study, we used transcranial magnetic stimulation to examine the ipsilateral silent period (iSP; a measure of transcallosal inhibition) in the biceps and triceps brachii during unilateral and bilateral isometric voluntary contractions. Healthy volunteers performed 10% of maximal isometric voluntary elbow flexion or extension with one arm while the contralateral arm remained at rest or performed 30% of maximal isometric voluntary elbow flexion or extension. The iSP was measured in the arm performing 10% contractions, and electromyographic (EMG) recordings were comparable across conditions. The iSP onset and duration in the biceps and triceps brachii were comparable. In both muscles, the iSP depth and area were increased during bilateral contractions of homologous agonist muscles (extension-extension and flexion-flexion) compared with a unilateral contraction, whereas during bilateral contractions of nonhomologous antagonist muscles (extension-flexion and flexion-extension), the iSP depth and area were decreased compared with a unilateral contraction, and sometimes facilitation of EMG was seen. This effect was never observed during bilateral activation of homologous muscles. The size of responses evoked by cervicomedullary electrical stimulation in the arm that made 10% contractions remained unchanged across conditions. Thus transcallosal inhibition targeting triceps and biceps brachii is upregulated by voluntary contraction of the contralateral agonist muscle and downregulated by voluntary contraction of the contralateral antagonist muscle. We speculate that these reciprocal task-dependent interactions between bilateral flexor and extensor arm regions of the motor cortex may contribute to coupling between the arms during motor behavior.

Can surface neuromuscular electrical stimulation of the wrist and hand combined with routine therapy facilitate recovery of arm function in patients with stroke?

OBJECTIVE

To investigate whether treatment with surface neuromuscular electrical stimulation to the wrist extensors improves recovery of arm function in severely disabled patients with stroke.

DESIGN

Single blinded randomized controlled trial.

SETTING

Acute stroke unit and stroke rehabilitation wards of a university hospital.

PARTICIPANTS

Patients with no upper limb function (Action Research Arm Test [ARAT] score 0) (N=90; mean age ± SD, 74±11y; 49% men) were recruited to the study within 6 weeks of stroke. Only 67 participants were alive at the end of the study and data from 66 of these people were analyzed.

INTERVENTIONS

Participants were randomized to surface neuromuscular electrical stimulation using surface electrical stimulators for 30 minutes, twice in a working day for 6 weeks in addition to standardized upper limb therapy or just standardized upper limb therapy.

MAIN OUTCOME MEASURE

The primary outcome measure was the ARAT score. Assessments were made at baseline and at 6, 12, 24, and 36 weeks after recruitment.

RESULTS

There were statistically significant improvements in measures of wrist extensor (mean difference 0.5; 95% confidence interval [CI], 0.0-1.0) and grip strength (mean difference 0.9; 95% CI, 0.1-1.7) over the treatment period. Arm function (ARAT score) was not significantly different between the groups over the treatment period at 6 weeks (mean difference 1.9; 95% CI, -2.9 to 6.8) or over the study period at 36 weeks (mean difference 6.4; 95% CI, -1.8 to 14.7), and the rate of recovery was not significantly different (mean difference 0.7; 95% CI, -0.2 to 1.6).

CONCLUSIONS

In patients with severe stroke, with no functional arm movement, electrical stimulation of wrist extensors improves muscle strength for wrist extension and grip, and larger studies are required to study its influence on arm function.

Impaired crossed facilitation of the corticospinal pathway after cervical spinal cord injury

In uninjured humans, it is well established that voluntary contraction of muscles on one side of the body can facilitate transmission in the contralateral corticospinal pathway. This crossed facilitatory effect may favor interlimb coordination and motor performance. Whether this aspect of corticospinal function is preserved after chronic spinal cord injury (SCI) is unknown. Here, using transcranial magnetic stimulation, we show in patients with chronic cervical SCI (C(5)-C(8)) that the size of motor evoked potentials (MEPs) in a resting intrinsic hand muscle remained unchanged during increasing levels of voluntary contraction with a contralateral distal or proximal arm muscle. In contrast, MEP size in a resting hand muscle was increased during the same motor tasks in healthy control subjects. The magnitude of voluntary electromyography was negatively correlated with MEP size after chronic cervical SCI and positively correlated in healthy control subjects. To examine the mechanisms contributing to MEP crossed facilitation we examined short-interval intracortical inhibition (SICI), interhemispheric inhibition (IHI), and motoneuronal behavior by testing F waves and cervicomedullary MEPs (CMEPs). During strong voluntary contractions SICI was unchanged after cervical SCI and decreased in healthy control subjects compared with rest. F-wave amplitude and persistence and CMEP size remained unchanged after cervical SCI and increased in healthy control subjects compared with rest. In addition, during strong voluntary contractions IHI was unchanged in cervical SCI compared with rest. Our results indicate that GABAergic intracortical circuits, interhemispheric glutamatergic projections between motor cortices, and excitability of index finger motoneurons are neural mechanisms underlying, at least in part, the lack of crossed corticospinal facilitation observed after SCI. Our data point to the spinal motoneurons as a critical site for modulating corticospinal transmission after chronic cervical SCI.

Afferent Regulation of Leg Motor Cortex Excitability After Incomplete Spinal Cord Injury

An incomplete spinal cord injury (SCI) impairs neural conduction along spared ascending sensory pathways to disrupt the control of residual motor movements. To characterize how SCI affects the activation of the motor cortex by spared ascending sensory pathways, we examined how stimulation of leg afferents facilitates the excitability of the motor cortex in subjects with incomplete SCI. Homo- and heteronymous afferents to the tibialis anterior (TA) representation in the motor cortex were electrically stimulated, and the responses were compared with uninjured controls. In addition, we examined if cortical excitability could be transiently increased by repetitively pairing stimulation of spared ascending sensory pathways with transcranial magnetic stimulation (TMS), an intervention termed paired associative stimulation (PAS). In uninjured subjects, activating the tibial nerve at the ankle 45–50 ms before a TMS pulse in a conditioning-test paradigm facilitated the motor-evoked potential (MEP) in the heteronymous TA muscle by twofold on average. In contrast, prior tibial nerve stimulation did not facilitate the TA MEP in individuals with incomplete SCI ( n = 8 SCI subjects), even in subjects with less severe injuries. However, we provide evidence that ascending sensory inputs from the homonymous common peroneal nerve (CPN) can, unlike the heteronymous pathways, facilitate the motor cortex to modulate the TA MEP ( n = 16 SCI subjects) but only in subjects with less severe injuries. Finally, by repetitively coupling CPN stimulation with coincident TA motor cortex activation during PAS, we show that 7 of 13 SCI subjects produced appreciable (>20%) facilitation of the MEP following the intervention. The increase in corticospinal tract excitability by PAS was transient (<20 min) and tended to be more prevalent in SCI subjects with stronger functional ascending sensory pathways.

Interactions between short latency afferent inhibition and long interval intracortical inhibition

Peripheral nerve stimulation inhibits the motor cortex and the process has been termed afferent inhibition. Short latency afferent inhibition (SAI) at interstimulus intervals (ISI) of approximately 20 ms likely involves central cholinergic transmission and was found to be altered in Alzheimer's disease and Parkinson's disease. Cholinergic and GABA(A) receptors are involved in mediating SAI. The effects of SAI on other intracortical inhibitory and facilitatory circuits have not been examined. The objective of the present study is to test how SAI interacts with long interval cortical inhibition (LICI), a cortical inhibitory circuit likely mediated by GABA(B) receptors. We studied 10 healthy volunteers. Surface electromyogram was recorded from the first dorsal interosseous muscle. SAI was elicited by median nerve stimulation at the wrist followed by transcranial magnetic stimulation (TMS) at ISI of N20 somatosensory evoked potential latency + 3 ms. The effects of different test motor evoked potential (MEP) amplitudes (0.2, 1, and 2 mV) were examined for LICI and SAI. Using paired and triple-pulse paradigms, the interactions between SAI and LICI were investigated. Both LICI and SAI decreased with increasing test MEP amplitude. Afferent stimulation that produced SAI decreased LICI. Thus, the present findings suggest that LICI and SAI have inhibitory interactions.

Afferent-induced facilitation of primary motor cortex excitability in the region controlling hand muscles in humans

Sensory inputs from cutaneous and limb receptors are known to influence motor cortex network excitability. Although most recent studies have focused on the inhibitory influences of afferent inputs on arm motor responses evoked by transcranial magnetic stimulation (TMS), facilitatory effects are rarely considered. In the present work, we sought to establish how proprioceptive sensory inputs modulate the excitability of the primary motor cortex region controlling certain hand and wrist muscles. Suprathreshold TMS pulses were preceded either by median nerve stimulation (MNS) or index finger stimulation with interstimulus intervals (ISIs) ranging from 20 to 200 ms (with particular focus on 40-80 ms). Motor-evoked potentials recorded in the abductor pollicis brevis (APB), first dorsalis interosseus and extensor carpi radialis muscles were strongly facilitated (by up to 150%) by MNS with ISIs of around 60 ms, whereas digit stimulation had only a weak effect. When MNS was delivered at the interval that evoked the optimal facilitatory effect, the H-reflex amplitude remained unchanged and APB motor responses evoked with transcranial electric stimulation were not increased as compared with TMS. Afferent-induced facilitation and short-latency intracortical inhibition (SICI) and intracortical facilitation (ICF) mechanisms are likely to interact in cortical circuits, as suggested by the strong facilitation observed when MNS was delivered concurrently with ICF and the reduction of SICI following MNS. We conclude that afferent-induced facilitation is a mechanism which probably involves muscle spindle afferents and should be considered when studying sensorimotor integration mechanisms in healthy and disease situations.

Sensory dysfunction following stroke: incidence, significance, examination, and intervention

Recent studies have provided evidence of the widespread incidence of sensory dysfunction following stroke. The importance of these findings lies in the association between sensory loss poststroke and poorer outcomes in motor capacity, functional abilities, length of inpatient stay, and quality of life. Since literature suggests that clinicians can use information about clients' sensory status to predict rehabilitation outcomes and select appropriate interventions, the accuracy of somatosensory assessment is extremely clinically relevant. However, many of the clinical tests that are commonly used to examine sensation have not been found to be valid or reliable. Emerging evidence supports the efficacy of several interventions that target the sensory systems. This article reviews the incidence, significance, examination, and interventions for sensory dysfunction following stroke and summarizes the important characteristics of interventions directed at somatosensation.

Peripheral sensory activation of cortical circuits in the leg motor cortex of man

Peripheral sensory afferents in the hand activate both excitatory and inhibitory intracortical circuits to potentially facilitate and prune descending motor commands. In this study, we characterized how afferent inputs modulate the excitability of cortical circuits in the leg area of the primary motor cortex by examining how stimulation of the tibial nerve (TN) at the ankle alters motor evoked potentials (MEPs) activated by transcranial magnetic stimulation (TMS). Resting MEPs in the tibialis anterior (TA) muscle were facilitated in response to heteronymous activation of the TN 45-50 ms earlier, whereas MEPs were inhibited at interstimulus intervals of 32.5-37.5 ms. Similar time-dependent modulation occurred in the soleus (SOL) muscle with stimulation of the homonymous posterior tibial nerve (PTN) at the knee. To determine the site of this afferent-evoked facilitation and inhibition (spinal or cortical), we compared the effects of afferent stimulation to responses evoked at subcortical sites. At interstimulus intervals where MEP facilitation was observed (near 50 ms), spinal H-reflexes and responses evoked from corticospinal tract stimulation at the brainstem were predominantly depressed by the sensory stimulus suggesting that the observed MEP facilitation was cortical in origin. At interstimulus intervals where MEP depression was observed (near 35 ms), brainstem evoked responses were depressed to a similar degree and, in contrast to the hand, this suggests that spinal rather than cortical circuits mediate short-latency afferent inhibition (SAI) of leg MEPs. When the MEP was facilitated by afferent inputs, short-interval intracortical inhibition (SICI) was reduced and intracortical facilitation (ICF) was increased, but long-interval intracortical inhibition (LICI) at a 100 ms interval was unchanged. In addition, sensory excitation increased the recruitment of early, middle and late descending corticospinal volleys as evidenced from increases in MEP facilitation at the corresponding I-wave periodicity. We propose that sensory activation from the leg has a diffuse and predominantly facilitatory effect on the leg primary motor cortex.

Dynamic influence of wrist flexion and extension on the intracortical inhibition of the first dorsal interosseus muscle during precision grip

This work questioned further the influence of wrist movements on the control of precision grip. Seated subjects wearing a full-arm orthosis with the wrist and hand free were instructed to maintain a thumb/index finger opposition corresponding to 15% of maximal voluntary contraction for the first dorsal interosseus (FDI). Paired-pulse transcranial magnetic stimulation eliciting conditioned MEPs of FDI was used to determine the modulation of short intracortical inhibition (SICI) during cyclic active and passive wrist flexion and extension and during a static condition (no wrist movement, hand in the neutral position). The FDI active motor threshold (AMT) and the conditioning stimulus (0.8 AMT) were assessed in each series of FDI SICI measurements and the test stimulus (TS) was adjusted to match the amplitudes of test FDI MEPs across conditions. An increase of FDI background EMG during active wrist flexion compared to extension in some subjects did not influence FDI SICI as tested at matched EMG levels in the static condition. FDI SICI was reduced during wrist flexion (whether active or passive) compared to wrist extension, the latter being of equivalent FDI SICI as in the static condition. We suggest that wrist flexion and precision grip could be linked in a functional proximo-distal synergy. Indeed, coupling the activity between M1 sites of wrist flexors and FDI muscle via cortico-cortical disinhibition of FDI site may help recruit the interjoint synergy. Also, the salience of afferent information from wrist muscles may contribute to the phase-dependent modulation of SICI in the preactivated FDI muscle.

Dynamic changes in corticospinal control of precision grip during wrist movements

This work tested the physiological basis underlying the control of a proximo-distal muscle coordination. Using transcranial magnetic stimulation (TMS) of the hand territory within the primary motor cortex (M1), we examined whether the corticospinal excitability of the first dorsal interosseus muscle (FDI, index abductor), engaged in a precision grip, was altered during wrist movements. To this end, 12 seated subjects maintained a pinch between the right index finger and the thumb and FDI motor evoked potentials (MEPs) were elicited under four conditions: (1) during active and (2) passive cyclic wrist flexion/extension, (3) in three positions of static wrist flexion and extension, respectively, and (4) at three levels of isometric force of wrist flexors (FCR) and extensors (ECR) respectively. FDI MEPs were normalized relative to the MEP/EMG linear relationship. They were facilitated during wrist flexion in the active and the passive conditions and this did not depend on FDI background EMG. Interestingly, the occurrence of the most facilitated FDI MEPs was correlated only with the peak of FCR activity. Also, the duration of the post-MEP silent periods normalized to FDI MEP amplitudes was shorter during wrist flexion compared to extension. We discussed the extent to which the dynamic influence of wrist flexion on FDI corticospinal excitability reflects the existence of a proximo-distal synergy between wrist flexion and precision grip and whether this synergy relies on the phase-dependent recruitment of common M1 networks between FCR and FDI muscles and on the salience of proprioceptive afferents from wrist muscles.

Lack of inhibitory interaction between somatosensory afferent inputs and intracortical inhibitory interneurons in focal hand dystonia

We looked for an impaired interaction in the primary motor cortex between intracortical inhibitory circuits and circuits fed by somatosensory inputs in patients with writer's cramp. Short-interval intracortical inhibition (sICI) to wrist extensor carpi radialis muscle (ECR) was conditioned by stimulation of antagonist muscle afferents and sICI to first dorsal interosseus (FDI) muscle by homotopic cutaneous afferents stimulation. sICI was assessed at rest and during a tonic contraction of the target muscle. Eighteen patients with writer's cramp (10 having a wrist dystonic posture in flexion during writing and 8 in extension) were compared to 14 control subjects. Peripheral inputs decreased sICI in control subjects. This decrease was lost in patients in both FDI and ECR, regardless of the wrist dystonic posture. By contrast, contraction-induced depression of sICI appeared dependant on the dystonic status of the muscle: depression of sICI to ECR was abolished in patients with wrist dystonic posture in flexion, but not in patients with dystonic posture in extension, sICI even giving way to motor-evoked potential facilitation. Loss of interaction between interneurons mediating sICI and peripheral inputs probably belongs to the initial abnormalities underlying dystonia. Lack of peripherally induced sICI modulation may oppose wrist and/or hand muscles synergies.

Modulation of short-latency intracortical inhibition in human primary motor cortex during synchronised versus syncopated finger movements

Rhythmic movements are inherently more stable and easier to perform when they are synchronised with a periodic stimulus, as opposed to syncopated between the beats of a pacing stimulus. Although this behavioural phenomenon is well documented, its neurophysiological basis is poorly understood. In a first experiment, we demonstrated that all healthy subjects (N=8) performing index finger abduction in time with an auditory metronome exhibited transitions from syncopation to synchronisation when the metronome tempo was scaled from 0.8 to 2.0 Hz. Subjects' mean transition frequency was 1.7+/-0.2 Hz. In a second experiment, we used paired-pulse transcranial magnetic stimulation to examine short-latency intracortical inhibition (sICI) directed towards the first dorsal interosseous (FDI) muscle in healthy subjects (N=9) who made synchronised and syncopated phasic finger movements in time with metronome pacing of 1.0 Hz. Despite the equivalence between the patterns in terms of task performance and corticospinal excitability of FDI at this movement frequency, there was significantly greater sICI during syncopation than during synchronisation. From this result, we infer that the stability of movement patterns may be contingent upon excitability of inhibitory networks within primary motor cortex.

Amplitude of muscle stretch modulates corticomotor gain during passive movement

Previous studies have shown that the excitability of corticomotor projections to forearm muscles exhibit phasic modulation during passive movement (flexion-extension) about the wrist joint. We examined the stimulus-response properties of flexor carpi radialis (FCR) and extensor carpi radialis (ECR) to transcranial magnetic stimulation (TMS) applied over the contralateral motor cortex while the wrist was moved passively at two different sinusoidal frequency-amplitude relationships. Movement velocity (and therefore, the rate of change in muscle length) at the time of stimulation was held constant. Motor evoked potential (MEP) amplitudes were facilitated during passive muscle shortening and suppressed during passive muscle lengthening with suppression being more evident at higher stimulation intensities. For both FCR and ECR, during the shortening phase, responses were facilitated during the large amplitude movement relative to the small amplitude movement. It is suggested that the altered gain may be related to the thixotropic properties of muscle.

Exercise-induced changes of motor excitability with and without sensory block

To explore interactions between the sensory and motor system, we investigated motor excitability changes following a motor exercise with and without an anesthetic block of cutaneous inputs overlying the target muscle. Transcranial magnetic stimulation (TMS) with a focal coil was applied to determine motor output maps, intracortical inhibition (ICI) and intracortical facilitation (ICF) of the first dorsal interosseous muscle (FDI) on both sides. Twelve subjects performed phasic right index finger adductions (frequency: 0.333 Hz) for 30 min. TMS measurements were performed before and after the motor task (Experiment 1). In Experiment 2, median and radial nerve were blocked with Ropivacaine injections at the right wrist prior to the motor exercise. TMS was applied before and after induction of anesthesia and after exercise. In Experiment 3, the same anesthetic block was applied and TMS was performed before and after induction of anesthesia and after additional 30 min of rest. In Experiment 1, right FDI motor output area was enlarged, its center of gravity moved posteriorly, and ICI was reduced after the exercise. In Experiment 2, anesthesia was associated with a shrinkage of right FDI motor output area. After exercise, right FDI motor output area enlarged again but was still significantly smaller than pre-anesthesia. In both experiments, TMS results of left FDI remained unchanged. In Experiment 3, the anesthesia-induced decrease of right FDI motor output area remained unchanged after the period of rest. We conclude that a simple motor task enhanced the cortical representation of the target muscle and reduced intracortical inhibition. An impairment of cutaneous afferents decreased the cortical representation of the target muscle. The decrease of motor excitability induced by the sensory deficit could only partially be reversed by the motor exercise.

Disinhibition in the human motor cortex is enhanced by synchronous upper limb movements

The phasic modulation of wrist flexor corticomotor disinhibition has previously been demonstrated during the flexion phase of rhythmical passive flexion-extension of the human wrist. Here we ask if rhythmical bimanual flexion-extension movements of the wrists of neurologically intact subjects, modulate inhibitory activity in the motor cortex. In the first experiment intracortical inhibition was assessed when one wrist was passively flexed and extended on its own, with the addition of the opposite limb voluntarily moving synchronously in a mirror symmetric pattern, and also in a near-symmetric asynchronous pattern. Two subsequent experiments investigated firstly the modulation of spinal reflex pathway activity during the same three movement conditions, and secondly the effect of contralateral wrist movement alone on the excitability of corticomotoneuronal pathways to a static test limb. When the wrist flexors of both upper limbs were shortening simultaneously (i.e. synchronously), intracortical inhibition associated with flexor representations was suppressed to a greater extent than when the two muscles were shortening asynchronously. The results of the three experiments indicate that modulation of inhibitory activity was taking place at the cortical level. These findings may have further application in the study of rehabilitation procedures where the effects of simultaneous activation of affected and unaffected upper limbs in hemiparetic patients are to be investigated.

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.